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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group D. Katz 2 Internet Draft Juniper Networks 3 D. Ward 4 Cisco Systems 5 Expires: January, 2005 July, 2004 7 BFD for Multihop Paths 8 draft-ietf-bfd-multihop-00.txt 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 Copyright Notice 33 Copyright (C) The Internet Society (2004). All Rights Reserved. 35 Abstract 37 This document describes the use of the Bidirectional Forwarding 38 Detection protocol (BFD) over multihop paths, including 39 unidirectional links. 41 Conventions used in this document 43 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 44 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 45 document are to be interpreted as described in RFC-2119 [KEYWORDS]. 47 1. Introduction 49 The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a 50 method for liveness detection of arbitrary paths between systems. 51 The BFD one-hop specification [BFD-1HOP] describes how to use BFD 52 across single hops of IPv4 and IPv6. 54 BFD can also be useful on arbitrary paths between systems, which may 55 span multiple network hops and follow unpredictable paths. 56 Furthermore, a pair of systems may have multiple paths between them 57 that may overlap. This document describes methods for using BFD in 58 such scenarios. 60 2. Issues 62 There are two primary issues in the use of BFD for multihop paths. 63 The first is security and spoofing; the one-hop spec describes a 64 lightweight method of avoiding spoofing by requiring a TTL/hop limit 65 of 255 on both transmit and receive, but this obviously does not work 66 across multiple hops. The utilization of BFD authentication 67 addresses this issue. 69 The more subtle issue is that of demultiplexing multiple BFD sessions 70 between the same pair of systems to the proper BFD session. In 71 particular, the first BFD packet received for a session may carry a 72 Your Discriminator value of zero, resulting in ambiguity as to which 73 session the packet should be associated. Once the discriminator 74 values have been exchanged, all further packets are demultiplexed to 75 the proper BFD session solely by the contents of the Your 76 Discriminator field. 78 The one-hop specification addresses this by requiring that multiple 79 sessions traverse independent physical or logical links--the first 80 packet is demultiplexed based on the link over which it was received. 81 In the more general case, this scheme cannot work, as two paths over 82 which BFD is running may overlap to an arbitrary degree (including 83 the first and/or last hop.) 85 3. Demultiplexing Packets 87 There are a number of possibilities for addressing the demultiplexing 88 issue which may be used, depending on the application. 90 3.1. Totally Arbitrary Paths 92 It may be desired to use BFD for liveness detection over paths for 93 which no part of the route is known (or if known, may not be stable.) 94 A straightforward approach to this problem is to limit BFD deployment 95 to a single session between a source/destination address pair. 96 Multiple sessions between the same pair of systems must have at least 97 one endpoint address distinct from one another. 99 In this scenario, the initial packet is demultiplexed to the 100 appropriate BFD session based on the source/destination address pair 101 when Your Discriminator is set to zero. 103 This approach is appropriate for general connectivity detection 104 between systems over routed paths, and is also useful for OSPF 105 Virtual Links [OSPFv2] [OSPFv3]. 107 3.2. Out-of-band Discriminator Signalling 109 Another approach to the demultiplexing problem is to signal the 110 discriminator values in each direction through an out-of-band 111 mechanism prior to establishing the BFD session. Once learned, the 112 discriminators are sent as usual in the BFD Control packets; no 113 packets with Your Discriminator set to zero are ever sent. This 114 method is used by the BFD MPLS specification [BFD-MPLS]. 116 This approach is advantageous because it allows BFD to be directed by 117 other system components that have knowledge of the paths in use, and 118 from BFD's perspective it is very simple. 120 The disadvantage is that it requires at least some level of BFD- 121 specific knowledge in parts of the system outside of BFD. 123 3.3. Unidirectional Links 125 Unidirectional links are classified as multihop paths because the 126 return path (which must exist at some level in order to make the link 127 useful) may be arbitrary, and the return paths for BFD sessions 128 protecting parallel unidirectional links may overlap or even be 129 identical. (If two unidirection links, one in each direction, are to 130 carry a single BFD session, this can be done using the single-hop 131 approach.) 133 Either of the two methods outlined earlier may be used in the 134 Unidirectional link case (as an MPLS LSP is in fact a unidirectional 135 link), but a more general solution can be done strictly within BFD 136 and without addressing limitations. 138 The approach is similar to the one-hop specification, since the 139 unidirectional link is a single hop. Let's define the two systems as 140 the Unidirectional Sender and the Unidirectional Receiver. In this 141 approach the Unidirectional Sender MUST operate in the Active role 142 (as defined in the base BFD specification), and the Unidirectional 143 Receiver MUST operate in the Passive role. 145 In the Passive role, by definition, the Unidirectional Receiver does 146 not transmit any BFD Control packets until it learns the 147 discriminator value in use by the other system (upon receipt of the 148 first BFD Control packet.) The Unidirectional Receiver demultiplexes 149 the first packet to the proper BFD session based on the physical or 150 logical link over which was received. This allows the receiver to 151 learn the remote discriminator value, which it then echoes back to 152 the sender in its own (arbitrarily routed) BFD Control packet, after 153 which time all packets are demultiplexed solely by discriminator. 155 4. Authentication 157 By their nature, multihop paths expose BFD to spoofing. 158 Implementations of BFD SHOULD utilize authentication over multihop 159 paths to help mitigate denial-of-service attacks. 161 Normative References 163 [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", 164 draft-ietf-bfd-base-00.txt, July, 2004. 166 [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single 167 Hop)", draft-ietf-bfd-v4v6-1hop-00.txt, July, 2004. 169 [BFD-MPLS] Aggarwal, R., and Kompella, K., "BFD for MPLS LSPs", 170 draft-ietf-bfd-mpls-00.txt, July, 2004. 172 [GTSM] Gill, V., et al, "The Generalized TTL Security Mechanism 173 (GTSM)", RFC 3682, February 2004. 175 [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate 176 Requirement Levels", RFC 2119, March 1997. 178 [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 180 [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. 182 Security Considerations 184 No additional security issues are raised in this document beyond 185 those that exist in the referenced BFD documents. 187 Authors' Addresses 189 Dave Katz 190 Juniper Networks 191 1194 N. Mathilda Ave. 192 Sunnyvale, California 94089-1206 USA 193 Phone: +1-408-745-2000 194 Email: dkatz@juniper.net 196 Dave Ward 197 Cisco Systems 198 170 W. Tasman Dr. 199 San Jose, CA 95134 USA 200 Phone: +1-408-526-4000 201 Email: dward@cisco.com 203 Full Copyright Notice 205 Copyright (C) The Internet Society (2004). All Rights Reserved. 207 This document and translations of it may be copied and furnished to 208 others, and derivative works that comment on or otherwise explain it 209 or assist in its implementation may be prepared, copied, published 210 and distributed, in whole or in part, without restriction of any 211 kind, provided that the above copyright notice and this paragraph are 212 included on all such copies and derivative works. However, this 213 document itself may not be modified in any way, such as by removing 214 the copyright notice or references to the Internet Society or other 215 Internet organizations, except as needed for the purpose of 216 developing Internet standards in which case the procedures for 217 copyrights defined in the Internet Standards process must be 218 followed, or as required to translate it into languages other than 219 English. 221 The limited permissions granted above are perpetual and will not be 222 revoked by the Internet Society or its successors or assigns. 224 This document and the information contained herein is provided on an 225 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 226 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 227 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 228 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 229 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." 231 Acknowledgement 233 Funding for the RFC Editor function is currently provided by the 234 Internet Society.