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Shallow 7 July 3, 2019 9 Constrained Application Protocol (CoAP) Hop-Limit Option 10 draft-ietf-core-hop-limit-04 12 Abstract 14 The presence of Constrained Application Protocol (CoAP) proxies may 15 lead to infinite forwarding loops, which is undesirable. To prevent 16 and detect such loops, this document specifies the Hop-Limit CoAP 17 option. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at https://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on January 4, 2020. 36 Copyright Notice 38 Copyright (c) 2019 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (https://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.1. Intended Usage . . . . . . . . . . . . . . . . . . . . . 2 55 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 3. Hop-Limit Option . . . . . . . . . . . . . . . . . . . . . . 3 57 4. HTTP-Mapping Considerations . . . . . . . . . . . . . . . . . 5 58 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 59 5.1. CoAP Response Code . . . . . . . . . . . . . . . . . . . 5 60 5.2. CoAP Option Number . . . . . . . . . . . . . . . . . . . 6 61 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 62 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 63 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 64 8.1. Normative References . . . . . . . . . . . . . . . . . . 7 65 8.2. Informative References . . . . . . . . . . . . . . . . . 7 66 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 68 1. Introduction 70 More and more applications are using the Constrained Application 71 Protocol (CoAP) [RFC7252] as a communication protocol between 72 involved application agents. For example, 73 [I-D.ietf-dots-signal-channel] specifies how CoAP is used as a 74 distributed denial-of-service (DDoS) attack signaling protocol for 75 seeking for help from DDoS mitigation providers. In such contexts, a 76 CoAP client can communicate directly with a server or indirectly via 77 proxies. 79 When multiple proxies are involved, infinite forwarding loops may be 80 experienced (e.g., routing misconfiguration, policy conflicts). To 81 prevent such loops, this document defines a new CoAP option, called 82 Hop-Limit (Section 3). Also, the document defines a new CoAP 83 Response Code (Section 5.1) to report loops together with relevant 84 diagnostic information to ease troubleshooting. 86 1.1. Intended Usage 88 The Hop-Limit option has originally been designed for a specific use 89 case [I-D.ietf-dots-signal-channel]. However, its intended usage is 90 general: CoAP proxies that do not have specific knowledge that proxy 91 forwarding loops are avoided in some other way, are expected to 92 implement this option and have it enabled by default. 94 Note that this means that a server that receives requests both via 95 proxies and directly from clients may see otherwise identical 96 requests with and without the Hop-Limit option included; servers with 97 internal caching will therefore also want to implement this option. 99 2. Terminology 101 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 102 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 103 "OPTIONAL" in this document are to be interpreted as described in BCP 104 14 [RFC2119][RFC8174] when, and only when, they appear in all 105 capitals, as shown here. 107 Readers should be familiar with the terms and concepts defined in 108 [RFC7252]. 110 3. Hop-Limit Option 112 The Hop-Limit option (see Section 5.2) is an elective option used to 113 detect and prevent infinite loops when proxies are involved. The 114 option is not repeatable. Therefore, any message carrying multiple 115 Hop-Limit options MUST be handled following the procedure specified 116 in Section 5.4.5 of [RFC7252]. 118 The value of the Hop-Limit option is encoded as an unsigned integer 119 (see Section 3.2 of [RFC7252]). This value MUST be between 1 and 255 120 inclusive. CoAP messages received with a Hop-Limit option set to '0' 121 or greater than '255' MUST be rejected by a CoAP server/proxy using 122 4.00 (Bad Request). 124 The Hop-Limit option is safe to forward. That is, a CoAP proxy which 125 does not understand the Hop-Limit option should forward it on. The 126 option is also part of the cache key. As such, a CoAP proxy which 127 does not understand the Hop-Limit option must follow the 128 recommendations in Section 5.7.1 of [RFC7252] for caching. Note that 129 loops which involve only such proxies won't be detected. 130 Nevertheless, the presence of such proxies won't prevent infinite 131 loop detection if at least one CoAP proxy which support the Hop-Limit 132 option is involved in the loop. 134 A CoAP proxy which understands the Hop-Limit option MAY be 135 instructed, using a configuration parameter, to insert a Hop-Limit 136 option when relaying a request which do not include the Hop-Limit 137 option. 139 The initial Hop-Limit value should be configurable. If no initial 140 value is explicitly provided, the default initial Hop-Limit value of 141 16 MUST be used. This value is chosen to be sufficiently large to 142 guarantee that a CoAP request would not be dropped in networks when 143 there were no loops, but not so large as to consume CoAP proxy 144 resources when a loop does occur. Lower values should be used with 145 caution and only in networks where topologies are known by the CoAP 146 client (or proxy) inserting the Hop-Limit option. 148 Because forwarding errors may occur if inadequate Hop-Limit values 149 are used, proxies at the boundaries of an administrative domain MAY 150 be instructed to remove or rewrite the value of Hop-Limit carried in 151 received messages (i.e., ignore the value of Hop-Limit received in a 152 message). This modification should be done with caution in case 153 proxy-forwarded traffic repeatedly crosses the administrative domain 154 boundary in a loop and so Hop-Limit detection gets broken. 156 Otherwise, a CoAP proxy which understands the Hop-Limit option MUST 157 decrement the value of the option by 1 prior to forwarding it. A 158 CoAP proxy which understands the Hop-Limit option MUST NOT use a 159 stored TBA1 (Hop Limit Reached) error response unless the value of 160 the Hop-Limit option in the presented request is less than or equal 161 to the value of the Hop-Limit option in the request used to obtain 162 the stored response. Otherwise, the CoAP proxy follows the behavior 163 in Section 5.6 of [RFC7252]. 165 Note: If a request with a given value of Hop-Limit failed to reach 166 a server because the hop limit is exhausted, then the same failure 167 will be observed if a less value of the Hop-Limit option is used 168 instead. 170 CoAP messages MUST NOT be forwarded if the Hop-Limit option is set to 171 '0' after decrement. Messages that cannot be forwarded because of 172 exhausted Hop-Limit SHOULD be logged with a TBA1 (Hop Limit Reached) 173 error response sent back to the CoAP peer. It is RECOMMENDED that 174 CoAP implementations support means to alert administrators about loop 175 errors so that appropriate actions are undertaken. 177 To ease debugging and troubleshooting, the CoAP proxy which detects a 178 loop includes its information in the diagnostic payload under the 179 conditions detailed in Section 5.5.2 of [RFC7252]. That information 180 MUST NOT include any space character. The information inserted by a 181 CoAP proxy can be, for example, a proxy name (e.g., p11.example.net), 182 proxy alias (e.g., myproxyalias), or IP address (e.g., 2001:db8::1). 184 Each intermediate proxy involved in relaying a TBA1 (Hop Limit 185 Reached) error message prepends its own information in the diagnostic 186 payload with a space character used as separator. Only one 187 information per proxy should appear in the diagnostic payload. Doing 188 so allows to limit the size of the TBA1 (Hop Limit Reached) error 189 message, and to ease correlation with hops count. Note that an 190 intermediate proxy prepends its information only if there is enough 191 space as determined by the Path MTU (Section 4.6 of [RFC7252]). If 192 not, an intermediate proxy forwards the TBA1 (Hop Limit Reached) 193 error message to the next hop without updating the diagnostic 194 payload. 196 4. HTTP-Mapping Considerations 198 This section focuses on the HTTP mappings specific to the CoAP 199 extension specified in this document. As a reminder, the basic 200 normative requirements on HTTP/CoAP mappings are defined in 201 Section 10 of [RFC7252]. The implementation guidelines for HTTP/CoAP 202 mappings are elaborated in [RFC8075]. 204 By default, the HTTP-to-CoAP Proxy inserts a Hop-Limit option 205 following the guidelines in Section 3. The HTTP-to-CoAP Proxy MAY be 206 instructed by policy to insert a Hop-Limit option only if a Via 207 (Section 5.7.1 of [RFC7230]) or CDN-Loop header field [RFC8586] is 208 present in the HTTP request. 210 The HTTP-to-CoAP Proxy uses 508 (Loop Detected) as the HTTP response 211 status code to map TBA1 (Hop Limit Reached). Furthermore, it maps 212 the diagnostic payload of TBA1 (Hop Limit Reached) as per Section 6.6 213 of [RFC8075]. 215 By default, the CoAP-to-HTTP Proxy inserts a Via header field in the 216 HTTP request if the CoAP request includes a Hop-Limit option. The 217 CoAP-to-HTTP Proxy MAY be instructed by policy to insert a CDN-Loop 218 header field instead of the Via header field. 220 The CoAP-to-HTTP Proxy maps the 508 (Loop Detected) HTTP response 221 status code to TBA1 (Hop Limit Reached). Moreover, the CoAP-to-HTTP 222 Proxy inserts its information following the guidelines in Section 3. 224 When both HTTP-to-CoAP and CoAP-to-HTTP proxies are involved, the 225 loop detection may get broken if the proxy-forwarded traffic 226 repeatedly crosses the HTTP-to-CoAP and CoAP-to-HTTP proxies. 227 Nevertheless, if the loop is within the CoAP or HTTP legs, the loop 228 detection is still functional. 230 5. IANA Considerations 232 5.1. CoAP Response Code 234 IANA is requested to add the following entry to the "CoAP Response 235 Codes" sub-registry available at https://www.iana.org/assignments/ 236 core-parameters/core-parameters.xhtml#response-codes: 238 +------+------------------+-----------+ 239 | Code | Description | Reference | 240 +------+------------------+-----------+ 241 | TBA1 | Hop Limit Reached| [RFCXXXX] | 242 +------+------------------+-----------+ 244 Table 1: CoAP Response Codes 246 This document suggests 5.08 as a code to be assigned for the new 247 response code. 249 Editorial Note: Please update TBA1 statements within the document 250 with the assigned code. 252 5.2. CoAP Option Number 254 IANA is requested to add the following entry to the "CoAP Option 255 Numbers" sub-registry available at https://www.iana.org/assignments/ 256 core-parameters/core-parameters.xhtml#option-numbers: 258 +--------+---+---+---+---+------------------+-----------+ 259 | Number | C | U | N | R | Name | Reference | 260 +--------+---+---+---+---+------------------+-----------+ 261 | TBA2 | | | | | Hop-Limit | [RFCXXXX] | 262 +--------+---+---+---+---+------------------+-----------+ 263 C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable 265 Table 2: CoAP Option Number 267 6. Security Considerations 269 Security considerations related to CoAP proxying are discussed in 270 Section 11.2 of [RFC7252]. 272 The diagnostic payload of a TBA1 (Hop Limit Reached) error message 273 may leak sensitive information revealing the topology of an 274 administrative domain. To prevent that, a CoAP proxy which is 275 located at the boundary of an administrative domain MAY be instructed 276 to strip the diagnostic payload or part of it before forwarding on 277 the TBA1 (Hop Limit Reached) response. 279 7. Acknowledgements 281 This specification was part of [I-D.ietf-dots-signal-channel]. Many 282 thanks to those who reviewed DOTS specifications. 284 Thanks to Klaus Hartke, Carsten Bormann, Peter van der Stok, and Jim 285 Schaad for the reviews. 287 Carsten Bormann provided the "Intended Usage" text. 289 8. References 291 8.1. Normative References 293 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 294 Requirement Levels", BCP 14, RFC 2119, 295 DOI 10.17487/RFC2119, March 1997, 296 . 298 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 299 Protocol (HTTP/1.1): Message Syntax and Routing", 300 RFC 7230, DOI 10.17487/RFC7230, June 2014, 301 . 303 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 304 Application Protocol (CoAP)", RFC 7252, 305 DOI 10.17487/RFC7252, June 2014, 306 . 308 [RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and 309 E. Dijk, "Guidelines for Mapping Implementations: HTTP to 310 the Constrained Application Protocol (CoAP)", RFC 8075, 311 DOI 10.17487/RFC8075, February 2017, 312 . 314 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 315 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 316 May 2017, . 318 8.2. Informative References 320 [I-D.ietf-dots-signal-channel] 321 K, R., Boucadair, M., Patil, P., Mortensen, A., and N. 322 Teague, "Distributed Denial-of-Service Open Threat 323 Signaling (DOTS) Signal Channel Specification", draft- 324 ietf-dots-signal-channel-34 (work in progress), May 2019. 326 [RFC8586] Ludin, S., Nottingham, M., and N. Sullivan, "Loop 327 Detection in Content Delivery Networks (CDNs)", RFC 8586, 328 DOI 10.17487/RFC8586, April 2019, 329 . 331 Authors' Addresses 333 Mohamed Boucadair 334 Orange 335 Rennes 35000 336 France 338 Email: mohamed.boucadair@orange.com 340 Tirumaleswar Reddy 341 McAfee, Inc. 342 Embassy Golf Link Business Park 343 Bangalore, Karnataka 560071 344 India 346 Email: kondtir@gmail.com 348 Jon Shallow 349 United Kingdom 351 Email: supjps-ietf@jpshallow.com