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Nottingham 3 Internet-Draft 8 June 2022 4 Intended status: Standards Track 5 Expires: 10 December 2022 7 Retrofit Structured Fields for HTTP 8 draft-ietf-httpbis-retrofit-04 10 Abstract 12 This specification nominates a selection of existing HTTP fields as 13 having syntax that is compatible with Structured Fields, so that they 14 can be handled as such (subject to certain caveats). 16 To accommodate some additional fields whose syntax is not compatible, 17 it also defines mappings of their semantics into new Structured 18 Fields. It does not specify how to negotiate their use. 20 About This Document 22 This note is to be removed before publishing as an RFC. 24 Status information for this document may be found at 25 https://datatracker.ietf.org/doc/draft-ietf-httpbis-retrofit/. 27 Discussion of this document takes place on the HTTP Working Group 28 mailing list (mailto:ietf-http-wg@w3.org), which is archived at 29 https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group 30 information can be found at https://httpwg.org/. 32 Source for this draft and an issue tracker can be found at 33 https://github.com/httpwg/http-extensions/labels/retrofit. 35 Status of This Memo 37 This Internet-Draft is submitted in full conformance with the 38 provisions of BCP 78 and BCP 79. 40 Internet-Drafts are working documents of the Internet Engineering 41 Task Force (IETF). Note that other groups may also distribute 42 working documents as Internet-Drafts. The list of current Internet- 43 Drafts is at https://datatracker.ietf.org/drafts/current/. 45 Internet-Drafts are draft documents valid for a maximum of six months 46 and may be updated, replaced, or obsoleted by other documents at any 47 time. It is inappropriate to use Internet-Drafts as reference 48 material or to cite them other than as "work in progress." 49 This Internet-Draft will expire on 10 December 2022. 51 Copyright Notice 53 Copyright (c) 2022 IETF Trust and the persons identified as the 54 document authors. All rights reserved. 56 This document is subject to BCP 78 and the IETF Trust's Legal 57 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 58 license-info) in effect on the date of publication of this document. 59 Please review these documents carefully, as they describe your rights 60 and restrictions with respect to this document. Code Components 61 extracted from this document must include Revised BSD License text as 62 described in Section 4.e of the Trust Legal Provisions and are 63 provided without warranty as described in the Revised BSD License. 65 Table of Contents 67 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 68 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 69 2. Compatible Fields . . . . . . . . . . . . . . . . . . . . . . 3 70 3. Mapped Fields . . . . . . . . . . . . . . . . . . . . . . . . 7 71 3.1. URLs . . . . . . . . . . . . . . . . . . . . . . . . . . 8 72 3.2. Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 8 73 3.3. ETags . . . . . . . . . . . . . . . . . . . . . . . . . . 9 74 3.4. Links . . . . . . . . . . . . . . . . . . . . . . . . . . 9 75 3.5. Cookies . . . . . . . . . . . . . . . . . . . . . . . . . 10 76 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 77 5. Security Considerations . . . . . . . . . . . . . . . . . . . 13 78 6. Normative References . . . . . . . . . . . . . . . . . . . . 13 79 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 81 1. Introduction 83 Structured Field Values for HTTP [STRUCTURED-FIELDS] introduced a 84 data model with associated parsing and serialization algorithms for 85 use by new HTTP field values. Fields that are defined as Structured 86 Fields can realise a number of benefits, including: 88 * Improved interoperability and security: precisely defined parsing 89 and serialisation algorithms are typically not available for 90 fields defined with just ABNF and/or prose. 92 * Reuse of common implementations: many parsers for other fields are 93 specific to a single field or a small family of fields. 95 * Canonical form: because a deterministic serialisation algorithm is 96 defined for each type, Structure Fields have a canonical 97 representation. 99 * Enhanced API support: a regular data model makes it easier to 100 expose field values as a native data structure in implementations. 102 * Alternative serialisations: While [STRUCTURED-FIELDS] defines a 103 textual serialisation of that data model, other, more efficient 104 serialisations of the underlying data model are also possible. 106 However, a field needs to be defined as a Structured Field for these 107 benefits to be realised. Many existing fields are not, making up the 108 bulk of header and trailer fields seen in HTTP traffic on the 109 internet. 111 This specification defines how a selection of existing HTTP fields 112 can be handled as Structured Fields, so that these benefits can be 113 realised -- thereby making them Retrofit Structured Fields. 115 It does so using two techniques. Section 2 lists compatible fields 116 -- those that can be handled as if they were Structured Fields due to 117 the similarity of their defined syntax to that in Structured Fields. 118 Section 3 lists mapped fields -- those whose syntax needs to be 119 transformed into an underlying data model which is then mapped into 120 that defined by Structured Fields. 122 Note that while implementations can parse and serialise compatible 123 fields as Structured Fields subject to the caveats in Section 2, a 124 sender cannot generate mapped fields from Section 3 and expect them 125 to be understood and acted upon by the recipient without prior 126 negotiation. This specification does not define such a mechanism. 128 1.1. Notational Conventions 130 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 131 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 132 "OPTIONAL" in this document are to be interpreted as described in 133 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 134 capitals, as shown here. 136 2. Compatible Fields 138 The HTTP fields listed in Table 1 can usually have their values 139 handled as Structured Fields according to the listed parsing and 140 serialisation algorithms in [STRUCTURED-FIELDS], subject to the 141 listed caveats. 143 The listed types are chosen for compatibility with the defined syntax 144 of the field as well as with actual internet traffic. However, not 145 all instances of these fields will successfully parse. This might be 146 because the field value is clearly invalid, or it might be because it 147 is valid but not parseable as a Structured Field. 149 An application using this specification will need to consider how to 150 handle such field values. Depending on its requirements, it might be 151 advisable to reject such values, treat them as opaque strings, or 152 attempt to recover a structured value from them in an ad hoc fashion. 154 +==================================+=================+ 155 | Field Name | Structured Type | 156 +==================================+=================+ 157 | Accept | List | 158 +----------------------------------+-----------------+ 159 | Accept-Encoding | List | 160 +----------------------------------+-----------------+ 161 | Accept-Language | List | 162 +----------------------------------+-----------------+ 163 | Accept-Patch | List | 164 +----------------------------------+-----------------+ 165 | Accept-Post | List | 166 +----------------------------------+-----------------+ 167 | Accept-Ranges | List | 168 +----------------------------------+-----------------+ 169 | Access-Control-Allow-Credentials | Item | 170 +----------------------------------+-----------------+ 171 | Access-Control-Allow-Headers | List | 172 +----------------------------------+-----------------+ 173 | Access-Control-Allow-Methods | List | 174 +----------------------------------+-----------------+ 175 | Access-Control-Allow-Origin | Item | 176 +----------------------------------+-----------------+ 177 | Access-Control-Expose-Headers | List | 178 +----------------------------------+-----------------+ 179 | Access-Control-Max-Age | Item | 180 +----------------------------------+-----------------+ 181 | Access-Control-Request-Headers | List | 182 +----------------------------------+-----------------+ 183 | Access-Control-Request-Method | Item | 184 +----------------------------------+-----------------+ 185 | Age | Item | 186 +----------------------------------+-----------------+ 187 | Allow | List | 188 +----------------------------------+-----------------+ 189 | ALPN | List | 190 +----------------------------------+-----------------+ 191 | Alt-Svc | Dictionary | 192 +----------------------------------+-----------------+ 193 | Alt-Used | Item | 194 +----------------------------------+-----------------+ 195 | Cache-Control | Dictionary | 196 +----------------------------------+-----------------+ 197 | CDN-Loop | List | 198 +----------------------------------+-----------------+ 199 | Clear-Site-Data | List | 200 +----------------------------------+-----------------+ 201 | Connection | List | 202 +----------------------------------+-----------------+ 203 | Content-Encoding | List | 204 +----------------------------------+-----------------+ 205 | Content-Language | List | 206 +----------------------------------+-----------------+ 207 | Content-Length | List | 208 +----------------------------------+-----------------+ 209 | Content-Type | Item | 210 +----------------------------------+-----------------+ 211 | Cross-Origin-Resource-Policy | Item | 212 +----------------------------------+-----------------+ 213 | Expect | Dictionary | 214 +----------------------------------+-----------------+ 215 | Expect-CT | Dictionary | 216 +----------------------------------+-----------------+ 217 | Forwarded | Dictionary | 218 +----------------------------------+-----------------+ 219 | Host | Item | 220 +----------------------------------+-----------------+ 221 | Keep-Alive | Dictionary | 222 +----------------------------------+-----------------+ 223 | Max-Forwards | Item | 224 +----------------------------------+-----------------+ 225 | Origin | Item | 226 +----------------------------------+-----------------+ 227 | Pragma | Dictionary | 228 +----------------------------------+-----------------+ 229 | Prefer | Dictionary | 230 +----------------------------------+-----------------+ 231 | Preference-Applied | Dictionary | 232 +----------------------------------+-----------------+ 233 | Retry-After | Item | 234 +----------------------------------+-----------------+ 235 | Sec-WebSocket-Extensions | List | 236 +----------------------------------+-----------------+ 237 | Sec-WebSocket-Protocol | List | 238 +----------------------------------+-----------------+ 239 | Sec-WebSocket-Version | Item | 240 +----------------------------------+-----------------+ 241 | Server-Timing | List | 242 +----------------------------------+-----------------+ 243 | Surrogate-Control | Dictionary | 244 +----------------------------------+-----------------+ 245 | TE | List | 246 +----------------------------------+-----------------+ 247 | Timing-Allow-Origin | List | 248 +----------------------------------+-----------------+ 249 | Trailer | List | 250 +----------------------------------+-----------------+ 251 | Transfer-Encoding | List | 252 +----------------------------------+-----------------+ 253 | Vary | List | 254 +----------------------------------+-----------------+ 255 | X-Content-Type-Options | Item | 256 +----------------------------------+-----------------+ 257 | X-Frame-Options | Item | 258 +----------------------------------+-----------------+ 259 | X-XSS-Protection | List | 260 +----------------------------------+-----------------+ 262 Table 1: Compatible Fields 264 Note the following caveats regarding compatibility: 266 Parameter and Dictionary keys: HTTP parameter names are case- 267 insensitive (per Section 5.6.6 of [HTTP]), but Structured Fields 268 require them to be all-lowercase. Although the vast majority of 269 parameters seen in typical traffic are all-lowercase, 270 compatibility can be improved by force-lowercasing parameters when 271 parsing. Likewise, many Dictionary-based fields (e.g., Cache- 272 Control, Expect-CT, Pragma, Prefer, Preference-Applied, Surrogate- 273 Control) have case-insensitive keys, and compatibility can be 274 improved by force-lowercasing them when parsing. 276 Parameter delimitation: The parameters rule in HTTP (see 277 Section 5.6.6 of [HTTP]) allows whitespace before the ";" 278 delimiter, but Structured Fields does not. Compatibility can be 279 improved by allowing such whitespace when parsing. 281 String quoting: Section 5.6.4 of [HTTP] allows backslash-escaping 282 most characters in quoted strings, whereas Structured Field 283 Strings only escape "\" and DQUOTE. Compatibility can be improved 284 by unescaping other characters before parsing. 286 Token limitations: In Structured Fields, tokens are required to 287 begin with an alphabetic character or "*", whereas HTTP tokens 288 allow a wider range of characters. This prevents use of mapped 289 values that begin with one of these characters. For example, 290 media types, field names, methods, range-units, character and 291 transfer codings that begin with a number or special character 292 other than "*" might be valid HTTP protocol elements, but will not 293 be able to be parsed as Structured Field Tokens. 295 Integer limitations: Structured Fields Integers can have at most 15 296 digits; larger values will not be able to be represented in them. 298 IPv6 Literals: Fields whose values contain IPv6 literal addresses 299 (such as CDN-Loop, Host, and Origin) are not able to be 300 represented as Structured Fields Tokens, because the brackets used 301 to delimit them are not allowed in Tokens. 303 Empty Field Values: Empty and whitespace-only field values are 304 considered errors in Structured Fields. For compatible fields, an 305 empty field indicates that the field should be silently ignored. 307 Alt-Svc: Some ALPN tokens (e.g., h3-Q43) do not conform to key's 308 syntax, and therefore cannot be represented as a Token. Since the 309 final version of HTTP/3 uses the h3 token, this shouldn't be a 310 long-term issue, although future tokens may again violate this 311 assumption. 313 Content-Length: Note that Content-Length is defined as a List 314 because it is not uncommon for implementations to mistakenly send 315 multiple values. See Section 8.6 of [HTTP] for handling 316 requirements. 318 Retry-After: Only the delta-seconds form of Retry-After can be 319 represented; a Retry-After value containing a http-date will need 320 to be converted into delta-seconds to be conveyed as a Structured 321 Field Value. 323 3. Mapped Fields 325 Some HTTP field values have syntax that cannot be successfully parsed 326 as Structured Fields. Instead, it is necessary to map them into a 327 separate Structured Field with an alternative name. 329 For example, the Date HTTP header field carries a date: 331 Date: Sun, 06 Nov 1994 08:49:37 GMT 332 Its value is more efficiently represented as an Integer number of 333 delta seconds from the Unix epoch (00:00:00 UTC on 1 January 1970, 334 minus leap seconds). Thus, the example above would be mapped to: 336 SF-Date: 784072177 338 As in Section 2, these fields are unable to carry values that are not 339 valid Structured Fields, and so an application using this 340 specification will need to how to support such values. Typically, 341 handling them using the original field name is sufficient. 343 Each field name listed below indicates a replacement field name and a 344 means of mapping its original value into a Structured Field. 346 3.1. URLs 348 The field names in Table 2 (paired with their mapped field names) 349 have values that can be mapped into Structured Fields by treating the 350 original field's value as a String. 352 +==================+=====================+ 353 | Field Name | Mapped Field Name | 354 +==================+=====================+ 355 | Content-Location | SF-Content-Location | 356 +------------------+---------------------+ 357 | Location | SF-Location | 358 +------------------+---------------------+ 359 | Referer | SF-Referer | 360 +------------------+---------------------+ 362 Table 2: URL Fields 364 For example, a Location field could be mapped as: 366 SF-Location: "https://example.com/foo" 368 3.2. Dates 370 The field names in Table 3 (paired with their mapped field names) 371 have values that can be mapped into Structured Fields by parsing 372 their payload according to Section 5.6.7 of [HTTP] and representing 373 the result as an Integer number of seconds delta from the Unix Epoch 374 (00:00:00 UTC on 1 January 1970, excluding leap seconds). 376 +=====================+===================+ 377 | Field Name | Mapped Field Name | 378 +=====================+===================+ 379 | Date | SF-Date | 380 +---------------------+-------------------+ 381 | Expires | SF-Expires | 382 +---------------------+-------------------+ 383 | If-Modified-Since | SF-IMS | 384 +---------------------+-------------------+ 385 | If-Unmodified-Since | SF-IUS | 386 +---------------------+-------------------+ 387 | Last-Modified | SF-LM | 388 +---------------------+-------------------+ 390 Table 3: Date Fields 392 For example, an Expires field could be mapped as: 394 SF-Expires: 1571965240 396 3.3. ETags 398 The field value of the ETag header field can be mapped into the SF- 399 ETag Structured Field by representing the entity-tag as a String, and 400 the weakness flag as a Boolean "w" parameter on it, where true 401 indicates that the entity-tag is weak; if 0 or unset, the entity-tag 402 is strong. 404 For example: 406 SF-ETag: "abcdef"; w=?1 408 If-None-Match's field value can be mapped into the SF-INM Structured 409 Field, which is a List of the structure described above. 411 For example: 413 SF-INM: "abcdef"; w=?1, "ghijkl" 415 3.4. Links 417 The field value of the Link header field [RFC8288] can be mapped into 418 the SF-Link List Structured Field by considering the URI-Reference as 419 a String, and link-param as Parameters. 421 For example: 423 SF-Link: "/terms"; rel="copyright"; anchor="#foo" 425 3.5. Cookies 427 The field values of the Cookie and Set-Cookie fields [COOKIES] can be 428 mapped into the SF-Cookie Structured Field (a List) and SF-Set-Cookie 429 Structured Field (a Dictionary), respectively. 431 In each case, cookie names are Tokens. Their values are Strings, 432 unless the value can be successfully parsed as the textual 433 representation of another, bare Item structured type (e.g., Byte 434 Sequence, Decimal, Integer, Token, or Boolean). 436 Set-Cookie parameters map to Parameters on the appropriate SF-Set- 437 Cookie member, with the parameter name being forced to lowercase. 438 Set-Cookie parameter values are Strings unless a specific type is 439 defined for them. This specification defines the parameter types in 440 Table 4. 442 +================+=================+ 443 | Parameter Name | Structured Type | 444 +================+=================+ 445 | HttpOnly | Boolean | 446 +----------------+-----------------+ 447 | Expires | Integer | 448 +----------------+-----------------+ 449 | Max-Age | Integer | 450 +----------------+-----------------+ 451 | Secure | Boolean | 452 +----------------+-----------------+ 453 | SameSite | Token | 454 +----------------+-----------------+ 456 Table 4: Set-Cookie Parameter Types 458 Expires is mapped to an Integer representation of parsed-cookie-date 459 (see Part x.x of [COOKIES]) expressed as a number of seconds delta 460 from the Unix Epoch (00:00:00 UTC on 1 January 1970, excluding leap 461 seconds). 463 Note that although this mapping is very similar to the syntax of 464 Cookie and Set-Cookie headers, cookies in both fields are separated 465 by commas, not semicolons, and multiple cookies can appear in each 466 field. 468 For example: 470 SF-Set-Cookie: lang="en-US"; expires="Wed, 09 Jun 2021 10:18:14 GMT"; 471 samesite=Strict; secure=?1 472 SF-Cookie: SID="31d4d96e407aad42", lang="en-US" 474 4. IANA Considerations 476 Please add the following note to the "Hypertext Transfer Protocol 477 (HTTP) Field Name Registry": 479 The "Structured Type" column indicates the type of the field (per 480 RFC8941), if any, and may be "Dictionary", "List" or "Item". A 481 prefix of "*" indicates that it is a retrofit type (i.e., not 482 natively Structured); see [this specification]. 484 Note that field names beginning with characters other than ALPHA 485 or "*" will not be able to be represented as a Structured Fields 486 Token, and therefore may be incompatible with being mapped into 487 fields that refer to it; see [this specification]. 489 Then, add a new column, "Structured Type", with the values from 490 Section 2 assigned to the nominated registrations, prefixing each 491 with "*" to indicate that it is a retrofit type. 493 Then, add the field names in Table 5, with the corresponding 494 Structured Type as indicated, a status of "permanent" and referring 495 to this document. 497 +=====================+=================+ 498 | Field Name | Structured Type | 499 +=====================+=================+ 500 | SF-Content-Location | String | 501 +---------------------+-----------------+ 502 | SF-Cookie | List | 503 +---------------------+-----------------+ 504 | SF-Date | Item | 505 +---------------------+-----------------+ 506 | SF-ETag | Item | 507 +---------------------+-----------------+ 508 | SF-Expires | Item | 509 +---------------------+-----------------+ 510 | SF-IMS | Item | 511 +---------------------+-----------------+ 512 | SF-INM | List | 513 +---------------------+-----------------+ 514 | SF-IUS | Item | 515 +---------------------+-----------------+ 516 | SF-Link | List | 517 +---------------------+-----------------+ 518 | SF-LM | Item | 519 +---------------------+-----------------+ 520 | SF-Location | String | 521 +---------------------+-----------------+ 522 | SF-Referer | String | 523 +---------------------+-----------------+ 524 | SF-Set-Cookie | Dictionary | 525 +---------------------+-----------------+ 527 Table 5: New Fields 529 Finally, add the indicated Structured Type for each existing registry 530 entry listed in Table 6. 532 +==========================================+=================+ 533 | Field Name | Structured Type | 534 +==========================================+=================+ 535 | Accept-CH | List | 536 +------------------------------------------+-----------------+ 537 | Cache-Status | List | 538 +------------------------------------------+-----------------+ 539 | CDN-Cache-Control | Dictionary | 540 +------------------------------------------+-----------------+ 541 | Cross-Origin-Embedder-Policy | Item | 542 +------------------------------------------+-----------------+ 543 | Cross-Origin-Embedder-Policy-Report-Only | Item | 544 +------------------------------------------+-----------------+ 545 | Cross-Origin-Opener-Policy | Item | 546 +------------------------------------------+-----------------+ 547 | Cross-Origin-Opener-Policy-Report-Only | Item | 548 +------------------------------------------+-----------------+ 549 | Origin-Agent-Cluster | Item | 550 +------------------------------------------+-----------------+ 551 | Priority | Dictionary | 552 +------------------------------------------+-----------------+ 553 | Proxy-Status | List | 554 +------------------------------------------+-----------------+ 556 Table 6: Existing Fields 558 5. Security Considerations 560 Section 2 identifies existing HTTP fields that can be parsed and 561 serialised with the algorithms defined in [STRUCTURED-FIELDS]. 562 Variances from existing parser behavior might be exploitable, 563 particularly if they allow an attacker to target one implementation 564 in a chain (e.g., an intermediary). However, given the considerable 565 variance in parsers already deployed, convergence towards a single 566 parsing algorithm is likely to have a net security benefit in the 567 longer term. 569 Section 3 defines alternative representations of existing fields. 570 Because downstream consumers might interpret the message differently 571 based upon whether they recognise the alternative representation, 572 implementations are prohibited from generating such fields unless 573 they have negotiated support for them with their peer. This 574 specification does not define such a mechanism, but any such 575 definition needs to consider the implications of doing so carefully. 577 6. Normative References 579 [COOKIES] Chen, L., Englehardt, S., West, M., and J. Wilander, 580 "Cookies: HTTP State Management Mechanism", Work in 581 Progress, Internet-Draft, draft-ietf-httpbis-rfc6265bis- 582 10, 24 April 2022, . 585 [HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP 586 Semantics", Work in Progress, Internet-Draft, draft-ietf- 587 httpbis-semantics-19, 12 September 2021, 588 . 591 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 592 Requirement Levels", BCP 14, RFC 2119, 593 DOI 10.17487/RFC2119, March 1997, 594 . 596 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 597 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 598 May 2017, . 600 [RFC8288] Nottingham, M., "Web Linking", RFC 8288, 601 DOI 10.17487/RFC8288, October 2017, 602 . 604 [STRUCTURED-FIELDS] 605 Nottingham, M. and P-H. Kamp, "Structured Field Values for 606 HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, 607 . 609 Author's Address 611 Mark Nottingham 612 Prahran 613 Australia 614 Email: mnot@mnot.net 615 URI: https://www.mnot.net/