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Fu 5 Expires: May 4, 2020 Bloomberg L.P. 6 November 1, 2019 8 BFD Encapsulated in Large Packets 9 draft-ietf-bfd-large-packets-02 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 May 4, 2020. 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 3.1. Implementations that do not support Large BFD Packets . . 3 67 3.2. Selecting MTU size to be detected . . . . . . . . . . . . 4 68 3.3. Detecting MTU mismatches . . . . . . . . . . . . . . . . 4 69 3.4. Equal Cost Multiple Paths (ECMP) or other Load Balancing 70 Considerations . . . . . . . . . . . . . . . . . . . . . 5 71 3.5. S-BFD . . . . . . . . . . . . . . . . . . . . . . . . . . 5 72 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 73 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 74 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 75 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 76 7.1. Normative References . . . . . . . . . . . . . . . . . . 6 77 7.2. Informative References . . . . . . . . . . . . . . . . . 7 78 Appendix A. Related Features . . . . . . . . . . . . . . . . . . 7 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 81 1. Introduction 83 The Bidirectional Forwarding Detection (BFD) [RFC5880] protocol is 84 commonly used to verify connectivity between two systems. However, 85 some applications may require that the Path MTU [RFC1191] between 86 those two systems meets a certain minimum criteria. When the Path 87 MTU decreases below the minimum threshold, those applications may 88 wish to consider the path unusable. 90 BFD may be encapsulated in a number of transport protocols. An 91 example of this is single-hop BFD [RFC5881]. In that case, the link 92 MTU configuration is typically enough to guarantee communication 93 between the two systems for that size MTU. BFD Echo mode 94 (Section 6.4 of [RFC5880]) is sufficient to permit verification of 95 the Path MTU of such directly connected systems. Previous proposals 96 ([I-D.haas-xiao-bfd-echo-path-mtu]) have been made for testing Path 97 MTU for such directly connected systems. However, in the case of 98 multi-hop BFD [RFC5883], this guarantee does not hold. 100 The encapsulation of BFD in multi-hop sessions is a simple UDP 101 packet. The BFD elements of procedure (Section 6.8.6 of [RFC5880]) 102 covers validating the BFD payload. However, the specification is 103 silent on the length of the encapsulation that is carrying the BFD 104 PDU. While it is most common that the transport protocol payload 105 (i.e. UDP) length is the exact size of the BFD PDU, this is not 106 required by the elements of procedure. This leads to the possibility 107 that the transport protocol length may be larger than the contained 108 BFD PDU. 110 2. BFD Encapsulated in Large Packets 112 Support for BFD between two systems is typically configured, even if 113 the actual session may be dynamically created by a client protocol. 114 A new BFD variable is defined in this document: 116 bfd.PaddedPduSize 117 The BFD transport protocol payload size is increased to this 118 value. The contents of this additional payload MUST be zero. 119 The minimum size of this variable MUST NOT be smaller than 120 permitted by the element of BFD procedure; 24 or 26 - see 121 Section 6.8.6 of [RFC5880]. 123 The Don't Fragment bit (Section 2.3 of [RFC0791]) of the IP payload, 124 when using IPv4 encapsulation, MUST be set. 126 3. Implementation and Deployment Considerations 128 3.1. Implementations that do not support Large BFD Packets 130 While this document proposes no change to the BFD protocol, 131 implementations may not permit arbitrarily padded transport PDUs to 132 carry BFD packets. While Section 6 of [RFC5880] warns against 133 excessive pedantry, implementations may not work with this mechanism 134 without additional support. 136 [RFC5880], section 6.8.6, discusses the procedures for receiving BFD 137 Control packets. When an implementation is incapable of processing 138 Large BFD Packets, it could manifest in one of two possible ways: 140 o A receiving BFD implementation is incapable of accepting Large BFD 141 Packets. This is identical to the packet being discarded. 143 o A receiving BFD implementation is capable of accepting Large BFD 144 Packets, but the Control packet is improperly rejected during 145 validation procedures. This is identical to the packet being 146 discarded. 148 In each of these cases, the BFD state machine would behave as if it 149 were not receiving Control packets and the implementation would 150 follow normal BFD procedures with regards to not having received 151 Control packets. 153 3.2. Selecting MTU size to be detected 155 Since the consideration is path MTU, BFD sessions using this feature 156 only need to use a bfd.PaddedPduSize appropriate to exercise the path 157 MTU for the desired application. This may be significantly smaller 158 than the system's link MTU; e.g. desired path MTU is 1500 bytes while 159 the interface MTU that BFD with large packets is running on is 9000 160 bytes. 162 In the case multiple BFD clients desire to test the same BFD 163 endpoints using different bfd.PaddedPduSize parameters, 164 implementations should select the largest bfd.PaddedPduSize parameter 165 from the configured sessions. This is similar to how implementations 166 of BFD select the most aggressive timing parameters for multiple 167 sessions to the same endpoint. 169 3.3. Detecting MTU mismatches 171 The accepted MTU for an interface is impacted by packet encapsulation 172 considerations at a given layer; e.g. layer 2, layer 3, tunnel, etc. 173 A common misconfiguration of interface parameters is inconsistent 174 MTU. In the presence of inconsistent MTU, it is possible for 175 applications to have unidirectional connectivity. 177 When it is necessary for an application using BFD with Large Packets 178 to test the bi-directional Path MTU, it is necessary to configure the 179 bfd.PaddedPduSize parameter on both sides of an interface. E.g., if 180 the desire is to verify a 1500 byte MTU in both directions on an 181 Ethernet or point to point link, each side of the BFD session must 182 have bfd.PaddedPduSize set to 1500. In the absence of such 183 consistent configuration, BFD with Large Packets may correctly 184 determine unidirectional connectivity at the tested MTU, but bi- 185 directional MTU may not be properly validated. 187 It should be noted that some interfaces may intentionally have 188 different MTUs. Setting the bfd.PaddedPduSize appropriately for each 189 side of the interface supports such scenarios. 191 3.4. Equal Cost Multiple Paths (ECMP) or other Load Balancing 192 Considerations 194 Various mechanisms are utilized to increase throughput between two 195 endpoints at various network layers. Such features include Link 196 Aggregate Groups (LAGs) or ECMP forwarding. Such mechanisms balance 197 traffic across multiple physical links while hiding the details of 198 that balacing from the higher networking layers. The details of that 199 balancing are highly implementation specific. 201 In the presence of such load balancing mechanisms, it is possible to 202 have member links that are not properly forwarding traffic. In such 203 circumstances, this will result in dropped traffic when traffic is 204 chosen to be load balanced across those member links. 206 Such load balancing mechanisms may not permit all link members to be 207 properly tested by BFD. This is because the BFD Control packets may 208 be forwarded only along links that are up. BFD on LAG, [RFC7130], 209 was developed to help cover one such scenario. However, for testing 210 forwarding over multiple hops, there is no such specified general 211 purpose BFD mechanism for exercising all links in an ECMP. This may 212 result in a BFD session being in the Up state while some traffic may 213 be dropped or otherwise negatively impacted along some component 214 links. 216 Some BFD implementations utilize their internal understanding of the 217 component links and their resultant forwarding to exercise BFD in 218 such a way to better test the ECMP members and to tie the BFD session 219 state to the health of that ECMP. Due to the implementation specific 220 load balancing, it is not possible to standardize such additional 221 mechanisms for BFD. 223 Mis-configuration of some member MTUs may lead to Load Balancing that 224 may have an inconsistent Path MTU depending on how the traffic is 225 balanced. While the intent of BFD with Large Packets is to verify 226 path MTU, it is subject to the same considerations above. 228 3.5. S-BFD 230 This mechanism also can be applied to other forms of BFD, including 231 S-BFD [RFC7880]. 233 4. Security Considerations 235 This document does not change the underlying security considerations 236 of the BFD protocol or its encapsulations. 238 5. IANA Considerations 240 This document introduces no additional considerations to IANA. 242 6. Acknowledgments 244 The authors would like to thank Les Ginsberg, Mahesh Jethandani, 245 Robert Raszuk, and Ketan Talaulikar, for their valuable feedback on 246 this proposal. 248 7. References 250 7.1. Normative References 252 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, 253 DOI 10.17487/RFC0791, September 1981, . 256 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 257 Requirement Levels", BCP 14, RFC 2119, 258 DOI 10.17487/RFC2119, March 1997, . 261 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 262 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 263 . 265 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 266 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 267 DOI 10.17487/RFC5881, June 2010, . 270 [RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 271 (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, 272 June 2010, . 274 [RFC7130] Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed., 275 Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional 276 Forwarding Detection (BFD) on Link Aggregation Group (LAG) 277 Interfaces", RFC 7130, DOI 10.17487/RFC7130, February 278 2014, . 280 [RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S. 281 Pallagatti, "Seamless Bidirectional Forwarding Detection 282 (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016, 283 . 285 7.2. Informative References 287 [I-D.haas-xiao-bfd-echo-path-mtu] 288 Haas, J. and M. Xiao, "Application of the BFD Echo 289 function for Path MTU Verification or Detection", draft- 290 haas-xiao-bfd-echo-path-mtu-01 (work in progress), July 291 2011. 293 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 294 DOI 10.17487/RFC1191, November 1990, . 297 [RFC3719] Parker, J., Ed., "Recommendations for Interoperable 298 Networks using Intermediate System to Intermediate System 299 (IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004, 300 . 302 Appendix A. Related Features 304 IS-IS [RFC3719] supports a Padding feature for its hellos. This 305 provides the ability to detect inconsistent link MTUs. 307 Authors' Addresses 309 Jeffrey Haas 310 Juniper Networks, Inc. 311 1133 Innovation Way 312 Sunnyvale, CA 94089 313 US 315 Email: jhaas@juniper.net 317 Albert Fu 318 Bloomberg L.P. 320 Email: afu14@bloomberg.net