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rfc:rfc4454

Network Working Group S. Singh Request for Comments: 4454 M. Townsley Category: Standards Track C. Pignataro

                                                         Cisco Systems
                                                              May 2006
               Asynchronous Transfer Mode (ATM) over
           Layer 2 Tunneling Protocol Version 3 (L2TPv3)

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, Version 3 (L2TPv3) defines an
 extensible tunneling protocol to transport layer 2 services over IP
 networks.  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.

Table of Contents

 1. Introduction ....................................................2
    1.1. Abbreviations ..............................................3
    1.2. Specification of Requirements ..............................3
 2. Control Connection Establishment ................................3
 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 ...................................................6
    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

Singh, et al. Standards Track [Page 1] RFC 4454 ATM over L2TPv3 May 2006

         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
         5.3.2. VPC Switching ......................................13
 6. ATM Maximum Concatenated Cells AVP .............................13
 7. OAM Emulation Required AVP .....................................14
 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 ............................................20
 11. Security Considerations .......................................21
 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 ...................................22
    12.4. ATM Alarm Status AVP Values ..............................22
    12.5. ATM-Specific Sublayer Bits ...............................23
 13. Acknowledgements ..............................................23
 14. References ....................................................23
    14.1. Normative References .....................................23
    14.2. Informative References ...................................24

1. Introduction

 This document describes the specifics of how to use the Layer 2
 Tunneling Protocol (L2TP) for Asynchronous Transfer Mode (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) messages to notify the peer
 L2TP Control Connection Endpoint (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].

Singh, et al. Standards Track [Page 2] RFC 4454 ATM over L2TPv3 May 2006

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 Checksum
 LAC     L2TP Access Concentrator (see [RFC3931])
 LCCE    L2TP Control Connection Endpoint (see [RFC3931])
 MSB     Most Significant Byte
 OAM     Operation, Administration, and Maintenance
 PE      Provider Edge
 PSN     Packet Switched Network
 PWE3    Pseudowire Emulation Edge to Edge
 RDI     Remote Defect Indicator
 SAR     Segmentation and Reassembly
 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

1.2. 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].

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 Start-Control-Connection-Request (SCCRQ) and corresponding
 Start-Control-Connection-Reply (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.

Singh, et al. Standards Track [Page 3] RFC 4454 ATM over L2TPv3 May 2006

 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 as
 well.

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 an
 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 delay the allocation of resources until absolutely
 necessary.
 The Circuit Status AVP (see Section 8) MUST be present in the
 Incoming-Call-Request (ICRQ) and Incoming-Call-Reply (ICRP) messages,
 and MAY be present in the SLI message for ATMPWs.
 The following figure shows how L2TP messages are exchanged to set up
 an ATMPW after the 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

Singh, et al. Standards Track [Page 4] RFC 4454 ATM over L2TPv3 May 2006

 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 Maximum
 Concatenated Cells 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 4-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
    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 Call-Disconnect-Notify
 (CDN) would have been immediately returned indicating that the
 circuit either was not provisioned or was not available at this LCCE.
 LCCE A SHOULD then exhibit a periodic retry mechanism.  If so, 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.

Singh, et al. Standards Track [Page 5] RFC 4454 ATM over L2TPv3 May 2006

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 the 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 that
 may be reported by any values identified in the Circuit Status AVP.
 The only exceptions to this are the initial ICRQ, ICRP, and CDN
 messages, which establish and tear down the L2TP session itself when
 the ATM circuit is created or deleted.  The SLI message may be sent
 from 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.

Singh, et al. Standards Track [Page 6] RFC 4454 ATM over L2TPv3 May 2006

  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                      |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Figure 1: General Format for ATM Encapsulation over L2TPv3 over IP
 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 an optional
 Cookie 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, an alternative to
 the Default L2-Specific Sublayer as mentioned in Section 4.6 of
 [RFC3931].  Four new flag bits (T, G, C, and U) are defined that
 concur with Section 8.2 of [PWE3ATM].
  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                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 2: ATM-Specific Sublayer Format
 The meaning of the fields of the ATM-Specific Sublayer is as follows:
  • S bit
    Definition of this bit is as per Section 4.6 of [RFC3931].

Singh, et al. Standards Track [Page 7] RFC 4454 ATM over L2TPv3 May 2006

  • B and E bits
    Definitions of these bits are as per Section 5.5 of [L2TPFRAG].
    If these bits are not used as per [L2TPFRAG], they MUST be set to
    0 upon transmission and ignored upon reception.
  • T (Transport type) bit
    Bit (T) of the ATM-Specific Sublayer 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, as described in Section 9.1).
  • G (EFCI) Bit
    The ingress LCCE device SHOULD set this bit to 1 if the Explicit
    Forward Congestion Indication (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 Cell Loss
    Priority (CLP) bit of any of the incoming ATM cells of the AAL5
    payload is set to 1, or if the CLP bit of the single ATM cell that
    is 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 CLP
    bit of all outgoing cells that transport the AAL5 CPCS-PDU to the
    value contained in this field.

Singh, et al. Standards Track [Page 8] RFC 4454 ATM over L2TPv3 May 2006

  • 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 ATM-Specific Sublayer.  The egress LCCE device
    SHOULD copy the U bit to the CPCS-UU Least Significant Bit (LSB)
    of the AAL5 payload.
    The Sequence Number field is used in sequencing, as described in
    Section 4.2.
 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.
 The ATM-Specific Sublayer is prepended to the AAL5-SDU.  The other
 cell mode encapsulation consists of the OPTIONAL ATM-Specific
 Sublayer, followed by a 4-byte ATM cell header and a 48-byte ATM
 cell-payload.

Singh, et al. Standards Track [Page 9] RFC 4454 ATM over L2TPv3 May 2006

5.1. ATM AAL5-SDU Mode

 In this mode, each AAL5 VC is mapped to an L2TP session.  The Ingress
 LCCE reassembles the AAL5 CPCS-SDU without the AAL5 trailer and any
 padding bytes.  Incoming EFCI, CLP, and C/R (if present) are carried
 in an ATM-Specific Sublayer across ATMPWs to the 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                         |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 3: ATM AAL5-SDU Mode Encapsulation
 If the 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 the 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 cells' 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 the 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 the 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 a 4-byte ATM cell header and a 48-byte ATM cell-
 payload; the HEC is not included.

Singh, et al. Standards Track [Page 10] RFC 4454 ATM over L2TPv3 May 2006

 In ATM Cell Mode encapsulation, the ATM-Specific Sublayer is
 OPTIONAL.  It can be included, if sequencing support is required.  It
 is left to the implementation to choose to signal the Default L2-
 Specific Sublayer or the 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)                |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 4: ATM Cell Mode Encapsulation
 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 a 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 the 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
 satisfy QOS or restoration requirements.

Singh, et al. Standards Track [Page 11] RFC 4454 ATM over L2TPv3 May 2006

 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.

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 a 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
 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 follows:
    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.

Singh, et al. Standards Track [Page 12] RFC 4454 ATM over L2TPv3 May 2006

 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.

5.3.2. 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 Concatenated Cells AVP", Attribute Type 86,
 indicates that the egress LCCE node can process a single PDU with
 concatenated cells up to 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 the egress LCCE is configured with a
 maximum number of cells to be concatenated by the ingress LCCE, it
 SHOULD signal this value to the ingress LCCE.

Singh, et al. Standards Track [Page 13] RFC 4454 ATM over L2TPv3 May 2006

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 an LCCE cannot support the
 transport of OAM cells across L2TP sessions.  If OAM cell emulation
 is configured or detected via some other means on one side, the other
 LCCE MUST support OAM cell emulation as well.
 This AVP is exchanged during session negotiation (in ICRQ and ICRP)
 or during the life of the session via SLI control messages.  If the
 other LCCE cannot 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 the 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 ATM 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 the SLI message
 with the Circuit Status AVP.

Singh, et al. Standards Track [Page 14] RFC 4454 ATM over L2TPv3 May 2006

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Circuit Status Reason     |            Alarm              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Circuit Status Reason is a 2-octet unsigned integer, and the
 Alarm Type is also a 2-octet 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 the SLI message to indicate 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

Singh, et al. Standards Track [Page 15] RFC 4454 ATM over L2TPv3 May 2006

 (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.
 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 bidirectional
 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 values of individual ATM cells
      that constitute 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 values 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.

Singh, et al. Standards Track [Page 16] RFC 4454 ATM over L2TPv3 May 2006

    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
      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 the
 following:
    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 cells' 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 for managing
      mismatched MTUs, as specified in Section 5 of [L2TPFRAG] or in
      the underlying PSN (IP, etc.) between tunnel endpoints as
      discussed in Section 4.1.4 of [RFC3931].  Only one of these
      methods SHOULD be used for a given AAL5-SDU Mode ATMPW.  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

Singh, et al. Standards Track [Page 17] RFC 4454 ATM over L2TPv3 May 2006

      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]).
    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 Cyclic Redundancy
      Check (CRC) 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].

Singh, et al. Standards Track [Page 18] RFC 4454 ATM over L2TPv3 May 2006

    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]).
    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 Constant Bit Rate (CBR) PVC
      to a class of service with tight loss and delay characteristics,
      such as an Expedited Forwarding (EF) Per-Hop Behavior (PHB) if
      the PSN is an IP DiffServ-enabled domain.  The following
      characteristics of ATMPWs operating in Cell Relay Mode include
      additional QoS considerations:
  1. ATM Cell transport VCC Pseudowires allow for mapping

multiple ATM VCCs to a single ATMPW. However, a user may

Singh, et al. Standards Track [Page 19] RFC 4454 ATM over L2TPv3 May 2006

           wish to map a single ATM VCC per ATMPW to satisfy QoS
           requirements (see Section 5.2.1).
  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 packet delay variation (PDV) that
 will impact the timing and data integrity of the ATMPW.  During
 intervals of acute 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 their 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 variable bit rate-real
 time (VBR-rt) 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 unspecified bit rate (UBR) 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 variable bit rate-non real
 time (VBR-nrt) 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.

Singh, et al. Standards Track [Page 20] RFC 4454 ATM over L2TPv3 May 2006

 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.

12. IANA Considerations

 The signaling mechanisms defined in this document rely upon the
 allocation of the 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

Singh, et al. Standards Track [Page 21] RFC 4454 ATM over L2TPv3 May 2006

    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].
 A new Result Code value for the CDN message is defined in Section 7.
 Following is a summary:
 Result Code AVP (Attribute Type 1) Values
 -----------------------------------------
 General Error Codes
       22 - Session not established due to other LCCE
            cannot 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 ATM Alarm Status AVP in the SLI message
 are defined in Section 8.1.  Additional values may be assigned by
 Expert Review [RFC2434].  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

Singh, et al. Standards Track [Page 22] RFC 4454 ATM over L2TPv3 May 2006

 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.
 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. Acknowledgements

 Thanks for the contributions 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 giving 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.

Singh, et al. Standards Track [Page 23] RFC 4454 ATM over L2TPv3 May 2006

14.2. Informative References

 [PWE3ATM]  Martini, L., "Encapsulation Methods for Transport of ATM
            Over MPLS Networks", Work in Progress, September 2005.
 [L2TPFRAG] Malis, A. and M. Townsley, "PWE3 Fragmentation and
            Reassembly", Work in Progress, November 2005.
 [FRF8.1]   "Frame Relay / ATM PVC Service Interworking Implementation
            Agreement (FRF 8.1)", Frame Relay Forum 2000.
 [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.

Singh, et al. Standards Track [Page 24] RFC 4454 ATM over L2TPv3 May 2006

Authors' Addresses

 Sanjeev Singh
 Cisco Systems
 170 W. Tasman Drive
 San Jose, CA  95134
 EMail: sanjeevs@cisco.com
 W. Mark Townsley
 Cisco Systems
 7025 Kit Creek Road
 PO Box 14987
 Research Triangle Park, NC 27709
 EMail: mark@townsley.net
 Carlos Pignataro
 Cisco Systems
 7025 Kit Creek Road
 PO Box 14987
 Research Triangle Park, NC 27709
 EMail: cpignata@cisco.com

Singh, et al. Standards Track [Page 25] RFC 4454 ATM over L2TPv3 May 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.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 Intellectual Property Rights or other rights that might be claimed to
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Singh, et al. Standards Track [Page 26]

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