idnits 2.17.1 draft-ietf-lpwan-coap-static-context-hc-13.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The abstract seems to contain references ([I-D.ietf-lpwan-ipv6-static-context-hc]), which it shouldn't. Please replace those with straight textual mentions of the documents in question. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords -- however, there's a paragraph with a matching beginning. Boilerplate error? RFC 2119 keyword, line 160: '... the CoAP header MAY be done in conjun...' RFC 2119 keyword, line 209: '...as found, then the packet MUST be sent...' RFC 2119 keyword, line 286: '...idirectional and MUST be elided during...' RFC 2119 keyword, line 296: '... The field SHOULD be elided if for i...' RFC 2119 keyword, line 333: '... Token Value MUST not be sent as a v...' (18 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Line 1070 has weird spacing: '...tkn piv kid...' == Line 1087 has weird spacing: '... mid tkn...' == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: Token Value MUST not be sent as a variable length residue to avoid ambiguity with Token Length. Therefore, Token Length value MUST be used to define the size of the residue. A specific function designated as "TKL" MUST be used in the Rule. During the decompression, this function returns the value contained in the Token Length field. -- The document date (March 05, 2020) is 1512 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '69' on line 1111 -- Looks like a reference, but probably isn't: '132' on line 1111 Summary: 2 errors (**), 0 flaws (~~), 4 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 lpwan Working Group A. Minaburo 3 Internet-Draft Acklio 4 Intended status: Standards Track L. Toutain 5 Expires: September 6, 2020 Institut MINES TELECOM; IMT Atlantique 6 R. Andreasen 7 Universidad de Buenos Aires 8 March 05, 2020 10 LPWAN Static Context Header Compression (SCHC) for CoAP 11 draft-ietf-lpwan-coap-static-context-hc-13 13 Abstract 15 This draft defines the way SCHC (Static Context Header Compression) 16 header compression can be applied to the CoAP protocol. SCHC is a 17 header compression mechanism adapted for constrained devices. SCHC 18 uses a static description of the header to reduce the redundancy and 19 the size of the information in the header. While 20 [I-D.ietf-lpwan-ipv6-static-context-hc] describes the SCHC 21 compression and fragmentation framework, and its application for 22 IPv6/UDP headers, this document applies the use of SCHC for CoAP 23 headers. The CoAP header structure differs from IPv6 and UDP since 24 CoAP uses a flexible header with a variable number of options, 25 themselves of variable length. The CoAP protocol messages format is 26 asymmetric: the request messages have a header format different from 27 the one in the response messages. This specification gives guidance 28 on how to apply SCHC to flexible headers and how to leverage the 29 asymmetry for more efficient compression Rules. 31 Status of This Memo 33 This Internet-Draft is submitted in full conformance with the 34 provisions of BCP 78 and BCP 79. 36 Internet-Drafts are working documents of the Internet Engineering 37 Task Force (IETF). Note that other groups may also distribute 38 working documents as Internet-Drafts. The list of current Internet- 39 Drafts is at https://datatracker.ietf.org/drafts/current/. 41 Internet-Drafts are draft documents valid for a maximum of six months 42 and may be updated, replaced, or obsoleted by other documents at any 43 time. It is inappropriate to use Internet-Drafts as reference 44 material or to cite them other than as "work in progress." 46 This Internet-Draft will expire on September 6, 2020. 48 Copyright Notice 50 Copyright (c) 2020 IETF Trust and the persons identified as the 51 document authors. All rights reserved. 53 This document is subject to BCP 78 and the IETF Trust's Legal 54 Provisions Relating to IETF Documents 55 (https://trustee.ietf.org/license-info) in effect on the date of 56 publication of this document. Please review these documents 57 carefully, as they describe your rights and restrictions with respect 58 to this document. Code Components extracted from this document must 59 include Simplified BSD License text as described in Section 4.e of 60 the Trust Legal Provisions and are provided without warranty as 61 described in the Simplified BSD License. 63 Table of Contents 65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 66 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 67 2. Applying SCHC to CoAP . . . . . . . . . . . . . . . . . . . . 4 68 3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 5 69 3.1. Differences between CoAP and UDP/IP . . . . . . . . . . . 5 70 4. Compression of CoAP header fields . . . . . . . . . . . . . . 6 71 4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7 72 4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7 73 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7 74 4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7 75 4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7 76 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8 77 5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8 78 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 8 79 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8 80 5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9 81 5.3.2. Variable number of path or query elements . . . . . . 10 82 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme 83 fields . . . . . . . . . . . . . . . . . . . . . . . . . 10 84 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path 85 and Location-Query fields . . . . . . . . . . . . . . . . 10 86 6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 10 87 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10 88 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11 89 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11 90 6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11 91 7. Examples of CoAP header compression . . . . . . . . . . . . . 12 92 7.1. Mandatory header with CON message . . . . . . . . . . . . 12 93 7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13 94 7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17 95 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 96 9. Security considerations . . . . . . . . . . . . . . . . . . . 27 97 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 98 11. Normative References . . . . . . . . . . . . . . . . . . . . 28 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 101 1. Introduction 103 CoAP [rfc7252] is a transfer protocol that implements a subset of 104 HTTP (Hypertext Transfer Protocol) and is optimized for REST-based 105 (Representational state transfer) services. Although CoAP was 106 designed for constrained devices, the size of a CoAP header still is 107 too large for the constraints of LPWAN (Low Power Wide Area Networks) 108 and some compression is needed to reduce the header size. 110 The [I-D.ietf-lpwan-ipv6-static-context-hc] defines SCHC, a header 111 compression mechanism for LPWAN network based on a static context. 112 The section 5 of the [I-D.ietf-lpwan-ipv6-static-context-hc] explains 113 the architecture where compression and decompression are done. The 114 context is known by both ends before transmission. The way the 115 context is configured or exchanged is out of the scope for this 116 document. 118 SCHC compresses and decompresses headers based on shared contexts 119 between devices. Each context consists of multiple Rules. Each rule 120 can match header fields and specific values or ranges of values. If 121 a rule matches, the matched header fields are substituted by the rule 122 ID and optionally some residual bits. Thus, different Rules may 123 correspond to different types of packets that a device expects to 124 send or receive. 126 A Rule describes the complete header of the packet with an ordered 127 list of fields descriptions, see section 7 of the 128 [I-D.ietf-lpwan-ipv6-static-context-hc], thereby each description 129 contains the field ID (FID), its length (FL) and its position (FP), a 130 direction indicator (DI) (upstream, downstream and bidirectional) and 131 some associated Target Values (TV). 133 A Matching Operator (MO) is associated to each header field 134 description. The rule is selected if all the MOs fit the TVs for all 135 fields of the incoming packet. 136 In that case, a Compression/Decompression Action (CDA) associated to 137 each field defines how the compressed and the decompressed values are 138 computed out of each other, for each of the header fields. 139 Compression mainly results in one of 4 actions: * send the field 140 value, * send nothing, * send some least significant bits of the 141 field or * send an index. After applying the compression there may 142 be some bits to be sent, these values are called Compression 143 Residues. 145 SCHC is a general concept mechanism that can be applied to different 146 protocols, the exact Rules to be used depend on the protocol and the 147 application, and CoAP differs from UDP and IPv6, see Section 3. 149 1.1. Terminology 151 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 152 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 153 "OPTIONAL" in this document are to be interpreted as described in BCP 154 14 [rfc2119][rfc8174] when, and only when, they appear in all 155 capitals, as shown here. 157 2. Applying SCHC to CoAP 159 The SCHC Compression rules can be applied to CoAP flows. SCHC 160 Compression of the CoAP header MAY be done in conjunction with the 161 lower layers (IPv6/UDP) or independently. The SCHC adaptation layers 162 as described in section 5 of [I-D.ietf-lpwan-ipv6-static-context-hc] 163 may be used as shown in Figure 1. 165 ^ +------------+ ^ +------------+ ^ +------------+ 166 | | CoAP | | | CoAP | inner | | CoAP | 167 | +------------+ v +------------+ x | OSCORE | 168 | | UDP | | DTLS | outer | +------------+ 169 | +------------+ +------------+ | | UDP | 170 | | IPv6 | | UDP | | +------------+ 171 v +------------+ +------------+ | | IPv6 | 172 | IPv6 | v +------------+ 173 +------------+ 175 Figure 1: rule scope for CoAP 177 Figure 1 shows some examples for CoAP architecture and the SCHC 178 rule's scope. 180 In the first example, a rule compresses the complete header stack 181 from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header 182 Compression Compressor/Decompressor) is performed at the Sender and 183 at the Receiver. 185 In the second example, an end-to-end encryption mechanisms is used 186 between the Sender and the Receiver. The SCHC compression is applied 187 in the CoAP layer compressing the CoAP header independently of the 188 other layers. The rule ID and the compression residue are encrypted 189 using a mechanism such as DTLS. Only the other end can decipher the 190 information. Layers below may also be compressed using other SCHC 191 rules (this is out of the scope of this document) as defined in the 192 SCHC [I-D.ietf-lpwan-ipv6-static-context-hc] document. 194 In the third example, OSCORE [rfc8613] is used. In this case, two 195 rulesets are used to compress the CoAP message. A first ruleset 196 focused on the inner header and is applied end to end by both ends. 197 A second ruleset compresses the outer header and the layers below and 198 is done between the Sender and the Receiver. 200 3. CoAP Compression with SCHC 202 SCHC with CoAP will be used exactly the same way as it is applied in 203 any protocol as IP or UDP with the difference that the fields 204 description needs to be defined based on both headers and target 205 values of the request and the responses. SCHC Rules description use 206 the direction information to optmize the compression by reducing the 207 number of Rules needed to compress traffic. CoAP compression follows 208 the [I-D.ietf-lpwan-ipv6-static-context-hc] scheme and as for other 209 protocols, if no valid Rule was found, then the packet MUST be sent 210 uncompressed using the RuleID dedicated to this purpose and the 211 Compression Residue is the complete header of the packet. See 212 section 6 of [I-D.ietf-lpwan-ipv6-static-context-hc]. 214 3.1. Differences between CoAP and UDP/IP 216 CoAP differs from IPv6 and UDP protocols on the following aspects: 218 o IPv6 and UDP are not request and response protocols as CoAP, and 219 so the same header fields are used in all packets for all 220 directions, with the value of some fields being swapped on the 221 return path (e.g. source and destination addresses fields). The 222 CoAP headers instead are asymmetric, the headers are different for 223 a request or a response. For example, the URI-path option is 224 mandatory in the request and is not found in the response, a 225 request may contain an Accept option and the response may contain 226 a Content option. 228 The [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a 229 Direction Indicator (DI) in the Field Description, which allows a 230 single Rule to process message headers differently depending on 231 the direction. 233 o Even when a field is "symmetric" (i.e. found in both directions) 234 the values carried in each direction are different. To performs 235 the compression a matching list in the TV might be use because 236 this allows reducing the range of expected values in a particular 237 direction and therefore reduces the size of the 238 compression residue. For instance, if a client sends only CON 239 requests, the type can be elided by compression and the answer may 240 use one single bit to carry either the ACK or RST type. In CoAP 241 some fields have the same behavior, for example the field Code can 242 have 0.0X code format value in the request and Y.ZZ code format in 243 the response. Through the direction indicator, a field 244 description in the Rules splits the possible field value in two 245 parts. Resulting in a smaller compression residue. 247 o In IPv6 and UDP, header fields have a fixed size, defined in the 248 Rule, which is not sent. In CoAP, some fields in the header have 249 a variable length, for example the Token size may vary from 0 to 8 250 bytes, the length is given by a field in the header. The CoAP 251 options are described using the Type-Length-Value encoding format. 253 Section 7.5.2 from [I-D.ietf-lpwan-ipv6-static-context-hc] offers 254 the possibility to define a function for the Field Length in the 255 Field Description to have knwoledge of the length before 256 compression. When doing SCHC compression of a variable length 257 field two cases may be raised after applying the CDA: * The result 258 of the compression is of fixed length and the compressed value is 259 sent in the residue. * Or the result of the compression is of 260 variable-length and in this case, the size is sent with the 261 compressed value in the residue. 263 o In CoAP headers, a field can appear several times. This is 264 typical for elements of a URI (path or queries). The SCHC 265 specification [I-D.ietf-lpwan-ipv6-static-context-hc] allows a 266 Field ID to appears several times in the rule, and uses the Field 267 Position (FP) to identify the correct instance, and thereby 268 removing the ambiguity of the matching operation. 270 o Field sizes defined in the CoAP protocol can be too large 271 regarding LPWAN traffic constraints. This is particularly true 272 for the Message ID field and the Token field. SCHC uses different 273 Matching operators (MO) to performs the compression, see section 274 7.4 of [I-D.ietf-lpwan-ipv6-static-context-hc]. In this case the 275 Most Significant Bits (MSB) MO can be applied to reduce the 276 information carried on LP 278 4. Compression of CoAP header fields 280 This section discusses the compression of the different CoAP header 281 fields. The CoAP compression with SCHC follows the Section 7.1 of 282 [I-D.ietf-lpwan-ipv6-static-context-hc]. 284 4.1. CoAP version field 286 CoAP version is bidirectional and MUST be elided during the SCHC 287 compression, since it always contains the same value. In the future, 288 if new versions of CoAP are defined, new rules will be needed to 289 avoid ambiguities between versions. 291 4.2. CoAP type field 293 The CoAP Protocol [rfc7252] has four type of messages: two request 294 (CON, NON); one response (ACK) and one empty message (RST). 296 The field SHOULD be elided if for instance a client is sending only 297 NON or only CON messages. For the RST message a dedicated Rule may 298 be needed. For other usages a mapping list can be used. 300 4.3. CoAP code field 302 The code field indicates the Request Method used in CoAP, a registry 303 is given in section 12.1 of [rfc7252]. The compression of the CoAP 304 code field follows the same principle as that of the CoAP type field. 305 If the device plays a specific role, the set of code values can be 306 split in two parts, the request codes with the 0 class and the 307 response values. 309 If the device only implements a CoAP client, the request code can be 310 reduced to the set of requests the client is able to process. 312 A mapping list can be used for known values, for other values the 313 field cannot be compressed an the value needs to be sent in the 314 residue. 316 4.4. CoAP Message ID field 318 The Message ID field can be compressed with the MSB(x) MO and the 319 Least Significant Bits (LSB) CDA, see section 7.4 of 320 [I-D.ietf-lpwan-ipv6-static-context-hc]. 322 4.5. CoAP Token fields 324 Token is defined through two CoAP fields, Token Length in the 325 mandatory header and Token Value directly following the mandatory 326 CoAP header. 328 Token Length is processed as any protocol field. If the value does 329 not change, the size can be stored in the TV and elided during the 330 transmission. Otherwise, it will have to be sent in the compression 331 residue. 333 Token Value MUST not be sent as a variable length residue to avoid 334 ambiguity with Token Length. Therefore, Token Length value MUST be 335 used to define the size of the residue. A specific function 336 designated as "TKL" MUST be used in the Rule. During the 337 decompression, this function returns the value contained in the Token 338 Length field. 340 5. CoAP options 342 5.1. CoAP Content and Accept options. 344 These fields are both unidirectional and MUST NOT be set to 345 bidirectional in a rule entry. 347 If a single value is expected by the client, it can be stored in the 348 TV and elided during the transmission. Otherwise, if several 349 possible values are expected by the client, a matching-list SHOULD be 350 used to limit the size of the residue. Otherwise, the value has to 351 be sent as a residue (fixed or variable length). 353 5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields 355 These fields are unidirectional and MUST NOT be set to bidirectional 356 in a rule DI entry. see section 7.1 of 357 [I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the 358 server to inform of the caching duration and is never found in client 359 requests. 361 If the duration is known by both ends, the value can be elided on the 362 LPWAN. 364 A matching list can be used if some well-known values are defined. 366 Otherwise these options can be sent as a residue (fixed or variable 367 length). 369 5.3. CoAP option Uri-Path and Uri-Query fields 371 These fields are unidirectional and MUST NOT be set to bidirectional 372 in a rule entry. They are used only by the client to access a 373 specific resource and are never found in server responses. 375 Uri-Path and Uri-Query elements are a repeatable options, the Field 376 Position (FP) gives the position in the path. 378 A Mapping list can be used to reduce the size of variable Paths or 379 Queries. In that case, to optimize the compression, several elements 380 can be regrouped into a single entry. Numbering of elements do not 381 change, MO comparison is set with the first element of the matching. 383 +-------------+--+--+--+--------+---------+-------------+ 384 | Field |FL|FP|DI| Target | Match | CDA | 385 | | | | | Value | Opera. | | 386 +-------------+--+--+--+--------+---------+-------------+ 387 |URI-Path | | 1|up|["/a/b",|equal |not-sent | 388 | | | | |"/c/d"] | | | 389 |URI-Path | | 3|up| |ignore |value-sent | 390 +-------------+--+--+--+--------+---------+-------------+ 392 Figure 2: complex path example 394 In Figure 2 a single bit residue can be used to code one of the 2 395 paths. If regrouping were not allowed, a 2 bits residue would be 396 needed. 398 5.3.1. Variable length Uri-Path and Uri-Query 400 When the length is not known at the rule creation, the Field Length 401 MUST be set to variable, and the unit is set to bytes. 403 The MSB MO can be applied to a Uri-Path or Uri-Query element. Since 404 MSB value is given in bit, the size MUST always be a multiple of 8 405 bits. 407 The length sent at the beginning of a variable length residue 408 indicates the size of the LSB in bytes. 410 For instance for a CORECONF path /c/X6?k="eth0" the rule can be set 411 to: 413 +-------------+---+--+--+--------+---------+-------------+ 414 | Field |FL |FP|DI| Target | Match | CDA | 415 | | | | | Value | Opera. | | 416 +-------------+---+--+--+--------+---------+-------------+ 417 |URI-Path | 8| 1|up|"c" |equal |not-sent | 418 |URI-Path |var| 2|up| |ignore |value-sent | 419 |URI-Query |var| 1|up|"k=" |MSB(16) |LSB | 420 +-------------+---+--+--+--------+---------+-------------+ 422 Figure 3: CORECONF URI compression 424 Figure 3 shows the parsing and the compression of the URI, where c is 425 not sent. The second element is sent with the length (i.e. 0x2 X 6) 426 followed by the query option (i.e. 0x05 "eth0"). 428 5.3.2. Variable number of path or query elements 430 The number of Uri-path or Uri-Query elements in a rule is fixed at 431 the rule creation time. If the number varies, several rules SHOULD 432 be created to cover all the possibilities. Another possibility is to 433 define the length of Uri-Path to variable and send a compression 434 residue with a length of 0 to indicate that this Uri-Path is empty. 435 This adds the 4 bits of the variable residue size. See section 7.5.2 436 [I-D.ietf-lpwan-ipv6-static-context-hc] 438 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields 440 These fields are unidirectional and MUST NOT be set to bidirectional 441 in a rule DI entry, see section 7.1 of the 442 [I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the 443 client to access a specific resource and are never found in server 444 response. 446 If the field value has to be sent, TV is not set, MO is set to 447 "ignore" and CDA is set to "value-sent". A mapping MAY also be used. 449 Otherwise, the TV is set to the value, MO is set to "equal" and CDA 450 is set to "not-sent". 452 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and 453 Location-Query fields 455 These fields are unidirectional. 457 These fields values cannot be stored in a rule entry. They MUST 458 always be sent with the compression residues. 460 6. SCHC compression of CoAP extension RFCs 462 6.1. Block 464 Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also 465 includes a fragmentation protocol. They are compatible. If a block 466 option is used, its content MUST be sent as a compression residue. 468 6.2. Observe 470 The [rfc7641] defines the Observe option. The TV is not set, MO is 471 set to "ignore" and the CDA is set to "value-sent". SCHC does not 472 limit the maximum size for this option (3 bytes). To reduce the 473 transmission size, either the device implementation MAY limit the 474 delta between two consecutive values, or a proxy can modify the 475 increment. 477 Since an RST message may be sent to inform a server that the client 478 does not require Observe response, a rule MUST allow the transmission 479 of this message. 481 6.3. No-Response 483 The [rfc7967] defines a No-Response option limiting the responses 484 made by a server to a request. If the value is known by both ends, 485 then TV is set to this value, MO is set to "equal" and CDA is set to 486 "not-sent". 488 Otherwise, if the value is changing over time, TV is not set, MO is 489 set to "ignore" and CDA to "value-sent". A matching list can also be 490 used to reduce the size. 492 6.4. OSCORE 494 OSCORE [rfc8613] defines end-to-end protection for CoAP messages. 495 This section describes how SCHC rules can be applied to compress 496 OSCORE-protected messages. 498 0 1 2 3 4 5 6 7 <--------- n bytes -------------> 499 +-+-+-+-+-+-+-+-+--------------------------------- 500 |0 0 0|h|k| n | Partial IV (if any) ... 501 +-+-+-+-+-+-+-+-+--------------------------------- 502 | | | 503 |<-- CoAP -->|<------ CoAP OSCORE_piv ------> | 504 OSCORE_flags 506 <- 1 byte -> <------ s bytes -----> 507 +------------+----------------------+-----------------------+ 508 | s (if any) | kid context (if any) | kid (if any) ... | 509 +------------+----------------------+-----------------------+ 510 | | | 511 | <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->| 513 Figure 4: OSCORE Option 515 The encoding of the OSCORE Option Value defined in Section 6.1 of 516 [rfc8613] is repeated in Figure 4. 518 The first byte is used for flags that specify the contents of the 519 OSCORE option. The 3 most significant bits of this byte are reserved 520 and always set to 0. Bit h, when set, indicates the presence of the 521 kid context field in the option. Bit k, when set, indicates the 522 presence of a kid field. The 3 least significant bits n indicate the 523 length of the piv (Partial Initialization Vector) field in bytes. 524 When n = 0, no piv is present. 526 The flag byte is followed by the piv field, kid context field and kid 527 field in this order and if present; the length of the kid context 528 field is encoded in the first byte denoting by s the length of the 529 kid context in bytes. 531 This specification recommends to identify the OSCORE Option and the 532 fields it contains. 534 Conceptually, it discerns up to 4 distinct pieces of information 535 within the OSCORE option: the flag bits, the piv, the kid context, 536 and the kid. It is thus recommended that the parser split the OSCORE 537 option into the 4 subsequent fields: 539 o CoAP OSCORE_flags, 541 o CoAP OSCORE_piv, 543 o CoAP OSCORE_kidctxt, 545 o CoAP OSCORE_kid. 547 These fields are shown superimposed on the OSCORE Option format in 548 Figure 4, the CoAP OSCORE_kidctxt field including the size bits s. 549 Their size SHOULD be reduced using SCHC compression. 551 7. Examples of CoAP header compression 553 7.1. Mandatory header with CON message 555 In this first scenario, the LPWAN compressor at the Network Gateway 556 side receives from an Internet client a POST message, which is 557 immediately acknowledged by the Device. For this simple scenario, 558 the rules are described Figure 5. 560 Rule ID 1 561 +-------------+--+--+--+------+---------+-------------++------------+ 562 | Field |FL|FP|DI|Target| Match | CDA || Sent | 563 | | | | |Value | Opera. | || [bits] | 564 +-------------+--+--+--+------+---------+-------------++------------+ 565 |CoAP version | | |bi| 01 |equal |not-sent || | 566 |CoAP Type | | |dw| CON |equal |not-sent || | 567 |CoAP Type | | |up|[ACK, | | || | 568 | | | | | RST] |match-map|matching-sent|| T | 569 |CoAP TKL | | |bi| 0 |equal |not-sent || | 570 |CoAP Code | | |bi|[0.00,| | || | 571 | | | | | ... | | || | 572 | | | | | 5.05]|match-map|matching-sent|| CC CCC | 573 |CoAP MID | | |bi| 0000 |MSB(7 ) |LSB || M-ID| 574 |CoAP Uri-Path| | |dw| path |equal 1 |not-sent || | 575 +-------------+--+--+--+------+---------+-------------++------------+ 577 Figure 5: CoAP Context to compress header without token 579 The version and Token Length fields are elided. The 26 method and 580 response codes defined in [rfc7252] has been shrunk to 5 bits using a 581 matching list. Uri-Path contains a single element indicated in the 582 matching operator. 584 SCHC Compression reduces the header sending only the Type, a mapped 585 code and the least significant bits of Message ID (9 bits in the 586 example above). 588 Note that a request sent by a client located in an Application Server 589 to a server located in the device, may not be compressed through this 590 rule since the MID will not start with 7 bits equal to 0. A CoAP 591 proxy, before the core SCHC C/D can rewrite the message ID to a value 592 matched by the rule. 594 7.2. OSCORE Compression 596 OSCORE aims to solve the problem of end-to-end encryption for CoAP 597 messages. The goal, therefore, is to hide as much of the message as 598 possible while still enabling proxy operation. 600 Conceptually this is achieved by splitting the CoAP message into an 601 Inner Plaintext and Outer OSCORE Message. The Inner Plaintext 602 contains sensitive information which is not necessary for proxy 603 operation. This, in turn, is the part of the message which can be 604 encrypted until it reaches its end destination. The Outer Message 605 acts as a shell matching the format of a regular CoAP message, and 606 includes all Options and information needed for proxy operation and 607 caching. This decomposition is illustrated in Figure 6. 609 CoAP options are sorted into one of 3 classes, each granted a 610 specific type of protection by the protocol: 612 o Class E: Encrypted options moved to the Inner Plaintext, 614 o Class I: Integrity-protected options included in the AAD for the 615 encryption of the Plaintext but otherwise left untouched in the 616 Outer Message, 618 o Class U: Unprotected options left untouched in the Outer Message. 620 Additionally, the OSCORE Option is added as an Outer option, 621 signalling that the message is OSCORE protected. This option carries 622 the information necessary to retrieve the Security Context with which 623 the message was encrypted so that it may be correctly decrypted at 624 the other end-point. 626 Original CoAP Message 627 +-+-+---+-------+---------------+ 628 |v|t|tkl| code | Msg Id. | 629 +-+-+---+-------+---------------+....+ 630 | Token | 631 +-------------------------------.....+ 632 | Options (IEU) | 633 . . 634 . . 635 +------+-------------------+ 636 | 0xFF | 637 +------+------------------------+ 638 | | 639 | Payload | 640 | | 641 +-------------------------------+ 642 / \ 643 / \ 644 / \ 645 / \ 646 Outer Header v v Plaintext 647 +-+-+---+--------+---------------+ +-------+ 648 |v|t|tkl|new code| Msg Id. | | code | 649 +-+-+---+--------+---------------+....+ +-------+-----......+ 650 | Token | | Options (E) | 651 +--------------------------------.....+ +-------+------.....+ 652 | Options (IU) | | OxFF | 653 . . +-------+-----------+ 654 . OSCORE Option . | | 655 +------+-------------------+ | Payload | 656 | 0xFF | | | 657 +------+ +-------------------+ 659 Figure 6: A CoAP message is split into an OSCORE outer and plaintext 661 Figure 6 shows the message format for the OSCORE Message and 662 Plaintext. 664 In the Outer Header, the original message code is hidden and replaced 665 by a default dummy value. As seen in sections 4.1.3.5 and 4.2 of the 666 [rfc8613], the message code is replaced by POST for requests and 667 Changed for responses when Observe is not used. If Observe is used, 668 the message code is replaced by FETCH for requests and Content for 669 responses. 671 The original message code is put into the first byte of the 672 Plaintext. Following the message code, the class E options comes and 673 if present the original message Payload is preceded by its payload 674 marker. 676 The Plaintext is now encrypted by an AEAD algorithm which integrity 677 protects Security Context parameters and eventually any class I 678 options from the Outer Header. Currently no CoAP options are marked 679 class I. The resulting Ciphertext becomes the new Payload of the 680 OSCORE message, as illustrated in Figure 7. 682 This Ciphertext is, as defined in RFC 5116, the concatenation of the 683 encrypted Plaintext and its authentication tag. Note that Inner 684 Compression only affects the Plaintext before encryption, thus we can 685 only aim to reduce this first, variable length component of the 686 Ciphertext. The authentication tag is fixed in length and considered 687 part of the cost of protection. 689 Outer Header 690 +-+-+---+--------+---------------+ 691 |v|t|tkl|new code| Msg Id. | 692 +-+-+---+--------+---------------+....+ 693 | Token | 694 +--------------------------------.....+ 695 | Options (IU) | 696 . . 697 . OSCORE Option . 698 +------+-------------------+ 699 | 0xFF | 700 +------+---------------------------+ 701 | | 702 | Ciphertext: Encrypted Inner | 703 | Header and Payload | 704 | + Authentication Tag | 705 | | 706 +----------------------------------+ 708 Figure 7: OSCORE message 710 The SCHC Compression scheme consists of compressing both the 711 Plaintext before encryption and the resulting OSCORE message after 712 encryption, see Figure 8. 714 This translates into a segmented process where SCHC compression is 715 applied independently in 2 stages, each with its corresponding set of 716 rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way 717 compression is applied to all fields of the original CoAP message. 719 Note that since the Inner part of the message can only be decrypted 720 by the corresponding end-point, this end-point will also have to 721 implement Inner SCHC Compression/Decompression. 723 Outer Message OSCORE Plaintext 724 +-+-+---+--------+---------------+ +-------+ 725 |v|t|tkl|new code| Msg Id. | | code | 726 +-+-+---+--------+---------------+....+ +-------+-----......+ 727 | Token | | Options (E) | 728 +--------------------------------.....+ +-------+------.....+ 729 | Options (IU) | | OxFF | 730 . . +-------+-----------+ 731 . OSCORE Option . | | 732 +------+-------------------+ | Payload | 733 | 0xFF | | | 734 +------+------------+ +-------------------+ 735 | Ciphertext |<---------\ | 736 | | | v 737 +-------------------+ | +-----------------+ 738 | | | Inner SCHC | 739 v | | Compression | 740 +-----------------+ | +-----------------+ 741 | Outer SCHC | | | 742 | Compression | | v 743 +-----------------+ | +-------+ 744 | | |Rule ID| 745 v | +-------+--+ 746 +--------+ +------------+ | Residue | 747 |Rule ID'| | Encryption | <--- +----------+--------+ 748 +--------+--+ +------------+ | | 749 | Residue' | | Payload | 750 +-----------+-------+ | | 751 | Ciphertext | +-------------------+ 752 | | 753 +-------------------+ 755 Figure 8: OSCORE Compression Diagram 757 7.3. Example OSCORE Compression 759 An example is given with a GET Request and its consequent CONTENT 760 Response from a device-based CoAP client to a cloud-based CoAP 761 server. A possible set of rules for the Inner and Outer SCHC 762 Compression is shown. A dump of the results and a contrast between 763 SCHC + OSCORE performance with SCHC + COAP performance is also 764 listed. This gives an approximation to the cost of security with 765 SCHC-OSCORE. 767 Our first example CoAP message is the GET Request in Figure 9 769 Original message: 770 ================= 771 0x4101000182bb74656d7065726174757265 773 Header: 774 0x4101 775 01 Ver 776 00 CON 777 0001 tkl 778 00000001 Request Code 1 "GET" 780 0x0001 = mid 781 0x82 = token 783 Options: 784 0xbb74656d7065726174757265 785 Option 11: URI_PATH 786 Value = temperature 788 Original msg length: 17 bytes. 790 Figure 9: CoAP GET Request 792 Its corresponding response is the CONTENT Response in Figure 10. 794 Original message: 795 ================= 796 0x6145000182ff32332043 798 Header: 799 0x6145 800 01 Ver 801 10 ACK 802 0001 tkl 803 01000101 Successful Response Code 69 "2.05 Content" 805 0x0001 = mid 806 0x82 = token 808 0xFF Payload marker 809 Payload: 810 0x32332043 812 Original msg length: 10 814 Figure 10: CoAP CONTENT Response 816 TheSCHC Rules for the Inner Compression include all fields that are 817 alreadypresent in a regular CoAP message. The methods described in 818 section Section 4 applies to these fields. As an example, see 819 Figure 11. 821 Rule ID 0 822 +---------------+--+--+-----------+-----------+-----------++------+ 823 | Field |FP|DI| Target | MO | CDA || Sent | 824 | | | | Value | | ||[bits]| 825 +---------------+--+--+-----------+-----------+-----------++------+ 826 |CoAP Code | |up| 1 | equal |not-sent || | 827 |CoAP Code | |dw|[69,132] | match-map |match-sent || c | 828 |CoAP Uri-Path | |up|temperature| equal |not-sent || | 829 |COAP Option-End| |dw| 0xFF | equal |not-sent || | 830 +---------------+--+--+-----------+-----------+-----------++------+ 832 Figure 11: Inner SCHC Rules 834 Figure 12 shows the Plaintext obtained for our example GET Request 835 and follows the process of Inner Compression and Encryption until we 836 end up with the Payload to be added in the outer OSCORE Message. 838 In this case the original message has no payload and its resulting 839 Plaintext can be compressed up to only 1 byte (size of the Rule ID). 840 The AEAD algorithm preserves this length in its first output, but 841 also yields a fixed-size tag which cannot be compressed and has to be 842 included in the OSCORE message. This translates into an overhead in 843 total message length, which limits the amount of compression that can 844 be achieved and plays into the cost of adding security to the 845 exchange. 847 ________________________________________________________ 848 | | 849 | OSCORE Plaintext | 850 | | 851 | 0x01bb74656d7065726174757265 (13 bytes) | 852 | | 853 | 0x01 Request Code GET | 854 | | 855 | bb74656d7065726174757265 Option 11: URI_PATH | 856 | Value = temperature | 857 |________________________________________________________| 859 | 860 | 861 | Inner SCHC Compression 862 | 863 v 864 _________________________________ 865 | | 866 | Compressed Plaintext | 867 | | 868 | 0x00 | 869 | | 870 | Rule ID = 0x00 (1 byte) | 871 | (No residue) | 872 |_________________________________| 874 | 875 | AEAD Encryption 876 | (piv = 0x04) 877 v 878 _________________________________________________ 879 | | 880 | encrypted_plaintext = 0xa2 (1 byte) | 881 | tag = 0xc54fe1b434297b62 (8 bytes) | 882 | | 883 | ciphertext = 0xa2c54fe1b434297b62 (9 bytes) | 884 |_________________________________________________| 886 Figure 12: Plaintext compression and encryption for GET Request 888 In Figure 13 the process is repeated for the example CONTENT 889 Response. The residue is 1 bit long. Note that since SCHC adds 890 padding after the payload, this misalignment causes the hexadecimal 891 code from the payload to differ from the original, even though it has 892 not been compressed. 894 On top of this, the overhead from the tag bytes is incurred as 895 before. 897 ________________________________________________________ 898 | | 899 | OSCORE Plaintext | 900 | | 901 | 0x45ff32332043 (6 bytes) | 902 | | 903 | 0x45 Successful Response Code 69 "2.05 Content" | 904 | | 905 | ff Payload marker | 906 | | 907 | 32332043 Payload | 908 |________________________________________________________| 910 | 911 | 912 | Inner SCHC Compression 913 | 914 v 915 __________________________________________ 916 | | 917 | Compressed Plaintext | 918 | | 919 | 0x001919902180 (6 bytes) | 920 | | 921 | 00 Rule ID | 922 | | 923 | 0b0 (1 bit match-map residue) | 924 | 0x32332043 >> 1 (shifted payload) | 925 | 0b0000000 Padding | 926 |__________________________________________| 928 | 929 | AEAD Encryption 930 | (piv = 0x04) 931 v 932 _________________________________________________________ 933 | | 934 | encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) | 935 | tag = 0xe9aef3f2461e0c29 (8 bytes) | 936 | | 937 | ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) | 938 |_________________________________________________________| 940 Figure 13: Plaintext compression and encryption for CONTENT Response 941 The Outer SCHC Rules (Figure 16) MUST process the OSCORE Options 942 fields. In Figure 14 and Figure 15 we show a dump of the OSCORE 943 Messages generated from our example messages once they have been 944 provided with the Inner Compressed Ciphertext in the payload. These 945 are the messages that have to be compressed by the Outer SCHC 946 Compression. 948 Protected message: 949 ================== 950 0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62 951 (25 bytes) 953 Header: 954 0x4102 955 01 Ver 956 00 CON 957 0001 tkl 958 00000010 Request Code 2 "POST" 960 0x0001 = mid 961 0x82 = token 963 Options: 964 0xd8080904636c69656e74 (10 bytes) 965 Option 21: OBJECT_SECURITY 966 Value = 0x0904636c69656e74 967 09 = 000 0 1 001 Flag byte 968 h k n 969 04 piv 970 636c69656e74 kid 972 0xFF Payload marker 973 Payload: 974 0xa2c54fe1b434297b62 (9 bytes) 976 Figure 14: Protected and Inner SCHC Compressed GET Request 978 Protected message: 979 ================== 980 0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29 981 (22 bytes) 983 Header: 984 0x6144 985 01 Ver 986 10 ACK 987 0001 tkl 988 01000100 Successful Response Code 68 "2.04 Changed" 990 0x0001 = mid 991 0x82 = token 993 Options: 994 0xd008 (2 bytes) 995 Option 21: OBJECT_SECURITY 996 Value = b'' 998 0xFF Payload marker 999 Payload: 1000 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) 1002 Figure 15: Protected and Inner SCHC Compressed CONTENT Response 1004 For the flag bits, a number of compression methods has been shown to 1005 be useful depending on the application. The simplest alternative is 1006 to provide a fixed value for the flags, combining MO equal and CDA 1007 not- sent. This saves most bits but could prevent flexibility. 1008 Otherwise, match-mapping could be used to choose from an interested 1009 number of configurations to the exchange. Otherwise, MSB could be 1010 used to mask off the 3 hard-coded most significant bits. 1012 Note that fixing a flag bit will limit the choice of CoAP Options 1013 that can be used in the exchange, since their values are dependent on 1014 certain options. 1016 The piv field lends itself to having a number of bits masked off with 1017 MO MSB and CDA LSB. This could be useful in applications where the 1018 message frequency is low such as that found in LPWAN technologies. 1019 Note that compressing the sequence numbers effectively reduces the 1020 maximum amount of sequence numbers that can be used in an exchange. 1021 Once this amount is exceeded, the OSCORE keys need to be re- 1022 established. 1024 The size s included in the kid context field MAY be masked off with 1025 CDA MSB. The rest of the field could have additional bits masked 1026 off, or have the whole field be fixed with MO equal and CDA not-sent. 1027 The same holds for the kid field. 1029 Figure 16 shows a possible set of Outer Rules to compress the Outer 1030 Header. 1032 Rule ID 0 1033 +-------------------+--+--+--------------+--------+---------++------+ 1034 | Field |FP|DI| Target | MO | CDA || Sent | 1035 | | | | Value | | ||[bits]| 1036 +-------------------+--+--+--------------+--------+---------++------+ 1037 |CoAP version | |bi| 01 |equal |not-sent || | 1038 |CoAP Type | |up| 0 |equal |not-sent || | 1039 |CoAP Type | |dw| 2 |equal |not-sent || | 1040 |CoAP TKL | |bi| 1 |equal |not-sent || | 1041 |CoAP Code | |up| 2 |equal |not-sent || | 1042 |CoAP Code | |dw| 68 |equal |not-sent || | 1043 |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | 1044 |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | 1045 |CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || | 1046 |CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP | 1047 |COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK | 1048 |COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || | 1049 |CoAP OSCORE_flags | |dw| b'' |equal |not-sent || | 1050 |CoAP OSCORE_piv | |dw| b'' |equal |not-sent || | 1051 |CoAP OSCORE_kid | |dw| b'' |equal |not-sent || | 1052 |COAP Option-End | |dw| 0xFF |equal |not-sent || | 1053 +-------------------+--+--+--------------+--------+---------++------+ 1055 Figure 16: Outer SCHC Rules 1057 These Outer Rules are applied to the example GET Request and CONTENT 1058 Response. The resulting messages are shown in Figure 17 and 1059 Figure 18. 1061 Compressed message: 1062 ================== 1063 0x001489458a9fc3686852f6c4 (12 bytes) 1064 0x00 Rule ID 1065 1489 Compression Residue 1066 458a9fc3686852f6c4 Padded payload 1068 Compression residue: 1069 0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding) 1070 mid tkn piv kid 1072 Payload 1073 0xa2c54fe1b434297b62 (9 bytes) 1075 Compressed message length: 12 bytes 1077 Figure 17: SCHC-OSCORE Compressed GET Request 1079 Compressed message: 1080 ================== 1081 0x0014218daf84d983d35de7e48c3c1852 (16 bytes) 1082 0x00 Rule ID 1083 14 Compression residue 1084 218daf84d983d35de7e48c3c1852 Padded payload 1085 Compression residue: 1086 0b0001 010 (7 bits -> 1 byte with padding) 1087 mid tkn 1089 Payload 1090 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) 1092 Compressed msg length: 16 bytes 1094 Figure 18: SCHC-OSCORE Compressed CONTENT Response 1096 For contrast, we compare these results with what would be obtained by 1097 SCHC compressing the original CoAP messages without protecting them 1098 with OSCORE. To do this, we compress the CoAP messages according to 1099 the SCHC rules in Figure 19. 1101 Rule ID 1 1102 +---------------+--+--+-----------+---------+-----------++--------+ 1103 | Field |FP|DI| Target | MO | CDA || Sent | 1104 | | | | Value | | || [bits] | 1105 +---------------+--+--+-----------+---------+-----------++--------+ 1106 |CoAP version | |bi| 01 |equal |not-sent || | 1107 |CoAP Type | |up| 0 |equal |not-sent || | 1108 |CoAP Type | |dw| 2 |equal |not-sent || | 1109 |CoAP TKL | |bi| 1 |equal |not-sent || | 1110 |CoAP Code | |up| 2 |equal |not-sent || | 1111 |CoAP Code | |dw| [69,132] |match-map|map-sent ||C | 1112 |CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM | 1113 |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT | 1114 |CoAP Uri-Path | |up|temperature|equal |not-sent || | 1115 |COAP Option-End| |dw| 0xFF |equal |not-sent || | 1116 +---------------+--+--+-----------+---------+-----------++--------+ 1118 Figure 19: SCHC-CoAP Rules (No OSCORE) 1120 This yields the results in Figure 20 for the Request, and Figure 21 1121 for the Response. 1123 Compressed message: 1124 ================== 1125 0x0114 1126 0x01 = Rule ID 1128 Compression residue: 1129 0b00010100 (1 byte) 1131 Compressed msg length: 2 1133 Figure 20: CoAP GET Compressed without OSCORE 1135 Compressed message: 1136 ================== 1137 0x010a32332043 1138 0x01 = Rule ID 1140 Compression residue: 1141 0b00001010 (1 byte) 1143 Payload 1144 0x32332043 1146 Compressed msg length: 6 1148 Figure 21: CoAP CONTENT Compressed without OSCORE 1150 As can be seen, the difference between applying SCHC + OSCORE as 1151 compared to regular SCHC + COAP is about 10 bytes of cost. 1153 8. IANA Considerations 1155 This document has no request to IANA. 1157 9. Security considerations 1159 This document does not have any more Security consideration than the 1160 ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc]. 1161 Variable length residues may be used to compress URI elements. They 1162 cannot produce a packet expansion either on the LPWAN network or in 1163 the Internet network after decompression. The length send is not 1164 used to indicate the information that should be reconstructed at the 1165 other end, but on the contrary the information sent as a Residue. 1166 Therefore, if a length is set to a high value, but the number of bits 1167 on the SCHC packet is smaller, the packet must be dropped by the 1168 decompressor. 1170 OSCORE compression is also based on the same compression method 1171 described in [I-D.ietf-lpwan-ipv6-static-context-hc]. The size of 1172 the Initialisation Vector residue size must be considered carefully. 1173 A too large value has a impact on the compression efficiency and a 1174 too small value will force the device to renew its key more often. 1175 This operation may be long and energy consuming. 1177 10. Acknowledgements 1179 The authors would like to thank Dominique Barthel, Carsten Bormann, 1180 Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov, 1181 Goran Selander. 1183 11. Normative References 1185 [I-D.ietf-lpwan-ipv6-static-context-hc] 1186 Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J. 1187 Zuniga, "Static Context Header Compression (SCHC) and 1188 fragmentation for LPWAN, application to UDP/IPv6", draft- 1189 ietf-lpwan-ipv6-static-context-hc-24 (work in progress), 1190 December 2019. 1192 [rfc2119] Bradner, S., "Key words for use in RFCs to Indicate 1193 Requirement Levels", BCP 14, RFC 2119, 1194 DOI 10.17487/RFC2119, March 1997, 1195 . 1197 [rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained 1198 Application Protocol (CoAP)", RFC 7252, 1199 DOI 10.17487/RFC7252, June 2014, 1200 . 1202 [rfc7641] Hartke, K., "Observing Resources in the Constrained 1203 Application Protocol (CoAP)", RFC 7641, 1204 DOI 10.17487/RFC7641, September 2015, 1205 . 1207 [rfc7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in 1208 the Constrained Application Protocol (CoAP)", RFC 7959, 1209 DOI 10.17487/RFC7959, August 2016, 1210 . 1212 [rfc7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T. 1213 Bose, "Constrained Application Protocol (CoAP) Option for 1214 No Server Response", RFC 7967, DOI 10.17487/RFC7967, 1215 August 2016, . 1217 [rfc8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1218 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1219 May 2017, . 1221 [rfc8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, 1222 "Object Security for Constrained RESTful Environments 1223 (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, 1224 . 1226 Authors' Addresses 1228 Ana Minaburo 1229 Acklio 1230 1137A avenue des Champs Blancs 1231 35510 Cesson-Sevigne Cedex 1232 France 1234 Email: ana@ackl.io 1236 Laurent Toutain 1237 Institut MINES TELECOM; IMT Atlantique 1238 2 rue de la Chataigneraie 1239 CS 17607 1240 35576 Cesson-Sevigne Cedex 1241 France 1243 Email: Laurent.Toutain@imt-atlantique.fr 1245 Ricardo Andreasen 1246 Universidad de Buenos Aires 1247 Av. Paseo Colon 850 1248 C1063ACV Ciudad Autonoma de Buenos Aires 1249 Argentina 1251 Email: randreasen@fi.uba.ar