Structured Field Values for HTTP
draft-ietf-httpbis-header-structure-17
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8941.
|
|
|---|---|---|---|
| Authors | Mark Nottingham , Poul-Henning Kamp | ||
| Last updated | 2020-04-12 (Latest revision 2020-03-15) | ||
| Replaces | draft-kamp-httpbis-structure, draft-ietf-httpbis-jfv, draft-nottingham-structured-headers | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-18)
by David Schinazi
Ready w/nits
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Tommy Pauly | ||
| Shepherd write-up | Show Last changed 2020-04-08 | ||
| IESG | IESG state | Became RFC 8941 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Barry Leiba | ||
| Send notices to | Tommy Pauly <tpauly@apple.com> |
draft-ietf-httpbis-header-structure-17
HTTP M. Nottingham
Internet-Draft Fastly
Intended status: Standards Track P-H. Kamp
Expires: September 16, 2020 The Varnish Cache Project
March 15, 2020
Structured Field Values for HTTP
draft-ietf-httpbis-header-structure-17
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 and trailer fields, known as "Structured Fields", or
"Structured Headers". It is intended for use by specifications of
new HTTP 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 September 16, 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 Fields . . . . . . . . . . . . . . . 5
3. Structured Data Types . . . . . . . . . . . . . . . . . . . . 8
3.1. Lists . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Inner Lists . . . . . . . . . . . . . . . . . . . . . 9
3.1.2. Parameters . . . . . . . . . . . . . . . . . . . . . 10
3.2. Dictionaries . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Items . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.1. Integers . . . . . . . . . . . . . . . . . . . . . . 13
3.3.2. Decimals . . . . . . . . . . . . . . . . . . . . . . 13
3.3.3. Strings . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.4. Tokens . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.5. Byte Sequences . . . . . . . . . . . . . . . . . . . 14
3.3.6. Booleans . . . . . . . . . . . . . . . . . . . . . . 15
4. Working With Structured Fields in HTTP . . . . . . . . . . . 15
4.1. Serializing Structured Fields . . . . . . . . . . . . . . 15
4.1.1. Serializing a List . . . . . . . . . . . . . . . . . 16
4.1.2. Serializing a Dictionary . . . . . . . . . . . . . . 18
4.1.3. Serializing an Item . . . . . . . . . . . . . . . . . 18
4.1.4. Serializing an Integer . . . . . . . . . . . . . . . 19
4.1.5. Serializing a Decimal . . . . . . . . . . . . . . . . 20
4.1.6. Serializing a String . . . . . . . . . . . . . . . . 20
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4.1.7. Serializing a Token . . . . . . . . . . . . . . . . . 21
4.1.8. Serializing a Byte Sequence . . . . . . . . . . . . . 21
4.1.9. Serializing a Boolean . . . . . . . . . . . . . . . . 22
4.2. Parsing Structured Fields . . . . . . . . . . . . . . . . 22
4.2.1. Parsing a List . . . . . . . . . . . . . . . . . . . 24
4.2.2. Parsing a Dictionary . . . . . . . . . . . . . . . . 25
4.2.3. Parsing an Item . . . . . . . . . . . . . . . . . . . 26
4.2.4. Parsing an Integer or Decimal . . . . . . . . . . . . 28
4.2.5. Parsing a String . . . . . . . . . . . . . . . . . . 30
4.2.6. Parsing a Token . . . . . . . . . . . . . . . . . . . 30
4.2.7. Parsing a Byte Sequence . . . . . . . . . . . . . . . 31
4.2.8. Parsing a Boolean . . . . . . . . . . . . . . . . . . 32
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
6. Security Considerations . . . . . . . . . . . . . . . . . . . 32
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
7.1. Normative References . . . . . . . . . . . . . . . . . . 32
7.2. Informative References . . . . . . . . . . . . . . . . . 33
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Appendix A. Frequently Asked Questions . . . . . . . . . . . . . 34
A.1. Why not JSON? . . . . . . . . . . . . . . . . . . . . . . 34
Appendix B. Implementation Notes . . . . . . . . . . . . . . . . 35
Appendix C. Changes . . . . . . . . . . . . . . . . . . . . . . 35
C.1. Since draft-ietf-httpbis-header-structure-15 . . . . . . 36
C.2. Since draft-ietf-httpbis-header-structure-14 . . . . . . 36
C.3. Since draft-ietf-httpbis-header-structure-13 . . . . . . 37
C.4. Since draft-ietf-httpbis-header-structure-12 . . . . . . 37
C.5. Since draft-ietf-httpbis-header-structure-11 . . . . . . 37
C.6. Since draft-ietf-httpbis-header-structure-10 . . . . . . 37
C.7. Since draft-ietf-httpbis-header-structure-09 . . . . . . 38
C.8. Since draft-ietf-httpbis-header-structure-08 . . . . . . 38
C.9. Since draft-ietf-httpbis-header-structure-07 . . . . . . 38
C.10. Since draft-ietf-httpbis-header-structure-06 . . . . . . 39
C.11. Since draft-ietf-httpbis-header-structure-05 . . . . . . 39
C.12. Since draft-ietf-httpbis-header-structure-04 . . . . . . 39
C.13. Since draft-ietf-httpbis-header-structure-03 . . . . . . 39
C.14. Since draft-ietf-httpbis-header-structure-02 . . . . . . 39
C.15. Since draft-ietf-httpbis-header-structure-01 . . . . . . 40
C.16. Since draft-ietf-httpbis-header-structure-00 . . . . . . 40
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
Specifying the syntax of new HTTP header (and trailer) 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
field author.
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Once a field is defined, bespoke parsers and serializers often need
to be written, because each field value 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 field values to address these problems. In
particular, it defines a generic, abstract model for them, along with
a concrete serialization for expressing that model in HTTP [RFC7230]
header and trailer fields.
A HTTP field that is defined as a "Structured Header" (or "Structured
Trailer", respectively; if the field can be either, it is a
"Structured Field") uses the types defined in this specification to
define its syntax and basic handling rules, thereby simplifying both
its definition by specification writers and handling by
implementations.
Additionally, future versions of HTTP can define alternative
serializations of the abstract model of these structures, allowing
fields 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 fields; the mechanisms described herein are only
intended to be used with those that explicitly opt into them.
Section 2 describes how to specify a Structured Field.
Section 3 defines a number of abstract data types that can be used in
Structured Fields.
Those abstract types can be serialized into and parsed from HTTP
field values using the algorithms described in Section 4.
1.1. Intentionally Strict Processing
This specification intentionally defines strict parsing and
serialization behaviors 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
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corrected by the producer early, and avoids both interoperability and
security issues that might otherwise result.
Note that as a result of this strictness, if a 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 field value 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 serialization
behaviors, and the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234] to illustrate expected syntax in HTTP header fields. In
doing so, it uses the VCHAR, SP, DIGIT, ALPHA and DQUOTE rules from
[RFC5234]. It also includes the tchar rule from [RFC7230].
When parsing from HTTP fields, implementations MUST follow the
algorithms, but MAY vary in implementation so as the behaviors are
indistinguishable from specified behavior. 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 serialization to HTTP 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 behavior so long as the
output still matches the ABNF.
2. Defining New Structured Fields
To specify a HTTP field as a Structured Field, its authors needs to:
o Reference this specification. Recipients and generators of the
field need to know that the requirements of this document are in
effect.
o Identify whether the field is a Structured Header (i.e., it can
only be used in the header section - the common case), a
Structured Field (only in the trailer section), or a Structured
Field (both).
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o Specify the type of the field value; either List (Section 3.1),
Dictionary (Section 3.2), 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.
Typically, this means that a field definition will specify the top-
level type - List, Dictionary 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 field definition explicitly allows them.
When parsing fails, the entire field is ignored (see Section 4.2); in
most situations, violating field-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, the field will by default be
ignored. If the field requires different error handling, this should
be explicitly specified.
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. To preserve forward
compatibility, field specifications are discouraged from defining the
presence of an unrecognized Parameter as an error condition.
To further assure that this extensibility is available in the future,
and to encourage consumers to use a complete parser implementation, a
field definition can specify that "grease" Parameters be added by
senders. A specification could stipulate that all Parameters that
fit a defined pattern are reserved for this use and then encourage
them to be sent on some portion of requests. This helps to
discourage recipients from writing a parser that does not account for
Parameters.
Specifications that use Dictionaries can also allow for forward
compatibility by requiring that the presence of - as well as value
and type associated with - unknown members be ignored. Later
specifications can then add additional members, specifying
constraints on them as appropriate.
An extension to a structured field can then require that an entire
field value be ignored by a recipient that understands the extension
if constraints on the value it defines are not met.
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A 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, field definitions can only use this
specification for the entire field value, not a portion thereof.
This specification defines minimums for the length or number of
various structures supported by implementations. It does not specify
maximum sizes in most cases, but authors should be aware that HTTP
implementations do impose various limits on the size of individual
fields, the total number of fields, and/or the size of the entire
header or trailer section.
Specifications can refer to a field name as a "structured header
name", "structured trailer name" or "structured field name" as
appropriate. Likewise, they can refer its field value as a
"structured header value", "structured trailer value" or "structured
field value" as necessary. Field definitions are encouraged to use
the 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:
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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 field 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 URL
for the message. See below for processing requirements.
"foourl" contains a URI-reference (Section 4.1 of [RFC3986]). If
its value is not a valid URI-reference, the entire header field
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 Fields. The ABNF provided represents the on-wire
format in HTTP field values.
In summary:
o There are three top-level types that a HTTP field 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).
o Both Items and Inner Lists can be parameterized with key/value
pairs.
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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 Parameterized (Section 3.1.2).
The ABNF for Lists in HTTP fields is:
sh-list = list-member *( *SP "," *SP list-member )
list-member = sh-item / inner-list
Each member is separated by a comma and optional whitespace. For
example, a field whose value is defined as a List of Strings could
look like:
Example-StrListHeader: "foo", "bar", "It was the best of times."
An empty List is denoted by not serializing the field at all.
Note that Lists can have their members split across multiple lines
inside a header or trailer section, as per Section 3.2.2 of
[RFC7230]; for example, the following are equivalent:
Example-Hdr: foo, bar
and
Example-Hdr: foo
Example-Hdr: bar
However, individual members of a List cannot be safely split between
across lines; see Section 4.2 for details.
Parsers MUST support Lists containing at least 1024 members. Field
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 Parameterized
(Section 3.1.2).
The ABNF for Inner Lists is:
inner-list = "(" *SP [ sh-item *( 1*SP sh-item ) *SP ] ")"
parameters
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Inner Lists are denoted by surrounding parenthesis, and have their
values delimited by a single space. A 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.
Field 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 the Parameters they occur within, and the
values are bare items (i.e., they themselves cannot be parameterized;
see Section 3.3).
The ABNF for Parameters is:
parameters = *( ";" *SP parameter )
parameter = param-name [ "=" param-value ]
param-name = key
key = ( lcalpha / "*" )
*( lcalpha / DIGIT / "_" / "-" / "." / "*" )
lcalpha = %x61-7A ; a-z
param-value = bare-item
A parameter is separated from its Item or Inner List and other
parameters by a semicolon. 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
serialized. For example:
Example-IntHeader: 1; a; b=?0
Note that this requirement is only on serialization; parsers are
still required to correctly handle the true value when it appears in
a parameter.
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Parsers MUST support at least 256 parameters on an Item or Inner
List, and support parameter keys with at least 64 characters. Field
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 Parameterized (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 is:
sh-dictionary = dict-member *( *SP "," *SP dict-member )
dict-member = member-name [ "=" member-value ]
member-name = key
member-value = sh-item / inner-list
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
serialized. For example, here both "b" and "c" are true:
Example-DictHeader: a=?0, b, c; foo=bar
Note that this requirement is only on serialization; parsers are
still required to correctly handle the true Boolean 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)
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
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As with lists, an empty Dictionary is represented by omitting the
entire field.
Typically, a 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 field's specification specifically disallows them.
Note that dictionaries can have their members split across multiple
lines inside a header or trailer section; for example, the following
are equivalent:
Example-Hdr: foo=1, bar=2
and
Example-Hdr: foo=1
Example-Hdr: bar=2
However, individual members of a Dictionary cannot be safely split
between lines; 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 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 is:
sh-item = bare-item parameters
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
or with Parameters:
Example-IntItemHeader: 5; foo=bar
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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 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 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
Note that the serialisation algorithm (Section 4.1.5) rounds input
with more than three digits of precision in the fractional component.
If an alternative rounding strategy is desired, this should be
specified by the header definition to occur before serialisation.
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 is:
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sh-string = DQUOTE *(chr) DQUOTE
chr = unescaped / escaped
unescaped = %x20-21 / %x23-5B / %x5D-7E
escaped = "\" ( DQUOTE / "\" )
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 - field
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) can be specified, along with a
character encoding (preferably [UTF-8]).
Parsers MUST support Strings (after any decoding) 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 field value serialization.
The ABNF for Tokens is:
sh-token = ( ALPHA / "*" ) *( tchar / ":" / "/" )
Parsers MUST support Tokens with at least 512 characters.
Note that 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 Fields.
The ABNF for a Byte Sequence is:
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sh-binary = ":" *(base64) ":"
base64 = ALPHA / DIGIT / "+" / "/" / "="
A Byte Sequence is delimited with colons and encoded using base64
([RFC4648], Section 4). For example:
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 Fields.
The ABNF for a Boolean is:
sh-boolean = "?" boolean
boolean = "0" / "1"
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 Fields in HTTP
This section defines how to serialize and parse Structured Fields in
field values, and protocols compatible with them (e.g., in HTTP/2
[RFC7540] before HPACK [RFC7541] is applied).
4.1. Serializing Structured Fields
Given a structure defined in this specification, return an ASCII
string suitable for use in a HTTP field 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 List, let output_string be the result of
running Serializing a List (Section 4.1.1) with the structure.
3. Else if the structure is a Dictionary, let output_string be the
result of running Serializing a Dictionary (Section 4.1.2) with
the structure.
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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 serialization.
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 field 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
Serializing 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 "," to output.
2. Append a single SP to output.
3. Return output.
4.1.1.1. Serializing 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 field 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.
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3. Append ")" to output.
4. Append the result of running Serializing Parameters
(Section 4.1.1.2) with list_parameters to output.
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 field value.
1. Let output be an empty string.
2. For each param_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 field value.
1. Convert input_key into a sequence of ASCII characters; if
conversion fails, fail serialization.
2. If input_key contains characters not in lcalpha, DIGIT, "_", "-",
".", or "*" fail serialization.
3. If the first character of input_key is not lcalpha or "*", fail
serialization.
4. Let output be an empty string.
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5. Append input_key to output.
6. Return output.
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 field 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 Boolean true:
1. Append the result of running Serializing Parameters
(Section 4.1.1.2) with parameters to output.
4. Otherwise:
1. Append "=" to output.
2. If member_value is an array, append the result of running
Serializing an Inner List (Section 4.1.1.1) with
(member_value, parameters) to output.
3. Otherwise, append the result of running Serializing an Item
(Section 4.1.3) with (member_value, parameters) to output.
5. If more members remain in input_dictionary:
1. Append "," to output.
2. Append a single SP to output.
6. Return output.
4.1.3. Serializing an Item
Given an Item as bare_item and Parameters as item_parameters, return
an ASCII string suitable for use in a HTTP field value.
1. Let output be an empty string.
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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.
4.1.3.1. Serializing a Bare Item
Given an Item as input_item, return an ASCII string suitable for use
in a HTTP field 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 serialization.
4.1.4. Serializing an Integer
Given an Integer as input_integer, return an ASCII string suitable
for use in a HTTP field 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
serialization.
2. Let output be an empty string.
3. If input_integer is less than (but not equal to) 0, append "-" to
output.
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4. Append input_integer's numeric value represented in base 10 using
only decimal digits to output.
5. Return output.
4.1.5. Serializing a Decimal
Given a decimal number as input_decimal, return an ASCII string
suitable for use in a HTTP field value.
1. If input_decimal is not a decimal number, fail serialization.
2. If input_decimal has more than three significant digits to the
right of the decimal point, round it to three decimal places,
rounding the final digit to the nearest value, or to the even
value if it is equidistant.
3. If input_decimal has more than 12 significant digits to the left
of the decimal point after rounding, fail serialization.
4. Let output be an empty string.
5. If input_decimal is less than (but not equal to) 0, append "-"
to output.
6. Append input_decimal's integer component represented in base 10
(using only decimal digits) to output; if it is zero, append
"0".
7. Append "." to output.
8. If input_decimal's fractional component is zero, append "0" to
output.
9. Otherwise, append the significant digits of input_decimal's
fractional component represented in base 10 (using only decimal
digits) to output.
10. Return output.
4.1.6. Serializing a String
Given a String as input_string, return an ASCII string suitable for
use in a HTTP field value.
1. Convert input_string into a sequence of ASCII characters; if
conversion fails, fail serialization.
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2. If input_string contains characters in the range %x00-1f or %x7f
(i.e., not in VCHAR or SP), fail serialization.
3. Let output be an empty string.
4. Append DQUOTE to output.
5. For each character char in input_string:
1. If char is "\" or DQUOTE:
1. Append "\" to output.
2. Append char to output.
6. Append DQUOTE to output.
7. Return output.
4.1.7. Serializing a Token
Given a Token as input_token, return an ASCII string suitable for use
in a HTTP field value.
1. Convert input_token into a sequence of ASCII characters; if
conversion fails, fail serialization.
2. If the first character of input_token is not ALPHA or "*", or the
remaining portion contains a character not in tchar, ":" or "/",
fail serialization.
3. Let output be an empty string.
4. Append input_token to output.
5. 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 field value.
1. If input_bytes is not a sequence of bytes, fail serialization.
2. Let output be an empty string.
3. Append ":" to output.
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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.
4.1.9. Serializing a Boolean
Given a Boolean as input_boolean, return an ASCII string suitable for
use in a HTTP field value.
1. If input_boolean is not a boolean, fail serialization.
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 Structured Fields
When a receiving implementation parses HTTP fields that are known to
be Structured Fields, 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
field's field-value (which is empty if that field is not present),
and field_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.
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3. If field_type is "list", let output be the result of running
Parsing a List (Section 4.2.1) with input_string.
4. If field_type is "dictionary", let output be the result of
running Parsing a Dictionary (Section 4.2.2) with input_string.
5. If field_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.
8. Otherwise, return output.
When generating input_bytes, parsers MUST combine all lines in the
same section (header or trailer) that case-insensitively match the
field name into one comma-separated field-value, as per [RFC7230],
Section 3.2.2; this assures that the entire field value is processed
correctly.
For Lists and Dictionaries, this has the effect of correctly
concatenating all of the field's lines, as long as individual members
of the top-level data structure are not split across multiple header
instances.
Strings split across multiple field lines 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 field lines because the inserted commas will cause parsing
to fail.
If parsing fails - including when calling another algorithm - the
entire field value MUST be ignored (i.e., treated as if the field
were not present in the section). 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 field; for example, an intermediary is not
required to strip a failing header field from a message before
forwarding it.
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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.
3. If input_string is empty, return members.
4. Consume the first character of input_string; if it is not
",", 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.
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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.
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.
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3. Otherwise:
1. Let value be Boolean true.
2. Let parameters be the result of running Parsing
Parameters Section 4.2.3.2 with input_string.
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.
7. Consume the first character of input_string; if it is not
",", 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.
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1. If the first character of input_string is a "-" or a DIGIT,
return the result of running Parsing an Integer or Decimal
(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.
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.
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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 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 one of lcalpha,
DIGIT, "_", "-", ".", 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.
4.2.4. Parsing an Integer or Decimal
Given an ASCII string as input_string, return an Integer or Decimal.
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.
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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.
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.
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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.
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.
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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.
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 behavior 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.
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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.
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 document has no actions for IANA.
6. Security Considerations
The size of most types defined by Structured Fields is not limited;
as a result, extremely large fields could be an attack vector (e.g.,
for resource consumption). Most HTTP implementations limit the sizes
of individual fields as well as the overall header or trailer section
size to mitigate such attacks.
It is possible for parties with the ability to inject new HTTP fields
to change the meaning of a Structured Field. 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>.
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[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>.
[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>.
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[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
[5] https://github.com/httpwg/wiki/wiki/Structured-Headers
[6] https://github.com/httpwg/structured-header-tests
Appendix A. Frequently Asked Questions
A.1. Why not JSON?
Earlier proposals for Structured Fields 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 field definition will find a need to
violate it.
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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 / serializer on field values.
Since a major goal for Structured Fields 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 fields.
Appendix B. Implementation Notes
A generic implementation of this specification should expose the top-
level serialize (Section 4.1) and parse (Section 4.2) 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 fields 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.
The serialization algorithm is defined in a way that it is not
strictly limited to the data types defined in Section 3 in every
case. For example, Decimals are designed to take broader input and
round to allowed values.
Appendix C. Changes
_RFC Editor: Please remove this section before publication._
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C.1. Since draft-ietf-httpbis-header-structure-15
o Editorial improvements.
o Use HTTP field terminology more consistently, in line with recent
changes to HTTP-core.
o String length requirements apply to decoded strings (#1051).
o Correctly round decimals in serialisation (#1043).
o Clarify input to serialisation algorithms (#1055).
o Omitted True dictionary value can have parameters (#1083).
o Keys can now start with '*' (#1068).
C.2. 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).
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C.3. 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).
o Expand the range of characters in token (#961).
o Disallow OWS before ";" delimiter in parameters (#961).
C.4. 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).
C.5. 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).
C.6. 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).
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o Allow inner lists in both dictionaries and lists; removes lists of
lists (#816).
o Subsume Parameterised Lists into Lists (#839).
C.7. 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).
C.8. 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).
C.9. 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).
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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).
C.10. 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.
C.11. 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.
C.12. Since draft-ietf-httpbis-header-structure-04
o Remove identifiers from item.
o Remove most limits on sizes.
o Refine number parsing.
C.13. Since draft-ietf-httpbis-header-structure-03
o Strengthen language around failure handling.
C.14. 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.
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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.
o Throw an error on trailing garbage.
C.15. Since draft-ietf-httpbis-header-structure-01
o Replaced with draft-nottingham-structured-headers.
C.16. 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
Acknowledgements
Many thanks to Matthew Kerwin for his detailed feedback and careful
consideration during the development of this specification.
Thanks also to Ian Clelland, Roy Fielding, Anne van Kesteren, Kazuho
Oku, Evert Pot, Julian Reschke, Martin Thomson, Mike West, and
Jeffrey Yasskin for their contributions.
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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