MPLS Generic Associated Channel
draft-ietf-mpls-tp-gach-gal-04

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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.

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [1].

Table of Contents

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

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 [11] (Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. Berger, “A Framework for MPLS in Transport Networks,” April 2010.). Some of these functions can be supported using existing tools such as Virtual Circuit Connectivity Verification (VCCV) [2] (Nadeau, T. and C. Pignataro, “Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires,” December 2007.), Bidirectional Forwarding Detection for MPLS LSPs (BFD-MPLS) [12] (Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, “BFD For MPLS LSPs,” June 2008.), LSP-Ping [13] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.), or BFD-VCCV [14] (Nadeau, T. and C. Pignataro, “Bidirectional Forwarding Detection (BFD) for the Pseudowire Virtual Circuit Connectivity Verification (VCCV),” July 2009.). 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 [15] (Vigoureux, M. and D. Ward, “Requirements for OAM in MPLS Transport Networks,” March 2010.). 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 also NOT depend on dynamic control plane functions.

VCCV [2] (Nadeau, T. and C. Pignataro, “Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires,” December 2007.) 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 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 [3] (Bryant, S., Swallow, G., Martini, L., and D. McPherson, “Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN,” February 2006.), 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 [13] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.) and BFD-MPLS [12] (Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, “BFD For MPLS LSPs,” June 2008.) 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 127/8 or 0:0:0:0:0:FFFF:127.0.0.0/104, respectively for IPv4 and IPv6. 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) [11] (Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. Berger, “A Framework for MPLS in Transport Networks,” April 2010.), 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 [13] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.) and [12] (Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, “BFD For MPLS LSPs,” June 2008.) 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.  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 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 stack.

1.3.  Scope

This document defines the encapsulation header for Sections, LSPs, and PWs associated control 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.

1.4.  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 [16] (Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., and S. Ueno, “MPLS-TP Requirements,” August 2009.) as follows (note that the terms 'Section' and ''Section Layer Network' are synonymous):

Concatenated Segment: A serial-compound link connection as defined in [17] (International Telecommunication Union, “Generic Functional Architecture of Transport Networks,” March 2000.). 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.

Section Layer Network: A section is a server layer (which may be MPLS-TP or a different technology) which provides for encapsulation and OAM of a client layer network. A section layer may provide for aggregation of multiple MPLS-TP clients. Note that G.805 [17] (International Telecommunication Union, “Generic Functional Architecture of Transport Networks,” March 2000.) 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.

2.  Generic Associated Channel Header

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

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 [3] (Bryant, S., Swallow, G., Martini, L., and D. McPherson, “Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN,” February 2006.) 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          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

Note that VCCV [2] (Nadeau, T. and C. Pignataro, “Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires,” December 2007.) 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 [4] (Martini, L., “IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3),” April 2006.) and RFC 4385 [3] (Bryant, S., Swallow, G., Martini, L., and D. McPherson, “Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN,” February 2006.). 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 [4] (Martini, L., “IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3),” April 2006.), but this document modifies the existing policy to accommodate a level of experimentation. See Section 8 (IANA Considerations) 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 structure of ACH TLVs that MAY follow an ACH TLV Header is defined and described in the following sections.

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

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

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           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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 (IANA Considerations) 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                             ~
~                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 [5] (Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, “MPLS Label Stack Encoding,” January 2001.) 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:

• differentiate specific packets (i.e., G-ACh packets) from others, such as user-plane ones,
• 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 [13] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.) and BFD-MPLS [12] (Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, “BFD For MPLS LSPs,” June 2008.) 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 127/8 or 0:0:0:0:0:FFFF:127.0.0.0/104, respectively for IPv4 and IPv6.

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.

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 [6] (Andersson, L. and R. Asati, “Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field,” February 2009.).

The Time-To-Live (TTL) field of the LSE that contains the GAL follows the definition and processing rules specified in [7] (Agarwal, P. and B. Akyol, “Time To Live (TTL) Processing in Multi-Protocol Label Switching (MPLS) Networks,” January 2003.).

4.2.1.1.  MPLS Label Switched Paths and Segments

The following figure (Figure 5 (Maintenance over a LSP)) 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 |
     +---+             +---+             +---+             +---+

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 the generation of G-ACh packets. A, B, C and D may also originate and process G-ACh packets.

The following figure (Figure 6 (G-ACh packet format for a LSP)) depicts the format of a MPLS-TP G-ACh packet 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)                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               ~
~                         G-ACh Message                         ~
~                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

To send a maintenance 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, adds an ACH to which it, pushes a GAL LSE and finally the LSP Label LSE.

• 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.
• The S bit of the GAL MUST be set according to its position in the label stack (see Section 4.2).
• The setting of the TC field of the GAL is application specific. See Section 4.2.1 for definition and processing rules.

The G-ACh message, the ACH or the GAL SHOULD NOT be modified towards the targeted destination. 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 (Maintenance over an MPLS Section)) depicts an example of an MPLS Section.

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

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 (G-ACh packet format for an MPLS Section)) 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.

 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                         ~
~                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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, adds an ACH to which it pushes a GAL LSE.

• 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.
• The S bit of the GAL MUST be set according to its position in the label stack. (see Section 4.2).
• The setting of the TC field of the GAL is application specific. See Section 4.2.1 for definition and processing rules.

The G-ACh message, the ACH and the GAL SHOULD NOT be modified 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 [18] (Ohta, H., “Assignment of the 'OAM Alert Label' for Multiprotocol Label Switching Architecture (MPLS) Operation and Maintenance (OAM) Functions,” November 2002.) describes the assignment of one of the reserved label values, defined in RFC 3032 [5] (Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, “MPLS Label Stack Encoding,” January 2001.), 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] (Ohta, H., “Assignment of the 'OAM Alert Label' for Multiprotocol Label Switching Architecture (MPLS) Operation and Maintenance (OAM) Functions,” November 2002.) 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:

• Unlike the mechanisms described and referenced in RFC 3429 [18] (Ohta, H., “Assignment of the 'OAM Alert Label' for Multiprotocol Label Switching Architecture (MPLS) Operation and Maintenance (OAM) Functions,” November 2002.), 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.
• 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] (Ohta, H., “Assignment of the 'OAM Alert Label' for Multiprotocol Label Switching Architecture (MPLS) Operation and Maintenance (OAM) Functions,” November 2002.).
• It has been reported that there are existing implementations and running deployments using the 'OAM Alert Label' as described in RFC 3429 [18] (Ohta, H., “Assignment of the 'OAM Alert Label' for Multiprotocol Label Switching Architecture (MPLS) Operation and Maintenance (OAM) Functions,” November 2002.). 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[8] (Rosen, E., Viswanathan, A., and R. Callon, “Multiprotocol Label Switching Architecture,” January 2001.).

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:

• It is not capable of processing packets on the Channel Type indicated by the ACH of the received packet.
• 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.
• The packet is received on an Experimental Channel Type that is locally disabled.
• 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.
• The ACH version is not recognized.

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

6.  Congestion Considerations

The congestion considerations detailed in RFC 5085 [2] (Nadeau, T. and C. Pignataro, “Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires,” December 2007.) apply.

7.  Security Considerations

The security considerations for the associated control channel are described in RFC 4385 [3] (Bryant, S., Swallow, G., Martini, L., and D. McPherson, “Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN,” February 2006.). Further security considerations MUST be described in the relevant associated channel type specification.

RFC 5085 [2] (Nadeau, T. and C. Pignataro, “Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires,” December 2007.) 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 in the "Multiprotocol Label Switching Architecture (MPLS) Label Values" registry, and suggests this value to be 13.

Channel Types for the Associated Channel Header are allocated from the IANA "PW Associated Channel Type" registry [4] (Martini, L., “IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3),” April 2006.). The PW Associated Channel Type registry is currently allocated based on the IETF consensus process, described in [9] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.). 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 [15] (Vigoureux, M. and D. Ward, “Requirements for OAM in MPLS Transport Networks,” March 2010.)) 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

32760 - 32767 : Experimental

Code points in the experimental range MUST be used according to the guidelines of RFC 3692 [10] (Narten, T., “Assigning Experimental and Testing Numbers Considered Useful,” January 2004.). 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 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]

IANA is requested create a new registry called the Associated Channel Header 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)

9.  Acknowledgments

The authors would 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.

10.  References

10.1. Normative References

10.2. Informative References

Authors' Addresses