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Versions: (draft-bocci-mpls-tp-gach-gal) 00 01 02 03 04 05 06 RFC 5586

MPLS Working Group                                         M. Bocci, Ed.
Internet-Draft                                         M. Vigoureux, Ed.
Updates: 3032, 4385, 5085                                 Alcatel-Lucent
(if approved)                                                 G. Swallow
Intended status: Standards Track                                 D. Ward
Expires: August 27, 2009                                       S. Bryant
                                                                   Cisco
                                                             R. Aggarwal
                                                        Juniper Networks
                                                       February 23, 2009


                 MPLS Generic Associated Channel header
                     draft-ietf-mpls-tp-gach-gal-02

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on August 27, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect



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   to this document.

Abstract

   This document generalises the applicability of the pseudowire (PW)
   Associated Channel Header (ACH), enabling the realization of a
   control channel associated to MPLS Label Switched Paths (LSPs) and
   MPLS Sections in addition to MPLS pseudowires.  In order to identify
   the presence of this Associated Channel Header in the label stack,
   this document also assigns one of the reserved MPLS label values to
   the Generic Associated channel Label (GAL), to be used as a label
   based exception mechanism.

Requirements Language

   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 [1].

































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Contributing Authors . . . . . . . . . . . . . . . . . . .  5
     1.2.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Generic Associated Channel Header  . . . . . . . . . . . . . .  6
     2.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Allocation of Channel Types  . . . . . . . . . . . . . . .  7
   3.  ACH TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  ACH TLV Payload Structure  . . . . . . . . . . . . . . . .  8
     3.2.  ACH TLV Header . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  ACH TLV Object . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Generalised Exception Mechanism  . . . . . . . . . . . . . . .  9
     4.1.  Relationship with Existing MPLS OAM Alert Mechanisms . . . 10
     4.2.  GAL Applicability and Usage  . . . . . . . . . . . . . . . 10
       4.2.1.  GAL Processing . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Relationship wth RFC 3429  . . . . . . . . . . . . . . . . 13
   5.  Compatability  . . . . . . . . . . . . . . . . . . . . . . . . 13
   6.  Congestion Considerations  . . . . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17























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1.  Introduction

   There is a need for Operations, Administration and Maintenance (OAM)
   mechanisms that can be used for fault detection, diagnostics,
   maintenance and other functions on a PW and an LSP.  These functions
   can be used between any two Label Edge Routers (LERs) / Label
   Switching Router (LSRs) or Terminating Provider Edge routers (T-PEs)
   / Switching Provider Edge routers (S-PEs) along the path of an LSP or
   PW respectively [15].  Some of these functions can be supported using
   existing tools such as Virtual Circuit Connectivity Verification
   (VCCV) [2], Bidirectional Forwarding Detection for MPLS LSPs (BFD-
   MPLS)[3], LSP-Ping [4], or BFD-VCCV [5].  However, a requirement has
   been indicated to augment this set of maintenance functions, in
   particular when MPLS networks are used for packet transport services
   and transport network operations [16].  Examples of these functions
   include performance monitoring, automatic protection switching, and
   support for management and signaling communication channels.  These
   tools MUST be applicable to, and function in essentially the same
   manner (from an operational point of view) on MPLS PWs, MPLS LSPs and
   MPLS Sections.  They MUST also operate in-band on the PW or LSP such
   that they do not depend on Packet Switched Network (PSN) routing or
   on user data traffic, and MUST also not depend on dynamic control
   plane functions.

   VCCV can use an Associated Channel Header (ACH) to provide a PW-
   associated control channel between a PW's end points, over which OAM
   and other control messages can be exchanged.  This document
   generalises the use of the ACH to enable the same associated control
   channel mechanism to be used for Sections, LSPs and PWs.  The
   associated control channel thus generalized is known as the Generic
   Associated Channel (G-ACh).  The ACH, specified in RFC 4385 [6], may
   be used with additional code points to support additional MPLS
   maintenance functions on the G-ACh.

   Generalizing the associated control channel mechanism to LSPs and
   Sections also requires a method to identify that a packet contains an
   ACH followed by a non-service payload.  Therefore, this document also
   defines a label based exception mechanism that serves to inform an
   LSR (or LER) that a packet it receives on an LSP or Section belongs
   to an associated control channel for that LSP or Section.

   RFC 4379 [4] and BFD-MPLS [3] define alert mechanisms that enable an
   MPLS LSR to identify and process MPLS OAM packets when these are
   encapsulated in an IP header.  These alert mechanisms are based on
   MPLS or PW label Time to Live (TTL) expiration and/or on the use of
   an IP destination address in the range 127/8.  These mechanisms are
   the default mechanisms for identifying MPLS OAM packets when
   encapsulated in an IP header.  However it may not always be possible



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   to use these mechanisms in some MPLS applications, e.g.  MPLS
   Transport Profile (MPLS-TP) [15], particularly when IP based
   demultiplexing cannot be used.  This document defines a mechanism
   that is RECOMMENDED for identifying and encapsulating MPLS OAM and
   other maintenance messages when IP based mechanisms such as those in
   [4] and [3] are not available.  This mechanism MAY be used in
   addition to IP-based mechanisms.

   The GAL mechanism is defined to work together with the ACH for LSPs
   and MPLS Sections.

   Note that, in this document, maintenance functions and packets should
   be understood in the broad sense.  That is, a set of maintenance and
   management mechanisms that include OAM, Automatic Protection
   Switching (APS), Signalling Communication Channel (SCC) and
   Management Communication Channel (MCC) messages.

   Also note that the GAL and ACH are applicable to MPLS in general.
   Their applicability to specific applications of MPLS is outside the
   scope of this document.

1.1.  Contributing Authors

   The editors gratefully acknowledge the contributions of Sami Boutros,
   Italo Busi, Marc Lasserre, Lieven Levrau and Siva Sivabalan

1.2.  Objectives

   This document defines a mechanism that provides a solution to the
   extended maintenance needs of emerging applications for MPLS.  It
   creates a generic control channel mechanism that may be applied to
   MPLS LSPs and Sections, while maintaining compatibility with the PW
   associated channel.  It also normalises the use of the ACH for PWs in
   a transport context, and defines a label based exception mechanism to
   alert LERs/LSRs of the presence of an ACH after the bottom of the
   stack.

1.3.  Scope

   This document defines the encapsulation header for LSP, MPLS Section
   and PW associated channel messages.

   It does not define how associated control channel capabilities are
   signaled or negotiated between LERs/LSRs or PEs, or the operation of
   various OAM functions.

   This document does not deprecate existing MPLS and PW OAM mechanisms.




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1.4.  Terminology

   ACH: Associated Channel Header

   G-ACh: Generic Associated Channel

   GAL: G-ACh Label

   Maintenance packet: Any packet containing a message belonging to a
   maintenance protocol that is carried on a PW, LSP or MPLS Section
   associated control channel.  Examples of such maintenance protocols
   include OAM functions, signaling communications or management
   communications.

   The terms 'Section' and 'Concatenated Segment' are defined in [17].


2.  Generic Associated Channel Header

   VCCV [2] defines three MPLS Control Channel (CC) Types that may be
   used to exchange OAM messages through a PW: CC Type 1 uses an ACH and
   is referred to as "In-band VCCV"; CC Type 2 uses the MPLS Router
   Alert Label to indicate VCCV packets and is referred to as "Out of
   Band VCCV"; CC Type 3 uses the TTL to force the packet to be
   processed by the targeted router control plane and is referred to as
   "MPLS PW Label with TTL == 1".

2.1.  Definition

   The use of the CC Type 1, previously limited to PWs, is here extended
   to also apply to LSPs and to Sections.  Note that for PWs, the PWE3
   control word [6] MUST be present in the encapsulation of user packets
   when the ACH is used to realize the associated control channel.

   The CC Type 1 control channel header is depicted in figure below:

    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 1: Associated Channel Header

   In the above figure, the first nibble is set to 0001b to indicate a
   control channel associated with a PW, an LSP or a Section.  The
   Version field is set to 0, as specified in RFC 4385 [6].  Bits 8 to
   14 of the G-ACH are reserved and MUST be set to 0 and ignored on



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   reception.

   Note that VCCV also includes mechanisms for negotiating the Control
   Channel and Connectivity Verification (i.e.  OAM functions) Types
   between PEs.  It is anticipated that similar mechanisms will be
   applied to LSPs.  Such application will require further
   specification.  However, such specification is beyond the scope of
   this document.

2.2.  Allocation of Channel Types

   The Channel Type field indicates the type of message carried on the
   associated control channel e.g.  IPv4 or IPv6 if IP demultiplexing is
   used for messages sent on the associated control channel, or OAM or
   other maintenance function if IP demultiplexing is not used.  For
   associated control channel packets where IP is not used as the
   multiplexer, the Channel Type SHOULD indicate the specific
   maintenance protocol carried in the associated control channel.

   Values for the Channel Type field currently used for VCCV are
   specified elsewhere, e.g. in RFC 4446 [7]and RFC 4385[6] .
   Additional Channel Type values and the associated maintenance
   functionality will be defined in other documents.  Each document
   specifying a protocol solution relying on the ACH MUST also specify
   the applicable Channel Type field value.

   Note that these values are allocated from the PW Associated Channel
   Type registry, but this document modifies the existing policy to
   accommodate a level of experimentation.  See Section 8 for further
   details.


3.  ACH TLVs

   In some applications of the "In-band VCCV" associated control channel
   it is necessary to include one or more ACH TLVs to provide additional
   context information to the maintenance packet.  One use of these ACH
   TLVs might be to identify the source and/or intended destination of
   the associated control channel maintenance message.  However, the use
   of this construct is not limited to providing addressing information
   nor is the applicability restricted to transport network
   applications.

   If the maintenance message MAY be preceded by one or more ACH TLVs,
   then this MUST be explicitly specified in the definition of an ACH
   Channel Type.  If the ACH Channel Type definition does state that one
   or more ACH TLVs MAY precede the maintenance message, an ACH TLV
   Header MUST follow the ACH.  If no ACH TLVs are required in a



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   specific associated control channel packet, but the Channel Type
   nevertheless defines that ACH TLVs MAY be used, an ACH TLV Header
   MUST be present but with a length field set to zero to indicate that
   no ACH TLV follow this header.

   If a channel type specification does not explicitly specify that ACH
   TLVs MAY be used, then an ACH TLV Header MUST NOT be used.

3.1.  ACH TLV Payload Structure

   This section defines and describes the structure of an ACH payload
   when an ACH TLV Header is present.  The structure of ACH TLVs that
   MAY follow an ACH TLV Header is defined and described in the
   following sections.

   The following figure (Figure 2) shows the structure of a G-ACh packet
   payload.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ACH                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         ACH TLV Header                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               ~
   ~                     zero or more ACH TLVs                     ~
   ~                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               ~
   ~                      Maintenance Message                      ~
   ~                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: ACH TLV Payload Structure

3.2.  ACH TLV Header

   The ACH TLV Header defines the length of the set of ACH TLVs that
   follow.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Length                 |          Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: ACH TLV Header

   The length field specifies the length in octets of the complete set



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   of TLVs including TLVs that follow the ACH TLV header.  A length of
   zero indicates that no ACH TLV follow this header.

   The reserved field is for future use and MUST be set to zero on
   transmission and ignored on reception.

3.3.  ACH TLV Object

   An ACH TLV consists of a 16-bit Type field, followed by a 16-bit
   Length field which specifies the number of octets of the Value field
   which follows the Length field.  This 32-bit word is followed by zero
   or more octets of Value information.  The format and semantics of the
   value information are defined by the TLV Type as recorded in the TLV
   Type registry.  See Section 8 for further details.  Note that the
   Value field of ACH TLVs MAY contain sub-TLV objects.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           TLV Type            |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               ~
   ~                             Value                             ~
   ~                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 4: ACH TLV Format


4.  Generalised Exception Mechanism

   Generalizing the associated channel mechanism to LSPs and Sections
   also requires a method to identify that a packet contains an ACH
   followed by a non-service payload.  This document specifies that a
   label is used for that purpose and calls this special label the G-ACh
   Label (GAL).  One of the reserved label values defined in RFC 3032
   [8] is assigned for this purpose.  The value of the label is to be
   allocated by IANA; this document suggests the value 13.

   The GAL provides an alert based exception mechanism to:

   o  differentiate specific packets (e.g. maintenance messages) from
      others, such as normal user-plane ones,

   o  indicate that the ACH appears immediately after the bottom of the
      label stack.

   The GAL MUST only be used where both of these purposes apply.



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4.1.  Relationship with Existing MPLS OAM Alert Mechanisms

   RFC 4379 [4] and BFD-MPLS [3] have defined alert mechanisms that
   enable a MPLS LSR to identify and process MPLS OAM packets when the
   OAM packets are encapsulated in an IP header.  These alert mechanisms
   are based on TTL expiration and/or use an IP destination address in
   the range 127/8.

   These alert mechanisms SHOULD be used in non MPLS-TP environments,
   although the mechanism defined in this document MAY also be used.

4.2.  GAL Applicability and Usage

   The GAL MUST only be used with LSPs, concatenated segments of LSPs,
   and with Sections.

   In MPLS-TP, the GAL MUST always be at the bottom of the label stack
   (i.e.  S bit set to 1).  However, in other MPLS environments, this
   document places no restrictions on where the GAL may appear within
   the label stack.

   The GAL MUST NOT appear in the label stack when transporting normal
   user-plane packets.  Furthermore, when present, the GAL MUST only
   appear once in the label stack.

4.2.1.  GAL Processing

   The Traffic Class (TC) field (formerly known as the EXP field) of the
   label stack entry containing the GAL follows the definition and
   processing rules specified and referenced in [9].

   The Time-To-Live (TTL) field of the label stack entry that contains
   the GAL follows the definition and processing rules specified in
   [10].

4.2.1.1.  MPLS Label Switched Paths and Segments

   The following figure (Figure 5) depicts two LERs (A and D) and two
   LSRs (B and C) for a given LSP which is established from A to D and
   switched in B and C.

        +---+             +---+             +---+             +---+
        | A |-------------| B |-------------| C |-------------| D |
        +---+             +---+             +---+             +---+

                 Figure 5: MPLS-TP maintenance over a LSP

   In this example, a G-ACh exists on an LSP that extends between LERs A



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   and D, via LSRs B and C. Only these nodes may insert, extract or
   process packets on this G-ACh.

   The following figure (Figure 6) depicts the format of a MPLS-TP
   maintenance message when used for an LSP.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               LSP Label               |  TC |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  GAL                  |  TC |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ACH                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  ACH TLV Header (if present)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               ~
   ~                     Zero or more ACH TLVs                     ~
   ~                           (if present)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               ~
   ~                      Maintenance Message                      ~
   ~                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 6: MPLS-TP maintenance message format for a LSP

   Note that it is possible that the LSP may be tunnelled in another LSP
   (e.g. if a MPLS Tunnel exists between B and C), and as such other
   labels may be present in the label stack.

   To send a maintenance message on the LSP associated control channel,
   the LER (A) generates a maintenance message, to which it MAY
   prepended an ACH TLV header and appropriate ACH TLVs, and with a ACH
   to which it pushes a GAL and finally the LSP label.

   o  The TTL field of the GAL MUST be set to at least 1.  The exact
      value of the TTL is application specific.

   o  The S bit of the GAL MUST be set according to its position in the
      label stack.

   o  The setting of the TC field is application specific.

   The maintenance message, the ACH or the GAL SHOULD NOT be modified
   towards the targeted destination.  Upon reception of the labelled
   packet, the targeted destination, after having checked both the LSP



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   label and GAL fields, SHOULD pass the whole maintenance message to
   the appropriate processing entity.

4.2.1.2.  MPLS Section

   The following figure (Figure 7) depicts an example of a MPLS Section.

                          +---+             +---+
                          | A |-------------| Z |
                          +---+             +---+

                Figure 7: Maintenance over an MPLS Section

   With regard to the MPLS Section, a G-ACh exists between A and Z. Only
   A and Z can insert, extract or process packets on this G-ACh.

   The following figure (Figure 8) depicts the format of a maintenance
   message when used for a MPLS Section.  The GAL MAY provide the
   exception mechanism for a control channel in its own right without
   being associated with a specific LSP, thus providing maintenance
   related communications across a specific link interconnecting two
   LSRs.  In this case, the GAL is the only label in the stack.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  GAL                  |  TC |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ACH                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  ACH TLV Header (if present)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               ~
   ~                     Zero or more ACH TLVs                     ~
   ~                         (if present)                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               ~
   ~                      Maintenance Message                      ~
   ~                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 8: Maintenance message format for a MPLS Section

   To send a maintenance message on a control channel associated to the
   Section, the head-end LSR (A) of the Section generates a maintenance
   message, to which it MAY prepend an ACH TLV Header and appropriate
   ACH TLVs, and with a ACH to which it pushes a GAL.




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   o  The TTL field of the GAL MUST be set to at least 1.  The exact
      value of the TTL is application specific.

   o  The S bit of the GAL MUST be set according to its position in the
      label stack.  For MPLS Sections, the S bit MUST be set to 1.

   o  The setting of the TC field is application specific.

   The maintenance message, the ACH and the GAL SHOULD NOT be modified
   towards the tail-end LSR (Z).  Upon reception of the labelled packet,
   the tail-end LSR (Z), after having checked the GAL fields, SHOULD
   pass the whole packet to the appropriate processing entity.

4.3.  Relationship wth RFC 3429

   RFC 3429 [18] describes the assignment of one of the reserved label
   values, defined in RFC 3032 [8], to the 'OAM Alert Label' that is
   used by user-plane MPLS OAM functions for the identification of MPLS
   OAM packets.  The value of 14 is used for that purpose.

   Both this document and RFC 3429 [18] therefore describe the
   assignment of reserved label values for similar purposes.  The
   rationale for the assignment of a new reserved label can be
   summarized as follows:

   o  Unlike the mechanisms described and referenced in RFC 3429 [18],
      MPLS-TP maintenance messages will not reside immediately after the
      GAL but instead behind the ACH, which itself resides after the
      bottom of the label stack.  This ensures that OAM, using the
      G-ACh, complies with RFC 4928 [11].

   o  The set of maintenance functions potentially operated in the
      context of the G-ACh is wider than the set of OAM functions
      referenced in RFC 3429 [18].

   o  It has been reported that there are existing implementations and
      running deployments using the 'OAM Alert Label' as described in
      RFC 3429 [18].  It is therefore not possible to modify the 'OAM
      Alert Label' allocation, purpose or usage.  Nevertheless, it is
      RECOMMENDED by this document that no further OAM extensions based
      on 'OAM Alert Label' (Label 14) usage be specified or developed.


5.  Compatability

   Procedures for handling a packet received with an invalid incoming
   label are specified in RFC 3031[12].




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   An LER, LSR or PE MUST discard received associated channel packets on
   which all of the MPLS or PW labels have been popped if any one of the
   following conditions is true:

   o  It is not capable of processing packets on the Channel Type
      indicated by the ACH of the received packet.

   o  It has not, through means outside the scope of this document,
      indicated to the sending LSR, LER or PE that it will process
      associated channel packets on the Channel Type indicated by the
      ACH of the received packet.

   o  If the ACH was indicated by the presence of a GAL, and the first
      nibble of the ACH of the received packet is not 0b0001.

   o  The ACH version is not recognised.

   In addition, it MAY increment an error counter and MAY also
   optionally issue a system and/or SNMP notification.


6.  Congestion Considerations

   The congestion considerations detailed in RFC 5085 [2] apply.


7.  Security Considerations

   The security considerations for the associated control channel are
   described in RFC 4385[6].  Further security considerations MUST be
   described in the relevant associated channel type specification.

   RFC 5085 [2] provides data plane related security considerations.
   These also apply to a G-ACh, whether the alert mechanism uses a GAL
   or only an ACH.


8.  IANA Considerations

   This document requests that IANA allocates a label value, to the GAL,
   from the pool of reserved labels, and suggests this value to be 13.

   Channel Types for the Associated Channel Header are allocated from
   the IANA PW Associated Channel Type registry [7].  The PW Associated
   Channel Type registry is currently allocated based on the IETF
   consensus process, described in [13].  This allocation process was
   chosen based on the consensus reached in the PWE3 working group that
   pseudowire associated channel mechanisms should be reviewed by the



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   IETF and only those that are consistent with the PWE3 architecture
   and requirements should be allocated a code point.

   However, a requirement has emerged (see [16]) to allow for
   optimizations or extensions to OAM and other control protocols
   running in an associated channel to be experimented without resorting
   to the IETF standards process, by supporting experimental code
   points.  This would prevent code points used for such functions from
   being used from the range allocated through the IETF standards and
   thus protects an installed base of equipment from potential
   inadvertent overloading of code points.  In order to support this
   requirement, this document requests that the code point allocation
   scheme for the PW Associated Channel Type be changed as follows:

   0 - 32751 : IETF Consensus

   32752 - 32767 : Experimental

   Code points in the experimental range MUST be used according to the
   guidelines of RFC 3692 [14].  Experimental OAM functions MUST be
   disabled by default.  The Channel Type value used for a given
   experimental OAM function MUST be configurable, and care MUST be
   taken to ensure that different OAM functions that are not inter-
   operable are configured to use different Channel Type values.

   The PW Associated Channel Type registry needs to be updated to
   include a column indicating whether the ACH is followed by a ACH TLV
   header (Yes/No).  There are two ACH Channel Type code-points
   currently assigned and in both cases no ACH TLV header is used.  Thus
   the new format of the PW Channel Type registry is:

   Registry:
   Value  Description                   TLV Follows  Reference
   -----  ----------------------------  -----------  ---------
   0x21   ACH carries an IPv4 packet    No           [RFC4385]
   0x57   ACH carries an IPv6 packet    No           [RFC4385]

                    Figure 9: PW Channel Type registry

   IANA is requested create a new registry called the Associated Channel
   TLV Registry.  The allocation policy for this registry is IETF
   consensus.  This registry MUST record the following information.
   There are no initial entries.

   Name       Type  Length   Description                  Reference
                   (octets)

                      Figure 10: PW ACH TLV registry



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9.  Acknowledgements

   The authors would like to thank all members of the teams (the Joint
   Working Team, the MPLS Interoperability Design Team in IETF and the
   MPLS-TP Ad-Hoc Team in ITU-T) involved in the definition and
   specification of MPLS Transport Profile.


10.  References

10.1.  Normative References

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

   [2]   Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
         Connectivity Verification (VCCV): A Control Channel for
         Pseudowires", RFC 5085, December 2007.

   [3]   Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, "BFD
         For MPLS LSPs", draft-ietf-bfd-mpls-07 (work in progress),
         June 2008.

   [4]   Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label
         Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.

   [5]   Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
         Detection (BFD) for the Pseudowire Virtual Circuit
         Connectivity Verification (VCCV)", draft-ietf-pwe3-vccv-bfd-03
         (work in progress), February 2009.

   [6]   Bryant, S., Swallow, G., Martini, L., and D. McPherson,
         "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use
         over an MPLS PSN", RFC 4385, February 2006.

   [7]   Martini, L., "IANA Allocations for Pseudowire Edge to Edge
         Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

   [8]   Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci,
         D., Li, T., and A. Conta, "MPLS Label Stack Encoding",
         RFC 3032, January 2001.

   [9]   Andersson, L. and R. Asati, "Multi-Protocol Label Switching
         (MPLS) label stack entry: "EXP" field renamed  to "Traffic
         Class" field", draft-ietf-mpls-cosfield-def-08 (work in
         progress), December 2008.

   [10]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in



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         Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
         January 2003.

   [11]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal Cost
         Multipath Treatment in MPLS Networks", BCP 128, RFC 4928,
         June 2007.

   [12]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
         Switching Architecture", RFC 3031, January 2001.

   [13]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

   [14]  Narten, T., "Assigning Experimental and Testing Numbers
         Considered Useful", BCP 82, RFC 3692, January 2004.

10.2.  Informative References

   [15]  Bocci, M., Bryant, S., and L. Levrau, "A Framework for MPLS in
         Transport Networks", draft-ietf-mpls-tp-framework-00 (work in
         progress), November 2008.

   [16]  Vigoureux, M., Ward, D., and M. Betts, "Requirements for OAM in
         MPLS Transport Networks",
         draft-ietf-mpls-tp-oam-requirements-00 (work in progress),
         December 2008.

   [17]  Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., and
         S. Ueno, "MPLS-TP Requirements",
         draft-ietf-mpls-tp-requirements-04 (work in progress),
         February 2009.

   [18]  Ohta, H., "Assignment of the 'OAM Alert Label' for
         Multiprotocol Label Switching Architecture (MPLS) Operation and
         Maintenance (OAM) Functions", RFC 3429, November 2002.


Authors' Addresses

   Matthew Bocci (editor)
   Alcatel-Lucent
   Voyager Place, Shoppenhangers Road
   Maidenhead, Berks  SL6 2PJ
   UK

   Email: matthew.bocci@alcatel-lucent.com





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   Martin Vigoureux (editor)
   Alcatel-Lucent
   Route de Villejust
   Nozay,   91620
   France

   Email: martin.vigoureux@alcatel-lucent.com


   George Swallow
   Cisco


   Email: swallow@cisco.com


   David Ward
   Cisco

   Email: dward@cisco.com


   Stewart Bryant
   Cisco

   Email: stbryant@cisco.com


   Rahul Aggarwal
   Juniper Networks

   Email: rahul@juniper.net



















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