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Network Working Group                                      M. Bocci, Ed.Request for Comments: 5586                             M. Vigoureux, Ed.Updates:3032,4385,5085                                 Alcatel-LucentCategory: Standards Track                                 S. Bryant, Ed.                                                           Cisco Systems                                                               June 2009MPLS Generic Associated ChannelStatus 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 toBCP 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 2009Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .31.1.  Objectives . . . . . . . . . . . . . . . . . . . . . . . .41.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .41.3.  Requirements Language and Terminology  . . . . . . . . . .52.  Generic Associated Channel Header  . . . . . . . . . . . . . .52.1.  Definition . . . . . . . . . . . . . . . . . . . . . . . .62.2.  Allocation of Channel Types  . . . . . . . . . . . . . . .63.  ACH TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . .73.1.  ACH TLV Payload Structure  . . . . . . . . . . . . . . . .73.2.  ACH TLV Header . . . . . . . . . . . . . . . . . . . . . .83.3.  ACH TLV Object . . . . . . . . . . . . . . . . . . . . . .84.  Generalized Exception Mechanism  . . . . . . . . . . . . . . .94.1.  Relationship with Existing MPLS OAM Alert Mechanisms . . .94.2.  GAL Applicability and Usage  . . . . . . . . . . . . . . .104.2.1.  GAL Processing . . . . . . . . . . . . . . . . . . . .104.3.  Relationship withRFC 3429 . . . . . . . . . . . . . . . .135.  Compatibility  . . . . . . . . . . . . . . . . . . . . . . . .146.  Congestion Considerations  . . . . . . . . . . . . . . . . . .157.  Major Contributing Authors . . . . . . . . . . . . . . . . . .158.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .159.  Security Considerations  . . . . . . . . . . . . . . . . . . .1510. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .1611. References . . . . . . . . . . . . . . . . . . . . . . . . . .1711.1. Normative References . . . . . . . . . . . . . . . . . . .1711.2. Informative References . . . . . . . . . . . . . . . . . .18Bocci, et al.               Standards Track                     [Page 2]

RFC 5586                     G-ACh and GAL                     June 20091.  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 inRFC4385 [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 areBocci, 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 20091.3.  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 inRFC 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 20092.1.  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 inRFC 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., inRFC 4446 [RFC4446] andRFC 4385   [RFC4385].  Additional Channel Type values and the associatedBocci, 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.  SeeSection 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 Payload3.2.  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 TLVBocci, et al.               Standards Track                     [Page 8]

RFC 5586                     G-ACh and GAL                     June 2009   Type registry.  SeeSection 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 Format4.  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 inRFC3032 [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 MechanismsRFC 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 20094.2.  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.  SeeSection 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 (seeSection 4.2).   o  The setting of the TC field of the GAL is application specific.      SeeSection 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.  SeeSection 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. (seeSection 4.2).   o  The setting of the TC field of the GAL is application specific.      SeeSection 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 withRFC 3429RFC 3429 [RFC3429] describes the assignment of one of the reserved   label values, defined inRFC 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 andRFC 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 inRFC 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 inRFC 3429 [RFC3429].   o  It has been reported that there are existing implementations and      running deployments using the "OAM Alert Label" as described inRFC 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 inRFC 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 inRFC 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.net8.  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 inRFC 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 200910.  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 ofRFC 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 Registry11.  References11.1.  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 2009Authors' 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.comBocci, et al.               Standards Track                    [Page 19]