Network Working Group                                 Sami Boutros (Ed.)
Internet Draft                                      Siva Sivabalan (Ed.)
Intended status: Standards Track                     Cisco Systems, Inc.
Updates: 6371 (if approved)
Expires: April 24, 2012                             Rahul Aggarwal (Ed.)
                                                            Arktan, Inc.

                                                  Martin Vigoureux (Ed.)

                                                        Xuehui Dai (Ed.)
                                                         ZTE Corporation

                                                        October 24, 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 RFC 6371 section 7.1.1 and 7.1.2.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   The list of current Internet-Drafts can be accessed at

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   The list of Internet-Draft Shadow Directories can be accessed 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
   ( in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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 to 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 an
     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 is locked and that a MEP on the transport path sends test
     data which it also validates on receipt.

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

   This document updates RFC 6371 section 7.1.1 [6].

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1.1. Updates RFC 6371

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

   That framework 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 a lock
   instruction is sent by management to both ends of the locked
   transport path and that 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

   MEG: Maintenance Entity Group

   MEP: Maintenance Entity Group End Point

   MIP: Maintenance Entity Group Intermediate Point

   MPLS-TP: MPLS Transport Profile

   MPLS-TP LSP: Bidirectional Label Switch Path

   TLV: Type Length Value

   TTL: Time To Live

   LI: Lock Instruct

   NMS: Network Management System

   Transport path: MPLS-TP LSP or MPLS PW

3. Lock Function

   Lock is used to request a MEP to 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.

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   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

   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 so 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 additionally 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 and so there is no protocol specification necessary.
   However, the Loopback function is dependent on the Lock function and
   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.

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   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 input port of an interface using an 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 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 is 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 transport path under test must be

   - 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
     is only possible on 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 is
     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

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     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 MEP.

   - 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 Associated Channel Header
   (ACh) described in [4]. it is identified by a new PW ACh Type of 0xHH
   (to be assigned by IANA).

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 maximum time between successive LI messages

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   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 a 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.

   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. 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 only 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 error.

   - If the transport path can be identified, but there is no return
     path (for example, the transport path was unidirectional) then the
     lock cannot be applied by the receiving MEP.

   - 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.

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   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 a MEP to bring the previously locked
   transport path back in service.

   When a MEP receives an Unlocked 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 ends.

   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 so 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 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], and so
   is dependent on the security of the G-ACh, itself. The G-ACh is a
   generalization of the Associated Channel defined in [8]. Thus, this
   document relies heavily on the security mechanisms provided for the
   Associated Channel and 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

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   noted that the channel provides end-to-end connectivity and SHOULD
   NOT be policed by transit nodes. Thus, there is no simple way of
   preventing any traffic 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 require 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

8. IANA Considerations

8.1. Pseudowire Associated Channel Type

   LI OAM requires a unique Associated Channel Type which is assigned by
   IANA from the Pseudowire Associated Channel Types Registry.

      Value        Description              TLV Follows  Reference
      -----------  -----------------------  -----------  ---------
      0xHH         LI                       No           [This.I-D]

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, and Adrian Farrel for their
   valuable comments.

10. References

10.1. Normative References

   [1]   Vigoureux, M., Ward, D., and M. Betts, "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]   D. Allan, et. al., Proactive Connectivity Verification,
         Continuity Check and Remote Defect indication for MPLS

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         Transport Profile draft-ietf-mpls-tp-cc-cv-rdi-06, work in
         progress, June 2010

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

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

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

10.2. Informative References

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

   [8]   S. Bryant, G. Swallow, L. Martini "Pseudowire Emulation Edge-
         to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC
         4385, Feb 2006.

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

Editors' Addresses

   Sami Boutros
   Cisco Systems, Inc.

   Siva Sivabalan
   Cisco Systems, Inc.

   Rahul Aggarwal
   Arktan, Inc

   Martin Vigoureux

   Xuehui Dai
   ZTE Corporation.

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Contributing Authors

   George Swallow
   Cisco Systems, Inc.

   David Ward
   Juniper Networks.

   Stewart Bryant
   Cisco Systems, Inc.

   Carlos Pignataro
   Cisco Systems, Inc.

   Eric Osborne
   Cisco Systems, Inc.

   Nabil Bitar

   Italo Busi

   Lieven Levrau

   Laurent Ciavaglia

   Bo Wu
   ZTE Corporation.

   Jian Yang
   ZTE Corporation.

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