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Katz 3 Internet Draft Juniper Networks 4 Intended status: Proposed Standard D. Ward 5 Cisco Systems 6 Expires: April, 2010 October 16, 2009 8 BFD for Multihop Paths 9 draft-ietf-bfd-multihop-08.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. Code Components extracted from this document must 43 include Simplified BSD License text as described in Section 4.e of 44 the Trust Legal Provisions and are provided without warranty as 45 described in the BSD License. 47 Abstract 49 This document describes the use of the Bidirectional Forwarding 50 Detection protocol (BFD) over multihop paths, including 51 unidirectional links. 53 Conventions used in this document 55 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 56 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 57 document are to be interpreted as described in RFC-2119 [KEYWORDS]. 59 1. Introduction 61 The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a 62 method for liveness detection of arbitrary paths between systems. 63 The BFD one-hop specification [BFD-1HOP] describes how to use BFD 64 across single hops of IPv4 and IPv6. 66 BFD can also be useful on arbitrary paths between systems, which may 67 span multiple network hops and follow unpredictable paths. 68 Furthermore, a pair of systems may have multiple paths between them 69 that may overlap. This document describes methods for using BFD in 70 such scenarios. 72 2. Issues 74 There are three primary issues in the use of BFD for multihop paths. 75 The first is security and spoofing; [BFD-1HOP] describes a 76 lightweight method of avoiding spoofing by requiring a TTL/hop limit 77 of 255 on both transmit and receive, but this obviously does not work 78 across multiple hops. The utilization of BFD authentication 79 addresses this issue. 81 The second, more subtle issue is that of demultiplexing multiple BFD 82 sessions between the same pair of systems to the proper BFD session. 83 In particular, the first BFD packet received for a session may carry 84 a Your Discriminator value of zero, resulting in ambiguity as to 85 which session the packet should be associated. Once the 86 discriminator values have been exchanged, all further packets are 87 demultiplexed to the proper BFD session solely by the contents of the 88 Your Discriminator field. 90 [BFD-1HOP] addresses this by requiring that multiple sessions 91 traverse independent physical or logical links--the first packet is 92 demultiplexed based on the link over which it was received. In the 93 more general case, this scheme cannot work, as two paths over which 94 BFD is running may overlap to an arbitrary degree (including the 95 first and/or last hop.) 97 Finally, the Echo function MUST NOT be used over multiple hops. 98 Intermediate hops would route the packets back to the sender, and 99 connectivity through the entire path would not be possible to verify. 101 3. Demultiplexing Packets 103 There are a number of possibilities for addressing the demultiplexing 104 issue which may be used, depending on the application. 106 3.1. Totally Arbitrary Paths 108 It may be desired to use BFD for liveness detection over paths for 109 which no part of the route is known (or if known, may not be stable.) 110 A straightforward approach to this problem is to limit BFD deployment 111 to a single session between a source/destination address pair. 112 Multiple sessions between the same pair of systems must have at least 113 one endpoint address distinct from one another. 115 In this scenario, the initial packet is demultiplexed to the 116 appropriate BFD session based on the source/destination address pair 117 when Your Discriminator is set to zero. 119 This approach is appropriate for general connectivity detection 120 between systems over routed paths, and is also useful for OSPF 121 Virtual Links [OSPFv2] [OSPFv3]. 123 3.2. Out-of-band Discriminator Signaling 125 Another approach to the demultiplexing problem is to signal the 126 discriminator values in each direction through an out-of-band 127 mechanism prior to establishing the BFD session. Once learned, the 128 discriminators are sent as usual in the BFD Control packets; no 129 packets with Your Discriminator set to zero are ever sent. This 130 method is used by the BFD MPLS specification [BFD-MPLS]. 132 This approach is advantageous because it allows BFD to be directed by 133 other system components that have knowledge of the paths in use, and 134 from the perspective of BFD implementation it is very simple. 136 The disadvantage is that it requires at least some level of BFD- 137 specific knowledge in parts of the system outside of BFD. 139 3.3. Unidirectional Links 141 Unidirectional links are classified as multihop paths because the 142 return path (which should exist at some level in order to make the 143 link useful) may be arbitrary, and the return paths for BFD sessions 144 protecting parallel unidirectional links may overlap or even be 145 identical. (If two unidirectional links, one in each direction, are 146 to carry a single BFD session, this can be done using the single-hop 147 approach.) 149 Either of the two methods outlined earlier may be used in the 150 Unidirectional link case, but a more general solution can be done 151 strictly within BFD and without addressing limitations. 153 The approach is similar to the one-hop specification, since the 154 unidirectional link is a single hop. Let's define the two systems as 155 the Unidirectional Sender and the Unidirectional Receiver. In this 156 approach the Unidirectional Sender MUST operate in the Active role 157 (as defined in the base BFD specification), and the Unidirectional 158 Receiver MUST operate in the Passive role. 160 In the Passive role, by definition, the Unidirectional Receiver does 161 not transmit any BFD Control packets until it learns the 162 discriminator value in use by the other system (upon receipt of the 163 first BFD Control packet.) The Unidirectional Receiver demultiplexes 164 the first packet to the proper BFD session based on the physical or 165 logical link over which was received. This allows the receiver to 166 learn the remote discriminator value, which it then echoes back to 167 the sender in its own (arbitrarily routed) BFD Control packet, after 168 which time all packets are demultiplexed solely by discriminator. 170 4. Encapsulation 172 The encapsulation of BFD Control packets for multihop application in 173 IPv4 and IPv6 is identical to that defined in [BFD-1HOP], except that 174 the UDP destination port MUST have a value of 4784. This can aid in 175 the demultiplexing and internal routing of incoming BFD packets. 177 5. Authentication 179 By their nature, multihop paths expose BFD to spoofing. As the 180 number of hops increase, the exposure to attack grows. As such, 181 implementations of BFD SHOULD utilize cryptographic authentication 182 over multihop paths to help mitigate denial-of-service attacks. 184 Normative References 186 [BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection", 187 draft-ietf-bfd-base-10.txt, October, 2009. 189 [BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single 190 Hop)", draft-ietf-bfd-v4v6-1hop-10.txt, October, 2009. 192 [KEYWORD] Bradner, S., "Key words for use in RFCs to Indicate 193 Requirement Levels", RFC 2119, March 1997. 195 Informative References 197 [BFD-MPLS] Aggarwal, R., Kompella, K., et al, "BFD for MPLS LSPs", 198 draft-ietf-bfd-mpls-07.txt, June, 2008. 200 [OSPFv2] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 202 [OSPFv3] Coltun, R., et al, "OSPF for IPv6", RFC 2740, December 1999. 204 Security Considerations 206 As the number of hops increases, BFD becomes further exposed to 207 attack. The use of strong forms of authentication is strongly 208 encouraged. 210 No additional security issues are raised in this document beyond 211 those that exist in the referenced BFD documents. 213 IANA Considerations 215 Port 4784 has been assigned by IANA for use with this protocol. 217 Authors' Addresses 219 Dave Katz 220 Juniper Networks 221 1194 N. Mathilda Ave. 222 Sunnyvale, California 94089-1206 USA 223 Phone: +1-408-745-2000 224 Email: dkatz@juniper.net 226 Dave Ward 227 Cisco Systems 228 170 W. Tasman Dr. 229 San Jose, CA 95134 USA 230 Phone: +1-408-526-4000 231 Email: dward@cisco.com 233 Changes from the previous draft 235 The fact that the port number was assigned by IANA was added. All 236 other changes are editorial in nature. 238 This document expires in April, 2010.