Network Working Group J. F. Reschke Internet-Draft greenbytes Intended status: Informational September 1, 2020 Expires: March 5, 2021 A JSON Encoding for HTTP Field Values draft-reschke-http-jfv-12 Abstract This document establishes a convention for use of JSON-encoded field values in HTTP fields. Editorial Note This note is to be removed before publishing as an RFC. Distribution of this document is unlimited. Although this is not a work item of the HTTPbis Working Group, comments should be sent to the Hypertext Transfer Protocol (HTTP) mailing list at ietf-http- wg@w3.org (mailto:ietf-http-wg@w3.org), which may be joined by sending a message with subject "subscribe" to ietf-http-wg- request@w3.org (mailto:ietf-http-wg- request@w3.org?subject=subscribe). Discussions of the HTTPbis Working Group are archived at . XML versions and latest edits for this document are available from . The changes in this draft are summarized in Appendix B.15. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." Reschke Expires March 5, 2021 [Page 1] Internet-Draft JSON Encoding for HTTP Field Values September 2020 This Internet-Draft will expire on March 5, 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Data Model and Format . . . . . . . . . . . . . . . . . . . . 4 3. Sender Requirements . . . . . . . . . . . . . . . . . . . . . 5 4. Recipient Requirements . . . . . . . . . . . . . . . . . . . 5 5. Using this Format in Field Definitions . . . . . . . . . . . 5 6. Deployment Considerations . . . . . . . . . . . . . . . . . . 6 7. Interoperability Considerations . . . . . . . . . . . . . . . 6 7.1. Encoding and Characters . . . . . . . . . . . . . . . . . 6 7.2. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.3. Object Constraints . . . . . . . . . . . . . . . . . . . 7 8. Internationalization Considerations . . . . . . . . . . . . . 7 9. Security Considerations . . . . . . . . . . . . . . . . . . . 7 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 10.1. Normative References . . . . . . . . . . . . . . . . . . 7 10.2. Informative References . . . . . . . . . . . . . . . . . 8 10.3. Specifications Using This Syntax (at some point of time) . . . . . . . . . . . . . . . . . . . . . . . . . 9 Appendix A. Use of JSON Field Value Encoding in the Wild . . . . 9 A.1. W3C Reporting API Specification . . . . . . . . . . . . . 9 A.2. W3C Clear Site Data Specification . . . . . . . . . . . . 9 A.3. W3C Feature Policy Specification . . . . . . . . . . . . 10 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 10 B.1. Since draft-reschke-http-jfv-00 . . . . . . . . . . . . . 10 B.2. Since draft-reschke-http-jfv-01 . . . . . . . . . . . . . 10 B.3. Since draft-reschke-http-jfv-02 . . . . . . . . . . . . . 10 B.4. Since draft-reschke-http-jfv-03 . . . . . . . . . . . . . 10 B.5. Since draft-reschke-http-jfv-04 . . . . . . . . . . . . . 10 B.6. Since draft-ietf-httpbis-jfv-00 . . . . . . . . . . . . . 10 B.7. Since draft-ietf-httpbis-jfv-01 . . . . . . . . . . . . . 11 B.8. Since draft-ietf-httpbis-jfv-02 . . . . . . . . . . . . . 11 Reschke Expires March 5, 2021 [Page 2] Internet-Draft JSON Encoding for HTTP Field Values September 2020 B.9. Since draft-reschke-http-jfv-05 . . . . . . . . . . . . . 11 B.10. Since draft-reschke-http-jfv-06 . . . . . . . . . . . . . 11 B.11. Since draft-reschke-http-jfv-07 . . . . . . . . . . . . . 11 B.12. Since draft-reschke-http-jfv-08 . . . . . . . . . . . . . 11 B.13. Since draft-reschke-http-jfv-09 . . . . . . . . . . . . . 11 B.14. Since draft-reschke-http-jfv-10 . . . . . . . . . . . . . 12 B.15. Since draft-reschke-http-jfv-11 . . . . . . . . . . . . . 12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction Defining syntax for new HTTP fields ([HTTP], Section 5) is non- trivial. Among the commonly encountered problems are: o There is no common syntax for complex field values. Several well- known fields do use a similarly looking syntax, but it is hard to write generic parsing code that will both correctly handle valid field values but also reject invalid ones. o The HTTP message format allows fields to repeat, so field syntax needs to be designed in a way that these cases are either meaningful, or can be unambiguously detected and rejected. o HTTP does not define a character encoding scheme ([RFC6365], Section 2), so fields are either stuck with US-ASCII ([RFC0020]), or need out-of-band information to decide what encoding scheme is used. Furthermore, APIs usually assume a default encoding scheme in order to map from octet sequences to strings (for instance, [XMLHttpRequest] uses the IDL type "ByteString", effectively resulting in the ISO-8859-1 character encoding scheme [ISO-8859-1] being used). (See Section 5.7 of [HTTP] for a summary of considerations for new fields.) This specification addresses the issues listed above by defining both a generic JSON-based ([RFC8259]) data model and a concrete wire format that can be used in definitions of new fields, where the goals were: o to be compatible with field recombination when fields occur multiple times in a single message (Section 5.1 of [HTTP]), and o not to use any problematic characters in the field value (non- ASCII characters and certain whitespace characters). Reschke Expires March 5, 2021 [Page 3] Internet-Draft JSON Encoding for HTTP Field Values September 2020 | *Note:* [HSTRUCT], a work item of the IETF HTTP Working Group, | is a different attempt to address this set of problems - it | tries to identify and formalize common field structures in | existing fields; the syntax defined over there would usually | lead to a more compact notation. 2. Data Model and Format In HTTP, fields with the same field name can occur multiple times within a single message (Section 5.1 of [HTTP]). When this happens, recipients are allowed to combine the field values using commas as delimiter. This rule matches nicely JSON's array format (Section 5 of [RFC8259]). Thus, the basic data model used here is the JSON array. Field definitions that need only a single value can restrict themselves to arrays of length 1, and are encouraged to define error handling in case more values are received (such as "first wins", "last wins", or "abort with fatal error message"). JSON arrays are mapped to field values by creating a sequence of serialized member elements, separated by commas and optionally whitespace. This is equivalent to using the full JSON array format, while leaving out the "begin-array" ('[') and "end-array" (']') delimiters. The ABNF character names and classes below are used (copied from [RFC5234], Appendix B.1): CR = %x0D ; carriage return HTAB = %x09 ; horizontal tab LF = %x0A ; line feed SP = %x20 ; space VCHAR = %x21-7E ; visible (printing) characters Characters in JSON strings that are not allowed or discouraged in HTTP field values - that is, not in the "VCHAR" definition - need to be represented using JSON's "backslash" escaping mechanism ([RFC8259], Section 7). The control characters CR, LF, and HTAB do not appear inside JSON strings, but can be used outside (line breaks, indentation etc.). These characters need to be either stripped or replaced by space characters (ABNF "SP"). Formally, using the HTTP specification's ABNF extensions defined in Section 5.5 of [HTTP]: Reschke Expires March 5, 2021 [Page 4] Internet-Draft JSON Encoding for HTTP Field Values September 2020 json-field-value = #json-field-item json-field-item = JSON-Text ; see [RFC8259], Section 2, ; post-processed so that only VCHAR characters ; are used 3. Sender Requirements To map a JSON array to an HTTP field value, process each array element separately by: 1. generating the JSON representation, 2. stripping all JSON control characters (CR, HTAB, LF), or replacing them by space ("SP") characters, 3. replacing all remaining non-VSPACE characters by the equivalent backslash-escape sequence ([RFC8259], Section 7). The resulting list of strings is transformed into an HTTP field value by combining them using comma (%x2C) plus optional SP as delimiter, and encoding the resulting string into an octet sequence using the US-ASCII character encoding scheme ([RFC0020]). 4. Recipient Requirements To map a set of HTTP field instances to a JSON array: 1. combine all field instances into a single field as per Section 5.1 of [HTTP], 2. add a leading begin-array ("[") octet and a trailing end-array ("]") octet, then 3. run the resulting octet sequence through a JSON parser. The result of the parsing operation is either an error (in which case the field values needs to be considered invalid), or a JSON array. 5. Using this Format in Field Definitions Specifications defining new HTTP fields need to take the considerations listed in Section 5.7 of [HTTP] into account. Many of these will already be accounted for by using the format defined in this specification. Reschke Expires March 5, 2021 [Page 5] Internet-Draft JSON Encoding for HTTP Field Values September 2020 Readers of HTTP-related specifications frequently expect an ABNF definition of the field value syntax. This is not really needed here, as the actual syntax is JSON text, as defined in Section 2 of [RFC8259]. A very simple way to use this JSON encoding thus is just to cite this specification - specifically the "json-field-value" ABNF production defined in Section 2 - and otherwise not to talk about the details of the field syntax at all. An alternative approach is just to repeat the ABNF-related parts from Section 2. This frees the specification from defining the concrete on-the-wire syntax. What's left is defining the field value in terms of a JSON array. An important aspect is the question of extensibility, e.g. how recipients ought to treat unknown field names. In general, a "must ignore" approach will allow protocols to evolve without versioning or even using entire new field names. 6. Deployment Considerations This JSON-based syntax will only apply to newly introduced fields, thus backwards compatibility is not a problem. That being said, it is conceivable that there is existing code that might trip over double quotes not being used for HTTP's quoted-string syntax (Section 5.4.1 of [HTTP]). 7. Interoperability Considerations The "I-JSON Message Format" specification ([RFC7493]) addresses known JSON interoperability pain points. This specification borrows from the requirements made over there: 7.1. Encoding and Characters This specification requires that field values use only US-ASCII characters, and thus by definition use a subset of UTF-8 (Section 2.1 of [RFC7493]). 7.2. Numbers Be aware of the issues around number precision, as discussed in Section 2.2 of [RFC7493]. Reschke Expires March 5, 2021 [Page 6] Internet-Draft JSON Encoding for HTTP Field Values September 2020 7.3. Object Constraints As described in Section 4 of [RFC8259], JSON parser implementations differ in the handling of duplicate object names. Therefore, senders MUST NOT use duplicate object names, and recipients SHOULD either treat field values with duplicate names as invalid (consistent with [RFC7493], Section 2.3) or use the lexically last value (consistent with [ECMA-262], Section 24.3.1.1). Furthermore, ordering of object members is not significant and can not be relied upon. 8. Internationalization Considerations In current versions of HTTP, field values are represented by octet sequences, usually used to transmit ASCII characters, with restrictions on the use of certain control characters, and no associated default character encoding, nor a way to describe it ([HTTP], Section 5). HTTP/2 does not change this. This specification maps all characters which can cause problems to JSON escape sequences, thereby solving the HTTP field internationalization problem. Future specifications of HTTP might change to allow non-ASCII characters natively. In that case, fields using the syntax defined by this specification would have a simple migration path (by just stopping to require escaping of non-ASCII characters). 9. Security Considerations Using JSON-shaped field values is believed to not introduce any new threads beyond those described in Section 12 of [RFC8259], namely the risk of recipients using the wrong tools to parse them. Other than that, any syntax that makes extensions easy can be used to smuggle information through field values; however, this concern is shared with other widely used formats, such as those using parameters in the form of name/value pairs. 10. References 10.1. Normative References Reschke Expires March 5, 2021 [Page 7] Internet-Draft JSON Encoding for HTTP Field Values September 2020 [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. F. Reschke, Ed., "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf-httpbis-semantics-11, August 27, 2020, . [RFC0020] Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, March 2015, . [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 8259, DOI 10.17487/RFC8259, December 2017, . 10.2. Informative References [ECMA-262] Ecma International, "ECMA-262 6th Edition, The ECMAScript 2015 Language Specification", Standard ECMA-262, June 2015, . [HSTRUCT] Nottingham, M. and P-H. Kamp, "Structured Field Values for HTTP", Work in Progress, Internet-Draft, draft-ietf- httpbis-header-structure-19, June 2020, . [ISO-8859-1] International Organization for Standardization, "Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1", ISO/ IEC 8859-1:1998, 1998. [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, . [XMLHttpRequest] WhatWG, "XMLHttpRequest", . Reschke Expires March 5, 2021 [Page 8] Internet-Draft JSON Encoding for HTTP Field Values September 2020 10.3. Specifications Using This Syntax (at some point of time) [CLEARSITE] West, M., "Clear Site Data", W3C Working Draft WD-clear- site-data-20171130, November 30, 2017, . Latest version available at . [FEATUREPOL] Clelland, I., "Feature Policy", W3C Editor's Draft , . [REPORTING] Creager, D., Grigorik, I., Meyer, P., and M. West, "Reporting API", W3C Working Draft WD-reporting- 1-20180925, September 25, 2018, . Latest version available at . Appendix A. Use of JSON Field Value Encoding in the Wild This section is to be removed before publishing as an RFC. Since work started on this document, various specifications have adopted this format. At least one of these moved away after the HTTP Working Group decided to focus on [HSTRUCT] (see thread starting at ). The sections below summarize the current usage of this format. A.1. W3C Reporting API Specification Defined in W3C Working Draft "Reporting API" (Section 3.1 of [REPORTING]). Still in use in latest working draft dated September 2018. A.2. W3C Clear Site Data Specification Used in earlier versions of "Clear Site Data". The current version replaces the use of JSON with a custom syntax that happens to be somewhat compatible with an array of JSON strings (see Section 3.1 of [CLEARSITE] and for feedback). Reschke Expires March 5, 2021 [Page 9] Internet-Draft JSON Encoding for HTTP Field Values September 2020 A.3. W3C Feature Policy Specification Originally defined in W3C document "Feature Policy" ([FEATUREPOL]), but switched to use of Structured Header Fields ([HSTRUCT]). Appendix B. Change Log This section is to be removed before publishing as an RFC. B.1. Since draft-reschke-http-jfv-00 Editorial fixes + working on the TODOs. B.2. Since draft-reschke-http-jfv-01 Mention slightly increased risk of smuggling information in header field values. B.3. Since draft-reschke-http-jfv-02 Mention Kazuho Oku's proposal for abbreviated forms. Added a bit of text about the motivation for a concrete JSON subset (ack Cory Benfield). Expand I18N section. B.4. Since draft-reschke-http-jfv-03 Mention relation to KEY header field. B.5. Since draft-reschke-http-jfv-04 Between June and December 2016, this was a work item of the HTTP working group (see ). Work (if any) continues now on . Changes made while this was a work item of the HTTP Working Group: B.6. Since draft-ietf-httpbis-jfv-00 Added example for "Accept-Encoding" (inspired by Kazuho's feedback), showing a potential way to optimize the format when default values apply. Reschke Expires March 5, 2021 [Page 10] Internet-Draft JSON Encoding for HTTP Field Values September 2020 B.7. Since draft-ietf-httpbis-jfv-01 Add interop discussion, building on I-JSON and ECMA-262 (see ). B.8. Since draft-ietf-httpbis-jfv-02 Move non-essential parts into appendix. Updated XHR reference. B.9. Since draft-reschke-http-jfv-05 Add meat to "Using this Format in Header Field Definitions". Add a few lines on the relation to "Key". Summarize current use of the format. B.10. Since draft-reschke-http-jfv-06 RFC 5987 is obsoleted by RFC 8187. Update CLEARSITE comment. B.11. Since draft-reschke-http-jfv-07 Update JSON and HSTRUCT references. FEATUREPOL doesn't use JSON syntax anymore. B.12. Since draft-reschke-http-jfv-08 Update HSTRUCT reference. Update notes about CLEARSITE and FEATUREPOL. B.13. Since draft-reschke-http-jfv-09 Update HSTRUCT and FEATUREPOL references. Update note about REPORTING. Changed category to "informational". Reschke Expires March 5, 2021 [Page 11] Internet-Draft JSON Encoding for HTTP Field Values September 2020 B.14. Since draft-reschke-http-jfv-10 Update HSTRUCT reference. B.15. Since draft-reschke-http-jfv-11 Update HSTRUCT reference. Update note about FEATUREPOL (now using Structured Fields). Reference [HTTP] instead if RFC723* and adjust (header) field terminology accordingly. Remove discussion about the relation to KEY (as that spec is dormant: ). Remove appendices "Examples" and "Discussion". Mark "Use of JSON Field Value Encoding in the Wild" for removal in RFC. Acknowledgements Thanks go to the Hypertext Transfer Protocol Working Group participants. Author's Address Julian F. Reschke greenbytes GmbH Hafenweg 16 48155 Münster Germany Email: julian.reschke@greenbytes.de URI: http://greenbytes.de/tech/webdav/ Reschke Expires March 5, 2021 [Page 12]