[Docs] [txt|pdf] [Tracker] [WG] [Email] [Nits] [IPR]

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

Network Working Group                                Rahul Aggarwal
Internet Draft                                       Kireeti Kompella
Expiration Date: January 2005                        Juniper Networks
                                                     Thomas D. Nadeau
                                                     George Swallow
                                                     Cisco Systems, Inc


                           BFD For MPLS LSPs


                       draft-ietf-bfd-mpls-00.txt



Status of this Memo


   By submitting this Internet-Draft, I certify that any applicable
   patent or IPR claims of which I am aware have been disclosed, and any
   of which I become aware will be disclosed, in accordance with RFC
   3668.


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''


   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt


   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.



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



draft-ietf-bfd-mpls-00.txt                                      [Page 1]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



   applicability of BFD in relation to LSP-Ping for this application. It
   also describes procedures for using BFD in this environment.



Conventions used in this document


   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 [KEYWORDS].



1. Introduction


   One desirable application of 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 [LSP-PING] 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 in this environment.



2. 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 [OAM-REQ].


2.1. 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) 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 [RSVP-TE]
     b) LDP IPv4/IPv6 prefix [LDP]
     c) VPN IPv4/IPv6 prefix [2547]




draft-ietf-bfd-mpls-00.txt                                      [Page 2]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



     d) Layer 2 VPN [L2-VPN]
     e) Layer 2 Circuit ID [LDP-PW]


   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 in the order
   of seconds. 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 [LSP-FR]. 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].


   We would like to point that the applicability of fast detection to
   MPLS LSPs needs more study and operational input.









draft-ietf-bfd-mpls-00.txt                                      [Page 3]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



2.2. 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) ECMP considerations. LSP-Ping makes it possible to exercise
   multiple alternate paths for a given LSP.


     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 re-synchronizing the MPLS LSP and FEC
   mappings.















draft-ietf-bfd-mpls-00.txt                                      [Page 4]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



3. Theory of Operation


   To use BFD for fault detection on a MPLS LSP a BFD session is
   established for that particular MPLS LSP. BFD control packets are
   sent along the same data path as the LSP being verified and are
   processed by the control plane of the egress LSR. If the LSP is
   associated with multiple FECs, a BFD session is 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 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 make
   the switchover exceeds the BFD fault detection interval a fault will
   be delcared 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.



4. 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 has to 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.



5. Session Establishment


   A BFD session is boot-strapped using LSP-Ping. 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
   carries the local discriminator assigned by the ingress LSR for the
   BFD session. This is subsequently used as the My Discriminator field
   in the BFD session packets sent by the ingress LSR.  The egress LSR
   responds with a echo reply message that carries the local
   discriminator assigned by it for the BFD session. This is
   subsequently used as the My Discriminator field in the BFD session




draft-ietf-bfd-mpls-00.txt                                      [Page 5]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



   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 sends 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. It sends
   control packets to the ingress LSR with the Your Discriminator field
   set to the local discriminator of the ingress LSR.  The ingress LSR
   can use this to demultiplex the BFD session.


5.1. 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. This TLV has a type TBD and 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.



6. Encapsulation


   BFD control packets sent by the ingress LSR are 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 maybe 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 TBD and a source port assigned by
   the sender. The source IP address is a routable address of the
   sender. The destination IP address is a (randomly chosen) address
   from 127/8. The IP TTL is 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 TBD.  The destination IP address and
   UDP port are 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 encapsulated in a
   MPLS label stack and 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. It may also be




draft-ietf-bfd-mpls-00.txt                                      [Page 6]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



   routed based on the destination IP address [BFD-MHOP].



7. Security Considerations


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



8. 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.



9. 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.



10. References


10.1. Normative References


   [BFD]      Katz, D., and Ward, D.,
              "Bidirectional Forwarding Detection",
              draft-katz-ward-bfd-02.txt, August 2003.


   [LSP-Ping] K. Kompella et. al., "Detecting MPLS Data Plane Failures",
              draft-ietf-mpls-lsp-ping-05.txt


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


10.2. Informative References


   [BFD-IP]   D. Katz, D. Ward, "BFD for IPv4 and IPv6 (Single Hop)",
              draft-katz-ward-bfd-v4v6-1hop-01.txt


   [BFD-MHOP] D. katz, D. Ward, "BFD for Multihop Paths",
              draft-ietf-bfd-multihop-00.txt


   [VCCV]     T. Nadeau, R. Aggarwal, "Pseudo Wire (PW) Virtual Circuit
              Connection Verification ((VCCV)",
              draft-ietf-pwe3-vccv-03.txt




draft-ietf-bfd-mpls-00.txt                                      [Page 7]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



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


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


   [2547]     E. Rosen, Y. Rekhter, "BGP/MPLS IP VPNs",
              draft-ietf-l3vpn-rfc2547bis-01.txt


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


   [LDP-PW]   L. Martini et. al.,"Pseudowire Setup and Maintenance
              using LDP",
              draft-ietf-pwe3-control-protocol-08.txt


   [OAM-MAP]  Nadeau, T., Morrow, M., Busschbach, P., et. al,
              Pseudo Wire (PW) OAM Message Mapping,
              draft-nadeau-pwe3-oam-msg-map-05.txt, January 2004


   [OAM-REQ]  Nadeau, T., et. al, "OAM Requirements for MPLS
              Networks", draft-ietf-mpls-oam-requirements-02.txt,
              June 2003.



11. 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


   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.




draft-ietf-bfd-mpls-00.txt                                      [Page 8]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



   300 Beaver Brook Road
   Boxborough , MA - 01719
   USA
   Email: swallow@cisco.com




12. Intellectual Property


   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
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   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.



13. Full Copyright Statement


   Copyright (C) The Internet Society (2004). 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 is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.








draft-ietf-bfd-mpls-00.txt                                      [Page 9]


Internet Draft         draft-ietf-bfd-mpls-00.txt              July 2004



14. Acknowledgement


   Funding for the RFC Editor function is currently provided by the
   Internet Society.
















































draft-ietf-bfd-mpls-00.txt                                     [Page 10]

Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/