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Versions: 00 01 02 03 04 RFC 4454

Network Working Group                                      Sanjeev Singh
Internet-Draft                                          W. Mark Townsley
Category: Standards Track                               Carlos Pignataro
Expiration Date: July 2006                                 Cisco Systems

                                                            January 2006

                            ATM over L2TPv3

                   draft-ietf-l2tpext-pwe3-atm-04.txt


Status of this Memo

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   applicable patent or other IPR claims of which he or she is aware
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Abstract


   The Layer 2 Tunneling Protocol, Version 3, (L2TPv3) defines an
   extensible tunneling protocol to transport layer 2 services over IP
   network. This document describes the specifics of how to use the L2TP
   control plane for Asynchronous Transfer Mode (ATM) Pseudowires and
   provides guidelines for transporting various ATM services over an IP
   network.






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   Contents

   Status of this Memo..........................................    1

   1. Introduction..............................................    3
      1.1 Abbreviations.........................................    3

   2. Control Connection Establishment..........................    4

   3. Session Establishment and ATM Circuit Status Notification.    4
      3.1 L2TPv3 Session Establishment..........................    4
      3.2 L2TPv3 Session Teardown...............................    6
      3.3 L2TPv3 Session Maintenance............................    6

   4. Encapsulation.............................................    7
      4.1 ATM-Specific Sublayer.................................    7
      4.2 Sequencing............................................    9

   5. ATM Transport.............................................    9
      5.1 ATM AAL5-SDU Mode.....................................   10
      5.2 ATM Cell Mode.........................................   10
         5.2.1 ATM VCC Cell-Relay Service.......................   11
         5.2.2 ATM VPC Cell-Relay Service.......................   12
         5.2.3 ATM Port Cell-Relay Service......................   12
      5.3 OAM Cell Support......................................   12
         5.3.1 VCC switching....................................   12
         5.3.1 VPC switching....................................   13

   6. ATM Maximum Concatenated Cells AVP........................   13

   7. OAM Emulation Required AVP................................   13

   8. ATM defects mapping and status notification...............   14
      8.1 ATM Alarm Status AVP..................................   14

   9. Applicability Statement...................................   15
      9.1 ATM AAL5-SDU Mode.....................................   16
      9.2 ATM Cell-Relay Mode...................................   18

   10. Congestion Control.......................................   19

   11. Security Considerations..................................   20

   12. IANA Considerations......................................   21
      12.1 L2-Specific Sublayer Type............................   21
      12.2 Control Message Attribute Value Pairs (AVPs).........   21
      12.3 Result Code AVP Values...............................   21
      12.4 ATM Alarm Status AVP Values..........................   22



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      12.5 ATM-Specific Sublayer bits...........................   22

   13. Acknowledgments..........................................   23

   14. References...............................................   23
      14.1 Normative References.................................   23
      14.2 Informative References...............................   23

   15. Authors' Addresses.......................................   25

Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  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].


1. Introduction

   This document describes the specifics of how to use the L2TP for ATM
   Pseudowires, including encapsulation, carrying various ATM services,
   such as, AAL5 SDU, ATM VCC/VPC/Port cell-relay over L2TP, and mapping
   ATM defects to L2TP Set Link Info (SLI) message to notify the peer
   LCCE.

   Any ATM specific AVPs or other L2TP constructs for ATM Pseudowire
   (ATMPW) support are defined here as well. Support for ATM Switched
   Virtual Path/Connection (SVP/SVC) and Soft Permanent Virtual
   Path/Connection (SPVP/SPVC) are outside the scope of this document.

   The reader is expected to be very familiar with the terminology and
   protocol constructs defined in [RFC3931].

1.1 Abbreviations

   AIS     Alarm Indication Signal
   ATMPW   ATM Pseudowire
   AVP     Attribute Value Pair
   CC      Continuity Check OAM Cell
   CE      Customer Edge
   HEC     Header Error Control
   LAC     L2TP Access Concentrator (See [RFC3931])
   LCCE    L2TP control connection endpoint (See [RFC3931])
   MSB     Most Significant Byte
   OAM     Operation, Administration, and Management
   PE      Provider Edge



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   PSN     Packet Service Network
   PWE3    Pseudowire Edge-to-edge emulation
   RDI     Remote Defect Indicator
   SDU     Service Data Unit
   SLI     Set Link Info, an L2TP control message
   SVC     Switched Virtual Connection
   SVP     Switched Virtual Path
   SPVC    Soft Permanent Virtual Connection
   SPVP    Soft Permanent Virtual Path
   VC      Virtual Circuit
   VCC     Virtual Channel Connection
   VCI     Virtual Channel Identifier
   VPC     Virtual Path Connection
   VPI     Virtual Path Identifier

2. Control Connection Establishment

   To emulate, ATM Pseudowires using L2TP, an L2TP Control Connection as
   described in Section 3.3 of [RFC3931] MUST be established.

   The SCCRQ and corresponding SCCRP MUST include the supported ATM
   Pseudowire Types (See Section 3.1), in the Pseudowire Capabilities
   List as defined in Section 5.4.3 of [RFC3931]. This identifies the
   control connection as able to establish L2TP sessions in support of
   the ATM Pseudowires.

   An LCCE MUST be able to uniquely identify itself in the SCCRQ and
   SCCRP messages via a globally unique value. By default, this is
   advertised via the structured Router ID AVP [RFC3931], though the
   unstructured Hostname AVP [RFC3931] MAY be used to identify LCCEs via
   this value.

3. Session Establishment and ATM Circuit Status Notification

   This section describes how L2TP ATMPWs or sessions are established
   between two LCCEs. This includes what will happen when an ATM Circuit
   (e.g. AAL5 PVC) is created, deleted or changes state when circuit
   state is in alarm.

3.1 L2TPv3 Session Establishment

   ATM Circuit (e.g. an AAL5 PVC) creation triggers establishment of a
   L2TP session using three-way handshake described in Section 3.4.1 of
   [RFC3931]. An LCCE MAY initiate the session immediately upon ATM
   circuit creation, or wait until the Circuit state transitions to
   ACTIVE before attempting to establish a session for the ATM circuit.
   It MAY be preferred to wait until Circuit status transitions to
   ACTIVE in order to avoid wasting L2TP resources.



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   The Circuit Status AVP (see Section 8) MUST be present in the ICRQ
   and ICRP messages, and MAY be present in the SLI message for ATMPWs.

   The following figure shows how L2TP messages are exchanged to setup
   an ATMPWs after ATM Circuit (e.g. an AAL5 PVC) becomes ACTIVE.

          LCCE (LAC) A                                  LCCE (LAC) B
      ------------------                            --------------------

       ATM Ckt Provisioned
                                                    ATM Ckt Provisioned
       ATM Ckt ACTIVE
                       ICRQ (status = 0x03) ---->
                                                    ATM Ckt ACTIVE
                       <----- ICRP (status = 0x03)
       L2TP session established
       OK to send data into PW

                       ICCN ----->
                                               L2TP session established
                                               OK to send data into PW

   The following signaling elements are required for the ATMPW
   establishment.

   a. Pseudowire Type: One of the supported ATM related PW Types should
      be present in the Pseudowire Type AVP of [RFC3931].

      0x0002  ATM AAL5 SDU VCC transport
      0x0003  ATM Cell transport Port Mode
      0x0009  ATM Cell transport VCC Mode
      0x000A  ATM Cell transport VPC Mode

   The above Cell-Relay modes can also signal the ATM Cell Concatenation
   AVP as described in Section 6.

   b. Remote End ID: Each PW is associated with a Remote End ID
      akin to the VC-ID in [PWE3ATM]. Two LCCEs of a PW would have the
      same Remote End ID and its format is described in Section 5.4.4
      of [RFC3931].

      This Remote End ID AVP MUST be present in the ICRQ in order for
      the remote LCCE to associate the session to the ATM Circuit. The
      Remote End Identifier AVP defined in [RFC3931] is of opaque form,
      though ATMPW implementations MAY simply use a four-octet value
      that is known to both LCCEs (either by direct configuration, or
      some other means). The exact method of how this value is
      configured, retrieved, discovered, or otherwise determined at



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      each LCCE is outside the scope of this document.

   As with the ICRQ, the ICRP is sent only after the ATM Circuit
   transitions to ACTIVE. If LCCE B had not been provisioned yet for the
   ATM Circuit identified in the ICRQ, a CDN would have been immediately
   returned indicating that the circuit was either not provisioned or is
   not available at this LCCE. LCCE A should then exhibit a periodic
   retry mechanism. The period and maximum number of retries MUST be
   configurable.

   An Implementation MAY send an ICRQ or ICRP before a PVC is ACTIVE, as
   long as the Circuit Status AVP reflects that the ATM Circuit is
   INACTIVE and an SLI is sent when the ATM Circuit becomes ACTIVE (see
   Section 8).

   The ICCN is the final stage in the session establishment. It confirms
   the receipt of the ICRP with acceptable parameters to allow
   bidirectional traffic.

3.2 L2TPv3 Session Teardown

   When an ATM Circuit is unprovisioned (deleted) at either LCCE, the
   associated L2TP session MUST be torn down via the CDN message defined
   in Section 3.4.3 of [RFC3931].

3.3 L2TPv3 Session Maintenance

   All sessions established by a given control connection utilize the
   L2TP Hello facility defined in Section 4.4 of [RFC3931] for session
   keepalive. This gives all sessions basic dead peer and path detection
   between LCCEs.

   If the control channel utilizing the Hello message is not in-band
   with data traffic over PSN, then other method MAY be used to detect
   the Session failure and it is left for further study.

   ATMPWs over L2TP use the Set Link Info (SLI) control message as
   defined in [RFC3931] to signal ATM Circuit Status between LCCEs after
   initial session establishment. This includes ACTIVE or INACTIVE
   notifications of the ATM Circuit, or any other parameters that may
   need to be shared between the LCCEs in order to provide proper PW
   emulation.

   The SLI message MUST be sent whenever there is a status change which
   may be reported by any values identified in the Circuit Status AVP.
   The only exception to this are the initial ICRQ, ICRP and CDN
   messages which establish and teardown the L2TP session itself when
   ATM circuit is created or deleted. The SLI message may be sent from



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   either LCCE at any time after the first ICRQ is sent (and perhaps
   before an ICRP is received, requiring the peer to perform a reverse
   Session ID lookup).

   The other application of the SLI message is to map the ATM OAM or
   physical layer alarms into Circuit Status AVP as described in Section
   8.

4. Encapsulation

   This section describes the general encapsulation format for ATM
   services over L2TP.

   Figure 1: General format for ATM encapsulation over L2TPv3 over IP

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     PSN Transport Header                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Session Header                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ATM-Specific Sublayer                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                      ATM Service Payload                      |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The PSN Transport header is specific to IP and its underlying
   transport header. This header is used to transport the encapsulated
   ATM payload through the IP network.

   The Session Header is a non-zero 32-bit session ID with optional
   cookies up to 64-bits. This Session ID is exchanged during session
   setup.

   The ATM Specific Sublayer is REQUIRED for AAL5 SDU mode and OPTIONAL
   for ATM Cell mode. Please refer to Section 4.1 for more details.

4.1 ATM-Specific Sublayer

   This section defines a new ATM-specific sublayer as, an alternative
   to default L2-Specific Sublayer as mentioned in Section 4.6 of
   [RFC3931].  Four new flag bits (T,G,C,U) are defined which concur
   with Section 8.2 of [PWE3ATM].

   Figure 2: ATM-Specific Sublayer Format



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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |x|S|B|E|T|G|C|U|          Sequence Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Definition of these four bits are as per Section 8.2 of [PWE3ATM] and
   also included here for reference.

      * S bit

      Definition of this bit is as per Section 4.6 of [RFC3931].

      * B and E bits

      Definition of these bits as per Section 5.5 of [L2TPFRAG]

      These bits are reserved and MUST be set to 0 upon transmission
      and ignored upon reception, unless otherwise, these bits are
      used as per [L2TPFRAG].

      * T (Transport type) bit

      Bit (T) of the control word indicates whether the packet
      contains an ATM admin cell or an AAL5 payload. If T = 1, the
      packet contains an ATM admin cell, encapsulated according to
      the VCC cell relay encapsulation of Section 5.2.
      If not set, the PDU contains an AAL5 payload. The ability to
      transport an ATM cell in the AAL5 SDU mode is intended to
      provide a means of enabling administrative functionality over
      the AAL5 VCC (though it does not endeavor to preserve user-cell
      and admin-cell arrival/transport ordering).

      * G (EFCI) Bit

      The ingress LCCE device SHOULD set this bit to 1 if the EFCI bit
      of the final cell of the incoming AAL5 payload is set to 1, or
      if the EFCI bit of the single ATM cell to be transported in
      the packet is set to 1.  Otherwise this bit SHOULD be set to
      0.  The egress LCCE device SHOULD set the EFCI bit of all the
      outgoing cells that transport the AAL5 payload to the value
      contained in this field.

      * C (CLP) Bit

      The ingress LCCE device SHOULD set this bit to 1 if the CLP bit
      of any of the incoming ATM cells of the AAL5 payload are set
      to 1, or if the CLP bit of the single ATM cell that is to be



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      transported in the packet is set to 1.  Otherwise this bit
      SHOULD be set to 0. The egress LCCE device SHOULD set the CLP
      bit of all outgoing cells that transport the AAL5 CPCS-PDU to
      the value contained in this field.


      * U (Command/Response) Bit

      When FRF.8.1 Frame Relay / ATM PVC Service Interworking (see
      [FRF8.1]) traffic is being transported, the CPCS-UU Least
      Significant Bit (LSB) of the AAL5 CPCS-PDU may contain the
      Frame Relay C/R bit.
      The ingress LCCE device SHOULD copy this bit to the U bit of
      the control word. The egress LCCE device SHOULD copy the
      U bit to the CPCS-UU Least Significant Bit (LSB) of the AAL5
      payload.

      The Sequence Number fields are described in Section 4.3

      In case of a reassembly timeout, the encapsulating LCCE should
      discard all component cells of the AAL5 frame.

      An additional enumeration is added to the L2-Specific Sublayer AVP
      to identify the ATM-Specific Sublayer:

         0 - There is no L2-Specific Sublayer present.
         1 - The Default L2-Specific Sublayer (defined in Section 4.6
             of [RFC3931]) is used.
         2 - The ATM-Specific Sublayer is used.

   The first two values are already defined in the L2TPv3 base
   specification [RFC3931].

4.2 Sequencing

   Data Packet Sequencing MAY be enabled for ATMPWs. The sequencing
   mechanisms described in [RFC3931] MUST be used to signal sequencing
   support. ATMPWs over L2TPv3 MUST request the presence of the ATM-
   Specific Sublayer when sequencing is enabled, and MAY request its
   presence at all times.

5. ATM Transport

   There are two encapsulations supported for ATM transport as described
   below.

   ATM Specific Sublayer is prepended to AAL5-SDU. The other Cell-mode
   encapsulation consists of the OPTIONAL ATM-Specific Sublayer and 4-



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   byte ATM Cell Header and 48-byte ATM Cell-payload.

5.1 ATM AAL5-SDU Mode

   In this mode each AAL5 VC is mapped to an L2TP session. Ingress LCCE
   reassembles AAL5 CPCS-SDU without AAL5 trailer and any padding bytes.
   Incoming EFCI, CLP and C/R (if present) are carried in ATM Specific
   sublayer across ATMPWs to egress LCCE. The processing of these bits
   on ingress and egress LCCEs is defined in Section 4.1.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |x|S|x|x|T|G|C|U|             Sequence Number                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                                                               |
   |                         AAL5 CPCS-SDU                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If ingress LCCE determines that an encapsulated AAL5 SDU exceeds the
   MTU size of the L2TPv3 session, then AAL5 SDU may be fragmented as
   per [L2TPFRAG] or underneath Transport layer (IP, etc).  F5 OAM cells
   that arrive during the reassembly of an AAL5 SDU are sent immediately
   on the PW followed by the AAL5 SDU payload. In this case OAM cell's
   relative order with respect to user data cells is not maintained.

   Performance Monitoring OAM, as specified in ITU-T 610 [I610-1],
   [I610-2], [I610-3] and security OAM cells as specified in [ATMSEC],
   should not be used in combination with AAL5 SDU mode. These cells MAY
   be dropped at ingress LCCE because cell sequence integrity is not
   maintained.

   The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
   Attribute Type 68, MUST be present in the ICRQ messages and MUST
   include the ATM AAL5 SDU VCC transport PW Type of 0x0002.

5.2 ATM Cell Mode

   In this mode, ATM cells skip the reassembly process at ingress LCCE.
   These cells are transported over an L2TP session, either as a single
   Cell or as concatenated cells, into a single packet. Each ATM Cell
   consists of 4 byte ATM cell header and 48-byte ATM Cell-payload, HEC
   is not included.

   In ATM Cell Mode encapsulation, ATM-Specific Sublayer is OPTIONAL.
   It can be included, if sequencing support is required. It is left to



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   the implementation to choose to signal Default L2-Specific Sublayer
   or ATM-Specific Sublayer.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |x|S|x|x|x|x|x|x|          Sequence Number (Optional)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        VPI            |           VCI                 |PTI  |C|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    ATM Cell Payload (48-bytes)                |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               "
                               "
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        VPI            |           VCI                 |PTI  |C|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    ATM Cell Payload (48-bytes)                |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the simplest case, this encapsulation can be used to transmit
   a single ATM cell per Pseudowire PDU. However, in order to
   provide better Pseudowire bandwidth efficiency, several ATM cells
   may be optionally encapsulated into single Pseudowire PDU.

   The maximum number of concatenated cells in a packet is limited by
   the MTU size of the session and also by the ability of egress
   LCCE to process them.  For more details about ATM Maximum
   Concatenated cells, please refer to Section 6.

5.2.1 ATM VCC Cell-Relay Service

   A VCC cell relay service may be provided by mapping an ATM Virtual
   Channel Connection to a single Pseudowire using cell mode
   encapsulation as defined in Section 5.2.

   An LCCE may map one or more VCCs to a single PW. However, a service
   provider may wish to provision a single VCC to a PW in order to
   satify QOS or restoration requirement.

   The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
   Attribute Type 68, MUST be present in the ICRQ messages and MUST
   include the ATM Cell transport VCC mode PW Type of 0x0009.




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5.2.2 ATM VPC Cell-Relay Service

   A Virtual Path Connection cell relay service may be provided by
   mapping an ATM Virtual Path Connection to single Pseudowire using
   cell mode encapsulation as defined in Section 5.2.

   An LCCE may map one or more VPCs to a single Pseudowire.

   The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
   Attribute Type 68, MUST be present in the ICRQ messages and MUST
   include the ATM Cell transport VPC mode PW Type of 0x000A.

5.2.3 ATM Port Cell-Relay Service

   ATM port cell relay service allows an ATM port to be connected to
   only another ATM port. All ATM cells that are received at the
   ingress ATM port on the LCCE, are encapsulated as per Section 5.2,
   into Pseudowire PDU and sent to peer LCCE.

   Each LCCE MUST discard any idle/unassigned cells received on an ATM
   port associated with ATMPWs.

   The Pseudowire Type AVP defined in Section 5.4.4 of [RFC3931],
   Attribute Type 68, MUST be present in the ICRQ messages and MUST
   include the ATM Cell transport Port mode PW Type of 0x0003.

5.3 OAM Cell Support

   The OAM cells are defined in [I610-1], [I610-2], [I610-3] and
   [ATMSEC] can be categorized as:

      a. Fault Management
      b. Performance monitoring and reporting
      c. Activation/deactivation
      d. System Management (e.g. security OAM cells).

   OAM Cells are always encapsulated using cell mode encapsulation,
   regardless of the encapsulation format used for user data.

5.3.1 VCC switching

   The LCCEs SHOULD be able to pass the F5 segment and end-to-end Fault
   Management, Resource Management (RM cells), Performance Management,
   Activation/deactivation and System Management OAM cells.

   F4 OAM cells are inserted or extracted at the VP link termination.
   These OAM cells are not seen at the VC link termination and are
   therefore not sent across the PW.



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5.3.1 VPC switching

   The LCCEs MUST be able to pass the F4 segment and end-to-end Fault
   Management, Resource Management (RM cells), Performance Management,
   Activation/deactivation and System Management OAM cells transparently
   according to [I610-1].

   F5 OAM cells are not inserted or extracted at the VP cross-connect.
   The LCCEs MUST be able to pass the F5 OAM cells transparently across
   the PW.

6. ATM Maximum Concatenated Cells AVP

   The "ATM Maximum Cells Concatenated AVP", Attribute type 86,
   indicates that the egress LCCE node can process a single PDU with
   concatenated cells upto a specified number of cells. An LCCE
   node transmitting concatenated cells on this PW MUST not exceed
   the maximum number of cells as specified in this AVP. This AVP
   is applicable only to ATM Cell-Relay PW Types (VCC, VPC, Port
   Cell-Relay). This Attribute value may not be same in both
   directions of the specific PW.

   The Attribute Value field for this AVP has the following format:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | ATM Maximum Concatenated Cells|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
   AVP MAY be set to 0, but MAY vary (see Section 5.2 of [RFC3931]).
   The length (before hiding) of this AVP is 8.

   This AVP is sent in an ICRQ, ICRP during session negotiation or via
   SLI control messages when LCCE changes the maximum number of
   Concatenated Cells configuration for a given ATM cell-relay Circuit.

   This AVP is OPTIONAL. If egress LCCE is configured with maximum
   number of cells to be concatenated by ingress LCEE, it should signal
   to ingress LCCE.

7. OAM Emulation Required AVP

   An "OAM Emulation Required AVP", Attribute type 87, MAY be needed to
   signal OAM Emulation in AAL5 SDU mode, if LCCE can not support
   transport of OAM cells across L2TP session. If OAM Cell Emulation is
   configured or detected via some other means on one side, the other



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   LCCE MUST support OAM Cell Emulation as well.

   This AVP is exchanged during session negotiation (in ICRQ, ICRP) or
   during life of the session via SLI control message. If the other LCCE
   can not support the OAM Cell Emulation, the associated L2TP session
   MUST be torn down via CDN message with result code 22.

   OAM Emulation AVP is a boolean AVP, having no Attribute Value. Its
   absence is FALSE and its presence is TRUE. This AVP MAY be hidden
   (the H bit MAY be 0 or 1). The M bit for this AVP SHOULD be set to 0,
   but MAY vary (see Section 5.2 of [RFC3931]). The Length (before
   hiding) of this AVP is 6.

8. ATM defects mapping and status notification

   ATM OAM alarms or circuit status is indicated via Circuit Status AVP
   as defined in  Section 5.4.5 of [RFC3931]. For reference, usage of
   this AVP is shown below.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved        |N|A|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Value is a 16 bit mask with the two least significant bits
   defined and the remaining bits are reserved for future use. Reserved
   bits MUST be set to 0 when sending, and ignored upon receipt.

   The A (Active) bit indicates whether the ATM Circuit is ACTIVE (1) or
   INACTIVE (0).

   The N (New) bit indicates whether the ATM circuit status indication
   is for a new Circuit (1) or an existing ATM Circuit (0).

8.1 ATM Alarm Status AVP

   An "ATM Alarm Status AVP", Attribute type 88, indicates the reason
   for the ATM circuit status and specific alarm type, if any, to its
   peer LCCE node. This OPTIONAL AVP MAY be present in SLI message with
   Circuit Status AVP.

   The Attribute Value field for this AVP has the following format:








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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Circuit Status Reason     |            Alarm              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Circuit status reason is a 2-octets unsigned integer and Alarm
   Type is also a 2-octets unsigned integer.

   This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
   AVP SHOULD be set to 0, but MAY vary (see Section 5.2 of [RFC3931]).
   The Length (before hiding) of this AVP is 10 octets.

   This AVP is sent in SLI message to indicate the additional
   information about the ATM circuit status.

   Circuit Status Reason values for the SLI message are as follows:

           0 - Reserved
           1 - No alarm or alarm cleared (default for Active Status)
           2 - Unspecified or unknown Alarm Received (default for
               Inactive Status)
           3 - ATM Circuit received F1 Alarm on ingress LCCE
           4 - ATM Circuit received F2 Alarm on ingress LCCE
           5 - ATM Circuit received F3 Alarm on ingress LCCE
           6 - ATM Circuit received F4 Alarm on ingress LCCE
           7 - ATM Circuit received F5 Alarm on ingress LCCE
           8 - ATM Circuit down due to ATM Port shutdown on Peer LCCE
           9 - ATM Circuit down due to loop-back timeout on ingress LCCE

   The general ATM Alarm failures are encoded as below:

           0 - Reserved
           1 - No Alarm type specified (default)
           2 - Alarm Indication Signal (AIS)
           3 - Remote Defect Indicator (RDI)
           4 - Loss of Signal (LOS)
           5 - Loss of pointer (LOP)
           6 - Loss of framer (LOF)
           7 - loopback cells (LB)
           8 - Continuity Check (CC)

9. Applicability Statement

   The ATM Pseudowire emulation described in this document allows for
   carrying various ATM services across an IP packet switched network
   (PSN). These ATM services can be PVC-based, PVP-based or Port-based.
   In all cases, ATMPWs operate in a point-to-point deployment model.



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   ATMPWs support two modes of encapsulation: ATM AAL5-SDU Mode and ATM
   Cell-Relay Mode. The following sections list their respective
   characteristics in relationship to the native service.

9.1 ATM AAL5-SDU Mode

   ATMPWs operating in AAL5-SDU Mode only support the transport of PVC-
   based services.  In this mode, the AAL5 CPCS-PDU from a single VCC is
   reassembled at the ingress LCCE, and the AAL5 CPCS-SDU (i.e., the
   AAL5 CPCS-PDU without CPCS-PDU Trailer or PAD octets, also referred
   to as AAL5 CPCS-PDU Payload) is transported over the Pseudowire.
   Therefore, Segmentation and Reassembly (SAR) functions are required
   at the LCCEs.  There is a one-to-one mapping between an ATM PVC and
   an ATMPW operating in AAL5-SDU Mode, supporting bi-directional
   transport of variable length frames. With the exception of optionally
   transporting OAM cells, only ATM Adaptation Layer (AAL) type 5 frames
   are carried in this mode, including Multiprotocol over AAL5 packets
   [RFC2684].

   The following considerations stem from ATM AAL5-SDU Mode Pseudowires
   not transporting the ATM cell headers and AAL5 CPCS-PDU Trailer (see
   Section 5.1):

      o An ATMPW operating in AAL5-SDU Mode conveys EFCI and CLP
        information using the G and C bits in the ATM-Specific Sublayer.
        In consequence, the EFCI and CLP value of individual ATM cells
        that consititute the AAL5 frame may be lost across the ATMPW,
        and CLP and EFCI transparency may not be maintained. The AAL5-
        SDU Mode does not preserve EFCI and CLP value for every ATM cell
        within the AAL5 PDU. The processing of these bits on ingress and
        egress is defined in Section 4.1.

      o Only the Least Significant Bit (LSB) from the CPCS-UU (User-to-
        User indication) field in the CPCS-PDU Trailer is transported
        using the ATM-Specific Sublayer (see Section 4.1).  This bit
        contains the Frame Relay C/R bit when FRF.8.1 Frame Relay / ATM
        PVC Service Interworking [FRF8.1] is used. The CPCS-UU field is
        not used in Multiprotocol Over AAL5 [RFC2684].  However,
        applications that transfer user to user information using the
        CPCS-UU octet would fail to operate.

      o The CPI (Common Part Indicator) field in the CPCS-PDU Trailer is
        also not transported across the ATMPW.  This does not affect
        Multiprotocol Over AAL5 applications since the field is used for
        alignment and MUST be coded as 0x00 [RFC2684].

      o The trailing CRC field in the CPCS-PDU is stripped at the
        ingress LCCE and not transported over the ATMPW operating in



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        AAL5-SDU Mode. It is in turn regenerated at the egress LCCE.
        Since the CRC has end-to-end significance, this means that
        errors introduced in the ATMPW payload during encapsulation or
        transit across the packet switched network may not be detected.
        To allow for payload integrity checking transparency on ATMPWs
        operating in AAL5-SDU Mode using L2TP over IP or L2TP over
        UDP/IP, the L2TPv3 session can utilize IPSec as specified in
        Section 4.1.3 of [RFC3931].

   Some additional characteristics of the AAL5-SDU Mode are:

      o The status of the ATM PVC is signaled between LCCEs using the
        Circuit Status AVP.  More granular cause values for the ATM
        circuit status and specific ATM alarm types are signaled using
        the ATM Alarm Status AVP (see Section 8.1).  Additionally, loss
        of connectivity between LCCEs can be detected by the L2TPv3
        keepalive mechanism (see Section 4.4 in [RFC3931]).

      o F5 OAM cell's relative order with respect to user data cells may
        not be maintained.  F5 OAM cells that arrive during the
        reassembly of an AAL5 SDU are sent immediately over the PW and
        before the AAL5 SDU payload. At egress, these OAM cells are sent
        before the cells that comprise the AAL5-SDU.  Therefore,
        applications that rely on cell sequence integrity between OAM
        and user data cells may not work. This includes Performance
        Monitoring and Security OAM cells (see Section 5.1). In
        addition, the AAL5-SDU service allows for OAM Emulation in which
        OAM cells are not transported over the ATMPW (see Section 7).
        This is advantageous for AAL5-SDU mode ATMPW implementations
        that do not support cell transport using the T-bit.

      o Fragmentation and Reassembly procedures may be used, both as
        specified in Section 5 of [L2TPFRAG] or in the underlying PSN
        (i.e., IP, etc) between tunnel endpoints as discussed in Section
        4.1.4 of [RFC3931]. The procedures described in [L2TPFRAG] can
        be used to support the maximum size of an AAL5 SDU, 2 ^ 16 - 1
        (65535) octets. However, relying on fragmentation on the
        L2TP/IPv4 packet between tunnel endpoints limits the maximum
        size of the AAL5 SDU that can be transported, because the
        maximum total length of an IPv4 datagram is already 65535
        octets. In this case, the maximum AAL5 SDU that can be
        transported is limited to 65535 minus the encapsulating headers,
        24-36 octets for L2TP-over-IPv4 or 36-48 octets for L2TP-over-
        UDP/IPv4.
        When the AAL5 payload is IPv4, an additional option is to
        fragment IP packets before tunnel encapsulation with L2TP/IP
        (see Section 4.1.4 of [RFC3931]).




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      o Sequencing may be enabled on the ATMPW using the ATM-Specific
        Sublayer Sequence Number field, to detect lost, duplicate, or
        out-of-order frames on a per-session basis (see Section 4.2).

      o Quality of Service characteristics such as throughput (cell
        rates), burst sizes and delay variation can be provided by
        leveraging Quality of Service features of the LCCEs and the
        underlying PSN, increasing the faithfulness of ATMPWs.  This
        includes mapping ATM service categories to a compatible PSN
        class of service.

9.2 ATM Cell-Relay Mode

   In this mode, no reassembly takes place at the ingress LCCE.  There
   are no SAR requirements for LCCEs.  Instead, ATM-Layer cells are
   transported over the ATMPW.  Consequently, all AAL types can be
   transported over ATMPWs operating in Cell-Relay Mode.  ATM Cell-Relay
   Pseudowires can operate in three different modes (see Section 5.2):
   ATM VCC, ATM VPC and ATM Port Cell-Relay Services.  The following are
   some of their characteristics:

      o The ATM cells transported over Cell-Relay Mode ATMPWs consist of
        a 4 byte ATM cell header and a 48-byte ATM Cell-payload (see
        Section 5.2).  The ATM Service Payload of a Cell-Relay Mode
        ATMPW is a multiple of 52 bytes.  The Header Error Checksum
        (HEC) in the ATM cell header containing a CRC (Cyclic Redundancy
        Check) calculated over the first 4 bytes of the ATM cell header
        is not transported. Accordingly, the HEC field may not
        accurately reflect errors on an end-to-end basis; errors or
        corruption in the 4-byte ATM cell header introduced in the ATMPW
        payload during encapsulation or transit across the PSN may not
        be detected. To allow for payload integrity checking
        transparency on ATMPWs operating in Cell-Relay Mode using L2TP
        over IP or L2TP over UDP/IP, the L2TPv3 session can utilize
        IPSec as specified in Section 4.1.3 of [RFC3931].

      o ATM PWs operating in Cell-Relay mode can transport a single ATM
        cell or multiple concatenated cells (see Section 6). Cell
        concatenation improves the bandwidth efficiency of the ATMPW (by
        decreasing the overhead) but introduces latency and delay
        variation.

      o The status of the ATM PVC is signaled between LCCEs using the
        Circuit Status AVP.  More granular cause values for the ATM
        circuit status and specific ATM alarm types are signaled using
        the ATM Alarm Status AVP (see Section 8.1).  Additionally, loss
        of connectivity between LCCEs can be detected by the L2TPv3
        keepalive mechanism (see Section 4.4 in [RFC3931]).



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      o ATM OAM cells are transported in the same fashion as user cells,
        and in the same order as they are received.  Therefore,
        applications that rely on cell sequence integrity between OAM
        and user data cells are not adversely affected. This includes
        performance management and security applications that utilize
        OAM cells (see Section 5.3).

      o The maximum number of concatenated cells is limited by the MTU
        size of the session (see Section 5.2 and Section 6).  Therefore,
        Fragmentation and Reassembly procedures are not used for Cell-
        Relay ATMPWs. Concatenating cells to then fragment the resulting
        packet defeats the purpose of cell concatenation. Concatenation
        of cells and fragmentation act as inverse functions, with
        additional processing but null net effect, and should not be
        used together.

      o Sequencing may be enabled on the ATMPW to detect lost,
        duplicate, or out-of-order packets on a per-session basis (see
        Section 4.2).

      o Quality of Service characteristics such as throughput (cell
        rates), burst sizes and delay variation can be provided by
        leveraging Quality of Service features of the LCCEs and the
        underlying PSN, increasing the faithfulness of ATMPWs.  This
        includes mapping ATM service categories to a compatible PSN
        class of service, and mapping CLP and EFCI bits to PSN classes
        of service.  For example, mapping a CBR PVC to a class of
        service with tight loss and delay characteristics, such as an EF
        PHB if the PSN is an IP DiffServ-enabled domain.  The following
        characteristics of ATMPWs operating in Cell-Relay mode include
        additional QoS considerations:

           - ATM Cell transport VCC Pseudowires allow for mapping
             multiple ATM VCCs to a single ATMPW. However a user may
             wish to map a single ATM VCC per ATMPW to satisfy QoS
             requirements (see Section 5.2.1).

           - Cell-Relay ATMPWs allow for concatenating multiple cells in
             a single Pseudowire PDU to improve bandwidth efficiency,
             but may introduce latency and delay variation.

10. Congestion Control

   As explained in [RFC3985], the PSN carrying the PW may be subject to
   congestion, with congestion characteristics depending on PSN type,
   network architecture, configuration, and loading. During congestion
   the PSN may exhibit packet loss and PDV that will impact the timing
   and data integrity of the ATMPW.  During intervals of acute



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   congestion, some Cell-Relay ATMPWs may not be able to maintain
   service.  The inelastic nature of some ATM services reduces the risk
   of congestion because the rates will not expand to consume all
   available bandwidth, but on the other hand those ATM services cannot
   arbitrarily reduce its load on the network to eliminate congestion
   when it occurs.

   Whenever possible, Cell-Relay ATMPWs should be run over traffic-
   engineered PSNs providing bandwidth allocation and admission control
   mechanisms.  IntServ-enabled domains providing the Guaranteed Service
   (GS) or DiffServ-enabled domains using Expedited Forwarding (EF) are
   examples of traffic-engineered PSNs. Such PSNs will minimize loss and
   delay while providing some degree of isolation of the Cell-Relay
   ATMPW's effects from neighboring streams.

   If the PSN is providing a best-effort service, then the following
   best-effort service congestion avoidance considerations apply: Those
   ATMPWs that carry constant bit rate (CBR) and VBR-rt (Variable Bit
   Rate-real time) services across the PSN will most probably not behave
   in a TCP-friendly manner prescribed by [RFC2914].  In the presence of
   services that reduce transmission rate, ATMPWs carrying CBR and VBR-
   rt traffic SHOULD be halted when acute congestion is detected, in
   order to allow for other traffic or the network infrastructure itself
   to continue.  ATMPWs carrying UBR (Unspecified Bit Rate) traffic,
   which are equivalent to best-effort IP service, need not be halted
   during acute congestion and MAY have cells delayed or dropped by the
   ingress PE if necessary.  ATMPWs carrying VBR-nrt (Variable Bit
   Rate-non real time) services may or may not behave in a TCP-friendly
   manner, depending on the end user application, but are most likely
   safe to continue operating, since the end-user application is
   expected to be delay-insensitive and may also be somewhat loss-
   insensitive.

   LCCEs SHOULD monitor for congestion (for example by measuring packet
   loss or as specified in Section 6.5 of [RFC3985]) in order to ensure
   that the ATM service may be maintained.  When severe congestion is
   detected (for example when enabling Sequencing and detecting that the
   packet loss is higher than a threshold) the ATM service SHOULD be
   terminated by tearing down the L2TP session via a CDN message.  The
   PW may be restarted by manual intervention, or by automatic means
   after an appropriate waiting time.

11. Security Considerations

   ATM over L2TPv3 is subject to the security considerations defined in
   [RFC3931]. There are no additional considerations specific to
   carrying ATM that are not present carrying other data link types.




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12. IANA Considerations

   The signaling mechanisms defined in this document rely upon the
   allocation of following ATM Pseudowire Types (see Pseudowire
   Capabilities List as defined in 5.4.3 of [RFC3931] and L2TPv3
   Pseudowire Types in 10.6 of [RFC3931]) by the IANA (number space
   created as part of publication of [RFC3931]):

      Pseudowire Types
      ----------------

      0x0002  ATM AAL5 SDU VCC transport
      0x0003  ATM Cell transparent Port Mode
      0x0009  ATM Cell transport VCC Mode
      0x000A  ATM Cell transport VPC Mode

12.1 L2-Specific Sublayer Type

   This number space is created and maintained per [RFC3931].

      L2-Specific Sublayer Type
      -------------------------

      2 - ATM L2-Specific Sublayer present

12.2 Control Message Attribute Value Pairs (AVPs)

   This number space is managed by IANA as per [BCP0068].

   A summary of the three new AVPs follows:

   Control Message Attribute Value Pairs

      Attribute
      Type        Description
      ---------   ----------------------------------
      86          ATM Maximum Concatenated Cells AVP
      87          OAM Emulation Required AVP
      88          ATM Alarm Status AVP


12.3 Result Code AVP Values

   This number space is managed by IANA as per [BCP0068].

   New Result Code value for the CDN message is defined in Section 7.
   Following is a summary:




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   Result Code AVP (Attribute Type 1) Values
   -----------------------------------------

   General Error Codes

         22 - Session not established due to other LCCE
              can not support the OAM Cell Emulation,

12.4 ATM Alarm Status AVP Values

   This is a new registry for IANA to maintain.

   New Attribute values for the SLI message is defined in Section 8.
   Following is a summary:

   ATM Alarm Status AVP (Attribute Type 88) Values
   -----------------------------------------------

   Circuit Status Reason values for the SLI message are as follows:

           0 - Reserved
           1 - No alarm or alarm cleared (default for Active Status)
           2 - Unspecified or unknown Alarm Received (default for
               Inactive Status)
           3 - ATM Circuit received F1 Alarm on ingress LCCE
           4 - ATM Circuit received F2 Alarm on ingress LCCE
           5 - ATM Circuit received F3 Alarm on ingress LCCE
           6 - ATM Circuit received F4 Alarm on ingress LCCE
           7 - ATM Circuit received F5 Alarm on ingress LCCE
           8 - ATM Circuit down due to ATM Port shutdown on Peer LCCE
           9 - ATM Circuit down due to loop-back timeout on ingress LCCE

   The general ATM Alarm failures are encoded as below:

           0 - Reserved
           1 - No Alarm type specified (default)
           2 - Alarm Indication Signal (AIS)
           3 - Remote Defect Indicator (RDI)
           4 - Loss of Signal (LOS)
           5 - Loss of pointer (LOP)
           6 - Loss of framer (LOF)
           7 - loopback cells (LB)
           8 - Continuity Check (CC)

12.5 ATM-Specific Sublayer bits

   This is a new registry for IANA to maintain.




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   The ATM-Specific Sublayer contains 8 bits in the low-order portion of
   the header. Reserved bits may be assigned by IETF Consensus
   [RFC2434].

      Bit 0 - Reserved
      Bit 1 - S (Sequence) bit
      Bit 2 - B (Fragmentation) bit
      Bit 3 - E (Fragmentation) bit
      Bit 4 - T (Transport type) bit
      Bit 5 - G (EFCI) bit
      Bit 6 - C (CLP) bit
      Bit 7 - U (Command/Response) bit



13. Acknowledgments

   Thanks for the contribution from Jed Lau, Pony Zhu, Prasad Yaditi,
   Durai and Jaya Kumar.

   Many Thanks to Srinivas Kotamraju for editorial review.

   Thanks to Shoou Yiu and Fred Shu for their valuable time to review
   this document.

14. References

14.1 Normative References

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

14.2 Informative References

   [PWE3ATM]  Martini, L., "Encapsulation Methods for Transport of ATM
              Over MPLS Networks", draft-ietf-pwe3-atm-encap-10 (work in
              progress), September 2005.

   [L2TPFRAG] Malis, A. and M. Townsley, "PWE3 Fragmentation and
              Reassembly", draft-ietf-pwe3-fragmentation-10 (work in
              progress), November 2005.

   [FRF8.1]   "Frame Relay / ATM PVC Service Interworking
              Implementation Agreement (FRF 8.1)", Frame Relay
              Forum 2000.



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   [BCP0068]  Townsley, W., "Layer Two Tunneling Protocol (L2TP)
              Internet Assigned Numbers Authority (IANA) Considerations
              Update", BCP 68, RFC 3438, December 2002.


   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [I610-1]   ITU-T Recommendation I.610 (1999): B-ISDN operation and
              maintenance principles and functions

   [I610-2]   ITU-T Recommendation I.610, Corrigendum 1 (2000):
              B-ISDN operation and maintenance principles and
              functions (corrigendum 1)

   [I610-3]   ITU-T Recommendation I.610, Amendment 1 (2000): B-ISDN
              operation and maintenance principles and functions
              (Amendment 1)

   [ATMSEC]   ATM Forum Specification, af-sec-0100.002 (2001): ATM
              Security Specification version 1.1

   [RFC2684]  Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation
              over ATM Adaptation Layer 5", RFC 2684, September 1999.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, September 2000.




















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15. Authors' Addresses

   Sanjeev Singh
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   sanjeevs@cisco.com

   W. Mark Townsley
   Cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709
   mark@townsley.net

   Carlos Pignataro
   Cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709
   cpignata@cisco.com






























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   this standard.  Please address the information to the IETF at
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Disclaimer of Validity

   This document and the information contained herein are provided on
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   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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.

Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.





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