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HTTP M. Nottingham
Internet-Draft Fastly
Intended status: Standards Track P-H. Kamp
Expires: July 31, 2020 The Varnish Cache Project
January 28, 2020
Structured Headers for HTTP
draft-ietf-httpbis-header-structure-15
Abstract
This document describes a set of data types and associated algorithms
that are intended to make it easier and safer to define and handle
HTTP header fields. It is intended for use by specifications of new
HTTP header fields that wish to use a common syntax that is more
restrictive than traditional HTTP field values.
Note to Readers
_RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at https://httpwg.github.io/
[2]; source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/header-structure
[3].
Tests for implementations are collected at https://github.com/httpwg/
structured-header-tests [4].
Implementations are tracked at https://github.com/httpwg/wiki/wiki/
Structured-Headers [5].
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 https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 31, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Intentionally Strict Processing . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Defining New Structured Headers . . . . . . . . . . . . . . . 5
3. Structured Data Types . . . . . . . . . . . . . . . . . . . . 7
3.1. Lists . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.1. Inner Lists . . . . . . . . . . . . . . . . . . . . . 8
3.1.2. Parameters . . . . . . . . . . . . . . . . . . . . . 9
3.2. Dictionaries . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Items . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Integers . . . . . . . . . . . . . . . . . . . . . . 12
3.3.2. Decimals . . . . . . . . . . . . . . . . . . . . . . 12
3.3.3. Strings . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.4. Tokens . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.5. Byte Sequences . . . . . . . . . . . . . . . . . . . 13
3.3.6. Booleans . . . . . . . . . . . . . . . . . . . . . . 14
4. Working With Structured Headers in HTTP Headers . . . . . . . 14
4.1. Serializing Structured Headers . . . . . . . . . . . . . 14
4.1.1. Serializing a List . . . . . . . . . . . . . . . . . 15
4.1.2. Serializing a Dictionary . . . . . . . . . . . . . . 17
4.1.3. Serializing an Item . . . . . . . . . . . . . . . . . 17
4.1.4. Serializing an Integer . . . . . . . . . . . . . . . 18
4.1.5. Serializing a Decimal . . . . . . . . . . . . . . . . 19
4.1.6. Serializing a String . . . . . . . . . . . . . . . . 19
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4.1.7. Serializing a Token . . . . . . . . . . . . . . . . . 20
4.1.8. Serializing a Byte Sequence . . . . . . . . . . . . . 20
4.1.9. Serializing a Boolean . . . . . . . . . . . . . . . . 21
4.2. Parsing Header Fields into Structured Headers . . . . . . 21
4.2.1. Parsing a List . . . . . . . . . . . . . . . . . . . 22
4.2.2. Parsing a Dictionary . . . . . . . . . . . . . . . . 24
4.2.3. Parsing an Item . . . . . . . . . . . . . . . . . . . 25
4.2.4. Parsing a Number . . . . . . . . . . . . . . . . . . 27
4.2.5. Parsing a String . . . . . . . . . . . . . . . . . . 28
4.2.6. Parsing a Token . . . . . . . . . . . . . . . . . . . 29
4.2.7. Parsing a Byte Sequence . . . . . . . . . . . . . . . 29
4.2.8. Parsing a Boolean . . . . . . . . . . . . . . . . . . 30
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
6. Security Considerations . . . . . . . . . . . . . . . . . . . 31
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.1. Normative References . . . . . . . . . . . . . . . . . . 31
7.2. Informative References . . . . . . . . . . . . . . . . . 32
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 33
Appendix B. Frequently Asked Questions . . . . . . . . . . . . . 33
B.1. Why not JSON? . . . . . . . . . . . . . . . . . . . . . . 33
B.2. Structured Headers don't "fit" my data. . . . . . . . . . 34
Appendix C. Implementation Notes . . . . . . . . . . . . . . . . 34
Appendix D. Changes . . . . . . . . . . . . . . . . . . . . . . 35
D.1. Since draft-ietf-httpbis-header-structure-14 . . . . . . 35
D.2. Since draft-ietf-httpbis-header-structure-13 . . . . . . 35
D.3. Since draft-ietf-httpbis-header-structure-12 . . . . . . 36
D.4. Since draft-ietf-httpbis-header-structure-11 . . . . . . 36
D.5. Since draft-ietf-httpbis-header-structure-10 . . . . . . 36
D.6. Since draft-ietf-httpbis-header-structure-09 . . . . . . 36
D.7. Since draft-ietf-httpbis-header-structure-08 . . . . . . 37
D.8. Since draft-ietf-httpbis-header-structure-07 . . . . . . 37
D.9. Since draft-ietf-httpbis-header-structure-06 . . . . . . 38
D.10. Since draft-ietf-httpbis-header-structure-05 . . . . . . 38
D.11. Since draft-ietf-httpbis-header-structure-04 . . . . . . 38
D.12. Since draft-ietf-httpbis-header-structure-03 . . . . . . 38
D.13. Since draft-ietf-httpbis-header-structure-02 . . . . . . 38
D.14. Since draft-ietf-httpbis-header-structure-01 . . . . . . 39
D.15. Since draft-ietf-httpbis-header-structure-00 . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
Specifying the syntax of new HTTP header fields is an onerous task;
even with the guidance in Section 8.3.1 of [RFC7231], there are many
decisions - and pitfalls - for a prospective HTTP header field
author.
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Once a header field is defined, bespoke parsers and serializers often
need to be written, because each header has slightly different
handling of what looks like common syntax.
This document introduces a set of common data structures for use in
definitions of new HTTP header field values to address these
problems. In particular, it defines a generic, abstract model for
header field values, along with a concrete serialisation for
expressing that model in HTTP [RFC7230] header fields.
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 by
specification writers and handling by implementations.
Additionally, future versions of HTTP can define alternative
serialisations of the abstract model of these structures, 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.
Section 2 describes how to specify a Structured Header.
Section 3 defines a number of abstract data types that can be used in
Structured Headers. Those abstract types can be serialized into and
parsed from HTTP headers using the algorithms described in Section 4.
1.1. Intentionally Strict Processing
This specification intentionally defines strict parsing and
serialisation behaviours using step-by-step algorithms; the only
error handling defined is to fail the operation altogether.
It is designed to encourage faithful implementation and therefore
good interoperability. Therefore, an implementation that tried to be
"helpful" by being more tolerant of input would make interoperability
worse, since that would create pressure on other implementations to
implement similar (but likely subtly different) workarounds.
In other words, strict processing is an intentional feature of this
specification; it allows non-conformant input to be discovered and
corrected by the producer early, and avoids both interoperability and
security issues that might otherwise result.
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Note that as a result of this strictness, if a header field is
appended to by multiple parties (e.g., intermediaries, or different
components in the sender), an error in one party's value is likely to
cause the entire header field to fail parsing.
1.2. Notational Conventions
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.
This document uses algorithms to specify parsing and serialisation
behaviours, and the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234] to illustrate expected syntax in HTTP header fields. In
doing so, uses the VCHAR, SP, DIGIT, ALPHA and DQUOTE rules from
[RFC5234]. It also includes the tchar rule from [RFC7230].
When parsing from HTTP header fields, implementations MUST follow the
algorithms, but MAY vary in implementation so as the behaviours are
indistinguishable from specified behaviour. If there is disagreement
between the parsing algorithms and ABNF, the specified algorithms
take precedence. In some places, the algorithms are "greedy" with
whitespace, but this should not affect conformance.
For serialisation to header fields, the ABNF illustrates the range of
acceptable wire representations with as much fidelity as possible,
and the algorithms define the recommended way to produce them.
Implementations MAY vary from the specified behaviour so long as the
output still matches the ABNF.
2. Defining New Structured Headers
To specify a HTTP header as a structured header, its authors needs
to:
o Reference this specification. Recipients and generators of the
header need to know that the requirements of this document are in
effect.
o Specify the type of the header field itself; either Dictionary
(Section 3.2), List (Section 3.1), or Item (Section 3.3).
o Define the semantics of those structures.
o Specify any additional constraints upon the structures used, as
well as the consequences when those constraints are violated.
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Typically, this means that a header definition will specify the top-
level type - Dictionary, List or Item - and then define its allowable
types, and constraints upon them. For example, a header defined as a
List might have all Integer members, or a mix of types; a header
defined as an Item might allow only Strings, and additionally only
strings beginning with the letter "Q". Likewise, inner lists are
only valid when a header definition explicitly allows them.
When Structured Headers parsing fails, the header is ignored (see
Section 4.2); in most situations, violating header-specific
constraints should have the same effect. Thus, if a header is
defined as an Item and required to be an Integer, but a String is
received, it will by default be ignored. If the header requires
different error handling, this should be explicitly specified.
However, both items and inner lists allow parameters as an
extensibility mechanism; this means that values can later be extended
to accommodate more information, if need be. As a result, header
specifications are discouraged from defining the presence of an
unrecognised parameter as an error condition.
To help assure that this extensibility is available in the future,
and to encourage consumers to use a fully capable Structured Headers
parser, a header definition can specify that "grease" parameters be
added by senders. For example, a specification could stipulate that
all parameters beginning with the letter 'q' are reserved for this
use.
Note that a header field definition cannot relax the requirements of
this specification because doing so would preclude handling by
generic software; they can only add additional constraints (for
example, on the numeric range of integers and decimals, the format of
strings and tokens, the types allowed in a dictionary's values, or
the number of items in a list). Likewise, header field definitions
can only use Structured Headers for the entire header field value,
not a portion thereof.
This specification defines minimums for the length or number of
various structures supported by Structured Headers implementations.
It does not specify maximum sizes in most cases, but header authors
should be aware that HTTP implementations do impose various limits on
the size of individual header fields, the total number of fields,
and/or the size of the entire header block.
Specifications can refer to a Structured Header's field-name as a
"structured header name" and its field-value as a "structured header
value" as necessary. Header definitions are encouraged to use the
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ABNF rules beginning with "sh-" defined in this specification; other
rules in this specification are not intended for their use.
For example, a fictitious Foo-Example header field might be specified
as:
42. Foo-Example Header
The Foo-Example HTTP header field conveys information about how
much Foo the message has.
Foo-Example is a Item Structured Header [RFCxxxx]. Its value MUST be
an Integer (Section Y.Y of [RFCxxxx]). Its ABNF is:
Foo-Example = sh-integer
Its value indicates the amount of Foo in the message, and MUST
be between 0 and 10, inclusive; other values MUST cause
the entire header to be ignored.
The following parameters are defined:
* A parameter whose name is "fooUrl", and whose value is a string
(Section Y.Y of [RFCxxxx]), conveying the Foo URLs
for the message. See below for processing requirements.
"fooUrl" contains a URI-reference (Section 4.1 of
[RFC3986], Section 4.1). If its value is not a valid URI-reference,
that URL MUST be ignored. If its value is a relative reference
(Section 4.2 of [RFC3986]), it MUST be resolved (Section 5 of
[RFC3986]) before being used.
For example:
Foo-Example: 2; foourl="https://foo.example.com/"
3. Structured Data Types
This section defines the abstract value types that can be composed
into Structured Headers. The ABNF provided represents the on-wire
format in HTTP headers.
In summary:
o There are three top-level types that a HTTP header can be defined
as; Lists, Dictionaries, and Items.
o Lists and Dictionaries are containers; their members can be Items
or Inner Lists (which are themselves lists of items).
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o Both Items and Inner Lists can be parameterised with key/value
pairs.
3.1. Lists
Lists are arrays of zero or more members, each of which can be an
item (Section 3.3) or an inner list (Section 3.1.1), both of which
can be parameterised (Section 3.1.2).
The ABNF for lists in HTTP headers is:
sh-list = list-member *( *SP "," *SP list-member )
list-member = sh-item / inner-list
In HTTP headers, each member is separated by a comma and optional
whitespace. For example, a header field whose value is defined as a
list of strings could look like:
Example-StrListHeader: "foo", "bar", "It was the best of times."
In HTTP headers, an empty list is denoted by not serialising the
header at all.
Note that lists can have their members split across multiple
instances inside a block of fields; for example, the following are
equivalent:
Example-Hdr: foo, bar
and
Example-Hdr: foo
Example-Hdr: bar
However, members of a list cannot be safely split between instances;
see Section 4.2 for details.
Parsers MUST support lists containing at least 1024 members. Header
specifications can constrain the types and cardinality of individual
list values as they require.
3.1.1. Inner Lists
An inner list is an array of zero or more items (Section 3.3). Both
the individual items and the inner-list itself can be parameterised
(Section 3.1.2).
The ABNF for inner-lists in HTTP headers is:
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inner-list = "(" *SP [ sh-item *( 1*SP sh-item ) *SP ] ")"
*parameter
In HTTP headers, inner lists are denoted by surrounding parenthesis,
and have their values delimited by a single space. A header field
whose value is defined as a list of inner-lists of strings could look
like:
Example-StrListListHeader: ("foo" "bar"), ("baz"), ("bat" "one"), ()
Note that the last member in this example is an empty inner list.
A header field whose value is defined as a list of inner-lists with
parameters at both levels could look like:
Example-ListListParam: ("foo"; a=1;b=2);lvl=5, ("bar" "baz");lvl=1
Parsers MUST support inner-lists containing at least 256 members.
Header specifications can constrain the types and cardinality of
individual inner-list members as they require.
3.1.2. Parameters
Parameters are an ordered map of key-values pairs that are associated
with an item (Section 3.3) or inner-list (Section 3.1.1). The keys
are unique within the scope of a map of parameters, and the values
are bare items (i.e., they themselves cannot be parameterised; see
Section 3.3).
The ABNF for parameters in HTTP headers is:
parameter = ";" *SP param-name [ "=" param-value ]
param-name = key
key = lcalpha *( lcalpha / DIGIT / "_" / "-" / "." / "*" )
lcalpha = %x61-7A ; a-z
param-value = bare-item
In HTTP headers, parameters are separated from their item or inner-
list and each other by semicolons. For example:
Example-ParamListHeader: abc;a=1;b=2; cde_456, (ghi;jk=4 l);q="9";r=w
Parameters whose value is Boolean true MUST omit that value when
serialised. For example:
Example-IntHeader: 1; a; b=?0
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Note that this requirement is only on serialisation; parsers are
still required to correctly handle the true value when it appears in
parameters.
Parsers MUST support at least 256 parameters on an item or inner-
list, and support parameter keys with at least 64 characters. Header
specifications can constrain the types and cardinality of individual
parameter names and values as they require.
3.2. Dictionaries
Dictionaries are ordered maps of name-value pairs, where the names
are short, textual strings and the values are items (Section 3.3) or
arrays of items, both of which can be parameterised (Section 3.1.2).
There can be zero or more members, and their names are unique in the
scope of the dictionary they occur within.
Implementations MUST provide access to dictionaries both by index and
by name. Specifications MAY use either means of accessing the
members.
The ABNF for dictionaries in HTTP headers is:
sh-dictionary = dict-member *( *SP "," *SP dict-member )
dict-member = member-name [ "=" member-value ]
member-name = key
member-value = sh-item / inner-list
In HTTP headers, members are separated by a comma with optional
whitespace, while names and values are separated by "=" (without
whitespace). For example:
Example-DictHeader: en="Applepie", da=:w4ZibGV0w6ZydGU=:
Members whose value is Boolean true MUST omit that value when
serialised, unless it has parameters. For example, here both "b" and
"c" are true, but "c"'s value is serialised because it has
parameters:
Example-DictHeader: a=?0, b, c=?1; foo=bar
Note that this requirement is only on serialisation; parsers are
still required to correctly handle the true value when it appears in
dictionary values.
A dictionary with a member whose value is an inner-list of tokens:
Example-DictListHeader: rating=1.5, feelings=(joy sadness)
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A dictionary with a mix of singular and list values, some with
parameters:
Example-MixDict: a=(1 2), b=3, c=4;aa=bb, d=(5 6);valid=?1
As with lists, an empty dictionary is represented in HTTP headers by
omitting the entire header field.
Typically, a header field specification will define the semantics of
dictionaries by specifying the allowed type(s) for individual member
names, as well as whether their presence is required or optional.
Recipients MUST ignore names that are undefined or unknown, unless
the header field's specification specifically disallows them.
Note that dictionaries can have their members split across multiple
instances inside a block of fields; for example, the following are
equivalent:
Example-Hdr: foo=1, bar=2
and
Example-Hdr: foo=1
Example-Hdr: bar=2
However, members of a dictionary cannot be safely split between
instances; see Section 4.2 for details.
Parsers MUST support dictionaries containing at least 1024 name/value
pairs, and names with at least 64 characters.
3.3. Items
An item is can be a integer (Section 3.3.1), decimal (Section 3.3.2),
string (Section 3.3.3), token (Section 3.3.4), byte sequence
(Section 3.3.5), or Boolean (Section 3.3.6). It can have associated
parameters (Section 3.1.2).
The ABNF for items in HTTP headers is:
sh-item = bare-item *parameter
bare-item = sh-integer / sh-decimal / sh-string / sh-token / sh-binary
/ sh-boolean
For example, a header field that is defined to be an Item that is an
integer might look like:
Example-IntItemHeader: 5
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or with parameters:
Example-IntItemHeader: 5; foo=bar
3.3.1. Integers
Integers have a range of -999,999,999,999,999 to 999,999,999,999,999
inclusive (i.e., up to fifteen digits, signed), for IEEE 754
compatibility ([IEEE754]).
The ABNF for integers in HTTP headers is:
sh-integer = ["-"] 1*15DIGIT
For example:
Example-IntegerHeader: 42
Note that commas in integers are used in this section's prose only
for readability; they are not valid in the wire format.
3.3.2. Decimals
Decimals are numbers with an integer and a fractional component. The
Integer component has at most 12 digits; the fractional component has
at most three digits.
The ABNF for decimals in HTTP headers is:
sh-decimal = ["-"] 1*12DIGIT "." 1*3DIGIT
For example, a header whose value is defined as a decimal could look
like:
Example-DecimalHeader: 4.5
3.3.3. Strings
Strings are zero or more printable ASCII [RFC0020] characters (i.e.,
the range %x20 to %x7E). Note that this excludes tabs, newlines,
carriage returns, etc.
The ABNF for strings in HTTP headers is:
sh-string = DQUOTE *(chr) DQUOTE
chr = unescaped / escaped
unescaped = %x20-21 / %x23-5B / %x5D-7E
escaped = "\" ( DQUOTE / "\" )
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In HTTP headers, strings are delimited with double quotes, using a
backslash ("\") to escape double quotes and backslashes. For
example:
Example-StringHeader: "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 characters after "\" MUST cause parsing to fail.
Unicode is not directly supported in strings, 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 content, a
byte sequence (Section 3.3.5) SHOULD be specified, along with a
character encoding (preferably [UTF-8]).
Parsers MUST support strings with at least 1024 characters.
3.3.4. Tokens
Tokens are short textual words; their abstract model is identical to
their expression in the HTTP header serialisation.
The ABNF for tokens in HTTP headers is:
sh-token = ( ALPHA / "\*" ) *( tchar / ":" / "/" )
Parsers MUST support tokens with at least 512 characters.
Note that a Structured Header token allows the characters as the
"token" ABNF rule defined in [RFC7230], with the exceptions that the
first character is required to be either ALPHA or "*", and ":" and
"/" are also allowed in subsequent characters.
3.3.5. Byte Sequences
Byte sequences can be conveyed in Structured Headers.
The ABNF for a byte sequence in HTTP headers is:
sh-binary = ":" *(base64) ":"
base64 = ALPHA / DIGIT / "+" / "/" / "="
In HTTP headers, a byte sequence is delimited with colons and encoded
using base64 ([RFC4648], Section 4). For example:
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Example-BinaryHdr: :cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg==:
Parsers MUST support byte sequences with at least 16384 octets after
decoding.
3.3.6. Booleans
Boolean values can be conveyed in Structured Headers.
The ABNF for a Boolean in HTTP headers is:
sh-boolean = "?" boolean
boolean = "0" / "1"
In HTTP headers, a boolean is indicated with a leading "?" character
followed by a "1" for a true value or "0" for false. For example:
Example-BoolHdr: ?1
4. Working With Structured Headers in HTTP Headers
This section defines how to serialize and parse Structured Headers in
header fields, and protocols compatible with them (e.g., in HTTP/2
[RFC7540] before HPACK [RFC7541] is applied).
4.1. Serializing Structured Headers
Given a structure defined in this specification, return an ASCII
string suitable for use in a HTTP header value.
1. If the structure is a Dictionary or List and its value is empty
(i.e., it has no members), do not serialize the field at all
(i.e., omit both the field-name and field-value).
2. If the structure is a Dictionary, let output_string be the result
of running Serializing a Dictionary (Section 4.1.2) with the
structure.
3. Else if the structure is a List, let output_string be the result
of running Serializing a List (Section 4.1.1) with the structure.
4. Else if the structure is an Item, let output_string be the result
of running Serializing an Item (Section 4.1.3) with the
structure.
5. Else, fail serialisation.
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6. Return output_string converted into an array of bytes, using
ASCII encoding [RFC0020].
4.1.1. Serializing a List
Given an array of (member_value, parameters) tuples as input_list,
return an ASCII string suitable for use in a HTTP header value.
1. Let output be an empty string.
2. For each (member_value, parameters) of input_list:
1. If member_value is an array, append the result of running
Serialising an Inner List (Section 4.1.1.1) with
(member_value, parameters) to output.
2. Otherwise, append the result of running Serializing an Item
(Section 4.1.3) with (member_value, parameters) to output.
3. If more member_values remain in input_list:
1. Append a COMMA to output.
2. Append a single SP to output.
3. Return output.
4.1.1.1. Serialising an Inner List
Given an array of (member_value, parameters) tuples as inner_list,
and parameters as list_parameters, return an ASCII string suitable
for use in a HTTP header value.
1. Let output be the string "(".
2. For each (member_value, parameters) of inner_list:
1. Append the result of running Serializing an Item
(Section 4.1.3) with (member_value, parameters) to output.
2. If more values remain in inner_list, append a single SP to
output.
3. Append ")" to output.
4. Append the result of running Serializing Parameters
Section 4.1.1.2 with list_parameters to output.
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5. Return output.
4.1.1.2. Serializing Parameters
Given an ordered dictionary as input_parameters (each member having a
param_name and a param_value), return an ASCII string suitable for
use in a HTTP header value.
1. Let output be an empty string.
2. For each parameter-name with a value of param_value in
input_parameters:
1. Append ";" to output.
2. Append the result of running Serializing a Key
(Section 4.1.1.3) with param_name to output.
3. If param_value is not Boolean true:
1. Append "=" to output.
2. Append the result of running Serializing a bare Item
(Section 4.1.3.1) with param_value to output.
3. Return output.
4.1.1.3. Serializing a Key
Given a key as input_key, return an ASCII string suitable for use in
a HTTP header value.
1. If input_key is not a sequence of characters, or contains
characters not in lcalpha, DIGIT, "_", "-", ".", or "*" fail
serialisation.
2. If the first character of input_key is not lcalpha, fail parsing.
3. Let output be an empty string.
4. Append input_key to output.
5. Return output.
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4.1.2. Serializing a Dictionary
Given an ordered dictionary as input_dictionary (each member having a
member_name and a tuple value of (member_value, parameters)), return
an ASCII string suitable for use in a HTTP header value.
1. Let output be an empty string.
2. For each member_name with a value of (member_value, parameters)
in input_dictionary:
1. Append the result of running Serializing a Key
(Section 4.1.1.3) with member's member_name to output.
3. If member_value is not Boolean true or parameters is not empty:
1. Append "=" to output.
1. If member_value is an array, append the result of running
Serialising an Inner List (Section 4.1.1.1) with
(member_value, parameters) to output.
2. Otherwise, append the result of running Serializing an
Item (Section 4.1.3) with (member_value, parameters) to
output.
4. If more members remain in input_dictionary:
1. Append a COMMA to output.
2. Append a single SP to output.
5. Return output.
4.1.3. Serializing an Item
Given an item bare_item and parameters item_parameters as input,
return an ASCII string suitable for use in a HTTP header value.
1. Let output be an empty string.
2. Append the result of running Serializing a Bare Item
Section 4.1.3.1 with bare_item to output.
3. Append the result of running Serializing Parameters
Section 4.1.1.2 with item_parameters to output.
4. Return output.
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4.1.3.1. Serialising a Bare Item
Given an item as input_item, return an ASCII string suitable for use
in a HTTP header value.
1. If input_item is an integer, return the result of running
Serializing an Integer (Section 4.1.4) with input_item.
2. If input_item is a decimal, return the result of running
Serializing a Decimal (Section 4.1.5) with input_item.
3. If input_item is a string, return the result of running
Serializing a String (Section 4.1.6) with input_item.
4. If input_item is a token, return the result of running
Serializing a Token (Section 4.1.7) with input_item.
5. If input_item is a Boolean, return the result of running
Serializing a Boolean (Section 4.1.9) with input_item.
6. If input_item is a byte sequence, return the result of running
Serializing a Byte Sequence (Section 4.1.8) with input_item.
7. Otherwise, fail serialisation.
4.1.4. Serializing an Integer
Given an integer as input_integer, return an ASCII string suitable
for use in a HTTP header value.
1. If input_integer is not an integer in the range of
-999,999,999,999,999 to 999,999,999,999,999 inclusive, fail
serialisation.
2. Let output be an empty string.
3. If input_integer is less than (but not equal to) 0, append "-" to
output.
4. Append input_integer's numeric value represented in base 10 using
only decimal digits to output.
5. Return output.
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4.1.5. Serializing a Decimal
Given a decimal_number as input_decimal, return an ASCII string
suitable for use in a HTTP header value.
1. Let output be an empty string.
2. If input_decimal is less than (but not equal to) 0, append "-" to
output.
3. Append input_decimal's integer component represented in base 10
(using only decimal digits) to output; if it is zero, append "0".
4. If the number of characters appended in the previous step is
greater than 12, fail serialisation.
5. Append "." to output.
6. If input_decimal's fractional component is zero, append "0" to
output.
7. Else if input_decimal's fractional component has up to three
digits, append them represented in base 10 (using only decimal
digits) to output.
8. Otherwise, append the first three digits of input_decimal's
fractional component (represented in base 10, using only decimal
digits) to output, rounding the final digit to the nearest value,
or to the even value if it is equidistant.
9. Return output.
4.1.6. Serializing a String
Given a string as input_string, return an ASCII string suitable for
use in a HTTP header value.
1. If input_string is not a sequence of characters, or contains
characters in the range %x00-1f or %x7f (i.e., is not in VCHAR or
SP), fail serialisation.
2. Let output be an empty string.
3. Append DQUOTE to output.
4. For each character char in input_string:
1. If char is "\" or DQUOTE:
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1. Append "\" to output.
2. Append char to output.
5. Append DQUOTE to output.
6. Return output.
4.1.7. Serializing a Token
Given a token as input_token, return an ASCII string suitable for use
in a HTTP header value.
1. If input_token is not a sequence of characters, the first
character is not ALPHA or "*", or the remaining contain a
character not in tchar, ":" or "/", fail serialisation.
2. Let output be an empty string.
3. Append input_token to output.
4. Return output.
4.1.8. Serializing a Byte Sequence
Given a byte sequence as input_bytes, return an ASCII string suitable
for use in a HTTP header value.
1. If input_bytes is not a sequence of bytes, fail serialisation.
2. Let output be an empty string.
3. Append ":" to output.
4. Append the result of base64-encoding input_bytes as per
[RFC4648], Section 4, taking account of the requirements below.
5. Append ":" to output.
6. Return output.
The encoded data is required to be padded with "=", as per [RFC4648],
Section 3.2.
Likewise, encoded data SHOULD have pad bits set to zero, as per
[RFC4648], Section 3.5, unless it is not possible to do so due to
implementation constraints.
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4.1.9. Serializing a Boolean
Given a Boolean as input_boolean, return an ASCII string suitable for
use in a HTTP header value.
1. If input_boolean is not a boolean, fail serialisation.
2. Let output be an empty string.
3. Append "?" to output.
4. If input_boolean is true, append "1" to output.
5. If input_boolean is false, append "0" to output.
6. Return output.
4.2. Parsing Header Fields into Structured Headers
When a receiving implementation parses HTTP header fields 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 array of bytes input_bytes that represents the chosen
header's field-value (which is empty if that header is not present),
and header_type (one of "dictionary", "list", or "item"), return the
parsed header value.
1. Convert input_bytes into an ASCII string input_string; if
conversion fails, fail parsing.
2. Discard any leading SP characters from input_string.
3. If header_type is "list", let output be the result of running
Parsing a List (Section 4.2.1) with input_string.
4. If header_type is "dictionary", let output be the result of
running Parsing a Dictionary (Section 4.2.2) with input_string.
5. If header_type is "item", let output be the result of running
Parsing an Item (Section 4.2.3) with input_string.
6. Discard any leading SP characters from input_string.
7. If input_string is not empty, fail parsing.
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8. Otherwise, return output.
When generating input_bytes, parsers MUST combine all instances of
the target header field into one comma-separated field-value, as per
[RFC7230], Section 3.2.2; this assures that the header is processed
correctly.
For Lists and Dictionaries, this has the effect of correctly
concatenating all instances of the header field, as long as
individual individual members of the top-level data structure are not
split across multiple header instances.
Strings split across multiple header instances will have
unpredictable results, because comma(s) and whitespace inserted upon
combination will become part of the string output by the parser.
Since concatenation might be done by an upstream intermediary, the
results are not under the control of the serializer or the parser.
Tokens, Integers, Decimals and Byte Sequences cannot be split across
multiple headers because the inserted commas will cause parsing to
fail.
If parsing fails - including when calling another algorithm - the
entire header field's value MUST be ignored (i.e., treated as if the
header field were not present in the message). This is intentionally
strict, to improve interoperability and safety, and specifications
referencing this document are not allowed to loosen this requirement.
Note that this requirement does not apply to an implementation that
is not parsing the header field; for example, an intermediary is not
required to strip a failing header field from a message before
forwarding it.
4.2.1. Parsing a List
Given an ASCII string as input_string, return an array of
(item_or_inner_list, parameters) tuples. input_string is modified to
remove the parsed value.
1. Let members be an empty array.
2. While input_string is not empty:
1. Append the result of running Parsing an Item or Inner List
(Section 4.2.1.1) with input_string to members.
2. Discard any leading SP characters from input_string.
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3. If input_string is empty, return members.
4. Consume the first character of input_string; if it is not
COMMA, fail parsing.
5. Discard any leading SP characters from input_string.
6. If input_string is empty, there is a trailing comma; fail
parsing.
3. No structured data has been found; return members (which is
empty).
4.2.1.1. Parsing an Item or Inner List
Given an ASCII string as input_string, return the tuple
(item_or_inner_list, parameters), where item_or_inner_list can be
either a single bare item, or an array of (bare_item, parameters)
tuples. input_string is modified to remove the parsed value.
1. If the first character of input_string is "(", return the result
of running Parsing an Inner List (Section 4.2.1.2) with
input_string.
2. Return the result of running Parsing an Item (Section 4.2.3) with
input_string.
4.2.1.2. Parsing an Inner List
Given an ASCII string as input_string, return the tuple (inner_list,
parameters), where inner_list is an array of (bare_item, parameters)
tuples. input_string is modified to remove the parsed value.
1. Consume the first character of input_string; if it is not "(",
fail parsing.
2. Let inner_list be an empty array.
3. While input_string is not empty:
1. Discard any leading SP characters from input_string.
2. If the first character of input_string is ")":
1. Consume the first character of input_string.
2. Let parameters be the result of running Parsing
Parameters (Section 4.2.3.2) with input_string.
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3. Return the tuple (inner_list, parameters).
3. Let item be the result of running Parsing an Item
(Section 4.2.3) with input_string.
4. Append item to inner_list.
5. If the first character of input_string is not SP or ")", fail
parsing.
4. The end of the inner list was not found; fail parsing.
4.2.2. Parsing a Dictionary
Given an ASCII string as input_string, return an ordered map whose
values are (item_or_inner_list, parameters) tuples. input_string is
modified to remove the parsed value.
1. Let dictionary be an empty, ordered map.
2. While input_string is not empty:
1. Let this_key be the result of running Parsing a Key
(Section 4.2.3.3) with input_string.
2. If the first character of input_string is "=":
1. Consume the first character of input_string.
2. Let member be the result of running Parsing an Item or
Inner List (Section 4.2.1.1) with input_string.
3. Otherwise:
1. Let value be Boolean true.
2. Let parameters be an empty, ordered map.
3. Let member be the tuple (value, parameters).
4. Add name this_key with value member to dictionary. If
dictionary already contains a name this_key (comparing
character-for-character), overwrite its value.
5. Discard any leading SP characters from input_string.
6. If input_string is empty, return dictionary.
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7. Consume the first character of input_string; if it is not
COMMA, fail parsing.
8. Discard any leading SP characters from input_string.
9. If input_string is empty, there is a trailing comma; fail
parsing.
3. No structured data has been found; return dictionary (which is
empty).
4.2.3. Parsing an Item
Given an ASCII string as input_string, return a (bare_item,
parameters) tuple. input_string is modified to remove the parsed
value.
1. Let bare_item be the result of running Parsing a Bare Item
(Section 4.2.3.1) with input_string.
2. Let parameters be the result of running Parsing Parameters
(Section 4.2.3.2) with input_string.
3. Return the tuple (bare_item, parameters).
4.2.3.1. Parsing a Bare Item
Given an ASCII string as input_string, return a bare item.
input_string is modified to remove the parsed value.
1. If the first character of input_string is a "-" or a DIGIT,
return the result of running Parsing a Number (Section 4.2.4)
with input_string.
2. If the first character of input_string is a DQUOTE, return the
result of running Parsing a String (Section 4.2.5) with
input_string.
3. If the first character of input_string is ":", return the result
of running Parsing a Byte Sequence (Section 4.2.7) with
input_string.
4. If the first character of input_string is "?", return the result
of running Parsing a Boolean (Section 4.2.8) with input_string.
5. If the first character of input_string is an ALPHA or "*", return
the result of running Parsing a Token (Section 4.2.6) with
input_string.
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6. Otherwise, the item type is unrecognized; fail parsing.
4.2.3.2. Parsing Parameters
Given an ASCII string as input_string, return an ordered map whose
values are bare items. input_string is modified to remove the parsed
value.
1. Let parameters be an empty, ordered map.
2. While input_string is not empty:
1. If the first character of input_string is not ";", exit the
loop.
2. Consume a ";" character from the beginning of input_string.
3. Discard any leading SP characters from input_string.
4. let param_name be the result of running Parsing a Key
(Section 4.2.3.3) with input_string.
5. Let param_value be Boolean true.
6. If the first character of input_string is "=":
1. Consume the "=" character at the beginning of
input_string.
2. Let param_value be the result of running Parsing a Bare
Item (Section 4.2.3.1) with input_string.
7. Append key param_name with value param_value to parameters.
If parameters already contains a name param_name (comparing
character-for-character), overwrite its value.
3. Return parameters.
4.2.3.3. Parsing a Key
Given an ASCII string as input_string, return a key. input_string is
modified to remove the parsed value.
1. If the first character of input_string is not lcalpha, fail
parsing.
2. Let output_string be an empty string.
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3. While input_string is not empty:
1. If the first character of input_string is not one of lcalpha,
DIGIT, "_", "-", ".", or "*", return output_string.
2. Let char be the result of removing the first character of
input_string.
3. Append char to output_string.
4. Return output_string.
4.2.4. Parsing a Number
Given an ASCII string as input_string, return a number. input_string
is modified to remove the parsed value.
NOTE: This algorithm parses both Integers (Section 3.3.1) and
Decimals (Section 3.3.2), and returns the corresponding structure.
1. Let type be "integer".
2. Let sign be 1.
3. Let input_number be an empty string.
4. If the first character of input_string is "-", consume it and
set sign to -1.
5. If input_string is empty, there is an empty integer; fail
parsing.
6. If the first character of input_string is not a DIGIT, fail
parsing.
7. While input_string is not empty:
1. Let char be the result of consuming the first character of
input_string.
2. If char is a DIGIT, append it to input_number.
3. Else, if type is "integer" and char is ".":
1. If input_number contains more than 12 characters, fail
parsing.
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2. Otherwise, append char to input_number and set type to
"decimal".
4. Otherwise, prepend char to input_string, and exit the loop.
5. If type is "integer" and input_number contains more than 15
characters, fail parsing.
6. If type is "decimal" and input_number contains more than 16
characters, fail parsing.
8. If type is "integer":
1. Parse input_number as an integer and let output_number be
the product of the result and sign.
2. If output_number is outside the range -999,999,999,999,999
to 999,999,999,999,999 inclusive, fail parsing.
9. Otherwise:
1. If the final character of input_number is ".", fail parsing.
2. If the number of characters after "." in input_number is
greater than three, fail parsing.
3. Parse input_number as a decimal number and let output_number
be the product of the result and sign.
10. Return output_number.
4.2.5. Parsing a String
Given an ASCII string as input_string, return an unquoted string.
input_string is modified to remove the parsed value.
1. Let output_string be an empty string.
2. If the first character of input_string is not DQUOTE, fail
parsing.
3. Discard the first character of input_string.
4. While input_string is not empty:
1. Let char be the result of consuming the first character of
input_string.
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2. If char is a backslash ("\"):
1. If input_string is now empty, fail parsing.
2. Let next_char be the result of consuming the first
character of input_string.
3. If next_char is not DQUOTE or "\", fail parsing.
4. Append next_char to output_string.
3. Else, if char is DQUOTE, return output_string.
4. Else, if char is in the range %x00-1f or %x7f (i.e., is not
in VCHAR or SP), fail parsing.
5. Else, append char to output_string.
5. Reached the end of input_string without finding a closing DQUOTE;
fail parsing.
4.2.6. Parsing a Token
Given an ASCII string as input_string, return a token. input_string
is modified to remove the parsed value.
1. If the first character of input_string is not ALPHA or "*", fail
parsing.
2. Let output_string be an empty string.
3. While input_string is not empty:
1. If the first character of input_string is not in tchar, ":"
or "/", return output_string.
2. Let char be the result of consuming the first character of
input_string.
3. Append char to output_string.
4. Return output_string.
4.2.7. Parsing a Byte Sequence
Given an ASCII string as input_string, return a byte sequence.
input_string is modified to remove the parsed value.
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1. If the first character of input_string is not ":", fail parsing.
2. Discard the first character of input_string.
3. If there is not a ":" character before the end of input_string,
fail parsing.
4. Let b64_content be the result of consuming content of
input_string up to but not including the first instance of the
character ":".
5. Consume the ":" character at the beginning of input_string.
6. If b64_content contains a character not included in ALPHA, DIGIT,
"+", "/" and "=", fail parsing.
7. Let binary_content be the result of Base 64 Decoding [RFC4648]
b64_content, synthesizing padding if necessary (note the
requirements about recipient behaviour below).
8. Return binary_content.
Because some implementations of base64 do not allow reject of encoded
data that is not properly "=" padded (see [RFC4648], Section 3.2),
parsers SHOULD NOT fail when it is not present, unless they cannot be
configured to do so.
Because some implementations of base64 do not allow rejection of
encoded data that has non-zero pad bits (see [RFC4648], Section 3.5),
parsers SHOULD NOT fail when it is present, unless they cannot be
configured to do so.
This specification does not relax the requirements in [RFC4648],
Section 3.1 and 3.3; therefore, parsers MUST fail on characters
outside the base64 alphabet, and on line feeds in encoded data.
4.2.8. Parsing a Boolean
Given an ASCII string as input_string, return a Boolean. input_string
is modified to remove the parsed value.
1. If the first character of input_string is not "?", fail parsing.
2. Discard the first character of input_string.
3. If the first character of input_string matches "1", discard the
first character, and return true.
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4. If the first character of input_string matches "0", discard the
first character, and return false.
5. No value has matched; fail parsing.
5. IANA Considerations
This draft has no actions for IANA.
6. Security Considerations
The size of most types defined by Structured Headers is not limited;
as a result, extremely large header fields could be an attack vector
(e.g., for resource consumption). Most HTTP implementations limit
the sizes of individual header fields as well as the overall header
block size to mitigate such attacks.
It is possible for parties with the ability to inject new HTTP header
fields to change the meaning of a Structured Header. In some
circumstances, this will cause parsing to fail, but it is not
possible to reliably fail in all such circumstances.
7. References
7.1. Normative References
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic",
IEEE 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
ISBN 978-1-5044-5924-2, July 2019,
<https://ieeexplore.ieee.org/document/8766229>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/std63>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/header-structure
[4] https://github.com/httpwg/structured-header-tests
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[5] https://github.com/httpwg/wiki/wiki/Structured-Headers
[6] https://github.com/httpwg/structured-header-tests
Appendix A. Acknowledgements
Many thanks to Matthew Kerwin for his detailed feedback and careful
consideration during the development of this specification.
Appendix B. Frequently Asked Questions
B.1. Why not JSON?
Earlier proposals for structured headers were based upon JSON
[RFC8259]. However, constraining its use to make it suitable for
HTTP header fields required senders and recipients to implement
specific additional handling.
For example, JSON has specification issues around large numbers and
objects with duplicate members. Although advice for avoiding these
issues is available (e.g., [RFC7493]), it cannot be relied upon.
Likewise, JSON strings are by default Unicode strings, which have a
number of potential interoperability issues (e.g., in comparison).
Although implementers can be advised to avoid non-ASCII content where
unnecessary, this is difficult to enforce.
Another example is JSON's ability to nest content to arbitrary
depths. Since the resulting memory commitment might be unsuitable
(e.g., in embedded and other limited server deployments), it's
necessary to limit it in some fashion; however, existing JSON
implementations have no such limits, and even if a limit is
specified, it's likely that some header field definition will find a
need to violate it.
Because of JSON's broad adoption and implementation, it is difficult
to impose such additional constraints across all implementations;
some deployments would fail to enforce them, thereby harming
interoperability. In short, if it looks like JSON, people will be
tempted to use a JSON parser / serialiser on header fields.
Since a major goal for Structured Headers is to improve
interoperability and simplify implementation, these concerns led to a
format that requires a dedicated parser and serializer.
Additionally, there were widely shared feelings that JSON doesn't
"look right" in HTTP headers.
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B.2. Structured Headers don't "fit" my data.
Structured headers intentionally limits the complexity of data
structures, to assure that it can be processed in a performant manner
with little overhead. This means that work is necessary to fit some
data types into them.
Sometimes, this can be achieved by creating limited substructures in
values, and/or using more than one header. For example, consider:
Example-Thing: name="Widget", cost=89.2, descriptions=(foo bar)
Example-Description: foo; url="https://example.net"; context=123,
bar; url="https://example.org"; context=456
Since the description contains an array of key/value pairs, we use a
List to represent them, with the token for each item in the array
used to identify it in the "descriptions" member of the Example-Thing
dictionary header.
When specifying more than one header, it's important to remember to
describe what a processor's behaviour should be when one of the
headers is missing.
If you need to fit arbitrarily complex data into a header, Structured
Headers is probably a poor fit for your use case.
Appendix C. Implementation Notes
A generic implementation of this specification should expose the top-
level parse (Section 4.2) and serialize (Section 4.1) functions.
They need not be functions; for example, it could be implemented as
an object, with methods for each of the different top-level types.
For interoperability, it's important that generic implementations be
complete and follow the algorithms closely; see Section 1.1. To aid
this, a common test suite is being maintained by the community at
https://github.com/httpwg/structured-header-tests [6].
Implementers should note that dictionaries and parameters are order-
preserving maps. Some headers may not convey meaning in the ordering
of these data types, but it should still be exposed so that
applications which need to use it will have it available.
Likewise, implementations should note that it's important to preserve
the distinction between tokens and strings. While most programming
languages have native types that map to the other types well, it may
be necessary to create a wrapper "token" object or use a parameter on
functions to assure that these types remain separate.
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Appendix D. Changes
_RFC Editor: Please remove this section before publication._
D.1. Since draft-ietf-httpbis-header-structure-14
o Editorial improvements.
o Allow empty dictionary values (#992).
o Change value of omitted parameter value to True (#995).
o Explain more about splitting dictionaries and lists across header
instances (#997).
o Disallow HTAB, replace OWS with spaces (#998).
o Change byte sequence delimiters from "*" to ":" (#991).
o Allow tokens to start with "*" (#991).
o Change Floats to fixed-precision Decimals (#982).
o Round the fractional component of decimal, rather than truncating
it (#982).
o Handle duplicate dictionary and parameter keys by overwriting
their values, rather than failing (#997).
o Allow "." in key (#1027).
o Check first character of key in serialisation (#1037).
o Talk about greasing headers (#1015).
D.2. Since draft-ietf-httpbis-header-structure-13
o Editorial improvements.
o Define "structured header name" and "structured header value"
terms (#908).
o Corrected text about valid characters in strings (#931).
o Removed most instances of the word "textual", as it was redundant
(#915).
o Allowed parameters on Items and Inner Lists (#907).
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o Expand the range of characters in token (#961).
o Disallow OWS before ";" delimiter in parameters (#961).
D.3. Since draft-ietf-httpbis-header-structure-12
o Editorial improvements.
o Reworked float serialisation (#896).
o Don't add a trailing space in inner-list (#904).
D.4. Since draft-ietf-httpbis-header-structure-11
o Allow * in key (#844).
o Constrain floats to six digits of precision (#848).
o Allow dictionary members to have parameters (#842).
D.5. Since draft-ietf-httpbis-header-structure-10
o Update abstract (#799).
o Input and output are now arrays of bytes (#662).
o Implementations need to preserve difference between token and
string (#790).
o Allow empty dictionaries and lists (#781).
o Change parameterized lists to have primary items (#797).
o Allow inner lists in both dictionaries and lists; removes lists of
lists (#816).
o Subsume Parameterised Lists into Lists (#839).
D.6. Since draft-ietf-httpbis-header-structure-09
o Changed Boolean from T/F to 1/0 (#784).
o Parameters are now ordered maps (#765).
o Clamp integers to 15 digits (#737).
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D.7. Since draft-ietf-httpbis-header-structure-08
o Disallow whitespace before items properly (#703).
o Created "key" for use in dictionaries and parameters, rather than
relying on identifier (#702). Identifiers have a separate minimum
supported size.
o Expanded the range of special characters allowed in identifier to
include all of ALPHA, ".", ":", and "%" (#702).
o Use "?" instead of "!" to indicate a Boolean (#719).
o Added "Intentionally Strict Processing" (#684).
o Gave better names for referring specs to use in Parameterised
Lists (#720).
o Added Lists of Lists (#721).
o Rename Identifier to Token (#725).
o Add implementation guidance (#727).
D.8. Since draft-ietf-httpbis-header-structure-07
o Make Dictionaries ordered mappings (#659).
o Changed "binary content" to "byte sequence" to align with Infra
specification (#671).
o Changed "mapping" to "map" for #671.
o Don't fail if byte sequences aren't "=" padded (#658).
o Add Booleans (#683).
o Allow identifiers in items again (#629).
o Disallowed whitespace before items (#703).
o Explain the consequences of splitting a string across multiple
headers (#686).
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D.9. Since draft-ietf-httpbis-header-structure-06
o Add a FAQ.
o Allow non-zero pad bits.
o Explicitly check for integers that violate constraints.
D.10. Since draft-ietf-httpbis-header-structure-05
o Reorganise specification to separate parsing out.
o Allow referencing specs to use ABNF.
o Define serialisation algorithms.
o Refine relationship between ABNF, parsing and serialisation
algorithms.
D.11. Since draft-ietf-httpbis-header-structure-04
o Remove identifiers from item.
o Remove most limits on sizes.
o Refine number parsing.
D.12. Since draft-ietf-httpbis-header-structure-03
o Strengthen language around failure handling.
D.13. Since draft-ietf-httpbis-header-structure-02
o Split Numbers into Integers and Floats.
o Define number parsing.
o Tighten up binary parsing and give it an explicit end delimiter.
o Clarify that mappings are unordered.
o Allow zero-length strings.
o Improve string parsing algorithm.
o Improve limits in algorithms.
o Require parsers to combine header fields before processing.
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o Throw an error on trailing garbage.
D.14. Since draft-ietf-httpbis-header-structure-01
o Replaced with draft-nottingham-structured-headers.
D.15. Since draft-ietf-httpbis-header-structure-00
o Added signed 64bit integer type.
o Drop UTF8, and settle on BCP137 ::EmbeddedUnicodeChar for h1-
unicode-string.
o Change h1_blob delimiter to ":" since "'" is valid t_char
Authors' Addresses
Mark Nottingham
Fastly
Email: mnot@mnot.net
URI: https://www.mnot.net/
Poul-Henning Kamp
The Varnish Cache Project
Email: phk@varnish-cache.org
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