Asymmetric Extended Route Optimization (AERO)Boeing Research & TechnologyP.O. Box 3707SeattleWA98124USAfltemplin@acm.orgI-DInternet-DraftThis document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). Nodes attached to
AERO links can exchange packets via trusted intermediate routers that
provide forwarding services to reach off-link destinations and route
optimization services for improved performance. AERO provides an IPv6
link-local address format that supports operation of the IPv6 Neighbor
Discovery (ND) protocol and links ND to IP forwarding. Dynamic link
selection, mobility management, quality of service (QoS) signaling and
route optimization are naturally supported through dynamic neighbor
cache updates, while IPv6 Prefix Delegation (PD) is supported by network
services such as the Dynamic Host Configuration Protocol for IPv6
(DHCPv6). AERO is a widely-applicable tunneling solution especially
well-suited to aviation services, mobile Virtual Private Networks (VPNs)
and other applications as described in this document.This document specifies the operation of IP over tunnel virtual links
using Asymmetric Extended Route Optimization (AERO). The AERO link can
be used for tunneling between neighboring nodes over either IPv6 or IPv4
networks, i.e., AERO views the IPv6 and IPv4 networks as equivalent
links for tunneling. Nodes attached to AERO links can exchange packets
via trusted intermediate routers that provide forwarding services to
reach off-link destinations and route optimization services for improved
performance .AERO provides an IPv6 link-local address format that supports
operation of the IPv6 Neighbor Discovery (ND)
protocol and links ND to IP forwarding. Dynamic link selection, mobility
management, quality of service (QoS) signaling and route optimization
are naturally supported through dynamic neighbor cache updates, while
IPv6 Prefix Delegation (PD) is supported by network services such as the
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) .A node's AERO interface can be configured over multiple underlying
interfaces. From the standpoint of ND, AERO interface neighbors
therefore may appear to have multiple link-layer addresses (i.e., the
addresses assigned to underlying interfaces). 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 is applicable to a wide variety of use cases. For example, it
can be used to coordinate the Virtual Private Network (VPN) links of
mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that
connect into a home enterprise network via public access networks using
services such as OpenVPN . AERO is also applicable
to aviation services for both manned and unmanned aircraft where the
aircraft is treated as a mobile node that can connect an Internet of
Things (IoT). Other applicable use cases are also in scope.The remainder of this document presents the AERO specification.The terminology in the normative references applies; the following
terms are defined within the scope of this document:an IPv6 control
message service for coordinating neighbor relationships between
nodes connected to a common link. The ND service used by AERO is
specified in .a networking
service for delegating IPv6 prefixes to nodes on the link. The
nominal PD service is DHCPv6 , however other services (e.g., alternate ND
options, network management, static configuration, etc.) are also
possible.a connected IPv6 or
IPv4 network topology over which the AERO link virtual overlay is
configured and native peer-to-peer communications are supported.
Example Internetworks include the global public Internet, private
enterprise networks, aviation networks, etc.a Non-Broadcast, Multiple Access
(NBMA) tunnel virtual overlay configured over an underlying
Internetwork. All nodes on the AERO link appear as single-hop
neighbors from the perspective of the virtual overlay even though
they may be separated by many underlying Internetwork hops. The AERO
mechanisms can also operate over native link types (e.g., Ethernet,
WiFi etc.) when a tunnel virtual overlay is not needed.a node's attachment to an AERO
link. Since the addresses assigned to an AERO interface are managed
for uniqueness, AERO interfaces do not require Duplicate Address
Detection (DAD) and therefore set the administrative variable
DupAddrDetectTransmits to zero .an IPv6 link-local address
constructed as specified in .a node that is connected to an AERO
link.a node that
requests IP PDs from one or more AERO Servers. Following PD, the
Client assigns an AERO address to the AERO interface for use in ND
exchanges with other AERO nodes. A node that acts as an AERO Client
on one AERO interface can also act as an AERO Server on a different
AERO interface.a node that
configures an AERO interface to provide default forwarding services
for AERO Clients. The Server assigns an administratively-provisioned
IPv6 link-local address to the AERO interface to support the
operation of the ND/PD services. An AERO Server can also act as an
AERO Relay.a node that
configures an AERO interface to relay IP packets between nodes on
the same AERO link and/or forward IP packets between the AERO link
and the native Internetwork. The Relay assigns an
administratively-provisioned IPv6 link-local address to the AERO
interface the same as for a Server. An AERO Relay can also act as an
AERO Server.a node that
provides proxying services for Clients that cannot associate
directly with Servers, e.g., when the Client is located in a secured
internal enclave and the Server is located in the external
Internetwork. The AERO Proxy is a conduit between the secured
enclave and the external Internetwork in the same manner as for
common web proxies, and behaves in a similar fashion as for ND
proxies .an AERO
interface endpoint that injects encapsulated packets into an AERO
link.an AERO
interface endpoint that receives encapsulated packets from an AERO
link.the same as defined for
Internetwork.a link that connects an AERO
node to the underlying network.an AERO node's interface
point of attachment to an underlying link.an IP address assigned to
an AERO node's underlying interface. When UDP encapsulation is used,
the UDP port number is also considered as part of the link-layer
address. Packets transmitted over an AERO interface use link-layer
addresses as encapsulation header source and destination addresses.
Destination link-layer addresses can be either "reachable" or
"unreachable" based on dynamically-changing network conditions.the source or
destination address of an encapsulated IP packet.an internal virtual or
external edge IP network that an AERO Client connects to the rest of
the network via the AERO interface. The Client sees each EUN as a
"downstream" network and sees the AERO interface as its point of
attachment to the "upstream" network.an IP prefix
associated with the AERO link and from which more-specific AERO
Client Prefixes (ACPs) are derived.an IP prefix derived
from an ASP and delegated to a Client, where the ACP prefix length
must be no shorter than the ASP prefix length and must be no longer
than 64 for IPv6 or 32 for IPv4.the lowest-numbered AERO
address from the first ACP delegated to the Client (see ).Throughout the document, the simple terms "Client", "Server",
"Relay" and "Proxy" refer to "AERO Client", "AERO Server", "AERO Relay"
and "AERO Proxy", respectively. Capitalization is used to distinguish
these terms from DHCPv6 client/server/relay .The terminology of DHCPv6 and IPv6 ND (including the
names of node variables, messages and protocol constants) is used
throughout this document. Also, the term "IP" is used to generically
refer to either Internet Protocol version, i.e., IPv4 or IPv6 .The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
Lower case uses of these words are not to be interpreted as carrying
RFC2119 significance.The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links: presents the AERO link
reference model. In this model:AERO Relay R1 aggregates AERO Service Prefix (ASP) A1, acts as
a default router for its associated Servers (S1 and S2), and
connects the AERO link to the rest of the Internetwork.AERO Servers S1 and S2 associate with Relay R1 and also act as
default routers for their associated Clients C1 and C2.AERO Clients C1 and C2 associate with Servers S1 and S2,
respectively. They receive AERO Client Prefix (ACP) 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 provides proxy services for AERO Clients in
secured enclaves that cannot associate directly with other AERO
link neighbors.Each node on the AERO link maintains an AERO interface
neighbor cache and an IP forwarding table the same as for any link.
Although the figure shows a limited deployment, in common operational
practice there may be many additional Relays, Servers, Clients and
Proxies.AERO Relays provide default forwarding services to AERO Servers.
Each Relay also peers with Servers and other Relays in a dynamic
routing protocol instance to discover the list of active ACPs (see
). Relays forward packets between neighbors
connected to the same AERO link and also forward packets between the
AERO link and the native Internetwork. Relays present the AERO link to
the native Internetwork as a set of one or more AERO Service Prefixes
(ASPs) and serve as a gateway between the AERO link and the
Internetwork. Relays maintain AERO interface neighbor cache entries
for Servers, and maintain an IP forwarding table entry for each AERO
Client Prefix (ACP). AERO Relays can also be configured to act as AERO
Servers.AERO Servers provide default forwarding services to AERO Clients.
Each Server also peers with Relays in a dynamic routing protocol
instance to advertise its list of associated ACPs (see ). Servers facilitate PD exchanges with Clients,
where each delegated prefix becomes an ACP taken from an ASP. Servers
forward packets between AERO interface neighbors, and maintain AERO
interface neighbor cache entries for Relays. They also maintain both
neighbor cache entries and IP forwarding table entries for each of
their associated Clients. AERO Servers can also be configured to act
as AERO Relays.AERO Clients act as requesting routers to receive ACPs through PD
exchanges with AERO Servers over the AERO link. Each Client can
associate with a single Server or with multiple Servers, e.g., for
fault tolerance, load balancing, etc. Each IPv6 Client receives at
least a /64 IPv6 ACP, and may receive even shorter prefixes.
Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
singleton IPv4 address), and may receive even shorter prefixes.
Clients maintain an AERO interface neighbor cache entry for each of
their associated Servers as well as for each of their correspondent
Clients.AERO Proxies provide a conduit for AERO Clients connected to
secured enclaves to associate with AERO link Servers. The Proxy can
either be explicit or transparent. In the explicit case, the Client
sends all of its control plane messages addressed to the Server to the
link-layer address of the Proxy. In the transparent case, the Client
sends all of its control plane messages to the Server's link-layer
address and the Proxy intercepts them before they leave the secured
enclave. In both cases, the Proxy forwards the Client's control and
data plane messages to and from the Client's current Server(s). The
Proxy may also discover a more direct route toward a target
destination via AERO route optimization, in which case future outbound
data packets would be forwarded via the more direct route. The Proxy
function is specified in .The AERO routing system comprises a private instance of the Border
Gateway Protocol (BGP) that is coordinated
between Relays and Servers and does not interact with either the
public Internet BGP routing system or the native Internetwork routing
system. Relays advertise only a small and unchanging set of ASPs to
the native Internetwork routing system instead of the full dynamically
changing set of ACPs.In a reference deployment, each AERO Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) using an AS Number (ASN) that is unique within the BGP instance,
and each Server further uses eBGP to peer with one or more Relays but
does not peer with other Servers. All Relays are members of the same
hub AS using a common ASN, and use iBGP to maintain a consistent view
of all active ACPs currently in service.Each Server maintains a working set of associated ACPs, and
dynamically announces new ACPs and withdraws departed ACPs in its eBGP
updates to Relays. Clients are expected to remain associated with
their current Servers for extended timeframes, however Servers SHOULD
selectively suppress updates for impatient Clients that repeatedly
associate and disassociate with them in order to dampen routing
churn.Each Relay configures a black-hole route for each of its ASPs. By
black-holing the ASPs, the Relay will maintain forwarding table
entries only for the ACPs that are currently active, and packets
destined to all other ACPs will correctly incur Destination
Unreachable messages due to the black hole route. Relays do not send
eBGP updates for ACPs to Servers, but instead only originate a default
route. In this way, Servers have only partial topology knowledge
(i.e., they know only about the ACPs of their directly associated
Clients) and they forward all other packets to Relays which have full
topology knowledge.Scaling properties of the AERO routing system are limited by the
number of BGP routes that can be carried by Relays. At the time of
this writing, the global public Internet BGP routing system manages
more than 500K routes with linear growth and no signs of router
resource exhaustion . Network emulation studies
have also shown that a single Relay can accommodate at least 1M
dynamically changing BGP routes even on a lightweight virtual machine,
i.e., and without requiring high-end dedicated router hardware.Therefore, assuming each Relay can carry 1M or more routes, this
means that at least 1M Clients can be serviced by a single set of
Relays. A means of increasing scaling would be to assign a different
set of Relays for each set of ASPs. In that case, each Server still
peers with one or more Relays, but the Server institutes route filters
so that it only sends BGP updates to the specific set of Relays that
aggregate the ASP. For example, if the ASP for the AERO link is
2001:db8::/32, a first set of Relays could service the ASP segment
2001:db8::/40, a second set of Relays could service
2001:db8:0100::/40, a third set could service 2001:db8:0200::/40,
etc.Assuming up to 1K sets of Relays, the AERO routing system can then
accommodate 1B or more ACPs with no additional overhead for Servers
and Relays (for example, it should be possible to service 1B /64 ACPs
taken from a /34 ASP and even more for shorter prefixes). In this way,
each set of Relays services a specific set of ASPs that they advertise
to the native Internetwork routing system, and each Server configures
ASP-specific routes that list the correct set of Relays as next hops.
This arrangement also allows for natural incremental deployment, and
can support small scale initial deployments followed by dynamic
deployment of additional Clients, Servers and Relays without
disturbing the already-deployed base.Note that in an alternate routing arrangement each set of Relays
could advertise an aggregated ASP for the link into the native
Internetwork routing system even though each Relay services only
smaller segments of the ASP. In that case, a Relay upon receiving a
packet with a destination address covered by the ASP segment of
another Relay can simply tunnel the packet to the other Relay. The
tradeoff then is the penalty for Relay-to-Relay tunneling compared
with reduced routing information in the native routing system.A full discussion of the BGP-based routing system used by AERO is
found in .AERO interface link-local address types include
administratively-provisioned addresses and AERO addresses.Administratively-provisioned addresses are allocated from the range
fe80::/96 and assigned to a Server or Relay's AERO interface.
Administratively-provisioned addresses MUST be managed for uniqueness
by the administrative authority for the AERO link. The address fe80::
is reserved as the IPv6 link-local subnet router anycast address, and
the address fe80::ffff:ffff is reserved as the "prefix-solicitation"
address used by Clients to bootstrap AERO address autoconfiguration.
These reserved addresses are therefore not available for general
assignment.An AERO address is an IPv6 link-local address with an embedded
prefix based on an ACP and associated with a Client's AERO interface.
AERO addresses remain stable as the Client moves between topological
locations, i.e., even if its link-layer addresses change.For IPv6, AERO addresses begin with the prefix fe80::/64 and
include in the interface identifier (i.e., the lower 64 bits) a 64-bit
prefix taken from one of the Client's IPv6 ACPs. For example, if the
AERO Client receives the IPv6 ACP:2001:db8:1000:2000::/56it constructs its corresponding AERO addresses as:fe80::2001:db8:1000:2000fe80::2001:db8:1000:2001fe80::2001:db8:1000:2002... etc. ...fe80::2001:db8:1000:20ffFor IPv4, AERO addresses are based on an IPv4-mapped IPv6
address formed from an IPv4 ACP and with a
Prefix Length of 96 plus the ACP prefix length. For example, for the
IPv4 ACP 192.0.2.32/28 the IPv4-mapped IPv6 ACP is:0:0:0:0:0:FFFF:192.0.2.16/124The Client then constructs its AERO addresses with the prefix
fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
in the interface identifier as:fe80::FFFF:192.0.2.16fe80::FFFF:192.0.2.17fe80::FFFF:192.0.2.18... etc. ...fe80:FFFF:192.0.2.31When the Server delegates ACPs to the Client, both the Server
and Client use the lowest-numbered AERO address from the first ACP
delegation as the "base" AERO address (for example, for the ACP
2001:db8:1000:2000::/56 the base AERO address is
fe80::2001:db8:1000:2000). The Client then assigns the base AERO
address to the AERO interface and uses it for the purpose of
maintaining the neighbor cache entry. The Server likewise uses the
AERO address as its index into the neighbor cache for this Client.If the Client has multiple AERO addresses (i.e., when there are
multiple ACPs and/or ACPs with short prefix lengths), the Client
originates ND messages using the base AERO address as the source
address and accepts and responds to ND messages destined to any of its
AERO addresses as equivalent to the base AERO address. In this way,
the Client maintains a single neighbor cache entry that may be indexed
by multiple AERO addresses.AERO interfaces use encapsulation (see: ) to exchange packets with neighbors attached to
the AERO link.AERO interfaces maintain a neighbor cache for tracking per-neighbor
state the same as for any interface. AERO interfaces use ND messages
including Neighbor Solicitation (NS), Neighbor Advertisement (NA),
Router Solicitation (RS), Router Advertisement (RA) and Redirect for
neighbor cache management. AERO interfaces use RS/RA messages with an
embedded PD message (e.g., see: ). AERO interfaces include
routing information in ND messages to support route optimization.AERO interface ND messages include one or more Source/Target
Link-Layer Address Options (S/TLLAOs) formatted as shown in :In this format:Type is set to '1' for SLLAO or '2' for TLLAO.Length is set to the constant value '5' (i.e., 5 units of 8
octets).X (proXy) is set to '1' in an S/TLLAO if the address
corresponds to a Proxy; otherwise, X is set to '0'.Reserved is set to the value '0' on transmission and ignored on
receipt.Interface ID is set to a 16-bit integer value corresponding to
an underlying interface of the AERO node. The value 255 is
reserved for Server-based route optimization (see: ).UDP Port Number and IP Address are set to the addresses used by
the AERO node when it sends encapsulated packets over the
specified underlying interface (or to '0' when the addresses are
left unspecified). When UDP is not used as part of the
encapsulation, UDP Port Number is set to '0'. When the
encapsulation IP address family is IPv4, IP Address is formed as
an IPv4-mapped IPv6 address as specified in .P(i) is a set of 64 Preference values that correspond to the 64
Differentiated Service Code Point (DSCP) values . Each P(i) is set to the value '0'
("disabled"), '1' ("low"), '2' ("medium") or '3' ("high") to
indicate a QoS preference level for packet forwarding
purposes.AERO interfaces may be configured over multiple underlying
interface connections to underlying links. For example, common mobile
handheld devices have both wireless local area network ("WLAN") and
cellular wireless links. These links are typically used "one at a
time" with low-cost WLAN preferred and highly-available cellular
wireless as a standby. In a more complex example, aircraft frequently
have many wireless data link types (e.g. satellite-based, cellular,
terrestrial, air-to-air directional, etc.) with diverse performance
and cost properties.A Client's underlying interfaces are classified as follows:Native interfaces connect to the open Internetwork, and have a
global IP address that is reachable from any open Internetwork
correspondent.NAT'ed interfaces connect to a closed network that is separated
from the open Internetwork by a Network Address Translator (NAT).
The NAT does not participate in any AERO control message
signaling, but the AERO Server can issue AERO control messages on
behalf of the Client.VPN'ed interfaces use security encapsulation over the
Internetwork to a Virtual Private Network (VPN) gateway that also
acts as an AERO Server. As with NAT'ed links, the AERO Server can
issue control messages on behalf of the Client.Proxy'ed interfaces connect to a closed network that is
separated from the open Internetwork by an AERO Proxy. Unlike
NAT'ed and VPN'ed interfaces, the AERO Proxy (rather than the
Server) can issue control message on behalf of the Client.Direct interfaces connect the Client directly to a peer without
crossing any networked paths. An example is a line-of-sight link
between a remote pilot and an unmanned aircraft.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 messages include only a single S/TLLAO
with Interface ID set to a constant value. In that case, the Client
would appear to have a single underlying 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 messages MAY include multiple S/TLLAOs --
each with an Interface ID that corresponds to a specific underlying
interface of the AERO node.When the Client includes an S/TLLAO for an underlying interface for
which it is aware that there is a NAT or Proxy on the path to the
Server, or when a node includes an S/TLLAO solely for the purpose of
announcing new QoS preferences, the node sets both UDP Port Number and
IP Address to 0 to indicate that the addresses are unspecified.When an ND message includes multiple S/TLLAOs, the first S/TLLAO
MUST correspond to the AERO node's underlying interface used to
transmit the message.When a Relay enables an AERO interface, it first assigns an
administratively-provisioned link-local address fe80::ID to the
interface. Each fe80::ID address MUST be unique among all AERO nodes
on the link. The Relay then engages in a dynamic routing protocol
session with one or more Servers and all other Relays on the link
(see: ), and advertises its assigned ASPs
into the native Internetwork.Each Relay subsequently maintains an IP forwarding table entry
for each active ACP covered by its ASP(s), and maintains neighbor
cache entries for all Servers on the link. Relays exchange NS/NA
messages with AERO link neighbors the same as for any AERO node.
However, Neighbor Unreachability Detection (NUD) (see: ) is optional since the dynamic routing protocol
already provides reachability confirmation.When a Server enables an AERO interface, it assigns an
administratively-provisioned link-local address fe80::ID the same as
for Relays. The Server further configures a service to facilitate PD
exchanges with AERO Clients. The Server maintains neighbor cache
entries for one or more Relays on the link, and manages per-Client
neighbor cache entries and IP forwarding table entries based on
control message exchanges. Each Server also engages in a dynamic
routing protocol with their neighboring Relays (see: ).When the Server receives an NS/RS message from a Client on the
AERO interface it authenticates the message and returns an NA/RA
message. The Server further provides a simple link-layer conduit
between AERO interface neighbors. In particular, when a packet sent
by a source Client arrives on the Server's AERO interface and is
destined to another AERO node, the Server forwards the packet from
within the AERO interface driver at the link layer without ever
disturbing the network layer.When a Client enables an AERO interface, it sends RS messages
with PD "Solicit" options over an underlying interface using the
prefix-solicitation address as the source network layer address and
all-routers as the destination network
layer address to obtain ACPs from one or more AERO Servers. Each
Server processes the message and returns an RA message with a PD
"Reply" option with the Server's link-layer address as the source
and the base AERO address as the destination network layer
addresses. In this way, the ND/PD control messages securely perform
all autoconfiguration operations in a single request/response
exchange.After the initial ND/PD message exchange, the Client can register
additional underlying interfaces with the Server by sending an RS
message over each underlying interface using its base AERO address
as the source network layer address and without including a PD
option. The Server will update its neighbor cache entry for the
Client and return an RA message.The Client maintains a neighbor cache entry for each of its
Servers and each of its active correspondent Clients. When the
Client receives ND messages on the AERO interface it updates or
creates neighbor cache entries, including link-layer address and QoS
preferences.When a Proxy enables an AERO interface, it maintains per-Client
proxy neighbor cache entries based on control message exchanges.
Proxies forward packets between their associated Clients and the
Clients' associated Servers.When the Proxy receives an RS message from a Client in the
secured enclave, it creates an incomplete proxy neighbor cache entry
and forwards the message to a Server selected by the Client while
using its own link-layer address as the source address. When the
Server returns an RA message, the Proxy completes the proxy neighbor
cache entry based on autoconfiguration information in the RA and
forwards the RA to the Client while using its own link-layer address
as the source address. The Client, Server and Proxy will then have
the necessary state for managing the proxyed neighbor
association.Each AERO interface maintains a conceptual neighbor cache that
includes an entry for each neighbor it communicates with on the AERO
link, the same as for any IPv6 interface .
AERO interface neighbor cache entries are said to be one of
"permanent", "static", "proxy" or "dynamic".Permanent neighbor cache entries are created through explicit
administrative action; they have no timeout values and remain in place
until explicitly deleted. AERO Relays maintain permanent neighbor
cache entries for Servers on the link, and AERO Servers maintain
permanent neighbor cache entries for Relays. Each entry maintains the
mapping between the neighbor's fe80::ID network-layer address and
corresponding link-layer address.Static neighbor cache entries are created and maintained through
ND/PD exchanges as specified in , and remain in
place for durations bounded by ND/PD lifetimes. AERO Servers maintain
static neighbor cache entries for each of their associated Clients,
and AERO Clients maintain static neighbor cache entries for each of
their associated Servers.Proxy neighbor cache entries are created and maintained by AERO
Proxies by gleaning information from Client/Server ND/PD exchanges,
and remain in place for durations bounded by ND/PD lifetimes. AERO
Proxies maintain proxy neighbor cache entries for each of their
associated Clients, and include pointers to the Client's current set
of Servers.Dynamic neighbor cache entries are created or updated based on
receipt of route optimization messages as specified in , and are garbage-collected when keepalive timers
expire. AERO nodes maintain dynamic neighbor cache entries for each of
their active correspondents with lifetimes based on ND messaging
constants.When a target AERO node receives a valid NS message with an AERO
source address, it returns an NA message and also creates or updates a
dynamic neighbor cache entry for the source network-layer and
link-layer addresses. The node then sets an "AcceptTime" variable in
the neighbor cache entry to ACCEPT_TIME seconds and uses this value to
determine whether packets received from the correspondent can be
accepted. The node resets AcceptTime when it receives a new ND
message, and otherwise decrements AcceptTime while no ND messages have
been received. It is RECOMMENDED that ACCEPT_TIME be set to the
default constant value 40 seconds to allow a 10 second window so that
the AERO route optimization procedure can converge before AcceptTime
decrements below FORWARD_TIME (see below).When a source AERO node receives a valid NA message with an AERO
source address that matches its NS message, it creates or updates a
dynamic neighbor cache entry for the target network-layer and
link-layer addresses. The node then sets a "ForwardTime" variable in
the neighbor cache entry to FORWARD_TIME seconds and uses this value
to determine whether packets can be forwarded directly to the
correspondent, i.e., instead of via a default route. The node resets
ForwardTime when it receives a new NA, and otherwise decrements
ForwardTime while no further NA messages have been received. It is
RECOMMENDED that FORWARD_TIME be set to the default constant value 30
seconds to match the default REACHABLE_TIME value specified in .The node also sets a "MaxRetry" variable to MAX_RETRY to limit the
number of keepalives sent when a correspondent may have gone
unreachable. It is RECOMMENDED that MAX_RETRY be set to 3 the same as
described for address resolution in Section 7.3.3 of .Different values for ACCEPT_TIME, FORWARD_TIME and MAX_RETRY MAY be
administratively set, if necessary, to better match the AERO link's
performance characteristics; however, if different values are chosen,
all nodes on the link MUST consistently configure the same values.
Most importantly, ACCEPT_TIME SHOULD be set to a value that is
sufficiently longer than FORWARD_TIME to allow the AERO route
optimization procedure to converge.When there may be a NAT between the Client and the Server, or if
the path from the Client to the Server should be tested for
reachability, the Client can send periodic RS messages to the Server
without a PD option to receive RA replies. The RS/RA messaging will
keep NAT state alive and test Server reachability without disturbing
the PD service.IP packets enter a node's AERO interface either from the network
layer (i.e., from a local application or the IP forwarding system) or
from the link layer (i.e., from the AERO tunnel virtual link). Packets
that enter the AERO interface from the network layer are encapsulated
and forwarded into the AERO link, i.e., they are tunneled to an AERO
interface neighbor. Packets that enter the AERO interface from the
link layer are either re-admitted into the AERO link or forwarded to
the network layer where they are subject to either local delivery or
IP forwarding. In all cases, the AERO interface itself MUST NOT
decrement the network layer TTL/Hop-count since its forwarding actions
occur below the network layer.AERO interfaces may have multiple underlying interfaces and/or
neighbor cache entries for neighbors with multiple Interface ID
registrations (see ). The AERO node uses
each packet's DSCP value to select an outgoing underlying interface
based on the node's own QoS preferences, and also to select a
destination link-layer address based on the neighbor's underlying
interface with the highest preference. If multiple outgoing interfaces
and/or neighbor interfaces have a preference of "high", the AERO node
sends one copy of the packet via each of the (outgoing / neighbor)
interface pairs; otherwise, the node sends a single copy of the packet
via the interface with the highest preference. AERO nodes keep track
of which underlying interfaces are currently "reachable" or
"unreachable", and only use "reachable" interfaces for forwarding
purposes.The following sections discuss the AERO interface forwarding
algorithms for Clients, Proxies, Servers and Relays. In the following
discussion, a packet's destination address is said to "match" if it is
a non-link-local address with a prefix covered by an ASP/ACP, or if it
is an AERO address that embeds an ACP, or if it is the same as an
administratively-provisioned link-local address.When an IP packet enters a Client's AERO interface from the
network layer the Client searches for a dynamic neighbor cache entry
that matches the destination. If there is a match, the Client uses
one or more "reachable" link-layer addresses in the entry as the
link-layer addresses for encapsulation and admits the packet into
the AERO link. Otherwise, the Client uses the link-layer address in
a static neighbor cache entry for a Server as the encapsulation
address (noting that there may be a Proxy on the path to the real
Server).When an IP packet enters a Client's AERO interface from the
link-layer, if the destination matches one of the Client's ACPs or
link-local addresses the Client decapsulates the packet and delivers
it to the network layer. Otherwise, the Client drops the packet and
MAY return a network-layer ICMP Destination Unreachable message
subject to rate limiting (see: ).When the Proxy receives a packet from a Client within the secured
enclave, the Proxy searches for a dynamic neighbor cache entry that
matches the destination. If there is a match, the Proxy uses one or
more "reachable" link-layer addresses in the entry as the link-layer
addresses for encapsulation and admits the packet into the AERO
link. Otherwise, the Proxy uses the link-layer address for one of
the Client's Servers as the encapsulation address.When the Proxy receives a packet from an AERO interface neighbor,
it searches for a proxy neighbor cache entry for a Client within the
secured enclave that matches the destination. If there is a match,
the Proxy forwards the packet to the Client. Otherwise, the Proxy
returns the packet to the neighbor, i.e., by reversing the source
and destination link-layer addresses.When an IP packet enters a Server's AERO interface from the
network layer, the Server searches for a static neighbor cache entry
for a Client that matches the destination. If there is a match, the
Server uses one or more link-layer addresses in the entry as the
link-layer addresses for encapsulation and admits the packet into
the AERO link. Otherwise, the Server uses the link-layer address in
a permanent neighbor cache entry for a Relay (selected through
longest-prefix match) as the link-layer address for
encapsulation.When an IP packet enters a Server's AERO interface from the link
layer, the Server processes the packet according to the
network-layer destination address as follows:if the destination matches one of the Server's own addresses
the Server decapsulates the packet and forwards it to the
network layer for local delivery.else, if the destination matches a static neighbor cache
entry for a Client the Server first determines whether the
neighbor is the same as the one it received the packet from. If
so, the Server drops the packet silently to avoid looping;
otherwise, the Server uses the neighbor's link-layer address(es)
as the destination for encapsulation and re-admits the packet
into the AERO link.else, the Server uses the link-layer address in a neighbor
cache entry for a Relay (selected through longest-prefix match)
as the link-layer address for encapsulation.When an IP packet enters a Relay's AERO interface from the
network layer, the Relay searches its IP forwarding table for an ACP
entry that matches the destination and otherwise searches for a
neighbor cache entry that matches the destination (e.g., for
administratively-provisioned link-local addresses). If there is a
match, the Relay uses the link-layer address in the corresponding
neighbor cache entry as the link-layer address for encapsulation and
forwards the packet into the AERO link. Otherwise, the Relay drops
the packet and (for non-link-local addresses) returns a
network-layer ICMP Destination Unreachable message subject to rate
limiting (see: ).When an IP packet enters a Relay's AERO interface from the
link-layer, the Relay processes the packet as follows:if the destination does not match an ASP, or if the
destination matches one of the Relay's own addresses, the Relay
decapsulates the packet and forwards it to the network layer
where it will be subject to either IP forwarding or local
delivery.else, if the destination matches an ACP entry in the IP
forwarding table, or if the destination matches the link-local
address in a permanent neighbor cache entry, the Relay first
determines whether the neighbor is the same as the one it
received the packet from. If so the Relay MUST drop the packet
silently to avoid looping; otherwise, the Relay uses the
neighbor's link-layer address as the destination for
encapsulation and re-admits the packet into the AERO link.else, the Relay drops the packet and (for non-link-local
addresses) returns an ICMP Destination Unreachable message
subject to rate limiting (see: ).When an AERO node receives a return packet such as generated by
an AERO Proxy (see ), it proceeds according to
the AERO link trust basis. Namely, the return packets have the same
trust profile as for link-layer Destination Unreachable messages. If
the node has sufficient trust basis to accept link-layer Destination
Unreachable messages, it can then process the return packet as
described in the following paragraph. Otherwise, the node SHOULD
drop the packet and treat it as an indication that a path may be
failing, and MAY use NUD to test the path for reachability.If the node has sufficient trust basis to accept return packets,
it searches for a dynamic neighbor cache entry that matches the
destination. If there is a match, the neighbor marks the
corresponding link-layer address as "unreachable", selects the
next-highest priority "reachable" link-layer address in the entry as
the link-layer address for encapsulation then (re)admits the packet
into the AERO link. If there are no "reachable" link-layer
addresses, the neighbor instead sets FowardTime in the dynamic
neighbor cache entry to 0. If the source address corresponds to one
of the neighbor's own addresses, the neighbor also forwards the
packet to the corresponding Server; otherwise, it drops the
packet.AERO interfaces encapsulate IP packets according to whether they
are entering the AERO interface from the network layer or if they are
being re-admitted into the same AERO link they arrived on. This latter
form of encapsulation is known as "re-encapsulation".The AERO interface encapsulates packets per the Generic UDP
Encapsulation (GUE) procedures in , or through an alternate
encapsulation format (see: ). For packets
entering the AERO interface from the network layer, the AERO interface
copies the "TTL/Hop Limit", "Type of Service/Traffic Class" , "Flow Label" (for
IPv6) and "Congestion Experienced" values in
the packet's IP header into the corresponding fields in the
encapsulation IP header. For packets undergoing re-encapsulation, the
AERO interface instead copies these values from the original
encapsulation IP header into the new encapsulation header, i.e., the
values are transferred between encapsulation headers and *not* copied
from the encapsulated packet's network-layer header. (Note especially
that by copying the TTL/Hop Limit between encapsulation headers the
value will eventually decrement to 0 if there is a (temporary) routing
loop.) For IPv4 encapsulation/re-encapsulation, the AERO interface
sets the DF bit as discussed in .When GUE encapsulation is used, the AERO interface next sets the
UDP source port to a constant value that it will use in each
successive packet it sends, and sets the UDP length field to the
length of the encapsulated packet plus 8 bytes for the UDP header
itself plus the length of the GUE header (or 0 if GUE direct IP
encapsulation is used). For packets sent to a Server or Relay, the
AERO interface sets the UDP destination port to 8060, i.e., the
IANA-registered port number for AERO. For packets sent to a Client,
the AERO interface sets the UDP destination port to the port value
stored in the neighbor cache entry for this Client. The AERO interface
then either includes or omits the UDP checksum according to the GUE
specification.Clients normally use the IP address of the underlying interface as
the encapsulation source address. If the underlying interface does not
have an IP address, however, the Client uses an IP address taken from
an ACP as the encapsulation source address (assuming the node has some
way of injecting the ACP into the underlying network routing system).
For IPv6 addresses, the Client normally uses the ACP Subnet Router
Anycast address .AERO interfaces decapsulate packets destined either to the AERO
node itself or to a destination reached via an interface other than
the AERO interface the packet was received on. Decapsulation is per
the procedures specified for the appropriate encapsulation format.AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes on the AERO link.
In particular:AERO Relays and Servers accept encapsulated packets with a
link-layer source address that matches a permanent neighbor cache
entry.AERO Servers accept authentic encapsulated ND messages from
Clients, and create or update a static neighbor cache entry for
the Client based on the specific message type.AERO Clients and Servers accept encapsulated packets if there
is a static neighbor cache entry with a link-layer address that
matches the packet's link-layer source address.AERO Clients and Servers accept encapsulated packets if there
is a dynamic neighbor cache entry with an AERO address that
matches the packet's network-layer source address, with a
link-layer address that matches the packet's link-layer source
address, and with a non-zero AcceptTime.AERO Proxies accept encapsulated packets if there is a proxy
neighbor cache entry that matches the packet's network-layer
destination address (i.e., the address of the Client) and
link-layer source address (i.e., the address of one of the
Client's Servers). When the proxy is configured to accept packets
originating from any address in the open Internetwork however
(e.g., from another Proxy), it omits the source address check.Note that this simple data origin authentication is effective
in environments in which link-layer addresses cannot be spoofed. In
other environments, each AERO message must include a signature that
the recipient can use to authenticate the message origin, e.g., as for
common VPN systems such as OpenVPN . In
environments where end systems use end-to-end security, however, it
may be sufficient to require signatures only for ND and ICMP control
plane messages and omit signatures for data plane messages.The AERO interface is the node's attachment to the AERO link. The
AERO interface acts as a tunnel ingress when it sends a packet to an
AERO link neighbor and as a tunnel egress when it receives a packet
from an AERO link neighbor. AERO interfaces observe the packet sizing
considerations for tunnels discussed in and as specified below.The Internet Protocol expects that IP packets will either be
delivered to the destination or a suitable Packet Too Big (PTB)
message returned to support the process known as IP Path MTU Discovery
(PMTUD) . However, PTB
messages may be crafted for malicious purposes such as denial of
service, or lost in the network . This can be
especially problematic for tunnels, where a condition known as a PMTUD
"black hole" can result. For these reasons, AERO interfaces employ
operational procedures that avoid interactions with PMTUD, including
the use of fragmentation when necessary.AERO interfaces observe two different types of fragmentation.
Source fragmentation occurs when the AERO interface (acting as a
tunnel ingress) fragments the encapsulated packet into multiple
fragments before admitting each fragment into the tunnel. Network
fragmentation occurs when an encapsulated packet admitted into the
tunnel by the ingress is fragmented by an IPv4 router on the path to
the egress. Note that a packet that incurs source fragmentation may
also incur network fragmentation.IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of
1280 bytes . Although IPv4 specifies a smaller
minimum link MTU of 68 bytes , AERO interfaces
also observe the IPv6 minimum for IPv4 even if encapsulated packets
may incur network fragmentation.IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500
bytes , while the minimum MRU for IPv4 is only
576 bytes (note that common IPv6 over IPv4
tunnels already assume a larger MRU than the IPv4 minimum).AERO interfaces therefore configure an MTU that MUST NOT be smaller
than 1280 bytes, MUST NOT be larger than the minimum MRU among all
nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
and SHOULD NOT be smaller than 1500 bytes. AERO interfaces also
configure a Maximum Segment Unit (MSU) as the maximum-sized
encapsulated packet that the ingress can inject into the tunnel
without source fragmentation. The MSU value MUST NOT be larger than
(MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
operational assurance that a larger size can traverse the link along
all paths.All AERO nodes MUST configure the same MTU/MSU values for reasons
cited in ; in
particular, multicast support requires a common MTU value among all
nodes on the link. All AERO nodes MUST configure an MRU large enough
to reassemble packets up to (MTU+ENCAPS) bytes in length; nodes that
cannot configure a large-enough MRU MUST NOT enable an AERO
interface.The network layer proceeds as follow when it presents an IP packet
to the AERO interface. For each IPv4 packet that is larger than the
AERO interface MTU and with the DF bit set to 0, the network layer
uses IPv4 fragmentation to break the packet into a minimum number of
non-overlapping fragments where the first fragment is no larger than
the MTU and the remaining fragments are no larger than the first. For
all other IP packets, if the packet is larger than the AERO interface
MTU, the network layer drops the packet and returns a PTB message to
the original source. Otherwise, the network layer admits each IP
packet or fragment into the AERO interface.For each IP packet admitted into the AERO interface, the interface
(acting as a tunnel ingress) encapsulates the packet. If the
encapsulated packet is larger than the AERO interface MSU the ingress
source-fragments the encapsulated packet into a minimum number of
non-overlapping fragments where the first fragment is no larger than
the MSU and the remaining fragments are no larger than the first. The
ingress then admits each encapsulated packet or fragment into the
tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
header in case any network fragmentation is necessary. The
encapsulated packets will be delivered to the egress, which
reassembles them into a whole packet if necessary.Several factors must be considered when fragmentation is needed.
For AERO links over IPv4, the IP ID field is only 16 bits in length,
meaning that fragmentation at high data rates could result in data
corruption due to reassembly misassociations . For AERO links over both
IPv4 and IPv6, studies have also shown that IP fragments are dropped
unconditionally over some network paths [I-D.taylor-v6ops-fragdrop].
In environments where IP fragmentation issues could result in
operational problems, the ingress SHOULD employ intermediate-layer
source fragmentation (see: and ) before appending the outer
encapsulation headers to each fragment. Since the encapsulation
fragment header reduces the room available for packet data, but the
original source has no way to control its insertion, the ingress MUST
include the fragment header length in the ENCAPS length even for
packets in which the header is absent.When an AERO node admits encapsulated packets into the AERO
interface, it may receive link-layer or network-layer error
indications.A link-layer error indication is an ICMP error message generated by
a router in the underlying network on the path to the neighbor or by
the neighbor itself. The message includes an IP header with the
address of the node that generated the error as the source address and
with the link-layer address of the AERO node as the destination
address.The IP header is followed by an ICMP header that includes an error
Type, Code and Checksum. Valid type values include "Destination
Unreachable", "Time Exceeded" and "Parameter Problem" . (AERO interfaces ignore
all link-layer IPv4 "Fragmentation Needed" and IPv6 "Packet Too Big"
messages since they only emit packets that are guaranteed to be no
larger than the IP minimum link MTU as discussed in .)The ICMP header is followed by the leading portion of the packet
that generated the error, also known as the "packet-in-error". For
ICMPv6, specifies that the packet-in-error
includes: "As much of invoking packet as possible without the ICMPv6
packet exceeding the minimum IPv6 MTU" (i.e., no more than 1280
bytes). For ICMPv4, specifies that the
packet-in-error includes: "Internet Header + 64 bits of Original Data
Datagram", however Section 4.3.2.3 updates
this specification by stating: "the ICMP datagram SHOULD contain as
much of the original datagram as possible without the length of the
ICMP datagram exceeding 576 bytes".The link-layer error message format is shown in (where, "L2" and "L3" refer to link-layer and
network-layer, respectively):The AERO node rules for processing these link-layer error
messages are as follows:When an AERO node receives a link-layer Parameter Problem
message, it processes the message the same as described as for
ordinary ICMP errors in the normative references .When an AERO node receives persistent link-layer Time Exceeded
messages, the IP ID field may be wrapping before earlier fragments
awaiting reassembly have been processed. In that case, the node
SHOULD begin including integrity checks and/or institute rate
limits for subsequent packets.When an AERO node receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its dynamic neighbor correspondents, the node
SHOULD process the message as an indication that a path may be
failing, and MAY initiate NUD over that path. If it receives
Destination Unreachable messages on many or all paths, the node
SHOULD set ForwardTime for the corresponding dynamic neighbor
cache entry to 0 and allow future packets destined to the
correspondent to flow through a default route.When an AERO Client receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its static neighbor Servers, the Client SHOULD
math 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 send a PD "Release" message
to 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 static neighbor Clients, the Server SHOULD
mark the path as unusable and use another path. If it receives
Destination Unreachable messages on multiple paths, the Server
should take no further actions unless it receives a PD "Release"
message or if the PD lifetime expires. In that case, the Server
MUST release the Client's delegated ACP, withdraw the ACP from the
AERO routing system and delete the neighbor cache entry.When an AERO Relay or Server receives link-layer Destination
Unreachable messages in response to an encapsulated packet that it
sends to one of its permanent neighbors, it treats the messages as
an indication that the path to the neighbor may be failing.
However, the dynamic routing protocol should soon reconverge and
correct the temporary outage.When an AERO Relay receives a packet for which the
network-layer destination address is covered by an ASP, if there is no
more-specific routing information for the destination the Relay drops
the packet and returns a network-layer Destination Unreachable message
subject to rate limiting. The Relay first writes the network-layer
source address of the original packet as the destination address of
the message and determines the next hop to the destination. If the
next hop is reached via the AERO interface, the Relay uses the IPv6
address "::" or the IPv4 address "0.0.0.0" as the source address of
the message, then encapsulates the message and forwards it to the next
hop within the AERO interface. Otherwise, the Relay uses one of its
non link-local addresses as the source address of the message and
forwards it via a link outside the AERO interface.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 of
the message and determines the next hop to the destination. If the
next hop is reached via the AERO interface, the node uses the IPv6
address "::" or the IPv4 address "0.0.0.0" as the source address of
the message, then encapsulates the message and forwards it to the next
hop within the AERO interface. Otherwise, the node uses one of its non
link-local addresses as the source address of the message and forwards
it via a link outside the AERO interface.When an AERO node receives any network-layer error message via the
AERO interface, it examines the network-layer destination address. If
the next hop toward the destination is via the AERO interface, the
node re-encapsulates and forwards the message to the next hop within
the AERO interface. Otherwise, if the network-layer source address is
the IPv6 address "::" or the IPv4 address "0.0.0.0", the node writes
one of its non link-local addresses as the source address,
recalculates the IP and/or ICMP checksums then forwards the message
via a link outside the AERO interface.AERO Router Discovery, Prefix Delegation and Autoconfiguration are
coordinated as discussed in the following Sections.Each AERO Server configures a PD service to facilitate Client
requests. Each Server is provisioned with a database of
ACP-to-Client ID mappings for all Clients enrolled in the AERO
system, as well as any information necessary to authenticate each
Client. The Client database is maintained by a central
administrative authority for the AERO link and securely distributed
to all Servers, e.g., via the Lightweight Directory Access Protocol
(LDAP) , via static configuration, etc.
Therefore, no Server-to-Server PD state synchronization is
necessary, and Clients can optionally hold separate PDs for the same
ACPs from multiple Servers. In this way, Clients can associate with
multiple Servers, and can receive new PDs from new Servers before
releasing PDs received from existing Servers. This provides the
Client with a natural fault-tolerance and/or load balancing
profile.AERO Clients and Servers use ND messages to maintain neighbor
cache entries. AERO Servers configure their AERO interfaces as
advertising interfaces, and therefore send unicast RA messages with
configuration information in response to a Client's RS message. The
RS/RA messaging is conducted in the same fashion as specified in
.AERO Clients and Servers include PD messages as options in the
RS/RA messages they exchange (see: ). Client-initiated PD
options are included in RS messages, and Server-initiated PD options
are included in RA messages. The unified ND/PD messages are
exchanged between Client and Server according to the prefix
management schedule determined by the PD service. The unified
messages can be protected using SEcure Neighbor Discovery (SEND)
.On Some AERO links, PD arrangements may be through some
out-of-band service such as network management, static
configuration, etc. In those cases, AERO nodes can use simple RS/RA
message exchanges with no explicit PD options. Instead, the RS/RA
messages use AERO addresses as a means of representing the delegated
prefixes, e.g., if a message includes a source address of
"fe80::2001:db8:1:2" then the recipient can infer that the sender
holds the prefix delegation "2001:db8:1:2::/64".The following sections specify the Client and Server
behavior.AERO Clients discover the link-layer addresses of AERO Servers
via static configuration (e.g., from a flat-file map of Server
addresses and locations), or through an automated means such as
Domain Name System (DNS) name resolution .
In the absence of other information, the Client resolves the DNS
Fully-Qualfied Domain Name (FQDN) "linkupnetworks.[domainname]"
where "linkupnetworks" is a constant text string and "[domainname]"
is a DNS suffix for the Client's underlying interface (e.g.,
"example.com"). After discovering the link-layer addresses, the
Client associates with one or more of the corresponding Servers.To associate with a Server, the Client acts as a requesting
router to request ACPs through a combined ND/PD message exchange.
The Client includes a PD "Solicit" message as an ND option in an RS
message with the prefix-solicitation address as the IPv6 source
address, all-routers multicast as the IPv6 destination address, the
address of the Client's underlying interface as the link-layer
source address and the link-layer address of the Server as the
link-layer destination address. (If the Client's underlying
interface does not have an IP address, the Client can use the ACP
Subnet Router Anycast address as the link-layer source address.)The Client next includes a "Client Identifier" and an "IA_PD"
(i.e., prefix request) code in the PD "Solicit" message. If the
Client is pre-provisioned with ACPs associated with the AERO
service, it MAY also include the ACPs in the "IA_PD" option to
indicate its preferences to the Server. The Client finally includes
any additional PD codes (e.g., "Rapid Commit").The Client next includes one or more SLLAOs in the RS message
formatted as described in to register its
link-layer address(es) with the Server. The first SLLAO MUST
correspond to the underlying interface over which the Client will
send the RS message. The Client MAY include additional SLLAOs
specific to other underlying interfaces, but if so it MUST have
assurance that there will be no NATs or Proxies on the paths to the
Server via those interfaces. (Otherwise, the Client can register
additional link-layer addresses with the Server by sending
subsequent NS/RS messages via different underlying interfaces after
the initial RS/RA exchange).The Client then sends the RS message to the AERO Server and waits
for an RA message reply (see ) while
retrying MAX_RETRY times until an RA is received. If no RA is
received, or if it receives an RA with Router Lifetime set to 0
and/or a "Reply" with no ACPs, the Client SHOULD discontinue
autoconfiguration attempts through this Server and try another
Server. Otherwise, the Client processes the ACPs in the embedded
"Reply" message.Next, the Client creates a static neighbor cache entry with the
Server's link-local address as the network-layer address and the
Server's encapsulation source address as the link-layer address. The
Client then autoconfigures AERO addresses for each of the delegated
ACPs and assigns them to the AERO interface.The Client next examines the P bit in the RA message flags field
. If the P bit value was 1, the Client
assumes that there is a NAT or Proxy on the path to the Server via
the interface over which it sent the RS message. In that case, the
Client sets UDP Port Number and IP Address to 0 in the S/TLLAOs of
any subsequent ND messages it sends to the Server over that
link.The Client also caches any ASPs included in Route Information
Options (RIOs) as ASPs to associate with
the AERO link, and assigns the MTU/MSU values in the MTU options to
its AERO interface while configuring an appropriate MRU. This
configuration information applies to the AERO link as a whole, and
all AERO nodes will receive the same values.Following autoconfiguration, the Client sub-delegates the ACPs to
its attached EUNs and/or the Client's own internal virtual
interfaces as described in . The Client subsequently
maintains its ACP delegations through each of its Servers by sending
RS "Renew", "Rebind", and/or "Release" messages. The Server will in
turn send RA "Reply" messages.After the Client registers its Interface IDs and their associated
UDP/IP addresses and 'P(i)' values, it may wish to change one or
more Interface ID registrations, e.g., if an underlying interface
changes address or becomes unavailable, if QoS preferences change,
etc. To do so, the Client prepares an unsolicited NA message to send
over any available underlying interface. The source and target
address of the NA message are set to the Client's AERO address, and
the destination address is set to all-nodes multicast. The NA MUST
include a TLLAO specific to the selected available underlying
interface as the first TLLAO and MAY include any additional TLLAOs
specific to other underlying interfaces. The Client includes fresh
'P(i)' values in each TLLAO to update the Server's neighbor cache
entry. If the Client wishes to update 'P(i)' values without updating
the link-layer address, it sets the UDP Port Number and IP Address
fields to 0. If the Client wishes to disable the interface, it sets
all 'P(i)' values to '0' ("disabled").If the Client wishes to discontinue use of a Server it issues an
RS "Release" message. When the Server processes the message, it
releases the ACP, deletes its neighbor cache entry for the Client,
withdraws the IP route from the routing system and returns an RA
"Reply".AERO Servers act as IPv6 routers and support a PD service on
their AERO links. AERO Servers arrange to add their encapsulation
layer IP addresses (i.e., their link-layer addresses) to a static
map of Server addresses for the link and/or the DNS resource records
for the FQDN "linkupnetworks.[domainname]" before entering
service.When an AERO Server receives a prospective Client's RS "Solicit"
message on its AERO interface, and the Server is too busy, it SHOULD
return an immediate RA "Reply" message with no ACPs and with Router
Lifetime set to 0. Otherwise, the Server authenticates the RS
message and processes the embedded "Solicit" option. The Server
first determines the correct ACPs to delegate to the Client by
searching the Client database. When the Server delegates the ACPs,
it also creates an IP forwarding table entry for each ACP so that
the AERO BGP-based routing system will propagate the ACPs to the
Relays that aggregate the corresponding ASP (see: ).Next, the Server prepares an RA "Reply" message that includes the
delegated ACPs. For IPv4 ACPs, the ACP is in IPv4-mapped IPv6
address format and with prefix length set as specified in . The Server then prepares an RA "Reply"
message using its link-local address (i.e., fe80::ID) as the
network-layer source address, the Client's base AERO address from
the first ACP as the network-layer destination address, the Server's
link-layer address as the source link-layer address, and the source
link-layer address of the RS message as the destination link-layer
address. The Server next sets the P flag in the RA message flags
field to 1 if the source link-layer address
in the RS message was different than the address in the first SLLAO
to indicate that there is a NAT or Proxy on the path; otherwise it
sets P to 0. The Server then includes one or more RIOs that encode
the ASPs for the AERO link. The Server also includes two MTU options
- the first MTU option includes the MTU for the link and the second
MTU option includes the MSU for the link (see ). The Server finally sends the RA "Reply" message
to the Client.The Server next creates a static neighbor cache entry for the
Client using the base AERO address as the network-layer address and
with lifetime set to no more than the smallest PD lifetime. Next,
the Server updates the neighbor cache entry link-layer address(es)
by recording the information in each SLLAO option indexed by the
Interface ID and including the UDP port number, IP address and P(i)
values. For the first SLLAO in the list, however, the Server records
the actual encapsulation source UDP and IP addresses instead of
those that appear in the SLLAO in case there was a NAT or Proxy in
the path.After the initial RS/RA exchange, the AERO Server maintains the
neighbor cache entry for the Client until the PD lifetimes expire.
If the Client issues an RS "Renew", the Server extends the PD
lifetimes. If the Client issues an RS "Release", or if the Client
does not issue a "Renew" before the lifetime expires, the Server
deletes the neighbor cache entry for the Client and withdraws the IP
routes from the AERO routing system. The Server processes these and
any other Client PD messages, and returns an RA "Reply". The Server
may also issue an unsolicited RA "Reconfigure" message to inform the
Client that it needs to renegotiate its PDs.When DHCPv6 is used as the PD service, AERO Clients and Servers
are always on the same link (i.e., the AERO link) from the
perspective of DHCPv6. However, in some implementations the DHCPv6
server and ND function may be located in separate modules. In that
case, the Server's AERO interface driver module can act as a
Lightweight DHCPv6 Relay Agent (LDRA) to relay PD messages to and from the DHCPv6 server
module.When the LDRA receives an authentic RS message, it extracts the
PD message option and wraps it in IPv6/UDP headers. It sets the
IPv6 source address to the source address of the RS message, sets
the IPv6 destination address to
'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to
values that will be understood by the DHCPv6 server.The LDRA then wraps the message in a Relay-Forward message
header and includes an Interface-ID option that includes enough
information to allow the LDRA to forward the resulting Reply
message back to the Client (e.g., the Client's link-layer
addresses, a security association identifier, etc.). The LDRA also
wraps the information in all of the SLLAO options from the RS
message into the Interface-ID option, then forwards the message to
the DHCPv6 server.When the DHCPv6 server prepares a Reply message, it wraps the
message in a Relay-Reply message and echoes the Interface-ID
option. The DHCPv6 server then delivers the Relay-Reply message to
the LDRA, which discards the Relay-Reply wrapper and IPv6/UDP
headers, then delivers the DHCPv6 message to be wrapped into an RA
response to the Client. The Server uses the information in the
Interface ID option to prepare the RA message and to cache the
link-layer addresses taken from the SLLAOs echoed in the
Interface-ID option.When a source Client forwards packets to a prospective
correspondent Client within the same AERO link domain (i.e., one for
which the packet's destination address is covered by an ASP), the
source Client MAY initiate an AERO link route optimization procedure
on behalf of any of its native underlying interfaces. The procedure is
based on an exchange of IPv6 ND messages using a chain of AERO Servers
and Relays as a trust basis.Although the Client is responsible for initiating route
optimization, the Server is the policy enforcement point that
determines whether route optimization is permitted. For example, on
some AERO links route optimization would allow traffic to circumvent
critical network-based traffic inspection points. In those cases, the
Server can simply discard any route optimization messages instead of
forwarding them.The following sections specify the AERO link route optimization
procedure. depicts the AERO link route
optimization reference operational scenario, using IPv6 addressing
as the example (while not shown, a corresponding example for IPv4
addressing can be easily constructed). The figure shows an AERO
Relay ('R1'), two AERO Servers ('S1', 'S2'), two AERO Clients ('C1',
'C2') and two ordinary IPv6 hosts ('H1', 'H2'):In , Relay ('R1') assigns
the administratively-provisioned link-local address fe80::1 to its
AERO interface with link-layer address L2(R1), Server ('S1') assigns
the address fe80::2 with link-layer address L2(S1),and Server ('S2')
assigns the address fe80::3 with link-layer address L2(S2). Servers
('S1') and ('S2') next arrange to add their link-layer addresses to
a published list of valid Servers for the AERO link.AERO Client ('C1') receives the ACP 2001:db8:0::/48 in an ND/PD
exchange via AERO Server ('S1') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(C1). Client ('C1') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::2 and
link-layer address L2(S1), then sub-delegates the ACP to its
attached EUNs. IPv6 host ('H1') connects to the EUN, and configures
the address 2001:db8:0::1.AERO Client ('C2') receives the ACP 2001:db8:1::/48 in an ND/PD
exchange via AERO Server ('S2') then assigns the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(C2). Client ('C2') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::3 and
link-layer address L2(S2), then sub-delegates the ACP to its
attached EUNs. IPv6 host ('H2') connects to the EUN, and configures
the address 2001:db8:1::1.Again, with reference to ,
when source host ('H1') sends a packet to destination host ('H2'),
the packet is first forwarded over the source host's attached EUN to
Client ('C1'). Client ('C1') then forwards the packet via its AERO
interface to Server ('S1') and also sends an NS message toward
Client ('C2') via Server ('S1').Server ('S1') then re-encapsulates and forwards both the packet
and the NS message out the same AERO interface toward Client ('C2')
via Relay ('R1'). When Relay ('R1') receives the packet and NS
message, it consults its forwarding table to discover Server ('S2')
as the next hop toward Client ('C2'). Relay ('R1') then forwards
both the packet and the NS message to Server ('S2'), which then
forwards them to Client ('C2').After Client ('C2') receives the NS message, it process the
message and creates or updates a dynamic neighbor cache entry for
Client ('C1'), then sends the NA response to the link-layer address
of Server ('S2'). When Server ('S2') receives the NA message it
re-encapsulates the message and forwards it on to Relay ('R1'),
which re-encapsulates and forwards the message on to Server ('S1')
which re-encapsulates and forwards the message on to Client
('C1').After Client ('C1') receives the NA message, it processes the
message and creates or updates a dynamic neighbor cache entry for
Client ('C2'). Thereafter, forwarding of packets from Client ('C1')
to Client ('C2') without involving any intermediate nodes is
enabled. The mechanisms that support this exchange are specified in
the following sections.When a Client forwards a packet with a source address from one of
its ACPs toward a destination address covered by an ASP (i.e.,
toward another AERO Client connected to the same AERO link), the
source Client MAY send an NS message forward toward the destination
Client via the Server.In the reference operational scenario, when Client ('C1')
forwards a packet toward Client ('C2'), it MAY also send an NS
message forward toward Client ('C2'), subject to rate limiting (see
Section 8.2 of ). Client ('C1') prepares the
NS message as follows:the link-layer source address is set to 'L2(C1)' (i.e., the
link-layer address of Client ('C1')).the link-layer destination address is set to 'L2(S1)' (i.e.,
the link-layer address of Server ('S1')).the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the base AERO address of Client ('C1')).the network-layer destination address is set to the AERO
address corresponding to the destination address of Client
('C2').the Type is set to 135.the Target Address is set to the destination address of the
packet that triggered route optimization.the message includes one or more SLLAOs set to appropriate
values for Client ('C1')'s native underlying interfaces.the message includes one or more RIOs that include Client
('C1')'s ACPs .the message SHOULD include a Timestamp option and a Nonce
option.Note that the act of sending NS messages is cited as "MAY", since
Client ('C1') may have advanced knowledge that the direct path to
Client ('C2') would be unusable or otherwise undesirable. If the
direct path later becomes unusable after the initial route
optimization, Client ('C1') simply allows packets to again flow
through Server ('S1').When Server ('S1') receives an NS message from Client ('C1'), it
first verifies that the SLLAOs in the NS are a proper subset of the
link-layer addresses in Client ('C1')'s neighbor cache entry. If the
Client's SLLAOs are not acceptable, Server ('S1') discards the
message. Otherwise, Server ('S1') verifies that Client ('C1') is
authorized to use the ACPs encoded in the RIOs of the NS and
discards the NS if verification fails.Server ('S1') then examines the network-layer destination address
of the NS to determine the next hop toward Client ('C2') by
searching for the AERO address in the neighbor cache. Since Client
('C2') is not one of its neighbors, Server ('S1') re-encapsulates
the NS and relays it via Relay ('R1') by changing the link-layer
source address of the message to 'L2(S1)' and changing the
link-layer destination address to 'L2(R1)'. Server ('S1') finally
forwards the re-encapsulated message to Relay ('R1') without
decrementing the network-layer TTL/Hop Limit field.When Relay ('R1') receives the NS message from Server ('S1') it
determines that Server ('S2') is the next hop toward Client ('C2')
by consulting its forwarding table. Relay ('R1') then
re-encapsulates the NS while changing the link-layer source address
to 'L2(R1)' and changing the link-layer destination address to
'L2(S2)'. Relay ('R1') then relays the NS via Server ('S2').When Server ('S2') receives the NS message from Relay ('R1') it
determines that Client ('C2') is a neighbor by consulting its
neighbor cache. Server ('S2') then re-encapsulates the NS while
changing the link-layer source address to 'L2(S2)' and changing the
link-layer destination address to 'L2(C2)'. Server ('S2') then
forwards the message to Client ('C2').When Client ('C2') receives the NS message, it accepts the NS
only if the message has a link-layer source address of one of its
Servers (e.g., L2(S2)). Client ('C2') further accepts the message
only if it is willing to serve as a route optimization target.In the reference operational scenario, when Client ('C2')
receives a valid NS message, it either creates or updates a dynamic
neighbor cache entry that stores the source address of the message
as the network-layer address of Client ('C1') , stores the
link-layer addresses found in the SLLAOs as the link-layer addresses
of Client ('C1'), and stores the ACPs encoded in the RIOs of the NS
as the ACPs for Client ('C1'). Client ('C2') then sets AcceptTime
for the neighbor cache entry to ACCEPT_TIME.After processing the message, Client ('C2') prepares an NA
message response as follows:the link-layer source address is set to 'L2(C2)' (i.e., the
link-layer address of Client ('C2')).the link-layer destination address is set to 'L2(S2)' (i.e.,
the link-layer address of Server ('S2')).the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the base AERO address of Client ('C2')).the network-layer destination address is set to
fe80::2001:db8:0:0 (i.e., the base AERO address of Client
('C1')).the Type is set to 136.The Target Address is set to the Target Address field in the
NS message.the message includes one or more TLLAOs set to appropriate
values for Client ('C2')'s native underlying interfaces.the message includes one or more RIOs that include Client
('C2')'s ACPs .the message SHOULD include a Timestamp option and MUST echo
the Nonce option received in the NS (i.e., if a Nonce option is
included).Client ('C2') then sends the NA message to Server ('S2').When Server ('S2') receives an NA message from Client ('C2'), it
first verifies that the TLLAOs in the NA are a proper subset of the
Interface IDs in Client ('C2')'s neighbor cache entry. If the
Client's TLLAOs are not acceptable, Server ('S2') discards the
message. Otherwise, Server ('S2') verifies that Client ('C2') is
authorized to use the ACPs encoded in the RIOs of the NA message. If
validation fails, Server ('S2') discards the NA.Server ('S2') then examines the network-layer destination address
of the NA to determine the next hop toward Client ('C1') by
searching for the AERO address in the neighbor cache. Since Client
('C1') is not a neighbor, Server ('S2') re-encapsulates the NA and
relays it via Relay ('R1') by changing the link-layer source address
of the message to 'L2(S2)' and changing the link-layer destination
address to 'L2(R1)'. Server ('S2') finally forwards the
re-encapsulated message to Relay ('R1') without decrementing the
network-layer TTL/Hop Limit field.When Relay ('R1') receives the NA message from Server ('S2') it
determines that Server ('S1') is the next hop toward Client ('C1')
by consulting its forwarding table. Relay ('R1') then
re-encapsulates the NA while changing the link-layer source address
to 'L2(R1)' and changing the link-layer destination address to
'L2(S1)'. Relay ('R1') then relays the NA via Server ('S1').When Server ('S1') receives the NA message from Relay ('R1') it
determines that Client ('C1') is a neighbor by consulting its
neighbor cache. Server ('S1') then re-encapsulates the NA while
changing the link-layer source address to 'L2(S1)' and changing the
link-layer destination address to 'L2(C1)'. Server ('S1') then
forwards the message to Client ('C1').When Client ('C1') receives the NA message, it first verifies the
Nonce value matches the value that it included in its NS message (if
any). If the Nonce values match, Client ('C1') then processes the
message as follows.In the reference operational scenario, when Client ('C1')
receives the NA message, it either creates or updates a dynamic
neighbor cache entry that stores the source address of the message
as the network-layer address of Client ('C2'), stores the link-layer
addresses found in the TLLAOs as the link-layer addresses of Client
('C2') and stores the ACPs encoded in the RIOs of the NA as the ACPs
for Client ('C2'). Client ('C1') then sets ForwardTime for the
neighbor cache entry to FORWARD_TIME.Now, Client ('C1') has a neighbor cache entry with a valid
ForwardTime value, while Client ('C2') has a neighbor cache entry
with a valid AcceptTime value. Thereafter, Client ('C1') may forward
ordinary network-layer data packets directly to Client ('C2')
without involving any intermediate nodes, and Client ('C2') can
verify that the packets came from an acceptable source. (In order
for Client ('C2') to forward packets to Client ('C1'), a
corresponding NS/NA message exchange is required in the reverse
direction; hence, the mechanism is asymmetric.)Route optimization may be initiated by the source Client by
sending NS messages with SLLAOs corresponding to its native
underlying interfaces. Route optimization for the source Client's
other interfaces may be initiated by Servers and/or Proxies. Each
node initiates route optimization by sending NS messages with SLLAOs
only for those underlying interfaces they are authoritative for.
Each node MUST consistently use the same Interface ID values to
denote the same interfaces. The Interface IDs are established and
maintained by the source Client's RS/RA exchanges.The target Client's Server serves as a route optimization target
if some or all of the target Client's underlying interfaces connect
via NATs, Proxies and/or VPNs. In that case, when the source sends
an NS message the target Server both forwards the NS toward a native
underlying interface of the target Client (if any) and prepares an
NA response the same as if it were the target Client (see: ). (This means that the source may receive
two separate NA messages - one from the target Server and one from
the target Client. The source must accept the union of the
information from both messages.)For non-native underlying interfaces, the target Server includes
a first TLLAO option in the NA with Interface ID set to 255 and
includes any additional TLLAOs corresponding to the Client's NATed,
Proxyed and/or VPNed underlying interfaces. The Server writes its
own link-layer address in TLLAOs corresponding to NATed and VPNed
underlying interfaces, and writes the link-layer address of the
Proxy in TLLAOs corresponding to Proxyed underlying interfaces
(while also setting the X flag). The Interface ID and QoS Preference
values in the TLLAOs are those supplied by the Client during the
initial RS/RA exchange and updated by any ensuing unsolicited NA
messages. The target Server must then maintain a dynamic neighbor
cache entry for the Client, but MUST NOT send BGP updates for
Clients discovered through dynamic route optimization.Thereafter, if the target Client moves to a new Server, the old
Server sends unsolicited NA messages with no TLLAOs (subject to rate
limiting) back to the source in response to data packets received
from a correspondent node while forwarding the packets themselves to
a Relay. The Relay will then either forward the packets to the new
Server if the target Client has moved, or drop the packets if the
target Client is no longer in the network. The source then allows
future packets destined to the target Client to again flow through
its own Server (or Relay). Note however that the old Server retains
the neighbor cache entry with its associated AcceptTime since there
may be many packets in flight. AcceptTime will then eventually
decrement to 0 once the correspondent node processes and acts on the
unsolicited NAs.When the target Client (or Proxy) sends unsolicited NA messages
to the target Server to update link-layer address and/or QoS
preferences, the target Server repeats the messages to any of its
dynamic neighbors while using its own link-layer and link-local
addresses as the source addresses. In this way, the target Server
acts as a link-scoped multicast repeater on behalf of the target
Client (or Proxy).(Note that instead of serving as the route optimization target
for Proxy interfaces, the target Server could instead forward the
source's NS messages and allow the Proxies to return NA messages,
i.e., the same as for Clients on native interfaces. That would mean
that the source could receive multiple NA messages from multiple
Proxies and, if some or all NA messages are lost, the source would
not be able to determine the full picture of the Client's Proxy
affiliations. If this alternate architecture is deemed appropriate
in some use cases, then the AERO Proxies could be employed to serve
as route optimization targets instead of depending on the Servers to
do so.)AERO nodes perform Neighbor Unreachability Detection (NUD) by
sending NS messages to elicit solicited NA messages from neighbors the
same as described in . NUD is performed either
reactively in response to persistent link-layer errors (see ) or proactively to update neighbor cache entry
timers and/or link-layer address information.When an AERO node sends an NS/NA message, it uses one of its
link-local addresses as the IPv6 source address and a link-local
address of the neighbor as the IPv6 destination address. When route
optimization directs a source AERO node to a target AERO node, the
source node SHOULD proactively test the direct path by sending an
initial NS message to elicit a solicited NA response. While testing
the path, the source node can optionally continue sending packets via
its default router, maintain a small queue of packets until target
reachability is confirmed, or (optimistically) allow packets to flow
directly to the target.While data packets are still flowing, the source node thereafter
periodically tests the direct path to the target node (see Section 7.3
of ) in order to keep dynamic neighbor cache
entries alive. When the target node receives a valid NS message, it
resets AcceptTime to ACCEPT_TIME and updates its cached link-layer
addresses (if necessary). When the source node receives a solicited NA
message, it resets ForwardTime to FORWARD_TIME and updates its cached
link-layer addresses (if necessary). If the source node is unable to
elicit a solicited NA response from the target node after MaxRetry
attempts, it SHOULD set ForwardTime to 0. Otherwise, the source node
considers the path usable and SHOULD thereafter process any link-layer
errors as an indication that the direct path to the target node has
either failed or has become intermittent.When ForwardTime for a dynamic neighbor cache entry expires, the
source node resumes sending any subsequent packets via a Server (or
Relay) and may (eventually) attempt to re-initiate the AERO route
optimization process. When AcceptTime for a dynamic neighbor cache
entry expires, the target node discards any subsequent packets
received directly from the source node. When both ForwardTime and
AcceptTime for a dynamic neighbor cache entry expire, the node deletes
the neighbor cache entry.Note that an AERO node may have multiple underlying interface paths
toward the target neighbor. In that case, the node SHOULD perform NUD
over each underlying interface and only consider the neighbor
unreachable if NUD fails over multiple underlying interface paths.AERO is an example of a Distributed Mobility Management (DMM)
service. Each AERO Server is responsible for only a subset of the
Clients on the AERO link, as opposed to a Centralized Mobility
Management (CMM) service where there is a single network service for
all Clients. AERO Clients coordinate with their regional Servers via
RS/RA exchanges to maintain the DMM profile, and the AERO routing
system tracks the current AERO Client/Server peering
relationships.Mobility management for AERO interfaces is accommodated by sending
unsolicited NA messages the same as for announcing link-layer address
changes for any interface that implements IPv6 ND . When a node sends an unsolicited NA message, it
sets the IPv6 source to its own link-local address, sets the IPv6
destination address to all-nodes multicast, sets the link-layer source
address to its own address and sets the link-layer destination address
to either a multicast address or the unicast link-layer address of a
neighbor. If the unsolicited NA message must be received by multiple
neighbors, the node sends multiple copies of the NA using a different
unicast link-layer destination address for each neighbor. Mobility
management considerations are specified in the following sections.When a Server receives packets with destination addresses that do
not match one of its static neighbor cache Clients, it forwards the
packets to a Relay and also returns an unsolicited NA message to the
sender with no TLLAOs. The packets will be delivered to the target
Client's new location, and the sender will realize that it needs to
deprecate its routing information that associated the target with
this Server.When a Client needs to change its link-layer addresses, e.g., due
to a mobility event, it sends unsolicited NAs to its neighbors using
the new link-layer address as the source address and with TLLAOs
that include the new Client UDP Port Number, IP Address and P(i)
values. If the Client sends the NA solely for the purpose of
updating QoS preferences without updating the link-layer address,
the Client sets the UDP Port Number and IP Address to 0.The Client MAY send up to MaxRetry unsolicited NA messages in
parallel with sending actual data packets in case one or more NAs
are lost. If all NAs are lost, the neighbor will eventually invoke
NUD by sending NS messages that include SLLAOs.When a Client needs to bring new underlying interfaces into
service (e.g., when it activates a new data link), it sends
unsolicited NAs to its neighbors using the new link-layer address as
the source address and with TLLAOs that include the new Client
link-layer information.When a Client needs to remove existing underlying interfaces from
service (e.g., when it de-activates an existing data link), it sends
unsolicited NAs to its neighbors with TLLAOs with all P(i) values
set to 0.If the Client needs to send the unsolicited NAs over an
underlying interface other than the one being removed from service,
it MUST include a current TLLAO for the sending interface as the
first TLLAO and include TLLAOs for any underlying interface being
removed from service as additional TLLAOs.AERO interface neighbors MAY provide a configuration option that
allows them to perform implicit mobility management in which no ND
messaging is used. In that case, the Client only transmits packets
over a single interface at a time, and the neighbor always observes
packets arriving from the Client from the same link-layer source
address.If the Client's underlying interface address changes (either due
to a readdressing of the original interface or switching to a new
interface) the neighbor immediately updates the neighbor cache entry
for the Client and begins accepting and sending packets to the
Client's new link-layer 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 associates with a new Server, it performs the
Client procedures specified in .When a Client disassociates with an existing Server, it sends an
RS "Release" message via a new Server with its base AERO address as
the network-layer source address and the
(administratively-provisioned) link-local address of the old Server
as the network-layer destination address. The new Server then caches
the Client's AERO address and "Release" message parameters (e.g.,
"transaction ID") and writes its own administratively-provisioned
link-local address as the network-layer source address. The new
Server then forwards the message to a Relay, which forwards the
message to the old Server.When the old Server receives the "Release", it releases the
Client's ACP prefix delegations and routes. The old Server then
deletes the Client's neighbor cache entry so that any in-flight
packets will be forwarded via a Relay to the new Server, which will
forward them to the Client. The old Server finally returns a "Reply"
message via a Relay to the new Server, which will decapsulate the
"Reply" message and forward it as an RA "Reply" to the Client.When the new Server forwards the "Reply" message, the Client can
delete both the default route and the neighbor cache entry for the
old Server. (Note that since messages may be lost in the network the
Client SHOULD retry until it gets an RA "Reply" indicating that the
RS "Release" was successful. If the Client does not receive a
"Reply" after MaxRetry attempts, the old Server may have failed and
the Client should discontinue its "Release" attempts.)Finally, Clients SHOULD NOT move rapidly between Servers in order
to avoid causing excessive oscillations in the AERO routing system.
Such oscillations could result in intermittent reachability for the
Client itself, while causing little harm to the network. 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, etc.When the underlying network does not support multicast, AERO
Clients map link-scoped multicast addresses to the link-layer address
of a Server, which acts as a multicast forwarding agent. The AERO
Client also serves as an IGMP/MLD Proxy for its EUNs and/or hosted
applications per while using the link-layer
address of the Server as the link-layer address for all multicast
packets.When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in for IPv4 underlying networks and use a TBD
site-scoped multicast mapping for IPv6 underlying networks. In that
case, border routers must ensure that the encapsulated site-scoped
multicast packets do not leak outside of the site spanned by the AERO
link.In some deployments, AERO Clients may be located in secured enclaves
(e.g., a corporate enterprise network, a radio access network, etc.)
that do not allow direct communications from the Client to a Server in
the outside Internetwork. In that case, the secured enclave can employ
an AERO Proxy.The AERO Proxy is located at the secured enclave perimeter and
listens for RS messages originating from or RA messages destined to AERO
Clients located within the enclave. The Proxy acts on these control
messages as follows:when the Proxy receives an RS message from a Client within the
secured enclave, it first authenticates the message then creates a
proxy neighbor cache entry for the Client in the INCOMPLETE State
and caches the Client and Server link-layer address along with any
identifying information including PD "transaction IDs", "Client
Identifiers", etc. and/or ND Nonce values. The Proxy then
re-encapsulates the message and forwards it to the Server indicated
by the destination link-layer address in the packet while
substituting its own external address as the source link-layer
address.when the Proxy receives an RA message from the Server, it matches
the message with the (INCOMPLETE) proxy neighbor cache entry. The
Proxy then caches the route information in the message as a mapping
from the Client's ACPs to the Client's address within the secured
enclave, and sets the neighbor cache entry state to REACHABLE. The
Proxy then re-encapsulates the message and forwards it to the
Client. At the same time, the Proxy sends an unsolicited NA message
including a TLLAO with the X flag set back to the Server to assert
that it is indeed a Proxy as opposed to an ordinary NAT. (In
environments where spoofing is a threat, the Proxy signs the NA
using SEND.)After the initial RS/RA handshake, the Proxy can send
unsolicited NA messages to the Client's Server(s) to update Server
neighbor cache entries on behalf of the Client. (For example, the Proxy
can send NA messages with a TLLAO with UDP Port Number and IP Address
set to 0 and with valid P(i) values to update the Server(s) with the
Client's new QoS preferences for that link). The Proxy also forwards any
unsolicited NA messages originating from the Client to the Client's
Server(s) (e.g. if the Client needs to announce new QoS preferences on
its own behalf), and forwards any data packets originating from the
Client to the Client's primary Server.At the same time, for data packets originating from a Client within
the enclave with destination addresses that match an ASP, the Proxy can
initiate route optimization by sending an NS message via the Server to
solicit an NA message from a target node on the path to the destination
Client the same as discussed in . The target
must deliver the NA message directly to the Proxy, i.e., instead of
relaying through the backward chain of Relays and Servers, since the
backward chain could deliver the NA to a different Proxy besides the one
that produced the NS. For this reason, the Proxy prepares an NS message
as specified in , but with its own
link-layer address as the link-layer source address and with a single
SLLAO containing its link-layer address and with the X flag set to
indicate that direct delivery is required.When the target receives the NS message, it creates a dynamic
neighbor cache entry in the ACCEPT state and returns an NA message
directly to the Proxy. When the target is a Client, it includes TLLAOs
in the NA message with link-layer addresses corresponding to its native
underling interfaces. When the target is a Server, it includes a first
TLLAO in the NA message with Interface ID set to 255 and with its own
link-layer address information, and also includes additional TLLAOs
corresponding to the destination Client's Proxyed, NATed or VPNed
underlying interfaces. (For NATed or VPNed underlying interfaces the
server writes its own link-layer address in the TLLAO, and for Proxyed
interfaces it writes the link-layer address of the Proxy.) When the
source Proxy receives the NA message, it creates a dynamic neighbor
cache entry in the FORWARD state that associates the TLLAOs of the NA
message as the next-hop toward the routes advertised in the NA RIOs.When a source Proxy sends route optimization NS messages toward the
target, it can include RIOs to assert specific routes, and the target
will only accept packets from the source Proxy with matching source
addresses. If the source Proxy wishes to assert a "wildcard" route, it
includes an RIO in the NS message with Prefix and Prefix Length set to
0. In that case, the target will either accept or ignore the NS based on
its configured trust policy. If the target accepts the NS, it will
accept all packets originating from the source Proxy regardless of their
source address.After the initial NS/NA exchange, the target may need to update the
neighbor cache entries for any source Proxies for which it holds a
dynamic neighbor cache entry in the ACCEPT state. The target therefore
sends unsolicited NA messages to announce any link layer changes. As a
result:the source Proxy may receive unsolicited NA messages with TLLAOs
with new UDP Port Number, IP Address and/or QoS preferences from the
target. In that case, the Proxy updates its neighbor cache entry and
forwards future outbound packets based on the new link layer
information.the source Proxy may receive reflected packets destined to the
link-layer address of a departed Client. In that case, the Proxy
proceeds as discussed in .the source Proxy may receive link-layer Destination Unreachable
messages in response to data packets it sends to one of the target
link-layer addresses. In that case, the Proxy processes the
link-layer error messages as an indication that the path may be
failing and proceeds as discussed in .After the NS/NA exchange, while data packets are still flowing
the source Proxy sends additional NS messages to the target using the
address in the target's first TLLAO as the destination. The NS message
will update the target's AcceptTime timer, and the resulting NA reply
will update the source Proxy's ForwardTime timer in their respective
neighbor cache entries.If at some later time the target Client departs from its secured
enclave, the Proxy sends unsolicited NAs to the Client's Servers to
announce the departure.When a Client's AERO interface is configured over a direct underlying
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 the QoS preferences associated
with its underling interfaces. If the direct underlying interface has
the highest QoS preference, then the Client's IP packets are transmitted
directly to the peer without going through an underlying network. If
other underlying interfaces have higher QoS preferences, then the
Client's IP packets are transmitted via a different underlying
interface, which may result in the inclusion of AERO Proxies, Servers
and Relays in the communications path. Direct underlying interfaces must
be tested periodically for reachability, e.g., via NUD, via periodic
unsolicited NAs, etc.IPv6 AERO links typically have ASPs that cover many candidate ACPs of
length /64 or shorter. However, in some cases it may be desirable to use
AERO over links that have only a /64 ASP. This can be accommodated by
treating all Clients on the AERO link as simple hosts that receive /128
prefix delegations.In that case, the Client sends an RS message to the Server the same
as for ordinary AERO links. The Server responds with an RA message that
includes one or more /128 prefixes (i.e., singleton addresses) that
include the /64 ASP prefix along with an interface identifier portion to
be assigned to the Client. The Client and Server then configure their
AERO addresses based on the interface identifier portions of the /128s
(i.e., the lower 64 bits) and not based on the /64 prefix (i.e., the
upper 64 bits).For example, if the ASP for the host-only IPv6 AERO link is
2001:db8:1000:2000::/64, each Client will receive one or more /128 IPv6
prefix delegations such as 2001:db8:1000:2000::1/128,
2001:db8:1000:2000::2/128, etc. When the Client receives the prefix
delegations, it assigns the AERO addresses fe80::1, fe80::2, etc. to the
AERO interface, and assigns the global IPv6 addresses (i.e., the /128s)
to either the AERO interface or an internal virtual interface such as a
loopback. In this arrangement, the Client conducts route optimization in
the same sense as discussed in .This specification has applicability for nodes that act as a Client
on an "upstream" AERO link, but also act as a Server on "downstream"
AERO links. More specifically, if the node acts as a Client to receive a
/64 prefix from the upstream AERO link it can then act as a Server to
provision /128s to Clients on downstream AERO links.An AERO implementation based on OpenVPN (https://openvpn.net/) was
announced on the v6ops mailing list on January 10, 2018. The latest
version is available at:
http://linkupnetworks.net/aero/AERO-OpenVPN-1.0.tgz.An initial public release of the AERO proof-of-concept source code
was announced on the intarea mailing list on August 21, 2015. The latest
version is available at:
http://linkupnetworks.net/aero/aero-3.0.3a.tgz.The IANA has assigned a 4-octet Private Enterprise Number "45282" for
AERO in the "enterprise-numbers" registry.The IANA has assigned the UDP port number "8060" for an earlier
experimental version of AERO . This document
obsoletes and claims the UDP port number "8060"
for all future use.No further IANA actions are required.AERO link security considerations are the same as for standard IPv6
Neighbor Discovery except that AERO improves on
some aspects. In particular, AERO uses a trust basis between Clients and
Servers, where the Clients only engage in the AERO mechanism when it is
facilitated by a trusted Server.NS and NA messages SHOULD include a Timestamp option (see Section 5.3
of ) that other AERO nodes can use to verify the
message time of origin. NS and RS messages SHOULD include a Nonce option
(see Section 5.3 of ) that recipients echo back
in corresponding responses. In cases where spoofing cannot be mitigated
through other means, however, all AERO IPv6 ND messages should employ
SEND , which also protects the PD information
embedded in RS/RA message options.AERO links must be protected against link-layer address spoofing
attacks in which an attacker on the link pretends to be a trusted
neighbor. Links that provide link-layer securing mechanisms (e.g., IEEE
802.1X WLANs) and links that provide physical security (e.g., enterprise
network wired LANs) provide a first line of defense, however AERO nodes
SHOULD also use securing services such as SEND for Client authentication
and network admission control. Following authenticated Client admission
and prefix delegation procedures, AERO nodes MUST ensure that the source
of data packets corresponds to the node to which the prefixes were
delegated.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.)AERO Clients, Servers and Relays on the open Internet are susceptible
to the same attack profiles as for any Internet nodes. For this reason,
IP security SHOULD be used when AERO is employed over
unmanaged/unsecured links using securing mechanisms such as IPsec , IKE and/or TLS . In some environments, however, the use of end-to-end
security from Clients to correspondent nodes (i.e., other Clients and/or
Internet nodes) could obviate the need for IP security between AERO
Clients, Servers and Relays.AERO Servers and Relays present targets for traffic amplification 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 Relays and Servers 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
can institute rate limits that protect Clients from receiving packet
floods that could DoS low data rate links.Security considerations for accepting link-layer ICMP messages and
reflected packets are discussed throughout the document.Discussions in the IETF, aviation standards communities and private
exchanges helped shape some of the concepts in this work. Individuals
who contributed insights include Mikael Abrahamsson, Mark Andrews, Fred
Baker, Bob Braden, Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph
Droms, Adrian Farrel, Sri Gundavelli, Brian Haberman, Bernhard Haindl,
Joel Halpern, Tom Herbert, Sascha Hlusiak, Lee Howard, Andre Kostur, Ted
Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski, Alexandru
Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie Volz, Ryuji
Wakikawa, Lloyd Wood and James Woodyatt. Members of the IESG also
provided valuable input during their review process that greatly
improved the document. Special thanks go to Stewart Bryant, Joel Halpern
and Brian Haberman for their shepherding guidance during the publication
of the AERO first edition.This work has further been encouraged and supported by Boeing
colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam
Brodie, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu
Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Ed King, Gene MacLean
III, Rob Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian Skeen, Mike
Slane, Carrie Spiker, Brendan Williams, Julie Wulff, Yueli Yang, Eric
Yeh and other members of the BR&T and BIT mobile networking teams.
Wayne Benson, Kyle Bae and Eric Yeh are especially acknowledged for
implementing the AERO functions as extensions to the public domain
OpenVPN distribution.Earlier works on NBMA tunneling approaches are found in .Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:The Internet Routing Overlay Network (IRON) Virtual Enterprise Traversal (VET) The Subnetwork Encapsulation and Adaptation Layer (SEAL) AERO, First Edition Note that these works cite numerous earlier efforts that are
not also cited here due to space limitations. The authors of those
earlier works are acknowledged for their insights.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.This work is aligned with the Boeing Research and Technology
(BR&T) autonomous systems networking program.http://en.wikipedia.org/wiki/TUN/TAPhttp://openvpn.netBGP in 2015, http://potaroo.netWhen GUE encapsulation is not needed, AERO can use common
encapsulations such as IP-in-IP , Generic Routing
Encapsulation (GRE) and
others. The encapsulation is therefore only differentiated from non-AERO
tunnels through the application of AERO control messaging and not
through, e.g., a well-known UDP port number.As for GUE encapsulation, alternate AERO encapsulation formats may
require encapsulation layer fragmentation. For simple IP-in-IP
encapsulation, an IPv6 fragment header is inserted directly between the
inner and outer IP headers when needed, i.e., even if the outer header
is IPv4. The IPv6 Fragment Header is identified to the outer IP layer by
its IP protocol number, and the Next Header field in the IPv6 Fragment
Header identifies the inner IP header version. For GRE encapsulation, a
GRE fragment header is inserted within the GRE header . shows the AERO IP-in-IP encapsulation format
before any fragmentation is applied: shows the AERO GRE encapsulation format
before any fragmentation is applied:Alternate encapsulation may be preferred in environments where GUE
encapsulation would add unnecessary overhead. For example, certain
low-bandwidth wireless data links may benefit from a reduced
encapsulation overhead.GUE encapsulation can traverse network paths that are inaccessible to
non-UDP encapsulations, e.g., for crossing Network Address Translators
(NATs). More and more, network middleboxes are also being configured to
discard packets that include anything other than a well-known IP
protocol such as UDP and TCP. It may therefore be necessary to determine
the potential for middlebox filtering before enabling alternate
encapsulation in a given environment.In addition to IP-in-IP, GRE and GUE, AERO can also use security
encapsulations such as IPsec and SSL/TLS. In that case, AERO control
messaging and route determination occur before security encapsulation is
applied for outgoing packets and after security decapsulation is applied
for incoming packets.AERO is especially well suited for use with VPN system encapsulations
such as OpenVPN .An encapsulation fragment header is inserted when the AERO tunnel
ingress needs to apply fragmentation to accommodate packets that must be
delivered without loss due to a size restriction. Fragmentation is
performed on the inner packet while encapsulating each inner packet
fragment in outer IP and encapsulation layer headers that differ only in
the fragment header fields.The fragment header can also be inserted in order to include a
coherent Identification value with each packet, e.g., to aid in
Duplicate Packet Detection (DPD). In this way, network nodes can cache
the Identification values of recently-seen packets and use the cached
values to determine whether a newly-arrived packet is in fact a
duplicate. The Identification value within each packet could further
provide a rough indicator of packet reordering, e.g., in cases when the
tunnel egress wishes to discard packets that are grossly out of
order.In some use cases, there may be operational assurance that no
fragmentation of any kind will be necessary, or that only occasional
large control messages will require fragmentation. In that case, the
encapsulation fragment header can be omitted and ordinary fragmentation
of the outer IP protocol version can be applied when necessary.On some platforms (e.g., popular cell phone operating systems), the
act of assigning a default IPv6 route and/or assigning an address to an
interface may not be permitted from a user application due to security
policy. Typically, those platforms include a TUN/TAP interface that acts as a point-to-point conduit between user
applications and the AERO interface. In that case, the Client can
instead generate a "synthesized RA" message. The message conforms to
and is prepared as follows:the IPv6 source address is the Client's AERO addressthe IPv6 destination address is all-nodes multicastthe Router Lifetime is set to a time that is no longer than the
ACP DHCPv6 lifetimethe message does not include a Source Link Layer Address Option
(SLLAO)the message includes a Prefix Information Option (PIO) with a /64
prefix taken from the ACP as the prefix for autoconfigurationThe Client then sends the synthesized RA message via the
TUN/TAP interface, where the operating system kernel will interpret it
as though it were generated by an actual router. The operating system
will then install a default route and use StateLess Address
AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP
interface. Methods for similarly installing an IPv4 default route and
IPv4 address on the TUN/TAP interface are based on synthesized DHCPv4
messages .AERO can be used in many different variations based on the specific
use case. The following sections discuss variations that adhere to the
AERO principles while allowing selective application of AERO
components.When Servers on the AERO link do not provide DHCPv6 services,
operation can still be accommodated through administrative
configuration of ACPs on AERO Clients. In that case, administrative
configurations of AERO interface neighbor cache entries on both the
Server and Client are also necessary. However, this may interfere with
the ability for Clients to dynamically change to new Servers, and can
expose the AERO link to misconfigurations unless the administrative
configurations are carefully coordinated.In some AERO link scenarios, there may be no Servers on the link
and/or no need for Clients to use a Server as an intermediary trust
anchor. In that case, each Client acts as a Server unto itself to
establish neighbor cache entries by performing direct Client-to-Client
IPv6 ND message exchanges, and some other form of trust basis must be
applied so that each Client can verify that the prospective neighbor
is authorized to use its claimed ACP.When there is no Server on the link, Clients must arrange to
receive ACPs and publish them via a secure alternate PD authority
through some means outside the scope of this document.In some environments, the AERO service may be useful for mobile
nodes that do not implement the AERO Client function and do not
perform encapsulation. For example, if the mobile node has a way of
injecting its ACP into the access subnetwork routing system an AERO
Server connected to the same access network can accept the ACP prefix
injection as an indication that a new mobile node has come onto the
subnetwork. The Server can then inject the ACP into the BGP routing
system the same as if an AERO Client/Server DHCPv6 PD exchange had
occurred. If the mobile node subsequently withdraws the ACP from the
access network routing system, the Server can then withdraw the ACP
from the BGP routing system.In this arrangement, AERO Servers and Relays are used in exactly
the same ways as for environments where DHCPv6 Client/Server exchanges
are supported. However, the access subnetwork routing systems must be
capable of accommodating rapid ACP injections and withdrawals from
mobile nodes with the understanding that the information must be
propagated to all routers in the system. Operational experience has
shown that this kind of routing system "churn" can lead to overall
instability and routing system inconsistency.In addition to the dynamic neighbor discovery procedures for AERO
link neighbors described above, AERO encapsulation can be applied to
manually-configured tunnels. In that case, the tunnel endpoints use an
administratively-provisioned link-local address and exchange NS/NA
messages the same as for dynamically-established tunnels.In some environments, AERO Servers and Relays may be connected by
dedicated point-to-point links, e.g., high speed fiberoptic leased
lines. In that case, the Servers and Relays can participate in the
AERO link the same as specified above but can avoid encapsulation over
the dedicated links. In that case, however, the links would be
dedicated for AERO and could not be multiplexed for both AERO and
non-AERO communications.A source Client may connect only to an IPvX underlying network,
while the target Client connects only to an IPvY underlying network.
In that case, the target and source Clients have no means for reaching
each other directly (since they connect to underlying networks of
different IP protocol versions) and so must ignore any route
optimization messages and continue to send packets via their
Servers.When an enterprise mobile node moves from a campus LAN connection
to a public Internet link, it must re-enter the enterprise via a
security gateway that has both a physical interface connection to the
Internet and a physical interface connection to the enterprise
internetwork. This most often entails the establishment of a Virtual
Private Network (VPN) link over the public Internet from the mobile
node to the security gateway. During this process, the mobile node
supplies the security gateway with its public Internet address as the
link-layer address for the VPN. The mobile node then acts as an AERO
Client to negotiate with the security gateway to obtain its ACP.In order to satisfy this need, the security gateway also operates
as an AERO Server with support for AERO Client proxying. In
particular, when a mobile node (i.e., the Client) connects via the
security gateway (i.e., the Server), the Server provides the Client
with an ACP in a DHCPv6 PD exchange the same as if it were attached to
an enterprise campus access link. The Server then replaces the
Client's link-layer source address with the Server's enterprise-facing
link-layer address in all AERO messages the Client sends toward
neighbors on the AERO link. The AERO messages are then delivered to
other nodes on the AERO link as if they were originated by the
security gateway instead of by the AERO Client. In the reverse
direction, the AERO messages sourced by nodes within the enterprise
network can be forwarded to the security gateway, which then replaces
the link-layer destination address with the Client's link-layer
address and replaces the link-layer source address with its own
(Internet-facing) link-layer address.After receiving the ACP, the Client can send IP packets that use an
address taken from the ACP as the network layer source address, the
Client's link-layer address as the link-layer source address, and the
Server's Internet-facing link-layer address as the link-layer
destination address. The Server will then rewrite the link-layer
source address with the Server's own enterprise-facing link-layer
address and rewrite the link-layer destination address with the target
AERO node's link-layer address, and the packets will enter the
enterprise network as though they were sourced from a node located
within the enterprise. In the reverse direction, when a packet sourced
by a node within the enterprise network uses a destination address
from the Client's ACP, the packet will be delivered to the security
gateway which then rewrites the link-layer destination address to the
Client's link-layer address and rewrites the link-layer source address
to the Server's Internet-facing link-layer address. The Server then
delivers the packet across the VPN to the AERO Client. In this way,
the AERO virtual link is essentially extended *through* the security
gateway to the point at which the VPN link and AERO link are
effectively grafted together by the link-layer address rewriting
performed by the security gateway. All AERO messaging services
(including route optimization and mobility signaling) are therefore
extended to the Client.In order to support this virtual link grafting, the security
gateway (acting as an AERO Server) must keep static neighbor cache
entries for all of its associated Clients located on the public
Internet. The neighbor cache entry is keyed by the AERO Client's AERO
address the same as if the Client were located within the enterprise
internetwork. The neighbor cache is then managed in all ways as though
the Client were an ordinary AERO Client. This includes the AERO IPv6
ND messaging signaling for Route Optimization and Neighbor
Unreachability Detection.Note that the main difference between a security gateway acting as
an AERO Server and an enterprise-internal AERO Server is that the
security gateway has at least one enterprise-internal physical
interface and at least one public Internet physical interface.
Conversely, the enterprise-internal AERO Server has only
enterprise-internal physical interfaces. For this reason security
gateway proxying is needed to ensure that the public Internet
link-layer addressing space is kept separate from the
enterprise-internal link-layer addressing space. This is afforded
through a natural extension of the security association caching
already performed for each VPN client by the security gateway.Changes from -81 to -82:Make DHCPv6 the default (but not exclusive) PD serviceSupport operation with no PD services nor ND Route Information
OptionsUpdates to AERO Proxy functionChanges from -80 to -81:Updates to Server and Proxy Extended Route OptimizationUpdates to AERO Proxy sectionCleanups and clarificationsChanges from -79 to -80:Substantial updates to AERO Proxy functionRemoved 'V' bit from SLLAO and replaced with 'X' bitAdded concept of Direct, Proxyed, NATed, VPNed and Native
underlying interfacesAdjusted route optimization text according to underrlying
interface typesChanges from -78 to -79:Neighbors now set UDP Port Number and IP Address in S/TLLAOs to 0
if the node is behind a NAT or otherwise does not wish to update its
link-layer address for this underlying interfaceIntroduced "proxy" as a new neighbor cache entry typeupdated GUE referencesmultipath considerations for error message handling and NUDChanges from -77 to -78:Added "V" bit to SLLAO flags field for NS messages. V=1 indicates
that the NA response must go through the reverse chain of Servers
and RelaysNow including DHCPv6 PD messages as IPv6 ND message optionsClarified the use of the "P" bit in the RA flags fieldUse of SEND to protect the combined DHCPv6/IPv6ND messagesProxy now treats a Client's Servers as the default routers (i.e.,
instead of using a Relay as the default).Changes from -76 to -77:Now using IPv6 ND NS/NA messaging for route optimization (no
longer using Predirect/Redirect)Now using combined IPv6 ND/DHCPv6 messaging so autoconfiguration
can be conducted in a single message exchangeIntroduced the AERO Proxy construct. Critical for applications
such as ATN/IPSChanges from -75 to -76:Bumped version number ahead of expiration deadlineChanges from -74 to -75:Bumped version number ahead of expiration deadline