Transmission of IP Packets over
Overlay Multilink Network (OMNI) InterfacesThe Boeing CompanyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftMobile nodes (e.g., aircraft of various configurations, terrestrial
vehicles, seagoing vessels, space systems, enterprise wireless devices,
pedestrians with cell phones, etc.) communicate with networked
correspondents over multiple access network data links and configure
mobile routers to connect end user networks. A multilink virtual
interface specification is presented that enables mobile nodes to
coordinate with a network-based mobility service and/or with other
mobile node peers. The virtual interface provides an adaptation layer
service that also applies for more static deployments such as enterprise
and home networks. This document specifies the transmission of IP
packets over Overlay Multilink Network (OMNI) Interfaces.Mobile nodes (e.g., aircraft of various configurations, terrestrial
vehicles, seagoing vessels, space systems, enterprise wireless devices,
pedestrians with cellphones, etc.) configure mobile routers with
multiple interface connections to wireless and/or wired-line data links.
These data links may have diverse performance, cost and availability
properties that can change dynamically according to mobility patterns,
flight phases, proximity to infrastructure, etc. The mobile router acts
as a Client of a network-based Mobility Service (MS) by configuring a
virtual interface over its underlay interface data link connections.Each Client configures a virtual interface (termed the "Overlay
Multilink Network Interface (OMNI)") as a thin layer over its underlay
network interfaces (which may themselves connect to virtual or physical
links). The OMNI interface is therefore the only interface abstraction
exposed to the IP layer and behaves according to the Non-Broadcast,
Multiple Access (NBMA) interface principle, while underlay interfaces
appear as link layer communication channels in the architecture. The
OMNI interface internally employs the "OMNI Adaptation Layer (OAL)" to
ensure that original IP packets are adapted to diverse underlay
interfaces with heterogeneous properties.The OMNI interface connects to a virtual overlay known as the "OMNI
link". The OMNI link multinet service spans one or more Internetworks
that may include private-use infrastructures (e.g., enterprise networks)
and/or the global public Internet itself. Together, OMNI and the OAL
provide the foundational elements required to support the "6M's of
modern Internetworking", including:Multilink – a Client's ability to coordinate multiple
diverse underlay interfaces as a single logical unit (i.e., the OMNI
interface) to achieve the required communications performance and
reliability objectives.Multinet – the ability to span the OMNI link over a segment
routing topology with multiple diverse administrative domain network
segments while maintaining seamless end-to-end communications
between mobile Clients and correspondents such as air traffic
controllers, fleet administrators, etc.Mobility – a Client’s ability to change network
points of attachment (e.g., moving between wireless base stations)
which may result in an underlay interface address change, but
without disruptions to ongoing communication sessions with peers
over the OMNI link.Multicast – the ability to send a single network
transmission that reaches multiple Clients belonging to the same
interest group, but without disturbing other Clients not subscribed
to the interest group.Multihop – a mobile Client vehicle-to-vehicle relaying
capability useful when multiple forwarding hops between vehicles may
be necessary to “reach back” to an infrastructure access
point connection to the OMNI link.MTU assurance – the ability to deliver packets of various
robust sizes between peers without loss due to a link size
restriction, and to dynamically adjust packets sizes to achieve the
optimal performance for each independent traffic flow.Client OMNI interfaces interact with the MS and/or other OMNI nodes
through IPv6 Neighbor Discovery (ND) control message exchanges . The MS consists of a distributed set of service
nodes (including Proxy/Servers and other infrastructure elements) that
also configure OMNI interfaces. Automatic Extended Route Optimization
(AERO) in particular provides a companion MS compatible with the OMNI
architecture . AERO discusses
details of ND message based route optimization, mobility management, and
multinet traversal while the fundamental aspects of OMNI link operation
are discussed in this document.Each OMNI interface provides a multilink nexus for exchanging inbound
and outbound traffic via selected underlay interface(s). The IP layer
sees the OMNI interface as a point of connection to the OMNI link. Each
OMNI link has one or more associated Mobility Service Prefixes (MSPs),
which are typically IP Global Unicast Address (GUA) prefixes assigned to
the link and from which Mobile Network Prefixes (MNPs) are derived. If
there are multiple OMNI links, the IP layer will see multiple OMNI
interfaces.Each Client receives an MNP through IPv6 ND control message exchanges
with Proxy/Servers over Access Networks (ANETs) and/or open
Internetworks (INETs). The Client sub-delegates the MNP to
downstream-attached End-user Networks (ENETs) independently of the
underlay interfaces selected for data transport. The Client acts as a
fixed or mobile router on behalf of peers on its ENETs, and uses OMNI
interface control messaging to coordinate with Hosts, Proxy/Servers
and/or other Clients. The Client iterates its control messaging over
each of the OMNI interface's ANET/INET underlay interfaces in order to
register each interface with the MS (see ). The
Client can also provide Proxy/Server-like services for a recursively
nested chain of other Clients located in downstream-attached ENETs.Clients may connect to multiple distinct OMNI links within the same
OMNI domain by configuring multiple OMNI interfaces, e.g., omni0, omni1,
omni2, etc. Each OMNI interface is configured over a set of underlay
interfaces and provides a nexus for Safety-Based Multilink (SBM)
operation. The IP layer applies SBM routing to select a specific OMNI
interface, then the selected OMNI interface applies Performance-Based
Multilink (PBM) internally to select appropriate underlay interfaces.
Applications select SBM topologies based on IP layer Segment Routing
, while each OMNI interface orchestrates PBM
internally based on OMNI layer Segment Routing.OMNI provides a link model suitable for a wide range of use cases.
For example, the International Civil Aviation Organization (ICAO)
Working Group-I Mobility Subgroup is developing a future Aeronautical
Telecommunications Network with Internet Protocol Services (ATN/IPS) and
has issued a liaison statement requesting IETF adoption in support of ICAO Document 9896 . The IETF IP Wireless Access in Vehicular
Environments (ipwave) working group has further included problem
statement and use case analysis for OMNI in a document now in AD
evaluation for RFC publication . Still other communities
of interest include AEEC, RTCA Special Committee 228 (SC-228) and NASA
programs that examine commercial aviation, Urban Air Mobility (UAM) and
Unmanned Air Systems (UAS). Pedestrians with handheld devices represent
another large class of potential OMNI users.This document specifies the transmission of IP packets and control
messages over OMNI interfaces. The operation of both IP protocol
versions (i.e., IPv4 and IPv6 ) is specified as the network layer data plane, while
OMNI interfaces use IPv6 ND messaging in the control plane independently
of the data plane protocol(s). OMNI interfaces also provide an OAL based
on encapsulation and fragmentation over heterogeneous underlay
interfaces as an adaptation sublayer between L3 and L2. Both OMNI and
the OAL are specified in detail throughout the remainder of this
document.The terminology in the normative references applies; especially, the
terms "link" and "interface" are the same as defined in the IPv6 and IPv6 Neighbor Discovery (ND) specifications. Additionally, this document assumes
the following IPv6 ND message types: Router Solicitation (RS), Router
Advertisement (RA), Neighbor Solicitation (NS), Neighbor Advertisement
(NA) and Redirect. Hosts, Clients and Proxy/Servers that implement IPv6
ND maintain per-neighbor state in Neighbor Cache Entries (NCEs). Each
NCE is indexed by the neighbor's network layer address(es) while the
neighbor's OAL encapsulation address provides context for Identification
verification.The Protocol Constants defined in Section 10 of are used in their same format and meaning in this
document. The terms "All-Routers multicast", "All-Nodes multicast" and
"Subnet-Router anycast" are the same as defined in (with Link-Local scope assumed).The term "IP" is used to refer collectively to either Internet
Protocol version (i.e., IPv4 or IPv6 ) when a specification at the layer in question
applies equally to either version.The following terms are defined within the scope of this
document:The Data Link layer in the OSI network
model. Also known as "layer-2", "link-layer", "sub-IP layer",
etc.The Network layer in the OSI network
model. Also known as "layer-3", "IP layer", etc.A mid-layer that adapts L3
to a diverse collection of L2 underlay interfaces and their
encapsulations. (No layer number is assigned, since numbering was an
artifact of the legacy reference model that need not carry forward
in the modern architecture.) The adaptation layer sees the upper
layer as "L3" and sees all lower layer encapsulations as "L2
encapsulations", which may include UDP, IP and true link-layer
(e.g., Ethernet, etc.) headers.a connected network
region (e.g., an aviation radio access network, satellite service
provider network, cellular operator network, WiFi network, etc.)
that connects Clients to the Mobility Service. Physical and/or data
link level security is assumed, and sometimes referred to as
"protected spectrum". Private enterprise networks and ground domain
aviation service networks may provide multiple secured IP hops
between the Client's point of connection and the nearest
Proxy/Server.a connected network
region with a coherent IP addressing plan that provides transit
forwarding services between ANETs and/or OMNI nodes that coordinate
with the Mobility Service over unprotected media. Since physical
and/or data link level security cannot always be assumed, security
must be applied by upper layers if necessary. The global public
Internet itself is an example.a simple or complex
"downstream" network that travels with the Client as a single
logical unit. The ENET could be as simple as a single link
connecting a single Host, or as complex as a large network with many
links, routers, bridges and Hosts. The ENET could also provide an
"upstream" link in a recursively-descending chain of additional
Clients and ENETs. In this way, an ENET of an upstream Client is
seen as the ANET of a downstream Client.a Client's attachment to
a link in an {A,I,E}NET.a "wildcard" term used when a given
specification applies equally to both ANET/INET cases. From the
Client's perspective, *NET interfaces are "upstream" interfaces that
connect the Client to the Mobility Service, while ENET interfaces
are "downstream" interfaces that the Client uses to connect
downstream ENETs, Hosts and/or other Clients.an ANET/INET/ENET
interface over which an OMNI interface is configured. The OMNI
interface is seen as a L3 interface by the IP layer, and each
underlay interface is seen as a L2 interface by the OMNI interface.
The underlay interface either connects directly to the physical
communications media or coordinates with another node where the
physical media is hosted.a Non-Broadcast, Multiple Access
(NBMA) virtual overlay configured over one or more INETs and their
connected ANETs/ENETs. An OMNI link may comprise multiple distinct
"segments" joined by L2 forwarding devices the same as for any link;
the addressing plans in each segment may be mutually exclusive and
managed by different administrative entities. Proxy/Servers and
other infrastructure elements extend the link to support
communications between Clients as single-hop neighbors.a node's attachment to an OMNI
link, and configured over one or more underlay interfaces. If there
are multiple OMNI links in an OMNI domain, a separate OMNI interface
is configured for each link. The OMNI interface configures a Maximum
Transmission Unit (MTU) and a Maximum Reassembly Unit (MRU) the same
as any interface.an OMNI interface
sublayer service that encapsulates original IP packets admitted into
the interface in an IPv6 header and/or subjects them to
fragmentation and reassembly. The OAL is also responsible for
generating MTU-related control messages as necessary, and for
providing addressing context for OMNI link SRT traversal. The OAL
presents a new layer in the Internet architecture known simply as
the "adaptation layer".an end user device that extends the OMNI
link over an ENET interface serviced by a Client. (As an
implementation matter, the Host either assigns the same IP address
from the ENET (underlay) interface to an (overlay) OMNI interface,
or configures an OMNI-like function as a virtual sublayer of the
ENET interface itself.) The IP addresses assigned to each Host ENET
interface remain stable even if the Client's upstream *NET interface
connections change.a network platform/device mobile
router that configures one or more OMNI interfaces over distinct
sets of underlay interfaces grouped as logical OMNI link units. The
Client coordinates with the Mobility Service via upstream networks
over *NET interfaces, and provides Proxy/Server services for Hosts
and other Clients on ENET interface downstream networks. The
Client's *NET interface addresses and performance characteristics
may change over time (e.g., due to node mobility, link quality,
etc.) while downstream-attached Hosts and other Clients see the ENET
as a stable ANET.a segment routing topology edge
node that configures an OMNI interface and connects Clients to the
Mobility Service. As a server, the Proxy/Server responds directly to
some Client IPv6 ND messages. As a proxy, the Proxy/Server forwards
other Client IPv6 ND messages to other Proxy/Servers and Clients. As
a router, the Proxy/Server provides a forwarding service for
ordinary data packets that may be essential in some environments and
a last resort in others. Proxy/Servers at ANET boundaries configure
both an ANET downstream interface and *NET upstream interface, while
INET-based Proxy/Servers configure only an INET interface.a
Proxy/Server connected to the source Client's *NET that forwards
packets sent by the source into the segment routing topology. FHS
Proxy/Servers also act as intermediate forwarding nodes to
facilitate RS/RA exchanges between Clients and Hub
Proxy/Servers.a
Proxy/Server connected to the target Client's *NET that forwards
packets received from the segment routing topology to the
target.a single Proxy/Server
selected by the Client that provides a designated router service for
all of the Client's*NET underlay networks. Since all Proxy/Servers
provide equivalent services, Clients normally select the first FHS
Proxy/Server they coordinate with to serve as the Hub. However, the
Hub can also be any available Proxy/Server for the OMNI link, i.e.,
and not necessarily one of the Client's FHS Proxy/Servers.a multinet
forwarding region configured over one or more INETs between the FHS
Proxy/Server and LHS Proxy/Server. The SRT spans the OMNI link on
behalf of source/target Client pairs using segment routing in a
manner outside the scope of this document (see: ).a mobile routing
service that tracks Client movements and ensures that Clients remain
continuously reachable even across mobility events. The MS consists
of the set of all Proxy/Servers and any other OMNI link supporting
infrastructure nodes. Specific MS details are out of scope for this
document, with an example found in .an aggregated
IP Global Unicast Address (GUA) prefix (e.g., 2001:db8::/32,
192.0.2.0/24, etc.) assigned to the OMNI link and from which
more-specific Mobile Network Prefixes (MNPs) are delegated. OMNI
link administrators typically obtain MSPs from an Internet address
registry, however private-use prefixes can also be used subject to
certain limitations (see: ). OMNI links that
connect to the global Internet advertise their MSPs to their
interdomain routing peers.a longer IP
prefix delegated from an MSP (e.g., 2001:db8:1000:2000::/56,
192.0.2.8/30, etc.) and assigned to a Client. Clients receive MNPs
from Proxy/Servers and sub-delegate them to routers, Hosts and other
Clients located in ENETs.a whole IP packet or
fragment admitted into the OMNI interface by the network layer prior
to OAL encapsulation and fragmentation, or an IP packet delivered to
the network layer by the OMNI interface following OAL decapsulation
and reassembly.an original IP packet encapsulated
in an IPv6 header (i.e., the OAL header) then submitted for OAL
fragmentation and reassembly.a portion of an OAL packet
following fragmentation but prior to encapsulation, or following
encapsulation but prior to OAL reassembly.an OAL packet that does
not require fragmentation is always encapsulated as an "atomic
fragment" with a Fragment Header with Fragment Offset and More
Fragments both set to 0, but with a valid Identification value.an encapsulated OAL
fragment following L2 encapsulation or prior to L2 decapsulation.
OAL sources and destinations exchange carrier packets over underlay
interfaces, and may be separated by one or more OAL intermediate
nodes. OAL intermediate nodes may perform re-encapsulation on
carrier packets by removing the L2 headers of the first hop network
and replacing them with new L2 headers for the next hop network.
(The term "carrier" honors agents of the service postulated by and .)an OMNI interface acts as an OAL
source when it encapsulates original IP packets to form OAL packets,
then performs OAL fragmentation and encapsulation to create carrier
packets.an OMNI interface acts as an
OAL destination when it decapsulates carrier packets, then performs
OAL reassembly and decapsulation to derive the original IP
packet.an OMNI interface acts
as an OAL intermediate node when it removes the L2 encapsulation
headers of carrier packets received on a first segment, then
re-encapsulates the carrier packets in new L2 encapsulation headers
and forwards them into the next segment.an IPv6 Neighbor Discovery option
providing multilink parameters for the OMNI interface as specified
in .the least
significant 64 bits of an IPv6 address, as specified in the IPv6
addressing architecture .an IPv6 address
beginning with fe80::/64 per the IPv6 addressing architecture and with either a 64-bit MNP (LLA-MNP) or a
56-bit random value (LLA-RND) encoded in the IID as specified in
.an IPv6 address
beginning with fd00::/8 followed by a 40-bit Global ID followed by a
16-bit Subnet ID per and with either a
64-bit MNP (ULA-MNP) or a 56-bit random value (ULA-RND) encoded in
the IID as specified in . (Note that
specifies a second form of ULAs based on
the prefix fc00::/8, which are referred to as "ULA-C" throughout
this document to distinguish them from the ULAs defined here.)a ULA beginning
with fd00::/16 followed by a 48-bit randomly-initialized value
followed by an MNP-based (TLA-MNP) or random (TLA-RND) IID as
specified in . Clients use TLAs to
bootstrap autoconfiguration in the presence of OMNI link
infrastructure or for sustained communications in the absence of
infrastructure. (Note that in some environments Clients can instead
use a (Hierarchical) Host Identity Tag ((H)HIT) instead of a TLA -
see: .)a TLA beginning
with fd00::/64 followed by an MNP-based (XLA-MNP) or random
(XLA-RND) IID as specified in . An XLA
is simply a TLA with an all-0 48-bit value following fd00::/16, and
can be used to supply a "wildcard match" for IPv6 ND cache entries,
a routing table entry for the OMNI link routing system, etc. (Note
that XLAs can also be statelessly formed from LLAs (and vice-versa)
simply by inverting prefix bits 7 and 8.)a Client OMNI interface's manner of
managing multiple diverse *NET underlay interfaces as a single
logical unit. The OMNI interface provides a single unified interface
to upper layers, while underlay interface selections are performed
on a per-packet basis considering traffic selectors such as DSCP,
flow label, application policy, signal quality, cost, etc. Multilink
selections are coordinated in both the outbound and inbound
directions based on source/target underlay interface pairs.an intermediate node's manner of
spanning multiple diverse IP Internetwork and/or private enterprise
network "segments" at the OAL layer below IP. Through intermediate
node concatenation of SRT network segments, multiple diverse
Internetworks (such as the global public IPv4 and IPv6 Internets)
can serve as transit segments in an end-to-end L2 forwarding path.
This OAL concatenation capability provides benefits such as
supporting IPv4/IPv6 transition and coexistence, joining multiple
diverse operator networks into a cooperative single service network,
etc. See: for further
information.an iterative relaying of IP packets
between Client's over an OMNI underlay interface technology (such as
omnidirectional wireless) without support of fixed infrastructure.
Multihop services entail Client-to-Client relaying within a
Mobile/Vehicular Ad-hoc Network (MANET/VANET) for Vehicle-to-Vehicle
(V2V) communications and/or for Vehicle-to-Infrastructure (V2I)
"range extension" where Clients within range of communications
infrastructure elements provide forwarding services for other
Clients.any action that results in a change
to a Client underlay interface address. The change could be due to,
e.g., a handover to a new wireless base station, loss of link due to
signal fading, an actual physical node movement, etc.A means for
ensuring fault tolerance through redundancy by connecting multiple
OMNI interfaces within the same domain to independent routing
topologies (i.e., multiple independent OMNI links).A means for
selecting one or more underlay interface(s) for packet transmission
and reception within a single OMNI interface.The set of all SBM/PBM OMNI links
that collectively provides services for a common set of MSPs. All
OMNI links within the same domain configure, advertise and respond
to the same OMNI IPv6 Anycast address(es).A
forwarding table on each OMNI source, destination and intermediate
node that includes Multilink Forwarding Vectors (MFV) with both next
hop forwarding instructions and context for reconstructing
compressed headers for specific underlay interface pairs used to
communicate with peers. See:
for further discussion.An MFIB
entry that includes soft state for each underlay interface pairwise
communication session between peers. MFVs are identified by both a
next-hop and previous-hop MFV Index (MFVI), with the next-hop
established based on an IPv6 ND solicitation and the previous hop
established based on the solicited IPv6 ND advertisement response.
See: for further
discussion.A
locally-unique 4 octet value that an OMNI node generates when it
creates an MFV, then advertises to either next-hop or previous-hop
nodes. OMNI intermediate nodes assign two distinct MFVIs for each
MFV and advertise one to next-hops and the other to previous-hops.
OMNI end systems assign and advertise a single MFVI. See: for further discussion.an IPv4 or IPv6 packet with a
Jumbo Payload option that includes a 32-bit length field to be used
instead of the 16-bit {Total, Payload} Length field (see: ). For IPv4, the Total Length field must be set to
the length of the IPv4 header only. For IPv6, the Payload Length
must be set to 0.a special form of an IP Jumbogram
with a segment length value included in the {Total, Payload} Length
field and also with a Jumbo Payload option (see: ).the IP address (and also the UDP port
number when UDP is used) that appears in (L2) encapsulation headers
in the data plane and in IPv6 ND OMNI option sub-options in the
control plane.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when, and only when,
they appear in all capitals, as shown here.An implementation is not required to internally use the architectural
constructs described here so long as its external behavior is consistent
with that described in this document.An OMNI interface is a virtual interface configured over one or more
underlay interfaces, which may be physical (e.g., an aeronautical radio
link, etc.) or virtual (e.g., an Internet or higher-layer "tunnel"). The
OMNI interface architectural layering model is the same as in , and augmented as shown in
. The IP layer therefore sees the OMNI interface
as a single L3 interface nexus for multiple underlay interfaces that
appear as L2 communication channels in the architecture.Each underlay interface provides an L2/L1 abstraction according to
one of the following models:ANET interfaces connect to a protected and secured ANET that is
separated from the open INET by Proxy/Servers. The ANET interface
may be either on the same L2 link segment as a Proxy/Server, or
separated from a Proxy/Server by multiple IP hops. (Note that NATs
may appear internally within an ANET or on the Proxy/Server itself
and may require NAT traversal the same as for the INET case.)INET interfaces connect to an INET either natively or through one
or several IPv4 Network Address Translators (NATs). Native INET
interfaces have global IP addresses that are reachable from any INET
correspondent. NATed INET interfaces typically configure private IP
addresses and connect to a private network behind one or more NATs
with the outermost NAT providing INET access.ENET interfaces connect a Client's downstream-attached networks,
where the Client provides forwarding services for ENET Host and
Client communications to remote peers. An ENET may be as simple as a
small stub network that travels with a mobile Client (e.g., an
Internet-of-Things) to as complex as a large private enterprise
network that the Client connects to a larger ANET or INET.
Downstream-attached Hosts and Clients see the ENET as an ANET and
see the (upstream) Client as a Proxy/Server.VPNed interfaces use security encapsulation over an underlay
network to a Client or Proxy/Server acting as a Virtual Private
Network (VPN) gateway. Other than the link-layer encapsulation
format, VPNed interfaces behave the same as for Direct
interfaces.Direct (aka "point-to-point") interfaces connect directly to a
Client or Proxy/Server without crossing any networked paths. An
example is a line-of-sight link between a remote pilot and an
unmanned aircraft.The OMNI interface forwards original IP packets from the
network layer (L3) using the OMNI Adaptation Layer (OAL) (see: ) as an encapsulation and fragmentation sublayer
service. This "OAL source" then further encapsulates the resulting OAL
packets/fragments in underlay network headers (e.g., UDP/IP, IP-only,
Ethernet-only, etc.) to create L2-encapsulated "carrier packets" for
transmission over underlay interfaces. The target OMNI interface
receives the carrier packets from underlay interfaces and discards the
L2 encapsulation headers. If the resulting OAL packets/fragments are
addressed to itself, the OMNI interface acts as an "OAL destination" and
performs reassembly if necessary, discards the OAL encapsulation, and
delivers the original IP packet to the network layer. If the OAL
fragments are addressed to another node, the OMNI interface instead acts
as an "OAL intermediate node" by re-encapsulating the carrier packets in
new underlay network L2 headers and forwarding them over an underlay
interface without reassembling or discarding the OAL encapsulation. The
OAL source and OAL destination are seen as "neighbors" on the OMNI link,
while OAL intermediate nodes provide a virtual bridging service that
joins the segments of a (multinet) Segment Routing Topology (SRT).The OMNI interface can forward original IP packets over underlay
interfaces while including/omitting various lower layer encapsulations
including OAL, UDP, IP and Ethernet (ETH) or other link-layer header.
The network layer can also access the underlay interfaces directly while
bypassing the OMNI interface entirely when necessary. This architectural
flexibility may be beneficial for underlay interfaces (e.g., some
aviation data links) for which encapsulation overhead may be a primary
consideration. OMNI interfaces that send original IP packets directly
over underlay interfaces without invoking the OAL can only reach peers
located on the same OMNI link segment. Source Clients can instead use
the OAL to coordinate with target Clients in the same or different OMNI
link segments by sending initial carrier packets to a First-Hop Segment
(FHS) Proxy/Server. The FHS Proxy/Sever then forwards the packets into
the SRT spanning tree, which transports them to a Last-Hop Segment (LHS)
Proxy/Server for the target Client.Original IP packets sent directly over underlay interfaces are
subject to the same path MTU related issues as for any Internetworking
path, and do not include per-packet identifications that can be used for
data origin verification and/or link-layer retransmissions. Original IP
packets presented directly to an underlay interface that exceed the
underlay network path MTU are dropped with an ordinary ICMPv6 Packet Too
Big (PTB) message returned. These PTB messages are subject to loss the same as for any non-OMNI IP interface.The OMNI interface encapsulation/decapsulation layering possibilities
are shown in below. Imaginary vertical
lines drawn between the Network Layer and Underlay interfaces in the
figure denote the encapsulation/decapsulation layering combinations
possible. Common combinations include IP-only (i.e., direct access to
underlay interfaces with or without using the OMNI interface), IP/IP,
IP/UDP/IP, IP/UDP/IP/ETH(ERNET), IP/OAL/UDP/IP, IP/OAL/UDP/ETH,
etc.The OMNI/OAL model gives rise to a number of opportunities:Clients receive MNPs from the MS, and coordinate with the MS
through IPv6 ND message exchanges with Proxy/Servers. Clients use
the MNP to construct a unique Link-Local Address (LLA-MNP) through
the algorithmic derivation specified in and assign the LLA to the OMNI interface.
Since LLA-MNPs are uniquely derived from an MNP, no Duplicate
Address Detection (DAD) or Multicast Listener Discovery (MLD)
messaging is necessary.since Temporary ULAs with random IIDs (TLA-RNDs) are
statistically unique, they can be used without DAD until an MNP is
obtained.underlay interfaces on the same L2 link segment as a Proxy/Server
do not require any L3 addresses (i.e., not even link-local) in
environments where communications are coordinated entirely over the
OMNI interface.as underlay interface properties change (e.g., link quality,
cost, availability, etc.), any active interface can be used to
update the profiles of multiple additional interfaces in a single
message. This allows for timely adaptation and service continuity
under dynamically changing conditions.coordinating underlay interfaces in this way allows them to be
represented in a unified MS profile with provisions for mobility and
multilink operations.exposing a single virtual interface abstraction to the IPv6 layer
allows for multilink operation (including QoS based link selection,
packet replication, load balancing, etc.) at L2 while still
permitting L3 traffic shaping based on, e.g., DSCP, flow label,
etc.the OMNI interface allows multinet traversal over the SRT when
communications across different administrative domain network
segments are necessary. This mode of operation would not be possible
via direct communications over the underlay interfaces
themselves.the OAL supports lossless and adaptive path MTU mitigations not
available for communications directly over the underlay interfaces
themselves. The OAL supports "packing" of multiple IP payload
packets within a single OAL "super-packet" and also supports
transmission of IP packets and parcels of all sizes up to and
including Jumbograms.the OAL applies per-packet identification values that allow for
link-layer reliability and data origin authentication.L3 sees the OMNI interface as a point of connection to the OMNI
link; if there are multiple OMNI links, L3 will see multiple OMNI
interfaces.Multiple independent OMNI interfaces can be used for increased
fault tolerance through Safety-Based Multilink (SBM), with
Performance-Based Multilink (PBM) applied within each interface.Multiple independent OMNI links can be joined together into a
single link without requiring renumbering of infrastructure
elements, since the ULAs assigned to the different links will be
mutually exclusive.the OMNI/OAL model supports transmission of a new form of IP
packets known as "IP Parcels" that improve performance and
efficiency for both upper layer protocols and networked paths.Note that even when the OMNI virtual interface is present,
applications can still access underlay interfaces either through the
network protocol stack using an Internet socket or directly using a raw
socket. This allows for intra-network (or point-to-point) communications
without invoking the OMNI interface and/or OAL. For example, when an
OMNI interface is configured over an underlay IP interface, applications
can still invoke intra-network IP communications directly over the
underlay interface as long as the communicating endpoints are not
subject to mobility dynamics. depicts the architectural model for a
source Client with an attached ENET connecting to the OMNI link via
multiple independent ANETs/INETs (i.e., *NETs). The Client's OMNI
interface sends IPv6 ND solicitation messages over available *NET
underlay interfaces using any necessary L2 encapsulations. The IPv6 ND
messages traverse the *NETs until they reach an FHS Proxy/Server (FHS#1,
FHS#2, ..., FHS#n), which returns an IPv6 ND advertisement message
and/or forwards a proxyed version of the message over the SRT to an LHS
Proxy/Server near the target Client (LHS#1, LHS#2, ..., LHS#m). The Hop
Limit in IPv6 ND messages is not decremented due to encapsulation;
hence, the source and target Client OMNI interfaces appear to be
attached to a common link.After the initial IPv6 ND message exchange, the source Client (as
well as any nodes on its attached ENETs) can send packets to the target
Client over the OMNI interface. OMNI interface multilink services will
forward the packets via FHS Proxy/Servers for the correct underlay
*NETs. The FHS Proxy/Server then forwards the packets over the SRT which
delivers them to an LHS Proxy/Server, and the LHS Proxy/Server in turn
forwards them to the target Client. (Note that when the source and
target Client are on the same SRT segment, the FHS and LHS Proxy/Servers
may be one and the same.)Clients select a Hub Proxy/Server (not shown in the figure), which
will often be one of their FHS Proxy/Servers but could also be any
Proxy/Server on the OMNI link. Clients then register all of their *NET
underlay interfaces with the Hub Proxy/Server via the FHS Proxy/Server
in a pure proxy role. The Hub Proxy/Server then provides a designated
router service for the Client, and the Client can quickly migrate to a
new Hub Proxy/Server if the first becomes unresponsive.Clients therefore use Proxy/Servers as gateways into the SRT to reach
OMNI link correspondents via a spanning tree established in a manner
outside the scope of this document. Proxy/Servers forward critical MS
control messages via the secured spanning tree and forward other
messages via the unsecured spanning tree (see Security Considerations).
When route optimization is applied as discussed in , Clients can instead forward directly
to SRT intermediate nodes (or directly to correspondents in the same SRT
segment) to reduce Proxy/Server load.Note: while not shown in the figure, a Client's ENET may connect many
additional Hosts and even other Clients in a recursive extension of the
OMNI link. This OMNI virtual link extension will be discussed more fully
throughout the document.The OMNI interface observes the link nature of tunnels, including the
Maximum Transmission Unit (MTU), Maximum Reassembly Unit (MRU) and the
role of fragmentation and reassembly . The OMNI interface is configured
over one or more underlay interfaces as discussed in , where the interfaces (and their associated underlay
network paths) may have diverse MTUs. OMNI interface considerations for
accommodating original IP packets of various sizes are discussed in the
following sections.IPv6 underlay interfaces are REQUIRED to configure a minimum MTU of
1280 octets and a minimum MRU of 1500 octets .
Therefore, the minimum IPv6 path MTU is 1280 octets since routers on the
path are not permitted to perform network fragmentation even though the
destination is required to reassemble more. The network therefore MUST
forward original IP packets of at least 1280 octets without generating
an IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) message . (While the source can apply "source fragmentation"
for locally-generated IPv6 packets up to 1500 octets and larger still if
it knows the destination configures a larger MRU, this does not affect
the minimum IPv6 path MTU.)IPv4 underlay interfaces are REQUIRED to configure a minimum MTU of
68 octets and a minimum MRU of 576 octets . Therefore, when the Don't
Fragment (DF) bit in the IPv4 header is set to 0 the minimum IPv4 path
MTU is 576 octets since routers on the path support network
fragmentation and the destination is required to reassemble at least
that much. The OMNI interface therefore MUST set DF to 0 in the IPv4
encapsulation headers of carrier packets that are no larger than 576
octets, and SHOULD set DF to 1 in larger carrier packets unless it has a
way to determine the encapsulation destination MRU and has carefully
considered the issues discussed in .When the network layer admits an original IP packet into the OMNI
interface the OAL prepends an IPv6 encapsulation header (see: ) where the 16-bit Payload Length field limits the
maximum-sized original IP packet to (2**16 -1) = 65535 octets; this is
also the maximum size that the OAL can accommodate with IPv6
fragmentation. The OMNI interface therefore sets an MTU and MRU of 65535
octets to support assured delivery of original packets no larger than
this size even if IPv6 fragmentation is required. (The OMNI interface
MAY set a larger MTU to support best-effort delivery for larger packets;
see below.) The OMNI interface then employs the OAL as an encapsulation
sublayer service to transform original IP packets into OAL
packets/fragments, and the OAL in turn uses underlay network
encapsulation to forward carrier packets over underlay interfaces (see:
).While the maximum-sized original IP packet that the OAL can
accommodate using IPv6 fragmentation is 65535 octets, OMNI interfaces
can forward still larger IPv6 packets as OAL "atomic fragments"
through the application of IPv6 Jumbograms .
For such larger packets, the OMNI interface performs OAL encapsulation
by appending an IPv6 header followed by an 8-octet Hop-By-Hop header
with Jumbo Payload option followed by a Routing Header of no more than
40-octets (if necessary) and finally followed by an 8-octet Fragment
Header.Since the Jumbo Payload option includes a 32-bit length field, OMNI
interfaces can therefore configure a larger IP MTU up to a maximum of
((2**32 - 1) - 8 - 40 - 8) = 4294967239 octets. In that case, the OAL
will still provide original IP packets no larger than 65535 with an
IPv6 fragmentation-based assured delivery service while larger IP
packets will receive a best-effort delivery service as atomic
fragments (note that the OAL destination is permitted to accept atomic
fragments that exceed the OMNI interface MRU).The OAL source forwards jumbo atomic fragments under the assumption
that upper and lower layers will employ sufficient integrity
assurance, noting that commonly-used 32-bit CRCs may be inadequate for
these larger sizes . If the packet is dropped
along the path to the OAL destination, the OAL source must arrange to
return a PTB "hard error" to the original source .This document notes that a Jumbogram service for IPv4 is also
specified in , where all
OMNI link aspects of the service are conducted in a similar fashion as
for IPv6 above.As specified in , an IP
Parcel is a variation of the IP Jumbogram construction beginning with
an IP header with the length of the first upper layer protocol segment
in the {Total, Payload} Length field, but with a Jumbo Payload option
with a length that may be the same as or larger than the length in the
IP header. The differences in these lengths determines the size and
number of upper layer protocol segments within the parcel.The IP Parcel format and transmission/reception procedures for OMNI
interfaces are specified in . End systems
that implement either the full OMNI interface (i.e., Clients) or
enough of the OAL to process parcels (i.e., Hosts) are permitted to
exchange parcels with consenting peers.When an OMNI interface forwards an original IP packet from the
network layer for transmission over one or more underlay interfaces, the
OMNI Adaptation Layer (OAL) acting as the OAL source applies
encapsulation to form OAL packets subject to fragmentation producing OAL
fragments suitable for L2 encapsulation and transmission as carrier
packets over underlay interfaces as described in .These carrier packets travel over one or more underlay networks
spanned by OAL intermediate nodes in the SRT, which re-encapsulate by
removing the L2 headers of the first underlay network and appending L2
headers appropriate for the next underlay network in succession. (This
process supports the multinet concatenation capability needed for
joining multiple diverse networks.) After re-encapsulation by zero or
more OAL intermediate nodes, the carrier packets arrive at the OAL
destination.When the OAL destination receives the carrier packets, it discards
the L2 headers and reassembles the resulting OAL fragments (if
necessary) into an OAL packet as described in .
The OAL destination next decapsulates the OAL packet to obtain the
original IP packet then delivers the original IP packet to the network
layer. The OAL source may be either the source Client or its FHS
Proxy/Server, while the OAL destination may be either the LHS
Proxy/Server or the target Client. Proxy/Servers (and SRT Gateways as
discussed in ) may also serve as
OAL intermediate nodes.The OAL presents an OMNI sublayer abstraction similar to ATM
Adaptation Layer 5 (AAL5). Unlike AAL5 which performs segmentation and
reassembly with fixed-length 53 octet cells over ATM networks, however,
the OAL uses IPv6 encapsulation, fragmentation and reassembly with
larger variable-length cells over heterogeneous underlay networks.
Detailed operations of the OAL are specified in the following
sections.When the network layer forwards an original IP packet into the OMNI
interface, the OAL source creates an "OAL packet" by prepending an
IPv6 OAL encapsulation header per but does
not decrement the Hop Limit/TTL of the original IP packet since
encapsulation occurs at a layer below IP forwarding. The OAL source
copies the "Type of Service/Traffic Class"
and "Explicit Congestion Notification (ECN)"
values in the original packet's IP header into the corresponding
fields in the OAL header, then sets the OAL header "Flow Label" as
specified in . The OAL source finally sets the
OAL header IPv6 Payload Length to the length of the original IP packet
and sets Hop Limit to a value that MUST NOT be larger than 63 yet is
still sufficiently large to enable loop-free forwarding over multiple
concatenated OMNI link intermediate hops.The OAL next selects OAL packet source and destination addresses.
Client OMNI interfaces set the OAL source address to a Unique Local
Address (ULA) based on the Mobile Network Prefix (ULA-MNP). When a
Client OMNI interface does not (yet) have a ULA prefix and/or an MNP
suffix, it can instead use a Temporary ULA (TLA) (or a (Hierarchical)
Host Identity Tag ((H)HIT - see: ) as an OAL
address. Finally, when the Client needs to express its MNP outside the
context of a specific ULA prefix, it can use an eXtended ULA (XLA).
Proxy/Server OMNI interfaces instead set the source address to a
Random ULA (ULA-RND) (see: ), but also
process packets with anycast and/or multicast OAL addresses that they
are configured to recognize.)The OAL source next selects a 32-bit OAL packet Identification
value as specified in . The OAL then calculates
a 2-octet OAL checksum using the algorithm specified in . The OAL source calculates the checksum over the
OAL packet beginning with a pseudo-header of the OAL header similar to
that found in Section 8.1 of , then extending
over the entire length of the original IP packet. The OAL
pseudo-header is formed as shown in :After calculating the checksum, the OAL source next fragments the
OAL packet if necessary while assuming the IPv4 minimum path MTU
(i.e., 576 octets) as the worst case for OAL fragmentation regardless
of the underlay interface IP protocol version since IPv6/IPv4 protocol
translation and/or IPv6-in-IPv4 encapsulation may occur in any
underlay network path. By initially assuming the IPv4 minimum even for
IPv6 underlay interfaces, the OAL source may produce smaller fragments
with additional encapsulation overhead but avoids loss due to
presenting an underlay interface with a carrier packet that exceeds
its MRU. Additionally, the OAL path could traverse multiple SRT
segments with intermediate OAL forwarding nodes performing
re-encapsulation where the L2 encapsulation of the previous segment is
replaced by the L2 encapsulation of the next segment which may be
based on a different IP protocol version and/or encapsulation
sizes.The OAL source therefore assumes a default minimum path MTU of 576
octets at each SRT segment for the purpose of generating OAL fragments
for L2 encapsulation and transmission as carrier packets. Each
successive SRT intermediate node may include either a 20 octet IPv4 or
40 octet IPv6 header, an 8 octet UDP header and in some cases an IP
security encapsulation (40 octets maximum assumed) during
re-encapsulation. Intermediate nodes at any SRT segment may also
insert or modify the Routing Header (40 octets maximum) following the
40 octet OAL IPv6 header and preceding the 8 octet Fragment Header.
Therefore, assuming a worst case of (40 + 40 + 8) = 88 octets for L2
encapsulations plus (40 + 40 + 8) = 88 octets for OAL encapsulation
leaves no less than (576 - 88 - 88) = 400 octets remaining to
accommodate a portion of the original IP packet/fragment. The OAL
source therefore sets a minimum Maximum Payload Size (MPS) of 400
octets as the basis for the minimum-sized OAL fragment that can be
assured of traversing all SRT segments without loss due to an MTU/MRU
restriction. The Maximum Fragment Size (MFS) for OAL fragmentation is
therefore determined by the MPS plus the size of the OAL encapsulation
headers.The OAL source SHOULD maintain "path MPS" values for individual OAL
destinations initialized to the minimum MPS and increased to larger
values if better information is known or discovered. For example, when
peers share a common underlay network link or a fixed path with a
known larger MTU, the OAL source can set path MPS to a larger size
(i.e., greater than 400 octets) as long as the peer reassembles before
re-encapsulating and forwarding (while re-fragmenting if necessary).
Also, if the OAL source has a way of knowing the maximum L2
encapsulation size for all SRT segments along the path it may be able
to increase path MPS to reserve additional room for payload data. Even
when OAL header compression is used, the OAL source must include the
uncompressed OAL header size in its path MPS calculation since it may
need to include a full header at any time.The OAL source can also optimistically set a larger path MPS and/or
actively probe individual OAL destinations to discover larger sizes
using packetization layer probes in a similar fashion as , but care must be taken to
avoid setting static values for dynamically changing paths leading to
black holes. The probe involves sending an OAL packet larger than the
current path MPS and receiving a small acknowledgement response (with
the possible receipt of link-layer error message when a probe is
lost). For this purpose, the OAL source can send an NS message with
one or more OMNI options with large PadN sub-options (see: ) and/or with a trailing large NULL packet in a
super-packet (see: ) in order to receive a
small NA response from the OAL destination. While observing the
minimum MPS will always result in robust and secure behavior, the OAL
source should optimize path MPS values when more efficient utilization
may result in better performance (e.g. for wireless aviation data
links). The OAL source should maintain separate path MPS values for
each (source, target) underlay interface pair for the same OAL
destination, since different underlay interface pairs may support
differing path MPS values.When the OAL source performs fragmentation, it SHOULD produce the
minimum number of non-overlapping fragments under current MPS
constraints, where each non-final fragment MUST be at least as large
as the minimum MPS, while the final fragment MAY be smaller. The OAL
source also converts all original IP packets no larger than the
current MPS (or larger than 65535 octets) into atomic fragments by
including a Fragment Header with Fragment Offset and More Fragments
both set to 0. The OAL source then inserts a Routing Header (if
necessary) following the IPv6 encapsulation header and before the
Fragment Header. If the original IP packet is larger than 65535, the
OAL source also inserts a Hop-By-Hop header with Jumbo Payload option
immediately following the IPv6 encapsulation header and before the
Routing Header (if necessary), then includes an (atomic) Fragment
Header. The header extension order for each fragment therefore appears
as the OAL IPv6 header followed by Hop-By-Hop header followed by
Routing Header followed by Fragment Header.The OAL source next appends the OAL checksum as the final two
octets of the final fragment while increasing its (Jumbo) Payload
Length by 2. If appending the checksum would cause the final fragment
to exceed the current MPS, the OAL source instead reduces this
"former" final fragment's Payload Length (PL) by (N*8 + (PL mod 8))
octets, where N is an integer that would result in a non-zero
reduction but without causing the former final fragment to become
smaller than the minimum MPS. The OAL source then creates a "new"
final fragment by copying the OAL IPv6 header and extension headers
from the former final fragment, then copying the (N*8 + (PL mod 8))
octets from the end of the former final fragment immediately following
the new final fragment extension headers. The OAL source then sets the
former final fragment's More Fragments flag to 1, increments the new
final fragment's fragment offset by the former final fragment's new
(PL / 8) and finally appends the checksum the same as discussed
above.Next, the OAL source replaces the IPv6 Fragment Header 1-octet
"Reserved" field (and for first fragments also the 2-bit "Reserved
Flags" field) with OMNI-specific encodings as shown in:For the first fragment, the OAL source sets the "(A)RQ" flag then
sets "Parcel ID", "(P)arcel" and "(S)ub-Parcels" as specified in . For each non-first fragment, the OAL source
instead sets the "(A)RQ" flag and writes a monotonically-increasing
"Ordinal" value between 1 and 127. Specifically, the OAL source writes
the ordinal number '1' for the first non-first fragment, '2' for the
second, '3' for the third, etc. up to the final fragment or the
ordinal value '127', whichever comes first. (For any additional
non-first fragments beyond ordinal '127', the OAL source instead
writes the value '0' in the Ordinal field and clears the "(A)RQ" flag.
The first fragment is implicitly always considered ordinal number '0'
even though the header does not include an explicit Ordinal
field.)The OAL source finally encapsulates the fragments in L2 headers to
form carrier packets and forwards them over an underlay interface,
while retaining the fragments and their ordinal numbers (i.e., #0, #1,
#2, etc. up to #127) for a brief period to support link-layer
retransmissions (see: ). OAL fragment and
carrier packet formats are shown in .Note: the minimum MPS assumes that any middleboxes (e.g.
IPv4 NATs) that connect private networks with path MTUs smaller than
576 octets must reassemble any fragmented (outbound) IPv4 carrier
packets sent by OAL sources before forwarding them to external
Internetworks since middleboxes that connect OAL destinations often
unconditionally drop (inbound) IPv4 fragments. However, when the path
MTU in the destination private network is small, the OAL destination
itself will be able to reassemble any IPv4 fragmentation that occurs
in the inbound path.The OAL source or intermediate node next encapsulates each OAL
fragment (with either full or compressed headers) in L2 encapsulation
headers to create a carrier packet. The OAL source or intermediate
node (i.e., the L2 source) includes a UDP header as the innermost
sublayer if NAT traversal and/or packet filtering middlebox traversal
are required; otherwise, the L2 source includes a full/compressed IP
header and/or an actual link-layer header (e.g., such as for
Ethernet-compatible links). The L2 source then appends any additional
encapsulation sublayer headers necessary and presents the resulting
carrier packet to an underlay interface, where the underlay network
conveys it to a next-hop OAL intermediate node or destination (i.e.,
the L2 destination).The L2 source encapsulates the OAL information immediately
following the innermost L2 sublayer header. If the first four bits of
the encapsulated OAL information following the innermost sublayer
header encode the value '6', the information must include an
uncompressed IPv6 header (plus extensions) followed by upper layer
protocol headers and data. If the first four bits encode the value
'4', an uncompressed IPv4 header (plus extensions) followed by upper
layer protocol headers and data follows. Otherwise, the first four
bits include a "Type" value, and the OAL information appears in an
alternate format as specified in (Types '0' and
'1' are currently specified while all other values are reserved for
future use). Carrier packets that contain an unrecognized Type value
are unconditionally dropped.The OAL node prepares the innermost L2 encapsulation header for OAL
packets as follows:For UDP encapsulation, the L2 source sets the UDP source port
to 8060 (i.e., the port number reserved for AERO/OMNI). When the
L2 destination is a Proxy/Server or Gateway, the L2 source sets
the UDP destination port to 8060; otherwise, the L2 source sets
the UDP destination port to its cached port number value for the
peer. The L2 source finally sets the UDP Length the same as
specified in . (If the OAL packet includes
an IP Jumbogram, the L2 source instead sets the UDP length to 0
and includes a Jumbo Payload option in the L2 IP header.)For IP encapsulation, the L2 source sets the IP {Protocol,
Next-Header} to TBD1 (see: IANA Considerations) and sets the
{Total, Payload} Length the same as specified in or . (If the OAL packet
includes a true Jumbogram, the L2 source includes a Jumbo Payload
option and sets {Total, Payload} Length plus the Jumbo Payload
length according to the OAL length information.)For direct encapsulations over Ethernet-compatible links, the
L2 source sets EtherType to TBD2 (see: IANA Considerations). Since
the Ethernet header does not include a length field, for the OMNI
EtherType the Ethernet header is followed by a four-octet Payload
Length field followed immediately by the encapsulated OAL
information. The Payload Length field encodes the length in octets
(in network byte order) of the OAL information exclusive of the
lengths of the Ethernet header and trailer.When an L2 source includes a UDP header, it SHOULD calculate and
include a UDP checksum in carrier packets with full OAL headers to
prevent mis-delivery, and MAY disable UDP checksums in carrier packets
with compressed OAL headers (see: ). If the L2
source discovers that a path is dropping carrier packets with UDP
checksums disabled, it should enable UDP checksums in future carrier
packets sent to the same L2 destination. If the L2 source discovers
that a path is dropping carrier packets that do not include a UDP
header, it should include a UDP header in future carrier packets.When an L2 source sends carrier packets with compressed OAL headers
and with UDP checksums disabled, mis-delivery due to corruption of the
4-octet Multilink Forwarding Vector Index (MFVI) is possible but
unlikely since the corrupted index would somehow have to match valid
state in the (sparsely-populated) Multilink Forwarding Information
Based (MFIB). In the unlikely event that a match occurs, an OAL
destination may receive a mis-delivered carrier packet but can
immediately reject packets with an incorrect Identification. If the
Identification value is somehow accepted, the OAL destination may
submit the mis-delivered carrier packet to the reassembly cache where
it will most likely be rejected due to incorrect reassembly
parameters. If a reassembly that includes the mis-delivered carrier
packets somehow succeeds (or, for atomic fragments) the OAL
destination will verify the OAL checksum to detect corruption.
Finally, any spurious data that somehow eludes all prior checks will
be detected and rejected by end-to-end upper layer integrity checks.
See: for further
discussion.For L2 encapsulations over IP, when the L2 source is also the OAL
source it next copies the "Type of Service/Traffic Class" and "Explicit Congestion Notification (ECN)" values in the OAL header into the corresponding
fields in the L2 IP header, then (for IPv6) set the L2 IPv6 header
"Flow Label" as specified in . The L2 source
then sets the L2 IP TTL/Hop Limit the same as for any host (i.e., it
does not copy the Hop Limit value from the OAL header) and finally
sets the source and destination IP addresses to direct the carrier
packet to the next hop. For carrier packets undergoing
re-encapsulation, the OAL intermediate node L2 source decrements the
OAL header Hop Limit and discards the carrier packet if the value
reaches 0. The L2 source then copies the "Type of Service/Traffic
Class" and "Explicit Congestion Notification (ECN)" values from the
previous hop L2 encapsulation header into the OAL header (if present),
then finally sets the source and destination IP addresses the same as
above.Following L2 encapsulation/re-encapsulation, the L2 source forwards
the resulting carrier packets over one or more underlay interfaces.
The underlay interfaces often connect directly to physical media on
the local platform (e.g., a laptop computer with WiFi, etc.), but in
some configurations the physical media may be hosted on a separate
Local Area Network (LAN) node. In that case, the OMNI interface can
establish a Layer-2 VLAN or a point-to-point tunnel (at a layer below
the underlay interface) to the node hosting the physical media. The
OMNI interface may also apply encapsulation at the underlay interface
layer (e.g., as for a tunnel virtual interface) such that carrier
packets would appear "double-encapsulated" on the LAN; the node
hosting the physical media in turn removes the LAN encapsulation prior
to transmission or inserts it following reception. Finally, the
underlay interface must monitor the node hosting the physical media
(e.g., through periodic keepalives) so that it can convey
up/down/status information to the OMNI interface.When an OMNI interface receives a carrier packet from an underlay
interface, it copies the ECN value from the L2 encapsulation headers
into the OAL header if the carrier packet contains a first-fragment.
The OMNI interface next discards the L2 encapsulation headers and
examines the OAL header of the enclosed OAL fragment. If the OAL
fragment is addressed to a different node, the OMNI interface (acting
as an OAL intermediate node) re-encapsulates and forwards while
decrementing the OAL Hop Limit as discussed in .
If the OAL fragment is addressed to itself, the OMNI interface (acting
as an OAL destination) accepts or drops the fragment based on the
(Source, Destination, Identification)-tuple and/or integrity
checks.The OAL destination next drops all non-final OAL fragments smaller
than the minimum MPS and all fragments that would overlap or leave
"holes" smaller than the minimum MPS with respect to other fragments
already received. The OAL destination updates a checklist of accepted
fragments of the same OAL packet that include an Ordinal number (i.e.,
Ordinals 0 through 127), but admits all accepted fragments into the
reassembly cache after first removing any extension headers except for
the fragment header itself. When the OAL destination receives the
final fragment (i.e., the one with More Fragments set to 0), it caches
the trailing checksum and reduces the Payload Length by 2. When
reassembly is complete, the OAL destination verifies the OAL packet
checksum and discards the packet if the checksum is incorrect. If the
OAL packet was accepted, the OAL destination finally removes the OAL
headers and delivers the original IP packet to the network layer.Carrier packets often travel over paths where all links in the path
include CRC-32 integrity checks for effective hop-by-hop error
detection for payload sizes up to 9180 octets ,
but other paths may traverse links (such as tunnels over IPv4) that do
not include adequate integrity protection. The OAL checksum therefore
allows OAL destinations to detect reassembly misassociation splicing
errors and/or carrier packet corruption caused by unprotected links
.The OAL checksum also provides algorithmic diversity with respect
to both lower layer CRCs and upper layer Internet checksums as part of
a complimentary multi-layer integrity assurance architecture. Any
corruption not detected by lower layer integrity checks is therefore
very likely to be detected by upper layer integrity checks that use
diverse algorithms.OAL sources that send carrier packets with full OAL headers include
a CRH-32 extension for segment-by-segment forwarding based on a
Multilink Forwarding Information Base (MFIB) in each OAL intermediate
node. OAL source, intermediate and destination nodes can instead
establish header compression state through IPv6 ND NS/NA message
exchanges. After an initial NS/NA exchange, OAL nodes can apply OAL
Header Compression to significantly reduce encapsulation overhead.Each OAL node establishes MFIB soft state entries known as
Multilink Forwarding Vectors (MVFs) which support both carrier packet
forwarding and OAL header compression/decompression. For FHS OAL
sources, each MFV is referenced by a single Multilink Forwarding
Vector Index (MFVI) that provides compression/decompression and
forwarding context for the next hop. For LHS OAL destinations, the MFV
is referenced by a single MFVI that provides context for the previous
hop. For OAL intermediate nodes, the MFV is referenced by two MFVIs -
one for the previous hop and one for the next hop.When an OAL node forwards carrier packets to a next hop, it can
include a full OAL header with a CRH-32 extension containing one or
more MVFIs. Whenever possible, however, the OAL node should instead
omit significant portions of the OAL header (including the CRH-32)
while applying OAL header compression. The full or compressed OAL
header follows immediately after the innermost L2 encapsulation (i.e.,
UDP, IP or L2) as discussed in . Two OAL
compressed header types (Types '0' and '1') are currently specified
below (note that the (A)RQ flag is always considered set and therefore
omitted from the compressed headers themselves).For OAL first-fragments (including atomic fragments), the OAL node
uses OMNI Compressed Header - Type 0 (OCH-0) format as shown in :The format begins with a 4-bit Type, a 6-bit Hop Limit, a
2-bit Explicit Congestion Notification (ECN) field, a 7-bit Parcel ID
and 5 flag bits. The format concludes with a 4-octet Identification
field followed (optionally) by a 4-octet MFVI field. The OAL node sets
Type to the value 0, sets Hop Limit to the minimum of the uncompressed
OAL header Hop Limit and 63, sets ECN the same as for an uncompressed
OAL header, and sets (Parcel ID, (P)arcel, (S)ub-parcels, (M)ore
Fragments, Identification) the same as for an uncompressed fragment
header. The OAL node finally sets Inde(X) and includes an MFVI if
necessary; otherwise, it clears Inde(X) and omits the MFVI. (The
(R)eserved flag is set to 0 on transmission and ignored on
reception.)The OAL first fragment (beginning with the original IP header) is
then included immediately following the OCH-0 header, and the L2
header length field is reduced by the difference in length between the
compressed headers and full-length OAL IPv6 and Fragment headers. The
OAL destination can therefore determine the Payload Length by
examining the L2 header length field and/or the length field(s) in the
original IP header. The OCH-0 format applies for first fragments only,
which are always regarded as ordinal fragment 0 even though no
explicit Ordinal field is included. The (A)RQ flag is always
implicitly set, and therefore omitted from the OCH-0 header.For OAL non-first fragments (i.e., those with non-zero Fragment
Offsets), the OAL uses OMNI Compressed Header - Type 1 (OCH-1) format
as shown in :The format begins with a 4-bit Type, a 6-bit Hop Limit, a
7-bit Ordinal, a 13-bit Fragment Offset and 2 flag bits. The format
concludes with a 4-octet Identification field followed (optionally) by
a 4-octet MFVI field. The OAL node sets Type to the value 1, sets Hop
Limit to the minimum of the uncompressed OAL header Hop Limit and 63,
and sets (Ordinal, Fragment Offset, (M)ore Fragments, Identification)
the same as for an uncompressed fragment header. If an MFVI is needed,
the OAL node finally sets Inde(X) and includes an MFVI; otherwise, the
node clears Inde(X) and omits the MFVI.The OAL non-first fragment body is then included immediately
following the OCH-1 header, and the L2 header length field is reduced
by the difference in length between the compressed headers and
full-length OAL IPv6 and Fragment headers. The OAL destination will
then be able to determine the Payload Length by examining the L2
header length field. The OCH-1 format applies for non-first fragments
only; therefore, the OAL source sets Ordinal to a monotonically
increasing value beginning with 1 for the first non-first fragment, 2
for the second non-first fragment, etc., up to and including the final
fragment. If more than 127 non-first fragments are included, these
additional fragments instead set Ordinal to 0. The (A)RQ flag is
always implicitly set, and therefore omitted from the OCH-1
header.When an OAL destination or intermediate node receives a carrier
packet, it determines the length of the encapsulated OAL information
by examining the length field of the innermost L2 header, verifies
that the innermost next header field indicates OMNI (see: ), then examines the first four bits immediately
following the innermost header. If the bits contain the value 4 or 6,
the OAL node processes the remainder as an uncompressed OAL/IP header.
If the bits contain a value 0 or 1, the OAL node instead processes the
remainder of the header as an OCH-0/1 as specified above.For carrier packets with OCH or full OAL headers addressed to
itself and with CRH-32 extensions, the OAL node then uses the MFVI to
locate the cached MFV which determines the next hop. During
forwarding, the OAL node changes the MFVI to the cached value for the
MVF next hop. If the OAL node is the destination, it instead
reconstructs the full OAL headers then adds the resulting OAL fragment
to the reassembly cache if the Identification is acceptable. (Note
that for carrier packets that include an OCH-0 with both the X and M
flags set to 0, the OAL node can instead locate forwarding state by
examining the original IP packet header information that appears
immediately after the OCH-0 header.)Note: OAL header compression does not interfere with checksum
calculation and verification, which must be applied according to the
full OAL pseudo-header per even when
compression is used.Note: The OCH-0/1 formats do not include the Traffic Class and Flow
Label information that appears in uncompressed OAL IPv6 headers.
Therefore, when OAL header compression state is initialized the
Traffic Class and Flow Label are considered fixed for as long as the
flow uses OCH-0/1 headers. If the flow requires frequent changes to
Traffic Class and/or Flow Label information, it can include
uncompressed OAL headers either continuously or periodically to update
header compression state.When an OAL source is unable to forward carrier packets directly to
an OAL destination without "tunneling" through a pair of OAL
intermediate nodes, the OAL source must regard the intermediate nodes
as ingress and egress tunnel endpoints. This will result in nested
OAL-in-OAL encapsulation in which the OAL source performs
fragmentation on the inner OAL packet then forwards the fragments to
the ingress tunnel endpoint which encapsulates each resulting OAL
fragment in an additional OAL header before performing fragmentation
following encapsulation.For example, if the OAL source has an NCE for the OAL destination
with MFVI 0x2376a7b5 and Identification 0x12345678 and the OAL ingress
tunnel endpoint has an NCE for the OAL egress tunnel endpoint with
MFVI 0xacdebf12 and Identification 0x98765432, the OAL source prepares
the carrier packets using compressed/uncompressed OAL headers that
include the MFVI and Identification corresponding to the OAL
destination and with L2 header information addressed to the next hop
toward the ingress tunnel endpoint. When the ingress tunnel endpoint
receives the carrier packet, it recognizes the current MFVI included
by the OAL source and determines the correct next hop MFVI.The ingress tunnel endpoint then discards the L2 headers from the
previous hop and encapsulates the original compressed/uncompressed OAL
header within a second compressed/uncompressed OAL header while
including the next-hop MVFI in the outer OAL encapsulation header and
omitting the MFVI in the inner header. The ingress tunnel endpoint
then includes L2 encapsulation headers with destinations appropriate
for the next hop on the path to the egress tunnel endpoint. The
encapsulation appears as shown in :Note that only a single OAL-in-OAL encapsulation layer is
supported, and that MFVIs appear only in the outer OAL header (i.e.,
either within a CRH-32 routing header when a full OAL header is used
or within an OCH header with X set to 0). The inner OAL header should
omit the CRH-32 header or use an OCH header with X set to 1,
respectively.Note that OAL/OCH encapsulation may cause the payloads of OAL
packets produced by the ingress tunnel endpoint to exceed the minimum
MPS by a small amount. If the ingress has assurance that the path to
the egress will include only links capable of transiting the resulting
(slightly larger) carrier packets it should forward without further
fragmentation. Otherwise, the ingress must perform fragmentation
following encapsulation to produce two fragments such that the size of
the first fragment matches the size of the original OAL packet, and
with the remainder in a second fragment. The egress tunnel endpoint
must then reassemble then decapsulate to arrive at the original OAL
packet which is then subject to further forwarding.The OAL encapsulates each original IP packet as an OAL packet then
performs fragmentation to produce one or more carrier packets with the
same 32-bit Identification value. In environments where spoofing is
not considered a threat, OMNI interfaces send OAL packets with
Identifications beginning with an unpredictable Initial Send Sequence
(ISS) value monotonically incremented (modulo
2**32) for each successive OAL packet sent to either a specific
neighbor or to any neighbor. (The OMNI interface may later change to a
new unpredictable ISS value as long as the Identifications are assured
unique within a timeframe that would prevent the fragments of a first
OAL packet from becoming associated with the reassembly of a second
OAL packet.) In other environments, OMNI interfaces should maintain
explicit per-neighbor send and receive windows to detect and exclude
spurious carrier packets that might clutter the reassembly cache as
discussed below.OMNI interface neighbors use TCP-like synchronization to maintain
windows with unpredictable ISS values incremented (modulo 2**32) for
each successive OAL packet and re-negotiate windows often enough to
maintain an unpredictable profile. OMNI interface neighbors exchange
IPv6 ND messages with OMNI options that include TCP-like information
fields to manage streams of OAL packets instead of streams of octets.
As a link-layer service, the OAL provides low-persistence best-effort
retransmission with no mitigations for duplication, reordering or
deterministic delivery. Since the service model is best-effort and
only control message sequence numbers are acknowledged, OAL nodes can
select unpredictable new initial sequence numbers outside of the
current window without delaying for the Maximum Segment Lifetime
(MSL).OMNI interface neighbors maintain current and previous window state
in IPv6 ND neighbor cache entries (NCEs) to support dynamic rollover
to a new window while still sending OAL packets and accepting carrier
packets from the previous windows. Each NCE is indexed by the
neighbor's ULA, while the OAL encapsulation ULA (which may be
different) provides context for Identification verification. OMNI
interface neighbors synchronize windows through asymmetric and/or
symmetric IPv6 ND message exchanges. When a node receives an IPv6 ND
message with new window information, it resets the previous window
state based on the current window then resets the current window based
on new and/or pending information.The IPv6 ND message OMNI option header extension sub-option
includes TCP-like information fields including Sequence Number,
Acknowledgement Number, Window and flags (see: ). OMNI interface neighbors maintain the following
TCP-like state variables in the NCE:OMNI interface neighbors "OAL A" and "OAL B" exchange IPv6 ND
messages per with OMNI options that include
TCP-like information fields. When OAL A synchronizes with OAL B, it
maintains both a current and previous SND.WND beginning with a new
unpredictable ISS and monotonically increments SND.NXT for each
successive OAL packet transmission. OAL A initiates synchronization by
including the new ISS in the Sequence Number of an authentic IPv6 ND
message with the SYN flag set and with Window set to M (up to 2**24)
as a tentative receive window size while creating a NCE in the
INCOMPLETE state if necessary. OAL A caches the new ISS as pending,
uses the new ISS as the Identification for OAL encapsulation, then
sends the resulting OAL packet to OAL B and waits up to RetransTimer
milliseconds to receive an IPv6 ND message response with the ACK flag
set (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).When OAL B receives the SYN, it creates a NCE in the STALE state if
necessary, resets its RCV variables, caches the tentative (send)
window size M, and selects a (receive) window size N (up to 2**24) to
indicate the number of OAL packets it is willing to accept under the
current RCV.WND. (The RCV.WND should be large enough to minimize
control message overhead yet small enough to provide an effective
filter for spurious carrier packets.) OAL B then prepares an IPv6 ND
message with the ACK flag set, with the Acknowledgement Number set to
OAL A's next sequence number, and with Window set to N. Since OAL B
does not assert an ISS of its own, it uses the IRS it has cached for
OAL A as the Identification for OAL encapsulation then sends the ACK
to OAL A.When OAL A receives the ACK, it notes that the Identification in
the OAL header matches its pending ISS. OAL A then sets the NCE state
to REACHABLE and resets its SND variables based on the Window size and
Acknowledgement Number (which must include the sequence number
following the pending ISS). OAL A can then begin sending OAL packets
to OAL B with Identification values within the (new) current SND.WND
for up to ReachableTime milliseconds or until the NCE is updated by a
new IPv6 ND message exchange. This implies that OAL A must send a new
SYN before sending more than N OAL packets within the current SND.WND,
i.e., even if ReachableTime is not nearing expiration. After OAL B
returns the ACK, it accepts carrier packets received from OAL A within
either the current or previous RCV.WND as well as any new authentic
NS/RS SYN messages received from OAL A even if outside the
windows.OMNI interface neighbors can employ asymmetric window
synchronization as described above using two independent (SYN ->
ACK) exchanges (i.e., a four-message exchange), or they can employ
symmetric window synchronization using a modified version of the TCP
three-way handshake as follows:OAL A prepares a SYN with an unpredictable ISS not within the
current SND.WND and with Window set to M as a tentative receive
window size. OAL A caches the new ISS and Window size as pending
information, uses the pending ISS as the Identification for OAL
encapsulation, then sends the resulting OAL packet to OAL B and
waits up to RetransTimer milliseconds to receive an ACK response
(retransmitting up to MAX_UNICAST_SOLICIT times if necessary).OAL B receives the SYN, then resets its RCV variables based on
the Sequence Number while caching OAL A's tentative receive Window
size M and a new unpredictable ISS outside of its current window
as pending information. OAL B then prepares a response with
Sequence Number set to the pending ISS and Acknowledgement Number
set to OAL A's next sequence number. OAL B then sets both the SYN
and ACK flags, sets Window to N and sets the OPT flag according to
whether an explicit concluding ACK is optional or mandatory. OAL B
then uses the pending ISS as the Identification for OAL
encapsulation, sends the resulting OAL packet to OAL A and waits
up to RetransTimer milliseconds to receive an acknowledgement
(retransmitting up to MAX_UNICAST_SOLICIT times if necessary).OAL A receives the SYN/ACK, then resets its SND variables based
on the Acknowledgement Number (which must include the sequence
number following the pending ISS) and OAL B's advertised Window N.
OAL A then resets its RCV variables based on the Sequence Number
and marks the NCE as REACHABLE. If the OPT flag is clear, OAL A
next prepares an immediate solicited NA message with the ACK flag
set, the Acknowledgement Number set to OAL B's next sequence
number, with Window set a value that may be the same as or
different than M, and with the OAL encapsulation Identification to
SND.NXT, then sends the resulting OAL packet to OAL B. If the OPT
flag is set and OAL A has OAL packets queued to send to OAL B, it
can optionally begin sending their carrier packets under the (new)
current SND.WND as implicit acknowledgements instead of returning
an explicit ACK. In that case, the tentative Window size M becomes
the current receive window size.OAL B receives the implicit/explicit acknowledgement(s) then
resets its SND state based on the pending/advertised values and
marks the NCE as REACHABLE. If OAL B receives an explicit
acknowledgement, it uses the advertised Window size and abandons
the tentative size. (Note that OAL B sets the OPT flag in the
SYN/ACK to assert that it will interpret timely receipt of carrier
packets within the (new) current window as an implicit
acknowledgement. Potential benefits include reduced delays and
control message overhead, but use case analysis is outside the
scope of this specification.)Following synchronization, OAL A and OAL B hold updated NCEs and
can exchange OAL packets with Identifications set to SND.NXT while the
state remains REACHABLE and there is available window capacity. Either
neighbor may at any time send a new SYN to assert a new ISS. For
example, if OAL A's current SND.WND for OAL B is nearing exhaustion
and/or ReachableTime is nearing expiration, OAL A continues to send
OAL packets under the current SND.WND while also sending a SYN with a
new unpredictable ISS. When OAL B receives the SYN, it resets its RCV
variables and may optionally return either an asymmetric ACK or a
symmetric SYN/ACK to also assert a new ISS. While sending SYNs, both
neighbors continue to send OAL packets with Identifications set to the
current SND.NXT then reset the SND variables after an acknowledgement
is received.While the optimal symmetric exchange is efficient, anomalous
conditions such as receipt of old duplicate SYNs can cause confusion
for the algorithm as discussed in Section 3.4 of . For this reason, the OMNI option header includes
an RST flag which OAL nodes set in solicited NA responses to ACKs
received with incorrect acknowledgement numbers. The RST procedures
(and subsequent synchronization recovery) are conducted exactly as
specified in .OMNI interfaces may set the PNG ("ping") flag when a reachability
confirmation outside the context of the IPv6 ND protocol is needed
(OMNI interfaces therefore most often set the PNG flag in
advertisement messages and ignore it in solicitation messages). When
an OMNI interface receives a PNG, it returns an unsolicited NA (uNA)
ACK with the PNG message Identification in the Acknowledgment, but
without updating RCV state variables. OMNI interfaces return unicast
uNA ACKs even for multicast PNG destination addresses, since OMNI link
multicast is based on unicast emulation.OMNI interfaces that employ the window synchronization procedures
described above observe the following requirements:OMNI interfaces MUST select new unpredictable ISS values that
are at least a full window outside of the current SND.WND.OMNI interfaces MUST set the initial SYN message Window field
to a tentative value to be used only if no concluding NA ACK is
sent.OMNI interfaces that receive advertisements with the PNG and/or
SYN flag set MUST NOT set the PNG and/or SYN flag in uNA
responses.OMNI interfaces that send advertisements with the PNG and/or
SYN flag set MUST ignore uNA responses with the PNG and/or SYN
flag set.OMNI interfaces MUST send IPv6 ND messages used for window
synchronization securely while using unpredictable initial
Identification values until synchronization is complete.Note: Although OMNI interfaces employ TCP-like window
synchronization and support uNA ACK responses to SYNs and PNGs, all
other aspects of the IPv6 ND protocol (e.g., control message
exchanges, NCE state management, timers, retransmission limits, etc.)
are honored exactly per .Note: Recipients of OAL-encapsulated IPv6 ND messages index the NCE
based on the message source address, which also determines the carrier
packet Identification window. However, IPv6 ND messages may contain a
message source address that does not match the OMNI encapsulation
source address when the recipient acts as a proxy.Note: OMNI interface neighbors apply the same send and receive
windows for all of their (multilink) underlay interface pairs that
exchange carrier packets. Each interface pair represents a distinct
underlay network path, and the set of paths traversed may be highly
diverse when multiple interface pairs are used. OMNI intermediate
nodes therefore SHOULD NOT cache window synchronization parameters in
IPv6 ND messages they forward since there is no way to ensure
network-wide middlebox state consistency.When the OAL source sends carrier packets to an OAL destination, it
should cache recently sent packets in case timely best-effort
selective retransmission is requested. The OAL destination in turn
maintains a checklist for the (Source, Destination,
Identification)-tuple of recently received carrier packets and notes
the ordinal numbers of OAL packet fragments already received (i.e., as
Frag #0, Frag #1, Frag #2, etc.). The timeframe for maintaining the
OAL source and destination caches determines the link persistence
(see: ).If the OAL destination notices some fragments missing after most
other fragments within the same link persistence timeframe have
already arrived, it may issue an Automatic Repeat Request (ARQ) with
Selective Repeat (SR) by sending a uNA message to the OAL source. The
OAL destination creates a uNA message with an OMNI option with one or
more Fragmentation Report (FRAGREP) sub-options that include a list of
(Identification, Bitmap)-tuples for fragments received and missing
from this OAL source (see: and ). The OAL destination includes an
authentication signature if necessary, performs OAL encapsulation
(with the its own address as the OAL source and the source address of
the message that prompted the uNA as the OAL destination) and sends
the message to the OAL source.When the OAL source receives the uNA message, it authenticates the
message then examines the FRAGREP. For each (Source, Destination,
Identification)-tuple, the OAL source determines whether it still
holds the corresponding carrier packets in its cache and retransmits
any for which the Bitmap indicates a loss event. For example, if the
Bitmap indicates that ordinal fragments #3, #7, #10 and #13 from the
OAL packet with Identification 0x12345678 are missing the OAL source
only retransmits carrier packets containing those fragments. When the
OAL destination receives the retransmitted carrier packets, it admits
the enclosed fragments into the reassembly cache and updates its
checklist. If some fragments are still missing, the OAL destination
may send a small number of additional uNA ARQ/SRs within the link
persistence timeframe.The OAL therefore provides a link-layer low-to-medium persistence
ARQ/SR service consistent with and Section
8.1 of . The service provides the benefit of
timely best-effort link-layer retransmissions which may reduce packet
loss and avoid some unnecessary end-to-end delays. This best-effort
network-based service therefore compliments higher layer end-to-end
protocols responsible for true reliability.When the OMNI interface forwards original IP packets from the
network layer, it invokes the OAL and returns internally-generated
ICMPv4 Fragmentation Needed or ICMPv6 Path
MTU Discovery (PMTUD) Packet Too Big (PTB)
messages as necessary. This document refers to both of these
ICMPv4/ICMPv6 message types simply as "PTBs", and introduces a
distinction between PTB "hard" and "soft" errors as discussed below
and also in .Ordinary PTB messages with ICMPv4 header "unused" field or ICMPv6
header Code field value 0 are hard errors that always indicate that a
packet has been dropped due to a real MTU restriction. However, the
OMNI interface can also forward large original IP packets via OAL
encapsulation and fragmentation while at the same time returning PTB
soft error messages (subject to rate limiting) if it deems the
original IP packet too large according to factors such as link
performance characteristics, number of fragments needed, reassembly
congestion, etc. This ensures that the path MTU is adaptive and
reflects the current path used for a given data flow. The OMNI
interface can therefore continuously forward packets without loss
while returning PTB soft error messages recommending a smaller size if
necessary. Original sources that receive the soft errors in turn
reduce the size of the packets they send (i.e., the same as for hard
errors), but can soon resume sending larger packets if the soft errors
subside.An OAL source sends PTB soft error messages by setting the ICMPv4
header "unused" field or ICMPv6 header Code field to the value 1 if
the packet was dropped or 2 if the packet was forwarded successfully.
The OAL source sets the PTB destination address to the original IP
packet source, and sets the source address to one of its OMNI
interface addresses that is routable from the perspective of the
original source. The OAL source then sets the MTU field to a value
smaller than the original packet size but no smaller than 576 for
ICMPv4 or 1280 for ICMPv6, writes the leading portion of the original
IP packet first fragment into the "packet in error" field, and returns
the PTB soft error to the original source. When the original source
receives the PTB soft error, it temporarily reduces the size of the
packets it sends the same as for hard errors but may seek to increase
future packet sizes dynamically while no further soft errors are
arriving. (If the original source does not recognize the soft error
code, it regards the PTB the same as a hard error but should heed the
retransmission advice given in suggesting
retransmission based on normal packetization layer retransmission
timers.)An OAL destination may experience reassembly cache congestion, and
can return uNA messages to the OAL source that originated the
fragments (subject to rate limiting) that include OMNI encapsulated
PTB messages with code 1 or 2. The OAL destination creates a uNA
message with an OMNI option containing an authentication message
sub-option if necessary followed optionally by a ICMPv6 Error
sub-option that encodes a PTB message with a reduced value and with
the leading portion an OAL first fragment containing the header of an
original IP packet whose source must be notified (see: ). The OAL destination encapsulates the leading
portion of the OAL first fragment (beginning with the OAL header) in
the PTB "packet in error" field, signs the message if an
authentication sub-option is included, performs OAL encapsulation
(with the its own address as the OAL source and the source address of
the message that prompted the uNA as the OAL destination) and sends
the message to the OAL source.When the OAL source receives the uNA message, it sends a
corresponding network layer PTB soft error to the original source to
recommend a smaller size. The OAL source crafts the PTB by extracting
the leading portion of the original IP packet from the OMNI
encapsulated PTB message (i.e., not including the OAL header) and
writes it in the "packet in error" field of a network layer PTB with
destination set to the original IP packet source and source set to one
of its OMNI interface addresses that is routable from the perspective
of the original source.Original sources that receive PTB soft errors can dynamically tune
the size of the original IP packets they to send to produce the best
possible throughput and latency, with the understanding that these
parameters may change over time due to factors such as congestion,
mobility, network path changes, etc. The receipt or absence of soft
errors should be seen as hints of when increasing or decreasing packet
sizes may be beneficial. The OMNI interface supports continuous
transmission and reception of packets of various sizes in the face of
dynamically changing network conditions. Moreover, since PTB soft
errors do not indicate a hard limit, original sources that receive
soft errors can resume sending larger packets without waiting for the
recommended 10 minutes specified for PTB hard errors . The OMNI interface
therefore provides an adaptive service that accommodates MTU diversity
especially well-suited for dynamic multilink environments.By default, the OAL source includes a 40-octet IPv6 encapsulation
header for each original IP packet during OAL encapsulation. The OAL
source also calculates then performs fragmentation such that a copy of
the 40-octet IPv6 header plus an 8-octet IPv6 Fragment Header is
included in each OAL fragment (when a Routing Header is added, the OAL
encapsulation headers become larger still). However, these
encapsulations may represent excessive overhead in some environments.
OAL header compression can dramatically reduce the amount of
encapsulation overhead, however a complimentary technique known as
"packing" (see: ) supports
encapsulation of multiple original IP packets and/or control messages
within a single OAL "super-packet".When the OAL source has multiple original IP packets to send to the
same OAL destination with total length no larger than the OAL
destination MRU, it can concatenate them into a super-packet
encapsulated in a single OAL header. Within the OAL super-packet, the
IP header of the first original IP packet (iHa) followed by its data
(iDa) is concatenated immediately following the OAL header, then the
IP header of the next original packet (iHb) followed by its data (iDb)
is concatenated immediately following the first original packet, etc.
with a trailing checksum field included in the final fragment. The OAL
super-packet format is transposed from and shown in :When the OAL source prepares a super-packet, it applies OAL
fragmentation, includes a trailing checksum in the final fragment,
applies L2 encapsulation to each fragment then sends the resulting
carrier packets to the OAL destination. When the OAL destination
receives the super-packet it sets aside the trailing checksum,
reassembles if necessary, then verifies the checksum while regarding
the remaining OAL header Payload Length as the sum of the lengths of
all payload packets. The OAL destination then selectively extracts
each original IP packet (e.g., by setting pointers into the
super-packet buffer and maintaining a reference count, by copying each
packet into a separate buffer, etc.) and forwards each packet to the
network layer. During extraction, the OAL determines the IP protocol
version of each successive original IP packet 'j' by examining the
four most-significant bits of iH(j), and determines the length of the
packet by examining the rest of iH(j) according to the IP protocol
version.When an OAL source prepares a super-packet that includes an IPv6 ND
message with an authentication signature or ICMPv6 message checksum as
the first original IP packet (i.e., iHa/iDa), it calculates the
authentication signature or checksum over the remainder of
super-packet. Security and integrity for forwarding initial protocol
data packets in conjunction with IPv6 ND messages used to establish
NCE state are therefore supported. (A common use case entails a path
MPS probe beginning with a signed IPv6 ND message followed by a NULL
IPv6 packet with a suitably large (Jumbo) Payload Length but with Next
Header set to 59 for "No Next Header".)OAL sources may send NULL OAL packets known as "bubbles" for the
purpose of establishing Network Address Translator (NAT) state on the
path to the OAL destination. The OAL source prepares a bubble by
crafting an OAL header with appropriate IPv6 source and destination
ULAs, with the IPv6 Next Header field set to the value 59 ("No Next
Header" - see ) and with only the trailing OAL
Checksum field (i.e., and no protocol data) immediately following the
IPv6 header.The OAL source includes a random Identification value then
encapsulates the OAL packet in L2 headers destined to either the
mapped address of the OAL destination's first-hop ingress NAT or the
L2 address of the OAL destination itself. When the OAL source sends
the resulting carrier packet, any egress NATs in the path toward the
L2 destination will establish state based on the activity but the
bubble will be harmlessly discarded by either an ingress NAT on the
path to the OAL destination or by the OAL destination itself.The bubble concept for establishing NAT state originated in and was later updated by .
OAL bubbles may be employed by mobility services such as .In light of the above, OAL sources, destinations and intermediate
nodes observe the following normative requirements:OAL sources MUST forward original IP packets either larger than
the OMNI interface MRU or smaller than the minimum MPS minus the
trailing checksum size as atomic fragments (i.e., and not as
multiple fragments).OAL sources MUST produce non-final fragments with payloads no
smaller than the minimum MPS during fragmentation.OAL intermediate nodes SHOULD and OAL destinations MUST
unconditionally drop any non-final OAL fragments with payloads
smaller than the minimum MPS.OAL destinations MUST drop any new OAL fragments with offset
and length that would overlap with other fragments and/or leave
holes smaller than the minimum MPS between fragments that have
already been received.Note: Under the minimum MPS, ordinary 1500 octet original IP
packets would require at most 4 OAL fragments, with each non-final
fragment containing 400 payload octets and the final fragment
containing 302 payload octets (i.e., the final 300 octets of the
original IP packet plus the 2 octet trailing checksum). For all packet
sizes, the likelihood of successful reassembly may improve when the
OMNI interface sends all fragments of the same fragmented OAL packet
consecutively over the same underlay interface pair instead of spread
across multiple underlay interface pairs. Finally, an assured
minimum/path MPS allows continuous operation over all paths including
those that traverse bridged L2 media with dissimilar MTUs.Note: Certain legacy network hardware of the past millennium was
unable to accept packet "bursts" resulting from an IP fragmentation
event - even to the point that the hardware would reset itself when
presented with a burst. This does not seem to be a common problem in
the modern era, where fragmentation and reassembly can be readily
demonstrated at line rate (e.g., using tools such as 'iperf3') even
over fast links on ordinary hardware platforms. Even so, while the OAL
destination is reporting reassembly congestion (see: ) the OAL source could impose "pacing" by inserting an
inter-fragment delay and increasing or decreasing the delay according
to congestion indications.As discussed in Section 3.7 of , there are
four basic threats concerning IPv6 fragmentation; each of which is
addressed by effective mitigations as follows:Overlapping fragment attacks - reassembly of overlapping
fragments is forbidden by ; therefore,
this threat does not apply to the OAL.Resource exhaustion attacks - this threat is mitigated by
providing a sufficiently large OAL reassembly cache and
instituting “fast discard” of incomplete reassemblies
that may be part of a buffer exhaustion attack. The reassembly
cache should be sufficiently large so that a sustained attack does
not cause excessive loss of good reassemblies but not so large
that (timer-based) data structure management becomes
computationally expensive. The cache should also be indexed based
on the arrival underlay interface such that congestion experienced
over a first underlay interface does not cause discard of
incomplete reassemblies for uncongested underlay interfaces.Attacks based on predictable fragment identification values -
in environments where spoofing is possible, this threat is
mitigated through the use of Identification windows beginning with
unpredictable values per . By maintaining
windows of acceptable Identifications, OAL neighbors can quickly
discard spurious carrier packets that might otherwise clutter the
reassembly cache. The OAL additionally provides an integrity check
to detect corruption that may be caused by spurious fragments
received with in-window Identification values.Evasion of Network Intrusion Detection Systems (NIDS) - since
the OAL source employs a robust MPS, network-based firewalls can
inspect and drop OAL fragments containing malicious data thereby
disabling reassembly by the OAL destination. However, since OAL
fragments may take different paths through the network (some of
which may not employ a firewall) each OAL destination must also
employ a firewall.IPv4 includes a 16-bit Identification (IP ID) field with only
65535 unique values such that at high data rates the field could wrap
and apply to new carrier packets while the fragments of old packets
using the same IP ID are still alive in the network . Since carrier packets sent via an IPv4 path with
DF=0 are normally no larger than 576 octets, IPv4 fragmentation is
possible only at small-MTU links in the path which should support data
rates low enough for safe reassembly . (IPv4
carrier packets larger than 576 octets with DF=0 may incur high data
rate reassembly errors in the path, but the OAL checksum provides OAL
destination integrity assurance.) Since IPv6 provides a 32-bit
Identification value, IP ID wraparound at high data rates is not a
concern for IPv6 fragmentation.Fragmentation security concerns for large IPv6 ND messages are
documented in . These concerns are addressed
when the OMNI interface employs the OAL instead of directly
fragmenting the IPv6 ND message itself. For this reason, OMNI
interfaces MUST NOT send IPv6 ND messages larger than the OMNI
interface MTU, and MUST employ OAL encapsulation and fragmentation for
IPv6 ND messages larger than the minimum/path MPS for this OAL
destination.Unless the path is secured at the network-layer or below (i.e., in
environments where spoofing is possible), OMNI interfaces MUST NOT
send ordinary carrier packets with Identification values outside the
current window and MUST secure IPv6 ND messages used for address
resolution or window state synchronization. OAL destinations SHOULD
therefore discard without reassembling any out-of-window OAL fragments
received over an unsecured path.OMNI Hosts are end systems that extend the OMNI link over ENET
underlay interfaces (i.e., either as an OMNI interface or as a
sublayer of the ENET interface itself). Each ENET is connected to the
rest of the OMNI link by a Client that receives an MNP delegation.
Clients delegate MNP addresses and/or sub-prefixes to ENET nodes
(i.e., Hosts, other Clients, routers and non-OMNI hosts) using
standard mechanisms such as DHCP and IPv6 Stateless Address AutoConfiguration
(SLAAC) . Clients forward packets between
their ENET Hosts and peers on external networks acting as routers
and/or OAL intermediate nodes.OMNI Hosts coordinate with Clients and/or other Hosts connected to
the same ENET using IP-encapsulated IPv6 ND messages. The IP
encapsulation headers and ND messages both use the MNP-based addresses
assigned to ENET underlay interfaces as source and destination
addresses (i.e., instead of ULAs). For IPv4 MNPs, the ND messages use
IPv4-Compatible IPv6 addresses in place of
the IPv4 addresses. (Note that IPv4-Compatible IPv6 addresses are
deprecated for all other uses by the aforementioned standard.)Hosts discover Clients by sending encapsulated RS messages using an
OMNI link IP anycast address (or the unicast address of the Client) as
the RS L2 encapsulation destination as specified in . The Client configures the IPv4 and/or IPv6 anycast
addresses for the OMNI link on its ENET interface and advertises the
address(es) into the ENET routing system. The Client then responds to
the encapsulated RS messages by sending an encapsulated RA message
that uses its ENET unicast address as the source. (To differentiate
itself from an INET border Proxy/Server, the Client sets the RA
message OMNI Interface Attributes sub-option LHS field to 0 for the
Host's interface index. When the RS message includes an L2 anycast
destination address, the Client also includes an Interface Attributes
sub-option for interface index 0 to inform the Host of its L2 unicast
address - see: for full details on the RS and
RA message contents.)Hosts coordinate with peer Hosts on the same ENET by sending
encapsulated NS messages to receive an NA reply. (Hosts determine
whether a peer is on the same ENET by matching the peer's IP address
with the MNP (sub)-prefix for the ENET advertised in the Client's RA
message .) Each ENET peer then creates a NCE
and synchronizes Identification windows the same as for OMNI link
neighbors, and the Host can then engage in OMNI link transactions with
the Client and/or other ENET Hosts. By coordinating with the Client in
this way, the Host treats the Client as if it were an ANET
Proxy/Server, and the Client provides the same services that a
Proxy/Server would provide. By coordinating with other Hosts, the peer
hosts can exchange large IP packets or parcels over the ENET using
IPv6 fragmentation if necessary.When a Host prepares an IP packet or parcel, it uses the IP address
of its native ENET interface as the source and the IP address of the
(remote) peer as the destination. The Host next performs parcel
segmentation if necessary (see: ) then
encapsulates the packet/parcel in an IP header of the version
supported by the ENET while setting the source to the same address and
destination to either the same address if the peer is on the local
ENET, or to the IP address of the Client otherwise. The Host can then
proceed to exchange packets/parcels with the destination, either
directly or via the Client as an intermediate node.The encapsulation procedures are coordinated per , except that the IP encapsulation header version
matches the native ENET IP protocol version and uses IPv6 GUA or
public/private IPv4 addresses instead of ULAs. The Host sets the
encapsulation IP header {Protocol, Next-Header} field to TBD1 to
indicate that this is an OAL encapsulation and not an ordinary
IP-in-IP encapsulation. When the inner header is IPv4-based, the Host
next translates the encapsulation header into an IPv6 header with
IPv4-Compatible addresses while setting the [IPv6 Traffic Class,
Payload Length, Next Header, Hop Limit] fields according to the IPv4
{Type of Service, Total Length, Protocol, TTL} fields, respectively,
while setting Flow Label to 0. The Host then calculates an OAL
checksum, writes the value as the final two octets of the encapsulated
packet then applies IPv6 fragmentation to the encapsulated packet to
produce IPv6 fragments no smaller than the MPS the same as described
in . If the original encapsulation IP header was
IPv4, the Host next translates the IPv6 encapsulation headers back to
IPv4 headers with Protocol value set to 44 since the immediately next
header is the IPv6 Fragment Header. The Host finally sends the IP
encapsulated fragments to the ENET peer.When the ENET peer receives IP encapsulated fragments, for IPv4 it
first translates the encapsulation headers back to IPv6 headers with
IPv4-Compatible addresses the same as above. The peer then reassembles
and verifies the OAL checksum. If the checksum is correct, the peer
next removes the encapsulation headers and applies parcel reassembly
if necessary. The peer then either delivers the encapsulated
packet/parcel to upper layers if the peer is the destination or
forwards the packet/parcel toward the final destination if the peer is
a Client acting as an intermediate node.Hosts and Clients that initiate OMNI-based packet/parcel
transactions should first test the path toward the final destination
using the parcel path qualification procedure specified in . An OMNI Host that sends and
receives parcels need not implement the full OMNI interface
abstraction but MUST implement enough of the OAL to be capable of
fragmenting and reassembling maximum-length encapsulated IP
packets/parcels and sub-parcels as discussed above and in the
following section.IP parcels are specified in , while details for their
application over OMNI interfaces is specified here. IP parcels are
formed by an OMNI Host or Client upper layer protocol entity
(identified by the "5-tuple" source IP address/port number,
destination IP address/port number and protocol number) when it
produces a protocol data unit containing the concatenation of up to 64
upper layer protocol segments. All non-final segments MUST be equal in
length while the final segment MUST NOT be larger but MAY be smaller.
Each non-final segment MUST be no larger than 65535 minus the length
of the IP header plus extensions, minus the length of the OAL
encapsulation header and trailer. The upper layer protocol then
presents the buffer and non-final segment size to the IP layer which
appends a single IP header (plus any extension headers) before
presenting the parcel to the OMNI Interface.For IPv4, the IP layer prepares the parcel by appending an IPv4
header with a Jumbo Payload option (see: ) where
"Jumbo Payload Length" is a 32-bit unsigned integer value (in network
byte order) set to the lengths of the IPv4 header plus all
concatenated segments. The IP layer next sets the IPv4 header DF bit
to 1, then sets the IPv4 header Total Length field to the length of
the IPv4 header plus the length of the first segment only. (Note: the
IP layer can form true IPv4 jumbograms (as opposed to parcels) by
instead setting the Total Length field to the length of the IPv4
header only.)For IPv6, the IP layer forms a parcel by appending an IPv6 header
with a Jumbo Payload option the same as for IPv4 above where "Jumbo
Payload Length" is set to the lengths of the IPv6 Hop-by-Hop Options
header and any other extension headers present plus all concatenated
segments. The IP layer next sets the IPv6 header Payload Length field
to the lengths of the IPv6 Hop-by-Hop Options header and any other
extension headers present plus the length of the first segment only.
(Note: the IP layer can form true IPv6 jumbograms (as opposed to
parcels) by instead setting the Payload Length field to 0.)An IP parcel therefore has the following structure:where J is the total number of segments (between 1 and 64),
L is the length of each non-final segment which MUST NOT be larger
than 65535 (minus headers as above) and K is the length of the final
segment which MUST NOT be larger than L. The values M and N are then
set to the length of the IP header plus extensions for IPv4 or to the
length of the extensions only for IPv6, then further calculated as
follows:M = M + ((J-1) ? L : K)N = N + (((J-1) * L) + K)Note: a "singleton" parcel is one that includes only the IP
header plus extensions with a single segment of length K, while a
"null" parcel is a singleton with K=0, i.e., a parcel consisting of
only the IP header plus extensions with no octets beyond.When the IP layer forwards a parcel, the OMNI interface invokes the
OAL which forwards it to either a Client as an intermediate node or
the final destination itself. The OAL source first assigns a
monotonically-incrementing (modulo 127) "Parcel ID" and subdivides the
parcel into sub-parcels no larger than the maximum of the path MTU to
the next hop or 64KB (minus the length of encapsulation headers). The
OAL source determines the number of segments of length L that can fit
into each sub-parcel under these size constraints, e.g. if the OAL
source determines that a sub-parcel can contain 3 segments of length
L, it creates sub-parcels with the first containing segments 1-3, the
second containing segments 4-6, etc. and with the final containing any
remaining segments. The OAL source then appends an identical IP header
plus extensions to each sub-parcel while resetting M and N in each
according to the above equations with J set to 3 and K set to L for
each non-final sub-parcel and with J set to the remaining number of
segments for the final sub-parcel.The OAL source next performs encapsulation on each sub-parcel with
destination set to the next hop address. If the next hop is reached
via an ANET/INET interface, the OAL source inserts an OAL header the
same as discussed in and sets the destination
to the ULA-MNP of the target Client. If the next hop is reached via an
ENET interface, the OAL source instead inserts an IP header of the
appropriate protocol version for the underlay ENET (i.e., even if the
encapsulation header is IPv4) and sets the destination to the ENET IP
address of the next hop. The OAL source inserts the encapsulation
header even if no actual fragmentation is needed and/or even if the
Jumbo Payload option is present.The OAL source next assigns an Identification number that is
monotonically-incremented for each consecutive sub-parcel, calculates
and appends the OAL checksum, then performs IPv6 fragmentation over
the sub-parcel if necessary to create fragments small enough to
traverse the path to the next hop. (If the encapsulation header is
IPv4, the OAL source first translates the encapsulation header into an
IPv6 header with IPv4-Compatible IPv6 addresses before performing the
fragmentation/reassembly operation while inserting an IPv6 Fragment
Header.) The OAL source then writes the "Parcel ID" and sets/clears
the "(P)arcel" and "(More) (S)ub-Parcels" bits in the Fragment Header
of the first fragment (see: ). (The OAL
source sets P to 1 for a parcel or to 0 for a non-parcel. When P is 1,
the OAL next sets S to 1 for non-final sub-parcels or to 0 if the
sub-parcel contains the final segment.) The OAL source then forwards
each IP encapsulated packet/fragment to the next hop (i.e., after
first translating the IPv6 encapsulation header back to IPv4 if
necessary).When the next hop receives the encapsulated IP fragments or whole
packets, it acts as an OAL destination and reassembles if necessary
(i.e., after first translating the IPv4 encapsulation header to IPv6
if necessary). If the P flag in the first fragment is 0, the OAL
destination then processes the reassembled entity as an ordinary IP
packet; otherwise it continues processing as a sub-parcel. If the OAL
destination is not the final destination, it retains the sub-parcels
along with their Parcel ID and Identification values for a brief time
in hopes of re-combining with peer sub-parcels of the same original
parcel identified by the 4-tuple consisting of the IP encapsulation
source and destination, Identification and Parcel ID. The OAL
destination re-combines peers by concatenating the segments included
in sub-parcels with the same Parcel ID and with Identification values
within 64 of one another to create a larger sub-parcel possibly even
as large as the entire original parcel. Order of concatenation is not
important, with the exception that the final sub-parcel (i.e., the one
with S set to 0) must occur as the final concatenation before
transmission. The OAL destination then appends a common IP header plus
extensions to each re-combined sub-parcel while resetting M and N in
each according to the above equations with J, K and L set
accordingly.When the current OAL destination is an intermediate node, it next
becomes an OAL source to forward the re-combined (sub-)parcel(s) to
the next hop toward the final destination using
encapsulation/translation the same as specified above. (Each such
intermediate node MUST ensure that the S flag remains set to 0 in the
sub-parcel that contains the final segment.) When the parcel or
sub-parcels arrive at the final OAL destination, it re-combines them
into the largest possible (sub)-parcels while honoring the S flag then
delivers them to upper layers which act on the enclosed 5-tuple
information supplied by the original source.Note: while the final destination may be tempted to re-combine the
sub-parcels of multiple different parcels with identical upper layer
protocol 5-tuples and with non-final segments of identical length,
this process could become complicated when the different parcels each
have final segments of diverse lengths. Since this could possibly
defeat any perceived performance advantages, the decision of whether
and how to perform inter-parcel concatenation is an implementation
matter.When the OMNI interface forwards original IP packets from the network
layer it first invokes the OAL to create OAL packets/fragments if
necessary, then includes any L2 encapsulations and finally engages the
native frame format of the underlay interface. For example, for
Ethernet-compatible interfaces the frame format is specified in , for aeronautical radio interfaces the frame format
is specified in standards such as ICAO Doc 9776 (VDL Mode 2 Technical
Manual), for various forms of tunnels the frame format is found in the
appropriate tunneling specification, etc.See for a map of the various L2
layering combinations possible. For any layering combination, the final
layer (e.g., UDP, IP, Ethernet, etc.) must have an assigned number and
frame format representation that is compatible with the selected
underlay interface. requires that nodes assign Link-Local
Addresses (LLAs) to all interfaces, and that routers use their LLAs as
the source address for RA and Redirect messages. OMNI interfaces honor
the first requirement, but do not honor the second since the OMNI link
could consist of the concatenation of multiple links with diverse ULA
prefixes (see ) but for which multiple
nodes might configure identical interface identifiers (IIDs). OMNI
interface LLAs are therefore considered only as context for IID
formation as discussed below and have no other operational role.OMNI interfaces assign IPv6 LLAs through pre-service administrative
actions. Clients assign "LLA-MNPs" with IIDs that embed the Client's
unique MNP, while Proxy/Servers assign "LLA-RNDs" that include a
randomly-generated IIDs generated as specified in . LLAs are configured as follows:IPv6 LLA-MNPs encode the most-significant 64 bits of an MNP
within the least-significant 64 bits of the IPv6 link-local prefix
fe80::/64, i.e., in the IID portion of the LLA. The LLA prefix
length is determined by adding 64 to the MNP prefix length. e.g.,
for the MNP 2001:db8:1000:2000::/56 the corresponding LLA-MNP prefix
is fe80::2001:db8:1000:2000/120. (The base LLA-MNP for each "/N"
prefix sets the final 128-N bits to 0, but all LLA-MNPs that match
the prefix are also accepted.) Non-MNP IPv6 prefix-based LLAs are
also represented the same as for LLA-MNPs, but include a GUA prefix
that is not properly covered by the MSP.IPv4-Compatible LLA-MNPs are constructed as fe80::{IPv4-Prefix},
i.e., the IID consists of 32 '0' bits followed by a 32 bit IPv4
address/prefix, which may be either public or private in
correspondence with the network layer addressing plan. The
IPv4-Compatible LLA-MNP prefix length is determined by adding 96 to
the IPv4 prefix length. For example, the IPv4-Compatible LLA-MNP for
192.0.2.0/24 is fe80::192.0.2.0/120, also written as
fe80::c000:0200/120. (The base LLA-MNP for each "/N" prefix sets the
final 128-N bits to 0, but all LLA-MNPs that match the prefix are
also accepted.) Non-MNP IPv4 prefix-based LLAs are also represented
the same as for LLA-MNPs, but include a GUA prefix that is not
properly covered by the MSP.LLA-RNDs are randomly-generated and assigned to Proxy/Servers and
other SRT infrastructure elements. They may also be assigned by
Clients to support the MNP delegation process. The upper 72 bits of
the LLA-RND encode the prefix fe80::/72, and the lower 56 bits
include a randomly-generated candidate pseudo-random value
configured as specified in ; if the most
significant 24 bits of the 56 bit candidate encodes the value '0',
the node generates a new candidate to obtain one with a different
most significant 24 bits to avoid overlap with IPv4-Compatible
LLAs.The address fe80::/128 (i.e., the LLA /64 prefix followed by an
all-zero IID) is considered the LLA Subnet Router Anycast
addressSince the prefix 0000::/8 is "Reserved by the IETF" , no MNPs can be allocated from that block ensuring
that there is no possibility for overlap between the different MNP and
RND LLA constructs discussed above.Since LLA-MNPs are based on the distribution of administratively
assured unique MNPs, and since LLA-RNDs are assumed unique through
pseudo-random assignment, OMNI interfaces set the autoconfiguration
variable DupAddrDetectTransmits to 0 .Note: If future protocol extensions relax the 64-bit boundary in IPv6
addressing, the additional prefix bits of an MNP could be encoded in
bits 16 through 63 of the LLA-MNP. (The most-significant 64 bits would
therefore still be in bits 64-127, and the remaining bits would appear
in bits 16 through 48.) However, this would interfere with the
relationship between OMNI LLAs and ULAs (see: ) and render many OMNI functions inoperable. The
analysis provided in furthermore suggests that
the 64-bit boundary will remain in the IPv6 architecture for the
foreseeable future.OMNI links use IPv6 Unique-Local Addresses (ULAs) as the source and
destination addresses in both IPv6 ND messages and OAL packet IPv6
encapsulation headers. ULAs are routable only within the scope of an
OMNI link, and are derived from the IPv6 Unique Local Address prefix
fd00::/8 (i.e., the prefix fc00::/7 followed by the L bit set to 1).
When the first 16 bits of the ULA encode the value fd00::/16, the
address is considered as either a Temporary ULA (TLA) or an eXtended ULA
(XLA) - see below. For all other ULAs, the 56 bits following fd00::/8
encode a 40-bit Global ID followed by a 16-bit Subnet ID as specified in
Section 3 of . All OMNI link ULA types finally
include a 64-bit value in the IID portion of the address ULA::/64 as
specified below.When a node configures a ULA for OMNI, it selects a 40-bit Global ID
for the OMNI link initialized to a candidate pseudo-random value as
specified in Section 3 of ; if the most
significant 8 bits of the candidate encodes the value '0', the node
selects a new candidate until it obtains one with a different most
significant 8 bits. All nodes on the same OMNI link use the same Global
ID, and statistical uniqueness of the pseudo-random Global ID provides a
unique OMNI link identifier allowing different links to be joined
together in the future without requiring renumbering.Next, for each logical segment of the same OMNI link the node selects
a 16-bit Subnet ID value between 0x0000 and 0xffff. Nodes on the same
logical segment configure the same Subnet ID, but nodes on different
segments of the same OMNI link can still exchange IPv6 ND messages as
single-hop neighbors even if they configure different Subnet IDs. When a
node moves to a different OMNI link segment, it resets the Global ID and
Subnet ID value according to the new segment but need not change the
IID.ULAs and their associated prefix lengths are configured in
correspondence with LLAs through stateless prefix translation where
"ULA-MNPs" simply copy the IIDs of their corresponding LLA-MNPs and
"ULA-RNDs" simply copy the IIDs of their corresponding LLA-RNDs. For
example, for the OMNI link ULA prefix fd{Global}:{Subnet}::/64:the ULA-MNP corresponding to the LLA-MNP fe80::2001:db8:1:2 with
a 56-bit MNP length is simply fd{Global}:{Subnet}:2001:db8:1:2/120
(where, the ULA prefix length becomes 64 plus the IPv6 MNP
length).the ULA-MNP corresponding to fe80::192.0.2.0 with a 28-bit MNP
length is simply fd{Global}:{Subnet}::192.0.2.0/124 (where, the ULA
prefix length becomes 96 plus the IPv4 MNP length).the ULA-RND corresponding to fe80::0012:3456:789a:bcde is simply
fd{Global}:{Subnet}::0012:3456:789a:bcde/128.the Subnet Router Anycast ULA corresponding to fe80::/128 is
simply fd{Global}:{Subnet}::/128.The ULA presents an IPv6 address format that is routable within the
OMNI link routing system and can be used to convey link-scoped (i.e.,
single-hop) IPv6 ND messages across multiple hops through IPv6
encapsulation . The OMNI link extends across one
or more underlying Internetworks to include all Proxy/Servers and other
service nodes. All Clients are also considered to be connected to the
OMNI link, however unnecessary encapsulations are omitted whenever
possible to conserve bandwidth (see: ).Clients can configure TLAs when they have no other ULA addresses by
setting the ULA prefix to fd00::/16 followed by a 48-bit
randomly-generated number followed by a random or MNP-based IID as
discussed in . XLAs are a special-case TLA
that use the prefix fd00::/64. (Note that XLAs can also be formed from
LLAs simply by inverting bits 7 and 8 of 'fe80' to form 'fd00'.)OMNI nodes use XLA-MNPs as "default" ULAs for representing MNPs in
the OMNI link routing system. Clients use {TLA,XLA}-MNPs when they
already know their MNP but need to express it outside the context of a
specific ULA prefix, and Proxy/Servers advertise XLA-MNPs into the OMNI
link routing system instead of advertising fully-qualified
{TLA,ULA}-MNPs and/or non-routable LLA-MNPs.{TLAs,XLAs} provide initial "bootstrapping" addresses while the
Client is in the process of procuring an MNP and/or identifying the ULA
prefix for the OMNI link segment; TLAs are not advertised into the OMNI
link routing system but can be used for Client-to-Client communications
within a single {A,I,E}NET when no OMNI link infrastructure is present.
Within each individual {A,I,E}NET, TLAs employ optimistic DAD principles
since they are statistically unique.Each OMNI link may be subdivided into SRT segments that often
correspond to different administrative domains or physical partitions.
Each SRT segment is identified by a different Subnet ID within the same
ULA ::/48 prefix. Multiple distinct OMNI links with different ULA ::/48
prefixes can also be joined together into a single unified OMNI link
through simple interconnection without requiring renumbering. In that
case, the (larger) unified OMNI link routing system may carry multiple
distinct ULA prefixes.OMNI nodes can use Segment Routing to
support efficient forwarding to destinations located in other OMNI link
segments. A full discussion of Segment Routing over the OMNI link
appears in .Note: IPv6 ULAs taken from the prefix fc00::/7 followed by the L bit
set to 0 (i.e., as fc00::/8) are never used for OMNI OAL addressing,
however the range could be used for MSP/MNP addressing under certain
limiting conditions (see: ). When used within the
context of OMNI, ULAs based on the prefix fc00::/8 are referred to as
"ULA-C's".Note: When they appear in the OMNI link routing table, ULA-RNDs
always use prefix lengths between /48 and /64 (or, /128) while XLA-MNPs
always use prefix lengths between /65 and /128. {TLA,ULA}-MNPs and
{TLA,XLA}-RNDs should never appear in the OMNI link routing table, but
may appear in {A,I,E}NET routing tables.OMNI domains use IP Global Unicast Address (GUA) prefixes as Mobility Service Prefixes (MSPs) from which Mobile
Network Prefixes (MNP) are delegated to Clients. Fixed correspondent
node networks reachable from the OMNI link are represented by non-MNP
GUA prefixes that are not derived from the MSP, but are treated in all
other ways the same as for MNPs.For IPv6, GUA MSPs are assigned by IANA
and/or an associated Regional Internet Registry (RIR) such that the OMNI
link can be interconnected to the global IPv6 Internet without causing
inconsistencies in the routing system. An OMNI link could instead use
ULAs with the 'L' bit set to 0 (i.e., from the "ULA-C" prefix
fc00::/8), however this would require
IPv6 NAT if the domain were ever connected to the global IPv6
Internet.For IPv4, GUA MSPs are assigned by IANA
and/or an associated RIR such that the OMNI link can be interconnected
to the global IPv4 Internet without causing routing inconsistencies. An
OMNI ANET/ENET could instead use private IPv4 prefixes (e.g.,
10.0.0.0/8, etc.) , however this would require
IPv4 NAT at the INET-to-ANET/ENET boundary. OMNI interfaces advertise
IPv4 MSPs into IPv6 routing systems as IPv4-Compatible IPv6 prefixes
(e.g., the IPv6 prefix for the IPv4 MSP
192.0.2.0/24 is ::192.0.2.0/120).OMNI interfaces assign the IPv4 anycast address TBD3 (see: IANA
Considerations), and IPv4 routers that configure OMNI interfaces
advertise the prefix TBD3/N into the routing system of other networks
(see: IANA Considerations). OMNI interfaces also configure global IPv6
anycast addresses formed according to as:2002:TBD3{32}:MSP{64}:Link-ID{16}where TBD3{32} is the 32 bit IPv4 anycast address and MSP{64} encodes
an MSP zero-padded to 64 bits (if necessary). For example, the OMNI IPv6
anycast address for MSP 2001:db8::/32 is
2002:TBD3{32}:2001:db8:0:0:{Link-ID}, the OMNI IPv6 anycast address for
MSP 192.0.2.0/24 is 2002:TBD3{32}::c000:0200:{Link-ID}, etc.).The16-bit Link-ID in the OMNI IPv6 anycast address identifies a
specific OMNI link within the domain that services the MSP. The special
Link-ID value '0' is a wildcard that matches all links, while all other
values identify specific links. Mappings between Link-ID values and the
ULA Global IDs assigned to OMNI links are outside the scope of this
document.OMNI interfaces assign OMNI IPv6 anycast addresses, and IPv6 routers
that configure OMNI interfaces advertise the corresponding prefixes into
the routing systems of other networks. An OMNI IPv6 anycast prefix is
formed the same as for any IPv6 prefix; for example, the prefix
2002:TBD3{32}:2001:db8::/80 matches all OMNI IPv6 anycast addresses
covered by the prefix. When IPv6 routers advertise OMNI IPv6 anycast
prefixes in this way, Clients can locate and associate with either a
specific OMNI link or any OMNI link within the domain that services the
MSP of interest.OMNI interfaces use OMNI IPv6 and IPv4 anycast addresses to support
Service Discovery in the spirit of , i.e., the
addresses are not intended for use in long-term transport protocol
sessions. Specific applications for OMNI IPv6 and IPv4 anycast addresses
are discussed throughout the document as well as in .OMNI Clients and Proxy/Servers that connect over open Internetworks
include a unique node identification value for themselves in the OMNI
options of their IPv6 ND messages (see: ). An
example identification value alternative is the Host Identity Tag (HIT)
as specified in , while Hierarchical HITs
(HHITs) may be more appropriate for
certain domains such as the Unmanned (Air) Traffic Management (UTM)
service for Unmanned Air Systems (UAS). Another example is the
Universally Unique IDentifier (UUID) which can
be self-generated by a node without supporting infrastructure with very
low probability of collision.When a Client is truly outside the context of any infrastructure, it
may have no MNP information at all. In that case, the Client can use a
TLA or (H)HIT as an IPv6 source/destination address for sustained
communications in Vehicle-to-Vehicle (V2V) and (multihop)
Vehicle-to-Infrastructure (V2I) scenarios. The Client can also propagate
the ULA/(H)HIT into the multihop routing tables of (collective)
Mobile/Vehicular Ad-hoc Networks (MANETs/VANETs) using only the vehicles
themselves as communications relays.When a Client connects via a protected-spectrum ANET, an alternate
form of node identification (e.g., MAC address, serial number, airframe
identification value, VIN, etc.) embedded in a ULA may be sufficient.
The Client can then include OMNI "Node Identification" sub-options (see:
) in IPv6 ND messages should the need to transmit
identification information over the network arise.OMNI interfaces maintain a neighbor cache for tracking per-neighbor
state and use the link-local address format specified in . IPv6 Neighbor Discovery (ND) messages sent over OMNI interfaces without
encapsulation observe the native underlay interface Source/Target
Link-Layer Address Option (S/TLLAO) format (e.g., for Ethernet the
S/TLLAO is specified in ). IPv6 ND messages sent
over OMNI interfaces using encapsulation do not include S/TLLAOs, but
instead include a new option type that encodes encapsulation addresses,
interface attributes and other OMNI link information. Hence, this
document does not define an S/TLLAO format but instead defines a new
option type termed the "OMNI option" designed for these purposes. (Note
that OMNI interface IPv6 ND messages sent without encapsulation may
include both OMNI options and S/TLLAOs, but the information conveyed in
each is mutually exclusive.)OMNI interfaces prepare IPv6 ND messages that include one or more
OMNI options (and any other IPv6 ND options) then completely populate
all option information. If the OMNI interface includes an authentication
signature, it sets the IPv6 ND message Checksum field to 0 and
calculates the authentication signature over the length of the entire
OAL packet or super-packet (beginning with a pseudo-header of the IPv6
ND message IPv6 header) but does not calculate/include the IPv6 ND
message checksum itself. Otherwise, the OMNI interface calculates the
standard IPv6 ND message checksum over the entire OAL packet or
super-packet and writes the value in the Checksum field. OMNI interfaces
verify authentication and/or integrity of each IPv6 ND message received
according to the specific check(s) included, and process the message
further only following verification.OMNI interface Clients such as aircraft typically have multiple
wireless data link types (e.g. satellite-based, cellular, terrestrial,
air-to-air directional, etc.) with diverse performance, cost and
availability properties. The OMNI interface would therefore appear to
have multiple L2 connections, and may include information for multiple
underlay interfaces in a single IPv6 ND message exchange. OMNI
interfaces manage their dynamically-changing multilink profiles by
including OMNI options in IPv6 ND messages as discussed in the following
subsections.OMNI options appear in IPv6 ND messages formatted as shown in :In this format:Type is set to TBD4 (see: IANA Considerations).Length is set to the number of 8 octet blocks in the option.
The value 0 is invalid, while the values 1 through 255 (i.e., 8
through 2040 octets, respectively) indicate the total length of
the OMNI option. If multiple OMNI option instances appear in the
same IPv6 ND message, the union of the contents of all OMNI
options is accepted unless otherwise qualified for specific
sub-options below.Sub-Options is a Variable-length field padded if necessary such
that the complete OMNI Option is an integer multiple of 8 octets
long. Sub-Options contains zero or more sub-options as specified
in .The OMNI option is included in all OMNI interface IPv6 ND
messages; the option is processed by receiving interfaces that
recognize it and otherwise ignored. The OMNI interface processes all
OMNI option instances received in the same IPv6 ND message in the
consecutive order in which they appear. The OMNI option(s) included in
each IPv6 ND message may include full or partial information for the
neighbor. The OMNI interface therefore retains the union of the
information in the most recently received OMNI options in the
corresponding NCE.Each OMNI option includes a Sub-Options block containing zero or
more individual sub-options. Each consecutive sub-option is
concatenated immediately following its predecessor. All sub-options
except Pad1 (see below) are in an OMNI-specific type-length-value
(TLV) format encoded as follows: Sub-Type is a 5-bit field that encodes the sub-option type.
Sub-option types defined in this document are:Sub-Types 15-29 are available for future assignment for
major protocol functions, while Sub-Type 30 supports scalable
extension to include other functions. Sub-Type 31 is reserved by
IANA.Sub-Length is an 11-bit field that encodes the length of the
Sub-Option Data in octets.Sub-Option Data is a block of data with format determined by
Sub-Type and length determined by Sub-Length. Note that each
individual sub-option may end on an arbitrary octet boundary,
whereas the OMNI option itself must include padding if necessary
for 8-octet alignment.The OMNI interface codes each sub-option with a 2 octet
header that includes Sub-Type in the most significant 5 bits followed
by Sub-Length in the next most significant 11 bits. Each sub-option
encodes a maximum Sub-Length value of 2038 octets minus the lengths of
the OMNI option header and any preceding sub-options. This allows
ample Sub-Option Data space for coding large objects (e.g., ASCII
strings, domain names, protocol messages, security codes, etc.), while
a single OMNI option is limited to 2040 octets the same as for any
IPv6 ND option.The OMNI interface codes initial sub-options in a first OMNI option
instance and subsequent sub-options in additional instances in the
same IPv6 ND message in the intended order of processing. The OMNI
interface can then code any remaining sub-options in additional IPv6
ND messages if necessary. Implementations must observe these size
limits and refrain from sending IPv6 ND messages larger than the OMNI
interface MTU.The OMNI interface processes all OMNI option Sub-Options received
in an IPv6 ND message while skipping over and ignoring any
unrecognized sub-options. The OMNI interface processes the Sub-Options
of all OMNI option instances in the consecutive order in which they
appear in the IPv6 ND message, beginning with the first instance and
continuing through any additional instances to the end of the message.
If an individual sub-option length would cause processing to exceed
the OMNI option instance and/or IPv6 ND message lengths, the OMNI
interface accepts any sub-options already processed and ignores the
remainder of that instance. The interface then processes any remaining
OMNI option instances in the same fashion to the end of the IPv6 ND
message.When an OMNI interface includes an authentication sub-option (e.g.,
see: ), it MUST appear as the first sub-option of
the first OMNI option which must appear immediately following the IPv6
ND message header (all other authentication sub-options are ignored).
If the IPv6 ND message is the first packet in a combined OAL
super-packet, the OMNI interface calculates the authentication
signature over the entire length of the super-packet, i.e., and not
just to the end of the IPv6 ND message itself. When the first
sub-option is not authentication, the OMNI interface instead
calculates the IPv6 ND message checksum over the entire length of the
packet/super-packet.When a Client OMNI interface prepares a secured unicast RS message,
it includes an Interface Attributes sub-option specific to the
underlay interface that will transmit the RS (see: ) immediately following the authentication and header
extension sub-options if present; otherwise as the first sub-option of
the first OMNI option which must appear immediately following the IPv6
ND message header. When a Client OMNI interface prepares a secured
unicast NS message, it instead includes a Multilink Forwarding
Parameters sub-option specific to the underlay interface that will
transmit the NS (see: ).Note: large objects that exceed the maximum Sub-Option Data length
are not supported under the current specification; if this proves to
be limiting in practice, future specifications may define support for
fragmenting large sub-options across multiple OMNI options within the
same IPv6 ND message (or even across multiple IPv6 ND messages, if
necessary).The following sub-option types and formats are defined in this
document:Sub-Type is set to 0. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Type is followed by 3 'x' bits, set to any value on
transmission (typically all-zeros) and ignored on reception.
Pad1 therefore consists of 1 octet with the most significant 5
bits set to 0, and with no Sub-Length or Sub-Option Data fields
following.If more than one octet of padding is required, the PadN
option, described next, should be used, rather than multiple Pad1
options.Sub-Type is set to 1. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N that encodes the number of padding
octets that follow.Sub-Option Data consists of N octets, set to any value on
transmission (typically all-zeros) and ignored on receipt.When a proxy forwards an IPv6 ND message with OMNI options,
it can employ PadN to cancel any sub-options (other than Pad1) that
should not be processed by the next hop by simply writing the value
'1' over the Sub-Type. When the proxy alters the IPv6 ND message
contents in this way, any included authentication and integrity
checks are invalidated. See: for a
discussion of IPv6 ND message authentication and integrity.IPv6 ND messages used for Prefix Length assertion, service
coordination and/or Window Synchronization include a Neighbor
Coordination sub-option. If a Neighbor Coordination sub-option is
included, it must appear immediately after the authentication
sub-option if present; otherwise, as the first (non-padding)
sub-option of the first OMNI option. If multiple Neighbor
Coordination sub-options are included (whether in a single OMNI
option or multiple), only the first is processed and all others are
ignored.The Neighbor Coordination sub-option is formatted as follows:Sub-Type is set to 2.Sub-Length is set to 14.The first two octets of Sub-Option Data contains a 1-octet
Prefix Length followed by a 1-octet flags field interpreted as
follows:Preflen is an 8 bit field that determines the length of
prefix associated with a ULA containing an MNP. Values 0
through 128 specify a valid prefix length (if any other
value appears the OMNI option must be ignored). For IPv6 ND
messages sent from a Client to the MS, Preflen applies to
the IPv6 source ULA and provides the length that the Client
is requesting from or asserting to the MS. For IPv6 ND
messages sent from the MS to the Client, Preflen applies to
the IPv6 destination ULA and indicates the length that the
MS is granting to the Client. For IPv6 ND messages sent
between MS endpoints, Preflen provides the length associated
with the source/target Client MNP that is subject of the ND
message and encodes the value 64 plus the length of the MNP.
(For example, if the MNP length is 56 then Preflen encodes
the value 120.) When an IPv6 ND RS/RA message sets Preflen
to 0, the recipient regards the message as a prefix release
indication.The N/A/U flags are set or cleared in Client RS messages
as directives to FHS and Hub Proxy/Servers and ignored in
all other IPv6 ND messages. When an FHS Proxy/Server
forwards or processes an RS with the N flag set, it responds
directly to NS Neighbor Unreachability Detection (NUD)
messages by returning NA(NUD) replies; otherwise, it
forwards NS(NUD) messages to the Client. When the Hub
Proxy/Server receives an RS with the A flag set, it responds
directly to NS Address Resolution (AR) messages by returning
NA(AR) replies; otherwise, it forwards NS(AR) messages to
the Client. When the Hub Proxy/Server receives an RS with
the U flag set, it maintains a Report List of recent NS(AR)
message sources for this Client and sends uNA messages to
all list members if any aspects of the Client's underlay
interfaces change. Proxy/Servers function according to the
N/A/U flag settings received in the most recent RS message
to support dynamic Client updates. In all IPv6 ND messages,
the remaining 5 flag bits are set to 0 on transmission and
ignored on reception.The remainder of Sub-Option Data contains a 4-octet Sequence
Number, followed by a 4-octet Acknowledgement Number, followed
by a 1-octet flags field followed by a 3-octet Window size
modeled from the Transmission Control Protocol (TCP) header
specified in Section 3.1 of . The (SYN,
ACK, RST) flags are used for TCP-like window synchronization,
while the TCP (URG, PSH, FIN) flags are not used and therefore
omitted. The (OPT, PNG) flags are OMNI-specific, and the
remaining flags are Reserved. Together, these fields support the
asymmetric and symmetric OAL window synchronization services
specified in .The Interface Attributes sub-option provides neighbors with
forwarding information for the multilink conceptual sending
algorithm discussed in . Neighbors use the
forwarding information to selecting among potentially multiple
candidate underlay interfaces that can be used to forward carrier
packets to the neighbor based on factors such as traffic selectors
and link quality. Interface Attributes further include link-layer
address information to be used for either direct INET encapsulation
for targets in the local SRT segment or spanning tree forwarding for
targets in remote SRT segments.OMNI nodes include Interface Attributes for some/all of a target
Client's underlay interfaces in NS/NA and uNA messages used to
publish Client information (see: ). At most one Interface Attributes
sub-option for each distinct omIndex may be included; if an NS/NA
message includes multiple Interface Attributes sub-options for the
same omIndex, the first is processed and all others are ignored.
OMNI nodes that receive NS/NA messages can use all of the included
Interface Attributes and/or Traffic Selectors to formulate a map of
the prospective target node as well as to seed the information to be
populated in a Multilink Forwarding Parameters sub-option (see:
).OMNI Clients and Proxy/Servers also include Interface Attributes
sub-options in RS/RA messages used to initialize, discover and
populate routing and addressing information. Each RS message MUST
contain exactly one Interface Attributes sub-option with an omIndex
corresponding to the Client's underlay interface used to transmit
the message, and each RA message MUST echo the same Interface
Attributes sub-option with any (proxyed) information populated by
the FHS Proxy/Server to provide operational context.OMNI Client RS and Proxy/Server RA messages MUST include the
Interface Attributes sub-option for the Client underlay interface in
the first OMNI option immediately following the Neighbor
Coordination and/or authentication sub-option(s) if present;
otherwise, immediately following the OMNI header. When an FHS
Proxy/Server receives an RS message destined to an anycast L2
address, it MUST include an Interface Attributes sub-option with
omIndex '0' that encodes its unicast L2 address relative to the
Client's underlay interface immediately after the Interface
Attributes sub-option in the solicited RA response. Any additional
Interface Attributes sub-options that appear in RS/RA messages are
ignored.The Interface Attributes sub-options are formatted as shown
below:Sub-Type is set to 3.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow.Sub-Option Data contains an "Interface Attributes" option
encoded as follows:omIndex is a 1-octet value corresponding to a specific
underlay interface. Client OMNI interfaces MUST number each
distinct underlay interface with an omIndex value between
'1' and '255' that represents a Client-specific 8-bit
mapping for the actual ifIndex value assigned by network
management , then set omIndex to
either a specific omIndex value or '0' to denote
"unspecified".omType is set to an 8-bit integer value corresponding to
the underlay interface identified by omIndex. The value
represents an OMNI interface-specific 8-bit mapping for the
actual IANA ifType value registered in the 'IANAifType-MIB'
registry [http://www.iana.org].Provider ID is set to an OMNI interface-specific 8-bit ID
value for the network service provider associated with this
omIndex.Link encodes a 4-bit link metric. The value '0' means the
link is DOWN, and the remaining values mean the link is UP
with metric ranging from '1' ("lowest") to '15'
("highest").Resvd is a 4-bit Reserved field set to 0 on transmission
and ignored on reception.FMT - a 3-bit "Forward/Mode/Type" code interpreted as
follows:The most significant two bits (i.e., "FMT-Forward"
and "FMT-Mode") are interpreted in conjunction with one
another. When FMT-Forward is clear, the LHS Proxy/Server
performs OAL reassembly and decapsulation to obtain the
original IP packet before forwarding. If the FMT-Mode
bit is clear, the LHS Proxy/Server then forwards the
original IP packet at layer 3; otherwise, it invokes the
OAL to re-encapsulate, re-fragment and forwards the
resulting carrier packets to the Client via the selected
underlay interface. When FMT-Forward is set, the LHS
Proxy/Server forwards unsecured OAL fragments to the
Client without reassembling, while reassembling secured
OAL fragments before re-fragmenting and forwarding to
the Client. If FMT-Mode is clear, all carrier packets
destined to the Client must always be forwarded through
the LHS Proxy/Server; otherwise the Client is eligible
for direct forwarding over the open INET where it may be
located behind one or more NATs.The least significant bit (i.e., "FMT-Type")
determines the length of the LHS Proxy/Server INADDR
field. If FMT-Type is clear, INADDR includes a 4-octet
IPv4 address; otherwise, a 16-octet IPv6 address. (Note:
the INADDR "short form" minimizes overhead for ND
messages that include many Interface Attributes
sub-options with IPv4 addresses.)SRT - a 5-bit Segment Routing Topology prefix length
value between 0 and 16 that (when added to 48) determines
the prefix length associated with the LHS ULA Subnet ID. For
example, the value 5 corresponds to the prefix ULA::/53.LHS Proxy/Server ULA/INADDR - the first 15 octets
following the "FMT/SRT" octet includes the 120 least
significant bits of the ULA of the LHS Proxy/Server on the
path from a source neighbor to the target Client's underlay
interface. (Note that the FMT/SRT code is replaced with the
value "fd" after processing to form a proper Proxy/Server
ULA.) When SRT and ULA are both set to 0, the LHS
Proxy/Server is considered unspecified in this IPv6 ND
message. FMT, SRT and LHS together provide guidance for the
OMNI interface forwarding algorithm. Specifically, if
SRT/LHS is located in the local OMNI link segment, then the
source can reach the target Client either through its
dependent Proxy/Server or through direct encapsulation
following NAT traversal according to FMT. Otherwise, the
target Client is located on a different SRT segment and the
path from the source must employ a combination of route
optimization and spanning tree hop traversals. INADDR
identifies the LHS Proxy/Server's INET-facing interface not
located behind NATs, therefore no UDP port number is
included since port number 8060 is used when the L2
encapsulation includes a UDP header. Instead, INADDR
includes only a 4-octet IPv4 or 16-octet IPv6 address with
type and length determined by FMT-Type. The IP address is
recorded in network byte order in ones-compliment
"obfuscated" form per .OMNI nodes include the Multilink Forwarding Parameters sub-option
in NS/NA messages used to coordinate with multilink route
optimization targets. If an NS message includes the sub-option, the
solicited NA response must also include the sub-option. The OMNI
node MUST include the sub-option in the first OMNI option
immediately following the Neighbor Coordination and/or
authentication message sub-option(s) if present. Otherwise, the OMNI
node MUST include the sub-option immediately following the OMNI
header. Each NS/NA message may contain at most one Multilink
Forwarding Parameters sub-option; if an NS/NA message contains
additional Multilink Forwarding Parameters sub-options, the first is
processed and all others are ignored.When an NS/NA message includes the sub-option, the FHS Client
omIndex MUST correspond to the underlay interface used to transmit
the message. When the NS/NA message also includes Interface
Attributes sub-options any that include the same FHS/LHS Client
omIndex are ignored while all others are processed.The Multilink Forwarding Parameters sub-option includes the
necessary state for establishing Multilink Forwarding Vectors (MFVs)
in the Multilink Forwarding Information Bases (MFIBs) of the OAL
source, destination and intermediate nodes in the path. The
sub-option also records addressing information for FHS/LHS nodes on
the path, including "INADDRs" which MUST be unicast IP encapsulation
addresses (i.e., and not anycast/multicast). The manner for
populating multilink forwarding information is specified in detail
in .The Multilink Forwarding Parameters sub-option is formatted as
shown in :Sub-Type is set to 4. If multiple instances appear in the
same message (i.e., whether in a single OMNI option or multiple)
the first instance is processed and all others are ignored.Sub-Length encodes the number of Sub-Option Data octets that
follow. The length includes all fields up to and including the
Tunnel Window Synchronization Parameters for all Job codes,
while including the remaining fields only for Job codes "0" and
"1" (see below).Sub-Option Data contains Multilink Forwarding Parameters as
follows:Reserved is a 1-octet reserved field set to 0 on
transmission and ignored on receipt.A/B and Job are fields that determine per-hop processing
of the MFVI List, where A is a 3-bit count of the number of
"A" MVFI List entries and B is a 3-bit count of the number
of "B" MVFI List entries (valid A/B values are 0-5). Job is
a 2-bit code interpreted as follows:'00' - "Initialize; Build B" - the FHS source sets
this code in an NS used to initialize MFV state (any
other messages that include this code MUST be dropped).
The FHS source first sets A/B to 0, and the FHS source
and each intermediate node along the path to the LHS
destination that processes the message creates a new
MFV. Each node that processes the message then assigns a
unique 4-octet "B" MFVI to the MVF and also writes the
value into list entry B, then increments B. When the
message arrives at the LHS destination, B will contain
the number of MFVI List "B" entries, with the FHS source
entry first, followed by entries for each consecutive
intermediate node and ending with an entry for the final
intermediate node (i.e., the list is populated in the
forward direction).'01' - "Follow B; Build A" - the LHS source sets this
code in a solicited NA response to a solicitation with
Job code "0" (any other messages that include this code
MUST be dropped). The LHS source first copies the MFVI
List and B value from the code "0" solicitation into
these fields and sets A to 0. The LHS source and each
intermediate node along the path to the FHS destination
that processes the message then uses MFVI List entry B
to locate the corresponding MFV. Each node that
processes the message then assigns a unique 4-octet "A"
MFVI to the MVF and also writes the value into list
entry B, then increments A and decrements B. When the
message arrives at the FHS destination, A will contain
the number of MFVI List "A" entries, with the LHS source
entry last, preceded by entries for each consecutive
intermediate node and beginning with an entry for the
final intermediate node (i.e., the list is populated in
the reverse direction).'10' - "Follow A; Record B" - the FHS node that sent
the original code "0" solicitation and received the
corresponding code "1" advertisement sets this code in
any subsequent NS/NA messages sent to the same LHS
destination. The FHS source copies the MVFI List and A
value from the code "1" advertisement into these fields
and sets B to 0. The FHS source and each intermediate
node along the path to the LHS destination that
processes the message then uses the "A" MFVI found at
list entry B to locate the corresponding MFV. Each node
that processes the message then writes the MVF's "B"
MFVI into list entry B, then decrements A and increments
B. When the message arrives at the LHS destination, B
will contain the number of MFVI List "B" entries
populated in the forward direction.'11' - "Follow B; Record A" - the LHS node that
received the original code "0" solicitation and sent the
corresponding code "1" advertisement sets this code in
any subsequent NS/NA messages sent to the same FHS
destination. The LHS source copies the MVFI List and B
values from the code "0" solicitation into these fields
and sets A to 0. The LHS source and each intermediate
node along the path to the FHS destination that
processes the message then uses the "B" MFVI List entry
found at list entry B to locate the corresponding MFV.
Each node that processes the message then writes the
MFV's "A" MFVI into list entry B, then increments A and
decrements B. When the message arrives at the FHS
destination, A will contain the number of MFVI List "A"
entries populated in the reverse direction.Job and A/B together determine the per-hop behavior
at each FHS/LHS source, intermediate node and destination
that processes an IPv6 ND message. When a Job code specifies
"Initialize", each FHS/LHS node that processes the message
creates a new MVF. When a Job code specifies "Build", each
node that processes the message assigns a new MFVI. When a
Job code specifies "Follow", each node that processes the
message uses an A/B MFVI List entry to locate an MFV (if the
MFV cannot be located, the node returns a parameter problem
and drops the message). Using this algorithm, FHS sources
that send code '00' solicitations and receive code '01'
advertisements discover only "A" information, while LHS
sources that receive code '00' solicitations and return code
'01' advertisements discover only "B" information. FHS/LHS
intermediate nodes can instead examine A, B and the MFVI
List to determine the number of previous hops, the number of
remaining hops, and the A/B MFVIs associated with the
previous/remaining hops. However, no intermediate nodes will
discover inappropriate A/B MFVIs for their location in the
multihop forwarding chain. See: for further discussion on
A/B MFVI processing.Multilink Forwarding Vector Index (MFVI) List is a
20-octet block that contains 5 consecutive 4-octet MFVI
entries. The FHS/LHS source and each intermediate node on
the path to the destination processes the list according to
the Job and A/B codes (see above). Note that the reason the
MFVI list contains at most 5 entries is that only the FHS
(Client, Proxy/Server, Gateway) and LHS (Client,
Proxy/Server, Gateway) nodes are eligible for OMNI link
route optimization resulting in at most 5 MFVIs "hops" that
must be exposed. All other OMNI link nodes (i.e., downstream
Clients that connect via an FHS/LHS Client) must forward
through their upstream-dependent OMNI link neighbors without
applying OMNI link route optimization.Tunnel Window Synchronization Parameters is a 12-octet
block that consists of a 4-octet Sequence Number followed by
a 4-octet Acknowledgement Number followed by a 1-octet Flags
field followed by a 3-octet Window field (i.e., the same as
for the OMNI header parameters). Tunnel endpoints use these
parameters for simultaneous middlebox window synchronization
in a single solicitation/advertisement message exchange.For Job codes '00' and '01' only, two trailing state
variable blocks are included for First-Hop Segment (FHS)
followed by Last-Hop Segment (LHS) network elements. When
present, each block encodes the following information:Client omIndex, omType, Provider ID and Resvd/Link
are 1-octet fields (at offset 0 from the beginning of
the Sub-Option Data) that include link parameters for
the Client underlay interface. These fields are
populated based on information discovered in Interface
Attributes sub-options included in earlier RS/RA and/or
NS/NA exchanges.FMT/SRT is a 1-octet field with a 5-bit SRT prefix
length that applies to all elements in the segment. The
FMT-Forward/Mode bits determine the characteristics of
the Proxy/Server relationship for this specific Client
underlay interface (i.e., the same as described in ), and the FMT-Type bits determine the IP
address version for all INADDR fields relative to this
SRT segment. Unlike the case for Interface Attributes,
all INADDR fields are always 16 bits in length
regardless of the IP protocol version with IPv4 INADDRs
encoded as IPv4-Compatible IPv6 addresses . (Note: the INADDR "long-form" is
used exclusively since there may be no a priori
knowledge of the IP address version used at each hop.)
The IP address is recoded in network byte order, and in
ones-compliment "obfuscated" form the same as described
in .Proxy/Server ULA/INADDR includes a 15 octet value
that encodes the 120 least significant bits of the
Proxy/Server ULA followed by a 16 octet INADDR. (Note
that the FMT/SRT code is replaced with the value "fd"
after processing to form a proper Proxy/Server ULA.)
INADDR identifies an open INET interface not located
behind NATs, therefore no UDP port number is included
since port number 8060 is used when the L2 encapsulation
includes a UDP header.Gateway ULA/INADDR encodes a 16 octet ULA followed by
a 16 octet INADDR exactly as for the Proxy/Server
ULA/INADDR. (Note that the Gateway ULA simply encodes
the value "fd" in the most significant bits, since the
FMT/SRT code applies to both the Proxy/Server and
Gateway.)When used in conjunction with Interface Attributes and/or
Multilink Forwarding Parameters information, the Traffic Selector
sub-option provides forwarding information for the multilink
conceptual sending algorithm discussed in .IPv6 ND messages include Traffic Selectors for some or all of the
source/target Client's underlay interfaces. Traffic Selectors for
some or all of a target Client's underlay interfaces are also
included in uNA messages used to publish Client information changes.
See: for more
information.Traffic Selectors must be honored by all implementations in the
format shown below:Sub-Type is set to 5. Each IPv6 ND message may contain zero
or more Traffic Selectors for each omIndex; when multiple
Traffic Selectors for the same omIndex appear, all are processed
and the cumulative information from all is accepted.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow.Sub-Option Data contains a "Traffic Selector" encoded as
follows:omIndex is a 1-octet value corresponding to a specific
underlay interface the same as specified above for Interface
Attributes and Multilink Forwarding Parameters above. The
OMNI options of a single message may include multiple
Traffic Selector sub-options; each with the same or
different omIndex values.TS Format is a 1-octet field that encodes a Traffic
Selector version per . If TS Format
encodes the value 1 or 2, the Traffic Selector includes IPv4
or IPv6 information, respectively. If TS Format encodes any
other value, the sub-option is ignored.The remainder of the sub-option includes a traffic
selector formatted per beginning
with the "Flags (A-N)" field, and with the Traffic Selector
IP protocol version coded in the TS Format field. If a
single interface identified by omIndex requires Traffic
Selectors for multiple IP protocol versions, or if a Traffic
Selector block would exceed the available space, the
remaining information is coded in additional Traffic
Selector sub-options that all encode the same omIndex.Sub-Type is set to 6. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow.Geo Type is a 1 octet field that encodes a type designator
that determines the format and contents of the Geo Coordinates
field that follows. The following types are currently
defined:0 - NULL, i.e., the Geo Coordinates field is
zero-length.A set of Geo Coordinates of length up to the remaining
available space for this OMNI option. New formats to be
specified in future documents and may include attributes such as
latitude/longitude, altitude, heading, speed, etc.The Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
sub-option may be included in the OMNI options of Client RS messages
and Proxy/Server RA messages. FHS Proxy/Servers that forward RS/RA
messages between a Client and an LHS Proxy/Server also forward
DHCPv6 Sub-Options unchanged. Note that DHCPv6 messages do not
include a Checksum field since integrity is protected by the IPv6 ND
message checksum, authentication signature and/or lower-layer
authentication and integrity checks.Sub-Type is set to 7. If multiple instances appear in OMNI
options of the same message the first is processed and all
others are ignored.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow. The 'msg-type' and 'transaction-id'
fields are always present; hence, the length of the DHCPv6
options is limited by the remaining available space for this
OMNI option.'msg-type' and 'transaction-id' are coded according to
Section 8 of .A set of DHCPv6 options coded according to Section 21 of
follows.The Host Identity Protocol (HIP) Message sub-option (when
present) provides authentication for IPv6 ND messages exchanged
between Clients and FHS Proxy/Servers over an open Internetwork. FHS
Proxy/Servers authenticate the HIP authentication signatures in
source Client IPv6 ND messages before securely forwarding them to
other OMNI nodes. LHS Proxy/Servers that receive secured IPv6 ND
messages from other OMNI nodes that do not already include a
security sub-option insert HIP authentication signatures before
forwarding them to the target Client.OMNI interfaces MUST include the HIP message (when present) as
the first sub-option of the first OMNI option, which MUST appear
immediately following the IPv6 ND message header. OMNI interfaces
can therefore easily locate the HIP message and verify the
authentication signature without applying deep inspection. OMNI
interfaces that receive IPv6 ND messages without a HIP (or other
authentication) sub-option as the first OMNI sub-option instead
verify the IPv6 ND message checksum.OMNI interfaces include the HIP message sub-option when they
forward IPv6 ND messages that require security over INET underlay
interfaces, i.e., where authentication and integrity is not already
assured by lower layers. The OMNI interface calculates the
authentication signature over the entire length of the OAL packet
(or super-packet) beginning with a pseudo-header of the IPv6 ND
message header and extending over the remainder of the OAL packet.
OMNI interfaces that process OAL packets that contain secured IPv6
ND messages verify the signature then either process the rest of the
message locally or forward a proxyed copy to the next hop.When a FHS Client inserts a HIP message sub-option in an NS/NA
message destined to a target in a remote spanning tree segment, it
must ensure that the insertion does not cause the message to exceed
the OMNI interface MTU. When the remote segment LHS Proxy/Server
forwards the NS/NA message from the spanning tree to the target
Client, it inserts a new HIP message sub-option if necessary while
overwriting or cancelling the (now defunct) HIP message sub-option
supplied by the FHS Client.If the defunct HIP sub-option size was smaller than the space
needed for the LHS Client HIP message (or, if no defunct HIP
sub-option is present), the LHS Proxy/Server adjusts the space
immediately following the OMNI header by copying the preceding
portion of the IPv6 ND message into buffer headroom free space or
copying the remainder of the IPv6 ND message into buffer tailroom
free space. The LHS Proxy/Server then insets the new HIP sub-option
immediately after the OMNI header and immediately before the next
sub-option while properly overwriting the defunct sub-option if
present.If the defunct HIP sub-option size was larger than the space
needed for the LHS Client HIP message, the LHS Proxy/Server instead
overwrites the existing sub-option and writes a single Pad1 or PadN
sub-option over the next 1-2 octets to cancel the remainder of the
defunct sub-option. If the LHS Proxy/Server cannot create sufficient
space through any means without causing the OMNI option to exceed
2040 octets or causing the IPv6 ND message to exceed the OMNI
interface MTU, it returns a suitable error (see: ) and drops the message.The HIP message sub-option is formatted as shown below:Sub-Type is set to 8. If multiple instances appear in OMNI
options of the same message the first is processed and all
others are ignored.Sub-Length is set to N, i.e., the length of the option in
octets beginning immediately following the Sub-Length field and
extending to the end of the HIP parameters. The length of the
entire HIP message is therefore limited by the remaining
available space for this OMNI option.The HIP message is coded per Section 5 of , except that the OMNI "Sub-Type" and
"Sub-Length" fields replace the first 2 octets of the HIP
message header (i.e., the Next Header and Header Length fields).
Also, since the IPv6 ND message is already protected by the
authentication signature and/or lower-layer authentication and
integrity checks, the HIP message Checksum field is replaced by
a Reserved field set to 0 on transmission and ignored on
reception.Note: In some environments, maintenance of a Host Identity
Tag (HIT) namespace may be unnecessary for securely associating an
OMNI node with an IPv6 address-based identity. In that case, IPv6
ULAs can be used instead of HITs in the authentication signature as
long as the address can be uniquely associated with the
Sender/Receiver.The Protocol Independent Multicast - Sparse Mode (PIM-SM) Message
sub-option may be included in the OMNI options of IPv6 ND messages.
PIM-SM messages are formatted as specified in Section 4.9 of , with the exception that the Checksum field is
replaced by a Reserved field (set to 0) since the IPv6 ND message is
already protected by the IPv6 ND message checksum, authentication
signature and/or lower-layer authentication and integrity checks.
The PIM-SM message sub-option format is shown in :Sub-Type is set to 9. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N, i.e., the length of the option in
octets beginning immediately following the Sub-Length field and
extending to the end of the PIM-SM message. The length of the
entire PIM-SM message is therefore limited by the remaining
available space for this OMNI option.The PIM-SM message is coded exactly as specified in Section
4.9 of , except that the Checksum field
is replaced by a Reserved field set to 0 on transmission and
ignored on reception. The "PIM Ver" field MUST encode the value
2, and the "Type" field encodes the PIM message type. (See
Section 4.9 of for a list of PIM-SM
message types and formats.)Fragmentation Report (FRAGREP) sub-options may be included in the
OMNI options of uNA messages sent from an OAL destination to an OAL
source. The message consists of (N / 20)-many (Identification,
Bitmap)-tuples which include the Identification values of OAL
fragments received plus a Bitmap marking the ordinal positions of
individual fragments received and fragments missing.Sub-Type is set to 10. If multiple instances appear in OMNI
options of the same message all are processed.Sub-Length is set to N, i.e., the length of the option in
octets beginning immediately following the Sub-Length field and
extending to the end of the sub-option. If N is not an integral
multiple of 20 octets, the sub-option is ignored. The length of
the entire sub-option should not cause the entire IPv6 ND
message to exceed the minimum IPv6 MTU.Identification (i) includes the IPv6 Identification value
found in the Fragment Header of a received OAL fragment. (Only
those Identification values included represent fragments for
which loss was unambiguously observed; any Identification values
not included correspond to fragments that were either received
in their entirety or may still be in transit.)Bitmap (i) includes an ordinal checklist of up to 128
fragments, with each bit set to 1 for a fragment received or 0
for a fragment missing. For example, for a 20-fragment OAL
packet with ordinal fragments #3, #10, #13 and #17 missing and
all other fragments received, Bitmap (i) encodes the
following:(Note that loss of an OAL atomic fragment is
indicated by a Bitmap(i) with all bits set to 0.)Sub-Type is set to 11. If multiple instances appear in OMNI
options of the same IPv6 ND message the first instance of a
specific ID-Type is processed and all other instances of the
same ID-Type are ignored. (It is therefore possible for a single
IPv6 ND message to convey multiple distinct Node Identifications
- each with a different ID-Type.)Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow. The ID-Type field is always present;
hence, the maximum Node Identification Value length is limited
by the remaining available space in this OMNI option.ID-Type is a 1 octet field that encodes the type of the Node
Identification Value. The following ID-Type values are currently
defined:0 - Universally Unique IDentifier (UUID) . Indicates that Node Identification Value
contains a 16 octet UUID.1 - Host Identity Tag (HIT) .
Indicates that Node Identification Value contains a 16 octet
HIT.2 - Hierarchical HIT (HHIT) . Indicates that Node
Identification Value contains a 16 octet HHIT.3 - Network Access Identifier (NAI) . Indicates that Node Identification Value
contains an N-1 octet NAI.4 - Fully-Qualified Domain Name (FQDN) . Indicates that Node Identification Value
contains an N-1 octet FQDN.5 - IPv6 Address. Indicates that Node Identification
contains a 16-octet IPv6 address that is not a (H)HIT. The
IPv6 address type is determined according to the IPv6
addressing architecture .6 - 252 - Unassigned.253-254 - Reserved for experimentation, as recommended in
.255 - reserved by IANA.Node Identification Value is an (N - 1) octet field encoded
according to the appropriate the "ID-Type" reference above.OMNI interfaces code Node Identification Values used for DHCPv6
messaging purposes as a DHCP Unique IDentifier (DUID) using the
"DUID-EN for OMNI" format with enterprise number 45282 (see: ) as shown in :In this format, the OMNI interface codes the ID-Type and Node
Identification Value fields from the OMNI sub-option following a 6
octet DUID-EN header, then includes the entire "DUID-EN for OMNI" in
a DHCPv6 message per .Sub-Type is set to 12. If multiple instances appear in OMNI
options of the same IPv6 ND message all are processed.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow.Sub-Option Data includes a one octet Type followed by a one
octet Code followed by an (N-2)-octet Message Body encoded
exactly as per Section 2.1 of . OMNI
interfaces include as much of the ICMPv6 error message body in
the sub-option as possible without causing the entire IPv6 ND
message to exceed the minimum IPv6 MTU. While all ICMPv6 error
message types are supported, OAL destinations in particular may
include ICMPv6 PTB messages in uNA messages to provide MTU
feedback information via the OAL source (see: ). Note: ICMPv6 informational messages must not
be included and must be ignored if received.Sub-Type is set to 13. If multiple instances appear in OMNI
options of the same IPv6 ND message, the first is processed and
all others are ignored.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow.The QUIC-TLS message encodes the QUIC and
TLS message parameters necessary to support QUIC connection
establishment.When present, the QUIC-TLS Message sub-option MUST appear
immediately after the header of the first OMNI option in the IPv6 ND
message; if the sub-option appears in any other location it MUST be
ignored. IPv6 ND solicitation and advertisement messages serve as
couriers to transport the QUIC and TLS parameters necessary to
establish a secured QUIC connection.OMNI Clients include a Proxy/Server Departure sub-option in RS
messages when they associate with a new FHS and/or Hub Proxy/Server
and need to send a departure indication to an old FHS and/or Hub
Proxy/Server. The Proxy/Server Departure sub-option is formatted as
shown below:Sub-Type is set to 14.Sub-Length is set to 32.Sub-Option Data contains the 16 octet ULA for the "Old FHS
Proxy/Server" followed by a 16 octet ULA for an "Old Hub
Proxy/Server. (If the Old FHS/Hub is unspecified, the
corresponding ULA instead includes the value 0.)Since the Sub-Type field is only 5 bits in length, future
specifications of major protocol functions may exhaust the remaining
Sub-Type values available for assignment. This document therefore
defines Sub-Type 30 as an "extension", meaning that the actual
Sub-Option type is determined by examining a 1 octet
"Extension-Type" field immediately following the Sub-Length field.
The Sub-Type Extension is formatted as shown in :Sub-Type is set to 30. If multiple instances appear in OMNI
options of the same message all are processed, where each
individual extension defines its own policy for processing
multiple of that type.Sub-Length is set to N that encodes the number of Sub-Option
Data octets that follow. The Extension-Type field is always
present, and the maximum Extension-Type Body length is limited
by the remaining available space in this OMNI option.Extension-Type contains a 1 octet Sub-Type Extension value
between 0 and 255.Extension-Type Body contains an N-1 octet block with format
defined by the given extension specification.Extension-Type values 0 and 1 are defined in the following
subsections, while Extension-Type values 2 through 252 are available
for assignment by future specifications which must also define the
format of the Extension-Type Body and its processing rules.
Extension-Type values 253 and 254 are reserved for experimentation,
as recommended in , and value 255 is
reserved by IANA.Sub-Type is set to 30.Sub-Length is set to N that encodes the number of
Sub-Option Data octets that follow. The Extension-Type and
Header Type fields are always present, and the Header Option
Value is limited by the remaining available space in this OMNI
option.Extension-Type is set to 0. Each instance encodes exactly
one header option per Section 5.1.1 of , with Ext-Type and Header Type representing
the first two octets of the option. If multiple instances of
the same Header Type appear in OMNI options of the same
message the first instance is processed and all others are
ignored. If Header Type indicates an Authentication
Encapsulation (see below), the entire sub-option MUST appear
as the first sub-option of the first OMNI option, which MUST
appear immediately following the IPv6 ND message header.Header Type and Header Option Value are coded exactly as
specified in Section 5.1.1 of ; the
following types are currently defined:0 - Origin Indication (IPv4) - value coded as a UDP
port number followed by a 4-octet IPv4 address both in
"obfuscated" form per Section 5.1.1 of .1 - Authentication Encapsulation - value coded per
Section 5.1.1 of .2 - Origin Indication (IPv6) - value coded per Section
5.1.1 of , except that the address
is a 16-octet IPv6 address instead of a 4-octet IPv4
address.Header Type values 3 through 252 are available for
assignment by future specifications, which must also define
the format of the Header Option Value and its processing
rules. Header Type values 253 and 254 are reserved for
experimentation, as recommended in ,
and value 255 is Reserved by IANA.Sub-Type is set to 30.Sub-Length is set to N that encodes the number of
Sub-Option Data octets that follow. The Extension-Type and
Trailer Type fields are always present, and the maximum-length
Trailer Option Value is limited by the remaining available
space in this OMNI option.Extension-Type is set to 1. Each instance encodes exactly
one trailer option per Section 4 of .
If multiple instances of the same Trailer Type appear in OMNI
options of the same message the first instance is processed
and all others ignored.Trailer Type and Trailer Option Value are coded exactly as
specified in Section 4 of ; the
following Trailer Types are currently defined:0 - Unassigned1 - Nonce Trailer - value coded per Section 4.2 of
.2 - Unassigned3 - Alternate Address Trailer (IPv4) - value coded per
Section 4.3 of .4 - Neighbor Discovery Option Trailer - value coded per
Section 4.4 of .5 - Random Port Trailer - value coded per Section 4.5
of .6 - Alternate Address Trailer (IPv6) - value coded per
Section 4.3 of , except that each
address is a 16-octet IPv6 address instead of a 4-octet
IPv4 address.Trailer Type values 7 through 252 are available for
assignment by future specifications, which must also define
the format of the Trailer Option Value and its processing
rules. Trailer Type values 253 and 254 are reserved for
experimentation, as recommended in ,
and value 255 is Reserved by IANA.The multicast address mapping of the native underlay interface
applies. The Client mobile router also serves as an IGMP/MLD Proxy for
its ENETs and/or hosted applications per .The Client uses Multicast Listener Discovery (MLDv2) to coordinate with Proxy/Servers, and underlay
network elements use MLD snooping . The Client
can also employ multicast routing protocols to coordinate with
network-based multicast sources as specified in .Since the OMNI link model is NBMA, OMNI links support link-scoped
multicast through iterative unicast transmissions to individual
multicast group members (i.e., unicast/multicast emulation).The Client's IPv6 layer selects the outbound OMNI interface according
to SBM considerations when forwarding original IP packets from local or
ENET applications to external correspondents. Each OMNI interface
maintains a neighbor cache the same as for any IPv6 interface, but
includes additional state for multilink coordination. Each Client OMNI
interface maintains default routes via Proxy/Servers discovered as
discussed in , and may configure more-specific
routes discovered through means outside the scope of this
specification.For each original IP packet it forwards, the OMNI interface selects
one or more source underlay interfaces based on PBM factors (e.g.,
traffic attributes, cost, performance, message size, etc.) and one or
more target underlay interfaces for the neighbor based on Interface
Attributes received in IPv6 ND messages (see: ).
Multilink forwarding may also direct packet replication across multiple
underlay interface pairs for increased reliability at the expense of
duplication. The set of all Interface Attributes and Traffic Selectors
received in IPv6 ND messages determines the multilink forwarding profile
for selecting target underlay interfaces.When the OMNI interface sends an original IP packet over a selected
source underlay interface, it first employs OAL encapsulation and
fragmentation as discussed in , then performs L2
encapsulation as directed by the appropriate MFV. The OMNI interface
also performs L2 encapsulation (following OAL encapsulation) when the
nearest Proxy/Server is located multiple hops away as discussed in .OMNI interface multilink service designers MUST observe the BCP
guidance in Section 15 in terms of implications
for reordering when original IP packets from the same flow may be spread
across multiple underlay interfaces having diverse properties.Clients may connect to multiple independent OMNI links within the
same or different OMNI domains to support SBM. The Client configures a
separate OMNI interface for each link so that multiple interfaces
(e.g., omni0, omni1, omni2, etc.) are exposed to the IP layer. Each
OMNI interface configures one or more OMNI anycast addresses (see:
), and the Client injects the corresponding
anycast prefixes into the ENET routing system. Multiple distinct OMNI
links can therefore be used to support fault tolerance, load
balancing, reliability, etc.Applications in ENETs can use Segment Routing to select the desired
OMNI interface based on SBM considerations. The application writes an
OMNI anycast address into the original IP packet's destination
address, and writes the actual destination (along with any additional
intermediate hops) into the Segment Routing Header. Standard IP
routing directs the packet to the Client's mobile router entity, where
the anycast address identifies the correct OMNI interface for next hop
forwarding. When the Client receives the packet, it replaces the IP
destination address with the next hop found in the Segment Routing
Header and forwards the message via the OMNI interface identified by
the anycast address.Note: The Client need not configure its OMNI interface indexes in
one-to-one correspondence with the global OMNI Link-IDs configured for
OMNI domain administration since the Client's indexes (i.e., omni0,
omni1, omni2, etc.) are used only for its own local interface
management.After a Proxy/Server has registered an MNP for a Client (see: ), the Proxy/Server will forward all packets destined
to an address within the MNP to the Client. The Client will under
normal circumstances then forward the packet to the correct
destination within its connected (downstream) ENETs.If at some later time the Client loses state (e.g., after a
reboot), it may begin returning packets with destinations
corresponding to its MNP to the Proxy/Server as its default router.
The Proxy/Server therefore drops any original IP packets received from
the Client with a destination address that corresponds to the Client's
MNP (i.e., whether ULA or GUA), and drops any carrier packets with
both source and destination address corresponding to the same Client's
MNP regardless of their origin.Clients engage the MS by sending RS messages with OMNI options under
the assumption that one or more Proxy/Server will process the message
and respond. The RS message is received by a FHS Proxy/Server, which may
in turn forward a proxyed copy of the RS to a Hub Proxy/Server located
on the same or different SRT segment. The Hub Proxy/Server then returns
an RA message either directly to the Client or via an FHS Proxy/Server
acting as a proxy.Clients and FHS Proxy/Servers include an authentication signature in
their RS/RA exchanges when necessary; otherwise, they calculate and
include a valid IPv6 ND message checksum (see: and ). FHS and Hub
Proxy/Server RS/RA message exchanges over the SRT secured spanning tree
instead always include the checksum and omit the authentication
signature. Clients and Proxy/Servers use the information included in
RS/RA messages to establish NCE state and OMNI link autoconfiguration
information as discussed in this section.For each underlay interface, the Client sends RS messages with OMNI
options to coordinate with a (potentially) different FHS Proxy/Server
for each interface but with a single Hub Proxy/Server. All Proxy/Servers
are identified by their ULA-RNDs and accept carrier packets addressed to
their anycast/unicast L2 INADDRs; the Hub Proxy/Server may be chosen
among any of the Client's FHS Proxy/Servers or may be any other
Proxy/Server for the OMNI link. Example ULA/INADDR discovery methods are
given in and include data link login
parameters, name service lookups, static configuration, a static "hosts"
file, etc. In the absence of other information, the Client can resolve
the DNS Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]"
where "linkupnetworks" is a constant text string and "[domainname]" is a
DNS suffix for the OMNI link (e.g., "example.com"). The name resolution
will retain a set of DNS resource records with the addresses of
Proxy/Servers for the domain.Each FHS Proxy/Server configures an ULA-RND based on a /64 ULA prefix
for the link/segment with randomly-generated Global ID to assure global
uniqueness then administratively assigned to FHS Proxy/Servers for the
link to assure global consistency. The Client can then configure
ULA-MNPs derived from the 64-bit ULA prefix assigned to a FHS
Proxy/Server for each underlay interface. The FHS Proxy/Servers
discovered over multiple of the Client's underlay interfaces may
configure the same or different ULA prefixes, and the Client's ULA-MNP
for each underlay interface will fall within the ULA (multilink) subnet
relative to each FHS Proxy/Server.Clients configure OMNI interfaces that observe the properties
discussed in previous sections. The OMNI interface and its underlay
interfaces are said to be in either the "UP" or "DOWN" state according
to administrative actions in conjunction with the interface connectivity
status. An OMNI interface transitions to UP or DOWN through
administrative action and/or through state transitions of the underlay
interfaces. When a first underlay interface transitions to UP, the OMNI
interface also transitions to UP. When all underlay interfaces
transition to DOWN, the OMNI interface also transitions to DOWN.When a Client OMNI interface transitions to UP, it sends RS messages
to register its MNP and an initial set of underlay interfaces that are
also UP. The Client sends additional RS messages to refresh lifetimes
and to register/deregister underlay interfaces as they transition to UP
or DOWN. The Client's OMNI interface sends initial RS messages over an
UP underlay interface with its XLA-MNP as the source (or with a TLA-RND
as the source if it does not yet have an MNP) and with destination set
to link-scoped All-Routers multicast or the ULA of a specific (Hub)
Proxy/Server. The OMNI interface includes an OMNI option per with an OMNI Neighbor Coordination sub-option with
(Preflen assertion, N/A/U flags and Window Synchronization parameters),
an Interface Attributes sub-option for the underlay interface, a DHCPv6
Solicit sub-option if necessary, and with any other necessary OMNI
sub-options such as authentication, Proxy/Server Departure, etc.The Client then calculates the authentication signature or checksum
and prepares to forward the RS over the underlay interface using OAL
encapsulation and fragmentation if necessary. If the Client uses OAL
encapsulation for RS messages sent to an unsynchronized FHS Proxy/Server
over an INET interface, the entire RS message must fit within a single
carrier packet (i.e., an atomic fragment) so that the FHS Proxy/Server
can verify the authentication signature without having to first
reassemble. The OMNI interface selects an Identification value (see:
), sets the OAL source address to the ULA-MNP
corresponding to the RS source if known (otherwise to a TLA-RND), sets
the OAL destination to an OMNI IPv6 anycast address or a known
Proxy/Server ULA, optionally includes a Nonce and/or Timestamp, then
performs fragmentation if necessary. When L2 encapsulation is used, the
Client includes the discovered FHS Proxy/Server INADDR or an anycast
address as the L2 destination then forwards the resulting carrier
packet(s) into the underlay network. Note that the Client does not yet
create a NCE, but instead caches the Identification, Nonce and/or
Timestamp values included in its RS message transmissions to match
against any received RA messages.When an FHS Proxy/Server receives the carrier packets containing an
RS it sets aside the L2 headers, verifies the Identifications and
reassembles if necessary, sets aside the OAL header, then verifies the
RS authentication signature or checksum. The FHS Proxy/Server then
creates/updates a NCE indexed by the Client's RS source address and
caches the OMNI Interface Attributes and any Traffic Selector
sub-options while also caching the L2 (UDP/IP) and OAL source and
destination address information. The FHS Proxy/Server next caches the
OMNI Neighbor Coordination sub-option Window Synchronization parameters
and N flag to determine its role in processing NS(NUD) messages (see:
) then examines the RS destination address. If
the destination matches its own ULA, the FHS Proxy/Server assumes the
Hub role and acts as the sole entry point for injecting the Client's
XLA-MNP into the OMNI link routing system (i.e., after performing any
necessary prefix delegation operations) while including a prefix length
and setting the prefix to fd00::/64 and suffix to the 64-bit MNP. The
FHS/Hub Proxy/Server then caches the OMNI Neighbor Coordination
sub-option A/U flags to determine its role in processing NS(AR) messages
and generating uNA messages (see: ).The FHS/Hub Proxy/Server then prepares to return an RA message
directly to the Client by first populating the Cur Hop Limit, Flags,
Router Lifetime, Reachable Time and Retrans Timer fields with values
appropriate for the OMNI link. The FHS/Hub Proxy/Server next includes as
the first RA message option an OMNI option with a neighbor coordination
sub-option with Window Synchronization information, an authentication
sub-option if necessary and a (proxyed) copy of the Client's original
Interface Attributes sub-option with its INET-facing interface
information written in the FMT/SRT and LHS Proxy/Server ULA/INADDR
fields. If the RS L2 destination IP address was anycast, the FHS/Hub
Proxy/Server next includes a second Interface Attributes sub-option with
omIndex set to '0' and with a unicast L2 IP address for its
Client-facing interface in the INADDR field.The FHS/Hub Proxy/Server next includes an Origin Indication
sub-option that includes the RS L2 source INADDR information (see: ), then includes any other necessary OMNI sub-options
(either within the same OMNI option or in additional OMNI options).
Following the OMNI option(s), the FHS/Hub Proxy/Server next includes any
other necessary RA options such as PIOs with (A; L=0) that include the
OMNI link MSPs , RIOs
with more-specific routes, Nonce and Timestamp options, etc. The FHS/Hub
Proxy/Server then sets the RA source address to its own ULA and
destination address to the Client's ULA-MNP (i.e., relative to the ULA
/64 prefix for its Client-facing underlay interface) while also
recording the corresponding XLA-MNP as an (alternate) index to the
Client NCE, then calculates the authentication signature or checksum.
The FHS/Hub Proxy/Server finally performs OAL encapsulation with source
set to its own ULA and destination set to the OAL source that appeared
in the RS, then calculates the OAL checksum, selects an appropriate
Identification, fragments if necessary, encapsulates each fragment in
appropriate L2 headers with source and destination address information
reversed from the RS L2 information and returns the resulting carrier
packets to the Client over the same underlay interface the RS arrived
on.When an FHS Proxy/Server receives an RS with a valid authentication
signature or checksum and with destination set to link-scoped
All-Routers multicast, it can either assume the Hub role itself the same
as above or act as a proxy and select the ULA of another Proxy/Server to
serve as the Hub. When an FHS Proxy/Server assumes the proxy role or
receives an RS with destination set to the ULA of another Proxy/Server,
it forwards the message while acting as a proxy. The FHS Proxy/Server
creates/updates a NCE for the Client (i.e., based on the RS source
address) and caches the OAL source, Window Synchronization, N flag,
Interface Attributes addressing information as above then writes its own
INET-facing FMT/SRT and LHS Proxy/Server ULA/INADDR information into the
appropriate Interface Attributes sub-option fields. The FHS Proxy/Server
then calculates and includes the checksum, performs OAL encapsulation
with source set to its own ULA and destination set to the ULA of the Hub
Proxy/Server, calculates the OAL checksum, selects an appropriate
Identification, fragments if necessary, encapsulates each fragment in
appropriate L2 headers and sends the resulting carrier packets into the
SRT secured spanning tree.When the Hub Proxy/Server receives the carrier packets, it discards
the L2 headers, reassembles if necessary to obtain the proxyed RS,
verifies checksums, then performs DHCPv6 Prefix Delegation (PD) to
obtain the Client's MNP if the RS source is a (TLA,XLA}-RND. The Hub
Proxy/Server then creates/updates a NCE for the Client's XLA-MNP and
caches any state (including the A/U flags, OAL addresses, Interface
Attributes information and Traffic Selectors), then finally performs
routing protocol injection. The Hub Proxy/Server then returns an RA that
echoes the Client's (proxyed) Interface Attributes sub-option and with
any RA parameters the same as specified for the FHS/Hub Proxy/Server
case above. The Hub Proxy/Server then sets the RA source address to its
own ULA and destination address to the RS source address; if the RS
source address is a {TLA,XLA}-RND, the Hub Proxy/Server also includes
the MNP in a DHCPv6 PD Reply OMNI sub-option. The Hub Proxy/Server next
calculates the checksum, then encapsulates the RA as an OAL packet with
source set to its own ULA and destination set to the ULA of the FHS
Proxy/Server that forwarded the RS. The Hub Proxy/Server finally
calculates the OAL checksum, selects an appropriate Identification,
fragments if necessary, encapsulates each fragment in appropriate L2
headers and sends the resulting carrier packets into the secured
spanning tree.When the FHS Proxy/Server receives the carrier packets it discards
the L2 headers, reassembles if necessary to obtain the RA message,
verifies checksums then updates the OMNI interface NCE for the Client
and creates/updates a NCE for the Hub. The FHS Proxy/Server then sets
the P flag in the RA flags field and proxys the
RA by changing the OAL source to its own ULA, changing the OAL
destination to the OAL address found in the Client's NCE, and changing
the RA destination address to the ULA-MNP of the Client relative to its
own /64 ULA prefix while also recording the corresponding XLA-MNP as an
alternate index into the Client NCE. (If the RA destination address was
a {TLA,XLA}-RND, the FHS Proxy Server determines the MNP by consulting
the DHCPv6 PD Reply message sub-option.) The FHS Proxy/Server next
includes Window Synchronization parameters responsive to those in the
Client's RS, an Interface Attributes sub-option with omIndex '0' and
with its unicast L2 IP address if necessary (see above), an Origin
Indication sub-option with the Client's cached INADDR and an
authentication sub-option if necessary. The FHS Proxy/Server finally
selects an Identification value per , calculates
the authentication signature or checksum, fragments if necessary,
encapsulates each fragment in L2 headers with addresses taken from the
Client's NCE and returns the resulting carrier packets via the same
underlay interface over which the RS was received.When the Client receives the carrier packets, it discards the L2
headers, reassembles if necessary and removes the OAL header to obtain
the RA message. The Client next verifies the authentication signature or
checksum, then matches the RA message with its previously-sent RS by
comparing the RS Sequence Number with the RA Acknowledgement Number and
also comparing the Nonce and/or Timestamp values if present. If the
values match, the Client then creates/updates OMNI interface NCEs for
both the Hub and FHS Proxy/Server and caches the information in the RA
message. In particular, the Client caches the RA source address as the
Hub Proxy/Server ULA and uses the OAL source address to configure both
an underlay interface-specific ULA for the Hub Proxy/Server and the ULA
of this FHS Proxy/Server. The Client then uses the ULA-MNP in the RA
destination address to configure its address within the ULA (multilink)
subnet prefix of the FHS Proxy/Server. If the Client has multiple
underlay interfaces, it creates additional FHS Proxy/Server NCEs and
ULA-MNPs as necessary when it receives RAs over those interfaces (noting
that multiple of the Client's underlay interfaces may be serviced by the
same or different FHS Proxy/Servers). The Client finally adds the Hub
Proxy/Server ULA to the default router list if necessary.For each underlay interface, the Client next caches the (filled-out)
Interface Attributes for its own omIndex and Origin Indication
information that it received in an RA message over that interface so
that it can include them in future NS/NA messages to provide neighbors
with accurate FMT/SRT/LHS information. (If the message includes an
Interface Attributes sub-option with omIndex '0', the Client also caches
the INADDR as the underlay network-local unicast address of the FHS
Proxy//Server via that underlay interface.) The Client then compares the
Origin Indication INADDR information with its own underlay interface
addresses to determine whether there may be NATs on the path to the FHS
Proxy/Server; if the INADDR information differs, the Client is behind a
NAT and must supply the Origin information in IPv6 ND message exchanges
with prospective neighbors on the same SRT segment. The Client finally
configures default routes and assigns the OMNI Subnet Router Anycast
address corresponding to the MNP (e.g., 2001:db8:1:2::) to the OMNI
interface.Following the initial exchange, the FHS Proxy/Server MAY later send
additional periodic and/or event-driven unsolicited RA messages per
. (The unsolicited RAs may be initiated either
by the FHS Proxy/Server itself or by the Hub via the FHS as a proxy.)
The Client then continuously manages its underlay interfaces according
to their states as follows:When an underlay interface transitions to UP, the Client sends an
RS over the underlay interface with an OMNI option with sub-options
as specified above.When an underlay interface transitions to DOWN, the Client sends
unsolicited NA messages over any UP underlay interface with an OMNI
option containing Interface Attributes sub-options for the DOWN
underlay interface with Link set to '0'. The Client sends isolated
unsolicited NAs when reliability is not thought to be a concern
(e.g., if redundant transmissions are sent on multiple underlay
interfaces), or may instead set the PNG flag in the OMNI header to
trigger a uNA reply.When the Router Lifetime for the Hub Proxy/Server nears
expiration, the Client sends an RS over any underlay interface to
receive a fresh RA from the Hub. If no RA messages are received over
a first underlay interface (i.e., after retrying), the Client marks
the underlay interface as DOWN and should attempt to contact the Hub
Proxy/Server via a different underlay interface. If the Hub
Proxy/Server is unresponsive over additional underlay interfaces,
the Client sends an RS message with destination set to the ULA of
another Proxy/Server which will then assume the Hub role.When all of a Client's underlay interfaces have transitioned to
DOWN (or if the prefix registration lifetime expires), the Hub
Proxy/Server withdraws the MNP the same as if it had received a
message with a release indication.The Client is responsible for retrying each RS exchange up to
MAX_RTR_SOLICITATIONS times separated by RTR_SOLICITATION_INTERVAL
seconds until an RA is received. If no RA is received over an UP
underlay interface (i.e., even after attempting to contact alternate
Proxy/Servers), the Client declares this underlay interface as DOWN.
When changing to a new FHS or Hub Proxy/Server, the Client also includes
a Proxy/Server Departure OMNI sub-option in new RS messages; the (new)
FHS Proxy/Server will in turn send uNA messages to the old FHS and/or
Hub Proxy/Server to announce the Client's departure as discussed in
.The IPv6 layer sees the OMNI interface as an ordinary IPv6 interface.
Therefore, when the IPv6 layer sends an RS message the OMNI interface
returns an internally-generated RA message as though the message
originated from an IPv6 router. The internally-generated RA message
contains configuration information consistent with the information
received from the RAs generated by the Hub Proxy/Server. Whether the
OMNI interface IPv6 ND messaging process is initiated from the receipt
of an RS message from the IPv6 layer or independently of the IPv6 layer
is an implementation matter. Some implementations may elect to defer the
OMNI interface internal RS/RA messaging process until an RS is received
from the IPv6 layer, while others may elect to initiate the process
proactively. Still other deployments may elect to administratively
disable IPv6 layer RS/RA messaging over the OMNI interface, since the
messages are not required to drive the OMNI interface internal RS/RA
process. (Note that this same logic applies to IPv4 implementations that
employ "ICMP Router Discovery" .)Note: The Router Lifetime value in RA messages indicates the time
before which the Client must send another RS message over this underlay
interface (e.g., 600 seconds), however that timescale may be
significantly longer than the lifetime the MS has committed to retain
the prefix registration (e.g., REACHABLETIME seconds). Proxy/Servers are
therefore responsible for keeping MS state alive on a shorter timescale
than the Client may be required to do on its own behalf.Note: On certain multicast-capable underlay interfaces, Clients
should send periodic unsolicited multicast NA messages and Proxy/Servers
should send periodic unsolicited multicast RA messages as "beacons" that
can be heard by other nodes on the link. If a node fails to receive a
beacon after a timeout value specific to the link, it can initiate
Neighbor Unreachability Detection (NUD) exchanges to test
reachability.Note: If a single FHS Proxy/Server services multiple of a Client's
underlay interfaces, Window Synchronization will initially be repeated
for the RS/RA exchange over each underlay interface, i.e., until the
Client discovers the many-to-one relationship. This will naturally
result in a single window synchronization that applies over the Client's
multiple underlay interfaces for the same FHS Proxy/Server.Note: Although the Client's FHS Proxy/Server is a first-hop segment
node from its own perspective, the Client stores the Proxy/Server's
FMT/SRT/ULA/INADDR as last-hop segment (LHS) information to supply to
neighbors. This allows both the Client and Hub Proxy/Server to supply
the information to neighbors that will perceive it as LHS information on
the return path to the Client.Note: The Hub Proxy/Server injects Client XLA-MNP into the OMNI link
routing system by simply creating a route-to-interface forwarding table
entry for fd00::{MNP}/N via the OMNI interface. The dynamic routing
protocol will notice the new entry and propagate the route to its peers.
If the Hub receives additional RS messages, it need not re-create the
forwarding table entry (nor disturb the dynamic routing protocol) if an
entry is already present. If the Hub ceases to receive RS messages from
any of the Client's interfaces, it removes the Client XLA-MNP from the
forwarding table (i.e., after a short delay) resulting in its removal
also from the routing system.Note: If the Client's initial RS message includes an anycast L2
destination address, the FHS Proxy/Server returns the solicited RA using
the same anycast address as the L2 source while including an Interface
Attributes sub-option with omIndex '0' and its true unicast address in
the INADDR. When the Client sends additional RS messages, it includes
this FHS Proxy/Server unicast address as the L2 destination and the FHS
Proxy/Server returns the solicited RA using the same unicast address as
the L2 source. This will ensure that RS/RA exchanges are not impeded by
any NATs on the path while avoiding long-term exposure of messages that
use an anycast address as the source.Note: The Origin Indication sub-option is included only by the FHS
Proxy/Server and not by the Hub (unless the Hub is also serving as an
FHS).Note: Clients should set the N/A/U flags consistently in successive
RS messages and only change those settings when an FHS/Hub Proxy/Server
service profile update is necessary.Note: After a Client has discovered its ULA-MNPs for a given set of
FHS Proxy/Servers, it should begin using its XLA-MNP as the IPv6 ND
message source address and ULA-MNP as the OAL source address in future
IPv6 ND messages and refrain from further use of TLAs. In any case, the
Client SHOULD NOT gratuitously configure and use large numbers of
additional TLAs, as doing so would simply result in address change churn
in neighbor cache entries with no operational advantages.Note: Although the Client adds the Hub Proxy/Server ULA to the
default router list, it also caches the ULAs of the FHS Proxy/Servers on
the path to the Hub over each underlying interface. When the Client
needs to send a packet to a default router, it therefore selects an ULA
corresponding to the selected interface which directs the packet to an
FHS Proxy/Server for that interface. The FHS Proxy/Server then forwards
the packet without disturbing the Hub.In environments where Identification window synchronization is
necessary, the RS/RA exchanges discussed above observe the principles
specified in . Window synchronization is
conducted between the Client and each FHS Proxy/Server used to contact
the Hub Proxy/Server, i.e., and not between the Client and the Hub.
This is due to the fact that the Hub Proxy/Server is responsible only
for forwarding control and data messages via the secured spanning tree
to FHS Proxy/Servers, and is not responsible for forwarding messages
directly to the Client under a synchronized window. Also, in the
reverse direction the FHS Proxy/Servers handle all default forwarding
actions without forwarding Client-initiated data to the Hub.When a Client needs to perform window synchronization via a new FHS
Proxy/Server, it sets the RS source address to its own {TLA,XLA}-MNP
(or a {TLA,XLA}-RND) and destination address to the ULA of the Hub
Proxy/Server (or to All-Routers multicast in an initial RS), then sets
the SYN flag and includes an initial Sequence Number for Window
Synchronization. The Client then performs OAL encapsulation using its
own ULA-MNP (or the TLA-RND) as the source and the ULA of the FHS
Proxy/Server as the destination and includes an Interface Attributes
sub-option then forwards the resulting carrier packets to the FHS
Proxy/Server. The FHS Proxy/Server then extracts the RS message and
caches the Window Synchronization parameters then re-encapsulates with
its own ULA as the source and the ULA of the Hub Proxy/Server as the
target.The FHS Proxy/Server then forwards the resulting carrier packets
via the secured spanning tree to the Hub Proxy/Server, which updates
the Client's Interface Attributes and returns a unicast RA message
with source set to its own ULA and destination set to the RS source
address and with the Client's Interface Attributes echoed. The Hub
Proxy/Server then performs OAL encapsulation using its own ULA as the
source and the ULA of the FHS Proxy/Server as the destination, then
forwards the carrier packets via the secured spanning tree to the FHS
Proxy/Server. The FHS Proxy/Server then proxys the message as
discussed in the previous section and includes responsive Window
Synchronization information. The FHS Proxy/Server then forwards the
message to the Client which updates its window synchronization
information for the FHS Proxy/Server as necessary.Following the initial RS/RA-driven window synchronization, the
Client can re-assert new windows with specific FHS Proxy/Servers by
performing NS/NA exchanges between its own XLA-MNPs and the ULAs of
the FHS Proxy/Servers without having to disturb the Hub.On some *NETs, a Client may be located multiple IP hops away from
the nearest OMNI link Proxy/Server. Forwarding through IP multihop
*NETs is conducted through the application of a routing protocol
(e.g., a MANET/VANET routing protocol over omni-directional wireless
interfaces, an inter-domain routing protocol in an enterprise network,
etc.). Example routing protocols optimized for MANET/VANET operations
include and which
operate according to the link model articulated in and subnet model articulated in .A Client located potentially multiple *NET hops away from the
nearest Proxy/Server prepares an RS message, sets the source address
to its XLA-MNP (or to a TLA-RND if it does not yet have an MNP), and
sets the destination to link-scoped All-Routers multicast or the
unicast ULA of a Proxy/Server the same as discussed above. The OMNI
interface then employs OAL encapsulation, sets the OAL source address
to a TLA and sets the OAL destination to an OMNI IPv6 anycast address
based on either a native IPv6 or IPv4-Compatible IPv6 prefix (see:
).For IPv6-enabled *NETs, if the underlay interface does not
configure an IPv6 GUA the Client injects the TLA into the IPv6
multihop routing system and forwards the message without further
encapsulation. Otherwise, the Client encapsulates the message in
UDP/IPv6 L2 headers, sets the source to the underlay interface IPv6
address and sets the destination to the same OMNI IPv6 anycast
address. The Client then forwards the message into the IPv6 multihop
routing system which conveys it to the nearest Proxy/Server that
advertises a matching OMNI IPv6 anycast prefix. If the nearest
Proxy/Server is too busy, it should forward (without Proxying) the
OAL-encapsulated RS to another nearby Proxy/Server connected to the
same IPv6 (multihop) network that also advertises the matching OMNI
IPv6 anycast prefix.For IPv4-only *NETs, the Client encapsulates the RS message in
UDP/IPv4 L2 headers, sets the source to the underlay interface IPv4
address and sets the destination to the OMNI IPv4 anycast address. The
Client then forwards the message into the IPv4 multihop routing system
which conveys it to the nearest Proxy/Server that advertises the
corresponding IPv4 prefix. If the nearest Proxy/Server is too busy
and/or does not configure the specified OMNI IPv6 anycast address, it
should forward (without Proxying) the OAL-encapsulated RS to another
nearby Proxy/Server connected to the same IPv4 (multihop) network that
configures the OMNI IPv6 anycast address. (In environments where
reciprocal RS forwarding cannot be supported, the first Proxy/Server
should instead return an RA based on its own MSP(s).)When an intermediate *NET hop that participates in the routing
protocol receives the encapsulated RS, it forwards the message
according to its routing tables (note that an intermediate node could
be a fixed infrastructure element such as a roadside unit or another
MANET/VANET node). This process repeats iteratively until the RS
message is received by a penultimate *NET hop within single-hop
communications range of a Proxy/Server, which forwards the message to
the Proxy/Server.When a Proxy/Server that configures the OMNI IPv6 anycast OAL
destination receives the message, it decapsulates the RS and assumes
either the Hub or FHS role (in which case, it forwards the RS to a
candidate Hub). The Hub Proxy/Server then prepares an RA message with
source address set to its own ULA and destination address set to the
RS source address if it is acting only as the Hub (or to the Client
ULA-MNP within its ULA subnet prefix if it is also acting as the FHS
Proxy/Server). The Hub Proxy/Server then performs OAL encapsulation
with the RA OAL source/destination set to the RS OAL
destination/source and forwards the RA either to the FHS Proxy/Server
or directly to the Client.When the Hub or FHS Proxy/Server forwards the RA to the Client, it
encapsulates the message in L2 encapsulation headers (if necessary)
with (src, dst) set to the (dst, src) of the RS L2 encapsulation
headers. The Proxy/Server then forwards the message to a *NET node
within communications range, which forwards the message according to
its routing tables to an intermediate node. The multihop forwarding
process within the *NET continues repetitively until the message is
delivered to the original Client, which decapsulates the message and
performs autoconfiguration the same as if it had received the RA
directly from a Proxy/Server on the same physical link. The Client
then injects the ULA-MNP into the IPv6 multihop routing system if
necessary, then begins using the ULA-MNP as its OAL source address and
suspends use of its TLA since it now has a unique address within the
FHS Proxy/Server's "Multilink Subnet".Note: When the RS message includes anycast OAL and/or L2
encapsulation destinations, the FHS Proxy/Server must use the same
anycast addresses as the OAL and/or L2 encapsulation sources to
support forwarding of the RA message and any initial data packets over
any NATs on the path. When the Client receives the RA, it will
discover its unicast ULA-MNP and/or L2 encapsulation addresses and can
forward future packets using the unicast (instead of anycast)
addresses to populate NAT state in the forward path. (If the Client
does not have immediate data to send to the FHS Proxy/Server, it can
instead send an OAL "bubble" - see .) After the
Client begins using unicast OAL/L2 encapsulation addresses in this
way, the FHS Proxy/Server should also begin using the same unicast
addresses in the reverse direction.Note: When an OMNI interface configures a TLA, any nodes that
forward an encapsulated RS message with the ULA as the OAL source must
not consider the message as being specific to a particular OMNI link.
TLAs can therefore also serve as the source and destination addresses
of unencapsulated IPv6 data communications within the local routing
region, and if the TLAs are injected into the local network routing
protocol their prefix length must be set to 128.When a Client is not pre-provisioned with an MNP (or, when the
Client requires additional MNP delegations), it requests the MS to
select MNPs on its behalf and set up the correct routing state. The
DHCPv6 service supports this requirement.When a Client requires the MS to select MNPs, it sends an RS
message with source set to a TLA-RND. If the Client requires only a
single MNP delegation, it can then include a OMNI Node Identification
sub-option plus an OMNI Neighbor Coordination sub-option with Preflen
set to the length of the desired MNP. If the Client requires multiple
MNP delegations and/or more complex DHCPv6 services, it instead
includes a DHCPv6 Message sub-option containing a Client Identifier,
one or more IA_PD options and a Rapid Commit option then sets the
'msg-type' field to "Solicit", and includes a 3 octet
'transaction-id'. The Client then sets the RS destination to
link-scoped All-Routers multicast and sends the message using OAL
encapsulation and fragmentation if necessary as discussed above.When the Hub Proxy/Server receives the RS message, it performs OAL
reassembly if necessary. Next, if the RS source is a TLA-RND and/or
the OMNI option includes a DHCPv6 message sub-option, the Hub
Proxy/Server acts as a "Proxy DHCPv6 Client" in a message exchange
with the locally-resident DHCPv6 server. If the RS did not contain a
DHCPv6 message sub-option, the Hub Proxy/Server generates a DHCPv6
Solicit message on behalf of the Client using an IA_PD option with the
prefix length set to the OMNI Neighbor Coordination header Preflen
value and with a Client Identifier formed from the OMNI option Node
Identification sub-option; otherwise, the Hub Proxy/Server uses the
DHCPv6 Solicit message contained in the OMNI option. The Hub
Proxy/Server then sends the DHCPv6 message to the DHCPv6 Server, which
delegates MNPs and returns a DHCPv6 Reply message with PD parameters.
(If the Hub Proxy/Server wishes to defer creation of Client state
until the DHCPv6 Reply is received, it can instead act as a
Lightweight DHCPv6 Relay Agent per by
encapsulating the DHCPv6 message in a Relay-forward/reply exchange
with Relay Message and Interface ID options. In the process, the Hub
Proxy/Server packs any state information needed to return an RA to the
Client in the Relay-forward Interface ID option so that the
information will be echoed back in the Relay-reply.)When the Hub Proxy/Server receives the DHCPv6 Reply, it creates
XLA-MNPs based on the delegated MNPs and creates OMNI interface
XLA-MNP forwarding table entries (i.e., to prompt the dynamic routing
protocol). The Hub Proxy/Server then sends an RA back to the FHS
Proxy/Server with the DHCPv6 Reply message included in an OMNI DHCPv6
message sub-option. The Hub Proxy/Server sets the RA destination
address to the RS source address, sets the RA source address to its
own ULA, performs OAL encapsulation and fragmentation, performs L2
encapsulation and sends the RA to the Client via the FHS Proxy/Server
as discussed above.When the FHS Proxy/Server receives the RA, it changes the RA
destination address to the ULA-MNP for the Client within its own ULA
subnet prefix then forwards the RA to the Client. When the Client
receives the RA, it reassembles and discards the OAL encapsulation
then creates a default route, assigns Subnet Router Anycast addresses
and uses the RA destination address or DHCPv6-delegated MNP to
automatically configure its primary ULA-MNP. The Client will then use
these primary MNP-based addresses as the source address of any IPv6 ND
messages it sends as long as it retains ownership of the MNP.Note: when the Hub Proxy/Server is also the FHS Proxy/Server, it
forwards the RA message directly to the Client with the destination
set to the Client's ULA-MNP (i.e., instead of forwarding via another
Proxy/Server).Clients can provide an OMNI link ingress point for other nodes on
their (downstream) ENETs that also act as Clients. When Client A has
already coordinated with an (upstream) ANET/INET Proxy/Server, Client
B on an ENET serviced by Client A can send OAL-encapsulated RS
messages with addresses set the same as specified in . When Client A receives the RS message, it infers
from the OAL encapsulation that Client B is seeking to establish
itself as a Client instead of just a simple ENET Host.Client A then returns an RA message the same as a Proxy/Server
would do as specified in except that it
instead uses its own ULA-MNP as the RA and OAL source addresses and
performs (recursive) DHCPv6 Prefix Delegation. The MNP delegation in
the RA message must be a sub-MNP from the MNP delegated to Client A.
For example, if Client A receives the MNP 2001:db8:1000::/48 it can
provide a sub-delegation such as 2001:db8:1000:2000::/56 to Client B.
Client B can in turn sub-delegate 2001:db8:1000:2000::/56 to its own
ENET(s), where there may be a further prospective Client C that would
in turn request OMNI link services via Client B.To support this Client-to-Client chaining, Clients send IPv6 ND
messages addressed to the OMNI link anycast address via their
ANET/INET (i.e., upstream) interfaces, but advertise the OMNI link
anycast address into their ENET (i.e., downstream) networks where
there may be further prospective Clients wishing to join the chain.
The ENET of the upstream Client is therefore seen as an ANET by
downstream Clients, and the upstream Client is seen as a Proxy/Server
by downstream Clients.If the underlay network link model is multiple access, the FHS
Proxy/Server is responsible for assuring that address duplication cannot
corrupt the neighbor caches of other nodes on the link. When the Client
sends an RS message on a multiple access underlay network, the
Proxy/Server verifies that the Client is authorized to use the address
and responds with an RA (or forwards the RS to the Hub) only if the
Client is authorized.After verifying Client authorization and returning an RA, the
Proxy/Server MAY return IPv6 ND Redirect messages to direct Clients
located on the same underlay network to exchange packets directly
without transiting the Proxy/Server. In that case, the Clients can
exchange packets according to their unicast L2 addresses discovered from
the Redirect message instead of using the dogleg path through the
Proxy/Server. In some underlay networks, however, such direct
communications may be undesirable and continued use of the dogleg path
through the Proxy/Server may provide better performance. In that case,
the Proxy/Server can refrain from sending Redirects, and/or Clients can
ignore them.*NETs SHOULD deploy Proxy/Servers in Virtual Router Redundancy
Protocol (VRRP) configurations so that service
continuity is maintained even if one or more Proxy/Servers fail. Using
VRRP, the Client is unaware which of the (redundant) FHS Proxy/Servers
is currently providing service, and any service discontinuity will be
limited to the failover time supported by VRRP. Widely deployed public
domain implementations of VRRP are available.Proxy/Servers SHOULD use high availability clustering services so
that multiple redundant systems can provide coordinated response to
failures. As with VRRP, widely deployed public domain implementations of
high availability clustering services are available. Note that
special-purpose and expensive dedicated hardware is not necessary, and
public domain implementations can be used even between lightweight
virtual machines in cloud deployments.In environments where fast recovery from Proxy/Server failure is
required, FHS Proxy/Servers SHOULD use proactive Neighbor Unreachability
Detection (NUD) in a manner that parallels Bidirectional Forwarding
Detection (BFD) to track Hub Proxy/Server
reachability. FHS Proxy/Servers can then quickly detect and react to
failures so that cached information is re-established through alternate
paths. Proactive NUD control messaging is carried only over
well-connected ground domain networks (i.e., and not low-end links such
as aeronautical radios) and can therefore be tuned for rapid
response.FHS Proxy/Servers perform proactive NUD for Hub Proxy/Servers for
which there are currently active Clients. If a Hub Proxy/Server fails,
the FHS Proxy/Server can quickly inform Clients of the outage by sending
multicast RA messages. The FHS Proxy/Server sends RA messages to Clients
with source set to the ULA of the Hub, with destination address set to
All-Nodes multicast (ff02::1) and with Router
Lifetime set to 0.The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA
messages separated by small delays . Any Clients
that have been using the (now defunct) Hub Proxy/Server will receive the
RA messages.When a Client connects to an *NET link for the first time, it sends
an RS message with an OMNI option. If the first hop router recognizes
the option, it responds according to the appropriate FHS/Hub
Proxy/Server role resulting in an RA message with an OMNI option
returned to the Client. The Client then engages this FHS Proxy/Sever
according to the OMNI link model specified above. If the first hop
router is a legacy IPv6 router, however, it instead returns an RA
message with no OMNI option and with a non-OMNI unicast source LLA as
specified in . In that case, the Client engages
the *NET according to the legacy IPv6 link model and without the OMNI
extensions specified in this document.If the *NET link model is multiple access, there must be assurance
that address duplication cannot corrupt the neighbor caches of other
nodes on the link. When the Client sends an RS message on a multiple
access *NET link with an OMNI option, first hop routers that recognize
the option ensure that the Client is authorized to use the address and
return an RA with a non-zero Router Lifetime only if the Client is
authorized. First hop routers that do not recognize the OMNI option
instead return an RA that makes no statement about the Client's
authorization to use the source address. In that case, the Client should
perform Duplicate Address Detection to ensure that it does not interfere
with other nodes on the link.An alternative approach for multiple access *NET links to ensure
isolation for Client-Proxy/Server communications is through link-layer
address mappings as discussed in . This
arrangement imparts a (virtual) point-to-point link model over the
(physical) multiple access link.Client OMNI interfaces configured over IPv6-enabled underlay
interfaces on an open Internetwork without an OMNI-aware first-hop
router receive IPv6 RA messages with no OMNI options, while OMNI
interfaces configured over IPv4-only underlay interfaces receive no IPv6
RA messages at all (but may receive IPv4 RA messages ). Client OMNI interfaces that receive RA messages
with OMNI options configure addresses, on-link prefixes, etc. on the
underlay interface that received the RA according to standard IPv6 ND
and address resolution conventions . Client OMNI interfaces configured over IPv4-only
underlay interfaces configure IPv4 address information on the underlay
interfaces using mechanisms such as DHCPv4 .Client OMNI interfaces configured over underlay interfaces connected
to open Internetworks can apply security services such as VPNs to
connect to a Proxy/Server, or can establish a direct link to the
Proxy/Server through some other means (see ).
In environments where an explicit VPN or direct link may be impractical
or undesirable, Client OMNI interfaces can instead send IPv6 ND messages
with OMNI options that include authentication signatures.OMNI interfaces use UDP/IP as L2 encapsulation headers for
transmission over open Internetworks with UDP service port number 8060
(see: and Section 3.6 of ) for both IPv4 and IPv6 underlay
interfaces. The OMNI interface submits original IP packets for OAL
encapsulation, then encapsulates the resulting OAL fragments in UDP/IP
L2 headers to form carrier packets. (The first four bits following the
UDP header determine whether the OAL headers are uncompressed/compressed
as discussed in .) The OMNI interface sets the UDP
length to the encapsulated OAL fragment length and sets the IP length to
an appropriate value at least as large as the UDP datagram.For Client-Proxy/Server (e.g., "Vehicle-to-Infrastructure (V2I)")
neighbor exchanges, the source must include an OMNI option with an
authentication sub-option in all IPv6 ND messages. The source can apply
HIP security services per using the IPv6 ND
message OMNI option as a "shipping container" to convey an
authentication signature in a (unidirectional) HIP "Notify" message. For
Client-Client (e.g., "Vehicle-to-Vehicle (V2V)") neighbor exchanges, two
Clients can exchange HIP "Initiator/Responder" messages coded in OMNI
options of multiple IPv6 NS/NA messages for mutual authentication
according to the HIP protocol. (Note: a simple Hashed Message
Authentication Code (HMAC) such as specified in
or the QUIC-TLS connection-oriented service can
be used as an alternate authentication service in some
environments.)When an OMNI interface includes an authentication sub-option, it must
appear as the first sub-option of the first OMNI option in the IPv6 ND
message which must appear immediately following the IPv6 ND message
header. When an OMNI interface prepares a HIP message sub-option, it
includes its own (H)HIT as the Sender's HIT and the neighbor's (H)HIT if
known as the Receiver's HIT (otherwise 0). If (H)HITs are not available
within the OMNI operational environment, the source can instead include
other IPv6 address types instead of (H)HITs as long as the Sender and
Receiver have some way to associate information embedded in the IPv6
address with the neighbor; such information could include a node
identifier, vehicle identifier, MAC address, etc.Before calculating the authentication signature, the source includes
any other necessary sub-options (such as Interface Attributes and Origin
Indication) and sets both the IPv6 ND message Checksum and
authentication signature fields to 0. The source then calculates the
authentication signature over the full length of the IPv6 ND message
beginning with a pseudo-header of the IPv6 header (i.e., the same as
specified in ) and extending over the length of
the message. (If the IPv6 ND message is part of an OAL super-packet, the
source instead calculates the authentication signature over the
remainder of the super-packet.) The source next writes the
authentication signature into the sub-option signature field and
forwards the message.After establishing a VPN or preparing for UDP/IP encapsulation, OMNI
interfaces send RS/RA messages for Client-Proxy/Server coordination
(see: ) and NS/NA messages for route
optimization, window synchronization and mobility management (see: ). These control plane messages must be
authenticated while other control and data plane messages are delivered
the same as for ordinary best-effort traffic with source address and/or
Identification window-based data origin verification. Upper layer
protocol sessions over OMNI interfaces that connect over open
Internetworks without an explicit VPN should therefore employ transport-
or higher-layer security to ensure authentication, integrity and/or
confidentiality.Clients should avoid using INET Proxy/Servers as general-purpose
routers for steady streams of carrier packets that do not require
authentication. Clients should instead perform route optimization to
coordinate with other INET nodes that can provide forwarding services
(or preferably coordinate directly with peer Clients directly) instead
of burdening the Proxy/Server. Procedures for coordinating with peer
Clients and discovering INET nodes that can provide better forwarding
services are discussed in .Clients that attempt to contact peers over INET underlay interfaces
often encounter NATs in the path. OMNI interfaces accommodate NAT
traversal using UDP/IP encapsulation and the mechanisms discussed in
. FHS Proxy/Servers include Origin
Indications in RA messages to allow Clients to detect the presence of
NATs.Note: Following the initial IPv6 ND message exchange, OMNI interfaces
configured over INET underlay interfaces maintain neighbor relationships
by transmitting periodic IPv6 ND messages with OMNI options that include
HIP "Update" and/or "Notify" messages. When HMAC authentication is used
instead of HIP, the Client and Proxy/Server exchange all IPv6 ND
messages with HMAC signatures included based on a shared-secret. When
QUIC-TLS connections are used, the Client and Proxy/Server observe
QUIC-TLS conventions .Note: OMNI interfaces configured over INET underlay interfaces should
employ the Identification window synchronization mechanisms specified in
in order to exclude spurious carrier packets
that might otherwise clutter the reassembly cache. This is especially
important in environments where carrier packet spoofing and/or
corruption is a threat.Note: NATs may be present on the path from a Client to its FHS
Proxy/Server, but never on the path from the FHS Proxy/Server to the Hub
where only INET and/or spanning tree hops occur. Therefore, the FHS
Proxy/Server does not communicate Client origin information to the Hub
where it would serve no purpose.In some use cases, it is desirable, beneficial and efficient for the
Client to receive a constant MNP that travels with the Client wherever
it moves. For example, this would allow air traffic controllers to
easily track aircraft, etc. In other cases, however (e.g., intelligent
transportation systems), the Client may be willing to sacrifice a
modicum of efficiency in order to have time-varying MNPs that can be
changed every so often to defeat adversarial tracking.The prefix delegation services discussed in
allows Clients that desire time-varying MNPs to obtain short-lived
prefixes to send RS messages with a {TLA,XLA}-RND source address and/or
with an OMNI option with DHCPv6 Option sub-options. The Client would
then be obligated to renumber its internal networks whenever its MNP
(and therefore also its OMNI address) changes. This should not present a
challenge for Clients with automated network renumbering services, but
may disrupt persistent sessions that would prefer to use a constant
address.Clients that generate (H)HITs but do not have pre-assigned MNPs can
request MNP delegations by issuing IPv6 ND messages that use the (H)HIT
instead of a TLA. For example, when a Client creates an RS message it
can set the source to a (H)HIT and destination to link-scoped
All-Routers multicast. The IPv6 ND message includes an OMNI option with
a HIP message sub-option, and need not include a Node Identification
sub-option if the Client's (H)HIT appears in the HIP message. The Client
then encapsulates the message in an IPv6 header with the (H)HIT as the
source address. The Client then sends the message as specified in .When the Hub Proxy/Server receives the RS message, it notes that the
source was a (H)HIT, then invokes the DHCPv6 protocol to request an MNP
prefix delegation while using the (H)HIT (in the form of a DUID) as the
Client Identifier. The Hub Proxy/Server then prepares an RA message with
source address set to its own ULA and destination set to the source of
the RS message. The Hub Proxy/Server next includes an OMNI option with a
HIP message sub-option and any DHCPv6 prefix delegation parameters. The
Proxy/Server finally encapsulates the RA in an OAL header with source
address set to its own ULA and destination set to the RS OAL source
address, then returns the encapsulated RA to the Client either directly
or by way of the FHS Proxy/Server as a proxy.Clients can also use (H)HITs and/or TLAs for direct Client-to-Client
communications outside the context of any OMNI link supporting
infrastructure. When two Clients encounter one another they can use
their (H)HITs and/or TLAs as original IPv6 packet source and destination
addresses to support direct communications. Clients can also inject
their (H)HITs and/or TLAs into an IPv6 multihop routing protocol to
enable multihop communications as discussed in . Clients can further exchange other IPv6 ND messages
using their (H)HITs and/or TLAs as source and destination addresses.Lastly, when Clients are within the coverage range of OMNI link
infrastructure a case could be made for injecting (H)HITs and/or TLAs
into the global MS routing system. For example, when the Client sends an
RS to an FHS Proxy/Server it could include a request to inject the
(H)HIT / TLA into the routing system instead of requesting an MNP prefix
delegation. This would potentially enable OMNI link-wide communications
using only (H)HITs or TLAs, and not MNPs. This document notes the
opportunity, but makes no recommendation.Clients assign LLAs to the OMNI interface, but do not use LLAs as
IPv6 ND message source/destination addresses nor for addressing ordinary
original IP packets exchanged with OMNI link neighbors.Clients use ULA-MNPs as source/destination IPv6 addresses in the
encapsulation headers of OAL packets and use XLA-MNPs as the IPv6 source
addresses of the IPv6 ND messages themselves. Clients use TLAs when an
MNP is not available, or as source/destination IPv6 addresses for
communications within a MANET/VANET local area. Clients can also use
(H)HITs instead of ULAs for local communications when operation outside
the context of a specific ULA domain and/or source address attestation
is necessary.Clients use MNP-based GUAs as original IP packet source and
destination addresses for communications with Internet destinations when
they are within range of OMNI link supporting infrastructure that can
inject the MNP into the routing system. Clients can also use MNP-based
GUAs within multihop routing regions that are currently disconnected
from infrastructure as long as the corresponding ULA-MNPs have been
injected into the routing system.Clients use anycast GUAs as OAL and/or L2 encapsulation destination
addresses for RS messages used to discover the nearest FHS Proxy/Server.
When the Proxy/Server returns a solicited RA, it must also use the same
anycast address as the RA OAL/L2 encapsulation source in order to
successfully traverse any NATs in the path. The Client should then
immediately transition to using the FHS Proxy/Server's discovered
unicast OAL/L2 address as the destination in order to minimize
dependence on the Proxy/Server's use of an anycast source address.An OAL destination or intermediate node may need to return
ICMPv6-like error messages (e.g., Destination Unreachable, Packet Too
Big, Time Exceeded, etc.) to an OAL source.
Since ICMPv6 error messages do not themselves include authentication
codes, OAL nodes can return error messages as an OMNI ICMPv6 Error
sub-option in a secured IPv6 ND uNA message.The following IANA actions are requested in accordance with and :The IANA is instructed to allocate an Internet Protocol number TBD1
from the 'protocol numbers' registry for the Overlay Multilink Network
Interface (OMNI) protocol. Guidance is found in (registration procedure is IESG Approval or
Standards Action).During final publication stages, the IESG will be requested to
procure an IEEE EtherType value TBD2 for OMNI according to the
statement found at
https://www.ietf.org/about/groups/iesg/statements/ethertypes/.Following this procurement, the IANA is instructed to register the
value TBD2 in the 'ieee-802-numbers' registry for Overlay Multilink
Network Interface (OMNI) encapsulation on Ethernet networks. Guidance
is found in (registration procedure is Expert
Review).The IANA is instructed to assign TBD3/N as an "OMNI IPv4 anycast"
address/prefix in the "IPv4 Special-Purpose Address" registry in a
similar fashion as for . The IANA is requested
to work with the authors to obtain a TBD3/N public IPv4 prefix,
whether through an RIR allocation, a delegation from IANA's "IPv4
Recovered Address Space" registry or through an unspecified third
party donation.The IANA is instructed to allocate an official Type number TBD4
from the "IPv6 Neighbor Discovery Option Formats" registry for the
OMNI option (registration procedure is RFC required). Implementations
set Type to 253 as an interim value .The IANA is instructed to allocate one Ethernet unicast address
TBD5 (suggested value '00-52-14') in the 'ethernet-numbers' registry
under "IANA Unicast 48-bit MAC Addresses" (registration procedure is
Expert Review). The registration should appear as follows:The IANA is instructed to assign two new Code values in the "ICMPv6
Code Fields: Type 2 - Packet Too Big" registry (registration procedure
is Standards Action or IESG Approval). The registry should appear as
follows:(Note: this registry also to be used to define values for
setting the "unused" field of ICMPv4 "Destination Unreachable -
Fragmentation Needed" messages.)The OMNI option defines a 5-bit Sub-Type field, for which IANA is
instructed to create and maintain a new registry entitled "OMNI Option
Sub-Type Values". Initial values are given below (registration
procedure is RFC required):The OMNI Geo Coordinates sub-option (see: )
contains an 8-bit Type field, for which IANA is instructed to create
and maintain a new registry entitled "OMNI Geo Coordinates Type
Values". Initial values are given below (registration procedure is RFC
required):The OMNI Node Identification sub-option (see: ) contains an 8-bit ID-Type field, for which IANA is
instructed to create and maintain a new registry entitled "OMNI Node
Identification ID-Type Values". Initial values are given below
(registration procedure is RFC required):The OMNI option defines an 8-bit Extension-Type field for Sub-Type
30 (Sub-Type Extension), for which IANA is instructed to create and
maintain a new registry entitled "OMNI Option Sub-Type Extension
Values". Initial values are given below (registration procedure is RFC
required):The OMNI Sub-Type Extension "RFC4380 UDP/IP Header Option" defines
an 8-bit Header Type field, for which IANA is instructed to create and
maintain a new registry entitled "OMNI RFC4380 UDP/IP Header Option".
Initial registry values are given below (registration procedure is RFC
required):The OMNI Sub-Type Extension for "RFC6081 UDP/IP Trailer Option"
defines an 8-bit Trailer Type field, for which IANA is instructed to
create and maintain a new registry entitled "OMNI RFC6081 UDP/IP
Trailer Option". Initial registry values are given below (registration
procedure is RFC required):The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO . This document
reclaims the UDP port number "8060" for 'aero' as the service port for
UDP/IP encapsulation. (Note that, although is
not widely implemented or deployed, any messages coded to that
specification can be easily distinguished and ignored since they
include an invalid ICMPv6 message type number '0'.) The IANA is
therefore instructed to update the reference for UDP port number
"8060" from "RFC6706" to "RFCXXXX" (i.e., this document) while
retaining the existing name 'aero'.The IANA has assigned a 4 octet Private Enterprise Number (PEN)
code "45282" in the "enterprise-numbers" registry. This document is
the normative reference for using this code in DHCP Unique IDentifiers
based on Enterprise Numbers ("DUID-EN for OMNI Interfaces") (see:
). The IANA is therefore instructed to change
the enterprise designation for PEN code "45282" from "LinkUp Networks"
to "Overlay Multilink Network Interface (OMNI)".The IANA has assigned the ifType code "301 - omni - Overlay
Multilink Network Interface (OMNI)" in accordance with Section 6 of
. The registration appears under the IANA
"Structure of Management Information (SMI) Numbers (MIB Module
Registrations) - Interface Types (ifType)" registry.No further IANA actions are required.Security considerations for IPv4 , IPv6 and IPv6 Neighbor Discovery
apply. OMNI interface IPv6 ND messages SHOULD include Nonce and
Timestamp options when transaction confirmation
and/or time synchronization is needed. (Note however that when OAL
encapsulation is used the (echoed) OAL Identification value can provide
sufficient transaction confirmation.)OMNI interfaces configured over secured ANET/ENET interfaces inherit
the physical and/or link-layer security properties (i.e., "protected
spectrum") of the connected networks. OMNI interfaces configured over
open INET interfaces can use symmetric securing services such as VPNs or
can by some other means establish a direct link. When a VPN or direct
link may be impractical or undesirable, however, the security services
specified in , or can be employed. While the OMNI link protects control
plane messaging, applications must still employ end-to-end transport- or
higher-layer security services to protect the data plane.Strong network layer security for control plane messages and
forwarding path integrity for data plane messages between Proxy/Servers
MUST be supported. In one example, the AERO service constructs an SRT spanning tree with
Proxy/Serves as leaf nodes and secures the spanning tree links with
network layer security mechanisms such as IPsec
or WireGuard . Secured control plane messages are
then constrained to travel only over the secured spanning tree paths and
are therefore protected from attack or eavesdropping. Other control and
data plane messages can travel over route optimized paths that do not
strictly follow the secured spanning tree, therefore end-to-end sessions
should employ transport- or higher-layer security services.
Additionally, the OAL Identification value can provide a first level of
data origin authentication to mitigate off-path spoofing in some
environments.Identity-based key verification infrastructure services such as iPSK
may be necessary for verifying the identities claimed by Clients. This
requirement should be harmonized with the manner in which (H)HITs are
attested in a given operational environment.Security considerations for specific access network interface types
are covered under the corresponding IP-over-(foo) specification (e.g.,
, , etc.).Security considerations for IPv6 fragmentation and reassembly are
discussed in . In environments where spoofing is
considered a threat, OMNI nodes SHOULD employ Identification window
synchronization and OAL destinations SHOULD configure an
(end-system-based) firewall.AERO/OMNI Release-3.2 was tagged on March 30, 2021, and is undergoing
internal testing. Additional internal releases expected within the
coming months, with first public release expected end of 1H2021.Many AERO/OMNI functions are implemented and undergoing final
integration. OAL fragmentation/reassembly buffer management code has
been cleared for public release.This document does not itself update other RFCs, but suggests that
the following could be updated through future IETF initiatives:Updates can be through, e.g., standards action, the errata
process, etc. as appropriate.The first version of this document was prepared per the consensus
decision at the 7th Conference of the International Civil Aviation
Organization (ICAO) Working Group-I Mobility Subgroup on March 22, 2019.
Consensus to take the document forward to the IETF was reached at the
9th Conference of the Mobility Subgroup on November 22, 2019. Attendees
and contributors included: Guray Acar, Danny Bharj, Francois
D´Humieres, Pavel Drasil, Nikos Fistas, Giovanni Garofolo,
Bernhard Haindl, Vaughn Maiolla, Tom McParland, Victor Moreno, Madhu
Niraula, Brent Phillips, Liviu Popescu, Jacky Pouzet, Aloke Roy, Greg
Saccone, Robert Segers, Michal Skorepa, Michel Solery, Stephane Tamalet,
Fred Templin, Jean-Marc Vacher, Bela Varkonyi, Tony Whyman, Fryderyk
Wrobel and Dongsong Zeng.The following individuals are acknowledged for their useful comments:
Amanda Baber, Stuart Card, Donald Eastlake, Adrian Farrel, Michael
Matyas, Robert Moskowitz, Madhu Niraula, Greg Saccone, Stephane Tamalet,
Eliot Lear, Eduard Vasilenko, Eric Vyncke. Pavel Drasil, Zdenek Jaron
and Michal Skorepa are especially recognized for their many helpful
ideas and suggestions. Akash Agarwal, Madhuri Madhava Badgandi, Sean
Dickson, Don Dillenburg, Joe Dudkowski, Vijayasarathy Rajagopalan, Ron
Sackman, Bhargava Raman Sai Prakash and Katherine Tran are acknowledged
for their hard work on the implementation and technical insights that
led to improvements for the spec.Discussions on the IETF 6man and atn mailing lists during the fall of
2020 suggested additional points to consider. The authors gratefully
acknowledge the list members who contributed valuable insights through
those discussions. Eric Vyncke and Erik Kline were the intarea ADs,
while Bob Hinden and Ole Troan were the 6man WG chairs at the time the
document was developed; they are all gratefully acknowledged for their
many helpful insights. Many of the ideas in this document have further
built on IETF experiences beginning in the 1990s, with insights from
colleagues including Ron Bonica, Brian Carpenter, Ralph Droms, Christian
Huitema, Thomas Narten, Dave Thaler, Joe Touch, Pascal Thubert, and many
others who deserve recognition.Early observations on IP fragmentation performance implications were
noted in the 1986 Digital Equipment Corporation (DEC) "qe reset"
investigation, where fragment bursts from NFS UDP traffic triggered
hardware resets resulting in communication failures. Jeff Chase, Fred
Glover and Chet Juzsczak of the Ultrix Engineering Group led the
investigation, and determined that setting a smaller NFS mount block
size reduced the amount of fragmentation and suppressed the resets.
Early observations on L2 media MTU issues were noted in the 1988 DEC
FDDI investigation, where Raj Jain, KK Ramakrishnan and Kathy Wilde
represented architectural considerations for FDDI networking in general
including FDDI/Ethernet bridging. Jeff Mogul (who led the IETF Path MTU
Discovery working group) and other DEC colleagues who supported these
early investigations are also acknowledged.Throughout the 1990's and into the 2000's, many colleagues supported
and encouraged continuation of the work. Beginning with the DEC Project
Sequoia effort at the University of California, Berkeley, then moving to
the DEC research lab offices in Palo Alto CA, then to Sterling Software
at the NASA Ames Research Center, then to SRI in Menlo Park, CA, then to
Nokia in Mountain View, CA and finally to the Boeing Company in 2005 the
work saw continuous advancement through the encouragement of many. Those
who offered their support and encouragement are gratefully
acknowledged.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Information Technology (BIT)
Mobility Vision Lab (MVL) program.Error Characteristics of Fiber Distributed Data Interface
(FDDI), IEEE Transactions on CommunicationsPerformance of Checksums and CRC's Over Real Data, IEEE/ACM
Transactions on Networking, Vol. 6, No. 5The OMNI Interface - An IPv6 Air/Ground Interface for Civil
Aviation, IETF Liaison Statement #1676,
https://datatracker.ietf.org/liaison/1676/WireGuard, Fast, Modern, Secure VPN Tunnel,
https://wireguard.com/ICAO Document 9896 (Manual on the Aeronautical
Telecommunication Network (ATN) using Internet Protocol Suite (IPS)
Standards and Protocol), Draft Edition 3 (work-in-progress)IPv4 Address Space Registry,
https://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xhtmlIPv6 Global Unicast Address Assignments,
https://www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6-unicast-address-assignments.xhtmlThe OAL Checksum Algorithm adopts the 8-bit Fletcher algorithm
specified in Appendix I of as also analyzed in
. declared historic for the reason that the algorithms had never
seen widespread use with TCP, however this document adopts the 8-bit
Fletcher algorithm for a different purpose. Quoting from Appendix I of
, the OAL Checksum Algorithm proceeds as
follows:"The 8-bit Fletcher Checksum Algorithm is calculated over a
sequence of data octets (call them D[1] through D[N]) by maintaining
2 unsigned 1's-complement 8-bit accumulators A and B whose contents
are initially zero, and performing the following loop where i ranges
from 1 to N:A := A + D[i]B := B + AIt can be shown that at the end of the loop A will contain
the 8-bit 1's complement sum of all octets in the datagram, and that
B will contain (N)D[1] + (N-1)D[2] + ... + D[N]."To calculate the OAL checksum, the above algorithm is applied over
the N-octet concatenation of the OAL pseudo-header and the encapsulated
IP packet or packets. Specifically, the algorithm is first applied over
the 40 octets of the OAL pseudo-header as data octets D[1] through
D[40], then continues over the entire length of the original IP
packet(s) as data octets D[41] through D[N].OMNI interface IPv6 ND messages are subject to authentication and
integrity checks at multiple levels. When an OMNI interface sends an
IPv6 ND message over an INET interface, it includes an authentication
sub-option with a valid signature but does not include an IPv6 ND
message checksum. The OMNI interface that receives the message verifies
the OAL checksum as a first-level integrity check, then verifies the
authentication signature (while ignoring the IPv6 ND message checksum)
to ensure IPv6 ND message authentication and integrity.When an OMNI interface sends an IPv6 ND message over an underlay
interface connected to a secured network, it omits the authentication
sub-option but instead calculates/includes an IPv6 ND message checksum.
The OMNI interface that receives the message applies any lower-layer
authentication and integrity checks, then verifies both the OAL checksum
and the IPv6 ND message checksum. (Note that optimized implementations
can verify both the OAL and IPv6 ND message checksums in a single pass
over the data.) When an OMNI interface sends IPv6 ND messages to a
synchronized neighbor, it includes an authentication sub-option only if
authentication is necessary; otherwise, it calculates/includes the IPv6
ND message checksum.When the OMNI interface calculates the authentication signature or
IPv6 ND message checksum, it performs the calculation beginning with a
pseudo-header of the IPv6 ND message header and extends over all
following OAL packet data. In particular, for OAL super-packets any
additional original IP packets included beyond the end of the IPv6 ND
message are simply considered as extensions of the IPv6 ND message for
the purpose of the calculation.OAL destinations discard carrier packets with unacceptable
Identifications and submit the encapsulated fragments in all others for
reassembly. The reassembly algorithm rejects any fragments with
unacceptable sizes, offsets, etc. and reassembles all others. Following
reassembly, the OAL checksum algorithm provides an integrity assurance
layer that compliments any integrity checks already applied by lower
layers as well as a first-pass filter for any checks that will be
applied later by upper layers.ICAO Doc 9776 is the "Technical Manual for VHF Data Link Mode 2"
(VDLM2) that specifies an essential radio frequency data link service
for aircraft and ground stations in worldwide civil aviation air traffic
management. The VDLM2 link type is "multicast capable" , but with considerable differences from common
multicast links such as Ethernet and IEEE 802.11.First, the VDLM2 link data rate is only 31.5Kbps - multiple orders of
magnitude less than most modern wireless networking gear. Second, due to
the low available link bandwidth only VDLM2 ground stations (i.e., and
not aircraft) are permitted to send broadcasts, and even so only as
compact layer 2 "beacons". Third, aircraft employ the services of ground
stations by performing unicast RS/RA exchanges upon receipt of beacons
instead of listening for multicast RA messages and/or sending multicast
RS messages.This beacon-oriented unicast RS/RA approach is necessary to conserve
the already-scarce available link bandwidth. Moreover, since the numbers
of beaconing ground stations operating within a given spatial range must
be kept as sparse as possible, it would not be feasible to have
different classes of ground stations within the same region observing
different protocols. It is therefore highly desirable that all ground
stations observe a common language of RS/RA as specified in this
document.Note that links of this nature may benefit from compression
techniques that reduce the bandwidth necessary for conveying the same
amount of data. The IETF lpwan working group is considering possible
alternatives: [https://datatracker.ietf.org/wg/lpwan/documents].Per , IPv6 ND messages may be sent to either
a multicast or unicast link-scoped IPv6 destination address. However,
IPv6 ND messaging should be coordinated between the Client and
Proxy/Server only without invoking other nodes on the underlay network.
This implies that Client-Proxy/Server control messaging should be
isolated and not overheard by other nodes on the link.To support Client-Proxy/Server isolation on some links, Proxy/Servers
can maintain an OMNI-specific unicast link-layer address ("MSADDR"). For
Ethernet-compatible links, this specification reserves one Ethernet
unicast address TBD5 (see: IANA Considerations). For non-Ethernet
statically-addressed links MSADDR is reserved per the assigned numbers
authority for the link-layer addressing space. For still other links,
MSADDR may be dynamically discovered through other means, e.g.,
link-layer beacons.Clients map the L3 addresses of all IPv6 ND messages they send (i.e.,
both multicast and unicast) to MSADDR instead of to an ordinary unicast
or multicast link-layer address. In this way, all of the Client's IPv6
ND messages will be received by Proxy/Servers that are configured to
accept packets destined to MSADDR. Note that multiple Proxy/Servers on
the link could be configured to accept packets destined to MSADDR, e.g.,
as a basis for supporting redundancy.Therefore, Proxy/Servers must accept and process packets destined to
MSADDR, while all other devices must not process packets destined to
MSADDR. This model has well-established operational experience in Proxy
Mobile IPv6 (PMIP) .<< RFC Editor - remove prior to publication >>Differences from earlier versions:Submit for RFC publication.