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Shallow 7 September 9, 2019 9 Constrained Application Protocol (CoAP) Hop-Limit Option 10 draft-ietf-core-hop-limit-05 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 March 12, 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. Debugging & Troubleshooting . . . . . . . . . . . . . . . . . 4 58 5. HTTP-Mapping Considerations . . . . . . . . . . . . . . . . . 5 59 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 60 6.1. CoAP Response Code . . . . . . . . . . . . . . . . . . . 6 61 6.2. CoAP Option Number . . . . . . . . . . . . . . . . . . . 6 62 7. Security Considerations . . . . . . . . . . . . . . . . . . . 6 63 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 64 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 65 9.1. Normative References . . . . . . . . . . . . . . . . . . 7 66 9.2. Informative References . . . . . . . . . . . . . . . . . 7 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 69 1. Introduction 71 More and more applications are using the Constrained Application 72 Protocol (CoAP) [RFC7252] as a communication protocol between 73 involved application agents. For example, 74 [I-D.ietf-dots-signal-channel] specifies how CoAP is used as a 75 signaling protocol between domains under distributed denial-of- 76 service (DDoS) attacks and DDoS mitigation providers. In such 77 contexts, a CoAP client can communicate directly with a server or 78 indirectly via proxies. 80 When multiple proxies are involved, infinite forwarding loops may be 81 experienced (e.g., routing misconfiguration, policy conflicts). To 82 prevent such loops, this document defines a new CoAP option, called 83 Hop-Limit (Section 3). Also, the document defines a new CoAP 84 Response Code (Section 6.1) to report loops together with relevant 85 diagnostic information to ease troubleshooting (Section 4). 87 1.1. Intended Usage 89 The Hop-Limit option has originally been designed for a specific use 90 case [I-D.ietf-dots-signal-channel]. However, its intended usage is 91 general: CoAP proxies that do not have specific knowledge that proxy 92 forwarding loops are avoided in some other way, are expected to 93 implement this option and have it enabled by default. 95 Note that this means that a server that receives requests both via 96 proxies and directly from clients may see otherwise identical 97 requests with and without the Hop-Limit option included; servers with 98 internal caching will therefore also want to implement this option. 100 2. Terminology 102 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 103 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 104 "OPTIONAL" in this document are to be interpreted as described in BCP 105 14 [RFC2119][RFC8174] when, and only when, they appear in all 106 capitals, as shown here. 108 Readers should be familiar with the terms and concepts defined in 109 [RFC7252]. 111 3. Hop-Limit Option 113 The Hop-Limit option (see Section 6.2) is an elective option used to 114 detect and prevent infinite loops when proxies are involved. The 115 option is not repeatable. Therefore, any message carrying multiple 116 Hop-Limit options MUST be handled following the procedure specified 117 in Section 5.4.5 of [RFC7252]. 119 The value of the Hop-Limit option is encoded as an unsigned integer 120 (see Section 3.2 of [RFC7252]). This value MUST be between 1 and 255 121 inclusive. CoAP messages received with a Hop-Limit option set to '0' 122 or greater than '255' MUST be rejected by a CoAP server/proxy using 123 4.00 (Bad Request). 125 The Hop-Limit option is safe to forward. That is, a CoAP proxy which 126 does not understand the Hop-Limit option should forward it on. The 127 option is also part of the cache key. As such, a CoAP proxy which 128 does not understand the Hop-Limit option must follow the 129 recommendations in Section 5.7.1 of [RFC7252] for caching. Note that 130 loops which involve only such proxies will not be detected. 131 Nevertheless, the presence of such proxies will not prevent infinite 132 loop detection if at least one CoAP proxy which support the Hop-Limit 133 option is involved in the loop. 135 A CoAP proxy which understands the Hop-Limit option MAY be 136 instructed, using a configuration parameter, to insert a Hop-Limit 137 option when relaying a request which do not include the Hop-Limit 138 option. 140 The initial Hop-Limit value should be configurable. If no initial 141 value is explicitly provided, the default initial Hop-Limit value of 142 16 MUST be used. This value is chosen to be sufficiently large to 143 guarantee that a CoAP request would not be dropped in networks when 144 there were no loops, but not so large as to consume CoAP proxy 145 resources when a loop does occur. Lower values should be used with 146 caution and only in networks where topologies are known by the CoAP 147 client (or proxy) inserting the Hop-Limit option. 149 Because forwarding errors may occur if inadequate Hop-Limit values 150 are used, proxies at the boundaries of an administrative domain MAY 151 be instructed to remove or rewrite the value of Hop-Limit carried in 152 received messages (i.e., ignore the value of Hop-Limit received in a 153 message). This modification should be done with caution in case 154 proxy-forwarded traffic repeatedly crosses the administrative domain 155 boundary in a loop and so Hop-Limit detection gets broken. 157 Otherwise, a CoAP proxy which understands the Hop-Limit option MUST 158 decrement the value of the option by 1 prior to forwarding it. A 159 CoAP proxy which understands the Hop-Limit option MUST NOT use a 160 stored TBA1 (Hop Limit Reached) error response unless the value of 161 the Hop-Limit option in the presented request is less than or equal 162 to the value of the Hop-Limit option in the request used to obtain 163 the stored response. Otherwise, the CoAP proxy follows the behavior 164 in Section 5.6 of [RFC7252]. 166 Note: If a request with a given value of Hop-Limit failed to reach 167 a server because the hop limit is exhausted, then the same failure 168 will be observed if a less value of the Hop-Limit option is used 169 instead. 171 CoAP messages MUST NOT be forwarded if the Hop-Limit option is set to 172 '0' after decrement. Messages that cannot be forwarded because of 173 exhausted Hop-Limit SHOULD be logged with a TBA1 (Hop Limit Reached) 174 error response sent back to the CoAP peer. It is RECOMMENDED that 175 CoAP implementations support means to alert administrators about loop 176 errors so that appropriate actions are undertaken. 178 4. Debugging & Troubleshooting 180 To ease debugging and troubleshooting, the CoAP proxy which detects a 181 loop includes its information in the diagnostic payload under the 182 conditions detailed in Section 5.5.2 of [RFC7252]. That information 183 MUST NOT include any space character. The information inserted by a 184 CoAP proxy can be, for example, a proxy name (e.g., p11.example.net), 185 proxy alias (e.g., myproxyalias), or IP address (e.g., 2001:db8::1). 187 Each intermediate proxy involved in relaying a TBA1 (Hop Limit 188 Reached) error message prepends its own information in the diagnostic 189 payload with a space character used as separator. Only one 190 information per proxy should appear in the diagnostic payload. Doing 191 so allows to limit the size of the TBA1 (Hop Limit Reached) error 192 message, and to ease correlation with hops count. Note that an 193 intermediate proxy prepends its information only if there is enough 194 space as determined by the Path MTU (Section 4.6 of [RFC7252]). If 195 not, an intermediate proxy forwards the TBA1 (Hop Limit Reached) 196 error message to the next hop without updating the diagnostic 197 payload. 199 5. HTTP-Mapping Considerations 201 This section focuses on the HTTP mappings specific to the CoAP 202 extension specified in this document. As a reminder, the basic 203 normative requirements on HTTP/CoAP mappings are defined in 204 Section 10 of [RFC7252]. The implementation guidelines for HTTP/CoAP 205 mappings are elaborated in [RFC8075]. 207 By default, the HTTP-to-CoAP Proxy inserts a Hop-Limit option 208 following the guidelines in Section 3. The HTTP-to-CoAP Proxy MAY be 209 instructed by policy to insert a Hop-Limit option only if a Via 210 (Section 5.7.1 of [RFC7230]) or CDN-Loop header field [RFC8586] is 211 present in the HTTP request. 213 The HTTP-to-CoAP Proxy uses 508 (Loop Detected) as the HTTP response 214 status code to map TBA1 (Hop Limit Reached). Furthermore, it maps 215 the diagnostic payload of TBA1 (Hop Limit Reached) as per Section 6.6 216 of [RFC8075]. 218 By default, the CoAP-to-HTTP Proxy inserts a Via header field in the 219 HTTP request if the CoAP request includes a Hop-Limit option. The 220 CoAP-to-HTTP Proxy MAY be instructed by policy to insert a CDN-Loop 221 header field instead of the Via header field. 223 The CoAP-to-HTTP Proxy maps the 508 (Loop Detected) HTTP response 224 status code to TBA1 (Hop Limit Reached). Moreover, the CoAP-to-HTTP 225 Proxy inserts its information following the guidelines in Section 4. 227 When both HTTP-to-CoAP and CoAP-to-HTTP proxies are involved, the 228 loop detection may get broken if the proxy-forwarded traffic 229 repeatedly crosses the HTTP-to-CoAP and CoAP-to-HTTP proxies. 230 Nevertheless, if the loop is within the CoAP or HTTP legs, the loop 231 detection is still functional. 233 6. IANA Considerations 234 6.1. CoAP Response Code 236 IANA is requested to add the following entry to the "CoAP Response 237 Codes" sub-registry available at https://www.iana.org/assignments/ 238 core-parameters/core-parameters.xhtml#response-codes: 240 +------+------------------+-----------+ 241 | Code | Description | Reference | 242 +------+------------------+-----------+ 243 | TBA1 | Hop Limit Reached| [RFCXXXX] | 244 +------+------------------+-----------+ 246 Table 1: CoAP Response Codes 248 This document suggests 5.08 as a code to be assigned for the new 249 response code. 251 Editorial Note: Please update TBA1 statements within the document 252 with the assigned code. 254 6.2. CoAP Option Number 256 IANA is requested to add the following entry to the "CoAP Option 257 Numbers" sub-registry available at https://www.iana.org/assignments/ 258 core-parameters/core-parameters.xhtml#option-numbers: 260 +--------+---+---+---+---+------------------+-----------+ 261 | Number | C | U | N | R | Name | Reference | 262 +--------+---+---+---+---+------------------+-----------+ 263 | TBA2 | | | | | Hop-Limit | [RFCXXXX] | 264 +--------+---+---+---+---+------------------+-----------+ 265 C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable 267 Table 2: CoAP Option Number 269 7. Security Considerations 271 Security considerations related to CoAP proxying are discussed in 272 Section 11.2 of [RFC7252]. 274 The diagnostic payload of a TBA1 (Hop Limit Reached) error message 275 may leak sensitive information revealing the topology of an 276 administrative domain. To prevent that, a CoAP proxy which is 277 located at the boundary of an administrative domain MAY be instructed 278 to strip the diagnostic payload or part of it before forwarding on 279 the TBA1 (Hop Limit Reached) response. 281 8. Acknowledgements 283 This specification was part of [I-D.ietf-dots-signal-channel]. Many 284 thanks to those who reviewed DOTS specifications. 286 Thanks to Klaus Hartke, Carsten Bormann, Peter van der Stok, Jim 287 Schaad, and Jaime Jimenez for the reviews. 289 Carsten Bormann provided the "Intended Usage" text. 291 9. References 293 9.1. Normative References 295 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 296 Requirement Levels", BCP 14, RFC 2119, 297 DOI 10.17487/RFC2119, March 1997, 298 . 300 [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer 301 Protocol (HTTP/1.1): Message Syntax and Routing", 302 RFC 7230, DOI 10.17487/RFC7230, June 2014, 303 . 305 [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 306 Application Protocol (CoAP)", RFC 7252, 307 DOI 10.17487/RFC7252, June 2014, 308 . 310 [RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and 311 E. Dijk, "Guidelines for Mapping Implementations: HTTP to 312 the Constrained Application Protocol (CoAP)", RFC 8075, 313 DOI 10.17487/RFC8075, February 2017, 314 . 316 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 317 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 318 May 2017, . 320 9.2. Informative References 322 [I-D.ietf-dots-signal-channel] 323 K, R., Boucadair, M., Patil, P., Mortensen, A., and N. 324 Teague, "Distributed Denial-of-Service Open Threat 325 Signaling (DOTS) Signal Channel Specification", draft- 326 ietf-dots-signal-channel-37 (work in progress), July 2019. 328 [RFC8586] Ludin, S., Nottingham, M., and N. Sullivan, "Loop 329 Detection in Content Delivery Networks (CDNs)", RFC 8586, 330 DOI 10.17487/RFC8586, April 2019, 331 . 333 Authors' Addresses 335 Mohamed Boucadair 336 Orange 337 Rennes 35000 338 France 340 Email: mohamed.boucadair@orange.com 342 Tirumaleswar Reddy 343 McAfee, Inc. 344 Embassy Golf Link Business Park 345 Bangalore, Karnataka 560071 346 India 348 Email: kondtir@gmail.com 350 Jon Shallow 351 United Kingdom 353 Email: supjps-ietf@jpshallow.com