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Versions: 00 01 02 03 04 05 06 07 RFC 5884

Network Working Group                                        R. Aggarwal
Internet Draft                                          Juniper Networks
Expiration Date: September 2007
                                                             K. Kompella
                                                        Juniper Networks

                                                               T. Nadeau
                                                     Cisco Systems, Inc.

                                                              G. Swallow
                                                     Cisco Systems, Inc.

                                                              March 2007


                           BFD For MPLS LSPs


                       draft-ietf-bfd-mpls-04.txt

Status of this Memo

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Abstract

   One desirable application of Bi-directional Forwarding Detection
   (BFD) is to detect a MPLS LSP data plane failure. LSP-Ping is an
   existing mechanism for detecting MPLS data plane failures and for
   verifying the MPLS LSP data plane against the control plane. BFD can
   be used for the former, but not for the latter. However the control
   plane processing required for BFD control packets is relatively
   smaller than the processing required for LSP-Ping messages. A
   combination of LSP-Ping and BFD can be used to provide faster data
   plane failure detection and/or make it possible to provide such
   detection on a greater number of LSPs. This document describes the
   applicability of BFD in relation to LSP-Ping for this application. It
   also describes procedures for using BFD in this environment.



Table of Contents

 1          Specification of requirements  .........................   3
 2          Introduction  ..........................................   3
 3          Applicability  .........................................   3
 3.1        BFD for MPLS LSPs: Motivation  .........................   3
 3.2        Using BFD in Conjunction with LSP-Ping  ................   5
 4          Theory of Operation  ...................................   5
 5          Initialization and Demultiplexing  .....................   6
 6          Session Establishment  .................................   7
 6.1        BFD Discriminator TLV in LSP-Ping  .....................   7
 7          Encapsulation  .........................................   8
 8          Security Considerations  ...............................   9
 9          IANA Considerations  ...................................   9
10          Acknowledgments  .......................................   9
11          References  ............................................   9
11.1        Normative References  ..................................   9
11.2        Informative References  ................................  10
12          Author Information  ....................................  10
13          Intellectual Property Statement  .......................  11
14          Full Copyright Statement  ..............................  12










Raggarwa                                                        [Page 2]


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1. Specification of requirements

   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 [RFC2119].


2. Introduction

   One desirable application of Bi-directional Forwarding Detection
   (BFD) is to track the liveliness of a Multi Protocol Label Switched
   (MPLS) Label Switched Path (LSP). In particular BFD can be used to
   detect a data plane failure in the forwarding path of a MPLS LSP.
   LSP-Ping [RFC4379] is an existing mechanism for detecting MPLS LSP
   data plane failures and for verifying the MPLS LSP data plane against
   the control plane. This document describes the applicability of BFD
   in relation to LSP-Ping for detecting MPLS LSP data plane failures.
   It also describes procedures for using BFD for detecting MPLS LSP
   data plane failures.


3. Applicability

   In the event of a MPLS LSP failing to deliver data traffic, it may
   not always be possible to detect the failure using the MPLS control
   plane. For instance the control plane of the MPLS LSP may be
   functional while the data plane may be mis-forwarding or dropping
   data. Hence there is a need for a mechanism to detect a data plane
   failure in the MPLS LSP path [RFC4377].


3.1. BFD for MPLS LSPs: Motivation

   LSP-Ping described in [LSP-Ping] is an existing mechanism for
   detecting a MPLS LSP data plane failure. In addition LSP-Ping also
   provides a mechanism for verifying the MPLS control plane against the
   data plane. This is done by ensuring that the LSP is mapped to the
   same Forwarding Equivalence Class (FEC), at the egress, as the
   ingress.

   BFD cannot be used for verifying the MPLS control plane against the
   data plane.  However BFD can be used to detect a data plane failure
   in the forwarding path of a MPLS LSP. The LSP may be  associated with
   any of the following FECs:
     a) RSVP IPv4/IPv6 Session [RFC3209]
     b) LDP IPv4/IPv6 prefix [RFC3036]
     c) VPN IPv4/IPv6 prefix [RFC4364]
     d) Layer 2 VPN [L2-VPN]



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     e) Layer 2 Circuit ID [RFC4447]

   LSP-Ping includes extensive control plane verification. BFD on the
   other hand was designed as a light-weight means of testing only the
   data plane. As a result, LSP-Ping is computationally more expensive
   than BFD for detecting MPLS LSP data plane faults. BFD is also more
   suitable for being implemented in hardware or firmware due to its
   fixed packet format. Thus the use of BFD for detecting MPLS LSP data
   plane faults has the following advantages:

     a) Support for fault detection for greater number of LSPs.

     b) Fast detection. Detection with sub-second granularity is
   considered as fast detection. LSP-Ping is intended to be used in an
   environment where fault detection messages are exchanged, either for
   diagnostic purposes or for infrequent periodic fault detection, in
   the order of tens of seconds or minutes. Hence its not appropriate
   for fast detection. BFD on the other hand is designed for sub-second
   fault detection intervals. Following are some potential cases when
   fast detection may be desirable for MPLS LSPs:

      1. In the case of a bypass LSP used for facility based link or
   node protection [RFC4090]. In this case the bypass LSP is essentially
   being used as an alternate link to protect one or more LSPs. It
   represents an aggregate and is used to carry data traffic belonging
   to one or more LSPs, when the link or the node being protected fails.
   Hence fast failure detection of the bypass LSP may be desirable
   particularly in the event of link or node failure when the data
   traffic is moved to the bypass LSP.

      2. MPLS Pseudo Wires (PW). Fast detection may be desired for MPLS
   PWs depending on i) the model used to layer the MPLS network with the
   layer 2 network. and ii) the service that the PW is emulating. For a
   non-overlay model between the layer 2 network and the MPLS network
   the provider may rely on PW fault detection to provide service status
   to the end-systems. Also in that case interworking scenarios such as
   ATM/Frame Relay interworking may force periodic PW fault detection
   messages. Depending on the requirements of the service that the MPLS
   PW is emulating, fast failure detection may be desirable. Use of BFD
   for PWs is further described in [VCCV] and [OAM-MAP].

   There may be other potential cases where fast failure detection is
   desired for MPLS LSPs.








Raggarwa                                                        [Page 4]


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3.2. Using BFD in Conjunction with LSP-Ping

   BFD can be used for MPLS LSP data plane fault detection. However it
   does not have all the funcitonality of LSP-Ping. In paticular it
   cannot be used for verifying the control plane against the data
   plane. LSP Ping performs the following functions that are outside the
   scope of BFD:

     a) Association of a LSP-Ping echo request message with a FEC. In
   the case of Penultimate Hop Popping (PHP), for a single label stack
   LSP, the only way to associate a fault detection message with a FEC
   is by carrying the FEC in the message. LSP-Ping provides this
   functionality. Next-hop label allocation also makes it necessary to
   carry the FEC in the fault detection message as the label alone is
   not sufficient to identify the LSP being verified. In addition to
   this presence of the FEC in the echo request message makes is
   possible to verify the control plane against the data plane at the
   egress LSR.

     b) Equal cost multi-path (ECMP) considerations. LSP-Ping traceroute
   makes it possible to probe multiple alternate paths for LDP IP FECs.

     c) Traceroute. LSP-Ping supports traceroute for a FEC and it can be
   used for fault isolation.

   Hence BFD is used in conjunction with LSP-Ping for MPLS LSP fault
   detection:

   i) LSP-Ping is used for boot-strapping the BFD session as described
   later in this document.

   ii) BFD is used to exchange fault detection (i.e. BFD session)
   packets at the required detection interval.

   iii) LSP-Ping is used to periodically verify the control plane
   against the data plane by ensuring that the LSP is mapped to the same
   FEC, at the egress, as the ingress.


4. Theory of Operation

   To use BFD for fault detection on a MPLS LSP a BFD session MUST be
   established for that particular MPLS LSP. BFD control packets MUST be
   sent along the same data path as the LSP being verified and are
   processed by the BFD processing module of the egress LSR. If the LSP
   is associated with multiple FECs, a BFD session SHOULD established
   for each FEC. For instance this may happen in the case of next-hop
   label allocation. Hence the operation is conceptually similar to the



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   data plane fault detection procedures of LSP-Ping.

   If MPLS fast-reroute is being used for the MPLS LSP the use of BFD
   for fault detection can result in false fault detections if the BFD
   fault detection interval is less than the MPLS fast-reroute
   switchover time. When MPLS fast-reroute is triggered because of a
   link or node failure BFD control packets will be dropped until
   traffic is switched on to the backup LSP. If the time taken to
   perform the switchover exceeds the BFD fault detection interval a
   fault will be declared even though the MPLS LSP is being locally
   repaired. To avoid this the BFD fault detection interval should be
   greater than the fast-reroute switchover time. An implementation
   SHOULD provide configuration options to control the BFD fault
   detection interval.

   If there are multiple alternate paths from an ingress LSR to an
   egress LSR for a LDP IP FEC, LSP-Ping traceroute MAY be used to
   determine each of these alternate paths. A BFD session SHOULD be
   established for each alternate path that is discovered.

   Periodic LSP-Ping echo request messages SHOULD be sent by the ingress
   LSR to the egress LSR along the same data path as the LSP. This is to
   periodically verify the control plane against the data plane by
   ensuring that the LSP is mapped to the same FEC, at the egress, as
   the ingress. The rate of generation of these LSP-Ping echo request
   messages SHOULD be significantly less than the rate of generation of
   the BFD control packets.  An implemetation MAY provide configuration
   options to control the rate of generation of the periodic LSP-Ping
   echo request messages.


5. Initialization and Demultiplexing

   A BFD session may be established for a FEC associated with a MPLS
   LSP. As desribed above in the case of PHP and next-hop label
   allocation the BFD control packet received by the egress LSR does not
   contain sufficient information to associate it with a BFD session.
   Hence the demultiplexing MUST be done using the remote discriminator
   field in the received BFD control packet. The exchange of BFD
   discriminators for this purpose is described in the next section.











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6. Session Establishment

   A BFD session is boot-strapped using LSP-Ping. This specification
   describes procedures only for BFD asynchronous mode. The initiation
   of fault detection for a particular <MPLS LSP, FEC> combination
   results in the exchange of LSP-Ping echo request and echo reply
   packets, in the ping mode, between the ingress and egress LSRs for
   that <MPLS LSP, FEC>. To establish a BFD session a LSP-Ping echo
   request message MUST carry the local discriminator assigned by the
   ingress LSR for the BFD session. This MUST subsequently be used as
   the My Discriminator field in the BFD session packets sent by the
   ingress LSR.

   On receipt of the LSP-Ping echo request message, the egress LSR MUST
   send a BFD control packet to the ingress LSR.  This BFD control
   packet MUST set the Your Discriminator field to the discriminator
   received from the ingress LSR in the LSP-Ping echo request message.
   The egress LSR MAY respond with a LSP-Ping echo reply message that
   carries the local discriminator assigned by it for the BFD session.
   The local discriminator assigned by the egress LSR MUST be used as
   the My Discriminator field in the BFD session packets sent by the
   egress LSR.

   Once the ingress LSR learns the local discriminator assigned by the
   egress LSR for a given BFD session, it MUST send a BFD control packet
   to the egress LSR with the Your Discriminator set to the local
   discriminator of the egress LSR. The egress LSR demultiplexes the BFD
   session based on the received Your Discriminator field. As mentioned
   above the egress LSR MUST send control packets to the ingress LSR
   with the Your Discriminator field set to the local discriminator of
   the ingress LSR. The ingress LSR uses this to demultiplex the BFD
   session.


6.1. BFD Discriminator TLV in LSP-Ping

   LSP-Ping echo request and echo reply messages carry a BFD
   discriminator TLV for the purpose of session establishment as
   described above. IANA is requested to assign a type value of 15 to
   this TLV. This TLV has a length of 4. The value contains the 4 byte
   local discriminator that the LSR, sending the LSP-Ping message,
   associates with the BFD session.

   If the BFD session is not in UP state, the periodic LSP-Ping echo
   request messages MUST include the BFD discriminator TLV.






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7. Encapsulation

   BFD control packets sent by the ingress LSR MUST be encapsulated in
   the MPLS label stack that corresponds to the FEC for which fault
   detection is being performed. If the label stack has a depth greater
   than one, the TTL of the inner MPLS label MAY be set to 1. This may
   be necessary for certain FECs to enable the egress LSR's control
   plane to receive the packet [LSP-Ping]. For MPLS PWs, alternatively,
   the presence of a fault detection message may be indicated by setting
   a bit in the control word [VCCV].

   The BFD control packet sent by the ingress LSR MUST be a UDP packet
   with a well known destination port 3784 [BFD-IP] and a source port
   assigned by the sender as per the procedures in [BFD-IP]. The source
   IP address is a routable address of the sender. The destination IP
   address is randomly chosen from the 127/8 range, with the following
   exception. If the FEC is a LDP IP FEC the ingress LSR may discover
   multiple alternate paths to the egress LSR for this FEC using LSP-
   ping traceroute. In this case the destination IP address, used in a
   BFD session established for one such alternate path, is the address
   in the 127/8 range discovered by LSP-ping traceroute [RFC4379] to
   exercise that particular alternate path.

   The IP TTL MUST be set to 1.

   BFD control packets sent by the egress LSR are UDP packets. The
   source IP address is a routable address of the replier; the source
   port is the well-known UDP port 3784.  The destination IP address and
   UDP port MUST be copied from the source IP address and UDP port of
   the control packet received from the ingress LSR.

   The BFD control packet sent by the egress LSR to the ingress LSR MAY
   be routed based on the destination IP address as per the procedures
   in [BFD-MHOP]. Or the BFD control packet sent by the egress LSR to
   the ingress LSR MAY be encapsulated in a MPLS label stack. If this is
   the case the presence of  the fault detection message is indicated as
   described above.  This may be the case if the FEC for which the fault
   detection is being perfomed corresponds to a bi-directional LSP or a
   MPLS PW. This may also be the case when there is a return LSP from
   the egress LSR to the ingress LSR.

   Note that once the BFD session for the MPLS LSP is UP, either end of
   the BFD session MUST NOT change the source IP address and the local
   discriminator values of the BFD control packets it generates, unless
   it first brings down the session. This implies that a LSR MUST ignore
   BFD packets for a given session, that is demultiplexed using the
   received Your Discriminator field, if the session is in UP state and
   if the My Discriminator or the Source IP address fields of the



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   received packet do not match the values associated with the session.


8. Security Considerations

   Security considerations discussed in [BFD], [BFD-MHOP] and [LSP-Ping]
   apply to this document.


9. IANA Considerations

   This document introduces a BFD discriminator TLV in LSP-Ping. This
   has to be assigned from the TLV type registry maintained by IANA.
   IANA is requested to assign a value of 15 to this TLV.


10. Acknowledgments

   We would like to thank Yakov Rekhter, Dave Katz and Ina Minei for
   contributing to the discussions that formed the basis of this
   document and for their comments. Thanks to Dimitri Papadimitriou for
   his comments and review.


11. References

11.1. Normative References

   [BFD]      Katz, D., and Ward, D., "Bidirectional Forwarding
   Detection",
              draft-ietf-bfd-base-06.txt.

   [BFD-IP]   D. Katz, D. Ward, "BFD for IPv4 and IPv6 (Single Hop)",
              draft-ietf-bfd-v4v6-1hop-05.txt

   [RFC4379]  K. Kompella et. al., "Detecting MPLS Data Plane Failures",
              RFC 4379, February 2006.

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











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11.2. Informative References

   [BFD-MHOP] D. katz, D. Ward, "BFD for Multihop Paths",
              draft-ietf-bfd-multihop-04.txt

   [VCCV]     T. Nadeau, C. Pignataro, R. Aggarwal,
              "Pseudo Wire (PW) Virtual Circuit Connection Verification
   ((VCCV)",
              draft-ietf-pwe3-vccv-13.txt

   [RFC3209]  Awduche, D., et. al, "RSVP-TE: Extensions to RSVP for LSP
              tunnels", RFC 3209, December 2001.

   [RFC4090]  P. Pan, et. al., "Fast Reroute Extensions to RSVP-TE for
              LSP Tunnels", May 2005.

   [RFC3036]  Andersson, L., et al, "LDP Specification", RFC 3036.

   [RFC4364]  E. Rosen, Y. Rekhter, "BGP/MPLS IP VPNs", RFC 4364,
              February 2006.

   [L2-VPN]   K. Kompella, et. al., "Layer 2 VPNs Over Tunnels",
              draft-kompella-ppvpn-l2vpn-03.txt

   [RFC4447]  L. Martini et. al.,"Pseudowire Setup and Maintenance
              using LDP", RFC 4447, April 2006.

   [OAM-MAP]  Nadeau, T., Morrow, M., Busschbach, P., et. al,
              Pseudo Wire (PW) OAM Message Mapping,
              draft-ietf-pwe3-oam-msg-map-02.txt, January 2004

   [RFC4377]  Nadeau, T., et. al, "OAM Requirements for MPLS
              Networks", RFC 4377, February 2006.


12. Author Information

   Rahul Aggarwal
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089
   Email: rahul@juniper.net

   Kireeti Kompella
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA 94089
   Email: kireeti@juniper.net



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Internet Draft         draft-ietf-bfd-mpls-04.txt             March 2007


   Thomas D. Nadeau
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxboro, MA 01719
   Phone: +1-978-936-1470
   Email: tnadeau@cisco.com

   George Swallow
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough , MA - 01719
   USA
   Email: swallow@cisco.com



13. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
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   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.












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14. Full Copyright Statement

   Copyright (C) The IETF Trust (2007).  This document is subject to the
   rights, licenses and restrictions contained in BCP 78, and except as
   set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE,






































Raggarwa                                                       [Page 12]


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