Bidirectional Forwarding Detection (BFD)

for MPLS Label Switched Paths (LSPs)

Abstract

One desirable application of Bidirectional Forwarding Detection (BFD) is to detect a Multiprotocol Label Switching (MPLS) Label Switched Path (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.

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/rfc5884.

Copyright Notice

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Table of Contents

   1. Introduction ....................................................3
   2. Specification of Requirements ...................................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 .............................................6
   5. Initialization and Demultiplexing ...............................7
   6. Session Establishment ...........................................7
      6.1. BFD Discriminator TLV in LSP Ping ..........................8
   7. Encapsulation ...................................................8
   8. Security Considerations .........................................9
   9. IANA Considerations ............................................10
   10. Acknowledgments ...............................................10
   11. References ....................................................10
      11.1. Normative References .....................................10
      11.2. Informative References ...................................10

1. Introduction

One desirable application of Bidirectional Forwarding Detection (BFD) is to track the liveness of a Multiprotocol Label Switching (MPLS) Label Switched Path (LSP). In particular, BFD can be used to detect a data plane failure in the forwarding path of an 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.

2. 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].

3. Applicability

In the event of an 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 [RFC4379] is an existing mechanism for detecting an 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 an MPLS LSP. The LSP may be associated with any of the following FECs:

      a) Resource Reservation Protocol (RSVP) LSP_Tunnel IPv4/IPv6
      
         Session [RFC3209]
      
      b) Label Distribution Protocol (LDP) IPv4/IPv6 prefix [RFC5036]
      
      c) Virtual Private Network (VPN) IPv4/IPv6 prefix [RFC4364]
      d) Layer 2 VPN [L2-VPN]

e) Pseudowires based on PWid FEC and Generalized PWid FEC

         [RFC4447]
      
      f) Border Gateway Protocol (BGP) labeled prefixes [RFC3107]

LSP Ping includes extensive control plane verification. BFD, on the other hand, was designed as a lightweight 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, it is 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 a 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 Pseudowires (PWs). 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.

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

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 functionality of LSP Ping. In particular, 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 an LSP Ping Echo request message with a FEC. In

the case of Penultimate Hop Popping (PHP) or when the egress Label Switching Router (LSR) distributes an explicit null label to the penultimate hop router, 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, presence of the FEC in the Echo request message makes it 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 bootstrapping 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 an 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 be 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 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 an 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 implementation MAY provide configuration options to control the rate of generation of the periodic LSP Ping Echo request messages.

To enable fault detection procedures specified in this document, for a particular MPLS LSP, this document requires the ingress and egress LSRs to be configured. This includes configuration for supporting BFD and LSP Ping as specified in this document. It also includes configuration that enables the ingress LSR to determine the method used by the egress LSR to identify Operations, Administration, and Maintenance (OAM) packets, e.g., whether the Time to Live (TTL) of the innermost MPLS label needs to be set to 1 to enable the egress

LSR to identify the OAM packet. For fault detection for MPLS PWs, this document assumes that the PW control channel type [RFC5085] is configured and the support of LSP Ping is also configured.

5. Initialization and Demultiplexing

A BFD session may be established for a FEC associated with an MPLS LSP. As described above, in the case of PHP or when the egress LSR distributes an explicit null label to the penultimate hop router, or 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.

6. Session Establishment

A BFD session is bootstrapped using LSP Ping. This specification describes procedures only for BFD asynchronous mode. BFD demand mode is outside the scope of this specification. Further, the use of the Echo function is outside the scope of this specification. 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, an 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, if the validation of the FEC in the LSP Ping Echo request message succeeds. 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 an 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.

The ingress LSR follows the procedures in [BFD] to send BFD Control packets to the egress LSR in response to the BFD Control packets received from the egress LSR. The BFD Control packets from the ingress to the egress LSR MUST set the local discriminator of the egress LSR, in the Your Discriminator field. 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 has assigned 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.

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 [RFC4379]. For MPLS PWs, alternatively, the presence of a fault detection message may be indicated by setting a bit in the control word [RFC5085].

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 MUST be randomly chosen from the 127/8 range for IPv4 and from the 0:0:0:0:0:FFFF:7F00/104 range for IPv6 with the following exception. If the FEC is an 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 for IPv4 or 0:0:0:0:0:FFFF:7F00/104 range for IPv6 discovered by LSP Ping traceroute [RFC4379] to exercise that particular alternate path.

The motivation for using the address range 127/8 is the same as specified in Section 2.1 of [RFC4379]. This is an exception to the behavior defined in [RFC1122].

The IP TTL or hop limit MUST be set to 1 [RFC4379].

BFD Control packets sent by the egress LSR are UDP packets. The source IP address is a routable address of the replier.

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]. If this is the case, the destination IP address MUST be set to the source IP address of the LSP Ping Echo request message, received by the egress LSR from the ingress LSR.

Or the BFD Control packet sent by the egress LSR to the ingress LSR MAY be encapsulated in an MPLS label stack. In this 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 performed corresponds to a bidirectional LSP or an MPLS PW. This may also be the case when there is a return LSP from the egress LSR to the ingress LSR. In this case, the destination IP address MUST be randomly chosen from the 127/8 range for IPv4 and from the 0:0:0:0:0:FFFF:7F00/104 range for IPv6.

The BFD Control packet sent by the egress LSR MUST have a well-known destination port 4784, if it is routed [BFD-MHOP], or it MUST have a well-known destination port 3784 [BFD-IP] if it is encapsulated in a MPLS label stack. The source port MUST be assigned by the egress LSR as per the procedures in [BFD-IP].

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 an LSR MUST ignore BFD packets for a given session, 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 received packet do not match the values associated with the session.

8. Security Considerations

Security considerations discussed in [BFD], [BFD-MHOP], and [RFC4379] apply to this document. For BFD Control packets sent by the ingress LSR or when the BFD Control packet sent by the egress LSR are encapsulated in an MPLS label stack, MPLS security considerations apply. These are discussed in [MPLS-SEC]. When BFD Control packets sent by the egress LSR are routed, the authentication considerations discussed in [BFD-MHOP] should be followed.

9. IANA Considerations

This document introduces a BFD discriminator TLV in LSP Ping. The BFD Discriminator has been assigned a value of 15 from the LSP Ping TLVs and sub-TLVs registry maintained by IANA.

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. Thanks to Carlos Pignataro for his comments and review.

11. References

11.1. Normative References

   [BFD]      Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", RFC 5880, June 2010.
   
   [BFD-IP]   Katz, D. and  D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
              2010.
   
   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

11.2. Informative References

   [BFD-MHOP] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, June 2010.
   
   [RFC5085]  Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
              Virtual Circuit Connectivity Verification (VCCV): A
              Control Channel for Pseudowires", RFC 5085, December 2007.
   
   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.
   
   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              May 2005.
   
   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, October 2007.
   
   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.
   
   [L2-VPN]   Kompella, K., Leelanivas, M., Vohra, Q., Achirica, J.,
              Bonica, R., Cooper, D., Liljenstolpe, C., Metz, E., Ould-
              Brahim, H., Sargor, C., Shah, H., Srinivasan, and Z.
              Zhang, "Layer 2 VPNs Over Tunnels", Work in Progress,
              February 2003.
   
   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447, April 2006.
   
   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, May 2001.
   
   [RFC4377]  Nadeau, T., Morrow, M., Swallow, G., Allan, D., and S.
              Matsushima, "Operations and Management (OAM) Requirements
              for Multi-Protocol Label Switched (MPLS) Networks", RFC
              4377, February 2006.
   
   [MPLS-SEC] Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", Work in Progress, October 2009.

Authors' Addresses

   Rahul Aggarwal
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   USA

EMail:

          rahul@juniper.net
   
   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   USA

EMail:

          kireeti@juniper.net

Thomas D. Nadeau
BT
BT Centre
81 Newgate Street
London EC1A 7AJ
UK

   EMail: tom.nadeau@bt.com
   
   George Swallow
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA  01719
   USA

EMail:

          swallow@cisco.com