Transmission of IP Packets over AERO Links
draft-templin-aerolink-28
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Author | Fred Templin | ||
| Last updated | 2014-06-27 | ||
| Replaced by | draft-templin-intarea-6706bis, draft-templin-intarea-6706bis | ||
| RFC stream | (None) | ||
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| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
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draft-templin-aerolink-28
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc6706 (if approved) June 27, 2014
Intended status: Standards Track
Expires: December 29, 2014
Transmission of IP Packets over AERO Links
draft-templin-aerolink-28.txt
Abstract
This document specifies the operation of IP over tunnel virtual Non-
Broadcast, Multiple Access (NBMA) 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 redirection services for
route optimization. AERO provides an IPv6 link-local address format
known as the AERO address that supports operation of the IPv6
Neighbor Discovery (ND) protocol and links IPv6 ND to IP routing.
Admission control and provisioning are supported by the Dynamic Host
Configuration Protocol for IPv6 (DHCPv6), and node mobility is
naturally supported through dynamic neighbor cache updates. Although
IPv6 ND messaging is used in the control plane, both IPv4 and IPv6
are supported in the data plane.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 29, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 5
3.1. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 5
3.2. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 6
3.3. AERO Interface Characteristics . . . . . . . . . . . . . 7
3.3.1. Coordination of Multiple Underlying Interfaces . . . 9
3.4. AERO Interface Neighbor Cache Maintenace . . . . . . . . 10
3.5. AERO Interface Data Origin Authentication . . . . . . . . 11
3.6. AERO Interface MTU Considerations . . . . . . . . . . . . 11
3.7. AERO Interface Encapsulation, Re-encapsulation and
Decapsulation . . . . . . . . . . . . . . . . . . . . . . 13
3.8. AERO Router Discovery, Prefix Delegation and Address
Configuration . . . . . . . . . . . . . . . . . . . . . . 14
3.8.1. AERO Client Behavior . . . . . . . . . . . . . . . . 14
3.8.2. AERO Server Behavior . . . . . . . . . . . . . . . . 16
3.9. AERO Redirection . . . . . . . . . . . . . . . . . . . . 17
3.9.1. Reference Operational Scenario . . . . . . . . . . . 17
3.9.2. Classical Redirection Approaches . . . . . . . . . . 19
3.9.3. Concept of Operations . . . . . . . . . . . . . . . . 20
3.9.4. Message Format . . . . . . . . . . . . . . . . . . . 20
3.9.5. Sending Predirects . . . . . . . . . . . . . . . . . 21
3.9.6. Processing Predirects and Sending Redirects . . . . . 23
3.9.7. Re-encapsulating and Relaying Redirects . . . . . . . 24
3.9.8. Processing Redirects . . . . . . . . . . . . . . . . 25
3.9.9. Server-Oriented Redirection . . . . . . . . . . . . . 25
3.10. Neighbor Reachability Maintenance . . . . . . . . . . . . 25
3.11. Mobility Management . . . . . . . . . . . . . . . . . . . 26
3.12. Encapsulation Protocol Version Considerations . . . . . . 28
3.13. Multicast Considerations . . . . . . . . . . . . . . . . 28
3.14. Operation on AERO Links Without DHCPv6 Services . . . . . 29
3.15. Operation on Server-less AERO Links . . . . . . . . . . . 29
4. Implementation Status . . . . . . . . . . . . . . . . . . . . 29
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
6. Security Considerations . . . . . . . . . . . . . . . . . . . 30
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
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8. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.1. Normative References . . . . . . . . . . . . . . . . . . 31
8.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. AERO Server and Relay Interworking . . . . . . . . . 34
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
This document specifies the operation of IP over tunnel virtual Non-
Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
Route Optimization (AERO). The AERO link can be used for tunneling
to neighboring nodes on 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 redirection services for route optimization
that addresses the requirements outlined in [RFC5522].
AERO provides an IPv6 link-local address format known as the AERO
address that supports operation of the IPv6 Neighbor Discovery (ND)
[RFC4861] protocol and links IPv6 ND to IP routing. Admission
control and provisioning are supported by the Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) [RFC3315], and node mobility
is naturally supported through dynamic neighbor cache updates.
Although IPv6 ND message signalling is used in the control plane,
both IPv4 and IPv6 are supported in the data plane. The remainder of
this document presents the AERO specification.
2. Terminology
The terminology in the normative references applies; the following
terms are defined within the scope of this document:
AERO link
a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
configured over a node's attached IPv6 and/or IPv4 networks. All
nodes on the AERO link appear as single-hop neighbors from the
perspective of the overlay IP layer.
AERO interface
a node's attachment to an AERO link.
AERO address
an IPv6 link-local address constructed as specified in Section 3.2
and assigned to a Client's AERO interface.
AERO node
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a node that is connected to an AERO link and that participates in
IPv6 ND over the link.
AERO Client ("Client")
a node that assigns an AERO address on an AERO interface and
receives an IP prefix delegation.
AERO Server ("Server")
a node that assigns the IPv6 link-local subnet router anycast
address (fe80::) and an administratively provisioned IPv6 link-
local unicast address on an AERO interface over which it can
provide default forwarding and redirection services for AERO
Clients.
AERO Relay ("Relay")
a node that relays IP packets between Servers on the same AERO
link, and/or that forwards IP packets between the AERO link and
the native Internetwork. An AERO Relay may or may not also be
configured as an AERO Server.
ingress tunnel endpoint (ITE)
an AERO interface endpoint that injects tunneled packets into an
AERO link.
egress tunnel endpoint (ETE)
an AERO interface endpoint that receives tunneled packets from an
AERO link.
underlying network
a connected IPv6 or IPv4 network routing region over which AERO
nodes tunnel IP packets.
underlying interface
an AERO node's interface point of attachment to an underlying
network.
link-layer address
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. Link-layer
addresses are used as the encapsulation header source and
destination addresses.
network layer address
the source or destination address of the encapsulated IP packet.
end user network (EUN)
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an internal virtual or external edge IP network that an AERO
Client connects to the AERO interface.
end user network prefix
an IP prefix delegated to an end user network.
aggregated prefix
an IP prefix assigned to the AERO link and from which end user
network prefixes are derived. (For example, and end user network
prefix 2001:db8:1:2::/64 is derived from the aggregated prefix
2001:db8::/32.)
Throughout the document, the simple terms "Client", "Server" and
"Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
respectively. Capitalization is used to distinguish these terms from
DHCPv6 client/server/relay. This is an important distinction, since
an AERO Server may be a DHCPv6 relay, and an AERO Relay may be a
DHCPv6 server.
The terminology of [RFC4861] (including the names of node variables
and protocol constants) applies to this document. Also throughout
the document, the term "IP" is used to generically refer to either
Internet Protocol version (i.e., IPv4 or IPv6).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Asymmetric Extended Route Optimization (AERO)
The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:
3.1. AERO Node Types
AERO Relays relay packets between nodes connected to the same AERO
link and also forward packets between the AERO link and the native
Internetwork. The relaying process entails re-encapsulation of IP
packets that were received from a first AERO node and are to be
forwarded without modification to a second AERO node.
AERO Servers provide default routing services to AERO Clients. AERO
Servers configure a DHCPv6 relay or server function and facilitate
DHCPv6 Prefix Delegation (PD) exchanges. An AERO Server may also act
as an AERO Relay.
AERO Clients act as requesting routers to receive IP prefixes through
a DHCPv6 PD exchange via AERO Servers over the AERO link. (Each
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client MAY associate with multiple Servers, but associating with many
Servers may result in excessive control message overhead.) Each IPv6
AERO Client receives at least a /64 IPv6 prefix delegation, and may
receive even shorter prefixes. Similarly, each IPv4 AERO Client
receives at least a /32 IPv4 prefix delegation (i.e., a singleton
IPv4 address), and may receive even shorter prefixes.
AERO Clients that act as routers sub-delegate portions of their
received prefix delegations to links on EUNs. End system
applications on AERO Clients that act as routers bind to EUN
interfaces (i.e., and not the AERO interface).
AERO Clients that act as ordinary hosts assign one or more IP
addresses taken from their received prefix delegations to the AERO
interface but DO NOT assign the delegated prefix itself to the AERO
interface. Instead, the Client assigns the delegated prefix to a
"black hole" route so that unused portions of the prefix are
nullified. End system applications on AERO Clients that act as hosts
bind directly to the AERO interface.
3.2. AERO Addresses
An AERO address is an IPv6 link-local address with an embedded IP
prefix and assigned to a Client's AERO interface. The AERO address
is formatted as follows:
fe80::[IP prefix]
For IPv6, the AERO address begins with the prefix fe80::/64 and
includes in its interface identifier the base prefix taken from the
Client's delegated IPv6 prefix. The base prefix is determined by
masking the delegated prefix with the prefix length. For example, if
the AERO Client receives the IPv6 prefix delegation:
2001:db8:1000:2000::/56
it constructs its AERO address as:
fe80::2001:db8:1000:2000
For IPv4, the AERO address begins with the prefix fe80::/96 and
includes in its interface identifier the base prefix taken from the
Client's delegated IPv4 prefix. For example, if the AERO Client
receives the IPv4 prefix delegation:
192.0.2.32/28
it constructs its AERO address as:
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fe80::192.0.2.32
The AERO address remains stable as the Client moves between
topological locations, i.e., even if its link-layer addresses change.
NOTE: In some cases, prospective neighbors may not have a priori
knowledge of the Client's delegated prefix length and may therefore
send initial IPv6 ND messages with an AERO destination address that
matches the delegated prefix but does not correspond to the base
prefix. In that case, the Client MUST accept the address as
equivalent to the base address, but then use the base address as the
source address of any IPv6 ND message replies. For example, if the
Client receives the IPv6 prefix delegation 2001:db8:1000:2000::/56
then subsequently receives an IPv6 ND message with destination
address fe80::2001:db8:1000:2001, it accepts the message but uses
fe80::2001:db8:1000:2000 as the source address of any IPv6 ND
replies.
3.3. AERO Interface Characteristics
AERO interfaces use IP-in-IPv6 encapsulation [RFC2473] to exchange
tunneled packets with AERO neighbors attached to an underlying IPv6
network, and use IP-in-IPv4 encapsulation [RFC2003][RFC4213] to
exchange tunneled packets with AERO neighbors attached to an
underlying IPv4 network. AERO interfaces can also operate over
secured tunnel types such as IPsec [RFC4301] or TLS [RFC5246]. When
Network Address Translator (NAT) traversal and/or filtering middlebox
traversal may be necessary, a UDP header is further inserted
immediately above the IP encapsulation header.
Servers assign the address fe80:: to their AERO interfaces as a link-
local Subnet Router Anycast address. Servers and Relays also assign
a link-local address fe80::ID to support the operation of the IPv6 ND
protocol and the inter-Server/Relay routing system (see: Appendix A).
Each fe80::ID address MUST be unique among all Servers and Relays on
the AERO link, and MUST NOT collide with any potential AERO addresses
(e.g., the addresses for Servers and Relays on the link could be
assigned as fe80::1, fe80::2, fe80::3, etc.). Servers accept IPV6 ND
messages with either fe80::ID or fe80:: as the IPv6 destination
address, but MUST use the fe80::ID address as the IPv6 source address
of any IPv6 ND messages they generate.
When a Client does not know the fe80::ID address of a Server, it can
use fe80:: as a temporary destination address in IPv6 ND messages.
The Client may also use fe80::, e.g., as the link-local address in a
neighbor cache entry for a Server when the Server's fe80::ID address
is not known in advance.
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When a Client enables an AERO interface, it invokes DHCPv6 PD using
the temporary IPv6 link-local source address
fe80::ffff:ffff:ffff:ffff. After the Client receives a prefix
delegation, it assigns the corresponding AERO address to the AERO
interface and deprecates the temporary address, i.e., the Client
invokes DHCPv6 to bootstrap the provisioning of a unique link-local
address before invoking IPv6 ND.
AERO interfaces maintain a neighbor cache and use an adaptation of
standard unicast IPv6 ND messaging. AERO interfaces use unicast
Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router
Solicitation (RS) and Router Advertisement (RA) messages the same as
for any IPv6 link. AERO interfaces use two redirection message types
-- the first known as a Predirect message and the second being the
standard Redirect message (see Section 3.9). AERO links further use
link-local-only addressing; hence, Clients ignore any Prefix
Information Options (PIOs) they may receive in RA messages.
AERO interface Redirect/Predirect messages include Target Link-Layer
Address Options (TLLAOs) formatted as shown in Figure 1:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID | Preference | UDP Port Number (or 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-- --+
| |
+-- IP Address --+
| |
+-- --+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: AERO Target Link-Layer Address Option (TLLAO) Format
In this format, Link ID is an integer value between 0 and 255
corresponding to an underlying interface of the target node, and
Preference is an integer value between 0 and 255 indicating the
node's preference for this underlying interface, with 0 being highest
preference and 255 being lowest. UDP Port Number and IP Address are
set to the addresses used by the target node when it sends
encapsulated packets over the underlying interface. When no UDP
encapsulation is used, UDP Port Number is set to 0. When the
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encapsulation IP address family is IPv4, IP Address is formed as an
IPv4-compatible IPv6 address [RFC4291], i.e., 96 bits of leading 0's
followed by a 32-bit IPv4 address
AERO interface Redirect/Predirect messages can both update and create
neighbor cache entries, including link-layer address information.
Redirect/Predirect messages SHOULD include a Timestamp option (see
Section 5.3 of [RFC3971]) that other AERO nodes can use to verify the
message time of origin.
AERO interface NS/NA/RS/RA messages used for neighbor reachability
verification update timers in existing neighbor cache entires but do
not update link-layer addresses nor create new neighbor cache
entries. AERO interface unsolicited NA messages are used to update a
neighbor's cached link-layer address for the sender, e.g., following
a link-layer address change due to node mobility. AERO interface NS/
RS messages SHOULD include a Nonce option (see Section 5.3 of
[RFC3971]) that the recipient echoes back in the corresponding NA/RA
response.
3.3.1. Coordination of Multiple Underlying Interfaces
AERO interfaces may be configured over multiple underlying
interfaces. For example, common 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, terrestrial, air-to-air directional, etc.) with
diverse performance and cost properties.
If a Client's multiple underlying interfaces are used "one at a time"
(i.e., all other interfaces are in standby mode while one interface
is active), then Predirect/Redirect messages include only a single
TLLAO with Link ID set to 0.
If the Client has multiple active underlying interfaces, then from
the perspective of IPv6 ND it would appear to have a single link-
local address with multiple link-layer addresses. In that case,
Predirect/Redirect messages MAY include multiple TLLAOs -- each with
a different Link ID that corresponds to an underlying interface of
the Client. Further details on coordination of multiple active
underlying interfaces are outside the scope of this specification.
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3.4. AERO Interface Neighbor Cache Maintenace
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 [RFC4861]. Neighbor cache
entries are created and maintained as follows:
When an AERO Server relays a DHCPv6 Reply message to an AERO Client,
it creates or updates a neighbor cache entry for the Client based on
the AERO address corresponding to the Client's prefix delegation as
the network-layer address and with the Client's encapsulation IP
address and UDP port number as the link-layer address.
When an AERO Client receives a DHCPv6 Reply message from an AERO
Server, it creates or updates a neighbor cache entry for the Server
based on the Reply message link-local source address as the network-
layer address, and the encapsulation IP source address and UDP source
port number as the link-layer address.
When an AERO Client receives a valid Predirect message it creates or
updates a neighbor cache entry for the Predirect target network-layer
and link-layer addresses, and also creates an IP forwarding table
entry for the predirected (source) prefix. The node then sets an
"AcceptTime" variable for the neighbor and uses this value to
determine whether messages received from the predirected neighbor can
be accepted.
When an AERO Client receives a valid Redirect message it creates or
updates a neighbor cache entry for the Redirect target network-layer
and link-layer addresses, and also creates an IP forwarding table
entry for the redirected (destination) prefix. The node then sets a
"ForwardTime" variable for the neighbor and uses this value to
determine whether packets can be sent directly to the redirected
neighbor. The node also maintains a constant value MAX_RETRY to
limit the number of keepalives sent when a neighbor may have gone
unreachable.
When an AERO Client receives a valid NS message it (re)sets
AcceptTime for the neighbor to ACCEPT_TIME.
When an AERO Client receives a valid solicited NA message, it
(re)sets ForwardTime for the neighbor to FORWARD_TIME. (When an AERO
Client receives a valid unsolicited NA message, it updates the
neighbor's link-layer address but DOES NOT reset ForwardTime.)
It is RECOMMENDED that FORWARD_TIME be set to the default constant
value 30 seconds to match the default REACHABLE_TIME value specified
for IPv6 ND [RFC4861].
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It is RECOMMENDED that ACCEPT_TIME be set to the default constant
value 40 seconds to allow a 10 second window so that the AERO
redirection procedure can converge before AcceptTime decrements below
FORWARD_TIME.
It is RECOMMENDED that MAX_RETRY be set to 3 the same as described
for IPv6 ND address resolution in Section 7.3.3 of [RFC4861].
Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be
administratively set, if necessary, to better match the AERO link's
performance characteristics; however, if different values are chosen,
all nodes on the link MUST consistently configure the same values.
In particular, ACCEPT_TIME SHOULD be set to a value that is
sufficiently longer than FORWARD_TIME to allow the AERO redirection
procedure to converge.
3.5. AERO Interface Data Origin Authentication
AERO nodes use a simple data origin authentication for encapsulated
packets they receive from other nodes. In particular, AERO nodes
accept encapsulated packets with a link-layer source address
belonging to one of their current AERO Servers and accept
encapsulated packets with a link-layer source address that is correct
for the network-layer source address.
The AERO node considers the link-layer source address correct for the
network-layer source address if there is an IP forwarding table entry
that matches the network-layer source address as well as a neighbor
cache entry corresponding to the next hop that includes the link-
layer address and AcceptTime is non-zero.
Note that this simple data origin authentication only applies to
environments in which link-layer addresses cannot be spoofed.
Additional security mitigations may be necessary in other
environments.
3.6. AERO Interface MTU Considerations
The AERO link Maximum Transmission Unit (MTU) is 64KB minus the
encapsulation overhead for IPv4 as the link-layer [RFC0791] and 4GB
minus the encapsulation overhead for IPv6 as the link layer
[RFC2675]. This is the most that IPv4 and IPv6 (respectively) can
convey within the constraints of protocol constants, but actual sizes
available for tunneling will frequently be much smaller.
The base tunneling specifications for IPv4 and IPv6 typically set a
static MTU on the tunnel interface to 1500 bytes minus the
encapsulation overhead or smaller still if the tunnel is likely to
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incur additional encapsulations on the path. This can result in path
MTU related black holes when packets that are too large to be
accommodated over the AERO link are dropped, but the resulting ICMP
Packet Too Big (PTB) messages are lost on the return path. As a
result, AERO nodes use the following MTU mitigations to accommodate
larger packets.
AERO nodes set their AERO interface MTU to the larger of the
underlying interface MTU minus the encapsulation overhead, and 1500
bytes. (If there are multiple underlying interfaces, the node sets
the AERO interface MTU according to the largest underlying interface
MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU
cannot be determined.) AERO nodes optionally cache other per-
neighbor MTU values in the underlying IP path MTU discovery cache
initialized to the underlying interface MTU.
AERO nodes admit packets that are no larger than 1280 bytes minus the
encapsulation overhead (*) as well as packets that are larger than
1500 bytes into the tunnel without fragmentation, i.e., as long as
they are no larger than the AERO interface MTU before encapsulation
and also no larger than the cached per-neighbor MTU following
encapsulation. For IPv4, the node sets the "Don't Fragment" (DF) bit
to 0 for packets no larger than 1280 bytes minus the encapsulation
overhead (*) and sets the DF bit to 1 for packets larger than 1500
bytes. If a large packet is lost in the path, the node may
optionally cache the MTU reported in the resulting PTB message or may
ignore the message, e.g., if there is a possibility that the message
is spurious.
For packets destined to an AERO node that are larger than 1280 bytes
minus the encapsulation overhead (*) but no larger than 1500 bytes,
the node uses IP fragmentation to fragment the encapsulated packet
into two pieces (where the first fragment contains 1024 bytes of the
original IP packet) then admits the fragments into the tunnel. If
the link-layer protocol is IPv4, the node admits each fragment into
the tunnel with DF set to 0 and subject to rate limiting to avoid
reassembly errors [RFC4963][RFC6864]. For both IPv4 and IPv6, the
node also sends a 1500 byte probe message (**) to the neighbor,
subject to rate limiting.
To construct a probe, the node prepares an NS message with a Nonce
option plus trailing padding octets added to a length of 1500 bytes
without including the length of the padding in the IPv6 Payload
Length field. The node then encapsulates the NS in the encapsulation
headers (while including the length of the padding in the
encapsulation header length fields), sets DF to 1 (for IPv4) and
sends the padded NS message to the neighbor. If the neighbor returns
an NA message with a correct Nonce value, the node may then send
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whole packets within this size range and (for IPv4) relax the rate
limiting requirement. (Note that the trailing padding SHOULD NOT be
included within the Nonce option itself but rather as padding beyond
the last option in the NS message; otherwise, the (large) Nonce
option would be echoed back in the solicited NA message and may be
lost at a link with a small MTU along the reverse path.)
AERO nodes MUST be capable of reassembling packets up to 1500 bytes
plus the encapsulation overhead length. It is therefore RECOMMENDED
that AERO nodes be capable of reassembling at least 2KB.
(*) Note that if it is known without probing that the minimum Path
MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then
MINMTU can be used instead of 1280 in the fragmentation threshold
considerations listed above.
(**) It is RECOMMENDED that no probes smaller than 1500 bytes be used
for MTU probing purposes, since smaller probes may be fragmented if
there is a nested tunnel somewhere on the path to the neighbor.
Probe sizes larger than 1500 bytes MAY be used, but may be
unnecessary since original sources are expected to implement
[RFC4821] when sending large packets.
3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation
AERO interfaces encapsulate IP packets according to whether they are
entering the AERO interface for the first time or if they are being
forwarded out the same AERO interface that they arrived on. This
latter form of encapsulation is known as "re-encapsulation".
AERO interfaces encapsulate packets per the specifications in
[RFC2003][RFC2473][RFC4213][RFC4301][RFC5246] except that the
interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class"
and "Congestion Experienced" values in the packet's IP header into
the corresponding fields in the encapsulation header. For packets
undergoing re-encapsulation, the AERO interface instead copies the
"TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion
Experienced" values in the original encapsulation header into the
corresponding fields in the new encapsulation header (i.e., the
values are transferred between encapsulation headers and *not* copied
from the encapsulated packet's network-layer header).
When AERO UDP encapsulation is used, the AERO interface encapsulates
the packet per the specifications in [RFC2003][RFC2473][RFC4213]
except that it inserts a UDP header between the encapsulation header
and the packet's IP header. The AERO interface sets the UDP source
port to a constant value that it will use in each successive packet
it sends, sets the UDP checksum field to zero (see:
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[RFC6935][RFC6936]) and sets the UDP length field to the length of
the IP packet plus 8 bytes for the UDP header itself. For packets
sent via a Server, the AERO interface sets the UDP destination port
to 8060 (i.e., the IANA-registered port number for AERO) when AERO-
only encapsulation is used. For packets sent to a neighboring
Client, the AERO interface sets the UDP destination port to the port
value stored in the neighbor cache entry for this neighbor.
The AERO interface next sets the IP protocol number in the
encapsulation header to the appropriate value for the first protocol
layer within the encapsulation (e.g., IPv4, IPv6, UDP, IPsec, etc.).
When IPv6 is used as the encapsulation protocol, the interface then
sets the flow label value in the encapsulation header the same as
described in [RFC6438]. When IPv4 is used as the encapsulation
protocol, the AERO interface sets the DF bit as discussed in
Section 3.6.
AERO interfaces decapsulate packets destined either to the node
itself or to a destination reached via an interface other than the
receiving AERO interface. When AERO UDP encapsulation is used (i.e.,
when a UDP header with destination port 8060 is present) the
interface examines the first octet of the encapsulated packet. If
the most significant four bits of the first octet encode the value
'0110' (i.e., the version number value for IPv6) or the value '0100'
(i.e., the version number value for IPv4), the packet is accepted and
the encapsulating UDP header is discarded; otherwise, the packet is
discarded.
Further decapsulation then proceeds according to the appropriate
tunnel type [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246].
3.8. AERO Router Discovery, Prefix Delegation and Address Configuration
3.8.1. AERO Client Behavior
AERO Clients discover the link-layer addresses of AERO Servers via
static configuration, or through an automated means such as DNS name
resolution. In the absence of other information, the Client resolves
the Fully-Qualified Domain Name (FQDN) "linkupnetworks.domainname",
where "domainname" is the DNS domain appropriate for the Client's
attached underlying network. 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 an IP prefix through DHCPv6 PD [RFC3315][RFC3633][RFC6355]
using fe80::ffff:ffff:ffff:ffff as the IPv6 source address (see
Section 3.3), 'All_DHCP_Relay_Agents_and_Servers' as the IPv6
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destination address and the link-layer address of the Server as the
link-layer destination address. The Client includes a DHCPv6 Unique
Identifier (DUID) in the Client Identifier option of its DHCPv6
messages (as well as a DHCPv6 authentication option if necessary) to
identify itself to the DHCPv6 server. If the Client is pre-
provisioned with an IP prefix associated with the AERO service, it
MAY also include the prefix in its DHCPv6 PD Request to indicate its
preferred prefix to the DHCPv6 server. The Client then sends the
encapsulated DHCPv6 request via an underlying interface.
When the Client receives its prefix delegation via a Reply from the
DHCPv6 server, it creates a neighbor cache entry with the Server's
link-local address (i.e., fe80::ID) as the network-layer address and
the Server's encapsulation address as the link-layer addresses.
Next, the Client assigns the AERO address derived from the delegated
prefix to the AERO interface and sub-delegates the prefix to nodes
and links within its attached EUNs (the AERO address thereafter
remains stable as the Client moves). The Client also sets both
AcceptTime and ForwardTime for each Server to the constant value
REACHABLE_TIME. The Client further renews its prefix delegation by
performing DHCPv6 Renew/Reply exchanges with its AERO address as the
IPv6 source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6
destination address, the link-layer address of a Server as the link-
layer destination address and the same DUID and authentication
information. If the Client wishes to associate with multiple
Servers, it can perform DHCPv6 Renew/Reply exchanges via each of the
Servers.
The Client then sends an RS message to each of its associated Servers
to receive an RA message with a default router lifetime and any other
link-specific parameters. When the Client receives an RA message, it
configures or updates a default route according to the default router
lifetime but ignores any Prefix Information Options (PIOs) included
in the RA message since the AERO link is link-local-only. The Client
further ignores any RS messages it might receive, since only Servers
may process RS messages.
The Client then sends periodic RS messages to each Server before
AcceptTime and ForwardTime expire to obtain new RA messages for
Neighbor Unreachability Detection (NUD), to refresh any network
state, and to update the default router lifetime and any other link-
specific parameters. When the Client receives a new RA message, it
resets AcceptTime and ForwardTime to REACHABLE_TIME. The Client can
also forward IP packets destined to networks beyond its local EUNs
via a Server as a default router. The Server may in turn return a
redirection message informing the Client of a neighbor on the AERO
link that is topologically closer to the final destination (see
Section 3.9).
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Note that, since the Client's AERO address is configured from the
unique DHCPv6 prefix delegation it receives, there is no need for
Duplicate Address Detection (DAD) on AERO links. Other nodes
maliciously attempting to hijack an authorized Client's AERO address
will be denied access to the network by the DHCPv6 server due to an
unacceptable link-layer address and/or security parameters (see:
Security Considerations).
3.8.2. AERO Server Behavior
AERO Servers configure a DHCPv6 relay function on their AERO links.
AERO Servers arrange to add their encapsulation layer IP addresses
(i.e., their link-layer addresses) to the DNS resource records for
the FQDN "linkupnetworks.domainname" before entering service.
When an AERO Server relays a prospective Client's DHCPv6 PD messages
to the DHCPv6 server, it wraps each message in a "Relay-forward"
message per [RFC3315] and includes a DHCPv6 Interface Identifier
option that encodes a value that identifies the AERO link to the
DHCPv6 server. Without creating internal state, the Server then
includes the Client's link-layer address in a DHCPv6 Client Link
Layer Address Option (CLLAO) [RFC6939] with the link-layer address
format shown in Figure 1 (i.e., Link ID followed by Preference
followed by UDP Port Number followed by IP Address). The Server sets
the CLLAO 'option-length' field to 22 (2 plus the length of the link-
layer address) and sets the 'link-layer type' field to TBD (see: IANA
Considerations). The Server finally includes a DHCPv6 Echo Request
Option (ERO) [RFC4994] that encodes the option code for the CLLAO in
a 'requested-option-code-n' field, then relays the message to the
DHCPv6 server. The CLLAO information will therefore subsequently be
echoed back in the DHCPv6 server's "Relay-reply" message.
When the DHCPv6 server issues the prefix delegation in a "Relay-
reply" message via the AERO Server (acting as a DHCPv6 relay), the
Server obtains the Client's link-layer address from the echoed CLLAO
option and also obtains the Client's delegated prefix from the
message. The Server then creates a neighbor cache entry for the
Client's AERO address with the Client's link-layer address as the
link-layer address for the neighbor cache entry. The neighbor cache
entry is created with both AcceptTime and ForwardTime set to
REACHABLE_TIME, since the Client will continue to send RS messages
within REACHABLE_TIME seconds as long as it wishes to remain
associated with this Server.
The Server also configures an IP forwarding table entry that lists
the Client's AERO address as the next hop toward the delegated IP
prefix with a lifetime derived from the DHCPv6 lease lifetime. The
Server finally injects the Client's prefix as an IP route into the
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inter-Server/Relay routing system (see: Appendix A) then relays the
DHCPv6 message to the Client while using fe80::ID as the IPv6 source
address, the link-local address found in the "peer address" field of
the Relay-reply message as the IPv6 destination address, and the
Client's link-layer address as the destination link-layer address.
Servers respond to NS/RS messages from Clients on their AERO
interfaces by returning an NA/RA message. The Server SHOULD NOT
include PIOs in the RA messages it sends to Clients, since the Client
will ignore any such options. When the Server receives an NS/RS
message from the Client, it resets AcceptTime and ForwardTime to
REACHABLE_TIME.
Servers ignore any RA messages they may receive from a Client, but
they MAY examine RA messages received from other Servers for
consistency verification purposes. Servers do not send NS messages
for the purpose of updating Client neighbor cache timers, since
Clients are responsible for refreshing neighbor cache state.
When the Server forwards a packet via the same AERO interface on
which it arrived, it initiates an AERO route optimization procedure
as specified in Section 3.9.
3.9. AERO Redirection
3.9.1. Reference Operational Scenario
Figure 2 depicts the AERO redirection reference operational scenario,
using IPv6 addressing as the example (while not shown, a
corresponding example for IPv4 addressing can be easily constructed).
The figure shows an AERO Server('A'), two AERO Clients ('B', 'C') and
three ordinary IPv6 hosts ('D', 'E', 'F'):
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.-(::::::::)
.-(:::: IP ::::)-. +-------------+
(:: Internetwork ::)--| Host F |
`-(::::::::::::)-' +-------------+
`-(::::::)-' 2001:db8:2::1
|
+--------------+
| AERO Server A|
| (D->B; E->C) |
+--------------+
fe80::ID
L2(A)
|
X-----+-----------+-----------+--------X
| AERO Link |
L2(B) L2(C)
fe80::2001:db8:0:0 fe80::2001:db8:1:0 .-.
+--------------+ +--------------+ ,-( _)-.
| AERO Client B| | AERO Client C| .-(_ IP )-.
| (default->A) | | (default->A) |--(__ EUN )
+--------------+ +--------------+ `-(______)-'
2001:DB8:0::/48 2001:DB8:1::/48 |
| 2001:db8:1::1
.-. +-------------+
,-( _)-. 2001:db8:0::1 | Host E |
.-(_ IP )-. +-------------+ +-------------+
(__ EUN )--| Host D |
`-(______)-' +-------------+
Figure 2: AERO Reference Operational Scenario
In Figure 2, AERO Server ('A') connects to the AERO link and connects
to the IP Internetwork, either directly or via an AERO Relay (not
shown). Server ('A') assigns the address fe80::ID to its AERO
interface with link-layer address L2(A). Server ('A') next arranges
to add L2(A) to a published list of valid Servers for the AERO link.
AERO Client ('B') receives the prefix 2001:db8:0::/48 in a DHCPv6 PD
exchange via AERO Server ('A') then assigns the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(B). Client ('B') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::ID and link-
layer address L2(A), then sub-delegates the prefix 2001:db8:0::/48 to
its attached EUNs. IPv6 host ('D') connects to the EUN, and
configures the address 2001:db8:0::1.
AERO Client ('C') receives the prefix 2001:db8:1::/48 in a DHCPv6 PD
exchange via AERO Server ('A') then assigns the address
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fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(C). Client ('C') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::ID and link-
layer address L2(A), then sub-delegates the prefix 2001:db8:1::/48 to
its attached EUNs. IPv6 host ('E') connects to the EUN, and
configures the address 2001:db8:1::1.
Finally, IPv6 host ('F') connects to a network outside of the AERO
link domain. Host ('F') configures its IPv6 interface in a manner
specific to its attached IPv6 link, and assigns the address
2001:db8:2::1 to its IPv6 link interface.
3.9.2. Classical Redirection Approaches
With reference to Figure 2, when the source host ('D') sends a packet
to destination host ('E'), the packet is first forwarded via the EUN
to AERO Client ('B'). Client ('B') then forwards the packet over its
AERO interface to AERO Server ('A'), which then re-encapsulates and
forwards the packet to AERO Client ('C'), where the packet is finally
forwarded to destination host ('E'). When Server ('A') re-
encapsulates and forwards the packet back out on its advertising AERO
interface, it must arrange to redirect Client ('B') toward Client
('C') as a better next-hop node on the AERO link that is closer to
the final destination. However, this redirection process applied to
AERO interfaces must be more carefully orchestrated than on ordinary
links since the parties may be separated by potentially many
underlying network routing hops.
Consider a first alternative in which Server ('A') informs Client
('B') only and does not inform Client ('C') (i.e., "classical
redirection"). In that case, Client ('C') has no way of knowing that
Client ('B') is authorized to forward packets from the claimed source
address, and it may simply elect to drop the packets. Also, Client
('B') has no way of knowing whether Client ('C') is performing some
form of source address filtering that would reject packets arriving
from a node other than a trusted default router, nor whether Client
('C') is even reachable via a direct path that does not involve
Server ('A').
Consider a second alternative in which Server ('A') informs both
Client ('B') and Client ('C') separately, via independent redirection
control messages (i.e., "augmented redirection"). In that case, if
Client ('B') receives the redirection control message but Client
('C') does not, subsequent packets sent by Client ('B') could be
dropped due to filtering since Client ('C') would not have a route to
verify the claimed source address. Also, if Client ('C') receives
the redirection control message but Client ('B') does not, subsequent
packets sent in the reverse direction by Client ('C') would be lost.
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Since both of these alternatives have shortcomings, a new redirection
technique (i.e., "AERO redirection") is needed.
3.9.3. Concept of Operations
Again, with reference to Figure 2, when source host ('D') sends a
packet to destination host ('E'), the packet is first forwarded over
the source host's attached EUN to Client ('B'), which then forwards
the packet via its AERO interface to Server ('A').
Server ('A') then re-encapsulates and forwards the packet out the
same AERO interface toward Client ('C') and also sends an AERO
"Predirect" message forward to Client ('C') as specified in
Section 3.9.5. The Predirect message includes Client ('B')'s
network- and link-layer addresses as well as information that Client
('C') can use to determine the IP prefix used by Client ('B') . After
Client ('C') receives the Predirect message, it process the message
and returns an AERO Redirect message destined for Client ('B') via
Server ('A') as specified in Section 3.9.6. During the process,
Client ('C') also creates or updates a neighbor cache entry for
Client ('B') and creates an IP forwarding table entry for Client
('B')'s prefix.
When Server ('A') receives the Redirect message, it re-encapsulates
the message and forwards it on to Client ('B') as specified in
Section 3.9.7. The message includes Client ('C')'s network- and
link-layer addresses as well as information that Client ('B') can use
to determine the IP prefix used by Client ('C'). After Client ('B')
receives the Redirect message, it processes the message as specified
in Section 3.9.8. During the process, Client ('B') also creates or
updates a neighbor cache entry for Client ('C') and creates an IP
forwarding table entry for Client ('C')'s prefix.
Following the above Predirect/Redirect message exchange, forwarding
of packets from Client ('B') to Client ('C') without involving Server
('A) as an intermediary is enabled. The mechanisms that support this
exchange are specified in the following sections.
3.9.4. Message Format
AERO Redirect/Predirect messages use the same format as for ICMPv6
Redirect messages depicted in Section 4.5 of [RFC4861], but also
include a new "Prefix Length" field taken from the low-order 8 bits
of the Redirect message Reserved field. (For IPv6, valid values for
the Prefix Length field are 0 through 64; for IPv4, valid values are
0 through 32.) The Redirect/Predirect messages are formatted as
shown in Figure 3:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (=137) | Code (=0/1) | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Target Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Destination Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 3: AERO Redirect/Predirect Message Format
3.9.5. Sending Predirects
When a Server forwards a packet from one of its associated Clients
toward another AERO Client connected to the same AERO link, the
Server sends a Predirect message forward toward the destination
Client instead of sending a Redirect message back to the source
Client.
In the reference operational scenario, when Server ('A') forwards a
packet sent by Client ('B') toward Client ('C'), it also sends a
Predirect message forward toward Client ('C'), subject to rate
limiting (see Section 8.2 of [RFC4861]). Server ('A') prepares the
Predirect message as follows:
o the link-layer source address is set to 'L2(A)' (i.e., the link-
layer address of Server ('A')).
o the link-layer destination address is set to 'L2(C)' (i.e., the
link-layer address of Client ('C')).
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o the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('B')).
o the network-layer destination address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C')).
o the Type is set to 137.
o the Code is set to 1 to indicate "Predirect".
o the Prefix Length is set to the length of the prefix to be applied
to the Target Address.
o the Target Address is set to fe80::2001:db8:0:0 (i.e., the AERO
address of Client ('B')).
o the Destination Address is set to the source address of the
originating packet that triggered the Predirection event. (If the
originating packet is an IPv4 packet, the address is constructed
in IPv4-compatible IPv6 address format).
o the message includes a TLLAO with Link ID and Preference set to
appropriate values for Client ('B')'s underlying interface, and
with UDP Port Number and IP Address set to 'L2(B)'.
o the message includes a Timestamp option.
o the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated to ensure that at least
the network-layer header is included but the size of the message
does not exceed 1280 bytes.
Note that the reference operational scenario applies to the case when
the source and destination Clients are associated with the same
Server. When the source and destination Clients are associated with
different Servers, the source Client's Server forwards the packets
and Predirect messages to a Relay which in turn forwards them toward
the destination Client. In that case, the Server sets the Predirect
link-layer destination address to the link-layer address of the
Relay.
Servers therefore require knowledge of all aggregated IP prefixes
associated with the AERO link so that they can determine when a
prospective destination Client is on-link. See Appendix A for a
discussion of AERO Server/Relay interworking.
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3.9.6. Processing Predirects and Sending Redirects
When Client ('C') receives a Predirect message, it accepts the
message only if the message has a link-layer source address of the
Server, i.e. 'L2(A)'. Client ('C') further accepts the message only
if it is willing to serve as a redirection target. Next, Client
('C') validates the message according to the ICMPv6 Redirect message
validation rules in Section 8.1 of [RFC4861], except that it accepts
the message even though the network-layer source address is not that
of it's current first-hop router.
In the reference operational scenario, when Client ('C') receives a
valid Predirect message, it either creates or updates a neighbor
cache entry that stores the Target Address of the message as the
network-layer address of Client ('B') and stores the link-layer
address found in the TLLAO as the link-layer address(es) of Client
('B'). Client ('C') then sets AcceptTime for the neighbor cache
entry to ACCEPT_TIME. Next, Client ('C') applies the Prefix Length
to the Destination Address and records the resulting prefix in its IP
forwarding table.
After processing the message, Client ('C') prepares a Redirect
message response as follows:
o the link-layer source address is set to 'L2(C)' (i.e., the link-
layer address of Client ('C')).
o the link-layer destination address is set to 'L2(A)' (i.e., the
link-layer address of Server ('A')).
o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('B')).
o the Type is set to 137.
o the Code is set to 0 to indicate "Redirect".
o the Prefix Length is set to the length of the prefix to be applied
to the Target Address.
o the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO
address of Client ('C')).
o the Destination Address is set to the destination address of the
originating packet that triggered the Redirection event. (If the
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originating packet is an IPv4 packet, the address is constructed
in IPv4-compatible IPv6 address format).
o the message includes a TLLAO with Link ID and Preference set to
appropriate values for Client ('C')'s underlying interface, and
with UDP Port Number and IP Address set to '0'.
o the message includes a Timestamp option.
o the message includes as much of the RHO copied from the
corresponding AERO Predirect message as possible such that at
least the network-layer header is included but the size of the
message does not exceed 1280 bytes.
After Client ('C') prepares the Redirect message, it sends the
message to Server ('A').
3.9.7. Re-encapsulating and Relaying Redirects
When Server ('A') receives a Redirect message from Client ('C'), it
validates the message according to the ICMPv6 Redirect message
validation rules in Section 8.1 of [RFC4861] and also verifies that
Client ('C') is authorized to use the Prefix Length in the Redirect
message when applied to the AERO address in the network-layer source
of the Redirect message by searching for the AERO address' embedded
prefix in the IP routing table. If validation fails, Server ('A')
discards the message; otherwise, it copies the correct UDP Port
number and IP Address for Client ('C') into the (previously empty)
TLLAO.
Server ('A') then examines the network-layer destination address of
the message to determine the next hop toward the prefix of Client
('B') by searching for the AERO address' embedded prefix in the IP
routing table. If the next hop is reached via the AERO interface,
Server ('A') re-encapsulates the Redirect and relays it on to Client
('B') by changing the link-layer source address of the message to
'L2(A)' and changing the link-layer destination address to 'L2(B)'.
Server ('A') finally forwards the re-encapsulated message to Client
('B') without decrementing the network-layer TTL/Hop Limit field.
While not shown in Figure 2, AERO Relays relay Redirect and Predirect
messages in exactly this same fashion described above (see:
Appendix A).
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3.9.8. Processing Redirects
When Client ('B') receives the Redirect message, it accepts the
message only if it has a link-layer source address of the Server,
i.e. 'L2(A)'. Next, Client ('B') validates the message according to
the ICMPv6 Redirect message validation rules in Section 8.1 of
[RFC4861], except that it accepts the message even though the
network-layer source address is not that of it's current first-hop
router. Following validation, Client ('B') then processes the
message as follows.
In the reference operational scenario, when Client ('B') receives the
Redirect message, it either creates or updates a neighbor cache entry
that stores the Target Address of the message as the network-layer
address of Client ('C') and stores the link-layer address found in
the TLLAO as the link-layer address of Client ('C'). Client ('B')
then sets the neighbor cache entry ForwardTime variable with timeout
value FORWARD_TIME. Next, Client ('B') applies the Prefix Length to
the Destination Address and records the resulting IP prefix in its IP
forwarding table.
Now, Client ('B') has an IP forwarding table entry for Client('C')'s
prefix and a neighbor cache entry with a valid ForwardTime value,
while Client ('C') has an IP forwarding table entry for Client
('B')'s prefix with a valid AcceptTime value. Thereafter, Client
('B') may forward ordinary network-layer data packets directly to
Client ("C") without involving Server ('A') and Client ('C') can
verify that the packets came from an acceptable source. (In order
for Client ('C') to forward packets to Client ('B') a corresponding
Predirect/Redirect message exchange is required in the reverse
direction.)
3.9.9. Server-Oriented Redirection
In some environments, the Server nearest the destination Client may
need to serve as the redirection target, e.g., if direct Client-to-
Client communications are not possible. In that case, the Server
prepares the Redirect message the same as if it were the destination
Client (see: Section 3.9.6), except that it writes its own link-layer
address in the TLLAO option.
3.10. Neighbor Reachability Maintenance
AERO nodes send unicast NS messages to elicit solicited NA messages
from neighbors the same as described for Neighbor Unreachability
Detection (NUD) in [RFC4861]. When an AERO node sends an NS/NA
message, it MUST use its link-local address as the IPv6 source
address and the link-local address of the neighbor as the IPv6
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destination address. When an AERO node receives an NS message or a
solicited NA message, it accepts the message if it has a neighbor
cache entry for the neighbor; otherwise, it ignores the message.
When a source Client is redirected to a target Client it SHOULD test
the direct path by sending an initial NS message to elicit a
solicited NA response. While testing the path, the source Client can
optionally continue sending packets via the Server, maintain a small
queue of packets until target reachability is confirmed, or
(optimistically) allow packets to flow directly to the target. The
source Client SHOULD thereafter continue to test the direct path to
the target Client (see Section 7.3 of [RFC4861]) periodically in
order to keep neighbor cache entries alive.
In particular, while the source Client is actively sending packets to
the target Client it SHOULD also send NS messages separated by
RETRANS_TIMER milliseconds in order to receive solicited NA messages.
If the source Client is unable to elicit a solicited NA response from
the target Client after MAX_RETRY attempts, it SHOULD set ForwardTime
to 0 and resume sending packets via the Server which may or may not
result in a new redirection event. Otherwise, the source Client
considers the path usable and SHOULD thereafter process any link-
layer errors as a hint that the direct path to the target Client has
either failed or has become intermittent.
When a target Client receives an NS message from a source Client, it
resets AcceptTime to ACCEPT_TIME if a neighbor cache entry exists;
otherwise, it discards the NS message.
When a source Client receives a solicited NA message from a target
Client, it resets ForwardTime to FORWARD_TIME if a neighbor cache
entry exists; otherwise, it discards the NA message.
When ForwardTime for a neighbor cache entry expires, the source
Client resumes sending any subsequent packets via the Server and may
(eventually) receive a new Redirect message. When AcceptTime for a
neighbor cache entry expires, the target Client discards any
subsequent packets received directly from the source Client. When
both ForwardTime and AcceptTime for a neighbor cache entry expire,
the Client deletes both the neighbor cache entry and the
corresponding IP forwarding table entry.
3.11. Mobility Management
When a Client needs to change its link-layer address, e.g., due to a
mobility event, it performs an immediate DHCPv6 Renew/Reply via each
of its Servers using the new link-layer address as the source. The
DHCPv6 Server will re-authenticate the Client and (assuming
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authentication succeeds) the DHCPv6 Renew/Reply exchange will update
each Server's neighbor cache.
Next, the Client sends unsolicited NA messages to each of its active
neighbors using the same procedures as specified in Section 7.2.6 of
[RFC4861], except that it sends the messages as unicast to each
neighbor via a Server instead of multicast. In this process, the
Client should send no more than MAX_NEIGHBOR_ADVERTISEMENT messages
separated by no less than RETRANS_TIMER seconds to each neighbor.
With reference to Figure 2, Client ('C') sends unicast unsolicited NA
messages to Client ('B') via Server ('A') as follows:
o the link-layer source address is set to 'L2(C)' (i.e., the link-
layer address of Client ('C')).
o the link-layer destination address is set to 'L2(A)' (i.e., the
link-layer address of Server ('A')).
o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('B')).
o the Type is set to 136.
o the Code is set to 0.
o the Solicited flag is set to 0.
o the Override flag is set to 1.
o the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO
address of Client ('C')).
o the message includes a TLLAO with Link ID and Preference set to
appropriate values for Client ('C')'s underlying interface, and
with UDP Port Number and IP Address set to '0'.
o the message includes a Timestamp option.
When Server ('A') receives the NA message, it relays the message in
the same way as described for relaying Redirect messages in
Section 3.9.7. In particular, Server ('A') copies the correct UDP
port number and IP address into the TLLAO, changes the link-layer
source address to its own address, changes the link-layer destination
address to the address of Client ('B'), then forwards the NA message
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based on an IP route matching the AERO address in the network-layer
destination address. When Client ('B') receives the NA message, it
accepts the message only if it already has a neighbor cache entry for
Client ('C') then updates the link-layer address for Client ('C')
based on the address in the TLLAO. However, Client ('B') MUST NOT
update ForwardTime since it has no way of knowing whether Client
('C') has updated AcceptTime.
When a Client associates with a new Server, it issues a new DHCPv6
Renew message via the new Server as the DHCPv6 relay. The new Server
then relays the message to the DHCPv6 server and processes the
resulting exchange. After the Client receives the resulting DHCPv6
Reply message, it sends an RS message to the new Server to receive a
new RA message.
When a Client disassociates with an existing Server, it sends a
"terminating RS" message to the old Server. The terminating RS
message is prepared exactly the same as for an ordinary RS message,
except that the Code field contains the value '1'. When the old
Server receives the terminating RS message, it withdraws the IP route
from the routing system and deletes the neighbor cache entry and IP
forwarding table entry for the Client. The old Server then returns
an RA message with default router lifetime set to 0 which the Client
can use to verify that the termination signal has been processed.
The client then deletes both the default route and the neighbor cache
entry for the old Server. (Note that the Client and the old Server
MAY impose a small delay before deleting the neighbor cache and IP
forwarding table entries so that any packets already in the system
can still be delivered to the Client.)
3.12. Encapsulation Protocol Version Considerations
A source Client may connect only to an IPvX underlying network, while
the target Client connects only to an IPvY underlying network. In
that case, the target and source Clients have no means for reaching
each other directly (since they connect to underlying networks of
different IP protocol versions) and so must ignore any redirection
messages and continue to send packets via the Server.
3.13. Multicast Considerations
When the underlying network does not support multicast, AERO nodes
map IPv6 link-scoped multicast addresses (including
'All_DHCP_Relay_Agents_and_Servers') to the link-layer address of a
Server.
When the underlying network supports multicast, AERO nodes use the
multicast address mapping specification found in [RFC2529] for IPv4
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underlying networks and use a direct multicast mapping for IPv6
underlying networks. (In the latter case, "direct multicast mapping"
means that if the IPv6 multicast destination address of the
encapsulated packet is "M", then the IPv6 multicast destination
address of the encapsulating header is also "M".)
3.14. Operation on AERO Links Without DHCPv6 Services
When the AERO link does not provide DHCPv6 services, operation can
still be accommodated through administrative configuration of
prefixes on AERO Clients. In that case, administrative
configurations of IP routes and AERO interface neighbor cache entries
on both the Server and Client are also necessary. However, this may
preclude the ability for Clients to dynamically change to new
Servers, and can expose the AERO link to misconfigurations unless the
administrative configurations are carefully coordinated.
3.15. Operation on Server-less AERO Links
In some AERO link scenarios, there may be no Servers on the link and/
or no need for Clients to use a Server as an intermediary trust
anchor. In that case, each Client acts as a Server unto itself to
establish neighbor cache entries and IP forwarding table entries by
performing direct Client-to-Client Predirect/Redirect exchanges, and
some other form of trust basis must be applied so that each Client
can verify that the prospective neighbor is authorized to use its
claimed prefix.
When there is no Server on the link, Clients must arrange to receive
prefix delegations and publish the delegations via a secure alternate
prefix delegation authority through some means outside the scope of
this document.
4. Implementation Status
An application-layer implementation is in progress.
5. IANA Considerations
The IANA is instructed to assign a new 2-octet Hardware Type number
for AERO in the "arp-parameters" registry per Section 2 of [RFC5494].
The number is assigned from the 2-octet Unassigned range with
Hardware Type "AERO" and with this document as the reference.
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6. Security Considerations
AERO link security considerations are the same as for standard IPv6
Neighbor Discovery [RFC4861] except that AERO improves on some
aspects. In particular, AERO uses a trust basis between Clients and
Servers, where the Clients only engage in the AERO mechanism when it
is facilitated by a trust anchor. AERO also uses DHCPv6
authentication for Client authentication and network admission
control.
AERO links must be protected against link-layer address spoofing
attacks in which an attacker on the link pretends to be a trusted
neighbor. Links that provide link-layer securing mechanisms (e.g.,
IEEE 802.1X WLANs) and links that provide physical security (e.g.,
enterprise network wired LANs) provide a first line of defense that
is often sufficient. In other instances, additional securing
mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec
[RFC4301] or TLS [RFC5246] may be necessary.
AERO Clients MUST ensure that their connectivity is not used by
unauthorized nodes on 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 unauthorized nodes via a NAT function.)
On some AERO links, establishment and maintenance of a direct path
between neighbors requires secured coordination such as through the
Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a
security association.
7. Acknowledgements
Discussions both on IETF lists and in private exchanges helped shape
some of the concepts in this work. Individuals who contributed
insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant,
Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha
Hlusiak, Lee Howard, Joe Touch and Bernie Volz. 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.
This work has further been encouraged and supported by Boeing
colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie,
Balaguruna Chidambaram, Wen Fang, Anthony Gregory, Jeff Holland, Ed
King, Gen MacLean, Kent Shuey, Mike Slane, Julie Wulff, Yueli Yang,
and other members of the BR&T and BIT mobile networking teams.
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Earlier works on NBMA tunneling approaches are found in
[RFC2529][RFC5214][RFC5569].
8. References
8.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
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[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, December 2011.
8.2. Informative References
[IRON] Templin, F., "The Internet Routing Overlay Network
(IRON)", Work in Progress, June 2012.
[RFC0879] Postel, J., "TCP maximum segment size and related topics",
RFC 879, November 1983.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
BCP 6, RFC 1930, March 1996.
[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC4994] Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski,
"DHCPv6 Relay Agent Echo Request Option", RFC 4994,
September 2007.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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[RFC5494] Arkko, J. and C. Pignataro, "IANA Allocation Guidelines
for the Address Resolution Protocol (ARP)", RFC 5494,
April 2009.
[RFC5522] Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
Route Optimization Requirements for Operational Use in
Aeronautics and Space Exploration Mobile Networks", RFC
5522, October 2009.
[RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, January 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
[RFC6204] Singh, H., Beebee, W., Donley, C., Stark, B., and O.
Troan, "Basic Requirements for IPv6 Customer Edge
Routers", RFC 6204, April 2011.
[RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based
DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August
2011.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, November 2011.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, July 2012.
[RFC6706] Templin, F., "Asymmetric Extended Route Optimization
(AERO)", RFC 6706, August 2012.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, February 2013.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935, April 2013.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, April 2013.
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[RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
Address Option in DHCPv6", RFC 6939, May 2013.
[RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation
with IPv6 Neighbor Discovery", RFC 6980, August 2013.
[RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
Address Selection Policy Using DHCPv6", RFC 7078, January
2014.
Appendix A. AERO Server and Relay Interworking
Figure 2 depicts a reference AERO operational scenario with a single
Server on the AERO link. In order to support scaling to larger
numbers of nodes, the AERO link can deploy multiple Servers and
Relays, e.g., as shown in Figure 4.
.-(::::::::)
.-(:::: IP ::::)-.
(:: Internetwork ::)
`-(::::::::::::)-'
`-(::::::)-'
|
+--------------+ +------+-------+ +--------------+
|AERO Server C | | AERO Relay D | |AERO Server E |
| (default->D) | | (A->C; G->E) | | (default->D) |
| (A->B) | +-------+------+ | (G->F) |
+-------+------+ | +------+-------+
| | |
X---+---+-------------------+------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
|AERO Client B | |AERO Client F |
| (default->C) | | (default->E) |
+--------------+ +--------------+
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. .-(_ IP )-.
(__ EUN ) (__ EUN )
`-(______)-' `-(______)-'
| |
+--------+ +--------+
| Host A | | Host G |
+--------+ +--------+
Figure 4: AERO Server/Relay Interworking
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In this example, Client ('B') associates with Server ('C'), while
Client ('F') associates with Server ('E'). Furthermore, Servers
('C') and ('E') do not associate with each other directly, but rather
have an association with Relay ('D') (i.e., a router that has full
topology information concerning its associated Servers and their
Clients). Relay ('D') connects to the AERO link, and also connects
to the native IP Internetwork.
When source host ('A') sends a packet toward destination host ('G'),
IP forwarding directs the packet through the EUN to Client ('B'),
which forwards the packet to Server ('C'). Server ('C') forwards
both the packet and a Predirect message through Relay ('D'). Relay
('D') then forwards both the original packet and Predirect to Server
('E'). When Server ('E') receives the packet and Predirect message,
it forwards them to Client ('F').
After processing the Predirect message, Client ('F') sends a Redirect
message to Server ('E'). Server ('E'), in turn, forwards the message
through Relay ('D') to Server ('C'). When Server ('C') receives the
Redirect message, it forwards the message to Client ('B') informing
it that host 'G's EUN can be reached via Client ('F'), thus
completing the AERO redirection.
The network-layer routing information shared between Servers and
Relays must be carefully coordinated. In particular, Relays require
full topology information, while individual Servers only require
partial topology information, i.e., they only need to know the set of
aggregated prefixes associated with the AERO link and the EUN
prefixes associated with their current set of associated Clients.
This can be accomplished in a number of ways, but a prominent example
is through the use of an internal instance of the Border Gateway
Protocol (BGP) [RFC4271] coordinated between Servers and Relays.
This internal BGP instance does not interact with the public Internet
BGP instance; therefore, the AERO link is presented to the IP
Internetwork as a small set of aggregated prefixes as opposed to the
full set of individual Client prefixes.
In a reference BGP arrangement, each AERO Server is configured as an
Autonomous System Border Router (ASBR) for a stub Autonomous System
(AS) (possibly using a private AS Number (ASN) [RFC1930]), and each
Server further peers with each Relay but does not peer with other
Servers. Each Server maintains a working set of associated Clients,
and dynamically announces new Client prefixes and withdraws departed
Client prefixes in its BGP updates. The Relays therefore discover
the full topology of the AERO link in terms of the working set of
Clients associated with each Server. Since Clients are expected to
remain associated with their current set of Servers for extended
timeframes, the amount of BGP control messaging between Servers and
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Relays should be minimal. However, Servers SHOULD dampen any route
oscillations caused by impatient Clients that repeatedly associate
and disassociate with the Server.
See [IRON] for further architectural discussion.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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