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Fu 5 Expires: August 30, 2019 Bloomberg L.P. 6 February 26, 2019 8 BFD Encapsulated in Large Packets 9 draft-ietf-bfd-large-packets-00 11 Abstract 13 The Bidirectional Forwarding Detection (BFD) protocol is commonly 14 used to verify connectivity between two systems. BFD packets are 15 typically very small. It is desirable in some circumstances to know 16 that not only is the path between two systems reachable, but also 17 that it is capable of carrying a payload of a particular size. This 18 document discusses thoughts on how to implement such a mechanism 19 using BFD in Asynchronous mode. 21 Requirements Language 23 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 24 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to 25 be interpreted as described in [RFC2119] only when they appear in all 26 upper case. They may also appear in lower or mixed case as English 27 words, without normative meaning. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on August 30, 2019. 46 Copyright Notice 48 Copyright (c) 2019 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 64 2. BFD Encapsulated in Large Packets . . . . . . . . . . . . . . 3 65 3. Implementation and Deployment Considerations . . . . . . . . 3 66 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 67 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 68 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 69 6.1. Normative References . . . . . . . . . . . . . . . . . . 4 70 6.2. Informative References . . . . . . . . . . . . . . . . . 5 71 Appendix A. Related Features . . . . . . . . . . . . . . . . . . 5 72 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 74 1. Introduction 76 The Bidirectional Forwarding Detection (BFD) [RFC5880] protocol is 77 commonly used to verify connectivity between two systems. However, 78 some applications may require that the Path MTU [RFC1191] between 79 those two systems meets a certain minimum criteria. When the Path 80 MTU decreases below the minimum threshold, those applications may 81 wish to consider the path unusable. 83 BFD may be encapsulated in a number of transport protocols. An 84 example of this is single-hop BFD [RFC5881]. In that case, the link 85 MTU configuration is typically enough to guarantee communication 86 between the two systems for that size MTU. BFD Echo mode 87 (Section 6.4 of [RFC5880]) is sufficient to permit verification of 88 the Path MTU of such directly connected systems. Previous proposals 89 ([I-D.haas-xiao-bfd-echo-path-mtu]) have been made for testing Path 90 MTU for such directly connected systems. However, in the case of 91 multi-hop BFD [RFC5883], this guarantee does not hold. 93 The encapsulation of BFD in multi-hop sessions is a simple UDP 94 packet. The BFD elements of procedure (Section 6.8.6 of [RFC5880]) 95 covers validating the BFD payload. However, the specification is 96 silent on the length of the encapsulation that is carrying the BFD 97 PDU. While it is most common that the transport protocol payload 98 (i.e. UDP) length is the exact size of the BFD PDU, this is not 99 required by the elements of procedure. This leads to the possibility 100 that the transport protocol length may be larger than the contained 101 BFD PDU. 103 2. BFD Encapsulated in Large Packets 105 Support for BFD between two systems is typically configured, even if 106 the actual session may be dynamically created by a client protocol. 107 A new BFD variable is defined in this document: 109 bfd.PaddedPduSize 110 The BFD transport protocol payload size is increased to this 111 value. The contents of this additional payload MUST be zero. 112 The minimum size of this variable MUST NOT be smaller than 113 permitted by the element of BFD procedure; 24 or 26 - see 114 Section 6.8.6 of [RFC5880]. 116 The Don't Fragment bit (Section 2.3 of [RFC0791]) of the IP payload, 117 when using IPv4 encapsulation, MUST be set. 119 3. Implementation and Deployment Considerations 121 While this document proposes no change to the BFD protocol, 122 implementations may not permit arbitrarily padded transport PDUs to 123 carry BFD packets. While Section 6 of [RFC5880] warns against 124 excessive pedantry, implementations may not work with this mechanism 125 without additional support. Additional changes to the base BFD 126 protocol may be required to permit negotiation of this functionality 127 and the padding value. 129 It is also worthy of note that even if an implementation can function 130 with larger transport PDUs, that additional packet size may have 131 impact on BFD scaling. Such systems may support a lower transmission 132 interval (bfd.DesiredMinTxInterval) when operating in large packet 133 mode. This interval may depend on the size of the transport PDU. 135 Given the impact on scaling larger PDU sizes may have on BFD 136 implementations, operators should consider applying it only in 137 situations where there is appropriate concern for path MTU. An 138 example of this is commercial WAN services. 140 Since the consideration is path MTU, BFD sessions using this feature 141 only need to use a bfd.PaddedPduSize appropriate to exercise the path 142 MTU for the desired application. This may be significantly smaller 143 than the system's link MTU; e.g. desired path MTU is 1500 bytes while 144 the interface MTU that BFD with large packets is running on is 9000 145 bytes. 147 This mechanism also can be applied to other forms of BFD, including 148 S-BFD [RFC7880]. 150 4. Security Considerations 152 This document does not change the underlying security considerations 153 of the BFD protocol or its encapsulations. 155 5. IANA Considerations 157 This document introduces no additional considerations to IANA. 159 6. References 161 6.1. Normative References 163 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 164 DOI 10.17487/RFC0791, September 1981, . 167 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 168 Requirement Levels", BCP 14, RFC 2119, 169 DOI 10.17487/RFC2119, March 1997, . 172 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 173 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 174 . 176 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 177 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 178 DOI 10.17487/RFC5881, June 2010, . 181 [RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 182 (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, 183 June 2010, . 185 [RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S. 186 Pallagatti, "Seamless Bidirectional Forwarding Detection 187 (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016, 188 . 190 6.2. Informative References 192 [I-D.haas-xiao-bfd-echo-path-mtu] 193 Haas, J. and M. Xiao, "Application of the BFD Echo 194 function for Path MTU Verification or Detection", draft- 195 haas-xiao-bfd-echo-path-mtu-01 (work in progress), July 196 2011. 198 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 199 DOI 10.17487/RFC1191, November 1990, . 202 [RFC3719] Parker, J., Ed., "Recommendations for Interoperable 203 Networks using Intermediate System to Intermediate System 204 (IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004, 205 . 207 Appendix A. Related Features 209 IS-IS [RFC3719] supports a Padding feature for its hellos. This 210 provides the ability to detect inconsistent link MTUs. 212 Authors' Addresses 214 Jeffrey Haas 215 Juniper Networks, Inc. 216 1133 Innovation Way 217 Sunnyvale, CA 94089 218 US 220 Email: jhaas@juniper.net 222 Albert Fu 223 Bloomberg L.P. 225 Email: afu14@bloomberg.net