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, 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. 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 Gateways that
are seen as AERO link neighbors. Each node's AERO interface uses an IPv6
link-local address format (known as the AERO address) 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 may 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 Gateway on the link for efficient communications. Fixed nodes
forward packets destined to other AERO nodes to the nearest Gateway,
which forwards them through the cloud. A mobile node's initial packets
are forwarded through the MAP, 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 Relays are interconnected in a secured private BGP overlay
routing instance known as the "SPAN". The SPAN provides a hybrid
routing/bridging service to join 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 Gateway/MAP is on the optimal route from any
correspondent node on the link, and provides a gateway between the
underlying Internetwork and the SPAN. To the underlying Internetwork,
the Gateway/MAP is the source of a route to its MSP, and hence uplink
traffic to the mobile node is naturally routed to the nearest
Gateway/MAP.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 . 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, or the public
Internet itself. For secured ANETs, link-layer security services
such as IEEE 802.1X and physical-layer security prevent unauthorized
access internally while border network-layer security services such
as firewalls and proxies prevent unauthorized outside access.a node's attachment to a link
in an ANET.an IP address assigned to a
node's interface connection to 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. (An INET partition is the same as
a SPAN segment 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.a Non-Broadcast, Multiple Access
(NBMA) tunnel virtual overlay configured over one or more underlying
INETs. Nodes on the AERO link appear as single-hop neighbors from
the perspective of the virtual overlay even though they may be
separated by many underlying INET hops. AERO links may be configured
over multiple underlying SPAN segments (see below).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.an IPv6 link-local address
assigned to an AERO interface and constructed as specified in .the lowest-numbered AERO
address aggregated by the MNP (see ).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
Gateway.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 to one or more ANETs and requests MNP PDs from AERO
Servers. The Client assigns a Client AERO address 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
services and a Mobility Anchor Point (MAP) for AERO Clients. The
Server assigns an administratively-provisioned AERO address to its
AERO interface to support the operation of the ND/PD services, and
advertises all of its associated MNPs via BGP peerings with
Relays.an AERO
Server that also provides forwarding services between nodes reached
via the AERO link and correspondents on other links. AERO Gateways
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 Gateway advertises the MSP(s) over INET interfaces,
and distributes all of its associated MNPs and non-MNP IP routes via
BGP peerings with Relays (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 Relay
forwards packets using standard IP forwarding. As a bridge, the
Relay forwards packets over the AERO link without decrementing the
IPv6 Hop Limit. AERO Relays peer with Servers and other Relays to
discover the full set of MNPs for the link as well as any non-MNPs
that are reachable via Gateways.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 .a
means for bridging disjoint INET partitions as segments of a unified
AERO link the same as for a bridged campus LAN. The SPAN is a
mid-layer IPv6 encapsulation service in the AERO routing system that
supports a unified AERO link view for all segments. Each segment in
the SPAN is a distinct INET partition.a global or unique
local /96 IPv6 prefix assigned to the AERO link to support SPAN
services.a sub-prefix of
the SPAN Service Prefix uniquely assigned to a single SPAN
segment.a global or unique local IPv6
address taken from a SPAN Partition Prefix and constructed as
specified in . SPAN addresses are statelessly
derived from AERO addresses, and vice-versa.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. From the
perspective of the AERO interface, the link-layer address is either
an INET address for intra-segment encapsulation or a SPAN address
for inter-segment encapsulation.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 Gateway connects to
the rest of the network via the AERO interface. The Client/Gateway
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.an AERO Server
that is currently tracking and reporting the mobility events of its
associated Mobile Node Clients.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 as a MAP on
behalf of a target MNP Client, or a Gateway for a non-MNP
destination.a geographically and/or
topologically referenced list of AERO addresses of all MAPs within
the same AERO link. There is a single MAP list for the entire AERO
link.a list of AERO/SPAN-to-INET address
mappings of all ROSes within the same SPAN segment. There is a
distinct ROS list for each segment.a
BGP-based overlay routing service coordinated by Servers and Relays
that tracks all MAP-to-Client associations.Throughout the document, the simple terms "Client", "Server",
"Relay", "Proxy" and "Gateway" refer to "AERO Client", "AERO Server",
"AERO Relay", "AERO Proxy" and "AERO Gateway", 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" 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: presents the 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 Relay R1 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). Relays
use the SPAN service to bridge disjoint segments of a partitioned
AERO link.AERO Servers S1 and S2 configure secured tunnels with Relay R1
and also act as Mobility Anchor Points (MAPs) and default routers
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 Relay R1 and
provides proxy services for AERO Clients in secured enclaves that
cannot associate directly with other AERO link neighbors.Each node on the AERO link maintains an AERO interface
neighbor cache and an IP forwarding table the same as for any link.
Although the figure shows a limited deployment, in common operational
practice there will normally be many additional Relays, Servers,
Clients and Proxys.AERO Relays provide hybrid routing/bridging services (as well as a
security trust anchor) for nodes on an AERO link. Relays 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 known as the SPAN header. The inner IP layer experiences
a virtual bridging service since the inner IP TTL/Hop Limit is not
decremented during forwarding. Each Relay also peers with Servers and
other Relays in a dynamic routing protocol instance to provide a
Distributed Mobility Management (DMM) service for the list of active
MNPs (see ). Relays 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. Relays configure secured
tunnels with Servers, Proxys and other Relays; 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 services and a Mobility
Anchor Point (MAP) for AERO Client Mobile Nodes (MNs). Each Server
also peers with Relays 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 AERO Clients to associate
with AERO 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 Gateways are Servers that provide forwarding services between
the AERO interface and INET/EUN interfaces. Gateways 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 Gateway
advertises the MSP(s) to INETs, and distributes all of its associated
MNPs and non-MNP IP routes via BGP peerings with Relays.AERO Relays, Servers, Proxys and Gateways 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 ANET interfaces, i.e., their ANET addresses
may change when the Client moves to a new ANET connection.The AERO routing system comprises a private instance of the Border
Gateway Protocol (BGP) that is coordinated
between Relays 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 Relays but
does not peer with other Servers. Each INET of a multi-segment AERO
link must include one or more Relays, which peer with the Servers and
Proxys within that INET. All Relays 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 Relays of
different INETs peer with one another using eBGP.Relays 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 Relay
configures a black-hole route for each of its MSPs. By black-holing
the MSPs, the Relay 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 Relays which
have full topology knowledge.Servers maintain a working set of associated MNPs, and dynamically
announce new MNPs and withdraw departed MNPs in eBGP updates to
Relays. Servers that are configured as Gateways also redistribute
non-MNP routes learned from non-AERO interfaces via their eBGP Relay
peerings.Clients are expected to remain associated with their current
Servers for extended timeframes, however Servers SHOULD selectively
suppress updates for impatient Clients that repeatedly associate and
disassociate with them in order to dampen routing churn. Servers that
are configured as Gateways advertise the MSPs via INET/EUN interfaces,
and forward packets between INET/EUN interfaces and the AERO interface
using standard IP forwarding.Scaling properties of the AERO routing system are limited by the
number of BGP routes that can be carried by Relays. As of 2015, the
global public Internet BGP routing system manages more than 500K
routes with linear growth and no signs of router resource exhaustion
. More recent network emulation studies have also
shown that a single Relay can accommodate at least 1M dynamically
changing BGP routes even on a lightweight virtual machine, i.e., and
without requiring high-end dedicated router hardware.Therefore, assuming each Relay can carry 1M or more routes, this
means that at least 1M Clients can be serviced by a single set of
Relays. A means of increasing scaling would be to assign a different
set of Relays for each set of MSPs. In that case, each Server still
peers with one or more Relays, but institutes route filters so that
BGP updates are only sent to the specific set of Relays that aggregate
the MSP. For example, if the MSP for the AERO link is 2001:db8::/32, a
first set of Relays could service the MSP 2001:db8::/40, a second set
of Relays could service 2001:db8:0100::/40, a third set could service
2001:db8:0200::/40, etc.Assuming up to 1K sets of Relays, the AERO routing system can then
accommodate 1B or more MNPs with no additional overhead (for example,
it should be possible to service 1B /64 MNPs taken from a /34 MSP and
even more for shorter prefixes). In this way, each set of Relays
services a specific set of MSPs that they advertise to the native
Internetwork routing system, and each Server configures MSP-specific
routes that list the correct set of Relays as next hops. This
arrangement also allows for natural incremental deployment, and can
support small scale initial deployments followed by dynamic deployment
of additional Clients, Servers and Relays without disturbing the
already-deployed base.Server and Relays can use the Bidirectional Forwarding Detection
(BFD) protocol to quickly detect link
failures that don’t result in interface state changes, BGP peer
failures, and administrative state changes. BFD is important in
environments where rapid response to failures is required for routing
reconvergence and, hence, communications continuity.A 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 .For IPv6 MNPs, the AERO routing system includes ordinary IPv6
routes. For IPv4 MNPs, the AERO routing system includes IPv6 routes
based on an IPv4-embedded IPv6 address format discussed in .A Client's AERO address is an IPv6 link-local address with an
interface identifier based on the Client's delegated MNP. Relay,
Server and Proxy AERO addresses are assigned from the range fe80::/96
and include an administratively-provisioned value in the lower 32
bits.For IPv6, Client AERO addresses begin with the prefix fe80::/64 and
include in the interface identifier (i.e., the lower 64 bits) a 64-bit
prefix taken from one of the Client's IPv6 MNPs. For example, if the
AERO Client receives the IPv6 MNP:2001:db8:1000:2000::/56it constructs its corresponding AERO addresses as:fe80::2001:db8:1000:2000fe80::2001:db8:1000:2001fe80::2001:db8:1000:2002... etc. ...fe80::2001:db8:1000:20ffFor IPv4, Client AERO addresses 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 192.0.2.32/28 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 AERO addresses with the prefix
fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
in the interface identifier as:fe80::FFFF:192.0.2.16fe80::FFFF:192.0.2.17fe80::FFFF:192.0.2.18... etc. ...fe80:FFFF:192.0.2.31Relay, Server and Proxy AERO addresses are allocated from the
range fe80::/96, and MUST be managed for uniqueness. The lower 32 bits
of the AERO address includes a unique integer value between 1 and
0xfffffffe (e.g., fe80::1, fe80::2, fe80::3, etc., fe80::ffff:fffe) as
assigned by the administrative authority for the link. If the link
spans multiple SPAN segments, the AERO addresses are assigned to each
segment in 1x1 correspondence with SPAN addresses (see: ). The address fe80:: is the IPv6 link-local
Subnet-Router anycast address, and the address fe80::ffff:ffff is
reserved as the unspecified AERO address.The lowest-numbered AERO address from a Client's MNP delegation
serves as the "base" AERO address (for example, for the MNP
2001:db8:1000:2000::/56 the base AERO address is
fe80::2001:db8:1000:2000). The Client then assigns the base AERO
address to the AERO interface and uses it for the purpose of
maintaining the neighbor cache entry. The Server likewise uses the
AERO address as its index into the neighbor cache for this Client.If the Client has multiple AERO addresses (i.e., when there are
multiple MNPs and/or MNPs with prefix lengths shorter than /64), the
Client originates ND messages using the base AERO address as the
source address and accepts and responds to ND messages destined to any
of its AERO addresses as equivalent to the base AERO address. In this
way, the Client maintains a single neighbor cache entry that may be
indexed by multiple AERO addresses.The Client's Subnet-Router anycast address can be statelessly
determined from its AERO address by simply transposing the AERO
address into the upper N bits of the Anycast address followed by 128-N
bits of zeroes. For example, for the AERO address fe80::2001:db8:1:2
the Subnet-Router anycast address is 2001:db8:1:2::.AERO addresses for mobile node Clients embed a MNP as discussed
above, while AERO addresses for non-MNP destinations are constructed
in exactly the same way. A Client AERO address therefore encodes
either an MNP if the prefix is reached via the SPAN or a non-MNP if
the prefix is reached via a Gateway.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 at all. 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 Relays.The same as for traditional campus LANs, multiple AERO link
segments can be joined into a single unified link via a virtual
bridging service termed the "SPAN". The SPAN performs link-layer
packet forwarding between segments (i.e., bridging) without
decrementing the network-layer TTL/Hop Limit. The SPAN model is
depicted in :To support the SPAN, AERO links require a reserved /64 IPv6 "SPAN
Service Prefix (SSP)". Although any routable IPv6 prefix can be used,
a Unique Local Address (ULA) prefix (e.g., fd00::/64) is recommended since border routers are commonly
configured to prevent packets with ULAs from being injected into the
AERO link by an external IPv6 node and from leaking out of the AERO
link to the outside world.Each segment in the SPAN assigns a unique sub-prefix of SSP::/96
termed a "SPAN Partition Prefix (SPP)". For example, a first segment
could assign fd00::1000/116, a second could assign fd00::2000/116, a
third could assign fd00::3000/116, etc. The administrative authorities
for each segment must therefore coordinate to assure
mutually-exclusive SPP assignments, but internal provisioning of the
SPP is an independent local consideration for each administrative
authority.A "SPAN address" is an address taken from a SPP and assigned to a
Relay, Server, Gateway or Proxy interface. SPAN addresses are formed
by simply replacing the upper portion of an administratively-assigned
AERO address with the SPP. For example, if the SPP is fd00::1000/116,
the SPAN address formed from the AERO address fe80::1001 is simply
fd00::1001.An "INET address" is an address of a node's interface connection to
an INET. AERO/SPAN/INET address mappings are maintained as permanent
neighbor cache entires as discussed in .AERO Relays serve as bridges to join multiple segments into a
unified AERO link over multiple diverse administrative domains. They
support the bridging function by first establishing forwarding table
entries for their SPPs either via standard BGP routing or static
routes. For example, if three Relays ('A', 'B' and 'C') from different
segments serviced the SPPs fd00::1000/116, fd00::2000/116 and
fd00::3000/116 respectively, then the forwarding tables in each Relay
are as follows:fd00::1000/116->local, fd00::2000/116->B,
fd00::3000/116->Cfd00::1000/116->A, fd00::2000/116->local,
fd00::3000/116->Cfd00::1000/116->A, fd00::2000/116->B,
fd00::3000/116->localThese forwarding table entries are permanent and never
change, since they correspond to fixed infrastructure elements in
their respective segments. This provides the basis for a link-layer
forwarding service that cannot be disrupted by routing updates due to
node mobility.With the SPPs in place in each Relay's forwarding table, control
and data packets sent between AERO nodes in different segments can
therefore be carried over the SPAN via encapsulation. For example,
when a source AERO node in segment A forwards a packet with IPv6
address 2001:db8:1:2::1 to a target AERO node in segment C with IPv6
address 2001:db8:1000:2000::1, it first encapsulates the packet in a
SPAN header with source SPAN address taken from fd00::1000/116 (e.g.,
fd00::1001) and destination SPAN address taken from fd00::3000/116
(e.g., fd00::3001). Next, it encapsulates the SPAN message 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 Relay (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
according to . A packet is said to be
"forwarded/sent into the SPAN" when it is encapsulated as described
above then forwarded via a secured tunnel to a neighboring Relay.This gives rise to a routing system that contains both MNP routes
that may change dynamically due to regional node mobility and SPAN
routes that never change. The Relays can therefore provide link-layer
bridging by sending packets into the SPAN 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.With reference to , for a Client's AERO
address the SPAN address is simply set to the Subnet-Router anycast
address. For non-link-local addresses, the destination SPAN address
may not be known in advance for the first few packets of a flow sent
via the SPAN. In that case, the SPAN destination address is set to the
original packet's destination, and the SPAN routing system will direct
the packet to the correct SPAN egress node. (In the above example, the
SPAN destination address is simply 2001:db8:1000:2000::1.)For IPv4 MNPs, Servers inject a "SPAN Compatibility Prefix (SCP)"
that embeds the MNP into the BGP routing system. The SCP begins with
the upper 64 bits of the SSP, followed by the constant string
"0000:FFFF" followed by the IPv4 MNP. For example, if the SSP is
fd00::/64 and the MNP is 192.0.2.0/24 then the SCP is
fd00::FFFF:192.0.2.0/120.This allows for encapsulation of IPv4 packets in IPv6 headers
with "SPAN Compatibility Addresses (SCAs)". In this example, the SCA
corresponding to the SCP is simply fd00::FFFF:192.0.2.0, and can be
used as the SPAN destination address for packets forwarded via the
SPAN. This allows for forwarding of initial IPv4 packets over IPv6
SPAN routes, followed by route optimization for direct
communications.AERO interfaces are virtual interfaces configured over one or more
underlying interfaces classified as follows:Native interfaces have global IP addresses that are reachable
from any INET correspondent. All Server, Gateway and Relay
interfaces are native interfaces, as are INET-facing interfaces of
Proxys.NATed interfaces connect to a private network behind a Network
Address Translator (NAT). The NAT does not participate in any AERO
control message signaling, but the Server can issue control
messages on behalf of the Client. 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. If no other periodic
messaging service is available, the Client can send RS messages to
receive RA replies from its Server(s).VPNed interfaces use security encapsulation to a Virtual
Private Network (VPN) server that also acts as an AERO Server. As
with NATed links, the Server can issue control messages on behalf
of the Client, but the Client need not send periodic keepalives in
addition to those already used to maintain the VPN connection.Proxyed interfaces connect to an ANET that is separated from
the open INET by an AERO Proxy. Unlike NATed and VPNed interfaces,
the Proxy can actively issue control messages on behalf of the
Client.Direct interfaces connect a Client directly to a neighbor
without crossing any ANET/INET paths. An example is a
line-of-sight link between a remote pilot and an unmanned
aircraft.AERO interfaces use encapsulation (see: ) to exchange packets with AERO link neighbors
over Native, NATed or VPNed interfaces. AERO interfaces do not use
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
quality information 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 folllowing information:ifIndex[i] - the same value as in the corresponding
ifIndex-tuple included in the OMNI option.V[i] - a bit that identifies the IP protocol version of the
address found in the Link Layer Address [i] field. The bit is set
to 0 for IPv4 or 1 for IPv6.Reserved[i] - MUST encode the value 0 on transmission, and
ignored on reception.Link Layer Address [i] - the IPv4 or IPv6 address used as the
encapsulation source address. The field is 4 bytes in length for
IPv4 or 16 bytes in length for IPv6.Port Number [i] - the upper layer protocol port number used as
the encapsulation source port, or 0 when no upper layer protocol
encapsulation is used. The field is 2 bytes in length.If an S/TLLAO is included, the first S/TLLAO ifIndex-tuple MUST
correspond to the first OMNI option ifIndex-tuple, and any additional
S/TLLAO ifIndex-tuples MUST correspond to a proper subset of the
remaining 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 corresponidng OMNI option
ifIndex-tuples, i.e., the OMNI option and S/TLLAO are companion
options 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 typically used "one at a time" with
low-cost WLAN preferred and highly-available cellular wireless as a
standby. 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 and set to a constant value. 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 a value that corresponds to a specific
interface. The OMNI option MUST include a first ifIndex-tuple that
corresponds to the interface over which the ND message is sent. 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.Relay, Server and Proxy AERO interfaces may be configured over one
or more secured tunnel interfaces. The AERO interface configures both
an AERO address and its corresponding SPAN address, while the
underlying secured tunnel interfaces are either unnumbered or
configure the same SPAN address. The AERO interface encapsulates each
IP packet in a SPAN header and presents the packet to the underlying
secured tunnel interface. For Relays that do not configure an AERO
interface, the secured tunnel interfaces themselves are exposed to the
IP layer with each interface configuring the Relay's SPAN address.
Routing protocols such as BGP therefore run directly over the Relay'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 SPAN address 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 Relays are discussed in the following sections.When a Server enables an AERO interface, it assigns AERO/SPAN
addresses and configures permanent neighbor cache entries for
neighbors in the same SPAN segment by consulting the ROS list for
the segment. The Server also configures secured tunnels with one or
more neighboring Relays and engages in a BGP routing protocol
session with each Relay.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.Gateways are simply Servers that run a dynamic routing protocol
between the AERO interface and INET/EUN interfaces (see: ). The Gateway 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 Gateway further provides an attachment point of the
AERO link to the non-MNP-based global topology.When a Proxy enables an AERO interface, it assigns AERO/SPAN
addresses and configures permanent neighbor cache entries the same
as for Servers. The Proxy also configures secured tunnels with one
or more neighboring Relays and maintains per-Client neighbor cache
entries based on control message exchanges.When a Client enables an AERO interface, it sends an RS message
with ND/PD parameters over an ANET interface 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.)After the initial ND/PD message exchange, the Client assigns AERO
addresses to the AERO interface based on the delegated prefix(es).
The Client can then register additional ANET interfaces with the
Server by sending an RS message over each ANET interface.AERO Relays 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 Relays is
therefore OPTIONAL, e.g., if an administrative interface is needed.
Relays configure secured tunnels with Servers, Proxys and other
Relays; they also configure AERO/SPAN addresses and permanent
neighbor cache entries the same as Servers. Relays engage in a BGP
routing protocol session with a subset of the Servers on the local
SPAN segment, and with other Relays on the SPAN (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 Servers and Proxys maintain permanent
neighbor cache entries for all other Servers and Proxys within the
same SPAN segment. Each entry maintains the mapping between the
neighbor's network-layer AERO address and corresponding INET address.
The list of all permanent neighbor cache entries for the SPAN segment
is maintained in the segment's ROS list.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 route optimization sources (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 target (e.g., a Client's
MAP) 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.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
"DEPARTTIME" 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 DEPARTTIME be set to the default
constant value REACHABLETIME 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 a target Server (acting as a MAP) receives a valid NS message
used for route optimization, it searches for a symmetric neighbor
cache entry for the target Client. The MAP 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 REPORTTIME seconds. The
MAP resets ReportTime when it receives a new authentic NS message, and
otherwise decrements ReportTime while no NS messages have been
received. It is RECOMMENDED that REPORTTIME be set to the default
constant value REACHABLETIME plus 10 seconds (40 seconds by default)
to allow a window for route optimization to converge before ReportTime
decrements below REACHABLETIME.When the ROS receives a solicited NA message response to its NS
message, 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 REACHABLETIME
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 REACHABLETIME be
set to the default constant value 30 seconds as specified in .The ROS also uses 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 DEPARTTIME, REPORTTIME, REACHABLETIME,
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, DEPARTTIME and REPORTTIME
SHOULD be set to a value that is sufficiently longer than
REACHABLETIME to avoid packet loss due to stale route optimization
state.Client AERO interfaces avoid 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 packet's IP header into the
corresponding fields in the 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 each packet in a SPAN header, then encapsulate the
resulting SPAN packet in an INET header 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 INET encapsulation format specific to the secured tunnel type (see:
). If the next hop is reached via an unsecured
underlying interface, the AERO interface instead uses Generic UDP
Encapsulation (GUE) or an
alternate minimal encapsulation format .When GUE 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 SPAN packet plus 8 bytes for the UDP header itself plus
the length of the GUE header (or 0 if GUE direct IP encapsulation is
used). For packets sent to a Server or Relay, 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 then either includes
or omits the UDP checksum according to the GUE specification.AERO interfaces observes 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 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 Relays, Servers and Proxys accept encapsulated data
packets and control messages received from secured tunnels.AERO Servers and Proxys accept encapsulated data packets and NS
messages used for Neighbor Unreachability Detection (NUD) received
from a source found in the ROS list.AERO Proxys and Clients accept packets that originate from
within the same secured ANET.AERO Clients and Gateways accept packets from downstream
network correspondents based on ingress filtering.AERO nodes silently drop any packets that do not satisfy the
above data origin authentication procedures. Further security
considerations are discussed .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 each packet's DSCP value (and/or other traffic discriminators
such as port number) to select an outgoing underlying interface 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 Relays. 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 AERO address).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).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 decapsulates the packet (if
necessary) and delivers it 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 searches for an asymmetric neighbor cache entry that
matches the destination and forwards the packet 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 via encapsulation.
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 Relay.else, the Proxy encapsulates and forwards the packet to a
Relay while using the packet's destination address as the SPAN
destination address. (If the destination is an AERO address, the
Proxy instead uses the corresponding Subnet-Router anycast
address for Client AERO addresses and the SPAN address for
administratively-provisioned AERO addresses.).When the Proxy receives an encapsulated data packet from an INET
neighbor or from a secured tunnel, it accepts the packet only if
data origin authentication succeeds and the SPAN destination address
is its own address. If the packet is a SPAN fragment, the Proxy then
adds the fragment to the reassembly buffer and returns if the
reassembly is still incomplete. Otherwise, the Proxy reassembles the
packet (if necessary) and continues processing.Next, the Proxy searches for a proxy neighbor cache entry that
matches the destination. If there is a proxy neighbor cache entry in
the REACHABLE state, the Proxy decapsulates and forwards the packet
to the Client. If the neighbor cache entry is in the DEPARTED state,
the Proxy instead re-encapsulates the packet and forwards it to a
Relay. If there is no neighbor cache entry, the Proxy instead
discards the packet.For control messages destined to a target Client's AERO address
that are received from a secured tunnel, the Server (acting as a
MAP) 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 reassembles if necessary and discards
the SPAN header (if the reassembly is incomplete, the Server instead
adds the fragment to the reassembly buffer and returns). The Server
then 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. If the current header is a
SPAN header, the Server reassembles if necessary and discards
the SPAN header. The Server then forwards the packet according
to the cached link-layer information, while using SPAN
encapsulation for the Client's Proxyed/Native interfaces, simple
INET encapsulation for NATed/VPNed interfaces, or no
encapsulation for Direct interfaces.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 SPAN address 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 SPAN segment or via a Relay
otherwise.else, if the destination is an AERO address that is not
assigned on the AERO interface the Server drops the packet.else, the Server (acting as a Gateway) releases the packet 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, 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 NATed/VPNed/Direct Client, it
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
Relay. For administratively-assigned AERO address destinations,
the Server uses the SPAN address corresponding to the
destination as the SPAN destination address. For Client AERO
address destinations, the Server uses the Subnet-Router anycast
address corresponding to the destination as the SPAN destination
address. For all others, the Server uses the packet's
destination IP address as the SPAN destination address.Relays forward packets over secured tunnels the same as any IP
router. When the Relay receives an encapsulated packet via a secured
tunnel, it removes the INET header and searches for a forwarding
table entry that matches the destination address in the next header.
The Relay then processes the packet as follows:if the destination matches one of the Relay's own addresses,
the Relay submits the packet for local delivery.else, if the destination matches a forwarding table entry the
Relay forwards the packet via a secured tunnel to the next hop.
If the destination matches an MSP without matching an MNP,
however, the Relay instead drops the packet and returns an ICMP
Destination Unreachable message subject to rate limiting (see:
).else, the Relay drops the packet and returns an ICMP
Destination Unreachable as above.As for any IP router, the Relay decrements the TTL/Hop
Limit when it forwards the packet. If the packet is encapsulated in
a SPAN header, only the Hop Limit in the SPAN header is decremented,
and not the TTL/Hop Limit in the inner packet header.The AERO interface is the node's attachment to the AERO link. For
AERO link neighbor underlying interface paths that do not require
encapsulation, the AERO interface sends unencapsulated IP packets. For
other paths, the AERO interface acts as a tunnel ingress when it sends
packets to the neighbor and as a tunnel egress when it receives
packets from the neighbor.AERO interfaces configure an MTU the same as for any IP interface,
however the MTU does not reflect the physical size of any links in the
path but rather determines the maximum size for reassembly. AERO
interfaces observe the packet sizing considerations for tunnels
discussed in and as
specified below.The Internet Protocol expects that IP packets will either be
delivered to the destination or a suitable Packet Too Big (PTB)
message returned to support the process known as IP Path MTU Discovery
(PMTUD) . However, PTB
messages may be crafted for malicious purposes or lost in the network
. This can be especially problematic for
tunnels, where a condition known as a PMTUD "black hole" can result.
For these reasons, AERO interfaces employ operational procedures that
avoid interactions with PMTUD, including the use of fragmentation when
necessary.AERO interfaces observe three different types of fragmentation.
Source fragmentation occurs when the AERO interface (acting as a
tunnel ingress) fragments the encapsulated packet into multiple
fragments before admitting each fragment into the tunnel. Network
fragmentation occurs when an encapsulated packet admitted into the
tunnel by the ingress is fragmented by an IPv4 router on the path to
the egress. Finally, link-layer fragmentation (aka link adaptation)
occurs at a layer below IP and is coordinated between underlying data
link endpoints.IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of
1280 bytes . Although IPv4 specifies a smaller
minimum link MTU of 68 bytes , AERO interfaces
also observe the IPv6 minimum for IPv4 even if encapsulated packets
may incur network fragmentation.IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500
bytes , while the minimum MRU for IPv4 is only
576 bytes (but, note that many standard IPv6
over IPv4 tunnel types already assume a larger MRU than the IPv4
minimum).AERO interfaces therefore configure an MTU that MUST NOT be smaller
than 1280 bytes, MUST NOT be larger than the minimum MRU among all
nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
and SHOULD NOT be smaller than 1500 bytes. AERO interfaces also
configure a Maximum Segment Unit (MSU) as the maximum-sized
encapsulated packet that the ingress can inject into the tunnel
without source fragmentation. The MSU value MUST NOT be larger than
1280 bytes if there is no operational assurance that a larger size can
traverse the link along all paths.The network layer proceeds as follows when it forwards an IP packet
to the AERO interface. For each IPv4 packet that is larger than the
AERO interface MTU and with DF set to 0, the network layer uses IPv4
fragmentation to break the packet into a minimum number of
non-overlapping fragments where the first fragment is no larger than
the MTU and the remaining fragments are no larger than the first. For
all other IP packets, if the packet is larger than the AERO interface
MTU, the network layer drops the packet and returns a PTB message to
the original source. Otherwise, the network layer admits each IP
packet or fragment into the AERO interface.For each IP packet admitted into AERO interface, if the neighbor is
reached via an underlying interface that does not require
encapsulation the AERO interface proceeds according to the underlying
interface MTU. If the packet is no larger than the underlying
interface MTU, the AERO interface presents the packet to the
underlying interface. Otherwise, for IPv4 packets with DF set to 0 the
AERO interface uses IPv4 fragmentation to break the packet into
fragments no larger than the underlying interface MTU. For other
packets, the AERO interface either performs link adaptation or drops
the packet and returns a PTB message to the original source. (If the
original source corresponds to a local application, the PTB would
appear to have originated from a router on the path when in fact it
was locally generated from within the AERO interface.) In the same
way, when a packet that has been admitted into the AERO link reaches a
target neighbor but is too large to be delivered over the final-hop
underlying interface, the target either performs link adaptation or
drops the packet and returns a PTB. Link adaptation is preferred in
both cases when possible to avoid packet loss.For underlying interfaces that require encapsulation, the AERO
interface (acting as a tunnel ingress) instead encapsulates the packet
and performs path MTU procedures according to the specific
encapsulation format. For INET interfaces, the ingress encapsulates
the packet in a SPAN header. If the SPAN packet is larger than the
MSU, the ingress source fragments the SPAN packet into a minimum
number of non-overlapping fragments where the first fragment is no
larger than the MSU and the remaining fragments are no larger than the
first. The ingress then encapsulates each SPAN packet/fragment in an
INET header and admits them into the tunnel. For IPv4, the ingress
sets the DF bit to 0 in the INET header in case any network
fragmentation is necessary. The encapsulated packets will be delivered
to the egress, which reassembles them into a whole packet if
necessary.By fragmenting at the SPAN layer instead of lower layers, standard
IPv6 fragmentation and reassembly ensures
that IPv4 issues such as data corruption due to reassembly
misassociations will not occur . The ingress sends each fragment with minimal delay
(i.e., in a multi-fragment burst) so that individual fragments are
unlikely to be diverted to different destinations due to routing
fluctuations.Since the SPAN header and IPv6 fragment extension header reduces
the room available for packet data, but the original source has no way
to control its insertion, the ingress MUST include their lengths in
ENCAPS even for packets in which the header is absent.In light of the above considerations, AERO interfaces MUST
configure an MTU of 9180 bytes (i.e., the same as specified in ). This means that the AERO interface MUST be
capable of reassembling original IP packets up to 9180 bytes in
length. When an IP packet is admitted into an AERO interface, the
interface encapsulates the packet using SPAN encapsulation and
fragments the encapsulated packet into fragments that are no larger
than 1280 bytes. The fragments will be reassembled by the tunnel
egress that services the final destination.The AERO interfaces of Clients behind Proxys MAY see underlying
interfaces with MTUs smaller than 9180 (but no smaller than the IP
minimum link MTU). If a Client's underlying interfaces configure a
diversity of MTUs (e.g., 1280, 1500, 4KB, 8KB, etc.) then neighbors
on the link would appear to have multiple MTUs. IPv6 Path MTU
Discovery accounts for this possibility
since MTU discovery must be performed even between nodes that appear
to be connected to the same link.Applications that cannot tolerate loss in the network due to MTU
restrictions should restrict themselves to sending packets no larger
than the IP minimum link MTU, i.e., even if the current path MTU
would appear to support a larger size. This is due to the fact that
routing changes could cause the path to traverse links with smaller
MTUs at any given point in time.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 Relay 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 Relay drops
the packet and returns a network-layer Destination Unreachable message
subject to rate limiting. The Relay 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 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 AERO
address 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 AERO address, it includes the AERO address 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 AERO address, it uses the AERO address as the
network-layer source address; otherwise, it uses the unspecified
AERO address (fe80::ffff:ffff) 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 a first ifIndex-tuple 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 with a
single link-layer address corresponding to the first OMNI option
ifIndex-tuple. The Client sets a "primary" flag in the OMNI option
if it wishes to enable proxy keepalives on this underlying
interface.The Client then sends the RS message (either directly via Direct
interfaces, via INET encapsulation for NATed interfaces, via a VPN
for VPNed interfaces, via a Proxy for proxyed interfaces or via a
Relay for native 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 AERO address 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, and caches
the other RA configuration information including Cur Hop Limit, M
and O flags, Reachable Time and Retrans Timer. The Client then
autoconfigures AERO addresses 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 AERO
address and with the initial OMNI option ifIndex-tuple corresponding
to the underlying interface. The Client sets a "primary" flag in the
OMNI option if it wishes to enable proxy keepalives on this
underlying interface.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 a first
ifIndex-tuple specific to the selected underlying interface, 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 AERO addresses 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 AERO address 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. If an SLLAO was present, the Server also compares the SLLAO
address information for the first ifIndex-tuple with the SPAN/INET
information to determine if there is a NAT on the path.Next, the Server prepares an RA message using its AERO address 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 the symmetric neighbor cache entry, and 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 an
ifIndex-tuple with ifIndex set to 0. 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 the Client's symmetric neighbor cache entry
set to expire after Router Lifetime 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, the Server
deletes the neighbor cache entry and withdraws the MNP without
delay.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 require a perimeter security
gateway to enable 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 AERO
address, 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 AERO address. 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 then replaces the SLLAO
in the RS message (if present) with a new SLLAO with a single
ifIndex-tuple matching the first ifIndex-tuple in the OMNI option
and with the Link Layer Address and Port Number fields set to the
Proxy's SPAN address. The Proxy finally encapsulates the (proxyed)
RS message in a SPAN header with destination set to the Server's
SPAN address 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 SPAN address as the link-layer address.
The Server then sends an RA message back to the Proxy via the
SPAN.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 ANET address, caches the
Server's advertised Router Lifetime and sets the neighbor cache
entry state to REACHABLE. The Proxy then replaces the RA SLLAO
with an SLLAO with its own ANET address, sets the P bit in the RA
flags field, sets the OMNI option "primary" flag according to the
cached value from the RS, optionally rewrites the Router Lifetime
and forwards the (proxyed) message to the Client. If the RA
included an MTU option, the Proxy rewrites the MTU value (if
necessary) to the minimum of the received MTU value and the MTU of
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 Relay via the SPAN. The Proxy instead forwards any
Client data destined to an asymmetric neighbor cache target directly
to the target according to the 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. If the "primary" flag was set, the Proxy performs periodic
RS/RA exchanges on the Client's behalf according to the cached Server
lifetime. 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
deletes the neighbor cache entry and 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
processsing 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 Relay. The Relay 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.When a neighbor cache entry transitions to DEPARTED, some of the
fragments of a multiple fragment packet may have already arrived at
the Proxy while others are en route to the Client's new location,
however no special attention in the reassembly algorithm is necessary
when re-routed packets are simply treated as loss. Since the fragments
of a multiple-fragment packet are sent in minimal inter-packet delay
bursts, such occasions will be rare.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 native 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 AERO address and with destination
address set to the AERO address 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 SPAN with the Proxy's AERO address as the source
and the AERO address 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.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, NATed and Direct interfaces, the Server
is the ROS.For Clients on Proxyed interfaces, the Proxy is the ROS.For Clients on native interfaces, the Client itself is the
ROS.For correspondent nodes on INET/EUN interfaces serviced by a
Gateway, the Gateway is the ROS.The route optimization procedure is conducted between the ROS and
the target Server/Gateway 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
Gateway.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 to receive a solicited NA message from the ROR.When the ROS sends an NS, it includes the AERO address of the ROS
as the source address (e.g., fe80::1) and the AERO address
corresponding to the data packet's destination address as the
destination address (e.g., if the destination address is
2001:db8:1:2::1 then the corresponding AERO address is
fe80::2001:db8:1:2). The NS message includes an OMNI option with a
single ifIndex-tuple with ifIndex set to 0, and an SLLAO with the
SPAN address of the ROS. The message also includes a Nonce and
Timestamp option if the ROS needs to correlate NA replies.The ROS then encapsulates the NS message in a SPAN header with
source set to its own SPAN address and destination set to the data
packet's destination address, then sends the message into the SPAN
without decrementing the network-layer TTL/Hop Limit field.When the Relay receives the NS 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 Relay then forwards the SPAN message toward
the ROR the same as for any IPv6 router. The final-hop Relay in the
SPAN will deliver the message via a secured tunnel to the ROR.When the ROR receives the NS message, it examines the AERO
destination 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 NS 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 message SPAN destination
address to the SPAN address of the Client's new Server, forwards
the message into the SPAN 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 destination AERO address of the NS, and
includes the Nonce value received in the NS plus the current
Timestamp.If the target belongs to an MNP Client, the ROR then includes an
OMNI option with prefix registration length set to the length of the
MNP; 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.)The ROR next includes a first ifIndex-tuple in the OMNI option
with ifIndex set to 0. If the target belongs to an MNP Client, the
ROR next includes additional ifIndex-tuples in the OMNI option for
each of the target Client's underlying interfaces with current
information for each interfaceThe ROR then includes a TLLAO option with ifIndex-tuples in
one-to-one correspondence with the tuples that appear in the OMNI
option. For NATed, VPNed and Direct interfaces, the link layer
addresses are the SPAN address of the ROR. For Proxyed interfaces,
the link-layer addresses are the SPAN addresses of the Proxy's INET
interfaces. For native interfaces, the link-layer addresses are the
SPAN addesses of the Client's native interfaces.The ROR finally encapsulates the NA message in a SPAN header with
source set to its own SPAN address and destination set to the source
SPAN address of the NS message, then forwards the message into the
SPAN without decrementing the network-layer TTL/Hop Limit field.When the Relay 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 Relay then forwards the SPAN-encapsulated
NA message toward the ROS the same as for any IPv6 router. The
final-hop Relay in the SPAN will deliver the message via a secured
tunnel to the ROS.When the ROS receives the solicited NA message, it caches the
source SPAN address then discards the INET and SPAN headers. The ROS
next verifies the Nonce and Timestamp values (if present), then
creates an asymmetric neighbor cache entry for the ROR and caches
all information found in the solicited NA 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 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 SPAN address of the ROR as the NS SPAN destination
address, and sends up to MAX_UNICAST_SOLICIT NS 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 the forward path is still
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
. NUD is performed either reactively in
response to persistent link-layer errors (see ) or proactively to confirm reachability. The NUD
algorithm is based on periodic authentic NS/NA 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 RS/RA exchanges may be considered
as acceptable hints of forward progress, while spurious data packets
should not be.When an ROR directs an ROS to a neighbor with one or more target
link-layer addresses, the ROS can proactively test each direct path by
sending an initial NS message to elicit a solicited NA response. While
testing the paths, the ROS can optionally continue sending packets via
the SPAN, maintain a small queue of packets until target reachability
is confirmed, or (optimistically) allow packets to flow via the direct
paths. In any case, the ROS should only consider the neighbor
unreachable if NUD fails over multiple target link-layer address
paths.When a ROS sends an NS message used for NUD, it uses its AERO
addresses as the IPv6 source address and the AERO address
corresponding to a target link-layer address as the destination. For
each target link-layer address, the source node encapsulates the NS
message in SPAN/INET headers with its own SPAN address as the source
and the SPAN address of the target as the destination, If the target
is located within the same SPAN segment, the source sets the INET
address of the target as the destination; otherwise, it sets the INET
address of a Relay as the destination. The source then forwards the
message into the SPAN.Paths that pass NUD tests are marked as "reachable", while those
that do not are marked as "unreachable". These markings inform the
AERO interface forwarding algorithm specified in .Proxys can perform NUD to verify Server reachability on behalf of
their proxyed Clients to reduce Client-initated control messaging
overhead.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 a Mobility Anchor Point (MAP) 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
perform the RS/RA exchanges on the Client's behalf.Mobility management considerations are specified in the following
sections.Servers acting as MAPs accommodate Client mobility and/or QoS
change events by sending unsolicited NA messages to each ROS in the
target Client's Report List. When a MAP sends an unsolicited NA
message, it sets the IPv6 source address to the Client's AERO
address and sets the IPv6 destination address to All-Nodes
multicast. The MAP also includes an OMNI option with prefix
registration information, with a first ifIndex-tuple with ifIndex
set to 0, and with additional ifIndex-tuples for the target Client's
remaining interfaces. The MAP then includes a TLLAO with
corresponding ifIndex-tuples, with the link layer address of the
first tuple set to the MAP's SPAN address and with link layer
addresses of the remaining tuples set to the corresponding target
SPAN addresses. The MAP finally encapsulates the message in a SPAN
header with source set to its own SPAN address and destination set
to the SPAN address of the ROS, then sends the message to a Relay
which in turn forwards it to the ROS.As discussed in Section 7.2.6 of , the
transmission and reception of unsolicited NA messages is unreliable
but provides a useful optimization. In well-connected Internetworks
with robust data links unsolicited NA messages will be delivered
with high probability, but in any case the MAP can optionally send
up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs to each ROS to
increase the likelihood that at least one will be received.When the ROS receives an unsolicited NA 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 unsolicited NA 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 state information.In addition to sending unsolicited NA messages to the current set
of ROSs for the Client, the MAP also sends unsolicited NAs to the
former link-layer address for any ifIndex-tuple for which the
link-layer address has changed. The NA messages update Proxys or
Servers that cannot easily detect (e.g., without active probing)
when a formerly-active Client has departed.When a Client needs to change its ANET 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 SPAN with an OMNI
option and SLLAO that include an ifIndex-tuple with the new link
quality and address information.Up to MAX_RTR_SOLICITATION 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
unsolicited NA 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 with appropriate link quality values and with an SLLAO (if
necessary) 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 message to its Server with an OMNI option with appropriate
link quality values.If the Client needs to send RS messages over an underlying
interface other than the one being removed from service, it MUST
include an ifIndex-tuple for the sending interface as the first
tuple and include additional 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 a notification identifier in the RS message
OMNI option per if
it wants the new Server to notify the old Server.When the new Server receives the Client's RS message, it responds
by returning an RA as specified in .
If the Client's RS includes a notification identifier, the new
Server also prepares an RS or unsolicited NA message to send to the
old Server. The RS/NA message includes the Client's AERO address as
the source address, the old Server's AERO address as the destination
address, and an OMNI option and S/TLLAO with an ifIndex-tuple with
ifIndex set to 0. The OMNI option includes a prefix release
indication, and the S/TLLAO includes the SPAN address of the new
Server. For RS messages, the new Server retries up to
MAX_RTR_SOLICITATIONS attempts until an RA is received. (Note that
the Client can alternatively send RS/NA messages with a release
indication to the old Server on its own behalf, however, this
additional Client messaging may be undesirable in some environments.
Note also that the choice of using RS or unsolicited NA is based on
the need for a reliable acknowledgement; in environments where
Router Lifetimes can be expected to be short, sending up to
MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs may be sufficient.)When the old Server processes the RS/NA, it changes the symmetric
neighbor cache entry state to DEPARTED, sets the link-layer address
of the Client to the address found in the S/TLLAO, and sets
DepartTime to DEPARTTIME seconds. For RS messages, the old Server
then returns an immediate RA message with Router Lifetime set to 0.
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 symmetric 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 prefix release
indication, with a single ifIndex-tuple with ifIndex set to 0 and
with the SPAN address 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. Subsequent data packets will then flow according to any
existing cached link-layer information and trigger a new NS/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 SPAN 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. Since the fragments of a
multiple-fragment packet are sent in a minimal inter-packet delay
burst, such occasions will be rare.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 Gateways 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/Gateway) "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 AERO address 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 SPAN address of X and destination address set to S then
forwards the message into the SPAN. The SPAN in turn forwards the
message to AERO Server/Gateway "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 AERO address
for the prefix that matches S as the network-layer source address
and TLLAOs with the SPAN addresses corresponding to any
ifIndex-tuples that are currently servicing S.When Y processes the Join/Prune message, if S located behind any
Native, Direct, VPNed or NATed 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 SPAN while continuing to use the AERO address of X
as the source address. Each Z* then updates its MRIB accordingly and
maintains the AERO address of X as the next hop in the reverse path.
Since the Relays in the SPAN 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
SPAN segment as X, the multicast data traffic sent to X directly
using SPAN/INET encapsulation instead of via a Relay.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 SPAN.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 AERO address 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 AERO address
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 SPAN 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 SPAN. X uses its own AERO address
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 SPAN address of X and destination address set to
R, then sends the message into the SPAN. 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
SPAN. R may then elect to send a PIM Join to Z* over the SPAN. 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 Relays, Servers and Proxys,
thereby providing redundancy in case of failures.The Relays, Servers and Proxys on each AERO link can assign AERO
and SPAN addresses 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 AERO address 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
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 label (e.g.,
colors such as Red, Green, Blue, etc.). In particular, the Client can
tag its RS messages with the appropriate label to cause the network to
select the desired VLAN.Clients that connect to multiple AERO interfaces can select the
outgoing interface appropriate for a given Red/Blue/Green/etc. 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
Relays.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 Relays on the link. Correspondent nodes then
include a "type 4" routing header with the Anycast address for the
AERO link as the IPv6 destination and with the address of the target
encoded as the "next segment" in the routing header . Standard IP routing
will then direct the packet to the nearest Relay for the correct AERO
link, which will replace the destination address with the target
address then forward the packet 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 Gateway NAT64 mapping caches. In that way, an IPv4 correspondent
node can send packets to the IPv4 address mapping of the target MN,
and the Gateway 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.The SPAN 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 SPAN segments. This can be
accomplished by incrementally deploying AERO Gateways on each INET
partition, with each Gateway 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
(bridged by the SPAN) 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 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 Gateways.Gateways that connect INETs/EUNs with dissimilar IP protocol
versions must employ a network address and protocol translation
function such as NAT64.In environments where rapid failure recovery is required, Servers
and Relays 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 Relays maintain BFD sessions in parallel with their BGP
peerings. If a Server or Relay 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 Relays 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.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.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.No further IANA actions are required.AERO Relays configure secured tunnels with AERO Servers and Proxys
within their local SPAN segments. Applicable secured tunnel alternatives
include IPsec , TLS/SSL , DTLS , etc. The AERO Relays
of all SPAN segments in turn configure secured tunnels for their
neighboring AERO Relays across the SPAN. Therefore, packets that
traverse the SPAN between any pair of AERO link neighbors are already
secured.AERO Servers, Gateways and Proxys targeted by a route optimization
may also receive packets directly from the INET partitions instead of
via the SPAN. For INET partitions that apply effective ingress filtering
to defeat source address spoofing, the simple data origin authentication
procedures in can be applied. This implies
that the ROS list must be maintained consistently by all route
optimization targets within the same INET partition, and that the ROS
list must be securely managed by the partition administrative
authority.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 via
the SPAN, 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 enclaves need not apply security
to their ND messages, since the messages will be intercepted by a
perimeter Proxy that applies security on its outward-facing interface.
AERO Clients located outside of secured enclaves SHOULD use symmetric
network and/or transport layer security services, but when there are
many prospective neighbors with dynamically changing connectivity an
asymmetric security service such as SEND may be needed (see: ).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 Relays 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 Relays 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 Gateways must implement ingress filtering to avoid a spoofing
attack in which spurious SPAN messages 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 and ROS lists 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.). The
ROS list can be conveyed to Servers and Proxys through administrative
action, secured file distribution, etc.Although public domain and commercial SEND implementations exist,
concerns regarding the strength of the cryptographic hash algorithm have
been documented .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, Ralph
Droms, Adrian Farrel, Nick Green, Sri Gundavelli, Brian Haberman,
Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak, Lee Howard,
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
to REACHABLETIME seconds 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 Relays in the
communications path. Direct interfaces must be tested periodically for
reachability, e.g., via NUD.AERO Clients that connect to the open Internetwork via either a
native or NATed interface can establish a VPN to securely connect to a
Server. Alternatively, the Client can exchange ND messages directly
with other AERO nodes on the same SPAN segment using INET
encapsulation only and without joining the SPAN. In that case,
however, the Client must apply asymmetric security for ND messages to
ensure routing and neighbor cache integrity (see: ).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 AERO addresses 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 AERO addresses 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.SEcure Neighbor Discovery (SEND) and
Cryptographically Generated Addresses (CGAs)
were designed to secure IPv6 ND messaging in environments where
symmetric network and/or transport-layer security services are
impractical (see: ). AERO nodes that use
SEND/CGA employ the following adaptations.When a source AERO node prepares a SEND-protected ND message, it
uses a link-local CGA as the IPv6 source address and writes the prefix
embedded in its AERO address (i.e., instead of fe80::/64) in the CGA
parameters Subnet Prefix field. When the neighbor receives the ND
message, it first verifies the message checksum and SEND/CGA
parameters while using the link-local prefix fe80::/64 (i.e., instead
of the value in the Subnet Prefix field) to match against the IPv6
source address of the ND message.The neighbor then derives the AERO address of the source by using
the value in the Subnet Prefix field as the interface identifier of an
AERO address. For example, if the Subnet Prefix field contains
2001:db8:1:2, the neighbor constructs the AERO address as
fe80::2001:db8:1:2. The neighbor then caches the AERO address in the
neighbor cache entry it creates for the source, and uses the AERO
address as the IPv6 destination address of any ND message replies.AERO Relays can be either Commercial off-the Shelf (COTS) standard
IP routers or virtual machines in the cloud. Relays must be
provisioned, supported and managed by the INET administrative
authority, and connected to the Relays of other INETs via inter-domain
peerings. Cost for purchasing, configuring and managing Relays 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 Relays, 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 Gateways can be any dedicated server or COTS router platform
connected to INETs and/or EUNs. The Gateway joins the SPAN and engages
in eBGP peering with one or more Relays as a stub AS. The Gateway then
injects its MNPs and/or non-MNP prefixes into the BGP routing system,
and provisions the prefixes to its downstream-attached networks. The
Gateway can perform ROS and MAP 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 REACHABLETIIME 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
AERO addresses 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-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 REACHABLETIME 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 tha paralles 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_ADVERTISEMENTS 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.