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 By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. Raggarwa [Page 1] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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] Raggarwa [Page 3] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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 Raggarwa [Page 5] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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. Raggarwa [Page 6] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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 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 . 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. Raggarwa [Page 7] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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 Raggarwa [Page 8] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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. Raggarwa [Page 9] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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 Raggarwa [Page 10] 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. 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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. Raggarwa [Page 11] Internet Draft draft-ietf-bfd-mpls-04.txt March 2007 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]