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.)¶

This specification addresses the issues listed above by defining both a generic JSON-based ([RFC7159]) data model and a concrete wire format that can be used in definitions of new header fields.¶

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 [RFC7159]). 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 ([RFC7159], 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 ([RFC7159], 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.¶

[rfc.comment.1: Explain what a definition of a new header field needs to do precisely to use this format] ¶

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

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 haeders, 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.2: Use of generic libs vs compactness of field values..] ¶

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]).¶

[rfc.comment.3: TBD, mention migration path to message format that is robust wrt UTF-8, or other binary encodings of JSON] ¶

[rfc.comment.4: TBD] ¶