Handling the TC and TTL fields in a Label Stack Entry when the Generic Associated Channel Label is Present
draft-vainshtein-mpls-gal-tc-ttl-handling-01

Network Working Group                                      A. Vainshtein
Internet-Draft                                               ECI Telecom
Updates: 5586 (if approved)                                 L. Andersson
Intended status: Standards Track            Huawei Technologies Co., Ltd
Expires: December 18, 2015                                     A. Farrel
                                                        Juniper Networks
                                                           June 16, 2015

 Handling the TC and TTL fields in a Label Stack Entry when the Generic
                  Associated Channel Label is Present
              draft-vainshtein-mpls-gal-tc-ttl-handling-01

Abstract

   This document clarifies handling of the Traffic Class (TC) and Time-
   to-Live (TTL) fields of a Label Stack Entry that contains the Generic
   Associated Channel (G-ACh) Label (GAL).  These clarifications are
   intended to aid interoperability of implementations.

   Original handling was defined in RFC 5586, and this document updates
   that RFC.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on December 18, 2015.

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
   3.  New Procedures  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  New Procedures for Handling the TC Field in an LSE That
           Contains the GAL  . . . . . . . . . . . . . . . . . . . .   4
     3.2.  New Procedures for Handling the TTL Field in an LSE
           Containing GAL  . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Scope of the new Procedures . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   [RFC5586] introduced an alert mechanism for the Generic Associated
   Channel (G-ACh) that uses a Generic Associated Channel Label (GAL).
   In particular, [RFC5586] allocated one of the values from the special
   purpose label space to be the GAL, specified that the Label Stack
   Entry (LSE) containing GAL must be always at the bottom of the label
   stack in the case of MPLS transport profile (MPLS-TP) Label Switched
   Paths (LSPs), and that G-ACh packets must not be forwarded based on
   the GAL.

   Per [RFC3032] each LSE contains, in addition to the label value and
   bottom-of-stack (BoS) flag, two additional fields:

   o  Traffic Class (TC) field - 3 bits (renamed from Experimental (EXP)
      field [RFC5462]).  [RFC5586] defined that the handling of this
      field in an LSE that contains the GAL is as specified and
      referenced in RFC 5462.

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   o  Time-to-Live (TTL) field - 8 bits.  [RFC5586] defined that the
      handling of this field in an LSE that contains the GAL is in
      accordance with [RFC3443].

   Implementations of [RFC5586] have encountered interoperability
   problems in their interpretation of these two fields when present in
   an LSE that contains the GAL.  Section 4.2.1.1 of [RFC5586] states:

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

   When this LSE becomes the top entry in the label stack (because the
   previous label has been popped) some receiving implementations have
   attempted to interpret the fields and this has resulted in errors,
   packet drops, or poor performance.  In particular, packets with an
   LSE with TTL set to zero have been dropped as "expired" while those
   with TTL set to one can be trapped to the generic (slow) MPLS
   exception handler with appropriate rate limiting before the GAL is
   noticed (which would otherwise result in trapping the packet to a
   fast OAM handler).  This document clarifies the rules for setting and
   processing them in the Label Stack Entry that includes the GAL.

   The above-mentioned references are not useful for the implementers
   and testers because they don't give enough information about the
   correct processing actions.  For example, [RFC5462] says only that
   the use of TC field for Quality of Service (QoS) and Explicit
   Congestion Notification (ECN) "is intended to be flexible'.  On the
   other hand, while [RFC3443] is very detailed with regard to
   processing of the TTL field, it mainly deals with issues that are
   irrelevant for an LSE that contains the GAL.

   This document defines handling of the TC and TTL fields in an LSE
   that contains GAL in an unambiguous way without referring to any
   other documents.  It updates [RFC5586] in that regard.

2.  Terminology

2.1.  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 [RFC2119].

2.2.  Abbreviations

   BoS: Bottom of Stack

   G-ACh: Generic Associated Channel

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   GAL: Generic Associated Channel Label

   LER: Label Edge Router

   LSE: Label Stack Entry

   LSP: Label Switching Path

   LSR: Label Switching Router

   PW: Pseudowire

   TC: Traffic Class field (formerly named EXP)

   TTL: Time-to-Live

3.  New Procedures

3.1.  New Procedures for Handling the TC Field in an LSE That Contains
      the GAL

   Setting the value of the TC field in an LSE that contains the GAL is
   done by the LER that originates the G-ACh packet and is a matter of
   local policy for that LER.  It is RECOMMENDED that implementations
   set the TC field of an LSE that contains the GAL to all zero (0b000).

   The LER that inspects an LSE that contains the GAL MUST ignore the
   value of the TC field.

3.2.  New Procedures for Handling the TTL Field in an LSE Containing GAL

   Setting the value of the TTL in an LSE that contains the GAL is done
   by the LER that originates the G-ACh packet and is a matter of local
   policy for that LER.  The LER that originates the G-ACh packet SHOULD
   NOT set this value to 0 or 1: this will avoid possible
   misinterpretation by the LER that inspects an LSE that contains the
   GAL if that LER does not comply with this document.  It is
   RECOMMENDED that implementations set the TTL of an LSE that contains
   the GAL to 255.

   The LER that examines an LSE that contains the GAL MUST ignore the
   value of the TTL field.

3.3.  Scope of the new Procedures

   [RFC5586] disallowed the use of the GAL in PWs, but that limitation
   was relaxed in [RFC6423].

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   The new procedures defined in this document for handling the TC field
   and the TTL field in an LSE that contains the GAL apply equally to
   all possible uses of the GAL including the so-called "Section G-ACh"
   where the GAL is the only label in the label stack, and the use of
   the GAL in LSPs and PWs.

4.  IANA Considerations

   This document makes no requests for IANA action.

5.  Security Considerations

   This document makes a minor update to the processing for MPLS packets
   containing the GAL and does not change any of the security
   fundamentals of MPLS.  For a discussion of security considerations
   relating to MPLS, please refer to [RFC5920].

   Note that the rules set out in this document specify that a receiver
   must ignore the values in the two MPLS LSE fields that are discussed.
   As such, this clarification removes a potential (and minor) attack
   vector where those fields could be malignly set and might cause
   incorrect action by the receiver.

6.  Acknowledgments

   The authors would like to thank Nadav Baadany whose question
   triggered this work, and Jeff Haas, Jie Dong, Mach Chen, and Huub van
   Helvoort for their comments on this document.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

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   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.

   [RFC6423]  Li, H., Martini, L., He, J., and F. Huang, "Using the
              Generic Associated Channel Label for Pseudowire in the
              MPLS Transport Profile (MPLS-TP)", RFC 6423, November
              2011.

7.2.  Informative References

   [RFC5920]  Fang, L., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010.

Authors' Addresses

   Alexander Vainshtein
   ECI Telecom
   Petah Tikva
   Israel

   Email: alexander.vainshtein@ecitele.com

   Loa Andersson
   Huawei Technologies Co., Ltd
   Stockholm
   Sweden

   Email: loa@mail01.huawei.com

   Adrian Farrel
   Juniper Networks

   Email: adrian@olddog.co.uk

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