Asymmetric Extended Route Optimization (AERO)Boeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftThis document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). AERO interfaces use
an IPv6 link-local address format that supports operation of the IPv6
Neighbor Discovery (ND) protocol and links ND to IP forwarding. Prefix
delegation/registration services are employed for network admission and
to manage the routing system. Multilink operation, mobility management,
quality of service (QoS) signaling and route optimization are naturally
supported through dynamic neighbor cache updates. Standard IP
multicasting services are also supported. AERO is a widely-applicable
mobile internetworking service especially well-suited to aviation
services, intelligent transportation systems, mobile Virtual Private
Networks (VPNs) and many other applications.Asymmetric Extended Route Optimization (AERO) fulfills the
requirements of Distributed Mobility Management (DMM) and route optimization for
aeronautical networking and other network mobility use cases such as
intelligent transportation systems. AERO is based on a Non-Broadcast,
Multiple Access (NBMA) virtual link model known as the AERO link. The
AERO link is a virtual overlay configured over one or more underlying
Internetworks, and nodes on the link can exchange IP packets via
tunneling. Multilink operation allows for increased reliability,
bandwidth optimization and traffic path diversity.The AERO service comprises Clients, Proxys, Servers and Relays that
are seen as AERO link neighbors as well as Bridges that interconnect
AERO link segments. Each node's AERO interface uses an IPv6 link-local
address format that supports operation of the IPv6 Neighbor Discovery
(ND) protocol and links ND to IP forwarding. A
node's AERO interface can be configured over multiple underlying
interfaces, and may therefore appear as a single interface with multiple
link-layer addresses. Each link-layer address is subject to change due
to mobility and/or QoS fluctuations, and link-layer address changes are
signaled by ND messaging the same as for any IPv6 link.AERO links provide a cloud-based service where mobile nodes may use
any Server acting as a Mobility Anchor Point (MAP) and fixed nodes may
use any Relay on the link for efficient communications. Fixed nodes
forward packets destined to other AERO nodes to the nearest Relay, which
forwards them through the cloud. A mobile node's initial packets are
forwarded through the Server, while direct routing is supported through
asymmetric extended route optimization while data packets are flowing.
Both unicast and multicast communications are supported, and mobile
nodes may efficiently move between locations while maintaining
continuous communications with correspondents and without changing their
IP Address.AERO Bridges are interconnected in a secured private BGP overlay
routing instance using encapsulation to provide a hybrid
routing/bridging service that joins the underlying Internetworks of
multiple disjoint administrative domains into a single unified AERO
link. Each AERO link instance is characterized by the set of Mobility
Service Prefixes (MSPs) common to all mobile nodes. The link extends to
the point where a Relay/Server is on the optimal route from any
correspondent node on the link, and provides a conduit between the
underlying Internetwork and the AERO link. To the underlying
Internetwork, the Relay/Server is the source of a route to the MSP, and
hence uplink traffic to the mobile node is naturally routed to the
nearest Relay/Server.AERO assumes the use of PIM Sparse Mode in support of multicast
communication. In support of Source Specific Multicast (SSM) when a
Mobile Node is the source, AERO route optimization ensures that a
shortest-path multicast tree is established with provisions for mobility
and multilink operation. In all other multicast scenarios there are no
AERO dependencies.AERO was designed for aeronautical networking for both manned and
unmanned aircraft, where the aircraft is treated as a mobile node that
can connect an Internet of Things (IoT). AERO is also applicable to a
wide variety of other use cases. For example, it can be used to
coordinate the Virtual Private Network (VPN) links of mobile nodes
(e.g., cellphones, tablets, laptop computers, etc.) that connect into a
home enterprise network via public access networks using services such
as OpenVPN . It can also be used to facilitate
vehicular and pedestrian communications services for intelligent
transportation systems. Other applicable use cases are also in
scope.The following numbered sections present the AERO specification. The
appendices at the end of the document are non-normative.The terminology in the normative references applies; the following
terms are defined within the scope of this document:an IPv6 control
message service for coordinating neighbor relationships between
nodes connected to a common link. AERO interfaces use the ND service
specified in .a networking
service for delegating IPv6 prefixes to nodes on the link. The
nominal PD service is DHCPv6 , however
alternate services (e.g., based on ND messaging) are also in scope
. Most notably, a minimal
form of PD known as "prefix registration" can be used if the Client
knows its prefix in advance and can represent it in the IPv6 source
address of an ND message.a node's first-hop data
link service network (e.g., a radio access network, cellular service
provider network, corporate enterprise network, etc.) that often
provides link-layer security services such as IEEE 802.1X and
physical-layer security prevent unauthorized access internally and
with border network-layer security services such as firewalls and
proxies that prevent unauthorized outside access.a node's attachment to a link
in an ANET.a connected IP network
topology with a coherent routing and addressing plan and that
provides a transit backbone service for ANET end systems. INETs also
provide an underlay service over which the AERO virtual link is
configured. Example INETs include corporate enterprise networks,
aviation networks, and the public Internet itself. When there is no
administrative boundary between an ANET and the INET, the ANET and
INET are one and the same.frequently, INETs such as
large corporate enterprise networks are sub-divided internally into
separate isolated partitions. Each partition is fully connected
internally but disconnected from other partitions, and there is no
requirement that separate partitions maintain consistent Internet
Protocol and/or addressing plans. (Each INET partition is seen as a
separate AERO link segment as discussed below.)a node's attachment to a link
in an INET.an IP address assigned to a
node's interface connection to an INET.the encapsulation of a
packet in an outer header or headers that can be routed within the
scope of the local INET partition.a Non-Broadcast, Multiple Access
(NBMA) virtual overlay over one or more underlying INETs manifested
by IPv6 encapsulation . The AERO link spans
underlying INET segments joined by virtual bridges in a spanning
tree the same as a bridged campus LAN. Nodes on the AERO link appear
as single-hop neighbors even though they may be separated by
multiple underlying INET hops, and can use Segment Routing to cause packets to visit selected waypoints on
the link.a node's attachment to an AERO
link. Since the addresses assigned to an AERO interface are managed
for uniqueness, AERO interfaces do not require Duplicate Address
Detection (DAD) and therefore set the administrative variable
'DupAddrDetectTransmits' to zero .an ANET or INET
interface over which an AERO interface is configured.a link local
IPv6 address per constructed as specified
in .a unique
local IPv6 address per constructed as
specified in . AERO ULAs are statelessly
derived from AERO LLAs, and vice-versa.an IP prefix
assigned to the AERO link and from which more-specific Mobile
Network Prefixes (MNPs) are derived.an IP prefix
allocated from an MSP and delegated to an AERO Client or Relay.a node that is connected to an AERO
link, or that provides services to other nodes on an AERO link.an AERO node
that connects over one or more underlying interfaces and requests
MNP PDs from AERO Servers. The Client assigns a Client LLA to the
AERO interface for use in ND exchanges with other AERO nodes and
forwards packets to correspondents according to AERO interface
neighbor cache state.an INET node
that configures an AERO interface to provide default forwarding and
mobility/multilink services for AERO Clients. The Server assigns an
administratively-provisioned LLA to its AERO interface to support
the operation of the ND/PD services, and advertises all of its
associated MNPs via BGP peerings with Bridges.an AERO Server
that also provides forwarding services between nodes reached via the
AERO link and correspondents on other links. AERO Relays are
provisioned with MNPs (i.e., the same as for an AERO Client) and run
a dynamic routing protocol to discover any non-MNP IP routes. In
both cases, the Relay advertises the MSP(s) to its downstream
networks, and distributes all of its associated MNPs and non-MNP IP
routes via BGP peerings with Bridges (i.e., the same as for an AERO
Server).a node that
provides hybrid routing/bridging services (as well as a security
trust anchor) for nodes on an AERO link. As a router, the Bridge
forwards packets using standard IP forwarding. As a bridge, the
Bridge forwards packets over the AERO link without decrementing the
IPv6 Hop Limit. AERO Bridges peer with Servers and other Bridges to
discover the full set of MNPs for the link as well as any non-MNPs
that are reachable via Relays.a node that
provides proxying services between Clients in an ANET and Servers in
external INETs. The AERO Proxy is a conduit between the ANET and
external INETs in the same manner as for common web proxies, and
behaves in a similar fashion as for ND proxies .an AERO
interface endpoint that injects encapsulated packets into an AERO
link.an AERO
interface endpoint that receives encapsulated packets from an AERO
link.an IP address used as an
encapsulation header source or destination address from the
perspective of the AERO interface. When an upper layer protocol
(e.g., UDP) is used as part of the encapsulation, the port number is
also considered as part of the link-layer address.the source or
destination address of an encapsulated IP packet presented to the
AERO interface.an internal virtual or
external edge IP network that an AERO Client or Relay connects to
the rest of the network via the AERO interface. The Client/Relay
sees each EUN as a "downstream" network, and sees the AERO interface
as the point of attachment to the "upstream" network.an AERO Client and all of
its downstream-attached networks that move together as a single
unit, i.e., an end system that connects an Internet of Things.a MN's on-board router
that forwards packets between any downstream-attached networks and
the AERO link.the AERO node
nearest the source that initiates route optimization. The ROS may be
a Server or Proxy acting on behalf of the source Client.the AERO
node nearest the target destination that responds to route
optimization requests. The ROR may be a Server acting on behalf of a
target MNP Client, or a Relay for a non-MNP destination.a geographically and/or
topologically referenced list of addresses of all Servers within the
same AERO link. There is a single MAP list for the entire AERO
link.a
BGP-based overlay routing service coordinated by Servers and Bridges
that tracks all Server-to-Client associations.the collective set of
all Servers, Proxys, Bridges and Relays that provide the AERO
Service to Clients.an individual
Server, Proxy, Bridge or Relay in the Mobility Service.Throughout the document, the simple terms "Client", "Server",
"Bridge", "Proxy" and "Relay" refer to "AERO Client", "AERO Server",
"AERO Bridge", "AERO Proxy" and "AERO Relay", respectively.
Capitalization is used to distinguish these terms from other common
Internetworking uses in which they appear without capitalization.The terminology of DHCPv6 and IPv6 ND (including the names of node variables, messages and
protocol constants) is used throughout this document. The terms
"All-Routers multicast", "All-Nodes multicast", "Solicited-Node
multicast" and "Subnet-Router anycast" are defined in (with Link-Local scope assumed). Also, the term "IP"
is used to generically refer to either Internet Protocol version, i.e.,
IPv4 or IPv6 .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.The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:AERO Bridges provide hybrid routing/bridging services (as well as a
security trust anchor) for nodes on an AERO link. Bridges use standard
IPv6 routing to forward packets both within the same INET partitions
and between disjoint INET partitions based on a mid-layer IPv6
encapsulation per . The inner IP layer
experiences a virtual bridging service since the inner IP TTL/Hop
Limit is not decremented during forwarding. Each Bridge also peers
with Servers and other Bridges in a dynamic routing protocol instance
to provide a Distributed Mobility Management (DMM) service for the
list of active MNPs (see ). Bridges present
the AERO link as a set of one or more Mobility Service Prefixes (MSPs)
but as link-layer devices need not connect directly to the AERO link
themselves unless an administrative interface is desired. Bridges
configure secured tunnels with Servers, Relays, Proxys and other
Bridges; they further maintain IP forwarding table entries for each
Mobile Network Prefix (MNP) and any other reachable non-MNP
prefixes.AERO Servers provide default forwarding and mobility/multilink
services for AERO Client Mobile Nodes (MNs). Each Server also peers
with Bridges in a dynamic routing protocol instance to advertise its
list of associated MNPs (see ). Servers
facilitate PD exchanges with Clients, where each delegated prefix
becomes an MNP taken from an MSP. Servers forward packets between AERO
interface neighbors and track each Client's mobility profiles.AERO Clients register their MNPs through PD exchanges with AERO
Servers over the AERO link, and distribute the MNPs to nodes on EUNs.
A Client may also be co-resident on the same physical or virtual
platform as a Server; in that case, the Client and Server behave as a
single functional unit.AERO Proxys provide a conduit for ANET Clients to associate with
Servers in external INETs. Client and Servers exchange control plane
messages via the Proxy acting as a bridge between the ANET/INET
boundary. The Proxy forwards data packets between Clients and the AERO
link according to forwarding information in the neighbor cache. The
Proxy function is specified in .AERO Relays are Servers that provide forwarding services between
the AERO interface and INET/EUN interfaces. Relays are provisioned
with MNPs the same as for an AERO Client, and also run a dynamic
routing protocol to discover any non-MNP IP routes. The Relay
advertises the MSP(s) to its connected networks, and distributes all
of its associated MNPs and non-MNP IP routes via BGP peerings with
Bridges.AERO Bridges, Servers, Proxys and Relays are critical
infrastructure elements in fixed (i.e., non-mobile) INET deployments
and hence have permanent and unchanging INET addresses. AERO Clients
are MNs that connect via underlying interfaces with addresses that may
change when the Client moves to a new network connection point. presents the basic AERO link
reference model: In this model:the AERO link is an overlay network service configured over
one or more underlying INET partitions which may be managed by
different administrative authorities and have incompatible
protocols and/or addressing plans.AERO Bridge B1 aggregates Mobility Service Prefix (MSP) M1,
discovers Mobile Network Prefixes (MNPs) X* and advertises the
MSP via BGP peerings over secured tunnels to Servers (S1, S2).
Bridges connect the disjoint segments of a partitioned AERO
link.AERO Servers/Relays S1 and S2 configure secured tunnels with
Bridge B1 and also provide mobility, multilink and default
router services for their associated Clients C1 and C2.AERO Clients C1 and C2 associate with Servers S1 and S2,
respectively. They receive Mobile Network Prefix (MNP)
delegations X1 and X2, and also act as default routers for their
associated physical or internal virtual EUNs. Simple hosts H1
and H2 attach to the EUNs served by Clients C1 and C2,
respectively.AERO Proxy P1 configures a secured tunnel with Bridge B1 and
provides proxy services for AERO Clients in secured enclaves
that cannot associate directly with other AERO link
neighbors.An AERO link configured over a single INET appears as a single
unified link with a consistent underlying network addressing plan.
In that case, all nodes on the link can exchange packets via simple
INET encapsulation, since the underlying INET is connected. In
common practice, however, an AERO link may be partitioned into
multiple "segments", where each segment is a distinct INET
potentially managed under a different administrative authority
(e.g., as for worldwide aviation service providers such as ARINC,
SITA, Inmarsat, etc.). Individual INETs may also themselves be
partitioned internally, in which case each internal partition is
seen as a separate segment.The addressing plan of each segment is consistent internally but
will often bear no relation to the addressing plans of other
segments. Each segment is also likely to be separated from others by
network security devices (e.g., firewalls, proxies, packet filtering
gateways, etc.), and in many cases disjoint segments may not even
have any common physical link connections. Therefore, nodes can only
be assured of exchanging packets directly with correspondents in the
same segment, and not with those in other segments. The only means
for joining the segments therefore is through inter-domain peerings
between AERO Bridges.The same as for traditional campus LANs, multiple AERO link
segments can be joined into a single unified link via a virtual
bridging service using a mid-layer IPv6 encpasulation per known as the "SPAN header" that supports
inter-segment forwarding (i.e., bridging) without decrementing the
network-layer TTL/Hop Limit. This bridging of AERO link segments is
shown in :Bridges, Servers, Relays and Proxys connect via secured INET
tunnels over their respecitve segments in a spanning tree topology
rooted at the Bridges. The secured spanning tree supports strong
authentication for IPv6 ND control messages and may also be used to
convey the initial data packets in a flow. Route optimization can
then be employed to cause data packets to take more direct paths
between AERO link neighbors without having to strictly follow the
spanning tree.Nodes on AERO links use the Link-Local Address (LLA) prefix
fe80::/10 to assign LLAs used for
network-layer addresses in IPv6 ND and data messages. A Client's LLA
is an IPv6 link-local address formed from the Client's delegated
MNP. Bridge, Server, Relay and Proxy LLAs are assigned from the
range fe80::/96 and include an administratively-provisioned value in
the lower 32 bits.IPv6 Client LLAs encode the Subnet-Router anycast address of a
MNP (or non-MNP globally routable prefix) within the
least-significant 112 bits of the IPv6 link-local prefix fe80::/16.
For example, for the MNP 2001:db8:1000:2000::/56 the corresponding
LLA is fe80:2001:db8:1000:2000::/72.IPv4-compatible Client LLAs are based on an IPv4-mapped IPv6
address formed from an IPv4 MNP and with a
prefix length of 96 plus the MNP prefix length. For example, for the
IPv4 MNP 192.0.2.16/28 the IPv4-mapped IPv6 MNP is:0:0:0:0:0:ffff:192.0.2.16/124 (also written as
0:0:0:0:0:ffff:c000:0210/124)The Client then constructs its LLA with the prefix
fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
in the interface identifier as: fe80::ffff:192.0.2.16.Mobility Service (MS) LLAs (used by Bridges, Servers, Relays and
Proxys) are allocated from the range fe80::/96, and MUST be managed
for uniqueness. The lower 32 bits of the LLA includes a unique
integer value between 1 and 0xfeffffff (e.g., fe80::1, fe80::2,
fe80::3, etc., fe80::feff:ffff) as assigned by the administrative
authority for the link. The address fe80:: is the IPv6 link-local
Subnet-Router anycast address, and the address range
fe80::ff00:0000/104 is reserved for future use.Finally, the address range fe80::/32 is used as the Teredo
service prefix for AERO according to the format in Section 4 of
(see for
further discussion).Nodes on AERO links use the Unique Local Address (ULA) prefix
fd80::/10 to form ULAs used for SPAN header
source and desitnation addresses. The prefix length intentionally
matches the IPv6 link-local prefix (fe80::/10), and enables a simple
stateless translation between LLAs and ULAs.AERO ULAs are formed by simply rewriting the upper bits of the
corresponding LLA as follows:the ULA formed from the IPv6 Client LLA
fe80:2001:db8:1000:2000:: is simply
fd80:2001:db8:1000:2000::the ULA formed from the IPv4-compatible Client LLA
fe80::ffff:192.0.2.1 is simply fd80::ffff:192.0.2.1the ULA formed from the MS LLA fe80::1001 is simply
fd80::1001the ULA prefix fd80::/32 is used as the Teredo service prefix
the same as for LLAs above.The AERO routing system comprises a private instance of the
Border Gateway Protocol (BGP) that is
coordinated between Bridges and Servers and does not interact with
either the public Internet BGP routing system or any underlying INET
routing systems.In a reference deployment, each Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) using an AS Number (ASN) that is unique within the BGP
instance, and each Server further uses eBGP to peer with one or more
Bridges but does not peer with other Servers. Each INET of a
multi-segment AERO link must include one or more Bridges, which peer
with the Servers and Proxys within that INET. All Bridges within the
same INET are members of the same hub AS using a common ASN, and use
iBGP to maintain a consistent view of all active MNPs currently in
service. The Bridges of different INETs peer with one another using
eBGP.Bridges advertise the AERO link's MSPs and any non-MNP routes to
each of their Servers. This means that any aggregated non-MNPs
(including "default") are advertised to all Servers. Each Bridge
configures a black-hole route for each of its MSPs. By black-holing
the MSPs, the Bridge will maintain forwarding table entries only for
the MNPs that are currently active, and packets destined to all
other MNPs will correctly incur Destination Unreachable messages due
to the black-hole route. In this way, Servers have only partial
topology knowledge (i.e., they know only about the MNPs of their
directly associated Clients) and they forward all other packets to
Bridges which have full topology knowledge.Each AERO link segment assigns a unique sub-prefix of fd80::/96
known as the ULA partition prefix. For example, a first segment
could assign fd80::1000/116, a second could assign fd80::2000/116, a
third could assign fd80::3000/116, etc. The administrative
authorities for each segment must therefore coordinate to assure
mutually-exclusive partiton prefix assignments, but internal
provisioning of each prefix is an independent local consideration
for each administrative authority.ULA partition prefixes are statitcally represented in Bridge
forwarding tables. Bridges join multiple segments into a unified
AERO link over multiple diverse administrative domains. They support
a bridging function by first establishing forwarding table entries
for their partiion prefixes either via standard BGP routing or
static routes. For example, if three Bridges ('A', 'B' and 'C') from
different segments serviced fd80::1000/116, fd80::2000/116 and
fd80::3000/116 respectively, then the forwarding tables in each
Bridge are as follows:fd80::1000/116->local, fd80::2000/116->B,
fd80::3000/116->Cfd80::1000/116->A, fd80::2000/116->local,
fd80::3000/116->Cfd80::1000/116->A, fd80::2000/116->B,
fd80::3000/116->localThese forwarding table entries are permanent and never
change, since they correspond to fixed infrastructure elements in
their respective segments.ULA Client prefixes are instead dynamically advertised in the
AERO link routing system by Servers and Relays that provide service
for their corresponding MNPs. For example, if three Servers ('D',
'E' and 'F') service the MNPs 2001:db8:1000:2000::/56,
2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing
system would include:fd80:2001:db8:1000:2000::/72fd80:2001:db8:3000:4000::/72fd80:2001:db8:5000:6000::/72A full discussion of the BGP-based routing system used by AERO is
found in . The system
provides for Distributed Mobility Management (DMM) per the
distributed mobility anchoring architecture .With the Client and partition prefixes in place in each Bridge's
forwarding table, control and data packets sent between AERO nodes
in different segments can therefore be carried over the via
mid-layer encapsulation using the SPAN header. For example, when a
source AERO node forwards a packet with IPv6 address 2001:db8:1:2::1
to a target AERO node with IPv6 address 2001:db8:1000:2000::1, it
first encapsulates the packet in a SPAN header with source address
set to fd80:2001:db8:1:2:: and destination address set to
fd80:2001:db8:1000:2000::. Next, it encapsulates the resulting SPAN
packet in an INET header with source address set to its own INET
address (e.g., 192.0.2.100) and destination set to the INET address
of a Bridge (e.g., 192.0.2.1).SPAN encapsulation is based on Generic Packet Tunneling in IPv6
; the encapsulation format in the above
example is shown in :In this format, the inner IP header and packet body are the
original IP packet, the SPAN header is an IPv6 header prepared
according to , and the INET header is
prepared as discussed in .This gives rise to a routing system that contains both Client
prefix routes that may change dynamically due to regional node
mobility and partion prefix routes that never change. The Bridges
can therefore provide link-layer bridging by sending packets over
the SRT instead of network-layer routing according to MNP routes. As
a result, opportunities for packet loss due to node mobility between
different segments are mitigated.In normal operations, IPv6 ND messages are conveyed over secured
paths between AERO link neighbors so that specific Proxys, Servers
or Relays can be addressed without being subject to mobility events.
Conversely, only the first few packets destined to Clients need to
traverse secured paths until route optimization can determine a more
direct path.The 16-bit sub-prefixes of fd80::/10 (e.g., fd80::/16, fd81::/16,
fd82::/16, etc.) identify distinct Segment Routing Topologies (SRTs)
(see: ). Each SRT is a mutually-exclusive AERO
link overlay instance using a mutually-exclusive set of ULAs, and
emulates a Virtual LAN (VLAN) service for the AERO link. In some
cases (e.g., when redundant topologies are needed for fault
tolerance and reliability) it may be beneficial to deploy multiple
SRTs that act as independent overlay instances. A communication
failure in one instance therefore will not affect communications in
other instances.Each SRT is identified by a distinct value in bits 10-15 of he
SSP fd80::10, i.e., as fd80::/16, fd81::/16, fd82::/16, etc. This
document asserts that up to four SRTs provide a level of safety
sufficient for critical communications such as civil aviation. Each
SRT is designated with a color that identifies a different AERO link
instance as follows:Red (default) - corresponds to the SSP fd80::/16Green - corresponds to the SSP fd81::/16Blue-1 - corresponds to the SSP fd82::/16Blue-2 - corresponds to SSP fd83::/16SSPs fd84::/16 through fdbf::/16 are reserved for future
use.Each AERO interface assigns an anycast ULA corresponding to
its SRT prefix. For example, the anycast ULA for the Green SRT is
simply fd81::. The anycast ULA is used for AERO interface
determination in Safety-Based Multilink (SBM) as discussed in . Each AERO interface
further applies Performance-Based Multilink (PBM) internally.An original IPv6 source can direct a packet to a specific SRT
ingress router for the AERO link by including a Segment Routing
Header (SRH) with the anycast ULA for the selected SRT as either the
IPv6 destination or as an intermediate segment ID within the SRH.
This allows the original source to determine the specific topology a
packet will traverse when there may be multiple alternatives to
choose from. This form of Segment Routing supports Safety-Based
Multilink (SBM), and can be exercised through general-purpose SRH
types such as .AERO nodes that insert a SPAN header can use Segment Routing
within the AERO link to influence the path of packets destined to
Clients on INET interfaces without causing all packets to traverse
the Server. When a Client, Proxy or Server has a packet to send to a
target discovered through route optimization located in the same
AERO link segment, it encapsulates the packet in a SPAN header with
the ULA of the target as the destination address, then uses the
target's Link Layer Address information for INET encapsulation.When a Client, Proxy or Server has a packet to send to a route
optimization target located in a different AERO link segment, it
encapsulates the packet in a SPAN header with the ULA of the
target's Server as the destination. The node also includes a SRH
with the MNP corresponding to the target ULA as the penultimate
Segment ID (SID) and with the IP encapsulation address of the target
as the ultimate SID. The node then forwards the packet via a secured
tunnel to a Bridge, which will eventually direct it to a Bridge on
the same segment as the target's Server.When a Bridge on the same segment as the target's Server receives
a SPAN-encapsulated packet destined to the target Server, it looks
ahead into the SRH to determine that the penultimate SID is set to
the target's MNP and the ultimate SID is set to the target's Link
Layer Address. The Bridge then advances the SPAN destination address
to the target's ULA and encapsulates the SPAN packet in an INET
header based on the target's Link Layer Address, then forwards the
packet to the target directly while bypassing the target's Server.
In this way, the Bridge participates in route optimization to
greatly reduce traffic load and suboptimal routing through the
target's Server.Segment Routing within the AERO link uses the AERO SRH format
shown in (note that a similar format is
defined by In this format:Next Header, Hdr Ext Len and Segments Left have the same
meaning as in . This document assigns a
new Routing Type value TBD.Immediately following are N Type/Length/Value Segment IDs as
follows:Type[i] indicates the type of SID that follows. The
following types are currently defined:0000 - SID is a 4 byte public IPv4 address followed
by a 2 byte port number0001 - SID is a 4 byte NATed IPv4 address followed by
a 2 byte port number0010 - SID is a 16 byte public IPv6 address followed
by a 2 byte port number0011 - SID is a 16 byte NATed IPv6 address followed
by a 2 byte port number0100 - SID is a 2 byte ID corresponding to an IPv6
router0101 - SID is a 4 byte ID corresponding to an IPv6
router0110 - 0111 - Reserved1000 - SID is a (Len+1) byte IPv4 MNP. Len SHOULD be
sized to accommodate the IPv4 prefix in the fewest
possible bytes, If the IPv4 prefix does not end on an
even byte boundary, the rightmost bits are set to 0.1001 - SID is a (Len+1) byte IPv6 MNP. Len SHOULD be
sized to accommdate the IPv6 prefix in the fewest
possible bytes. If the IPv6 prefix does not end on an
even byte boundary, the rightmost bits are set to 0.1010 - 1111 - ReservedLen[i] is a value that, when added to 1, indicates the
length of the SID that follows. Note that Len is exclusive
of the 2 byte port number included for Types 0000 - 0011.
For example, for Type 0010 Len encodes the value 15, yet a 2
byte trailing port number follows immediately after the
SID.SID[i] is an ID of (Len +1) bytes in length formatted
according to Type. For Type values less than 0100, the SID
is followed by a 2 byte port number whose length is NOT
included in Len.Trailing padding added if necessary to create an integral
number of 8 octet blocks.AERO interfaces are virtual interfaces configured over one or more
underlying interfaces classified as follows: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. All Server, Relay and Bridge interfaces
are native interfaces, as are INET-facing interfaces of Proxys.
NATed INET interfaces connect to a private network behind one or
more NATs that provide INET access. Clients that are behind a NAT
are required to send periodic keepalive messages to keep NAT state
alive when there are no data packets flowing.Proxyed interfaces connect to an ANET that is separated from
the open INET by an AERO Proxy. Proxys can actively issue control
messages over the INET on behalf of the Client to reduce ANET
congestion. Clients connected to Proxyed interfaces receive RAs
with the P flag set to 1.VPNed interfaces use security encapsulation over the INET to a
Virtual Private Network (VPN) server that also acts as an AERO
Server. Other than the link-layer encapsulation format, VPNed
interfaces behave the same as Direct interfaces.Direct interfaces connect a Client directly to a Server without
crossing any ANET/INET paths. An example is a line-of-sight link
between a remote pilot and an unmanned aircraft. The same Client
considerations apply as for VPNed interfaces.AERO interfaces use SPAN encapsulation as necessary as discussed in
. AERO interfaces use link-layer
encapsulation (see: ) to exchange packets
with AERO link neighbors over INET or VPNed interfaces. AERO
interfaces do not use link-layer encapsulation over Proxyed and Direct
underlying interfaces.AERO interfaces maintain a neighbor cache for tracking per-neighbor
state the same as for any interface. AERO interfaces use ND messages
including Router Solicitation (RS), Router Advertisement (RA),
Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for
neighbor cache management.AERO interfaces send ND messages with an Overlay Multilink Network
Interface (OMNI) option formatted as specified in . The OMNI option includes
prefix registration information and "ifIndex-tuples" containing link
information parameters for the AERO interface's underlying
interfaces.When encapsulation is used, AERO interface ND messages MAY also
include an AERO Source/Target Link-Layer Address Option (S/TLLAO)
formatted as shown in :In this format, Type and Length are set the same as specified for
S/TLLAOs in , with trailing zero padding
octets added as necessary to produce an integral number of 8 octet
blocks. The S/TLLAO includes N ifIndex-tuples in correspondence to
ifIndex-tuples that appear in the OMNI option. Each ifIndex-tuple
includes the following information:ifIndex[i] - the same value as in the corresponding
ifIndex-tuple included in the OMNI option.SRT[i] - a 3-bit "Segment Routing Topology" value (see: ) coded as follows:000 - Red001 - Green010 - Blue-1011 - Blue-2100 - 111 - ReservedLHS[i] - a 3-bit "LookaHead Segments" value that encodes the
number (from 0 to 7) of entries in Segment Routing List [i].FMT[i] - a 2-bit "Format" code. Determines the format of the
Link Layer Address [i] field as follows:00 - Link Layer Address [i] encodes a Teredo-format LLA for
a node behind a NAT.01 - Link Layer Address [i] encodes a Teredo-format LLA for
a node on the open INET.10 - Link Layer Address [i] encodes an IPv6 address for a
node behind a NAT..11 - Link Layer Address [i] encodes an IPv6 address for a
node on the open INET.Segment Routing List [i] - Includes LHS[i]-many 16 byte ULAs
corresponding to the Segment IDs (SIDs) that must be visited prior
to forwarding to Link Layer Address [i]. The ultimate SID appears
first, followed by the penultimate SID second, etc.Link Layer Address [i] - Included according to FMT[i], and
identifies the link-layer address of the source/target. The IP
address and port number (for IPv4) are recorded in ones-compliment
"obfuscated" form per .Port Number [i] - Present only for IPv6 (i.e., when FMT[i] is
10 or 11). When present, the field is 2 bytes in length and
immediately follows Link Layer Address [i], with the value
recorded in obfuscated form.If an S/TLLAO is included, any ifIndex-tuples correspond to a
proper subset of the OMNI option ifIndex-tuples. Any S/TLLAO
ifIndex-tuple having an ifIndex value that does not appear in an OMNI
option ifindex-tuple is ignored. If the same ifIndex value appears in
multiple ifIndex-tuples, the first tuple is processed and the
remaining tuples are ignored. Any S/TLLAO ifIndex-tuples can therefore
be viewed as inter-dependent extensions of their corresponding OMNI
option ifIndex-tuples, i.e., the OMNI option and S/TLLAO are
companions that are interpreted in conjunction with each other.A Client's AERO interface may be configured over multiple
underlying interface connections. For example, common mobile handheld
devices have both wireless local area network ("WLAN") and cellular
wireless links. These links are often used "one at a time" with
low-cost WLAN preferred and highly-available cellular wireless as a
standby, but a simultaneous-use capability could provide benefits. In
a more complex example, aircraft frequently have many wireless data
link types (e.g. satellite-based, cellular, terrestrial, air-to-air
directional, etc.) with diverse performance and cost properties.If a Client's multiple underlying interfaces are used "one at a
time" (i.e., all other interfaces are in standby mode while one
interface is active), then ND message OMNI options include only a
single ifIndex-tuple set to constant values. In that case, the Client
would appear to have a single interface but with a dynamically
changing link-layer address.If the Client has multiple active underlying interfaces, then from
the perspective of ND it would appear to have multiple link-layer
addresses. In that case, ND message OMNI options MAY include multiple
ifIndex-tuples - each with values that correspond to a specific
interface. Every ND message need not include all OMNI and/or S/TLLAO
ifIndex-tuples; for any ifIndex-tuple not included, the neighbor
considers the status as unchanged.Bridge, Server and Proxy AERO interfaces may be configured over one
or more secured tunnel interfaces. The AERO interface configures both
an LLA and its corresponding ULA, while the underlying secured tunnel
interfaces are either unnumbered or configure the same ULA. The AERO
interface encapsulates each IP packet in a SPAN header and presents
the packet to the underlying secured tunnel interface. For Bridges
that do not configure an AERO interface, the secured tunnel interfaces
themselves are exposed to the IP layer with each interface configuring
the Bridge's ULA. Routing protocols such as BGP therefore run directly
over the Bridge's secured tunnel interfaces. For nodes that configure
an AERO interface, routing protocols such as BGP run over the AERO
interface but do not employ SPAN encapsulation. Instead, the AERO
interface presents the routing protocol messages directly to the
underlying secured tunnels without applying encapsulation and while
using the ULA as the source address. This distinction must be honored
consistently according to each node's configuration so that the IP
forwarding table will associate discovered IP routes with the correct
interface.AERO Servers, Proxys and Clients configure AERO interfaces as their
point of attachment to the AERO link. AERO nodes assign the MSPs for
the link to their AERO interfaces (i.e., as a "route-to-interface") to
ensure that packets with destination addresses covered by an MNP not
explicitly assigned to a non-AERO interface are directed to the AERO
interface.AERO interface initialization procedures for Servers, Proxys,
Clients and Bridges are discussed in the following sections.When a Server enables an AERO interface, it assigns an LLA/ULA
appropriate for the given AERO link segment. The Server also
configures secured tunnels with one or more neighboring Bridges and
engages in a BGP routing protocol session with each Bridge.The AERO interface provides a single interface abstraction to the
IP layer, but internally comprises multiple secured tunnels as well
as an NBMA nexus for sending encapsulated data packets to AERO
interface neighbors. The Server further configures a service to
facilitate ND/PD exchanges with AERO Clients and manages per-Client
neighbor cache entries and IP forwarding table entries based on
control message exchanges.Relays are simply Servers that run a dynamic routing protocol to
redistribute routes between the AERO interface and INET/EUN
interfaces (see: ). The Relay provisions
MNPs to networks on the INET/EUN interfaces (i.e., the same as a
Client would do) and advertises the MSP(s) for the AERO link over
the INET/EUN interfaces. The Relay further provides an attachment
point of the AERO link to a non-MNP-based global topology.When a Proxy enables an AERO interface, it assigns an LLA/ULA and
configures permanent neighbor cache entries the same as for Servers.
The Proxy also configures secured tunnels with one or more
neighboring Bridges and maintains per-Client neighbor cache entries
based on control message exchanges.When a Client enables an AERO interface, it sends RS messages
with ND/PD parameters over its underlying interfaces to a Server in
the MAP list, which returns an RA message with corresponding
parameters. (The RS/RA messages may pass through a Proxy in the case
of a Client's Proxyed interface, or through one or more NATs in the
case of a Client's INET interface.)AERO Bridges need not connect directly to the AERO link, since
they operate as link-layer forwarding devices instead of network
layer routers. Configuration of AERO interfaces on Bridges is
therefore OPTIONAL, e.g., if an administrative interface is needed.
Bridges configure secured tunnels with Servers, Proxys and other
Bridges; they also configure LLAs/ULAs and permanent neighbor cache
entries the same as Servers. Bridges engage in a BGP routing
protocol session with a subset of the Servers and other Bridges on
the spanning tree (see: ).Each AERO interface maintains a conceptual neighbor cache that
includes an entry for each neighbor it communicates with on the AERO
link per . AERO interface neighbor cache
entries are said to be one of "permanent", "symmetric", "asymmetric"
or "proxy".Permanent neighbor cache entries are created through explicit
administrative action; they have no timeout values and remain in place
until explicitly deleted. AERO Bridges maintain permanent neighbor
cache entries for their associated Proxys and Servers (and
vice-versa). Each entry maintains the mapping between the neighbor's
network-layer LLA and corresponding INET address.Symmetric neighbor cache entries are created and maintained through
RS/RA exchanges as specified in , and remain in
place for durations bounded by ND/PD lifetimes. AERO Servers maintain
symmetric neighbor cache entries for each of their associated Clients,
and AERO Clients maintain symmetric neighbor cache entries for each of
their associated Servers. The list of all Servers on the AERO link is
maintained in the link's MAP list.Asymmetric neighbor cache entries are created or updated based on
route optimization messaging as specified in , and are garbage-collected when keepalive timers
expire. AERO ROSs maintain asymmetric neighbor cache entries for
active targets with lifetimes based on ND messaging constants.
Asymmetric neighbor cache entries are unidirectional since only the
ROS (and not the ROR) creates an entry.Proxy neighbor cache entries are created and maintained by AERO
Proxys when they process Client/Server ND/PD exchanges, and remain in
place for durations bounded by ND/PD lifetimes. AERO Proxys maintain
proxy neighbor cache entries for each of their associated Clients.
Proxy neighbor cache entries track the Client state and the address of
the Client's associated Server(s).To the list of neighbor cache entry states in Section 7.3.2 of
, Proxy and Server AERO interfaces add an
additional state DEPARTED that applies to symmetric and proxy neighbor
cache entries for Clients that have recently departed. The interface
sets a "DepartTime" variable for the neighbor cache entry to
"DEPART_TIME" seconds. DepartTime is decremented unless a new ND
message causes the state to return to REACHABLE. While a neighbor
cache entry is in the DEPARTED state, packets destined to the target
Client are forwarded to the Client's new location instead of being
dropped. When DepartTime decrements to 0, the neighbor cache entry is
deleted. It is RECOMMENDED that DEPART_TIME be set to the default
constant value REACHABLE_TIME plus 10 seconds (40 seconds by default)
to allow a window for packets in flight to be delivered while stale
route optimization state may be present.When an ROR receives an authentic NS message used for route
optimization, it searches for a symmetric neighbor cache entry for the
target Client. The ROR then returns a solicited NA message without
creating a neighbor cache entry for the ROS, but creates or updates a
target Client "Report List" entry for the ROS and sets a "ReportTime"
variable for the entry to REPORT_TIME seconds. The ROR resets
ReportTime when it receives a new authentic NS message, and otherwise
decrements ReportTime while no authentic NS messages have been
received. It is RECOMMENDED that REPORT_TIME be set to the default
constant value REACHABLE_TIME plus 10 seconds (40 seconds by default)
to allow a window for route optimization to converge before ReportTime
decrements below REACHABLE_TIME.When the ROS receives a solicited NA message response to its NS
message used for route optimization, it creates or updates an
asymmetric neighbor cache entry for the target network-layer and
link-layer addresses. The ROS then (re)sets ReachableTime for the
neighbor cache entry to REACHABLE_TIME seconds and uses this value to
determine whether packets can be forwarded directly to the target,
i.e., instead of via a default route. The ROS otherwise decrements
ReachableTime while no further solicited NA messages arrive. It is
RECOMMENDED that REACHABLE_TIME be set to the default constant value
30 seconds as specified in .AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the
number of NS keepalives sent when a correspondent may have gone
unreachable, the value MAX_RTR_SOLICITATIONS to limit the number of RS
messages sent without receiving an RA and the value
MAX_NEIGHBOR_ADVERTISEMENT to limit the number of unsolicited NAs that
can be sent based on a single event. It is RECOMMENDED that
MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and
MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in .Different values for DEPART_TIME, REPORT_TIME, REACHABLE_TIME,
MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and
MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if
different values are chosen, all nodes on the link MUST consistently
configure the same values. Most importantly, DEPART_TIME and
REPORT_TIME SHOULD be set to a value that is sufficiently longer than
REACHABLE_TIME to avoid packet loss due to stale route optimization
state.In some instances, AERO interfaces insert a mid-layer IPv6 header
known as the SPAN header as discussed in the following sections. After
either inserting or omitting the SPAN header, the AERO interface
inserts an outer encapsulation header as discussed below.AERO interfaces avoid outer encapsulation over Direct underlying
interfaces and Proxyed underlying interfaces for which the first-hop
access router is AERO-aware. Other AERO interfaces encapsulate packets
according to whether they are entering the AERO interface from the
network layer or if they are being re-admitted into the same AERO link
they arrived on. This latter form of encapsulation is known as
"re-encapsulation".For packets entering the AERO interface from the network layer, the
AERO interface copies the "TTL/Hop Limit", "Type of Service/Traffic
Class" , "Flow Label" (for IPv6) and "Congestion Experienced" values in the inner packet's IP header into the
corresponding fields in the SPAN and outer encapsulation
header(s).For packets undergoing re-encapsulation, the AERO interface instead
copies these values from the original encapsulation header into the
new encapsulation header, i.e., the values are transferred between
encapsulation headers and *not* copied from the encapsulated packet's
network-layer header. (Note especially that by copying the TTL/Hop
Limit between encapsulation headers the value will eventually
decrement to 0 if there is a (temporary) routing loop.) For IPv4
encapsulation/re-encapsulation, the AERO interface sets the DF bit as
discussed in .AERO interfaces configured over INET underlying interfaces
encapsulate packets in INET headers according to the next hop
determined in the forwarding algorithm in . If
the next hop is reached via a secured tunnel, the AERO interface uses
an encapsulation format specific to the secured tunnel type (see:
). If the next hop is reached via an unsecured
INET interface, the AERO interface instead uses UDP/IP encapsulation
according to the Teredo format specified in
and as extended in .When Teredo encapsulation is used, the AERO interface next sets the
UDP source port to a constant value that it will use in each
successive packet it sends, and sets the UDP length field to the
length of the encapsulated packet plus 8 bytes for the UDP header
itself plus the length of any included Teredo extension headers or
trailers. The encapsulated packet may be either IPv6 or IPv4, as
distinguished by the version number found in the first four bits.For Teredo-encapsulated packets sent to a Server, Relay or Bridge,
the AERO interface sets the UDP destination port to 8060, i.e., the
IANA-registered port number for AERO. For packets sent to a Client,
the AERO interface sets the UDP destination port to the port value
stored in the neighbor cache entry for this Client. The AERO interface
finally includes/omits the UDP checksum according to .AERO interfaces observe the packet sizing and fragmentation
considerations found in .AERO interfaces decapsulate packets destined either to the AERO
node itself or to a destination reached via an interface other than
the AERO interface the packet was received on. When the encapsulated
packet arrives in multiple SPAN fragments, the AERO interface
reassembles as discussed in . Further
decapsulation steps are performed according to the appropriate
encapsulation format specification.AERO nodes employ simple data origin authentication procedures. In
particular:AERO Bridges, Servers and Proxys accept encapsulated data
packets and control messages received from the spanning tree.AERO Proxys and Clients accept packets that originate from
within the same secured ANET.AERO Clients and Relays accept packets from downstream network
correspondents based on ingress filtering.AERO Clients, Relays and Servers verify the outer UDP/IP
encapsulation addresses according to the Teredo specification
.AERO nodes silently drop any packets that do not satisfy the
above data origin authentication procedures. Further security
considerations are discussed in .IPv6 underlying interfaces are REQUIRED to configure a minimum
Maximum Transmission Unit (MTU) of 1280 bytes . The minimum MTU for IPv4 underlying interfaces is
only 68 bytes , meaning that a packet smaller
than the IPv6 MTU may require fragmentation when IPv4 encapsulation is
used. Therefore, the Don't Fragment (DF) bit in the IPv4 encapsulation
header MUST be set to 0.The AERO interface configures an MTU of 9180 bytes ; the size is therefore not a reflection of the
underlying interface MTUs, but rather determines the largest packet
the AERO interface can forward or reassemble. The AERO interface
therefore accommodates IP packets up to 9180 bytes while generating
IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) messages as necessary (see below).AERO interfaces employ SPAN encapsulation and
fragmentation/reassembly per to accommodate
the 9180 byte MTU. The AERO interface returns internally-generated PTB
messages for packets admitted into the interface that it deems too
large (e.g., according to link performance characteristics, reassembly
cost, etc.) while either dropping or forwarding the packet as
necessary. The AERO interface performs PMTUD even if the destination
appears to be on the same link since intermediate AERO link nodes may
return a PTB. This ensures that the path MTU is adaptive and reflects
the current path used for a given data flow.AERO nodes perform SPAN encapsulation and fragmentation/reassembly
as follows:When a node's AERO interface sends a packet over a Proxyed,
VPNed or Direct underlying interface, it sends without SPAN
encapsulation if the packet is no larger than the underlying
interface MTU. Otherwise, it inserts a SPAN header with source
address set to the node's own ULA and destination set to the ULA
of the link-layer peer Proxy, Server or Client on the underlying
interface. The AERO interface then uses IPv6 fragmentation to
break the packet into a minimum number of non-overlapping
fragments, where the largest fragment size is determined by the
underlying interface MTU and the smallest fragment is no smaller
than 640 bytes. The AERO interface then sends the fragments to the
link-layer peer, which reassembles before forwarding toward the
final destination.When a node's AERO interface sends a packet over an INET
underlying interface, it sends encapsulated packets no larger than
1280 bytes without a SPAN header if the destination is reached via
an INET address within the same AERO link segment. Otherwise, it
inserts a SPAN header with source address set to the node's ULA,
destination set to the ULA of the next hop AERO node toward the
final destination and (if necessary) with a SRH with the remaining
Segment IDs on the path to the final destination. The AERO
interface then uses IPv6 fragmentation to break the encapsulated
packet into a minimum number of non-overlapping fragments, where
the largest fragment size (including both SPAN and INET
encapsulation) is 1280 bytes and the smallest fragment is no
smaller than 640 bytes. The AERO interface then encapsulates the
SPAN fragments in INET headers and sends them to the SPAN
destination, which reassembles before forwarding toward the final
destination.In order to avoid a "tiny fragment" attack, AERO interfaces
unconditionally drop all SPAN fragments smaller than 640 bytes. In
order to set the correct context for reassembly, the AERO interface
that inserts a SPAN header MUST also be the one that inserts the IPv6
Fragment Header Identification value. Although all fragments of the
same fragmented SPAN packet are typically sent via the same underlying
interface, this is not strictly required since all fragments will
arrive at the AERO interface that performs reassembly even if they
travel over different paths.Note that the AERO interface can forward large packets via
encapsulation and fragmentation while at the same time returning
advisory PTB messages, e.g., subject to rate limiting. The receiving
node that performs reassembly can also send advisory PTB messages if
reassembly conditions become unfavorable. The AERO interface can
therefore continuously forward large packets without loss while
returning advisory messages recommending a smaller size (but no
smaller than 1280). Advisory PTB messages are differentiated from PTB
messages that report loss by setting the Code field in the ICMPv6
message header to the value 1. This document therefore updates and .IP packets enter a node's AERO interface either from the network
layer (i.e., from a local application or the IP forwarding system) or
from the link layer (i.e., from an AERO interface neighbor). All
packets entering a node's AERO interface first undergo data origin
authentication as discussed in . Packets that
satisfy data origin authentication are processed further, while all
others are dropped silently.Packets that enter the AERO interface from the network layer are
forwarded to an AERO interface neighbor. Packets that enter the AERO
interface from the link layer are either re-admitted into the AERO
link or forwarded to the network layer where they are subject to
either local delivery or IP forwarding. In all cases, the AERO
interface itself MUST NOT decrement the network layer TTL/Hop-count
since its forwarding actions occur below the network layer.AERO interfaces may have multiple underlying interfaces and/or
neighbor cache entries for neighbors with multiple ifIndex-tuple
registrations (see ). The AERO interface
uses traffic classifiers (e.g., DSCP value, port number, etc.) to
select an outgoing underlying interface for each packet based on the
node's own QoS preferences, and also to select a destination
link-layer address based on the neighbor's underlying interface with
the highest preference. AERO implementations SHOULD allow for QoS
preference values to be modified at runtime through network
management.If multiple outgoing interfaces and/or neighbor interfaces have a
preference of "high", the AERO node replicates the packet and sends
one copy via each of the (outgoing / neighbor) interface pairs;
otherwise, the node sends a single copy of the packet via an interface
with the highest preference. AERO nodes keep track of which underlying
interfaces are currently "reachable" or "unreachable", and only use
"reachable" interfaces for forwarding purposes.The following sections discuss the AERO interface forwarding
algorithms for Clients, Proxys, Servers and Bridges. In the following
discussion, a packet's destination address is said to "match" if it is
the same as a cached address, or if it is covered by a cached prefix
(which may be encoded in an LLA).When an IP packet enters a Client's AERO interface from the
network layer the Client searches for an asymmetric neighbor cache
entry that matches the destination. If there is a match, the Client
uses one or more "reachable" neighbor interfaces in the entry for
packet forwarding. If there is no asymmetric neighbor cache entry,
the Client instead forwards the packet toward a Server (the packet
is intercepted by a Proxy if there is a Proxy on the path). The
Client encapsulates the packet in a SPAN header and fragments if
necessary according to MTU requirements (see: ).When an IP packet enters a Client's AERO interface from the
link-layer, if the destination matches one of the Client's MNPs or
link-local addresses the Client reassembles and decapsulates as
necessary and delivers the inner packet to the network layer.
Otherwise, the Client drops the packet and MAY return a
network-layer ICMP Destination Unreachable message subject to rate
limiting (see: ).For control messages originating from or destined to a Client,
the Proxy intercepts the message and updates its proxy neighbor
cache entry for the Client. The Proxy then forwards a (proxyed) copy
of the control message. (For example, the Proxy forwards a proxied
version of a Client's NS/RS message to the target neighbor, and
forwards a proxied version of the NA/RA reply to the Client.)When the Proxy receives a data packet from a Client within the
ANET, the Proxy reassembles and re-fragments if necessary then
searches for an asymmetric neighbor cache entry that matches the
destination and forwards as follows:if the destination matches an asymmetric neighbor cache
entry, the Proxy uses one or more "reachable" neighbor
interfaces in the entry for packet forwarding using SPAN
encapsulation and including a SRH if necessary according to the
cached TLLAO information. If the neighbor interface is in the
same SPAN segment, the Proxy forwards the packet directly to the
neighbor; otherwise, it forwards the packet to a Bridge.else, the Proxy uses SPAN encapsulation and forwards the
packet to a Bridge while using the ULA corresponding to the
packet's destination as the SPAN destination address.When the Proxy receives an encapsulated data packet from an INET
neighbor or from a secured tunnel from a Bridge, it accepts the
packet only if data origin authentication succeeds and if there is a
proxy neighbor cache entry that matches the inner destination. Next,
the Proxy reassembles the packet (if necessary) and continues
processing.Next if reassembly is complete and the neighbor cache state is
REACHABLE, the Proxy returns a PTB if necessary (see: ) then either drops or forwards the packet to the
Client while performing SPAN encapsulation and re-fragmentation to
the ANET MTU size if necessary. If the neighbor cache entry state is
DEPARTED, the Proxy instead changes the SPAN destination address to
the address of the new Server and forwards it to a Bridge while
performing re-fragmentation to 1280 bytes if necessary.For control messages destined to a target Client's LLA that are
received from a secured tunnel, the Server intercepts the message
and sends an appropriate response on behalf of the Client. (For
example, the Server sends an NA message reply in response to an NS
message directed to one of its associated Clients.) If the Client's
neighbor cache entry is in the DEPARTED state, however, the Server
instead forwards the packet to the Client's new Server as discussed
in .When the Server receives an encapsulated data packet from an INET
neighbor or from a secured tunnel, it accepts the packet only if
data origin authentication succeeds. If the SPAN destination address
is its own address, the Server continues processing as follows:if the destination matches a symmetric neighbor cache entry
in the REACHABLE state the Server prepares the packet for
forwarding to the destination Client. The Server first
reassembles (if necessary) and forwards the packet (while
re-fragmenting if necessary) as specified in .else, if the destination matches a symmetric neighbor cache
entry in the DEPARETED state the Server re-encapsulates the
packet and forwards it using the ULA of the Client's new Server
as the destination.else, if the destination matches an asymmetric neighbor cache
entry, the Server uses one or more "reachable" neighbor
interfaces in the entry for packet forwarding via the local INET
if the neighbor is in the same AERO link segment or using SPAN
encapsulation and Segment Routing if necessary with the final
destination set to the neighbor's ULA otherwise.else, if the destination is an LLA that is not assigned on
the AERO interface the Server drops the packet.else, the Server (acting as a Relay) reassembles if
necessary, decapsulates the packet and releases it to the
network layer for local delivery or IP forwarding. Based on the
information in the forwarding table, the network layer may
return the packet to the same AERO interface in which case
further processing occurs as below. (Note that this arrangement
accommodates common implementations in which the IP forwarding
table is not accessible from within the AERO interface. If the
AERO interface can directly access the IP forwarding table (such
as for in-kernel implementations) the forwarding table lookup
can instead be performed internally from within the AERO
interface itself.)When the Server's AERO interface receives a data packet
from the network layer or from a VPNed or Direct Client, it performs
SPAN encapsulation and fragmentation if necessary, then processes
the packet according to the network-layer destination address as
follows:if the destination matches a symmetric or asymmetric neighbor
cache entry the Server processes the packet as above.else, the Server encapsulates the packet and forwards it to a
Bridge using its own ULA as the source and the ULA corresponding
to the destination as the destination.Bridges forward SPAN-encapsulated packets over secured tunnels
the same as any IP router. When the Bridge receives a
SPAN-encapsulated packet via a secured tunnel, it removes the outer
INET header and searches for a forwarding table entry that matches
the SPAN destination address. The Bridge then processes the packet
as follows:if the destination is the ULA of a Server located in the
local AERO link segment, the Bridge checks for a SRH. If there
is a SRH with the ULA of the final destination as the
penultimate ID and with a Link Layer Address of the final
destination as the ultimate ID, the Bridge copies the ULA of the
final destination into the destination address. If the Link
Layer Address does not indicate the presence of a NAT, the
Bridge then forwards the packet directly to the Link Layer
Address using link-layer (UDP/IP) encapsulation. Otherwise, the
Bridge forwards the packet directly to the Server.if the destination matches one of the Bridge's own addresses,
the Bridge submits the packet for local delivery.else, if the destination matches a forwarding table entry the
Bridge forwards the packet via a secured tunnel to the next hop.
If the destination matches an MSP without matching an MNP,
however, the Bridge instead drops the packet and returns an ICMP
Destination Unreachable message subject to rate limiting (see:
).else, the Bridge drops the packet and returns an ICMP
Destination Unreachable as above.As for any IP router, the Bridge decrements the TTL/Hop
Limit when it forwards the packet. Therefore, only the Hop Limit in
the SPAN header is decremented, and not the TTL/Hop Limit in the
inner packet header.When an AERO node admits a packet into the AERO interface, it may
receive link-layer or network-layer error indications.A link-layer error indication is an ICMP error message generated by
a router in the INET on the path to the neighbor or by the neighbor
itself. The message includes an IP header with the address of the node
that generated the error as the source address and with the link-layer
address of the AERO node as the destination address.The IP header is followed by an ICMP header that includes an error
Type, Code and Checksum. Valid type values include "Destination
Unreachable", "Time Exceeded" and "Parameter Problem" . (AERO interfaces ignore
all link-layer IPv4 "Fragmentation Needed" and IPv6 "Packet Too Big"
messages since they only emit packets that are guaranteed to be no
larger than the IP minimum link MTU as discussed in .)The ICMP header is followed by the leading portion of the packet
that generated the error, also known as the "packet-in-error". For
ICMPv6, specifies that the packet-in-error
includes: "As much of invoking packet as possible without the ICMPv6
packet exceeding the minimum IPv6 MTU" (i.e., no more than 1280
bytes). For ICMPv4, specifies that the
packet-in-error includes: "Internet Header + 64 bits of Original Data
Datagram", however Section 4.3.2.3 updates
this specification by stating: "the ICMP datagram SHOULD contain as
much of the original datagram as possible without the length of the
ICMP datagram exceeding 576 bytes".The link-layer error message format is shown in (where, "L2" and "L3" refer to link-layer and
network-layer, respectively):The AERO node rules for processing these link-layer error
messages are as follows:When an AERO node receives a link-layer Parameter Problem
message, it processes the message the same as described as for
ordinary ICMP errors in the normative references .When an AERO node receives persistent link-layer Time Exceeded
messages, the IP ID field may be wrapping before earlier fragments
awaiting reassembly have been processed. In that case, the node
should begin including integrity checks and/or institute rate
limits for subsequent packets.When an AERO node receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its asymmetric neighbor correspondents, the node
should process the message as an indication that a path may be
failing, and optionally initiate NUD over that path. If it
receives Destination Unreachable messages over multiple paths, the
node should allow future packets destined to the correspondent to
flow through a default route and re-initiate route
optimization.When an AERO Client receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its symmetric neighbor Servers, the Client should
mark the path as unusable and use another path. If it receives
Destination Unreachable messages on many or all paths, the Client
should associate with a new Server and release its association
with the old Server as specified in .When an AERO Server receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its symmetric neighbor Clients, the Server should
mark the underlying path as unusable and use another underlying
path.When an AERO Server or Proxy receives link-layer Destination
Unreachable messages in response to an encapsulated packet that it
sends to one of its permanent neighbors, it treats the messages as
an indication that the path to the neighbor may be failing.
However, the dynamic routing protocol should soon reconverge and
correct the temporary outage.When an AERO Bridge receives a packet for which the
network-layer destination address is covered by an MSP, if there is no
more-specific routing information for the destination the Bridge drops
the packet and returns a network-layer Destination Unreachable message
subject to rate limiting. The Bridge writes the network-layer source
address of the original packet as the destination address and uses one
of its non link-local addresses as the source address of the
message.When an AERO node receives an encapsulated packet for which the
reassembly buffer it too small, it drops the packet and returns a
network-layer Packet Too Big (PTB) message. The node first writes the
MRU value into the PTB message MTU field, writes the network-layer
source address of the original packet as the destination address and
writes one of its non link-local addresses as the source address.AERO Router Discovery, Prefix Delegation and Autoconfiguration are
coordinated as discussed in the following Sections.Each AERO Server on the link configures a PD service to
facilitate Client requests. Each Server is provisioned with a
database of MNP-to-Client ID mappings for all Clients enrolled in
the AERO service, as well as any information necessary to
authenticate each Client. The Client database is maintained by a
central administrative authority for the AERO link and securely
distributed to all Servers, e.g., via the Lightweight Directory
Access Protocol (LDAP) , via static
configuration, etc. Clients receive the same service regardless of
the Servers they select.AERO Clients and Servers use ND messages to maintain neighbor
cache entries. AERO Servers configure their AERO interfaces as
advertising NBMA interfaces, and therefore send unicast RA messages
with a short Router Lifetime value (e.g., ReachableTime seconds) in
response to a Client's RS message. Thereafter, Clients send
additional RS messages to keep Server state alive.AERO Clients and Servers include PD parameters in RS/RA messages
(see for ND/PD
alternatives). The unified ND/PD messages are exchanged between
Client and Server according to the prefix management schedule
required by the PD service. If the Client knows its MNP in advance,
it can instead employ prefix registration by including its LLA as
the source address of an RS message and with an OMNI option with
valid prefix registration information for the MNP. If the Server
(and Proxy) accept the Client's MNP assertion, they inject the
prefix into the routing system and establish the necessary neighbor
cache state.The following sections specify the Client and Server
behavior.AERO Clients discover the addresses of Servers in a similar
manner as described in . Discovery methods
include static configuration (e.g., from a flat-file map of Server
addresses and locations), or through an automated means such as
Domain Name System (DNS) name resolution .
Alternatively, the Client can discover Server addresses through a
layer 2 data link login exchange, or through a unicast RA response
to a multicast/anycast RS as described below. 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 AERO link (e.g., "example.com").To associate with a Server, the Client acts as a requesting
router to request MNPs. The Client prepares an RS message with PD
parameters and includes a Nonce and Timestamp option if the Client
needs to correlate RA replies. If the Client already knows the
Server's LLA, it includes the LLA as the network-layer destination
address; otherwise, it includes the link-scoped All-Routers
multicast (ff02::2) or Subnet-Router anycast (fe80::) address as the
network-layer destination. If the Client already knows its own LLA,
it uses the LLA as the network-layer source address; otherwise, it
uses the unspecified IPv6 address (::/128) as the network-layer
source address.The Client next includes an OMNI option in the RS message to
register its link-layer information with the Server. The Client sets
the OMNI option prefix registration information according to the
MNP, and includes an ifIndex-tuple with S set to '1' corresponding
to the underlying interface over which the Client will send the RS
message. The Client MAY include additional ifIndex-tuples specific
to other underlying interfaces. The Client MAY also include an SLLAO
corresponding to the OMNI option ifIndex-tuple with S set to
'1'.The Client then sends the RS message (either directly via Direct
interfaces, via a VPN for VPNed interfaces, via a Proxy for proxyed
interfaces or via INET encapsulation for INET interfaces) and waits
for an RA message reply (see ). The
Client retries up to MAX_RTR_SOLICITATIONS times until an RA is
received. If the Client receives no RAs, or if it receives an RA
with Router Lifetime set to 0, the Client SHOULD abandon this Server
and try another Server. Otherwise, the Client processes the PD
information found in the RA message.Next, the Client creates a symmetric neighbor cache entry with
the Server's LLA as the network-layer address and the Server's
encapsulation and/or link-layer addresses as the link-layer address.
The Client records the RA Router Lifetime field value in the
neighbor cache entry as the time for which the Server has committed
to maintaining the MNP in the routing system via this underlying
interface, and caches the other RA configuration information
including Cur Hop Limit, M and O flags, Reachable Time and Retrans
Timer. The Client then autoconfigures LLAs for each of the delegated
MNPs and assigns them to the AERO interface. The Client also caches
any MSPs included in Route Information Options (RIOs) as MSPs to associate with the AERO link, and
assigns the MTU value in the MTU option to the underlying
interface.The Client then registers additional underlying interfaces with
the Server by sending RS messages via each additional interface. The
RS messages include the same parameters as for the initial RS/RA
exchange, but with destination address set to the Server's LLA.Following autoconfiguration, the Client sub-delegates the MNPs to
its attached EUNs and/or the Client's own internal virtual
interfaces as described in
to support the Client's downstream attached "Internet of Things
(IoT)". The Client subsequently sends additional RS messages over
each underlying interface before the Router Lifetime received for
that interface expires.After the Client registers its underlying interfaces, it may wish
to change one or more registrations, e.g., if an interface changes
address or becomes unavailable, if QoS preferences change, etc. To
do so, the Client prepares an RS message to send over any available
underlying interface. The RS includes an OMNI option with prefix
registration information specific to its MNP, with an ifIndex-tuple
specific to the selected underlying interface with S set to '1', and
with any additional ifIndex-tuples specific to other underlying
interfaces. The Client includes fresh ifIndex-tuple values to update
the Server's neighbor cache entry. When the Client receives the
Server's RA response, it has assurance that the Server has been
updated with the new information.If the Client wishes to discontinue use of a Server it issues an
RS message over any underlying interface with an OMNI option with a
prefix release indication. When the Server processes the message, it
releases the MNP, sets the symmetric neighbor cache entry state for
the Client to DEPARTED and returns an RA reply with Router Lifetime
set to 0. After a short delay (e.g., 2 seconds), the Server
withdraws the MNP from the routing system.AERO Servers act as IP routers and support a PD service for
Clients. Servers arrange to add their LLAs to a static map of Server
addresses for the link and/or the DNS resource records for the FQDN
"linkupnetworks.[domainname]" before entering service. Server
addresses should be geographically and/or topologically referenced,
and made available for discovery by Clients on the AERO link.When a Server receives a prospective Client's RS message on its
AERO interface, it SHOULD return an immediate RA reply with Router
Lifetime set to 0 if it is currently too busy or otherwise unable to
service the Client. Otherwise, the Server authenticates the RS
message and processes the PD parameters. The Server first determines
the correct MNPs to delegate to the Client by searching the Client
database. When the Server delegates the MNPs, it also creates a
forwarding table entry for each MNP so that the MNPs are propagated
into the routing system (see: ). For IPv6,
the Server creates an IPv6 forwarding table entry for each MNP. For
IPv4, the Server creates an IPv6 forwarding table entry with the
SPAN Compatibility Prefix (SCP) corresponding to the IPv4
address.The Server next creates a symmetric neighbor cache entry for the
Client using the base LLA as the network-layer address and with
lifetime set to no more than the smallest PD lifetime. Next, the
Server updates the neighbor cache entry by recording the information
in each ifIndex-tuple in the RS OMNI option. The Server also records
the actual SPAN/INET addresses in the neighbor cache entry.Next, the Server prepares an RA message using its LLA as the
network-layer source address and the network-layer source address of
the RS message as the network-layer destination address. The Server
sets the Router Lifetime to the time for which it will maintain both
this underlying interface individually and the symmetric neighbor
cache entry as a whole. The Server also sets Cur Hop Limit, M and O
flags, Reachable Time and Retrans Timer to values appropriate for
the AERO link. The Server includes the delegated MNPs, any other PD
parameters and an OMNI option with no ifIndex-tuples. The Server
then includes one or more RIOs that encode the MSPs for the AERO
link, plus an MTU option (see ). The Server
finally forwards the message to the Client using SPAN/INET, INET, or
NULL encapsulation as necessary.After the initial RS/RA exchange, the Server maintains a
ReachableTime timer for each of the Client's underlying interfaces
individually (and for the Client's symmetric neighbor cache entry
collectively) set to expire after ReachableTime seconds. If the
Client (or Proxy) issues additional RS messages, the Server sends an
RA response and resets ReachableTime. If the Server receives an ND
message with PD release indication it sets the Client's symmetric
neighbor cache entry to the DEPARTED state and withdraws the MNP
from the routing system after a short delay (e.g., 2 seconds). If
ReachableTime expires before a new RS is received on an individual
underlying interface, the Server marks the interface as DOWN. If
ReachableTime expires before any new RS is received on any
individual underlying interface, the Server sets the symmetric
neighbor cache entry state to STALE and sets a 10 second timer. If
the Server has not received a new RS or ND message with PD release
indication before the 10 second timer expires, it deletes the
neighbor cache entry and withdraws the MNP from the routing
system.The Server processes any ND/PD messages pertaining to the Client
and returns an NA/RA reply in response to solicitations. The Server
may also issue unsolicited RA messages, e.g., with PD reconfigure
parameters to cause the Client to renegotiate its PDs, with Router
Lifetime set to 0 if it can no longer service this Client, etc.
Finally, If the symmetric neighbor cache entry is in the DEPARTED
state, the Server deletes the entry after DepartTime expires.Note: Clients SHOULD notify former Servers of their departures,
but Servers are responsible for expiring neighbor cache entries and
withdrawing routes even if no departure notification is received
(e.g., if the Client leaves the network unexpectedly). Servers
SHOULD therefore set Router Lifetime to ReachableTime seconds in
solicited RA messages to minimize persistent stale cache information
in the absence of Client departure notifications. A short Router
Lifetime also ensures that proactive Client/Server RS/RA messaging
will keep any NAT state alive (see above).Note: All Servers on an AERO link MUST advertise consistent
values in the RA Cur Hop Limit, M and O flags, Reachable Time and
Retrans Timer fields the same as for any link, since unpredictable
behavior could result if different Servers on the same link
advertised different values.When DHCPv6 is used as the ND/PD service back end, AERO Clients
and Servers are always on the same link (i.e., the AERO link) from
the perspective of DHCPv6. However, in some implementations the
DHCPv6 server and ND function may be located in separate modules.
In that case, the Server's AERO interface module can act as a
Lightweight DHCPv6 Relay Agent (LDRA) to relay PD messages to and from the DHCPv6 server
module.When the LDRA receives an authentic RS message, it extracts the
PD message parameters and uses them to construct an
IPv6/UDP/DHCPv6 message. It sets the IPv6 source address to the
source address of the RS message, sets the IPv6 destination
address to 'All_DHCP_Relay_Agents_and_Servers' and sets the UDP
fields to values that will be understood by the DHCPv6 server.The LDRA then wraps the message in a DHCPv6 'Relay-Forward'
message header and includes an 'Interface-Id' option that includes
enough information to allow the LDRA to forward the resulting
Reply message back to the Client (e.g., the Client's link-layer
addresses, a security association identifier, etc.). The LDRA also
wraps the OMNI option and SLLAO into the Interface-Id option, then
forwards the message to the DHCPv6 server.When the DHCPv6 server prepares a Reply message, it wraps the
message in a 'Relay-Reply' message and echoes the Interface-Id
option. The DHCPv6 server then delivers the Relay-Reply message to
the LDRA, which discards the Relay-Reply wrapper and IPv6/UDP
headers, then uses the DHCPv6 message to construct an RA response
to the Client. The Server uses the information in the Interface-Id
option to prepare the RA message and to cache the link-layer
addresses taken from the OMNI option and SLLAO echoed in the
Interface-Id option.Clients may connect to ANETs that deploy perimeter security
services to facilitate communications to Servers in outside INETs. In
that case, the ANET can employ an AERO Proxy. The Proxy is located at
the ANET/INET border and listens for RS messages originating from or
RA messages destined to ANET Clients. The Proxy acts on these control
messages as follows:when the Proxy receives an RS message from a new ANET Client,
it first authenticates the message then examines the network-layer
destination address. If the destination address is a Server's LLA,
the Proxy proceeds to the next step. Otherwise, if the destination
is All-Routers multicast or Subnet-Router anycast, the Proxy
selects a "nearby" Server that is likely to be a good candidate to
serve the Client and replaces the destination address with the
Server's LLA. Next, the Proxy creates a proxy neighbor cache entry
and caches the Client and Server link-layer addresses along with
the OMNI option information and any other identifying information
including Transaction IDs, Client Identifiers, Nonce values, etc.
The Proxy finally encapsulates the (proxyed) RS message in a SPAN
header with source set to the Proxy's ULA and destination set to
the Server's ULA then forwards the message into the SPAN.when the Server receives the RS, it authenticates the message
then creates or updates a symmetric neighbor cache entry for the
Client with the Proxy's ULA as the link-layer address. The Server
then sends an RA message back to the Proxy via the spanning
tree.when the Proxy receives the RA, it authenticates the message
and matches it with the proxy neighbor cache entry created by the
RS. The Proxy then caches the PD route information as a mapping
from the Client's MNPs to the Client's link-layer address, caches
the Server's advertised Router Lifetime and sets the neighbor
cache entry state to REACHABLE. The Proxy then sets the P bit in
the RA flags field, optionally rewrites the Router Lifetime and
forwards the (proxyed) message to the Client. The Proxy finally
includes an MTU option (if necessary) with an MTU to use for the
underlying ANET interface.After the initial RS/RA exchange, the Proxy forwards any
Client data packets for which there is no matching asymmetric neighbor
cache entry to a Bridge using SPAN encapsulation with its own ULA as
the source and the ULA corresponding to the Client as the destination.
The Proxy instead forwards any Client data destined to an asymmetric
neighbor cache target directly to the target according to the
SPAN/link-layer information - the process of establishing asymmetric
neighbor cache entries is specified in .While the Client is still attached to the ANET, the Proxy sends NS,
RS and/or unsolicited NA messages to update the Server's symmetric
neighbor cache entries on behalf of the Client and/or to convey QoS
updates. This allows for higher-frequency Proxy-initiated RS/RA
messaging over well-connected INET infrastructure supplemented by
lower-frequency Client-initiated RS/RA messaging over constrained ANET
data links.If the Server ceases to send solicited advertisements, the Proxy
sends unsolicited RAs on the ANET interface with destination set to
All-Nodes multicast (ff02::1) and with Router Lifetime set to zero to
inform Clients that the Server has failed. Although the Proxy engages
in ND exchanges on behalf of the Client, the Client can also send ND
messages on its own behalf, e.g., if it is in a better position than
the Proxy to convey QoS changes, etc. For this reason, the Proxy marks
any Client-originated solicitation messages (e.g. by inserting a Nonce
option) so that it can return the solicited advertisement to the
Client instead of processing it locally.If the Client becomes unreachable, the Proxy sets the neighbor
cache entry state to DEPARTED and retains the entry for DepartTime
seconds. While the state is DEPARTED, the Proxy forwards any packets
destined to the Client to a Bridge via SPAN encapsulation with the
Client's current Server as the destination. The Bridge in turn
forwards the packets to the Client's current Server. When DepartTime
expires, the Proxy deletes the neighbor cache entry and discards any
further packets destined to this (now forgotten) Client.In some ANETs that employ a Proxy, the Client's MNP can be injected
into the ANET routing system. In that case, the Client can send data
messages without encapsulation so that the ANET routing system
transports the unencapsulated packets to the Proxy. This can be very
beneficial, e.g., if the Client connects to the ANET via low-end data
links such as some aviation wireless links.If the first-hop ANET access router is AERO-aware, the Client can
avoid encapsulation for both its control and data messages. When the
Client connects to the link, it can send an unencapsulated RS message
with source address set to its LLA and with destination address set to
the LLA of the Client's selected Server or to All-Routers multicast or
Subnet-Router anycast. The Client includes an OMNI option formatted as
specified in .The Client then sends the unencapsulated RS message, which will be
intercepted by the AERO-Aware access router. The access router then
encapsulates the RS message in an ANET header with its own address as
the source address and the address of a Proxy as the destination
address. The access router further remembers the address of the Proxy
so that it can encapsulate future data packets from the Client via the
same Proxy. If the access router needs to change to a new Proxy, it
simply sends another RS message toward the Server via the new Proxy on
behalf of the Client.In some cases, the access router and Proxy may be one and the same
node. In that case, the node would be located on the same physical
link as the Client, but its message exchanges with the Server would
need to pass through a security gateway at the ANET/INET border. The
method for deploying access routers and Proxys (i.e. as a single node
or multiple nodes) is an ANET-local administrative consideration.In environments where fast recovery from Server failure is
required, Proxys SHOULD use proactive Neighbor Unreachability
Detection (NUD) to track Server reachability in a similar fashion as
for Bidirectional Forwarding Detection (BFD) . Proxys can then quickly detect and react to
failures so that cached information is re-established through
alternate paths. The NUD control messaging is carried only over
well-connected ground domain networks (i.e., and not low-end
aeronautical radio links) and can therefore be tuned for rapid
response.Proxys perform proactive NUD with Servers for which there are
currently active ANET Clients by sending continuous NS messages in
rapid succession, e.g., one message per second. The Proxy sends the
NS message via the spanning tree with the Proxy's LLA as the source
and the LLA of the Server as the destination. When the Proxy is also
sending RS messages to the Server on behalf of ANET Clients, the
resulting RA responses can be considered as equivalent hints of
forward progress. This means that the Proxy need not also send a
periodic NS if it has already sent an RS within the same period. If
the Server fails (i.e., if the Proxy ceases to receive
advertisements), the Proxy can quickly inform Clients by sending
multicast RA messages on the ANET interface.The Proxy sends RA messages on the ANET interface with source
address set to the Server's address, destination address set to
All-Nodes multicast, and Router Lifetime set to 0. The Proxy SHOULD
send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small
delays . Any Clients on the ANET that had
been using the failed Server will receive the RA messages and
associate with a new Server.In environments where Client messaging over ANETs is
bandwidth-limited and/or expensive, Clients can enlist the services
of the Proxy to coordinate with multiple Servers in a single RS/RA
message exchange. The Client can send a single RS message to
All-Routers multicast that includes the ID's of multiple Servers in
MS-Register sub-options of the OMNI option.When the Proxy receives the RS and processes the OMNI option, it
performs a separate RS/RA exchange with each MS-Register Server.
When it has received the RA messages, it creates an "aggregate" RA
message to return to the Client with an OMNI option with each
responding Server's ID recorded in an MS-Register sub-option.Clients can thereafter employ efficient point-to-multipoint
Server coordination under the assistance of the Proxy to
dramatically reduce the number of messages sent over the ANET while
enlisting the support of multiple Servers for fault tolerance.
Clients can further include MS-Release suboptions in RS messages to
request the Proxy to release from former Servers via the procedures
discussed in .The OMNI interface specification provides further
discussion of the Client/Proxy RS/RA messaging involved in
point-to-multipoint coordination.While data packets are flowing between a source and target node,
route optimization SHOULD be used. Route optimization is initiated by
the first eligible Route Optimization Source (ROS) closest to the
source as follows:For Clients on VPNed and Direct interfaces, the Server is the
ROS.For Clients on Proxyed interfaces, the Proxy is the ROS.For Clients on INET interfaces, the Client itself is the
ROS.For correspondent nodes on INET/EUN interfaces serviced by a
Relay, the Relay is the ROS.The route optimization procedure is conducted between the ROS and
the target Server/Relay acting as a Route Optimization Responder (ROR)
in the same manner as for IPv6 ND Address Resolution and using the
same NS/NA messaging. The target may either be a MNP Client serviced
by a Server, or a non-MNP correspondent reachable via a Relay.The procedures are specified in the following sections.While data packets are flowing from the source node toward a
target node, the ROS performs address resolution by sending an NS
message for Address Resolution (NS(AR)) to receive a solicited NA
message from the ROR. When the ROS sends an NS(AR), it includes:the LLA of the ROS as the source address.the data packet's destination as the Target Address.the Solicited-Node multicast address
formed from the lower 24 bits of the data packet's destination
as the destination address, e.g., for 2001:db8:1:2::10:2000 the
NS destination address is ff02:0:0:0:0:1:ff10:2000.The NS(AR) message includes an OMNI option with no
ifIndex-tuples and no SLLAO, such that the target will not create a
neighbor cache entry.The ROS then encapsulates the NS(AR) message in a SPAN header
with source set to its own ULA and destination set to the ULA
corresponding to the packet's final destination, then sends the
message into the spanning tree without decrementing the
network-layer TTL/Hop Limit field.When the Bridge receives the NS(AR) message from the ROS, it
discards the INET header and determines that the ROR is the next hop
by consulting its standard IPv6 forwarding table for the SPAN header
destination address. The Bridge then forwards the message toward the
ROR via the spanning tree the same as for any IPv6 router. The
final-hop Bridge in the spanning tree will deliver the message via a
secured tunnel to the ROR.When the ROR receives the NS(AR) message, it examines the Target
Address to determine whether it has a neighbor cache entry and/or
route that matches the target. If there is no match, the ROR drops
the message. Otherwise, the ROR continues processing as follows:if the target belongs to an MNP Client neighbor in the
DEPARTED state the ROR changes the NS(AR) message SPAN
destination address to the ULA of the Client's new Server,
forwards the message into the spanning tree and returns from
processing.If the target belongs to an MNP Client neighbor in the
REACHABLE state, the ROR instead adds the AERO source address to
the target Client's Report List with time set to ReportTime.If the target belongs to a non-MNP route, the ROR continues
processing without adding an entry to the Report List.The ROR then prepares a solicited NA message to send back
to the ROS but does not create a neighbor cache entry. The ROR sets
the NA source address to the LLA corresponding to the target, sets
the Target Address to the target of the solicitation, and sets the
destination address to the source of the solicitation.The ROR then includes an OMNI option with prefix registration
length set to the length of the MNP if the target is an MNP Client;
otherwise, set to the maximum of the non-MNP prefix length and 64.
(Note that a /64 limit is imposed to avoid causing the ROS to set
short prefixes (e.g., "default") that would match destinations for
which the routing system includes more-specific prefixes.)If the target is an MNP Client, the ROR next includes
ifIndex-tuples in the OMNI option for each of the target Client's
underlying interfaces with current information for each interface
and with the S flag set to 0. The ROR then includes a TLLAO with
ifIndex-tuples in one-to-one correspondence with the tuples that
appear in the OMNI option.The ROR sets the Link Layer Address and Port Number (if
necessary) to its own INET address for VPNed and Direct interfaces
or to the INET address of the Proxy for Proxyed interface, then
includes its own ULA or the ULA of the Proxy as the ultimate Segment
Routing List entry. For INET interfaces, the ROR instead sets the
Link Layer Address and Port Number (if necessary) to the Client's
INET address then sets its own ULA in the penultimate Segment
Routing List entry and sets the target's ULA in the ultimate Segment
Routing List entry.The ROR then sets the NA message R flag to 1 (as a router), S
flag to 1 (as a response to a solicitation), and O flag to 0 (as a
proxy). The ROR finally encapsulates the NA message in a SPAN header
with source set to its own ULA and destination set to the source ULA
of the NS(AR) message, then forwards the message into the spanning
tree without decrementing the network-layer TTL/Hop Limit field.When the Bridge receives the NA message from the ROR, it discards
the INET header and determines that the ROS is the next hop by
consulting its standard IPv6 forwarding table for the SPAN header
destination address. The Bridge then forwards the SPAN-encapsulated
NA message toward the ROS the same as for any IPv6 router. The
final-hop Bridge in the spanning tree will deliver the message via a
secured tunnel to the ROS.When the ROS receives the solicited NA message, it processes the
message the same as for standard IPv6 Address Resolution . In the process, it caches the source ULA then
creates an asymmetric neighbor cache entry for the ROR and caches
all information found in the OMNI and TLLAO options. The ROS finally
sets the asymmetric neighbor cache entry lifetime to ReachableTime
seconds.Following route optimization, the ROS forwards future data
packets destined to the target via the addresses found in the cached
link-layer information. The route optimization is shared by all
sources that send packets to the target via the ROS, i.e., and not
just the source on behalf of which the route optimization was
initiated.While new data packets destined to the target are flowing through
the ROS, it sends additional NS(AR) messages to the ROR before
ReachableTime expires to receive a fresh solicited NA message the
same as described in the previous sections (route optimization
refreshment strategies are an implementation matter, with a
non-normative example given in ). The ROS
uses the cached ULA of the ROR as the NS(AR) SPAN destination
address, and sends up to MAX_MULTICAST_SOLICIT NS(AR) messages
separated by 1 second until an NA is received. If no NA is received,
the ROS assumes that the current ROR has become unreachable and
deletes the neighbor cache entry. Subsequent data packets will
trigger a new route optimization per to
discover a new ROR while initial data packets travel over a
suboptimal route.If an NA is received, the ROS then updates the asymmetric
neighbor cache entry to refresh ReachableTime, while (for MNP
destinations) the ROR adds or updates the ROS address to the target
Client's Report List and with time set to ReportTime. While no data
packets are flowing, the ROS instead allows ReachableTime for the
asymmetric neighbor cache entry to expire. When ReachableTime
expires, the ROS deletes the asymmetric neighbor cache entry. Any
future data packets flowing through the ROS will again trigger a new
route optimization.The ROS may also receive unsolicited NA messages from the ROR at
any time (see: ). If there is an asymmetric
neighbor cache entry for the target, the ROS updates the link-layer
information but does not update ReachableTime since the receipt of
an unsolicited NA does not confirm that any forward paths are
working. If there is no asymmetric neighbor cache entry, the ROS
simply discards the unsolicited NA.In this arrangement, the ROS holds an asymmetric neighbor cache
entry for the ROR, but the ROR does not hold an asymmetric neighbor
cache entry for the ROS. The route optimization neighbor
relationship is therefore asymmetric and unidirectional. If the
target node also has packets to send back to the source node, then a
separate route optimization procedure is performed in the reverse
direction. But, there is no requirement that the forward and reverse
paths be symmetric.AERO nodes perform Neighbor Unreachability Detection (NUD) per
either reactively in response to persistent
link-layer errors (see ) or proactively to
confirm reachability. The NUD algorithm is based on periodic control
message exchanges. The algorithm may further be seeded by ND hints of
forward progress, but care must be taken to avoid inferring
reachability based on spoofed information. For example, authentic IPv6
ND message exchanges may be considered as acceptable hints of forward
progress, while spurious data packets should not be.AERO Servers, Proxys and Relays can use standard NS/NA NUD
exchanges sent over the spanning tree to securely test reachability
without risk of DoS attacks from nodes pretending to be a neighbor;
Proxys can further perform NUD to securely verify Server reachability
on behalf of their proxyed Clients. However, a means for a ROS to test
the unsecured forward directions of target route optimized paths is
also necessary.When an ROR directs an ROS to a neighbor with one or more target
link-layer addresses, the ROS can proactively test each such unsecured
route optimized path by sending "loopback" NS(NUD) messages. While
testing the paths, the ROS can optionally continue to send packets via
the spanning tree, maintain a small queue of packets until target
reachability is confirmed, or (optimistically) allow packets to flow
via the route optimized paths.When the ROS sends a loopback NS(NUD) message, it uses its LLA as
both the IPv6 source and destination address, and the MNP
Subnet-Router anycast address as the Target Address. The ROS includes
a Nonce and Timestamp option, then encapsulates the message in
SPAN/INET headers with its own ULA as the source and the ULA of the
route optimization target as the destination. The ROS then forwards
the message to the target (either directly to the link layer address
of the target if the target is in the same AERO link segment, or via a
Bridge if the target is in a different AERO link segment).When the route optimization target receives the NS(NUD) message, it
notices that the IPv6 destination address is the same as the source
address. It then reverses the SPAN source and destination addresses
and returns the message to the ROS (either directly or via the
spanning tree). The route optimization target does not decrement the
NS(NUD) message IPv6 Hop-Limit in the process, since the message has
not exited the AERO link.When the ROS receives the NS(NUD) message, it can determine from
the Nonce, Timestamp and Target Address that the message originated
from itself and that it transited the forward path. The ROS need not
prepare an NA response, since the destination of the response would be
itself and testing the route optimization path again would be
redundant.The ROS marks route optimization target paths that pass these NUD
tests as "reachable", and those that do not as "unreachable". These
markings inform the AERO interface forwarding algorithm specified in
.Note that to avoid a DoS vector nodes MUST NOT return loopback
NS(NUD) messages received from an unsecured link-layer source via the
spanning tree.AERO is a Distributed Mobility Management (DMM) service. Each
Server is responsible for only a subset of the Clients on the AERO
link, as opposed to a Centralized Mobility Management (CMM) service
where there is a single network mobility collective entity for all
Clients. Clients coordinate with their associated Servers via RS/RA
exchanges to maintain the DMM profile, and the AERO routing system
tracks all current Client/Server peering relationships.Servers provide default routing and mobility/multilink services for
their dependent Clients. Clients are responsible for maintaining
neighbor relationships with their Servers through periodic RS/RA
exchanges, which also serves to confirm neighbor reachability. When a
Client's underlying interface address and/or QoS information changes,
the Client is responsible for updating the Server with this new
information. Note that for Proxyed interfaces, however, the Proxy can
also perform some RS/RA exchanges on the Client's behalf.Mobility management considerations are specified in the following
sections.Servers accommodate Client mobility/multilink and/or QoS change
events by sending unsolicited NA (uNA) messages to each ROS in the
target Client's Report List. When a Server sends a uNA message, it
sets the IPv6 source address to the Client's LLA, sets the
destination address to All-Nodes multicast and sets the Target
Address to the Client's Subnet-Router anycast address. The Server
also includes an OMNI option with prefix registration information
and with ifIndex-tuples for the target Client's remaining interfaces
with S set to 0. The Server then includes a TLLAO with corresponding
ifIndex-tuples prepared the same as for the initial route
optimization event. The Server sets the NA R flag to 1, the S flag
to 0 and the O flag to 0, then encapsulates the message in a SPAN
header with source set to its own ULA and destination set to the ULA
of the ROS and sends the message into the spanning tree.As discussed in Section 7.2.6 of , the
transmission and reception of uNA messages is unreliable but
provides a useful optimization. In well-connected Internetworks with
robust data links uNA messages will be delivered with high
probability, but in any case the Server can optionally send up to
MAX_NEIGHBOR_ADVERTISEMENT uNAs to each ROS to increase the
likelihood that at least one will be received.When the ROS receives a uNA message, it ignores the message if
there is no existing neighbor cache entry for the Client. Otherwise,
it uses the included OMNI option and TLLAO information to update the
neighbor cache entry, but does not reset ReachableTime since the
receipt of an unsolicited NA message from the target Server does not
provide confirmation that any forward paths to the target Client are
working.If uNA messages are lost, the ROS may be left with stale address
and/or QoS information for the Client for up to ReachableTime
seconds. During this time, the ROS can continue sending packets
according to its stale neighbor cache information. When
ReachableTime is close to expiring, the ROS will re-initiate route
optimization and receive fresh link-layer address information.In addition to sending uNA messages to the current set of ROSs
for the Client, the Server also sends uNAs to the former link-layer
address for any ifIndex-tuple for which the link-layer address has
changed. The uNA messages update Proxys that cannot easily detect
(e.g., without active probing) when a formerly-active Client has
departed.When a Client needs to change its underlying interface addresses
and/or QoS preferences (e.g., due to a mobility event), either the
Client or its Proxys send RS messages to the Server via the spanning
tree with an OMNI option that includes an ifIndex-tuple with S set
to 1 and with the new link quality and address information.Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel
with sending actual data packets in case one or more RAs are lost.
If all RAs are lost, the Client SHOULD re-associate with a new
Server.When the Server receives the Client's changes, it sends uNA
messages to all nodes in the Report List the same as described in
the previous section.When a Client needs to bring new underlying interfaces into
service (e.g., when it activates a new data link), it sends an RS
message to the Server via the underlying interface with an OMNI
option that includes an ifIndex-tuple with S set to 1 and
appropriate link quality values and with link-layer address
information for the new link.When a Client needs to remove existing underlying interfaces from
service (e.g., when it de-activates an existing data link), it sends
an RS or uNA message to its Server with an OMNI option with
appropriate link quality values.If the Client needs to send RS/uNA messages over an underlying
interface other than the one being removed from service, it MUST
include ifIndex-tuples with appropriate link quality values for any
underlying interfaces being removed from service.When a Client associates with a new Server, it performs the
Client procedures specified in . The
Client also includes MS-Release identifiers in the RS message OMNI
option per if it
wants the new Server to notify any old Servers from which the Client
is departing.When the new Server receives the Client's RS message, it returns
an RA as specified in and sends up to
MAX_NEIGHBOR_ADVERTIISEMENT uNA messages to any old Servers listed
in OMNI option MS-Release identifiers. Each uNA message includes the
Client's LLA as the source address, the old Server's LLA as the
destination address, and an OMNI option with the Register/Release
bit set to 0. The new Server wraps the uNA in a SPAN header with its
own ULA as the source and the old Server's ULA as the destination,
then sends the message into the spanning tree.When an old Server receives the uNA, it changes the Client's
neighbor cache entry state to DEPARTED, sets the link-layer address
of the Client to the new Server's ULA, and resets DepartTime. After
a short delay (e.g., 2 seconds) the old Server withdraws the
Client's MNP from the routing system. After DepartTime expires, the
old Server deletes the Client's neighbor cache entry.The old Server also sends unsolicited NA messages to all ROSs in
the Client's Report List with an OMNI option with a single
ifIndex-tuple with ifIndex set to 0 and S set to '1', and with the
ULA of the new Server in a companion TLLAO. When the ROS receives
the NA, it caches the address of the new Server in the existing
asymmetric neighbor cache entry and marks the entry as STALE for a
period of 10 seconds after which the cache entry is deleted. While
in the STALE state, subsequent data packets flow according to any
existing cached link-layer information and trigger a new NS(AR)/NA
exchange via the new Server.Clients SHOULD NOT move rapidly between Servers in order to avoid
causing excessive oscillations in the AERO routing system. Examples
of when a Client might wish to change to a different Server include
a Server that has gone unreachable, topological movements of
significant distance, movement to a new geographic region, movement
to a new AERO link segment, etc.When a Client moves to a new Server, some of the fragments of a
multiple fragment packet may have already arrived at the old Server
while others are en route to the new Server, however no special
attention in the reassembly algorithm is necessary when re-routed
fragments are simply treated as loss.The AERO Client provides an IGMP (IPv4) or
MLD (IPv6) proxy service for its EUNs and/or
hosted applications . The Client forwards
IGMP/MLD messages over any of its underlying interfaces for which
group membership is required. The IGMP/MLD messages may be further
forwarded by a first-hop ANET access router acting as an
IGMP/MLD-snooping switch , then ultimately
delivered to an AERO Proxy/Server acting as a Protocol Independent
Multicast - Sparse-Mode (PIM-SM, or simply "PIM") Designated Router
(DR) . AERO Relays also act as PIM routers
(i.e., the same as AERO Proxys/Servers) on behalf of nodes on INET/EUN
networks. The behaviors identified in the following sections
correspond to Source-Specific Multicast (SSM) and Any-Source Multicast
(ASM) operational modes.When an ROS (i.e., an AERO Proxy/Server/Relay) "X" acting as PIM
router receives a Join/Prune message from a node on its downstream
interfaces containing one or more ((S)ource, (G)roup) pairs, it
updates its Multicast Routing Information Base (MRIB) accordingly.
For each S belonging to a prefix reachable via X's non-AERO
interfaces, X then forwards the (S, G) Join/Prune to any PIM routers
on those interfaces per .For each S belonging to a prefix reachable via X's AERO
interface, X originates a separate copy of the Join/Prune for each
(S,G) in the message using its own LLA as the source address and
ALL-PIM-ROUTERS as the destination address. X then encapsulates each
message in a SPAN header with source address set to the ULA of X and
destination address set to S then forwards the message into the
spanning tree, which delivers it to AERO Server/Relay "Y" that
services S. At the same time, if the message was a Join, X sends a
route-optimization NS message toward each S the same as discussed in
. The resulting NAs will return the LLA
for the prefix that matches S as the network-layer source address
and TLLAOs with the ULA corresponding to any ifIndex-tuples that are
currently servicing S.When Y processes the Join/Prune message, if S located behind any
INET, Direct, or VPNed interfaces Y acts as a PIM router and updates
its MRIB to list X as the next hop in the reverse path. If S is
located behind any Proxys "Z"*, Y also forwards the message to each
Z* over the spanning tree while continuing to use the LLA of X as
the source address. Each Z* then updates its MRIB accordingly and
maintains the LLA of X as the next hop in the reverse path. Since
the Bridges do not examine network layer control messages, this
means that the (reverse) multicast tree path is simply from each Z*
(and/or Y) to X with no other multicast-aware routers in the path.
If any Z* (and/or Y) is located on the same AERO link segment as X,
the multicast data traffic sent to X directly using SPAN/INET
encapsulation instead of via a Bridge.Following the initial Join/Prune and NS/NA messaging, X maintains
an asymmetric neighbor cache entry for each S the same as if X was
sending unicast data traffic to S. In particular, X performs
additional NS/NA exchanges to keep the neighbor cache entry alive
for up to t_periodic seconds . If no new
Joins are received within t_periodic seconds, X allows the neighbor
cache entry to expire. Finally, if X receives any additional
Join/Prune messages for (S,G) it forwards the messages to each Y and
Z* in the neighbor cache entry over the spanning tree.At some later time, Client C that holds an MNP for source S may
depart from a first Proxy Z1 and/or connect via a new Proxy Z2. In
that case, Y sends an unsolicited NA message to X the same as
specified for unicast mobility in . When X
receives the unsolicited NA message, it updates its asymmetric
neighbor cache entry for the LLA for source S and sends new Join
messages to any new Proxys Z2. There is no requirement to send any
Prune messages to old Proxys Z1 since source S will no longer source
any multicast data traffic via Z1. Instead, the multicast state for
(S,G) in Proxy Z1 will soon time out since no new Joins will
arrive.After some later time, C may move to a new Server Y2 and depart
from old Sever Y1. In that case, Y1 sends Join messages for any of
C's active (S,G) groups to Y2 while including its own LLA as the
source address. This causes Y2 to include Y1 in the multicast
forwarding tree during the interim time that Y1's symmetric neighbor
cache entry for C is in the DEPARTED state. At the same time, Y1
sends an unsolicited NA message to X with an OMNI option and TLLAO
with ifIndex-tuple set to 0 and a release indication to cause X to
release its asymmetric neighbor cache entry. X then sends a new Join
message to S via the spanning tree and re-initiates route
optimization the same as if it were receiving a fresh Join message
from a node on a downstream link.When an ROS X acting as a PIM router receives a Join/Prune from a
node on its downstream interfaces containing one or more (*,G)
pairs, it updates its Multicast Routing Information Base (MRIB)
accordingly. X then forwards a copy of the message to the Rendezvous
Point (RP) R for each G over the spanning tree. X uses its own LLA
as the source address and ALL-PIM-ROUTERS as the destination
address, then encapsulates each message in a SPAN header with source
address set to the ULA of X and destination address set to R, then
sends the message into the spanning tree. At the same time, if the
message was a Join X initiates NS/NA route optimization the same as
for the SSM case discussed in .For each source S that sends multicast traffic to group G via R,
the Proxy/Server Z* for the Client that aggregates S encapsulates
the packets in PIM Register messages and forwards them to R via the
spanning tree, which may then elect to send a PIM Join to Z*. This
will result in an (S,G) tree rooted at Z* with R as the next hop so
that R will begin to receive two copies of the packet; one native
copy from the (S, G) tree and a second copy from the pre-existing
(*, G) tree that still uses PIM Register encapsulation. R can then
issue a PIM Register-stop message to suppress the
Register-encapsulated stream. At some later time, if C moves to a
new Proxy/Server Z*, it resumes sending packets via PIM Register
encapsulation via the new Z*.At the same time, as multicast listeners discover individual S's
for a given G, they can initiate an (S,G) Join for each S under the
same procedures discussed in . Once the
(S,G) tree is established, the listeners can send (S, G) Prune
messages to R so that multicast packets for group G sourced by S
will only be delivered via the (S, G) tree and not from the (*, G)
tree rooted at R. All mobility considerations discussed for SSM
apply.Bi-Directional PIM (BIDIR-PIM) provides
an alternate approach to ASM that treats the Rendezvous Point (RP)
as a Designated Forwarder (DF). Further considerations for BIDIR-PIM
are out of scope.An AERO Client can connect to multiple AERO links the same as for
any data link service. In that case, the Client maintains a distinct
AERO interface for each link, e.g., 'aero0' for the first link,
'aero1' for the second, 'aero2' for the third, etc. Each AERO link
would include its own distinct set of Bridges, Servers and Proxys,
thereby providing redundancy in case of failures.The Bridges, Servers and Proxys on each AERO link can assign AERO
and ULAs that use the same or different numberings from those on other
links. Since the links are mutually independent there is no
requirement for avoiding inter-link address duplication, e.g., the
same LLA such as fe80::1000 could be used to number distinct nodes
that connect to different AERO links.Each AERO link could utilize the same or different ANET
connections. The links can be distinguished at the link-layer via the
SSP in a similar fashion as for Virtual Local Area Network (VLAN)
tagging (e.g., IEEE 802.1Q) and/or through assignment of distinct sets
of MSPs on each link. This gives rise to the opportunity for
supporting multiple redundant networked paths, where each VLAN is
distinguished by a different SRT color (see: ). In
particular, the Client can tag its RS messages with the appropriate
label to cause the network to select the desired VLAN.The Client's IP layer can select the outgoing AERO interface
appropriate for a given traffic profile while (in the reverse
direction) correspondent nodes must have some way of steering their
packets destined to a target via the correct AERO link.In a first alternative, if each AERO link services different MSPs,
then the Client can receive a distinct MNP from each of the links. IP
routing will therefore assure that the correct Red/Green/Blue/etc.
network is used for both outbound and inbound traffic. This can be
accomplished using existing technologies and approaches, and without
requiring any special supporting code in correspondent nodes or
Bridges.In a second alternative, if each AERO link services the same MSP(s)
then each link could assign a distinct "AERO Link Anycast" address
that is configured by all Bridges on the link. Correspondent nodes can
then perform Segment Routing to select the correct SRT, which will
then direct the packet over multiple hops to the target.AERO Client MNs and INET correspondent nodes consult the Domain
Name System (DNS) the same as for any Internetworking node. When
correspondent nodes and Client MNs use different IP protocol versions
(e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain A
records for IPv4 address mappings to MNs which must then be populated
in Relay NAT64 mapping caches. In that way, an IPv4 correspondent node
can send packets to the IPv4 address mapping of the target MN, and the
Relay will translate the IPv4 header and destination address into an
IPv6 header and IPv6 destination address of the MN.When an AERO Client registers with an AERO Server, the Server can
return the address(es) of DNS servers in RDNSS options . The DNS server provides the IP addresses of other
MNs and correspondent nodes in AAAA records for IPv6 or A records for
IPv4.SPAN encapsulation ensures that dissimilar INET partitions can be
joined into a single unified AERO link, even though the partitions
themselves may have differing protocol versions and/or incompatible
addressing plans. However, a commonality can be achieved by
incrementally distributing globally routable (i.e., native) IP
prefixes to eventually reach all nodes (both mobile and fixed) in all
AERO link segments. This can be accomplished by incrementally
deploying AERO Relays on each INET partition, with each Relay
distributing its MNPs and/or discovering non-MNP prefixes on its INET
links.This gives rise to the opportunity to eventually distribute native
IP addresses to all nodes, and to present a unified AERO link view
even if the INET partitions remain in their current protocol and
addressing plans. In that way, the AERO link can serve the dual
purpose of providing a mobility/multilink service and a transition
service. Or, if an INET partition is transitioned to a native IP
protocol version and addressing scheme that is compatible with the
AERO link MNP-based addressing scheme, the partition and AERO link can
be joined by Relays.Relays that connect INETs/EUNs with dissimilar IP protocol versions
may need to employ a network address and protocol translation function
such as NAT64.In environments where rapid failure recovery is required, Servers
and Bridges SHOULD use Bidirectional Forwarding Detection (BFD) . Nodes that use BFD can quickly detect and react to
failures so that cached information is re-established through
alternate nodes. BFD control messaging is carried only over
well-connected ground domain networks (i.e., and not low-end radio
links) and can therefore be tuned for rapid response.Servers and Bridges maintain BFD sessions in parallel with their
BGP peerings. If a Server or Bridge fails, BGP peers will quickly
re-establish routes through alternate paths the same as for common BGP
deployments. Similarly, Proxys maintain BFD sessions with their
associated Bridges even though they do not establish BGP peerings with
them.Proxys SHOULD use proactive NUD for Servers for which there are
currently active ANET Clients in a manner that parallels BFD, i.e., by
sending unicast NS messages in rapid succession to receive solicited
NA messages. When the Proxy is also sending RS messages on behalf of
ANET Clients, the RS/RA messaging can be considered as equivalent
hints of forward progress. This means that the Proxy need not also
send a periodic NS if it has already sent an RS within the same
period. If a Server fails, the Proxy will cease to receive
advertisements and can quickly inform Clients of the outage by sending
multicast RA messages on the ANET interface.The Proxy sends multicast RA messages with source address set to
the Server's address, destination address set to All-Nodes multicast,
and Router Lifetime set to 0. The Proxy SHOULD send
MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays
. Any Clients on the ANET interface that have
been using the (now defunct) Server will receive the RA messages and
associate with a new Server.AERO Clients that connect to the open Internet via INET interfaces
can establish a VPN or direct link to securely connect to a Server in
a "tethered" arrangement with all of the Client's traffic transiting
the Server. Alternatively, the Client can associate with an INET
Server using UDP/IP encapsulation and asymmetric securing services as
discussed in the following sections.When a Client's AERO interface enables an INET underlying
interface, it first determines whether the interface is likely to be
behind a NAT. For IPv4, the Client might assume it is on the open
Internet if the INET address is not a special-use IPv4 address per
. Similarly for IPv6, the Client might assume
it is on the open Internet if the INET address is not a link-local
or unique-local IPv6
address.The Client then prepares a UDP/IP-encapsulated RS message with IPv6
source address set to its LLA, with IPv6 destination set to
All-Routers multicast and with an OMNI option with underlying
interface parameters. If the Client believes that it is on the open
Internet, it SHOULD also include an SLLAO set according to the address
used for INET encapsulation (otherwise, it MAY omit the SLLAO). If the
underlying address is IPv4, the Client includes a Teredo address using the prefix fe80::/32 with the Server's IPv4
address, and with the IP address and Port Number used for INET
encapsulation written in obfuscated form and with FMT set to '01'
(INET) as discussed in . If the underlying
interface address is IPv6, the Client instead includes the IPv6
address and Port number in obfuscated form and sets FMT to '11'
(INET). The Client finally includes a Teredo Authentication option per
to provide message authentication, sets the
UDP/IP source to its INET address and UDP port, sets the UDP/IP
destination to the Server's INET address and the AERO service port
number (8060), then sends the message to the Server.When the Server receives the RS, it authenticates the message and
registers the Client's MNP and INET interface information according to
the OMNI option parameters. If the RS message includes an SLLAO, the
Server compares the encapsulation IP address and UDP port number with
the (unobfuscated) SLLAO values. If the values are the same, the
Server caches the Client's information as "INET" addresses meaning
that the Client is likely to accept direct messages without requiring
NAT traversal exchanges. If the values are different (or, if there was
no SLLAO) the Server instead caches the Client's information as "NAT"
addresses meaning that NAT traversal exchanges may be necessary.The Server then returns an RA message with IPv6 source and
destination set corresponding to the addresses in the RS, and with a
Teredo Authentication option. For IPv4, the Server also includes an
IPv4 Teredo Origin option per with the mapped
and obfuscated IPv4 address and port number observed in the
encapsulation headers. For IPv6, the Server instead includes an IPv6
Teredo Origin option per with the mapped
and obfuscated observed IPv6 address and port number (note that the
value 0x02 in the second octet differentiates from other Teredo option
types).When the Client receives the RA message, it compares the mapped IP
address and port from the Teredo Origin option with its own address.
If the addresses are the same, the Client assumes the open Internet /
Cone NAT principle; if the addresses are different, the Client instead
assumes that further Server qualification procedures are necessary to
detect the type of NAT and proceeds according to standard Teredo
procedures.After the Client has registered its INET interfaces in such RS/RA
exchanges it sends periodic RS messages to receive fresh RA messages
before the Router Lifetime received on each INET interface expires.
The Client also maintains default routes via its Servers, i.e., the
same as described in earlier sections.When the Client sends messages to target IP addresses, it also
invokes route optimization per using IPv6
ND address resolution messaging. The Client sends the NS(AR) message
to the Server wrapped in a UDP/IP header with a Teredo Authentication
option with the NS source address set to the Client's LLA and
destination address set to the target solicited node multicast
address. The Server authenticates the message and sends a
corresponding NS(AR) message over the spanning tree the same as if it
were the ROS, but with the SPAN source address set to the Server's ULA
and destination set to the ULA of the target. When the ROR receives
the NS(AR), it adds the Server's ULA and Client's LLA to the target's
Report List, and returns an NA with OMNI and TLLAO information for the
target. The Server then returns a UDP/IP encapsulated NA message with
a Teredo Authentication option to the Client.Following route optimization, for targets in the same AERO link
segment if the target's TLLAO addresss is on the open INET, the Client
forwards data packets directly to the target INET address. If the
target's TLLAO address is behind a NAT, the Client first establishes
NAT state for the Link Layer Address using the "bubble" mechanisms
specified in . The
Client continues to send data packets via its Server until NAT state
is populated, then begins forwarding packets via the direct path
through the NAT to the target. For targets in different AERO link
segments, the Client inserts a Segment Routing header and forwards
data packets to the Bridge that returned the NA message.The ROR may return uNAs via the Server if the target moves, and the
Server will send corresponding Teredo Authentication-protected uNAs to
the Client. The Client can also send "loopback" NS(NUD) messages to
test forward path reachability even though there is no security
association between the Client and the target.The Client sends Teredo UDP/IP encapsulated IPv6 packets no larger
than 1280 bytes in one piece. In order to accommodate larger IPv6
packets (up to the AERO interface 9180 MTU), the Client inserts a SPAN
header with source set to its own ULA and destination set to the ULA
of the target and uses IPv6 fragmentation according to . The Client then encapsulates each fragment in a
UDP/IP header and sends the fragments to the next hop.In some environments, use of the Teredo Authentication option
alone may be sufficient for assuring IPv6 ND message authentication
between Clients and Servers. When additional protection is
necessary, nodes should employ SEcure Neighbor Discovery (SEND)
with Cryptographically-Generated Addresses
(CGA) .When SEND/CGA are used, the Client prepares RS messages with its
link-local CGA as the IPv6 source and All-Routers as the IPv6
Destination, includes any SEND options and wraps the message in a
SPAN header. The Client sets the SPAN source address to its own ULA
and sets the SPAN destination address to the "All-Routers" ULA. The
Client then wraps the RS message in UDP/IP headers according to the
Teredo format and sends the message to the Server.When the Server receives the message, it first verifies the
Teredo Authentication option (if present) then uses the SPAN source
address to determine the MNP of the Client. The Server then
processes the SEND options to authenticate the RS message and
prepares an RA message response. The Server prepares the RA with its
own link-local CGA and the CGA of the Client as the IPv6 source and
destination, includes any SEND options and wraps the message in a
SPAN header. The Server sets the SPAN source address to its own ULA
and sets the SPAN destination address to the Client's ULA. The
Server then wraps the RA message in UDP/IP headers according to the
Teredo format and sends the message to the Client. Thereafter, the
Client/Server send additional RS/RA messages to maintain their
association and any NAT state.The Client and Server also may exchange NS/NA messages using
their own CGA as the source and with SPAN encapsulation as above.
When a Client sends an NS(AR), it sets the IPv6 source to its CGA
and sets the IPv6 destination to the Solicited-Node Multicast
address of the target. The Client then wraps the message in a SPAN
header with its own ULA as the source and the ULA of the target as
the destination and sends it to the Server. The Server authenticates
the message, then changes the IPv6 source address to the Client's
LLA, removes the SEND options, and sends a corresponding NS(AR) into
the spanning tree. When the Server receives the corresponding
SPAN-encapsulated NA, it changes the IPv6 destination address to the
Client's CGA, inserts SEND options, then wraps the message in UDP/IP
headers and sends it to the Client.When a Client sends a uNA, it sets the IPv6 source address to its
own CGA and sets the IPv6 destination address to All-Nodes
multicast, includes SEND options, wraps the message in SPAN and
UDP/IP headers and sends the message to the Server. The Server
authenticates the message, then changes the IPv6 address to the
Client's LLA, removes the SEND options and forwards the message the
same as discussed in . In the reverse
direction, when the Server forwards a uNA to the Client, it changes
the IPv6 address to its own CGA and inserts SEND options then
forwards the message to the Client.When a Client sends an NS(NUD), it sets both the IPv6 source and
destination address to its own LLA, wraps the message in a SPAN
header and UDP/IP headers, then sends the message directly to the
peer which will loop the message back. In this case alone, the
Client does not use the Server as a trust broker for forwarding the
ND message.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 MN 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 DHCPv6-PD service offers a way for Clients that desire
time-varying MNPs to obtain short-lived prefixes (e.g., on the order
of a small number of minutes). In that case, the identity of the
Client would not be bound to the MNP but rather the Client's identity
would be bound to the DHCPv6 Device Unique Identifier (DUID) and used
as the seed for Prefix Delegation. The Client would then be obligated
to renumber its internal networks whenever its MNP (and therefore also
its LLA) changes. This should not present a challenge for Clients with
automated network renumbering services, however presents limits for
the durations of ongoing sessions that would prefer to use a constant
address.An AERO implementation based on OpenVPN (https://openvpn.net/) was
announced on the v6ops mailing list on January 10, 2018 and an initial
public release of the AERO proof-of-concept source code was announced on
the intarea mailing list on August 21, 2015.As of 4/1/2020, more recent updated implementations are under
internal development and testing with plans to release in the near
future.The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO . This document
obsoletes and claims the UDP port number "8060"
for all future use.The IANA is instructed to assign an IPv6 Routing Type code (TBD) in
the ipv6-parameters registry .No further IANA actions are required.AERO Bridges configure secured tunnels with AERO Servers, Realys and
Proxys within their local AERO link segments. Applicable secured tunnel
alternatives include IPsec , TLS/SSL , DTLS , WireGuard, etc. The
AERO Bridges of all AERO link segments in turn configure secured tunnels
for their neighboring AERO Bridges in a spanning tree topology.
Therefore, control messages exchanged between any pair of AERO link
neighbors on the spanning tree are already secured.AERO Servers, Relays and Proxys targeted by a route optimization may
also receive data packets directly from arbitrary nodes in INET
partitions instead of via the spanning tree. For INET partitions that
apply effective ingress filtering to defeat source address spoofing, the
simple data origin authentication procedures in can be applied.For INET partitions that cannot apply effective ingress filtering,
the two options for securing communications include 1) disable route
optimization so that all traffic is conveyed over secured tunnels, or 2)
enable on-demand secure tunnel creation between INET partition
neighbors. Option 1) would result in longer routes than necessary and
traffic concentration on critical infrastructure elements. Option 2)
could be coordinated by establishing a secured tunnel on-demand instead
of performing an NS/NA exchange in the route optimization procedures.
Procedures for establishing on-demand secured tunnels are out of
scope.AERO Clients that connect to secured ANETs need not apply security to
their ND messages, since the messages will be intercepted by a perimeter
Proxy that applies security on its INET-facing interface. AERO Clients
connected to the open INET can use symmetric network and/or transport
layer security services such as VPNs or can by some other means
establish a direct link. When a VPN or direct link may be impractical,
however, an asymmetric security service such as SEcure Neighbor
Discovery (SEND) with Cryptographically
Generated Addresses (CGAs) and/or the Teredo
Authentication option may be necessary.Application endpoints SHOULD use application-layer security services
such as TLS/SSL, DTLS or SSH to assure the same
level of protection as for critical secured Internet services. AERO
Clients that require host-based VPN services SHOULD use symmetric
network and/or transport layer security services such as IPsec, TLS/SSL,
DTLS, etc. AERO Proxys and Servers can also provide a network-based VPN
service on behalf of the Client, e.g., if the Client is located within a
secured enclave and cannot establish a VPN on its own behalf.AERO Servers and Bridges present targets for traffic amplification
Denial of Service (DoS) attacks. This concern is no different than for
widely-deployed VPN security gateways in the Internet, where attackers
could send spoofed packets to the gateways at high data rates. This can
be mitigated by connecting Servers and Bridges over dedicated links with
no connections to the Internet and/or when connections to the Internet
are only permitted through well-managed firewalls. Traffic amplification
DoS attacks can also target an AERO Client's low data rate links. This
is a concern not only for Clients located on the open Internet but also
for Clients in secured enclaves. AERO Servers and Proxys can institute
rate limits that protect Clients from receiving packet floods that could
DoS low data rate links.AERO Relays must implement ingress filtering to avoid a spoofing
attack in which spurious messages with ULA addresses are injected into
an AERO link from an outside attacker. AERO Clients MUST ensure that
their connectivity is not used by unauthorized nodes on their EUNs to
gain access to a protected network, i.e., AERO Clients that act as
routers MUST NOT provide routing services for unauthorized nodes. (This
concern is no different than for ordinary hosts that receive an IP
address delegation but then "share" the address with other nodes via
some form of Internet connection sharing such as tethering.)The MAP list MUST be well-managed and secured from unauthorized
tampering, even though the list contains only public information. The
MAP list can be conveyed to the Client in a similar fashion as in (e.g., through layer 2 data link login messaging,
secure upload of a static file, DNS lookups, etc.).Although public domain and commercial SEND implementations exist,
concerns regarding the strength of the cryptographic hash algorithm have
been documented .SRH authentication facilities are specified in .Security considerations for accepting link-layer ICMP messages and
reflected packets are discussed throughout the document.Discussions in the IETF, aviation standards communities and private
exchanges helped shape some of the concepts in this work. Individuals
who contributed insights include Mikael Abrahamsson, Mark Andrews, Fred
Baker, Bob Braden, Stewart Bryant, Brian Carpenter, Wojciech Dec, Pavel
Drasil, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli, Brian
Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak,
Lee Howard, Zdenek Jaron, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy
Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru
Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie Volz, Ryuji
Wakikawa, Tony Whyman, Lloyd Wood and James Woodyatt. Members of the
IESG also provided valuable input during their review process that
greatly improved the document. Special thanks go to Stewart Bryant, Joel
Halpern and Brian Haberman for their shepherding guidance during the
publication of the AERO first edition.This work has further been encouraged and supported by Boeing
colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam
Brodie, John Bush, Balaguruna Chidambaram, Irene Chin, Bruce Cornish,
Claudiu Danilov, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad
Farooqui, Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury, Greg
Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew, Gene
MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Vijay Rajagopalan, Greg
Saccone, Rod Santiago, Kent Shuey, Brian Skeen, Mike Slane, Carrie
Spiker, Katie Tran, Brendan Williams, Amelia Wilson, Julie Wulff, Yueli
Yang, Eric Yeh and other members of the Boeing mobility, networking and
autonomy teams. Kyle Bae, Wayne Benson, Katie Tran and Eric Yeh are
especially acknowledged for implementing the AERO functions as
extensions to the public domain OpenVPN distribution.Earlier works on NBMA tunneling approaches are found in .Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:The Internet Routing Overlay Network (IRON) Virtual Enterprise Traversal (VET) The Subnetwork Encapsulation and Adaptation Layer (SEAL) AERO, First Edition Note that these works cite numerous earlier efforts that are
not also cited here due to space limitations. The authors of those
earlier works are acknowledged for their insights.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 Commercial Airplanes (BCA)
Internet of Things (IoT) and autonomy programs.This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.http://openvpn.netBGP in 2015, http://potaroo.netWhen GUE encapsulation is not needed, AERO can use common
encapsulations such as IP-in-IP , Generic Routing
Encapsulation (GRE) and
others. The encapsulation is therefore only differentiated from non-AERO
tunnels through the application of AERO control messaging and not
through, e.g., a well-known UDP port number.As for GUE encapsulation, alternate AERO encapsulation formats may
require encapsulation layer fragmentation. For simple IP-in-IP
encapsulation, an IPv6 fragment header is inserted directly between the
inner and outer IP headers when needed, i.e., even if the outer header
is IPv4. The IPv6 Fragment Header is identified to the outer IP layer by
its IP protocol number, and the Next Header field in the IPv6 Fragment
Header identifies the inner IP header version. For GRE encapsulation, a
GRE fragment header is inserted within the GRE header . shows the AERO IP-in-IP encapsulation format
before any fragmentation is applied: shows the AERO GRE encapsulation format
before any fragmentation is applied:Alternate encapsulation may be preferred in environments where GUE
encapsulation would add unnecessary overhead. For example, certain
low-bandwidth wireless data links may benefit from a reduced
encapsulation overhead.GUE encapsulation can traverse network paths that are inaccessible to
non-UDP encapsulations, e.g., for crossing Network Address Translators
(NATs). More and more, network middleboxes are also being configured to
discard packets that include anything other than a well-known IP
protocol such as UDP and TCP. It may therefore be necessary to determine
the potential for middlebox filtering before enabling alternate
encapsulation in a given environment.In addition to IP-in-IP, GRE and GUE, AERO can also use security
encapsulations such as IPsec, TLS/SSL, DTLS, etc. In that case, AERO
control messaging and route determination occur before security
encapsulation is applied for outgoing packets and after security
decapsulation is applied for incoming packets.AERO is especially well suited for use with VPN system encapsulations
such as OpenVPN .AERO can be applied to a multitude of Internetworking scenarios, with
each having its own adaptations. The following considerations are
provided as non-normative guidance:Route optimization as discussed in
results in the route optimization source (ROS) creating an asymmetric
neighbor cache entry for the target neighbor. The neighbor cache entry
is maintained for at most ReachableTime seconds and then deleted
unless updated. In order to refresh the neighbor cache entry lifetime
before the ReachableTime timer expires, the specification requires
implementations to issue a new NS/NA exchange to reset ReachableTime
while data packets are still flowing. However, the decision of when to
initiate a new NS/NA exchange and to perpetuate the process is left as
an implementation detail.One possible strategy may be to monitor the neighbor cache entry
watching for data packets for (ReachableTime - 5) seconds. If any data
packets have been sent to the neighbor within this timeframe, then
send an NS to receive a new NA. If no data packets have been sent,
wait for 5 additional seconds and send an immediate NS if any data
packets are sent within this "expiration pending" 5 second window. If
no additional data packets are sent within the 5 second window, delete
the neighbor cache entry.The monitoring of the neighbor data packet traffic therefore
becomes an asymmetric ongoing process during the neighbor cache entry
lifetime. If the neighbor cache entry expires, future data packets
will trigger a new NS/NA exchange while the packets themselves are
delivered over a longer path until route optimization state is
re-established.AERO interface neighbors MAY provide a configuration option that
allows them to perform implicit mobility management in which no ND
messaging is used. In that case, the Client only transmits packets
over a single interface at a time, and the neighbor always observes
packets arriving from the Client from the same link-layer source
address.If the Client's underlying interface address changes (either due to
a readdressing of the original interface or switching to a new
interface) the neighbor immediately updates the neighbor cache entry
for the Client and begins accepting and sending packets according to
the Client's new address. This implicit mobility method applies to use
cases such as cellphones with both WiFi and Cellular interfaces where
only one of the interfaces is active at a given time, and the Client
automatically switches over to the backup interface if the primary
interface fails.When a Client's AERO interface is configured over a Direct
interface, the neighbor at the other end of the Direct link can
receive packets without any encapsulation. In that case, the Client
sends packets over the Direct link according to QoS preferences. If
the Direct interface has the highest QoS preference, then the Client's
IP packets are transmitted directly to the peer without going through
an ANET/INET. If other interfaces have higher QoS preferences, then
the Client's IP packets are transmitted via a different interface,
which may result in the inclusion of Proxys, Servers and Bridges in
the communications path. Direct interfaces must be tested periodically
for reachability, e.g., via NUD.IPv6 AERO links typically have MSPs that aggregate many candidate
MNPs of length /64 or shorter. However, in some cases it may be
desirable to use AERO over links that have only a /64 MSP. This can be
accommodated by treating all Clients on the AERO link as simple hosts
that receive /128 prefix delegations.In that case, the Client sends an RS message to the Server the same
as for ordinary AERO links. The Server responds with an RA message
that includes one or more /128 prefixes (i.e., singleton addresses)
that include the /64 MSP prefix along with an interface identifier
portion to be assigned to the Client. The Client and Server then
configure their LLAs based on the interface identifier portions of the
/128s (i.e., the lower 64 bits) and not based on the /64 prefix (i.e.,
the upper 64 bits).For example, if the MSP for the host-only IPv6 AERO link is
2001:db8:1000:2000::/64, each Client will receive one or more /128
IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
2001:db8:1000:2000::2/128, etc. When the Client receives the prefix
delegations, it assigns the LLAs fe80::1, fe80::2, etc. to the AERO
interface, and assigns the global IPv6 addresses (i.e., the /128s) to
either the AERO interface or an internal virtual interface such as a
loopback. In this arrangement, the Client conducts route optimization
in the same sense as discussed in .This specification has applicability for nodes that act as a Client
on an "upstream" AERO link, but also act as a Server on "downstream"
AERO links. More specifically, if the node acts as a Client to receive
a /64 prefix from the upstream AERO link it can then act as a Server
to provision /128s to Clients on downstream AERO links.AERO Bridges can be either Commercial off-the Shelf (COTS) standard
IP routers or virtual machines in the cloud. Bridges must be
provisioned, supported and managed by the INET administrative
authority, and connected to the Bridges of other INETs via
inter-domain peerings. Cost for purchasing, configuring and managing
Bridges is nominal even for very large AERO links.AERO Servers can be standard dedicated server platforms, but most
often will be deployed as virtual machines in the cloud. The only
requirements for Servers are that they can run the AERO user-level
code and have at least one network interface connection to the INET.
As with Bridges, Servers must be provisioned, supported and managed by
the INET administrative authority. Cost for purchasing, configuring
and managing Servers is nominal especially for virtual Servers hosted
in the cloud.AERO Proxys are most often standard dedicated server platforms with
one network interface connected to the ANET and a second interface
connected to an INET. As with Servers, the only requirements are that
they can run the AERO user-level code and have at least one interface
connection to the INET. Proxys must be provisioned, supported and
managed by the ANET administrative authority. Cost for purchasing,
configuring and managing Proxys is nominal, and borne by the ANET
administrative authority.AERO Relays can be any dedicated server or COTS router platform
connected to INETs and/or EUNs. The Relay connects to the AERO link
and engages in eBGP peering with one or more Bridges as a stub AS. The
Relay then injects its MNPs and/or non-MNP prefixes into the BGP
routing system, and provisions the prefixes to its downstream-attached
networks. The Relay can perform ROS/ROR services the same as for any
Server, and can route between the MNP and non-MNP address spaces.AERO Servers may appear as a single point of failure in the
architecture, but such is not the case since all Servers on the link
provide identical services and loss of a Server does not imply
immediate and/or comprehensive communication failures. Although
Clients typically associate with a single Server at a time, Server
failure is quickly detected and conveyed by Bidirectional Forward
Detection (BFD) and/or proactive NUD allowing Clients to migrate to
new Servers.If a Server fails, ongoing packet forwarding to Clients will
continue by virtue of the asymmetric neighbor cache entries that have
already been established in route optimization sources (ROSs). If a
Client also experiences mobility events at roughly the same time the
Server fails, unsolicited NA messages may be lost but proxy neighbor
cache entries in the DEPARTED state will ensure that packet forwarding
to the Client's new locations will continue for up to DepartTime
seconds.If a Client is left without a Server for an extended timeframe
(e.g., greater than ReachableTime seconds) then existing asymmetric
neighbor cache entries will eventually expire and both ongoing and new
communications will fail. The original source will continue to
retransmit until the Client has established a new Server relationship,
after which time continuous communications will resume.Therefore, providing many Servers on the link with high
availability profiles provides resilience against loss of individual
Servers and assurance that Clients can establish new Server
relationships quickly in event of a Server failure.The AERO architectural model is client / server in the control
plane, with route optimization in the data plane. The same as for
common Internet services, the AERO Client discovers the addresses of
AERO Servers and selects one Server to connect to. The AERO service is
analogous to common Internet services such as google.com, yahoo.com,
cnn.com, etc. However, there is only one AERO service for the link and
all Servers provide identical services.Common Internet services provide differing strategies for
advertising server addresses to clients. The strategy is conveyed
through the DNS resource records returned in response to name
resolution queries. As of January 2020 Internet-based 'nslookup'
services were used to determine the following:When a client resolves the domainname "google.com", the DNS
always returns one A record (i.e., an IPv4 address) and one AAAA
record (i.e., an IPv6 address). The client receives the same
addresses each time it resolves the domainname via the same DNS
resolver, but may receive different addresses when it resolves the
domainname via different DNS resolvers. But, in each case, exactly
one A and one AAAA record are returned.When a client resolves the domainname "ietf.org", the DNS
always returns one A record and one AAAA record with the same
addresses regardless of which DNS resolver is used.When a client resolves the domainname "yahoo.com", the DNS
always returns a list of 4 A records and 4 AAAA records. Each time
the client resolves the domainname via the same DNS resolver, the
same list of addresses are returned but in randomized order (i.e.,
consistent with a DNS round-robin strategy). But, interestingly,
the same addresses are returned (albeit in randomized order) when
the domainname is resolved via different DNS resolvers.When a client resolves the domainname "amazon.com", the DNS
always returns a list of 3 A records and no AAAA records. As with
"yahoo.com", the same three A records are returned from any
worldwide Internet connection point in randomized order.The above example strategies show differing approaches to
Internet resilience and service distribution offered by major Internet
services. The Google approach exposes only a single IPv4 and a single
IPv6 address to clients. Clients can then select whichever IP protocol
version offers the best response, but will always use the same IP
address according to the current Internet connection point. This means
that the IP address offered by the network must lead to a
highly-available server and/or service distribution point. In other
words, resilience is predicated on high availability within the
network and with no client-initiated failovers expected (i.e., it is
all-or-nothing from the client's perspective). However, Google does
provide for worldwide distributed service distribution by virtue of
the fact that each Internet connection point responds with a different
IPv6 and IPv4 address. The IETF approach is like google
(all-or-nothing from the client's perspective), but provides only a
single IPv4 or IPv6 address on a worldwide basis. This means that the
addresses must be made highly-available at the network level with no
client failover possibility, and if there is any worldwide service
distribution it would need to be conducted by a network element that
is reached via the IP address acting as a service distribution
point.In contrast to the Google and IETF philosophies, Yahoo and Amazon
both provide clients with a (short) list of IP addresses with Yahoo
providing both IP protocol versions and Amazon as IPv4-only. The order
of the list is randomized with each name service query response, with
the effect of round-robin load balancing for service distribution.
With a short list of addresses, there is still expectation that the
network will implement high availability for each address but in case
any single address fails the client can switch over to using a
different address. The balance then becomes one of function in the
network vs function in the end system.The same implications observed for common highly-available services
in the Internet apply also to the AERO client/server architecture.
When an AERO Client connects to one or more ANETs, it discovers one or
more AERO Server addresses through the mechanisms discussed in earlier
sections. Each Server address presumably leads to a fault-tolerant
clustering arrangement such as supported by Linux-HA, Extended Virtual
Synchrony or Paxos. Such an arrangement has precedence in common
Internet service deployments in lightweight virtual machines without
requiring expensive hardware deployment. Similarly, common Internet
service deployments set service IP addresses on service distribution
points that may relay requests to many different servers.For AERO, the expectation is that a combination of the Google/IETF
and Yahoo/Amazon philosophies would be employed. The AERO Client
connects to different ANET access points and can receive 1-2 Server
LLAs at each point. It then selects one AERO Server address, and
engages in RS/RA exchanges with the same Server from all ANET
connections. The Client remains with this Server unless or until the
Server fails, in which case it can switch over to an alternate Server.
The Client can likewise switch over to a different Server at any time
if there is some reason for it to do so. So, the AERO expectation is
for a balance of function in the network and end system, with fault
tolerance and resilience at both levels.<< RFC Editor - remove prior to publication >>Changes from draft-templin-intarea-6706bis-48 to
draft-templin-intrea-6706bis-49:SPAN Anycast address and SBM/PBM concepts introduced.Changes from draft-templin-intarea-6706bis-47 to
draft-templin-intrea-6706bis-48:SEND/CGA.Changes from draft-templin-intarea-6706bis-46 to
draft-templin-intrea-6706bis-47:Major changes to align with Teredo, including changed AERO
"Relay" to "Bridge", and changed AERO "Gateway" to "Relay". The term
"[Rr]elay" now refers to exactly the same thing in both AERO and
Teredo.Changed to use Teredo message authentication instead of SEND.Changes from draft-templin-intarea-6706bis-42 to
draft-templin-intrea-6706bis-43:Segment Routing.Changes from draft-templin-intarea-6706bis-39 to
draft-templin-intrea-6706bis-40:Teredo.Changes from draft-templin-intarea-6706bis-38 to
draft-templin-intrea-6706bis-39:Major clarifications and simplifications of SPAN
fragmentation/reassembly.Revised AERO address format to support prefix lengths up to
112.New method for forming SPAN Client Prefixes and population in the
routing system.Updates RFC4443 to set a new value in the ICMP PTB Code
field.Changes from draft-templin-intarea-6706bis-35 to
draft-templin-intrea-6706bis-36:Clients in the open Internet secured using SEND/CGA.Changes from draft-templin-intarea-6706bis-32 to
draft-templin-intrea-6706bis-33:Updated Proxy discussion with "point-to-multipoint" server
coordinationSignificant updates to Address Resolution and NUD to include
correct addresses in messagesDifferentiate between NS(AR) and NS(NUD) as their addresses and
use cases differ.Changes from draft-templin-intarea-6706bis-30 to
draft-templin-intrea-6706bis-31:Added "advisory PTB messages" under FAA SE2025 contract number
DTFAWA-15-D-00030.Changes from draft-templin-intarea-6706bis-29 to
draft-templin-intrea-6706bis-30:Deprecate "primary" concept. Now, RS/RA keepalives are maintained
over *all* underlying interfaces (i.e., and not just one
primary).Changes from draft-templin-intarea-6706bis-28 to
draft-templin-intrea-6706bis-29:Changed OMNI interface citation to
"draft-templin-6man-omni-interface"Changed SPAN Service Prefix to fd80::/10.Changed S/TLLAO format to include 'S' bit for ifIndex
corresponding to the underlying interface that is Source of ND
message.Updated Path MTUChanges from draft-templin-intarea-6706bis-27 to
draft-templin-intrea-6706bis-28:MTU and fragmentation.Changes from draft-templin-intarea-6706bis-26 to
draft-templin-intrea-6706bis-27:MTU and fragmentation.SPAN Service Prefix set to fd00::/10Client SPAN addresses defined.Changes from draft-templin-intarea-6706bis-25 to
draft-templin-intrea-6706bis-26:MTU and RA configuration information updated.Changes from draft-templin-intarea-6706bis-24 to
draft-templin-intrea-6706bis-25:Added concept of "primary" to allow for proxyed RS/RA over only
selected underlying interfaces.General Cleanup.Changes from draft-templin-intarea-6706bis-23 to
draft-templin-intrea-6706bis-24:OMNI interface spec now a normative reference.Use REACHABLE_TIME as the nominal Router Lifetime to return in
RAs.General cleanup.Changes from draft-templin-intarea-6706bis-22 to
draft-templin-intrea-6706bis-23:Choice of using either RS/RA or unsolicited NA for old Server
notification.General cleanup.Changes from draft-templin-intarea-6706bis-21 to
draft-templin-intrea-6706bis-22:Tightened up text on Proxy.Removed unnecessarily restrictive texts.General cleanup.Changes from draft-templin-intarea-6706bis-20 to
draft-templin-intrea-6706bis-21:Clarified relationship between OMNI and S/TLLAO
ifIndex-tuples.Important text in Section 13.15.3 on Servers timing out Clients
that have gone silent without sending a departure notification.New text on RS/RA as "hints of forward progress" for proactive
NUD.Changes from draft-templin-intarea-6706bis-19 to
draft-templin-intrea-6706bis-20:Included new route optimization source and destination addressing
strategy. Now, route optimization maintenance uses the address of
the existing Server instead of the data packet destination address
so that less pressure is placed on the BGP routing system
convergence time and Server constancy is supported.Included new method for releasing from old MSE without requiring
Client messaging.Included references to new OMNI interface spec (including the
OMNI option).New appendix on AERO Client/Server architecture.Changes from draft-templin-intarea-6706bis-18 to
draft-templin-intrea-6706bis-19:Changed Proxy/Server keepalives to use "proactive NUD" in a
manner that parallels BFDChanges from draft-templin-intarea-6706bis-17 to
draft-templin-intrea-6706bis-18:Discuss how AERO option is used in relation to S/TLLAOsNew text on Bidirectional Forwarding Detection (BFD)Cleaned up usage (and non-usage) of unsolicited NAsNew appendix on Server failuresChanges from draft-templin-intarea-6706bis-15 to
draft-templin-intrea-6706bis-17:S/TLLAO now includes multiple link-layer addresses within a
single option instead of requiring multiple optionsNew unsolicited NA message to inform the old link that a Client
has moved to a new linkChanges from draft-templin-intarea-6706bis-14 to
draft-templin-intrea-6706bis-15:MTU and fragmentationNew details in movement to new ServerChanges from draft-templin-intarea-6706bis-13 to
draft-templin-intrea-6706bis-14:Security based on secured tunnels, ingress filtering, MAP list
and ROS listChanges from draft-templin-intarea-6706bis-12 to
draft-templin-intrea-6706bis-13:New paragraph in Section 3.6 on AERO interface layering over
secured tunnelsRemoved extraneous text in Section 3.7Added new detail to the forwarding algorithm in Section 3.9Clarified use of fragmentationRoute optimization now supported for both MNP and non-MNP-based
prefixesRelays are now seen as link-layer elements in the
architecture.Built out multicast section in detail.New Appendix on implementation considerations for route
optimization.Changes from draft-templin-intarea-6706bis-11 to
draft-templin-intrea-6706bis-12:Introduced Gateways as a new AERO element for connecting
Correspondent Nodes on INET linksIntroduced terms "Access Network (ANET)" and "Internetwork
(INET)"Changed "ASP" to "MSP", and "ACP" to "MNP"New figure on the relation of Segments to the SPAN and AERO
linkNew "S" bit in S/TLLAO to indicate the "Source" S/TLLAO as
opposed to additional S/TLLAOsChanged Interface ID for Servers from 255 to 0xffffSignificant updates to Route Optimization, NUD, and Mobility
ManagementNew Section on MulticastNew Section on AERO Clients in the open InternetworkNew Section on Operation over multiple AERO links (VLANs over the
SPAN)New Sections on DNS considerations and Transition
considerationsChanges from draft-templin-intarea-6706bis-10 to
draft-templin-intrea-6706bis-11:Added The SPANChanges from draft-templin-intarea-6706bis-09 to
draft-templin-intrea-6706bis-10:Orphaned packets in flight (e.g., when a neighbor cache entry is
in the DEPARTED state) are now forwarded at the link layer instead
of at the network layer. Forwarding at the network layer can result
in routing loops and/or excessive delays of forwarded packets while
the routing system is still reconverging.Update route optimization to clarify the unsecured nature of the
first NS used for route discoveryMany cleanups and clarifications on ND messaging parametersChanges from draft-templin-intarea-6706bis-08 to
draft-templin-intrea-6706bis-09:Changed PRL to "MAP list"For neighbor cache entries, changed "static" to "symmetric", and
"dynamic" to "asymmetric"Specified Proxy RS/RA exchanges with Servers on behalf of
ClientsAdded discussion of unsolicited NAs in Section 3.16, and included
forward reference to Section 3.18Added discussion of AERO Clients used as critical infrastructure
elements to connect fixed networks.Added network-based VPN under security considerationsChanges from draft-templin-intarea-6706bis-07 to
draft-templin-intrea-6706bis-08:New section on AERO-Aware Access RouterChanges from draft-templin-intarea-6706bis-06 to
draft-templin-intrea-6706bis-07:Added "R" bit for release of PDs. Now have a full RS/RA service
that can do PD without requiring DHCPv6 messaging over-the-airClarifications on solicited vs unsolicited NAsClarified use of MAX_NEIGHBOR_ADVERTISEMENT for the purpose of
increase reliabilityChanges from draft-templin-intarea-6706bis-05 to
draft-templin-intrea-6706bis-06:Major re-work and simplification of Route Optimization
functionAdded Distributed Mobility Management (DMM) and Mobility Anchor
Point (MAP) terminologyNew section on "AERO Critical Infrastructure Element
Considerations" demonstrating low overall cost for the serviceminor text revisions and deletionsremoved extraneous appendicesChanges from draft-templin-intarea-6706bis-04 to
draft-templin-intrea-6706bis-05:New Appendix E on S/TLLAO Extensions for special-purpose links.
Discussed ATN/IPS as example.New sentence in introduction to declare appendices as
non-normative.Changes from draft-templin-intarea-6706bis-03 to
draft-templin-intrea-6706bis-04:Added definitions for Potential Router List (PRL) and secure
enclaveIncluded text on mapping transport layer port numbers to network
layer DSCP valuesAdded reference to DTLS and DMM Distributed Mobility Anchoring
working group documentReworked Security ConsiderationsUpdated references.Changes from draft-templin-intarea-6706bis-02 to
draft-templin-intrea-6706bis-03:Added new section on SEND.Clarifications on "AERO Address" section.Updated references and added new reference for RFC8086.Security considerations updates.General text clarifications and cleanup.Changes from draft-templin-intarea-6706bis-01 to
draft-templin-intrea-6706bis-02:Note on encapsulation avoidance in Section 4.Changes from draft-templin-intarea-6706bis-00 to
draft-templin-intrea-6706bis-01:Remove DHCPv6 Server Release procedures that leveraged the old
way Relays used to “route” between Server link-local
addressesRemove all text relating to Relays needing to do any
AERO-specific operationsProxy sends RS and receives RA from Server using SEND. Use CGAs
as source addresses, and destination address of RA reply is to the
AERO address corresponding to the Client’s ACP.Proxy uses SEND to protect RS and authenticate RA (Client does
not use SEND, but rather relies on subnetwork security. When the
Proxy receives an RS from the Client, it creates a new RS using its
own addresses as the source and uses SEND with CGAs to send a new RS
to the Server.Emphasize distributed mobility managementAERO address-based RS injection of ACP into underlying routing
system.Changes from draft-templin-aerolink-82 to
draft-templin-intarea-6706bis-00:Document use of NUD (NS/NA) for reliable link-layer address
updates as an alternative to unreliable unsolicited NA. Consistent
with Section 7.2.6 of RFC4861.Server adds additional layer of encapsulation between outer and
inner headers of NS/NA messages for transmission through Relays that
act as vanilla IPv6 routers. The messages include the AERO Server
Subnet Router Anycast address as the source and the Subnet Router
Anycast address corresponding to the Client's ACP as the
destination.Clients use Subnet Router Anycast address as the encapsulation
source address when the access network does not provide a
topologically-fixed address.