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rfc:rfc4667

Network Working Group W. Luo Request for Comments: 4667 Cisco Systems, Inc. Category: Standards Track September 2006

        Layer 2 Virtual Private Network (L2VPN) Extensions
               for Layer 2 Tunneling Protocol (L2TP)

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.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 The Layer 2 Tunneling Protocol (L2TP) provides a standard method for
 setting up and managing L2TP sessions to tunnel a variety of L2
 protocols.  One of the reference models supported by L2TP describes
 the use of an L2TP session to connect two Layer 2 circuits attached
 to a pair of peering L2TP Access Concentrators (LACs), which is a
 basic form of Layer 2 Virtual Private Network (L2VPN).  This document
 defines the protocol extensions for L2TP to set up different types of
 L2VPNs in a unified fashion.

Table of Contents

 1. Introduction ....................................................2
    1.1. Specification of Requirements ..............................2
 2. Network Reference Model .........................................2
 3. Forwarder Identifier ............................................3
 4. Protocol Components .............................................4
    4.1. Control Messages ...........................................4
    4.2. Existing AVPs for L2VPN ....................................4
    4.3. New AVPs for L2VPN .........................................5
    4.4. AVP Interoperability .......................................7
 5. Signaling Procedures ............................................7
    5.1. Overview ...................................................7
    5.2. Pseudowire Tie Detection ...................................8
    5.3. Generic Algorithm ..........................................9
 6. IANA Considerations ............................................12

Luo Standards Track [Page 1] RFC 4667 L2VPN Extensions for L2TP September 2006

 7. Security Considerations ........................................12
 8. Acknowledgement ................................................13
 9. References .....................................................13
    9.1. Normative References ......................................13
    9.2. Informative References ....................................13

1. Introduction

 [RFC3931] defines a dynamic tunneling mechanism to carry multiple
 Layer 2 protocols besides Point-to-Point Protocol (PPP), the only
 protocol supported in [RFC2661], over a packet-based network.  The
 baseline protocol supports various types of applications, which have
 been highlighted in the different Layer 2 Tunneling Protocol (L2TP)
 reference models in [RFC3931].  An L2TP Access Concentrator (LAC) is
 an L2TP Control Connection Endpoint (LCCE) that cross-connects
 attachment circuits and L2TP sessions.  Layer 2 Virtual Private
 Network (L2VPN) applications are typically in the scope of the LAC-
 LAC reference model.
 This document discusses the commonalities and differences among L2VPN
 applications with respect to using L2TPv3 as the signaling protocol.
 In this document, the acronym "L2TP" refers to L2TPv3 or L2TP in
 general.

1.1. 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].

2. Network Reference Model

 In the LAC-LAC reference model, a LAC serves as a cross-connect
 between attachment circuits and L2TP sessions.  Each L2TP session
 acts as an emulated circuit, also known as pseudowire.  A pseudowire
 is used to bind two "forwarders" together.  For different L2VPN
 applications, different types of forwarders are defined.
 In the L2VPN framework [L2VPNFW], a LAC is a Provider Edge (PE)
 device.  LAC and PE are interchangeable terms in the context of this
 document.  Remote systems in the LAC-LAC reference model are Customer
 Edge (CE) devices.

Luo Standards Track [Page 2] RFC 4667 L2VPN Extensions for L2TP September 2006

 +----+  L2  +----+                      +----+  L2  +----+
 | CE |------| PE |....[core network]....| PE |------| CE |
 +----+      +----+                      +----+      +----+
                  |<- emulated service ->|
       |<----------------- L2 service -------------->|
                L2VPN Network Reference Model
 In a simple cross-connect application, an attachment circuit is a
 forwarder directly bound to a pseudowire.  It is a one-to-one
 mapping.  Traffic received from the attachment circuit on a local PE
 is forwarded to the remote PE through the pseudowire.  When the
 remote PE receives traffic from the pseudowire, it forwards the
 traffic to the corresponding attachment circuit on its end.  The
 forwarding decision is based on the attachment circuit or pseudowire
 demultiplexing identifier.
 With Virtual Private LAN Service (VPLS), a Virtual Switching Instance
 (VSI) is a forwarder connected to one or more attachment circuits and
 pseudowires.  A single pseudowire is used to connect a pair of VSIs
 on two peering PEs.  Traffic received from an attachment circuit or a
 pseudowire is first forwarded to the corresponding VSI based on the
 attachment circuit or pseudowire demultiplexing identifier.  The VSI
 performs additional lookup to determine where to further forward the
 traffic.
 With Virtual Private Wire Service (VPWS), attachment circuits are
 grouped into "colored pools".  Each pool is a forwarder and is
 connected through a network of point-to-point cross-connects.  The
 data forwarding perspective is identical to the cross-connect
 application.  However, constructing colored pools involves more
 complicated signaling procedures.

3. Forwarder Identifier

 A forwarder identifier is assigned to each forwarder on a given PE
 and is unique in the context of the PE.  It is defined as the
 concatenation of an Attachment Group Identifier (AGI) and an
 Attachment Individual Identifier (AII), denoted as <AGI, AII>.  The
 AGI is used to group a set of forwarders together for signaling
 purposes.  An AII is used to distinguish forwarders within a group.
 AII can be unique on a per-platform or per-group basis.
 As far as the signaling procedures are concerned, a forwarder
 identifier is an arbitrary string of bytes.  It is up to
 implementations to decide the values for AGI and AII.

Luo Standards Track [Page 3] RFC 4667 L2VPN Extensions for L2TP September 2006

 When connecting two forwarders together, both MUST have the same AGI
 as part of their forwarder identifiers.  The AII of the source
 forwarder is known as the Source AII (SAII), and the AII of the
 target forwarder is known as the Target AII (TAII).  Therefore, the
 source forwarder and target forwarder can be denoted as <AGI, SAII>
 and <AGI, TAII>, respectively.

4. Protocol Components

4.1. Control Messages

 L2TP defines two sets of session management procedures: incoming call
 and outgoing call.  Even though it is entirely possible to use the
 outgoing call procedures for signaling L2VPNs, the incoming call
 procedures have some advantages in terms of the relevance of the
 semantics.  [PWE3L2TP] gives more details on why the incoming call
 procedures are more appropriate for setting up pseudowires.
 The signaling procedures for L2VPNs described in the following
 sections are based on the Control Connection Management and the
 Incoming Call procedures, defined in Sections 3.3 and 3.4.1 of
 [RFC3931], respectively.  L2TP control message types are defined in
 Section 3.1 of [RFC3931].  This document references the following
 L2TP control messages:
   Start-Control-Connection-Request (SCCRQ)
   Start-Control-Connection-Reply   (SCCRP)
   Incoming-Call-Request            (ICRQ)
   Incoming-Call-Reply              (ICRP)
   Incoming-Call-Connected          (ICCN)
   Set-Link-Info                    (SLI)

4.2. Existing AVPs for L2VPN

 The following Attribute Value Pairs (AVPs), defined in Sections
 5.4.3, 5.4.4, and 5.4.5 of [RFC3931], are used for signaling L2VPNs.
 Router ID
    The Router ID sent in SCCRQ and SCCRP during control connection
    setup establishes the unique identity of each PE.
 Pseudowire Capabilities List
    The Pseudowire Capabilities List sent in the SCCRQ and SCCRP
    indicates the pseudowire types supported by the sending PE.  It
    merely serves as an advertisement to the receiving PE.  Its
    content should not affect the control connection setup.

Luo Standards Track [Page 4] RFC 4667 L2VPN Extensions for L2TP September 2006

    Before a local PE initiates a session of a particular pseudowire
    type to a remote PE, it MUST examine whether the remote PE has
    advertised this pseudowire type in this AVP and SHOULD NOT attempt
    to initiate the session if the intended pseudowire type is not
    supported by the remote PE.
 Pseudowire Type
    The Pseudowire Type sent in ICRQ signals the intended pseudowire
    type to the receiving PE.  The receiving PE checks it against its
    local pseudowire capabilities list.  If it finds a match, it
    responds with an ICRP without a Pseudowire Type AVP, which
    implicitly acknowledges its acceptance of the intended pseudowire.
    If it does not find a match, it MUST respond with a Call-
    Disconnect-Notify (CDN), with an "unsupported pseudowire type"
    result code.
 L2-Specific Sublayer
    The L2-Specific Sublayer can be sent in ICRQ, ICRP, and ICCN.  If
    the receiving PE supports the specified L2-Specific Sublayer, it
    MUST include the identified L2-Specific Sublayer in its data
    packets sent to the sending PE.  Otherwise, it MUST reject the
    connection by sending a CDN to the sending PE.
 Circuit Status
    The Circuit Status is sent in both ICRQ and ICRP to inform the
    receiving PE about the circuit status on the sending PE.  It can
    also be sent in ICCN and SLI to update the status.
 Remote End Identifier
    The TAII value is encoded in the Remote End ID AVP and sent in
    ICRQ along with the optional AGI to instruct the receiving PE to
    bind the proposed pseudowire to the forwarder that matches the
    specified forwarder identifier.

4.3. New AVPs for L2VPN

 Attachment Group Identifier
    The AGI AVP, Attribute Type 89, is an identifier used to associate
    a forwarder to a logical group.  The AGI AVP is used in
    conjunction with the Local End ID AVP and Remote End ID AVP, which
    encode the SAII and TAII, respectively, to identify a specific
    forwarder.  When the AGI AVP is omitted in the control messages or
    contains a zero-length value, the forwarders are considered to use

Luo Standards Track [Page 5] RFC 4667 L2VPN Extensions for L2TP September 2006

    the default AGI.  Note that there is only one designated default
    AGI value for all forwarders.
    The Attribute Value field for this AVP has the following format:
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |M|H|0|0|0|0|    Length         |              0                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               89              |      AGI (variable length)    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    The AGI field is a variable-length field.  This AVP MAY be present
    in ICRQ.
    This AVP MAY be hidden (the H bit MAY be 0 or 1).  The hiding of
    AVP attribute values is defined in Section 5.3 of [RFC3931].  The
    M bit for this AVP SHOULD be set to 0.  The Length (before hiding)
    of this AVP is 6 octets plus the length of the AGI field.
 Local End ID
    The Local End ID AVP, Attribute Type 90, encodes the SAII value.
    The SAII may also be used in conjunction with the TAII to detect
    pseudowire ties.  When it is omitted in the control messages, it
    is assumed that it has the same value as the TAII.
    The Attribute Value field for this AVP has the following format:
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |M|H|0|0|0|0|    Length         |              0                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               90              |       SAII (variable length)  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    The SAII field is a variable-length field.  This AVP MAY be
    present in ICRQ.
    This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for
    this AVP SHOULD be set to 0.  The Length (before hiding) of this
    AVP is 6 octets plus the length of the SAII field.
 Interface Maximum Transmission Unit
    The Interface Maximum Transmission Unit (MTU) AVP, Attribute Type

Luo Standards Track [Page 6] RFC 4667 L2VPN Extensions for L2TP September 2006

    91, indicates the MTU in octets of a packet that can be sent out
    from the CE-facing interface.  The MTU values of a given
    pseudowire, if advertised in both directions, MUST be identical.
    If they do not match, the pseudowire SHOULD NOT be established.
    When this AVP is omitted in the control messages in either
    direction, it is assumed that the remote PE has the same interface
    MTU as the local PE for the pseudowire being signaled.
    The Attribute Value field for this AVP has the following format:
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |M|H|0|0|0|0|    Length         |              0                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               91              |          Interface MTU        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    The Interface MTU field is a 2-octet integer value.  This AVP MAY
    be present in ICRQ and ICRP.  When a PE receives an Interface MTU
    AVP with an MTU value different from its own, it MAY respond with
    a CDN with a new result code indicating the disconnect cause.
      23 - Mismatching interface MTU
    This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for
    this AVP SHOULD be set to 0.  The Length (before hiding) of this
    AVP is 8 octets.

4.4. AVP Interoperability

 To ensure interoperability, the mandatory (M) bit settings of the
 existing AVPs used in L2VPN applications should be the same as those
 specified in [RFC3931].  The generic M-bit processing is described in
 Section 5.2 of [RFC3931].  Setting the M-bit of the new AVPs to 1
 will impair interoperability.

5. Signaling Procedures

5.1. Overview

 Assume that a PE assigns a forwarder identifier to one of its local
 forwarders and that it knows it needs to set up a pseudowire to a
 remote forwarder on a remote PE that has a certain Forwarder ID.
 This knowledge can be obtained either through manual configuration or
 some auto-discovery procedure.
 Before establishing the intended pseudowire, each pair of peering PEs

Luo Standards Track [Page 7] RFC 4667 L2VPN Extensions for L2TP September 2006

 exchanges control connection messages to establish a control
 connection.  Each advertises its supported pseudowire types, as
 defined in [PWE3IANA], in the Pseudowire Capabilities List AVP.
 After the control connection is established, the local PE examines
 whether the remote PE supports the pseudowire type it intends to set
 up.  Only if the remote PE supports the intended pseudowire type
 should it initiate a pseudowire connection request.
 When the local PE receives an ICRQ for a pseudowire connection, it
 examines the forwarder identifiers encoded in the AGI, SAII, and TAII
 in order to determine the following:
  1. Whether it has a local forwarder with the forwarder identifier

value specified in the ICRQ.

  1. Whether the remote forwarder with the forwarder identifier

specified in the ICRQ is allowed to connect with this local

     forwarder.
 If both conditions are met, it sends an ICRP to the remote PE to
 accept the connection request.  If either of the two conditions
 fails, it sends a CDN to the remote PE to reject the connection
 request.
 The local PE can optionally include a result code in the CDN to
 indicate the disconnect cause.  The possible result codes are
   24 - Attempt to connect to non-existent forwarder
   25 - Attempt to connect to unauthorized forwarder

5.2. Pseudowire Tie Detection

 Conceivably in the network reference models, as either PE may
 initiate a pseudowire to another PE at any time, the PEs could end up
 initiating a pseudowire to each other simultaneously.  In order to
 avoid setting up duplicated pseudowires between two forwarders, each
 PE must be able to independently detect such a pseudowire tie.  The
 following procedures need to be followed to detect a tie:
 If both TAII and SAII are present in the ICRQ, the receiving PE
 compares the TAII and SAII against the SAII and TAII previously sent
 to the sending PE.  If the received TAII matches the sent SAII and
 the received SAII matches the sent TAII, a tie is detected.
 If only the TAII is present in the ICRQ, the SAII is assumed to have
 the same value as the TAII.  The receiving PE compares the received
 TAII with the SAII that it previously sent to the sending PE.  If the

Luo Standards Track [Page 8] RFC 4667 L2VPN Extensions for L2TP September 2006

 SAII in that ICRQ is also omitted, then the value encoded in the sent
 TAII is used for comparison.  If they match, a tie is detected.
 If the AGI is present, it is first prepended to the TAII and SAII
 values before the tie detection occurs.
 Once a tie is discovered, the PE uses the standard L2TP tie breaking
 procedure, as described in Section 5.4.4 of [RFC3931], to disconnect
 the duplicated pseudowire.

5.3. Generic Algorithm

 The following uses a generic algorithm to illustrate the protocol
 interactions when constructing an L2VPN using L2TP signaling.
 Each PE first forms a list, SOURCE_FORWARDERS, consisting of all
 local forwarders of a given VPN.  Then it puts all local forwarders
 that need to be interconnected and all remote forwarders of the same
 VPN into another list, TARGET_FORWARDERS.  The formation of the
 network topology depends on the content in the SOURCE_FORWARDERS and
 TARGET_FORWARDERS lists.  These two lists can be constructed by
 manual configuration or some auto-discovery procedure.
 The algorithm is used to set up a full mesh of interconnections
 between SOURCE_FORWARDERS and TARGET_FORWARDERS.  An L2VPN is formed
 when the algorithm is finished in every participating PE of this
 L2VPN.
   1.  Pick the next forwarder, from SOURCE_FORWARDERS.  If no
       forwarder is available for processing, the processing is
       complete.
   2.  Pick the next forwarder, from TARGET_FORWARDERS.  If no
       forwarder is available for processing, go back to step 1.
   3.  If the two forwarders are associated with different Router
       IDs, a pseudowire must be established between them.  Proceed
       to step 6.
   4.  Compare the <AGI, AII> values of the two forwarders.  If
       they match, the source and target forwarders are the same,
       so no more action is necessary.  Go back to step 2.
   5.  As the source and target forwarders both reside on the local
       PE, no pseudowire is needed.  The PE simply creates a local
       cross-connect between the two forwarders.  Go back to step 2.
   6.  As the source and target forwarders reside on different PEs,

Luo Standards Track [Page 9] RFC 4667 L2VPN Extensions for L2TP September 2006

       a pseudowire must be established between them.  The PE first
       examines whether the source forwarder has already established a
       pseudowire to the target forwarder.  If so, go back to step 2.
   7.  If no pseudowire is already established between the source and
       target forwarders, the local PE obtains the address of the
       remote PE and establishes a control connection to the remote
       PE if one does not already exist.
   8.  The local PE sends an ICRQ to the remote PE.  The AGI, TAII,
       and SAII values are encoded in the AGI AVP, the Remote End ID
       AVP, and the Local End ID AVP, respectively.
   9.  If the local PE receives a response corresponding to the
       ICRQ it just sent, proceed to step 10.  Otherwise, if the
       local PE receives an ICRQ from the same remote PE, proceed
       to step 11.
   10. The local PE receives a response from the remote PE.  If
       it is a CDN, go back to step 2.  If it's an ICRP, the local
       PE binds the source forwarder to the pseudowire and sends
       an ICCN to the remote PE.  Go back to step 2.
   11. If the local PE receives an ICRQ from the same remote PE,
       it needs to perform session tie detection, as described in
       Section 5.2.  If a session tie is detected, the PE performs
       tie breaking.
   12. If the local PE loses the tie breaker, it sends a CDN with
       the result code that indicates that the disconnection is due to
       losing the tie breaker.  Proceed to step 14.
   13. If the local PE wins the tie breaker, it ignores the remote
       PE's ICRQ, but acknowledges receipt of the control message
       and continues waiting for the response from the remote PE.
       Go to step 10.
   14. The local PE determines whether it should accept the
       connection request, as described in Section 5.1.
       If it accepts the ICRQ, it sends an ICRP to the remote PE.
   15. The local PE receives a response from the remote PE.  If
       it is a CDN, go back to step 2.  If it is an ICCN, the local
       PE binds the source forwarder to the pseudowire, go back
       to step 2.
 The following diagram illustrates the above procedure:

Luo Standards Track [Page 10] RFC 4667 L2VPN Extensions for L2TP September 2006

  1. ——–> Pick Next

| Source Forwarder

        |                 |
        |                 |
        |                 v                  N
        |        Found Source Forwarder? ----------> End
        |                 |
        |              Y  |
        |                 v
        |              Pick Next     <--------------------------------
        |           Target Forwarder                                 |
        |                 |                                          |
        |                 |                                          |
        |  N              v                                          |
        -------- Found Target Forwarder?                             |
                          |                                          |
                       Y  |                                          |
                          v             Y                        Y   |
                    Same Router ID? ------> Same Forwarder ID? ------|
                          |                         |                |
                       N  |                      N  |                |
                          |                         v                |
                          |                      Create Local -------|
                          v                      Cross-connect       |
                  Pseudowire Already    Y                            |
                  Established Between -------------------------------|
                  Source and Target?                                 |
                          |                                          |
                       N  |                                          |
                          v                                          |
               Local Initiates Pseudowire                            |
             Connection Request to Remote                            |
                          |                                          |
                          |                                          |
                          v                                          |
    ------->    Local Wait for Message                               |
    |           ----- from Remote   --------------                   |
    |           |                                |                   |
    |           |                                |                   |
    |           v                                v                   |
    |   Local Receives Pseudowire      Local Receives Pseudowire     |
    |     Connection Request             Connection Response         |
    |       from Remote                     from Remote              |
    |           |                                |                   |
    |           |                                |                   |
    |           v                                v             N     |
    |   Perform Pseudowire              Connection Accepted? --------|
    |   Tie Detection                            |                   |

Luo Standards Track [Page 11] RFC 4667 L2VPN Extensions for L2TP September 2006

    |           |                             Y  |                   |
    |           |                                v                   |
    |           |                        Local Binds Source ---------|
    |           |                      Forwarder to Pseudowire       |
    |           |                                                    |
    |           v             N                  N                   |
    |       Tie Detected?  -----> Accept Remote ----->  Reject ------|
    |           |             Connection Request?    Remote Request  |
    |        Y  |                        ^   |                       |
    |           v                        |   |   Y                   |
    |   Perform Tie Breaking             |   ------>  Local Binds ----
    |           |                        |         Source Forwarder
    |           |                        |           to Pseudowire
    |           v             N          |
    |   Won Tie Breaking?  ------>   Disconnect
    |           |                  Local Connection
    |        Y  |
    |           v
    ------ Ignore Remote
          Connection Request

6. IANA Considerations

 The IANA registry procedure in this document follows that in Section
 10 of [RFC3931].  The IANA has assigned the following new values for
 existing registries managed by IANA.
 This document defines three new L2TP control message Attribute Value
 Pairs (AVPs) that have been assigned by the IANA.  These are
 described in Section 4.3 and are summarized below:
   89 - Attachment Group Identifier
   90 - Local End Identifier
   91 - Interface Maximum Transmission Unit
 Sections 4.3 and 5.1 define three new result codes for the CDN
 message that have been assigned by the IANA:
   23 - Mismatching interface MTU
   24 - Attempt to connect to non-existent forwarder
   25 - Attempt to connect to unauthorized forwarder

7. Security Considerations

 This specification does not introduce any additional security
 considerations beyond those discussed in [RFC3931] and [L2VPNFW].

Luo Standards Track [Page 12] RFC 4667 L2VPN Extensions for L2TP September 2006

8. Acknowledgement

 The author would like to thank Mark Townsley and Carlos Pignataro for
 their valuable input.

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
            Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

9.2. Informative References

 [PWE3IANA] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
            Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
 [L2VPNFW]  Andersson L., Ed. and E. Rosen, Ed., "Framework for Layer
            2 Virtual Private Networks (L2VPNs)", RFC 4664, September
            2006.
 [PWE3L2TP] W. Townsley, "Pseudowires and L2TPv3", Work in Progress.
 [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
            G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
            RFC 2661, August 1999.

Author's Address

 Wei Luo
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 EMail: luo@cisco.com

Luo Standards Track [Page 13] RFC 4667 L2VPN Extensions for L2TP September 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Luo Standards Track [Page 14]

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