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Versions: 00 01 02 03 04 05 06 RFC 4385

Network Working Group                                         S. Bryant
Internet Draft                                               G. Swallow
Expiration Date: August 2005                              Cisco Systems
                                                           D. McPherson
                                                         Arbor Networks

                                                          February 2005


                PWE3 Control Word for use over an MPLS PSN

                         draft-ietf-pwe3-cw-02.txt




Status of this Memo

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Abstract

   This document describes the preferred designs of the PWE3 Control
   Word, and the PW Associated Channel Header. The design of these
   fields is chosen so that an MPLS LSR performing MPLS payload
   inspection will not confuse a PWE3 payload with an IP payload.

Conventions used in this document






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   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 the 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 PWE3 packets, the packets in the PW may not all follow
   the same path though 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 distinguishes a PW payload from
   an IP payload carried over an MPLS PSN.

2.   PWE3 Packet Identification

   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 SHOULD 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.

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

   If the first nibble of a PWE3 packet carried over an MPLS PSN has a
   value of 0, it starts with a PW-CW. If the first nibble of a packet
   carried over an MPLS PSN has a value of 1, it starts with a PW-ACH.
   The use of any other first nibble value for a PWE3 packet carried
   over an MPLS PSN is deprecated.

   A PW carried over an MPLS PSN that uses the contents of the MPLS
   payload to select the ECMP path SHOULD employ the PW Control Word
   described in Section 3 for data, and the PW Associated Channel
   Header described in Section 4 for channel associated traffic. These
   fields MUST immediately follow the bottom of the MPLS label stack.








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3.   Generic PW Control Word

   The PW MPLS Control Word 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: PW Control Word


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

   When a PW-CW is used, it SHOULD have the following preferred form:


    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: PWE3 Preferred Control Word

   The meaning of the fields of the PWE3 Preferred Control Word (Figure
   2) are as follows:

   Flags (bits 4 to 7):

          These bits are available for per-payload signalling.  Their
          definition is encapsulation specific.

   FRG (bits 8 and 9):

          These bits are used when fragmenting a PW payload. Their use
          is described in [FRAG] which is currently 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):

          If the PW payload size could be less than 64 bytes, and is
          either variable, or unknown to the CE-bound PE, the length
          field is used to indicate the size of a PW payload that might





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          have been padded to the minimum Ethernet MAC frame size
          during its transit across the PSN. If the MPLS payload
          (defined as the PW-CW + the PW payload + any additional PW
          headers) is less than 63 bytes, the length MUST be set to the
          length of the MPLS payload.

          Otherwise the length MUST be set to 0.

   Sequence number (Bit 16 to 31):

          If the sequence number is not used, it is set to zero by the
          sender and ignored by the receiver.  Otherwise it specifies
          the sequence number of a packet.  A circular list of sequence
          numbers is used.  A sequence number takes a value from 1 to
          65535 (2**16-1). The sequence number window size for packet
          acceptance is dependent on the parameters of the PSN, and
          SHOULD be configurable. The mechanism used by the
          decapsulating PE to (re)acquire the correct sequence number
          is implementation dependent.

4.   PW Associated Channel

   For some features of PWs, such as OAM, 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 "PWE
   channel type" as used in subsection 5.1.2 of [RFC3985]

   When MPLS is used as the PSN, the PW Assocated Channel 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| FmtID |   Reserved    |         Channel Type          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 3: PW Associated Channel Header

   The meaning of the fields in the PW Associated Channel Header
   (Figure 3) are as follows:

   FmtID:

          Format ID for the remaining 3 octets of the header. A FmtID
          of 0 indicates that the 3 octets are as depicted above.

   Reserved:

          Must be sent as 0, and ignored on receive.






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   Channel Type:

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

   Bits 0..3 MUST be 0x01, and hence differ from the first four bits of
   an IP packet [BCP]. This provides the necessary MPLS payload
   discrimination.

   Note that L2TPv3 has its own mechanisms for providing this
   associated channel.

5.   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].

6.   Security Considerations

   An application using PW Associated Channel to provide an OAM [VCCV]
   or other message channel MUST be aware that this 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 using this as a mechanism to disrupt the
   operation of the PW or the PE, and to stop this channel being used
   as a conduit to deliver packets elsewhere.

   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.



7.   Acknowledgements

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

8.   Intellectual Property Statement


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   rights might or might not be available; nor does it represent that






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   it has made any independent effort to identify any such rights.
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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
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   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



10.    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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11.    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-00.txt>, September 2004,
              Work in Progress.

   [FRAG]    Malis, A., Townsley, M., "PWE3 Fragmentation and
              Reassembly", Internet Draft, <draft-ietf-pwe3-
              fragmentation-08.txt>, February 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-
              07.txt>, October 2004, 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. , To be published.
   [VCCV]    Nadeau, T., Aggarwal, T., " Pseudo Wire (PW) Virtual
              Circuit Connection Verification (VCCV)", Internet
              Draft, <draft-ietf-pwe3-vccv-04.txt>, Feb. 2005,
              Work in Progress.






12.    Authors' Addresses


   Stewart Bryant
   Cisco Systems,
   250, Longwater,






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   Green Park,
   Reading, RG2 6GB,
   United Kingdom.             Email: stbryant@cisco.com

   Danny McPherson
   Arbor Networks              Email: danny@arbor.net

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













































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