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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 4618

Network Working Group                                       Luca Martini
Internet Draft                                             Eric C. Rosen
Expiration Date: 0 2004                              Cisco Systems, Inc.

Giles Heron                                                   Toby Smith
Andrew G. Malis                                    Laurel Networks, Inc.

Yeongil Seo
KT Technology Lab

                                                                  0 2004

Encapsulation Methods for Transport of PPP/HDLC Over IP and MPLS Networks


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

   The list of Internet-Draft Shadow Directories can be accessed at


   A Pseudowire (PW) can be used to carry PPP, or HDLC Protocol Data
   Units over an IP or MPLS network without terminating the PPP/HDLC
   protocol. This enables service providers to offer "emulated" HDLC, or
   PPP link services over existing IP or MPLS networks. This document
   specifies the encapsulation of PPP/HDLC PDUs within a pseudowire.

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Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Intellectual Property Statement  ........................   2
    3      Introduction  ...........................................   3
    4      General encapsulation method  ...........................   5
    4.1    The Control Word  .......................................   5
    4.1.1  Setting the sequence number  ............................   6
    4.1.2  Processing the sequence number  .........................   6
    4.2    MTU Requirements  .......................................   7
    5      Protocol-Specific Details  ..............................   8
    5.1    HDLC  ...................................................   8
    5.2    PPP  ....................................................   8
    6      Using an MPLS Label as the Demultiplexer Field  .........   9
    6.1    MPLS Shim EXP Bit Values  ...............................   9
    6.2    MPLS Shim S Bit Value  ..................................   9
    7      Security Considerations  ................................   9
    8      Full Copyright Statement  ...............................   9
    9      References  .............................................  10
   10      Author Information  .....................................  11

1. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119

2. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of

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   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

3. Introduction

   A PPP/HDLC Pseudowire (PW) allows PPP/HDLC Protocol Data Units (PDUs)
   to be carried over an IP network or an MPLS network. In addressing
   the issues associated with carrying a PPP/HDLC PDU over a PSN, this
   document assumes that a Pseudowire (PW) has been set up by some means
   outside the scope of this document. This may be via manual
   configuration, or a signaling protocol such as that defined in [1] or
   [7]. As described in [8], this PW may be tunneled through an MPLS,
   IPv4 or IPv6 PSN.

   The following figure describe the reference models which are derived
   from [8] to support the HDLC/PPP PW emulated services.

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         |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudo Wire ------>|          |
         |          |                            |          |
         |          |    |<-- PSN Tunnel -->|    |          |
         |          V    V                  V    V          |
         V   AC     +----+                  +----+    AC    V
   +-----+    |     | PE1|==================| PE2|     |    +-----+
   |     |----------|............PW1.............|----------|     |
   | CE1 |    |     |    |                  |    |     |    | CE2 |
   |     |----------|............PW2.............|----------|     |
   +-----+  ^ |     |    |==================|    |     | ^  +-----+
         ^  |       +----+                  +----+     | |  ^
         |  |   Provider Edge 1         Provider Edge 2  |  |
         |  |                                            |  |
   Customer |                                            | Customer
   Edge 1   |                                            | Edge 2
            |                                            |
            |                                            |
      native HDLC/PPP service                       native HDLC/PPP service

      Figure 1: PWE3 HDLC/PPP Interface Reference Configuration

   This document specifies the emulated PW encapsulation for PPP, and
   HDLC.  Although different layer 2 protocols require different
   information to be carried in this encapsulation, an attempt has been
   made to make the encapsulation as common as possible for all layer 2
   protocols. Other layer 2 protocols are described in separate
   documents. [4] [5] [6]

   This document also specifies the way in which the demultiplexer field
   is added to the emulated PW encapsulation when an MPLS label is used
   as the demultiplexer field. QoS related issues are not discussed in
   this document.  For the purpose of this document PE1 will be defined
   as the ingress router, and PE2 as the egress router. A layer 2 PDU
   will be received at PE1, encapsulated at PE1, transported,
   decapsulated at PE2, and transmitted out of PE2.

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4. General encapsulation method

4.1. The Control Word

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

        -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.

   When carrying HDLC/PPP over an IP or MPLS backbone sequentiality may
   need to be preserved.  The OPTIONAL control word defined here
   addresses this requirement.  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].  The control word 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|0 0 0 0|B|E|   Length  |     Sequence Number           |

                   Figure 2: MPLS PWE3 Control Word

   In the above diagram the first 4 bits are the PID as defined in [8]

   B and E are fragmentation bits and their functionality is specified
   in [9].

   The next 4 bits provide space for carrying protocol specific flags.
   These are not used for HDLC/PPP and they They MUST be set to 0 when
   transmitting, and MUST be ignored upon receipt.

   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

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   service provider's egress router, it may be desirable to remove the
   padding before forwarding the packet.

   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 an unsequenced packet.

4.1.1. Setting the sequence number

   For a given PW, and a pair of routers PE1 and PE2, if PE1 supports
   frame sequencing then the following procedures should be used:

     - the initial frame transmitted on the PW MUST use sequence number
     - subsequent frames MUST increment the sequence number by one for
       each frame
     - 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.

4.1.2. Processing the sequence number

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

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

   When a frame is received on that PW, the sequence number should be
   processed as follows:

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

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

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

     - otherwise the frame is out of order.

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

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

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

   A simple extension of the above processing algorithm can be used to
   detect lost packets.

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

4.2. MTU Requirements

   The network MUST be configured with an MTU that is sufficient to
   transport the largest encapsulation frames. If MPLS is used as the
   tunneling protocol, for example, this is likely to be 12 or more
   bytes greater than the largest frame size. Other tunneling protocols
   may have longer headers and require larger MTUs. The methodology
   described in [9] MAY be used to fragment encapsulated frames that
   exeeed the PSN MTU. However if [9] is not used then if the ingress
   router determines that an encapsulated layer 2 PDU exceeds the MTU of
   the PSN tunnel through which it must be sent, the PDU MUST be

   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.

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5. Protocol-Specific Details

5.1. HDLC

   HDLC mode provides port to port transport of HDLC encapsulated
   traffic. The HDLC PDU is transported in its entirety, including the
   HDLC address, control and protocol fields, but excluding HDLC flags
   and the FCS. Bit/Byte stuffing is undone. 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 HDLC mode is suitable for port to port transport of Frame Relay
   UNI or NNI traffic. It must be noted, however, that this mode is
   transparent to the FECN, BECN and DE bits. Since all packets are
   passed in a largely transparent manner over the HDLC PW, any protocol
   which has HDLC-like framing may utilize the HDLC PW mode, including
   PPP, Frame-Relay, X.25, etc.  Exceptions include cases where direct
   access to the HDLC interface is required, or modes which operate on
   the flags, FCS, or bit/byte unstuffing that is performed before
   sending the HDLC PDU over the PW. An example of this is PPP ACCM

5.2. PPP

   PPP mode provides point to point transport of PPP encapsulated
   traffic, as specified in [3]. The PPP PDU is transported in its
   entirety, including the protocol field (whether compressed using PFC
   or not), but excluding any media-specific framing information, such
   as HDLC address and control fields or FCS.  Since media-specific
   framing is not carried the following options will not operate
   correctly if the PPP peers attempt to negotiate them:

     - Frame Check Sequence (FCS) Alternatives
     - Address-and-Control-Field-Compression (ACFC)
     - Asynchronous-Control-Character-Map (ACCM)

   Note also that PW LSP Interface MTU negotiation as specified in [1]
   is not affected by PPP MRU advertisement. Thus if a PPP peer sends a
   PDU with a length in excess of that negotiated for the PW tunnel that
   PDU will be discarded by the ingress router.

   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.

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6. Using an MPLS Label as the Demultiplexer Field

   To use an MPLS label as the demultiplexer field, a 32-bit label stack
   entry [2] is simply prepended to the emulated PW encapsulation, and
   hence will appear as the bottom label of an MPLS label stack. This
   label may be called the "PW label". The particular emulated pseudo-
   wire identified by a particular label value must be agreed by the
   ingress and egress LSRs, either by signaling (e.g, via the methods of
   [1]) or by configuration. Other fields of the label stack entry are
   set as follows.

6.1. MPLS Shim EXP Bit Values

   If it is desired to carry Quality of Service information, the Quality
   of Service information SHOULD be represented in the EXP field of the
   PW label.  If more than one MPLS label is imposed by the ingress LSR,
   the EXP field of any labels higher in the stack SHOULD also carry the
   same value.

6.2. 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 PW label is at the bottom of the stack.

7. Security Considerations

   The ethernet pseudowire type is subject to all of the general
   security considerations discussed in [8][1]. This document specifies
   only encapsulations, and not the protocols that may be 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.

8. Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

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9. References

   [1] "Transport of Layer 2 Frames Over MPLS",
        Martini, L., et al., draft-ietf-pwe3-control-protocol-06.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] "The Point-to-Point Protocol (PPP)", RFC 1661.

   [4] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP
   and MPLS Networks",
       draft-ietf-pwe3-atm-encap-02.txt ( work in progress )

   [5] "Encapsulation Methods for Transport of Ethernet Frames Over
        Networks", draft-ietf-pwe3-ethernet-encap-06.txt. ( work in
   progress )

   [6] "Frame Relay over Pseudo-Wires", draft-ietf-pwe3-frame-relay-
   01.txt. (
       work in progress )

   [7] "Layer Two Tunneling Protocol (Version 3) "L2TPv3"J. Lau, M.
       A. Valencia, G. Zorn, I. Goyret, G. Pall, A. Rubens, B. Palter, ,
   work in
       progress, draft-ietf-l2tpext-l2tp-base-12.txt, March  2004.

   [8] "PWE3 Architecture" Bryant, et al., draft-ietf-pwe3-arch-07.txt (
       in progress ), June 2003.

   [9] "PWE3 Fragmentation and Reassembly", A. Malis,W. M. Townsley,
       draft-ietf-pwe3-fragmentation-05.txt ( work in progress )
   February 2004

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10. Author Information

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112
   e-mail: lmartini@cisco.com

   Giles Heron
   Abbey Place
   24-28 Easton Street
   High Wycombe
   HP11 1NT
   e-mail: giles.heron@tellabs.com

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

   Toby Smith
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: tob@laurelnetworks.com

   Andrew G. Malis
   90 Rio Robles Dr.
   San Jose, CA 95134
   e-mail: Andy.Malis@tellabs.com

   Yeongil Seo
   463-1 KT Technology Lab
   Jeonmin-dong Yusung-gu
   Daegeon, Korea
   email: syi1@kt.co.kr

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