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

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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.

Bryant, et al.              Standards Track                     [Page 1]
RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006

   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.

Bryant, et al.              Standards Track                     [Page 2]
RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006