Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN
RFC 4385

Network Working Group                                          S. Bryant
Request for Comments: 4385                                    G. Swallow
Category: Standards Track                                     L. Martini
                                                           Cisco Systems
                                                            D. McPherson
                                                          Arbor Networks
                                                           February 2006

               Pseudowire Emulation Edge-to-Edge (PWE3)
                 Control Word for Use over an MPLS PSN

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes the preferred design of a Pseudowire
   Emulation Edge-to-Edge (PWE3) Control Word to be used over an MPLS
   packet switched network, and the Pseudowire 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.

1.  Introduction

   The standard MPLS encapsulations have no explicit protocol
   identifier.  In order for a pseudowire (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 Operations and Management (OAM) [VCCV] mechanism from
   correctly monitoring the PW.

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   This document specifies how the PW control word 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 Pseudowire Associated Channel Header.

1.1.  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].

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] [RFC2460] 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), which 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, this indicates that the packet starts with a PWMCW.  If
   the first nibble of a packet 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 that prevents packet
   misordering.  A suitable mechanism is the PWMCW described in Section
   3 for data, and the PWACH described in Section 5 for channel-
   associated traffic.

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

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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):

          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.

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   Length (bits 10 to 15):

          When the PSN path between the PEs includes an Ethernet
          segment, 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 the non-
   normative Appendix.

   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
   packet sequencing and packet sequencing is enabled for the PW, then
   the following procedures MUST be used:

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

     o Subsequent packets MUST increment the sequence number by one for
       each packet.

     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 packet sequencing is disabled, then the sequence
   number field in the control word MUST be set to zero for all packets
   transmitted on the PW.

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4.2.  Processing the Sequence Number

   If a router PE2 supports receive sequence number processing, and
   packet 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 packet is received on that PW, the sequence number SHOULD be
   processed as follows:

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

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

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

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

     o Otherwise the packet is out of order.

   If the packet is found to be in order, it MAY be delivered
   immediately.

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

   Packets that are received out of order MAY either be dropped or
   reordered.  The choice between dropping or reordering an out-of-
   sequence packet 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.

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5.  PW Associated Channel

   For some PW features, an associated channel is required.  An
   associated channel is a channel that is multiplexed in the PW with
   user traffic, and thus follows the same path through the PSN as user
   traffic.  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.

6.  IANA Considerations

   IANA has 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.  The
   value 0x57 indicates that the Associated Channel carries an IPv6
   packet.

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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.

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9.  Normative References

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

10.  Informative References

   [BCP]      Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", Work in
              Progress, September 2005.

   [FRAG]     Malis, A. and M. Townsley, "PWE3 Fragmentation and
              Reassembly", Work in Progress, November 2005.

   [IANA]     Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", Work in Progress, November 2005.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

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

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [VCCV]     Nadeau, T. and R. Aggarwal, "Pseudowire Virtual Circuit
              Connectivity Verification (VCCV)", Work in Progress,
              August 2005.

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Appendix.  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     /* packet sequence number
   unsigned16 EXPECTED = 1 /* expected sequence number
                           /* initialized to one
   boolean sequencingDisabled
   boolean dropOutOfOrder  /* policy on in-window out of sequence
                           /* packets

   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;

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

       if (sequencingDisabled) then begin
           /* A packet 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
           processPacket();
           updateExpected();
           return;
       end;

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       /* 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
           /* packet is in the window, but there are late/missing
           /* packets
           if (dropOutOfOrder) then begin
               /* policy is to receive immediately, dropping
               /* out of sequence packets
               processPacket();
               updateExpected();
               return;
           end else begin
               /* policy is to wait for late packets
               processMissingPackets();
               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
           /* packet is in the window, but there are late/missing
           /* packets

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

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

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Authors' Addresses

   Stewart Bryant
   Cisco Systems,
   250, Longwater,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom.

   EMail: stbryant@cisco.com

   George Swallow
   Cisco Systems, Inc.
   1414 Massachusetts Ave
   Boxborough, MA 01719

   EMail:  swallow@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

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