A JSON Encoding for HTTP Field Values

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

• 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.
• The HTTP message format allows field lines 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 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 16.3 of [HTTP] for a summary of considerations for new fields.)¶

• to be compatible with field recombination when field lines occur multiple times in a single message (Section 5.3 of [HTTP]), and
• not to use any problematic characters in the field value (non-ASCII characters and certain whitespace characters).

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.¶

  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").¶

  json-field-value = #json-field-item
  json-field-item  = JSON-Text
                   ; see [RFC8259], Section 2,
                   ; post-processed so that only VCHAR characters
                   ; are used

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]).¶

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.¶

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.¶

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]).¶

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).¶

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.¶

See https://github.com/reschke/json-fields for a proof-of-concept (in development).¶

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