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 following numbered sections present the AERO specification. The
appendices at the end of the document are non-normative.The terminology in the normative references applies; the following
terms are defined within the scope of this document:an IPv6 control
message service for coordinating neighbor relationships between
nodes connected to a common link. 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. 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 tunneling 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 PDs from one or more AERO Servers. Following PD, the Client
assigns a Client AERO address to the AERO interface for use in ND
exchanges with other AERO nodes. A 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
and a Mobility Anchor Point (MAP) for AERO Clients. The Server
assigns an administratively-provisioned AERO address to the AERO
interface to support the operation of the ND/PD services. 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. AERO
Relays are standard IP routers that do not require any AERO-specific
functions.a node that
provides proxying services, 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.the lowest-numbered AERO
address from the first ACP delegated to the Client (see ).a private access network
(e.g., a corporate enterprise network, radio access network,
cellular service provider network, etc.) with secured links and
perimeters. Link-layer security services such as IEEE 802.1X and
physical-layer security such as campus wired LANs prevent
unauthorized access from within the enclave, while border
network-layer security services such as firewalls and proxies
prevent unauthorized access from the external Internetwork.a geographically
and/or topologically referenced list of IP addresses of Servers for
the AERO link.an AERO Server
that is currently tracking and reporting the mobility events of its
associated Clients.an
overlay routing service coordinated by Servers and Relays that
tracks all MAP-to-Client associations.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 terms Mobility Anchor Point (MAP) and Distributed Mobility
Management (DMM) are used in the same sense as standard Internetworking
terminology.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 for AERO Servers. Each Relay also peers with Servers and
other Relays in a dynamic routing protocol instance to provide a
Distributed Mobility Management (DMM) service for the list of active
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 and Mobility Anchor Point
(MAP) services for 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, and
track each Client's mobility profiles.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 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 .AERO Relays, Servers and Proxies are critical infrastructure
elements in fixed (i.e., non-mobile) deployments. Relays, Servers and
Proxies must use public link-layer addresses that do not change and
can be reached from any correspondent in the underlying Internetwork
(i.e., in the same fashion as for popular Internet services). AERO
Clients may be mobile, and may not have any public link-layer
addresses, e.g., if they are located behind NATs or Proxies.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 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.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 . The system provides
for Distributed Mobility Management (DMM) per the distributed mobility
anchoring architecture .A Client's AERO address is an IPv6 link-local address with an
interface identifier based on the Client's delegated ACP. Relay,
Server and Proxy AERO addresses are assigned from the range fe80::/96
and include an administratively-provisioned value in the lower 32
bits.For IPv6, Client AERO addresses begin with the prefix fe80::/64 and
include in the interface identifier (i.e., the lower 64 bits) a 64-bit
prefix taken from one of the Client's IPv6 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, Client 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:192.0.2.31Relay, Server and Proxy AERO addresses are allocated from the
range fe80::/96, and MUST be managed for uniqueness by the
administrative authority for the link. For interfaces that assign
static IPv4 addresses, the lower 32 bits of the AERO address includes
the IPv4 address, e.g., for the IPv4 address 192.0.2.1 the
corresponding AERO address is fe80::192.0.2.1. For other interfaces,
the lower 32 bits of the AERO address includes a unique integer value,
e.g., fe80::1, fe80::2, fe80::3, etc. The address fe80:: is reserved
as the IPv6 link-local Subnet Router Anycast address , and the address fe80::ffff:ffff is reserved as the
unspecified AERO address; hence, these values are not available for
administrative assignment. (Note that a special link-local-format
unspecified address is defined for AERO to satisfy PD services that
require a link-local source address.)When the Server delegates ACPs to the Client, the lowest-numbered
AERO address from the first ACP delegation serves 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 prefix lengths shorter than /64), the
Client originates ND messages using the base AERO address as the
source address and accepts and responds to ND messages destined to any
of its AERO addresses as equivalent to the base AERO address. In this
way, the Client maintains a single neighbor cache entry that may be
indexed by multiple AERO addresses.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 links additionally require a reserved IPv6 prefix to support
encapsulated forwarding of IPv6 ND messages between Servers on the
link. Although any non-link-local IPv6 prefix could be reserved for
this purpose, a Unique Local Address (ULA) prefix would be desireable since it is not routable
outside of the AERO link. For example, if the reserved (ULA) prefix is
fd00:db8::/64 the AERO Server Subnet Router Anycast Address is
fd00:db8::.A full discussion of the AERO addressing service is found in .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), Neighbor Advertisement (NA) and Redirect
for neighbor cache management.AERO interface ND messages include one or more Source/Target
Link-Layer Address Options (S/TLLAOs) formatted as shown in :In this format:Type is set to '1' for SLLAO or '2' for TLLAO.Length is set to the constant value '5' (i.e., 5 units of 8
octets).Prefix Length is set to the ACP prefix length if the ND message
source address is a Client AERO address, or 128 if the source is
an administratively-provisioned AERO address. For example, if the
source address is fe80::2001:db8:1:0 and the ACP is
2001:db8:1:0::/56, Prefix Length is set to the value 56.
Conversely, if the source address is fe80::1, Prefix Length is set
to 128. If the ND message contains multiple S/TLLAOs, only the
Prefix Length value in the first S/TLLAO is consulted and the
values in other S/TLLAOs are ignored.R (the "Release" bit) is set to '1' in the S/TLLAOs of any ND
message for each Interface ID that is to be released from service;
otherwise, set to '0'. If the R value in the first S/TLLAO is '1',
the recipient deletes the neighbor cache entry entirely and (in
the case of RS/RA messages) also releases the corresponding
PD.X (the "proXy" bit) is set to '1' in the SLLAO of RS/RA
messages by the Proxy when there is a Proxy in the path;
otherwise, set to '0'. If the ND message contains multiple SLLAOs,
only the X value in the first SLLAO is consulted and the values in
other SLLAOs are ignored.N (the "NAT" bit) is set to '1' in the SLLAO of RA messages by
the Server if there is a NAT in the path; otherwise, set to '0'.
If the ND message contains multiple SLLAOs, only the N value in
the first SLLAO is consulted and the values in other SLLAOs are
ignored.Reservd 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. Once the node has
assigned an Interface ID to an underlying interface, the
assignment must remain unchanged until the node fully detaches
from the AERO link. The value '255' is reserved as the AERO Server
interface ID, i.e., Servers MUST use Interface ID '255', and
Clients MUST number their Interface IDs with values in the range
of 0-254.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 Preferences that correspond to the 64
Differentiated Service Code Point (DSCP) values . Each P(i) is set to the value '0'
("disabled"), '1' ("low"), '2' ("medium") or '3' ("high") to
indicate a QoS preference level for packet forwarding
purposes.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 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 can also issue control
messages 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 at the
network layer and must instead be derived from the link-layer
encapsulation headers.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 AERO 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 AERO 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. The Server also engages in a dynamic
routing protocol with its neighboring Relays (see: ).When the Server receives an NS/RS message on the AERO interface
it authenticates the message and returns a solicited NA/RA message.
(When the Server receives an unsolicited NA message, it likewise
authenticates the message and processes it locally.) 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 at the link layer without ever disturbing the network
layer.When a Proxy enables an AERO interface, it assigns an
administratively-provisioned address fe80::ID the same as for Relays
and Servers. The Proxy further maintains per-Client proxy neighbor
cache entries based on control message exchanges. Proxies forward
packets between their associated Clients and each Client's
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 proxyed 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 proxyed 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.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. (The RS/RA messages may pass through a Proxy on the path
in the case of a Client's Proxyed interface.) 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 assigns AERO
addresses to the AERO interface based on the delegated prefix(es).
The Client can then 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 target 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 there is a NAT on the path, the Client must send periodic
messages to keep NAT state alive. If no other periodic messaging
service is available, the Client can send RS messages to receive RA
replies from its Server(s).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 route optimization sources maintain dynamic neighbor
cache entries for each of their active target Clients with lifetimes
based on ND messaging constants. Dynamic neighbor cache entries are
asymmetric since only the route optimization source (i.e., and not the
target Client) creates an entry.When a target AERO Server (acting as a Mobility Anchor Point (MAP))
receives a valid NS message used for route optimization, it searches
for a static neighbor cache entry for the target Client. The Server
then returns a solicited NA message, and adds the link-layer address
of the source to a "Report List" associated with the Client's static
neighbor cache entry. The Server then sets a "ReportTime" variable for
the Report list entry to REPORTTIME seconds. The Server resets
ReportTime when it receives a new NS message, and otherwise decrements
ReportTime while no NS messages have been received. It is RECOMMENDED
that REPORTTIME be set to the default constant value 40 seconds to
allow a 10 second window so that the AERO route optimization procedure
can converge before ReportTime decrements below REACHABLETIME (see
below).When the route optimization source receives a solicited NA message
response to its NS message, it creates or updates a dynamic neighbor
cache entry for the target network-layer and link-layer addresses. The
node then (re)sets "ReachableTime" for the neighbor cache entry to
REACHABLETIME seconds and uses this value to determine whether packets
can be forwarded directly to the target, i.e., instead of via a
default route. The node otherwise decrements ReachableTime while no
further solicited NA messages arrive. It is RECOMMENDED that
REACHABLETIME be set to the default constant value 30 seconds as
specified in .The route optimization source also uses the value
MAX_UNICAST_SOLICIT to limit the number of NS keepalives sent when a
correspondent may have gone unreachable, the value
MAX_RTR_SOLICITATIONS to limit the number of RS messages sent without
receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT to limit the
number of unsolicited NAs that can be sent based on a single event. It
is RECOMMENDED that MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and
MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in .Different values for REPORTTIME, REACHABLETIME,
MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and
MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if
different values are chosen, all nodes on the link MUST consistently
configure the same values. Most importantly, REPORTTIME SHOULD be set
to a value that is sufficiently longer than REACHABLETIME to allow the
AERO route optimization procedure to converge.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. AERO implementations SHOULD
allow for QoS preference values to be modified at runtime through
network management.If multiple outgoing interfaces and/or neighbor interfaces have a
preference of "high", the AERO node 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 AERO 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 local 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, if the destination matches a dynamic neighbor cache
entry for a target Client, the Server forwards the packet
according to the interface ID settings in the dynamic neighbor
cache entry.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.Relays forward packets the same as any IP router. When the Relay
receives an encapsulated packet from a Server via the AERO link, it
removes the encapsulation header and searches for a forwarding table
entry that matches the destination address in the next IP header.
When the Relay receives an unencapsulated packet from a node outside
the AERO link, it performs the same forwarding table lookup. The
Relay then 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
submits the packet for 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 encapsulates and forwards the packet using the
neighbor's link-layer address as the destination for
encapsulation.else, the Relay drops the packet and returns an ICMP
Destination Unreachable message subject to rate limiting (see:
).As for any IP router, the Relay decrements the TTL/Hop
Count when it forwards the packet.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 ReachableTime in the dynamic neighbor cache
entry to 0. 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 forwarding table for an entry that matches the inner
destination address. If there is a forwarding 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.See for further discussion on the nature
of return packets.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: , , , , etc.). 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 .When GUE encapsulation is not available, encapsulation between
Servers and Relays can use standard mechanisms such as Generic Routing
Encapsulation (GRE) , GRE-in-UDP 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 some environments,
however, it may be sufficient to require signatures only for ND
control plane messages (see: ) and omit
signatures for data plane messages.The AERO interface is the node's attachment to the AERO link. The
AERO interface acts as a tunnel ingress when it sends a packet to an
AERO link neighbor and as a tunnel egress when it receives a packet
from an AERO link neighbor. AERO interfaces observe the packet sizing
considerations for tunnels discussed in and as specified below.The Internet Protocol expects that IP packets will either be
delivered to the destination or a suitable Packet Too Big (PTB)
message returned to support the process known as IP Path MTU Discovery
(PMTUD) . However, PTB
messages may be crafted for malicious purposes such as denial of
service, or lost in the network . This can be
especially problematic for tunnels, where a condition known as a PMTUD
"black hole" can result. For these reasons, AERO interfaces employ
operational procedures that avoid interactions with PMTUD, including
the use of fragmentation when necessary.AERO interfaces observe two different types of fragmentation.
Source fragmentation occurs when the AERO interface (acting as a
tunnel ingress) fragments the encapsulated packet into multiple
fragments before admitting each fragment into the tunnel. Network
fragmentation occurs when an encapsulated packet admitted into the
tunnel by the ingress is fragmented by an IPv4 router on the path to
the egress. Note that 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 ReachableTime 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 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. If the Client
knows its ACP in advance, it can include its AERO address as the
source address of an RS message and with an SLLAO with a valid
Prefix Length for the ACP. If the Server (and Proxy) accept the
Client's ACP assertion, they inject the prefix into the routing
system and establish the necessary neighbor cache state. If the
Client does not know its ACP in advance, or if it wishes to engage
in a formal PD exchange, it can use an explicit PD service such as
DHCPv6.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 included in the first S/TLLAO in the message).The following sections specify the Client and Server
behavior.AERO Clients discover the link-layer addresses of AERO Servers in
the Potential Router List (PRL) 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. 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 unspecified AERO 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, and the Prefix Length field MUST be set to the
length of the Client's ACP if known, or '0' otherwise. The Client
MAY include additional SLLAOs specific to other underlying
interfaces, but if so it sets their 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 up to MAX_RTR_SOLICITATIONS times until an RA is received.
If the Client receives no RAs, or if it receives an RA with Router
Lifetime set to 0 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
address in the first SLLAO as the link-layer address. The Client
records the lifetime value in the RA Router Lifetime field as the
time for which the Server has committed to maintaining the ACP in
the routing system. The Client then autoconfigures AERO addresses
for each of the delegated ACPs and assigns them to the AERO
interface.The Client next examines the X and N bits in the first SLLAO of
the RA message. If the X bit value is '1' the Client infers that
there is a Proxy on the path via the interface over which it sent
the RS message, and if the N bit value is '1' the Client infers that
there is a NAT on the path. If N is '1', the Client SHOULD set UDP
Port Number and IP Address to 0 in the first S/TLLAO 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 additional 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 RS message to send over any
available underlying interface. The RS MUST include a SLLAO specific
to the selected available underlying interface as the first SLLAO
and MAY include any additional SLLAOs specific to other underlying
interfaces. The Client includes fresh 'P(i)' values in each SLLAO to
update the Server's neighbor cache entry. If the Client wishes to
update '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"). When the Client receives the Server's RS response, it
has assurance that the Server has been updated with the new
information.If the Client wishes to discontinue use of a Server it issues an
RS message with PD parameters that will cause the Server to release
the Client. The Client can request that the Server release the ACP
by setting the 'R' bit in the first SLLAO in the RS message. When
the Server processes the message, it releases the ACP, deletes its
neighbor cache entry for the Client, withdraws the IP route from the
routing system and returns an RA reply 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. The
list of Server addresses should be geographically and/or
topologically referenced, and forms the Potential Router List (PRL)
for the AERO link.When an AERO Server receives a prospective Client's RS message
with PD parameters on its AERO interface, it first checks the
network-layer source address and Prefix Length in the first SLLAO to
determine whether the Client already knows in advance the ACP it
wishes to receive. If the Server is too busy, it SHOULD return an
immediate RA reply with no SLLAOs 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, any other PD parameters and an SLLAO with the
Server's link-layer address and with Interface ID set to 255. 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 N flag 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 on the
path, and sets the X flag to 1 if the first SLLAO in the RS message
also had X set to 1. 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 in the RS 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 in the path.
The Server also records the value of the X bit to indicate whether
there is a Proxy on 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 (or Proxy) issues additional RS messages with PD
renewal parameters, the Server extends the PD lifetimes. If the
Client (or Proxy) issues an RS with PD release parameters (e.g., by
including a first SLLAO with R set to 1), or if the Client (or
Proxy) 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 module can act as a
Lightweight DHCPv6 Relay Agent (LDRA) to relay PD messages to and from the DHCPv6 server
module.When the LDRA receives an authentic RS message, it extracts the
PD message parameters and uses them to construct an
IPv6/UDP/DHCPv6 message. It sets the IPv6 source address to the
source address of the RS message, sets the IPv6 destination
address to 'All_DHCP_Relay_Agents_and_Servers' and sets the UDP
fields to values that will be understood by the DHCPv6 server.The LDRA then wraps the message in a DHCPv6 'Relay-Forward'
message header and includes an 'Interface-Id' option that includes
enough information to allow the LDRA to forward the resulting
Reply message back to the Client (e.g., the Client's link-layer
addresses, a security association identifier, etc.). The LDRA also
wraps the information in all of the SLLAOs from the RS message
into the Interface-Id option, then forwards the message to the
DHCPv6 server.When the DHCPv6 server prepares a Reply message, it wraps the
message in a 'Relay-Reply' message and echoes the Interface-Id
option. The DHCPv6 server then delivers the Relay-Reply message to
the LDRA, which discards the Relay-Reply wrapper and IPv6/UDP
headers, then uses the DHCPv6 message to construct an RA response
to the Client. The Server uses the information in the Interface-Id
option to prepare the RA message and to cache the link-layer
addresses taken from the SLLAOs echoed in the Interface-Id
option.In some environments, Clients may be located in secured subnetwork
enclaves 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 Proxy is located at the secured enclave perimeter and listens
for encapsulated RS messages originating from or RA messages destined
to 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 addresses along with
any identifying information including Transaction IDs, Client
Identifiers, Nonce values, etc. The Proxy then examines the
address in the first SLLAO of the RS message. If the address is
different than the Client link-layer address, the Proxy notes that
the Client is behind a NAT. The Proxy then re-encapsulates the RS
message using its own external address as the source link-layer
address, sets the X flag in the first SLLAO to '1', changes the
address in the first SLLAO to its own external address, and
forwards the message to the Server.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 re-encapsulates the RA message using its
own internal address as the source link-layer address, sets the X
flag in the first SLLAO to '1', sets the N flag in the fiirst
SLLAO to '1' if the Client is behind a NAT, and forwards the
message to the Client.After the initial RS/RA exchange, the Proxy forwards data
packets between the Client and Server with the Server acting as the
Client's default router. The Proxy can send RS messages to the
Client's Server(s) to update Server neighbor cache entries on behalf
of the Client, e.g., to refresh neighbor cache entry lifetimes and/or
to convey QoS updates. The Proxy also forwards any Client data packets
for which there is no matching dynamic neighbor cache entry to the
"eldest" of the Client's Servers, i.e., the first among possibly
multiple Servers selected by the Client. If the eldest Server becomes
unresponsive, the Proxy sends future data packets via the next-eldest
Server, etc.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 messages
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 represents a form of "header
compression", meaning that the MTU should be sized based on the size
of full encapsulated messages even if most messages are sent
unencapsulated.While data packets are flowing from a source Client to a target
Client that are both holders of ACPs belonging to the same AERO link,
route optimization SHOULD be used to avoid sending traffic through
sub-optimal routes that consume expensive resources. Route
optimization is conducted on a per-interface basis based on the source
Client's available underlying interfaces, and may need to involve
Proxies and Servers in the process.Route optimization is initiated by the first eligible AERO node
closest to the source Client (i.e., the route optimization source) as
follows:For VPNed, NATed and Direct underlying interfaces, the Server
is the route optimization source.For Proxyed underlying interfaces, the Proxy is the route
optimization source.For native underlying interfaces, the Client itself is the
route optimization source.While the source Client sends data packets toward a target Client,
the route optimization source also sends an NS message to receive a
solicited NA message from a target Server acting as a Mobility Anchor
Point (MAP) for the target Client. The route optimization process
parallels IPv6 ND Address Resolution.The NS includes the AERO address of the route optimization source
as the network-layer source address, the AERO address corresponding to
the data packet's destination address as the network-layer destination
address, and the route optimization source address as the link-layer
source address. For Clients and Proxies as the route optimization
source, the address of the Client's Server is used as the link-layer
destination address. For Servers as the route optimization source, the
address of a Relay is used as the link-layer destination address. The
NS message also includes a single SLLAO with the route optimization
source address in the UDP Port Number and IP address fields. Finally,
the NS message includes a Timestamp and Nonce option that can be used
to match against the corresponding solicited NA.When the source Server forwards or originates the NS message, it
inserts an additional mid-layer IP encapsulation header between the NS
message link-layer and network-layer headers. 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 the
target AERO address as the destination address. The source Server then
changes the link-layer source address to its own address and the
link-layer destination address to the address of a Relay. The Server
finally forwards the message to the Relay without decrementing the
network-layer TTL/Hop Limit field.When the Relay receives the double-encapsulated NS message from the
source Server, it discards the link-layer header(s) and determines
that the target Server is the next hop toward the target Client by
consulting its standard IP forwarding table for the Client Subnet
Router Anycast destination address. The Relay then encapsulates and
forwards the message to the target Server the same as for any IP
router.When the target Server receives the double-encapsulated NS message
from the Relay, it removes the link-layer and mid-layer headers and
examines the network-layer destination address to determine whether
the target Client is one of its static neighbors. If the target Client
is not a static neighbor, the target Server discards the NS and
prepares a solicited NA message with no TLLAOs to send back to the
route optimization source. The target Server then encapsulates the
solicited NA message, sets the link-layer source address to its own
address and sets the link-layer destination address to the address
found in the NS SLLAO. The target Server finally includes the Nonce
value received in the NS plus the current Timestamp, then sends the
solicited NA message to the route optimization source.Otherwise, the target Server adds the link-layer address found in
the NS SLLAO to the "Report" list for the target Client's neighbor
cache entry with timer set to ReportTime seconds, but it does not
create or update a neighbor cache entry. The target Server then
prepares a solicited NA message to send back to the route optimization
source. The solicited NA message includes a first TLLAO with the
target Server's address in the IP address and UDP Port Number, with
Interface ID set to '255', with all P(i) values set to "low" and with
"Prefix Length" set to the prefix length of the target Client's ACP.
The solicited NA message then includes additional TLLAOs for all of
the target Client's underlying interfaces. For NATed, VPNed and Direct
interfaces, the TLLAO addresses are the address of the Server. For
Proxyed interfaces, the TLLAO addresses are the addresses of the
Proxies, and for native interfaces the TLLAO addresses are the
addresses of the Client. The target Server then encapsulates the
solicited NA message, sets the link-layer source address to its own
address and sets the link-layer destination address to the address
found in the NS SLLAO. The target Server finally includes the Nonce
value received in the NS plus the current Timestamp, then sends the
solicited NA message to the route optimization source.When the route optimization source receives the solicited NA
message, it verifies the Nonce and Timestamp. If there are no TLLAOs,
the route optimization source discards the message; otherwise, it
creates a dynamic neighbor cache entry for the target Client and
caches all address, Interface ID, P(i) and Prefix Length information
found in the solicited NA TLLAOs. The route optimization source also
sets the neighbor cache entry state to REACHABLE and sets the timeout
value to ReachableTime. Future data packets that flow through the
route optimization source will then go directly to the target Client
instead of traveling through a dogleg route involving unnecessary
Servers and/or Relays. The route optimization further is shared by all
sources that send packets to the target Client, i.e., and not just the
original source Client.While new data packets destined to the target are flowing through
the route optimization source, it sends additional NS messages to the
target Server before ReachableTime expires to receive a fresh
solicited NA message. The route optimization source then updates the
dynamic neighbor cache entry to refresh ReachableTime, while the
target Server updates the target Client's static neighbor cache entry
to refresh ReportTime. While no data packets are flowing, the route
optimization source instead allows the dynamic neighbor cache entry to
expire. Following expiration, future data packets flowing through the
route optimization source will again trigger a new route optimization
exchange while initial data packets travel over a suboptimal route via
Servers and/or Relays.In this arrangement, the route optimization source holds a dynamic
neighbor cache entry for the target, but the target does not hold a
dynamic neighbor cache entry for the route optimization source. The
route optimization neighbor relationship is therefore asymmetric and
unidirectional. If the target Client also has packets to send back to
the source Client, then a separate route optimization procedure is
required in the reverse direction. But, there is no requirement that
the forward and reverse paths be symmetric.AERO nodes perform Neighbor Unreachability Detection (NUD) by
sending NS messages to elicit solicited NA messages from the target
node the same as described in . NUD is
performed either reactively in response to persistent link-layer
errors (see ) or proactively to confirm
reachability in the forward direction.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.Note that AERO nodes may have multiple underlying interface paths
toward the target neighbor. In that case, NUD SHOULD be performed over
each underlying interface individually and the node should only
consider the neighbor unreachable if NUD fails over multiple
underlying interface paths.Underlying interface paths that pass NUD tests are marked as
"reachable", while those that do not are marked as "unreachable".
These markings inform the AERO interface forwarding algorithm
specified in .AERO is an example of a Distributed Mobility Management (DMM)
service. Each Server is responsible for only a subset of the Clients
on the AERO link, as opposed to a Centralized Mobility Management
(CMM) service where there is a single network mobility service for all
Clients. Clients coordinate with their associated Servers via RS/RA
exchanges to maintain the DMM profile, and the AERO routing system
tracks all current Client/Server peering relationships.Servers provide a Mobility Anchor Point (MAP) for their dependent
Clients. Clients are responsible for maintaining neighbor
relationships with their Servers through periodic RS/RA exchanges,
which also serves to confirm Client reachability. When a Client's
underlying interface address and/or QoS information changes, the
Client is responsible for updating the Server with this new
information. (Note that for Proxyed interfaces, however, the Proxy can
perform the RS/RA exchanges on the Client's behalf.)Mobility management considerations are specified in the following
sections.AERO Servers accommodate mobility and/or QoS change events by
sending unsolicited NA messages to all route optimization sources
for the target Client. When a Server sends an unsolicited NA
message, it sets the IPv6 source address to the Client's AERO
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 route optimization source in the
Client's Report list. The Server also includes a first TLLAO for its
own Interface ID, and includes additional TLLAOs for all of the
target Client's Interface IDs. If there are multiple route
optimization sources, the node sends identical unicast copies of the
unsolicited NA to each source.As for the hot-swap of interface cards discusssed in Section
7.2.6 of , the transmission and reception of
unsolicited NA messages is unreliable but provides a useful
optimization. The Server can optionally send up to
MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs to each route
optimization source to increase the likelihood that at least one
will be received.When a route optimization source receives an unsolicited NA
message, it ignores the message if there is no existing neighbor
cache entry for the Client. Otherwise, it uses the included TLLAOs
to update the address and QoS information in the neighbor cache
entry, but does not reset ReachableTime since the receipt of an
unsolicited NA message from the target Server does not provide
confirmation that any forward paths to the target Client are
working.If unsolicited NA messages are lost, the route optimization
source may be left with stale address and/or QoS information for the
Client for up to ReachableTime seconds. During this time, the route
optimization source can continue sending packets to the target
Client according to its current neighbor cache information but may
receive persistent Destination Unreachable messages and/or
unsolicited NA messages with no TLLAOs as discussed in . In that case, the route optimization source
SHOULD delete the dynamic neighbor cache entry and re-initiate route
optimization immediately instead of waiting for ReachableTime to
expire.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 which delivers them to the target Client's
current location. If the source is not one of its static neighbor
Clients, the Server also returns an unsolicited NA message to the
source with no TLLAOs - the sender will then realize that it needs
to delete its neighbor cache entry that associated the target with
this Server and re-initiate route optimization.When a Proxy receives packets with destination addresses that do
not match one of its proxy neighbor cache Clients, if forwards the
packets to a Server which delivers them to the target Client's
current location. If the source is not one of its proxy neighbor
Clients, the Proxy also returns an unsolicited NA message to the
source with no TLLAOs the same as described for Servers above.When a Client receives packets with destination addresses that do
not match one of its ACPs, it drops the packets and returns an
unsolicited NA message to the source with no TLLAOs.When a Client needs to change its link-layer addresses and/or QoS
preferences (e.g., due to a mobility event), either the Client or
Proxy sends RS messages to its Servers using the new link-layer
address as the source address and with TLLAOs that include the new
Client UDP Port Number, IP Address and P(i) values. If the RS
messages are sent solely for the purpose of updating QoS preferences
without updating the link-layer address, the UDP Port Number and IP
Address are set to 0.Up to MAX_RTR_SOLICITATION RS messages MAY be sent in parallel
with sending actual data packets in case one or more RAs are lost.
If all RAs are lost, the Client SHOULD re-associate with a new
Server.When a Client needs to bring new underlying interfaces into
service (e.g., when it activates a new data link), it sends RS
messages to its Servers using the new link-layer address as the
source address and with TLLAOs that include the new Client
link-layer information.When a Client needs to remove existing underlying interfaces from
service (e.g., when it de-activates an existing data link), it sends
RS messages to its Servers with SLLAOs with all P(i) values set to
0.If the Client needs to send RS messages over an underlying
interface other than the one being removed from service, it MUST
include a current SLLAO for the sending interface as the first SLLAO
and include SLLAOs 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 MAX_RTR_SOLICITATIONS attempts,
the old Server may have failed and the Client should discontinue its
release attempts. When the old Server processes the PD release, it
sends unsolicited NA messages with no TLLAOs to all route
optimization sources in the Client's Report list.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, movement to a new geographic region, 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.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 Proxies, Servers and
Relays in the communications path. Direct underlying interfaces must be
tested periodically for reachability, e.g., via NUD.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.SEcure Neighbor Discovery (SEND) and
Cryptographically Generated Addresses (CGAs)
were designed to secure IPv6 ND messaging in environments where
symmetric network and/or transport-layer security services are
impractical (see: ). AERO nodes that use SEND/CGA
employ the following adaptations.When a source AERO node prepares a SEND-protected ND message, it uses
a link-local CGA as the IPv6 source address and writes the prefix
embedded in its AERO address (i.e., instead of fe80::/64) in the CGA
parameters Subnet Prefix field. When the neighbor receives the ND
message, it first verifies the message checksum and SEND/CGA parameters
while using the link-local prefix fe80::/64 (i.e., instead of the value
in the Subnet Prefix field) to match against the IPv6 source address of
the ND message.The neighbor then derives the AERO address of the source by using the
value in the Subnet Prefix field as the interface identifier of an AERO
address. For example, if the Subnet Prefix field contains 2001:db8:1:2,
the neighbor constructs the AERO address as fe80::2001:db8:1:2. The
neighbor then caches the AERO address in the neighbor cache entry it
creates for the source, and uses the AERO address as the IPv6
destination address of any ND message replies.AERO Relays are low-end to midrange Commercial off-the Shelf (COTS)
standard IP routers with no AERO code. Relays must be provisioned,
supported and managed by the AERO Link Service Provider. Cost for
purchasing, configuring and managing Relays is nominal even for very
large AERO links.AERO Servers can be standard dedicated server platforms, but most
often will be deployed as virtual machines in the cloud. The only
requirements for Servers are that they can run the AERO user-level code
and have at least one network interface with a public IP address. As
with Relays, Servers must be provisioned, supported and managed by the
AERO Link Service Provider. Cost for purchasing, configuring and
managing Servers is nominal especially for virtual Servers hosted in the
cloud.AERO Proxies are most often standard dedicated server platforms with
one network interface connected to the secured enclave and a second
interface connected to the public Internetwork. As with Servers, the
only requirements are that they can run the AERO user-level code and
have at least one interface with a public IP address. Proxies must be
provisioned, supported and managed by the administrative authority for
the secured enclave. Cost for purchasing, configuring and managing
Proxies is nominal, and borne by the secured enclave administrative
authority.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-2.0.tgz.An initial public release of the AERO proof-of-concept source code
was announced on the intarea mailing list on August 21, 2015. The latest
version is available at:
http://linkupnetworks.net/aero/aero-4.0.0.tgz.A survey of public domain and commercial SEND implementations is
available at
https://www.ietf.org/mail-archive/web/its/current/msg02758.html.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 include considerations for both the
data plane and the control plane.Data plane security considerations are the same as for ordinary
Internet communications. Application endpoints in AERO Clients and their
EUNs SHOULD use application-layer security services such as TLS/SSL
, DTLS or SSH to assure the same level of protection as for
critical secured Internet services such as online banking. AERO Clients
that require VPN access to enterprise networks SHOULD use symmetric
network and/or transport layer security services such as TLS/SSL, DTLS,
IPsec , etc.Control plane security considerations are the same as for standard
IPv6 Neighbor Discovery , except that the PRL
also improves security by providing AERO Clients with a list of trusted
Servers. As fixed infrastructure elements, AERO Proxies and Servers
SHOULD pre-configure security associations for one another (e.g., using
pre-placed keys) and use symmetric network and/or transport layer
security services such as IPsec, TLS/SSL or DTLS to secure ND messages.
AERO Clients that connect to secured enclaves need not apply security to
their ND messages, since the messages will be intercepted by an enclave
perimeter Proxy. AERO Clients located outside of secured enclaves SHOULD
use symmetric network and/or transport layer security to secure their ND
exchanges with Servers, but when there are many prospective neighbors
with dynamically changing connectivity an asymmetric security service
such as SEND may be needed (see: ).AERO Servers and Relays present targets for traffic amplification
Denial of Service (DoS) attacks. This concern is no different than for
widely-deployed VPN security gateways in the Internet, where attackers
could send spoofed packets to the gateways at high data rates. This can
be mitigated by connecting 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 and Proxies can institute
rate limits that protect Clients from receiving packet floods that could
DoS low data rate links.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.)Although public domain and commercial SEND implementations exist,
concerns regarding the strength of the cryptographic hash algorithm have
been documented .The PRL MUST be well-managed and secured from unauthorized tampering,
even though the list includes only public information.Security considerations for accepting link-layer ICMP messages and
reflected packets are discussed throughout the document.Discussions in the IETF, aviation standards communities and private
exchanges helped shape some of the concepts in this work. Individuals
who contributed insights include Mikael Abrahamsson, Mark Andrews, Fred
Baker, Bob Braden, Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph
Droms, Adrian Farrel, Nick Green, Sri Gundavelli, Brian Haberman,
Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak, Lee Howard,
Andre Kostur, Hubert Kuenig, Ted Lemon, Andy Malis, Satoru Matsushima,
Tomek Mrugalski, Madhu Niraula, Alexandru Petrescu, Behcet Saikaya,
Michal Skorepa, Joe Touch, Bernie Volz, Ryuji Wakikawa, Tony Whyman,
Lloyd Wood and James Woodyatt. Members of the IESG also provided
valuable input during their review process that greatly improved the
document. Special thanks go to Stewart Bryant, Joel Halpern and Brian
Haberman for their shepherding guidance during the publication of the
AERO first edition.This work has further been encouraged and supported by Boeing
colleagues including Kyle Bae, M. Wayne Benson, Dave Bernhardt, Cam
Brodie, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu
Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Seth Jahne, Ed King,
Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Greg Saccone, Kent
Shuey, Brian Skeen, Mike Slane, Carrie Spiker, Brendan Williams, Julie
Wulff, Yueli Yang, Eric Yeh and other members of the BR&T and BIT
mobile networking teams. Kyle Bae, Wayne Benson and Eric Yeh are
especially acknowledged for implementing the AERO functions as
extensions to the public domain OpenVPN distribution.Earlier works on NBMA tunneling approaches are found in .Many of the constructs presented in this second edition of AERO are
based on the author's earlier works, including:The Internet Routing Overlay Network (IRON) Virtual Enterprise Traversal (VET) The Subnetwork Encapsulation and Adaptation Layer (SEAL) AERO, First Edition Note that these works cite numerous earlier efforts that are
not also cited here due to space limitations. The authors of those
earlier works are acknowledged for their insights.This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.This work is aligned with the Boeing autonomy program.http://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, TLS/SSL, DTLS, etc. In that case, AERO
control messaging and route determination occur before security
encapsulation is applied for outgoing packets and after security
decapsulation is applied for incoming packets.AERO is especially well suited for use with VPN system encapsulations
such as OpenVPN .The AERO S/TLLAO format specified in
includes a Length value of 5 (i.e., 5 units of 8 octets). However,
special-purpose links may extend the basic format to include additional
fields and a Length value larger than 5.For example, adaptation of AERO to the Aeronautical
Telecommunications Network with Internet Protocol Services (ATN/IPS)
includes link selection preferences based on transport port numbers in
addition to the existing DSCP-based preferences. ATN/IPS nodes maintain
a map of transport port numbers to 64 possible preference fields, e.g.,
TCP port 22 maps to preference field 8, TCP port 443 maps to preference
field 20, UDP port 8060 maps to preference field 34, etc. The extended
S/TLLAO format for ATN/IPS is shown in , where
the Length value is 7 and the 'Q(i)' fields provide link preferences for
the corresponding transport port number.<< RFC Editor - remove prior to publication >>Changes from draft-templin-intarea-6706bis-06 to
draft-templin-intrea-6706bis-07:Added "R" bit for release of PDs. Now have a full RS/RA service
that can do PD without requiring DHCPv6 messaging over-the-airClarifications on solicited vs unsolicited NAsClarified use of MAX_NEIGHBOR_ADVERTISEMENTS for the purpose of
increase reliabilityChanges from draft-templin-intarea-6706bis-05 to
draft-templin-intrea-6706bis-06:Major re-work and simplification of Route Optimization
functionAdded Distributed Mobility Management (DMM) and Mobility Anchor
Point (MAP) terminologyNew section on "AERO Critical Infrastructure Element
Considerations" demonstrating low overall cost for the serviceminor text revisions and deletionsremoved extraneous appendicesChanges from draft-templin-intarea-6706bis-04 to
draft-templin-intrea-6706bis-05:New Appendix E on S/TLLAO Extensions for special-purpose links.
Discussed ATN/IPS as example.New sentence in introduction to declare appendices as
non-normative.Changes from draft-templin-intarea-6706bis-03 to
draft-templin-intrea-6706bis-04:Added definitions for Potential Router List (PRL) and secure
enclaveIncluded text on mapping transport layer port numbers to network
layer DSCP valuesAdded reference to DTLS and DMM Distributed Mobility Anchoring
working group documentReworked Security ConsiderationsUpdated references.Changes from draft-templin-intarea-6706bis-02 to
draft-templin-intrea-6706bis-03:Added new section on SEND.Clarifications on "AERO Address" section.Updated references and added new reference for RFC8086.Security considerations updates.General text clarifications and cleanup.Changes from draft-templin-intarea-6706bis-01 to
draft-templin-intrea-6706bis-02:Note on encapsulation avoidance in Section 4.Changes from draft-templin-intarea-6706bis-00 to
draft-templin-intrea-6706bis-01:Remove DHCPv6 Server Release procedures that leveraged the old
way Relays used to “route” between Server link-local
addressesRemove all text relating to Relays needing to do any
AERO-specific operationsProxy sends RS and receives RA from Server using SEND. Use CGAs
as source addresses, and destination address of RA reply is to the
AERO address corresponding to the Client’s ACP.Proxy uses SEND to protect RS and authenticate RA (Client does
not use SEND, but rather relies on subnetwork security. When the
Proxy receives an RS from the Client, it creates a new RS using its
own addresses as the source and uses SEND with CGAs to send a new RS
to the Server.Emphasize distributed mobility managementAERO address-based RS injection of ACP into underlying routing
system.Changes from draft-templin-aerolink-82 to
draft-templin-intarea-6706bis-00:Document use of NUD (NS/NA) for reliable link-layer address
updates as an alternative to unreliable unsolicited NA. Consistent
with Section 7.2.6 of RFC4861.Server adds additional layer of encapsulation between outer and
inner headers of NS/NA messages for transmission through Relays that
act as vanilla IPv6 routers. The messages include the AERO Server
Subnet Router Anycast address as the source and the Subnet Router
Anycast address corresponding to the Client's ACP as the
destination.Clients use Subnet Router Anycast address as the encapsulation
source address when the access network does not provide a
topologically-fixed address.