AII Aggregation: Attachment Individual Identifier (AII) Types for Aggregation
A new pseudowire endpoint identifier that lets providers summarize thousands of endpoints into one value.
Metz, C., Martini, L., Balus, F., and J. Sugimoto, "Attachment Individual Identifier (AII) Types for Aggregation", RFC 5003, DOI 10.17487/RFC5003, September 2007, <https://www.rfc-editor.org/info/rfc5003>.
@misc{rfc5003,
series = {Request for Comments},
number = 5003,
howpublished = {RFC 5003},
publisher = {RFC Editor},
doi = {10.17487/RFC5003},
url = {https://www.rfc-editor.org/info/rfc5003},
author = {C. Metz and L. Martini and F. Balus and J. Sugimoto},
title = {{Attachment Individual Identifier (AII) Types for Aggregation}},
pagetotal = 7,
year = 2007,
month = sep,
} RFC text
Contents
Network Working Group C. Metz
Request for Comments: 5003 L. Martini
Category: Standards Track Cisco Systems Inc.
F. Balus
Alcatel-Lucent
J. Sugimoto
Nortel Networks
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.
Abstract
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 ....................................................2
2. Specification of Requirements ...................................3
3. Structure for the New AII Type ..................................3
3.1. AII Type 1 .................................................3
3.2. AII Type 2 .................................................3
4. IANA Considerations .............................................5
5. Security Considerations .........................................5
6. Acknowledgments .................................................5
7. Normative References ............................................5
8. Informative References ..........................................5
1. Introduction
[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
document.
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
AII values.
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
12.
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
source.
6. Acknowledgments
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.
Authors' Addresses
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
EMail: lmartini@cisco.com
Chris Metz
Cisco Systems, Inc.
3700 Cisco Way
San Jose, Ca. 95134
EMail: chmetz@cisco.com
Florin Balus
Alcatel-Lucent
701 East Middlefield Rd.
Mountain View, CA 94043
EMail: florin.balus@alcatel-lucent.com
Jeff Sugimoto
Nortel Networks
3500 Carling Ave.
Ottawa, Ontario, CANADA
EMail: sugimoto@nortel.com
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AI ELI5
On this page
§1The Endpoint Identifier Problem
RFC 4447 defines how to set up point-to-point pseudowires (PWs) between two provider edge (PE) nodes using LDP signaling. The signaling messages carry a forwarding equivalence class (FEC) element with the PW type and an endpoint identifier, which is used to select the PW forwarder that binds the PW to the attachment circuit at each end.
Two FEC element types exist for this: the PWid FEC (type 128) and the Generalized ID (GID) FEC (type 129). The PWid FEC carries a fixed-length 32-bit value, and the same value must be configured on the local and remote PE before setup. The GID FEC instead carries TLV fields for attachment individual identifiers (AIIs) which, together with an attachment group identifier (AGI), name the endpoints.
The identifier on the local PE is the source attachment identifier (SAI) and the one on the remote PE is the target attachment identifier (TAI). They can differ, which helps when the local PE has to learn remote TAI values first, for example through MP-BGP auto-discovery.
To connect two ends of a private wire, each end needs a name tag. The old way needed both ends to agree on one number in advance. The newer way lets each end have its own tag.
§1Why Aggregation and Global Uniqueness
The GID FEC lets the AII be structured to suit an application. An AII structure that lets many individual values be identified as one could significantly cut the load on AII distribution mechanisms such as MP-BGP and the memory PEs need to store them. The RFC notes that PWE3 signaling messages will always include a fully qualified AII value.
A globally unique AII would help PW management and security in large inter-AS and inter-provider environments. Providers would not have to worry about AII values overlapping during provisioning, or about AII network address translation boxes during signaling. Such values could aid troubleshooting and could be subjected to source-validity checks. An AII derived automatically from a provider's existing IP address space can simplify provisioning.
The two goals are summarization of many discrete AIIs into a single summary value, and global uniqueness if the provider wants it.
If many networks pick names on their own, two might pick the same name and cause a mix-up. Giving every name a piece that is unique to its owner prevents that, and grouping names saves space.
§3AII Type 1 and AII Type 2
AII Type 1 had already been allocated by IANA for provisioning models that need a fixed-length 32-bit value. That value is unique on the local PE.
AII Type 2 is the new structure. It allows varying levels of summarization, so it is no longer necessary to distribute or configure every individual AII (which could number in the tens of thousands or more) on local PEs before setting up PWs to remote PEs. How and where the aggregation is done, and how the result is distributed as reachability information, is not covered by this document.
Type 2 combines three things to make globally unique values: a provider's globally unique identifier (Global ID), a 32-bit prefix and a 4-octet attachment circuit identifier (AC ID).
Type 1 is a plain number that only makes sense inside one building. Type 2 adds the company, the building and the room, so the address works anywhere.
§3.2The AII Type 2 Layout
The TLV starts with the type, 0x02, and a length, which is the length of the value field in octets and is set to 12. The value has three fields.
- Global ID: a 4-octet value unique to the provider. It can hold the provider's 2-octet or 4-octet Autonomous System Number (ASN) and is expected to come from the ASN of the autonomous system hosting the PEs. A 2-octet AS number means the two high-order octets MUST be zero. The RFC notes that this use of the ASN has nothing to do with the ASN's role in protocols such as BGP.
- Prefix: a 32-bit value assigned by the provider, or derived automatically from the PE's /32 IPv4 loopback address. For IP reachability it need not be related to the IPv4 space used in the provider's IGP or BGP.
- AC ID: a fixed-length 4-octet field that further identifies an attachment circuit on the PE, so circuits that share a prefix can be told apart.
The tag has three parts: which company, which group of connections, and which single connection inside that group. A big group can be pointed at using only the first two parts.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | AII Type | 1 byte | 0x02 | Set to 02. |
LENGTH | Length | 1 byte | 12 | Length of the value field in octets. |
GLOBAL-ID | Global ID | 4 bytes | Unique to the provider; can be derived from the provider's ASN. | |
PREFIX | Prefix | 4 bytes | Assigned by the provider or derived from the PE's /32 IPv4 loopback address. | |
AC-ID | AC ID | 4 bytes | Identifies an individual attachment circuit that shares a prefix. |
§5Security and IANA
AII values appear in AII distribution protocols and in PW signaling protocols, and are subject to various authentication schemes (the RFC names MD5) if desired. Using global ID values such as an ASN in the inter-provider case could allow a form of source-validation checking, to ensure that an AII (aggregated or explicit) came from a legitimate source.
On the IANA side, a value was allocated from the Attachment Individual Identifier (AII) Type registry defined in RFC 4446. The value for this AII type is 0x02.
Because each tag contains the owner's unique number, a receiver can check that a tag really came from the network it claims to come from.