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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 4717

Network Working Group                                       Luca Martini
Internet Draft                              Level 3 Communications, LLC.
Expiration Date: April 2004

Jeremy Brayley                                             Matthew Bocci
Laurel Networks, Inc.                                            Alcatel

Eric C. Rosen                                            Ghassem Koleyni
Cisco Systems, Inc.                                     Nortel Networks.

                                                            October 2003


Encapsulation Methods for Transport of ATM Cells/Frame Over IP and MPLS Networks


                    draft-ietf-pwe3-atm-encap-03.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   A framework for providing various Layer 1 and Layer 2 services over a
   Packet Switched Network has been described in [3]. This draft
   provides encapsulation formats and guidelines for transporting a
   variety of ATM services over a PSN.







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Table of Contents

    1      Specification of Requirements  ..........................   3
    2      Introduction  ...........................................   3
    3      Terminology  ............................................   4
    4      General encapsulation method  ...........................   6
    4.1    MPLS Shim S Bit Value  ..................................   6
    4.2    MPLS Shim TTL Values  ...................................   6
    4.3    The Control Word  .......................................   7
    4.3.1  The Generic Control Word  ...............................   7
    4.3.2  The Preferred Control Word  .............................   9
    4.3.3  Setting the sequence number field in the control word  ..   9
    4.3.4  Processing the sequence number field in the control word  ...10
    4.4    MTU Requirements  .......................................  11
    5      ATM  ....................................................  11
    5.1    Applicability  ..........................................  12
    5.1.1  ATM N to 1 Cell Mode  ...................................  12
    5.1.2  ATM One-to-One Cell Encapsulation  ......................  14
    5.1.3  AAL5 SDU Frame Encapsulation  ...........................  14
    5.1.4  AAL5 PDU Frame Encapsulation  ...........................  15
    5.2    ATM OAM Cell Support  ...................................  16
    5.2.1  VCC Case  ...............................................  16
    5.2.2  VPC Case  ...............................................  16
    5.2.3  Defect Handling  ........................................  17
    6      ATM N-to-one Cell Mode  .................................  18
    6.1    ATM N-to-one Service Encapsulation  .....................  18
    7      ATM One-to-one Cell Mode  ...............................  21
    7.1    ATM One-to-one Service Encapsulation  ...................  21
    7.2    Sequence Number  ........................................  22
    7.3    ATM VCC Cell Transport Service  .........................  22
    7.4    ATM VPC Services  .......................................  24
    7.4.1  ATM VPC Cell Transport Services  ........................  24
    8      ATM AAL5 CPCS-SDU Mode  .................................  25
    8.1    Transparent AAL5 SDU Frame Encapsulation  ...............  26
    9      AAL5 PDU frame mode  ....................................  27
    9.1    Transparent AAL5 PDU Frame Encapsulation  ...............  27
    9.2    Fragmentation  ..........................................  29
    9.2.1  Procedures in the ATM-to-PSN Direction  .................  29
    9.2.2  Procedures in the PSN-to-ATM Direction  .................  30
   10      Mapping of ATM and PSN Classes of Service  ..............  30
   11      Security Considerations  ................................  31
   12      Intellectual Property Disclaimer  .......................  31
   13      References  .............................................  31
   14      Author Information  .....................................  32





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1. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119


2. Introduction

   Packet Switched Networks (PSNs) have the potential to reduce the
   complexity of a service providers 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:
     - Leveraging of the existing systems and services to provide
       increased capacity from a packet switched core.
     - Preserving existing network operational processes and procedures
       used to maintain the legacy services.
     - 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 L2TP
       and MPLS.  It lists ATM specific requirements and provides
       encapsulation formats and semantics for connecting ATM edge
       networks through a core packet network using L2TP or MPLS. The
       techniques described in this draft will allow ATM service
       providers to take advantage of new technologies in the core in
       order to provide ATM multi-services.

       Figure 1, below displays the ATM services reference model. This
       model is adapted from [3].


















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                         |<----- Pseudo Wire ---->|
                         |                        |
                         |  |<-- 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

       QoS related issues are not discussed in this draft.  This draft
       describes two methods of ATM cell encapsulation, One-to-one mode
       and N-to-one mode.  This draft describes two methods of AAL5
       encapsulation, PDU mode and SDU mode.


3. Terminology

   One-to-one mode: The One-to-one mode specifies an encapsulation
   method which maps one ATM VCC (or one ATM VPC) to one Pseudo Wire.

   N-to-one mode (N >= 1): The N-to-one mode specifies an encapsulation
   method which maps one or more ATM VCCs (or one or more ATM VPCs) to
   one Pseudo Wire.

   Packet Switched Network - A Packet Switched Network (PSN) is an IP or
   MPLS network.

   Pseudo Wire Emulation Edge to Edge - Pseudo Wire Emulation Edge to
   Edge (PWE3) is a mechanism that emulates the essential attributes of
   a service (such as a T1 leased line or Frame Relay) over a PSN.

   Customer Edge - A Customer Edge (CE) is 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 - A Provider Edge (PE) is a device that provides PWE3
   to a CE.




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   Pseudo Wire - A Pseudo Wire (PW) is a connection between two PEs
   carried over a PSN. The PE provides the adaptation between the CE and
   the PW.

   Pseudo Wire PDU - A Pseudo Wire PDU is a PDU sent on the PW that
   contains all of the data and control information necessary to provide
   the desired service.

   PSN Tunnel - A PSN Tunnel is 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
   Pseudo Wire PDU (ATM to PSN direction.)

   Egress - The point where the ATM service is decapsulated from a
   Pseudo Wire PDU (PSN to ATM direction.)

   CTD -  Cell Transfer Delay

   MTU -  Maximum Transmission 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.












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4. General encapsulation method

   This section describes the general encapsulation format for ATM over
   PSN pseudo wires.

    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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Pseudo Wire 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 (L2TP or MPLS). This header is used to transport
   the encapsulated ATM information through the packet switched core.

   The Pseudo Wire Header identifies a particular ATM service on a
   tunnel. In case of MPLS the Pseudo Wire Header is the MPLS label at
   the bottom of the MPLS label stack. In the Case of L2TP the Pseudo
   Wire Header is the L2TP header.

   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.


4.1. MPLS Shim S Bit Value

   The ingress 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.


4.2. MPLS Shim TTL Values

   The setting of the TTL value in the PW label is application
   dependent, however in a strict point to point application the TTL
   SHOULD be appropriately set to 2.







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4.3. The Control Word

   There are four requirements that may need to be satisfied when
   transporting layer 2 protocols over an IP or MPLS backbone [8]:

        -i. Sequentiality may need to be preserved.
       -ii. Small packets may need to be padded in order to be
            transmitted on a medium where the minimum transport unit is
            larger than the actual packet size.
      -iii. Control bits carried in the header of the layer 2 frame may
            need to be transported.
       -iv. To allow accurate packet inspection in an MPLS PSN, and/or
            to operate correctly over MPLS PSNs that have deployed
            equal-cost multiple-path load-balancing, a PW packet MUST
            NOT alias an IP packet.

   The PWE3 architecture document describes a generic control word and a
   preferred control word. This document makes use of both of these
   control words depending on the encapsulation mode. Both of these
   control words addresses all of the above requirements.

   For some encapsulation modes, the control word is REQUIRED, and for
   others OPTIONAL. Where the control word is OPTIONAL implementations
   MUST support sending no control word, and MAY support sending a
   control word.

   In all cases the egress router must be aware of whether the ingress
   router will send a control word over a specific virtual circuit. This
   may be achieved by configuration of the routers, or by signaling, for
   example as defined in [1].

   If the Pseudo Wire traverses a network link that requires a minimum
   frame size such as Ethernet as a practical example, with a minimum
   frame size of 64 octets, then such links will apply padding to the
   Pseudo Wire 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.


4.3.1. The Generic Control Word

   This control word is used in the following encapsulation modes:

     - ATM 1 to 1 Cell Mode






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     - AAL5 PDU Frame Mode

   The PWE3 architecture document [8] 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                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The detailed structure for the ATM 1 to 1 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 when
   transmitting in order to satisfy the IP aliasing requirement (iv) in
   Section 4.1. 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 alghorithm 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 and so no padding is applied in Ethernet links in the PSN.








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4.3.2. The Preferred Control Word

   This control word is used in the following encapsulation modes:
     - ATM N to 1 Cell Mode
     - 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, from left to right, the first 4 bits MUST be
   set to 0 when transmitting in order to satisfy the IP aliasing
   requirement (iv) in Section 4.1. They MUST be ignored by the
   receiving PE.

   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.

   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 an unsequenced packet.


4.3.3. 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 following procedures should be
   used:




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     - the initial packet transmitted on the emulated VC MUST use
       sequence number 1

     - subsequent packets MUST increment the sequence number by one for
       each packet

     - when the transmit sequence number reaches the maximum 16 bit
       value (65535) the sequence number MUST wrap to 1

   If the transmitting router PE1 does not support sequence number
   processing, then the sequence number field in the control word MUST
   be set to 0.


4.3.4. Processing the sequence number field in the control word

   This section applies to the sequence number field of both the Generic
   and Preferred Control Words.

   If a router PE2 supports receive sequence number processing, then the
   following procedures should be used:

   When an emulated VC is initially set up, the "expected sequence
   number" associated with it MUST be initialized to 1.

   When a packet is received on that emulated VC, the sequence number
   should be processed as follows:

     - if the sequence number on the packet is 0, then the packet passes
       the sequence number check

     - otherwise if the packet sequence number >= the expected sequence
       number and the packet sequence number - the expected sequence
       number < 32768, then the packet is in order.

     - otherwise if the packet sequence number < the expected sequence
       number and the expected sequence number - the packet sequence
       number >= 32768, then the packet is in order.

     - otherwise the packet is out of order.

   If a packet passes the sequence number check, or is in order then, it
   can be delivered immediately. If the packet is in order, then the
   expected sequence number should be set using the algorithm:

   expected_sequence_number := packet_sequence_number + 1 mod 2**16
   if (expected_sequence_number = 0) then expected_sequence_number:= 1;




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   Packets which are received out of order MAY be dropped or reordered
   at the discretion of the receiver.

   If a router PE2 does not support receive sequence number processing,
   then the sequence number field MAY be ignored.


4.4. 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
   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. ATM

   This Draft 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 Pseudo-
   Wire. 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 Pseudo-Wire. 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.










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5.1. Applicability

   Three deployment models are supported by the encapsulations described
   in this document:
        -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.
       -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.
      -iii. AAL5: 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 [5] or the ATM transfer
            capability [6] should be preserved.


5.1.1. ATM N to 1 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 PSN tunnel. However, a service provider may wish to provision
   a single VCC to a PSN tunnel in order to satisfy QoS or restoration
   requirements.

   The encapsulation also supports the binding of multiple VCCs/VPCs to
   a single Pseudo Wire.  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:
        -i. Supports all ATM Adaptation Layers Types.
       -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.



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      -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.
       -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.
        -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 [5] or
            ATM transfer capability [6], should be preserved.

   The limitations of the ATM N-to-one cell encapsulation are:
       -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.
      -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 a L2 FCS failure.
     -viii. There is no currently defined method to support EPD/PPD on
            the PSN.
       -ix. There are currently no OAM mechanisms defined for the PSN
            like those defined for ATM. Therefore the methods for the
            detection/consequent-actions of failures in the PSN are not
            specified. This also means that QoS/availability metrics
            cannot be specified for the PSN.
        -x. If cells are concatenated from multiple VCCs or VPCs, then
            the PSN packet must receive treatment by the PSN to support
            the highest QoS of the ATM VCCs/VPCs carried.
       -xi. Cell encapsulation only supports point-to-point LSPs.
            Multipoint-to-point and point-to-multi-point are for further
            study (FFS).
      -xii. When PSN is MPLS network, to have bidirectional
            connectivity, as required in ATM, two LSPs should be
            configured, one for each direction (ATM-to-MPLS and MPLS-
            to-ATM) of the ATM connection.
     -xiii. 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.






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5.1.2. 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 of N to one mode, when only one ATM connection is
   carried on a PW, apply.


5.1.3. AAL5 SDU Frame Encapsulation

   This OPTIONAL encapsulation supports the AAL5 model.

   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 RFC 2684
   encapsulated IP PDUs across a PSN.

   The AAL5-SDU encapsulation requires Segmentation and Reassembly 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 Pseudo Wire to the egress PE. Herein lies another
   advantage of the AAL5-SDU encapsulation.

   The limitations of the AAL5 SDU encapsulation are:
        -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.
       -ii. 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 ATM security applications cannot be supported. This
            also means that the AAL5 CRC may not be used to accurately
            check for errors on the end-to-end ATM VCC.



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      -iii. 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.
       -iv. 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 [5].
        -v. 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.


5.1.4. 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.

   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.




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   The limitations of the AAL5 SDU mode apply to this mode, with the
   exception of limitations (i) and (ii). Limitation (v) can be negated
   through the use of fragmentation.



5.2. ATM OAM Cell Support

5.2.1. 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 [7].

   The PEs SHOULD be able to pass the following OAM cells transparently:
     - F5 AIS (segment and end-to-end)
     - F5 RDI (segment and end-to-end)
     - F5 loopback (segment and end-to-end)
     - Resource Management
     - Performance Management
     - Continuity Check
     - Security

   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.


5.2.2. 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
   [7].

   The PEs SHOULD be able to process and pass the following OAM cells
   transparently according to [7]:
     - F4 AIS (segment and end-to-end)
     - F4 RDI (segment and end-to-end)
     - F4 loopback (segment and end-to-end)

   F5 OAM are not inserted or extracted here. The PEs MUST be able to
   pass the following OAM cells transparently:  F5 AIS (segment and
   end-to-end)
     - F5 RDI (segment and end-to-end)
     - F5 loopback (segment and end-to-end)
     - Resource Management






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     - Performance Management
     - Continuity Check
     - Security

   The OAM cell MAY be encapsulated together with other user data cells
   if multiple cell encapsulation is used.


5.2.3. Defect Handling

   Figure 3 illustrates four possible locations for defects on the PWE3
   service:
     - (a) On the ATM connection from CE to PE
     - (b) On the ATM side of the PW
     - (c) On the PSN side of the PE
     - (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

   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 Pseudo Wire specific maintenance
   mechanism such as the PW status message defined in [1].
   Alternatively, for example, the ingress PE MAY withdraw the Pseudo



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   Wire (VC 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 ingress PE cannot support the generation of OAM cells, it MAY
   notify the egress PE using a Pseudo Wire specific maintenance
   mechanism such as the PW status message defined in [1].
   Alternatively, for example, the ingress PE MAY withdraw the Pseudo
   Wire (VC label) associated with the service. Upon receiving such a
   notification, the egress PE SHOULD generate the appropriate F5 AIS.

   If the PW in one direction fails, then the complete bidirectional
   service is considered to have failed.


6. ATM N-to-one Cell Mode

   The N-to-one mode (N >= 1) described in this Draft 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.


6.1. ATM N-to-one Service Encapsulation

   This section describes the general encapsulation format for ATM over
   PSN pseudo wires.

    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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Pseudo Wire 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 (L2TP or MPLS). This header is used to transport
   the encapsulated ATM information through the packet switched core.



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   The Pseudo Wire Header identifies a particular ATM service on a
   tunnel.  Non-ATM services may also be carried on the PSN tunnel.

   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.

   The ATM Service Payload is specific to the service being offered via
   the Pseudo Wire. 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 [4] section 3.1.1, but without the trailer
   byte. The length of each frame, without the encapsulation headers, is
   a multiple of 52 bytes long. 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 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
   may be signaled, for example using the methods described in [1]. 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 bits in the control word are not used, and 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 the CLP bit from zero
   to one 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:









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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  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 pseudo-wire
       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.

     * 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 HEC) of the incoming cell.




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7. ATM One-to-one Cell Mode

   The One-to-one mode described in this Draft 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
   Pseudo-Wire.


7.1. ATM One-to-one Service Encapsulation

   This section describes the general encapsulation format for ATM over
   PSN pseudo wires, such as IP, L2TP, or MPLS. The specifics pertaining
   to each packet technology are covered in later sections.  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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Pseudo Wire 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

   The PSN Transport Header depends on the packet technology: IP, L2TP
   or MPLS.  This header is used to transport the encapsulated ATM
   information through the packet switched core. This header is always
   present if the Pseudo Wire is MPLS.

   The Pseudo Wire Header depends on the packet technology: IP, L2TP or
   MPLS.  It identifies a particular ATM service within the PSN tunnel.

   The generic control word is inserted after the Pseudo Wire Header.
   The presence of the control word is MANDATORY.

   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



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   being encapsulated.


7.2. Sequence Number

   The sequence number is not required for all services.

   Treatment of the sequence number is according to previous sections
   "Setting the sequence number", and "Processing the sequence number".


7.3. ATM VCC Cell Transport Service

   The VCC cell transport service is characterized by the mapping of a
   single ATM VCC (VPI/VCI) to a Pseudo Wire. 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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Pseudo Wire 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

     * 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.




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     * 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 Pseudo Wire PDU.  The ingress
   and egress PE SHOULD agree to a maximum number of cells in a single
   Pseudo Wire PDU.  This agreement may be accomplished via a Pseudo
   Wire 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.

    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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Pseudo Wire 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



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7.4. ATM VPC Services

   The VPC service is defined by mapping a single VPC (VPI) to a Pseudo
   Wire.  As such it emulates as 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 Pseudo Wire header.  As a VPC
   service, the egress PE MUST NOT change the VCI field.


7.4.1. 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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Pseudo Wire 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



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   encapsulate multiple ATM cells into a Pseudo Wire PDU.  The ingress
   and egress PE SHOULD agree to a maximum number of cells in a single
   Pseudo Wire PDU.  This agreement may be accomplished via a Pseudo
   Wire specific signaling mechanism or via static configuration.

   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.

    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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Pseudo Wire 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


8. ATM AAL5 CPCS-SDU Mode

   The AAL5 payload VCC service defines a mapping between the payload of
   an AAL5 VCC and a single Pseudo Wire.  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 Pseudo Wire.  Therefore, once the ingress PE reassembles the AAL5



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   CPCS-PDU, the PE discards the PAD and CPCS-PDU trailer then inserts
   the resulting payload into a Pseudo Wire 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 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
   single AAL5 CPCS-PDU are sent immediately on the Pseudo Wire 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).


8.1. 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 VCC
       cell relay encapsulation, figure 8.  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



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       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
       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 [3]
       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.


9. AAL5 PDU frame mode

   The AAL5 payload PDU service is OPTIONAL.


9.1. 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 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



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   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.

   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)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Pseudo Wire 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 Pseudo Wire 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



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       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.

     * 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.


9.2. Fragmentation

   The ingress PE may not always be able to reassemble a full AAL5
   frame. This may be due to the AAL5 PDU exceeding the Pseudo Wire MTU
   or when 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.



9.2.1. Procedures in the ATM-to-PSN Direction

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






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     - 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.
     - The E and C bits of the fragment shall be set as defined earlier
       in section 9.
     - 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).


9.2.2. Procedures in the PSN-to-ATM Direction

   The following procedures shall apply:
     - The 3-bit PTI field of each ATM cell header is constructed as
       follows:
            -i. The most significant bit is set to 0, indicating a user
                data cell.
           -ii. The middle bit is set to the E bit value of the
                fragment.
          -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.
           -iv. The least significant PTI bit is set to 0 for all other
                cells in the PSN frame.
     - The CLP bit of each ATM cell header is set to the value of the C
       bit of the control byte in Figure 12.
     - When a fragment is received, each constituent ATM cell is sent in
       correct order.


10. Mapping of ATM and PSN Classes of Service

   This section is informational.

   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 Diff-Serv
   framework, a class of service based on the EF PHB is a good
   candidate.

   Furthermore, ATM service categories have support for multiple
   conformance definitions [5]. 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.




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   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
   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 which
   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
   details in sections 6 through 9 for each encapsulation mode.


11. Security Considerations

   This document specifies only encapsulations, and not the protocols
   used to carry the encapsulated packets across the PSN.  Each such
   protocol may have its own set of security issues, 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 then a native ATM service.


12. Intellectual Property Disclaimer

   This document is being submitted for use in IETF standards
   discussions.


13. References

   [1] "Transport of Layer 2 Frames Over MPLS", draft-ietf-pwe3-
   control-protocol-03.txt. ( work in progress )

   [2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
        Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032

   [3] "Requirements for Peudo Wire Emulation Edge-to-Edge (PWE3",
   draft-ietf-pwe3-requirements-05.txt. ( work in Progress )

   [4] ATM Forum Specification fb-fbatm-0151.000 (2000) ,Frame Based ATM
   over SONET/SDH Transport (FAST)

   [5] ATM Forum Specification af-tm-0121.000 (1999), Traffic Management
   Specification Version 4.1.



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   [6] ITU-T Recommendation I.371 (2000), Traffic control and congestion
   control in B-ISDN.

   [7] ITU-T Recommendation I.610, (1999), B-ISDN operation and
   maintenance principles and functions.

   [8] "PWE3 Architecture", draft-ietf-pwe3-arch-04.txt. (work in
   progress)


14. Author Information


   Luca Martini
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: luca@level3.net


   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: nna@level3.net


   Giles Heron
   PacketExchange Ltd.
   The Truman Brewery
   91 Brick Lane
   LONDON E1 6QL
   United Kingdom
   e-mail: giles@packetexchange.net


   Dimitri Stratton Vlachos
   Mazu Networks, Inc.
   125 Cambridgepark Drive
   Cambridge, MA 02140
   e-mail: d@mazunetworks.com


   Dan Tappan
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   e-mail: tappan@cisco.com



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   Jayakumar Jayakumar,
   Cisco Systems Inc.
   225, E.Tasman, MS-SJ3/3,
   San Jose , CA, 95134
   e-mail: jjayakum@cisco.com


   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   E-mail: erosen@cisco.com


   Steve Vogelsang
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: sjv@laurelnetworks.com


   Jeremy Brayley
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: jbrayley@laurelnetworks.com


   Gerald de Grace
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: gdegrace@laurelnetworks.com


   John Shirron
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: jshirron@laurelnetworks.com






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   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   e-mail: Andy.Malis@vivacenetworks.com


   Vinai Sirkay
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   e-mail: sirkay@technologist.com


   Chris Liljenstolpe
   Cable & Wireless
   11700 Plaza America Drive
   Reston, VA 20190
   e-mail: chris@cw.net


   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   e-mail: kireeti@juniper.net


   Ghassem Koleyni
   Nortel Networks
   P O Box 3511, Station C Ottawa, Ontario,
   K1Y 4H7 Canada
   e-mail: ghassem@nortelnetworks.com


   John Fischer
   Alcatel
   600 March Rd
   Kanata, ON, Canada. K2K 2E6
   e-mail: john.fischer@alcatel.com


   Matthew Bocci
   Alcatel
   Grove House, Waltham Road Rd
   White Waltham, Berks, UK. SL6 3TN
   e-mail: matthew.bocci@alcatel.co.uk



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   Mustapha Aissaoui
   Alcatel
   600 March Rd
   Kanata, ON, Canada. K2K 2E6
   e-mail: mustapha.aissaoui@alcatel.com


   Tom Walsh
   Lucent Technologies
   1 Robbins Road
   Westford, MA 01886 USA
   e-mail: tdwalsh@lucent.com


   John Rutemiller
   Marconi Networks
   1000 Marconi Drive
   Warrendale, PA 15086
   e-mail: John.Rutemiller@marconi.com


   Rick Wilder
   Masergy Communications
   2901 Telestar Ct.
   Falls Church, VA 22042
   e-mail: rwilder@masergy.com


   Laura Dominik
   Qwest Communications, Inc.
   600 Stinson Blvd.
   Minneapolis, MN, 55413
   Email: ldomini@qwest.com

















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