path: root/doc/rfc/rfc6428.txt

Internet Engineering Task Force (IETF)                     D. Allan, Ed.
Request for Comments: 6428                                      Ericsson
Category: Standards Track                                G. Swallow, Ed.
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                           J. Drake, Ed.
                                                                 Juniper
                                                           November 2011


       Proactive Connectivity Verification, Continuity Check, and
        Remote Defect Indication for the MPLS Transport Profile

Abstract

   Continuity Check, Proactive Connectivity Verification, and Remote
   Defect Indication functionalities are required for MPLS Transport
   Profile (MPLS-TP) Operations, Administration, and Maintenance (OAM).

   Continuity Check monitors a Label Switched Path for any loss of
   continuity defect.  Connectivity Verification augments Continuity
   Check in order to provide confirmation that the desired source is
   connected to the desired sink.  Remote Defect Indication enables an
   end point to report, to its associated end point, a fault or defect
   condition that it detects on a pseudowire, Label Switched Path, or
   Section.

   This document specifies specific extensions to Bidirectional
   Forwarding Detection (BFD) and methods for proactive Continuity
   Check, Continuity Verification, and Remote Defect Indication for
   MPLS-TP pseudowires, Label Switched Paths, and Sections using BFD as
   extended by this memo.

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
   http://www.rfc-editor.org/info/rfc6428.






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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.

Table of Contents

   1. Introduction ....................................................3
   2. Conventions Used in This Document ...............................3
      2.1. Terminology ................................................3
      2.2. Requirements Language ......................................5
   3. MPLS-TP CC, Proactive CV, and RDI Mechanism Using BFD ...........5
      3.1. Existing Capabilities ......................................5
      3.2. CC, CV, and RDI Overview ...................................5
      3.3. ACH Code Points for CC and Proactive CV ....................6
      3.4. MPLS-TP BFD CC Message Format ..............................7
      3.5. MPLS-TP BFD Proactive CV Message Format ....................8
           3.5.1. Section MEP-ID ......................................9
           3.5.2. LSP MEP-ID ..........................................9
           3.5.3. PW End Point MEP-ID ................................10
      3.6. BFD Session in MPLS-TP Terminology ........................10
      3.7. BFD Profile for MPLS-TP ...................................11
           3.7.1. Session Initiation and Modification ................12
           3.7.2. Defect Entry Criteria ..............................13
           3.7.3. Defect Entry Consequent Action .....................14
           3.7.4. Defect Exit Criteria ...............................14
           3.7.5. State Machines .....................................15
           3.7.6. Configuration of MPLS-TP BFD Sessions ..............17
           3.7.7. Discriminator Values ...............................17
   4. Configuration Considerations ...................................18
   5. IANA Considerations ............................................18
   6. Security Considerations ........................................19
   7. References .....................................................19
      7.1. Normative References ......................................19
      7.2. Informative References ....................................20
   8. Acknowledgments ................................................20
   9. Contributing Authors ...........................................21




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1.  Introduction

   In traditional transport networks, circuits are provisioned on two or
   more switches.  Service providers need Operations, Administration,
   and Maintenance (OAM) tools to detect mis-connectivity and loss of
   continuity of transport circuits.  Both pseudowires (PWs) and MPLS-TP
   Label Switched Paths (LSPs) [12] emulating traditional transport
   circuits need to provide the same Continuity Check (CC), proactive
   Continuity Verification (CV), and Remote Defect Indication (RDI)
   capabilities as required in RFC 5860 [3].  This document describes
   the use of Bidirectional Forwarding Detection (BFD) [4] for CC,
   proactive CV, and RDI of a PW, LSP, or Sub-Path Maintenance Entity
   (SPME) between two Maintenance Entity Group End Points (MEPs).

   As described in RFC 6371 [13], CC and CV functions are used to detect
   loss of continuity (LOC) and unintended connectivity between two MEPs
   (e.g., mis-merging or mis-connectivity or unexpected MEP).

   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 LSPs and is associated with proactive CC and CV BFD
   control packet generation.

   This document specifies the BFD extension and behavior to satisfy the
   CC, proactive CV monitoring, and the RDI functional requirements for
   both co-routed and associated bidirectional LSPs.  Supported
   encapsulations include Generic Associated Channel Label (GAL) /
   Generic Associated Channel (G-ACh), Virtual Circuit Connectivity
   Verification (VCCV), and UDP/IP.  Procedures for unidirectional
   point-to-point (P2P) and point-to-multipoint (P2MP) LSPs are for
   further study.

   This document utilizes identifiers for MPLS-TP systems as defined in
   RFC 6370 [9].  Work is ongoing in the ITU-T to define a globally-
   unique semantic for ITU Carrier Codes (ICCs), and future work may
   extend this document to utilize ICCs as identifiers for MPLS-TP
   systems.

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

2.  Conventions Used in This Document

2.1.  Terminology

   ACH: Associated Channel Header

   BFD: Bidirectional Forwarding Detection



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   CC: Continuity Check

   CV: Connectivity Verification

   GAL: Generic Associated Channel Label

   G-ACh: Generic Associated Channel

   LDI: Link Down Indication

   LKI: Lock Instruct

   LKR: Lock Report

   LSP: Label Switched Path

   LSR: Label Switching Router

   ME:  Maintenance Entity

   MEG: Maintenance Entity Group

   MEP: Maintenance Entity Group End Point

   MIP: Maintenance Entity Group Intermediate Point

   MPLS: Multiprotocol Label Switching

   MPLS-OAM: MPLS Operations, Administration and Maintenance

   MPLS-TP: MPLS Transport Profile

   MPLS-TP LSP: Unidirectional or bidirectional Label Switched Path
   representing a circuit

   MS-PW: Multi-Segment Pseudowire

   NMS: Network Management System

   OAM: Operations, Administration, and Maintenance [14]

   PW: Pseudowire

   PDU: Protocol Data Unit

   P/F: Poll/Final

   RDI: Remote Defect Indication



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   SPME: Sub-Path Maintenance Entity

   TTL: Time To Live

   TLV: Type Length Value

   VCCV: Virtual Circuit Connectivity Verification

2.2.   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 RFC 2119 [1].

3.  MPLS-TP CC, Proactive CV, and RDI Mechanism Using BFD

   This document describes procedures for achieve combined CC, CV, and
   RDI functionality within a single MPLS-TP MEG using BFD.  This
   augments the capabilities that can be provided for MPLS-TP LSPs using
   existing specified tools and procedures.

3.1.  Existing Capabilities

   A CC-only mode may be provided via protocols and procedures described
   in RFC 5885 [7] with ACH channel 7.  These procedures may be applied
   to bidirectional LSPs (via the use of the GAL) as well as PWs.

   Implementations may also interoperate with legacy equipment by
   implementing RFC 5884 [8] for LSPs and RFC 5085 [10] for PWs, in
   addition to the procedures documented in this memo.  In accordance
   with RFC 5586 [2], when BFD control packets are encapsulated in an IP
   header, the fields in the IP header are set as defined in RFC 5884
   [8].  When IP encapsulation is used, CV mis-connectivity defect
   detection can be performed by inferring a globally unique source on
   the basis of the combination of the source IP address and My
   Discriminator fields.

3.2.  CC, CV, and RDI Overview

   The combined CC, CV, and RDI functionality for MPLS-TP is achieved by
   multiplexing CC and CV PDUs within a single BFD session.  The CV PDUs
   are augmented with a Source MEP-ID TLV to permit mis-connectivity
   detection to be performed by sink MEPs.

   The interleaving of PDUs is achieved via the use of distinct
   encapsulations and code points for generic associated channel (G-ACh)
   encapsulated BFD depending on whether the PDU format is CC or CV:




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   o  CC format: defines a new code point in the Associated Channel
      Header (ACH) described in RFC 5586 [2].  This format supports
      Continuity Check and RDI functionalities.

   o  CV format: defines a new code point in the Associated Channel
      Header (ACH) described in RFC 5586 [2].  The ACH with "MPLS-TP
      Proactive CV" code point indicates that the message is an MPLS-TP
      BFD proactive CV message, and information for CV processing is
      appended in the form of the Source MEP-ID TLV.

   RDI is communicated via the BFD diagnostic field in BFD CC messages,
   and the diagnostic code field in CV messages MUST be ignored.  It is
   not a distinct PDU.  As per [4], a sink MEP SHOULD encode a
   diagnostic code of "1 - Control Detection Time Expired" when the time
   since the last received BFD control packet exceeds the detection
   time, which is equal to the remote system's Transmit Interval
   multiplied by the remote system's Detect Multiplier (which is set to
   3 in this document).  A sink MEP SHOULD encode a diagnostic code of
   "5 - Path Down" as a consequence of the sink MEP receiving LDI.  A
   sink MEP MUST encode a diagnostic code of "9 - mis-connectivity
   defect" when CV PDU processing indicates a mis-connectivity defect.
   A sink MEP that has started sending diagnostic code 5 SHOULD NOT
   change it to 1 when the detection timer expires.

3.3.  ACH Code Points for CC and Proactive CV

   Figure 1 illustrates the G-ACh encoding for BFD CC-CV-RDI
   functionality.

    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     |      BFD CC/CV Code Point     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: ACH Indication of MPLS-TP CC/CV/RDI

   The first nibble (0001b) indicates the G-ACh as per RFC 5586 [2].

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

   The code point is either

   - BFD CC code point = 0x0022, or

   - BFD proactive CV code point = 0x0023.





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   CC and CV PDUs apply to all pertinent MPLS-TP structures, including
   PWs, MPLS LSPs (including SPMEs), and Sections.

   CC and CV operations are simultaneously employed on a maintenance
   entity (ME) within a single BFD session.  The expected usage is that
   normal operation is to send CC BFD protocol data units (PDUs)
   interleaved with a CV BFD PDU (augmented with a Source MEP-ID and
   identified as requiring additional processing by the different ACh
   channel types).  The insertion interval for CV PDUs is one per
   second.  Detection of a loss of continuity defect occurs when the
   time since the last received BFD control packet exceeds the detection
   time, which is equal to the session periodicity times the remote
   system's Detect Multiplier (which is set to 3 for the CC code point).
   Mis-connectivity defects are detected in a maximum of one second.

3.4.  MPLS-TP BFD CC Message Format

   The format of an MPLS-TP CC 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0
   0 0 1|Version|     Flags     |      BFD CC Code point        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
   | ~                  BFD Control Packet                           ~ |
   | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2: MPLS-TP CC Message

   As shown in Figure 2, the MPLS-TP CC message consists of the BFD
   control packet as defined in [4] pre-pended by the G-ACh.




















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3.5.  MPLS-TP BFD Proactive CV Message Format

   The format of an MPLS-TP CV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 1|Version|     Flags     |       BFD CV Code Point       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                  BFD Control Packet                           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                      Source MEP-ID TLV                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: MPLS-TP CV Message

   As shown in Figure 3, the MPLS-TP CV message consists of the BFD
   control packet as defined in [4], pre-pended by the ACH and appended
   by a Source MEP-ID TLV.

   A Source MEP-ID TLV is encoded as a 2-octet field that specifies a
   Type, followed by a 2-octet Length field, followed by a variable-
   length Value field.  A BFD session will only use one encoding of the
   Source ID TLV.

   The length in the BFD control packet is as per [4]; the length of the
   Source MEP-ID TLV is not included.  There are three possible Source
   MEP TLVs (corresponding to the MEP-IDs defined in [9]).  The type
   fields are:

      0 - Section MEP-ID

      1 - LSP MEP-ID

      2 - PW MEP-ID

   When the GAL is used, the TTL field of the GAL MUST be set to at
   least 1, and the GAL MUST be the end of stack label (S=1) as per [2].

   A node MUST NOT change the value in the Source MEP-ID TLV.

   When digest-based authentication is used, the Source ID TLV MUST NOT
   be included in the digest.




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3.5.1.  Section MEP-ID

   The IP-compatible MEP-ID for MPLS-TP Sections is the interface ID.
   The format of the Section MEP-ID TLV is:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type                         |  Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       MPLS-TP Global_ID                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    MPLS-TP Node Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    MPLS-TP Interface Number                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4: Section MEP-ID TLV Format

   Where the Type is of value '0'.  The Length is the length of the
   value fields.  The MPLS-TP Global_ID, Node Identifier, and Interface
   Numbers are as per [9].

3.5.2.  LSP MEP-ID

   The fields for the LSP MEP-ID are as defined in [9].  This is
   applicable to both LSPs and SPMEs.  This consists of the 32-bit MPLS-
   TP Global_ID, the 32-bit Node Identifier, followed by the 16-bit
   Tunnel_Num (that MUST be unique within the context of the Node
   Identifier), and the 16-bit LSP_NUM (that MUST be unique within the
   context of the Tunnel_Num).  The format of the TLV is:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type                         |  Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       MPLS-TP Global_ID                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    MPLS-TP Node Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tunnel_Num          |            LSP_Num            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: LSP MEP-ID TLV Format






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   Where the type is of value '1'.  The length is the length of the
   value fields.  The MPLS-TP Global_ID, Node Identifier, Tunnel_Num,
   and LSP_Num are as per [9].

3.5.3.  PW End Point MEP-ID

   The fields for the MPLS-TP PW End Point MEP-ID are as defined in [9].
   The format of the TLV is:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type                         |  Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       MPLS-TP Global_ID                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    MPLS-TP Node Identifier                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             AC_ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   AGI Type    |  AGI Length   |      AGI Value                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                    AGI  Value (contd.)                        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: PW End Point MEP-ID TLV Format

   Where the type is value '2'.  The length is the length of the
   following data: the Global_ID, Node Identifier, and Attachment
   Circuit ID (AC_ID) are as per [9].  The Attachment Group Identifier
   (AGI) Type is as per [6], and the AGI Length is the length of the AGI
   value field.

3.6.  BFD Session in MPLS-TP Terminology

   A BFD session corresponds to a CC and proactive CV OAM instance in
   MPLS-TP terminology.  A BFD session is enabled when the CC and
   proactive CV functionality are enabled on a configured Maintenance
   Entity (ME).

   When the CC and proactive CV functionality are disabled on an ME, the
   BFD session transitions to the ADMIN DOWN state, and the BFD session
   ends.

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




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   All BFD state changes and P/F exchanges MUST be done using CC
   packets.  P/F and session state information in CV packets MUST be
   ignored.

3.7.  BFD Profile for MPLS-TP

   BFD operates in asynchronous mode utilizing the encapsulation defined
   in Section 3 for all sessions in a given MEG.  For LSPs, SPMEs, and
   Sections, this is GAL/G-ACh-encapsulated BFD using the code points
   specified in Section 3.3.  For PWs, this is G-ACh or GAL/G-ACh-
   encapsulated BFD using the code points specified in Section 3.3.  In
   this mode, the BFD control packets are periodically sent at a
   configurable time rate.  This rate is a fixed value common for both
   directions of MEG for the lifetime of the MEG.

   This document specifies bidirectional BFD for P2P transport LSPs;
   hence, all BFD packets MUST be sent with the M bit clear.

   There are two modes of operation for bidirectional LSPs: one in which
   the session state of both directions of the LSP is coordinated, and
   one constructed from BFD sessions in such a way that the two
   directions operate independently but are still part of the same MEG.
   A single bidirectional BFD session is used for coordinated operation.
   Two independent BFD sessions are used for independent operation.  It
   should be noted that independent operation treats session state and
   defect state as independent entities.  For example, an independent
   session can be in the UP state while receiving RDI.  For a
   coordinated session, the session state will track the defect state.

   In coordinated mode, an implementation SHOULD NOT reset
   bfd.RemoteDiscr until it is exiting the DOWN state.

   In independent mode, an implementation MUST NOT reset bfd.RemoteDiscr
   upon transitioning to the DOWN state.

   Overall operation is as specified in RFC 5880 [4] and augmented for
   MPLS in RFC 5884 [8].  Coordinated operation is as described in [4].
   Independent operation requires clarification of two aspects of [4].
   Independent operation is characterized by the setting of
   bfd.MinRxInterval to zero by the MEP that is typically the session
   originator (referred to as the source MEP), and there will be a
   session originator at either end of the bidirectional LSP.  Each
   source MEP will have a corresponding sink MEP that has been
   configured to a transmission interval of zero.







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   This memo specifies a preferred interpretation of the base
   specification on how a MEP behaves with a BFD transmit rate set to
   zero.  One interpretation is that no periodic messages on the reverse
   component of the bidirectional LSP originate with that MEP; it will
   only originate messages on a state change.

   The first clarification is that, when a state change occurs, a MEP
   set to a transmit rate of zero sends BFD control messages with a one-
   second period on the reverse component until such time that the state
   change is confirmed by the session peer.  At this point, the MEP set
   to a transmit rate of zero can resume quiescent behavior.  This adds
   robustness to all state transitions in the RxInterval=0 case.

   The second clarification is that the originating MEP (the one with a
   non-zero bfd.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 that include the RDI diagnostic
   code as the state change is never confirmed.  This adds robustness to
   the exchange of RDI on a unidirectional failure (for both session
   types DOWN with a diagnostic of either control detection period
   expired or neighbor signaled session down offering RDI
   functionality).

   A further extension to the base specification is that there are
   additional OAM protocol exchanges that act as inputs to the BFD state
   machine.  These are the Link Down Indication [5] and the Lock
   Instruct/Lock Report transactions, the Lock Report interaction being
   optional.

3.7.1.  Session Initiation and Modification

   Session initiation occurs starting from MinRx = 1 second, MinTx >= 1
   second, and the detect multiplier = 3.

   Once in the UP state, Poll/Final discipline is used to modify the
   periodicity of control message exchange from their default rates to
   the desired rates and to set the detect multiplier to 3.

   Note that in the Poll/Final process a receiver of a new timer value
   with a poll flag can reject the timer value by tearing the session,
   or it can return its preferred timer value with the final flag.  Note
   also that the receiver of a new timer value with a final flag can
   reject the timer value by tearing the session, or it can return its
   preferred timer value with the poll flag.

   Once the desired rate has been reached using the Poll/Final
   mechanism, implementations SHOULD NOT attempt further rate
   modification.



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   In the rare circumstance where an operator has a reason to further
   change session parameters, beyond the initial migration from default
   values, Poll/Final discipline can be used with the caveat that a peer
   implementation may consider a session change unacceptable and/or
   bring the BFD session down via the use of the ADMIN DOWN state.

3.7.2.  Defect Entry Criteria

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

   The following conditions cause a MEP to enter the defect state for CC
   PDUs (in no particular order):

      1. BFD session times out (loss of continuity defect).

      2. Receipt of a Link Down Indication or Lock Report.

   The following will cause the MEP to enter the mis-connectivity defect
   state for CV operation (again, not in any particular order):

      1. BFD control packets are received with an unexpected
         encapsulation (mis-connectivity defect), these include:

         -  receiving an IP encoded CC or CV BFD control packet on an
            LSP configured to use GAL/G-ACh, or
         -  vice versa
         (Note there are other possibilities that can also alias as an
         OAM packet.)

      2. Receipt of an unexpected globally unique Source MEP identifier
         (mis-connectivity defect).  Note that as each encoding of the
         Source MEP-ID TLV contains unique information (there is no
         mechanical translation possible between MEP-ID formats),
         receipt of an unexpected Source MEP-ID type is the same as
         receiving an unexpected value.

      3. Receipt of a session discriminator that is not in the local BFD
         database in the Your Discriminator field (mis-connectivity
         defect).

      4. Receipt of a session discriminator that is in the local
         database but does not have the expected label (mis-connectivity
         defect).




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      5. If BFD authentication is used, receipt of a message with
         incorrect authentication information (password, MD5 digest, or
         SHA1 hash).

   The effective defect hierarchy (order of checking) is:

      1. Receiving nothing.

      2. Receiving Link Down Indication, e.g., a local link failure, an
         MPLS-TP LDI, or Lock Report.

      3. Receiving from an incorrect source (determined by whatever
         means).

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

      5. Receiving BFD control packets in all discernable ways correct.

3.7.3. 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
   peer using CC messages.

   The blocking of traffic as a consequent action MUST be driven only by
   a defect's consequent action as specified in Section 5.1.1.2 of RFC
   6371 [13].

   When the defect is mis-connectivity, the Section, LSP, or PW
   termination will silently discard all non-OAM traffic received.  The
   sink MEP will also send a defect indication back to the source MEP
   via the use of a diagnostic code of mis-connectivity defect (9).

3.7.4.  Defect Exit Criteria

3.7.4.1.  Exit from a Loss of Continuity Defect

   For a coordinated session, exit from a loss of connectivity defect is
   as described in Figure 7, which updates RFC 5880 [4].

   For an independent session, exit from a loss of connectivity defect
   occurs upon receipt of a well-formed BFD control packet from the peer
   MEP as described in Figures 8 and 9.







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3.7.4.2.  Exit from a Mis-Connectivity Defect

   Exit from a mis-connectivity defect state occurs when no CV messages
   with mis-connectivity defects have been received for a period of 3.5
   seconds.

3.7.5.  State Machines

   The following state machines update RFC 5880 [4].  They have been
   modified to include LDI and LKR as specified in [5] as inputs to the
   state machine and to clarify the behavior for independent mode.  LKR
   is an optional input.

   The coordinated session state machine has been augmented to indicate
   LDI and optionally LKR as inputs to the state machine.  For a session
   that is in the UP state, receipt of LDI or optionally LKR will
   transition the session into the DOWN state.

                             +--+
                             |  | UP, ADMIN DOWN, TIMER, LDI, LKR
                             |  V
               DOWN        +------+  INIT
              +------------|      |------------+
              |            | DOWN |            |
              |  +-------->|      |<--------+  |
              |  |         +------+         |  |
              |  |         MIS-CONNECTIVITY,|  |
              |  |               ADMIN DOWN,|  |
              |  |ADMIN DOWN,          DOWN,|  |
              |  |TIMER               TIMER,|  |
              V  |LDI,LKR           LDI,LKR |  V
            +------+                      +------+
       +----|      |                      |      |----+
   DOWN|    | INIT |--------------------->|  UP  |    |INIT, UP
       +--->|      | INIT, UP             |      |<---+
            +------+                      +------+

   Figure 7: MPLS CC State Machine for Coordinated Session Operation

   For independent mode, there are two state machines: one for the
   source MEP (which requested bfd.MinRxInterval=0) and one for the sink
   MEP (which agreed to bfd.MinRxInterval=0).

   The source MEP will not transition out of the UP state once
   initialized except in the case of a forced ADMIN DOWN.  Hence, LDI
   and optionally LKR do not enter into the state machine transition
   from the UP state, but do enter into the INIT and DOWN states.




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                             +--+
                             |  | UP, ADMIN DOWN, TIMER, LDI, LKR
                             |  V
               DOWN        +------+  INIT
              +------------|      |------------+
              |            | DOWN |            |
              |  +-------->|      |<--------+  |
              |  |         +------+         |  |
              |  |                          |  |
              |  |ADMIN DOWN     ADMIN DOWN |  |
              |  |TIMER,                    |  |
              |  |LDI,                      |  |
              V  |LKR                       |  V
            +------+                      +------+
       +----|      |                      |      |----+
   DOWN|    | INIT |--------------------->|  UP  |    | INIT, UP, DOWN,
       +--->|      | INIT, UP             |      |<---+ LDI, LKR
            +------+                      +------+

            Figure 8: MPLS CC State Machine for Source MEP for
                       Independent Session Operation

   The sink MEP state machine (for which the transmit interval has been
   set to zero) is modified to:

   1) Permit direct transition from DOWN to UP once the session has been
      initialized.  With the exception of via the ADMIN DOWN state, the
      source MEP will never transition from the UP state; hence, in
      normal unidirectional fault scenarios, it will never transition to
      the INIT state.





















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                                +--+
                                |  | ADMIN DOWN, TIMER, LDI, LKR
                                |  V
                  DOWN        +------+  INIT, UP
                 +------------|      |------------+
                 |            | DOWN |            |
                 |  +-------->|      |<--------+  |
                 |  |         +------+         |  |
                 |  |         MIS-CONNECTIVITY,|  |
                 |  |               ADMIN DOWN,|  |
                 |  |ADMIN DOWN,    TIMER,     |  |
                 |  |TIMER,         DOWN,      |  |
                 |  |LDI,           LDI,       |  V
                 V  |LKR            LKR        |  |
               +------+                      +------+
          +----|      |                      |      |----+
      DOWN|    | INIT |--------------------->|  UP  |    |INIT, UP
          +--->|      | INIT, UP             |      |<---+
               +------+                      +------+

               Figure 9: MPLS CC State Machine for the Sink MEP
                      for Independent Session Operation

3.7.6.  Configuration of MPLS-TP BFD Sessions

   The configuration of MPLS-TP BFD session parameters and the
   coordination of the same between the source and sink MEPs are out of
   scope of this memo.

3.7.7.  Discriminator Values

   In the BFD control packet, the discriminator values either are local
   to the sink MEP or have no significance (when not known).

   The My Discriminator field MUST be set to a non-zero value (which can
   be a fixed value).  The transmitted Your Discriminator value MUST
   reflect back the received value of the My Discriminator field or be
   set to zero if that value is not known.

   Per Section 7 of RFC 5884 [8], a node MUST NOT change the value of
   the My Discriminator field for an established BFD session.










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4.  Configuration Considerations

   The following is an example set of configuration parameters for a BFD
   session:

      Mode and Encapsulation
      ----------------------
      RFC 5884 - BFD CC in UDP/IP/LSP
      RFC 5885 - BFD CC in G-ACh
      RFC 5085 - UDP/IP in G-ACh
       MPLS-TP - CC/CV in GAL/G-ACh or G-ACh

   For MPLS-TP, the following additional parameters need to be
   configured:

   1) Session mode, coordinated or independent
   2) CC periodicity
   3) The MEP-ID for the MEPs at either end of the LSP
   4) Whether authentication is enabled (and if so, the associated
      parameters)

   The discriminators used by each MEP, both bfd.LocalDiscr and
   bfd.RemoteDiscr, can optionally be configured or locally assigned.
   Finally, a detect multiplier of 3 is directly inferred from the code
   points.

5.  IANA Considerations

   IANA has allocated two channel types from the "Pseudowire Associated
   Channel Types" registry in RFC 4385 [15].

      0x0022   MPLS-TP CC message
      0x0023   MPLS-TP CV message

   IANA has created a "CC/CV MEP-ID TLV" registry.  The parent registry
   is the "Pseudowire Associated Channel Types" registry of RFC 4385
   [15].  All code points within this registry shall be allocated
   according to the "Standards Action" procedures as specified in [11].
   The items tracked in the registry will be the type, associated name,
   and reference.

   The initial values are:

      0 - Section MEP-ID
      1 - LSP MEP-ID
      2 - PW MEP-ID





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   IANA has assigned the following code point from the "Bidirectional
   Forwarding Detection (BFD) Parameters" registry, "BFD Diagnostic
   Codes" subregistry [4]:

      9 - mis-connectivity defect

6.  Security Considerations

   The use of CV improves network integrity by ensuring traffic is not
   "leaking" between LSPs.

   Base BFD foresees an optional authentication section (see Section 6.7
   of [4]) that can be applied to this application.  Although the Source
   MEP-ID TLV is not included in the BFD authentication digest, there is
   a chain of trust such that the discriminator associated with the
   digest is also associated with the expected MEP-ID; this will prevent
   impersonation of CV messages in this application.

   This memo specifies the use of globally unique identifiers for MEP-
   IDs.  This provides absolutely authoritative detection of persistent
   leaking of traffic between LSPs.  Non-uniqueness can result in
   undetected leaking in the scenario where two LSPs with common MEP-IDs
   are misconnected.  This would be considered undesirable but rare; it
   would also be difficult to exploit for malicious purposes as, at a
   minimum, both a network end point and a node that was a transit point
   for the target MEG would need to be compromised.

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

   [3]   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.

   [4]   Katz, D. and D. Ward, "Bidirectional Forwarding Detection
         (BFD)", RFC 5880, June 2010.

   [5]   Swallow, G., Ed., Fulignoli, A., Ed., Vigoureux, M., Ed.,
         Boutros, S., and D. Ward, "MPLS Fault Management Operations,
         Administration, and Maintenance (OAM)", RFC 6427, November
         2011.



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   [6]   Martini, L., "IANA Allocations for Pseudowire Edge to Edge
         Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

   [7]   Nadeau, T., Ed., and C. Pignataro, Ed., "Bidirectional
         Forwarding Detection (BFD) for the Pseudowire Virtual Circuit
         Connectivity Verification (VCCV)", RFC 5885, June 2010.

   [8]   Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
         "Bidirectional Forwarding Detection (BFD) for MPLS Label
         Switched Paths (LSPs)", RFC 5884, June 2010.

   [9]   Bocci, M., Swallow, G., and E. Gray, "MPLS Transport Profile
         (MPLS-TP) Identifiers", RFC 6370, September 2011.

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

   [11]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

7.2.  Informative References

   [12]  Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau, L.,
         and L. Berger, "A Framework for MPLS in Transport Networks",
         RFC 5921, July 2010.

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

   [14]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu, D., and
         S. Mansfield, "Guidelines for the Use of the "OAM" Acronym in
         the IETF", BCP 161, RFC 6291, June 2011.

   [15]  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.

8.  Acknowledgments

   Nitin Bahadur, Rahul Aggarwal, Tom Nadeau, Nurit Sprecher, and Yaacov
   Weingarten also contributed to this document.








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9.  Contributing Authors

   Annamaria Fulignoli
   Ericsson
   EMail: annamaria.fulignoli@ericsson.com

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

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

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

   David Ward
   Juniper
   EMail: dward@juniper.net

   Robert Rennison
   ECI Telecom
   EMail: robert.rennison@ecitele.com

Editors' Addresses

   Dave Allan
   Ericsson
   EMail: david.i.allan@ericsson.com

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

   John Drake
   Juniper
   EMail: jdrake@juniper.net












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