Network Working Group C. Metz
Request for Comments: 5003 L. Martini
Category: Standards Track Cisco Systems Inc.
September 2007 Attachment Individual Identifier (AII) Types for Aggregation
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
The signaling protocols used to establish point-to-point pseudowires
include type-length-value (TLV) fields that identify pseudowire
endpoints called attachment individual identifiers (AIIs). This
document defines AII structures in the form of new AII TLV fields
that support AII aggregation for improved scalability and Virtual
Private Network (VPN) auto-discovery. It is envisioned that this
would be useful in large inter-domain virtual private wire service
networks where pseudowires are established between selected local and
remote provider edge (PE) nodes based on customer need.
Table of Contents
1. Introduction ....................................................22. Specification of Requirements ...................................33. Structure for the New AII Type ..................................33.1. AII Type 1 .................................................33.2. AII Type 2 .................................................34. IANA Considerations .............................................55. Security Considerations .........................................56. Acknowledgments .................................................57. Normative References ............................................58. Informative References ..........................................5
[RFC4447] defines the signaling mechanisms for establishing point-
to-point pseudowires (PWs) between two provider edge (PE) nodes.
When a PW is set up, the LDP signaling messages include a forwarding
equivalence class (FEC) element containing information about the PW
type and an endpoint identifier used in the selection of the PW
forwarder that binds the PW to the attachment circuit at each end.
There are two types of FEC elements defined for this purpose: PWid
FEC (type 128) and the Generalized ID (GID) FEC (type 129). The PWid
FEC element includes a fixed-length 32-bit value called the PWid that
serves as an endpoint identifier. The same PWid value must be
configured on the local and remote PE prior to PW setup.
The GID FEC element includes TLV fields for attachment individual
identifiers (AIIs) that, in conjunction with an attachment group
identifier (AGI), serve as PW endpoint identifiers. The endpoint
identifier on the local PE (denoted as <AGI, source AII, or SAII>) is
called the source attachment identifier (SAI) and the endpoint
identifier on the remote PE (denoted as <AGI, target AII, or TAII>)
is called the target attachment identifier (TAI). The SAI and TAI
can be distinct values. This is useful for applications and
provisioning models where the local PE (with a particular SAI) does
not know and must somehow learn (e.g., via Multiprotocol BGP (MP-BGP)
auto-discovery) of remote TAI values prior to launching PW setup
messages towards the remote PE.
The use of the GID FEC TLV provides the flexibility to structure
(source or target) AII values to best fit the needs of a particular
application or provisioning model [L2VPN-SIG]. For example, an AII
structure that enables many individual AII values to be identified as
a single value could significantly reduce the burden on AII
distribution mechanisms (e.g., MP-BGP) and on PE memory needed to
store this AII information. It should be noted that Pseudowire
Emulation Edge-to-Edge (PWE3) signaling messages will always include
a fully qualified AII value.
An AII that is globally unique would facilitate PW management and
security in large inter-AS (autonomous system) and inter-provider
environments. Providers would not have to worry about AII value
overlap during provisioning or the need for AII network address
translation (NAT) boxes during signaling. Globally unique AII values
could aid in troubleshooting and could be subjected to source-
validity checks during AII distribution and signaling. An AII
automatically derived from a provider's existing IP address space can
simplify the provisioning process.
This document defines an AII structure based on [RFC4447] that:
o Enables many discrete attachment individual identifiers to be
summarized into a single AII summary value. This will enhance
scalability by reducing the burden on AII distribution mechanisms
and on PE memory.
o Ensures global uniqueness if desired by the provider. This will
facilitate Internet-wide PW connectivity and provide a means for
providers to perform source validation on the AII distribution
(e.g., MP-BGP) and signaling (e.g., LDP) channels.
This is accomplished by defining new AII types and the associated
formats of the value field.
2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Structure for the New AII Type
[RFC4447] defines the format of the GID FEC TLV and the use and
semantics of the attachment group identifier (AGI).
3.1. AII Type 1
AII Type 1 has been allocated by IANA for use with provisioning
models requiring a fixed-length 32-bit value [L2VPN-SIG]. This value
is unique on the local PE.
3.2. AII Type 2
The AII Type 2 structure permits varying levels of AII summarization
to take place, thus reducing the scaling burden on the aforementioned
AII distribution mechanisms and PE memory. In other words, it no
longer becomes necessary to distribute or configure all individual
AII values (which could number in the tens of thousands or more) on
local PEs prior to establishing PWs to remote PEs. The details of
how and where the aggregation of AII values is performed and then
distributed as AII reachability information are not discussed in this
AII Type 2 uses a combination of a provider's globally unique
identifier (Global ID), a 32-bit prefix field, and a 4-octet
attachment circuit identifier (AC ID) field to create globally unique
The encoding of AII Type 2 is shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| AII Type=02 | Length | Global ID |
| Global ID (contd.) | Prefix |
| Prefix (contd.) | AC ID |
| AC ID |
Figure 1. AII Type 2 TLV Structure
o AII Type = 0x02
o Length = length of value field in octets. The length is set to
o Global ID = This is a 4-octet field containing a value that is
unique to the provider. The global ID can contain the 2-octet or
4-octet value of the provider's Autonomous System Number (ASN).
It is expected that the global ID will be derived from the
globally unique ASN of the autonomous system hosting the PEs
containing the actual AIIs. The presence of a global ID based on
the provider's ASN ensures that the AII will be globally unique.
If the global ID is derived from a 2-octet AS number, then the
two high-order octets of this 4-octet field MUST be set to zero.
Please note that the use of the provider's ASN as a global ID
DOES NOT have anything at all to do with the use of the ASN in
protocols such as BGP.
o Prefix = The 32-bit prefix is a value assigned by the provider or
it can be automatically derived from the PE's /32 IPv4 loopback
address. Note that, for IP reachability, it is not required that
the 32-bit prefix have any association with the IPv4 address
space used in the provider's IGP or BGP.
o Attachment Circuit (AC) ID = This is a fixed-length 4-octet field
used to further refine identification of an attachment circuit on
the PE. The inclusion of the AC ID is used to identify
individual attachment circuits that share a common prefix.
4. IANA Considerations
IANA has allocated a value from the "Attachment Individual Identifier
(AII) Type" registry defined in [RFC4446].
The value for this AII type is 0x02.
5. Security Considerations
AII values appear in AII distribution protocols [L2VPN-SIG] and PW
signaling protocols [RFC4447] and are subject to various
authentication schemes (i.e., MD5) if so desired.
The use of global ID values (e.g., ASN) in the inter-provider case
could enable a form of source-validation checking to ensure that the
AII value (aggregated or explicit) originated from a legitimate
Thanks to Carlos Pignataro, Scott Brim, Skip Booth, George Swallow,
and Bruce Davie for their input into this document.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
G. Heron, "Pseudowire Setup and Maintenance Using the
Label Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to
Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
8. Informative References
[L2VPN-SIG] Rosen, E., Luo, W., Davie, B., and V. Radoaca,
"Provisioning, Autodiscovery, and Signaling in L2VPNs",
Work in Progress, May 2006.
Cisco Systems, Inc.
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Cisco Systems, Inc.
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Ottawa, Ontario, CANADA
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