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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Katz 3 Internet Draft Juniper Networks 4 D. Ward 5 Cisco Systems 6 Expires: September, 2005 March, 2005 8 BFD for Multihop Paths 9 draft-ietf-bfd-multihop-02.txt 11 Status of this Memo 13 By submitting this Internet-Draft, I certify that any applicable 14 patent or other IPR claims of which I am aware have been disclosed, 15 or will be disclosed, and any of which I become aware will be 16 disclosed, in accordance with RFC 3668. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/1id-abstracts.html 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html 34 Copyright Notice 36 Copyright (C) The Internet Society (2005). All Rights Reserved. 38 Abstract 40 This document describes the use of the Bidirectional Forwarding 41 Detection protocol (BFD) over multihop paths, including 42 unidirectional links. 44 Conventions used in this document 46 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 47 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 48 document are to be interpreted as described in RFC-2119 [KEYWORDS]. 50 1. Introduction 52 The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a 53 method for liveness detection of arbitrary paths between systems. 54 The BFD one-hop specification [BFD-1HOP] describes how to use BFD 55 across single hops of IPv4 and IPv6. 57 BFD can also be useful on arbitrary paths between systems, which may 58 span multiple network hops and follow unpredictable paths. 59 Furthermore, a pair of systems may have multiple paths between them 60 that may overlap. This document describes methods for using BFD in 61 such scenarios. 63 2. Issues 65 There are two primary issues in the use of BFD for multihop paths. 66 The first is security and spoofing; [BFD-1HOP] describes a 67 lightweight method of avoiding spoofing by requiring a TTL/hop limit 68 of 255 on both transmit and receive, but this obviously does not work 69 across multiple hops. The utilization of BFD authentication 70 addresses this issue. 72 The more subtle issue is that of demultiplexing multiple BFD sessions 73 between the same pair of systems to the proper BFD session. In 74 particular, the first BFD packet received for a session may carry a 75 Your Discriminator value of zero, resulting in ambiguity as to which 76 session the packet should be associated. Once the discriminator 77 values have been exchanged, all further packets are demultiplexed to 78 the proper BFD session solely by the contents of the Your 79 Discriminator field. 81 [BFD-1HOP] addresses this by requiring that multiple sessions 82 traverse independent physical or logical links--the first packet is 83 demultiplexed based on the link over which it was received. In the 84 more general case, this scheme cannot work, as two paths over which 85 BFD is running may overlap to an arbitrary degree (including the 86 first and/or last hop.) 88 3. Demultiplexing Packets 90 There are a number of possibilities for addressing the demultiplexing 91 issue which may be used, depending on the application. 93 3.1. Totally Arbitrary Paths 95 It may be desired to use BFD for liveness detection over paths for 96 which no part of the route is known (or if known, may not be stable.) 97 A straightforward approach to this problem is to limit BFD deployment 98 to a single session between a source/destination address pair. 99 Multiple sessions between the same pair of systems must have at least 100 one endpoint address distinct from one another. 102 In this scenario, the initial packet is demultiplexed to the 103 appropriate BFD session based on the source/destination address pair 104 when Your Discriminator is set to zero. 106 This approach is appropriate for general connectivity detection 107 between systems over routed paths, and is also useful for OSPF 108 Virtual Links [OSPFv2] [OSPFv3]. 110 3.2. Out-of-band Discriminator Signalling 112 Another approach to the demultiplexing problem is to signal the 113 discriminator values in each direction through an out-of-band 114 mechanism prior to establishing the BFD session. Once learned, the 115 discriminators are sent as usual in the BFD Control packets; no 116 packets with Your Discriminator set to zero are ever sent. This 117 method is used by the BFD MPLS specification [BFD-MPLS]. 119 This approach is advantageous because it allows BFD to be directed by 120 other system components that have knowledge of the paths in use, and 121 from BFD's perspective it is very simple. 123 The disadvantage is that it requires at least some level of BFD- 124 specific knowledge in parts of the system outside of BFD. 126 3.3. Unidirectional Links 128 Unidirectional links are classified as multihop paths because the 129 return path (which must exist at some level in order to make the link 130 useful) may be arbitrary, and the return paths for BFD sessions 131 protecting parallel unidirectional links may overlap or even be 132 identical. (If two unidirection links, one in each direction, are to 133 carry a single BFD session, this can be done using the single-hop 134 approach.) 136 Either of the two methods outlined earlier may be used in the 137 Unidirectional link case, but a more general solution can be done 138 strictly within BFD and without addressing limitations. 140 The approach is similar to the one-hop specification, since the 141 unidirectional link is a single hop. Let's define the two systems as 142 the Unidirectional Sender and the Unidirectional Receiver. In this 143 approach the Unidirectional Sender MUST operate in the Active role 144 (as defined in the base BFD specification), and the Unidirectional 145 Receiver MUST operate in the Passive role. 147 In the Passive role, by definition, the Unidirectional Receiver does 148 not transmit any BFD Control packets until it learns the 149 discriminator value in use by the other system (upon receipt of the 150 first BFD Control packet.) The Unidirectional Receiver demultiplexes 151 the first packet to the proper BFD session based on the physical or 152 logical link over which was received. This allows the receiver to 153 learn the remote discriminator value, which it then echoes back to 154 the sender in its own (arbitrarily routed) BFD Control packet, after 155 which time all packets are demultiplexed solely by discriminator. 157 4. Authentication 159 By their nature, multihop paths expose BFD to spoofing. 160 Implementations of BFD SHOULD utilize authentication over multihop 161 paths to help mitigate denial-of-service attacks. 163 Normative References 165 [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", 166 draft-ietf-bfd-base-02.txt, March, 2005. 168 [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single 169 Hop)", draft-ietf-bfd-v4v6-1hop-02.txt, March, 2005. 171 [BFD-MPLS] Aggarwal, R., and Kompella, K., "BFD for MPLS LSPs", 172 draft-ietf-bfd-mpls-01.txt, February, 2005. 174 [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate 175 Requirement Levels", RFC 2119, March 1997. 177 [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 179 [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. 181 Security Considerations 183 No additional security issues are raised in this document beyond 184 those that exist in the referenced BFD documents. 186 Authors' Addresses 188 Dave Katz 189 Juniper Networks 190 1194 N. Mathilda Ave. 191 Sunnyvale, California 94089-1206 USA 192 Phone: +1-408-745-2000 193 Email: dkatz@juniper.net 195 Dave Ward 196 Cisco Systems 197 170 W. Tasman Dr. 198 San Jose, CA 95134 USA 199 Phone: +1-408-526-4000 200 Email: dward@cisco.com 202 Changes from the previous draft 204 No changes were made other than updating references. 206 Full Copyright Notice 208 Copyright (C) The Internet Society (2005). This document is subject 209 to the rights, licenses and restrictions contained in BCP 78, and 210 except as set forth therein, the authors retain all their rights. 212 This document and the information contained herein are provided on an 213 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 214 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 215 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 216 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 217 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 218 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 220 Acknowledgement 222 Funding for the RFC Editor function is currently provided by the 223 Internet Society. 225 This document expires in September, 2005.