Structured Field Values for HTTP
RFC 8941



Internet Engineering Task Force (IETF)                     M. Nottingham
Request for Comments: 8941                                        Fastly
Category: Standards Track                                      P-H. Kamp
ISSN: 2070-1721                                The Varnish Cache Project
                                                           February 2021

                    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 that wish to use a common
   syntax that is more restrictive than traditional HTTP field values.

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/rfc8941.

Copyright Notice

   Copyright (c) 2021 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
     1.1.  Intentionally Strict Processing
     1.2.  Notational Conventions
   2.  Defining New Structured Fields
   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
   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
     4.2.  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
   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
   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 8.3.1 of [RFC7231],
   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 [RFC7230]
   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 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 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 and OWS rules from [RFC7230].

   When parsing from HTTP fields, implementations MUST have behavior
   that is indistinguishable from following the algorithms.  If there is
   disagreement between the parsing algorithms and ABNF, the specified
   algorithms take precedence.

   For serialization to HTTP fields, the ABNF illustrates their expected
   wire representations, and 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.

   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.  Subsequent
   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.

   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 "sf-" defined in this specification;
   other rules in this specification are not intended to be used in
   field definitions.

   For example, a fictitious Foo-Example header field might be specified
   as:

   |  42.  Foo-Example Header
   |  
   |  The Foo-Example HTTP header field conveys information about how
   |  much Foo the message has.
   |  
   |  Foo-Example is an Item Structured Header [RFC8941].  Its value
   |  MUST be an Integer (Section 3.3.1 of [RFC8941]).  Its ABNF is:
   |  
   |        Foo-Example = sf-integer
   |  
   |  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 [RFC8941]), 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 types for Structured Fields.  The
   ABNF provided represents the on-wire format in HTTP field values.

   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).

   The ABNF for Lists in HTTP fields is:

   sf-list       = list-member *( OWS "," OWS list-member )
   list-member   = sf-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 Tokens could
   look like:

   Example-List: sugar, tea, rum

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

   Note that Lists can have their members split across multiple lines of
   the same header or trailer section, as per Section 3.2.2 of
   [RFC7230]; 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).

   The ABNF for Inner Lists is:

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

   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.

   The ABNF for Parameters is:

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

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

   The ABNF for Dictionaries is:

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

   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

   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 that are
   undefined or unknown, unless the field's specification specifically
   disallows them.

   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), or a Boolean (Section 3.3.6).  It can have
   associated parameters (Section 3.1.2).

   The ABNF for Items is:

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

   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].

   The ABNF for Integers is:

   sf-integer = ["-"] 1*15DIGIT

   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.

   The ABNF for decimals is:

   sf-decimal  = ["-"] 1*12DIGIT "." 1*3DIGIT

   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 header 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.

   The ABNF for Strings is:

   sf-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-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.

   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 [STD63]).

   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:

   sf-token = ( ALPHA / "*" ) *( tchar / ":" / "/" )

   For example:

   Example-Token: foo123/456

   Parsers MUST support Tokens with at least 512 characters.

   Note that Token allows the same 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:

   sf-binary = ":" *(base64) ":"
   base64    = ALPHA / DIGIT / "+" / "/" / "="

   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.

   The ABNF for a Boolean is:

   sf-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-Boolean: ?1

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

4.  Working with Structured Fields in HTTP

   This section defines how to serialize and parse Structured Fields in
   textual HTTP field values and other encodings compatible with them
   (e.g., in HTTP/2 [RFC7540] before compression with HPACK [RFC7541]).

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.  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.

4.2.  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 header value.

   1.  Convert input_bytes into an ASCII string input_string; if
       conversion fails, fail parsing.

   2.  Discard any leading SP characters from input_string.

   3.  If 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
   [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.  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 -- 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 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.  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.  Parse input_number as an integer and let output_number be
            the product of the result and sign.

   9.   Otherwise:

        1.  If the final character of input_number is ".", fail parsing.

        2.  If the number of characters after "." in input_number is
            greater than three, fail parsing.

        3.  Parse input_number as a decimal number and let output_number
            be the product of the result and sign.

   10.  Return output_number.

4.2.5.  Parsing a String

   Given an ASCII string as input_string, return an unquoted String.
   input_string is modified to remove the parsed value.

   1.  Let output_string be an empty string.

   2.  If the first character of input_string is not DQUOTE, fail
       parsing.

   3.  Discard the first character of input_string.

   4.  While input_string is not empty:

       1.  Let char be the result of consuming the first character of
           input_string.

       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 [RFC4648],
   Sections 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 IANA actions.

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>.

   [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",
              DOI 10.1109/IEEESTD.2019.8766229, IEEE 754-2019, July
              2019, <https://ieeexplore.ieee.org/document/8766229>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

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

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/info/rfc7541>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [STD63]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003,
              <https://www.rfc-editor.org/info/std63>.

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.

   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 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.

   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.

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
   Prahran VIC
   Australia

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

   Poul-Henning Kamp
   The Varnish Cache Project

   Email: phk@varnish-cache.org