MPLS Working Group                                       Dave Allan, Ed.
Internet Draft                                                 Ericsson
Intended status: Standards Track
Expires: November 2010                                George Swallow Ed.
                                                      Cisco Systems, Inc

                                                          John Drake Ed.

                                                            May 11, 2010

      Proactive Connection Verification, Continuity Check and Remote
               Defect indication for MPLS Transport Profile


   Continuity Check (CC), Proactive Connectivity Verification (CV) and
   Remote Defect Indication (RDI) functionalities required for are MPLS-
   TP OAM.

   Continuity Check monitors the integrity of the continuity of the path
   for any loss of continuity defect. Connectivity verification monitors
   the integrity of the routing of the path between sink and source for
   any connectivity issues. RDI enables an End Point to report, to its
   associated End Point, a fault or defect condition that it detects on
   a PW, LSP or Section.

   This document specifies methods for proactive CV, CC, and RDI for
   MPLS-TP Label Switched Path (LSP), PWs and Sections using
   Bidirectional Forwarding Detection (BFD).

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [1].

Status of this Memo

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

   Internet-Drafts are working documents of the Internet
   Engineering Task Force (IETF), its areas, and its working

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   groups.  Note that other groups may also distribute working
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Table of Contents

   1. Introduction..........................................3
   1.1. Authors.............................................3
   2. Conventions used in this document........................4
   2.1. Terminology.........................................4
   2.2. Issues for discussion.................................4
   3. MPLS CC, proactive CV and RDI Mechanism using BFD...........5
   3.1. ACH code points for CC and proactive CV..................5
   3.2. MPLS BFD CC Message format.............................6
   3.3. MPLS BFD proactive CV Message format....................6
   3.4. BFD Session in MPLS-TP terminology......................7
   3.5. BFD Profile for MPLS-TP...............................7
   3.5.1. Session initiation..................................8
   3.5.2. Defect entry criteria...............................8
   3.5.3. Defect entry consequent action .......................9
   3.5.4. Defect exit criteria................................9
   3.5.5. Configuration of MPLS-TP BFD sessions.................10

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   3.5.6. Discriminator values...............................10
   4. Acknowledgments.......................................10
   5. IANA Considerations...................................10
   6. Security Considerations................................11
   7. References...........................................11
   7.1. Normative References.................................11
   7.2. Informative References...............................11

1. Introduction

   In traditional transport networks, circuits are provisioned on two or
   more switches. Service Providers (SP) need OAM tools to detect mis-
   connectivity and loss of continuity of transport circuits. Both PWs
   and MPLS-TP LSPs [6] emulating traditional transport circuits need to
   provide the same CC and proactive CV capabilities as required in
   draft-ietf-mpls-tp-oam-requirements[3]. This document describes the
   use of BFD for CC, proactive CV, and RDI of a PW, LSP or PST between
   two Maintenance Entity Group End Points (MEPs).

   As described in [8], Continuity Check (CC) and Proactive Connectivity
   Verification (CV) functions are used to detect loss of continuity
   (LOC), and unintended connectivity between two MEPs (e.g. mismerging
   or misconnection or unexpected MEP).

   The Remote Defect Indication (RDI) is an indicator that is
   transmitted by a MEP to communicate to its peer MEP that a signal
   fail condition exists. RDI is only used for bidirectional connections
   and is associated with proactive CC & CV packet generation.

   This document specifies the BFD extension and behavior to satisfy the
   CC, proactive CV monitoring and the RDI functional requirements for
   bi-directional paths. Procedures for uni-directional paths are for
   further study.

   The mechanisms specified in this document are restricted to BFD
   asynchronous mode.

1.1. Authors

David Allan, John Drake, George Swallow, Annamaria Fulignoli, Sami
Boutros, Siva Sivabalan, David Ward, Martin Vigoureux.

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2. Conventions used in this document

2.1. Terminology

ACH: Associated Channel Header

BFD: Bidirectional Forwarding Detection

CV: Connection Verification

GAL: Generalized Alert Label

LSR: Label Switching Router

MEG: Maintenance Entity Group

MEP: Maintenance Entity Group End Point

MIP: Maintenance Entity Group Intermediate Point

MPLS-OAM: MPLS Operations, Administration and Maintenance

MPLS-TP: MPLS Transport Profile

MPLS-TP LSP: Uni-directional or Bidirectional Label Switch Path
representing a circuit

MS-PW: Multi-Segment PseudoWire

NMS: Network Management System

PW: Pseudo Wire

RDI: Remote Defect Indication.

TTL: Time To Live

TLV: Type Length Value

2.2. Issues for discussion

   1) Requirement for additional BFD diagnostic codes

              1. When periodicity of CV cannot be supported

              2. For mis-connectivity defect

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   2) Do we continue to separate CC and CV as separate functions, or
      collapse them into a single CC+CV behavior given CV is a superset
      of CC?

   3) Is receipt of an unexpected discriminator really a problem?

3. MPLS CC, proactive CV and RDI Mechanism using BFD

   This document proposes distinct encapsulations and code points for
   BFD depending on whether the mode of operation is CC or CV:

  o  CC mode: defines a new code point in the Associated Channel Header
     (ACH) described in [2].In this mode Continuity Check and RDI
     functionalities are supported.

  o  CV mode: defines a new code point in the Associated Channel Header
     (ACH) described in [2]. Under MPLS label stack, the ACH with "MPLS
     Proactive CV" code point indicates that the message is an MPLS BFD
     proactive CV and CC message.

  o  RDI: is communicated via the BFD state field in BFD CC and CV
     messages. It is not a distinct PDU.

3.1. ACH code points for CC and proactive CV

    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
   |0 0 0 1|Version|     Flags     |0xHH   BFD CC/CV Code Point    |

       Figure 1: ACH Indication of MPLS-TP Connection Verification

   The first nibble (0001b) indicates the ACH.

   The version and the flags are set to 0 as specified in [2].

   The code point is either

   - BFD CC code point = 0xHH. [HH to be assigned by IANA from the PW
   Associated Channel Type registry.] or,

   - BFD proactive CV code point = 0xHH. [HH to be assigned by IANA from
   the PW Associated Channel Type registry.]

   Both CC and CV modes apply to PWs, MPLS LSPs (including tandem
   connection monitoring), and Sections.

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   It's possible to run BFD in CC mode on some transport paths and BFD
   in CV mode on other transport paths. For a given Maintenance Entity
   Group (MEG) only one mode can be used.  A MEP that is configured to
   support CC mode and receives CV BFD packets, or vice versa, MUST
   consider them as an unexpected packet, i.e. detect a mis-connectivity

3.2. MPLS BFD CC Message format

   The format of an MPLS CC Message format 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
   |0 0 0 1|Version|     Flags     |    0xHH BFD CC Code point     |
   |                                                               |
   ~                  BFD Control Packet                           ~
   |                                                               |
                     Figure 2: MPLS CC Message

3.3. MPLS BFD proactive CV Message format

   The format of an MPLS CV Message format is shown below, ACH TLVs [5]
   MUST precede the BFD control packet.

    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
   |0 0 0 1|Version|     Flags     |    0xHH  BFD CV Code Point    |
   |                    ACH TLV Header                             |
   |                                                               |
   ~          Unique MEP-ID of source of the BFD packet            ~
   |                                                               |
   |                                                               |
   ~                  BFD Control Packet                           ~
   |                                                               |

                     Figure 3: MPLS CV Message

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   As shown in Figure 3, BFD Control packet as defined in [4] is
   transmitted as MPLS labeled packets along with ACH, ACH TLV Header
   defined in Section 3 of RFC 5586 and one ACH TLV object carrying the
   unique MEP Identifier of the source of the BFD packet defined in [7]

   When GAL label is used, the TTL field of the GAL MUST be set to at
   least 1, and the GAL will be the end of stack label (S=1).

3.4. BFD Session in MPLS-TP terminology

   A BFD session corresponds to a CC or a proactive CV OAM instance in
   MPLS-TP terminology.

   A BFD session is enabled when the CC or proactive CV functionality is
   enabled on a configured Maintenance Entity (ME) or in the case of an
   associated bi-directional path, pair of Maintenance Entities.

   On a Sink MEP, a BFD session can be in DOWN, INIT or UP state as
   detailed in [4].

   When on a ME the CC or proactive CV functionality is disabled, the
   BFD session transitions to the ADMIN DOWN State and the BFD session

   A new BFD session is initiated when the operator enables or re-
   enables the CC or CV functionality on the same ME.

3.5. BFD Profile for MPLS-TP

   BFD MUST operate in asynchronous mode. In this mode, the BFD Control
   packets are periodically sent at configurable time rate. This rate is
   typically a fixed value for the lifetime of the session. In the rare
   circumstance where an operator has a reason to change session
   parameters, poll/final discipline is used.

   The transport profile is designed to operate independent of the
   control plane; hence the C bit SHOULD be set.

   This document specifies bi-directional BFD for p2p transport paths,
   hence the M bit MUST be clear.

   There are two modes of operation for bi-directional paths. One in
   which both directions of the path fate share and one constructed from
   BFD sessions in such a way that the two directions operate
   independently. A single bi-directional BFD session is used for fate
   sharing operation. Two independent BFD sessions are used for
   independent operation.

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   Fate sharing operation is as described in [4]. Independent operation
   requires clarification of two aspects of [4]. Independent operation
   is characterized by the setting of MinRxInterval to zero by the MEP
   that is typically the session originator, and there will be a session
   originator at either end of the bi-directional path.

   The base spec is unclear on aspects of how a session with a BFD
   source set to zero interval behaves. One interpretation is that no
   periodic messages originate with that source, it will only originate
   messages on a state change.

   The first clarification is that when a state change occurs a zero
   interval source send BFD control messages with a one second period
   until such time that the state change is confirmed by the session
   peer. At this point the zero interval source can resume quiescent
   behavior. This adds robustness to all state transitions in the
   RxInterval=0 case.

   The second is that the originating MEP (the one with a non-zero
   TxInterval) will ignore a DOWN state received from a zero interval
   peer. This means that the zero interval peer will continue to send
   DOWN state messages as the state change is never confirmed. This adds
   robustness to the exchange of RDI indication on a uni-directional
   failure (for both session types DOWN with a diagnostic of control
   detection period expired offering RDI functionality).

   The normal usage is that 1:1 protected paths must use fate sharing,
   and independent operation applies to 1+1 protected paths.

3.5.1. Session initiation

   In all scenarios a BFD session starts with both ends in the DOWN
   state. DOWN state messages exchanged include the desired Tx and Rx
   rates for the session. If a node cannot support the Min Tx rate
   desired by a peer MEP it does not transition from down to the INIT
   state and sends a diagnostic code (TBD) indicating that the requested
   Tx rate cannot be supported.

   Otherwise once a transition from DOWN to INIT has occurred, the
   session progresses as per [4].

3.5.2. Defect entry criteria

   There are further defect criteria beyond that defined in [4] to
   consider given the possibility of mis-connectivity and mis-
   configuration defects. The result is the criteria for a path
   direction to transition from the defect free state to a defect state
   is a superset of that in the BFD base specification [4].

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   The following conditions case a MEP to enter the defect state:
     1. BFD session times out (Loss of Continuity defect),
     2. BFD control packets are received with an unexpected
        encapsulation (Mis-connectivity defect), these include
          - a PW receiving a packet with a GAL
          - an LSP receiving an IP header instead of a GAL
          (note there are other possibilities but these can also alias
     3. Receipt of an unexpected globally unique Source MEP identifier
        (Mis-connectivity defect),
     4. Receipt of an unexpected session discriminator (Mis-connectivity
     5. Receipt of an unexpected M bit (Session Mis-configuration
   The effective defect hierarchy (order of checking) is

     1. Receiving nothing

     2. Receiving from an incorrect source (determined by whatever

     3. Receiving from a correct source (as near as can be determined),
        but with incorrect session information)

     4. Receiving control packets in all discernable ways correct.

3.5.3. Defect entry consequent action

   Upon defect entry a sink MEP will assert signal fail into any client
   (sub-)layers. It will also communicate session DOWN to its session

   The blocking of traffic as consequent action MUST be driven only by a
   defect's consequent action as specified in draft-ietf-mpls-tp-oam-
   framework Error! Reference source not found. section
   When the defect is mis-braching, the transport path termination will
   silently discard all non-oam traffic received.

3.5.4. Defect exit criteria

   Exit from a Loss of continuity defect

   For a fate sharing session exit from a loss of connectivity defect is
   as described in [4].

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   For an independent session, exit from a loss of connectivity defect
   occurs upon receipt of a well formed control packet from the peer

   Exit from a session mis-configuration defect

   [editors: for a future version of the document]

   Exit from a mis-connectivity defect

   The exit criteria for a mis-connectivity defect is determined by the
   maximum of the set of min Rx session time times the multiplier that
   have been received. A session can transition from DOWN to UP
   (independent mode) or DOWN to INIT (fate sharing mode) when both
   correctly formed control packets are being exchanged, and no mis-
   connected control packets have been received in the specified

3.5.5. Configuration of MPLS-TP BFD sessions

   [Editors note, for a future revision of the document]

3.5.6. Discriminator values

   MPLS labels at peer MEPs are used to provide context for the received
   BFD packets.

   In the BFD control packet the discriminator values have either local
   or no significance.

   My Discriminator field MUST be set to a nonzero value (it can be a
   fixed value), the transmitted your discriminator value MUST reflect
   back the received value of My discriminator field or be set to 0 if
   that value is not known.

4. Acknowledgments

   To be added in a later version of this document

5. IANA Considerations

   To be added in a later version of this document

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6. Security Considerations

   The security considerations for the authentication TLV need further

   Base BFD foresees an optional authentication section (see [4]
   section 6.7); that can be extended also to the tool proposed in
   this document.

   Authentication methods that require checksum calculation on the
   outgoing packet must extend the checksum also on the ME
   Identifier Section. This is possible but seems uncorrelated with
   the solution proposed in this document: it could be better to
   use the simple password authentication method.

7. References

7.1. Normative References

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

  [2]   Bocci, M. et al., " MPLS Generic Associated Channel ", RFC
        5586 , June 2009

  [3]   Vigoureux, M., Betts, M. and D. Ward, "Requirements for
        OAM in MPLS Transport Networks", draft-ietf-mpls-tp-oam-
        requirements-06 (work in progress), March 2010

  [4]   Katz, D. and D. Ward, "Bidirectional Forwarding
        Detection", draft-ietf-bfd-base-11 (work in progress),
        February 2009

  [5]   Boutros, S. et al., "Definition of ACH TLV Structure",
        draft-ietf-mpls-tp-ach-tlv-02 (work in progress), March

7.2. Informative References

  [6]   Bocci, M., et al., "A Framework for MPLS in Transport
        Networks", draft-ietf-mpls-tp-framework-12, (work in
        progress), May 2010

  [7]   Bocci, M. and G. Swallow, "MPLS-TP Identifiers", draft-
        swallow-mpls-tp-identifiers-02 (work in progress), March

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  [8]   Allan, D., Busi, I. and B. Niven-Jenkins, "MPLS-TP OAM
        Framework", draft-ietf-mpls-tp-oam-framework-06 (work in
        progress), April 2010

   Authors' Addresses

   Dave Allan

   John Drake

   George Swallow
   Cisco Systems, Inc.

   Annamaria Fulignoli

   Sami Boutros
   Cisco Systems, Inc.

   Martin Vigoureux

   Siva Sivabalan
   Cisco Systems, Inc.

   David Ward

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