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 IP
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 alternate services (e.g., based on ND
messaging) are also in scope .a connected IP 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.an IP router
that can relay IP packets between AERO Servers and/or forward IP
packets between the AERO link and the native Internetwork. Relays
are standard IP routers that can be purchased from any major network
equipment supplier.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 are standard IP routers that 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 tunnels with
neighboring Servers, and maintain an IP forwarding table entry for
each AERO Client Prefix (ACP).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 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 transparent conduit for AERO Clients
connected to secured enclaves to associate with AERO link Servers. 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. 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. As of 2015, the
global public Internet BGP routing system manages more than 500K
routes with linear growth and no signs of router resource exhaustion
. More recent network emulation studies have also
shown that a single Relay can accommodate at least 1M dynamically
changing BGP routes even on a lightweight virtual machine, i.e., and
without requiring high-end dedicated router hardware.Therefore, assuming each Relay can carry 1M or more routes, this
means that at least 1M Clients can be serviced by a single set of
Relays. A means of increasing scaling would be to assign a different
set of Relays for each set of 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 Relay and Server AERO interfaces.
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
(i.e., the same as for any IPv6 link), 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 addresses that embed an IPv6 prefix can be statelessly
transformed into an IPv6 Subnet Router Anycast address and vice-versa.
For example, for the AERO address fe80::2001:db8:2000:3000 the
corresponding Subnet Router Anycast address is 2001:db8:2000:3000::.
In the same way, for the IPv6 Subnet Router Anycast address
2001:db8:1:2:: the corresponding AERO address is fe80::2001:db8:1:2.
In other words, the low-order 64 bits of an AERO address can be used
as the high-order 64 bits of a Subnet Router Anycast address, and
vice-versa.AERO interfaces additionally reserve an IPv6 prefix to support IPv6
ND message exchanges between Servers. A Unique Local Address (ULA)
prefix would be a good candidate for the
reserved prefix, since it is not routable outside of the AERO link. An
address with interface identifier set to 0 taken from the reserved
prefix is used as the AERO Server Subnet Router Anycast address. For
example, if the reserved prefix is the ULA prefix fd00:db8::/64 the
AERO Server Subnet Router Anycast Address is fd00:db8::.AERO interfaces use encapsulation (see: ) to exchange packets with neighbors attached to
the AERO link.AERO interfaces maintain a neighbor cache for tracking per-neighbor
state the same as for any interface. AERO interfaces use ND messages
including Router Solicitation (RS), Router Advertisement (RA),
Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for
neighbor cache management.AERO interface ND messages include one or more Source/Target
Link-Layer Address Options (S/TLLAOs) formatted as shown in :In this format:Type is set to '1' for SLLAO or '2' for TLLAO.Length is set to the constant value '5' (i.e., 5 units of 8
octets).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.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.NATed 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.VPNed interfaces use security encapsulation over the
Internetwork to a Virtual Private Network (VPN) gateway that also
acts as an AERO Server. As with NATed links, the AERO Server can
issue control messages on behalf of the Client.Proxyed interfaces connect to a closed network that is
separated from the open Internetwork by an AERO Proxy. Unlike
NATed and VPNed 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 neighbor
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).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
ND/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 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 ND/PD parameters over an underlying interface to one or more
AERO Servers, which return RA messages with corresponding PD
parameters. See for
the types of ND/PD parameters that can be included in the RS/RA
message exchanges.After the initial ND/PD message exchange, the Client can register
additional underlying interfaces with the Server by sending a simple
RS message (i.e., one with no PD parameters) over each underlying
interface using its base AERO address as the source network layer
address. The Server will update its neighbor cache entry for the
Client and return a simple 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 sends a corresponding RS 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 sends a corresponding 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 proxy
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 their associated Relays and Servers on the link, and
AERO Servers maintain permanent neighbor cache entries for their
associated 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 when they process 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.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 used for route
optimization, 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 a "ReportTime" variable in
the neighbor cache entry to REPORT_TIME seconds. The node resets
ReportTime when it receives a new NS message, and otherwise decrements
ReportTime while no NS messages have been received. It is RECOMMENDED
that REPORT_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 ReportTime decrements below FORWARD_TIME (see
below).When a source AERO node receives a valid NA message 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
arrive. 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 REPORT_TIME, FORWARD_TIME and MAX_RETRY MAY be
administratively set; however, if different values are chosen, all
nodes on the link MUST consistently configure the same values. Most
importantly, REPORT_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 or Proxy 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 PD parameters to receive RA replies. The RS/RA messaging will
keep NAT/Proxy 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 and re-admitting the packet
into the AERO link.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 the same as for any IP router. 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 to the AERO neighbor.
Otherwise, the Relay drops the packet and 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, (i.e., when it receives a packet from a Server via a
tunnel) 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 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 returns an ICMP
Destination Unreachable message subject to rate limiting (see:
).When an AERO Server receives a return packet from 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 Server
has sufficient trust basis to accept link-layer Destination
Unreachable messages, it can then process the return packet by
searching for a dynamic neighbor cache entry that matches the
destination. If there is a match, the Server 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
Server instead sets ForwardTime in the dynamic neighbor cache entry
to 0 (noting that ReportTime may still be non-zero). Otherwise, the
Server SHOULD drop the packet and treat it as an indication that a
path may be failing, and MAY use Neighbor Unreachability Detection
(NUD) (see: ) to test the path for
reachability.When an AERO Relay receives a return packet from an AERO Server,
it searches its routing table for an entry that matches the inner
destination address. If there is a routing table entry that lists a
different Server as the next hop, the Relay forwards the packet to
the different Server; otherwise, the Relay 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 (e.g., 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 .Encapsulation between Servers and Relays can use standard
mechanisms such as Generic Routing Encapsulation (GRE) and IPSec so that Relays
can be standard IP routers with no AERO-specific mechanisms.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 (either directly or via a Proxy), 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 Proxies accept encapsulated packets if there is a proxy
neighbor cache entry that matches the packet's network-layer
address.Each packet should include a signature that the recipient can
use to authenticate the message origin, e.g., as for common VPN
systems such as OpenVPN . In environments where
source address spoofing is not considered a threat, however, it may be
sufficient to require signatures only for ND 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
mark the path as unusable and use another path. If it receives
Destination Unreachable messages on many or all paths, the Client
SHOULD associate with a new Server and release its association
with the old Server as specified in .When an AERO Server receives persistent link-layer Destination
Unreachable messages in response to encapsulated packets that it
sends to one of its static neighbor Clients, the Server SHOULD
mark the underlying path as unusable and use another underlying
path. If it receives Destination Unreachable messages on multiple
paths, the Server should take no further actions unless it
receives a receives an explicit ND/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 writes the network-layer source
address of the original packet as the destination address and uses one
of its non link-local addresses as the source address of the
message.When an AERO node receives an encapsulated packet for which the
reassembly buffer it too small, it drops the packet and returns a
network-layer Packet Too Big (PTB) message. The node first writes the
MRU value into the PTB message MTU field, writes the network-layer
source address of the original packet as the destination address and
writes one of its non link-local addresses as the source address.AERO Router Discovery, Prefix Delegation and Autoconfiguration are
coordinated as discussed in the following Sections.Each AERO Server 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.
Thereafter, Clients send additional RS messages to the Server's
unicast address to refresh prefix and/or router lifetimes.AERO Clients and Servers include PD parameters in the RS/RA
messages they exchange (see: ). The unified ND/PD
messages are exchanged between Client and Server according to the
prefix management schedule required by the PD service.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::/N" (where 'N' is the
prefix length common to all ACPs for the link).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-Qualified 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 an ND/PD message exchange. The Client
sends an RS message with PD parameters and with 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 already knows its own AERO address, it uses
the AERO address as the IPv6 source address; otherwise, it uses the
prefix-solicitation address as the source address. If the Client's
underlying interface connects to a subnetwork that supports ACP
injection, the Client can use the ACP's Subnet Router Anycast
address as the link-layer source address.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 sets the UDP
Port Number and IP Address fields to 0. The Client can instead
register additional link-layer addresses with the Server by sending
additional RS messages including SLLAOs via other 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 the Client
receives no RAs, or if it receives an RA with Router Lifetime set to
0 and/or with no ACP PD parameters, the Client SHOULD discontinue
autoconfiguration attempts through this Server and try another
Server. Otherwise, the Client processes the ACPs found in the RA
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
infers 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
to support the Client's downstream attached "Internet of Things
(IoT)". The Client subsequently maintains its ACP delegations
through each of its Servers by sending RS messages with PD
parameters to receive corresponding RA 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 target address of the
NA message is set to the Client's link-local 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 message with PD parameters that will cause the Server to release
the Client. 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 containing
any necessary PD parameters.AERO Servers act as IPv6 routers and support a PD service for
Clients. 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 message
with PD parameters on its AERO interface, and the Server is too
busy, it SHOULD return an immediate RA reply with no ACPs and with
Router Lifetime set to 0. Otherwise, the Server authenticates the RS
message and processes the PD parameters. 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 message that includes the
delegated ACPs and any other PD parameters. The Server then returns
the RA message using its link-local address as the network-layer
source address, the network-layer source address of the RS message
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 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 additional RS messages with PD renewal
parameters, the Server extends the PD lifetimes. If the Client
issues an RS with PD release parameters, or if the Client does not
issue a renewal 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 message with PD reconfigure parameters to
inform the Client that it needs to renegotiate its PDs.When DHCPv6 is used as the ND/PD service back end, AERO Clients
and Servers are always on the same link (i.e., the AERO link) from
the perspective of DHCPv6. However, in some implementations the
DHCPv6 server and ND function may be located in separate modules.
In that case, the Server's AERO interface 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 parameters and uses them to fabricate an
IPv6/UDP/DHCPv6 message. It sets the IPv6 source address to the
source address of the RS message, sets the IPv6 destination
address to 'All_DHCP_Relay_Agents_and_Servers' and sets the UDP
fields to values that will be understood by the DHCPv6 server.The LDRA then wraps the message in a 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 uses the DHCPv6 message to fabricate 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.
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 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 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 SLLAOs set to appropriate values for the
Client ('C1')'s underlying interfaces The first SLLAO serves as
the "Report-To" address for the Client, which is the address to
which the target will announce mobility events and/or other
dynamic updates.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.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') then inserts an
additional layer of encapsulation between the outer IP header and
the NS message proper. This mid-layer IP header uses the AERO Server
Subnet Router Anycast address as the source address and the Subnet
Router Anycast address corresponding to Client ("C2")'s AERO address
as the destination address (in this case, C2's Subnet Router Anycast
address is 2001:db8:1:0::). The Server then forwards this
double-encapsulated NS message to 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 message to Relay ('R1') without decrementing
the network-layer TTL/Hop Limit field.When Relay ('R1') receives the double-encapsulated NS message
from Server ('S1') it discards the outer IP header and determines
that Server ('S2') is the next hop toward Client ('C2') by
consulting its standard IP forwarding table for the Client Subnet
Router Anycast destination address. Relay ('R1') then encapsulates
and forwards the message to Server ('S2') the same as for any IP
router.When Server ('S2') receives the double-encapsulated NS message
from Relay ('R1') it removes the mid-layer IP header and determines
that Client ('C2') is a neighbor on a native underlying interface by
consulting its neighbor cache for Client ('C2')'s AERO address.
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') and stores the
link-layer addresses found in the SLLAOs as the link-layer addresses
of Client ('C1'). Client ('C2') then sets ReportTime for the
neighbor cache entry to REPORT_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(C1)' (i.e.,
the link-layer address of Client ('C1')).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 was
present).Client ('C2') then sends the NA message to Client ('C1').When Client ('C1') receives the NA message, it first verifies
that the NA matches the original NS message. 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 ReportTime value. Thereafter, Client ('C1') may forward
ordinary network-layer data packets directly to Client ('C2')
without involving any intermediate nodes, and Client ('C2') can
dynamically report any changes in link-layer information by sending
unsolicited NA messages. (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.)The source Client itself may initiate route optimization if the
Client has only native interfaces. If the source Client has Direct,
NATed, Proxyed or VPNed interfaces, route optimization must instead
be initiated by the source Server. The source Server MUST include an
SLLAO with a "Report-To" address in the route optimization NS
messages it sends. The "Report-To" address must be one of the source
Server's globally routable IP addresses.In the same way, the target Client may serve as a route
optimization target if it has only native interfaces. If some or all
of the target Client's underlying interfaces are Direct, NATed,
Proxyed or VPNed the target Server must instead serve as the route
optimization target. In that case, when the source Server sends an
NS message the target Server prepares an NA response the same as if
it were the target Client (see: ).When the target Server sends an NA response to a route
optimization NS, it includes a Timestamp option, any necessary
security options, and TLLAOs corresponding to the target Client's
underlying interfaces. The target Server writes the link-layer
address of the Client in TLLAOs corresponding to native underlying
interfaces, writes the link-layer address of the Proxy in TLLAOs
corresponding to Proxyed underlying interfaces and writes its own
link-layer address in TLLAOs corresponding to other interfaces. The
Interface ID and QoS Preference values in the TLLAOs are those
supplied by the target Client during ND exchanges with the target
Server. The target Server then establishes a dynamic neighbor cache
entry for the source with ReportTime set to REPORT_TIME seconds and
with a "Report-To" address set to the address of the source.When the source Server receives the NA response, it creates or
updates a dynamic neighbor cache entry for the target with
ForwardTime set to FORWARD_TIME seconds and with the information
provided in the TLLAOs as the link-layer addresses and preference
values for the Client. The source Server then translates the
solicited NA message into an unsolicited NA message by changing the
source address to its own link-local address, changing the
destination address to all-nodes multicast, recalculating checksums
and any security options, and including the Timestamp option as it
appeared in the original solicited NA. The source Server then
retains this message for subsequent transmission to any source
neighbors that send packets to the target within the current
ForwardTime window.While ForwardTime is greater than 0, the source Server sends
unsolicited NA messages (subject to rate limiting) in response to
data packets from source Clients or Proxies that are destined to the
target Client. The unsolicited NA messages update source Client and
Proxy dynamic neighbor cache entries with ForwardTime set to
FORWARD_TIME minus the difference between the current time and the
NA Timestamp. Subsequent packets from the source destined to the
target Client then travel via the route-optimized path instead of
through the dogleg path through Servers and Relays.Following route optimization, when the target Client (or Proxy)
sends unsolicited NA messages to the target Server to update
link-layer addresses and/or QoS preferences, the target Server
translates the messages the same as described above and repeats them
to any of its neighbors with non-zero ReportTime. The source Server
in turn translates the messages and repeats them to any of their
source Clients or Proxys to which they recently sent NAs.If the target Client moves to a new Server, the old Server sends
immediate unsolicited NA messages with no TLLAOs to any of its
dynamic neighbors with non-zero ReportTime, and retains the dynamic
neighbor cache entry until ReportTime expires. While ReportTime is
non-zero, 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.
When the source receives the unsolicited NAs with no TLLAOs, it
allows future packets destined to the target Client to again flow
through its own Server (or Relay).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. (NS messages may include
SLLAOs and NA messages may include TLLAOs in order to update
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 ReportTime to REPORT_TIME and updates its cached link-layer
addresses (if necessary). When the source node receives a
corresponding 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 an 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 may be failing.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 ReportTime for a dynamic neighbor cache
entry expires, the target node ceases to send dynamic mobility and QoS
updates to the source node. When both ForwardTime and ReportTime 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 mobility
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.AERO interfaces accommodate mobility management by sending
unsolicited NA messages the same as for announcing link-layer address
changes for any interface that implements IPv6 ND . (In environments where reliability is a concern,
AERO interfaces can send immediate NS messages to receive solicited NA
messages, i.e., they can skip the unreliable unsolicited NA messaging
step and move directly to a reliable NS/NA exchange. This comes at a
penalty of at least one round trip.)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
delete 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. (For neighbors that are Servers, the Client can instead
initiate an RS/RA exchange.) 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. (For neighbors that are Servers, the Client
can instead initiate an RS/RA exchange.)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. (For neighbors that are Servers, the Client can instead
initiate an RS/RA exchange.)If the Client needs to send ND messages 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 . The
Client then sends RS messages with PD release parameters to the old
Server to release itself from that Server's domain. If the Client
does not receive an RA reply after MaxRetry attempts, the old Server
may have failed and the Client should discontinue its release
attempts.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 no 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 environments, AERO Clients may be located in secured
subnetwork enclaves (e.g., corporate enterprise networks, radio access
networks, cellular service provider networks, 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 Transaction IDs, Client
Identifiers, Nonce values, etc. The Proxy then creates a new RS
message using its own link-local address as the source and with an
RIO that includes the Client's ACP. The Proxy then forwards the
message to the Server indicated by the destination link-layer
address in the original RS while using its own external address as
the source link-layer address.when the Server receives the RS message, it authenticates the
message then creates a static neighbor cache entry for the Client
with the Proxy's address as the link-layer address. The Server then
sends an RA message back to the Proxy.when the Proxy receives the RA message, it matches the message
with the RS that created 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 creates a new RA message using the cached
Client information and forwards it to the Client.After the initial RS/RA handshake, the Proxy forwards data
packets between the Client and Server with the Server acting as the
Client's default router. The Proxy can send ND messages to the Client's
Server(s) to update Server neighbor cache entries on behalf of the
Client. (For example, the Proxy can send unsolicited 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 control and data messages
originating from the Client to the Client's primary Server.At some time after data packets have been flowing from the Client to
the Server, the Proxy may receive unsolicited NA messages from the
Server with TLLAOs corresponding to a target Client. The Proxy
establishes a dynamic neighbor cache entry for the target with
ForwardTime set to FORWARD_TIME and allows future data packets destined
to the target to flow directly according to the link-layer address
information instead of through the Server. The Proxy may at some later
point receive additional NA messages with TLLAOs, and if so resets
ForwardTime and updates its cached link-layer address information. If
the Proxy receives no further NA messages, or if it receives NA messages
with no TLLAOs, it deletes the dynamic neighbor cache entry.In some subnetworks that employ a Proxy, the Client's ACP can be
injected into the underlying network routing system. In that case, the
Client can send data messages without encapsulation so that the native
underlying network routing system transports the unencapsulated packets
to the Proxy. This can be very beneficial, e.g., if the Client connects
to the network via low-end data links such as some aviation wireless
links. In that case, however, the Client's control message are still
sent encapsulated so as to supply the Proxy with the address of the
Server and to transport IPv6 ND messages without decrementing the
hop-count. In summary, the interface becomes one where control messages
are encapsulated while data messages are either unencapsulated or
encapsulated according to the specific use case. This encapsulation
avoidance can be seen as a form of "header compression", meaning that
the MTU should be sized based on the size of full encapsulated messages
even if most messages are sent unencapsulated.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-1.2.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-4.0.0.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 NA and RA responses.In cases where spoofing cannot be mitigated through other means, AERO
IPv6 ND messages should employ SEcure Neighbor Discovery (SEND) , which also protects the PD information embedded in
RS/RA message options. In order to apply SEND, AERO nodes use
Cryptographically Generated Addresses (CGAs) as
the source addresses of secured ND messages.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 authentication and
network admission control.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
application-layer 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.AERO Relays and Servers MUST discard packets with AERO Server Subnet
Router Anycast as the source address originating from any node other
than a permanent neighbor. This is to avoid a message injection spoofing
attack from an off-link attacker.Security considerations for accepting link-layer ICMP messages and
reflected packets are discussed throughout the document.Discussions in the IETF, aviation standards communities and private
exchanges helped shape some of the concepts in this work. Individuals
who contributed insights include Mikael Abrahamsson, Mark Andrews, Fred
Baker, Bob Braden, Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph
Droms, Adrian Farrel, Nick Green, Sri Gundavelli, Brian Haberman,
Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak, Lee Howard,
Andre Kostur, Ted Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski,
Alexandru Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie
Volz, Ryuji Wakikawa, Tony Whyman, Lloyd Wood and James Woodyatt.
Members of the IESG also provided valuable input during their review
process that greatly improved the document. Special thanks go to Stewart
Bryant, Joel Halpern and Brian Haberman for their shepherding guidance
during the publication of the AERO first edition.This work has further been encouraged and supported by Boeing
colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam
Brodie, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu
Danilov, Wen Fang, Anthony Gregory, Jeff Holland, 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.
Kyle Bae, Wayne Benson and Eric Yeh are especially acknowledged for
implementing the AERO functions as extensions to the public domain
OpenVPN distribution.Earlier works on NBMA tunneling approaches are found in .Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:The Internet Routing Overlay Network (IRON) Virtual Enterprise Traversal (VET) The Subnetwork Encapsulation and Adaptation Layer (SEAL) AERO, First Edition Note that these works cite numerous earlier efforts that are
not also cited here due to space limitations. The authors of those
earlier works are acknowledged for their insights.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.This work is aligned with the Boeing 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 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 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 draft-templin-intarea-6706bis-01 to
draft-templin-intrea-6706bis-02:Note on encapsulation avoidance in Section 4.Changes from draft-templin-intarea-6706bis-00 to
draft-templin-intrea-6706bis-01:Remove DHCPv6 Server Release procedures that leveraged the old
way Relays used to “route” between Server link-local
addressesRemove all text relating to Relays needing to do any
AERO-specific operationsProxy sends RS and receives RA from Server using SEND. Use CGAs
as source addresses, and destination address of RA reply is to the
AERO address corresponding to the Client’s ACP.Proxy uses SEND to protect RS and authenticate RA (Client does
not use SEND, but rather relies on subnetwork security. When the
Proxy receives an RS from the Client, it creates a new RS using its
own addresses as the source and uses SEND with CGAs to send a new RS
to the Server.Emphasize distributed mobility managementAERO address-based RS injection of ACP into underlying routing
system.Changes from draft-templin-aerolink-82 to
draft-templin-intarea-6706bis-00:Document use of NUD (NS/NA) for reliable link-layer address
updates as an alternative to unreliable unsolicited NA. Consistent
with Section 7.2.6 of RFC4861.Server adds additional layer of encapsulation between outer and
inner headers of NS/NA messages for transmission through Relays that
act as vanilla IPv6 routers. The messages include the AERO Server
Subnet Router Anycast address as the source and the Subnet Router
Anycast address corresponding to the Client's ACP as the
destination.Clients use Subnet Router Anycsat address as the encapsulation
source address when the access network does not provide a
topologically-fixed address.