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 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 tunneling
solution 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 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 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 should
extend 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 form of PD
known as "Prefix Assertion" can be used if the prefix can be
represented 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 an Internetworking backbone service. 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.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.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 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 one or
more AERO Servers. Following PD, the Client assigns a Client AERO
address to the AERO interface for use in ND exchanges with other
AERO nodes. A Client can also be deployed on the same platform as a
Server, and a node that acts as a Client on one AERO interface can
also act as an AERO Server on a different AERO interface.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 the
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 INETs as segments (or, partitions) 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.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 AERO link
segment.a global or unique local IPv6
address taken from a SPAN Partition Prefix.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 UDP encapsulation is used,
the UDP 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.a geographically and/or
topologically referenced list of IP addresses of Servers for the
AERO link.a
BGP-based overlay routing service coordinated by Servers and Relays
that tracks all MAP-to-Client associations.the AERO node
nearest the source that initiates route optimization. The ROS may be
a Server, Proxy or in some cases even the Client itself.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.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. 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
Lower case uses of these words are not to be interpreted as carrying
RFC2119 significance.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 INETs 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 (i.e., INETs) 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 and between
disjoint INETs 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 receive MNPs through PD exchanges with AERO Servers
over the AERO link, and distribute the MNPs to nodes on EUNs. Each
Client can associate with a single Server or with multiple Servers
(e.g., for fault tolerance, load balancing, etc). 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
and without the need for any Client/Server control messaging.AERO Proxys provide a conduit for AERO Clients in ANETs to
associate with AERO Servers in external INETs. Client and Servers
exchange control plane messages via the Proxy, which intercepts them
before they leave the ANET. The Proxy forwards data packets to and
from Clients 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.For IPv6 MNPs, the AERO routing system includes only IPv6 routes.
For IPv4 MNPs, the AERO routing system includes both IPv4 routes and
also IPv6 routes based on the IPv4-mapped IPv6 address corresponding
to the MNP and with prefix length set to 96 plus the length of the
IPv4 prefix. (For example, if the IPv4 MNP is 192.0.2.0/24 then the
IPv4-mapped prefix is 0:0:0:0:0:FFFF:192.0.2.0/120.)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 segment 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.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 .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:192.0.2.16/124The 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 (e.g., fe80::1,
fe80::2, fe80::3, etc.) as assigned by the administrative authority
for the link. If the link spans multiple segments (i.e., multiple
INETs), the AERO addresses are assigned to each INET in 1x1
correspondence with SPAN addresses (see: ). The
address fe80:: is reserved as the IPv6 link-local Subnet Router
Anycast address , and the address
fe80::ffff:ffff is reserved as the unspecified AERO address; hence,
these values are not available general assignment.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 zeros. For example, for the AERO address fe80::2001:db8:1:2
the subnet router anycast address is 2001:db8:1:2::/64.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 is 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.In the simplest case, 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 encapsulation with INET addresses, 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 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 /96 IPv6 "SPAN
Service Prefix (SSP)". Although any routable IPv6 prefix can be used,
a Unique Local Address (ULA) prefix (e.g., fd00::/96) 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 the SSP
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 a local consideration for each administrative authority.A "SPAN address" is an address taken from a SPP and assigned to a
Relay, Server 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 segment. 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 node in segment A forwards a packet with IPv6 address
2001:db8:1:2::1 to a destination 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.)AERO interfaces use encapsulation (see: ) to exchange packets with neighbors attached to
the AERO link.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 interface ND messages include one or more Source/Target
Link-Layer Address Options (S/TLLAOs) formatted as shown in :In this format:Type is set to '1' for SLLAO or '2' for TLLAO.Length is set to the constant value '5' (i.e., 5 units of 8
octets).Prefix Length is set to the MNP prefix length in an ND message
for the Client AERO address found in the source (RS), destination
(RA) or target (NA) address; otherwise set to 0. If the message
contains multiple SLLAOs, only the Prefix Length value in the
SLLAO with S set to 1 is consulted and the values in other SLLAOs
are ignored.S (the 'Source' bit) is set to '1' in the S/TLLAO of an ND
message that corresponds to the ANET/INET interface over which the
ND message is sent, and set to 0 in all other S/TLLAOs.R (the "Release" bit) is set to '1' in an S/TLLAO in an RS/NA
sent for the purpose of departing from a Server; otherwise, set to
'0'. The recipient places the corresponding neighbor cache entry
in the DEPARTED state. For RS message, the recipient then releases
the corresponding PD and returns an RA with Router Lifetime set to
'0'D (the "Disable" bit) is set to '1' in the S/TLLAOs of an RS/NA
message for each Interface ID that is to be disabled in the
neighbor cache entry; otherwise, set to '0'.X (the "proXy" bit) is set to '1' in the SLLAO of an RS/RA
message by the Proxy when there is a Proxy in the path; otherwise,
set to '0'. If the message contains multiple SLLAOs, only the X
value in the first SLLAO is consulted and the values in other
SLLAOs are ignored.N (the "(Network Address) Translator (NAT)" bit) is set to '1'
in the SLLAO of an RA message by the Server if there is a
translator in the path; otherwise, set to '0'. If the message
contains multiple SLLAOs, only the N value in the first SLLAO is
consulted and the values in other SLLAOs are ignored.Resvd is set to the value '0' on transmission and ignored on
receipt.Interface ID is set to a 16-bit integer value corresponding to
an AERO node's ANET/INET interface. Once the node has assigned an
Interface ID to an ANET interface, the assignment must remain
unchanged until the node fully detaches from the AERO link. The
value 0xffff is reserved as the Server's INET Interface ID, i.e.,
Servers MUST use Interface ID 0xffff, and Clients MUST number
their ANET Interface IDs with values in the range of 0-0xfffe.Port Number and Link Layer Address are set to the encapsulation
addresses required to send packets via the target node (or to '0'
when the addresses are left unspecified). When UDP is not used as
part of the encapsulation, Port Number is set to '0'. When the
encapsulation IP address family is IPv4, IP Address is formed as
an IPv4-mapped IPv6 address as specified in .P(i) is a set of Preferences that correspond to the 64
Differentiated Service Code Point (DSCP) values . Each P(i) is set to the value '0'
("disabled"), '1' ("low"), '2' ("medium") or '3' ("high") to
indicate a QoS preference level for packet forwarding
purposes.A Client's AERO interface may be configured over multiple ANET
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.A Client's ANET interfaces are classified as follows:Native interfaces connect to the open INET, and have a global
IP address that is reachable from any INET correspondent.NATed interfaces connect to an ANET 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 over the ANET 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 the 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.If a Client's multiple ANET interfaces are used "one at a time"
(i.e., all other interfaces are in standby mode while one interface is
active), then ND messages include only a single S/TLLAO with Interface
ID set to a constant value. In that case, the Client would appear to
have a single ANET interface but with a dynamically changing ANET
address.If the Client has multiple active ANET interfaces, then from the
perspective of ND it would appear to have multiple link-layer
addresses. In that case, ND messages MAY include multiple S/TLLAOs --
each with an Interface ID that corresponds to a specific ANET
interface. S must be set to 1 in the S/TLLAO corresponding to the AERO
node's ANET interface used to transmit the message and set to 0 in all
other S/TLLAOs.When the Client includes an S/TLLAO for an ANET interface for which
it is aware that there is a NAT on the path to the Server, or when a
node includes an S/TLLAO solely for the purpose of announcing new QoS
preferences, the node MAY set both Port Number and Link-Layer Address
to 0 to indicate that the addresses are unspecified at the network
layer and must instead be derived from the link-layer encapsulation
headers.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 also configure the same SPAN
address. The AERO interface encapsulates each packet in a SPAN header
if necessary 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 same SPAN address. Routing protocols
such as BGP therefore run directly over the 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 packets
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. 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.When a Proxy enables an AERO interface, it assigns AERO/SPAN
addresses the same as for Servers. The Proxy further maintains
permanent neighbor cache entries for neighbors in the same SPAN
segment, and maintains per-Client neighbor cache entries based on
control message exchanges.When a Client enables an AERO interface, it sends RS messages
with ND/PD parameters over an ANET interface to one or more Servers,
which return RA messages with corresponding PD 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 desired.
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
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 as well as for their neighboring Relays. (Relays
also maintain permanent neighbor cache entries for their neighbors if
they configure an AERO interface.) Each entry maintains the mapping
between the neighbor's network-layer AERO address and corresponding
INET address.Symmetric neighbor cache entries are created and maintained through
RS/RA exchanges as specified in , and remain in
place for durations bounded by ND/PD lifetimes. AERO Servers maintain
symmetric neighbor cache entries for each of their associated Clients,
and AERO Clients maintain symmetric neighbor cache entries for each of
their associated Servers.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 each of their active target Clients with
lifetimes based on ND messaging constants. Asymmetric neighbor cache
entries are unidirectional since only the ROS and not the target
(i.e., the 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 state of
each of the Client's associated Servers.To the list of neighbor cache entry states in Section 7.3.2 of
, 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 40
seconds to allow for packets in flight to be delivered while stale
route optimization state may be present.When a target Server (acting as a Mobility Anchor Point (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 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 40 seconds to allow a 10 second window so that route
optimization can 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.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). Packets
that enter the AERO interface from the network layer are encapsulated
and forwarded into the AERO link, i.e., they are tunneled 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 ANET/INET interfaces
and/or neighbor cache entries for neighbors with multiple Interface ID
registrations (see ). The AERO interface
uses each packet's DSCP value (and/or port number) to select an
outgoing ANET/INET interface based on the node's own QoS preferences,
and also to select a destination link-layer address based on the
neighbor's ANET/INET 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 the
interface with the highest preference. AERO nodes keep track of which
ANET/INET interfaces are currently "reachable" or "unreachable", and
only use "reachable" interfaces for forwarding purposes.For control messages, the source node encapsulates the message in
SPAN/INET headers and forwards the message into the SPAN. For data
packets, if the neighboring node can only be reached via the SPAN (or,
if it is not yet know that the neighboring node is within the local
segment) the source node forwards them into the SPAN. Otherwise, the
source node encapsulates packets in only an INET header for
transmission within the local segment.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 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 to a Server (which may be
intercepted by a Proxy).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 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 as the
Proxy, the Proxy uses simple INET encapsulation; otherwise the
Proxy forwards the packet into the SPAN.else, the Proxy forwards the packet into the SPAN 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 the
INET, it 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 forwards the packet to the Client; if the
neighbor cache entry is in the DEPARTED state, the Proxy instead
forwards the packet to the Client's Server and returns an
unsolicited NA message as discussed in . If
there is no neighbor cache entry, the Proxy discards the packet.For control messages destined to a target Client that are
received from the SPAN, 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 via the SPAN in
response to an NS message directed to one of its associated
Clients.)For control messages originating from a source Client that are
received from the SPAN, the Server sends an appropriate response to
the same Client. (For example, the Server sends an RA message reply
via the SPAN in response to an RS message sourced by one of its
associated Clients.)When the Server's AERO interface receives a data packet from the
link-layer (i.e., from an INET neighbor or from a SPAN secured
tunnel), it decapsulates and processes the packet according to the
network-layer destination address as follows:if the destination matches a symmetric neighbor cache entry
the Server forwards the packet according to the neighbor cache
state and link-layer address information. If the neighbor cache
entry is in the REACHABLE state, the Server forwards the packet
according to the cached link-layer information. If the neighbor
cache entry is in the DEPARTED state, the Server instead
forwards the packet to the Client's new Server as discussed in
. If the packet is destined to the same
Client from which it arrived, however, the Server must forward
the packet via a different "reachable" neighbor interface than
the one the packet arrived on. If there are no "reachable"
neighbor interfaces, the Server drops the packet.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
encapsulation.else, if the destination is an administrative AERO address
that is not assigned on the AERO interface the Server forwards
the packet into the SPAN while using the SPAN address
corresponding to the destination as the SPAN destination
address. If the packet arrived from the SPAN, however, the
Server instead drops the packet to avoid looping.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, 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 forwards the packet into the SPAN. For AERO
address destinations, the Server uses the SPAN address
corresponding to the destination as the SPAN destination
address; for others, the Server uses the packet's destination IP
address as the SPAN destination addressRelays 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 route to an MNP 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 drops the packet and returns an ICMP Destination
Unreachable message subject to rate limiting (see: ).else, if the destination matches a route to a non-MSP the
Relay forwards the packet via a secured tunnel to the next
hop.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 inner packet header.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". Note that
Clients can avoid encapsulation when the first-hop access router is
AERO-aware.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 .The AERO interface encapsulates the packet according to the next
hop determined in the forwarding algorithm in . If the next hop is reached via a secured tunnel,
the AERO interface encapsulates the packet in a SPAN header and 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 encapsulates the
packet per the Generic UDP Encapsulation (GUE) procedures in , or through an alternate
encapsulation format (see: ).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 encapsulated 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.As the final aspect of encapsulation, the AERO interface 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 for
encapsulated packets they receive from other nodes on the AERO link.
In particular:AERO Relays, Servers and Proxys accept data packets and control
messages received from Relays via secured tunnels.AERO Clients and Servers accept encapsulated packets if there
is a symmetric neighbor cache entry with a link-layer address that
matches the packet's link-layer source address.AERO Proxys accept encapsulated packets if there is a proxy
neighbor cache entry that matches the packet's network-layer
address.Each packet should include a signature that the recipient can
use to authenticate the message origin, e.g., as for common VPN
systems such as OpenVPN . In some environments,
however, it may be sufficient to require signatures only for ND
control plane messages (see: ) and omit
signatures for data plane messages.The AERO interface is the node's attachment to the AERO link. The
AERO interface acts as a tunnel ingress when it sends a packet to an
AERO link neighbor and as a tunnel egress when it receives a packet
from an AERO link neighbor. 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 such as denial of
service, 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 two 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. Note that an IPv4 packet that incurs source fragmentation
may also incur network fragmentation.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
(MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
operational assurance that a larger size can traverse the link along
all paths.All AERO nodes MUST configure the same MTU value for reasons cited
in ; in particular,
multicast support requires a common MTU value among all nodes on the
link. All AERO nodes MUST configure an MRU large enough to reassemble
packets up to (MTU+ENCAPS) bytes in length; nodes that cannot
configure a large-enough MRU MUST NOT enable an AERO interface. For
example, for an MTU of 1500 bytes (or slightly larger) an appropriate
MRU might be 2KB.The network layer proceeds as follows when it presents an IP packet
to the AERO interface. For each IPv4 packet that is larger than the
AERO interface MTU and with the DF bit 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 the AERO interface, the interface
(acting as a tunnel ingress) encapsulates the packet. If the
encapsulated packet is larger than the MSU the ingress
source-fragments the encapsulated 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 admits each encapsulated packet or fragment into the
tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
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.Several factors must be considered when fragmentation is needed.
For AERO links over IPv4, the IP ID field is only 16 bits in length,
meaning that fragmentation at high data rates could result in data
corruption due to reassembly misassociations . In environments where IP
fragmentation issues could result in operational problems, the ingress
SHOULD employ intermediate-layer source fragmentation (see: and ) before appending the outer
encapsulation headers to each fragment. Since the encapsulation
fragment header reduces the room available for packet data, but the
original source has no way to control its insertion, the ingress MUST
include the fragment header length in the ENCAPS length even for
packets in which the header is absent.When an AERO node admits encapsulated packets 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 MAY initiate NUD over that path. If it receives
Destination Unreachable messages on many or all paths, the node
SHOULD set ReachableTime for the corresponding asymmetric neighbor
cache entry to 0 and allow future packets destined to the
correspondent to flow through a default route.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. If it receives Destination Unreachable messages on multiple
paths, the Server should take no further actions unless it
receives an explicit ND/PD release message or if the PD lifetime
expires. In that case, the Server MUST release the Client's
delegated MNP, withdraw the MNP from the AERO routing system and
delete the neighbor cache entry.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. Therefore, no Server-to-Server PD state
synchronization is necessary, and Clients can optionally hold
separate PDs for the same MNPs from multiple Servers. Clients can
receive new PDs from new Servers before releasing PDs received from
existing Servers for service continuity. Clients receive the same
service regardless of the Servers they select, although selecting
Servers that are topologically nearby may provide better
routing.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
configuration information in response to a Client's RS message.
Thereafter, Clients send additional RS messages to refresh prefix
and/or router lifetimes.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 include its AERO address as the source address of an RS
message and with an SLLAO with a valid Prefix Length 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 can discover the INET and AERO addresses of Servers
in the MAP list via 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 . 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 Client's ANET
interface (e.g., "example.com"). Alternatively, the Client can
discover the Server's address through a multicast RS as described
below.To associate with a Server, the Client acts as a requesting
router to request MNPs. The Client prepares an RS message with PD
parameters (e.g., with an SLLAO with non-zero Prefix Length). If the
Client already knows the Server's AERO address, it includes the AERO
address as the network-layer destination address; otherwise, it
includes all-routers multicast (ff02::2) as the network-layer
destination address. 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 SLLAO in the RS message formatted as
described in to register its link-layer
information with the Server. The SLLAO corresponding to the ANET
interface over which the Client will send the RS message MUST set S
to 1. The Client MAY include additional SLLAOs specific to other
underlying interfaces, but if so it MUST set their S, Port Number
and Link Layer Address fields to 0. If the Client is connected to an
ANET for which encapsulation is required, the Client finally
encapsulates the RS message in an ANET header with its own ANET
address as the source address and the INET address of the Server as
the destination.The Client then sends the RS message (either via a VPN for VPNed
interfaces, via a Proxy for proxyed interfaces or via the SPAN for
native interfaces) and waits for an RA message reply (see ) while retrying 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
address in the first SLLAO as the Server's INET 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. 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 its AERO interface while configuring an appropriate
MRU.The Client then registers additional ANET interfaces with the
Server by sending RS messages via each additional ANET 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 an SLLAO specific to the ANET interface. (The RS
messages include PD parameters the same as for the initial exchange
so that the additional ANETs can register the PD information.)The Client examines the X and N bits in the SLLAO with S set to 1
in each RA message it receives. If X is 1 the Client infers that
there is a Proxy on the path, and if N is 1 the Client infers that
there is a NAT on the path. If N is 1, the Client SHOULD set Port
Number and Link-Layer Address to 0 in the first S/TLLAO of any
subsequent ND messages it sends to the Server over that link.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 maintains its MNP delegations
through each of its Servers by sending additional RS messages before
Router Lifetime expires.After the Client registers its ANET interfaces, it may wish to
change one or more registrations, e.g., if an ANET 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
ANET interface. The RS MUST include an SLLAO with S set to 1 for the
selected ANET interface and MAY include any additional SLLAOs
specific to other ANET interfaces. The Client includes fresh P(i)
values in each SLLAO to update the Server's neighbor cache entry. If
the Client wishes to update only the P(i) values, it sets the Port
Number and Link-Layer Address fields to 0. If the Client wishes to
disable the underlying interface, it sets D to 1. 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 associate with multiple Servers, it
repeats the same procedures above for each additional Server. If the
Client wishes to discontinue use of a Server it issues an RS message
over any underlying interface with an SLLAO with R set to 1 . When
the Server processes the message, it releases the MNP, sets the
symmetric neighbor cache entry state for the Client to DEPARTED,
withdraws the IP route from the routing system and returns an RA
reply with Router Lifetime set to 0.AERO Servers act as IP routers and support a PD service for
Clients. Servers arrange to add their AERO and INET 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. The list of Server addresses should be geographically
and/or topologically referenced, and forms the MAP list for 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 an IP
forwarding table entry for each MNP so that the MNPs are propagated
into the routing system (see: ). For IPv6,
the Server creates a single IPv6 forwarding table entry for each
MNP. For IPv4, the Server creates both an IPv4 forwarding table
entry and an IPv6 forwarding table entry with the IPv4-mapped IPv6
address 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 SLLAO in the RS indexed by the Interface ID and
including the Port Number, Link Layer Address and P(i) values. For
the SLLAO with S set to 1, however, the Server records the actual
INET header source addresses instead of those that appear in the
SLLAO in case there was a NAT in the path. The Server also records
the value of the X bit to indicate whether there is a Proxy 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 includes the delegated MNPs, any other PD parameters and
an SLLAO with the Link Layer Address set to the Server's SPAN
address and with Interface ID set to 0xffff. The Server then
includes one or more RIOs that encode the MSPs for the AERO link,
plus an MTU option for the link MTU (see ).
The Server finally encapsulates the message in a SPAN header with
source address set to its own SPAN address and destination address
set to the Client's (or Proxy's) SPAN address, then forwards the
message into the SPAN.After the initial RS/RA exchange, the Server maintains the
symmetric neighbor cache entry for the Client. If the Client (or
Proxy) issues additional NS/RS messages, the Server resets
ReachableTime. If the Client (or Proxy) issues an RS with PD release
parameters (e.g., by including an SLLAO with R set to 1), or if the
Client becomes unreachable, the Server sets the Client's symmetric
neighbor cache entry to the DEPARTED state and withdraws the IP
routes from the AERO routing system.The Server processes these and any other Client ND/PD messages,
and returns an NA/RA reply. 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.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 information in all of the SLLAOs from the RS message
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 SLLAOs echoed in the Interface-Id
option.Clients may connect to ANETs that do not support direct
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 RS message
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 the Proxy
selects a "nearby" Server that is likely to be a good candidate to
serve the Client and replaces the RS destination address with the
Server's AERO address. Next, the Proxy creates a proxy neighbor
cache entry and caches the Client and Server addresses along with
any identifying information including Transaction IDs, Client
Identifiers, Nonce values, etc. The Proxy then examines the
address in the RS message SLLAO with S set to 1. If the address is
different than the Client's ANET address, the Proxy notes that the
Client is behind a NAT. The Proxy then sets the X to 1 and changes
the Link Layer Address to its own SPAN address. The Proxy finally
encapsulates the RS message in SPAN/INET headers using the SPAN
address of the Server as the SPAN destination address and the INET
address of a Relay as the INET destination address. The Proxy then
forwards the message to the Server via the SPAN.when the Server receives the RS message, 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 with a single SLLAO
back to the Proxy via the SPAN.when the Proxy receives the RA message, it matches the message
with the RS that created the proxy neighbor cache entry. The Proxy
then caches the PD route information as a mapping from the
Client's MNPs to the Client's ANET address, and sets the neighbor
cache entry state to REACHABLE. The Proxy then changes the SLLAO
Link Layer Address to its own ANET address, sets X to 1, sets N to
1 if the Client is behind a NAT, then re-encapsulates the RA
message in an ANET header and forwards it to the Client.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. Finally, the Proxy 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 continues
to send NS/RS messages to update each Server's symmetric neighbor
cache entries on behalf of the Client and/or to convey QoS updates. If
the Server ceases to send solicited NA/RA responses, the Proxy marks
the Server as unreachable and sends an unsolicited RA with Router
Lifetime set to zero to inform the Client that this Server is no
longer able to provide Service. If the Client becomes unreachable, the
Proxy sets the neighbor cache entry state to DEPARTED and sends an RS
message to each Server with an SLLAO with D set to 1 and with
Interface ID set to the Client's interface ID so that the Server will
de-register this Interface ID. Although the Proxy engages in these 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.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. This encapsulation
avoidance represents a form of "header compression", meaning that the
MTU should be sized based on the size of full encapsulated messages
even if most messages are sent unencapsulated.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. The Client includes an SLLAO with Interface ID,
Prefix Length and P(i) information but with Port Number and Link-Layer
Address set to 0.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.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 MAP 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 no SLLAOs, but SHOULD
include a Timestamp and Nonce option.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 it into the SPAN without
decrementing the network-layer TTL/Hop Limit field.When the Relay receives the (double-encapsulated) 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 IP router. The
final-hop Relay in the SPAN will deliver the message via a secured
tunnel to the ROR.When the ROR receives the (double-encapsulated) NS message, it
examines the AERO destination address to determine whether it has a
route that matches the target; if not, it drops the NS message and
returns from processing. Next, if the target belongs to an MNP
Client in the DEPARTED state the ROR (acting as a MAP) changes the
NS message SPAN destination address to the address of the Client's
new MAP, forwards the message into the SPAN and returns from
processing. If the target belongs to an MNP Client 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, 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. The ROR next includes a TLLAO with Interface ID set to
0xffff, with S set to 1, with all P(i) values set to "low", and with
Link Layer Address set to the ROR's SPAN address. If the target
belongs to an MNP Client, the ROR sets the Prefix Length to the MNP
prefix length; otherwise, it sets Prefix Length 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. Note also that prefix lengths longer than
/64 are out of scope for this specification.)If the target belongs to an MNP Client, the ROR next includes
additional TLLAOs for all of the target Client's Interface IDs. For
NATed, VPNed and Direct interfaces, the TLLAO Link Layer Addresses
are the SPAN address of the ROR. For Proxyed interfaces, the TLLAO
Link Layer Addresses are the SPAN addresses of the target Client's
Proxys, and for native interfaces the TLLAO Link Layer Addresses are
the SPAN addresses of the target Client.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 (double-encapsulated) 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 (double-encapsulated) solicited NA
message, it discards the INET and SPAN headers. The ROS next
verifies the Nonce and Timestamp values, then creates an asymmetric
neighbor cache entry for the ROR and caches all information found in
the solicited NA TLLAOs. 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 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. Future data packets flowing through
the ROS will again trigger a new route optimization exchange while
initial data packets travel over a suboptimal route via Servers
and/or Relays.The ROS may also receive unsolicited NA messages from the ROR at
any time. 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 route optimization source
simply discards the unsolicited NA. Cases in which unsolicited NA
messages are generated are specified in .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 may further be seeded by ND hints of forward progress, but
care must be taken to avoid inferring reachability based on spoofed
information.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, if the address is not located within
the same AERO link segment the source node encapsulates the NS message
in a SPAN header with its own SPAN address as the source and the SPAN
address of the target as the destination, then forwards the message
into the SPAN. If the target address is located within the same
segment, however, the source node omits the SPAN header and
encapsulates the message in an INET header with its own INET address
as the source and the INET address of the target as the destination,
then sends the message directly to the target.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 so that the Clients need not engage in NUD
messaging themselves.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 service 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.RORs acting as MAPs accommodate mobility and/or QoS change events
by sending an unsolicited NA message 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 (ff02::1). The
MAP also includes a TLLAO with Interface ID 0xffff, S set to 1 and
Link Layer address set to the MAP's SPAN address, and includes
additional TLLAOs for all of the target Client's Interface IDs with
Link Layer Addresses set to the corresponding 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 into the SPAN.As for the hot-swap of interface cards 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 an 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 TLLAOs to update the Link Layer
Address and QoS information in 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 to the target Client according to its current
neighbor cache information but may receive persistent unsolicited NA
messages as discussed in .When a Server acting as a MAP receives packets with destination
addresses that match a symmetric neighbor cache entry in the
DEPARTED state, it forwards the packets to the SPAN address
corresponding to the Client's new MAP. If the ROS is in the Report
List, the old MAP also sends an unsolicited NA message via the SPAN
(subject to rate limiting) with a TLLAO with Interface ID 0xffff and
with R set to 1. When the ROS receives the NA, it SHOULD delete the
asymmetric neighbor cache entry and re-initiate route
optimization.When a Proxy receives packets with destination addresses that
match a proxy neighbor cache entry in the DEPARTED state, it
forwards the packets to one of the target Client's MAPs. If the ROS
is not one of its proxy neighbor Clients, the Proxy also returns an
unsolicited NA message via the SPAN (subject to rate limiting) with
a single TLLAO with the target Client's Interface ID and with D set
to 1. The ROS will then realize that it needs to mark its neighbor
cache entry Interface ID for the Proxy as "unreachable", and SHOULD
re-initiate route optimization while continuing to forward packets
according to the remaining neighbor cache entry state.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 its Servers via the SPAN with SLLAOs
that include the new Client Port Number, Link Layer Address and P(i)
values. If the RS messages are sent solely for the purpose of
updating QoS preferences, Port Number and Link-Layer Address are set
to 0.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 ANET interfaces into service
(e.g., when it activates a new data link), it sends RS messages to
its Servers via the ANET interface with SLLAOs that include the new
Client Link Layer Address information.When a Client needs to remove existing ANET interfaces from
service (e.g., when it de-activates an existing data link), it sends
RS messages to its Servers with SLLAOs with D set to 1.If the Client needs to send RS messages over an ANET interface
other than the one being removed from service, it MUST include a
current SLLAO with S set to 1 for the sending interface and include
additional SLLAOs for any ANET interfaces being removed from
service.When a Client associates with a new Server, it performs the
Client procedures specified in . The
Client then sends an RS message over any working ANET interface with
destination set to the old Server's AERO address and with an SLLAO
with R set to 1 to fully release itself from the old Server. The
SLLAO also includes the SPAN address of the new Server in the Link
Layer Address. If the Client does not receive an RA reply after
MAX_RTR_SOLICITATIONS attempts over multiple ANET interfaces, the
old Server may have failed and the Client should discontinue its
release attempts.When the old Server processes the RS, it sends unsolicited NA
messages with a single TLLAO with Interface ID set to 0xffff and
with R and S set to 1 to all ROSs in the Client's Report List. The
Server also changes the symmetric neighbor cache entry state to
DEPARTED, sets the link-layer address of the Client to the address
found in the RS SLLAO (i.e., the SPAN address of the new Server),
and sets a timer to DepartTime seconds. The old Server then returns
an RA message to the Client with Router Lifetime set to 0. After
DepartTime seconds expires, the old Server deletes the symmetric
neighbor cache entry.When the Client receives the RA message with Router Lifetime set
to 0, it still must inform each of its remaining Proxys that it has
released the old Server from service. To do so, it sends an RS over
each remaining proxyed ANET interface with destination set to the
old Server's AERO address and with no SLLAO. The Proxy will mark
this Server as DEAPARTED and return an immediate RA without first
performing an RS/RA exchange with the old 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 segment, etc.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 ANET 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 Interface
IDs 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 can use
simple INET encapsulation and need not go over the SPAN.Following the intial 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 Interface ID 0xffff and
R set to 1 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.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 Internetwork 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: ).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 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 the same as defined in IEEE 802.1Q. 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.This can be accomplished by introducing an "AERO Link Anycast"
address that is configured by all Relays connected to the same AERO
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.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 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 returns
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 segments can be joined into a
single unified AERO link, even though the INET segments 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 segments. This can be accomplished
by incrementally deploying AERO Gateways on each INET segment, 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 segments 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 segment is transitioned to a native IP protocol version and
addressing scheme that is compatible with the AERO link MNP-based
addressing scheme, the INET segment and AERO link can be joined by IP
standard routers.Gateways that connect INETs/EUNs with dissimilar IP protocol versions
must employ a network address and protocol translation function such as
NAT64.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 latest versions are
available at: http://linkupnetworks.net/aero.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 independent of the AERO link. Secured
tunnel encapsulation 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, the bridging service provided by Relays is secured, and
security considerations for the exchange of data plane and control plane
messages between AERO link neighbors are discussed in the following
paragraphs.Data plane security considerations are the same as for ordinary
Internet communications. Application endpoints in AERO Clients and their
EUNs 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.Control plane security considerations are the same as for standard
IPv6 Neighbor Discovery . As fixed
infrastructure elements, AERO Servers and Proxys configure secured
tunnels with one or more Relays on their SPAN segments using symmetric
network and/or transport layer security services such as IPsec, TLS/SSL
or DTLS. The AERO Relays of all SPAN segments in turn configure secured
tunnels with their neighboring AERO Relays. 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. AERO Clients located
outside of secured enclaves SHOULD use symmetric network and/or
transport layer security to secure their ND exchanges with Servers, but
when there are many prospective neighbors with dynamically changing
connectivity an asymmetric security service such as SEND may be needed
(see: ).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 Relays must implement ingress filtering to avoid a spoofing
attack in which spurious SPAN messages are injected into an AERO link
from an outside attacker. Restricting access to the link can be achieved
by having Internetwork border routers implement ingress filtering to
discard encapsulated packets injected into the link by an outside
agent.AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on their EUNs to gain access to a protected network,
i.e., AERO Clients that act as routers MUST NOT provide routing services
for unauthorized nodes. (This concern is no different than for ordinary
hosts that receive an IP address delegation but then "share" the address
with other nodes via some form of Internet connection sharing such as
tethering.)The MAP list MUST be well-managed and secured from unauthorized
tampering, even though the list contains only public information. The
MAP list can be conveyed to the Client, e.g., through secure upload of a
static file, through DNS lookups, 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, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu
Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Seth Jahne, Ed King,
Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Greg Saccone, Kent
Shuey, Brian Skeen, Mike Slane, Carrie Spiker, Brendan Williams, Julie
Wulff, Yueli Yang, Eric Yeh and other members of the BR&T and BIT
mobile networking teams. Kyle Bae, Wayne Benson 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 Information Technology (BIT)
MobileNet program.This work is aligned with the Boeing autonomy 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 .The AERO S/TLLAO format specified in
includes a Length value of 5 (i.e., 5 units of 8 octets). However,
special-purpose links may extend the basic format to include additional
fields and a Length value larger than 5.For example, adaptation of AERO to the Aeronautical
Telecommunications Network with Internet Protocol Services (ATN/IPS)
includes link selection preferences based on transport port numbers in
addition to the existing DSCP-based preferences. ATN/IPS nodes maintain
a map of transport port numbers to 64 possible preference fields, e.g.,
TCP port 22 maps to preference field 8, TCP port 443 maps to preference
field 20, UDP port 8060 maps to preference field 34, etc. The extended
S/TLLAO format for ATN/IPS is shown in , where
the Length value is 7 and the 'Q(i)' fields provide link preferences for
the corresponding transport port number.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 ANET 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
ANET 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.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 REACHABLE_TIME 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
REACHABLE_TIME 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 (REACHABLE_TIME - 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.<< RFC Editor - remove prior to publication >>Changes 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.