Internet-Draft Multiple GAL April 2021
Mirsky, et al. Expires 30 October 2021 [Page]
MPLS Working Group
5586 (if approved)
Intended Status:
Standards Track
G. Mirsky
ZTE Corp.
H. van Helvoort
Individual Contributor
S. Bryant
Futurewei Technologies Inc.
A. Vainshtein
Ribbon Communications Inc.
I. Busi

Number of Generic Associated Channel Labels in the MPLS Label Stack


This document describes the requirements for using multiple Generic Associated Channel Labels (GALs) in an MPLS label stack. As a result, the document updates RFC 5586 by removing the restriction imposed on the usage of GAL that limits the number of GAL in the MPLS label stack to one.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 30 October 2021.

Table of Contents

1. Introduction

[RFC5085] defined the associated channel mechanism and the Associated Channel Header (ACH) for exchange of control, management, and Operations, Administration, and Maintenance (OAM) messages in Pseudowires (PWs). [RFC5586] generalized that associated channel mechanism and the ACH for use in Sections, Label Switched Paths (LSPs), and PWs as the Generic Associated Channel (G-ACh) and introduced the generalized label-based exception mechanism using the Generic Associated Channel Label (GAL).

[RFC5586] restricted the number of times a GAL can appear in an MPLS label stack to one time only. This document updates [RFC5586] by removing that restriction for non-MPLS-TP networks.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. Number of GAL in the MPLS Label Stack

[RFC5586] has limited the number of GALs in an MPLS label stack:

In some MPLS networks, e.g., when realizing Service Function Chaining with MPLS-based forwarding plane [RFC8595], putting more than a single GAL in the MPLS label stack can simplify the processing of OAM packets and, as a result, improve the performance. An extension of the MPLS Echo Request and Reply protocol [RFC8029] in such an environment is discussed in [I-D.lm-mpls-sfc-path-verification]. Because it is expected that a general Service Function does not support processing of MPLS echo request messages, a GAL being used within a basic unit of MPLS label stack to indicate that the payload is ACH-encapsulated OAM message. And in the label-stacking case, multiple basic units on the MPLS label stack, and, consequently, GALs could be placed in an MPLS label stack. Thus, this document removes the limit on the number of GALs present in an MPLS label stack by changing the statement in [RFC5586] as follows:

[RFC5586] requires that when GAL is at the bottom of the label stack, it is followed by an ACH:

This document updates [RFC5586] by extending that requirement for environments when GAL is not at the bottom of the label stack as follows:

4. Processing GAL when not at the Bottom of the Label Stack

[Ed.note: Describe GAL processing by transit and egress nodes. Illustrate the transformation of the MPLS label stack as a packet transits through the domain.]

5. IANA Considerations

This document makes no request for IANA allocations. This section should be removed before publication.

6. Security Considerations

There are no further security considerations than those in [RFC5586].

7. Acknowledgments


8. References

8.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual Circuit Connectivity Verification (VCCV): A Control Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085, , <>.
Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., "MPLS Generic Associated Channel", RFC 5586, DOI 10.17487/RFC5586, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Farrel, A., Bryant, S., and J. Drake, "An MPLS-Based Forwarding Plane for Service Function Chaining", RFC 8595, DOI 10.17487/RFC8595, , <>.

8.2. Informative References

Yao, L. and G. Mirsky, "MPLS-based Service Function Path(SFP) Consistency Verification", Work in Progress, Internet-Draft, draft-lm-mpls-sfc-path-verification-02, , <>.
Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N., Aldrin, S., and M. Chen, "Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures", RFC 8029, DOI 10.17487/RFC8029, , <>.

Authors' Addresses

Greg Mirsky
ZTE Corp.
Huub van Helvoort
Individual Contributor
Stewart Bryant
Futurewei Technologies Inc.
Alexander Vainshtein
Ribbon Communications Inc.
Italo Busi