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rfc:rfc4717

Network Working Group L. Martini Request for Comments: 4717 J. Jayakumar Category: Standards Track Cisco Systems, Inc.

                                                              M. Bocci
                                                               Alcatel
                                                           N. El-Aawar
                                           Level 3 Communications, LLC
                                                            J. Brayley
                                                      ECI Telecom Inc.
                                                            G. Koleyni
                                                       Nortel Networks
                                                         December 2006
              Encapsulation Methods for Transport of
        Asynchronous Transfer Mode (ATM) over MPLS Networks

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 IETF Trust (2006).

Abstract

 An Asynchronous Transfer Mode (ATM) Pseudowire (PW) is used to carry
 ATM cells over an MPLS network.  This enables service providers to
 offer "emulated" ATM services over existing MPLS networks.  This
 document specifies methods for the encapsulation of ATM cells within
 a pseudowire.  It also specifies the procedures for using a PW to
 provide an ATM service.

Martini, et al. Standards Track [Page 1] RFC 4717 Encapsulation for ATM over MPLS December 2006

Table of Contents

 1. Introduction ....................................................3
 2. Specification of Requirements ...................................4
 3. Applicability Statement .........................................4
 4. Terminology .....................................................4
 5. General Encapsulation Method ....................................6
    5.1. The Control Word ...........................................6
         5.1.1. The Generic Control Word ............................7
         5.1.2. The Preferred Control Word ..........................8
         5.1.3. Setting the Sequence Number Field in the
                Control Word ........................................9
    5.2. MTU Requirements ...........................................9
    5.3. MPLS Shim S Bit Value .....................................10
    5.4. MPLS Shim TTL Values ......................................10
 6. Encapsulation Mode Applicability ...............................10
    6.1. ATM N-to-One Cell Mode ....................................11
    6.2. ATM One-to-One Cell Encapsulation .........................13
    6.3. AAL5 SDU Frame Encapsulation ..............................13
    6.4. AAL5 PDU Frame Encapsulation ..............................14
 7. ATM OAM Cell Support ...........................................15
    7.1. VCC Case ..................................................15
    7.2. VPC Case ..................................................16
    7.3. SDU/PDU OAM Cell Emulation Mode ...........................16
    7.4. Defect Handling ...........................................17
 8. ATM N-to-One Cell Mode .........................................18
    8.1. ATM N-to-One Service Encapsulation ........................19
 9. ATM One-to-One Cell Mode .......................................21
    9.1. ATM One-to-One Service Encapsulation ......................21
    9.2. Sequence Number ...........................................22
    9.3. ATM VCC Cell Transport Service ............................22
    9.4. ATM VPC Services ..........................................24
         9.4.1. ATM VPC Cell Transport Services ....................25
 10. ATM AAL5 CPCS-SDU Mode ........................................26
    10.1. Transparent AAL5 SDU Frame Encapsulation .................27
 11. AAL5 PDU Frame Mode ...........................................28
    11.1. Transparent AAL5 PDU Frame Encapsulation .................28
    11.2. Fragmentation ............................................30
         11.2.1. Procedures in the ATM-to-PSN Direction ............30
         11.2.2. Procedures in the PSN-to-ATM Direction ............31
 12. Mapping of ATM and PSN Classes of Service .....................31
 13. ILMI Support ..................................................32
 14. ATM-Specific Interface Parameter Sub-TLVs .....................32
 15. Congestion Control ............................................32
 16. Security Considerations .......................................33
 17. Normative References ..........................................34
 18. Informative References ........................................34
 19. Significant Contributors ......................................36

Martini, et al. Standards Track [Page 2] RFC 4717 Encapsulation for ATM over MPLS December 2006

1. Introduction

 Packet Switched Networks (PSNs) have the potential to reduce the
 complexity of a service provider's infrastructure by allowing
 virtually any existing digital service to be supported over a single
 networking infrastructure.  The benefit of this model to a service
 provider is threefold:
  1. i. Leveraging of the existing systems and services to provide

increased capacity from a packet-switched core.

  1. ii. Preserving existing network operational processes and

procedures used to maintain the legacy services.

  1. iii. Using the common packet-switched network infrastructure to

support both the core capacity requirements of existing

          services and the requirements of new services supported
          natively over the packet-switched network.
 This document describes a method to carry ATM services over MPLS.  It
 lists ATM-specific requirements and provides encapsulation formats
 and semantics for connecting ATM edge networks through a packet-
 switched network using MPLS.
 Figure 1, below, displays the ATM services reference model.  This
 model is adapted from [RFC3985].
                   |<----- Pseudowire ----->|
                   |                        |
                   |  |<-- PSN Tunnel -->|  |
      ATM Service  V  V                  V  V  ATM Service
           |     +----+                  +----+     |
 +----+    |     | PE1|==================| PE2|     |    +----+
 |    |----------|............PW1.............|----------|    |
 | CE1|    |     |    |                  |    |     |    |CE2 |
 |    |----------|............PW2.............|----------|    |
 +----+    |     |    |==================|    |     |    +----+
      ^          +----+                  +----+     |    ^
      |      Provider Edge 1         Provider Edge 2     |
      |                                                  |
      |<-------------- Emulated Service ---------------->|
 Customer                                                Customer
 Edge 1                                                  Edge 2
                 Figure 1: ATM Service Reference Model

Martini, et al. Standards Track [Page 3] RFC 4717 Encapsulation for ATM over MPLS December 2006

2. Specification of Requirements

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

3. Applicability Statement

 The ATM over PW service is not intended to perfectly emulate a
 traditional ATM service, but it can be used for applications that
 need an ATM transport service.
 The following are notable differences between traditional ATM service
 and the protocol described in this document:
  1. ATM cell ordering can be preserved using the OPTIONAL sequence

field in the control word; however, implementations are not

     required to support this feature.  The use of this feature may
     impact other ATM quality of service (QoS) commitments.
  1. The QoS model for traditional ATM can be emulated. However, the

detailed specification of ATM QoS emulation is outside the scope

     of this document.  The emulation must be able to provide the
     required ATM QoS commitments for the end-user application.
  1. The ATM flow control mechanisms are transparent to the MPLS

network and cannot reflect the status of the MPLS network.

  1. Control plane support for ATM SVCs, SVPs, SPVCs, and SPVPs is

outside the scope of this document.

 Note that the encapsulations described in this specification are
 identical to those described in [Y.1411] and [Y.1412].

4. Terminology

 One-to-one mode: specifies an encapsulation method that maps one ATM
 Virtual Channel Connection (VCC) (or one ATM Virtual Path Connection
 (VPC)) to one pseudowire.
 N-to-one mode (N >= 1): specifies an encapsulation method that maps
 one or more ATM VCCs (or one or more ATM VPCs) to one pseudowire.
 Packet-Switched Network (PSN): an IP or MPLS network.
 Pseudowire Emulation Edge to Edge (PWE3): a mechanism that emulates
 the essential attributes of a service (such as a T1 leased line or
 Frame Relay) over a PSN.

Martini, et al. Standards Track [Page 4] RFC 4717 Encapsulation for ATM over MPLS December 2006

 Customer Edge (CE): a device where one end of a service originates
 and/or terminates.  The CE is not aware that it is using an emulated
 service rather than a native service.
 Provider Edge (PE): a device that provides PWE3 to a CE.
 Pseudowire (PW): a connection between two PEs carried over a PSN.
 The PE provides the adaptation between the CE and the PW.
 Pseudowire PDU: a PDU sent on the PW that contains all of the data
 and control information necessary to provide the desired service.
 PSN Tunnel: a tunnel inside which multiple PWs can be nested so that
 they are transparent to core PSN devices.
 PSN Bound: the traffic direction where information from a CE is
 adapted to a PW, and PW-PDUs are sent into the PSN.
 CE Bound: the traffic direction where PW-PDUs are received on a PW
 from the PSN, re-converted back in the emulated service, and sent out
 to a CE.
 Ingress: the point where the ATM service is encapsulated into a
 pseudowire PDU (ATM to PSN direction).
 Egress: the point where the ATM service is decapsulated from a
 pseudowire PDU (PSN to ATM direction).
 CTD: Cell Transfer Delay.
 MTU: Maximum Transmission Unit.
 SDU: Service Data Unit.
 OAM: Operations And Maintenance.
 PVC: Permanent Virtual Connection.  An ATM connection that is
 provisioned via a network management interface.  The connection is
 not signaled.
 VCC: Virtual Circuit Connection.  An ATM connection that is switched
 based on the cell header's VCI.
 VPC: Virtual Path Connection.  An ATM connection that is switched
 based on the cell header's VPI.
 Additional terminology relevant to pseudowires and Layer 2 Virtual
 Private Networking (L2VPN) in general may be found in [RFC4026].

Martini, et al. Standards Track [Page 5] RFC 4717 Encapsulation for ATM over MPLS December 2006

5. General Encapsulation Method

 This section describes the general encapsulation format for ATM over
 PSN pseudowires.
  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 (As Required)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Pseudowire Header                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     ATM Control Word                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     ATM Service Payload                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Figure 2: General format for ATM encapsulation over PSNs
 The PSN Transport Header depends on the particular tunneling
 technology in use.  This header is used to transport the encapsulated
 ATM information through the packet-switched core.
 The Pseudowire Header identifies a particular ATM service on a
 tunnel.  In case of MPLS, the pseudowire header is one or more MPLS
 labels at the bottom of the MPLS label stack.
 The ATM Control Word is inserted before the ATM service payload.  It
 may contain a length and sequence number in addition to certain
 control bits needed to carry the service.

5.1. The Control Word

 The Control Words defined in this section are based on the Generic PW
 MPLS Control Word as defined in [RFC4385].  They provide the ability
 to sequence individual frames on the PW, avoidance of equal-cost
 multiple-path load-balancing (ECMP) [RFC2992], and OAM mechanisms
 including VCCV [VCCV].
 [RFC4385] states, "If a PW is sensitive to packet misordering and is
 being carried over an MPLS PSN that uses the contents of the MPLS
 payload to select the ECMP path, it MUST employ a mechanism which
 prevents packet misordering."  This is necessary because ECMP
 implementations may examine the first nibble after the MPLS label
 stack to determine whether or not the labelled packet is IP.  Thus,
 if the VPI of an ATM connection carried over the PW using N-to-one
 cell mode encapsulation, without a control word present, begins with
 0x4 or 0x6, it could be mistaken for an IPv4 or IPv6 packet.  This

Martini, et al. Standards Track [Page 6] RFC 4717 Encapsulation for ATM over MPLS December 2006

 could, depending on the configuration and topology of the MPLS
 network, lead to a situation where all packets for a given PW do not
 follow the same path.  This may increase out-of-order frames on a
 given PW, or cause OAM packets to follow a different path than actual
 traffic (see section 4.4.3 on Frame Ordering).
 The features that the control word provides may not be needed for a
 given ATM PW.  For example, ECMP may not be present or active on a
 given MPLS network, strict frame sequencing may not be required, etc.
 If this is the case, and the control word is not REQUIRED by the
 encapsulation mode for other functions (such as length or the
 transport of ATM protocol specific information), the control word
 provides little value and is therefore OPTIONAL.  Early ATM PW
 implementations have been deployed that do not include a control word
 or the ability to process one if present.  To aid in backwards
 compatibility, future implementations MUST be able to send and
 receive frames without a control word present.
 In all cases, the egress PE MUST be aware of whether the ingress PE
 will send a control word over a specific PW.  This may be achieved by
 configuration of the PEs, or by signaling, as defined in [RFC4447].
 If the pseudowire traverses a network link that requires a minimum
 frame size (Ethernet is a practical example), with a minimum frame
 size of 64 octets, then such links will apply padding to the
 pseudowire PDU to reach its minimum frame size.  In this case, the
 control word must include a length field set to the PDU length.  A
 mechanism is required for the egress PE to detect and remove such
 padding.

5.1.1. The Generic Control Word

 This control word is used in the following encapsulation modes:
  1. ATM One-to-one Cell Mode
  2. AAL5 PDU Frame Mode
 The PWE3 control word document [RFC4385] provides the following
 structure for the generic control word:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0|          Specified by PW Encapsulation                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Martini, et al. Standards Track [Page 7] RFC 4717 Encapsulation for ATM over MPLS December 2006

 The detailed structure for the ATM One-to-one Cell Mode and for the
 AAL5 PDU Frame Mode is as follows:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Resvd |        Sequence Number        | ATM Specific  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In the above diagram, the first 4 bits MUST be set to 0 to indicate
 PW data.  They MUST be ignored by the receiving PE.
 The next four bits are reserved and MUST be set to 0 upon
 transmission and ignored upon reception.
 The next 16 bits provide a sequence number that can be used to
 guarantee ordered packet delivery.  The processing of the sequence
 number field is OPTIONAL.
 The sequence number space is a 16-bit, unsigned circular space.  The
 sequence number value 0 is used to indicate that the sequence number
 check algorithm is not used.
 The last 8 bits provide space for carrying ATM-specific flags.  These
 are defined in the protocol-specific details below.
 There is no requirement for a length field for the One-to-one Cell
 and PDU Frame modes because the PSN PDU is always greater than 64
 bytes; therefore, no padding is applied in Ethernet links in the PSN.

5.1.2. The Preferred Control Word

 This control word is used in the following encapsulation modes:
  1. ATM N-to-one Cell Mode
  2. AAL5 SDU Frame Mode
 It is defined as follows:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Flags |Res|   Length  |     Sequence Number           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In the above diagram, the first 4 bits MUST be set to 0 to indicate
 PW data.  They MUST be ignored by the receiving PE.

Martini, et al. Standards Track [Page 8] RFC 4717 Encapsulation for ATM over MPLS December 2006

 The next 4 bits provide space for carrying protocol-specific flags.
 These are defined in the protocol-specific details below.
 The next 6 bits provide a length field, which is used as follows:  If
 the packet's length (defined as the length of the layer 2 payload
 plus the length of the control word) is less than 64 bytes, the
 length field MUST be set to the packet's length.  Otherwise, the
 length field MUST be set to zero.  The value of the length field, if
 non-zero, can be used to remove any padding.  When the packet reaches
 the service provider's egress router, it may be desirable to remove
 the padding before forwarding the packet.  Note that the length field
 is not used in the N-to-one mode and MUST be set to 0.
 The last 16 bits provide a sequence number that can be used to
 guarantee ordered packet delivery.  The processing of the sequence
 number field is OPTIONAL.
 The sequence number space is a 16-bit, unsigned circular space.  The
 sequence number value 0 is used to indicate that the sequence number
 check algorithm is not used.

5.1.3. Setting the Sequence Number Field in the Control Word

 This section applies to the sequence number field of both the Generic
 and Preferred Control Words.
 For a given emulated VC and a pair of routers PE1 and PE2, if PE1
 supports packet sequencing, then the sequencing procedures defined in
 [RFC4385] MUST be used.
 Packets that are received out of order MAY be dropped or reordered at
 the discretion of the receiver.
 A simple extension of the processing algorithm in [RFC4385] MAY be
 used to detect lost packets.
 If a PE router negotiated not to use receive sequence number
 processing, and it received a non-zero sequence number, then it
 SHOULD send a PW status message indicating a receive fault and
 disable the PW.

5.2. MTU Requirements

 The network MUST be configured with an MTU that is sufficient to
 transport the largest encapsulation frames.  If MPLS is used as the
 tunneling protocol, for example, this is likely to be 12 or more
 bytes greater than the largest frame size.  Other tunneling protocols
 may have longer headers and require larger MTUs.  If the ingress

Martini, et al. Standards Track [Page 9] RFC 4717 Encapsulation for ATM over MPLS December 2006

 router determines that an encapsulated layer 2 PDU exceeds the MTU of
 the tunnel through which it must be sent, the PDU MUST be dropped.
 If an egress router receives an encapsulated layer 2 PDU whose
 payload length (i.e., the length of the PDU itself without any of the
 encapsulation headers) exceeds the MTU of the destination layer 2
 interface, the PDU MUST be dropped.

5.3. MPLS Shim S Bit Value

 The ingress label switching router (LSR), PE1, MUST set the S bit of
 the PW label to a value of 1 to denote that the VC label is at the
 bottom of the stack.  For more information on setting the S Bit, see
 [RFC3032].

5.4. MPLS Shim TTL Values

 The setting of the TTL value in the PW label is application
 dependent.  In any case, [RFC3032] TTL processing procedure,
 including handling of expired TTLs, MUST be followed.

6. Encapsulation Mode Applicability

 This document defines two methods for encapsulation of ATM cells,
 namely, One-to-one mode and N-to-one mode.
 The N-to-one mode (N >= 1) specifies an encapsulation method that
 maps one or more ATM VCCs (or one or more ATM VPCs) to one
 pseudowire.  This is the only REQUIRED mode.  One format is used for
 both the VCC or VPC mapping to the tunnel.  The 4-octet ATM header is
 unaltered in the encapsulation; thus, the VPI/VCI is always present.
 Cells from one or more VCCs (or one or more VPCs) may be
 concatenated.
 The One-to-one mode specifies an encapsulation method that maps one
 ATM VCC or one ATM VPC to one pseudowire.  For VCCs, the VPI/VCI is
 not included.  For VPCs, the VPI is not included.  Cells from one VCC
 or one VPC may be concatenated.  This mode is OPTIONAL.
 Furthermore, different OPTIONAL encapsulations are supported for ATM
 AAL5 transport: one for ATM AAL5 SDUs, and another for ATM AAL5 PDUs.
 Three deployment models are supported by the encapsulations described
 in this document:
  1. i. Single ATM Connection: A PW carries the cells of only one

ATM VCC or VPC. This supports both the transport of

          multiservice ATM and L2VPN service over a PSN for all AAL
          types.

Martini, et al. Standards Track [Page 10] RFC 4717 Encapsulation for ATM over MPLS December 2006

  1. ii. Multiple ATM Connections: A PW carries the cells of multiple

ATM VCCs and/or VPCs. This also supports both the transport

          of multiservice ATM and L2VPN service over a PSN for all AAL
          types.
  1. iii. AAL5: A PW carries the AAL5 frames of only one ATM VCC. A

large proportion of the data carried on ATM networks is

          frame based and therefore uses AAL5.  The AAL5 mapping takes
          advantage of the delineation of higher-layer frames in the
          ATM layer to provide increased bandwidth efficiency compared
          with the basic cell mapping.  The nature of the service, as
          defined by the ATM service category [TM4.0] or the ATM
          transfer capability [I.371], should be preserved.

6.1. ATM N-to-One Cell Mode

 This encapsulation supports both the Single and Multiple ATM
 Connection deployment models.  This encapsulation is REQUIRED.
 The encapsulation allows multiple VCCs/VPCs to be carried within a
 single pseudowire.  However, a service provider may wish to provision
 a single VCC to a pseudowire in order to satisfy QoS or restoration
 requirements.
 The encapsulation also supports the binding of multiple VCCs/VPCs to
 a single pseudowire.  This capability is useful in order to make more
 efficient use of the PW demultiplexing header space as well as to
 ease provisioning of the VCC/VPC services.
 In the simplest case, this encapsulation can be used to transmit a
 single ATM cell per PSN PDU.  However, in order to provide better PSN
 bandwidth efficiency, several ATM cells may optionally be
 encapsulated in a single PSN PDU.  This process is called cell
 concatenation.
 The encapsulation has the following attributes:
  1. i. Supports all ATM Adaptation Layer Types.
  1. ii. Non-terminating OAM/Admin cells are transported among the

user cells in the same order as they are received. This

          requirement enables the use of various performance
          management and security applications.

Martini, et al. Standards Track [Page 11] RFC 4717 Encapsulation for ATM over MPLS December 2006

  1. iii. In order to gain transport efficiency on the PSN, multiple

cells may be encapsulated in a single PW PDU. This process

          is called cell concatenation.  How many cells to insert or
          how long to wait for cell arrival before sending a PW PDU is
          an implementation decision.  Cell concatenation adds latency
          and delay variation to a cell relay service.
  1. iv. The CLP bit from each cell may be mapped to a corresponding

marking on the PW PDU. This allows the drop precedence to

          be preserved across the PSN.
  1. v. If the Single ATM connection deployment model is used, then

it is simpler to provide an ATM layer service. The nature

          of the service, as defined by the ATM service category
          [TM4.0] or ATM transfer capability [I.371], should be
          preserved.
 The limitations of the ATM N-to-one cell encapsulation are:
  1. vi. There is no currently defined method to translate the

forward congestion indication (EFCI) to a corresponding

          function in the PSN.  Nor is there a way to translate PSN
          congestion to the EFCI upon transmission by the egress PE.
  1. vii. The ATM cell header checksum can detect a 2-bit error or

detect and correct a single-bit error in the cell header.

          Analogous functionality does not exist in most PSNs.  A
          single bit error in a PW PDU will most likely cause the
          packet to be dropped due to an L2 Frame Check Sequence (FCS)
          failure.
  1. viii. Cells can be concatenated from multiple VCCs or VPCs

belonging to different service categories and QoS

          requirements.  In this case, the PSN packet must receive
          treatment by the PSN to support the highest QoS of the ATM
          VCCs/VPCs carried.
  1. ix. Cell encapsulation only supports point-to-point Label

Switched Paths (LSPs). Multipoint-to-point and point-to-

          multi-point are for further study (FFS).
  1. x. The number of concatenated ATM cells is limited by the MTU

size and the cell transfer delay (CTD) and cell delay

          variation (CDV) objectives of multiple ATM connections that
          are multiplexed into a single PW.

Martini, et al. Standards Track [Page 12] RFC 4717 Encapsulation for ATM over MPLS December 2006

6.2. ATM One-to-One Cell Encapsulation

 This OPTIONAL encapsulation supports the Single ATM Connection
 deployment model.
 Like the N-to-one cell encapsulation mode, the One-to-one mode
 supports cell concatenation.  The advantage of this encapsulation is
 that it utilizes less bandwidth that the N-to-one encapsulation, for
 a given number of concatenated cells.  Since only one ATM VCC or VPC
 is carried on a PW, the VCI and/or VPI of the ATM VCC or VPC can be
 derived from the context of the PW using the PW label.  These fields
 therefore do not need to be encapsulated for a VCC, and only the VCI
 needs to be encapsulated for a VPC.  This encapsulation thus allows
 service providers to achieve a higher bandwidth efficiency on PSN
 links than the N-to-one encapsulation for a given number of
 concatenated cells.
 The limitations vi, vii, ix, and x of N-to-one mode apply.

6.3. AAL5 SDU Frame Encapsulation

 This OPTIONAL encapsulation supports the AAL5 model.  This mode
 allows the transport of ATM AAL5 CSPS-SDUs traveling on a particular
 ATM PVC across the network to another ATM PVC.  This encapsulation is
 used by a PW of type 0x0002 "ATM AAL5 SDU VCC transport" as allocated
 in [RFC4446].
 The AAL5 SDU encapsulation is more efficient for small AAL5 SDUs than
 the VCC cell encapsulations.  In turn, it presents a more efficient
 alternative to the cell relay service when carrying [RFC2684]-
 encapsulated IP PDUs across a PSN.
 The AAL5-SDU encapsulation requires Segmentation and Reassembly (SAR)
 on the PE-CE ATM interface.  This SAR function is provided by common
 off-the-shelf hardware components.  Once reassembled, the AAL5-SDU is
 carried via a pseudowire to the egress PE.  Herein lies another
 advantage of the AAL5-SDU encapsulation.
 The limitations of the AAL5 SDU encapsulation are:
  1. i. If an ATM OAM cell is received at the ingress PE, it is sent

before the cells of the surrounding AAL5 frame. Therefore,

          OAM cell reordering may occur, which may cause certain ATM
          OAM performance monitoring and ATM security applications to
          operate incorrectly.

Martini, et al. Standards Track [Page 13] RFC 4717 Encapsulation for ATM over MPLS December 2006

  1. ii. If the ALL5 PDU is scrambled using ATM security standards, a

PE will not be able to extract the ALL5 SDU, and therefore

          the whole PDU will be dropped.
  1. iii. The AAL5 PDU CRC is not transported across the PSN. The CRC

must therefore be regenerated at the egress PE since the CRC

          has end-to-end significance in ATM security.  This means
          that the AAL5 CRC may not be used to accurately check for
          errors on the end-to-end ATM VCC.
  1. iv. The Length of AAL5 frame may exceed the MTU of the PSN.

This requires fragmentation, which may not be available to

          all nodes at the PW endpoint.
  1. v. This mode does not preserve the value of the CLP bit for

every ATM cell within an AAL5 PDU. Therefore, transparency

          of the CLP setting may be violated.  Additionally, tagging
          of some cells may occur when tagging is not allowed by the
          conformance definition [TM4.0].
  1. vi. This mode does not preserve the EFCI state for every ATM

cell within an AAL5 PDU. Therefore, transparency of the

          EFCI state may be violated.

6.4. AAL5 PDU Frame Encapsulation

 This OPTIONAL encapsulation supports the AAL5 model.
 The primary application supported by AAL5 PDU frame encapsulation
 over PSN is the transparent carriage of ATM layer services that use
 AAL5 to carry higher-layer frames.  The main advantage of this AAL5
 mode is that it is transparent to ATM OAM and ATM security
 applications.
 One important consideration is to allow OAM information to be treated
 as in the original network.  This encapsulation mode allows this
 transparency while performing AAL5 frame encapsulation.  This mode
 supports fragmentation, which may be performed in order to maintain
 the position of the OAM cells with respect to the user cells.
 Fragmentation may also be performed to maintain the size of the
 packet carrying the AAL5 PDU within the MTU of the link.
 Fragmentation provides a means for the PE to set the size of the PW
 packet to a different value than that of the original AAL5 PDU.  This
 means that the PE has control on the delay and jitter provided to the
 ATM cells.

Martini, et al. Standards Track [Page 14] RFC 4717 Encapsulation for ATM over MPLS December 2006

 The whole AAL5-PDU is encapsulated.  In this case, all necessary
 parameters, such as CPCS-UU (CPCS User-to-User indicator), CPI
 (Common Part Indicator), Length (Length of the CPCS-SDU) and CRC
 (Cyclic Redundancy Check), are transported as part of the payload.
 Note that carrying of the full PDU also allows the simplification of
 the fragmentation operation since it is performed at cell boundaries
 and the CRC in the trailer of the AAL5 PDU can be used to check the
 integrity of the PDU.
 Reassembly is not required at the egress PE for the PSN-to-ATM
 direction.
 The limitations v and vi of the AAL5 SDU mode apply to this mode as
 well.

7. ATM OAM Cell Support

7.1. VCC Case

 In general, when configured for ATM VCC service, both PEs SHOULD act
 as a VC switch, in accordance with the OAM procedures defined in
 [I.610].
 The PEs SHOULD be able to pass the following OAM cells transparently:
  1. F5 Alarm Indication Signal (AIS) (segment and end-to-end)
  2. F5 Remote Defect Indicator (RDI) (segment and end-to-end)
  3. F5 loopback (segment and end-to-end)
  4. Resource Management
  5. Performance Management
  6. Continuity Check
  7. Security
 However, if configured to be an administrative segment boundary, the
 PE SHOULD terminate and process F5 segment 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 PSN.
 When the PE is operating in AAL5 CPCS-SDU transport mode if it does
 not support transport of ATM cells, the PE MUST discard incoming MPLS
 frames on an ATM PW that contain a PW label with the T bit set.

Martini, et al. Standards Track [Page 15] RFC 4717 Encapsulation for ATM over MPLS December 2006

7.2. VPC Case

 When configured for a VPC cell relay service, both PEs SHOULD act as
 a VP cross-connect in accordance with the OAM procedures defined in
 [I.610].
 The PEs SHOULD be able to process and pass the following OAM cells
 transparently according to [I.610]:
  1. F4 AIS (segment and end-to-end)
  2. F4 RDI (segment and end-to-end)
  3. F4 loopback (segment and end-to-end)
 However, if configured to be an administrative segment boundary, the
 PE SHOULD terminate and process F4 segment OAM cells.
 F5 OAM are not inserted or extracted here.  The PEs MUST be able to
 pass the following OAM cells transparently:
  1. F5 AIS (segment and end-to-end)
  2. F5 RDI (segment and end-to-end)
  3. F5 loopback (segment and end-to-end)
  4. Resource Management
  5. Performance Management
  6. Continuity Check
  7. Security
 The OAM cell MAY be encapsulated together with other user data cells
 if multiple cell encapsulation is used.

7.3. SDU/PDU OAM Cell Emulation Mode

 A PE operating in ATM SDU or PDU transport mode that does not support
 transport of OAM cells across a PW MAY provide OAM support on ATM
 PVCs using the following procedures:
  1. Loopback cells response
     If an F5 end-to-end OAM cell is received from an ATM VC, by
     either PE that is transporting this ATM VC, with a loopback
     indication value of 1, and the PE has a label mapping for the ATM
     VC, then the PE MUST decrement the loopback indication value and
     loop back the cell on the ATM VC.  Otherwise, the loopback cell
     MUST be discarded by the PE.

Martini, et al. Standards Track [Page 16] RFC 4717 Encapsulation for ATM over MPLS December 2006

  1. AIS alarm
     If an ingress PE, PE1, receives an AIS F4/F5 OAM cell, it MUST
     notify the remote PE of the failure.  The remote PE, PE2, MUST in
     turn send F5 OAM AIS cells on the respective PVCs.  Note that if
     the PE supports forwarding of OAM cells, then the received OAM
     AIS alarm cells MUST be forwarded along the PW as well.
  1. Interface failure
     If the PE detects a physical interface failure, or the interface
     is administratively disabled, the PE MUST notify the remote PE
     for all VCs associated with the failure.
  1. PSN/PW failure detection
     If the PE detects a failure in the PW, by receiving a label
     withdraw for a specific PW ID, or the targeted Label Distribution
     Protocol (LDP) session fails, or a PW status TLV notification is
     received, then a proper AIS F5 OAM cell MUST be generated for all
     the affected ATM PVCs.  The AIS OAM alarm will be generated on
     the ATM output port of the PE that detected the failure.

7.4. Defect Handling

 Figure 3 illustrates four possible locations for defects on the PWE3
 service:
  1. (a) On the ATM connection from CE to PE
  2. (b) On the ATM side of the PW
  3. © On the PSN side of the PE
  4. (d) In the PSN
                 +----+                  +----+
 +----+          | PE1|==================| PE2|          +----+
 |    |---a------|b..c........PW1...d.........|----------|    |
 | CE1|          |    |                  |    |          |CE2 |
 |    |----------|............PW2.............|----------|    |
 +----+          |    |==================|    |          +----+
      ^          +----+                  +----+          ^
      |      Provider Edge 1         Provider Edge 2     |
      |                                                  |
      |<-------------- Emulated Service ---------------->|
 Customer                                                Customer
 Edge 1                                                  Edge 2
                      Figure 3: Defect Locations

Martini, et al. Standards Track [Page 17] RFC 4717 Encapsulation for ATM over MPLS December 2006

 For failures at (a) or (b), in the VPC case, the ingress PE MUST be
 able to generate an F4 AIS upon reception of a lower-layer defect
 (such as LOS).  In the VCC case, the ingress PE SHOULD be able to
 generate an F5 AIS upon reception of a corresponding F4 AIS or
 lower-layer defect (such as LOS).  These messages are sent across the
 PSN.
 For failures at (c) or (d), in the VCC case, the egress PE SHOULD be
 able to generate an F5 AIS based on a PSN failure (such as a PSN
 tunnel failure or LOS on the PSN port).  In the VPC case, the egress
 PE SHOULD be able to generate an F4 AIS based on a PSN failure (such
 as a PSN tunnel failure or LOS on the PSN port).
 If the ingress PE cannot support the generation of OAM cells, it MAY
 notify the egress PE using a pseudowire-specific maintenance
 mechanism such as the PW status message defined in [RFC4447].
 Alternatively, for example, the ingress PE MAY withdraw the
 pseudowire (PW label) label associated with the service.  Upon
 receiving such a notification, the egress PE SHOULD generate the
 appropriate F4 AIS (for VPC) or F5 AIS (for VCC).
 If the PW in one direction fails, then the complete bidirectional
 service is considered to have failed.

8. ATM N-to-One Cell Mode

 The N-to-one mode (N >= 1) described in this document allows a
 service provider to offer an ATM PVC- or SVC-based service across a
 network.  The encapsulation allows multiple ATM VCCs or VPCs to be
 carried within a single PSN tunnel.  A service provider may also use
 N-to-one mode to provision either one VCC or one VPC on a tunnel.
 This section defines the VCC and VPC cell relay services over a PSN
 and their applicability.

Martini, et al. Standards Track [Page 18] RFC 4717 Encapsulation for ATM over MPLS December 2006

8.1. ATM N-to-One Service Encapsulation

 This section describes the general encapsulation format for ATM over
 PSN pseudowires.
  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 (As Required)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     pseudowire Header                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Flags |Res|   Length  |     Sequence Number           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     ATM Service Payload                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Figure 4: General format for ATM encapsulation over PSNs
 The PSN Transport Header depends on the particular tunneling
 technology in use.  This header is used to transport the encapsulated
 ATM information through the packet-switched core.
 The Pseudowire Header identifies a particular ATM service on a
 tunnel.  Non-ATM services may also be carried on the PSN tunnel.
 As shown above, in Figure 4, the ATM Control Word is inserted before
 the ATM service payload.  It may contain a length field and a
 sequence number field in addition to certain control bits needed to
 carry the service.
 The ATM Service Payload is specific to the service being offered via
 the pseudowire.  It is defined in the following sections.
 In this encapsulation mode, ATM cells are transported individually.
 The encapsulation of a single ATM cell is the only REQUIRED
 encapsulation for ATM.  The encapsulation of more than one ATM cell
 in a PSN frame is OPTIONAL.
 The ATM cell encapsulation consists of an OPTIONAL control word and
 one or more ATM cells, each consisting of a 4-byte ATM cell header
 and the 48-byte ATM cell payload.  This ATM cell header is defined as
 in the FAST encapsulation [FBATM] section 3.1.1, but without the
 trailer byte.  The length of each frame, without the encapsulation
 headers, is a multiple of 52 bytes.  The maximum number of ATM cells
 that can be fitted in a frame, in this fashion, is limited only by
 the network MTU and by the ability of the egress router to process
 them.  The ingress router MUST NOT send more cells than the egress

Martini, et al. Standards Track [Page 19] RFC 4717 Encapsulation for ATM over MPLS December 2006

 router is willing to receive.  The number of cells that the egress
 router is willing to receive may either be configured in the ingress
 router or be signaled, for example using the methods described later
 in this document and in [RFC4447].  The number of cells encapsulated
 in a particular frame can be inferred by the frame length.  The
 control word is OPTIONAL.  If the control word is used, then the flag
 and length bits in the control word are not used.  These bits MUST be
 set to 0 when transmitting, and MUST be ignored upon receipt.
 The EFCI and CLP bits are carried across the network in the ATM cell
 header.  The edge routers that implement this document MAY, when
 either adding or removing the encapsulation described herein, change
 the EFCI bit from zero to one in order to reflect congestion in the
 network that is known to the edge router, and change the CLP bit from
 zero to one in order to reflect marking from edge policing of the ATM
 Sustained Cell Rate.  The EFCI and CLP bits SHOULD NOT be changed
 from one to zero.
 This diagram illustrates an encapsulation of two ATM cells:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Control word ( Optional )                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          VPI          |              VCI              | PTI |C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  ATM Payload ( 48 bytes )                     |
 |                          "                                    |
 |                          "                                    |
 |                          "                                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          VPI          |              VCI              | PTI |C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  ATM Payload ( 48 bytes )                     |
 |                          "                                    |
 |                          "                                    |
 |                          "                                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 5: Multiple Cell ATM Encapsulation
  • When multiple VCCs or VPCs are transported in one pseudowire,

VPI/VCI values MUST be unique. When the multiple VCCs or VPCs

     are from different a physical transmission path, it may be
     necessary to assign unique VPI/VCI values to the ATM connections.
     If they are from the same physical transmission path, the VPI/VCI
     values are unique.

Martini, et al. Standards Track [Page 20] RFC 4717 Encapsulation for ATM over MPLS December 2006

  • VPI
     The ingress router MUST copy the VPI field from the incoming cell
     into this field.  For particular emulated VCs, the egress router
     MAY generate a new VPI and ignore the VPI contained in this
     field.
  • VCI
     The ingress router MUST copy the VCI field from the incoming ATM
     cell header into this field.  For particular emulated VCs, the
     egress router MAY generate a new VCI.
  • PTI & CLP (C bit)
     The PTI and CLP fields are the PTI and CLP fields of the incoming
     ATM cells.  The cell headers of the cells within the packet are
     the ATM headers (without Header Error Check (HEC) field) of the
     incoming cell.

9. ATM One-to-One Cell Mode

 The One-to-one mode described in this document allows a service
 provider to offer an ATM PVC- or SVC-based service across a network.
 The encapsulation allows one ATM VCC or VPC to be carried within a
 single pseudowire.

9.1. ATM One-to-One Service Encapsulation

 This section describes the general encapsulation format for ATM over
 pseudowires on an MPLS PSN.  Figure 6 provides a general format for
 encapsulation of ATM cells into packets.
  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 (As Required)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Pseudowire Header                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Resvd |    Optional Sequence Number   | ATM Specific  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     ATM Service Payload                       |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 6: General format for One-to-one mode encapsulation over PSNs

Martini, et al. Standards Track [Page 21] RFC 4717 Encapsulation for ATM over MPLS December 2006

 The MPLS PSN Transport Header depends on how the MPLS network is
 configured.  The Pseudowire Header identifies a particular ATM
 service within the PSN tunnel created by the PSN Transport Header.
 This header is used to transport the encapsulated ATM information
 through the packet-switched core.
 The generic control word is inserted after the Pseudowire Header.
 The presence of the control word is REQUIRED.
 The ATM Specific Header is inserted before the ATM service payload.
 The ATM Specific Header contains control bits needed to carry the
 service.  These are defined in the ATM service descriptions below.
 The length of ATM Specific Header may not always be one octet.  It
 depends on the service type.
 The ATM payload octet group is the payload of the service that is
 being encapsulated.

9.2. Sequence Number

 The sequence number is not required for all services.
 Treatment of the sequence number is according to section 5.1.3.

9.3. ATM VCC Cell Transport Service

 The VCC cell transport service is characterized by the mapping of a
 single ATM VCC (VPI/VCI) to a pseudowire.  This service is fully
 transparent to the ATM Adaptation Layer.  The VCC single cell
 transport service is OPTIONAL.  This service MUST use the following
 encapsulation 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               PSN Transport Header (As Required)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Pseudowire Header                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0| Resvd |  Optional Sequence Number     |M|V|Res| PTI |C|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                   ATM Cell Payload ( 48 bytes )               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 7: Single ATM VCC Cell Encapsulation

Martini, et al. Standards Track [Page 22] RFC 4717 Encapsulation for ATM over MPLS December 2006

  • M (transport mode) bit
     Bit (M) of the control byte indicates whether the packet contains
     an ATM cell or a frame payload.  If set to 0, the packet contains
     an ATM cell.  If set to 1, the PDU contains an AAL5 payload.
  • V (VCI present) bit
     Bit (V) of the control byte indicates whether the VCI field is
     present in the packet.  If set to 1, the VCI field is present for
     the cell.  If set to 0, no VCI field is present.  In the case of
     a VCC, the VCI field is not required.  For VPC, the VCI field is
     required and is transmitted with each cell.
  • Reserved bits
     The reserved bits should be set to 0 at the transmitter and
     ignored upon reception.
  • PTI Bits
     The 3-bit Payload Type Identifier (PTI) incorporates ATM Layer
     PTI coding of the cell.  These bits are set to the value of the
     PTI of the encapsulated ATM cell.
  • C (CLP) Bit
     The Cell Loss Priority (CLP) field indicates CLP value of the
     encapsulated cell.
 For increased transport efficiency, the ingress PE SHOULD be able to
 encapsulate multiple ATM cells into a pseudowire PDU.  The ingress
 and egress PE MUST agree to a maximum number of cells in a single
 pseudowire PDU.  This agreement may be accomplished via a
 pseudowire-specific signaling mechanism or via static configuration.
 When multiple cells are encapsulated in the same PSN packet, the
 ATM-specific byte MUST be repeated for each cell.  This means that 49
 bytes are used to encapsulate each 53 byte ATM cell.

Martini, et al. Standards Track [Page 23] RFC 4717 Encapsulation for ATM over MPLS December 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 (As Required)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Pseudowire Header                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Resvd |  Optional Sequence Number     |M|V|Res| PTI |C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                   ATM Cell Payload ( 48 bytes )               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |M|V|Res| PTI |C|                                               |
 +-+-+-+-+-+-+-+-+                                               |
 |                   ATM Cell Payload ( 48 bytes )               |
 |                                                               |
 |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               |
 +-+-+-+-+-+-+-+-+
             Figure 8: Multiple ATM VCC Cell Encapsulation

9.4. ATM VPC Services

 The VPC service is defined by mapping a single VPC (VPI) to a
 pseudowire.  As such, it emulates a Virtual Path cross-connect across
 the PSN.  All VCCs belonging to the VPC are carried transparently by
 the VPC service.
 The egress PE may choose to apply a different VPI other than the one
 that arrived at the ingress PE.  The egress PE MUST choose the
 outgoing VPI based solely upon the pseudowire header.  As a VPC
 service, the egress PE MUST NOT change the VCI field.

Martini, et al. Standards Track [Page 24] RFC 4717 Encapsulation for ATM over MPLS December 2006

9.4.1. ATM VPC Cell Transport Services

 The ATM VPC cell transport service is OPTIONAL.
 This service MUST use the following cell mode encapsulation:
  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 (As Required)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Pseudowire Header                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Resvd |  Optional Sequence Number     |M|V|Res| PTI |C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             VCI               |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                                                               |
 |                   ATM Cell Payload ( 48 bytes )               |
 |                                                               |
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 9: Single Cell VPC Encapsulation
 The ATM control byte contains the same information as in the VCC
 encapsulation except for the VCI field.
  • VCI Bits
     The 16-bit Virtual Circuit Identifier (VCI) incorporates ATM
     Layer VCI value of the cell.
 For increased transport efficiency, the ingress PE SHOULD be able to
 encapsulate multiple ATM cells into a pseudowire PDU.  The ingress
 and egress PE MUST agree to a maximum number of cells in a single
 pseudowire PDU.  This agreement may be accomplished via a
 pseudowire-specific signaling mechanism or via static configuration.
 If the Egress PE supports cell concatenation, the ingress PE MUST
 only concatenate cells up to the "Maximum Number of concatenated ATM
 cells in a frame" interface parameter sub-TLV as received as part of
 the control protocol [RFC4447].
 When multiple ATM cells are encapsulated in the same PSN packet, the
 ATM-specific byte MUST be repeated for each cell.  This means that 51
 bytes are used to encapsulate each 53-byte ATM cell.

Martini, et al. Standards Track [Page 25] RFC 4717 Encapsulation for ATM over MPLS December 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 (As Required)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Pseudowire Header                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Resvd |  Optional Sequence Number     |M|V|Res| PTI |C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             VCI               |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                                                               |
 |                   ATM Cell Payload (48 bytes)                 |
 |                                                               |
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |M|V|Res| PTI |C|        VCI    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   VCI         |                                               |
 +-+-+-+-+-+-+-+-+                                               |
 |                   ATM Cell Payload (48 bytes)                 |
 |                                                               |
 |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               |
 +-+-+-+-+-+-+-+-+
              Figure 10: Multiple Cell VPC Encapsulation

10. ATM AAL5 CPCS-SDU Mode

 The AAL5 payload VCC service defines a mapping between the payload of
 an AAL5 VCC and a single pseudowire.  The AAL5 payload VCC service
 requires ATM segmentation and reassembly support on the PE.
 The AAL5 payload CPCS-SDU service is OPTIONAL.
 Even the smallest TCP packet requires two ATM cells when sent over
 AAL5 on a native ATM device.  It is desirable to avoid this padding
 on the pseudowire.  Therefore, once the ingress PE reassembles the
 AAL5 CPCS-PDU, the PE discards the PAD and CPCS-PDU trailer, and then
 the ingress PE inserts the resulting payload into a pseudowire PDU.
 The egress PE MUST regenerate the PAD and trailer before transmitting
 the AAL5 frame on the egress ATM port.
 This service does allow the transport of OAM and RM cells, but it
 does not attempt to maintain the relative order of these cells with
 respect to the cells that comprise the AAL5 CPCS-PDU.  All OAM cells,
 regardless of their type, that arrive during the reassembly of a

Martini, et al. Standards Track [Page 26] RFC 4717 Encapsulation for ATM over MPLS December 2006

 single AAL5 CPCS-PDU are sent immediately on the pseudowire using
 N-to-one cell encapsulation, followed by the AAL5 payload.
 Therefore, the AAL5 payload VCC service will not be suitable for ATM
 applications that require strict ordering of OAM cells (such as
 performance monitoring and security applications).

10.1. Transparent AAL5 SDU Frame Encapsulation

 The AAL5 CPCS-SDU is prepended by the following header:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Res  |T|E|C|U|Res|  Length   |   Sequence Number (Optional)  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                              "                                |
 |                     ATM cell or AAL5 CPCS-SDU                 |
 |                              "                                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 11: AAL5 CPCS-SDU Encapsulation
 The AAL5 payload service encapsulation requires the ATM control word.
 The Flag bits are described below.
  • Res (Reserved)
     These bits are reserved and MUST be set to 0 upon transmission
     and ignored upon reception.
  • 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 N-to-
     one cell relay encapsulation, Figure 4.  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).
  • E (EFCI) Bit
     The ingress router, PE1, SHOULD set this bit to 1 if the EFCI bit
     of the final cell of those that transported the AAL5 CPCS-SDU 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

Martini, et al. Standards Track [Page 27] RFC 4717 Encapsulation for ATM over MPLS December 2006

     SHOULD be set to 0.  The egress router, PE2, SHOULD set the EFCI
     bit of all cells that transport the AAL5 CPCS-SDU to the value
     contained in this field.
  • C (CLP) Bit
     The ingress router, PE1, SHOULD set this bit to 1 if the CLP bit
     of any of the ATM cells that transported the AAL5 CPCS-SDU is set
     to 1, or if the CLP 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 router, PE2, SHOULD set the CLP bit of all cells
     that transport the AAL5 CPCS-SDU to the value contained in this
     field.
  • U (Command/Response Field) Bit
     When FRF.8.1 Frame Relay/ATM PVC Service Interworking [RFC3916]
     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 router, PE1, SHOULD copy this bit to the U bit of the
     control word.  The egress router, PE2, SHOULD copy the U bit to
     the CPCS-UU Least Significant Bit (LSB) of the AAL5 CPCS PDU.

11. AAL5 PDU Frame Mode

 The AAL5 payload PDU service is OPTIONAL.

11.1. Transparent AAL5 PDU Frame Encapsulation

 In this mode, the ingress PE encapsulates the entire CPCS-PDU
 including the PAD and trailer.
 This mode MAY support fragmentation procedures described in the
 "Fragmentation" section below, in order to maintain OAM cell
 sequencing.
 Like the ATM AAL5 payload VCC service, the AAL5 transparent VCC
 service is intended to be more efficient than the VCC cell transport
 service.  However, the AAL5 transparent VCC service carries the
 entire AAL5 CPCS-PDU, including the PAD and trailer.  Note that the
 AAL5 CPCS-PDU is not processed, i.e., an AAL5 frame with an invalid
 CRC or length field will be transported.  One reason for this is that
 there may be a security agent that has scrambled the ATM cell
 payloads that form the AAL5 CPCS-PDU.
 This service supports all OAM cell flows by using a fragmentation
 procedure that ensures that OAM cells are not repositioned in respect
 to AAL5 composite cells.

Martini, et al. Standards Track [Page 28] RFC 4717 Encapsulation for ATM over MPLS December 2006

 The AAL5 transparent VCC service is OPTIONAL.
 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 (As Required)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Pseudowire Header                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0| Resvd |   Optional Sequence Number    |M|V| Res |U|E|C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             "                                 |
 |                        AAL5 CPCS-PDU                          |
 |                      (n * 48 bytes)                           |
 |                             "                                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 12: AAL5 transparent service encapsulation
 The generic control word is inserted after the Pseudowire Header.
 The presence of the control word is MANDATORY.
 The M, V, Res, and C bits are as defined earlier for VCC One-to-one
 cell mode.
  • U Bit
     This field indicates whether this frame contains the last cell of
     an AAL5 PDU and represents the value of the ATM User-to-User bit
     for the last ATM cell of the PSN frame.  Note: The ATM User-to-
     User bit is the least significant bit of the PTI field in the ATM
     header.  This field is used to support the fragmentation
     functionality described later in this section.
  • E (EFCI) bit
     This field is used to convey the EFCI state of the ATM cells.
     The EFCI state is indicated in the middle bit of each ATM cell's
     PTI field.
     ATM-to-PSN direction (ingress): The EFCI field of the control
     byte is set to the EFCI state of the last cell of the AAL5 PDU or
     AAL5 fragment.
     PSN-to-ATM direction (egress): The EFCI state of all constituent
     cells of the AAL5 PDU or AAL5 fragment is set to the value of the
     EFCI field in the control byte.

Martini, et al. Standards Track [Page 29] RFC 4717 Encapsulation for ATM over MPLS December 2006

  • C (CLP) bit
     This field is used to convey the cell loss priority of the ATM
     cells.
     ATM-to-PSN direction (ingress): The CLP field of the control byte
     is set to 1 if any of the constituent cells of the AAL5 PDU or
     AAL5 fragment has its CLP bit set to 1; otherwise, this field is
     set to 0.
     PSN-to-ATM direction (egress): The CLP bit of all constituent
     cells for an AAL5 PDU or AAL5 fragment is set to the value of the
     CLP field in the control byte.  The payload consists of the
     re-assembled AAL5 CPCS-PDU, including the AAL5 padding and
     trailer or the AAL5 fragment.

11.2. Fragmentation

 The ingress PE may not always be able to reassemble a full AAL5
 frame.  This may be because the AAL5 PDU exceeds the pseudowire MTU
 or because OAM cells arrive during reassembly of the AAL5 PDU.  In
 these cases, the AAL5 PDU shall be fragmented.  In addition,
 fragmentation may be desirable to bound ATM cell delay.
 When fragmentation occurs, the procedures described in the following
 subsections shall be followed.

11.2.1. Procedures in the ATM-to-PSN Direction

 The following procedures shall apply while fragmenting AAL5 PDUs:
  1. Fragmentation shall always occur at cell boundaries within the

AAL5 PDU.

  1. Set the UU bit to the value of the ATM User-to-User bit in the

cell header of the most recently received ATM cell.

  1. The E and C bits of the fragment shall be set as defined in

section 9.

  1. If the arriving cell is an OAM or an RM cell, send the current

PSN frame and then send the OAM or RM cell using One-to-one

     single cell encapsulation (VCC).

Martini, et al. Standards Track [Page 30] RFC 4717 Encapsulation for ATM over MPLS December 2006

11.2.2. Procedures in the PSN-to-ATM Direction

 The following procedures shall apply:
  1. The 3-bit PTI field of each ATM cell header is constructed as

follows:

  1. i. The most significant bit is set to 0, indicating a user data

cell.

  1. ii. The middle bit is set to the E bit value of the fragment.
  1. iii. The least significant bit for the last ATM cell in the PSN

frame is set to the value of the UU bit of Figure 12.

  1. iv. The least significant PTI bit is set to 0 for all other

cells in the PSN frame.

  1. The CLP bit of each ATM cell header is set to the value of the C

bit of the control byte in Figure 12.

  1. When a fragment is received, each constituent ATM cell is sent in

correct order.

12. Mapping of ATM and PSN Classes of Service

 This section is provided for informational purposes, and for guidance
 only.  This section should not be considered part of the standard
 proposed in this document.
 When ATM PW service is configured over a PSN, the ATM service
 category of a connection SHOULD be mapped to a compatible class of
 service in the PSN network.  A compatible class of service maintains
 the integrity of the service end to end.  For example, the CBR
 service category SHOULD be mapped to a class of service with
 stringent loss and delay objectives.  If the PSN implements the IP
 Diffserv framework, a class of service based on the EF PHB is a good
 candidate.
 Furthermore, ATM service categories have support for multiple
 conformance definitions [TM4.0].  Some are CLP blind (e.g., CBR),
 meaning that the QoS objectives apply to the aggregate CLP0+1
 conforming cell flow.  Some are CLP significant (e.g., VBR.3),
 meaning that the QoS objectives apply to the CLP0 conforming cell
 flow only.
 When the PSN is MPLS based, a mapping between the CLP bit and the EXP
 field can be performed to provide visibility of the cell loss

Martini, et al. Standards Track [Page 31] RFC 4717 Encapsulation for ATM over MPLS December 2006

 priority in the MPLS network.  The actual value to be marked in the
 EXP field depends on the ATM service category, the ATM conformance
 definition, and the type of tunnel LSP used (E-LSP or L-LSP).  The
 details of this mapping are outside the scope of this document.
 Operators have the flexibility to design a specific mapping that
 satisfies their own requirements.
 In both the ATM-to-PSN and PSN-to-ATM directions, the method used to
 transfer the CLP and EFCI information of the individual cells into
 the ATM-specific field, or flags, of the PW packet is described in
 detail in sections 6 through 9 for each encapsulation mode.

13. ILMI Support

 An MPLS edge PE MAY provide an ATM Integrated Local Management
 Interface (ILMI) to the ATM edge switch.  If an ingress PE receives
 an ILMI message indicating that the ATM edge switch has deleted a VC,
 or if the physical interface goes down, it MUST send a PW status
 notification message for all PWs associated with the failure.  When a
 PW label mapping is withdrawn, or PW status notification message is
 received, the egress PE MUST notify its client of this failure by
 deleting the VC using ILMI.

14. ATM-Specific Interface Parameter Sub-TLVs

 The Interface parameter TLV is defined in [RFC4447], and the IANA
 registry with initial values for interface parameter sub-TLV types is
 defined in [RFC4446], but the ATM PW-specific interface parameter is
 specified as follows:
  1. 0x02 Maximum Number of concatenated ATM cells.
     A 2-octet value specifying the maximum number of concatenated ATM
     cells that can be processed as a single PDU by the egress PE.  An
     ingress PE transmitting concatenated cells on this PW can
     concatenate a number of cells up to the value of this parameter,
     but MUST NOT exceed it.  This parameter is applicable only to PW
     types 3, 9, 0x0a, 0xc, [RFC4446], and 0xd and is REQUIRED for
     these PWC types.  This parameter does not need to match in both
     directions of a specific PW.

15. 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 that will impact the service carried
 by the ATM PW.  In addition, since ATM PWs carry a variety of

Martini, et al. Standards Track [Page 32] RFC 4717 Encapsulation for ATM over MPLS December 2006

 services across the PSN, including but not restricted to TCP/IP, they
 may or may not behave in a TCP-friendly manner prescribed by
 [RFC2914].  In the presence of services that reduce transmission
 rate, ATM PWs may thus consume more than their fair share and in that
 case SHOULD be halted.
 Whenever possible, ATM PWs 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 EF (expedited forwarding) are examples
 of traffic-engineered PSNs.  Such PSNs will minimize loss and delay
 while providing some degree of isolation of the ATM PW's effects from
 neighboring streams.
 It should be noted that when transporting ATM, Diffserv-enabled
 domains may use AF (Assured Forwarding) and/or DF (Default
 Forwarding) instead of EF, in order to place less burden on the
 network and gain additional statistical multiplexing advantage.  In
 particular, Table 1 of Appendix "V" in [ATM-MPLS] contains a detailed
 mapping between ATM classes and Diffserv classes.
 The PEs SHOULD monitor for congestion (by using explicit congestion
 notification, [VCCV], or by measuring packet loss) in order to ensure
 that the service using the ATM PW may be maintained.  When a PE
 detects significant congestion while receiving the PW PDUs, the PE
 MAY use RM cells for ABR connections to notify the remote PE.
 If the PW has been set up using the protocol defined in [RFC4447],
 then procedures specified in [RFC4447] for status notification can be
 used to disable packet transmission on the ingress PE from the egress
 PE.  The PW may be restarted by manual intervention, or by automatic
 means after an appropriate waiting time.

16. Security Considerations

 This document specifies only encapsulations, not the protocols used
 to carry the encapsulated packets across the PSN.  Each such protocol
 may have its own set of security issues [RFC4447][RFC3985], but those
 issues are not affected by the encapsulations specified herein.  Note
 that the security of the transported ATM service will only be as good
 as the security of the PSN.  This level of security might be less
 rigorous than a native ATM service.

Martini, et al. Standards Track [Page 33] RFC 4717 Encapsulation for ATM over MPLS December 2006

17. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
            Heron, "Pseudowire Setup and Maintenance Using the Label
            Distribution Protocol (LDP)", RFC 4447, April 2006.
 [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
            Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
            Encoding", RFC 3032, January 2001.
 [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
            Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
 [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
            "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
            Use over an MPLS PSN", RFC 4385, February 2006.

18. Informative References

 [FBATM]    ATM Forum Specification af-fbatm-0151.000 (2000), "Frame
            Based ATM over SONET/SDH Transport (FAST)"
 [TM4.0]    ATM Forum Specification af-tm-0121.000 (1999), "Traffic
            Management Specification Version 4.1"
 [I.371]    ITU-T Recommendation I.371 (2000), "Traffic control and
            congestion control in B-ISDN".
 [I.610]    ITU-T Recommendation I.610, (1999), "B-ISDN operation and
            maintenance principles and functions".
 [Y.1411]   ITU-T Recommendation Y.1411 (2003), ATM-MPLS Network
            Interworking - Cell Mode user Plane Interworking
 [Y.1412]   ITU-T Recommendation Y.1412 (2003), ATM-MPLS network
            interworking - Frame mode user plane interworking
 [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
            Edge (PWE3) Architecture", RFC 3985, March 2005.
 [RFC3916]  Xiao, X., McPherson, D., and P. Pate, "Requirements for
            Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
            September 2004.

Martini, et al. Standards Track [Page 34] RFC 4717 Encapsulation for ATM over MPLS December 2006

 [RFC4026]  Andersson, L. and T. Madsen, "Provider Provisioned Virtual
            Private Network (VPN) Terminology", RFC 4026, March 2005.
 [VCCV]     Nadeau, T., Pignataro, C., and R. Aggarwal, "Pseudowire
            Virtual Circuit Connectivity Verification (VCCV)", Work in
            Progress, June 2006.
 [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
            Algorithm", RFC 2992, November 2000.
 [ATM-MPLS] ATM Forum Specification af-aic-0178.001, "ATM-MPLS Network
            Interworking Version 2.0", August 2003.
 [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
            2914, September 2000.
 [RFC2684]  Grossman, D. and J. Heinanen, "Multiprotocol Encapsulation
            over ATM Adaptation Layer 5", RFC 2684, September 1999.

Martini, et al. Standards Track [Page 35] RFC 4717 Encapsulation for ATM over MPLS December 2006

19. Significant Contributors

 Giles Heron
 Tellabs
 Abbey Place
 24-28 Easton Street
 High Wycombe
 Bucks
 HP11 1NT
 UK
 EMail: giles.heron@tellabs.com
 Dimitri Stratton Vlachos
 Mazu Networks, Inc.
 125 Cambridgepark Drive
 Cambridge, MA 02140
 EMail: d@mazunetworks.com
 Dan Tappan
 Cisco Systems, Inc.
 1414 Massachusetts Avenue
 Boxborough, MA 01719
 EMail: tappan@cisco.com
 Eric C. Rosen
 Cisco Systems, Inc.
 1414 Massachusetts Avenue
 Boxborough, MA 01719
 EMail: erosen@cisco.com
 Steve Vogelsang
 ECI Telecom
 Omega Corporate Center
 1300 Omega Drive
 Pittsburgh, PA 15205
 EMail: stephen.vogelsang@ecitele.com
 Gerald de Grace
 ECI Telecom
 Omega Corporate Center
 1300 Omega Drive
 Pittsburgh, PA 15205
 EMail: gerald.degrace@ecitele.com

Martini, et al. Standards Track [Page 36] RFC 4717 Encapsulation for ATM over MPLS December 2006

 John Shirron
 ECI Telecom
 Omega Corporate Center
 1300 Omega Drive
 Pittsburgh, PA 15205
 EMail: john.shirron@ecitele.com
 Andrew G. Malis
 Verizon Communications
 40 Sylvan Road
 Waltham, MA
 EMail: andrew.g.malis@verizon.com
 Phone: 781-466-2362
 Vinai Sirkay
 Redback Networks
 300 Holger Way
 San Jose, CA 95134
 EMail: vsirkay@redback.com
 Chris Liljenstolpe
 Alcatel
 11600 Sallie Mae Dr.
 9th Floor
 Reston, VA 20193
 EMail: chris.liljenstolpe@alcatel.com
 Kireeti Kompella
 Juniper Networks
 1194 N. Mathilda Ave
 Sunnyvale, CA 94089
 EMail: kireeti@juniper.net
 John Fischer
 Alcatel
 600 March Rd
 Kanata, ON, Canada. K2K 2E6
 EMail: john.fischer@alcatel.com

Martini, et al. Standards Track [Page 37] RFC 4717 Encapsulation for ATM over MPLS December 2006

 Mustapha Aissaoui
 Alcatel
 600 March Rd
 Kanata, ON, Canada. K2K 2E6
 EMail: mustapha.aissaoui@alcatel.com
 Tom Walsh
 Lucent Technologies
 1 Robbins Road
 Westford, MA 01886 USA
 EMail: tdwalsh@lucent.com
 John Rutemiller
 Marconi Networks
 1000 Marconi Drive
 Warrendale, PA 15086
 EMail: John.Rutemiller@marconi.com
 Rick Wilder
 Alcatel
 45195 Business Court
 Loudoun Gateway II Suite 300
 M/S STERV-SMAE
 Sterling, VA 20166
 EMail: Rick.Wilder@alcatel.com
 Laura Dominik
 Qwest Communications, Inc.
 600 Stinson Blvd.
 Minneapolis, MN 55413
 Email: ldomini@qwest.com

Martini, et al. Standards Track [Page 38] RFC 4717 Encapsulation for ATM over MPLS December 2006

Authors' Addresses

 Luca Martini
 Cisco Systems, Inc.
 9155 East Nichols Avenue, Suite 400
 Englewood, CO 80112
 EMail: lmartini@cisco.com
 Jayakumar Jayakumar
 Cisco Systems, Inc.
 225 E.Tasman, MS-SJ3/3
 San Jose, CA 95134
 EMail: jjayakum@cisco.com
 Matthew Bocci
 Alcatel
 Grove House, Waltham Road Rd
 White Waltham, Berks, UK. SL6 3TN
 EMail: matthew.bocci@alcatel.co.uk
 Nasser El-Aawar
 Level 3 Communications, LLC.
 1025 Eldorado Blvd.
 Broomfield, CO 80021
 EMail: nna@level3.net
 Jeremy Brayley
 ECI Telecom Inc.
 Omega Corporate Center
 1300 Omega Drive
 Pittsburgh, PA 15205
 EMail: jeremy.brayley@ecitele.com
 Ghassem Koleyni
 Nortel Networks
 P O Box 3511, Station C Ottawa, Ontario,
 K1Y 4H7 Canada
 EMail: ghassem@nortelnetworks.com

Martini, et al. Standards Track [Page 39] RFC 4717 Encapsulation for ATM over MPLS December 2006

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 contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement

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Martini, et al. Standards Track [Page 40]

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