Asymmetric Extended Route Optimization (AERO)Boeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftThis document specifies the operation of IP over Overlay Multilink
Network (OMNI) interfaces using the Asymmetric Extended Route
Optimization (AERO) internetworking and mobility management service.
AERO/OMNI use an IPv6 link-local address format that supports operation
of the IPv6 Neighbor Discovery (ND) protocol and links ND to IP
forwarding. Prefix delegation/registration services are employed for
network admission and to manage the routing system. Multilink operation,
mobility management, quality of service (QoS) signaling and route
optimization are naturally supported through dynamic neighbor cache
updates. Standard IP multicasting services are also supported. AERO is a
widely-applicable mobile internetworking service especially well-suited
to aviation services, intelligent transportation systems, mobile Virtual
Private Networks (VPNs) and many other applications.Asymmetric Extended Route Optimization (AERO) fulfills the
requirements of Distributed Mobility Management (DMM) and route optimization for
aeronautical networking and other network mobility use cases such as
intelligent transportation systems. AERO is an internetworking and
mobility management service that employs the Overlay Multilink Network
Interface (OMNI)
Non-Broadcast, Multiple Access (NBMA) virtual link model. The OMNI link
is a virtual overlay configured over one or more underlying
Internetworks, and nodes on the link can exchange IP packets as
single-hop neighbors via encapsulation. The OMNI Adaptation Layer (OAL)
supports multilink operation for increased reliability, bandwidth
optimization and traffic path selection while accommodating Maximum
Transmission Unit (MTU) diversity.The AERO service comprises Clients, Proxys, Servers and Relays that
are seen as OMNI link neighbors as well as Bridges that interconnect
OMNI link segments. Each node's OMNI interface uses an IPv6 link-local
address format that supports operation of the IPv6 Neighbor Discovery
(ND) protocol and links ND to IP forwarding. A
node's OMNI interface can be configured over multiple underlying
interfaces, and may therefore appear as a single interface with multiple
link-layer addresses. Each link-layer address is subject to change due
to mobility and/or QoS fluctuations, and link-layer address changes are
signaled by ND messaging the same as for any IPv6 link.AERO provides a cloud-based service where mobile node Clients may use
any Server acting as a Mobility Anchor Point (MAP) and fixed nodes may
use any Relay on the link for efficient communications. Fixed nodes
forward packets destined to other AERO nodes to the nearest Relay, which
forwards them through the cloud. A mobile node's initial packets are
forwarded through the Server, while direct routing is supported through
asymmetric extended route optimization while data packets are flowing.
Both unicast and multicast communications are supported, and mobile
nodes may efficiently move between locations while maintaining
continuous communications with correspondents and without changing their
IP Address.AERO Bridges are interconnected in a secured private BGP overlay
routing instance using encapsulation to provide a hybrid
routing/bridging service that joins the underlying Internetworks of
multiple disjoint administrative domains into a single unified OMNI
link. Each OMNI link instance is characterized by the set of Mobility
Service Prefixes (MSPs) common to all mobile nodes. The link extends to
the point where a Relay/Server is on the optimal route from any
correspondent node on the link, and provides a conduit between the
underlying Internetwork and the OMNI link. To the underlying
Internetwork, the Relay/Server is the source of a route to the MSP, and
hence uplink traffic to the mobile node is naturally routed to the
nearest Relay/Server.AERO assumes the use of PIM Sparse Mode in support of multicast
communication. In support of Source Specific Multicast (SSM) when a
Mobile Node is the source, AERO route optimization ensures that a
shortest-path multicast tree is established with provisions for mobility
and multilink operation. In all other multicast scenarios there are no
AERO dependencies.AERO was designed for aeronautical networking for both manned and
unmanned aircraft, where the aircraft is treated as a mobile node that
can connect an Internet of Things (IoT). AERO is also applicable to a
wide variety of other use cases. For example, it can be used to
coordinate the links of mobile nodes (e.g., cellphones, tablets, laptop
computers, etc.) that connect into a home enterprise network via public
access networks using tunneling software such as OpenVPN with VPN or non-VPN services enabled according to the
appropriate security model. AERO can also be used to facilitate
terrestrial vehicular and urban air mobility (as well as pedestrian
communication services) for future intelligent transportation systems
. Other applicable use cases are
also in scope.The following numbered sections present the AERO specification. The
appendices at the end of the document are non-normative.The terminology in the normative references applies; especially, the
terminology in the OMNI specification is used extensively
throughout. 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 uses the ND service specified
in .a networking service
for delegating IPv6 prefixes to nodes on the link. The nominal
service is DHCPv6 , however alternate
services (e.g., based on ND messaging) are also in scope. Most
notably, a minimal form of prefix delegation known as "prefix
registration" can be used if the Client knows its prefix in advance
and can represent it in the IPv6 source address of an ND
message.a node's first-hop data
link service network (e.g., a radio access network, cellular service
provider network, corporate enterprise network, etc.) that often
provides link-layer security services such as IEEE 802.1X and
physical-layer security prevent unauthorized access internally and
with border network-layer security services such as firewalls and
proxies that prevent unauthorized outside access.a node's attachment to a link
in an ANET.a connected IP network
topology with a coherent routing and addressing plan and that
provides a transit backbone service for ANET end systems. INETs also
provide an underlay service over which the AERO virtual link is
configured. Example INETs include corporate enterprise networks,
aviation networks, and the public Internet itself. When there is no
administrative boundary between an ANET and the INET, the ANET and
INET are one and the same.frequently, INETs such as
large corporate enterprise networks are sub-divided internally into
separate isolated partitions. Each partition is fully connected
internally but disconnected from other partitions, and there is no
requirement that separate partitions maintain consistent Internet
Protocol and/or addressing plans. (Each INET partition is seen as a
separate OMNI link segment as discussed below.)a node's attachment to a link
in an INET.an IP address assigned to a
node's interface connection to an INET.the encapsulation of a
packet in an outer header or headers that can be routed within the
scope of the local INET partition.the same as defined in , and manifested by IPv6
encapsulation . The OMNI link spans
underlying INET segments joined by virtual bridges in a spanning
tree the same as a bridged campus LAN. AERO nodes on the OMNI link
appear as single-hop neighbors even though they may be separated by
multiple underlying INET hops, and can use Segment Routing to cause packets to visit selected waypoints on
the link.a node's attachment to an OMNI
link. Since the addresses assigned to an OMNI interface are managed
for uniqueness, OMNI interfaces do not require Duplicate Address
Detection (DAD) and therefore set the administrative variable
'DupAddrDetectTransmits' to zero .an OMNI interface
process whereby packets admitted into the interface are wrapped in a
mid-layer IPv6 header and fragmented/reassembled if necessary to
support the OMNI link Maximum Transmission Unit (MTU). The OAL is
also responsible for generating MTU-related control messages as
necessary, and for providing addressing context for spanning
multiple segments of a bridged OMNI link.a link local
IPv6 address per constructed as specified
in .an IPv6
address from the IPv6 ULA prefix fd00::/8 ,
and constructed as specified in . OMNI ULAs are
statelessly derived from OMNI LLAs, and vice-versa.an ANET or INET
interface over which an OMNI interface is configured.an aggregated
IP Global Unicast Address (GUA) prefix (e.g., 2001:db8::/32,
192.0.2.0/24, etc.) assigned to the OMNI link and from which
more-specific Mobile Network Prefixes (MNPs) are delegated. OMNI
link administrators typically obtain MSPs from an Internet address
registry, however private-use prefixes can alternatively be used
subject to certain limitations (see: ). OMNI links that connect
to the global Internet advertise their MSPs to their interdomain
routing peers.a longer IP
prefix delegated from an MSP (e.g., 2001:db8:1000:2000::/56,
192.0.2.8/30, etc.) and assigned to an AERO Client or Relay.a node that is connected to an OMNI
link and participates in the AERO internetworking and mobility
service.an AERO node
that connects over one or more underlying interfaces and requests
MNP delegation/registration service from AERO Servers. The Client
assigns a Client LLA to the OMNI interface for use in ND exchanges
with other AERO nodes and forwards packets to correspondents
according to OMNI interface neighbor cache state.an INET node
that configures an OMNI interface to provide default forwarding and
mobility/multilink services for AERO Clients. The Server assigns an
administratively-provisioned LLA to its OMNI interface to support
the operation of the ND services, and advertises all of its
associated MNPs via BGP peerings with Bridges.an AERO Server
that also provides forwarding services between nodes reached via the
OMNI link and correspondents on other links. AERO Relays are
provisioned with MNPs (i.e., the same as for an AERO Client) and run
a dynamic routing protocol to discover any non-MNP IP GUA routes in
service on its connected INET links. In both cases, the Relay
advertises the MSP(s) to its downstream networks, and distributes
all of its associated MNPs and non-MNP IP GUA routes via BGP
peerings with Bridges (i.e., the same as for an AERO Server).a node that
provides hybrid routing/bridging services (as well as a security
trust anchor) for nodes on an OMNI link. As a router, the Bridge
forwards packets using standard IP forwarding. As a bridge, the
Bridge forwards packets over the OMNI link without decrementing the
IPv6 Hop Limit. AERO Bridges peer with Servers and other Bridges to
discover the full set of MNPs for the link as well as any non-MNP IP
GUA routes that are reachable via Relays.a node that
provides proxying services between Clients in an ANET and Servers in
external INETs. The AERO Proxy is a conduit between the ANET and
external INETs in the same manner as for common web proxies, and
behaves in a similar fashion as for ND proxies . A node may be configured to act as either a
Proxy and/or a Server, depending on Client Server selection
criteria.an OMNI
interface endpoint that injects encapsulated packets into an OMNI
link.an OMNI
interface endpoint that receives encapsulated packets from an OMNI
link.an IP address used as an
encapsulation header source or destination address from the
perspective of the OMNI interface. When an upper layer protocol
(e.g., UDP) is used as part of the encapsulation, the port number is
also considered as part of the link-layer address.the source or
destination address of an encapsulated IP packet presented to the
OMNI interface.an internal virtual or
external edge IP network that an AERO Client or Relay connects to
the rest of the network via the OMNI interface. The Client/Relay
sees each EUN as a "downstream" network, and sees the OMNI 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 OMNI link. The MR is the MN entity that hosts the AERO
Client.the AERO node
nearest the source that initiates route optimization. The ROS may be
a Server or Proxy acting on behalf of the source Client.the AERO
node nearest the target destination that responds to route
optimization requests. The ROR may be a Server acting on behalf of a
target MNP Client, or a Relay for a non-MNP destination.a geographically and/or
topologically referenced list of addresses of all Servers within the
same OMNI link. There is a single MAP list for the entire OMNI
link.a
BGP-based overlay routing service coordinated by Servers and Bridges
that tracks all Server-to-Client associations.the collective set of
all Servers, Proxys, Bridges and Relays that provide the AERO
Service to Clients.an individual
Server, Proxy, Bridge or Relay in the Mobility Service.Throughout the document, the simple terms "Client", "Server",
"Bridge", "Proxy" and "Relay" refer to "AERO Client", "AERO Server",
"AERO Bridge", "AERO Proxy" and "AERO Relay", respectively.
Capitalization is used to distinguish these terms from other common
Internetworking uses in which they appear without capitalization.The terminology of DHCPv6 and IPv6 ND (including the names of node variables, messages and
protocol constants) is used throughout this document. The terms
"All-Routers multicast", "All-Nodes multicast", "Solicited-Node
multicast" and "Subnet-Router anycast" are defined in . Also, the term "IP" is used to generically refer to
either Internet Protocol version, i.e., IPv4 or
IPv6 .The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when, and only when,
they appear in all capitals, as shown here.The following sections specify the operation of IP over OMNI links
using the AERO service:AERO Clients are Mobile Nodes (MNs) that connect via underlying
interfaces with addresses that may change when the Client moves to a
new network connection point. AERO Clients register their Mobile
Network Prefixes (MNPs) with the AERO service, and distribute the MNPs
to nodes on EUNs. AERO Bridges, Servers, Proxys and Relays are
critical infrastructure elements in fixed (i.e., non-mobile) INET
deployments and hence have permanent and unchanging INET addresses.
Together, they constitute the AERO service which provides an OMNI link
virtual overlay for connecting AERO Clients.AERO Bridges provide hybrid routing/bridging services (as well as a
security trust anchor) for nodes on an OMNI link. Bridges use standard
IPv6 routing to forward packets both within the same INET partitions
and between disjoint INET partitions based on a mid-layer IPv6
encapsulation per . The inner IP layer
experiences a virtual bridging service since the inner IP TTL/Hop
Limit is not decremented during forwarding. Each Bridge also peers
with Servers and other Bridges in a dynamic routing protocol instance
to provide a Distributed Mobility Management (DMM) service for the
list of active MNPs (see ). Bridges present
the OMNI link as a set of one or more Mobility Service Prefixes (MSPs)
and configure secured tunnels with Servers, Relays, Proxys and other
Bridges; they further maintain IP forwarding table entries for each
MNP and any other reachable non-MNP prefixes.AERO Servers provide default forwarding and mobility/multilink
services for AERO Client Mobile Nodes (MNs). Each Server also peers
with Bridges in a dynamic routing protocol instance to advertise its
list of associated MNPs (see ). Servers
facilitate prefix delegation/registration exchanges with Clients,
where each delegated prefix becomes an MNP taken from an MSP. Servers
forward packets between OMNI interface neighbors and track each
Client's mobility profiles. Servers may further act as Servers for
some sets of Clients and as Proxies for others.AERO Proxys provide a conduit for ANET Clients to associate with
Servers in external INETs. Client and Servers exchange control plane
messages via the Proxy acting as a bridge between the ANET/INET
boundary. The Proxy forwards data packets between Clients and the OMNI
link according to forwarding information in the neighbor cache. The
Proxy function is specified in . Proxys may
further act as Proxys for some sets of Clients and as Servers for
others.AERO Relays are Servers that provide forwarding services between
the OMNI interface and INET/EUN interfaces. Relays are provisioned
with MNPs the same as for an AERO Client, and also run a dynamic
routing protocol to discover any non-MNP IP routes. The Relay
advertises the MSP(s) to its connected networks, and distributes all
of its associated MNPs and non-MNP IP GUA routes via BGP peerings with
Bridges presents the basic OMNI link
reference model: In this model:the OMNI link is an overlay network service configured over
one or more underlying INET partitions which may be managed by
different administrative authorities and have incompatible
protocols and/or addressing plans.AERO Bridge B1 aggregates Mobility Service Prefix (MSP) M1,
discovers Mobile Network Prefixes (MNPs) X* and advertises the
MSP via BGP peerings over secured tunnels to Servers (S1, S2).
Bridges connect the disjoint segments of a partitioned OMNI
link.AERO Servers/Relays S1 and S2 configure secured tunnels with
Bridge B1 and also provide mobility, multilink and default
router services for their associated Clients C1 and C2.AERO Clients C1 and C2 associate with Servers S1 and S2,
respectively. They receive Mobile Network Prefix (MNP)
delegations X1 and X2, and also act as default routers for their
associated physical or internal virtual EUNs. Simple hosts H1
and H2 attach to the EUNs served by Clients C1 and C2,
respectively.AERO Proxy P1 configures a secured tunnel with Bridge B1 and
provides proxy services for AERO Clients in secured enclaves
that cannot associate directly with other OMNI link
neighbors.An OMNI link configured over a single INET appears as a single
unified link with a consistent underlying network addressing plan.
In that case, all nodes on the link can exchange packets via simple
INET encapsulation, since the underlying INET is connected. In
common practice, however, an OMNI link may be partitioned into
multiple "segments", where each segment is a distinct INET
potentially managed under a different administrative authority
(e.g., as for worldwide aviation service providers such as ARINC,
SITA, Inmarsat, etc.). Individual INETs may also themselves be
partitioned internally, in which case each internal partition is
seen as a separate segment.The addressing plan of each segment is consistent internally but
will often bear no relation to the addressing plans of other
segments. Each segment is also likely to be separated from others by
network security devices (e.g., firewalls, proxies, packet filtering
gateways, etc.), and in many cases disjoint segments may not even
have any common physical link connections. Therefore, nodes can only
be assured of exchanging packets directly with correspondents in the
same segment, and not with those in other segments. The only means
for joining the segments therefore is through inter-domain peerings
between AERO Bridges.The same as for traditional campus LANs, multiple OMNI link
segments can be joined into a single unified link via a virtual
bridging service using the OMNI Adaptation Layer (OAL) which inserts
a mid-layer IPv6 encapsulation per that
supports inter-segment forwarding (i.e., bridging) without
decrementing the network-layer TTL/Hop Limit. This bridging of OMNI
link segments is shown in :Bridges, Servers, Relays and Proxys connect via secured INET
tunnels over their respective segments in a spanning tree topology
rooted at the Bridges. The secured spanning tree supports strong
authentication for IPv6 ND control messages and may also be used to
convey the initial data packets in a flow. Route optimization can
then be employed to cause data packets to take more direct paths
between OMNI link neighbors without having to strictly follow the
spanning tree.AERO nodes on OMNI links use the Link-Local Address (LLA) prefix
fe80::/64 to assign LLAs used for
network-layer addresses in link-scoped IPv6 ND and data messages.
They also use the Unique Local Address (ULA) prefix fd00::/8
followed by a pseudo-random 40-bit global ID to form the prefix
[ULA]::/48, then include a 16-bit OMNI link identifier '*' to form
the prefix [ULA*]::/64 to assign OAL addresses. Finally, the MSPs
and MNPs used by AERO nodes are typically based on Global Unicast
Addresses (GUAs), but in some cases may be based on private-use
addresses. See for
a full specification of LLAs, ULAs and GUAs used by AERO nodes on
OMNI links.The AERO routing system comprises a private instance of the
Border Gateway Protocol (BGP) that is
coordinated between Bridges and Servers and does not interact with
either the public Internet BGP routing system or any underlying INET
routing systems.In a reference deployment, each Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) using an AS Number (ASN) that is unique within the BGP
instance, and each Server further uses eBGP to peer with one or more
Bridges but does not peer with other Servers. Each INET of a
multi-segment OMNI link must include one or more Bridges, which peer
with the Servers and Proxys within that INET. All Bridges within the
same INET are members of the same hub AS using a common ASN, and use
iBGP to maintain a consistent view of all active MNPs currently in
service. The Bridges of different INETs peer with one another using
eBGP.Bridges advertise the OMNI link's MSPs and any non-MNP routes to
each of their Servers. This means that any aggregated non-MNPs
(including "default") are advertised to all Servers. Each Bridge
configures a black-hole route for each of its MSPs. By black-holing
the MSPs, the Bridge will maintain forwarding table entries only for
the MNPs that are currently active, and packets destined to all
other MNPs will correctly incur Destination Unreachable messages due
to the black-hole route. In this way, Servers have only partial
topology knowledge (i.e., they know only about the MNPs of their
directly associated Clients) and they forward all other packets to
Bridges which have full topology knowledge.Each OMNI link segment assigns a unique sub-prefix of [ULA*]::/96
known as the ULA partition prefix. For example, a first segment
could assign [ULA*]::1000/116, a second could assign
[ULA*]::2000/116, a third could assign [ULA*]::3000/116, etc. The
administrative authorities for each segment must therefore
coordinate to assure mutually-exclusive partition prefix
assignments, but internal provisioning of each prefix is an
independent local consideration for each administrative authority.
For each partition prefix, the Bridge(s) that connect that segment
assign the all-zero's address of the prefix as a Subnet Router
Anycast address. For example, the Subnet Router Anycast address for
[ULA*]::1000/116 is simply [ULA*]::1000.ULA partition prefixes are statically represented in Bridge
forwarding tables. Bridges join multiple segments into a unified
OMNI link over multiple diverse administrative domains. They support
a bridging function by first establishing forwarding table entries
for their partition prefixes either via standard BGP routing or
static routes. For example, if three Bridges ('A', 'B' and 'C') from
different segments serviced [ULA*]::1000/116, [ULA*]::2000/116 and
[ULA*]::3000/116 respectively, then the forwarding tables in each
Bridge are as follows:[ULA*]::1000/116->local,
[ULA*]::2000/116->B, [ULA*]::3000/116->C[ULA*]::1000/116->A,
[ULA*]::2000/116->local, [ULA*]::3000/116->C[ULA*]::1000/116->A, [ULA*]::2000/116->B,
[ULA*]::3000/116->localThese forwarding table entries are permanent and never
change, since they correspond to fixed infrastructure elements in
their respective segments.ULA Client prefixes are instead dynamically advertised in the
AERO routing system by Servers and Relays that provide service for
their corresponding MNPs. For example, if three Servers ('D', 'E'
and 'F') service the MNPs 2001:db8:1000:2000::/56,
2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing
system would include:[ULA*]:2001:db8:1000:2000/120[ULA*]:2001:db8:3000:4000/120[ULA*]:2001:db8:5000:6000/120A full discussion of the BGP-based routing system used by AERO is
found in .With the Client and partition prefixes in place in Bridge
forwarding tables, the OMNI interface sends control and data packets
toward AERO destination nodes located in different OMNI link
segments over the spanning tree via mid-layer encapsulation using
the OMNI Adaptation Layer (OAL) header based on Generic Packet
Tunneling in IPv6 . When necessary, the OMNI
interface also includes an OMNI Routing Header (ORH) as an extension
to the OAL header if final segment forwarding information is
available, e.g., in the neighbor cache. The ORH is formatted as
shown in :In this format:Next Header identifies the type of header immediately
following the ORH.Hdr Ext Len is the length of the Routing header in 8-octet
units (not including the first 8 octets), with trailing padding
added if necessary to produce an integral number of 8-octet
units.Routing Type is set to TBD (see IANA Considerations).Segments Left is omitted, and replaced by a 5-bit SRT and
3-bit FMT field.SRT - a 5-bit Segment Routing Topology prefix length value
that (when added to 96) determines the prefix length to apply to
the ULA formed from concatenating [ULA*]::/96 with the 32 bit
LHS MSID value that follows. For example, the value 16
corresponds to the prefix length 112.FMT - a 3-bit "Framework/Mode/Type" code corresponding to the
included Link Layer Address as follows:When the most significant bit (i.e., "Framework") is set
to 0, L2ADDR is the INET encapsulation address of the
Server/Proxy named in the LHS; otherwise, it is the address
for the target Client, and the Destination Suffix is
included.When the next most significant bit (i.e., "Mode") is set
to 0, the Source/Target L2ADDR is on the open INET;
otherwise, it is (likely) located behind a Network Address
Translator (NAT).When the least significant bit (i.e., "Type") is set to
0, L2ADDR includes a UDP Port Number followed by an IPv4
address; otherwise, it includes a UDP Port Number followed
by an IPv6 address.LHS - the 32 bit MSID of a service node in the Last Hop
Segment on the path to the target. When SRT and LHS are both set
to 0, the LHS is considered unspecified. When SRT is set to 0
and LHS is non-zero, the prefix length is set to 128. SRT and
LHS provide guidance to the OMNI interface forwarding algorithm.
Specifically, if SRT/LHS is located in the local OMNI link
segment then the OMNI interface can omit OAL/ORH encapsulation
and send directly to the target using INET encapsulation
according to FMT/L2ADDR; else, it must perform INET/OAL/ORH
encapsulation and forward according to the OMNI link spanning
tree.Link Layer Address (L2ADDR) - Formatted according to FMT, and
identifies the link-layer address (i.e., the encapsulation
address) of the source/target. The UDP Port Number appears in
the first two octets and the IP address appears in the next 4
octets for IPv4 or 16 octets for IPv6. The Port Number and IP
address are recorded in ones-compliment "obfuscated" form per
. The OMNI interface forwarding
algorithm uses FMT/L2ADDR to determine the INET encapsulation
address for local forwarding when SRT/LHS is located in the same
OMNI link segment.Destination Suffix is a 64-bit field included only for
OAL-encapsulated packets that are destined directly to the ULA
of the Client (i.e., according to the FMT code). When present,
Destination Suffix encodes the 64-bit ULA suffix for the Client
that will receive packet. For example, if the Client ULA is
[ULA*]:2001:db8:1:2 then the Destination suffix encodes the
value 2001:db8:1:2.Null Padding contains zero-valued octets as necessary to pad
the ORH to an integral number of 8-octet units.When an AERO node uses OAL encapsulation for a packet with
addresses such as 2001:db8:1:2::1 and 2001:db8:1234:5678::1, it sets
the OAL header source address to its own ULA address (e.g.,
[ULA*]::1000:2000). The node also sets the destination address to
the ULA of the Client (e.g., [ULA*]::2001:db8:1234:5678) when the
Client is addressed directly; otherwise, it sets the destination
address to the ULA of the Client's Proxy/Server (e.g.,
[ULA*]::4321:9876). If the neighbor cache includes Last Hop Segment
information for the target destination, the node next inserts an ORH
immediately following the OAL header while including the correct
SRT, FMT, LHS, L2ADDR and (if necessary) Destination Suffix
information. Next, the node overwrites the OAL header destination
address with the LHS Subnet Router Anycast address (for example, for
LHS 1000:2000 with SRT 16, the Subnet Router Anycast address is
[ULA*]::1000:0000).The node then fragments the OAL/ORH packet if necessary, with
each resulting fragment including the OAL/ORH headers while only the
first fragment includes the original IPv6 header. The node finally
encapsulates each resulting OAL/ORH packet/fragment in an INET
header with source address set to its own INET address (e.g.,
192.0.2.100) and destination set to the INET address of a Bridge
(e.g., 192.0.2.1).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 OAL header is an IPv6 header prepared
according to , the ORH is a Routing Header
extension of the OAL header, and the INET header is prepared as
discussed in .This gives rise to a routing system that contains both Client
prefix routes that may change dynamically due to regional node
mobility and partition prefix routes that rarely if ever change. The
Bridges can therefore provide link-layer bridging by sending packets
over the spanning tree instead of network-layer routing according to
MNP routes. As a result, opportunities for packet loss due to node
mobility between different segments are mitigated.In normal operations, IPv6 ND messages are conveyed over secured
paths between OMNI link neighbors so that specific Proxys, Servers
or Relays can be addressed without being subject to mobility events.
Conversely, only the first few packets destined to Clients need to
traverse secured paths until route optimization can determine a more
direct path.Note: An IPv6 "minimal encapsulation" format (i.e., an IPv6
variant of ) based on extensions to the ORH
was considered, analyzed and rejected. In the approach, the ORH
would be inserted as an extension header to the original IPv6 packet
header. The IPv6 destination address would then be written into the
ORH, and the ULA corresponding to the destination would be
overwritten in the IPv6 destination address. This would seemingly
convey enough forwarding information so that OAL encapsulation could
be avoided. However, this "minimal encapsulation" IPv6 packet would
then have a non-ULA source address and ULA destination address, an
incorrect value in upper layer protocol checksums, and a Hop Limit
that is decremented within the spanning tree when it should not be.
The insertion and removal of the ORH would also entail rewriting the
Payload Length and Next Header fields - again, invalidating upper
layer checksums. These irregularities would result in implementation
challenges and the potential for operational issues, e.g., since
actionable ICMPv6 error reports could not be delivered to the
original source. In order to address the issues, still more
information such as the original IPv6 source address could be
written into the ORH. However, with the additional information the
benefit of the "minimal encapsulation" savings quickly diminishes,
and becomes overshadowed by the implementation and operational
irregularities.The 64-bit sub-prefixes of [ULA]::/48 identify up to 2^16
distinct Segment Routing Topologies (SRTs). Each SRT is a
mutually-exclusive OMNI link overlay instance using a distinct set
of ULAs, and emulates a Virtual LAN (VLAN) service for the OMNI
link. In some cases (e.g., when redundant topologies are needed for
fault tolerance and reliability) it may be beneficial to deploy
multiple SRTs that act as independent overlay instances. A
communication failure in one instance therefore will not affect
communications in other instances.Each SRT is identified by a distinct value in bits 48-63 of
[ULA]::/48, i.e., as [ULA0]::/64, [ULA1]::/64, [ULA2]::/64, etc.
This document asserts that up to four SRTs provide a level of safety
sufficient for critical communications such as civil aviation. Each
SRT is designated with a color that identifies a different OMNI link
instance as follows:Red - corresponds to [ULA0]::/64Green - corresponds to [ULA1]::/64Blue-1 - corresponds to [ULA2]::/64Blue-2 - corresponds to [ULA3]::/64the remaining [ULA*]::/64 sub prefixes are available for
additional SRTs.Each OMNI interface is identified by a unique interface
name (e.g., omni0, omni1, omni2, etc.) and assigns an anycast ULA
corresponding to its SRT prefix. For example, the anycast ULA for
the Green SRT is simply [ULA1]::. The anycast ULA is used for OMNI
interface determination in Safety-Based Multilink (SBM) as discussed
in . Each OMNI
interface further applies Performance-Based Multilink (PBM)
internally.An original IPv6 source can direct an IPv6 packet to an AERO node
by including a standard IPv6 Segment Routing Header (SRH) with the anycast ULA for the selected SRT as
either the IPv6 destination or as an intermediate hop within the
SRH. This allows the original source to determine the specific OMNI
link topology a packet will traverse when there may be multiple
alternatives.When the AERO node processes the SRH and forwards the packet to
the correct OMNI interface, the OMNI interface writes the next IPv6
address from the SRH into the IPv6 destination address and
decrements Segments Left. If decrementing would cause Segments Left
to become 0, the OMNI interface deletes the SRH before forwarding.
This form of Segment Routing supports Safety-Based Multilink
(SBM).AERO node OMNI interfaces can insert OAL/ORH headers for Segment
Routing within the OMNI link to influence the paths of packets
destined to targets in remote segments without requiring all packets
to traverse strict spanning tree paths.When an AERO node's OMNI interface has a packet to send to a
target discovered through route optimization located in the same
OMNI link segment, it encapsulates the packet in OAL/ORH headers if
necessary as discussed above. The node then uses the target's Link
Layer Address (L2ADDR) information for INET encapsulation.When an AERO node's OMNI interface has a packet to send to a
route optimization target located in a remote OMNI link segment, it
encapsulates the packet in OAL/ORH headers as discussed above while
forwarding the packet to a Bridge with destination set to the Subnet
Router Anycast address for the final OMNI link segment.When a Bridge receives a packet destined to its Subnet Router
Anycast address with an OAL/ORH with SRT/LHS values corresponding to
the local segment, it examines the L2ADDR according to FMT and
removes the ORH from the packet. If the ORH includes a saved
Destination Suffix, the Bridge then writes the corresponding ULA
into the OAL destination address; otherwise, it writes the ULA
corresponding to the SRT/LHS fields into the destination. The Bridge
then encapsulates the packet in an INET header according to L2ADDR
and forwards the packet within the INET either to the LHS
Server/Proxy or directly to the destination itself. In this way, the
Bridge participates in route optimization to reduce traffic load and
suboptimal routing through strict spanning tree paths.OMNI interfaces are virtual interfaces configured over one or more
underlying interfaces classified as follows:INET interfaces connect to an INET either natively or through
one or several IPv4 Network Address Translators (NATs). Native
INET interfaces have global IP addresses that are reachable from
any INET correspondent. All Server, Relay and Bridge interfaces
are native interfaces, as are INET-facing interfaces of Proxys.
NATed INET interfaces connect to a private network behind one or
more NATs that provide INET access. Clients that are behind a NAT
are required to send periodic keepalive messages to keep NAT state
alive when there are no data packets flowing.ANET interfaces connect to an ANET that is separated from the
open INET by a Proxy. Proxys can actively issue control messages
over the INET on behalf of the Client to reduce ANET
congestion.VPNed interfaces use security encapsulation over the INET to a
Virtual Private Network (VPN) server that also acts as a Server or
Proxy. Other than the link-layer encapsulation format, VPNed
interfaces behave the same as Direct interfaces.Direct interfaces connect a Client directly to a Server or
Proxy without crossing any ANET/INET paths. An example is a
line-of-sight link between a remote pilot and an unmanned
aircraft. The same Client considerations apply as for VPNed
interfaces.OMNI interfaces use OAL/ORH encapsulation as necessary as discussed
in . OMNI interfaces use link-layer
encapsulation (see: ) to exchange packets
with OMNI link neighbors over INET or VPNed interfaces as well as over
ANET interfaces for which the Client and Proxy may be multiple IP hops
away. OMNI interfaces do not use link-layer encapsulation over Direct
underlying interfaces or ANET interfaces when the Client and Proxy are
known to be on the same underlying link.OMNI interfaces maintain a neighbor cache for tracking per-neighbor
state the same as for any interface. OMNI interfaces use ND messages
including Router Solicitation (RS), Router Advertisement (RA),
Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for
neighbor cache management.OMNI interfaces send ND messages with an OMNI option formatted as
specified in . The
OMNI option includes prefix registration information and Interface
Attributes containing link information parameters for the OMNI
interface's underlying interfaces. Each OMNI option may include
multiple Interface Attributes sub-options, each identified by an
ifIndex value.A Client's OMNI interface may be configured over multiple
underlying interface connections. For example, common mobile handheld
devices have both wireless local area network ("WLAN") and cellular
wireless links. These links are often used "one at a time" with
low-cost WLAN preferred and highly-available cellular wireless as a
standby, but a simultaneous-use capability could provide benefits. In
a more complex example, aircraft frequently have many wireless data
link types (e.g. satellite-based, cellular, terrestrial, air-to-air
directional, etc.) with diverse performance and cost properties.If a Client's multiple underlying interfaces are used "one at a
time" (i.e., all other interfaces are in standby mode while one
interface is active), then ND message OMNI options include only a
single Interface Attributes sub-option set to constant values. In that
case, the Client would appear to have a single interface but with a
dynamically changing link-layer address.If the Client has multiple active underlying interfaces, then from
the perspective of ND it would appear to have multiple link-layer
addresses. In that case, ND message OMNI options MAY include multiple
Interface Attributes sub-options - each with values that correspond to
a specific interface. Every ND message need not include Interface
Attributes for all underlying interfaces; for any attributes not
included, the neighbor considers the status as unchanged.Bridge, Server and Proxy OMNI interfaces may be configured over one
or more secured tunnel interfaces. The OMNI interface configures both
an LLA and its corresponding ULA, while the underlying secured tunnel
interfaces are either unnumbered or configure the same ULA. The OMNI
interface encapsulates each IP packet in OAL/ORH headers and presents
the packet to the underlying secured tunnel interface. Routing
protocols such as BGP that run over the OMNI interface do not employ
OAL/ORH encapsulation, but rather present the routing protocol
messages directly to the underlying secured tunnels while using the
ULA as the source address. This distinction must be honored
consistently according to each node's configuration so that the IP
forwarding table will associate discovered IP routes with the correct
interface.AERO Servers, Proxys and Clients configure OMNI interfaces as their
point of attachment to the OMNI link. AERO nodes assign the MSPs for
the link to their OMNI 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-OMNI interface are directed to the OMNI
interface.OMNI interface initialization procedures for Servers, Proxys,
Clients and Bridges are discussed in the following sections.When a Server enables an OMNI interface, it assigns an LLA/ULA
appropriate for the given OMNI link segment. The Server also
configures secured tunnels with one or more neighboring Bridges and
engages in a BGP routing protocol session with each Bridge.The OMNI 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 OMNI
interface neighbors. The Server further configures a service to
facilitate ND exchanges with AERO Clients and manages per-Client
neighbor cache entries and IP forwarding table entries based on
control message exchanges.Relays are simply Servers that run a dynamic routing protocol to
redistribute routes between the OMNI interface and INET/EUN
interfaces (see: ). The Relay provisions
MNPs to networks on the INET/EUN interfaces (i.e., the same as a
Client would do) and advertises the MSP(s) for the OMNI link over
the INET/EUN interfaces. The Relay further provides an attachment
point of the OMNI link to a non-MNP-based global topology.When a Proxy enables an OMNI interface, it assigns an LLA/ULA and
configures permanent neighbor cache entries the same as for Servers.
The Proxy also configures secured tunnels with one or more
neighboring Bridges and maintains per-Client neighbor cache entries
based on control message exchanges. Importantly Proxys are often
configured to act as Servers, and vice-versa.When a Client enables an OMNI interface, it sends RS messages
with ND parameters over its underlying interfaces to a Server, which
returns an RA message with corresponding parameters. (The RS/RA
messages may pass through a Proxy in the case of a Client's ANET
interface, or through one or more NATs in the case of a Client's
INET interface.)AERO Bridges configure an OMNI interface and assign the ULA
Subnet Router Anycast address for each OMNI link segment they
connect to. Bridges configure secured tunnels with Servers, Proxys
and other Bridges; they also configure LLAs/ULAs and permanent
neighbor cache entries the same as Servers. Bridges engage in a BGP
routing protocol session with a subset of the Servers and other
Bridges on the spanning tree (see: ).Each OMNI interface maintains a conceptual neighbor cache that
includes an entry for each neighbor it communicates with on the OMNI
link per . OMNI interface neighbor cache
entries are said to be one of "permanent", "symmetric", "asymmetric"
or "proxy".Permanent neighbor cache entries are created through explicit
administrative action; they have no timeout values and remain in place
until explicitly deleted. AERO Bridges maintain permanent neighbor
cache entries for their associated Proxys/Servers (and vice-versa).
Each entry maintains the mapping between the neighbor's network-layer
LLA and corresponding INET address.Symmetric neighbor cache entries are created and maintained through
RS/RA exchanges as specified in , and remain in
place for durations bounded by prefix 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 ROSs maintain asymmetric neighbor cache entries for
active targets with lifetimes based on ND messaging constants.
Asymmetric neighbor cache entries are unidirectional since only the
ROS (and not the ROR) creates an entry.Proxy neighbor cache entries are created and maintained by AERO
Proxys when they process Client/Server ND exchanges, and remain in
place for durations bounded by ND and prefix lifetimes. AERO Proxys
maintain proxy neighbor cache entries for each of their associated
Clients. Proxy neighbor cache entries track the Client state and the
address of the Client's associated Server(s).To the list of neighbor cache entry states in Section 7.3.2 of
, Proxy and Server OMNI interfaces add an
additional state DEPARTED that applies to symmetric and proxy neighbor
cache entries for Clients that have recently departed. The interface
sets a "DepartTime" variable for the neighbor cache entry to
"DEPART_TIME" seconds. DepartTime is decremented unless a new ND
message causes the state to return to REACHABLE. While a neighbor
cache entry is in the DEPARTED state, packets destined to the target
Client are forwarded to the Client's new location instead of being
dropped. When DepartTime decrements to 0, the neighbor cache entry is
deleted. It is RECOMMENDED that DEPART_TIME be set to the default
constant value REACHABLE_TIME plus 10 seconds (40 seconds by default)
to allow a window for packets in flight to be delivered while stale
route optimization state may be present.When an ROR receives an authentic NS message used for route
optimization, it searches for a symmetric neighbor cache entry for the
target Client. The ROR then returns a solicited NA message without
creating a neighbor cache entry for the ROS, but creates or updates a
target Client "Report List" entry for the ROS and sets a "ReportTime"
variable for the entry to REPORT_TIME seconds. The ROR resets
ReportTime when it receives a new authentic NS message, and otherwise
decrements ReportTime while no authentic NS messages have been
received. It is RECOMMENDED that REPORT_TIME be set to the default
constant value REACHABLE_TIME plus 10 seconds (40 seconds by default)
to allow a window for route optimization to converge before ReportTime
decrements below REACHABLE_TIME.When the ROS receives a solicited NA message response to its NS
message used for route optimization, it creates or updates an
asymmetric neighbor cache entry for the target network-layer and
link-layer addresses. The ROS then (re)sets ReachableTime for the
neighbor cache entry to REACHABLE_TIME seconds and uses this value to
determine whether packets can be forwarded directly to the target,
i.e., instead of via a default route. The ROS otherwise decrements
ReachableTime while no further solicited NA messages arrive. It is
RECOMMENDED that REACHABLE_TIME be set to the default constant value
30 seconds as specified in .AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the
number of NS keepalives sent when a correspondent may have gone
unreachable, the value MAX_RTR_SOLICITATIONS to limit the number of RS
messages sent without receiving an RA and the value
MAX_NEIGHBOR_ADVERTISEMENT to limit the number of unsolicited NAs that
can be sent based on a single event. It is RECOMMENDED that
MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and
MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in .Different values for DEPART_TIME, REPORT_TIME, REACHABLE_TIME,
MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and
MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if
different values are chosen, all nodes on the link MUST consistently
configure the same values. Most importantly, DEPART_TIME and
REPORT_TIME SHOULD be set to a value that is sufficiently longer than
REACHABLE_TIME to avoid packet loss due to stale route optimization
state.OMNI interface IPv6 ND messages include OMNI options with Interface Attributes
that provide Link-Layer Address and QoS Preference information for
the neighbor's underlying interfaces. This information is stored in
the neighbor cache and provides the basis for the forwarding
algorithm specified in . The information is
cumulative and reflects the union of the OMNI information from the
most recent ND messages received from the neighbor; it is therefore
not required that each ND message contain all neighbor
information.The OMNI option Interface Attributes for each underlying
interface includes a two-part "Link-Layer Address" consisting of a
simple IP encapsulation address determined by the FMT and L2ADDR
fields and an OAL ULA determined by the SRT and LHS fields. If the
neighbor is located in the local OMNI link segment (and, if any
necessary NAT state has been established) forwarding via simple IP
encapsulation can be used; otherwise, OAL encapsulation must be
used. Underlying interfaces are further selected based on their
associated preference values "high", "medium", "low" or
"disabled".Note: the OMNI option is distinct from any Source/Target
Link-Layer Address Options (S/TLLAOs) that may appear in an ND
message according to the appropriate IPv6 over specific link layer
specification (e.g., ). If both an OMNI
option and S/TLLAO appear, the former pertains to encapsulation
addresses while the latter pertains to the native L2 address format
of the underlying media.As discussed in Section 4.4 of NA
messages include three flag bits R, S and O. OMNI interface NA
messages treat the flags as follows:R: The R ("Router") flag is set to 1 in the NA messages sent
by all AERO/OMNI node types. Simple hosts that would set R to 0
do not occur on the OMNI link itself, but may occur on the
downstream links of Clients and Relays.S: The S ("Solicited") flag is set exactly as specified in
Section 4.4. of , i.e., it is set to 1
for Solicited NAs and set to 0 for Unsolicited NAs (both unicast
and multicast).O: The O ("Override") flag is set to 0 for solicited proxy
NAs and set to 1 for all other solicited and unsolicited NAs.
For further study is whether solicited NAs for anycast targets
apply for OMNI links. Since OMNI LLAs must be uniquely assigned
to Clients to support correct ND protocol operation, however, no
role is currently seen for assigning the same OMNI LLA to
multiple Clients.The OMNI Adaptation Layer (OAL) inserts mid-layer IPv6 headers
known as the OAL/ORH headers when necessary as discussed in the
following sections. After either inserting or omitting the OAL/ORH
headers, the OMNI interface also inserts or omits an outer
encapsulation header as discussed below.OMNI interfaces avoid outer encapsulation over Direct underlying
interfaces and ANET underlying interfaces for which the Client and
Proxy are connected to the same underlying link. Otherwise, OMNI
interfaces encapsulate packets according to whether they are entering
the OMNI interface from the network layer or if they are being
re-admitted into the same OMNI link they arrived on. This latter form
of encapsulation is known as "re-encapsulation".For packets entering the OMNI interface from the network layer, the
OMNI interface copies the "TTL/Hop Limit", "Type of Service/Traffic
Class" , "Flow Label" (for IPv6) and "Congestion Experienced" values in the inner packet's IP header into the
corresponding fields in the OAL and outer encapsulation header(s).For packets undergoing re-encapsulation, the OMNI 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.)OMNI interfaces configured over ANET underlying interfaces which
employ a different IP protocol version (and/or when the Client and
Proxy may be separated by multiple ANET IP hops) use IP-in-IP
encapsulation so that the inner packet can traverse the ANET without
decrementing the TTL/Hop-Limit. IPv6 underlying ANET interfaces use
encapsulation, while IPv4 interfaces use the
appropriate encapsulation per one of .OMNI interfaces configured over INET underlying interfaces
encapsulate packets in INET headers according to the next hop
determined in the forwarding algorithm in . If
the next hop is reached via a secured tunnel, the OMNI interface uses
an encapsulation format specific to the secured tunnel type (see:
). If the next hop is reached via an unsecured
INET interface, the OMNI interface instead uses UDP/IP encapsulation
per and as extended in .When UDP/IP encapsulation is used, the OMNI interface next sets the
UDP source port to a constant value that it will use in each
successive packet it sends, and sets the UDP length field to the
length of the encapsulated packet plus 8 bytes for the UDP header
itself plus the length of any included extension headers or trailers.
The encapsulated packet may be either IPv6 or IPv4, as distinguished
by the version number found in the first four bits.For UDP/IP-encapsulated packets sent to a Server, Relay or Bridge,
the OMNI interface sets the UDP destination port to 8060, i.e., the
IANA-registered port number for AERO. For packets sent to a Client,
the OMNI interface sets the UDP destination port to the port value
stored in the neighbor cache entry for this Client. The OMNI interface
finally includes/omits the UDP checksum according to .OMNI interfaces decapsulate packets destined either to the AERO
node itself or to a destination reached via an interface other than
the OMNI interface the packet was received on. When the encapsulated
packet arrives in multiple OAL fragments, the OMNI interface
reassembles as discussed in . Further
decapsulation steps are performed according to the appropriate
encapsulation format specification.AERO nodes employ simple data origin authentication procedures. In
particular:AERO Bridges, Servers and Proxys accept encapsulated data
packets and control messages received from the (secured) spanning
tree.AERO Proxys and Clients accept packets that originate from
within the same secured ANET.AERO Clients and Relays accept packets from downstream network
correspondents based on ingress filtering.AERO Clients, Relays and Servers verify the outer UDP/IP
encapsulation addresses according to .AERO nodes silently drop any packets that do not satisfy the
above data origin authentication procedures. Further security
considerations are discussed in .The OMNI interface observes the link nature of tunnels, including
the Maximum Transmission Unit (MTU), Maximum Reassembly Unit (MRU) and
the role of fragmentation and reassembly . The OMNI interface employs an
OMNI Adaptation Layer (OAL) for accommodating multiple underlying
links with diverse MTUs. The functions of the OAL and the OMNI
interface MTU/MRU are specified in Section 5 of , with MTU/MRU both set to
the constant value 9180 bytes.When the network layer presents an IP packet to the OMNI interface,
the OAL encapsulates the packet in OAL/ORH headers. The OAL then
fragments the encapsulated packet if necessary such that the OAL/ORH
headers appear in each fragment while the original IP packet header
appears only in the first fragment. The OAL then transmits each
OAL/ORH packet/fragment over an underlying interface connected to
either a physical link such as Ethernet, WiFi and the like or a
virtual link such as an Internet or higher-layer tunnel (see the
definition of link in ).Note: Although the ORH may be removed by a Bridge on the path (see:
), this does not interfere with the
destination's ability to reassemble in the event that the packet was
fragmented. This is due to the fact that the ORH is not included in
the fragmentable part; therefore, its removal does not invalidate the
offset values in any fragment headers.IP packets enter a node's OMNI 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 OMNI interface neighbor). All
packets entering a node's OMNI interface first undergo data origin
authentication as discussed in . Packets that
satisfy data origin authentication are processed further, while all
others are dropped silently. The OMNI interface OAL wraps accepted
packets in OAL/ORH headers if necessary as discussed above.Packets that enter the OMNI interface from the network layer are
forwarded to an OMNI interface neighbor. Packets that enter the OMNI
interface from the link layer are either re-admitted into the OMNI
link or forwarded to the network layer where they are subject to
either local delivery or IP forwarding. In all cases, the OMNI
interface itself MUST NOT decrement the network layer TTL/Hop-count
since its forwarding actions occur below the network layer.OMNI interfaces may have multiple underlying interfaces and/or
neighbor cache entries for neighbors with multiple underlying
interfaces (see ). The OMNI interface uses
interface attributes and/or traffic classifiers (e.g., DSCP value,
port number, etc.) to select an outgoing underlying interface for each
packet based on the node's own QoS preferences, and also to select a
destination link-layer address based on the neighbor's underlying
interface with the highest preference. AERO implementations SHOULD
allow for QoS preference values to be modified at runtime through
network management.If multiple outgoing interfaces and/or neighbor interfaces have a
preference of "high", the AERO node replicates the packet and sends
one copy via each of the (outgoing / neighbor) interface pairs;
otherwise, the node sends a single copy of the packet via an interface
with the highest preference. AERO nodes keep track of which underlying
interfaces are currently "reachable" or "unreachable", and only use
"reachable" interfaces for forwarding purposes.The following sections discuss the OMNI interface forwarding
algorithms for Clients, Proxys, Servers and Bridges. In the following
discussion, a packet's destination address is said to "match" if it is
the same as a cached address, or if it is covered by a cached prefix
(which may be encoded in an LLA).When an IP packet enters a Client's OMNI interface from the
network layer the Client searches for an asymmetric neighbor cache
entry that matches the destination. If there is a match, the Client
uses one or more "reachable" neighbor interfaces in the entry for
packet forwarding. If there is no asymmetric neighbor cache entry,
the Client instead forwards the packet toward a Server (the packet
is intercepted by a Proxy if there is a Proxy on the path). The
Client encapsulates the packet in OAL/ORH headers if necessary and
fragments according to MTU requirements (see: ).When an IP packet enters a Client's OMNI interface from the
link-layer, if the destination matches one of the Client's MNPs or
link-local addresses the Client reassembles and decapsulates as
necessary and delivers the inner packet to the network layer.
Otherwise, the Client drops the packet and MAY return a
network-layer ICMP Destination Unreachable message subject to rate
limiting (see: ).For control messages originating from or destined to a Client,
the Proxy intercepts the message and updates its proxy neighbor
cache entry for the Client. The Proxy then forwards a (proxyed) copy
of the control message. (For example, the Proxy forwards a proxied
version of a Client's NS/RS message to the target neighbor, and
forwards a proxied version of the NA/RA reply to the Client.)When the Proxy receives a data packet from a Client within the
ANET, the Proxy reassembles and re-fragments if necessary then
searches for an asymmetric neighbor cache entry that matches the
destination and forwards as follows:if the destination matches an asymmetric neighbor cache
entry, the Proxy uses one or more "reachable" neighbor
interfaces in the entry for packet forwarding using OAL/ORH
encapsulation if necessary according to the cached link-layer
address information. If the neighbor interface is in the same
OMNI link segment, the Proxy forwards the packet directly to the
neighbor; otherwise, it forwards the packet to a Bridge.else, the Proxy uses OAL/ORH encapsulation and forwards the
packet to a Bridge while using the ULA corresponding to the
packet's destination as the destination address.When the Proxy receives an encapsulated data packet from an INET
neighbor or from a secured tunnel from a Bridge, it accepts the
packet only if data origin authentication succeeds and if there is a
proxy neighbor cache entry that matches the inner destination. Next,
the Proxy reassembles the packet (if necessary) and continues
processing. If the reassembly is complete and the neighbor cache
state is REACHABLE, the Proxy then returns a PTB if necessary (see:
) then either drops or forwards the packet
to the Client while performing OAL/ORH encapsulation and
re-fragmentation if necessary. If the neighbor cache entry state is
DEPARTED, the Proxy instead changes the destination address to the
address of the new Server and forwards it to a Bridge while
performing OAL/ORH re-fragmentation if necessary.For control messages destined to a target Client's LLA that are
received from a secured tunnel, the Server intercepts the message
and sends a Proxyed response on behalf of the Client. (For example,
the Server sends a Proxyed NA message reply in response to an NS
message directed to one of its associated Clients.) If the Client's
neighbor cache entry is in the DEPARTED state, however, the Server
instead forwards the packet to the Client's new Server as discussed
in .When the Server receives an encapsulated data packet from an INET
neighbor or from a secured tunnel, it accepts the packet only if
data origin authentication succeeds. The Server then continues
processing as follows:if the network layer destination matches a symmetric neighbor
cache entry in the REACHABLE state the Server prepares the
packet for forwarding to the destination Client. The Server
first reassembles (if necessary) and forwards the packet (while
re-fragmenting if necessary) as specified in .else, if the destination matches a symmetric neighbor cache
entry in the DEPARETED state the Server re-encapsulates the
packet and forwards it using the ULA of the Client's new Server
as the destination.else, if the destination matches an asymmetric neighbor cache
entry, the Server uses one or more "reachable" neighbor
interfaces in the entry for packet forwarding via the local INET
if the neighbor is in the same OMNI link segment or using
OAL/ORH encapsulation if necessary with the final destination
set to the neighbor's ULA otherwise.else, if the destination matches a non-MNP route in the IP
forwarding table or an LLA assigned to the Server's OMNI
interface, the Server reassembles if necessary, decapsulates the
packet and releases it to the network layer for local delivery
or IP forwarding.else, the Server drops the packet.When the Server's OMNI interface receives a data packet
from the network layer or from a VPNed or Direct Client, it performs
OAL/ORH encapsulation and fragmentation if necessary, then processes
the packet according to the network-layer destination address as
follows:if the destination matches a symmetric or asymmetric neighbor
cache entry the Server processes the packet as above.else, the Server encapsulates the packet in OLA/ORH headers
and forwards it to a Bridge using its own ULA as the source and
the ULA corresponding to the destination as the destination.Bridges forward OAL/ORH-encapsulated packets over secured tunnels
the same as any IP router. When the Bridge receives an
OAL/ORH-encapsulated packet via a secured tunnel, it removes the
outer INET header and searches for a forwarding table entry that
matches the destination address. The Bridge then processes the
packet as follows:if the destination matches its ULA Subnet Router Anycast
address, the Bridge determines if the next header is an ORH. If
so, the Bridge removes the ORH from the packet while
decrementing the OAL header Payload Length field. If the ORH
includes a Destination Suffix the Bridge also writes the ULA
formed from the Destination Suffix into the OAL header
destination address; otherwise, it writes the ULA formed from
the SRT/LHS values. Next, the Bridge examines the FMT to
determine if the target is behind a NAT. If no NAT is indicated,
the Bridge forwards the packet directly to the L2ADDR using
link-layer (UDP/IP) encapsulation. If a NAT is indicated, the
Bridge MAY perform NAT traversal procedures by sending bubbles
per . The Bridge then either applies
AERO route optimization after NAT traversal procedures have
converged, or simply forwards the packet directly to the Server
indicated by SRT/LHS.if the destination matches one of the Bridge's own addresses,
the Bridge submits the packet for local delivery.else, if the destination matches a forwarding table entry the
Bridge forwards the packet via a secured tunnel to the next hop.
If the destination matches an MSP without matching an MNP,
however, the Bridge instead drops the packet and returns an ICMP
Destination Unreachable message subject to rate limiting (see:
).else, the Bridge drops the packet and returns an ICMP
Destination Unreachable as above.As for any IP router, the Bridge decrements the TTL/Hop
Limit when it forwards the packet. Therefore, when an OAL header is
present only the Hop Limit in the OAL header is decremented and not
the TTL/Hop Limit in the inner packet header. Bridges do not insert
OAL/ORH headers themselves; instead, they act as IPv6 routers and
forward packets based on the destination address found in the
headers of packets they receive.When an AERO node admits a packet into the OMNI 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" . (OMNI interfaces ignore
all link-layer IPv4 "Fragmentation Needed" and IPv6 "Packet Too Big"
messages since they only emit packets that are guaranteed to be no
larger than the IP minimum link MTU as discussed in .)The ICMP header is followed by the leading portion of the packet
that generated the error, also known as the "packet-in-error". For
ICMPv6, specifies that the packet-in-error
includes: "As much of invoking packet as possible without the ICMPv6
packet exceeding the minimum IPv6 MTU" (i.e., no more than 1280
bytes). For ICMPv4, specifies that the
packet-in-error includes: "Internet Header + 64 bits of Original Data
Datagram", however Section 4.3.2.3 updates
this specification by stating: "the ICMP datagram SHOULD contain as
much of the original datagram as possible without the length of the
ICMP datagram exceeding 576 bytes".The link-layer error message format is shown in (where, "L2" and "L3" refer to link-layer and
network-layer, respectively):The AERO node rules for processing these link-layer error
messages are as follows:When an AERO node receives a link-layer Parameter Problem
message, it processes the message the same as described as for
ordinary ICMP errors in the normative references .When an AERO node receives persistent link-layer Time Exceeded
messages, the IP ID field may be wrapping before earlier fragments
awaiting reassembly have been processed. In that case, the node
should begin including integrity checks and/or institute rate
limits for subsequent packets.When an AERO node receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its asymmetric neighbor correspondents, the node
should process the message as an indication that a path may be
failing, and optionally initiate NUD over that path. If it
receives Destination Unreachable messages over multiple paths, the
node should allow future packets destined to the correspondent to
flow through a default route and re-initiate route
optimization.When an AERO Client receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its symmetric neighbor Servers, the Client should
mark the path as unusable and use another path. If it receives
Destination Unreachable messages on many or all paths, the Client
should associate with a new Server and release its association
with the old Server as specified in .When an AERO Server receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its symmetric neighbor Clients, the Server should
mark the underlying path as unusable and use another underlying
path.When an AERO Server or Proxy receives link-layer Destination
Unreachable messages in response to an encapsulated packet that it
sends to one of its permanent neighbors, it treats the messages as
an indication that the path to the neighbor may be failing.
However, the dynamic routing protocol should soon reconverge and
correct the temporary outage.When an AERO Bridge receives a packet for which the
network-layer destination address is covered by an MSP, if there is no
more-specific routing information for the destination the Bridge drops
the packet and returns a network-layer Destination Unreachable message
subject to rate limiting. The Bridge writes the network-layer source
address of the original packet as the destination address and uses one
of its non link-local addresses as the source address of the
message.When an AERO node receives an encapsulated packet for which the
reassembly buffer it too small, it drops the packet and returns a
network-layer Packet Too Big (PTB) message. The node first writes the
MRU value into the PTB message MTU field, writes the network-layer
source address of the original packet as the destination address and
writes one of its non link-local addresses as the source address.AERO Router Discovery, Prefix Delegation and Autoconfiguration are
coordinated as discussed in the following Sections.Each AERO Server on the OMNI link is configured to facilitate
Client prefix delegation/registration 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 OMNI link
and securely distributed to all Servers, e.g., via the Lightweight
Directory Access Protocol (LDAP) , via
static configuration, etc. Clients receive the same service
regardless of the Servers they select.AERO Clients and Servers use ND messages to maintain neighbor
cache entries. AERO Servers configure their OMNI interfaces as
advertising NBMA interfaces, and therefore send unicast RA messages
with a short Router Lifetime value (e.g., ReachableTime seconds) in
response to a Client's RS message. Thereafter, Clients send
additional RS messages to keep Server state alive.AERO Clients and Servers include prefix delegation and/or
registration parameters in RS/RA messages (see ). The ND messages are
exchanged between Client and Server according to the prefix
management schedule required by the service. If the Client knows its
MNP in advance, it can employ prefix registration by including its
LLA as the source address of an RS message and with an OMNI option
with valid prefix registration information for the MNP. If the
Server (and Proxy) accept the Client's MNP assertion, they inject
the prefix into the routing system and establish the necessary
neighbor cache state.The following sections specify the Client and Server
behavior.AERO Clients discover the addresses of Servers in a similar
manner as described in . Discovery methods
include static configuration (e.g., from a flat-file map of Server
addresses and locations), or through an automated means such as
Domain Name System (DNS) name resolution .
Alternatively, the Client can discover Server addresses through a
layer 2 data link login exchange, or through a unicast RA response
to a multicast/anycast RS as described below. In the absence of
other information, the Client can resolve the DNS Fully-Qualified
Domain Name (FQDN) "linkupnetworks.[domainname]" where
"linkupnetworks" is a constant text string and "[domainname]" is a
DNS suffix for the OMNI link (e.g., "example.com").To associate with a Server, the Client acts as a requesting
router to request MNPs. The Client prepares an RS message with
prefix management parameters and includes a Nonce and Timestamp
option if the Client needs to correlate RA replies. If the Client
already knows the Server's LLA, it includes the LLA as the
network-layer destination address; otherwise, it includes
(link-local) All-Routers multicast as the network-layer destination.
If the Client already knows its own LLA, it uses the LLA as the
network-layer source address; otherwise, it uses an OMNI Temporary
LLA as the network-layer source address and includes a DHCP Unique
Identifier (DUID) sub-option in the OMNI option (see: ).The Client next includes an OMNI option in the RS message to
register its link-layer information with the Server. The Client sets
the OMNI option prefix registration information according to the
MNP, and includes Interface Attributes corresponding to the
underlying interface over which the Client will send the RS message.
The Client MAY include additional Interface Attributes specific to
other underlying interfaces.The Client then sends the RS message (either directly via Direct
interfaces, via a VPN for VPNed interfaces, via a Proxy for ANET
interfaces or via INET encapsulation for INET interfaces) and waits
for an RA message reply (see ). The
Client retries up to MAX_RTR_SOLICITATIONS times until an RA is
received. If the Client receives no RAs, or if it receives an RA
with Router Lifetime set to 0, the Client SHOULD abandon this Server
and try another Server. Otherwise, the Client processes the prefix
information found in the RA message.Next, the Client creates a symmetric neighbor cache entry with
the Server's LLA as the network-layer address and the Server's
encapsulation and/or link-layer addresses as the link-layer address.
The Client records the RA Router Lifetime field value in the
neighbor cache entry as the time for which the Server has committed
to maintaining the MNP in the routing system via this underlying
interface, and caches the other RA configuration information
including Cur Hop Limit, M and O flags, Reachable Time and Retrans
Timer. The Client then autoconfigures LLAs for each of the delegated
MNPs and assigns them to the OMNI interface. The Client also caches
any MSPs included in Route Information Options (RIOs) as MSPs to associate with the OMNI link, and
assigns the MTU value in the MTU option to the underlying
interface.The Client then registers additional underlying interfaces with
the Server by sending RS messages via each additional interface. The
RS messages include the same parameters as for the initial RS/RA
exchange, but with destination address set to the Server's LLA.Following autoconfiguration, the Client sub-delegates the MNPs to
its attached EUNs and/or the Client's own internal virtual
interfaces as described in
to support the Client's downstream attached "Internet of Things
(IoT)". The Client subsequently sends additional RS messages over
each underlying interface before the Router Lifetime received for
that interface expires.After the Client registers its underlying interfaces, it may wish
to change one or more registrations, e.g., if an interface changes
address or becomes unavailable, if QoS preferences change, etc. To
do so, the Client prepares an RS message to send over any available
underlying interface. The RS includes an OMNI option with prefix
registration information specific to its MNP, with Interface
Attributes specific to the selected underlying interface, and with
any additional Interface Attributes specific to other underlying
interfaces. When the Client receives the Server's RA response, it
has assurance that the Server has been updated with the new
information.If the Client wishes to discontinue use of a Server it issues an
RS message over any underlying interface with an OMNI option with a
prefix release indication. When the Server processes the message, it
releases the MNP, sets the symmetric neighbor cache entry state for
the Client to DEPARTED and returns an RA reply with Router Lifetime
set to 0. After a short delay (e.g., 2 seconds), the Server
withdraws the MNP from the routing system.AERO Servers act as IP routers and support a prefix
delegation/registration service for Clients. Servers arrange to add
their LLAs to a static map of Server addresses for the link and/or
the DNS resource records for the FQDN "linkupnetworks.[domainname]"
before entering service. Server addresses should be geographically
and/or topologically referenced, and made available for discovery by
Clients on the OMNI link.When a Server receives a prospective Client's RS message on its
OMNI 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 prefix delegation/registration parameters.
The Server first determines the correct MNPs to provide to the
Client by searching the Client database. When the Server returns the
MNPs, it also creates a forwarding table entry for the ULA
corresponding to each MNP so that the MNPs are propagated into the
routing system (see: ). For IPv6, the Server
creates an IPv6 forwarding table entry for each MNP. For IPv4, the
Server creates an IPv6 forwarding table entry with the
IPv4-compatibility ULA prefix corresponding to the IPv4 address.The Server next creates a symmetric neighbor cache entry for the
Client using the base LLA as the network-layer address and with
lifetime set to no more than the smallest prefix lifetime. Next, the
Server updates the neighbor cache entry by recording the information
in each Interface Attributes sub-option in the RS OMNI option. The
Server also records the actual OAL/INET addresses in the neighbor
cache entry.Next, the Server prepares an RA message using its LLA as the
network-layer source address and the network-layer source address of
the RS message as the network-layer destination address. The Server
sets the Router Lifetime to the time for which it will maintain both
this underlying interface individually and the symmetric neighbor
cache entry as a whole. The Server also sets Cur Hop Limit, M and O
flags, Reachable Time and Retrans Timer to values appropriate for
the OMNI link. The Server includes the MNPs, any other prefix
management parameters and an OMNI option with no Interface
Attributes. The Server then includes one or more RIOs that encode
the MSPs for the OMNI link, plus an MTU option (see ). The Server finally forwards the message to the
Client using OAL/INET, INET, or NULL encapsulation as necessary.After the initial RS/RA exchange, the Server maintains a
ReachableTime timer for each of the Client's underlying interfaces
individually (and for the Client's symmetric neighbor cache entry
collectively) set to expire after ReachableTime seconds. If the
Client (or Proxy) issues additional RS messages, the Server sends an
RA response and resets ReachableTime. If the Server receives an ND
message with a prefix release indication it sets the Client's
symmetric neighbor cache entry to the DEPARTED state and withdraws
the MNP from the routing system after a short delay (e.g., 2
seconds). If ReachableTime expires before a new RS is received on an
individual underlying interface, the Server marks the interface as
DOWN. If ReachableTime expires before any new RS is received on any
individual underlying interface, the Server sets the symmetric
neighbor cache entry state to STALE and sets a 10 second timer. If
the Server has not received a new RS or ND message with a prefix
release indication before the 10 second timer expires, it deletes
the neighbor cache entry and withdraws the MNP from the routing
system.The Server processes any ND messages pertaining to the Client and
returns an NA/RA reply in response to solicitations. The Server may
also issue unsolicited RA messages, e.g., with reconfigure
parameters to cause the Client to renegotiate its prefix
delegation/registrations, with Router Lifetime set to 0 if it can no
longer service this Client, etc. Finally, If the symmetric neighbor
cache entry is in the DEPARTED state, the Server deletes the entry
after DepartTime expires.Note: Clients SHOULD notify former Servers of their departures,
but Servers are responsible for expiring neighbor cache entries and
withdrawing routes even if no departure notification is received
(e.g., if the Client leaves the network unexpectedly). Servers
SHOULD therefore set Router Lifetime to ReachableTime seconds in
solicited RA messages to minimize persistent stale cache information
in the absence of Client departure notifications. A short Router
Lifetime also ensures that proactive Client/Server RS/RA messaging
will keep any NAT state alive (see above).Note: All Servers on an OMNI link MUST advertise consistent
values in the RA Cur Hop Limit, M and O flags, Reachable Time and
Retrans Timer fields the same as for any link, since unpredictable
behavior could result if different Servers on the same link
advertised different values.When a Client is not pre-provisioned with an OMNI LLA
containing a MNP, it will need for the Server to select one or
more MNPs on its behalf and set up the correct state in the AERO
routing service. (A Client with a pre-provisioned MNP may also
request the Server to select additional MNPs.) The DHCPv6 service
is used to support this requirement.When a Client needs to have the Server select MNPs, it sends an
RS message with an OMNI option that includes a DHCPv6 message
suboption with DHCPv6 Prefix Delegation (DHCPv6-PD) parameters.
When the Server receives the RS message, it extracts the DHCPv6-PD
message from the OMNI option.The Server then acts as a "Proxy DHCPv6 Client" in a message
exchange with the locally-resident DHCPv6 server, which delegates
MNPs and returns a DHCPv6-PD Reply message. (If the Server wishes
to defer creation of MN state until the DHCPv6-PD Reply is
received, it can instead act as a Lightweight DHCPv6 Relay Agent
per by encapsulating the DHCPv6-PD
message in a Relay-forward/reply exchange with Relay Message and
Interface ID options.)When the Server receives the DHCPv6-PD Reply, it adds a route
to the routing system and creates an OMNI MN LLA based on the
delegated MNP. The Server then sends an RA back to the Client with
the (newly-created) OMNI MN LLA as the destination address and
with the DHCPv6-PD Reply message coded in the OMNI option. When
the Client receives the RA, it creates a default route, assigns
the Subnet Router Anycast address and sets its OMNI LLA based on
the delegated MNP.Clients may connect to protected-spectrum ANETs that employ
physical and/or link-layer security services to facilitate
communications to Servers in outside INETs. In that case, the ANET can
employ an AERO Proxy. The Proxy is located at the ANET/INET border and
listens for RS messages originating from or RA messages destined to
ANET Clients. The Proxy acts on these control messages as follows:when the Proxy receives an RS message from a new ANET Client,
it first authenticates the message then examines the network-layer
destination address. If the destination address is a Server's LLA,
the Proxy proceeds to the next step. Otherwise, if the destination
is (link-local) All-Routers multicast, the Proxy selects a
"nearby" Server that is likely to be a good candidate to serve the
Client and replaces the destination address with the Server's LLA.
Next, the Proxy creates a proxy neighbor cache entry and caches
the Client and Server link-layer addresses along with the OMNI
option information and any other identifying information including
Transaction IDs, Client Identifiers, Nonce values, etc. The Proxy
finally encapsulates the (proxyed) RS message in an OAL header
with source set to the Proxy's ULA and destination set to the
Server's ULA. The Proxy also includes an OMNI header with an
Interface Attributes option that includes its own INET address
plus a unique Port Number for this Client, then forwards the
message into the OMNI link spanning tree.when the Server receives the RS, it authenticates the message
then creates or updates a symmetric neighbor cache entry for the
Client with the Proxy's ULA, INET address and Port Number as the
link-layer address information. The Server then sends an RA
message back to the Proxy via the spanning tree.when the Proxy receives the RA, it authenticates the message
and matches it with the proxy neighbor cache entry created by the
RS. The Proxy then caches the prefix information as a mapping from
the Client's MNPs to the Client's link-layer address, caches the
Server's advertised Router Lifetime and sets the neighbor cache
entry state to REACHABLE. The Proxy then optionally rewrites the
Router Lifetime and forwards the (proxyed) message to the Client.
The Proxy finally includes an MTU option (if necessary) with an
MTU to use for the underlying ANET interface.After the initial RS/RA exchange, the Proxy forwards any
Client data packets for which there is no matching asymmetric neighbor
cache entry to a Bridge using OAL encapsulation with its own ULA as
the source and the ULA corresponding to the Client as the destination.
The Proxy instead forwards any Client data destined to an asymmetric
neighbor cache target directly to the target according to the
OAL/link-layer information - the process of establishing asymmetric
neighbor cache entries is specified in .While the Client is still attached to the ANET, the Proxy sends NS,
RS and/or unsolicited NA messages to update the Server's symmetric
neighbor cache entries on behalf of the Client and/or to convey QoS
updates. This allows for higher-frequency Proxy-initiated RS/RA
messaging over well-connected INET infrastructure supplemented by
lower-frequency Client-initiated RS/RA messaging over constrained ANET
data links.If the Server ceases to send solicited advertisements, the Proxy
sends unsolicited RAs on the ANET interface with destination set to
(link-local) All-Nodes multicast and with Router Lifetime set to zero
to inform Clients that the Server has failed. Although the Proxy
engages in ND exchanges on behalf of the Client, the Client can also
send ND messages on its own behalf, e.g., if it is in a better
position than the Proxy to convey QoS changes, etc. For this reason,
the Proxy marks any Client-originated solicitation messages (e.g. by
inserting a Nonce option) so that it can return the solicited
advertisement to the Client instead of processing it locally.If the Client becomes unreachable, the Proxy sets the neighbor
cache entry state to DEPARTED and retains the entry for DepartTime
seconds. While the state is DEPARTED, the Proxy forwards any packets
destined to the Client to a Bridge via OAL encapsulation with the
Client's current Server as the destination. The Bridge in turn
forwards the packets to the Client's current Server. When DepartTime
expires, the Proxy deletes the neighbor cache entry and discards any
further packets destined to this (now forgotten) Client.In some ANETs that employ a Proxy, the Client's MNP can be injected
into the ANET routing system. In that case, the Client can send data
messages without encapsulation so that the ANET routing system
transports the unencapsulated packets to the Proxy. This can be very
beneficial, e.g., if the Client connects to the ANET via low-end data
links such as some aviation wireless links.If the first-hop ANET access router is on the same underlying link
and recognizes the AERO/OMNI protocol, the Client can avoid
encapsulation for both its control and data messages. When the Client
connects to the link, it can send an unencapsulated RS message with
source address set to its LLA and with destination address set to the
LLA of the Client's selected Server or to (link-local) All-Routers
multicast. The Client includes an OMNI option formatted as specified
in .The Client then sends the unencapsulated RS message, which will be
intercepted by the AERO-Aware access router. The access router then
encapsulates the RS message in an ANET header with its own address as
the source address and the address of a Proxy as the destination
address. The access router further remembers the address of the Proxy
so that it can encapsulate future data packets from the Client via the
same Proxy. If the access router needs to change to a new Proxy, it
simply sends another RS message toward the Server via the new Proxy on
behalf of the Client.In some cases, the access router and Proxy may be one and the same
node. In that case, the node would be located on the same physical
link as the Client, but its message exchanges with the Server would
need to pass through a security gateway at the ANET/INET border. The
method for deploying access routers and Proxys (i.e. as a single node
or multiple nodes) is an ANET-local administrative consideration.Clients may need to connect directly to Servers via INET, Direct
and VPNed interfaces (i.e., non-ANET interfaces). If the Client's
underlying interfaces all connect via the same INET partition, then
it can connect to a single controlling Server via all
interfaces.If some Client interfaces connect via different INET partitions,
however, the Client still selects a set of controlling Servers and
sends RS messages via their directly-connected Servers while using
the LLA of the controlling Server as the destination.When a Server receives an RS with destination set to the LLA of a
controlling Server, it acts as a Proxy to forward the message to the
controlling Server while forwarding the corresponding RA reply to
the Client.In environments where fast recovery from Server failure is
required, Proxys SHOULD use proactive Neighbor Unreachability
Detection (NUD) to track Server reachability in a similar fashion as
for Bidirectional Forwarding Detection (BFD) . Proxys can then quickly detect and react to
failures so that cached information is re-established through
alternate paths. The NUD control messaging is carried only over
well-connected ground domain networks (i.e., and not low-end
aeronautical radio links) and can therefore be tuned for rapid
response.Proxys perform proactive NUD with Servers for which there are
currently active ANET Clients by sending continuous NS messages in
rapid succession, e.g., one message per second. The Proxy sends the
NS message via the spanning tree with the Proxy's LLA as the source
and the LLA of the Server as the destination. When the Proxy is also
sending RS messages to the Server on behalf of ANET Clients, the
resulting RA responses can be considered as equivalent hints of
forward progress. This means that the Proxy need not also send a
periodic NS if it has already sent an RS within the same period. If
the Server fails (i.e., if the Proxy ceases to receive
advertisements), the Proxy can quickly inform Clients by sending
multicast RA messages on the ANET interface.The Proxy sends RA messages on the ANET interface with source
address set to the Server's address, destination address set to
(link-local) All-Nodes multicast, and Router Lifetime set to 0. The
Proxy SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated
by small delays . Any Clients on the ANET
that had been using the failed Server will receive the RA messages
and associate with a new Server.In environments where Client messaging over ANETs is
bandwidth-limited and/or expensive, Clients can enlist the services
of the Proxy to coordinate with multiple Servers in a single RS/RA
message exchange. The Client can send a single RS message to
(link-local) All-Routers multicast that includes the ID's of
multiple Servers in MS-Register sub-options of the OMNI option.When the Proxy receives the RS and processes the OMNI option, it
sends a separate RS to each MS-Register Server ID. When the Proxy
receives an RA, it can optionally return an immediate "singleton" RA
to the Client or record the Server's ID for inclusion in a pending
"aggregate" RA message. The Proxy can then return aggregate RA
messages to the Client including multiple Server IDs in order to
conserve bandwidth. Each RA includes a proper subset of the Server
IDs from the original RS message, and the Proxy must ensure that the
message contents of each RA are consistent with the information
received from the (aggregated) Servers.Clients can thereafter employ efficient point-to-multipoint
Server coordination under the assistance of the Proxy to reduce the
number of messages sent over the ANET while enlisting the support of
multiple Servers for fault tolerance. Clients can further include
MS-Release sub-options in IPv6 ND messages to request the Proxy to
release from former Servers via the procedures discussed in .The OMNI interface specification provides further
discussion of the Client/Proxy RS/RA messaging involved in
point-to-multipoint coordination.While data packets are flowing between a source and target node,
route optimization SHOULD be used. Route optimization is initiated by
the first eligible Route Optimization Source (ROS) closest to the
source as follows:For Clients on VPNed and Direct interfaces, the Server is the
ROS.For Clients on ANET interfaces, the Proxy is the ROS.For Clients on INET interfaces, the Client itself is the
ROS.For correspondent nodes on INET/EUN interfaces serviced by a
Relay, the Relay is the ROS.The route optimization procedure is conducted between the ROS and
the target Server/Relay acting as a Route Optimization Responder (ROR)
in the same manner as for IPv6 ND Address Resolution and using the
same NS/NA messaging. The target may either be a MNP Client serviced
by a Server, or a non-MNP correspondent reachable via a Relay.The procedures are specified in the following sections.While data packets are flowing from the source node toward a
target node, the ROS performs address resolution by sending an NS
message for Address Resolution (NS(AR)) to receive a solicited NA
message from the ROR. When the ROS sends an NS(AR), it includes:the LLA of the ROS as the source address.the data packet's destination as the Target Address.the Solicited-Node multicast address
formed from the lower 24 bits of the data packet's destination
as the destination address, e.g., for 2001:db8:1:2::10:2000 the
NS destination address is ff02:0:0:0:0:1:ff10:2000.The NS(AR) message includes an OMNI option with no
Interface Attributes, such that the target will not create a
neighbor cache entry. The Prefix Length in the OMNI option is set to
the Prefix Length associated with the ROS's LLA.The ROS then encapsulates the NS(AR) message in an OAL header
with source set to its own ULA and destination set to the ULA
corresponding to the target, then sends the message into the
spanning tree without decrementing the network-layer TTL/Hop Limit
field. (When the ROS is a Client, it instead securely sends the
NS(AR) to one of its current Servers by including an Authentication
option per . The Server then forwards the
message into the spanning tree on behalf of the Client, while
setting the IPv6 source address and the OAL source address to the
LLA and ULA of the Client, respectively.)When the Bridge receives the NS(AR) message from the ROS, it
discards the INET header and determines that the ROR is the next hop
by consulting its standard IPv6 forwarding table for the OAL header
destination address. The Bridge then forwards the message toward the
ROR via the spanning tree the same as for any IPv6 router. The
final-hop Bridge in the spanning tree will deliver the message via a
secured tunnel to the ROR.When the ROR receives the NS(AR) message, it examines the Target
Address to determine whether it has a neighbor cache entry and/or
route that matches the target. If there is no match, the ROR drops
the message. Otherwise, the ROR continues processing as follows:if the target belongs to an MNP Client neighbor in the
DEPARTED state the ROR changes the NS(AR) message OAL
destination address to the ULA of the Client's new Server,
forwards the message into the spanning tree and returns from
processing.If the target belongs to an MNP Client neighbor in the
REACHABLE state, the ROR instead adds the AERO source address to
the target Client's Report List with time set to ReportTime.If the target belongs to a non-MNP route, the ROR continues
processing without adding an entry to the Report List.The ROR then prepares a solicited NA message to send back
to the ROS but does not create a neighbor cache entry. The ROR sets
the NA source address to the LLA corresponding to the target, sets
the Target Address to the target of the solicitation, and sets the
destination address to the source of the solicitation. The ROR then
includes an OMNI option with Prefix Length set to the length
associated with the LLA.If the target is an MNP Client, the ROR next includes Interface
Attributes in the OMNI option for each of the target Client's
underlying interfaces with current information for each interface
and with the S/T-ifIndex field in the OMNI header set to 0 to
indicate that the message originated from the ROR and not the
Client.For each Interface Attributes sub-option, the ROR sets the L2ADDR
according to its own INET address for VPNed or Direct interfaces, to
the INET address of the Proxy or to the Client's INET address for
INET interfaces. The ROR then includes the lower 32 bits of its own
ULA (or the ULA of the Proxy) as the LHS, encodes the ULA prefix
length code in the SRT field and sets the FMT code accordingly as
specified in .The ROR then sets the NA message R flag to 1 (as a router), S
flag to 1 (as a response to a solicitation), and O flag to 0 (as a
proxy). The ROR finally encapsulates the NA message in an OAL header
with source set to its own ULA and destination set to the source ULA
of the NS(AR) message, then forwards the message into the spanning
tree without decrementing the network-layer TTL/Hop Limit field.When the Bridge receives the NA message from the ROR, it discards
the INET header and determines that the ROS is the next hop by
consulting its standard IPv6 forwarding table for the OAL header
destination address. The Bridge then forwards the OAL-encapsulated
NA message toward the ROS the same as for any IPv6 router. The
final-hop Bridge in the spanning tree will deliver the message via a
secured tunnel to the ROS.When the ROS receives the solicited NA message, it processes the
message the same as for standard IPv6 Address Resolution . In the process, it caches the source ULA then
creates an asymmetric neighbor cache entry for the target and caches
all information found in the OMNI option. The ROS finally sets the
asymmetric neighbor cache entry lifetime to ReachableTime seconds.
(When the ROS is a Client, the solicited NA message will first be
delivered via the spanning tree to one of its current Servers, which
then securely forwards the message to the Client by including an
Authentication option per .Following route optimization, the ROS forwards future data
packets destined to the target via the addresses found in the cached
link-layer information. The route optimization is shared by all
sources that send packets to the target via the ROS, i.e., and not
just the source on behalf of which the route optimization was
initiated.While new data packets destined to the target are flowing through
the ROS, it sends additional NS(AR) messages to the ROR before
ReachableTime expires to receive a fresh solicited NA message the
same as described in the previous sections (route optimization
refreshment strategies are an implementation matter, with a
non-normative example given in ). The ROS
uses the cached ULA of the ROR as the NS(AR) OAL destination address
(i.e., instead of using the ULA corresponding to the target as was
the case for the initial NS(AR)), and sends up to
MAX_MULTICAST_SOLICIT NS(AR) messages separated by 1 second until an
NA is received. If no NA is received, the ROS assumes that the
current ROR has become unreachable and deletes the target neighbor
cache entry. Subsequent data packets will trigger a new route
optimization with an NS with OAL destination address set to the ULA
corresponding to the target per to discover
a new ROR while initial data packets travel over a suboptimal
route.If an NA is received, the ROS then updates the asymmetric
neighbor cache entry to refresh ReachableTime, while (for MNP
destinations) the ROR adds or updates the ROS address to the
target's Report List and with time set to ReportTime. While no data
packets are flowing, the ROS instead allows ReachableTime for the
asymmetric neighbor cache entry to expire. When ReachableTime
expires, the ROS deletes the asymmetric neighbor cache entry. Any
future data packets flowing through the ROS will again trigger a new
route optimization.The ROS may also receive unsolicited NA messages from the ROR at
any time (see: ). If there is an asymmetric
neighbor cache entry for the target, the ROS updates the link-layer
information but does not update ReachableTime since the receipt of
an unsolicited NA does not confirm that any forward paths are
working. If there is no asymmetric neighbor cache entry, the ROS
simply discards the unsolicited NA.In this arrangement, the ROS holds an asymmetric neighbor cache
entry for the target via the ROR, but the ROR does not hold an
asymmetric neighbor cache entry for the ROS. The route optimization
neighbor relationship is therefore asymmetric and unidirectional. If
the target node also has packets to send back to the source node,
then a separate route optimization procedure is performed in the
reverse direction. But, there is no requirement that the forward and
reverse paths be symmetric.AERO nodes perform Neighbor Unreachability Detection (NUD) per
either reactively in response to persistent
link-layer errors (see ) or proactively to
confirm reachability. The NUD algorithm is based on periodic control
message exchanges. The algorithm may further be seeded by ND hints of
forward progress, but care must be taken to avoid inferring
reachability based on spoofed information. For example, authentic IPv6
ND message exchanges may be considered as acceptable hints of forward
progress, while spurious data packets should not be.AERO Servers, Proxys and Relays can use (OAL-encapsulated) standard
NS/NA NUD exchanges sent over the OMNI link spanning tree to securely
test reachability without risk of DoS attacks from nodes pretending to
be a neighbor (these NS/NA(NUD) messages use the unicast LLAs and ULAs
of the two parties involved in the NUD test the same as for standard
IPv6 ND). Proxys can further perform NUD to securely verify Server
reachability on behalf of their proxyed Clients. However, a means for
an ROS to test the unsecured forward directions of target route
optimized paths is also necessary.When an ROR directs an ROS to a neighbor with one or more target
link-layer addresses, the ROS can proactively test each such unsecured
route optimized path by sending "loopback" NS(NUD) messages. While
testing the paths, the ROS can optionally continue to send packets via
the spanning tree, maintain a small queue of packets until target
reachability is confirmed, or (optimistically) allow packets to flow
via the route optimized paths.When the ROS sends a loopback NS(NUD) message, it uses its own LLA
as both the IPv6 source and destination address, and the MNP
Subnet-Router anycast address as the Target Address. The ROS includes
a Nonce and Timestamp option, then encapsulates the message in
OAL/INET headers with its own ULA as the source and the ULA of the
route optimization target as the destination. The ROS then forwards
the message to the target (either directly to the L2ADDR of the target
if the target is in the same OMNI link segment, or via a Bridge if the
target is in a different OMNI link segment).When the route optimization target receives the NS(NUD) message, it
notices that the IPv6 destination address is the same as the source
address. It then reverses the OAL header source and destination
addresses and returns the message to the ROS (either directly or via
the spanning tree). The route optimization target does not decrement
the NS(NUD) message IPv6 Hop-Limit in the process, since the message
has not exited the OMNI link.When the ROS receives the NS(NUD) message, it can determine from
the Nonce, Timestamp and Target Address that the message originated
from itself and that it transited the forward path. The ROS need not
prepare an NA response, since the destination of the response would be
itself and testing the route optimization path again would be
redundant.The ROS marks route optimization target paths that pass these NUD
tests as "reachable", and those that do not as "unreachable". These
markings inform the OMNI interface forwarding algorithm specified in
.Note that to avoid a DoS vector nodes MUST NOT return loopback
NS(NUD) messages received from an unsecured link-layer source via the
(secured) spanning tree.AERO is a Distributed Mobility Management (DMM) service. Each
Server is responsible for only a subset of the Clients on the OMNI
link, as opposed to a Centralized Mobility Management (CMM) service
where there is a single network mobility collective entity for all
Clients. Clients coordinate with their associated Servers via RS/RA
exchanges to maintain the DMM profile, and the AERO routing system
tracks all current Client/Server peering relationships.Servers provide default routing and mobility/multilink services for
their dependent Clients. Clients are responsible for maintaining
neighbor relationships with their Servers through periodic RS/RA
exchanges, which also serves to confirm neighbor reachability. When a
Client's underlying interface address and/or QoS information changes,
the Client is responsible for updating the Server with this new
information. Note that when there is a Proxy in the path, the Proxy
can also perform some RS/RA exchanges on the Client's behalf.Mobility management messaging is based on the transmission and
reception of unsolicited Neighbor Advertisement (uNA) messages. Each
uNA message sets the IPv6 destination address to (link-local)
All-Nodes multicast to convey a general update of Interface Attributes
to (possibly) multiple recipients, or to a specific unicast LLA to
announce a departure event to a specific recipient. Implementations
must therefore examine the destination address to determine the nature
of the mobility event (i.e., update vs departure).Mobility management considerations are specified in the following
sections.Servers accommodate Client mobility, multilink and/or QoS change
events by sending unsolicited NA (uNA) messages to each ROS in the
target Client's Report List. When a Server sends a uNA message, it
sets the IPv6 source address to the Client's LLA, sets the
destination address to (link-local) All-Nodes multicast and sets the
Target Address to the Client's Subnet-Router anycast address. The
Server also includes an OMNI option with Prefix Length set to the
length associated with the Client's LLA, with Interface Attributes
for the target Client's underlying interfaces and with the OMNI
header S/T-ifIndex set to 0. The Server then sets the NA R flag to
1, the S flag to 0 and the O flag to 1, then encapsulates the
message in an OAL header with source set to its own ULA and
destination set to the ULA of the ROS and sends the message into the
spanning tree.As discussed in Section 7.2.6 of , the
transmission and reception of uNA messages is unreliable but
provides a useful optimization. In well-connected Internetworks with
robust data links uNA messages will be delivered with high
probability, but in any case the Server can optionally send up to
MAX_NEIGHBOR_ADVERTISEMENT uNAs to each ROS to increase the
likelihood that at least one will be received.When the ROS receives a uNA message prepared as above, it ignores
the message if there is no existing neighbor cache entry for the
Client. Otherwise, it uses the included OMNI option information to
update the neighbor cache entry, but does not reset ReachableTime
since the receipt of an unsolicited NA message from the target
Server does not provide confirmation that any forward paths to the
target Client are working.If uNA messages are lost, the ROS may be left with stale address
and/or QoS information for the Client for up to ReachableTime
seconds. During this time, the ROS can continue sending packets
according to its stale neighbor cache information. When
ReachableTime is close to expiring, the ROS will re-initiate route
optimization and receive fresh link-layer address information.In addition to sending uNA messages to the current set of ROSs
for the Client, the Server also sends uNAs to the ULA associated
with the link-layer address for any underlying interface for which
the link-layer address has changed. These uNA messages update an old
Proxy/Server that cannot easily detect (e.g., without active
probing) when a formerly-active Client has departed. When the Server
sends the uNA, it sets the IPv6 source address to the Client's LLA,
sets the destination address to the old Proxy/Server's LLA, and sets
the Target Address to the Client's Subnet-Router anycast address.
The Server also includes an OMNI option with Prefix Length set to
the length associated with the Client's LLA, with Interface
Attributes for the changed underlying interface, and with the OMNI
header S/T-ifIndex set to 0. The Server then sets the NA R flag to
1, the S flag to 0 and the O flag to 1, then encapsulates the
message in an OAL header with source set to its own ULA and
destination set to the ULA of the old Proxy/Server and sends the
message into the spanning tree.When a Client needs to change its underlying interface addresses
and/or QoS preferences (e.g., due to a mobility event), either the
Client or its Proxys send RS messages to the Server via the spanning
tree with an OMNI option that includes Interface attributes with the
new link quality and address information.Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel
with sending actual data packets in case one or more RAs are lost.
If all RAs are lost, the Client SHOULD re-associate with a new
Server.When the Server receives the Client's changes, it sends uNA
messages to all nodes in the Report List the same as described in
the previous section.When a Client needs to bring new underlying interfaces into
service (e.g., when it activates a new data link), it sends an RS
message to the Server via the underlying interface with an OMNI
option that includes Interface Attributes with appropriate link
quality values and with link-layer address information for the new
link.When a Client needs to deactivate an existing underlying
interface, it sends an RS or uNA message to its Server with an OMNI
option with appropriate Interface Attribute values - in particular,
the link quality value 0 assures that neighbors will cease to use
the link.If the Client needs to send RS/uNA messages over an underlying
interface other than the one being deactivated, it MUST include
Interface Attributes with appropriate link quality values for any
underlying interfaces being deactivated.Note that when a Client deactivates an underlying interface,
neighbors that have received the RS/uNA messages need not purge all
references for the underlying interface from their neighbor cache
entries. The Client may reactivate or reuse the underlying interface
and/or its ifIndex at a later point in time, when it will send
RS/uNA messages with fresh Interface Attributes to update any
neighbors.The Client performs the procedures specified in when it first associates with a new Server
or renews its association with an existing Server. The Client also
includes MS-Release identifiers in the RS message OMNI option per
if it wants the new
Server to notify any old Servers from which the Client is
departing.When the new Server receives the Client's RS message, it returns
an RA as specified in and sends up to
MAX_NEIGHBOR_ADVERTIISEMENT uNA messages to any old Servers listed
in OMNI option MS-Release identifiers. When the new Server sends a
uNA message, it sets the IPv6 source address to the Client's LLA,
sets the destination address to the old Server's LLA, and sets the
Target Address to the Client's Subnet-Router anycast address. The
new Server also includes an OMNI option with Prefix Length set to
the length associated with the Client's LLA, with Interface
Attributes for its own underlying interface, and with the OMNI
header S/T-ifIndex set to 0. The new Server then sets the NA R flag
to 1, the S flag to 0 and the O flag to 1, then encapsulates the
message in an OAL header with source set to its own ULA and
destination set to the ULA of the old Server and sends the message
into the spanning tree.When an old Server receives the uNA, it changes the Client's
neighbor cache entry state to DEPARTED, sets the link-layer address
of the Client to the new Server's ULA, and resets DepartTime. After
a short delay (e.g., 2 seconds) the old Server withdraws the
Client's MNP from the routing system. After DepartTime expires, the
old Server deletes the Client's neighbor cache entry.The old Server also iteratively forwards a copy of the uNA
message to each ROS in the Client's Report List by changing the OAL
destination address to the ULA of the ROS while leaving all other
fields of the message unmodified. When the ROS receives the uNA, it
examines the Target address to determine the correct asymmetric
neighbor cache entry and verifies that the IPv6 destination address
matches the old Server. The ROS then caches the IPv6 source address
as the new Server for the existing asymmetric neighbor cache entry
and marks the entry as STALE. While in the STALE state, the ROS
allows new data packets to flow according to any existing cached
link-layer information and sends new NS(AR) messages using its own
ULA as the OAL source and the ULA of the new Server as the OAL
destination address to elicit NA messages that reset the asymmetric
neighbor cache entry state to REACHABLE. If no new NA message is
received for 10 seconds while in the STALE state, the ROS deletes
the neighbor cache entry.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 OMNI link segment, etc.When a Client moves to a new Server, some of the fragments of a
multiple fragment packet may have already arrived at the old Server
while others are en route to the new Server, however no special
attention in the reassembly algorithm is necessary when re-routed
fragments are simply treated as loss.The AERO Client provides an IGMP (IPv4) or
MLD (IPv6) proxy service for its EUNs and/or
hosted applications . The Client forwards
IGMP/MLD messages over any of its underlying interfaces for which
group membership is required. The IGMP/MLD messages may be further
forwarded by a first-hop ANET access router acting as an
IGMP/MLD-snooping switch , then ultimately
delivered to an AERO Proxy/Server acting as a Protocol Independent
Multicast - Sparse-Mode (PIM-SM, or simply "PIM") Designated Router
(DR) . AERO Relays also act as PIM routers
(i.e., the same as AERO Proxys/Servers) on behalf of nodes on INET/EUN
networks. The behaviors identified in the following sections
correspond to Source-Specific Multicast (SSM) and Any-Source Multicast
(ASM) operational modes.When an ROS (i.e., an AERO Proxy/Server/Relay) "X" acting as PIM
router receives a Join/Prune message from a node on its downstream
interfaces containing one or more ((S)ource, (G)roup) pairs, it
updates its Multicast Routing Information Base (MRIB) accordingly.
For each S belonging to a prefix reachable via X's non-OMNI
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 OMNI
interface, X originates a separate copy of the Join/Prune for each
(S,G) in the message using its own LLA as the source address and
ALL-PIM-ROUTERS as the destination address. X then encapsulates each
message in an OAL header with source address set to the ULA of X and
destination address set to S then forwards the message into the
spanning tree, which delivers it to AERO Server/Relay "Y" that
services S. At the same time, if the message was a Join, X sends a
route-optimization NS message toward each S the same as discussed in
. The resulting NAs will return the LLA
for the prefix that matches S as the network-layer source address
and with an OMNI option with the ULA corresponding to any underlying
interfaces that are currently servicing S.When Y processes the Join/Prune message, if S located behind any
INET, Direct, or VPNed interfaces Y acts as a PIM router and updates
its MRIB to list X as the next hop in the reverse path. If S is
located behind any Proxys "Z"*, Y also forwards the message to each
Z* over the spanning tree while continuing to use the LLA of X as
the source address. Each Z* then updates its MRIB accordingly and
maintains the LLA of X as the next hop in the reverse path. Since
the Bridges do not examine network layer control messages, this
means that the (reverse) multicast tree path is simply from each Z*
(and/or Y) to X with no other multicast-aware routers in the path.
If any Z* (and/or Y) is located on the same OMNI link segment as X,
the multicast data traffic sent to X directly using OAL/INET
encapsulation instead of via a Bridge.Following the initial Join/Prune and NS/NA messaging, X maintains
an asymmetric neighbor cache entry for each S the same as if X was
sending unicast data traffic to S. In particular, X performs
additional NS/NA exchanges to keep the neighbor cache entry alive
for up to t_periodic seconds . If no new
Joins are received within t_periodic seconds, X allows the neighbor
cache entry to expire. Finally, if X receives any additional
Join/Prune messages for (S,G) it forwards the messages to each Y and
Z* in the neighbor cache entry over the spanning tree.At some later time, Client C that holds an MNP for source S may
depart from a first Proxy Z1 and/or connect via a new Proxy Z2. In
that case, Y sends an unsolicited NA message to X the same as
specified for unicast mobility in . When X
receives the unsolicited NA message, it updates its asymmetric
neighbor cache entry for the LLA for source S and sends new Join
messages to any new Proxys Z2. There is no requirement to send any
Prune messages to old Proxys Z1 since source S will no longer source
any multicast data traffic via Z1. Instead, the multicast state for
(S,G) in Proxy Z1 will soon time out since no new Joins will
arrive.After some later time, C may move to a new Server Y2 and depart
from old Sever Y1. In that case, Y1 sends Join messages for any of
C's active (S,G) groups to Y2 while including its own LLA as the
source address. This causes Y2 to include Y1 in the multicast
forwarding tree during the interim time that Y1's symmetric neighbor
cache entry for C is in the DEPARTED state. At the same time, Y1
sends an unsolicited NA message to X with an OMNI option with
S/T-ifIndex in the header set to 0 and a release indication to cause
X to release its asymmetric neighbor cache entry. X then sends a new
Join message to S via the spanning tree and re-initiates route
optimization the same as if it were receiving a fresh Join message
from a node on a downstream link.When an ROS X acting as a PIM router receives a Join/Prune from a
node on its downstream interfaces containing one or more (*,G)
pairs, it updates its Multicast Routing Information Base (MRIB)
accordingly. X then forwards a copy of the message to the Rendezvous
Point (RP) R for each G over the spanning tree. X uses its own LLA
as the source address and ALL-PIM-ROUTERS as the destination
address, then encapsulates each message in an OAL header with source
address set to the ULA of X and destination address set to R, then
sends the message into the spanning tree. At the same time, if the
message was a Join X initiates NS/NA route optimization the same as
for the SSM case discussed in .For each source S that sends multicast traffic to group G via R,
the Proxy/Server Z* for the Client that aggregates S encapsulates
the packets in PIM Register messages and forwards them to R via the
spanning tree, which may then elect to send a PIM Join to Z*. This
will result in an (S,G) tree rooted at Z* with R as the next hop so
that R will begin to receive two copies of the packet; one native
copy from the (S, G) tree and a second copy from the pre-existing
(*, G) tree that still uses PIM Register encapsulation. R can then
issue a PIM Register-stop message to suppress the
Register-encapsulated stream. At some later time, if C moves to a
new Proxy/Server Z*, it resumes sending packets via PIM Register
encapsulation via the new Z*.At the same time, as multicast listeners discover individual S's
for a given G, they can initiate an (S,G) Join for each S under the
same procedures discussed in . Once the
(S,G) tree is established, the listeners can send (S, G) Prune
messages to R so that multicast packets for group G sourced by S
will only be delivered via the (S, G) tree and not from the (*, G)
tree rooted at R. All mobility considerations discussed for SSM
apply.Bi-Directional PIM (BIDIR-PIM) provides
an alternate approach to ASM that treats the Rendezvous Point (RP)
as a Designated Forwarder (DF). Further considerations for BIDIR-PIM
are out of scope.An AERO Client can connect to multiple OMNI links the same as for
any data link service. In that case, the Client maintains a distinct
OMNI interface for each link, e.g., 'omni0' for the first link,
'omni1' for the second, 'omni2' for the third, etc. Each OMNI link
would include its own distinct set of Bridges, Servers and Proxys,
thereby providing redundancy in case of failures.Each OMNI link could utilize the same or different ANET
connections. The links can be distinguished at the link-layer via the
SRT prefix in a similar fashion as for Virtual Local Area Network
(VLAN) tagging (e.g., IEEE 802.1Q) and/or through assignment of
distinct sets of MSPs on each link. This gives rise to the opportunity
for supporting multiple redundant networked paths, with each VLAN
distinguished by a different SRT "color" (see: ).The Client's IP layer can select the outgoing OMNI interface
appropriate for a given traffic profile while (in the reverse
direction) correspondent nodes must have some way of steering their
packets destined to a target via the correct OMNI link.In a first alternative, if each OMNI link services different MSPs,
then the Client can receive a distinct MNP from each of the links. IP
routing will therefore assure that the correct Red/Green/Blue/etc.
network is used for both outbound and inbound traffic. This can be
accomplished using existing technologies and approaches, and without
requiring any special supporting code in correspondent nodes or
Bridges.In a second alternative, if each OMNI link services the same MSP(s)
then each link could assign a distinct "OMNI link Anycast" address
that is configured by all Bridges on the link. Correspondent nodes can
then perform Segment Routing to select the correct SRT, which will
then direct the packet over multiple hops to the target.AERO Client MNs and INET correspondent nodes consult the Domain
Name System (DNS) the same as for any Internetworking node. When
correspondent nodes and Client MNs use different IP protocol versions
(e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain A
records for IPv4 address mappings to MNs which must then be populated
in Relay NAT64 mapping caches. In that way, an IPv4 correspondent node
can send packets to the IPv4 address mapping of the target MN, and the
Relay will translate the IPv4 header and destination address into an
IPv6 header and IPv6 destination address of the MN.When an AERO Client registers with an AERO Server, the Server can
return the address(es) of DNS servers in RDNSS options . The DNS server provides the IP addresses of other
MNs and correspondent nodes in AAAA records for IPv6 or A records for
IPv4.OAL encapsulation ensures that dissimilar INET partitions can be
joined into a single unified OMNI link, even though the partitions
themselves may have differing protocol versions and/or incompatible
addressing plans. However, a commonality can be achieved by
incrementally distributing globally routable (i.e., native) IP
prefixes to eventually reach all nodes (both mobile and fixed) in all
OMNI link segments. This can be accomplished by incrementally
deploying AERO Relays on each INET partition, with each Relay
distributing its MNPs and/or discovering non-MNP IP GUA 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 OMNI link view
even if the INET partitions remain in their current protocol and
addressing plans. In that way, the OMNI link can serve the dual
purpose of providing a mobility/multilink service and a transition
service. Or, if an INET partition is transitioned to a native IP
protocol version and addressing scheme that is compatible with the
OMNI link MNP-based addressing scheme, the partition and OMNI link can
be joined by Relays.Relays that connect INETs/EUNs with dissimilar IP protocol versions
may need to employ a network address and protocol translation function
such as NAT64 .In environments where rapid failure recovery is required, Servers
and Bridges SHOULD use Bidirectional Forwarding Detection (BFD) . Nodes that use BFD can quickly detect and react to
failures so that cached information is re-established through
alternate nodes. BFD control messaging is carried only over
well-connected ground domain networks (i.e., and not low-end radio
links) and can therefore be tuned for rapid response.Servers and Bridges maintain BFD sessions in parallel with their
BGP peerings. If a Server or Bridge fails, BGP peers will quickly
re-establish routes through alternate paths the same as for common BGP
deployments. Similarly, Proxys maintain BFD sessions with their
associated Bridges even though they do not establish BGP peerings with
them.Proxys SHOULD use proactive NUD for Servers for which there are
currently active ANET Clients in a manner that parallels BFD, i.e., by
sending unicast NS messages in rapid succession to receive solicited
NA messages. When the Proxy is also sending RS messages on behalf of
ANET Clients, the RS/RA messaging can be considered as equivalent
hints of forward progress. This means that the Proxy need not also
send a periodic NS if it has already sent an RS within the same
period. If a Server fails, the Proxy will cease to receive
advertisements and can quickly inform Clients of the outage by sending
multicast RA messages on the ANET interface.The Proxy sends multicast RA messages with source address set to
the Server's address, destination address set to (link-local)
All-Nodes multicast, and Router Lifetime set to 0. The Proxy SHOULD
send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small
delays . Any Clients on the ANET interface
that have been using the (now defunct) Server will receive the RA
messages and associate with a new Server.AERO Clients that connect to the open Internet via INET interfaces
can establish a VPN or direct link to securely connect to a Server in
a "tethered" arrangement with all of the Client's traffic transiting
the Server. Alternatively, the Client can associate with an INET
Server using UDP/IP encapsulation and asymmetric securing services as
discussed in the following sections.When a Client's OMNI interface enables an INET underlying
interface, it first determines whether the interface is likely to be
behind a NAT. For IPv4, the Client assumes it is on the open Internet
if the INET address is not a special-use IPv4 address per . Similarly for IPv6, the Client assumes it is on
the open Internet if the INET address is not a link-local or unique-local IPv6
address.The Client then prepares a UDP/IP-encapsulated RS message with IPv6
source address set to its LLA, with IPv6 destination set to
(link-local) All-Routers multicast and with an OMNI option with
underlying interface attributes. If the Client believes that it is on
the open Internet, it SHOULD include Interface Attributes with the
L2ADDR used for INET encapsulation (otherwise, it MAY omit L2ADDR). If
the underlying address is IPv4, the Client includes the Port Number
and IPv4 address written in obfuscated form
as discussed in . If the underlying
interface address is IPv6, the Client instead includes the Port Number
and IPv6 address in obfuscated form. The Client finally includes an
Authentication option per to provide message
authentication, sets the UDP/IP source to its INET address and UDP
port, sets the UDP/IP destination to the Server's INET address and the
AERO service port number (8060), then sends the message to the
Server.When the Server receives the RS, it authenticates the message and
registers the Client's MNP and INET interface information according to
the OMNI option parameters. If the RS message includes an L2ADDR in
the OMNI option, the Server compares the encapsulation IP address and
UDP port number with the (unobfuscated) values. If the values are the
same, the Server caches the Client's information as "INET" addresses
meaning that the Client is likely to accept direct messages without
requiring NAT traversal exchanges. If the values are different (or, if
the OMNI option did not include an L2ADDR) the Server instead caches
the Client's information as "NAT" addresses meaning that NAT traversal
exchanges may be necessary.The Server then returns an RA message with IPv6 source and
destination set corresponding to the addresses in the RS, and with an
Authentication option per . For IPv4, the
Server also includes an Origin option per
with the mapped and obfuscated Port Number and IPv4 address observed
in the encapsulation headers. For IPv6, the Server instead includes an
IPv6 Origin option per with the mapped
and obfuscated observed Port Number and IPv6 address (note that the
value 0x02 in the second octet differentiates from other option types).When the Client receives the RA message, it compares the mapped
Port Number and IP address from the Origin option with its own
address. If the addresses are the same, the Client assumes the open
Internet / Cone NAT principle; if the addresses are different, the
Client instead assumes that further qualification procedures are
necessary to detect the type of NAT and proceeds according to standard
procedures.After the Client has registered its INET interfaces in such RS/RA
exchanges it sends periodic RS messages to receive fresh RA messages
before the Router Lifetime received on each INET interface expires.
The Client also maintains default routes via its Servers, i.e., the
same as described in earlier sections.When the Client sends messages to target IP addresses, it also
invokes route optimization per using IPv6
ND address resolution messaging. The Client sends the NS(AR) message
to the Server wrapped in a UDP/IP header with an Authentication option
with the NS source address set to the Client's LLA and destination
address set to the target solicited node multicast address. The Server
authenticates the message and sends a corresponding NS(AR) message
over the spanning tree the same as if it were the ROS, but with the
OAL source address set to the Server's ULA and destination set to the
ULA of the target. When the ROR receives the NS(AR), it adds the
Server's ULA and Client's LLA to the target's Report List, and returns
an NA with OMNI option information for the target. The Server then
returns a UDP/IP encapsulated NA message with an Authentication option
to the Client.Following route optimization for targets in the same OMNI link
segment, if the target's L2ADDR is on the open INET, the Client
forwards data packets directly to the target INET address. If the
target is behind a NAT, the Client first establishes NAT state for the
L2ADDR using the "bubble" mechanisms specified in . The Client continues to
send data packets via its Server until NAT state is populated, then
begins forwarding packets via the direct path through the NAT to the
target. For targets in different OMNI link segments, the Client uses
OAL/ORH encapsulation and forwards data packets to the Bridge that
returned the NA message.The ROR may return uNAs via the Server if the target moves, and the
Server will send corresponding Authentication-protected uNAs to the
Client. The Client can also send "loopback" NS(NUD) messages to test
forward path reachability even though there is no security association
between the Client and the target.The Client sends UDP/IP encapsulated IPv6 packets no larger than
1280 bytes in one piece. In order to accommodate larger IPv6 packets
(up to the OMNI interface MTU), the Client inserts an OAL header with
source set to its own ULA and destination set to the ULA of the target
and uses IPv6 fragmentation according to . The
Client then encapsulates each fragment in a UDP/IP header and sends
the fragments to the next hop.In some use cases, it is desirable, beneficial and efficient for
the Client to receive a constant MNP that travels with the Client
wherever it moves. For example, this would allow air traffic
controllers to easily track aircraft, etc. In other cases, however
(e.g., intelligent transportation systems), the MN may be willing to
sacrifice a modicum of efficiency in order to have time-varying MNPs
that can be changed every so often to defeat adversarial tracking.The DHCPv6 service offers a way for Clients that desire
time-varying MNPs to obtain short-lived prefixes (e.g., on the order
of a small number of minutes). In that case, the identity of the
Client would not be bound to the MNP but rather the Client's identity
would be bound to the DHCPv6 Device Unique Identifier (DUID) and used
as the seed for Prefix Delegation. The Client would then be obligated
to renumber its internal networks whenever its MNP (and therefore also
its LLA) changes. This should not present a challenge for Clients with
automated network renumbering services, however presents limits for
the durations of ongoing sessions that would prefer to use a constant
address.An early 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.AERO Release-3.0.2 was tagged on October 15, 2020, and is undergoing
internal testing. Additional releases expected Q42020, with first public
release expected before year-end.The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO . This document
obsoletes and claims the UDP port number "8060"
for all future use.The IANA is instructed to assign a new type value TBD in the IPv6
Routing Types registry.No further IANA actions are required.AERO Bridges configure secured tunnels with AERO Servers, Relays and
Proxys within their local OMNI link segments. Applicable secured tunnel
alternatives include IPsec , TLS/SSL , DTLS , WireGuard , etc. The AERO Bridges of all OMNI link segments in turn
configure secured tunnels for their neighboring AERO Bridges in a
spanning tree topology. Therefore, control messages exchanged between
any pair of OMNI link neighbors on the spanning tree are already
secured.AERO Servers, Relays and Proxys targeted by a route optimization may
also receive data packets directly from arbitrary nodes in INET
partitions instead of via the spanning tree. For INET partitions that
apply effective ingress filtering to defeat source address spoofing, the
simple data origin authentication procedures in can be applied.For INET partitions that require strong security in the data plane,
two options for securing communications include 1) disable route
optimization so that all traffic is conveyed over secured tunnels, or 2)
enable on-demand secure tunnel creation between INET partition
neighbors. Option 1) would result in longer routes than necessary and
traffic concentration on critical infrastructure elements. Option 2)
could be coordinated by establishing a secured tunnel on-demand instead
of performing an NS/NA exchange in the route optimization procedures.
Procedures for establishing on-demand secured tunnels are out of
scope.AERO Clients that connect to secured ANETs need not apply security to
their ND messages, since the messages will be intercepted by a perimeter
Proxy that applies security on its INET-facing interface as part of the
spanning tree (see above). AERO Clients connected to the open INET can
use symmetric network and/or transport layer security services such as
VPNs or can by some other means establish a direct link. When a VPN or
direct link may be impractical, however, an asymmetric security service
such as the Authentication option specified in
should be applied. The Authentication option requires a unique Client
identifier, which can be obtained per the Universally Unique IDentifier
(UUID) service and also used as a DHCP Unique
Identifier (DUID) per .Application endpoints SHOULD use application-layer security services
such as TLS/SSL, DTLS or SSH to assure the same
level of protection as for critical secured Internet services. AERO
Clients that require host-based VPN services SHOULD use symmetric
network and/or transport layer security services such as IPsec, TLS/SSL,
DTLS, etc. AERO Proxys and Servers can also provide a network-based VPN
service on behalf of the Client, e.g., if the Client is located within a
secured enclave and cannot establish a VPN on its own behalf.AERO Servers and Bridges present targets for traffic amplification
Denial of Service (DoS) attacks. This concern is no different than for
widely-deployed VPN security gateways in the Internet, where attackers
could send spoofed packets to the gateways at high data rates. This can
be mitigated by connecting Servers and Bridges over dedicated links with
no connections to the Internet and/or when connections to the Internet
are only permitted through well-managed firewalls. Traffic amplification
DoS attacks can also target an AERO Client's low data rate links. This
is a concern not only for Clients located on the open Internet but also
for Clients in secured enclaves. AERO Servers and Proxys can institute
rate limits that protect Clients from receiving packet floods that could
DoS low data rate links.AERO Relays must implement ingress filtering to avoid a spoofing
attack in which spurious messages with ULA addresses are injected into
an OMNI link from an outside attacker. AERO Clients MUST ensure that
their connectivity is not used by unauthorized nodes on their EUNs to
gain access to a protected network, i.e., AERO Clients that act as
routers MUST NOT provide routing services for unauthorized nodes. (This
concern is no different than for ordinary hosts that receive an IP
address delegation but then "share" the address with other nodes via
some form of Internet connection sharing such as tethering.)The MAP list MUST be well-managed and secured from unauthorized
tampering, even though the list contains only public information. The
MAP list can be conveyed to the Client in a similar fashion as in (e.g., through layer 2 data link login messaging,
secure upload of a static file, DNS lookups, etc.).SRH authentication facilities are specified in .Security considerations for accepting link-layer ICMP messages and
reflected packets are discussed throughout the document.Security considerations for IPv6 fragmentation and reassembly are
discussed in .Discussions in the IETF, aviation standards communities and private
exchanges helped shape some of the concepts in this work. Individuals
who contributed insights include Mikael Abrahamsson, Mark Andrews, Fred
Baker, Bob Braden, Stewart Bryant, Brian Carpenter, Wojciech Dec, Pavel
Drasil, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli, Brian
Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak,
Lee Howard, Zdenek Jaron, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy
Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru
Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie Volz, Ryuji
Wakikawa, Tony Whyman, Lloyd Wood and James Woodyatt. Members of the
IESG also provided valuable input during their review process that
greatly improved the document. Special thanks go to Stewart Bryant, Joel
Halpern and Brian Haberman for their shepherding guidance during the
publication of the AERO first edition.This work has further been encouraged and supported by Boeing
colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam
Brodie, John Bush, Balaguruna Chidambaram, Irene Chin, Bruce Cornish,
Claudiu Danilov, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad
Farooqui, Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury, Greg
Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew, Gene
MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Vijay Rajagopalan, Greg
Saccone, Rod Santiago, Kent Shuey, Brian Skeen, Mike Slane, Carrie
Spiker, Katie Tran, Brendan Williams, Amelia Wilson, Julie Wulff, Yueli
Yang, Eric Yeh and other members of the Boeing mobility, networking and
autonomy teams. Kyle Bae, Wayne Benson, Katie Tran and Eric Yeh are
especially acknowledged for implementing the AERO functions as
extensions to the public domain OpenVPN distribution.Earlier works on NBMA tunneling approaches are found in .Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:The Internet Routing Overlay Network (IRON) Virtual Enterprise Traversal (VET) The Subnetwork Encapsulation and Adaptation Layer (SEAL) AERO, First Edition Note that these works cite numerous earlier efforts that are
not also cited here due to space limitations. The authors of those
earlier works are acknowledged for their insights.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Commercial Airplanes (BCA)
Internet of Things (IoT) and autonomy programs.This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.http://openvpn.netBGP in 2015, http://potaroo.netWireguard, https://www.wireguard.comWireguardAERO can be applied to a multitude of Internetworking scenarios, with
each having its own adaptations. The following considerations are
provided as non-normative guidance:Route optimization as discussed in
results in the route optimization source (ROS) creating an asymmetric
neighbor cache entry for the target neighbor. The neighbor cache entry
is maintained for at most ReachableTime seconds and then deleted
unless updated. In order to refresh the neighbor cache entry lifetime
before the ReachableTime timer expires, the specification requires
implementations to issue a new NS/NA exchange to reset ReachableTime
while data packets are still flowing. However, the decision of when to
initiate a new NS/NA exchange and to perpetuate the process is left as
an implementation detail.One possible strategy may be to monitor the neighbor cache entry
watching for data packets for (ReachableTime - 5) seconds. If any data
packets have been sent to the neighbor within this timeframe, then
send an NS to receive a new NA. If no data packets have been sent,
wait for 5 additional seconds and send an immediate NS if any data
packets are sent within this "expiration pending" 5 second window. If
no additional data packets are sent within the 5 second window, delete
the neighbor cache entry.The monitoring of the neighbor data packet traffic therefore
becomes an asymmetric ongoing process during the neighbor cache entry
lifetime. If the neighbor cache entry expires, future data packets
will trigger a new NS/NA exchange while the packets themselves are
delivered over a longer path until route optimization state is
re-established.OMNI interface neighbors MAY provide a configuration option that
allows them to perform implicit mobility management in which no ND
messaging is used. In that case, the Client only transmits packets
over a single interface at a time, and the neighbor always observes
packets arriving from the Client from the same link-layer source
address.If the Client's underlying interface address changes (either due to
a readdressing of the original interface or switching to a new
interface) the neighbor immediately updates the neighbor cache entry
for the Client and begins accepting and sending packets according to
the Client's new address. This implicit mobility method applies to use
cases such as cellphones with both WiFi and Cellular interfaces where
only one of the interfaces is active at a given time, and the Client
automatically switches over to the backup interface if the primary
interface fails.When a Client's OMNI interface is configured over a Direct
interface, the neighbor at the other end of the Direct link can
receive packets without any encapsulation. In that case, the Client
sends packets over the Direct link according to QoS preferences. If
the Direct interface has the highest QoS preference, then the Client's
IP packets are transmitted directly to the peer without going through
an ANET/INET. If other interfaces have higher QoS preferences, then
the Client's IP packets are transmitted via a different interface,
which may result in the inclusion of Proxys, Servers and Bridges in
the communications path. Direct interfaces must be tested periodically
for reachability, e.g., via NUD.AERO Bridges can be either Commercial off-the Shelf (COTS) standard
IP routers or virtual machines in the cloud. Bridges must be
provisioned, supported and managed by the INET administrative
authority, and connected to the Bridges of other INETs via
inter-domain peerings. Cost for purchasing, configuring and managing
Bridges is nominal even for very large OMNI links.AERO Servers can be standard dedicated server platforms, but most
often will be deployed as virtual machines in the cloud. The only
requirements for Servers are that they can run the AERO user-level
code and have at least one network interface connection to the INET.
As with Bridges, Servers must be provisioned, supported and managed by
the INET administrative authority. Cost for purchasing, configuring
and managing Servers is nominal especially for virtual Servers hosted
in the cloud.AERO Proxys are most often standard dedicated server platforms with
one network interface connected to the ANET and a second interface
connected to an INET. As with Servers, the only requirements are that
they can run the AERO user-level code and have at least one interface
connection to the INET. Proxys must be provisioned, supported and
managed by the ANET administrative authority. Cost for purchasing,
configuring and managing Proxys is nominal, and borne by the ANET
administrative authority.AERO Relays can be any dedicated server or COTS router platform
connected to INETs and/or EUNs. The Relay connects to the OMNI link
and engages in eBGP peering with one or more Bridges as a stub AS. The
Relay then injects its MNPs and/or non-MNP prefixes into the BGP
routing system, and provisions the prefixes to its downstream-attached
networks. The Relay can perform ROS/ROR services the same as for any
Server, and can route between the MNP and non-MNP address spaces.AERO Servers may appear as a single point of failure in the
architecture, but such is not the case since all Servers on the link
provide identical services and loss of a Server does not imply
immediate and/or comprehensive communication failures. Although
Clients typically associate with a single Server at a time, Server
failure is quickly detected and conveyed by Bidirectional Forward
Detection (BFD) and/or proactive NUD allowing Clients to migrate to
new Servers.If a Server fails, ongoing packet forwarding to Clients will
continue by virtue of the asymmetric neighbor cache entries that have
already been established in route optimization sources (ROSs). If a
Client also experiences mobility events at roughly the same time the
Server fails, unsolicited NA messages may be lost but proxy neighbor
cache entries in the DEPARTED state will ensure that packet forwarding
to the Client's new locations will continue for up to DepartTime
seconds.If a Client is left without a Server for an extended timeframe
(e.g., greater than ReachableTime seconds) then existing asymmetric
neighbor cache entries will eventually expire and both ongoing and new
communications will fail. The original source will continue to
retransmit until the Client has established a new Server relationship,
after which time continuous communications will resume.Therefore, providing many Servers on the link with high
availability profiles provides resilience against loss of individual
Servers and assurance that Clients can establish new Server
relationships quickly in event of a Server failure.The AERO architectural model is client / server in the control
plane, with route optimization in the data plane. The same as for
common Internet services, the AERO Client discovers the addresses of
AERO Servers and selects one Server to connect to. The AERO service is
analogous to common Internet services such as google.com, yahoo.com,
cnn.com, etc. However, there is only one AERO service for the link and
all Servers provide identical services.Common Internet services provide differing strategies for
advertising server addresses to clients. The strategy is conveyed
through the DNS resource records returned in response to name
resolution queries. As of January 2020 Internet-based 'nslookup'
services were used to determine the following:When a client resolves the domainname "google.com", the DNS
always returns one A record (i.e., an IPv4 address) and one AAAA
record (i.e., an IPv6 address). The client receives the same
addresses each time it resolves the domainname via the same DNS
resolver, but may receive different addresses when it resolves the
domainname via different DNS resolvers. But, in each case, exactly
one A and one AAAA record are returned.When a client resolves the domainname "ietf.org", the DNS
always returns one A record and one AAAA record with the same
addresses regardless of which DNS resolver is used.When a client resolves the domainname "yahoo.com", the DNS
always returns a list of 4 A records and 4 AAAA records. Each time
the client resolves the domainname via the same DNS resolver, the
same list of addresses are returned but in randomized order (i.e.,
consistent with a DNS round-robin strategy). But, interestingly,
the same addresses are returned (albeit in randomized order) when
the domainname is resolved via different DNS resolvers.When a client resolves the domainname "amazon.com", the DNS
always returns a list of 3 A records and no AAAA records. As with
"yahoo.com", the same three A records are returned from any
worldwide Internet connection point in randomized order.The above example strategies show differing approaches to
Internet resilience and service distribution offered by major Internet
services. The Google approach exposes only a single IPv4 and a single
IPv6 address to clients. Clients can then select whichever IP protocol
version offers the best response, but will always use the same IP
address according to the current Internet connection point. This means
that the IP address offered by the network must lead to a
highly-available server and/or service distribution point. In other
words, resilience is predicated on high availability within the
network and with no client-initiated failovers expected (i.e., it is
all-or-nothing from the client's perspective). However, Google does
provide for worldwide distributed service distribution by virtue of
the fact that each Internet connection point responds with a different
IPv6 and IPv4 address. The IETF approach is like google
(all-or-nothing from the client's perspective), but provides only a
single IPv4 or IPv6 address on a worldwide basis. This means that the
addresses must be made highly-available at the network level with no
client failover possibility, and if there is any worldwide service
distribution it would need to be conducted by a network element that
is reached via the IP address acting as a service distribution
point.In contrast to the Google and IETF philosophies, Yahoo and Amazon
both provide clients with a (short) list of IP addresses with Yahoo
providing both IP protocol versions and Amazon as IPv4-only. The order
of the list is randomized with each name service query response, with
the effect of round-robin load balancing for service distribution.
With a short list of addresses, there is still expectation that the
network will implement high availability for each address but in case
any single address fails the client can switch over to using a
different address. The balance then becomes one of function in the
network vs function in the end system.The same implications observed for common highly-available services
in the Internet apply also to the AERO client/server architecture.
When an AERO Client connects to one or more ANETs, it discovers one or
more AERO Server addresses through the mechanisms discussed in earlier
sections. Each Server address presumably leads to a fault-tolerant
clustering arrangement such as supported by Linux-HA, Extended Virtual
Synchrony or Paxos. Such an arrangement has precedence in common
Internet service deployments in lightweight virtual machines without
requiring expensive hardware deployment. Similarly, common Internet
service deployments set service IP addresses on service distribution
points that may relay requests to many different servers.For AERO, the expectation is that a combination of the Google/IETF
and Yahoo/Amazon philosophies would be employed. The AERO Client
connects to different ANET access points and can receive 1-2 Server
LLAs at each point. It then selects one AERO Server address, and
engages in RS/RA exchanges with the same Server from all ANET
connections. The Client remains with this Server unless or until the
Server fails, in which case it can switch over to an alternate Server.
The Client can likewise switch over to a different Server at any time
if there is some reason for it to do so. So, the AERO expectation is
for a balance of function in the network and end system, with fault
tolerance and resilience at both levels.<< RFC Editor - remove prior to publication >>Changes from draft-templin-intarea-6706bis-61 to
draft-templin-intrea-6706bis-62:New sub-section on OMNI Neighbor Interface AttributesChanges from draft-templin-intarea-6706bis-59 to
draft-templin-intrea-6706bis-60:Removed all references to S/TLLAO - all Interface Attributes are
now maintained completely in the OMNI option.Changes from draft-templin-intarea-6706bis-58 to
draft-templin-intrea-6706bis-59:The term "Relay" used in older draft versions is now "Bridge".
"Relay" now refers to what was formally called: "Gateway".Fine-grained cleanup of Forwarding Algorithm; IPv6 ND message
addressing; OMNI Prefix Lengths, etc.Changes from draft-templin-intarea-6706bis-54 to
draft-templin-intrea-6706bis-55:Updates on Segment Routing and S/TLLAO contents.Various editorials and addressing cleanups.Changes from draft-templin-intarea-6706bis-52 to
draft-templin-intrea-6706bis-53:Normative reference to the OMNI spec, and remove portions that
are already specified in OMNI.Renamed "AERO interface/link" to "OMIN interface/link" throughout
the document.Truncated obsolete back section matter.