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Versions: 00 01 02 03 04 05 06 RFC 4385

INTERNET DRAFT   PWE3 Control Word for use over an MPLS PSN   Oct 2005



Network Working Group                                         S. Bryant
Internet Draft                                               G. Swallow
Expiration Date: April 2006                                  L. Martini
                                                          Cisco Systems
                                                           D. McPherson
                                                         Arbor Networks

                                                           October 2005

                PWE3 Control Word for use over an MPLS PSN

                         draft-ietf-pwe3-cw-06.txt




Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   This document describes the preferred design of a PWE3 Control Word
   to be use over an MPLS packet switched network, and the Pseudo Wire
   Associated Channel Header. The design of these fields is chosen so
   that an MPLS Label Switching Router performing MPLS payload
   inspection will not confuse a PWE3 payload with an IP payload.







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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 [RFC2119].

1.   Introduction

   The standard MPLS encapsulations have no explicit protocol
   identifier. In order for a pseudo wire (PW) [RFC3985] to operate
   correctly over an MPLS packet switched network (PSN) that performs
   MPLS payload inspection, a PW packet must not appear to a label
   switching router (LSR) as if it were an IP packet [BCP]. An example
   of an LSR that performs MPLS payload inspection is one that is
   performing equal-cost multiple-path load-balancing (ECMP) [RFC2992].
   If ECMP were performed on PW packets, the packets in the PW may not
   all follow the same path through the PSN. This may result in
   misordered packet delivery to the egress PE. The inability to ensure
   that all packets belonging to a PW follow the same path may also
   prevent the PW OAM [VCCV] mechanism from correctly monitoring the
   PW.

   This draft specifies how a PW header is used to distinguish a PW
   payload from an IP payload carried over an MPLS PSN. It then
   describes the preferred design of a PW Control Word to be use over
   an MPLS PSN, and the Pseudo Wire Associated Channel Header.

2.   Avoiding ECMP

   A PW that is carried over an MPLS PSN that uses the contents of the
   MPLS payload to select the ECMP path may be subjected to packet
   misordering [BCP]. In cases where the application using the PW is
   sensitive to packet misordering, or where packet misordering will
   disrupt the operation of the PW, it is necessary to prevent the PW
   being subjected to ECMP.

   All IP packets [RFC791][RFC1883] start with a version number that is
   checked by LSRs performing MPLS payload inspection. To prevent the
   incorrect processing of packets carried within a PW, PW packets
   carried over an MPLS PSN MUST NOT start with the value 4 (IPv4) or
   the value 6 (IPv6) in the first nibble [BCP], as those are assumed
   to carry normal IP payloads.

   This document defines a PW header and two general formats of that
   header. These two formats are the PW MPLS Control Word (PWMCW) which
   is used for data passing across the PW, and a PW Associated Channel
   Header (PWACH) that can be used for functions such as OAM.

   If the first nibble of a PW packet carried over an MPLS PSN has a
   value of 0, it starts with a PWMCW. If the first nibble of a packet






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   carried over an MPLS PSN has a value of 1, it starts with a PWACH.
   The use of any other first nibble value for a PW packet carried over
   an MPLS PSN is deprecated.

   If a PW is sensitive to packet misordering and is being carried over
   an MPLS PSN that uses the contents of the MPLS payload to select the
   ECMP path, it MUST employ a mechanism which prevents packet
   misordering. A suitable mechanism is the PWMCW described in Section
   3 for data, and the PWACH described in Section 4 for channel
   associated traffic.

   The PWMCW or the PWACH MUST immediately follow the bottom of the
   MPLS label stack.

3.   Generic PW MPLS Control Word

   The Generic PW MPLS Control Word (PWMCW) is shown in Figure 1.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0|          Specified by PW Encapsulation                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 1: Generic PW MPLS Control Word


   The PW set-up protocol or configuration mechanism determines whether
   a PW uses a PWMCW. Bits 0..3 differ from the first four bits of an
   IP packet [BCP] and hence provide the necessary MPLS payload
   discrimination.

   When a PWMCW is used, it MUST adhere to the Generic format
   illustrated in Figure 1 above. To provide consistency between the
   designs of different types of PW, it SHOULD also use the following
   preferred format:


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0| Flags |FRG|  Length   | Sequence Number               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 2: Preferred PW MPLS Control Word

   The meaning of the fields of the Preferred PW MPLS Control Word
   (Figure 2) is as follows:

   Flags (bits 4 to 7):





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          These bits MAY be used by for per-payload signaling. Their
          semantics MUST be defined in the PW specification.

   FRG (bits 8 and 9):

          These bits are used when fragmenting a PW payload. Their use
          is described in [FRAG] which is currently a work in progress.
          When the PW is of a type that will never need payload
          fragmentation, these bits may be used as general purpose
          flags.

   Length (bits 10 to 15):

          When the PSN path between the PEs includes an Ethernet, the
          PW packet arriving at the CE-bound PE from the PSN may
          include padding appended by the Ethernet Data Link Layer. The
          CE-bound PE uses the length field to determine the size of
          the padding added by the PSN, and hence extract the PW
          payload from the PW packet.

          If the MPLS payload is less than 64 bytes, the length field
          MUST be set to the length of the PW payload plus the length
          of the PWMCW. Otherwise it MUST be set to zero.

   Sequence number (Bit 16 to 31):

          The sequence number implements the sequencing function
          [RFC3985]. The use of this field is described in Section 4.

4.   Sequencing

   The sequence number mechanism is PW specific. The PW encapsulation
   specification MAY define a sequence number mechanism to be used, or
   it may indicate that the mechanism described here is to be used. A
   pseudo-code description of this mechanism is given in non-normative
   Appendix 1.

   The sequence number mechanism described here uses a circular
   unsigned 16 bit number space that excludes the value zero.

4.1    Setting the Sequence Number

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

     o The initial frame transmitted on the PW MUST be sent with
       sequence number one.








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     o Subsequent frames MUST increment the sequence number by one for
       each frame.

     o The sequence number that follows 65535 (maximum unsigned 16 bit
       number) is one.

   If the transmitting router PE1 does not support sequence number
   processing, or frame sequencing is disabled, then the sequence
   number field in the control word MUST be set to zero for all frames
   transmitted on the PW.

4.2    Processing the sequence number

   If a router PE2 supports receive sequence number processing, and
   frame sequencing is enabled for this PW, then the following
   procedure is used:

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

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

     o If the sequence number on the frame is zero, the sequence
       integrity of the packets cannot be determined. In this case, the
       received frame is considered to be in order.

     o Otherwise if the frame sequence number equals the expected
       sequence number, the frame is in order.

     o Otherwise if the frame sequence number is greater than the
       expected sequence number, and the frame sequence number minus
       the expected sequence number is less than 32768, the frame is
       within the allowed receive sequence number window. The
       implementation MAY treat the packet as is in order.

     o Otherwise if the frame sequence number is less than the expected
       sequence number and the expected sequence number minus the frame
       sequence number is greater than or equal to 32768, the frame is
       within the allowed receive sequence number window. The
       implementation MAY treat the packet as is in order.

     o Otherwise the frame is out of order.

   If the frame is in order, it can be delivered immediately.

   If the frame sequence number was not zero, then the expected
   sequence number is set to the frame sequence number plus one. The
   expected sequence number that follows 65535 (maximum unsigned 16 bit
   number) is one.






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   Frames which are received out of order MAY either be dropped or
   reordered. The choice between dropping or re-ordering an out of
   sequence frame is at the discretion of the receiver.

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

5.   PW Associated Channel

   For some PW features, an associated channel is required. An
   associated channel is a channel that is multiplexed over the PW so
   that it follows exactly the same path through the PSN as the PW.
   Note that the use of the term "channel" is not a "PW channel type"
   as used in subsection 5.1.2 of [RFC3985]

   When MPLS is used as the PSN, the PW Associated Channel (PWAC) is
   identified by the following header:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 1|Version|   Reserved    |         Channel Type          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 3: PW Associated Channel Header

   The meanings of the fields in the PW Associated Channel Header
   (PWACH) (Figure 3) are:

   Version:

          This is the version number of the PWACH. This specification
          defines version 0.

   Reserved:

          MUST be sent as 0, and ignored on reception.

   Channel Type:

          The PW Associated Channel Type is defined in the IANA PW
          Associated Channel Type registry [IANA].

   Bits 0..3 MUST be 0001. This allows the packet to be distinguished
   from an IP packet [BCP] and from a PW data packet.










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6.   IANA considerations

   IANA needs to set up a registry of "Pseudowire Associated Channel
   Types". These are 16-bit values. Registry entries are assigned by
   using the "IETF Consensus" policy defined in [RFC2434]. The value
   0X21 indicates that the Associated Channel carries an IPv4 packet.

7.   Security Considerations

   An application using a PW Associated Channel must be aware that the
   channel can potentially be misused. Any application using the
   Associated Channel MUST therefore fully consider the resultant
   security issues, and provide mechanisms to prevent an attacker from
   using this as a mechanism to disrupt the operation of the PW or the
   PE, and to stop this channel from being used as a conduit to deliver
   packets elsewhere. The selection of a suitable security mechanism
   for an application using a PW Associated Channel is outside the
   scope of this document.

   If a PW has been configured to operate without a CW, the PW
   Associated Channel Type mechanism described in the document MUST NOT
   be used. This is to prevent user payloads being fabricated in such a
   way that they mimic the PW Associated Channel Header, and thereby
   provide a method of attacking the application that is using the
   Associated Channel.

8.   Acknowledgements

   The authors wish to thank David Allan, Thomas Nadeau, Yaakov Stein,
   and Mark Townsley for their input to this work.

9.   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.


10.     Full copyright statement

   Copyright (C) The Internet Society (2005). 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.

11.    Normative References

   Internet-drafts are works in progress available from
   http://www.ietf.org/internet-drafts/

   [RFC791]   RFC-791: DARPA Internet Program, Protocol
               Specification, ISI, September 1981.

   [RFC1883]  RFC-1883: Internet Protocol, Version 6 (IPv6), S.
               Deering, et al, December 1995

























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12.    Informative References


   Internet-drafts are works in progress available from
   <http://www.ietf.org/internet-drafts/>

   [BCP]     Swallow, G. et al, "Avoiding Equal Cost Multipath
              Treatment in MPLS Networks", Internet Draft
              <draft-ietf-mpls-ecmp-bcp-01.txt>, July 2005, Work
              in Progress.

   [FRAG]    Malis, A., Townsley, M., "PWE3 Fragmentation and
              Reassembly", Internet Draft, <draft-ietf-pwe3-
              fragmentation-09.txt>, September 2005, Work in
              Progress.

   [IANA]    Martini, L., Townsley M., "IANA Allocations for
              pseudo Wire Edge to Edge Emulation (PWE3) ",
              Internet Draft, <draft-ietf-pwe3-iana-allocation-
              12.txt>, September 2005, Work in Progress.

   [RFC2434] RFC-2434: Guidelines for Writing an IANA
              Considerations Section in RFCs, Narten, T.,
              Alvestrand, H., October 1998

   [RFC2992] RFC-2992:  Analysis of an Equal-Cost Multi-Path
              Algorithm, C. Hopps, November 2000

   [RFC3985] RFC-3985: PWE3 Architecture, Bryant, S. ed., Pate,
              P. ed., March 2005





















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13.    Authors' Addresses


   Stewart Bryant
   Cisco Systems,
   250, Longwater,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom.             Email: stbryant@cisco.com

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

   Danny McPherson
   Arbor Networks, Inc.        Email: danny@arbor.net

   George Swallow
   Cisco Systems, Inc.
   1414 Massachusetts Ave
   Boxborough, MA 01719        Email:  swallow@cisco.com

14.    Appendix 1 Sequence Number Processing

   This appendix is non-normative.

   This appendix provides a pseudo-code description of the sequence
   number processing mechanism described in Section 4.2.

   unsigned16 RECEIVED     /* frame sequence number
   unsigned16 EXPECTED = 1 /* expected sequence number
                           /* initialized to one
   boolean sequencingDisabled
   boolean dropOutOfOrder  /* policy on in-window out of sequence
                           /* frames

   updateExpected()
   begin
       EXPECTED := RECEIVED + 1;
       /* Because EXPECTED is an unsigned16 it will wrap
       /* from 65535 to 0
       /* zero is skipped
       if (EXPECTED = 0)
           EXPECTED := 1;
       return;
   end;








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   On receipt of a PW packet from PSN:
   begin
       if (RECEIVED = 0) then begin
           processFrame();
           return;
       end;

       if (sequencingDisabled) then begin
           /* A frame was received with non-zero sequence number, but
           /* sequencing is disabled
           indicateReceiveFault();
           disablePW();
           return;
       end;

       /* The received sequence is the expected sequence number
       if ((RECEIVED = EXPECTED) then begin
           /* packet is in order
           processFrame();
           updateExpected();
           return;
       end;

       /* Test for received sequence number is greater than
       /* the expected sequence number and is within the
       /* allowed receive sequence number window
       if ((RECEIVED > EXPECTED) and
           ((RECEIVED - EXPECTED) < 32768) then begin
           /* frame is in the window, but there are late/missing
           /* frames
           if (dropOutOfOrder) then begin
               /* policy is to receive immediately, dropping
               /* out of sequence frames
               processFrame();
               updateExpected();
               return;
           end else begin
               /* policy is to wait for late packets
               processMissingFrames();
               return;
           end;
       end;

       /* Test for the received sequence is less than the
       /* expected sequence number and is within the allowed
       /* receive sequence number window
       if ((RECEIVED < EXPECTED) and
           ((EXPECTED - RECEIVED) >= 32768) then begin
           /* frame is in the window, but there are late/missing
           /* frames






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           if (dropOutOfOrder) then begin
               /* policy is to receive immediately, dropping
               /* out of sequence frames
               processFrame();
               updateExpected();
               return;
           end else begin
               /* policy is to wait for late packets
               processMissingFrames();
               return;
           end;
       end;

       /* Received packet was outside the allowed receive
       /* sequence number window
       processOutOfWindow();
   end;







































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