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

Internet Draft                                             Eric C. Rosen
Expiration Date: November 2005                       Cisco Systems, Inc.

Giles Heron
Andrew G. Malis
Tellabs




                                                                May 2005


   Encapsulation Methods for Transport of PPP/HDLC Over MPLS Networks


               draft-ietf-pwe3-hdlc-ppp-encap-mpls-05.txt

Status of this Memo

   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.


   Internet-Drafts are working documents of the Internet Engineering
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Abstract

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



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   encapsulation of PPP/HDLC PDUs within a pseudo wire.


Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   2
    3      General encapsulation method  ...........................   4
    3.1    The Control Word  .......................................   4
    3.1.1  Setting the sequence number  ............................   5
    3.1.2  Processing the sequence number  .........................   5
    3.2    MTU Requirements  .......................................   6
    4      Protocol-Specific Details  ..............................   6
    4.1    HDLC  ...................................................   6
    4.2    Frame Relay Port Mode  ..................................   7
    4.3    PPP  ....................................................   9
    5      Using an MPLS Label as the Demultiplexer Field  .........   9
    5.1    MPLS Shim EXP Bit Values  ...............................  10
    5.2    MPLS Shim S Bit Value  ..................................  10
    6      IANA Considerations  ....................................  10
    7      Security Considerations  ................................  10
    8      Intellectual Property Statement  ........................  10
    9      Full Copyright Statement  ...............................  11
   10      Normative References  ...................................  11
   11      Informative References  .................................  12
   12      Author Information  .....................................  12
   13      Contributing Author Information  ........................  13





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

   A PPP/HDLC Pseudowire (PW) allows PPP/HDLC Protocol Data Units (PDUs)
   to be carried over an MPLS network. In addressing the issues
   associated with carrying a PPP/HDLC PDU over an MPLS network, 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 using the signaling protocol such as that defined
   in [CONTROL].



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

         |<-------------- 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, however QoS related issues are not discussed in this document.
   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. [ATM]
   [ETHER] [FRAME] For the purpose of the discussion in 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 on the
   attachment circuit of PE2.













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3. General encapsulation method

3.1. The Control Word

   There are three requirements that may need to be satisfied when
   transporting layer 2 protocols over an 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 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 [CONTROL].  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|Res|   Length  |     Sequence Number           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: MPLS PWE3 Control Word

   In the above diagram the first 4 bits are set to 0 in indicate PW
   data [ARCH].

   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 not
   zero, is used to remove any padding that may have been added by the
   MPLS network. When the packet reaches the service provider's egress
   router any padding MUST be removed before forwarding the packet.



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   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.


3.1.1. 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
       1
     - 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.


3.1.2. 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.

     - 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.





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     - 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 PE router negotiated not to use receive sequence number
   processing, and it received a non zero sequence number, then it
   SHOULD send a PW status message indicating a receive fault, and
   disable the PW.


3.2. MTU Requirements

   The network MUST be configured with an MTU that is sufficient to
   transport the largest encapsulation frames. When MPLS is used as the
   tunneling protocol, for example, this is likely to be 12 or more
   bytes greater than the largest frame size. The methodology described
   in [FRAG] MAY be used to fragment encapsulated frames that exeeed the
   PSN MTU. However if [FRAG] 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 dropped.

   If a packet is received, on the attachment circuit, that exceeds the
   interface MTU paramater value [CONTROL] , it MUST be dropped.


4. Protocol-Specific Details

4.1. 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.



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   When the PE detect a status change in the attachment circuit status,
   such as an attachment circuit physical link failure, or the AC is
   administratively disabled, the PE MUST send the appropriate PW status
   notification message that corresponds to the HDLC AC status. In a
   similar manner, the local PW status MUST also be refleceted in a
   respective PW status notification message as described in [CONTROL].

   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
   negotiation.

   The PW of type 0x0006 "HDLC" will be used to transport HDLC frames
   while PW of type 0x0007 "PPP" will be used to transport PPP frames.
   The IANA allocation registry of "Pseudowire Type" is defined in the
   iana allocation document for PWs [IANA] along with initial allocated
   values.


4.2. Frame Relay Port Mode

   Figure 3 illustrates the concept of frame relay port mode or many-
   to-one mapping which is an OPTIONAL capability.

   Figure 3 (a) shows two frame relay devices physically connected with
   a frame relay UNI or NNI. Between their two ports P1 and P2, n frame
   relay VCs are configured.

   Figure 3 (b) shows the replacement of the physical frame relay
   interface with a pair of PEs and a PW between them. The interface
   between a FR device and a PE is either a FR UNI or NNI. The set of n
   FR VCs between the two FR ports P1 and P2 which are controlled by the
   same signaling channel using DLCI=0, are mapped into one PW. Hence
   with port mode we have many-to-one mapping between FR VCs and a PW.











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              +------+                          +-------+
              | FR   |                          |   FR  |
              |device|         FR UNI/NNI       | device|
              |    [P1]------------------------[P2]     |
              |      |      carrying n FR VCs   |       |
              +------+            |             +-------+
                                  |
                 [Pn]: A port     |
                                  | (a) FR interface between two
                                  |     FR devices
                                  |
                                  V
                    |<---------------------------->|
                    |                              |
                     +----+                  +----+
   +------+          |    |     One PW       |    |         +------+
   |      |          |    |==================|    |         |      |
   |  FR  |    FR    | PE1| carrying n FR VCs| PE2|    FR   |  FR  |
   |device|----------|    |                  |    |---------|device|
   | CE1  | UNI/NNI  |    |                  |    | UNI/NNI | CE2  |
   +------+          +----+                  +----+         +------+
          |                                                 |
          |<----------------------------------------------->|
                                  n FR VCs

              (b) Pseudo-wires replacing the FR interface

         Figure 3 - Concept of frame relay port-to-port mode

   FR VCs are not visible individually to a PE; there is no
   configuration of individual FR VC in a PE. A PE processes the set of
   FR VCs assigned to a port as an aggregate.

   FR port mode provides transport between two PEs of a complete FR
   frame using the same encapsulation as described above for HDLC mode.

   Althought frame relay port mode shares the same encapsulation as HDLC
   mode, a different PW type is allocated in [IANA]: 0x000F  Frame-Relay
   Port mode.

   All other aspects of this PW type are identical to the HDLC PW
   encapsulation described above.









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4.3. PPP

   PPP mode provides point to point transport of PPP encapsulated
   traffic, as specified in [PPP]. 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
   [CONTROL] 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.

   When the PE detect a status change in the attachment circuit status,
   such as an attachment circuit physical link failure, or the AC is
   administratively disabled, the PE MUST send the appropriate PW status
   notification message that corresponds to the HDLC AC status. It
   should be noted that PPP negotiation status is transparent to the PW,
   and MUST not be communicated to the remote MPLS PE. In a similar
   manner, the local PW status MUST also be reflected in a respective PW
   status notification message as described in [CONTROL].



5. Using an MPLS Label as the Demultiplexer Field

   To use an MPLS label as the demultiplexer field, a 32-bit label stack
   entry [MPLSENCAP] 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 PW identified by a particular label value must be agreed by
   the ingress and egress LSRs, either by signaling (e.g, via the
   methods of [CONTROL]) or by configuration. Other fields of the label
   stack entry are set as follows.







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5.1. 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 MUST also carry the
   same value.


5.2. 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.


6. IANA Considerations

   This document has no IANA Actions.


7. Security Considerations

   The ethernet pseudowire type is subject to all of the general
   security considerations discussed in [ARCH][CONTROL]. This document
   specifies only encapsulations, and not the protocols that may be used
   to carry the encapsulated packets across the MPLS network. Each such
   protocol may have its own set of security issues, but those issues
   are not affected by the encapsulations specified herein.


8. 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
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.




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   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-
   ipr@ietf.org.

   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.


9. 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
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


10. Normative References

   [CONTROL] "Pseudowire Setup and Maintenance using LDP",
        Martini, L., et al., draft-ietf-pwe3-control-protocol-16.txt,
        ( work in progress ), April 2005
   [MPLSENCAP] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D.
   Tappan, G. Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032

[IANA] "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)"
   Martini,Townsley, draft-ietf-pwe3-iana-allocation-09.txt (work in
   progress), April 2004













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11. Informative References

   [PPP] "The Point-to-Point Protocol (PPP)", RFC 1661.

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

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

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

   [ARCH] "PWE3 Architecture" Bryant, et al.,RFC3985

   [FRAG] "PWE3 Fragmentation and Reassembly", A. Malis,W. M. Townsley,
        draft-ietf-pwe3-fragmentation-08.txt ( work in progress )
        February 2005


12. Author Information


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


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


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



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



13. Contributing Author Information


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


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


























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