MPLS Working Group                                             G. Mirsky
Internet-Draft                                               J. Tantsura
Intended status: Standards Track                                Ericsson
Expires: August 16, 2015                                   I. Varlashkin
                                                                  Google
                                                                 M. Chen
                                                                  Huawei
                                                       February 12, 2015


     Bidirectional Forwarding Detection (BFD) Directed Return Path
                   draft-mirsky-mpls-bfd-directed-02

Abstract

   Bidirectional Forwarding Detection (BFD) is expected to monitor bi-
   directional paths.  When a BFD session monitors in its forward
   direction an explicitly routed path there is a need to be able to
   direct far-end BFD peer to use specific path as reverse direction of
   the BFD session.

Status of This Memo

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   This Internet-Draft will expire on August 16, 2015.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Direct Reverse BFD Path . . . . . . . . . . . . . . . . . . .   4
     3.1.  Case of MPLS Data Plane . . . . . . . . . . . . . . . . .   4
       3.1.1.  BFD Reverse Path TLV  . . . . . . . . . . . . . . . .   4
       3.1.2.  Segment Routing Tunnel sub-TLV  . . . . . . . . . . .   5
     3.2.  Case of IPv6 Data Plane . . . . . . . . . . . . . . . . .   5
     3.3.  Bootstrapping BFD session with BFD Reverse Path over
           Segment Routed tunnel . . . . . . . . . . . . . . . . . .   6
     3.4.  Return Codes  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Use Case Scenario . . . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  TLV . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Return Codes  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The [RFC5880], [RFC5881], and the [RFC5883] established BFD protocol
   for IP networks and the [RFC5884] set rules of using BFD Asynchronous
   mode over IP/MPLS LSPs.  All standards implicitly assume that the
   far-end BFD peer will use the best route regardless of route being
   used to send BFD control packets towards it.  As result, if the near-
   end BFD peer sends its BFD control packets over explicit path that is
   diverging from the best route, then reverse direction of the BFD
   session is likely not to be on co-routed bi-directional path with the
   forward direction of the BFD session.  And because BFD control
   packets are not guaranteed to cross the same links and nodes in both
   directions detection of Loss of Continuity (LoC) defect in forward
   direction may demonstrate positive negatives.

   This document proposes to use BFD Return Path TLV extension to LSP
   Ping [RFC4379] to instruct the far-end BFD peer to use explicit path
   for its BFD control packets associated with the particular BFD



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   session.  As a special case, forward and reverse directions of the
   BFD session can form bi-directional co-routed associated channel.

1.1.  Conventions used in this document

1.1.1.  Terminology

   BFD: Bidirectional Forwarding Detection

   MPLS: Multiprotocol Label Switching

   LSP: Label Switching Path

   LoC: Loss of Continuity

1.1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

2.  Problem Statement

   BFD is best suited to monitor bi-directional co-routed paths.  In
   most cases, in IP and IP/MPLS networks the best route between two IP
   nodes is likely to be co-routed in the stable network environment so
   that implicit BF D requirement is being fulfilled.  If BFD is tasked
   to monitor unidirectional explicitly routed path, e.g.  MPLS LSP, its
   control packets in forward direction would be in-band due to
   mechanism defined in [RFC5884] and [RFC5586].  But the reverse
   direction of the BFD session would still follow the best route and
   that presents following problems in regard to detecting defects on
   the unidirectional explicit path:

   o  failure detection on the reverse path cannot be interpreted as bi-
      directional failure and thus trigger, for example, protection
      switchover of the forward direction;

   o  if reverse direction is in Down state, the head-end node would not
      receive indication of forward direction failure from its far-end
      peer.

   To address these challenges the far-end BFD peer should be instructed
   to use specific path for its control packets.






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3.  Direct Reverse BFD Path

3.1.  Case of MPLS Data Plane

   LSP ping, defined in [RFC4379], uses BFD Discriminator TLV [RFC5884]
   to bootstrap a BFD session over an MPLS LSP.  This document defines a
   new TLV, BFD Reverse Path TLV, that MUST contain a single sub-TLV
   that can be used to carry information about reverse path for the
   specified in BFD Discriminator TLV session.

3.1.1.  BFD Reverse Path TLV

   The BFD Reverse Path TLV is an optional TLV within the LSP ping
   protocol.  However, if used, the BFD Discriminator TLV MUST be
   included in an Echo Request message as well.  If the BFD
   Discriminator TLV is not present when the BFD Reverse Path TLV is
   included, then it MUST be treated as malformed Echo Request, as
   described in [RFC4379].

   The BFD Reverse Path TLV carries the specified path that BFD control
   packets of the BFD session referenced in the BFD Discriminator TLV
   are required to follow.  The format of the BFD Reverse Path TLV is as
   presented in Figure 1.

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   BFD Reverse Path TLV Type   |          Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Reverse Path                         |
    ~                                                             ~
    |                                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 1: BFD Reverse Path TLV

   BFD Reverse Path TLV Type is 2 octets in length and value to be
   assigned by IANA.

   Length is 2 octets in length and defines the length in octets of the
   Reverse Path field.

   Reverse Path field contains a sub-TLV.  Any Target FEC sub-TLV,
   already or in the future defined, from IANA sub-registry Sub-TLVs for
   TLV Types 1, 16, and 21 of MPLS LSP Ping Parameters registry MAY be
   used in this field.  Only one sub-TLV MUST be included in the Reverse
   Path TLV.  If more than one sub-TLVs are present in the Reverse Path




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   TLV, then only the first sub-TLV MUST be used and the rest MUST be
   silently discarded.

   If the egress LSR fails to establish the BFD session because path
   specified in the Reverse Path TLV is not known, the egress MAY
   establish the BFD session over IP network [RFC5884] and MAY send Echo
   Reply with the Reverse Path TLV received and the return code set to
   "Failed to establish the BFD session.  The specified reverse path was
   not found" (TBD4) Section 3.4.  If the egress LSR cannot find path
   specified in the Reverse Path TLV and does not establish BFD session
   per RFC 5884, it MUST send Echo Reply with the Reverse Path TLV
   received and the return code set to "Failed to establish the BFD
   session.  The specified reverse path was not found".

3.1.2.  Segment Routing Tunnel sub-TLV

   With MPLS data plane explicit path can be either Static or RSVP-TE
   LSP, or Segment Routing tunnel.  In case of Static or RSVP-TE LSP
   [RFC7110] defined sub-TLVs to identify explicit return path.  For the
   Segment Routing with MPLS data plane case a new sub-TLV is defined in
   this document as presented in Figure 2.

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  SegRouting MPLS sub-TLV Type |          Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Label Stack Element                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Label Stack Element                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                                                             ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 2: Segment Routing MPLS Tunnel sub-TLV

   The Segment Routing Tunnel sub-TLV Type is two octets in length, and
   will be allocated by IANA.

   The Segment Routing Tunnel sub-TLV MAY be used in Reply Path TLV
   defined in [RFC7110]

3.2.  Case of IPv6 Data Plane

   IPv6 can be data plane of choice for Segment Routed tunnels
   [I-D.previdi-6man-segment-routing-header].  In such networks the BFD
   Reverse Path TLV described in Section 3.1.1 can be used as well.  IP
   networks, unlike IP/MPLS, do not require use of LSP ping with BFD



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   Discriminator TLV[RFC4379] to bootstrap BFD session.  But to specify
   reverse path of a BFD session in IPv6 environment the BFD
   Discriminator TLV MUST be used along with the BFD Reverse Path TLV.
   The BFD Reverse Path TLV in IPv6 network MUST include sub-TLV.

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  SegRouting IPv6 sub-TLV Type |          Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                             |
    |                         IPv6 Prefix                         |
    |                                                             |
    |                                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                             |
    |                         IPv6 Prefix                         |
    |                                                             |
    |                                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                                                             ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 3: Segment Routing IPv6 Tunnel sub-TLV

3.3.  Bootstrapping BFD session with BFD Reverse Path over Segment
      Routed tunnel

   As discussed in [I-D.kumarkini-mpls-spring-lsp-ping] introduction of
   Segment Routing network domains with MPLS dataplane adds three new
   sub-TLVs that may be used with Target FEC TLV.  Section 6.1 addresses
   use of new sub-TLVs in Target FEC TLV in LSP ping and LSP traceroute.
   For the case of LSP ping the [I-D.kumarkini-mpls-spring-lsp-ping]
   states that:

   "Initiator MUST include FEC(s) corresponding to the destination
   segment.

   Initiator MAY include FECs corresponding to some or all of segments
   imposed in the label stack by the initiator to communicate the
   segments traversed.  "

   When LSP ping is used to bootstrap BFD session this document updates
   this and defines that LSP Ping MUST include the FEC corresponding to
   the destination segment and SHOULD NOT include FECs corresponding to
   some or all of segment imposed by the initiator.  Operationally such
   restriction would not cause any problem or uncertainty as LSP ping




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   with FECs corresponding to some or all segments or traceroute may
   preceed the LSP ping that bootstraps the BFD session.

3.4.  Return Codes

   This document defines the following Return Codes:

   o  Failed to establish the BFD session.  The specified reverse path
      was not found, (TBD4) - the specified reverse path was not found,
      failed to establish the BFD session.  When a specified reverse
      path is not available at the egress LSR, an Echo Reply with the
      return code set to "Failed to establish the BFD session.  The
      specified reverse path was not found."  MAY be sent back to the
      initiator . (Section 3.1.1)

4.  Use Case Scenario

   In network presented in Figure 4 node A monitors two tunnels to node
   H: A-B-C-D-G-H and A-B-E-F-G-H.  To bootstrap BFD session to monitor
   the first tunnel, node A MUST include BFD Discriminator TLV with
   Discriminator value N and MAY include BFD Reverse Path TLV that
   references H-G-D-C-B-A tunnel.  To bootstrap BFD session to monitor
   the second tunnel, node A MUST include BFD Discriminator TLV with
   Discriminator value M and MAY include BFD Reverse Path TLV that
   references H-G-F-E-B-A tunnel.

           C---------D
           |         |
   A-------B         G-----H
           |         |
           E---------F

                Figure 4: Use Case for BFD Reverse Path TLV

   If an operator needs node H to monitor path to node A, e.g.
   H-G-D-C-B-A tunnel, then by looking up list of known Reverse Paths it
   MAY find and use existing BFD sessions.

5.  IANA Considerations

5.1.  TLV

   The IANA is requested to assign a new value for BFD Reverse Path TLV
   from the "Multiprotocol Label Switching Architecture (MPLS) Label
   Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and
   sub-TLVs" sub-registry.





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            +----------+----------------------+---------------+
            | Value    | Description          | Reference     |
            +----------+----------------------+---------------+
            | X (TBD1) | BFD Reverse Path TLV | This document |
            +----------+----------------------+---------------+

                     Table 1: New BFD Reverse Type TLV

5.2.  Sub-TLV

   The IANA is requested to assign two new sub-TLV types from
   "Multiprotocol Label Switching Architecture (MPLS) Label Switched
   Paths (LSPs) Ping Parameters - TLVs" registry, "Sub-TLVs for TLV
   Types 1, 16, and 21" sub-registry.

    +----------+-------------------------------------+---------------+
    | Value    | Description                         | Reference     |
    +----------+-------------------------------------+---------------+
    | X (TBD2) | Segment Routing MPLS Tunnel sub-TLV | This document |
    | X (TBD3) | Segment Routing IPv6 Tunnel sub-TLV | This document |
    +----------+-------------------------------------+---------------+

                Table 2: New Segment Routing Tunnel sub-TLV

5.3.  Return Codes

   The IANA is requested to assign a new Return Code value from the
   "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs)
   Ping Parameters" registry, "Return Codes" sub-registry, as follows
   using a Standards Action value.

   +----------+----------------------------------------+---------------+
   | Value    | Description                            | Reference     |
   +----------+----------------------------------------+---------------+
   | X (TBD4) | Failed to establish the BFD session.   | This document |
   |          | The specified reverse path was not     |               |
   |          | found.                                 |               |
   +----------+----------------------------------------+---------------+

                         Table 3: New Return Code

6.  Security Considerations

   Sequirity considerations discussed in [RFC5880], [RFC5884], and
   [RFC4379], apply to this document.






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7.  Acknowledgements

8.  Normative References

   [I-D.kumarkini-mpls-spring-lsp-ping]
              Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini,
              S., Gredler, H., and M. Chen, "Label Switched Path (LSP)
              Ping/Trace for Segment Routing Networks Using MPLS
              Dataplane", draft-kumarkini-mpls-spring-lsp-ping-02 (work
              in progress), October 2014.

   [I-D.previdi-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., and I. Leung, "IPv6
              Segment Routing Header (SRH)", draft-previdi-6man-segment-
              routing-header-05 (work in progress), January 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
              2010.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, June 2010.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, June 2010.

   [RFC7110]  Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
              "Return Path Specified Label Switched Path (LSP) Ping",
              RFC 7110, January 2014.








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Authors' Addresses

   Greg Mirsky
   Ericsson

   Email: gregory.mirsky@ericsson.com


   Jeff  Tantsura
   Ericsson

   Email: jeff.tantsura@ericsson.com


   Ilya Varlashkin
   Google

   Email: Ilya@nobulus.com


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com



























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