A JSON Encoding for HTTP Header Field Values

This document establishes a convention for use of JSON-encoded field values in HTTP header fields.¶

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Defining syntax for new HTTP header fields ([RFC7230], Section 3.2) is non-trivial. Among the commonly encountered problems are: ¶

• There is no common syntax for complex field values. Several well-known header 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.
• The HTTP message format allows header 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.
• HTTP/1.1 does not define a character encoding scheme ([RFC6365], Section 2), so header 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 8.3.1 of [RFC7231] for a summary of considerations for new header fields.)¶

• to be compatible with header field recombination when fields occur multiple times in a single message (Section 3.2.2 of [RFC7230]), and
• not to use any problematic characters in the field value (non-ASCII characters and certain whitespace characters).

In HTTP, header fields with the same field name can occur multiple times within a single message (Section 3.2.2 of [RFC7230]). 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.¶

Header 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 header 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").¶

To map a JSON array to an HTTP header 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]).¶

To map a set of HTTP header field instances to a JSON array: ¶

1. combine all header field instances into a single field as per Section 3.2.2 of [RFC7230],
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 header field values needs to be considered invalid), or a JSON array.¶

Specifications defining new HTTP header fields need to take the considerations listed in Section 8.3.1 of [RFC7231] 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.¶

This JSON-based syntax will only apply to newly introduced header 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 3.2.6 of [RFC7230]).¶

In HTTP/1.1, header 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 ([RFC7230], Section 3.2). HTTP/2 does not change this.¶

This specification maps all characters which can cause problems to JSON escape sequences, thereby solving the HTTP header field internationalization problem.¶

Future specifications of HTTP might change to allow non-ASCII characters natively. In that case, header fields using the syntax defined by this specification would have a simple migration path (by just stopping to require escaping of non-ASCII characters).¶

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.¶

This section shows how some of the existing HTTP header fields would look like if they would use the format defined by this specification.¶

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 <https://lists.w3.org/Archives/Public/ietf-http-wg/2016OctDec/0505.html>).¶

The sections below summarize the current usage of this format.¶

[KEY] aims to improve the cacheability of responses that vary based on certain request header fields, addressing lack of granularity in the existing "Vary" response header field ([RFC7231], Section 7.1.4). If the JSON-based format described by this document gains popularity, it might be useful to add a JSON-aware "Key Parameter" (see Section 2.3 of [KEY]).¶

This approach uses a default of "JSON array", using implicit array markers. An alternative would be a default of "JSON object". This would simplify the syntax for non-list-typed header fields, but all the benefits of having the same data model for both types of header fields would be gone. A hybrid approach might make sense, as long as it doesn't require any heuristics on the recipient's side.¶

[rfc.comment.1: Use of generic libs vs compactness of field values..] ¶

Thanks go to the Hypertext Transfer Protocol Working Group participants. ¶