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Katz 3 Internet Draft Juniper Networks 4 Intended status: Proposed Standard D. Ward 5 Cisco Systems 6 Expires: August, 2009 February 5, 2009 8 BFD for Multihop Paths 9 draft-ietf-bfd-multihop-07.txt 11 Status of this Memo 13 This Internet-Draft is submitted to IETF in full conformance with the 14 provisions of BCP 78 and BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/1id-abstracts.html 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html 32 Copyright Notice 34 Copyright (c) 2009 IETF Trust and the persons identified as the 35 document authors. All rights reserved. 37 This document is subject to BCP 78 and the IETF Trust's Legal 38 Provisions Relating to IETF Documents 39 (http://trustee.ietf.org/license-info) in effect on the date of 40 publication of this document. Please review these documents 41 carefully, as they describe your rights and restrictions with respect 42 to this document. 44 Abstract 46 This document describes the use of the Bidirectional Forwarding 47 Detection protocol (BFD) over multihop paths, including 48 unidirectional links. 50 Conventions used in this document 52 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 53 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 54 document are to be interpreted as described in RFC-2119 [KEYWORDS]. 56 1. Introduction 58 The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a 59 method for liveness detection of arbitrary paths between systems. 60 The BFD one-hop specification [BFD-1HOP] describes how to use BFD 61 across single hops of IPv4 and IPv6. 63 BFD can also be useful on arbitrary paths between systems, which may 64 span multiple network hops and follow unpredictable paths. 65 Furthermore, a pair of systems may have multiple paths between them 66 that may overlap. This document describes methods for using BFD in 67 such scenarios. 69 2. Issues 71 There are three primary issues in the use of BFD for multihop paths. 72 The first is security and spoofing; [BFD-1HOP] describes a 73 lightweight method of avoiding spoofing by requiring a TTL/hop limit 74 of 255 on both transmit and receive, but this obviously does not work 75 across multiple hops. The utilization of BFD authentication 76 addresses this issue. 78 The second, more subtle issue is that of demultiplexing multiple BFD 79 sessions between the same pair of systems to the proper BFD session. 80 In particular, the first BFD packet received for a session may carry 81 a Your Discriminator value of zero, resulting in ambiguity as to 82 which session the packet should be associated. Once the 83 discriminator values have been exchanged, all further packets are 84 demultiplexed to the proper BFD session solely by the contents of the 85 Your Discriminator field. 87 [BFD-1HOP] addresses this by requiring that multiple sessions 88 traverse independent physical or logical links--the first packet is 89 demultiplexed based on the link over which it was received. In the 90 more general case, this scheme cannot work, as two paths over which 91 BFD is running may overlap to an arbitrary degree (including the 92 first and/or last hop.) 94 Finally, the Echo function MUST NOT be used over multiple hops. 95 Intermediate hops would route the packets back to the sender, and 96 connectivity through the entire path would not be possible to verify. 98 3. Demultiplexing Packets 100 There are a number of possibilities for addressing the demultiplexing 101 issue which may be used, depending on the application. 103 3.1. Totally Arbitrary Paths 105 It may be desired to use BFD for liveness detection over paths for 106 which no part of the route is known (or if known, may not be stable.) 107 A straightforward approach to this problem is to limit BFD deployment 108 to a single session between a source/destination address pair. 109 Multiple sessions between the same pair of systems must have at least 110 one endpoint address distinct from one another. 112 In this scenario, the initial packet is demultiplexed to the 113 appropriate BFD session based on the source/destination address pair 114 when Your Discriminator is set to zero. 116 This approach is appropriate for general connectivity detection 117 between systems over routed paths, and is also useful for OSPF 118 Virtual Links [OSPFv2] [OSPFv3]. 120 3.2. Out-of-band Discriminator Signaling 122 Another approach to the demultiplexing problem is to signal the 123 discriminator values in each direction through an out-of-band 124 mechanism prior to establishing the BFD session. Once learned, the 125 discriminators are sent as usual in the BFD Control packets; no 126 packets with Your Discriminator set to zero are ever sent. This 127 method is used by the BFD MPLS specification [BFD-MPLS]. 129 This approach is advantageous because it allows BFD to be directed by 130 other system components that have knowledge of the paths in use, and 131 from the perspective of BFD implementation it is very simple. 133 The disadvantage is that it requires at least some level of BFD- 134 specific knowledge in parts of the system outside of BFD. 136 3.3. Unidirectional Links 138 Unidirectional links are classified as multihop paths because the 139 return path (which should exist at some level in order to make the 140 link useful) may be arbitrary, and the return paths for BFD sessions 141 protecting parallel unidirectional links may overlap or even be 142 identical. (If two unidirectional links, one in each direction, are 143 to carry a single BFD session, this can be done using the single-hop 144 approach.) 146 Either of the two methods outlined earlier may be used in the 147 Unidirectional link case, but a more general solution can be done 148 strictly within BFD and without addressing limitations. 150 The approach is similar to the one-hop specification, since the 151 unidirectional link is a single hop. Let's define the two systems as 152 the Unidirectional Sender and the Unidirectional Receiver. In this 153 approach the Unidirectional Sender MUST operate in the Active role 154 (as defined in the base BFD specification), and the Unidirectional 155 Receiver MUST operate in the Passive role. 157 In the Passive role, by definition, the Unidirectional Receiver does 158 not transmit any BFD Control packets until it learns the 159 discriminator value in use by the other system (upon receipt of the 160 first BFD Control packet.) The Unidirectional Receiver demultiplexes 161 the first packet to the proper BFD session based on the physical or 162 logical link over which was received. This allows the receiver to 163 learn the remote discriminator value, which it then echoes back to 164 the sender in its own (arbitrarily routed) BFD Control packet, after 165 which time all packets are demultiplexed solely by discriminator. 167 4. Encapsulation 169 The encapsulation of BFD Control packets for multihop application in 170 IPv4 and IPv6 is identical to that defined in [BFD-1HOP], except that 171 the UDP destination port MUST have a value of 4784. This can aid in 172 the demultiplexing and internal routing of incoming BFD packets. 174 5. Authentication 176 By their nature, multihop paths expose BFD to spoofing. As the 177 number of hops increase, the exposure to attack grows. As such, 178 implementations of BFD SHOULD utilize cryptographic authentication 179 over multihop paths to help mitigate denial-of-service attacks. 181 Normative References 183 [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", 184 draft-ietf-bfd-base-09.txt, February, 2009. 186 [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single 187 Hop)", draft-ietf-bfd-v4v6-1hop-09.txt, February, 2009. 189 [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate 190 Requirement Levels", RFC 2119, March 1997. 192 Informative References 194 [BFD-MPLS] Aggarwal, R., Kompella, K., et al, "BFD for MPLS LSPs", 195 draft-ietf-bfd-mpls-07.txt, June, 2008. 197 [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 199 [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. 201 Security Considerations 203 As the number of hops increases, BFD becomes further exposed to 204 attack. The use of strong forms of authentication is strongly 205 encouraged. 207 No additional security issues are raised in this document beyond 208 those that exist in the referenced BFD documents. 210 IANA Considerations 212 This document has no actions for IANA. 214 Authors' Addresses 216 Dave Katz 217 Juniper Networks 218 1194 N. Mathilda Ave. 219 Sunnyvale, California 94089-1206 USA 220 Phone: +1-408-745-2000 221 Email: dkatz@juniper.net 223 Dave Ward 224 Cisco Systems 225 170 W. Tasman Dr. 226 San Jose, CA 95134 USA 227 Phone: +1-408-526-4000 228 Email: dward@cisco.com 230 Changes from the previous draft 232 A note was added that the Echo function is not available for multihop 233 BFD. All other changes are editorial in nature. 235 This document expires in August, 2009.