Network Working Group                                            H. Ohta
Request for Comments: 3429                                           NTT
Category: Informational                                    November 2002


                Assignment of the 'OAM Alert Label' for
           Multiprotocol Label Switching Architecture (MPLS)
              Operation and Maintenance (OAM) Functions

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document describes the assignment of one of the reserved label
   values defined in RFC 3032 (MPLS label stack encoding) to the
   'Operation and Maintenance (OAM) Alert Label' that is used by user-
   plane Multiprotocol Label Switching Architecture (MPLS) OAM functions
   for identification of MPLS OAM packets.

1. Introduction

   This document describes the assignment of one of the reserved label
   values defined in RFC 3032 (MPLS label stack encoding [2]) to the
   'OAM Alert Label' that is used by user-plane MPLS OAM functions for
   identification of MPLS OAM packets as described in the ITU-T
   Recommendation Y.1711 [1] (on MPLS OAM functions).

2. OAM functions

   MPLS OAM (Operation and Maintenance) functions provide necessary
   tools for network operators to operate and maintain the networks.
   MPLS OAM functionality is required at the MPLS layer, and more
   specifically at each MPLS level, independent of OAM functionality
   provided by the lower layers (SONET/SDH, etc.).  The objectives of
   the OAM functions include the following:

   -  Defect and failure detection: Defect/failures affecting the
      transport of user information are detected by continuous or
      periodic checking.  As a result, maintenance event information or
      appropriate alarms will be produced.

   -  Reporting the defect/failure information: Defect information is
      given to other management entities (e.g., Operation Support
      System) in order to provide the appropriate indications to the
      maintenance staff for maintaining the Quality of Service (QoS)
      level offered to customers.

   -  Defect/failure localization: Determination by internal or external
      test systems of a failed entity is performed if defect information
      is insufficient.

   -  Performance monitoring: Performance (packet losses, transfer
      delay, bit errors, etc.) of the user information transport is
      measured in order to estimate the transport integrity.

3. OAM Packet Identification

   The user-plane MPLS OAM mechanisms as described in the ITU-T
   Recommendation Y.1711 [1] uses a special label called 'OAM Alert
   Label' to differentiate OAM packets from the normal user packets.
   One of the reserved label values defined in RFC 3032 (MPLS label
   stack encoding [2]) is assigned to 'OAM Alert Label'.  A value of 14
   is used for this purpose.

4. MPLS OAM work in ITU-T SG13

   ITU-T Study Group 13, Question 3/13 is progressing work on user-plane
   MPLS OAM and has produced the following documents:

   (1) Recommendation Y.1710 (Requirements for OAM functionality for
       MPLS networks) [3]

   (2) Corrigendum 1 to Recommendation Y.1710 [4]

   (3) Recommendation Y.1711 (OAM mechanisms for MPLS networks) [1]

   (4) Draft Recommendation Y.1720 (Protection switching for MPLS
       networks) [6] relies on OAM mechanisms in Y.1711, under last call
       as of Nov. 2002.

5. Considerations on penultimate hop popping (PHP)

   In response to concerns raised during IETF meetings and in related
   discussions, this section provides an explanation on how MPLS OAM
   functions defined in ITU-T Recommendation Y.1711 [1] are applied to
   MPLS networks where PHP is in effect.

5.1 Scope of ITU-T Recommendation Y.1711

   The scope of ITU-T Recommendation Y.1711 includes application to both
   non-PHP and PHP cases as quoted below [1].

   "1 Scope
   This Recommendation provides mechanisms for user-plane OAM (Operation
   and Maintenance) functionality in MPLS networks according to the
   requirements and principles given in Recommendation Y.1710.  OAM
   functions specified in this Recommendation can be applied to both
   non-PHP and PHP cases unless otherwise stated.  The current version
   of this recommendation is designed primarily to support
   point-to-point and multipoint-to-point explicit routed LSPs
   (ER-LSPs)."

5.2 Applicability of MPLS OAM to PHP

   There are two cases where PHP is used:

   Case 1: The ultimate node is an MPLS LSR, and implements both MPLS
   control-plane and data-plane, but is not able to perform 2 lookups at
   line rate.  So it asks the penultimate node to pop the top label
   (rather than swapping it), using the MPLS reserved label 3 (implicit
   null label) as per defined in RFC 3032 [2].

   Case 2: The ultimate node has no MPLS label look up and processing
   capability and does not recognize labeled packets.  This node asks
   for PHP, using the MPLS reserved label 3 (implicit null label) as
   defined in RFC 3032 [2].

   Currently, MPLS OAM functions defined in ITU-T Recommendation Y.1711
   [1] can only be applied to Case 1.  The next subsection describes the
   node behavior in Case 1.  Application for Case 2 needs further study.
   Also, application to carrier supporting carrier scenarios is for
   future study.

5.3 Node behavior when OAM functions are activated

   Where the ultimate LSR is an MPLS LSR and PHP is in effect, the
   penultimate LSR pops the top label and forwards the OAM packet (with
   the OAM label and the OAM payload intact) to the ultimate LSR [5].

   -  If the ultimate LSR supports MPLS OAM, it understands that a
      received packet with an OAM label on top is an OAM packet, since
      the original top label has been removed by the penultimate LSR.
      It also knows the ingress LSR that originated the MPLS OAM packet
      from the TTSI (Trail Termination Source Identifier) value of the

      received MPLS OAM packet.  TTSI is a unique identifier for ingress
      LSR that is contained in MPLS OAM packets (see ITU-T
      Recommendation Y.1711 [1]).

   -  If the ultimate LSR does not support MPLS OAM, the OAM packet is
      discarded as per section 3.18 of RFC 3031 [5].

6. IANA Considerations

   The IANA has reserved the use of the MPLS label value of 14 as the
   'OAM Alert Label'.  See section 3 for additional information.

7. Security Considerations

   This document does not raise any security issues that are not already
   present in either the MPLS architecture or in the architecture of the
   network layer protocol contained within the encapsulation.

   OAM functions could enhance the security of MPLS networks.  For
   example, Connectivity Verification (CV) function defined in ITU-T
   Recommendation Y.1711 [1] can detect mis-connections, and therefore
   can prevent customers' traffic being exposed to other customers.

8. Acknowledgements

   The author wishes to thank Shahram Davari with PMC-Sierra, Neil
   Harrison with British Telecom, Monique Morrow, Thomas D. Nadeau, Hari
   Rakotoranto and Chip Sharp with Cisco Systems, Khalid Ahmad and David
   Allan with Nortel Networks, and Mina Azad with Azad-Mohtaj Consulting
   for their valuable contributions and discussions.

9. Normative References

   [1] ITU-T Recommendation Y.1711, "OAM mechanism for MPLS networks",
       November 2002.

   [2] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinaccia, D.,
       Li, T. and A. Conta, "MPLS label stack encoding", RFC 3032,
       January 2001.

   [3] ITU-T recommendation Y.1710, "Requirements for OAM functionality
       for MPLS networks" July 2001.

   [4] ITU-T Corrigendum 1 to Recommendation Y.1710, November 2002.

   [5] Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label
       Switching Architecture", RFC 3031, January 2001.

10. Informative Reference

   [6] ITU-T Draft Recommendation Y.1720, "Protection switching for MPLS
       networks", under last call as of November 2002.

11. Author's Address

   Hiroshi OHTA
   NTT
   3-9-11 Midori-Cho, Musashino-Shi
   Tokyo 180-8585 Japan

   Phone: +81 422 59 3617
   Fax:   +81 422 59 3787
   EMail: ohta.hiroshi@lab.ntt.co.jp

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