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rfc:rfc5586

Network Working Group M. Bocci, Ed. Request for Comments: 5586 M. Vigoureux, Ed. Updates: 3032, 4385, 5085 Alcatel-Lucent Category: Standards Track S. Bryant, Ed.

                                                         Cisco Systems
                                                             June 2009
                  MPLS Generic Associated Channel

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) 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
 Provisions Relating to IETF Documents in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.

Abstract

 This document generalizes 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.

Bocci, et al. Standards Track [Page 1] RFC 5586 G-ACh and GAL June 2009

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.3.  Requirements Language and Terminology  . . . . . . . . . .  5
 2.  Generic Associated Channel Header  . . . . . . . . . . . . . .  5
   2.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.2.  Allocation of Channel Types  . . . . . . . . . . . . . . .  6
 3.  ACH TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.1.  ACH TLV Payload Structure  . . . . . . . . . . . . . . . .  7
   3.2.  ACH TLV Header . . . . . . . . . . . . . . . . . . . . . .  8
   3.3.  ACH TLV Object . . . . . . . . . . . . . . . . . . . . . .  8
 4.  Generalized Exception Mechanism  . . . . . . . . . . . . . . .  9
   4.1.  Relationship with Existing MPLS OAM Alert Mechanisms . . .  9
   4.2.  GAL Applicability and Usage  . . . . . . . . . . . . . . . 10
     4.2.1.  GAL Processing . . . . . . . . . . . . . . . . . . . . 10
   4.3.  Relationship with RFC 3429 . . . . . . . . . . . . . . . . 13
 5.  Compatibility  . . . . . . . . . . . . . . . . . . . . . . . . 14
 6.  Congestion Considerations  . . . . . . . . . . . . . . . . . . 15
 7.  Major Contributing Authors . . . . . . . . . . . . . . . . . . 15
 8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
 9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
 10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
   11.2. Informative References . . . . . . . . . . . . . . . . . . 18

Bocci, et al. Standards Track [Page 2] RFC 5586 G-ACh and GAL June 2009

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 pseudowire (PW) and a Label
 Switched Path (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
 [MPLS-TP].  Some of these functions can be supported using existing
 tools such as Virtual Circuit Connectivity Verification (VCCV)
 [RFC5085], Bidirectional Forwarding Detection for MPLS LSPs (BFD-
 MPLS) [BFD-MPLS], LSP-Ping [RFC4379], or BFD-VCCV [BFD-VCCV].
 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 [OAM-REQ].
 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 traffic, and MUST
 NOT depend on dynamic control plane functions.
 VCCV [RFC5085] can use an Associated Channel Header (ACH) to provide
 a PW associated control channel between a PW's endpoints, over which
 OAM and other control messages can be exchanged.  This document
 generalizes the applicability 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 [RFC4385], may be used with additional code points to support
 additional MPLS maintenance functions on the G-ACh.
 Generalizing the applicability of the ACH 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.  The label used for that purpose is one
 of the MPLS reserved labels and is referred to as the GAL (G-ACh
 Label).  The GAL mechanism is defined to work together with the ACH
 for LSPs and MPLS Sections.
 RFC 4379 [RFC4379] and BFD-MPLS [BFD-MPLS] 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

Bocci, et al. Standards Track [Page 3] RFC 5586 G-ACh and GAL June 2009

 based, for example, on Time To Live (TTL) expiration and/or on the
 use of an IP destination address in the range of 127.0.0.0/8 or 0:0:
 0:0:0:FFFF:127.0.0.0/104 for IPv4 and IPv6, respectively.  These
 mechanisms are the default mechanisms for identifying MPLS OAM
 packets when encapsulated in an IP header.  However, it may not
 always be possible to use these mechanisms in some MPLS applications,
 e.g., MPLS Transport Profile (MPLS-TP) [MPLS-TP], 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 used in [RFC4379] and [BFD-MPLS] are not available.  Yet, this
 mechanism MAY be used in addition to IP-based mechanisms.
 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), Signaling Communication Channel (SCC), and
 Management Communication Channel (MCC) messages.
 Also note that the GAL and ACH are applicable to MPLS and PWs in
 general.  This document specifies general mechanism and uses MPLS-TP
 as an example application.  The application of the GAL and ACH to
 other specific MPLS uses is outside the scope of this document.

1.1. 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 normalizes 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
 label stack.

1.2. Scope

 This document defines the encapsulation header for Section, LSP, and
 PW associated control channel messages.
 This document does not define how associated control channel
 capabilities are signaled or negotiated between LERs/LSRs or between
 PEs, nor does it define the operation of various OAM functions.
 This document does not deprecate existing MPLS and PW OAM mechanisms.

Bocci, et al. Standards Track [Page 4] RFC 5586 G-ACh and GAL June 2009

1.3. Requirements Language and Terminology

 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].
 This document uses the following additional terminology:
 ACH: Associated Channel Header
 G-ACh: Generic Associated Channel
 GAL: G-ACh Label
 G-ACh packet: Any packet containing a message belonging to a protocol
 that is carried on a PW, LSP, or MPLS Section associated control
 channel.  Examples include maintenance protocols such as OAM
 functions, signaling communications, or management communications.
 The terms "Section" and "Concatenated Segment" are defined in
 [TP-REQ] as follows (note that the terms "Section" and "Section Layer
 Network" are synonymous):
 Section Layer Network: A section layer is a server layer (which may
 be MPLS-TP or a different technology) that provides for the transfer
 of the section layer client information between adjacent nodes in the
 transport path layer or transport service layer.  Note that G.805
 [G805] defines the section layer as one of the two layer networks in
 a transmission media layer network.  The other layer network is the
 physical media layer network.
 Concatenated Segment: A serial-compound link connection as defined in
 [G805].  A concatenated segment is a contiguous part of an LSP or
 multi-segment PW that comprises a set of segments and their
 interconnecting nodes in sequence.

2. Generic Associated Channel Header

 VCCV [RFC5085] defines three 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".

Bocci, et al. Standards Track [Page 5] RFC 5586 G-ACh and GAL June 2009

2.1. Definition

 The use of the ACH, previously limited to PWs, is here generalized to
 also apply to LSPs and to Sections.  Note that for PWs, the PWE3
 control word [RFC4385] MUST be present in the encapsulation of user
 packets when the ACH is used to realize the associated control
 channel.
 The ACH used by CC Type 1 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, LSP, or Section.  The Version
 field is set to 0, as specified in RFC 4385 [RFC4385].  Bits 8 to 15
 of the ACH are reserved and MUST be set to 0 and ignored on
 reception.  Bits 16 to 31 are used to encode the possible Channel
 Types.  This 16-bit field is in network byte order.
 Note that VCCV [RFC5085] also includes mechanisms for negotiating the
 Control Channel and Connectivity Verification (i.e., OAM function)
 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.
 The G-ACh MUST NOT be used to transport user traffic.

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 indicates the specific 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 [RFC4446] and RFC 4385
 [RFC4385].  Additional Channel Type values and the associated

Bocci, et al. Standards Track [Page 6] RFC 5586 G-ACh and GAL June 2009

 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 [RFC4446], but this document modifies the existing
 policy to accommodate a level of experimentation.  See Section 10 for
 further details.

3. ACH TLVs

 In some applications of the generalized associated control channel,
 it is necessary to include one or more ACH TLVs to provide additional
 context information to the G-ACh packet.  One use of these ACH TLVs
 might be to identify the source and/or intended destination of the
 associated channel 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 G-ACh 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 G-ACh message, an ACH TLV Header MUST follow
 the ACH.  If no ACH TLVs are required in a specific associated
 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 an ACH Channel Type specification does not explicitly specify that
 ACH TLVs MAY be used, then the 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 following figure (Figure 2) shows the structure of a G-ACh packet
 payload.

Bocci, et al. Standards Track [Page 7] RFC 5586 G-ACh and GAL June 2009

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                              ACH                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         ACH TLV Header                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               ~
 ~                     zero or more ACH TLVs                     ~
 ~                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               ~
 ~                        G-ACh Message                          ~
 ~                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 2: G-ACh Packet Payload

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
 of TLVs including sub-TLVs that follow the ACH TLV Header.  A length
 of zero indicates that no ACH TLV follow this header.  Note that no
 padding is required for the set of ACH TLVs.
 The Reserved field is for future use and MUST be set to zero on
 transmission and ignored on reception.

3.3. ACH TLV Object

 ACH TLVs MAY follow an ACH TLV Header.  The structure of ACH TLVs is
 defined and described in this section.
 An ACH TLV consists of a 16-bit Type field, followed by a 16-bit
 Length field that 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

Bocci, et al. Standards Track [Page 8] RFC 5586 G-ACh and GAL June 2009

 Type registry.  See Section 10 for further details.  Note that the
 Value field of ACH TLVs MAY contain sub-TLVs.  Note that no padding
 is required for individual TLVs or sub-TLVs.
  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. Generalized Exception Mechanism

 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.  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 [RFC3032] is assigned for this purpose.  IANA assigned the value
 13 to the GAL.
 The GAL provides an alert based exception mechanism to:
 o  differentiate specific packets (i.e., G-ACh packets) from others,
    such as 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 these purposes apply.

4.1. Relationship with Existing MPLS OAM Alert Mechanisms

 RFC 4379 [RFC4379] and BFD-MPLS [BFD-MPLS] 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, for example, on Time To Live (TTL) expiration and/or on the
 use of an IP destination address in the range of 127.0.0.0/8 or 0:0:
 0:0:0:FFFF:127.0.0.0/104 for IPv4 and IPv6, respectively.
 These mechanisms are the default mechanisms for identifying MPLS OAM
 packets when encapsulated in an IP header although the mechanism
 defined in this document MAY also be used.

Bocci, et al. Standards Track [Page 9] RFC 5586 G-ACh and GAL June 2009

4.2. GAL Applicability and Usage

 In MPLS-TP, the GAL MUST be used with packets on a G-ACh on LSPs,
 Concatenated Segments of LSPs, and with Sections, and MUST NOT be
 used with PWs.  It 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 or its use with PWs.  Where the GAL is at the bottom
 of the label stack (i.e., S bit set to 1), then it MUST always be
 followed by an ACH.
 The GAL MUST NOT appear in the label stack when transporting normal
 user-plane packets.  Furthermore, when present, the GAL MUST NOT
 appear more than once in the label stack.
 A receiving LSR, LER, or PE MUST NOT forward a G-ACh packet to
 another node based on the GAL label.

4.2.1. GAL Processing

 The Traffic Class (TC) field (formerly known as the EXP field) of the
 Label Stack Entry (LSE) containing the GAL follows the definition and
 processing rules specified and referenced in [RFC5462].
 The Time-To-Live (TTL) field of the LSE that contains the GAL follows
 the definition and processing rules specified in [RFC3443].

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 that is established from A to D and
 switched in B and C.
      +---+             +---+             +---+             +---+
      | A |-------------| B |-------------| C |-------------| D |
      +---+             +---+             +---+             +---+
                   Figure 5: Maintenance over an LSP
 In this example, a G-ACh exists on the LSP that extends between LERs
 A and D, via LSRs B and C.  Only A and D may initiate new G-ACh
 packets.  A, B, C, and D may process and respond to G-ACh packets.
 The following figure (Figure 6) depicts the format of an MPLS-TP
 G-ACh packet when used for an LSP.

Bocci, et al. Standards Track [Page 10] RFC 5586 G-ACh and GAL June 2009

  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)                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               ~
 ~                         G-ACh Message                         ~
 ~                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 6: G-ACh Packet Format for an LSP
 Note that it is possible that the LSP may be tunneled in another LSP
 (e.g., if an MPLS Tunnel exists between B and C), and as such other
 LSEs may be present in the label stack.
 To send a G-ACh message on the LSP associated control channel, the
 LER (A) generates a G-ACh message, to which it MAY prepend an ACH TLV
 Header and appropriate ACH TLVs.  It then adds an ACH, onto which it
 pushes a GAL LSE.  Finally, the LSP Label LSE is pushed onto the
 resulting packet.
 o  The TTL field of the GAL LSE MUST be set to at least 1.  The exact
    value of the TTL is application specific.  See Section 4.2.1 for
    definition and processing rules.
 o  The S bit of the GAL MUST be set according to its position in the
    label stack (see Section 4.2).
 o  The setting of the TC field of the GAL is application specific.
    See Section 4.2.1 for definition and processing rules.
 LSRs MUST NOT modify the G-ACh message, the ACH or the GAL towards
 the targeted destination.

Bocci, et al. Standards Track [Page 11] RFC 5586 G-ACh and GAL June 2009

 Note:  This is because once a G-ACh packet has been sent on an LSP,
    no node has visibility of it unless the LSP label TTL expires or
    the GAL is exposed when the LSP label is popped.  If this is at
    the targeted destination, for example, indicated by an address in
    an ACH TLV, then processing can proceed as specified below.  If
    this is not the targeted destination, but the node has agreed to
    process packets on that ACH channel, then the processing applied
    to the packet is out of scope of this document.
 Upon reception of the labeled packet, the targeted destination, after
 having checked both the LSP Label and GAL LSEs fields, SHOULD pass
 the whole packet to the appropriate processing entity.

4.2.1.2. MPLS Section

 The following figure (Figure 7) depicts an example of an 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 G-ACh packet
 when used for an 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.

Bocci, et al. Standards Track [Page 12] RFC 5586 G-ACh and GAL June 2009

  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)                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               ~
 ~                         G-ACh message                         ~
 ~                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 8: G-ACh Packet Format for an MPLS Section
 To send a G-ACh message on a control channel associated to the
 Section, the head-end LSR (A) of the Section generates a G-ACh
 message, to which it MAY prepend an ACH TLV Header and appropriate
 ACH TLVs.  Next, the LSR adds an ACH.  Finally, it pushes a GAL LSE.
 o  The TTL field of the GAL MUST be set to at least 1.  The exact
    value of the TTL is application specific.  See Section 4.2.1 for
    definition and processing rules.
 o  The S bit of the GAL MUST be set according to its position in the
    label stack. (see Section 4.2).
 o  The setting of the TC field of the GAL is application specific.
    See Section 4.2.1 for definition and processing rules.
 Intermediate nodes of the MPLS Section MUST NOT modify the G-ACh
 message, the ACH and the GAL towards the tail-end LSR (Z).  Upon
 reception of the G-ACh packet, the tail-end LSR (Z), after having
 checked the GAL LSE fields, SHOULD pass the whole packet to the
 appropriate processing entity.

4.3. Relationship with RFC 3429

 RFC 3429 [RFC3429] describes the assignment of one of the reserved
 label values, defined in RFC 3032 [RFC3032], 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.

Bocci, et al. Standards Track [Page 13] RFC 5586 G-ACh and GAL June 2009

 Both this document and RFC 3429 [RFC3429] 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
    [RFC3429], G-ACh messages will not reside immediately after the
    GAL but instead behind the ACH, which itself resides after the
    bottom of the label stack.
 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 [RFC3429].
 o  It has been reported that there are existing implementations and
    running deployments using the "OAM Alert Label" as described in
    RFC 3429 [RFC3429].  It is therefore not possible to modify the
    "OAM Alert Label" allocation, purpose, or usage.  Nevertheless, it
    is RECOMMENDED that no further OAM extensions based on "OAM Alert
    Label" (Label 14) usage be specified or developed.

5. Compatibility

 Procedures for handling a packet received with an invalid incoming
 label are specified in RFC 3031 [RFC3031].
 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  The packet is received on an Experimental Channel Type that is
    locally disabled.
 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 0001b.
 o  The ACH version is not recognized.

Bocci, et al. Standards Track [Page 14] RFC 5586 G-ACh and GAL June 2009

 In addition, the LER, LSR, or PE MAY increment an error counter and
 MAY also issue a system and/or Simple Network Management Protocol
 (SNMP) notification.

6. Congestion Considerations

 The congestion considerations detailed in RFC 5085 [RFC5085] apply.

7. Major Contributing Authors

 The editors would like to thank George Swallow, David Ward, and Rahul
 Aggarwal who made a major contribution to the development of this
 document.
    George Swallow
    Cisco Systems
    Email: swallow@cisco.com
    David Ward
    Cisco Systems
    Email: dward@cisco.com
    Rahul Aggarwal
    Juniper Networks
    Email: rahul@juniper.net

8. Acknowledgments

 The editors gratefully acknowledge the contributions of Sami Boutros,
 Italo Busi, Marc Lasserre, Lieven Levrau, and Siva Sivabalan.
 The authors would also like to thank Malcolm Betts, ITU-T Study Group
 15, and 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 the MPLS
 Transport Profile.

9. Security Considerations

 The security considerations for the associated control channel are
 described in RFC 4385 [RFC4385].  Further security considerations
 MUST be described in the relevant associated channel type
 specification.
 RFC 5085 [RFC5085] provides data plane related security
 considerations.  These also apply to a G-ACh, whether the alert
 mechanism uses a GAL or only an ACH.

Bocci, et al. Standards Track [Page 15] RFC 5586 G-ACh and GAL June 2009

10. IANA Considerations

 IANA allocated label value 13 to the GAL from the pool of reserved
 labels in the "Multiprotocol Label Switching Architecture (MPLS)
 Label Values" registry.
 Channel Types for the Associated Channel Header are allocated from
 the IANA "PW Associated Channel Type" registry [RFC4446].  The PW
 Associated Channel Type registry is currently allocated based on the
 IETF consensus process (termed "IETF Review" in [RFC5226]).  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 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 [OAM-REQ]) 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, IANA has changed the code point allocation scheme for
 the PW Associated Channel Type be changed as follows:
 0 - 32751 : IETF Review
 32760 - 32767 : Experimental
 Code points in the experimental range MUST be used according to the
 guidelines of RFC 3692 [RFC3692].  Functions using experimental G-ACh
 code points 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 has been 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:

Bocci, et al. Standards Track [Page 16] RFC 5586 G-ACh and GAL June 2009

 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 created a new registry called the Associated Channel Header TLV
 Registry.  The allocation policy for this registry is IETF review.
 This registry MUST record the following information.  There are no
 initial entries.
 Name       Type  Length   Description                  Reference
                 (octets)
                      Figure 10: ACH TLV Registry

11. References

11.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3031]   Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
             Label Switching Architecture", RFC 3031, January 2001.
 [RFC3032]   Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
             Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
             Encoding", RFC 3032, January 2001.
 [RFC3443]   Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
             in Multi-Protocol Label Switching (MPLS) Networks",
             RFC 3443, January 2003.
 [RFC3692]   Narten, T., "Assigning Experimental and Testing Numbers
             Considered Useful", BCP 82, RFC 3692, January 2004.
 [RFC4385]   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.
 [RFC4446]   Martini, L., "IANA Allocations for Pseudowire Edge to
             Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

Bocci, et al. Standards Track [Page 17] RFC 5586 G-ACh and GAL June 2009

 [RFC5085]   Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
             Connectivity Verification (VCCV): A Control Channel for
             Pseudowires", RFC 5085, December 2007.
 [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 5226,
             May 2008.
 [RFC5462]   Andersson, L. and R. Asati, "Multiprotocol Label
             Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
             to "Traffic Class" Field", RFC 5462, February 2009.

11.2. Informative References

 [BFD-MPLS]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
             "BFD For MPLS LSPs", Work in Progress, June 2008.
 [BFD-VCCV]  Nadeau, T. and C. Pignataro, "Bidirectional Forwarding
             Detection (BFD) for the Pseudowire Virtual Circuit
             Connectivity Verification (VCCV)", Work in Progress,
             May 2009.
 [G805]      International Telecommunication Union, "Generic
             Functional Architecture of Transport Networks", ITU-
             T G.805, March 2000.
 [MPLS-TP]   Bocci, M., Bryant, S., and L. Levrau, "A Framework for
             MPLS in Transport Networks", Work in Progress,
             November 2008.
 [OAM-REQ]   Vigoureux, M., Ed., Ward, D., Ed., and M. Betts, Ed.,
             "Requirements for OAM in MPLS Transport Networks", Work
             in Progress, March 2009.
 [RFC3429]   Ohta, H., "Assignment of the 'OAM Alert Label' for
             Multiprotocol Label Switching Architecture (MPLS)
             Operation and Maintenance (OAM) Functions", RFC 3429,
             November 2002.
 [RFC4379]   Kompella, K. and G. Swallow, "Detecting Multi-Protocol
             Label Switched (MPLS) Data Plane Failures", RFC 4379,
             February 2006.
 [TP-REQ]    Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M.,
             Ed., Sprecher, N., and S. Ueno, "MPLS-TP Requirements",
             Work in Progress, May 2009.

Bocci, et al. Standards Track [Page 18] RFC 5586 G-ACh and GAL June 2009

Authors' Addresses

 Matthew Bocci (editor)
 Alcatel-Lucent
 Voyager Place, Shoppenhangers Road
 Maidenhead, Berks  SL6 2PJ
 UK
 EMail: matthew.bocci@alcatel-lucent.com
 Martin Vigoureux (editor)
 Alcatel-Lucent
 Route de Villejust
 Nozay,   91620
 France
 EMail: martin.vigoureux@alcatel-lucent.com
 Stewart Bryant (editor)
 Cisco Systems
 EMail: stbryant@cisco.com

Bocci, et al. Standards Track [Page 19]

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