Network Working Group D. Katz Internet Draft Juniper Networks D. Ward Cisco Systems Expires: January, 2005 July, 2004 BFD for IPv4 and IPv6 (Single Hop) draft-ietf-bfd-v4v6-1hop-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Katz, Ward [Page 1] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 Abstract This document describes the use of the Bidirectional Forwarding Detection protocol over IPv4 and IPv6 for single IP hops. It further describes the use of BFD with OSPFv2, OSPFv3, and IS-IS. Comments on this draft should be directed to rtg-bfd@ietf.org. 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 very desirable application for BFD [BFD] is to track IPv4 and IPv6 connectivity between directly-connected systems. This could be used to supplant the detection mechanisms in IS-IS and OSPF, or to monitor router-host connectivity, among other applications. This document describes the particulars necessary to use BFD in this environment, and describes how BFD can be used in conjunction OSPFv2 [OSPFv2], OSPFv3 [OSPFv3], and IS-IS [ISIS]. 2. Applications and Limitations This application of BFD can be used by any pair of systems communicating via IPv4 and/or IPv6 across a single IP hop that can be associated with an incoming interface. This includes, but is not limited to, physical media, virtual circuits, and tunnels. Each BFD session between a pair of systems MUST traverse a separate path in both directions. If BFD is to be used in conjunction with both IPv4 and IPv6 on a particular link, a separate BFD session MUST be established for each protocol (and thus encapsulated by that protocol) over that link. Katz, Ward [Page 2] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 3. Initialization and Demultiplexing In this application, there will be only a single BFD session between two systems over a given interface (logical or physical) for a particular protocol. The BFD session must be bound to this interface. As such, both sides of a session MUST take the "Active" role (sending initial BFD Control packets with a zero value of Your Discriminator) and any BFD packet from the remote machine with a zero value of Your Discriminator MUST be associated with the session bound to the remote system, interface, and protocol. 4. Encapsulation 4.1. BFD for IPv4 In the case of IPv4, BFD Control packets MUST be transmitted in UDP packets with destination port 3784, within an IPv4 packet. The source port MUST be in the range 49152 through 65535. The same UDP source port number MUST be used for all BFD Control packets associated with a particular session. The source port number SHOULD be unique among all BFD sessions on the system. If more than 16384 BFD sessions are simultaneously active, UDP source port numbers MAY be reused on multiple sessions, but the number of distinct uses of the same UDP source port number SHOULD be minimized. An implementation MAY use the UDP port source number to aid in demultiplexing incoming BFD Control packets, but ultimately the mechanisms in [BFD] MUST be used to demultiplex incoming packets to the proper session. BFD Echo packets MUST be transmitted in UDP packets with destination UDP port 3785 in an IPv4 packet. The setting of the UDP source port is outside the scope of this specification. The destination address MUST be chosen in such a way as to cause the remote system to forward the packet back to the local system. The source address MUST be chosen in such a way as to preclude the remote system from generating ICMP Redirect messages (in particular, the source address MUST NOT be part of the subnet bound to the interface over which the BFD Echo packet is being transmitted.) 4.2. BFD for IPv6 In the case of IPv6, BFD Control packets MUST be transmitted in UDP packets with destination port 3784, within an IPv6 packet. The source port MUST be in the range 49152 through 65535. The same UDP source port number MUST be used for all BFD Control packets Katz, Ward [Page 3] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 associated with a particular session. The source port number SHOULD be unique among all BFD sessions on the system. If more than 16384 BFD sessions are simultaneously active, UDP source port numbers MAY be reused on multiple sessions, but the number of distinct uses of the same UDP source port number SHOULD be minimized. An implementation MAY use the UDP port source number to aid in demultiplexing incoming BFD Control packets, but ultimately the mechanisms in [BFD] MUST be used to demultiplex incoming packets to the proper session. BFD Echo packets MUST be transmitted in UDP packets with destination UDP port 3785 in an IPv6 packet. The setting of the UDP source port is outside the scope of this specification. The source and destination addresses MUST both be associated with the local system. The destination address MUST be chosen in such a way as to cause the remote system to forward the packet back to the local system. 5. TTL/Hop Count Issues All BFD Control packets for sessions operating according to this specification MUST be sent with a TTL or Hop Count value of 255. All received BFD Control packets that are demultiplexed to sessions operating according to this specification MUST be discarded if the received TTL or Hop Count is not equal to 255. A discussion of this mechanism can be found in [GTSM]. 6. Addressing Issues On a subnetted network, BFD Control packets MUST be transmitted with source and destination addresses that are part of the subnet (addressed from and to interfaces on the subnet.) On an addressed but unsubnetted point-to-point link, BFD Control packets MUST be transmitted with source and destination addresses that match the addresses configured on that link. On an unnumbered point-to-point link, the source address of a BFD Control packet MUST NOT be used to identify the session. This means that the initial BFD packet MUST be accepted with any source address, and that subsequent BFD packets MUST be demultiplexed solely by the My Discriminator field (as is always the case.) This allows the source address to change if necessary. Note that the TTL/Hop Count check described in section 5 precludes the BFD packets from having come from any source other than the immediate neighbor. Katz, Ward [Page 4] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 7. BFD for use with OSPFv2, OSPFv3, and IS-IS The two versions of OSPF, as well as IS-IS, all suffer from an architectural limitation, namely that their Hello protocols are limited in the granularity of failure their detection times. In particular, OSPF has a minimum detection time of two seconds, and IS- IS has a minimum detection time of one second. BFD MAY be used to achieve arbitrarily small detection times for these protocols by supplanting the Hello protocols used in each case. 7.1. Session Establishment The mechanism by which a BFD session is established in this environment is outside the scope of this specification. An obvious choice would be to use the discovery mechanism inherent in the Hello protocols in OSPF and IS-IS to bootstrap the establishment of a BFD session. Any BFD sessions established to support OSPF and IS-IS across a single IP hop MUST operate in accordance with the rest of this document. If multiple routing protocols wish to establish BFD sessions with the same remote system for the same data protocol, all MUST share a single BFD session. 7.2. Session Parameters The setting of the various timing parameters and modes in this application are outside the scope of this specification. Note that all protocols sharing a session will operate using the same parameters. The mechanism for choosing the parameters among those desired by the various protocols are outside the scope of this specification. 7.3. Interactions with OSPF and IS-IS without Graceful Restart When a BFD session transitions from Up to Failing, action SHOULD be taken in the routing protocol to signal the lack of connectivity for the data protocol (IPv4 or IPv6) over which BFD is running. If only one data protocol is being advertised in the routing protocol Hello, or if multiple protocols are being advertised but the protocols must share a common topology, a Hello protocol timeout SHOULD be emulated Katz, Ward [Page 5] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 for the associated OSPF neighbors and/or IS-IS adjacencies. If multiple data protocols are advertised in the routing protocol Hello, and the routing protocol supports different topologies for each data protocol, the failing data protocol SHOULD no longer be advertised in Hello packets in order to signal a lack of connectivity for that protocol. If a BFD session never reaches Up state (possibly because the remote system does not support BFD), this MUST NOT preclude the establishment of an OSPF neighbor or an IS-IS adjacency. 7.4. Interactions with OSPF and IS-IS with Graceful Restart The Graceful Restart functions in OSPF [OSPF-GRACE] and IS-IS [ISIS- GRACE] are predicated on the existence of a separate forwarding plane that does not necessarily share fate with the control plane in which the routing protocols operate. In particular, the assumption is that the forwarding plane can continue to function while the protocols restart and sort things out. BFD implementations announce via the Control Plane Independent (C) bit whether or not BFD shares fate with the control plane. This information is used to determine the actions to be taken in conjunction with Graceful Restart. If BFD does not share its fate with the control plane on either system, it can be used to determine whether Graceful Restart is NOT viable (the forwarding plane is not operating.) In this situation, if a BFD session fails while graceful restart is taking place, and BFD is independent of the control plane on the local system, and the remote system has been transmitting BFD Control packets with the C bit set, the graceful restart SHOULD be aborted and the topology change made visible to the network as outlined in section 7.3. If BFD shares its fate with the control plane on either system (either the local system shares fate with the control plane, or the remote system is transmitting BFD packets with the C bit set to zero), it is not useful during graceful restart, as the BFD session is likely to fail regardless of the state of the forwarding plane. In this situation, if a BFD session fails while graceful restart is taking place (or if the BFD session failure triggers a graceful restart event), the graceful restart SHOULD be allowed to complete and the topology change should not be made visible to the network as outlined in section 7.3. Katz, Ward [Page 6] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 7.5. OSPF Virtual Links If it is desired to use BFD for failure detction of OSPF Virtual Links, the mechanism described in [BFD-MULTI] MUST be used, since OSPF Virtual Links may traverse an arbitrary number of hops. BFD Authentication SHOULD be used and is strongly encouraged. 8. BFD for use with Tunnels A number of mechanisms are available to tunnel IPv4 and IPv6 over arbitrary topologies. If the tunnel mechanism does not decrement the TTL or hop count of the network protocol carried within, the mechanism described in this document may be used to provide liveness detection for the tunnel. The BFD Authentication mechanism SHOULD be used and is strongly encouraged. Normative References [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", draft-ietf-bfd-base-00.txt, July, 2004. [BFD-MULTI] Katz, D., and Ward, D., "BFD for Multihop Paths", draft- ietf-bfd-multihop-00.txt, July, 2004. [GTSM] Gill, V., et al, "The Generalized TTL Security Mechanism (GTSM)", RFC 3682, February 2004. [ISIS] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990. [ISIS-GRACE] Shand, M., and Ginsberg, L., "Restart signaling for IS- IS", draft-ietf-isis-restart-05.txt, January 2004. [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. [OSPF-GRACE] Moy, J., et al, "Graceful OSPF Restart", RFC 3623, November 2003. Katz, Ward [Page 7] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 Security Considerations In this application, the use of TTL=255 on transmit and receive is viewed as supplying equivalent security characteristics to other protocols used in the infrastructure, as it is not trivially spoofable. The security implications of this mechanism are further discussed in the GTSM specification. The security implications of the use of BFD Authentication are discussed in the base BFD specification. Authors' Addresses Dave Katz Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, California 94089-1206 USA Phone: +1-408-745-2000 Email: dkatz@juniper.net Dave Ward Cisco Systems 170 W. Tasman Dr. San Jose, CA 95134 USA Phone: +1-408-526-4000 Email: dward@cisco.com Changes from the previous draft The only significant changes to this version are the addition of language describing tunnels and OSPF virtual links. All other changes are editorial in nature. Katz, Ward [Page 8] Internet Draft BFD for IPv4 and IPv6 (Single Hop) July, 2004 Full Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. 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