Internet Engineering Task Force (IETF) M. Nottingham

Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 9651 Cloudflare
Obsoletes: 8941 P-H. Kamp
Category: Standards Track The Varnish Cache Project
ISSN: 2070-1721 September 2024

Structured Field Values for HTTP

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",
"Structured Headers", or "Structured Trailers". It is intended for
use by specifications of new HTTP fields.

This document obsoletes RFC 8941.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9651.

Copyright Notice

Copyright (c) 2024 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Intentionally Strict Processing
1.2. Notational Conventions
2. Defining New Structured Fields
2.1. Example
2.2. Error Handling
2.3. Preserving Extensibility
2.4. Using New Structured Types in Extensions
3. Structured Data Types
3.1. Lists
3.1.1. Inner Lists
3.1.2. Parameters
3.2. Dictionaries
3.3. Items
3.3.1. Integers
3.3.2. Decimals
3.3.3. Strings
3.3.4. Tokens
3.3.5. Byte Sequences
3.3.6. Booleans
3.3.7. Dates
3.3.8. Display Strings
4. Working with Structured Fields in HTTP
4.1. Serializing Structured Fields
4.1.1. Serializing a List
4.1.2. Serializing a Dictionary
4.1.3. Serializing an Item
4.1.4. Serializing an Integer
4.1.5. Serializing a Decimal
4.1.6. Serializing a String
4.1.7. Serializing a Token
4.1.8. Serializing a Byte Sequence
4.1.9. Serializing a Boolean
4.1.10. Serializing a Date
4.1.11. Serializing a Display String
4.2. Parsing Structured Fields
4.2.1. Parsing a List
4.2.2. Parsing a Dictionary
4.2.3. Parsing an Item
4.2.4. Parsing an Integer or Decimal
4.2.5. Parsing a String
4.2.6. Parsing a Token
4.2.7. Parsing a Byte Sequence
4.2.8. Parsing a Boolean
4.2.9. Parsing a Date
4.2.10. Parsing a Display String
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Frequently Asked Questions
A.1. Why Not JSON?
Appendix B. Implementation Notes
Appendix C. ABNF
Appendix D. Changes from RFC 8941
Acknowledgements
Authors' Addresses

1. Introduction

Specifying the syntax of new HTTP header (and trailer) fields is an
onerous task; even with the guidance in Section 16.3.2 of [HTTP],
there are many decisions -- and pitfalls -- for a prospective HTTP
field author.

Once a field is defined, bespoke parsers and serializers often need
to be written, because each field value has a 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 [HTTP]
header and trailer fields.

An HTTP field that is defined as a "Structured Header" or "Structured
Trailer" (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 that model 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 fields 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 entire operation altogether.

It is designed to encourage faithful implementation and 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.

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 the VCHAR, SP, DIGIT, ALPHA, and DQUOTE rules from
[RFC5234] to specify characters and/or their corresponding ASCII
bytes, depending on context. It uses the tchar and OWS rules from
[HTTP] for the same purpose.

This document uses algorithms to specify parsing and serialization
behaviors. When parsing from HTTP fields, implementations MUST have
behavior that is indistinguishable from following the algorithms.

For serialization to HTTP fields, the algorithms define the
recommended way to produce them. Implementations MAY vary from the
specified behavior so long as the output is still correctly handled
by the parsing algorithm described in Section 4.2.

2. Defining New Structured Fields

To specify an HTTP field as a Structured Field, its authors need to:

* Normatively reference this specification. Recipients and
generators of the field need to know that the requirements of this
document are in effect.

* Identify whether the field is a Structured Header (i.e., it can
only be used in the header section -- the common case), a
Structured Trailer (only in the trailer section), or a Structured
Field (both).

* Specify the type of the field value; either List (Section 3.1),
Dictionary (Section 3.2), or Item (Section 3.3).

* Define the semantics of the field value.

* Specify any additional constraints upon the field value, 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", or strings
in lowercase. Likewise, Inner Lists (Section 3.1.1) are only valid
when a field definition explicitly allows them.

Fields that use the Display String type are advised to carefully
specify their allowable Unicode code points; for example, specifying
the use of a profile from [PRECIS].

Field definitions can only use this specification for the entire
field value, not a portion thereof.

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.

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.

2.1. Example

A fictitious Foo-Example header field might be specified as:

| 42. Foo-Example Header Field
|
| The Foo-Example HTTP header field conveys information about how
| much Foo the message has.
|
| Foo-Example is an Item Structured Header Field [RFC9651]. Its
| value MUST be an Integer (Section 3.3.1 of [RFC9651]).
|
| Its value indicates the amount of Foo in the message, and it MUST
| be between 0 and 10, inclusive; other values MUST cause the entire
| header field to be ignored.
|
| The following parameter is defined:
|
| * A parameter whose key is "foourl", and whose value is a String
| (Section 3.3.3 of [RFC9651]), 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/"

2.2. Error Handling

When parsing fails, the entire field is ignored (see Section 4.2).
Field definitions cannot override this 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).

When field-specific constraints are violated, the entire field is
also ignored, unless the field definition defines other handling
requirements. For example, if a header field is defined as an Item
and required to be an Integer, but a String is received, it should be
ignored unless that field's definition explicitly specifies
otherwise.

2.3. Preserving Extensibility

Structured Fields are designed to be extensible because experience
has shown that, even when it is not foreseen, it is often necessary
to modify and add to the allowable syntax and semantics of a field in
a controlled fashion.

Both Items and Inner Lists allow Parameters as an extensibility
mechanism; this means that their 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.

Field specifications are required to be either an Item, List, or
Dictionary to preserve extensibility. Fields that erroneously
defined as another type (e.g., Integer) are assumed to be Items
(i.e., they allow Parameters).

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 keys be ignored. Subsequent
specifications can then add additional keys, 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.

2.4. Using New Structured Types in Extensions

Because a field definition needs to reference a specific RFC for
Structured Fields, the types available for use in its value are
limited to those defined in that RFC. For example, a field whose
definition references this document can have a value that uses the
Date type (Section 3.3.7), whereas a field whose definition
references RFC 8941 cannot because it will be treated as invalid (and
therefore discarded) by implementations of that specification.

This limitation also applies to future extensions to a field; for
example, a field that is defined with a reference to RFC 8941 cannot
use the Date type because some recipients might still be using a
parser based on RFC 8941 to process it.

However, this document is designed to be backward compatible with RFC
8941; a parser that implements the requirements here can also parse
valid Structured Fields whose definitions reference RFC 8941.

Upgrading a Structured Fields implementation to support a newer
revision of the specification (such as this document) brings the
possibility that some field values that were invalid according to the
earlier RFC might become valid when processed.

For example, a field instance might contain a syntactically valid
Date (Section 3.3.7), even though that field's definition does not
accommodate Dates. An implementation based on RFC 8941 would fail
parsing such a field instance because it is not defined in that
specification. If that implementation were upgraded to this
specification, parsing would now succeed. In some cases, the
resulting Date value will be rejected by field-specific logic, but
values in fields that are otherwise ignored (such as extension
parameters) might not be detected, and the field might subsequently
be accepted and processed.

3. Structured Data Types

This section provides an overview of the abstract types that
Structured Fields use and gives a brief description and examples of
how each of those types are serialized into textual HTTP fields.
Section 4 specifies the details of how they are parsed from and
serialized into textual HTTP fields.

In summary:

* There are three top-level types that an HTTP field can be defined
as: Lists, Dictionaries, and Items.

* Lists and Dictionaries are containers; their members can be Items
or Inner Lists (which are themselves arrays of Items).

* Both Items and Inner Lists can be Parameterized 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 Parameterized (Section 3.1.2).

An empty List is denoted by not serializing the field at all. This
implies that fields defined as Lists have a default empty value.

When serialized as a textual HTTP field, each member is separated by
a comma and optional whitespace. For example, a field whose value is
defined as a List of Tokens could look like:

Example-List: sugar, tea, rum

Note that Lists can have their members split across multiple lines of
the same header or trailer section, as per Section 5.3 of [HTTP]; for
example, the following are equivalent:

Example-List: sugar, tea, rum

and

Example-List: sugar, tea
Example-List: rum

However, individual members of a List cannot be safely split between
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).

When serialized in a textual HTTP field, Inner Lists are denoted by
surrounding parenthesis, and their values are delimited by one or
more spaces. A field whose value is defined as a List of Inner Lists
of Strings could look like:

Example-List: ("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-List: ("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-value 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 the Parameters they occur within, and
the values are bare items (i.e., they themselves cannot be
parameterized; see Section 3.3).

Implementations MUST provide access to Parameters both by index and
by key. Specifications MAY use either means of accessing them.

Note that parameters are ordered, and parameter keys cannot contain
uppercase letters.

When serialized in a textual HTTP field, a Parameter is separated
from its Item or Inner List and other Parameters by a semicolon. For
example:

Example-List: abc;a=1;b=2; cde_456, (ghi;jk=4 l);q="9";r=w

Parameters whose value is Boolean (see Section 3.3.6) true MUST omit
that value when serialized. For example, the "a" parameter here is
true, while the "b" parameter is false:

Example-Integer: 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.

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 order of individual parameters, as
well as their values' types as required.

3.2. Dictionaries

Dictionaries are ordered maps of key-value pairs, where the keys 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 keys are unique in the
scope of the Dictionary they occur within.

Implementations MUST provide access to Dictionaries both by index and
by key. Specifications MAY use either means of accessing the
members.

As with Lists, an empty Dictionary is represented by omitting the
entire field. This implies that fields defined as Dictionaries have
a default empty value.

Typically, a field specification will define the semantics of
Dictionaries by specifying the allowed type(s) for individual members
by their keys, as well as whether their presence is required or
optional. Recipients MUST ignore members whose keys are undefined or
unknown, unless the field's specification specifically disallows
them.

When serialized as a textual HTTP field, members are ordered as
serialized and separated by a comma with optional whitespace. Member
keys cannot contain uppercase characters. Keys and values are
separated by "=" (without whitespace). For example:

Example-Dict: en="Applepie", da=:w4ZibGV0w6ZydGU=:

Note that in this example, the final "=" is due to the inclusion of a
Byte Sequence; see Section 3.3.5.

Members whose value is Boolean (see Section 3.3.6) true MUST omit
that value when serialized. For example, here both "b" and "c" are
true:

Example-Dict: 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-Dict: rating=1.5, feelings=(joy sadness)

A Dictionary with a mix of Items and Inner Lists, some with
parameters:

Example-Dict: a=(1 2), b=3, c=4;aa=bb, d=(5 6);valid

Note that Dictionaries can have their members split across multiple
lines of the same header or trailer section; for example, the
following are equivalent:

Example-Dict: foo=1, bar=2

and

Example-Dict: foo=1
Example-Dict: 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 key/value
pairs and keys with at least 64 characters. Field specifications can
constrain the order of individual Dictionary members, as well as
their values' types as required.

3.3. Items

An Item can be an Integer (Section 3.3.1), a Decimal (Section 3.3.2),
a String (Section 3.3.3), a Token (Section 3.3.4), a Byte Sequence
(Section 3.3.5), a Boolean (Section 3.3.6), or a Date
(Section 3.3.7). It can have associated parameters (Section 3.1.2).

For example, a header field that is defined to be an Item that is an
Integer might look like:

Example-Integer: 5

or with parameters:

Example-Integer: 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].

For example:

Example-Integer: 42

Integers larger than 15 digits can be supported in a variety of ways;
for example, by using a String (Section 3.3.3), a Byte Sequence
(Section 3.3.5), or a parameter on an Integer that acts as a scaling
factor.

While it is possible to serialize Integers with leading zeros (e.g.,
"0002", "-01") and signed zero ("-0"), these distinctions may not be
preserved by implementations.

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.

For example, a header whose value is defined as a Decimal could look
like:

Example-Decimal: 4.5

While it is possible to serialize Decimals with leading zeros (e.g.,
"0002.5", "-01.334"), trailing zeros (e.g., "5.230", "-0.40"), and
signed zero (e.g., "-0.0"), these distinctions may not be preserved
by implementations.

Note that the serialization 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 field definition to occur before serialization.

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.

Non-ASCII characters are not directly supported in Strings because
they cause a number of interoperability issues, and -- with few
exceptions -- field values do not require them.

When it is necessary for a field value to convey non-ASCII content, a
Display String (Section 3.3.8) can be specified.

When serialized in a textual HTTP field, Strings are delimited with
double quotes, using a backslash ("\") to escape double quotes and
backslashes. For example:

Example-String: "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.

Parsers MUST support Strings (after any decoding) with at least 1024
characters.

3.3.4. Tokens

Tokens are short textual words that begin with an alphabetic
character or "*", followed by zero to many token characters, which
are the same as those allowed by the "token" ABNF rule defined in
[HTTP] plus the ":" and "/" characters.

For example:

Example-Token: foo123/456

Parsers MUST support Tokens with at least 512 characters.

Note that Tokens are defined largely for compatibility with the data
model of existing HTTP fields and may require additional steps to use
in some implementations. As a result, new fields are encouraged to
use Strings.

3.3.5. Byte Sequences

Byte Sequences can be conveyed in Structured Fields.

When serialized in a textual HTTP field, a Byte Sequence is delimited
with colons and encoded using base64 ([RFC4648], Section 4). For
example:

Example-ByteSequence: :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.

When serialized in a textual HTTP field, a Boolean is indicated with
a leading "?" character followed by a "1" for a true value or "0" for
false. For example:

Example-Boolean: ?1

Note that in Dictionary (Section 3.2) and Parameter (Section 3.1.2)
values, Boolean true is indicated by omitting the value.

3.3.7. Dates

Date values can be conveyed in Structured Fields.

Dates have a data model that is similar to Integers, representing a
(possibly negative) delta in seconds from 1970-01-01T00:00:00Z,
excluding leap seconds. Accordingly, their serialization in textual
HTTP fields is similar to that of Integers, distinguished from them
with a leading "@".

For example:

Example-Date: @1659578233

Parsers MUST support Dates whose values include all days in years 1
to 9999 (i.e., -62,135,596,800 to 253,402,214,400 delta seconds from
1970-01-01T00:00:00Z).

3.3.8. Display Strings

Display Strings are similar to Strings, in that they consist of zero
or more characters, but they allow Unicode scalar values (i.e., all
Unicode code points except for surrogates), unlike Strings.

Display Strings are intended for use in cases where a value is
displayed to end users and therefore may need to carry non-ASCII
content. It is NOT RECOMMENDED that they be used in situations where
a String (Section 3.3.3) or Token (Section 3.3.4) would be adequate
because Unicode has processing considerations (e.g., normalization)
and security considerations (e.g., homograph attacks) that make it
more difficult to handle correctly.

Note that Display Strings do not indicate the language used in the
value; that can be done separately if necessary (e.g., with a
parameter).

In textual HTTP fields, Display Strings are represented in a manner
similar to Strings, except that non-ASCII characters are percent-
encoded; there is a leading "%" to distinguish them from Strings.

For example:

Example-DisplayString: %"This is intended for display to %c3%bcsers."

See Section 6 for additional security considerations when handling
Display Strings.

4. Working with Structured Fields in HTTP

This section defines how to serialize and parse the abstract types
defined by Section 3 into textual HTTP field values and other
encodings compatible with them (e.g., in HTTP/2 [HTTP/2] before
compression with HPACK [HPACK]).

4.1. Serializing Structured Fields

Given a structure defined in this specification, return an ASCII
string suitable for use in an 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.

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 an 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 an 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.

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_key and a param_value), return an ASCII string suitable for use
in an HTTP field value.

1. Let output be an empty string.

2. For each param_key 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_key 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
an 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.

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_key and a tuple value of (member_value, parameters)), return
an ASCII string suitable for use in an HTTP field value.

1. Let output be an empty string.

2. For each member_key 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_key to output.

2. If member_value is Boolean true:

1. Append the result of running Serializing Parameters
(Section 4.1.1.2) with parameters to output.

3. 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.

4. If more members remain in input_dictionary:

1. Append "," to output.

2. Append a single SP to output.

3. 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 an HTTP field 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.

4.1.3.1. Serializing a Bare Item

Given an Item as input_item, return an ASCII string suitable for use
in an 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 Byte Sequence, return the result of running
Serializing a Byte Sequence (Section 4.1.8) with input_item.

6. If input_item is a Boolean, return the result of running
Serializing a Boolean (Section 4.1.9) with input_item.

7. If input_item is a Date, return the result of running Serializing
a Date (Section 4.1.10) with input_item.

8. If input_item is a Display String, return the result of running
Serializing a Display String (Section 4.1.11) with input_item.

9. Otherwise, fail serialization.

4.1.4. Serializing an Integer

Given an Integer as input_integer, return an ASCII string suitable
for use in an 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.

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 an 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 an HTTP field value.

1. Convert input_string into a sequence of ASCII characters; if
conversion fails, fail serialization.

2. If input_string contains characters in the range %x00-1f or %x7f-
ff (i.e., not in VCHAR or SP), fail serialization.

3. Let output be the string DQUOTE.

4. For each character char in input_string:

1. If char is "\" or DQUOTE:

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 an 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 an 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.

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 an 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.

4.1.10. Serializing a Date

Given a Date as input_date, return an ASCII string suitable for use
in an HTTP field value.

1. Let output be "@".

2. Append to output the result of running Serializing an Integer
with input_date (Section 4.1.4).

3. Return output.

4.1.11. Serializing a Display String

Given a sequence of Unicode code points as input_sequence, return an
ASCII string suitable for use in an HTTP field value.

1. If input_sequence is not a sequence of Unicode code points, fail
serialization.

2. Let byte_array be the result of applying UTF-8 encoding
(Section 3 of [UTF8]) to input_sequence. If encoding fails, fail
serialization.

3. Let encoded_string be a string containing "%" followed by DQUOTE.

4. For each byte in byte_array:

1. If byte is %x25 ("%"), %x22 (DQUOTE), or in the ranges
%x00-1f or %x7f-ff:

1. Append "%" to encoded_string.

2. Let encoded_byte be the result of applying base16
encoding (Section 8 of [RFC4648]) to byte, with any
alphabetic characters converted to lowercase.

3. Append encoded_byte to encoded_string.

2. Otherwise, decode byte as an ASCII character and append the
result to encoded_string.

5. Append DQUOTE to encoded_string.

6. Return encoded_string.

Note that [UTF8] prohibits the encoding of code points between U+D800
and U+DFFF (surrogates); if they occur in input_sequence,
serialization will fail.

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 as input_bytes that represent 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 field 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 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 field lines in
the same section (header or trailer) that case-insensitively match
the field name into one comma-separated field-value, as per
Section 5.2 of [HTTP]; 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 field
instances. The parsing algorithms for both types allow tab
characters, since these might be used to combine field lines by some
implementations.

Strings split across multiple field lines will have unpredictable
results, because one or more commas (with optional whitespace) 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, even when they are both
under the control of the same party.

Tokens, Integers, Decimals, and Byte Sequences cannot be split across
multiple field lines because the inserted commas will cause parsing
to fail.

Parsers MAY fail when processing a field value spread across multiple
field lines, when one of those lines does not parse as that field.
For example, a parsing handling an Example-String field that's
defined as an sf-string is allowed to fail when processing this field
section:

Example-String: "foo
Example-String: bar"

If parsing fails, either the entire field value MUST be ignored
(i.e., treated as if the field were not present in the section), or
alternatively the complete HTTP message MUST be treated as malformed.
This is intentionally strict to improve interoperability and safety,
and field specifications that use Structured Fields 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 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 OWS 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 OWS 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.

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 the result of running Parsing
Parameters (Section 4.2.3.2) with input_string.

3. Let member be the tuple (value, parameters).

4. If dictionary already contains a key this_key (comparing
character for character), overwrite its value with member.

5. Otherwise, append key this_key with value member to
dictionary.

6. Discard any leading OWS characters from input_string.

7. If input_string is empty, return dictionary.

8. Consume the first character of input_string; if it is not
",", fail parsing.

9. Discard any leading OWS characters from input_string.

10. If input_string is empty, there is a trailing comma; fail
parsing.

3. No structured data has been found; return dictionary (which is
empty).

Note that when duplicate Dictionary keys are encountered, all but the
last instance are ignored.

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 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 an ALPHA or "*", return
the result of running Parsing a Token (Section 4.2.6) with
input_string.

4. If the first character of input_string is ":", return the result
of running Parsing a Byte Sequence (Section 4.2.7) with
input_string.

5. If the first character of input_string is "?", return the result
of running Parsing a Boolean (Section 4.2.8) with input_string.

6. If the first character of input_string is "@", return the result
of running Parsing a Date (Section 4.2.9) with input_string.

7. If the first character of input_string is "%", return the result
of running Parsing a Display String (Section 4.2.10) with
input_string.

8. 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 the ";" character from the beginning of input_string.

3. Discard any leading SP characters from input_string.

4. Let param_key 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. If parameters already contains a key param_key (comparing
character for character), overwrite its value with
param_value.

8. Otherwise, append key param_key with value param_value to
parameters.

3. Return parameters.

Note that when duplicate parameter keys are encountered, all but the
last instance are ignored.

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.

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. Let output_number be an Integer that is the result of
parsing input_number as an integer.

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. Let output_number be a Decimal that is the result of parsing
input_number as a decimal number.

10. Let output_number be the product of output_number and sign.

11. 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.

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-ff (i.e., it 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.

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 base64-decoding [RFC4648]
b64_content, synthesizing padding if necessary (note the
requirements about recipient behavior below). If base64 decoding
fails, parsing fails.

8. Return binary_content.

Because some implementations of base64 do not allow rejection of
encoded data that is not properly "=" padded (see [RFC4648],
Section 3.2), parsers SHOULD NOT fail when "=" padding 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 non-zero pad bits are present, unless
they cannot be configured to do so.

This specification does not relax the requirements in Sections 3.1
and 3.3 of [RFC4648]; 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.

4.2.9. Parsing a Date

Given an ASCII string as input_string, return a Date. 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. Let output_date be the result of running Parsing an Integer or
Decimal (Section 4.2.4) with input_string.

4. If output_date is a Decimal, fail parsing.

5. Return output_date.

4.2.10. Parsing a Display String

Given an ASCII string as input_string, return a sequence of Unicode
code points. input_string is modified to remove the parsed value.

1. If the first two characters of input_string are not "%" followed
by DQUOTE, fail parsing.

2. Discard the first two characters of input_string.

3. Let byte_array be an empty byte array.

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 in the range %x00-1f or %x7f-ff (i.e., it is not
in VCHAR or SP), fail parsing.

3. If char is "%":

1. Let octet_hex be the result of consuming two characters
from input_string. If there are not two characters, fail
parsing.

2. If octet_hex contains characters outside the range
%x30-39 or %x61-66 (i.e., it is not in 0-9 or lowercase
a-f), fail parsing.

3. Let octet be the result of hex decoding octet_hex
(Section 8 of [RFC4648]).

4. Append octet to byte_array.

4. If char is DQUOTE:

1. Let unicode_sequence be the result of decoding byte_array
as a UTF-8 string (Section 3 of [UTF8]). Fail parsing if
decoding fails.

2. Return unicode_sequence.

5. Otherwise, if char is not "%" or DQUOTE:

1. Let byte be the result of applying ASCII encoding to
char.

2. Append byte to byte_array.

5. Reached the end of input_string without finding a closing DQUOTE;
fail parsing.

5. IANA Considerations

IANA has added the following note to the "Hypertext Transfer Protocol
(HTTP) Field Name Registry":

| The "Structured Type" column indicates the type of the field (per
| RFC 9651), if any, and may be "Dictionary", "List", or "Item".
|
| Note that field names beginning with characters other than ALPHA
| or "*" will not be able to be represented as a Structured Fields
| Token and therefore may be incompatible with being mapped into
| field values that refer to it.

A new column, "Structured Type", has been added to the registry.

The indicated Structured Type for each existing registry entry listed
in Table 1 has also been added.

+==========================================+=================+
| Field Name | Structured Type |
+==========================================+=================+
| Accept-CH | List |
+------------------------------------------+-----------------+
| Cache-Status | List |
+------------------------------------------+-----------------+
| CDN-Cache-Control | Dictionary |
+------------------------------------------+-----------------+
| Cross-Origin-Embedder-Policy | Item |
+------------------------------------------+-----------------+
| Cross-Origin-Embedder-Policy-Report-Only | Item |
+------------------------------------------+-----------------+
| Cross-Origin-Opener-Policy | Item |
+------------------------------------------+-----------------+
| Cross-Origin-Opener-Policy-Report-Only | Item |
+------------------------------------------+-----------------+
| Origin-Agent-Cluster | Item |
+------------------------------------------+-----------------+
| Priority | Dictionary |
+------------------------------------------+-----------------+
| Proxy-Status | List |
+------------------------------------------+-----------------+

Table 1: Existing Fields

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.

The Display String type can convey any possible Unicode code point
without sanitization; for example, they might contain unassigned code
points, control points (including NUL), or noncharacters. Therefore,
applications consuming Display Strings need to consider strategies
such as filtering or escaping untrusted content before displaying it.
See [PRECIS] and [UNICODE-SECURITY].

7. References

7.1. Normative References

[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.

[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>.

[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>.

[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.

7.2. Informative References

[HPACK] 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>.

[HTTP/2] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.

[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229,
ISBN 978-1-5044-5924-2, July 2019,
<https://ieeexplore.ieee.org/document/8766229>.

[PRECIS] Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
Internationalized Strings in Application Protocols",
RFC 8264, DOI 10.17487/RFC8264, October 2017,
<https://www.rfc-editor.org/info/rfc8264>.

[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>.

[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.

[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>.

[UNICODE-SECURITY]
Davis, M. and M. Suignard, "Unicode Security
Considerations", Unicode Technical Report #36, 19
September 2014,
<https://www.unicode.org/reports/tr36/tr36-15.html>.
Latest version available at
<https://www.unicode.org/reports/tr36/>.

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 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.

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-field-tests>.

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 it will
be available to applications that need to use it.

Likewise, implementations should note that it's important to preserve
the distinction between Tokens and Strings. While most programming
languages have built-in 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.

Implementations are allowed to limit the size of different
structures, subject to the minimums defined for each type. When a
structure exceeds an implementation limit, that structure fails
parsing or serialization.

Appendix C. ABNF

This section uses the Augmented Backus-Naur Form (ABNF) notation
[RFC5234] to illustrate the expected syntax of Structured Fields.
However, it cannot be used to validate their syntax because it does
not capture all requirements.

This section is non-normative. If there is disagreement between the
parsing algorithms and ABNF, the specified algorithms take
precedence.

sf-list = list-member *( OWS "," OWS list-member )
list-member = sf-item / inner-list

inner-list = "(" *SP [ sf-item *( 1*SP sf-item ) *SP ] ")"
parameters

parameters = *( ";" *SP parameter )
parameter = param-key [ "=" param-value ]
param-key = key
key = ( lcalpha / "*" )
*( lcalpha / DIGIT / "_" / "-" / "." / "*" )
lcalpha = %x61-7A ; a-z
param-value = bare-item

sf-dictionary = dict-member *( OWS "," OWS dict-member )
dict-member = member-key ( parameters / ( "=" member-value ))
member-key = key
member-value = sf-item / inner-list

sf-item = bare-item parameters
bare-item = sf-integer / sf-decimal / sf-string / sf-token
/ sf-binary / sf-boolean / sf-date / sf-displaystring

sf-integer = ["-"] 1*15DIGIT
sf-decimal = ["-"] 1*12DIGIT "." 1*3DIGIT
sf-string = DQUOTE *( unescaped / "%" / bs-escaped ) DQUOTE
sf-token = ( ALPHA / "*" ) *( tchar / ":" / "/" )
sf-binary = ":" base64 ":"
sf-boolean = "?" ( "0" / "1" )
sf-date = "@" sf-integer
sf-displaystring = "%" DQUOTE *( unescaped / "\" / pct-encoded )
DQUOTE

base64 = *( ALPHA / DIGIT / "+" / "/" ) *"="

unescaped = %x20-21 / %x23-24 / %x26-5B / %x5D-7E
bs-escaped = "\" ( DQUOTE / "\" )

pct-encoded = "%" lc-hexdig lc-hexdig
lc-hexdig = DIGIT / %x61-66 ; 0-9, a-f

Appendix D. Changes from RFC 8941

This revision of the "Structured Field Values for HTTP" specification
has made the following changes:

* Added the Date Structured Type. (Section 3.3.7)

* Stopped encouraging use of ABNF in definitions of new Structured
Fields. (Section 2)

* Moved ABNF to an informative appendix. (Appendix C)

* Added a "Structured Type" column to the "Hypertext Transfer
Protocol (HTTP) Field Name Registry". (Section 5)

* Refined parse failure handling. (Section 4.2)

* Added the Display String Structured Type. (Section 3.3.8)

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
Cloudflare
Prahran VIC
Australia
Email: [email protected]
URI: https://www.mnot.net/

Poul-Henning Kamp
The Varnish Cache Project
Email: [email protected]