Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720, September 22, 2014
rfc6179, rfc6706 (if
approved)
Intended status: Standards Track
Expires: March 26, 2015
Transmission of IP Packets over AERO Links
draft-templin-aerolink-39.txt
Abstract
This 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
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 forwarding. 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 DHCPv6 and 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 March 26, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Asymmetric Extended Route Optimization (AERO) . . . . . . . . 6
3.1. AERO Link Reference Model . . . . . . . . . . . . . . . . 6
3.2. AERO Node Types . . . . . . . . . . . . . . . . . . . . . 7
3.3. AERO Addresses . . . . . . . . . . . . . . . . . . . . . 8
3.4. AERO Interface Characteristics . . . . . . . . . . . . . 9
3.4.1. Coordination of Multiple Underlying Interfaces . . . 11
3.5. AERO Interface Neighbor Cache Maintenace . . . . . . . . 11
3.6. AERO Interface Sending Algorithm . . . . . . . . . . . . 13
3.7. AERO Interface Encapsulation, Re-encapsulation and
Decapsulation . . . . . . . . . . . . . . . . . . . . . . 15
3.8. AERO Interface Data Origin Authentication . . . . . . . . 16
3.9. AERO Interface MTU and Fragmentation . . . . . . . . . . 17
3.10. AERO Interface Error Handling . . . . . . . . . . . . . . 20
3.11. AERO Router Discovery, Prefix Delegation and Address
Configuration . . . . . . . . . . . . . . . . . . . . . . 24
3.11.1. AERO DHCPv6 Service Model . . . . . . . . . . . . . 24
3.11.2. AERO Client Behavior . . . . . . . . . . . . . . . . 24
3.11.3. AERO Server Behavior . . . . . . . . . . . . . . . . 27
3.12. AERO Relay/Server Routing System . . . . . . . . . . . . 29
3.13. AERO Redirection . . . . . . . . . . . . . . . . . . . . 30
3.13.1. Reference Operational Scenario . . . . . . . . . . . 30
3.13.2. Concept of Operations . . . . . . . . . . . . . . . 31
3.13.3. Message Format . . . . . . . . . . . . . . . . . . . 32
3.13.4. Sending Predirects . . . . . . . . . . . . . . . . . 32
3.13.5. Re-encapsulating and Relaying Predirects . . . . . . 34
3.13.6. Processing Predirects and Sending Redirects . . . . 35
3.13.7. Re-encapsulating and Relaying Redirects . . . . . . 36
3.13.8. Processing Redirects . . . . . . . . . . . . . . . . 37
3.13.9. Server-Oriented Redirection . . . . . . . . . . . . 37
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3.14. Neighbor Unreachability Detection (NUD) . . . . . . . . . 38
3.15. Mobility Management . . . . . . . . . . . . . . . . . . . 39
3.15.1. Announcing Link-Layer Address Changes . . . . . . . 39
3.15.2. Bringing New Links Into Service . . . . . . . . . . 41
3.15.3. Removing Existing Links from Service . . . . . . . . 41
3.15.4. Moving to a New Server . . . . . . . . . . . . . . . 41
3.16. Encapsulation Protocol Version Considerations . . . . . . 42
3.17. Multicast Considerations . . . . . . . . . . . . . . . . 42
3.18. Operation on AERO Links Without DHCPv6 Services . . . . . 42
3.19. Operation on Server-less AERO Links . . . . . . . . . . . 43
3.20. Proxy AERO . . . . . . . . . . . . . . . . . . . . . . . 43
3.21. Extending AERO Links Through Security Gateways . . . . . 44
3.22. Extending IPv6 AERO Links to the Internet . . . . . . . . 46
4. Implementation Status . . . . . . . . . . . . . . . . . . . . 49
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
6. Security Considerations . . . . . . . . . . . . . . . . . . . 49
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 50
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 51
8.1. Normative References . . . . . . . . . . . . . . . . . . 51
8.2. Informative References . . . . . . . . . . . . . . . . . 52
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 55
1. Introduction
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 to 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 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 forwarding. 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 DHCPv6 and IPv6 ND message signalling is used in the control
plane, both IPv4 and IPv6 can be used in the data plane. The
remainder of this document presents the AERO specification.
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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 virtual overlay.
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
a node that is connected to an AERO link and that participates in
IPv6 ND and DHCPv6 messaging over the link.
AERO Client ("Client")
a node that applies an AERO address to an AERO interface and
receives an IP prefix via a DHCPv6 Prefix Delegation (PD) exchange
with one or more AERO Servers.
AERO Server ("Server")
a node that configures an AERO interface to provide default
forwarding and DHCPv6 services for AERO Clients. The Server
applies the IPv6 link-local subnet router anycast address (fe80::)
to the AERO interface and also applies an administratively
assigned IPv6 link-local unicast address used for operation of
DHCPv6 and the IPv6 ND protocol.
AERO Relay ("Relay")
a node that configures an AERO interface to relay IP packets
between nodes on the same AERO link and/or forward IP packets
between the AERO link and the native Internetwork. The Relay
applies an administratively assigned IPv6 link-local unicast
address to the AERO interface the same as for a Server.
ingress tunnel endpoint (ITE)
an AERO interface endpoint that injects tunneled packets into an
AERO link.
egress tunnel endpoint (ETE)
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an AERO interface endpoint that receives tunneled packets from an
AERO link.
underlying network
a connected IPv6 or IPv4 network routing region over which the
tunnel virtual overlay is configured. A typical example is an
enterprise network.
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)
an internal virtual or external edge IP network that an AERO
Client connects to the rest of the network via the AERO interface.
AERO Service Prefix (ASP)
an IP prefix associated with the AERO link and from which AERO
Client Prefixes (ACPs) are derived (for example, the IPv6 ACP
2001:db8:1:2::/64 is derived from the IPv6 ASP 2001:db8::/32).
AERO Client Prefix (ACP)
a more-specific IP prefix taken from an ASP and delegated to a
Client.
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.
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].
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3. Asymmetric Extended Route Optimization (AERO)
The following sections specify the operation of IP over Asymmetric
Extended Route Optimization (AERO) links:
3.1. AERO Link Reference Model
.-(::::::::)
.-(:::: IP ::::)-.
(:: Internetwork ::)
`-(::::::::::::)-'
`-(::::::)-'
|
+--------------+ +--------+-------+ +--------------+
|AERO Server S1| | AERO Relay R1 | |AERO Server S2|
| Nbr: C1; R1 | | Nbr: S1; S2 | | Nbr: C2; R1 |
| default->R1 | |(H1->S1; H2->S2)| | default->R1 |
| H1->C1 | +--------+-------+ | H2->C2 |
+-------+------+ | +------+-------+
| | |
X---+---+-------------------+------------------+---+---X
| AERO Link |
+-----+--------+ +--------+-----+
|AERO Client C1| |AERO Client C2|
| Nbr: S1 | | Nbr: S2 |
| default->S1 | | default->S2 |
+--------------+ +--------------+
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IP )-. .-(_ IP )-.
(__ EUN ) (__ EUN )
`-(______)-' `-(______)-'
| |
+--------+ +--------+
| Host H1| | Host H2|
+--------+ +--------+
Figure 1: AERO Link Reference Model
Figure 1 above presents the AERO link reference model. In this
model:
o Relay R1 acts as a default router for its associated Servers S1
and S2, and connects the AERO link to the rest of the IP
Internetwork
o Servers S1 and S2 associate with Relay R1 and also act as default
routers for their associated Clients C1 and C2.
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o Clients C1 and C2 associate with Servers S1 and S2, respectively
and also act as default routers for their associated EUNs
o Hosts H1 and H2 attach to the EUNs served by Clients C1 and C2,
respectively
In common operational practice, there may be many additional Relays,
Servers and Clients.
3.2. AERO Node Types
AERO Relays provide default forwarding services to AERO Servers.
Relays forward packets between Servers 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).
Each Relay advertises the ASPs for the AERO link into the native IP
Internetwork and serves as a gateway between the AERO link and the
Internetwork. AERO Relays maintain an AERO interface neighbor cache
entry for each AERO Server, and maintain an IP forwarding table entry
for each AERO Client Prefix (ACP).
AERO Servers provide default forwarding services to AERO Clients.
Each Server also peers with each Relay in a dynamic routing protocol
instance to advertise its list of associated ACPs. Servers configure
a DHCPv6 server function to facilitate Prefix Delegation (PD)
exchanges with Clients. Each delegated prefix becomes an ACP taken
from an ASP. Servers forward packets between Clients and Relays, as
well as between Clients and other Clients associated with the same
Server. AERO Servers maintain an AERO interface neighbor cache entry
for each AERO Relay. They also maintain both a neighbor cache entry
and an IP forwarding table entry for each of their associated
Clients.
AERO Clients act as requesting routers to receive ACPs through DHCPv6
PD exchanges with AERO Servers over the AERO link and sub-delegate
portions of their ACPs to EUN interfaces. (Each Client MAY associate
with a single Server or with multiple Servers, e.g., for fault
tolerance and/or load balancing.) 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. AERO Clients
maintain an AERO interface neighbor cache entry for each of their
associated Servers as well as for each of their correspondent
Clients.
AERO Clients that act as hosts typically configure a TUN/TAP
interface as a point-to-point linkage between the IP layer and the
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AERO interface. The IP layer therefore sees only the TUN/TAP
interface, while the AERO interface provides an intermediate conduit
between the TUN/TAP interface and the underlying interfaces. AERO
Clients that act as hosts assign one or more IP addresses from their
ACPs to the TUN/TAP interface.
3.3. AERO Addresses
An AERO address is an IPv6 link-local address with an embedded ACP
and assigned to a Client's AERO interface. The AERO address is
formed as follows:
fe80::[ACP]
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 IPv6 ACP. The base prefix is determined by masking the ACP
with the prefix length. For example, if the AERO Client receives the
IPv6 ACP:
2001:db8:1000:2000::/56
it constructs its AERO address as:
fe80::2001:db8:1000:2000
For IPv4, the AERO address is formed from the lower 64 bits of an
IPv4-mapped IPv6 address [RFC4291] that includes the base prefix
taken from the Client's IPv4 ACP. For example, if the AERO Client
receives the IPv4 ACP:
192.0.2.32/28
it constructs its AERO address as:
fe80::FFFF: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 advanced
knowledge of the Client's ACP length and may therefore send initial
IPv6 ND messages with an AERO destination address that matches the
ACP 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 ACP
2001:db8:1000:2000::/56 then subsequently receives an IPv6 ND message
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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.
3.4. 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 coordinate 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.
AERO interfaces maintain a neighbor cache, and AERO Clients and
Servers 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, AERO nodes
ignore any Prefix Information Options (PIOs) they may receive in RA
messages over an AERO interface.
AERO interface ND messages include one or more Target Link-Layer
Address Options (TLLAOs) formatted as shown in Figure 2:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-- --+
| |
+-- IP Address --+
| |
+-- --+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: AERO Target Link-Layer Address Option (TLLAO) Format
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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 255 being the
highest preference, 1 being the lowest, and 0 meaning "link
disabled"). 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 the encapsulation IP address family is
IPv4, IP Address is formed as an IPv4-mapped IPv6 address [RFC4291].
When a Relay enables an AERO interface, it assigns an
administratively assigned link-local address fe80::ID to the
interface. Each fe80::ID address MUST be unique among all Relays and
Servers on the link, and MUST NOT collide with any potential AERO
addresses. The addresses are typically taken from the range
fe80::/96, e.g., as fe80::1, fe80::2, fe80::3, etc. The Relay also
maintains an IP forwarding table entry for each Client-Server
association and maintains a neighbor cache entry for each Server on
the link. Relays do not require the use of IPv6 ND messaging for
reachability determination since Relays and Servers engage in a
dynamic routing protocol over the AERO interface. At a minimum,
however, Relays respond to NS messages by returning an NA.
When a Server enables an AERO interface, it assigns the address
fe80:: to the interface as a link-local Subnet Router Anycast
address, and also assigns an administratively assigned link-local
address fe80::ID the same as for Relays. (The Server then accepts
DHCPv6 and IPv6 ND solicitation messages destined to either the
fe80:: or fe80::ID addresses, but always uses fe80::ID as the source
address in the replies it generates.) The Server further configures
a DHCPv6 server function to facilitate DHCPv6 PD exchanges with AERO
Clients. The Server maintains a neighbor cache entry for each Relay
on the link, and manages per-Client neighbor cache entries and IP
forwarding table entries based on DHCPv6 exchanges. When the Server
receives an NS/RS message on the AERO interface it returns an NA/RA
message but does not update the neighbor cache. Each Server also
engages in a dynamic routing protocol with all Relays on the link.
Finally, the Server provides a simple conduit between Clients and
Relays, or between Clients and other Clients. Therefore, packets
enter the Server's AERO interface from the link layer and are
forwarded back out the link layer without ever leaving the AERO
interface and therefore without ever disturbing the network layer.
When a Client enables an AERO interface, it invokes DHCPv6 PD to
receive an ACP from an AERO Server. Next, it assigns the
corresponding AERO address to the AERO interface and creates a
neighbor cache entry for the Server, i.e., the PD exchange bootstraps
the provisioning of a unique link-local address. The Client
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maintains a neighbor cache entry for each of its Servers and each of
its active correspondent Clients. When the Client receives Redirect/
Predirect messages on the AERO interface it updates or creates
neighbor cache entries, including link-layer address information.
Unsolicited NA messages update the cached link-layer addresses for
correspondent Clients (e.g., following a link-layer address change
due to node mobility) but do not create new neighbor cache entries.
NS/NA messages used for Neighbor Unreachability Detection (NUD)
update timers in existing neighbor cache entires but do not update
link-layer addresses nor create new neighbor cache entries. Finally,
the Client need not maintain any IP forwarding table entries for its
Servers or correspondent Clients. Instead, it can set a single
"route-to-interface" default route in the IP forwarding table
pointing to the AERO interface, and all forwarding decisions can be
made within the AERO interface based on neighbor cache entries. (On
systems in which adding a default route would violate security
policy, the default route could instead be installed via a
"synthesized RA", e.g., as discussed in Section 3.11.2.)
3.4.1. Coordination of Multiple Underlying Interfaces
AERO interfaces may be configured over multiple underlying
interfaces. 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, 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 Redirect, Predirect and unsolicited NA messages
include only a single TLLAO with Link ID set to a constant value.
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,
Redirect, Predirect and unsolicited NA messages MAY include multiple
TLLAOs -- each with a different Link ID that corresponds to a
specific underlying interface of the Client.
3.5. 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]. AERO interface
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neighbor cache entires are said to be one of "permanent", "static" 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 a permanent
neighbor cache entry for each Server on the link, and AERO Servers
maintain a permanent neighbor cache entry for each Relay on the link.
Static neighbor cache entries are created though DHCPv6 PD exchanges
and remain in place for durations bounded by prefix lifetimes. AERO
Servers maintain a static neighbor cache entry for each of their
associated Clients, and AERO Clients maintain a static neighbor cache
for each of their associated Servers. When an AERO Server sends a
DHCPv6 Reply message response to a Client's DHCPv6 Solicit/Request or
Renew message, it creates or updates a static neighbor cache entry
based on the Client's AERO address as the network-layer address, the
prefix lifetime as the neighbor cache entry lifetime, the Client's
encapsulation IP address and UDP port number as the link-layer
address and the prefix length as the length to apply to the AERO
address. When an AERO Client receives a DHCPv6 Reply message from a
Server, it creates or updates a static neighbor cache entry based on
the Reply message link-local source address as the network-layer
address, the prefix lifetime as the neighbor cache entry lifetime,
and the encapsulation IP source address and UDP source port number as
the link-layer address.
Dynamic neighbor cache entries are created based on receipt of an
IPv6 ND message, and are garbage-collected if not used within a short
timescale. AERO Clients maintain dynamic neighbor cache entries for
each of their active correspondent Clients with lifetimes based on
IPv6 ND messaging constants. When an AERO Client receives a valid
Predirect message it creates or updates a dynamic neighbor cache
entry for the Predirect target network-layer and link-layer addresses
plus prefix length. The node then sets an "AcceptTime" variable in
the neighbor cache entry and uses this value to determine whether
packets received from the correspondent can be accepted. When an
AERO Client receives a valid Redirect message it creates or updates a
dynamic neighbor cache entry for the Redirect target network-layer
and link-layer addresses plus prefix length. The Client then sets a
"ForwardTime" variable in the neighbor cache entry and uses this
value to determine whether packets can be sent directly to the
correspondent. The Client also maintains a "MaxRetry" variable to
limit the number of keepalives sent when a correspondent may have
gone unreachable.
For dynamic neighbor cache entries, when an AERO Client receives a
valid NS message it (re)sets AcceptTime for the neighbor to
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ACCEPT_TIME. When an AERO Client receives a valid solicited NA
message, it (re)sets ForwardTime for the neighbor to FORWARD_TIME and
sets MaxRetry to MAX_RETRY. When an AERO Client receives a valid
unsolicited NA message, it updates the correspondent's link-layer
addresses but DOES NOT reset AcceptTime, ForwardTime or MaxRetry.
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].
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.
Most importantly, ACCEPT_TIME SHOULD be set to a value that is
sufficiently longer than FORWARD_TIME to allow the AERO redirection
procedure to converge.
3.6. AERO Interface Sending Algorithm
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 admitted into the AERO link, i.e., they are
tunnelled to an AERO interface neighbor. Packets that enter the AERO
interface from the link layer are either re-admitted into the AERO
link or delivered to the network layer where they are subject to
either local delivery or IP forwarding. Since each AERO node has
only partial information about neighbors on the link, AERO interfaces
may forward packets with link-local destination addresses at a layer
below the network layer. This means that AERO nodes act as both IP
routers and sub-IP layer forwarding agents. AERO interface sending
considerations for Clients, Servers and Relays are given below.
When an IP packet enters a Client's AERO interface from the network
layer, if the destination is covered by an ASP the Client searches
for a dynamic neighbor cache entry with a non-zero ForwardTime and an
AERO address that matches the packet's destination address. (The
destination address may be either an address covered by the
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neighbor's ACP or the (link-local) AERO address itself.) If there is
a match, the Client uses a link-layer address in the entry as the
link-layer address for encapsulation then admits the packet into the
AERO link. If there is no match, the Client instead uses the link-
layer address of a neighboring Server as the link-layer address for
encapsulation.
When an IP packet enters a Server's AERO interface from the link
layer, if the destination is covered by an ASP the Server searches
for a static neighbor cache entry with an AERO address that matches
the packet's destination address. (The destination address may be
either an address covered by the neighbor's ACP or the AERO address
itself.) If there is a match, the Server uses a link-layer address
in the entry as the link-layer address for encapsulation and re-
admits the packet into the AERO link. If there is no match, the
Server instead uses the link-layer address in any permanent neighbor
cache entry as the link-layer address for encapsulation.
When an IP packet enters a Relay's AERO interface from the network
layer, the Relay searches its IP forwarding table for an entry that
is covered by an ASP and also matches the destination. If there is a
match, the Relay uses the link-layer address in the neighbor cache
entry for the next-hop Server as the link-layer address for
encapsulation and admits the packet into the AERO link. When an IP
packet enters a Relay's AERO interface from the link-layer, if the
destination is not a link-local address and is does not match an ASP
the Relay removes the packet from the AERO interface and uses IP
forwarding to forward the packet to the Internetwork. If the
destination address is a link-local or non-link-local address that
matches an ASP, and there is a more-specific ACP entry in the IP
forwarding table, the Relay uses the link-layer address in the
corresponding neighbor cache entry for the next-hop Server as the
link-layer address for encapsulation and re-admits the packet into
the AERO link. When an IP packet enters a Relay's AERO interface
from either the network layer or link-layer, and the packet's
destination address matches an ASP but there is no more-specific ACP
entry, the Relay drops the packet and returns an ICMP Destination
Unreachable message (see: Section 3.10).
When an AERO Server receives a packet from a Relay via the AERO
interface, the Server MUST NOT forward the packet back to the same or
a different Relay.
When an AERO Relay receives a packet from a Server via the AERO
interface, the Relay MUST NOT forward the packet back to the same
Server.
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When an AERO node re-admits a packet into the AERO link without
involving the network layer, the node MUST NOT decrement the network
layer TTL/Hop-count.
Note that in the above that the link-layer address for encapsulation
may be determined through consulting either the neighbor cache or the
IP forwarding table. IP forwarding is therefore linked to IPv6 ND
via the AERO address.
3.7. AERO Interface Encapsulation, Re-encapsulation and Decapsulation
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".
AERO interfaces encapsulate packets per the base tunneling
specifications (e.g., [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246],
etc.) 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).
The AERO interface encapsulates the packet per the base tunneling
specification 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 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. For packets sent to a correspondent Client, the AERO interface
sets the UDP destination port to the port value stored in the
neighbor cache entry for this correspondent. The AERO interface also
sets the UDP checksum field to zero (see: [RFC6935][RFC6936]) unless
an integrity check is required (see: Section 3.9).
The AERO interface next sets the IP protocol number in the
encapsulation header to 17 (i.e., the IP protocol number for UDP).
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
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protocol, the AERO interface sets the DF bit as discussed in
Section 3.9.
AERO interfaces decapsulate packets destined either to the node
itself or to a destination reached via an interface other than the
AERO interface the packet was received on. When the AERO interface
receives a UDP packet, it examines the first octet of the
encapsulated packet. The packet is accepted 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). Otherwise, the packet is accepted if the
first octet encodes a valid IP protocol number per the IANA
"protocol-numbers" registry that matches a supported encapsulation
type. Otherwise, the packet is discarded.
Further decapsulation then proceeds according to the appropriate base
tunneling specification.
3.8. AERO Interface Data Origin Authentication
AERO nodes employ simple data origin authentication procedures for
encapsulated packets they receive from other nodes on the AERO link.
In particular:
o AERO Relays and Servers accept encapsulated packets with a link-
layer source address that matches a permanent neighbor cache
entry.
o AERO Servers accept authentic encapsulated DHCPv6 messages, and
create or update a static neighbor cache entry for the source
based on the specific message type.
o AERO Servers accept encapsulated packets if there is a static
neighbor cache entry with an AERO address that matches the
packet's network-layer source address and with a link-layer
address that matches the packet's link-layer source address.
o AERO Clients accept encapsulated packets if there is a static
neighbor cache entry with a link-layer source address that matches
the packet's link-layer source address.
o AERO Clients and Servers accept encapsulated packets if there is a
dynamic neighbor cache entry with an AERO address that matches the
packet's network-layer source address, with a link-layer address
that matches the packet's link-layer source address, and with a
non-zero AcceptTime.
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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.
3.9. AERO Interface MTU and Fragmentation
The AERO interface is the node's point of attachment to the AERO
link. AERO links over IP networks have a maximum link MTU of 64KB
minus the encapsulation overhead (i.e., "64KB-ENCAPS"), since the
maximum packet size in the base IP specifications is 64KB
[RFC0791][RFC2460]. AERO interfaces therefore set a maximum MTU of
64KB-ENCAPS. (Note that AERO links over IPv6 networks have a
theoretical maximum link MTU of 4GB-ENCAPS [RFC2675], however IPv6
Jumbograms are considered optional for IPv6 nodes [RFC6434] and
therefore out of scope for this document.)
IPv6 specifies a minimum link MTU of 1280 bytes [RFC2460]. This is
the minimum packet size an AERO interface MUST be capable of
forwarding without returning an ICMP Packet Too Big (PTB) message.
Although IPv4 specifies a smaller minimum link MTU of 68 bytes
[RFC0791], AERO interfaces also observe a 1280 byte minimum for IPv4.
Additionally, the vast majority of links in the Internet configure an
MTU of at least 1500 bytes. Hosts have therefore become conditioned
to expect that IP packets up to 1500 bytes in length will either be
delivered to the final destination or a suitable ICMP Packet Too Big
(PTB) message returned, however such PTB messages are often lost
[RFC2923]. Therefore, AERO interfaces MUST set a minimum MTU of 1500
bytes, meaning that they MUST pass IP packets of at least 1500 bytes
even if some fragmentation is necessary.
PTB messages may be generated by the IP layer of the AERO node if the
packet is too large to enter the AERO interface, from within the AERO
interface itself if the packet is larger than 1500 bytes and also
larger than the MTU of the underlying interface to be used for
tunneling minus ENCAPS, or from a router within the AERO link (i.e.,
the "tunnel") after the encapsulated packet has been admitted. The
latter condition would result in a link-layer (L2) PTB message
delivered to the AERO interface, while the former two conditions
would result in a network-layer (L3) PTB message delivered to the
original source.
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
dangerous reassembly misassociations [RFC6864][RFC4963]. For AERO
links over both IPv4 and IPv6, studies have shown that IP fragments
may be dropped unconditionally over some Internet paths [I-D.taylor-
v6ops-fragdrop]. For these reasons, when fragmentation is needed it
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is performed within the AERO interface itself before the fragments
are encapsulated and admitted into the tunnel. This fragmentation is
supported through the insertion of an IPv6 Fragment Header [RFC2460]
that is not associated with either the encapsulating nor encapsulated
IP headers. Since the IPv6 Fragment Header reduces the room
available for packet data, but the source node has no way to control
its insertion, the Fragment Header length MUST be included in
"ENCAPS" even for packets in which the header does not appear.
The AERO interface therefore admits encapsulated packets into the
tunnel (using fragmentation as necessary) as follows:
o For IP packets that are no larger than (1280-ENCAPS) bytes, the
AERO interface admits the packet into the tunnel without
fragmentation. For IPv4 AERO links, the AERO interface sets the
Don't Fragment (DF) bit to 0 so that these packets will be
deterministically delivered even if there is a restricting link in
the path and also calculates the UDP checksum over the
encapsulated packet. The tunnel egress will then verify the
checksum as an integrity check to detect reassembly errors.
o For IP packets that are larger than (1280-ENCAPS) bytes but no
larger than 1500 bytes, the AERO interface prepends an IPv6
Fragment Header before the packet header. Next, the AERO
interface uses the fragmentation algorithm in [RFC2460] to break
the packet into two pieces where the first piece is no larger than
1024 bytes and the second piece is no larger than the first.
(This fragmentation is conducted without a leading IPv6 header;
hence, the AERO interface must keep track of the fragment lengths
through some other means.) The AERO interface then encapsulates
both pieces (and for IPv4 sets the DF bit to 0 and calculates the
UDP checksum) then admits them into the tunnel.
o For IPv4 packets that are larger than 1500 bytes and with the DF
bit set to 0, the AERO interface uses ordinary IP fragmentation to
break the packet into a minimum number of fragments where the
first fragment is no larger than 1024 bytes and all other
fragments are no larger than the first fragment. The AERO
interface then encapsulates each fragment (and for IPv4 sets the
DF bit to 0 and calculates the UDP checksum) then admits the
fragments into the tunnel. These encapsulated fragments will be
deterministically delivered to the final destination.
o For all other IP packets, if the packet is larger than the AERO
interface MTU the AERO node drops the packet and returns an L3 PTB
message with MTU set to the AERO interface MTU; otherwise, the
node admits the packet into the AERO interface. Next, if the
packet length is larger than the MTU of the underlying interface
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to be used for tunneling minus ENCAPS, the AERO interface drops
the packet and returns an L3 PTB message with MTU set to the
larger of 1500 or the underlying interface MTU minus ENCAPS.
Otherwise, the AERO interface encapsulates the packet and admits
it into the tunnel without fragmentation (and for IPv4 sets the DF
bit to 1) and translates any L2 PTB messages it may receive from
the network into corresponding L3 PTB messages to send to the
original source as specified in Section 3.10. Since both L2 and
L3 PTB messages may be either lost or contain insufficient
information, however, it is RECOMMENDED that sources that send
unfragmentable IP packets larger than 1500 bytes use Packetization
Layer Path MTU Discovery (PLPMTUD) [RFC4821].
While sending packets according to the above specifications, the
source AERO interface (i.e., the tunnel ingress) MAY also send 1500
byte probe packets to the destination AERO interface (i.e., the
tunnel egress) to determine whether the probes can traverse the
tunnel without fragmentation. If the probes succeed, the tunnel
ingress can begin sending packets that are larger than 1280-ENCAPS
bytes but no larger than 1500 bytes without fragmentation (and for
IPv4 with DF set to 1). Since the path MTU within the tunnel may
fluctuate due to routing changes, the tunnel ingress SHOULD
continually send additional probes subject to rate limiting in case
L2 PTB messages are lost. If the path MTU within the tunnel later
becomes insufficient, the tunnel ingress MUST resume fragmentation.
To construct a probe, the tunnel ingress 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 tunnel ingress then encapsulates the padded NS
message in the encapsulation headers (and for IPv4 sets DF to 1) then
sends the message to the tunnel egress. If the tunnel egress returns
a solicited NA message with a matching Nonce option, the tunnel
ingress deems the probe successful. Note that the tunnel ingress
SHOULD NOT include the trailing padding 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.
When the tunnel egress receives the fragments of a fragmented packet,
it reassembles them into a whole packet per the reassembly algorithm
in [RFC2460] then discards the IPv6 fragment header. The tunnel
egress MUST be capable of reassembling packets up to 1500+ENCAPS
bytes in length, hence it is RECOMMENDED that the tunnel egress be
capable of reassembling at least 2KB.
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As an exception to the above procedures, IPv6 ND and DHCPv6 messages
of all sizes MUST be accommodated even if some fragmentation is
necessary. These packets are therefore accommodated through a
modification of the second rule in the above algorithm as follows:
o For IPv6 ND and DHCPv6 messages that are larger than (1280-ENCAPS)
bytes, the AERO interface prepends an IPv6 Fragment Header before
the message header. Next, the AERO interface uses the
fragmentation algorithm in [RFC2460] to break the packet into a
minimum number of pieces where the first piece is no larger than
1024 bytes and the remaining pieces are no larger than the first.
The AERO interface then encapsulates both pieces (and for IPv4
sets the DF bit to 0 and calculates the UDP checksum) and admits
them into the tunnel.
In that case, the tunnel egress MAY be required to reassemble
fragmented IPv6 ND or DHCPv6 messages that are larger than 2KB-ENCAPS
but no larger than 64KB-ENCAPS.
3.10. AERO Interface Error Handling
When an AERO node admits encapsulated packets into the AERO
interface, it may receive link-layer (L2) or network-layer (L3) error
indications.
An L2 error indication is an ICMP error message generated by a router
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. For ICMPv6 [RFC4443], the error Types
include "Destination Unreachable", "Packet Too Big (PTB)", "Time
Exceeded" and "Parameter Problem". For ICMPv4 [RFC0792], the error
Types include "Destination Unreachable", "Fragmentation Needed" (a
Destination Unreachable Code that is analogous to the ICMPv6 PTB),
"Time Exceeded" and "Parameter Problem".
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, [RFC4443] 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, [RFC0792] specifies that the packet-in-error includes:
"Internet Header + 64 bits of Original Data Datagram", however
[RFC1812] Section 4.3.2.3 updates this specification by stating: "the
ICMP datagram SHOULD contain as much of the original datagram as
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possible without the length of the ICMP datagram exceeding 576
bytes".
The L2 error message format is shown in Figure 3:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| L2 IP Header of |
| error message |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2 ICMP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
~ ~ P
| IP and other encapsulation | a
| headers of original L3 packet | c
~ ~ k
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ e
~ ~ t
| IP header of |
| original L3 packet | i
~ ~ n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~ e
| Upper layer headers and | r
| leading portion of body | r
| of the original L3 packet | o
~ ~ r
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
Figure 3: AERO Interface L2 Error Message Format
The AERO node rules for processing these L2 error messages is as
follows:
o When an AERO node receives an L2 "Parameter Problem", it processes
the message the same as described as for ordinary ICMP errors in
the normative references [RFC0792][RFC4443].
o When an AERO node receives persistent L2 Time Exceeded messages,
the IP ID field may be wrapping before earlier fragments have been
processed due to high data rates. Since the AERO node includes a
UDP integrity check, however, it MAY ignore the messages and
continue sending packets without rate limiting.
o When an AERO Client receives persistent L2 Destination Unreachable
messages in response to tunneled packets that it sends to one of
its dynamic neighbor correspondents, the Client SHOULD test the
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path to the correspondent using Neighbor Unreachability Detection
(NUD) (see Section 3.14). If NUD fails, the Client SHOULD set
ForwardTime for the corresponding dynamic neighbor cache entry to
0 and allow future packets destined to the correspondent to flow
through a Server.
o When an AERO Client receives persistent L2 Destination Unreachable
messages in response to tunneled packets that it sends to one of
its static neighbor Servers, the Client SHOULD test the path to
the Server using NUD. If NUD fails, the Client SHOULD delete the
neighbor cache entry and attempt to associate with a new Server.
o When an AERO Server receives persistent L2 Destination Unreachable
messages in response to tunneled packets that it sends to one of
its static neighbor Clients, the Server SHOULD test the path to
the Client using NUD. If NUD fails, the Server SHOULD cancel the
DHCPv6 PD lease for the Client's ACP, withdraw its route for the
ACP from the AERO routing system and delete the neighbor cache
entry (see Sections 3.11 and 3.12).
o When an AERO Relay or Server receives an L2 Destination
Unreachable message in response to a tunneled packet that it sends
to one of its permanent neighbors, it discards the message since
the routing system is likely in a temporary transitional state
that will soon re-converge.
o When an AERO node receives an L2 PTB message, it translates the
message into an L3 PTB message if possible (*) and forwards the
message toward the original source as described below.
To translate an L2 PTB message to an L3 PTB message, the AERO node
first caches the MTU field value of the L2 ICMP header. The node
next discards the L2 IP and ICMP headers, and also discards the
encapsulation headers of the original L3 packet. Next the node
encapsulates the included segment of the original L3 packet in an L3
IP and ICMP header, and sets the ICMP header Type and Code values to
appropriate values for the L3 IP protocol. In the process, the node
writes the maximum of 1500 bytes and (L2 MTU - ENCAPS) into the MTU
field of the L3 ICMP header.
The node next writes the IP source address of the original L3 packet
as the destination address of the L3 PTB message and determines the
next hop to the destination. If the next hop is reached via the AERO
interface, the node uses the IPv6 address "::" or the IPv4 address
"0.0.0.0" as the IP source address of the L3 PTB message. Otherwise,
the node uses one of its non link-local addresses as the source
address of the L3 PTB message. The node finally calculates the ICMP
checksum over the L3 PTB message and writes the Checksum in the
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corresponding field of the L3 ICMP header. The L3 PTB message
therefore is formatted as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| L3 IP Header of |
| error message |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L3 ICMP Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
~ ~ p
| IP header of | k
| original L3 packet | t
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ i
~ ~ n
| Upper layer headers and |
| leading portion of body | e
| of the original L3 packet | r
~ ~ r
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
Figure 4: AERO Interface L3 Error Message Format
After the node has prepared the L3 PTB message, it either forwards
the message via a link outside of the AERO interface without
encapsulation, or encapsulates and forwards the message to the next
hop via the AERO interface.
When an AERO Relay receives an L3 packet for which the 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 an L3 Destination Unreachable message. The Relay first
writes the IP source address of the original L3 packet as the
destination address of the L3 Destination Unreachable message and
determines the next hop to the destination. If the next hop is
reached via the AERO interface, the Relay uses the IPv6 address "::"
or the IPv4 address "0.0.0.0" as the IP source address of the L3
Destination Unreachable message and forwards the message to the next
hop within the AERO interface. Otherwise, the Relay uses one of its
non link-local addresses as the source address of the L3 Destination
Unreachable message and forwards the message via a link outside the
AERO interface.
When an AERO node receives any L3 error message via the AERO
interface, it examines the destination address in the L3 IP header of
the message. If the next hop toward the destination address of the
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error message is via the AERO interface, the node re-encapsulates and
forwards the message to the next hop within the AERO interface.
Otherwise, if the source address in the L3 IP header of the message
is the IPv6 address "::" or the IPv4 address "0.0.0.0", the node
writes one of its non link-local addresses as the source address of
the L3 message and recalculates the IP and/or ICMP checksums. The
node finally forwards the message via a link outside of the AERO
interface.
(*) Note that in some instances the packet-in-error field of an L2
PTB message may not include enough information for translation to an
L3 PTB message. In that case, the AERO interface simply discards the
L2 PTB message. It can therefore be said that translation of L2 PTB
messages to L3 PTB messages can provide a useful optimization when
possible, but is not critical for sources that correctly use PLPMTUD.
3.11. AERO Router Discovery, Prefix Delegation and Address
Configuration
3.11.1. AERO DHCPv6 Service Model
Each AERO Server configures a DHCPv6 server function to facilitate PD
requests from Clients. Each Server is pre-configured with an
identical list of ACP-to-Client ID mappings for all Clients enrolled
in the AERO system, as well as any information necessary to
authenticate Clients. The configuration information is maintained by
a central administrative authority for the AERO link and securely
propagated to all Servers whenever a new Client is enrolled or an
existing Client is withdrawn.
With these identical configurations, each Server can function
independently of all other Servers, including the maintenance of
active leases. Therefore, no Server-to-Server DHCPv6 state
synchronization is necessary, and Clients can optionally hold
separate leases for the same ACP from multiple Servers.
In this way, Clients can easily associate with multiple Servers, and
can receive new leases from new Servers before deprecating leases
held through old Servers. This enables a graceful "make-before-
break" capability.
3.11.2. 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 "linkupnetworks" is a constant text string and "[domainname]"
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is the connection-specific DNS suffix for the Client's underlying
network connection (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 an ACP through a DHCPv6 PD exchange[RFC3315][RFC3633] in
which the Client's Solicit/Request messages use the IPv6
"unspecified" address (i.e., "::") as the IPv6 source address,
'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address
and the link-layer address of the Server as the link-layer
destination address. The Client also includes a Client Identifier
option with a DHCP Unique Identifier (DUID) plus any necessary
authentication options to identify itself to the DHCPv6 server, and
includes a Client Link Layer Address Option (CLLAO) [RFC6939] with
the format shown in Figure 5:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_CLIENT_LINKLAYER_ADDR | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| link-layer type (16 bits) | Link ID | Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: AERO Client Link-Layer Address Option (CLLAO) Format
The Client sets the CLLAO 'option-length' field to 4 and sets the
'link-layer type' field to TBD1 (see: IANA Considerations), then
includes appropriate Link ID and Preference values for the underlying
interface over which the Solicit/Request will be issued (note that
these are the same values that would be included in a TLLAO as shown
in Figure 2). If the Client is pre-provisioned with an ACP
associated with the AERO service, it MAY also include the ACP in the
Solicit/Request message Identity Association (IA) option to indicate
its preferred ACP to the DHCPv6 server. The Client then sends the
encapsulated DHCPv6 request via the underlying interface.
When the Client receives its ACP and the set of ASPs via a DHCPv6
Reply from the AERO Server, it creates a static neighbor cache entry
with the Server's link-local address as the network-layer address and
the Server's encapsulation address as the link-layer address. The
Client then records the lifetime for the ACP in the neighbor cache
entry and marks the neighbor cache entry as "default", i.e., the
Client considers the Server as a default router. If the Reply
message contains a Vendor-Specific Information Option (see:
Section 3.10.3) the Client also caches each ASP in the option.
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The Client then applies the AERO address to the AERO interface and
sub-delegates the ACP to nodes and links within its attached EUNs
(the AERO address thereafter remains stable as the Client moves).
The Client also assigns a default IP route to the AERO interface as a
route-to-interface, i.e., with no explicit next-hop. The next hop
will then be determined after a packet has been submitted to the AERO
interface by inspecting the neighbor cache (see above).
On some platforms (e.g., popular cell phone operating systems), the
act of assigning a default IPv6 route to the AERO interface may not
be permitted from a user application due to security policy.
Typically, those platforms include a TUN/TAP interface that acts as a
point-to-point conduit between user applications and the AERO
interface. In that case, the Client can instead generate a
"synthesized RA" message. The message conforms to [RFC4861] and is
prepared as follows:
o the IPv6 source address is fe80::
o the IPv6 destination address is all-nodes multicast
o the Router Lifetime is set to a time that is no longer than the
ACP DHCPv6 lifetime
o the message does not include a Source Link Layer Address Option
(SLLAO)
o the message includes a Prefix Information Option (PIO) with a /64
prefix taken from the ACP as the prefix for autoconfiguration
The Client then sends the synthesized RA message via the TUN/TAP
interface, where the operating system kernel will interpret it as
though it were generated by an actual router. The operating system
will then install a default route and use StateLess Address
AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP
interface. Methods for similarly installing an IPv4 default route
and IPv4 address on the TUN/TAP interface are based on synthesized
DHCPv4 messages [RFC2131]. Note that in this method, the Client
appears as a mobility proxy for applications that bind to the (point-
to-point) TUN/TAP interface. The arrangement can be likened to a
Proxy AERO scenario in which the mobile node and Client are located
within the same physical platform (see Section 3.20 for further
details on Proxy AERO).
The Client subsequently renews its ACP delegation through each of its
Servers 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,
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the link-layer address of a Server as the link-layer destination
address and the same Client identifier, authentication options and
CLLAO option as was used in the initial PD request. Note that if the
Client does not issue a DHCPv6 Renew before the Server has terminated
the lease (e.g., if the Client has been out of touch with the Server
for a considerable amount of time), the Server's Reply will report
NoBinding and the Client must re-initiate the DHCPv6 PD procedure.
If the Client sends synthesized RA and/or DHCPv4 messages (see
above), it also sends a new synthesized message when issuing a DHCPv6
Renew or when re-initiating the DHCPv6 PD procedure.
Since the Client's AERO address is configured from the unique ACP
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).
AERO Clients ignore the IP address and UDP port number in any S/TLLAO
options in ND messages they receive directly from another AERO
Client, but examine the Link ID and Preference values to match the
message with the correct link-layer address information.
When a source Client forwards a packet to a prospective destination
Client (i.e., one for which the packet's destination address is
covered by an ASP), the source Client initiates an AERO route
optimization procedure as specified in Section 3.13.
3.11.3. AERO Server Behavior
AERO Servers configure a DHCPv6 server 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 receives a prospective Client's DHCPv6 PD
Solicit/Request message, it first authenticates the message. If
authentication succeeds, the Server determines the correct ACP to
delegate to the Client by matching the Client's DUID within an online
directory service (e.g., LDAP). The Server then delegates the ACP
and creates a static neighbor cache entry for the Client's AERO
address with lifetime set to no more than the lease lifetime and the
Client's link-layer address as the link-layer address for the Link ID
specified in the CLLAO option. The Server then creates an IP
forwarding table entry so that the AERO routing system will propagate
the ACP to all Relays (see: Section 3.12). Finally, the Server sends
a DHCPv6 Reply message to the Client while using fe80::ID as the IPv6
source address, the Client's AERO address as the IPv6 destination
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address, and the Client's link-layer address as the destination link-
layer address. The Server also includes a Server Unicast option with
server-address set to fe80::ID so that all future Client/Server
transactions will be link-local-only unicast over the AERO link.
When the Server sends the DHCPv6 Reply message, it also includes a
DHCPv6 Vendor-Specific Information Option with 'enterprise-number'
set to "TBD2" (see: IANA Considerations). The option is formatted as
shown in[RFC3315] and with the AERO enterprise-specific format shown
in Figure 6:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_VENDOR_OPTS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| enterprise-number ("TBD2") |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ASP (1) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ASP (2) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ASP (3) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. (etc.) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: AERO Vendor-Specific Information Option
Per Figure 6, the option includes one or more ASP. The ASP field
contains the IP prefix as it would appear in the interface identifier
portion of the corresponding AERO address (see: Section 3.3). For
IPv6, valid values for the Prefix Length field are 0 through 64; for
IPv4, valid values are 0 through 32.
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After the initial DHCPv6 PD exchange, the AERO Server maintains the
neighbor cache entry for the Client as long as the lease lifetime
remains current. If the Client issues a Renew/Reply exchange, the
Server extends the lifetime. If the Client issues a Release/Reply
exchange, or if the Client does not issue a Renew/Reply within the
lease lifetime, the Server deletes the neighbor cache entry for the
Client and withdraws the IP route from the AERO routing system.
3.12. AERO Relay/Server Routing System
Relays require full topology information of all Client/Server
associations, while individual Servers only require partial topology
information, i.e., they only need to know the ACPs associated with
their current set of associated Clients. This is accomplished
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 ASPs as opposed to the full set of
individual ACPs.
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. Similarly, Relays need not peer with each other, since they
will receive all updates from all Servers and will therefore have a
consistent view of the AERO link ACP delegations.
Each Server maintains a working set of associated Clients, and
dynamically announces new ACPs and withdraws departed ACPs in its BGP
updates to Relays (this is typically accomplished via a "redistribute
static" routing directive). Relays do not send BGP updates to
Servers, however, such that the BGP route reporting is unidirectional
from the Servers to the Relays.
The Relays therefore discover the full topology of the AERO link in
terms of the working set of ACPs associated with each Server, while
the Servers only discover the ACPs of their associated Clients.
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 Relays should be minimal. However, BGP
peers SHOULD dampen any route oscillations caused by impatient
Clients that repeatedly associate and disassociate with Servers.
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3.13. AERO Redirection
3.13.1. Reference Operational Scenario
Figure 7 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 Relay ('R1'), two AERO Servers ('S1', 'S2'),
two AERO Clients ('C1', 'C2') and two ordinary IPv6 hosts ('H1',
'H2'):
+--------------+ +--------------+ +--------------+
| Server S1 | | Relay R1 | | Server S2 |
+--------------+ +--------------+ +--------------+
fe80::2 fe80::1 fe80::3
L2(S1) L2(R1) L2(S2)
| | |
X-----+-----+------------------+-----------------+----+----X
| AERO Link |
L2(A) L2(B)
fe80::2001:db8:0:0 fe80::2001:db8:1:0
+--------------+ +--------------+
|AERO Client C1| |AERO Client C2|
+--------------+ +--------------+
2001:DB8:0::/48 2001:DB8:1::/48
| |
.-. .-.
,-( _)-. 2001:db8:0::1 2001:db8:1::1 ,-( _)-.
.-(_ IP )-. +---------+ +---------+ .-(_ IP )-.
(__ EUN )--| Host H1 | | Host H2 |--(__ EUN )
`-(______)-' +---------+ +---------+ `-(______)-'
Figure 7: AERO Reference Operational Scenario
In Figure 7, Relay ('R1') applies the address fe80::1 to its AERO
interface with link-layer address L2(R1), Server ('S1') applies the
address fe80::2 with link-layer address L2(S1),and Server ('S2')
applies the address fe80::3 with link-layer address L2(S2). Servers
('S1') and ('S2') next arrange to add their link-layer addresses to a
published list of valid Servers for the AERO link.
AERO Client ('C1') receives the ACP 2001:db8:0::/48 in a DHCPv6 PD
exchange via AERO Server ('S1') then applies the address
fe80::2001:db8:0:0 to its AERO interface with link-layer address
L2(C1). Client ('C1') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::2 and link-
layer address L2(S1), then sub-delegates the ACP to its attached
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EUNs. IPv6 host ('H1') connects to the EUN, and configures the
address 2001:db8:0::1.
AERO Client ('C2') receives the ACP 2001:db8:1::/48 in a DHCPv6 PD
exchange via AERO Server ('S2') then applies the address
fe80::2001:db8:1:0 to its AERO interface with link-layer address
L2(C2). Client ('C2') configures a default route and neighbor cache
entry via the AERO interface with next-hop address fe80::3 and link-
layer address L2(S2), then sub-delegates the ACP to its attached
EUNs. IPv6 host ('H1') connects to the EUN, and configures the
address 2001:db8:1::1.
3.13.2. Concept of Operations
Again, with reference to Figure 7, when source host ('H1') sends a
packet to destination host ('H2'), the packet is first forwarded over
the source host's attached EUN to Client ('C1'). Client ('C1') then
forwards the packet via its AERO interface to Server ('S1') and also
sends a Predirect message toward Client ('C2') via Server ('S1').
Server ('S1') then re-encapsulates and forwards both the packet and
the Predirect message out the same AERO interface toward Client
('C2') via Relay ('R1').
When Relay ('R1') receives the packet and Predirect message, it
consults its forwarding table to discover Server ('S2') as the next
hop toward Client ('C2'). Relay ('R1') then forwards both the packet
and the Predirect message to Server ('S2'), which then forwards them
to Client ('C2').
After Client ('C2') receives the Predirect message, it process the
message and returns a Redirect message toward Client ('C1') via
Server ('S2'). During the process, Client ('C2') also creates or
updates a dynamic neighbor cache entry for Client ('C1').
When Server ('S2') receives the Redirect message, it re-encapsulates
the message and forwards it on to Relay ('R1'), which forwards the
message on to Server ('S1') which forwards the message on to Client
('C1'). After Client ('C1') receives the Redirect message, it
processes the message and creates or updates a dynamic neighbor cache
entry for Client ('C2').
Following the above Predirect/Redirect message exchange, forwarding
of packets from Client ('C1') to Client ('C2') without involving any
intermediate nodes is enabled. The mechanisms that support this
exchange are specified in the following sections.
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3.13.3. 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 8:
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 8: AERO Redirect/Predirect Message Format
3.13.4. Sending Predirects
When a Client forwards a packet with a source address from one of its
ACPs toward a destination address covered by an ASP (i.e., toward
another AERO Client connected to the same AERO link), the source
Client MAY send a Predirect message forward toward the destination
Client via the Server.
In the reference operational scenario, when Client ('C1') forwards a
packet toward Client ('C2'), it MAY also send a Predirect message
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forward toward Client ('C2'), subject to rate limiting (see
Section 8.2 of [RFC4861]). Client ('C1') prepares the Predirect
message as follows:
o the link-layer source address is set to 'L2(C1)' (i.e., the link-
layer address of Client ('C1')).
o the link-layer destination address is set to 'L2(S1)' (i.e., the
link-layer address of Server ('S1')).
o the network-layer source address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('C1')).
o the network-layer destination address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C2')).
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
assigned to the Target Address.
o the Target Address is set to fe80::2001:db8:0:0 (i.e., the AERO
address of Client ('C1')).
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 one or more TLLAOs with Link ID and
Preference set to appropriate values for Client ('C1')'s
underlying interfaces, and with UDP Port Number and IP Address set
to 0'.
o the message SHOULD include a Timestamp option and a Nonce option.
o the message includes a Redirected Header Option (RHO) that
contains the originating packet truncated if necessary 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 act of sending Predirect messages is cited as "MAY",
since Client ('C1') may have advanced knowledge that the direct path
to Client ('C2') would be unusable or otherwise undesirable. If the
direct path later becomes unusable after the initial route
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optimization, Client ('C1') simply allows packets to again flow
through Server ('S1').
3.13.5. Re-encapsulating and Relaying Predirects
When Server ('S1') receives a Predirect message from Client ('C1'),
it first verifies that the TLLAOs in the Predirect are a proper
subset of the Link IDs in Client ('C1')'s neighbor cache entry. If
the Client's TLLAOs are not acceptable, Server ('S1') discards the
message. Otherwise, Server ('S1') validates the message according to
the ICMPv6 Redirect message validation rules in Section 8.1 of
[RFC4861], except that the Predirect has Code=1. Server ('S1') also
verifies that Client ('C1') is authorized to use the Prefix Length in
the Predirect when applied to the AERO address in the network-layer
source address by searching for the AERO address in the neighbor
cache. If validation fails, Server ('S1') discards the Predirect;
otherwise, it copies the correct UDP Port numbers and IP Addresses
for Client ('C1')'s links into the (previously empty) TLLAOs.
Server ('S1') then examines the network-layer destination address of
the Predirect to determine the next hop toward Client ('C2') by
searching for the AERO address in the neighbor cache. Since Client
('C2') is not one of its neighbors, Server ('S1') re-encapsulates the
Predirect and relays it via Relay ('R1') by changing the link-layer
source address of the message to 'L2(S1)' and changing the link-layer
destination address to 'L2(R1)'. Server ('S1') finally forwards the
re-encapsulated message to Relay ('R1') without decrementing the
network-layer TTL/Hop Limit field.
When Relay ('R1') receives the Predirect message from Server ('S1')
it determines that Server ('S2') is the next hop toward Client ('C2')
by consulting its forwarding table. Relay ('R1') then re-
encapsulates the Predirect while changing the link-layer source
address to 'L2(R1)' and changing the link-layer destination address
to 'L2(S2)'. Relay ('R1') then relays the Predirect via Server
('S2').
When Server ('S2') receives the Predirect message from Relay ('R1')
it determines that Client ('C2') is a neighbor by consulting its
neighbor cache. Server ('S2') then re-encapsulates the Predirect
while changing the link-layer source address to 'L2(S2)' and changing
the link-layer destination address to 'L2(C2)'. Server ('S2') then
forwards the message to Client ('C2').
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3.13.6. Processing Predirects and Sending Redirects
When Client ('C2') receives the Predirect message, it accepts the
Predirect only if the message has a link-layer source address of one
of its Servers (e.g., L2(S2)). Client ('C2') further accepts the
message only if it is willing to serve as a redirection target.
Next, Client ('C2') 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 Code=1 and even though the
network-layer source address is not that of it's current first-hop
router.
In the reference operational scenario, when Client ('C2') receives a
valid Predirect message, it either creates or updates a dynamic
neighbor cache entry that stores the Target Address of the message as
the network-layer address of Client ('C1') , stores the link-layer
addresses found in the TLLAOs as the link-layer addresses of Client
('C1') and stores the Prefix Length as the length to be applied to
the network-layer address for forwarding purposes. Client ('C2')
then sets AcceptTime for the neighbor cache entry to ACCEPT_TIME.
After processing the message, Client ('C2') prepares a Redirect
message response as follows:
o the link-layer source address is set to 'L2(C2)' (i.e., the link-
layer address of Client ('C2')).
o the link-layer destination address is set to 'L2(S2)' (i.e., the
link-layer address of Server ('S2')).
o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C2')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('C1')).
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 ('C2')).
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 one or more TLLAOs with Link ID and
Preference set to appropriate values for Client ('C2')'s
underlying interfaces, and with UDP Port Number and IP Address set
to '0'.
o the message SHOULD include a Timestamp option and MUST echo the
Nonce option received in the Predirect (i.e., if a Nonce option is
included).
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 ('C2') prepares the Redirect message, it sends the
message to Server ('S2').
3.13.7. Re-encapsulating and Relaying Redirects
When Server ('S2') receives a Redirect message from Client ('C2'), it
first verifies that the TLLAOs in the Redirect are a proper subset of
the Link IDs in Client ('C2')'s neighbor cache entry. If the
Client's TLLAOs are not acceptable, Server ('S2') discards the
message. Otherwise, Server ('S2') validates the message according to
the ICMPv6 Redirect message validation rules in Section 8.1 of
[RFC4861]. Server ('S2') also verifies that Client ('C2') is
authorized to use the Prefix Length in the Redirect when applied to
the AERO address in the network-layer source address by searching for
the AERO address in the neighbor cache. If validation fails, Server
('S2') discards the Predirect; otherwise, it copies the correct UDP
Port numbers and IP Addresses for Client ('C2')'s links into the
(previously empty) TLLAOs.
Server ('S2') then examines the network-layer destination address of
the Predirect to determine the next hop toward Client ('C2') by
searching for the AERO address in the neighbor cache. Since Client
('C2') is not a neighbor, Server ('S2') re-encapsulates the Predirect
and relays it via Relay ('R1') by changing the link-layer source
address of the message to 'L2(S2)' and changing the link-layer
destination address to 'L2(R1)'. Server ('S2') finally forwards the
re-encapsulated message to Relay ('R1') without decrementing the
network-layer TTL/Hop Limit field.
When Relay ('R1') receives the Predirect message from Server ('S2')
it determines that Server ('S1') is the next hop toward Client ('C1')
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by consulting its forwarding table. Relay ('R1') then re-
encapsulates the Predirect while changing the link-layer source
address to 'L2(R1)' and changing the link-layer destination address
to 'L2(S1)'. Relay ('R1') then relays the Predirect via Server
('S1').
When Server ('S1') receives the Predirect message from Relay ('R1')
it determines that Client ('C1') is a neighbor by consulting its
neighbor cache. Server ('S1') then re-encapsulates the Predirect
while changing the link-layer source address to 'L2(S1)' and changing
the link-layer destination address to 'L2(C1)'. Server ('S1') then
forwards the message to Client ('C1').
3.13.8. Processing Redirects
When Client ('C1') receives the Redirect message, it accepts the
message only if it has a link-layer source address of one of its
Servers (e.g., ''L2(S1)'). Next, Client ('C1') 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 ('C1') then processes
the message as follows.
In the reference operational scenario, when Client ('C1') receives
the Redirect message, it either creates or updates a dynamic neighbor
cache entry that stores the Target Address of the message as the
network-layer address of Client ('C2'), stores the link-layer
addresses found in the TLLAOs as the link-layer addresses of Client
('C2') and stores the Prefix Length as the length to be applied to
the network-layer address for forwarding purposes. Client ('C1')
then sets ForwardTime for the neighbor cache entry to FORWARD_TIME.
Now, Client ('C1') has a neighbor cache entry with a valid
ForwardTime value, while Client ('C2') has a neighbor cache entry
with a valid AcceptTime value. Thereafter, Client ('C1') may forward
ordinary network-layer data packets directly to Client ('C2') without
involving any intermediate nodes, and Client ('C2') can verify that
the packets came from an acceptable source. (In order for Client
('C2') to forward packets to Client ('C1'), a corresponding
Predirect/Redirect message exchange is required in the reverse
direction; hence, the mechanism is asymmetric.)
3.13.9. Server-Oriented Redirection
In some environments, the Server nearest the target 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
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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. The Server must then maintain a
neighbor cache entry for the redirected source Client.
3.14. Neighbor Unreachability Detection (NUD)
AERO nodes perform Neighbor Unreachability Detection (NUD) by sending
unicast NS messages to elicit solicited NA messages from neighbors
the same as described in [RFC4861]. NUD is performed either
reactively in response to persistent L2 errors (see Section 3.10) or
proactively to refresh existing neighbor cache entries.
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 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
proactively 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 proactively
test the direct path to the target Client (see Section 7.3 of
[RFC4861]) periodically in order to keep dynamic 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 one of its Servers. 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. If ForwardTime is non-zero,
the target Client then sends a solicited NA message to the link-layer
address of the source Client; otherwise, it sends the solicited NA
message to the link-layer address of one of its Servers.
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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 dynamic neighbor cache entry expires, the
source Client resumes sending any subsequent packets via a Server and
may (eventually) attempt to re-initiate the AERO redirection process.
When AcceptTime for a dynamic neighbor cache entry expires, the
target Client discards any subsequent packets received directly from
the source Client. When both ForwardTime and AcceptTime for a
dynamic neighbor cache entry expire, the Client deletes the neighbor
cache entry.
3.15. Mobility Management
3.15.1. Announcing Link-Layer Address Changes
When a Client needs to change its link-layer address, e.g., due to a
mobility event, it performs an immediate DHCPv6 Rebind/Reply exchange
via each of its Servers using the new link-layer address as the
source and with a CLLAO that includes the correct Link ID and
Preference values. If authentication succeeds, the Server then
update its neighbor cache and sends a DHCPv6 Reply. Note that if the
Client does not issue a DHCPv6 Rebind before the Server has
terminated the lease (e.g., if the Client has been out of touch with
the Server for a considerable amount of time), the Server's Reply
will report NoBinding and the Client must re-initiate the DHCPv6 PD
procedure.
Next, the Client sends unsolicited NA messages to each of its
correspondent Client 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 7, when Client ('C2') needs to change its
link-layer address it sends unicast unsolicited NA messages to Client
('C1') via Server ('S2') as follows:
o the link-layer source address is set to 'L2(C2)' (i.e., the link-
layer address of Client ('C2')).
o the link-layer destination address is set to 'L2(S2)' (i.e., the
link-layer address of Server ('S2')).
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o the network-layer source address is set to fe80::2001:db8:1:0
(i.e., the AERO address of Client ('C2')).
o the network-layer destination address is set to fe80::2001:db8:0:0
(i.e., the AERO address of Client ('C1')).
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 ('C2')).
o the message includes one or more TLLAOs with Link ID and
Preference set to appropriate values for Client ('C2')'s
underlying interfaces, and with UDP Port Number and IP Address set
to '0'.
o the message SHOULD include a Timestamp option.
When Server ('S1') receives the NA message, it relays the message in
the same way as described for relaying Redirect messages in
Section 3.12.7. In particular, Server ('S1') copies the correct UDP
port numbers and IP addresses into the TLLAOs, changes the link-layer
source address to its own address, changes the link-layer destination
address to the address of Relay ('R1'), then forwards the NA message
via the relaying chain the same as for a Redirect.
When Client ('C1') receives the NA message, it accepts the message
only if it already has a neighbor cache entry for Client ('C2') then
updates the link-layer addresses for Client ('C2') based on the
addresses in the TLLAOs. However, Client ('C1') MUST NOT update
ForwardTime since Client ('C2') will not have updated AcceptTime.
Note that these unsolicited NA messages are unacknowledged; hence,
Client ('C2') has no way of knowing whether Client ('C1') has
received them. If the messages are somehow lost, however, Client
('C1') will soon learn of the mobility event via the NUD procedures
specified in Section 3.14.
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3.15.2. Bringing New Links Into Service
When a Client needs to bring a new underlying interface into service
(e.g., when it activates a new data link), it performs an immediate
Rebind/Reply exchange via each of its Servers using the new link-
layer address as the source address and with a CLLAO that includes
the new Link ID and Preference values. If authentication succeeds,
the Server then updates its neighbor cache and sends a DHCPv6 Reply.
The Client MAY then send unsolicited NA messages to each of its
correspondent Clients to inform them of the new link-layer address as
described in Section 3.15.1.
3.15.3. Removing Existing Links from Service
When a Client needs to remove an existing underlying interface from
service (e.g., when it de-activates an existing data link), it
performs an immediate Rebind/Reply exchange via each of its Servers
over any available link with a CLLAO that includes the deprecated
Link ID and a Preference value of 0. If authentication succeeds, the
Server then updates its neighbor cache and sends a DHCPv6 Reply. The
Client SHOULD then send unsolicited NA messages to each of its
correspondent Clients to inform them of the deprecated link-layer
address as described in Section 3.15.1.
3.15.4. Moving to a New Server
When a Client associates with a new Server, it performs the Client
procedures specified in Section 3.10.
When a Client disassociates with an existing Server, it sends a
DHCPv6 Release message to the unicast link-local network layer
address of the old Server. The Client SHOULD send the message via a
new Server (i.e., by setting the link-layer destination address to
the address of the new Server) in case the old Server is unreachable
at the link layer, e.g., if the old Server is in a different network
partition. The new Server will forward the message to a Relay, which
will in turn forward the message to the old Server.
When the old Server receives the DHCPv6 Release, it first
authenticates the message. If authentication succeeds, the old
Server withdraws the IP route from the AERO routing system and
deletes the neighbor cache entry for the Client. (The old Server MAY
impose a small delay before deleting the neighbor cache entry so that
any packets already in the system can still be delivered to the
Client.) The old Server then returns a DHCPv6 Reply message via a
Relay. The Client can then use the Reply message to verify that the
termination signal has been processed, and can delete both the
default route and the neighbor cache entry for the old Server. (Note
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that the Server's Reply to the Client's Release message may be lost,
e.g., if the AERO routing system has not yet converged. Since the
Client is responsible for reliability, however, it will retry until
it gets an indication that the Release was successful.)
Clients SHOULD NOT move rapidly between Servers in order to avoid
causing excessive oscillations in the AERO routing system. Such
oscillations could result in intermittent reachability for the Client
itself, while causing little harm to the network due to routing
protocol dampening. Examples of when a Client might wish to change
to a different Server include a Server that has gone unreachable,
topological movements of significant distance, etc.
3.16. 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.
3.17. 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
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".)
3.18. Operation on AERO Links Without DHCPv6 Services
When Servers on the AERO link do not provide DHCPv6 services,
operation can still be accommodated through administrative
configuration of ACPs on AERO Clients. In that case, administrative
configurations of AERO interface neighbor cache entries on both the
Server and Client are also necessary. However, this may interfere
with the ability for Clients to dynamically change to new Servers,
and can expose the AERO link to misconfigurations unless the
administrative configurations are carefully coordinated.
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3.19. 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 by performing direct Client-to-
Client IPv6 ND message exchanges, and some other form of trust basis
must be applied so that each Client can verify that the prospective
neighbor is authorized to use its claimed ACP.
When there is no Server on the link, Clients must arrange to receive
ACPs and publish them via a secure alternate prefix delegation
authority through some means outside the scope of this document.
3.20. Proxy AERO
Proxy Mobile IPv6 (PMIPv6) [RFC5213][RFC5844] presents a localized
mobility management scheme for use within an access network domain.
It is typically used in cellular wireless service provider networks,
and allows mobile nodes to receive and retain a stable IP address
without needing to implement any special mobility protocols. In the
PMIPv6 architecture, access network devices known as Mobility Access
Gateways (MAGs) provide mobile nodes with an access link abstraction
and receive prefixes for the mobile nodes from a Local Mobility
Anchor (LMA).
The AERO Client (acting as a MAG) can similarly provide proxy
services for mobile nodes that do not participate in AERO messaging.
The proxy Client presents an access link abstraction to mobile nodes,
and performs DHCPv6 PD exchanges over the AERO interface with an AERO
Server (acting as an LMA) to receive a prefix for address
provisioning of the mobile node.
When a mobile node comes onto an access link presented by a proxy
Client, the Client authenticates the node and obtains a unique
identifier that it can use as the DUID in its DHCPv6 PD messages to
the Server. When the Server delegates a prefix, the Client creates a
new AERO address for the mobile node and assigns the delegated prefix
to the mobile node's access link. The Client then generates address
autoconfiguration messages (e.g., IPv6 RA, DHCPv6, DHCPv4, etc.) over
the access link and configures itself as a default router for the
mobile node. Since the Client may serve many such mobile nodes
simultaneously, it may configure multiple AERO addresses, i.e., one
for each mobile node.
When two mobile nodes are associated with the same proxy Client, the
Client can forward traffic between the mobiles without involving the
Server since it configures the AERO addresses of each mobile and
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therefore also has the necessary routing information. When two
mobiles are associated with different Clients, the first mobile
node's Client can initiate standard AERO route optimization using the
mobile's AERO address as the source for route optimization messaging.
This may result in a route optimization where the first mobile node's
Client discovers a direct path to the second mobile node's Client.
When a mobile node moves to a new proxy Client, the old proxy Client
issues a DHCPv6 Release message and sends unsolicited NA messages to
any of the mobile node's correspondents the same as specified for
announcing link-layer address changes in Section 3.15.1. However,
since the old Client has no way of knowing where the mobile has moved
to, it sets the Code field in the NA message to 1. When the
correspondent receives such an NA message, it deletes the neighbor
cache entry for the departed mobile node and again allows packets to
flow through its Server.
In addition to the use of DHCPv6 PD signaling, the AERO approach
differs from PMIPv6 in its use of the NBMA virtual link model instead
of point-to-point tunnels. This provides a more agile interface for
Client-to-Server coordinations, and also facilitates simple route
optimization. The AERO routing system is also arranged in such a
fashion that Clients get the same service from any Server they happen
to associate with. This provides a natural fault tolerance and load
balancing capability such as desired for distributed mobility
management. All other considerations are the same as specified in
[RFC5213][RFC5844].
3.21. Extending AERO Links Through Security Gateways
When an enterprise mobile device moves from a campus LAN connection
to a public Internet link, it must re-enter the enterprise via a
security gateway that has both a physical interface connection to the
Internet and a physical interface connection to the enterprise
internetwork. This most often entails the establishment of a Virtual
Private Network (VPN) link over the public Internet from the mobile
device to the security gateway. During this process, the mobile
device supplies the security gateway with its public Internet address
as the link-layer address for the VPN. The mobile device then acts
as an AERO Client to negotiate with the security gateway to obtain
its ACP.
In order to satisfy this need, the security gateway also operates as
an AERO Server with support for AERO Client proxying. In particular,
when a mobile device (i.e., the Client) connects via the security
gateway (i.e., the Server), the Server provides the Client with an
ACP in a DHCPv6 PD exchange the same as if it were attached to an
enterprise campus access link. The Server then replaces the Client's
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link-layer source address with the Server's enterprise-facing link-
layer address in all AERO messages the Client sends toward neighbors
on the AERO link. The AERO messages are then delivered to other
devices on the AERO link as if they were originated by the security
gateway instead of by the AERO Client. In the reverse direction, the
AERO messages sourced by devices within the enterprise network can be
forwarded to the security gateway, which then replaces the link-layer
destination address with the Client's link-layer address and replaces
the link-layer source address with its own (Internet-facing) link-
layer address.
After receiving the ACP, the Client can send IP packets that use an
address taken from the ACP as the network layer source address, the
Client's link-layer address as the link-layer source address, and the
Server's Internet-facing link-layer address as the link-layer
destination address. The Server will then rewrite the link-layer
source address with the Server's own enterprise-facing link-layer
address and rewrite the link-layer destination address with the
target AERO node's link-layer address, and the packets will enter the
enterprise network as though they were sourced from a device located
within the enterprise. In the reverse direction, when a packet
sourced by a node within the enterprise network uses a destination
address from the Client's ACP, the packet will be delivered to the
security gateway which then rewrites the link-layer destination
address to the Client's link-layer address and rewrites the link-
layer source address to the Server's Internet-facing link-layer
address. The Server then delivers the packet across the VPN to the
AERO Client. In this way, the AERO virtual link is essentially
extended *through* the security gateway to the point at which the VPN
link and AERO link are effectively grafted together by the link-layer
address rewriting performed by the security gateway. All AERO
messaging services (including route optimization and mobility
signaling) are therefore extended to the Client.
In order to support this virtual link grafting, the security gateway
(acting as an AERO Server) must keep static neighbor cache entries
for all of its associated Clients located on the public Internet.
The neighbor cache entry is keyed by the AERO Client's AERO address
the same as if the Client were located within the enterprise
internetwork. The neighbor cache is then managed in all ways as
though the Client were an ordinary AERO Client. This includes the
AERO IPv6 ND messaging signaling for Route Optimization and Neighbor
Unreachability Detection.
Note that the main difference between a security gateway acting as an
AERO Server and an enterprise-internal AERO Server is that the
security gateway has at least one enterprise-internal physical
interface and at least one public Internet physical interface.
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Conversely, the enterprise-internal AERO Server has only enterprise-
internal physical interfaces. For this reason security gateway
proxying is needed to ensure that the public Internet link-layer
addressing space is kept separate from the enterprise-internal link-
layer addressing space. This is afforded through a natural extension
of the security association caching already performed for each VPN
client by the security gateway.
3.22. Extending IPv6 AERO Links to the Internet
When an IPv6 host ('H1') with an address from an ACP owned by AERO
Client ('C1') sends packets to a correspondent IPv6 host ('H2'), the
packets eventually arrive at the IPv6 router that owns ('H2')s
prefix. This IPv6 router may or may not be an AERO Client ('C2')
either within the same home network as ('C1') or in a different home
network.
If Client ('C1') is currently located outside the boundaries of its
home network, it will connect back into the home network via a
security gateway acting as an AERO Server. The packets sent by
('H1') via ('C1') will then be forwarded through the security gateway
then through the home network and finally to ('C2') where they will
be delivered to ('H2'). This could lead to sub-optimal performance
when ('C2') could instead be reached via a more direct route without
involving the security gateway.
Consider the case when host ('H1') has the IPv6 address
2001:db8:1::1, and Client ('C1') has the ACP 2001:db8:1::/64 with
underlying IPv6 Internet address of 2001:db8:1000::1. Also, host
('H2') has the IPv6 address 2001:db8:2::1, and Client ('C2') has the
ACP 2001:db8:2::/64 with underlying IPv6 Internet address of
2001:db8:2000::1. While Client ('C1') may not initially know whether
('C2') is in fact an AERO Client, it can attempt route optimization
using an approach similar to the Return Routability procedure
specified for Mobile IPv6 (MIPv6) [RFC6275]. In order to support
this process, both Clients MUST intercept and decapsulate packets
that have a subnet router anycast address corresponding to any of the
/64 prefixes covered by their respective ACPs.
To initiate the process, Client ('C1') creates a specially-crafted
encapsulated AERO Predirect message that will be routed through its
home network then through ('C2')s home network and finally to ('C2')
itself. Client ('C1') prepares the initial message in the exchange
as follows:
o The encapsulating IPv6 header source address is set to
2001:db8:1:: (i.e., the IPv6 subnet router anycast address for
('C1')s ACP)
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o The encapsulating IPv6 header destination address is set to
2001:db8:2:: (i.e., the presumed IPv6 subnet router anycast
address for ('C2')s ACP)
o The encapsulating IPv6 header is followed by a UDP header with
source and destination port set to 8060
o The encapsulated IPv6 header source address is set to
fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))
o The encapsulated IPv6 header destination address is set to
fe80::2001:db8:2:0 (i.e., the presumed AERO address for ('C2'))
o The encapsulated AERO Predirect message includes all of the
securing information that would occur in a MIPv6 "Home Test Init"
message (format TBD)
Client ('C1') then further encapsulates the message in the
encapsulating headers necessary to convey the packet to the security
gateway (e.g., through IPsec encapsulation) so that the message now
appears "double-encapsulated". ('C1') then sends the message to the
security gateway, which re-encapsulates and forwards it over the home
network from where it will eventually reach ('C2').
At the same time, ('C1') creates and sends a second encapsulated AERO
Predirect message that will be routed through the IPv6 Internet
without involving the security gateway. Client ('C1') prepares the
message as follows:
o The encapsulating IPv6 header source address is set to
2001:db8:1000:1 (i.e., the Internet IPv6 address of ('C1'))
o The encapsulating IPv6 header destination address is set to
2001:db8:2:: (i.e., the presumed IPv6 subnet router anycast
address for ('C2')s ACP)
o The encapsulating IPv6 header is followed by a UDP header with
source and destination port set to 8060
o The encapsulated IPv6 header source address is set to
fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))
o The encapsulated IPv6 header destination address is set to
fe80::2001:db8:2:0 (i.e., the presumed AERO address for ('C2'))
o The encapsulated AERO Predirect message includes all of the
securing information that would occur in a MIPv6 "Care-of Test
Init" message (format TBD)
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If ('C2') is indeed an AERO Client, it will receive both Predirect
messages through its home network. ('C2') then return a
corresponding Redirect for each of the Predirect messages with the
source and destination addresses in the inner and outer headers
reversed. The first message includes all of the securing information
that would occur in a MIPv6 "Home Test" message, while the second
message includes all of the securing information that would occur in
a MIPv6 "Care-of Test" message (formats TBD).
When ('C1') receives the Redirect messages, it performs the necessary
security procedures per the MIPv6 specification. It then prepares an
encapsulated NS message that includes the same source and destination
addresses as for the "Care-of Test Init" Predirect message, and
includes all of the securing information that would occur in a MIPv6
"Binding Update" message (format TBD) and sends the message to
('C2').
When ('C2') receives the NS message, if the securing information is
correct it creates or updates a neighbor cache entry for ('C1') with
fe80::2001:db8:1:0 as the network-layer address, 2001:db8:1000::1 as
the link-layer address and with AcceptTime set to ACCEPT_TIME.
('C2') then sends an encapsulated NA message back to ('C1') that
includes the same source and destination addresses as for the "Care-
of Test" Redirect message, and includes all of the securing
information that would occur in a MIPv6 "Binding Acknowledgement"
message (format TBD) and sends the message to ('C1').
When ('C1') receives the NA message, it creates or updates a neighbor
cache entry for ('C2') with fe80::2001:db8:2:0 as the network-layer
address and 2001:db8:2:: as the link-layer address and with
ForwardTime set to FORWARD_TIME, thus completing the route
optimization in the forward direction.
('C1') subsequently forwards encapsulated packets with outer source
address 2001:db8:1000::1, with outer destination address
2001:db8:2::, with inner source address taken from the 2001:db8:1::,
and with inner destination address taken from 2001:db8:2:: due to the
fact that it has a securely-established neighbor cache entry with
non-zero ForwardTime. ('C2') subsequently accepts any such
encapsulated packets due to the fact that it has a securely-
established neighbor cache entry with non-zero AcceptTime..
In order to keep neighbor cache entries alive, ('C1') periodically
sends additional NS messages to ('C2') and receives any NA responses.
If ('C1') moves to a different point of attachment after the initial
route optimization, it sends a new secured NS message to ('C2') as
above to update ('C2')s neighbor cache.
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If ('C2') has packets to send to ('C1'), it performs a corresponding
route optimization in the opposite direction following the same
procedures described above. In the process, the already-established
unidirectional neighbor cache entries within ('C1') and ('C2') are
updated to include the now-bidirectional information. In particular,
the AcceptTime and ForwardTime variables for both neighbor cache
entries are updated to non-zero values, and the link-layer address
for ('C1')s neighbor cache entry for ('C2') is reset to
2001:db8:2000::1.
Note that two AERO Clients can use full security protocol messaging
instead of Return Routability, e.g., if strong authentication and/or
confidentiality are desired. In that case, security protocol key
exchanges such as specified for MOBIKE [RFC4555] would be used to
establish security associations and neighbor cache entries between
the AERO clients. Thereafter, AERO NS/NA messaging can be used to
maintain neighbor cache entries, test reachability, and to announce
mobility events. If reachability testing fails, e.g., if both
Clients move at roughly the same time, the Clients can tear down the
security association and neighbor cache entries and again allow
packets to flow through their home network (which may result in a new
route optimization event).
4. Implementation Status
An application-layer implementation is in progress.
5. IANA Considerations
IANA is instructed to assign a new 2-octet Hardware Type number
"TBD1" 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.
IANA is instructed to assign a 4-octet Enterprise Number "TBD2" for
AERO in the "enterprise-numbers" registry per [RFC3315].
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. Unless there is some other means
of authenticating the Client's identity (e.g., link-layer security),
AERO nodes SHOULD also use DHCPv6 securing services (e.g., DHCPv6
authentication, Secure DHCPv6 [I-D.ietf-dhc-sedhcpv6], etc.) for
Client authentication and network admission control.
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AERO Redirect, Predirect and unsolicited NA 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 Predirect, NS and
RS messages SHOULD include a Nonce option (see Section 5.3 of
[RFC3971]) that recipients echo back in corresponding responses.
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 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 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, Mark Andrews, Fred Baker,
Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph Droms, Sri
Gundavelli, Brian Haberman, Joel Halpern, Sascha Hlusiak, Lee Howard,
Andre Kostur, Ted Lemon, 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, Claudiu Danilov, Wen Fang, Anthony Gregory,
Jeff Holland, Ed King, Gen MacLean, Kent Shuey, Brian Skeen, 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
[I-D.ietf-dhc-sedhcpv6]
Jiang, S., Shen, S., Zhang, D., and T. Jinmei, "Secure
DHCPv6 with Public Key", draft-ietf-dhc-sedhcpv6-03 (work
in progress), June 2014.
[RFC0879] Postel, J., "TCP maximum segment size and related topics",
RFC 879, November 1983.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
BCP 6, RFC 1930, March 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[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.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
2923, September 2000.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[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.
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[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006.
[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.
[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[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.
[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.
[RFC5844] Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
Mobile IPv6", RFC 5844, May 2010.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
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[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.
[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 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.
[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.
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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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