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rfc:rfc6428

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.

Allan, et al. Standards Track [Page 1] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

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

Allan, et al. Standards Track [Page 2] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

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

Allan, et al. Standards Track [Page 3] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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

Allan, et al. Standards Track [Page 4] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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:

Allan, et al. Standards Track [Page 5] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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
  1. BFD CC code point = 0x0022, or
  1. BFD proactive CV code point = 0x0023.

Allan, et al. Standards Track [Page 6] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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.

Allan, et al. Standards Track [Page 7] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

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.

Allan, et al. Standards Track [Page 8] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

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

Allan, et al. Standards Track [Page 9] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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.

Allan, et al. Standards Track [Page 10] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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.

Allan, et al. Standards Track [Page 11] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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.

Allan, et al. Standards Track [Page 12] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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:
  1. receiving an IP encoded CC or CV BFD control packet on an

LSP configured to use GAL/G-ACh, or

  1. 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).

Allan, et al. Standards Track [Page 13] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

    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.

Allan, et al. Standards Track [Page 14] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

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.

Allan, et al. Standards Track [Page 15] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

                           +--+
                           |  | 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.

Allan, et al. Standards Track [Page 16] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

                              +--+
                              |  | 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.

Allan, et al. Standards Track [Page 17] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

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

Allan, et al. Standards Track [Page 18] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 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.

Allan, et al. Standards Track [Page 19] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

 [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.

Allan, et al. Standards Track [Page 20] RFC 6428 CC, CV, and RDI for MPLS-TP November 2011

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

Allan, et al. Standards Track [Page 21]

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