Network Working GroupJ. Reschke
Intended status: InformationalFebruary 22, 2021
Expires: August 26, 2021

A JSON Encoding for HTTP Field Values


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, which may be joined by sending a message with subject "subscribe" to

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.16.

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

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This Internet-Draft will expire on August 26, 2021.

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1. Introduction

Defining syntax for new HTTP fields ([HTTP], Section 5) is non-trivial. Among the commonly encountered problems are:

(See Section 16.3 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:

2. Data Model and Format

In HTTP, field lines with the same field name can occur multiple times within a single message (Section 5.3 of [HTTP]). When this happens, recipients are allowed to combine the field line values using commas as delimiter, forming a combined "field value". 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 5.6.1 of [HTTP]:

  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 line values to a JSON array:

  1. combine all field line values into a single field value as per Section 5.3 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 16.3 of [HTTP] into account. Many of these will already be accounted for by using the format defined in this specification.

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 5.6.4 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].

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

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).

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

10.2. Informative References

Ecma International, “ECMA-262 6th Edition, The ECMAScript 2015 Language Specification”, Standard ECMA-262, June 2015, <>.
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.
Hoffman, P. and J. Klensin, “Terminology Used in Internationalization in the IETF”, BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, <>.
Nottingham, M. and P-H. Kamp, “Structured Field Values for HTTP”, RFC 8941, DOI 10.17487/RFC8941, February 2021, <>.
WhatWG, “XMLHttpRequest”, <>.

10.3. Specifications Using This Syntax (at some point of time)

West, M., “Clear Site Data”, W3C Working Draft WD-clear-site-data-20171130, November 2017, <>.
Latest version available at <>.
Clelland, I., “Feature Policy”, W3C Editor's Draft, <>.
Creager, D., Grigorik, I., Meyer, P., and M. West, “Reporting API”, W3C Working Draft WD-reporting-1-20180925, September 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 [RFC8941] (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).

A.3. W3C Feature Policy Specification

Originally defined in W3C document "Feature Policy" ([FEATUREPOL]), but switched to use of Structured Header Fields ([RFC8941]).

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.

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".

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.

B.16. Since draft-reschke-http-jfv-12 📄 🔍

Update HTTP reference and update terminology some more.

Update HSTRUCT reference (now RFC 8941).


Thanks go to the Hypertext Transfer Protocol Working Group participants.

Author's Address

Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Münster, 48155