This document describes Structured Headers, a way of simplifying HTTP header field definition and parsing. It is intended for use by new specifications of HTTP header fields. This includes revisions of existing specifications when doing so does not cause interoperability issues.
Discussion of this draft takes place on the HTTP working group mailing list (email@example.com), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.
RFC EDITOR: please remove this section before publication
Working Group information can be found at https://httpwg.github.io/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/header-structure.
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Specifying the syntax of new HTTP header fields is an onerous task; even with the guidance in [RFC7231], Section 8.3.1, there are many decisions – and pitfalls – for a prospective HTTP header field author.
Likewise, bespoke parsers often need to be written for specific HTTP headers, because each has slightly different handling of what looks like common syntax.
This document introduces structured HTTP header field values (hereafter, Structured Headers) to address these problems. Structured Headers define a generic, abstract model for data, along with a concrete serialisation for expressing that model in textual HTTP headers, as used by HTTP/1 [RFC7230] and HTTP/2 [RFC7540].
HTTP headers that are defined as Structured Headers use the types defined in this specification to define their syntax and basic handling rules, thereby simplifying both their definition and parsing.
Additionally, future versions of HTTP can define alternative serialisations of the abstract model of Structured Headers, allowing headers that use it to be transmitted more efficiently without being redefined.
Note that it is not a goal of this document to redefine the syntax of existing HTTP headers; the mechanisms described herein are only intended to be used with headers that explicitly opt into them.
To specify a header field that uses Structured Headers, see Section 2.
Section 4 defines a number of abstract data types that can be used in Structured Headers, of which only three are allowed at the “top” level: lists, dictionaries, or items.
Those abstract types can be serialised into textual headers – such as those used in HTTP/1 and HTTP/2 – using the algorithms described in Section 3.
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
HTTP headers that use Structured Headers need to be defined to do so explicitly; recipients and generators need to know that the requirements of this document are in effect. The simplest way to do that is by referencing this document in its definition.
The field’s definition will also need to specify the field-value’s allowed syntax, in terms of the types described in Section 4, along with their associated semantics.
Field definitions MUST NOT relax or otherwise modify the requirements of this specification; doing so would preclude handling by generic software.
However, field definitions are encouraged to clearly state additional constraints upon the syntax, as well as the consequences when those constraints are violated.
# FooExample Header The FooExample HTTP header field conveys a list of numbers about how much Foo the sender has. FooExample is a Structured header [RFCxxxx]. Its value MUST be a dictionary ([RFCxxxx], Section Y.Y). The dictionary MUST contain: * A member whose key is "foo", and whose value is an integer ([RFCxxxx], Section Y.Y), indicating the number of foos in the message. * A member whose key is "bar", and whose value is a string ([RFCxxxx], Section Y.Y), conveying the characteristic bar-ness of the message. If the parsed header field does not contain both, it MUST be ignored.
Note that empty header field values are not allowed by the syntax, and therefore will be considered errors.
When a receiving implementation parses textual HTTP header fields (e.g., in HTTP/1 or HTTP/2) that are known to be Structured Headers, it is important that care be taken, as there are a number of edge cases that can cause interoperability or even security problems. This section specifies the algorithm for doing so.
Given an ASCII string input_string that represents the chosen header’s field-value, return the parsed header value. Note that input_string may incorporate multiple header lines combined into one comma-separated field-value, as per [RFC7230], Section 3.2.2.
Note that in the case of lists and dictionaries, this has the effect of combining multiple instances of the header field into one. However, for singular items and parameterised labels, it has the effect of selecting the first value and ignoring any subsequent instances of the field, as well as extraneous text afterwards.
Additionally, note that the effect of the parsing algorithms as specified is generally intolerant of syntax errors; if one is encountered, the typical response is to throw an error, thereby discarding the entire header field value. This includes any non-ASCII characters in input_string.
This section defines the abstract value types that can be composed into Structured Headers, along with the textual HTTP serialisations of them.
Abstractly, numbers are integers with an optional fractional part. They have a maximum of fifteen digits available to be used in one or both of the parts, as reflected in the ABNF below; this allows them to be stored as IEEE 754 double precision numbers (binary64) ([IEEE754]).
The textual HTTP serialisation of numbers allows a maximum of fifteen digits between the integer and fractional part, along with an optional “-“ indicating negative numbers.
number = ["-"] ( "." 1*15DIGIT / DIGIT "." 1*14DIGIT / 2DIGIT "." 1*13DIGIT / 3DIGIT "." 1*12DIGIT / 4DIGIT "." 1*11DIGIT / 5DIGIT "." 1*10DIGIT / 6DIGIT "." 1*9DIGIT / 7DIGIT "." 1*8DIGIT / 8DIGIT "." 1*7DIGIT / 9DIGIT "." 1*6DIGIT / 10DIGIT "." 1*5DIGIT / 11DIGIT "." 1*4DIGIT / 12DIGIT "." 1*3DIGIT / 13DIGIT "." 1*2DIGIT / 14DIGIT "." 1DIGIT / 15DIGIT ) integer = ["-"] 1*15DIGIT unsigned = 1*15DIGIT
integer and unsigned are defined as conveniences to specification authors; if their use is specified and their ABNF is not matched, a parser MUST consider it to be invalid.
For example, a header whose value is defined as a number could look like:
Abstractly, strings are ASCII strings [RFC0020], excluding control characters (i.e., the range 0x20 to 0x7E). Note that this excludes tabs, newlines and carriage returns. They may be at most 1024 characters long.
The textual HTTP serialisation of strings uses a backslash (“") to escape double quotes and backslashes in strings.
string = DQUOTE 1*1024(char) DQUOTE char = unescaped / escape ( DQUOTE / "\" ) unescaped = %x20-21 / %x23-5B / %x5D-7E escape = "\"
For example, a header whose value is defined as a string could look like:
ExampleStringHeader: "hello world"
Note that strings only use DQUOTE as a delimiter; single quotes do not delimit strings. Furthermore, only DQUOTE and “" can be escaped; other sequences MUST generate an error.
Unicode is not directly supported in Structured Headers, because it causes a number of interoperability issues, and – with few exceptions – header values do not require it.
When it is necessary for a field value to convey non-ASCII string content, binary content (Section 4.5) SHOULD be specified, along with a character encoding (most likely, UTF-8).
Given an ASCII string input_string, return an unquoted string. input_string is modified to remove the parsed value.
Labels are short (up to 256 characters) textual identifiers; their abstract model is identical to their expression in the textual HTTP serialisation.
label = lcalpha *255( lcalpha / DIGIT / "_" / "-"/ "*" / "/" ) lcalpha = %x61-7A ; a-z
Note that labels can only contain lowercase letters.
For example, a header whose value is defined as a label could look like:
Given an ASCII string input_string, return a label. input_string is modified to remove the parsed value.
Parameterised Labels are labels (Section 4.3) with up to 256 parameters; each parameter has a label and an optional value that is an item (Section 4.6). Ordering between parameters is not significant, and duplicate parameters MUST be considered an error.
The textual HTTP serialisation uses semicolons (“;”) to delimit the parameters from each other, and equals (“=”) to delimit the parameter name from its value.
parameterised = label *256( OWS ";" OWS label [ "=" item ] )
ExampleParamHeader: abc; a=1; b=2; c
Given an ASCII string input_string, return a label with an mapping of parameters. input_string is modified to remove the parsed value.
Arbitrary binary content up to 16K in size can be conveyed in Structured Headers.
The textual HTTP serialisation indicates their presence by a leading “*”, with the data encoded using Base 64 Encoding [RFC4648], without padding (as “=” might be confused with the use of dictionaries).
binary = "*" 1*21846(base64) base64 = ALPHA / DIGIT / "+" / "/"
For example, a header whose value is defined as binary content could look like:
Given an ASCII string input_string, return binary content. input_string is modified to remove the parsed value.
item = number / string / label / binary
Given an ASCII string input_string, return an item. input_string is modified to remove the parsed value.
Dictionaries are unordered maps of key-value pairs, where the keys are labels (Section 4.3) and the values are items (Section 4.6). There can be between 1 and 1024 members, and keys are required to be unique.
In the textual HTTP serialisation, keys and values are separated by “=” (without whitespace), and key/value pairs are separated by a comma with optional whitespace.
dictionary = label "=" item *1023( OWS "," OWS label "=" item )
For example, a header field whose value is defined as a dictionary could look like:
ExampleDictHeader: foo=1.23, en="Applepie", da=*w4ZibGV0w6ZydGUK
Typically, a header field specification will define the semantics of individual keys, as well as whether their presence is required or optional. Recipients MUST ignore keys that are undefined or unknown, unless the header field’s specification specifically disallows them.
Given an ASCII string input_string, return a mapping of (label, item). input_string is modified to remove the parsed value.
In the textual HTTP serialisation, each member is separated by a comma and optional whitespace.
list = list_member 1*1024( OWS "," OWS list_member ) list_member = item / parameterised
For example, a header field whose value is defined as a list of labels could look like:
ExampleLabelListHeader: foo, bar, baz_45
and a header field whose value is defined as a list of parameterised labels could look like:
ExampleParamListHeader: abc/def; g="hi";j, klm/nop
Given an ASCII string input_string, return a list of items. input_string is modified to remove the parsed value.
This draft has no actions for IANA.
Replaced with draft-nottingham-structured-headers.
Added signed 64bit integer type.
Drop UTF8, and settle on BCP137 ::EmbeddedUnicodeChar for h1-unicode-string.
Change h1_blob delimiter to “:” since “’” is valid t_char