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RFC 6435 - MPLS Transport Profile Lock Instruct and Loopback Fun

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Internet Engineering Task Force (IETF)                   S. Boutros, Ed.
Request for Comments: 6435                             S. Sivabalan, Ed.
Updates: 6371                                        Cisco Systems, Inc.
Category: Standards Track                               R. Aggarwal, Ed.
ISSN: 2070-1721                                             Arktan, Inc.
                                                       M. Vigoureux, Ed.
                                                             X. Dai, Ed.
                                                         ZTE Corporation
                                                           November 2011

      MPLS Transport Profile Lock Instruct and Loopback Functions


   Two useful Operations, Administration, and Maintenance (OAM)
   functions in a transport network are "lock" and "loopback".  The lock
   function enables an operator to lock a transport path such that it
   does not carry client traffic, but can continue to carry OAM messages
   and may carry test traffic.  The loopback function allows an operator
   to set a specific node on the transport path into loopback mode such
   that it returns all received data.

   This document specifies the lock function for MPLS networks and
   describes how the loopback function operates in MPLS networks.

   This document updates Sections 7.1.1 and 7.1.2 of RFC 6371.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   Two useful Operations, Administration, and Maintenance (OAM)
   functions in a transport network are "lock" and "loopback".  This
   document discusses these functions in the context of MPLS networks.

   -  The lock function enables an operator to lock a transport path
      such that it does not carry client traffic.  As per RFC 5860 [1],
      lock is an administrative state in which it is expected that no
      client traffic may be carried.  However, test traffic and OAM
      messages can still be mapped onto the locked transport path.  The
      lock function may be applied to the Label Switched Paths (LSPs),
      Pseudowires (PWs) (including multi-segment Pseudowires) (MS-PWs),
      and bidirectional MPLS Sections as defined in RFC 5960 [9]).

   -  The loopback function allows an operator to set a specific node on
      a transport path into loopback mode such that it returns all
      received data.  Loopback can be applied at a Maintenance Entity
      Group End Point (MEP) or a Maintenance Entity Group Intermediate
      Point (MIP) on a co-routed bidirectional LSP, on a PW, or on a
      bidirectional MPLS Section.  It can also be applied at a MEP on an
      associated bidirectional LSP.

      Loopback is used to test the integrity of the transport path to
      and from the node that is performing loopback.  It requires that
      the transport path be locked and that a MEP on the transport path
      send test data that it also validates on receipt.

   This document specifies the lock function for MPLS networks and
   describes how the loopback function operates in MPLS networks.

1.1.  Updates RFC 6371

   This document updates Sections 7.1.1 and 7.1.2 of RFC 6371 [6].

   The framework in RFC 6371 makes the assumption that the Lock Instruct
   message is used to independently enable locking and requires a
   response message.

   The mechanism defined in this document requires that when a lock
   instruction is sent by management to both ends of the locked
   transport path, the Lock Instruct message does not require a

2.  Terminology and Conventions

2.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [2].

2.2.  Acronyms and Terms

   ACH: Associated Channel Header

   LI: Lock Instruct

   MEG: Maintenance Entity Group

   MEP: Maintenance Entity Group End Point

   MIP: Maintenance Entity Group Intermediate Point

   MPLS-TP: MPLS Transport Profile

   NMS: Network Management System

   TLV: Type Length Value

   Transport path: MPLS-TP LSP or PW

   TTL: Time To Live

3.  Lock Function

   Lock is used to request that a MEP take a transport path out of
   service for administrative reasons.  For example, Lock can be used to
   allow some form of maintenance to be done for a transport path.  Lock

   is also a prerequisite of the loopback function described in Section
   4.  The NMS or a management process initiates a Lock by sending a
   Lock command to a MEP.  The MEP takes the transport path out of
   service, that is, it stops injecting or forwarding traffic onto the
   transport path.

   To properly lock a transport path (for example, to ensure that a
   loopback test can be performed), both directions of the transport
   path must be taken out of service; therefore, a Lock command is sent
   to the MEPs at both ends of the path.  This ensures that no traffic
   is sent in either direction.  Thus, the lock function can be realized
   entirely using the management plane.

   However, dispatch of messages in the management plane to the two MEPs
   may present coordination challenges.  It is desirable that the lock
   be achieved in a coordinated way within a tight window, and this may
   be difficult with a busy management plane.  In order to provide
   additional coordination, an LI OAM message can also be sent.  A MEP
   locks a transport path when it receives a command from a management
   process or when it receives an LI message as described in Section 6.

   This document defines an LI message for MPLS OAM.  The LI message is
   based on a new ACH Type as well as an existing TLV.  This is a common
   mechanism applicable to lock LSPs, PWs, and bidirectional MPLS

4.  Loopback Function

   This section provides a description of the loopback function within
   an MPLS network.  This function is achieved through management
   commands, so there is no protocol specification necessary.  However,
   the loopback function is dependent on the lock function, so it is
   appropriate to describe it in this document.

   The loopback function is used to test the integrity of a transport
   path from a MEP up any other node in the same MEG.  This is achieved
   by setting the target node into loopback mode, and transmitting a
   pattern of test data from the MEP.  The target node loops all
   received data back toward the originator, and the MEP extracts the
   test data and compares it with what it sent.

   Loopback is a function that enables a receiving MEP or MIP to return
   traffic to the sending MEP when in the loopback state.  This state
   corresponds to the situation where, at a given node, a forwarding
   plane loop is configured, and the incoming direction of a transport
   path is cross-connected to the outgoing reverse direction.
   Therefore, except in the case of early TTL expiry, traffic sent by
   the source will be received by that source.

   Data-plane loopback is an out-of-service function, as required in
   Section 2.2.5 of RFC 5860 [1].  This function loops back all traffic
   (including user data and OAM).  The traffic can be originated from
   one internal point at the ingress of a transport path within an
   interface or inserted from an input port of an interface using
   external test equipment.  The traffic is looped back unmodified
   (other than normal per-hop processing such as TTL decrement) in the
   direction of the point of origin by an interface at either an
   intermediate node or a terminating node.

   It should be noted that the data-plane loopback function itself is
   applied to data-plane loopback points residing on different
   interfaces from MIPs/MEPs.  All traffic (including both payload and
   OAM) received on the looped back interface is sent on the reverse
   direction of the transport path.

   For data-plane loopback at an intermediate point in a transport path,
   the loopback needs to be configured to occur at either the ingress or
   egress interface.  This is done using management.

   The management plane can be used to configure the loopback function.
   The management plane must ensure that the two MEPs are locked before
   it requests setting MEP or MIP in the loopback state.

   The nature of test data and the use of loopback traffic to measure
   packet loss, delay, and delay variation are outside the scope of this

4.1.  Operational Prerequisites

   Obviously, for the loopback function to operate, there are several

   -  There must be a return path, so the transport path under test must
      be bidirectional.

   -  The node in loopback mode must be on both the forward and return
      paths.  This is possible for all MEPs and MIPs on a co-routed
      bidirectional LSP, on a PW, or on a bidirectional MPLS Section,
      but it is only possible for MEPs on associated bidirectional LSPs.

   -  The transport path cannot deliver client data when one of its
      nodes is in loopback mode, so it is important that the transport
      path be locked before loopback is enabled.

   -  Management-plane coordination between the node in loopback mode
      and the MEP sending test data is required.  The MEP must not send
      test data until loopback has been properly configured because this
      would result in the test data continuing toward the destination.

   -  The TTL of the test packets must be set sufficiently large to
      account for both directions of the transport path under test;
      otherwise, the packets will not be returned to the originating

   -  OAM messages intended for delivery to nodes along the transport
      path under test can be delivered by correct TTL expiry.  However,
      OAM messages cannot be delivered to points beyond the loopback
      node until the loopback condition is lifted.

5.  Lock Instruct Message

5.1.  Message Identification

   The Lock Instruct message is carried in the Generic ACH described in
   [4].  It is identified by a new PW ACH Type of 0x0026.

5.2.  LI Message Format

   The format of an LI message is shown 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
   | Vers  | Reserved                              | Refresh Timer |
   |                        MEP Source ID TLV                      |
   ~                                                               ~

            Figure 1: MPLS Lock Instruct Message Format

   Version: The Version number is currently 1.  (Note: the version
   number is to be incremented whenever a change is made that affects
   the ability of an implementation to correctly parse or process the
   message.  These changes include any syntactic or semantic changes
   made to any of the fixed fields, or to any Type-Length-Value (TLV) or
   sub- TLV assignment or format that is defined at a certain version
   number.  The version number may not need to be changed if an optional
   TLV or sub-TLV is added.)

   Reserved: The Reserved field MUST be set to zero on transmission and
   SHOULD be ignored on receipt.

   Refresh Timer: The Refresh Timer is the maximum time between
   successive LI messages specified in seconds.  The default value is 1.
   The value 0 is not permitted.  When a lock is applied, a refresh
   timer is chosen.  This value MUST NOT be changed for the duration of
   that lock.  A node receiving an LI message with a changed refresh
   timer MAY ignore the new value and continue to apply the old value.

   MEP Source ID TLV: This is one of the three MEP Source ID TLVs
   defined in [3] and identifies the MEP that originated the LI message.

6.  Operation of the Lock Function

6.1.  Locking a Transport Path

   When a MEP receives a Lock command from an NMS or through some other
   management process, it MUST take the transport path out of service.
   That is, it MUST stop injecting or forwarding traffic onto the LSP,
   PW, or bidirectional Section that has been locked.

   If rapid coordination of lock state is to be achieved (as described
   in Section 3) then as soon as the transport path has been locked, the
   MEP MUST send an LI message targeting the MEP at the other end of the
   locked transport path. In this case, the source MEP MUST set the
   Refresh Timer value in the LI message and MUST retransmit the LI
   message at the frequency indicated by the value set.

   When locking a transport path, the NMS or management process is
   required to send a Lock command to both ends of the transport path.
   Thus, a MEP may receive either the management command or an LI
   message first.  A MEP MUST take the transport path out of service
   immediately in either case, but sends LI messages itself after it has
   received a management Lock command.  Thus, a MEP is locked if either
   Lock was requested by management (and, as a result, the MEP is
   sending LI messages) or it is receiving LI messages from the remote

   Note that a MEP that receives an LI message MUST identify the correct
   transport path and validate the message.  The label stack on the
   received message is used to identify the transport path to be locked:

   -  If no matching label binding exists, then there is no
      corresponding transport path and the received LI message is in

   -  If the transport path can be identified, but there is no return
      path (for example, the transport path was unidirectional) then
      (obviously) the receiving MEP cannot apply a lock to the return

   -  If the transport path is suitable for locking but the Source MEP-
      ID identifies an unexpected MEP for the MEG to which the receiving
      MEP belongs, the received LI message is in error.

   When an errored LI message is received, the receiving MEP MUST NOT
   apply a lock.  A MEP receiving errored LI messages SHOULD perform
   local diagnostic actions (such as counting the messages) and MAY log
   the messages.

   A MEP keeps a transport path locked as long as it is either receiving
   the periodic LI messages or has an in-force Lock command from
   management (see Section 6.2 for an explanation of unlocking a MEP).
   Note that in some scenarios (such as the use of loopback as described
   in Section 4), LI messages will not continue to be delivered on a
   locked transport path.  This is why a transport path is considered
   locked while there is an in-force Lock command from a management
   process regardless of whether LI messages are being received.

6.2.  Unlocking a Transport Path

   Unlock is used to request that a MEP bring the previously locked
   transport path back in service.

   When a MEP receives an Unlock command from a management process, it
   MUST cease sending LI messages.  However, as described in Section
   6.1, if the MEP is still receiving LI messages, the transport path
   MUST remain out of service.  Thus, to unlock a transport path, the
   management process has to send an Unlock command to the MEPs at both

   When a MEP has been unlocked and has not received an LI message for a
   multiple of 3.5 times the Refresh Timer on the LI message (or has
   never received an LI message), the MEP unlocks the transport path and
   puts it back into service.

7.  Security Considerations

   MPLS-TP is a subset of MPLS and builds upon many of the aspects of
   the security model of MPLS.  MPLS networks make the assumption that
   it is very hard to inject traffic into a network, and it is equally
   hard to cause traffic to be directed outside the network.  For more
   information on the generic aspects of MPLS security, see [7].

   This document describes a protocol carried in the G-ACh [4], so it is
   dependent on the security of the G-ACh, itself.  The G-ACh is a
   generalization of the Pseudowire Associated Channel defined in [8].
   Thus, this document relies heavily on the security mechanisms
   provided for the Associated Channel as described in [4] and [8].

   A specific concern for the G-ACh is that is can be used to provide a
   covert channel.  This problem is wider than the scope of this
   document and does not need to be addressed here, but it should be
   noted that the channel provides end-to-end connectivity and SHOULD
   NOT be policed by transit nodes.  Thus, there is no simple way to
   prevent traffic from being carried in the G-ACh between consenting

   A good discussion of the data-plane security of an associated channel
   may be found in [5].  That document also describes some mitigation

   It should be noted that the G-ACh is essentially connection-oriented,
   so injection or modification of control messages specified in this
   document requires the subversion of a transit node.  Such subversion
   is generally considered hard in MPLS networks, and impossible to
   protect against at the protocol level.  Management-level techniques
   are more appropriate.

8.  IANA Considerations

8.1.  Pseudowire Associated Channel Type

   LI OAM requires a unique Associated Channel Type that has been
   assigned by IANA in the "Pseudowire Associated Channel Types"

      Value        Description              TLV Follows  Reference
      -----------  -----------------------  -----------  ---------
      0x0026       LI                       No           [RFC6435]

9.  Acknowledgements

   The authors would like to thank Loa Andersson, Yoshinori Koike,
   Alessandro D'Alessandro Gerardo, Shahram Davari, Greg Mirsky, Yaacov
   Weingarten, Liu Guoman, Matthew Bocci, Adrian Farrel, and Jia He for
   their valuable comments.

10.  References

10.1.  Normative References

   [1] Vigoureux, M., Ed., Ward, D., Ed., and M. Betts, Ed.,
       "Requirements for Operations, Administration, and Maintenance
       (OAM) in MPLS Transport Networks", RFC 5860, May 2010.

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

   [3] Allan, D., Ed., Swallow, G., Ed., and J. Drake, Ed., "Proactive
       Connectivity Verification, Continuity Check, and Remote Defect
       Indication for the MPLS Transport Profile", RFC 6428, November

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

   [5] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire Virtual
       Circuit Connectivity Verification (VCCV): A Control Channel for
       Pseudowires", RFC 5085, December 2007.

   [6] Busi, I., Ed., and D. Allan, Ed., "Operations, Administration,
       and Maintenance Framework for MPLS-Based Transport Networks", RFC
       6371, September 2011.

10.2.  Informative References

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

   [8] 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.

   [9] Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
       Transport Profile Data Plane Architecture", RFC 5960, August

Contributing Authors

   George Swallow
   Cisco Systems, Inc.
   EMail: swallow@cisco.com

   David Ward
   Juniper Networks.
   EMail: dward@juniper.net

   Stewart Bryant
   Cisco Systems, Inc.
   EMail: stbryant@cisco.com

   Carlos Pignataro
   Cisco Systems, Inc.
   EMail: cpignata@cisco.com

   Eric Osborne
   Cisco Systems, Inc.
   EMail: eosborne@cisco.com

   Nabil Bitar
   EMail: nabil.bitar@verizon.com

   Italo Busi
   EMail: italo.busi@alcatel-lucent.com

   Lieven Levrau
   EMail: lieven.levrau@alcatel-lucent.com

   Laurent Ciavaglia
   EMail: laurent.ciavaglia@alcatel-lucent.com

   Bo Wu
   ZTE Corporation.
   EMail: wu.bo@zte.com.cn

   Jian Yang
   ZTE Corporation.
   EMail: yang_jian@zte.com.cn

Editors' Addresses

   Sami Boutros
   Cisco Systems, Inc.
   EMail: sboutros@cisco.com

   Siva Sivabalan
   Cisco Systems, Inc.
   EMail: msiva@cisco.com

   Rahul Aggarwal
   Arktan, Inc
   EMail: raggarwa_1@yahoo.com

   Martin Vigoureux
   EMail: martin.vigoureux@alcatel-lucent.com

   Xuehui Dai
   ZTE Corporation.
   EMail: dai.xuehui@zte.com.cn


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