[Docs] [txt|pdf] [Tracker] [Email] [Nits]

Versions: 00

Transport Working Group                            Matthew Bocci
Internet Draft                                     Mustapha Aissaoui
Expiration Date: November 2002                     John Fischer
                                                   Alcatel

                                                   Ghassem Koleyni
                                                   Nortel Networks




                                                   May 2002


     Applicability Statement for AAL5 Transparent Frame Encapsulation
                                 over PSN

           <draft-bocci-pwe3-app-frame-over-psn-00.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].


   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.

1. Abstract

   This draft provides an applicability statement for the transparent
   AAL5 PDU frame mode encapsulation in draft-fischer [4].

   Draft-fischer describes methods to carry ATM services over IP, L2TP
   or MPLS. The PSN (e.g. MPLS) is used to transport ATM layer services
   such as those defined by ITU-T as ATM transfer capabilities [2] and
   the ATM Forum as ATM service categories [3]. The basic requirement is
   to transparently transport ATM VCC service related information (e.g.,
   traffic parameters, QoS, OAM, etc.) over the Pseudo Wire (PW), over
   the packet network. Transparent PDU frame mode enables bandwidth


Bocci, et al.           Expires  November 2002               [Page 1]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

   efficiency gains to be realized for ATM VCCs that use AAL5, yet still
   provide full ATM transparency, including the correct sequencing of
   OAM cells on an AAL5 flow.

   Table of contents

   1. Abstract........................................................1
   2. Introduction....................................................2
   3. Conventions used in this document...............................3
   4. Terminology.....................................................3
   5. Applicability Statement.........................................4
   5.1 Applicability..................................................4
   5.2 Implementation and deployment considerations...................6
   5.3. Limitations...................................................6
   6 ATM Service Encapsulation........................................7
   6.1 Length and Sequence Number.....................................7
   6.1.1 Setting the length field.....................................8
   6.1.2 Processing the length field..................................9
   6.1.3 Setting the sequence number..................................9
   6.1.4 Processing the sequence number...............................9
   7 ATM VCC Services................................................10
   7.1 Transparent AAL5 PDU Frame Service............................10
   7.1.1 OAM Cell Support............................................12
   7.1.2 Fragmentation...............................................12
   7.1.2.1 Procedures in the ATM-to-MPLS Direction...................12
   7.1.2.2 Procedures in the MPLS-to-ATM Direction...................13
   8 ILMI support....................................................13
   9 QoS considerations..............................................13
   10 ATM Pseudo-Wire over MPLS specific requirements................15
   10.1 MPLS Transport Label.........................................16
   10.2 MPLS Pseudo Wire Label.......................................16
   11 ATM Pseudo-Wire over L2TP specific requirements................17
   11.1 L2TP Session ID..............................................18
   11.2 Cookie.......................................................18
   12 ATM Pseudo-Wire over IP specific requirements..................18
   12.1 C, K, and S bits.............................................19
   12.2 Protocol Type field..........................................19
   12.3 Key Field....................................................19
   12.4 GRE Sequence Number Field....................................19
   13. Security Considerations.......................................20
   14. References....................................................20
   15. Acknowledgement...............................................21
   16. Author's Addresses............................................21

2. Introduction

   This  draft  provides  an  applicability  statement  for  the  AAL5
   transparent frame mode encapsulation in  draft-fischer [4].

   Draft-fischer describes methods to carry ATM services over an IP,
   L2TP or MPLS based PSN.   The PSN is used to transport ATM layer
   services such as those defined by ITU-T as ATM transfer capabilities

Bocci, et al.            Expires November 2002                [Page 2]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

   [2] and by the ATM Forum as ATM service categories [3]. The basic
   requirement is to transparently transport the ATM VCC service related
   information (e.g., traffic parameters, QoS, OAM, etc.) over the
   Pseudo Wire (PW), over the packet network. The transparent AAL5 PDU
   mode is intended to be more efficient than the basic cell mode of
   draft-fischer [4], yet still provide full ATM transparency including
   the correct sequencing of OAM cells on an AAL5 flow.

   Section  5  of  this  draft  presents  the  applicability  statement.
   Sections 6 to 13 are taken from draft-fischer [4]. They provide the
   details of the encapsulation methods as well as the OAM, ILMI, and
   QoS procedures for the basic cell mode.

3. Conventions used in this document

   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 [16].


4. Terminology
  Packet Switched Network - A Packet Switched Network (PSN) is a
  network using IP, MPLS or L2TP as the unit of switching.

  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 an
  emulated service originates and terminates.  The CE is not aware that
  it is using an emulated service rather than a "real" service.

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

  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 network devices.

  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 de-capsulated from a
  Pseudo Wire PDU (PSN to ATM direction.)


Bocci, et al.            Expires November 2002                [Page 3]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


   CTD    Cell Transfer Delay
   MTU    Maximum Transfer Unit
   OAM    Operations, Administration, and Maintenance.
   PVC    Permanent Virtual Connection. An ATM connection that is
          provisioning via a network management interface.  The
          connection is not signalled.
   VCC    Virtual Circuit Connection.  An ATM connection that is
          switched based on the cell header's VCI.


5. Applicability Statement

5.1 Applicability

   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 PDU frame mode takes advantage
   of the delineation of higher layer frames in the ATM layer to provide
   increased bandwidth efficiency compared with the basic cell
   encapsulation mode of draft-fischer [4]. The nature of the service,
   as defined by the ATM service category [3] or the ATM transfer
   capability [2] should be preserved. To provide this, the basic
   requirement of the ATM-PSN interworking function is to map the AAL5
   PDU frames belonging to a VCC, together with any related OAM and
   protocol control information, into a PW.


   Two network applications that utilize the PDU frame mode
   encapsulation described in draft-fischer [4] are:

   a. The transport of multi-service ATM over a packet core network
   where AAL5 is used as the adaptation layer. Many service providers
   have multiple service networks and the Operational Support System
   capabilities needed to support these existing service offerings.
   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 benefits of this model to a service provider are threefold:

        i.      Leveraging of the existing systems and services to
        provide increased capacity from a packet switched core.
        ii.     Preserving existing network operational processes and
        procedures used to maintain the legacy services e.g. ATM OAM and
        ATM security.
        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.


Bocci, et al.            Expires November 2002                [Page 4]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


   b. L2 VPN service over a PSN infrastructure. In this case, VPN sites
   are connected through ATM VCCs, as in today's L2 VPNs. The
   transparent PDU frame mode encapsulation allows the VPN service
   provider to transparently extend this L2 connectivity over its PSN
   while achieving bandwidth efficiency gains over the basic cell mode
   and supporting ATM layer applications of the VPN customer, such as
   ATM security. The advantage is for the service provider to combine L2
   and L3 services over the same PSN.

   Figure 1 shows the reference model for carrying ATM services over a
   PSN.  This model is adapted from [6].



                    |<------- Pseudo Wire ------>|
                    |                            |
                    |    |<-- PSN Tunnel -->|    |
                    V    V                  V    V
         ATM Service+----+                  +----+ ATM Service
   +-----+    |     | PE1|==================| PE2|     |    +-----+
   |     |----------|............PW1.............|----------|     |
   | CE1 |    |     |    |                  |    |     |    | CE2 |
   |     |----------|............PW2.............|----------|     |
   +-----+    |     |    |==================|    |     |    +-----+
         ^    |     +----+                  +----+     |    ^
         |    |    Provider                Provider    |    |
         |    |     Edge 1                  Edge 2     |    |
         |                                                  |
         |<-------------- Emulated Service ---------------->|

   Customer                                                Customer
    Edge 1                                                  Edge 2

                 Figure 1: ATM-over-PSN Service Reference Model


   An ATM VCC is carried over a PW. The PW corresponding to any VCC may
   be further tunneled in a transport PSN tunnel to achieve multiplexing
   gain and bandwidth efficiency.

   When the QoS considerations in Section 10 are respected, this ATM
   over PSN service encapsulation method provides end users with the
   same quality of service on any given VCC as per corresponding SLA,
   traffic contracts and the QoS commitments for that connection.

   One important consideration to make when interworking is to allow
   OAM information to be treated as in the original network. The
   interworking function allows this transparency while performing AAL5
   frame encapsulation. Fragmentation may be performed in order to
   maintain the position of the OAM cells with respect to the user
   cells.

Bocci, et al.            Expires November 2002                [Page 5]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


   Fragmentation may also be performed to maintain the size of the
   packet carrying the AAL5 PDU within the MTU of the link.

   Cell Loss priority (CLP) field conveys the priority of the cell in
   the connection. The Explicit Forward Congestion Indicator (EFCI)
   field conveys the congestion state of ATM network. Information on
   both of these fields is obtained from the ATM cell header. CLP and
   EFCI fields are both part of the ATM service specific information
   header.

   For ease of operation and to achieve transparency, 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 reassembled fragments.


5.2 Implementation and deployment considerations

   AAL5 transparent mode is only applicable to services that use AAL5 to
   carry higher layer frames over ATM VCCs.

5.3. Limitations

   AAL5 frame encapsulation only supports point-to-point LSPs.  Multi-
   point-to-point and point-to-multi-point are for further study (FFS).

   To have bi-directional 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.

   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.

   The maximum number of cells of an AAL5 PDU that may be reassembled
   before transport across the PSN may be limited by the cell transfers
   delay (CTD) and cell delay variation (CDV) objectives of the
   connection.

   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.




Bocci, et al.            Expires November 2002                [Page 6]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

   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 ATM 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. All
   encapsulation formats and procedures contained in the following
   sections are from draft-fischer [4].

   Figure 2 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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Optional Length and Sequence Number       | ATM Specific  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     ATM Service Payload                       |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 2: General format for ATM 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 Length and Sequence Number is inserted after the Pseudo Wire
   Header.  This field is optional.

   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.

6.1 Length and Sequence Number

Bocci, et al.            Expires November 2002                [Page 7]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


   The length and sequence number are not required for all services.
   Length and sequence number are to satisfy these requirements.

   - Order may need to be preserved.
   - 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.

   The one-octet Length indicates length of the packet payload that
   includes, length of the length field, Sequence number length, the ATM
   specific header length and the payload length (i.e., Pseudo Wire
   PDU). The Length field is set to 0 by the ingress PE if not used and
   is ignored by the egress PE.

   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
   length field MUST be set to the PDU length. A mechanism is required
   for the egress PE to detect and remove such padding.

   The Sequence Number is a 2-octet field that may be used to track
   packet order delivery. This field is set to 0 by the ingress PE if
   not used and is ignored by the egress PE. The sequence number space
   is a 16-bit, unsigned circular space. Processing of the sequence
   number field is OPTIONAL.

   In all cases the egress PE MUST be aware of whether the ingress PE
   will send the length and sequence number over a specific Pseudo Wire.
   This may be achieved using static configuration or using Pseudo Wire
   specific signaling.

   Length field is not required for cell mode


6.1.1 Setting the length field

   The length field is required to enable the egress PE to remove any
   padding that may have resulted if the pseudo-wire traversed a network
   link that enforces a minimum frame size (e.g. Ethernet). Ethernet
   applies padding to frames that are smaller than 64 bytes, where this
   includes a minimum of 18 bytes for the Ethernet header and trailer.

   The length field represents the size of the packet in bytes including
   the length, sequence number, ATM specific header and ATM service
   payload.  If the size of the packet is larger than can be
   represented by the length field, the field MUST be set to 0.  In
   addition, the length field MAY be set to 0 if the size of the packet
   prevents any padding from being applied.



Bocci, et al.            Expires November 2002                [Page 8]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

   The AAL5 SDU Frame service is the only service that can generate
   packets smaller than the Ethernet minimum and MUST set the length
   field accordingly.  The length field MUST either be set to the size
   of the packet if the size is less than 46 bytes or to 0.

   All cell transport services MUST always set the length field to 0 to
   indicate to the remote PE that no padding was applied.

6.1.2 Processing the length field

   Since length field is not used for cell mode, no processing is
   required.

6.1.3 Setting the sequence number

   The following procedures SHOULD be used by the ingress PE if
   sequencing is desired for a given ATM service:

   - the initial PDU transmitted on the Pseudo Wire MUST use sequence
   number 1
   - the PE MUST increment the sequence number by one for each
   subsequent PDU
   - when the transmit sequence number reaches the maximum 16 bit value
   (65535) the sequence number MUST wrap to 1

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


6.1.4 Processing the sequence number

   If the egress PE supports receive sequence number processing, then
   the following procedures SHOULD be used:

   When an ATM service is initially created, the "expected sequence
   number" associated with it MUST be initialized to 1.

   When a PDU is received on the Pseudo Wire associated with the ATM
   service, the sequence number SHOULD be processed as follows:

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

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

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

    - otherwise the PDU is out of order.

Bocci, et al.            Expires November 2002                [Page 9]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


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

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

   Pseudo Wire PDUs that are received out of order MAY be dropped or
   reordered at the discretion of the egress PE.

   If the egress PE does not support receive sequence number processing,
   then the sequence number field MAY be ignored.


7 ATM VCC Services

This section defines ATM AAL5 VCC services that may be supported over
the PSN.

7.1 Transparent AAL5 PDU Frame Service

   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 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                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Optional Length and Sequence Number        |M|V|Res|Frg|E|C|

Bocci, et al.            Expires November 2002               [Page 10]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +                             "                                 |
   |                        AAL5 CPCS-PDU                          |
   |                      (n * 48 bytes)                           |
   |                             "                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


               Figure 3: AAL5 transparent service encapsulation

   The first octet following the Pseudo Wire Header carries control
   information.  The M, V, Res, and C bits are as defined earlier for
   VCC cell mode.

         * Frg (Fragmentation) Bits

         This field is used to support the fragmentation functionality
         described later in this section.

          - 11 Single Segment Message (Beginning and End of Message)
          - 10 Beginning of Message
          - 00 Continuation of Message
          - 01 End of Message

         * 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.

         * 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,

Bocci, et al.            Expires November 2002               [Page 11]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

           including the AAL5 padding and trailer or the AAL5 fragment.

7.1.1 OAM Cell Support

   When configured for the AAL5 transparent VCC service, both PE's
   SHOULD act as a VC switch, in accordance with the OAM procedures
   defined in [5].

   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

   The PEs SHALL use the single ATM VCC cell mode encapsulation (Section
   6.1, draft-fischer[4]) when passing an OAM cell.

   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).

   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).

   If the ingress PE cannot support the generation of OAM cells, it MAY
   notify the egress PE using a Pseudo Wire specific maintenance
   mechanism to be defined.  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.

7.1.2 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.

   If no fragmentation occurs, then the fragmentation bits are set to 11
   (SSM, Single Segment Message).

   When fragmentation occurs, the procedures described in the following
   subsections shall be followed.


7.1.2.1 Procedures in the ATM-to-MPLS Direction

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

Bocci, et al.            Expires November 2002               [Page 12]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

         AAL5 PDU.
       - For the first fragment, the FRG bits shall be set to 10 (BOM,
         Beginning Of Message).
       - For the last fragment, the FRG bits shall be set to 01 (EOM,
         End Of Message).
       - For all other fragments, the FRG bits shall be set to 00 (COM,
         Continuation Of Message).
       - The E and C bits of the fragment shall be set as defined
         earlier in section 7.

7.1.2.2 Procedures in the MPLS-to-ATM Direction

   The following procedures shall apply:
       - The 3-bit PTI field of each ATM cell header is constructed as
         follows:
             + The most significant bit is set to 0, indicating a user
               data cell.
             + The middle bit is set to the E bit value of the
               fragment.
             + The least significant bit is set to 1 for the last ATM
               cell of a fragment where the FRG bits are 01 (EOM) or
               11(SSM); otherwise this bit is set to 0.
       - The C bit of each ATM cell header is set to the value of the C
            bit of the control byte in Figure 5.

8 ILMI support

   Integrated Local Management Interface (ILMI) typically is used in ATM
   networks for neighbor resource availability detection, address
   registration, auto-configuration, and loss of connectivity detection.
   ILMI messages are sent as SNMP PDU's within ATM AAL5 cells.

   A PE MAY provide an ATM ILMI to its attached CE. If the ingress PE
   receives an ILMI message indicating that the ATM service (VCC) is
   down, it MAY use a Pseudo Wire specific mechanism to notify the
   egress PE of the ATM service status.  For example, a PE using an MPLS
   based Pseudo Wire may withdraw its advertised VC label.

   When receiving such a notification, the egress PE MAY use ILMI to
   signal the ATM service status to its attached CE.

9 QoS considerations

   This section provides guidelines for the provision of QoS for the
   individual ATM PWs over the PSN.  The objective is to provide the
   ability to support the traffic contracts and the QoS commitments made
   to the ATM connections [8].  This section is informational and the
   provided guidelines SHOULD be treated as good practices and the
   mappings as examples only.

   When ATM PW service is configured over a PSN, each ATM service
   category SHOULD be mapped to a compatible class of service in the PSN

Bocci, et al.            Expires November 2002               [Page 13]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

   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 to
   provide QoS classes, a class of service based on the EF PHB is a good
   candidate.

   Furthermore, ATM service categories have support for multiple
   conformance definitions.  A conformance definition specifies the
   conformance of cells of a connection at an interface, e.g., public
   UNI, in relation to the conformance algorithm and corresponding
   parameters specified in the connection traffic descriptor [15].  For
   example, the conformance definition specifies if the requested QoS
   parameters, e.g., CLR, apply to the aggregate CLP0+1 conforming cell
   flow or to the CLP0 conforming flow only.  Thus, the conformance
   definition SHOULD be respected in the selected PSN class of service.

   For example, a connection CLP1 cell flow in a VBR.3 conformance
   definition is treated as excess traffic in the ATM network and thus
   has no QoS guarantees associated with it. This flow SHOULD be
   provided a treatment no better than the treatment of the CLP0 cell
   flow in the PSN. This does not mean however that the PSN network
   should mirror the various conformance definitions of the ATM service
   categories.

   In the remainder of this section, it is assumed that the PSN
   implements IP Diff-Serv framework to provide QoS.

   All ATM traffic management functions specified in [3] are applicable
   for the ATM part of the ATM PW in the ingress PE and egress PE. In
   the ATM-to-PSN direction, each ATM connection MAY be policed and/or
   shaped to conform to its traffic descriptor in the ATM endpoint of
   the ATM PW in the PE.  Whenever allowed by the conformance
   definition, a non-conforming CLP0 cell may be turned into a CLP1 cell
   and becomes conforming.  Connection admission SHOULD be applied to
   make sure sufficient resources are available to carry the ATM PW over
   the transport LSP.  The mapping of ATM service category and
   conformance definition to the Diff-Serv PHB determines the outgoing
   PHB.  This is the PHB to be applied to the cells or packets of the
   ATM PW in the ingress PE and egress PE and inside the PSN.  The PSN
   transport header of the outgoing PSN packet SHOULD be marked to
   identify the selected PHB.  This consists of marking the DS field in
   the IP header in the case of IP PSN, or the EXP field in the
   transport shim header in the case of a MPLS PSN.


   Figure 4 provides an example of mapping ATM service category and
   conformance definition to a Diff-Serv PHB.


    ATM Service    Conformance     CLP      Diff-Serv   DSCP
    Category       Definition      Setting  PHB         Marking

Bocci, et al.            Expires November 2002               [Page 14]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

    ----------------------------------------------------------------
    CBR            CBR.1           0/1      EF          101110
    rt-VBR         VBR.1           0/1      EF          101110
                   VBR.2/VBR.3     0        AF11        001010
                                   1        AF12        001100
    nrt-VBR        VBR.1           0/1      AF21        010010
                   VBR.2/VBR.3     0        AF21        010010
                                   1        AF22        010100
    ABR            ABR             0        AF31        011010
    UBR+MDCR       UBR.1/UBR.2     0/1      AF31        011010
    GFR            GFR.1/GFR.2     0        AF31        011010
                                   1        AF32        011100
    UBR            UBR.1/UBR.2     0/1      DF          000000

       Figure 4: Example of ATM Service Category to PHB Mapping


   Note that an alternative mapping may not distinguish between the
   conformance definitions in a given ATM service category. In this
   case, the CLP0 and CLP1 flows of a ATM connection are provided with
   the same QoS in the PSN. As an example, all conformance definitions
   of the nrt-VBR service category MAY be mapped to the AF21 PHB in
   Figure 9.

   When the PSN is MPLS based, the selected PHB for the ATM PW is
   conveyed in different ways depending if the transport LSP is an L-
   LSP or an E-LSP [15].  In the case of an L-LSP, the PHB Scheduling
   Class is signaled at the transport LSP establishment and is therefore
   inferred from the transport label value.  The Drop Precedence of the
   individual PW packets is conveyed in the EXP field of the transport
   LSP shim header.  In the case of an E-LSP, the PHB is conveyed in the
   EXP field of the transport LSP shim header.  The actual values to be
   marked in the EXP field to reflect the example in Figure 9 is outside
   the scope of this document.

   In the presence of congestion, the PE MAY mark the EFCI bit and MAY
   perform selective cell discard based on CLP setting, if allowed by
   the conformance definition, and in accordance with [15].  The method
   used to transfer the CLP and EFCI information of the individual cells
   into the ATM specific field of the PW packet is described in details
   in sections 7 and 8.

   In the PSN-to-ATM direction, the ATM service category and conformance
   definition is obtained from the context accessed via the pseudo wire
   label of the ATM PW.  The information needed to reconstruct the ATM
   header, including the setting of the CLP and EFCI fields, for the
   individual cells is contained in the ATM specific information field
   of the PW packet.  The method used to determine the CLP and EFCI
   information of the individual cells from the ATM specific information
   field of the PW packet is described in details in sections 6 and 7.

10 ATM Pseudo-Wire over MPLS specific requirements

Bocci, et al.            Expires November 2002               [Page 15]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


   Figure 5 provides a general format for interworking between ATM and
   MPLS.  The ATM information is encapsulated inside two MPLS labels as
   defined in [9].

   The Pseudo Wire Endpoint uses a unique MPLS label, the pseudo wire
   label, to identify each direction of an ATM connection.  This label
   allows the PWE to access context information for the connection.  As
   an example, the context may contain the information regarding
   connection type such as VCC or VPI/VCI value that need to be inserted
   into the ATM cell header in the MPLS-to-ATM direction.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    MPLS Transport Label                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   MPLS Pseudo Wire Label                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Optional Length and Sequence Number       | ATM Specific  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     ATM Service Payload                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 5: Format for ATM PW over a MPLS PSN

10.1 MPLS Transport Label

   The 4-octet MPLS transport label identifies an LSP used to transport
   traffic between two ATM-MPLS pseudo wire endpoints.  This label is
   used to switch the transport LSP between core LSRs. Since an MPLS LSP
   is unidirectional, for the case of bi-directional traffic there will
   be two different pseudo wire LSPs, one for each direction of the
   connection.  These may have different label values. Setting of the
   EXP and TTL is for further study.  The S bit is set to 0 since this
   is not the last label in the MPLS label stack.

10.2 MPLS Pseudo Wire Label

   The 4-octet interworking label uniquely identifies one pseudo wire
   LSP, carried inside a MPLS transport LSP.  The pseudo wire label has
   the structure of a standard MPLS shim header.  More than one pseudo
   wire LSP may be supported by one MPLS transport LSP.  The pseudo wire
   endpoint provides the association between the ATM connection or the
   ATM port and MPLS LSP by means of the 20-bit label field of the
   pseudo wire LSP.  In this association, in the ATM-to-MPLS direction a
   mapping of the VCI/VPI of the ATM connection or the Port to the 20-
   bit label is performed, while in the MPLS-to-ATM direction the 20-bit
   label is mapped to a VPI/VCI of the ATM connection or to a Port. This
   association is signalled or provisioned between the two pseudo-wire
   endpoints.

Bocci, et al.            Expires November 2002               [Page 16]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002



   Since an MPLS LSP is unidirectional, for the case of bi-directional
   ATM VCCs there will be two different pseudo wire LSPs, one for each
   direction of the connection.  These may have different label values.
   Setting of the EXP and TTL is for further study.  The S bit is set to
   1 since this is the last label in the bottom of the MPLS stack. The
   pseudo wire label is not visible to the LSRs within the MPLS core
   network.

11 ATM Pseudo-Wire over L2TP specific requirements

   Figure 6 provides a general format for interworking between ATM and
   L2TP.  This draft refers to L2TPv3, or L2TP base, as described in
   [11].  L2TP base extends the original L2TP [12] to operate directly
   over an IP PSN and to further generalize the control procedures to
   carry any tunneled Layer 2 protocol other than PPP.  Any further
   reference to L2TP in this document assumes L2TPv3 or L2TP base as
   specified in [11].

   The ATM information is encapsulated inside a L2TP tunnel packet. The
   L2TP tunnel is setup over an IPv4 PSN. The IPv4 protocol in the IPv4
   header is set to 115 to indicate that the L2TP packet is encapsulated
   in an IPv4 packet [11]. Furthermore, L2TP can operate over IP or over
   UDP. The use of either mode is outside the scope of this document.
   The encapsulation format shown in Figure 11 represents the common
   headers for carrying L2TP packet over UDP and IP. If UDP is used,
   additional header information is required and is defined in [11].

   The Pseudo Wire Endpoint uses a unique L2TP session ID to identify
   each direction of an ATM connection. This session ID is local to each
   PE and allows the PWE to identify each ATM PW in the L2TP tunnel in
   order to access context information for the ATM connection. As an
   example, the context may contain the information regarding connection
   type such as VCC VPI/VCI value that need to be inserted into the ATM
   cell header in the L2TP-to-ATM direction. Multiple PWs with locally
   unique Session IDs at each PE can be multiplexed into a L2TP tunnel.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    L2TP Session ID                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Cookie (optional, up to 64 bits)               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Optional Length and Sequence Number       | ATM Specific  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     ATM Service Payload                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Bocci, et al.            Expires November 2002               [Page 17]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

             Figure 6: Format for ATM PW over a L2TP PSN

11.1 L2TP Session ID

   This is 32-bit field containing a non-zero identifier for a session,
   or a PW in this case. L2TP sessions are named by identifiers that
   have local significance only at each PE [11].

   Each PE for the life of the session will give different Session IDs
   the same PW. Multiple PWs with locally unique Session IDs at each PE
   can be multiplexed into a L2TP tunnel. When the L2TP control
   connection is used for session establishment, Session IDs are
   selected and exchanged as Local Session ID Attribute Value Pairs
   (AVPs) during the creation of a PW. A session ID of zero is reserved
   for the carriage of L2TP control messages [11].

11.2 Cookie

   The optional Cookie field contains a variable length (maximum 64
   bits), long word-aligned value used to check the association of a
   received packet with the PW identified by the Session ID. The Cookie
   MUST be configured with a random value utilizing all bits in the
   field [11].  The Cookie provides an additional level of guarantee,
   beyond the Session ID lookup, that a packet has been directed to the
   proper PW identified by the Session ID.

   When the L2TP control connection is used for PW session
   establishment, random Cookie values are selected and exchanged as
   Assigned Cookie AVPs during the creation of a PW.  The maximum size
   of the Cookie field is 64 bits.

12 ATM Pseudo-Wire over IP specific requirements

   Figure 7 provides a general format for carrying an ATM PW over an IP
   PSN. This is an alternative encapsulation to the one using L2TP, as
   described in Section 11. The GRE encapsulation is used as specified
   in [13] and [14]. The IPv4 protocol in the IPv4 header is set to 47
   to indicate that the GRE packet is encapsulated in an IPv4 packet
   [13].

   The ATM information is encapsulated inside a GRE/IP packet. The
   Pseudo Wire Endpoint uses a unique GRE Key to identify each direction
   of an ATM connection. As an example, the context may contain the
   information regarding connection type such as VCC VPI/VCI value that
   need to be inserted into the ATM cell header in the IP-to-ATM
   direction. Multiple PWs with unique GRE Keys can be multiplexed into
   a GRE/IP tunnel.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    IPv4 Header (N words)                      |

Bocci, et al.            Expires November 2002               [Page 18]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |C| |K|S| Reserved0       | Ver |         Protocol Type         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Key                                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 GRE Sequence Number (Optional)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Optional Length and Sequence Number       | ATM Specific  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     ATM Service Payload                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 7: Format for ATM PW over an IP PSN

12.1 C, K, and S bits

      The Checksum field in the GRE header is not required for carrying
      ATM PW over IP PSN. Therefore the C bit is set to zero.

      The Key field in the GRE header is always used (see Section 12.3).
      Therefore, the K bit is always set to 1.

      If the GRE Sequence Number field is used, then the value of the K
      bit is set to 1. Otherwise, its value is set to zero.



12.2 Protocol Type field

   The Protocol Type field is set to a number to be allocated by IEEE
   for the purpose of encapsulating ATM PW over GRE.

12.3 Key Field

   The Key field contains a four-octet number that is inserted by the
   transmitting PE. The Key field is used by the receiving PE for
   identifying an individual ATM PW within a GRE/IP tunnel. Multiple PWs
   with unique GRE Keys can be multiplexed into a GRE/IP tunnel. The
   method by which the Key field value is negotiated between the
   transmitting PE and a receiving PE is further study.

12.4 GRE Sequence Number Field

   If the Sequence Number Present bit is set to 1, then it indicates
   that the Sequence Number field is present in the GRE header.
   Otherwise, the Sequence Number field is not present in the GRE
   header. The use of the Sequence Number field MUST comply to [14].

   A specific ATM PWE network may choose to rely on the GRE protocol to
   track in-order delivery of ATM PW packets. Another way of tracking

Bocci, et al.            Expires November 2002               [Page 19]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002

   in-order delivery is to use the PW Sequence number field as explained
   in Section 5.1.


13. Security Considerations

   No additional security issues are introduced in this document.  As
   ATM encapsulation to MPLS packet is related to MPLS. AAL5 frame
   encapsulation shares the security concerns associated with MPLS.

14. References

   [1}  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

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

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

   [4] IETF draft-fischer-pwe3-atm-service-03.txt(March 2002, work in
       progress), PWE3: ATM service description

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

   [6] IETF draft-ietf-pwe3-requirements-02.txt (November 2001, work in
       progress,), Requirements for pseudo Wire Emulation Edge-to-Edge
       (PWE3).

   [7] IETF draft-martini-l2circuit-encap-mpls-04.txt Martini (November
      2001, Work in Progress), Encapsulation Methods for Transport of
      Layer 2 Frames Over IP and MPLS Networks.

   [8] IETF draft-koleyni-pwe3-atm-over-mpls-04.txt (January 2002, Work
      in Progress), Requirements and framework for ATM network
      interworking over MPLS

   [9] IETF RFC 3032 (2001), MPLS Label Stack Encoding.

   [10] ATM Forum af-aic-0178.000 (2001), ATM-MPLS Network Interworking.

   [11] IETF draft-ietf-l2tpext-l2tp-base-01.txt (October 2001, Work in
        Progress), Layer Two Tunneling Protocol (L2TP).

   [12] IETF RFC 2661 (1999), Layer Two Tunneling Layer Two Tunneling
        Protocol (L2TP).

   [13] IETF RFC 2784(2000), Generic Routing Encapsulation (GRE).

   [14] IETF RFC 2890(2000), Key and Sequence Number Extensions to GRE.

Bocci, et al.            Expires November 2002               [Page 20]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


   [15] IETF draft-ietf-mpls-diff-ext-09.txt (April 2001, Work in
        Progress), MPLS Support of Differentiated Services.

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


15. Acknowledgements

The authors like to acknowledge the contribution to this work by: TBD

16. Author's Addresses

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

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

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

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
















Bocci, et al.            Expires November 2002               [Page 21]


Internet Draft  draft-bocci-pwe3-app-frame-over-psn-00.txt    May 2002


Full Copyright Statement

   "Copyright (C) The Internet Society (May 2002). All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet  organizations,  except  as  needed  for  the  purpose  of
   developing Internet standards in which case the procedures for
   copyrights  defined  in  the  Internet  Standards  process  must  be
   followed, or as required to translate it into





































Bocci, et al.            Expires November 2002               [Page 22]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/