PWE3 Working Group                                           Chris Metz
     Internet Draft                                             Luca Martini
     Expires: August 2006                                      Cisco Systems
                                                                Florin Balus
                                                               Jeff Sugimoto
                                                             Nortel Networks
                                                           February 25, 2006
              Pseudowire Attachment Identifiers for Aggregation and VPN
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     Copyright Notice
        Copyright (C) The Internet Society (2006).  All Rights Reserved.
        The signaling protocols used to establish point-to-point pseudowires
        include type-length-value (TLV) fields that identify pseudowire
        endpoints called attachment individual identifiers (AII). This
        document defines AII structures in the form of new AII type-length-
        value fields that support AII aggregation for improved scalability
        and VPN autodiscovery. 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 PE
        nodes based on customer need.
     Conventions used in this document
        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 RFC-2119 Error!
        Reference source not found..
     Table of Contents
        1. Introduction...................................................2
        2. Structure for the New AII Type.................................4
           2.1. AII Type 1................................................4
           2.2. AII Type 2................................................4
        3. IANA Considerations............................................6
        4. Security Considerations........................................6
        5. Acknowledgments................................................6
        Author's Addresses................................................7
        Intellectual Property Statement...................................8
        Disclaimer of Validity............................................8
        Copyright Statement...............................................8
     1. Introduction
        [PWE3-CONTROL] 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
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        the PW forwarder that binds the PW to the attachment circuit at each
        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 (AII) 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 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 particular application or
        provisioning model needs [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 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 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
        This document defines an AII structure based on [PWE3-CONTROL] 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.
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        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. Structure for the New AII Type
        [PWE3-CONTROL] defines the format of the GID FEC TLV and the use and
        semantics of the attachment group identifier (AGI).
     2.1. 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.
     2.2. 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 an optional 4-octet
        attachment circuit identifier field to create globally unique AII
        The encoding of AII Type 2 is shown in figure 1.
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         0                   1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        |  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 8
           if the AC ID is NULL and 12 if the AC ID is non-null.
        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 PE hosting the AIIs is present in an autonomous system
           where the provider is not running BGP, chooses not to expose this
           information or does not wish to use the global ID, then the global
           ID field MUST be set to zero. If the global ID is derived from a
           2-octet AS number, then the high-order 4 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.
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        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 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 for IP reachability.
        o  Attachment Circuit (AC) ID = This is a fixed length four 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.
           If the AC ID is not present then the AC ID field MUST be null and
           the AII Length field is set to 8. If the AC ID is present then the
           length field is set to 12 octets.
     3. IANA Considerations
        This document requests that IANA allocate a value from the
        "Attachment Individual Identifier (AII) Type" registry defined in
        The suggested value for this AII type is 0x02.
     4. Security Considerations
        AII values appear in AII distribution protocols [MP-BGP-AUTO-DISC]
        and PW signaling protocols [PWE3-CONTROL] 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
     5. Acknowledgments
        Thanks to Carlos Pignataro, Scott Brim, Skip Booth, George Swallow
        and Bruce Davie for their input into this document.
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        [PWE3-CONTROL], “Pseudowire Setup and Maintenance using LDP”,
                  draft-ietf-pwe3-control-protocol-17.txt, June 2005
        [IANA], "IANA Allocations for pseudo Wire Edge to Edge Emulation
                  (PWE3)" Martini,Townsley, draft-ietf-pwe3-iana-allocation-
                  15.txt, work in progress), November 2005
        [L2VPN-SIG], “Provisioning Models and Endpoint Identifiers in L2VPN
                  Signaling”, draft-ietf-l2vpn-signaling-06.txt, Sept. 2005
        [MP-BGP-AUTO-DISC], “Using BGP as an Auto-Discovery Mechanism for
                  Layer-3 and Layer-2 VPNs”, Ould-Brahim, H. et al, draft-
                  ietf-l3vpn-bgpvpn-auto-06.txt, June 2005
     Author's Addresses
     Chris Metz
     Cisco Systems, Inc.
     3700 Cisco Way
     San Jose, Ca. 95134
     Luca Martini
     Cisco Systems, Inc.
     9155 East Nichols Avenue, Suite 400
     Englewood, CO, 80112
     Florin Balus
     3500 Carling Ave.
     Ottawa, Ontario, CANADA
     Jeff Sugimoto
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     3500 Carling Ave.
     Ottawa, Ontario, CANADA
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