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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft-ietf-httpbis-header-structure-19" category="std"> number="8941" submissionType="IETF" category="std" consensus="true" obsoletes="" updates="" xml:lang="en" tocInclude="true" sortRefs="true" symRefs="true" tocDepth="3" version="3">
  <!-- xml2rfc v2v3 conversion 2.46.0 -->
  <front>
    <title>Structured Field Values for HTTP</title>
    <seriesInfo name="RFC" value="8941"/>
    <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
      <organization>Fastly</organization>
      <address>
	<postal>
	  <city>Prahran</city>
	  <region>VIC</region>
	  <country>Australia</country>
	</postal>
        <email>mnot@mnot.net</email>
        <uri>https://www.mnot.net/</uri>
      </address>
    </author>
    <author initials="P-H." surname="Kamp" fullname="Poul-Henning Kamp">
      <organization>The Varnish Cache Project</organization>
      <address>
        <email>phk@varnish-cache.org</email>
      </address>
    </author>
    <date /> month="February" year="2021"/>
    <area>Applications and Real-Time</area>
    <workgroup>HTTP</workgroup>
    <keyword>Internet-Draft</keyword>
    <abstract>
      <t>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”, "Structured Fields", "Structured
      Headers", or “Structured Trailers”. "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.</t>
    </abstract>

    <note title="Note to Readers">

<t><spanx style="emph">RFC EDITOR: please remove this section before publication</spanx></t>

<t>Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at <eref target="https://lists.w3.org/Archives/Public/ietf-http-wg/">https://lists.w3.org/Archives/Public/ietf-http-wg/</eref>.</t>

<t>Working Group information can be found at <eref target="https://httpwg.github.io/">https://httpwg.github.io/</eref>; source code and issues list for this draft can be found at <eref target="https://github.com/httpwg/http-extensions/labels/header-structure">https://github.com/httpwg/http-extensions/labels/header-structure</eref>.</t>

<t>Tests for implementations are collected at <eref target="https://github.com/httpwg/structured-field-tests">https://github.com/httpwg/structured-field-tests</eref>.</t>

<t>Implementations are tracked at <eref target="https://github.com/httpwg/wiki/wiki/Structured-Headers">https://github.com/httpwg/wiki/wiki/Structured-Headers</eref>.</t>

    </note>
  </front>
  <middle>
    <section anchor="introduction" title="Introduction"> anchor="introduction">
      <name>Introduction</name>
      <t>Specifying the syntax of new HTTP header (and trailer) fields is an
      onerous task; even with the guidance in Section 8.3.1 of <xref target="RFC7231"/>, target="RFC7231" section="8.3.1"/>, there are many decisions – -- and
      pitfalls – -- for a prospective HTTP field author.</t>
      <t>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.</t>
      <t>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 <xref target="RFC7230"/> header and trailer fields.</t>

<t>A
      <t>An HTTP field that is defined as a “Structured Header” "Structured Header" or “Structured Trailer” "Structured
      Trailer" (if the field can be either, it is a “Structured Field”) "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.</t>
      <t>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.</t>
      <t>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.</t>
      <t><xref target="specify"/> describes how to specify a
      Structured Field.</t>
      <t><xref target="types"/> defines a number of abstract
      data types that can be used in Structured Fields.</t>
      <t>Those abstract types can be serialized into and parsed from HTTP
      field values using the algorithms described in <xref target="text"/>.</t>
      <section anchor="strict" title="Intentionally anchor="strict">
        <name>Intentionally Strict Processing"> Processing</name>
        <t>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.</t>
        <t>It is designed to encourage faithful implementation and therefore
	good interoperability. Therefore, an implementation that tried to be
	helpful by being more tolerant of input would make interoperability
	worse, since that would create pressure on other implementations to
	implement similar (but likely subtly different) workarounds.</t>
        <t>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, early and avoids both interoperability and
	security issues that might otherwise result.</t>
        <t>Note that as a result of this strictness, if a field is appended to
	by multiple parties (e.g., intermediaries, intermediaries or different components in
	the sender), an error in one party’s party's value is likely to cause the
	entire field value to fail parsing.</t>
      </section>
      <section anchor="notational-conventions" title="Notational Conventions">

<t>The anchor="notational-conventions">
        <name>Notational Conventions</name>
        <t>
    The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
    "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>",
    "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and “OPTIONAL” "<bcp14>OPTIONAL</bcp14>" in this document are
    to be interpreted as
    described in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.</t> here.
        </t>
        <t>This document uses algorithms to specify parsing and serialization behaviors,
	behaviors and the Augmented Backus-Naur Form (ABNF) notation of <xref target="RFC5234"/> to illustrate expected syntax in
	HTTP header fields. In doing so, it uses the VCHAR, SP, DIGIT, ALPHA ALPHA,
	and DQUOTE rules from <xref target="RFC5234"/>. It
	also includes the tchar and OWS rules from <xref target="RFC7230"/>.</t>
        <t>When parsing from HTTP fields, implementations MUST <bcp14>MUST</bcp14> have behavior
	that is indistinguishable from following the algorithms. If there is
	disagreement between the parsing algorithms and ABNF, the specified
	algorithms take precedence.</t>
        <t>For serialization to HTTP fields, the ABNF illustrates their
	expected wire representations, and the algorithms define the
	recommended way to produce them. Implementations MAY <bcp14>MAY</bcp14> vary from the
	specified behavior so long as the output is still correctly handled by
	the parsing algorithm.</t> algorithm described in <xref target="text-parse"/>.</t>
      </section>
    </section>
    <section anchor="specify" title="Defining anchor="specify">
      <name>Defining New Structured Fields"> Fields</name>
      <t>To specify a an HTTP field as a Structured Field, its authors needs need to:</t>

<t><list style="symbols">
  <t>Normatively
      <ul>
        <li>Normatively reference this specification. Recipients and
	generators of the field need to know that the requirements of this
	document are in effect.</t>
  <t>Identify effect.</li>
        <li>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).</t>
  <t>Specify
	(both).</li>
        <li>Specify the type of the field value; either List (<xref target="list"/>), Dictionary (<xref target="dictionary"/>), or Item (<xref target="item"/>).</t>
  <t>Define target="item"/>).</li>
        <li>Define the semantics of the field value.</t>
  <t>Specify value.</li>
        <li>Specify any additional constraints upon the field value, as well
	as the consequences when those constraints are violated.</t>
</list></t> violated.</li>
      </ul>
      <t>Typically, this means that a field definition will specify the
      top-level type – -- List, Dictionary Dictionary, or Item – -- and then define its
      allowable types, 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”, "Q", or strings in
      lowercase. Likewise, Inner Lists (<xref target="inner-list"/>) are only valid when a field definition explicitly
      allows them.</t>
      <t>When parsing fails, the entire field is ignored (see <xref target="text-parse"/>); 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.</t>
      <t>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 parameter as an error condition.</t>
      <t>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 "grease" parameters be added by
      senders. A specification could stipulate that all Parameters 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.</t>
      <t>Specifications that use Dictionaries can also allow for forward
      compatibility by requiring that the presence of – -- as well as value and
      type associated with – -- unknown members be ignored. Later Subsequent specifications
      can then add additional members, specifying constraints on them as
      appropriate.</t>
      <t>An extension to a structured field 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.</t>
      <t>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 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.</t>
      <t>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.</t>
      <t>Specifications can refer to a field name as a “structured "structured header name”, “structured
      name", "structured trailer name” name", or “structured "structured field name” name" as
      appropriate. Likewise, they can refer its field value as a “structured "structured
      header value”, “structured value", "structured trailer value” value", or “structured "structured field value” value" as
      necessary.
Field definitions are encouraged to use the ABNF rules
      beginning with “sf-“ "sf-" defined in this specification; other rules in this
      specification are not intended for their use.</t> to be used in field definitions.</t>
      <t>For example, a fictitious Foo-Example header field might be specified
      as:</t>

<figure><artwork type="example"><![CDATA[
--8<--
42.

<blockquote>
<t>42. Foo-Example Header

The Header</t>

<t>The Foo-Example HTTP header field conveys information about how
much Foo the message has.

Foo-Example has.</t>

<t>Foo-Example is a an Item Structured Header [RFCxxxx]. [RFC8941]. Its value MUST be
an Integer (Section Y.Y 3.3.1 of [RFCxxxx]). [RFC8941]). Its ABNF is: is:</t>

<artwork>
   Foo-Example = sf-integer

Its
</artwork>

<t>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 ignored.</t>

<t>The following parameters are defined:
* A Parameter parameter is defined:</t>

<ul>
<li>A parameter whose name key is "foourl", and whose value is a String
  (Section Y.Y 3.3.3 of [RFCxxxx]), [RFC8941]), conveying the Foo URL
  for the message. See below for processing requirements.

"foourl" requirements.</li>
</ul>

<t>"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: used.</t>

<t>For example:</t>

<artwork>
   Foo-Example: 2; foourl="https://foo.example.com/"
-->8--
]]></artwork></figure>
</artwork>
</blockquote>

    </section>
    <section anchor="types" title="Structured anchor="types">
      <name>Structured Data Types"> Types</name>
      <t>This section defines the abstract types for Structured Fields. The
      ABNF provided represents the on-wire format in HTTP field values.</t>
      <t>In summary:</t>

<t><list style="symbols">
  <t>There
      <ul>
        <li>There are three top-level types that a an HTTP field can be defined
	as: Lists, Dictionaries, and Items.</t>
  <t>Lists Items.</li>
        <li>Lists and Dictionaries are containers; their members can be Items
	or Inner Lists (which are themselves arrays of Items).</t>
  <t>Both Items).</li>
        <li>Both Items and Inner Lists can be parameterized Parameterized with key/value pairs.</t>
</list></t> pairs.</li>
      </ul>
      <section anchor="list" title="Lists"> anchor="list">
        <name>Lists</name>
        <t>Lists are arrays of zero or more members, each of which can be an
	Item (<xref target="item"/>) or an Inner List (<xref target="inner-list"/>), both of which can be
	Parameterized (<xref target="param"/>).</t>
        <t>The ABNF for Lists in HTTP fields is:</t>

<figure><artwork type="abnf"><![CDATA[
        <sourcecode type="abnf">
sf-list       = list-member *( OWS "," OWS list-member )
list-member   = sf-item / inner-list
]]></artwork></figure>
</sourcecode>
        <t>Each member is separated by a comma and optional whitespace. For
	example, a field whose value is defined as a List of Strings Tokens could
	look like:</t>

<figure><artwork type="example"><![CDATA[
Example-StrList: "foo", "bar", "It was the best of times."
]]></artwork></figure>
        <sourcecode type="http-message">
Example-List: sugar, tea, rum
	</sourcecode>
        <t>An empty List is denoted by not serializing the field at all. This
	implies that fields defined as Lists have a default empty value.</t>
        <t>Note that Lists can have their members split across multiple lines inside a
	of the same header or trailer section, as per Section 3.2.2 of <xref target="RFC7230"/>; target="RFC7230" section="3.2.2"/>; for example, the following are
	equivalent:</t>

<figure><artwork type="example"><![CDATA[
Example-Hdr: foo, bar
]]></artwork></figure>
        <sourcecode type="http-message">
Example-List: sugar, tea, rum
	</sourcecode>
        <t>and</t>

<figure><artwork type="example"><![CDATA[
Example-Hdr: foo
Example-Hdr: bar
]]></artwork></figure>
        <sourcecode type="http-message">
Example-List: sugar, tea
Example-List: rum
	</sourcecode>
        <t>However, individual members of a List cannot be safely split
	between across lines; see <xref target="text-parse"/>
	for details.</t>
        <t>Parsers MUST <bcp14>MUST</bcp14> support Lists containing at least 1024 members. Field
	specifications can constrain the types and cardinality of individual
	List values as they require.</t>
        <section anchor="inner-list" title="Inner Lists"> anchor="inner-list">
          <name>Inner Lists</name>
          <t>An Inner List is an array of zero or more Items (<xref target="item"/>). Both the individual Items and the
	  Inner List itself can be Parameterized (<xref target="param"/>).</t>
          <t>The ABNF for Inner Lists is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
inner-list    = "(" *SP [ sf-item *( 1*SP sf-item ) *SP ] ")"
                parameters
]]></artwork></figure>
</sourcecode>
          <t>Inner Lists are denoted by surrounding parenthesis, and have
	  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:</t>

<figure><artwork type="example"><![CDATA[
Example-StrListList:
          <sourcecode type="http-message">
Example-List: ("foo" "bar"), ("baz"), ("bat" "one"), ()
]]></artwork></figure>
</sourcecode>
          <t>Note that the last member in this example is an empty Inner List.</t>
          <t>A header field whose value is defined as a List of Inner Lists
	  with Parameters at both levels could look like:</t>

<figure><artwork type="example"><![CDATA[
Example-ListListParam:
          <sourcecode type="http-message">
Example-List: ("foo"; a=1;b=2);lvl=5, ("bar" "baz");lvl=1
]]></artwork></figure>
</sourcecode>
          <t>Parsers MUST <bcp14>MUST</bcp14> support Inner Lists containing at least 256
	  members. Field specifications can constrain the types and
	  cardinality of individual Inner List members as they require.</t>
        </section>
        <section anchor="param" title="Parameters"> anchor="param">
          <name>Parameters</name>
          <t>Parameters are an ordered map of key-value pairs that are
	  associated with an Item (<xref target="item"/>) or
	  Inner List (<xref target="inner-list"/>).
          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 <xref target="item"/>).</t>
<t>Implementations <bcp14>MUST</bcp14> provide access to Parameters both by index and
   by key.  Specifications <bcp14>MAY</bcp14> use either means of accessing them.</t>
          <t>The ABNF for Parameters is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
parameters    = *( ";" *SP parameter )
parameter     = param-name param-key [ "=" param-value ]
param-name
param-key     = key
key           = ( lcalpha / "*" )
                *( lcalpha / DIGIT / "_" / "-" / "." / "*" )
lcalpha       = %x61-7A ; a-z
param-value   = bare-item
]]></artwork></figure>
</sourcecode>
          <t>Note that Parameters parameters are ordered as serialized, and Parameter parameter
	  keys cannot contain uppercase letters. A parameter is separated from
	  its Item or Inner List and other parameters by a semicolon. For
	  example:</t>

<figure><artwork type="example"><![CDATA[
Example-ParamList:
          <sourcecode type="http-message">
Example-List: abc;a=1;b=2; cde_456, (ghi;jk=4 l);q="9";r=w
]]></artwork></figure>
</sourcecode>
          <t>Parameters whose value is Boolean (see <xref target="boolean"/>) true MUST <bcp14>MUST</bcp14> omit that value when serialized. For
	  example, the “a” "a" parameter here is true, while the “b” "b" parameter is
	  false:</t>

<figure><artwork type="example"><![CDATA[
Example-Int:
          <sourcecode type="http-message">
Example-Integer: 1; a; b=?0
]]></artwork></figure>
</sourcecode>
          <t>Note that this requirement is only on serialization; parsers are
	  still required to correctly handle the true value when it appears in
	  a parameter.</t>
          <t>Parsers MUST <bcp14>MUST</bcp14> 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, parameters, as
	  well as their values’ values' types as required.</t>
        </section>
      </section>
      <section anchor="dictionary" title="Dictionaries"> anchor="dictionary">
        <name>Dictionaries</name>
        <t>Dictionaries are ordered maps of name-value key-value pairs, where the names keys
	are short textual strings and the values are Items (<xref target="item"/>) or arrays of Items, both of which
	can be Parameterized (<xref target="param"/>). There
	can be zero or more members, and their names keys are unique in the scope
	of the Dictionary they occur within.</t>
        <t>Implementations MUST <bcp14>MUST</bcp14> provide access to Dictionaries both by index
	and by name. key. Specifications MAY <bcp14>MAY</bcp14> use either means of accessing the
	members.</t>
        <t>The ABNF for Dictionaries is:</t>

<figure><artwork type="abnf"><![CDATA[
        <sourcecode type="abnf">
sf-dictionary  = dict-member *( OWS "," OWS dict-member )
dict-member    = member-name [ member-key ( parameters / ( "=" member-value ]
member-name ))
member-key     = key
member-value   = sf-item / inner-list
]]></artwork></figure>
</sourcecode>
        <t>Members are ordered as serialized, serialized and separated by a comma with
	optional whitespace. Member names keys cannot contain uppercase
	characters. Names Keys and values are separated by “=” "=" (without
	whitespace). For example:</t>

<figure><artwork type="example"><![CDATA[
        <sourcecode type="http-message">
Example-Dict: en="Applepie", da=:w4ZibGV0w6ZydGU=:
]]></artwork></figure>
</sourcecode>
        <t>Note that in this example, the final “=” "=" is due to the inclusion of
	a Byte Sequence; see <xref target="binary"/>.</t>
        <t>Members whose value is Boolean (see <xref target="boolean"/>) true MUST <bcp14>MUST</bcp14> omit that value when serialized. For
	example, here both “b” "b" and “c” "c" are true:</t>

<figure><artwork type="example"><![CDATA[
        <sourcecode type="http-message">
Example-Dict: a=?0, b, c; foo=bar
]]></artwork></figure>
</sourcecode>
        <t>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.</t>
        <t>A Dictionary with a member whose value is an Inner List of Tokens:</t>

<figure><artwork type="example"><![CDATA[
Example-DictList:
        <sourcecode type="http-message">
Example-Dict: rating=1.5, feelings=(joy sadness)
]]></artwork></figure>
</sourcecode>
        <t>A Dictionary with a mix of Items and Inner Lists, some with Parameters:</t>

<figure><artwork type="example"><![CDATA[
Example-MixDict: parameters:</t>
        <sourcecode type="http-message">
Example-Dict: a=(1 2), b=3, c=4;aa=bb, d=(5 6);valid
]]></artwork></figure>
</sourcecode>
        <t>As with lists, Lists, an empty Dictionary is represented by omitting the
	entire field. This implies that fields defined as Dictionaries have a
	default empty value.</t>
        <t>Typically, a field specification will define the semantics of
	Dictionaries by specifying the allowed type(s) for individual members
	by their names, keys, as well as whether their presence is required or
	optional. Recipients MUST <bcp14>MUST</bcp14> ignore names members whose keys that are undefined or unknown,
	unless the field’s field's specification specifically disallows them.</t>
        <t>Note that Dictionaries can have their members split across multiple
	lines inside a of the same header or trailer section; for example, the following
	are equivalent:</t>

<figure><artwork type="example"><![CDATA[
Example-Hdr:
        <sourcecode type="http-message">
Example-Dict: foo=1, bar=2
]]></artwork></figure>
</sourcecode>
        <t>and</t>

<figure><artwork type="example"><![CDATA[
Example-Hdr:
        <sourcecode type="http-message">
Example-Dict: foo=1
Example-Hdr:
Example-Dict: bar=2
]]></artwork></figure>
</sourcecode>
        <t>However, individual members of a Dictionary cannot be safely split
	between lines; see <xref target="text-parse"/> for
	details.</t>
        <t>Parsers MUST <bcp14>MUST</bcp14> support Dictionaries containing at least 1024 name/value pairs,
	key/value pairs and names keys with at least 64 characters. Field
	specifications can constrain the order of individual Dictionary
	members, as well as their values’ values' types as required.</t>
      </section>
      <section anchor="item" title="Items"> anchor="item">
        <name>Items</name>
        <t>An Item can be a an Integer (<xref target="integer"/>), a Decimal (<xref target="decimal"/>), a String (<xref target="string"/>), a Token (<xref target="token"/>),
	a Byte Sequence (<xref target="binary"/>), or a Boolean
	(<xref target="boolean"/>). It can have associated Parameters
	parameters (<xref target="param"/>).</t>
        <t>The ABNF for Items is:</t>

<figure><artwork type="abnf"><![CDATA[
        <sourcecode type="abnf">
sf-item   = bare-item parameters
bare-item = sf-integer / sf-decimal / sf-string / sf-token
            / sf-binary / sf-boolean
]]></artwork></figure>
</sourcecode>
        <t>For example, a header field that is defined to be an Item that is
	an Integer might look like:</t>

<figure><artwork type="example"><![CDATA[
Example-IntItemHeader:
        <sourcecode type="http-message">
Example-Integer: 5
]]></artwork></figure>
</sourcecode>
        <t>or with Parameters:</t>

<figure><artwork type="example"><![CDATA[
Example-IntItem: parameters:</t>
        <sourcecode type="http-message">
Example-Integer: 5; foo=bar
]]></artwork></figure>
</sourcecode>
        <section anchor="integer" title="Integers"> anchor="integer">
          <name>Integers</name>
          <t>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 (<xref target="IEEE754"/>).</t> <xref target="IEEE754"/>.</t>
          <t>The ABNF for Integers is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
sf-integer = ["-"] 1*15DIGIT
]]></artwork></figure>
</sourcecode>
          <t>For example:</t>

<figure><artwork type="example"><![CDATA[
          <sourcecode type="http-message">
Example-Integer: 42
]]></artwork></figure>
</sourcecode>
          <t>Integers larger than 15 digits can be supported in a variety of
	  ways; for example, by using a String (<xref target="string"/>), a Byte Sequence (<xref target="binary"/>), or a parameter on an Integer that acts as a
	  scaling factor.</t>
          <t>While it is possible to serialise serialize Integers with leading zeros
	  (e.g., “0002”, “-01”) "0002", "-01") and signed zero (“-0”), ("-0"), these distinctions may
	  not be preserved by implementations.</t>
          <t>Note that commas in Integers are used in this section’s section's prose
	  only for readability; they are not valid in the wire format.</t>
        </section>
        <section anchor="decimal" title="Decimals"> anchor="decimal">
          <name>Decimals</name>
          <t>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.</t>
          <t>The ABNF for decimals is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
sf-decimal  = ["-"] 1*12DIGIT "." 1*3DIGIT
]]></artwork></figure>
</sourcecode>
          <t>For example, a header whose value is defined as a Decimal could
	  look like:</t>

<figure><artwork type="example"><![CDATA[
          <sourcecode type="http-message">
Example-Decimal: 4.5
]]></artwork></figure>
</sourcecode>
          <t>While it is possible to serialise serialize Decimals with leading zeros
	  (e.g., “0002.5”, “-01.334”), "0002.5", "-01.334"), trailing zeros (e.g., “5.230”, “-0.40”), "5.230",
	  "-0.40"), and signed zero (e.g., “-0.0”), "-0.0"), these distinctions may not
	  be preserved by implementations.</t>
          <t>Note that the serialisation serialization algorithm (<xref target="ser-decimal"/>) rounds input with more than three digits of
	  precision in the fractional component. If an alternative rounding
	  strategy is desired, this should be specified by the header
	  definition to occur before serialisation.</t> serialization.</t>
        </section>
        <section anchor="string" title="Strings"> anchor="string">
          <name>Strings</name>
          <t>Strings are zero or more printable ASCII <xref target="RFC0020"/> characters (i.e., the range %x20 to %x7E). Note
	  that this excludes tabs, newlines, carriage returns, etc.</t>
          <t>The ABNF for Strings is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
sf-string = DQUOTE *chr DQUOTE
chr       = unescaped / escaped
unescaped = %x20-21 / %x23-5B / %x5D-7E
escaped   = "\" ( DQUOTE / "\" )
]]></artwork></figure>
</sourcecode>
          <t>Strings are delimited with double quotes, using a backslash (“\”) ("\")
	  to escape double quotes and backslashes. For example:</t>

<figure><artwork type="example"><![CDATA[
          <sourcecode type="http-message">
Example-String: "hello world"
]]></artwork></figure>
</sourcecode>
          <t>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 "\" <bcp14>MUST</bcp14> cause parsing to fail.</t>
          <t>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.</t>
          <t>When it is necessary for a field value to convey non-ASCII
	  content, a Byte Sequence (<xref target="binary"/>)
	  can be specified, along with a character encoding (preferably UTF-8 <xref target="UTF-8"/>).</t> target="STD63"/>).</t>
          <t>Parsers MUST <bcp14>MUST</bcp14> support Strings (after any decoding) with at least
	  1024 characters.</t>
        </section>
        <section anchor="token" title="Tokens"> anchor="token">
          <name>Tokens</name>
          <t>Tokens are short textual words; their abstract model is identical
	  to their expression in the HTTP field value serialization.</t>
          <t>The ABNF for Tokens is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
sf-token = ( ALPHA / "*" ) *( tchar / ":" / "/" )
]]></artwork></figure>
</sourcecode>
          <t>For example:</t>

<figure><artwork type="example"><![CDATA[
          <sourcecode type="http-message">
Example-Token: foo123/456
]]></artwork></figure>
</sourcecode>
          <t>Parsers MUST <bcp14>MUST</bcp14> support Tokens with at least 512 characters.</t>
          <t>Note that Token allows the same characters as the “token” "token" ABNF
	  rule defined in <xref target="RFC7230"/>, with the
	  exceptions that the first character is required to be either ALPHA
	  or “*”, "*", and “:” ":" and “/” "/" are also allowed in subsequent
	  characters.</t>
        </section>
        <section anchor="binary" title="Byte Sequences"> anchor="binary">
          <name>Byte Sequences</name>
          <t>Byte Sequences can be conveyed in Structured Fields.</t>
          <t>The ABNF for a Byte Sequence is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
sf-binary = ":" *(base64) ":"
base64    = ALPHA / DIGIT / "+" / "/" / "="
]]></artwork></figure>
</sourcecode>
          <t>A Byte Sequence is delimited with colons and encoded using base64
	  (<xref target="RFC4648"/>, Section 4). target="RFC4648" sectionFormat="comma" section="4"/>). For
	  example:</t>

<figure><artwork type="example"><![CDATA[
Example-Binary:
          <sourcecode type="http-message">
Example-ByteSequence: :cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg==:
]]></artwork></figure>
</sourcecode>
          <t>Parsers MUST <bcp14>MUST</bcp14> support Byte Sequences with at least 16384 octets
	  after decoding.</t>
        </section>
        <section anchor="boolean" title="Booleans"> anchor="boolean">
          <name>Booleans</name>
          <t>Boolean values can be conveyed in Structured Fields.</t>
          <t>The ABNF for a Boolean is:</t>

<figure><artwork type="abnf"><![CDATA[
          <sourcecode type="abnf">
sf-boolean = "?" boolean
boolean    = "0" / "1"
]]></artwork></figure>
</sourcecode>
          <t>A Boolean is indicated with a leading “?” "?" character followed by a “1”
	  "1" for a true value or “0” "0" for false. For example:</t>

<figure><artwork type="example"><![CDATA[
Example-Bool:
          <sourcecode type="http-message">
Example-Boolean: ?1
]]></artwork></figure>
</sourcecode>
          <t>Note that in Dictionary (<xref target="dictionary"/>) and Parameter (<xref target="param"/>) values, Boolean true is indicated by omitting
	  the value.</t>
        </section>
      </section>
    </section>
    <section anchor="text" title="Working With anchor="text">
      <name>Working with Structured Fields in HTTP"> HTTP</name>
      <t>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 <xref target="RFC7540"/> before
      compression with HPACK <xref target="RFC7541"/>).</t>
      <section anchor="text-serialize" title="Serializing anchor="text-serialize">
        <name>Serializing Structured Fields"> Fields</name>
        <t>Given a structure defined in this specification, return an ASCII
	string suitable for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>If
        <ol>
          <li>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).</t>
  <t>If field-value).</li>
          <li>If the structure is a List, let output_string be the result of
	  running Serializing a List (<xref target="ser-list"/>) with the structure.</t>
  <t>Else structure.</li>
          <li>Else, if the structure is a Dictionary, let output_string be the
	  result of running Serializing a Dictionary (<xref target="ser-dictionary"/>) with the structure.</t>
  <t>Else structure.</li>
          <li>Else, if the structure is an Item, let output_string be the
	  result of running Serializing an Item (<xref target="ser-item"/>) with the structure.</t>
  <t>Else, structure.</li>
          <li>Else, fail serialization.</t>
  <t>Return serialization.</li>
          <li>Return output_string converted into an array of bytes, using
	  ASCII encoding <xref target="RFC0020"/>.</t>
</list></t> target="RFC0020"/>.</li>
        </ol>
        <section anchor="ser-list" title="Serializing anchor="ser-list">
          <name>Serializing a List"> List</name>
          <t>Given an array of (member_value, parameters) tuples as
	  input_list, return an ASCII string suitable for use in a an HTTP field
	  value.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let output be an empty string.</t> string.</li>
            <li>
              <t>For each (member_value, parameters) of input_list:
  <list style="numbers">
      <t>If
              </t>
              <ol>
                <li>If member_value is an array, append the result of running
		Serializing an Inner List (<xref target="ser-innerlist"/>) with (member_value, parameters) to output.</t>
      <t>Otherwise,
		output.</li>
                <li>Otherwise, append the result of running Serializing an
		Item (<xref target="ser-item"/>) with
		(member_value, parameters) to output.</t> output.</li>
                <li>
                  <t>If more member_values remain in input_list:
      <list style="numbers">
          <t>Append “,”
                  </t>
                  <ol>
                    <li>Append "," to output.</t>
          <t>Append output.</li>
                    <li>Append a single SP to output.</t>
        </list></t>
    </list></t>
  <t>Return output.</t>
</list></t> output.</li>
                  </ol>
                </li>
              </ol>
            </li>
            <li>Return output.</li>
          </ol>
          <section anchor="ser-innerlist" title="Serializing anchor="ser-innerlist">
            <name>Serializing an Inner List"> List</name>
            <t>Given an array of (member_value, parameters) tuples as
	    inner_list, and parameters as list_parameters, return an ASCII
	    string suitable for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>Let
            <ol>
              <li>Let output be the string “(“.</t> "(".</li>
              <li>
                <t>For each (member_value, parameters) of inner_list:
  <list style="numbers">
      <t>Append
                </t>
                <ol>
                  <li>Append the result of running Serializing an Item (<xref target="ser-item"/>) with (member_value,
		  parameters) to output.</t>
      <t>If output.</li>
                  <li>If more values remain in inner_list, append a single SP to output.</t>
    </list></t>
  <t>Append “)” output.</li>
                </ol>
              </li>
              <li>Append ")" to output.</t>
  <t>Append output.</li>
              <li>Append the result of running Serializing Parameters (<xref target="ser-params"/>) with list_parameters to output.</t>
  <t>Return output.</t>
</list></t>
	      output.</li>
              <li>Return output.</li>
            </ol>
          </section>
          <section anchor="ser-params" title="Serializing Parameters"> anchor="ser-params">
            <name>Serializing Parameters</name>
            <t>Given an ordered Dictionary as input_parameters (each member
	    having a param_name param_key and a param_value), return an ASCII string
	    suitable for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>Let
            <ol>
              <li>Let output be an empty string.</t> string.</li>
              <li>
                <t>For each param_name param_key with a value of param_value in input_parameters:
  <list style="numbers">
      <t>Append “;”
                </t>
                <ol>
                  <li>Append ";" to output.</t>
      <t>Append output.</li>
                  <li>Append the result of running Serializing a Key (<xref target="ser-key"/>) with param_name param_key to output.</t>
		  output.</li>
                  <li>
                    <t>If param_value is not Boolean true:
      <list style="numbers">
          <t>Append “=”
                    </t>
                    <ol>
                      <li>Append "=" to output.</t>
          <t>Append output.</li>
                      <li>Append the result of running Serializing a bare Item
		      (<xref target="ser-bare-item"/>) with
		      param_value to output.</t>
        </list></t>
    </list></t>
  <t>Return output.</t>
</list></t> output.</li>
                    </ol>
                  </li>
                </ol>
              </li>
              <li>Return output.</li>
            </ol>
          </section>
          <section anchor="ser-key" title="Serializing anchor="ser-key">
            <name>Serializing a Key"> Key</name>
            <t>Given a key as input_key, return an ASCII string suitable for
	    use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>Convert
            <ol>
              <li>Convert input_key into a sequence of ASCII characters; if
	      conversion fails, fail serialization.</t>
  <t>If serialization.</li>
              <li>If input_key contains characters not in lcalpha, DIGIT, “_”, “-“, “.”, "_",
	      "-", ".", or “*” "*", fail serialization.</t>
  <t>If serialization.</li>
              <li>If the first character of input_key is not lcalpha or “*”, "*",
	      fail serialization.</t>
  <t>Let serialization.</li>
              <li>Let output be an empty string.</t>
  <t>Append string.</li>
              <li>Append input_key to output.</t>
  <t>Return output.</t>
</list></t> output.</li>
              <li>Return output.</li>
            </ol>
          </section>
        </section>
        <section anchor="ser-dictionary" title="Serializing anchor="ser-dictionary">
          <name>Serializing a Dictionary"> Dictionary</name>
          <t>Given an ordered Dictionary as input_dictionary (each member
	  having a member_name member_key and a tuple value of (member_value,
	  parameters)), return an ASCII string suitable for use in a an HTTP
	  field value.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let output be an empty string.</t> string.</li>
            <li>
              <t>For each member_name member_key with a value of (member_value, parameters) in input_dictionary:
  <list style="numbers">
      <t>Append
              </t>
              <ol>
                <li>Append the result of running Serializing a Key (<xref target="ser-key"/>) with member’s member_name member's member_key
		to output.</t> output.</li>
                <li>
                  <t>If member_value is Boolean true:
      <list style="numbers">
          <t>Append
                  </t>
                  <ol>
                    <li>Append the result of running Serializing Parameters
		    (<xref target="ser-params"/>) with
		    parameters to output.</t>
        </list></t> output.</li>
                  </ol>
                </li>
                <li>
                  <t>Otherwise:
      <list style="numbers">
          <t>Append “=”
                  </t>
                  <ol>
                    <li>Append "=" to output.</t>
          <t>If output.</li>
                    <li>If member_value is an array, append the result of
		    running Serializing an Inner List (<xref target="ser-innerlist"/>) with
		    (member_value, parameters) to output.</t>
          <t>Otherwise, output.</li>
                    <li>Otherwise, append the result of running Serializing an
		    Item (<xref target="ser-item"/>) with
		    (member_value, parameters) to output.</t>
        </list></t> output.</li>
                  </ol>
                </li>
                <li>
                  <t>If more members remain in input_dictionary:
      <list style="numbers">
          <t>Append “,”
                  </t>
                  <ol>
                    <li>Append "," to output.</t>
          <t>Append output.</li>
                    <li>Append a single SP to output.</t>
        </list></t>
    </list></t>
  <t>Return output.</t>
</list></t> output.</li>
                  </ol>
                </li>
              </ol>
            </li>
            <li>Return output.</li>
          </ol>
        </section>
        <section anchor="ser-item" title="Serializing anchor="ser-item">
          <name>Serializing an Item"> Item</name>
          <t>Given an Item as bare_item and Parameters as item_parameters,
	  return an ASCII string suitable for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let output be an empty string.</t>
  <t>Append string.</li>
            <li>Append the result of running Serializing a Bare Item <xref target="ser-bare-item"/> (<xref target="ser-bare-item"/>) with bare_item to output.</t>
  <t>Append
	    output.</li>
            <li>Append the result of running Serializing Parameters <xref target="ser-params"/> (<xref target="ser-params"/>) with item_parameters to output.</t>
  <t>Return output.</t>
</list></t>
	    output.</li>
            <li>Return output.</li>
          </ol>
          <section anchor="ser-bare-item" title="Serializing anchor="ser-bare-item">
            <name>Serializing a Bare Item"> Item</name>
            <t>Given an Item as input_item, return an ASCII string suitable
	    for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>If
            <ol>
              <li>If input_item is an Integer, return the result of running
	      Serializing an Integer (<xref target="ser-integer"/>) with input_item.</t>
  <t>If input_item.</li>
              <li>If input_item is a Decimal, return the result of running
	      Serializing a Decimal (<xref target="ser-decimal"/>) with input_item.</t>
  <t>If input_item.</li>
              <li>If input_item is a String, return the result of running
	      Serializing a String (<xref target="ser-string"/>) with input_item.</t>
  <t>If input_item.</li>
              <li>If input_item is a Token, return the result of running
	      Serializing a Token (<xref target="ser-token"/>) with input_item.</t>
  <t>If input_item.</li>
              <li>If input_item is a Boolean, Byte Sequence, return the result of
	      running Serializing a Boolean Byte Sequence (<xref target="ser-boolean"/>) target="ser-binary"/>) with input_item.</t>
  <t>If input_item.</li>
              <li>If input_item is a Byte Sequence, Boolean, return the result of running
	      Serializing a Byte Sequence Boolean (<xref target="ser-binary"/>) target="ser-boolean"/>) with input_item.</t>
  <t>Otherwise,
	      input_item.</li>
              <li>Otherwise, fail serialization.</t>
</list></t> serialization.</li>

            </ol>
          </section>
        </section>
        <section anchor="ser-integer" title="Serializing anchor="ser-integer">
          <name>Serializing an Integer"> Integer</name>
          <t>Given an Integer as input_integer, return an ASCII string
	  suitable for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>If
          <ol>
            <li>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.</t>
  <t>Let
	    serialization.</li>
            <li>Let output be an empty string.</t>
  <t>If string.</li>
            <li>If input_integer is less than (but not equal to) 0, append “-“ "-"
	    to output.</t>
  <t>Append input_integer’s output.</li>
            <li>Append input_integer's numeric value represented in base 10
	    using only decimal digits to output.</t>
  <t>Return output.</t>
</list></t> output.</li>
            <li>Return output.</li>
          </ol>
        </section>
        <section anchor="ser-decimal" title="Serializing anchor="ser-decimal">
          <name>Serializing a Decimal"> Decimal</name>
          <t>Given a decimal number as input_decimal, return an ASCII string
	  suitable for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>If
          <ol>
            <li>If input_decimal is not a decimal number, fail serialization.</t>
  <t>If serialization.</li>
            <li>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.</t>
  <t>If equidistant.</li>
            <li>If input_decimal has more than 12 significant digits to the
	    left of the decimal point after rounding, fail serialization.</t>
  <t>Let serialization.</li>
            <li>Let output be an empty string.</t>
  <t>If string.</li>
            <li>If input_decimal is less than (but not equal to) 0, append “-“ "-"
	    to output.</t>
  <t>Append input_decimal’s output.</li>
            <li>Append input_decimal's integer component represented in base
	    10 (using only decimal digits) to output; if it is zero, append “0”.</t>
  <t>Append “.”
	    "0".</li>
            <li>Append "." to output.</t>
  <t>If input_decimal’s output.</li>
            <li>If input_decimal's fractional component is zero, append “0” "0" to output.</t>
  <t>Otherwise,
	    output.</li>
            <li>Otherwise, append the significant digits of input_decimal’s input_decimal's
	    fractional component represented in base 10 (using only decimal
	    digits) to output.</t>
  <t>Return output.</t>
</list></t> output.</li>
            <li>Return output.</li>
          </ol>
        </section>
        <section anchor="ser-string" title="Serializing anchor="ser-string">
          <name>Serializing a String"> String</name>
          <t>Given a String as input_string, return an ASCII string suitable
	  for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>Convert
          <ol>
            <li>Convert input_string into a sequence of ASCII characters; if
	    conversion fails, fail serialization.</t>
  <t>If serialization.</li>
            <li>If input_string contains characters in the range %x00-1f or %x7f
	    %x7f-ff (i.e., not in VCHAR or SP), fail serialization.</t>
  <t>Let serialization.</li>
            <li>Let output be the string DQUOTE.</t> DQUOTE.</li>
            <li>
              <t>For each character char in input_string:
  <list style="numbers">
              </t>
              <ol>
                <li>
                  <t>If char is “\” "\" or DQUOTE:
      <list style="numbers">
          <t>Append “\” to output.</t>
        </list></t>
      <t>Append
                  </t>
                  <ol>
                    <li>Append "\" to output.</li>
                  </ol>
                </li>
                <li>Append char to output.</t>
    </list></t>
  <t>Append output.</li>
              </ol>
            </li>
            <li>Append DQUOTE to output.</t>
  <t>Return output.</t>
</list></t> output.</li>
            <li>Return output.</li>
          </ol>
        </section>
        <section anchor="ser-token" title="Serializing anchor="ser-token">
          <name>Serializing a Token"> Token</name>
          <t>Given a Token as input_token, return an ASCII string suitable for
	  use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>Convert
          <ol>
            <li>Convert input_token into a sequence of ASCII characters; if
	    conversion fails, fail serialization.</t>
  <t>If serialization.</li>
            <li>If the first character of input_token is not ALPHA or “*”, "*", or
	    the remaining portion contains a character not in tchar, “:” ":", or “/”,
	    "/", fail serialization.</t>
  <t>Let serialization.</li>
            <li>Let output be an empty string.</t>
  <t>Append string.</li>
            <li>Append input_token to output.</t>
  <t>Return output.</t>
</list></t> output.</li>
            <li>Return output.</li>
          </ol>
        </section>
        <section anchor="ser-binary" title="Serializing anchor="ser-binary">
          <name>Serializing a Byte Sequence"> Sequence</name>
          <t>Given a Byte Sequence as input_bytes, return an ASCII string
	  suitable for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>If
          <ol>
            <li>If input_bytes is not a sequence of bytes, fail serialization.</t>
  <t>Let serialization.</li>
            <li>Let output be an empty string.</t>
  <t>Append “:” string.</li>
            <li>Append ":" to output.</t>
  <t>Append output.</li>
            <li>Append the result of base64-encoding input_bytes as per <xref target="RFC4648"/>, Section 4, target="RFC4648" sectionFormat="comma" section="4"/>, taking account of
	    the requirements below.</t>
  <t>Append “:” to output.</t>
  <t>Return output.</t>
</list></t> below.</li>
            <li>Append ":" to output.</li>
            <li>Return output.</li>
          </ol>
          <t>The encoded data is required to be padded with “=”, "=", as per <xref target="RFC4648"/>, Section 3.2.</t> target="RFC4648" sectionFormat="comma" section="3.2"/>.</t>
          <t>Likewise, encoded data SHOULD <bcp14>SHOULD</bcp14> have pad bits set to zero, as per
	  <xref target="RFC4648"/>, Section 3.5, target="RFC4648" sectionFormat="comma" section="3.5"/>, unless it is
	  not possible to do so due to implementation constraints.</t>
        </section>
        <section anchor="ser-boolean" title="Serializing anchor="ser-boolean">
          <name>Serializing a Boolean"> Boolean</name>
          <t>Given a Boolean as input_boolean, return an ASCII string suitable
	  for use in a an HTTP field value.</t>

<t><list style="numbers">
  <t>If
          <ol>
            <li>If input_boolean is not a boolean, fail serialization.</t>
  <t>Let serialization.</li>
            <li>Let output be an empty string.</t>
  <t>Append “?” string.</li>
            <li>Append "?" to output.</t>
  <t>If output.</li>
            <li>If input_boolean is true, append “1” "1" to output.</t>
  <t>If output.</li>
            <li>If input_boolean is false, append “0” "0" to output.</t>
  <t>Return output.</t>
</list></t> output.</li>
            <li>Return output.</li>
          </ol>
        </section>
      </section>
      <section anchor="text-parse" title="Parsing anchor="text-parse">
        <name>Parsing Structured Fields"> Fields</name>
        <t>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.</t>
        <t>Given an array of bytes as input_bytes that represents represent the chosen field’s
	field's field-value (which is empty if that field is not present), present) and
	field_type (one of “dictionary”, “list”, "dictionary", "list", or “item”), "item"), return the parsed
	header value.</t>

<t><list style="numbers">
  <t>Convert
        <ol>
          <li>Convert input_bytes into an ASCII string input_string; if
	  conversion fails, fail parsing.</t>
  <t>Discard parsing.</li>
          <li>Discard any leading SP characters from input_string.</t>
  <t>If input_string.</li>
          <li>If field_type is “list”, "list", let output be the result of running
	  Parsing a List (<xref target="parse-list"/>) with input_string.</t>
  <t>If
	  input_string.</li>
          <li>If field_type is “dictionary”, "dictionary", let output be the result of
	  running Parsing a Dictionary (<xref target="parse-dictionary"/>) with input_string.</t>
  <t>If input_string.</li>
          <li>If field_type is “item”, "item", let output be the result of running
	  Parsing an Item (<xref target="parse-item"/>) with input_string.</t>
  <t>Discard
	  input_string.</li>
          <li>Discard any leading SP characters from input_string.</t>
  <t>If input_string.</li>
          <li>If input_string is not empty, fail parsing.</t>
  <t>Otherwise, parsing.</li>
          <li>Otherwise, return output.</t>
</list></t> output.</li>
        </ol>
        <t>When generating input_bytes, parsers MUST <bcp14>MUST</bcp14> 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 <xref target="RFC7230"/>, Section 3.2.2; target="RFC7230" sectionFormat="comma" section="3.2.2"/>; this assures that
	the entire field value is processed correctly.</t>
        <t>For Lists and Dictionaries, this has the effect of correctly
	concatenating all of the field’s 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.</t>
        <t>Strings split across multiple field lines will have unpredictable
	results, because comma(s) and whitespace inserted upon combination 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.</t>
        <t>Tokens, Integers, Decimals Decimals, and Byte Sequences cannot be split
	across multiple field lines because the inserted commas will cause
	parsing to fail.</t>
        <t>Parsers MAY <bcp14>MAY</bcp14> 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 that's
	defined as a an sf-string is allowed to fail when processing this field
	section:</t>

<figure><artwork type="example"><![CDATA[
        <sourcecode type="http-message">
Example-String: "foo
Example-String: bar"
]]></artwork></figure>
</sourcecode>
        <t>If parsing fails – -- including when calling another algorithm – -- the
	entire field value MUST <bcp14>MUST</bcp14> 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.</t>
        <t>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.</t>
        <section anchor="parse-list" title="Parsing anchor="parse-list">
          <name>Parsing a List"> List</name>
          <t>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.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let members be an empty array.</t> array.</li>
            <li>
              <t>While input_string is not empty:
  <list style="numbers">
      <t>Append
              </t>
              <ol>
                <li>Append the result of running Parsing an Item or Inner List
		(<xref target="parse-item-or-list"/>) with
		input_string to members.</t>
      <t>Discard members.</li>
                <li>Discard any leading OWS characters from input_string.</t>
      <t>If input_string.</li>
                <li>If input_string is empty, return members.</t>
      <t>Consume members.</li>
                <li>Consume the first character of input_string; if it is not “,”,
		",", fail parsing.</t>
      <t>Discard parsing.</li>
                <li>Discard any leading OWS characters from input_string.</t>
      <t>If input_string.</li>
                <li>If input_string is empty, there is a trailing comma; fail parsing.</t>
    </list></t>
  <t>No parsing.</li>
              </ol>
            </li>
            <li>No structured data has been found; return members (which is empty).</t>
</list></t> empty).</li>
          </ol>
          <section anchor="parse-item-or-list" title="Parsing anchor="parse-item-or-list">
            <name>Parsing an Item or Inner List"> List</name>
            <t>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, item or an array of (bare_item, parameters)
	    tuples. input_string is modified to remove the parsed value.</t>

<t><list style="numbers">
  <t>If
            <ol>
              <li>If the first character of input_string is “(“, "(", return the
	      result of running Parsing an Inner List (<xref target="parse-innerlist"/>) with input_string.</t>
  <t>Return
	      input_string.</li>
              <li>Return the result of running Parsing an Item (<xref target="parse-item"/>) with input_string.</t>
</list></t> input_string.</li>
            </ol>
          </section>
          <section anchor="parse-innerlist" title="Parsing anchor="parse-innerlist">
            <name>Parsing an Inner List"> List</name>
            <t>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.</t>

<t><list style="numbers">
  <t>Consume
            <ol>
              <li>Consume the first character of input_string; if it is not “(“,
	      "(", fail parsing.</t>
  <t>Let parsing.</li>
              <li>Let inner_list be an empty array.</t> array.</li>
              <li>
                <t>While input_string is not empty:
  <list style="numbers">
      <t>Discard
                </t>
                <ol>
                  <li>Discard any leading SP characters from input_string.</t> input_string.</li>
                  <li>
                    <t>If the first character of input_string is “)”:
      <list style="numbers">
          <t>Consume ")":
                    </t>
                    <ol>
                      <li>Consume the first character of input_string.</t>
          <t>Let input_string.</li>
                      <li>Let parameters be the result of running Parsing
		      Parameters (<xref target="parse-param"/>) with input_string.</t>
          <t>Return input_string.</li>
                      <li>Return the tuple (inner_list, parameters).</t>
        </list></t>
      <t>Let parameters).</li>
                    </ol>
                  </li>
                  <li>Let item be the result of running Parsing an Item (<xref target="parse-item"/>) with input_string.</t>
      <t>Append
		  input_string.</li>
                  <li>Append item to inner_list.</t>
      <t>If inner_list.</li>
                  <li>If the first character of input_string is not SP or “)”, ")",
		  fail parsing.</t>
    </list></t>
  <t>The parsing.</li>
                </ol>
              </li>
              <li>The end of the inner list Inner List was not found; fail parsing.</t>
</list></t> parsing.</li>
            </ol>
          </section>
        </section>
        <section anchor="parse-dictionary" title="Parsing anchor="parse-dictionary">
          <name>Parsing a Dictionary"> Dictionary</name>
          <t>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.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let dictionary be an empty, ordered map.</t> map.</li>
            <li>
              <t>While input_string is not empty:
  <list style="numbers">
      <t>Let
              </t>
              <ol>
                <li>Let this_key be the result of running Parsing a Key (<xref target="parse-key"/>) with input_string.</t> input_string.</li>
                <li>
                  <t>If the first character of input_string is “=”:
      <list style="numbers">
          <t>Consume "=":
                  </t>
                  <ol>
                    <li>Consume the first character of input_string.</t>
          <t>Let input_string.</li>
                    <li>Let member be the result of running Parsing an Item or
		    Inner List (<xref target="parse-item-or-list"/>) with input_string.</t>
        </list></t> input_string.</li>
                  </ol>
                </li>
                <li>
                  <t>Otherwise:
      <list style="numbers">
          <t>Let
                  </t>
                  <ol>
                    <li>Let value be Boolean true.</t>
          <t>Let true.</li>
                    <li>Let parameters be the result of running Parsing
		    Parameters <xref target="parse-param"/> (<xref target="parse-param"/>)
		    with input_string.</t>
          <t>Let input_string.</li>
                    <li>Let member be the tuple (value, parameters).</t>
        </list></t>
      <t>Add name this_key with value member to dictionary. If parameters).</li>
                  </ol>
                </li>
                <li>If dictionary already contains a name key this_key (comparing character-for-character), character for character),  overwrite its value.</t>
      <t>Discard value with member.</li>
		<li>Otherwise, append key this_key with value member to dictionary.</li>
                <li>Discard any leading OWS characters from input_string.</t>
      <t>If input_string.</li>
                <li>If input_string is empty, return dictionary.</t>
      <t>Consume dictionary.</li>
                <li>Consume the first character of input_string; if it is not “,”,
		",", fail parsing.</t>
      <t>Discard parsing.</li>
                <li>Discard any leading OWS characters from input_string.</t>
      <t>If input_string.</li>
                <li>If input_string is empty, there is a trailing comma; fail parsing.</t>
    </list></t>
  <t>No parsing.</li>
              </ol>
            </li>
            <li>No structured data has been found; return dictionary (which is empty).</t>
</list></t> empty).</li>
          </ol>
          <t>Note that when duplicate Dictionary keys are encountered, this has the effect of ignoring all but
         the last instance.</t> instance are ignored.</t>
        </section>
        <section anchor="parse-item" title="Parsing anchor="parse-item">
          <name>Parsing an Item"> Item</name>
          <t>Given an ASCII string as input_string, return a (bare_item,
	  parameters) tuple. input_string is modified to remove the parsed
	  value.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let bare_item be the result of running Parsing a Bare Item
	    (<xref target="parse-bare-item"/>) with input_string.</t>
  <t>Let
	    input_string.</li>
            <li>Let parameters be the result of running Parsing Parameters
	    (<xref target="parse-param"/>) with input_string.</t>
  <t>Return
	    input_string.</li>
            <li>Return the tuple (bare_item, parameters).</t>
</list></t> parameters).</li>
          </ol>
          <section anchor="parse-bare-item" title="Parsing anchor="parse-bare-item">
            <name>Parsing a Bare Item"> Item</name>
            <t>Given an ASCII string as input_string, return a bare
	    Item. input_string is modified to remove the parsed value.</t>

<t><list style="numbers">
  <t>If
            <ol>
              <li>If the first character of input_string is a “-“ "-" or a DIGIT,
	      return the result of running Parsing an Integer or Decimal
	      (<xref target="parse-number"/>) with input_string.</t>
  <t>If
	      input_string.</li>
              <li>If the first character of input_string is a DQUOTE, return
	      the result of running Parsing a String (<xref target="parse-string"/>) with input_string.</t>
  <t>If
	      input_string.</li>
              <li>If the first character of input_string is “:”, an ALPHA or "*",
	      return the result of running Parsing a Byte Sequence Token (<xref target="parse-binary"/>) target="parse-token"/>) with input_string.</t>
  <t>If input_string.</li>
              <li>If the first character of input_string is “?”, ":", return the
	      result of running Parsing a Boolean Byte Sequence (<xref target="parse-boolean"/>) target="parse-binary"/>) with input_string.</t>
  <t>If
	      input_string.</li>
              <li>If the first character of input_string is an ALPHA or “*”, "?", return the
	      result of running Parsing a Token Boolean (<xref target="parse-token"/>) target="parse-boolean"/>) with input_string.</t>
  <t>Otherwise,
	      input_string.</li>
              <li>Otherwise, the item type is unrecognized; fail parsing.</t>
</list></t> parsing.</li>
            </ol>
          </section>
          <section anchor="parse-param" title="Parsing Parameters"> anchor="parse-param">
            <name>Parsing Parameters</name>
            <t>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.</t>

<t><list style="numbers">
  <t>Let
            <ol>
              <li>Let parameters be an empty, ordered map.</t> map.</li>
              <li>
                <t>While input_string is not empty:
  <list style="numbers">
      <t>If
                </t>
                <ol>
                  <li>If the first character of input_string is not “;”, ";", exit the loop.</t>
      <t>Consume a “;” loop.</li>
                  <li>Consume the ";" character from the beginning of input_string.</t>
      <t>Discard input_string.</li>
                  <li>Discard any leading SP characters from input_string.</t>
      <t>let param_name input_string.</li>
                  <li>Let param_key be the result of running Parsing a Key
		  (<xref target="parse-key"/>) with input_string.</t>
      <t>Let
		  input_string.</li>
                  <li>Let param_value be Boolean true.</t> true.</li>
                  <li>
                    <t>If the first character of input_string is “=”:
      <list style="numbers">
          <t>Consume "=":
                    </t>
                    <ol>
                      <li>Consume the “=” "=" character at the beginning of input_string.</t>
          <t>Let input_string.</li>
                      <li>Let param_value be the result of running Parsing a
		      Bare Item (<xref target="parse-bare-item"/>) with input_string.</t>
        </list></t>
      <t>Append key param_name with value param_value to parameters. If input_string.</li>
                    </ol>
                  </li>
                  <li>If parameters already contains a name param_name key param_key (comparing character-for-character), character for character), overwrite its value.</t>
    </list></t>
  <t>Return parameters.</t>
</list></t> value with param_value.</li>
		  <li>Otherwise, append key param_key with value param_value to parameters.</li>
		</ol>
              </li>
              <li>Return parameters.</li>
            </ol>
            <t>Note that when duplicate Parameter parameter keys are encountered, this has the effect of ignoring
            all but the last instance.</t> instance are ignored.</t>
          </section>
          <section anchor="parse-key" title="Parsing anchor="parse-key">
            <name>Parsing a Key"> Key</name>
            <t>Given an ASCII string as input_string, return a
	    key. input_string is modified to remove the parsed value.</t>

<t><list style="numbers">
  <t>If
            <ol>
              <li>If the first character of input_string is not lcalpha or “*”,
	      "*", fail parsing.</t>
  <t>Let parsing.</li>
              <li>Let output_string be an empty string.</t> string.</li>
              <li>
                <t>While input_string is not empty:
  <list style="numbers">
      <t>If
                </t>
                <ol>
                  <li>If the first character of input_string is not one of
		  lcalpha, DIGIT, “_”, “-“, “.”, "_", "-", ".", or “*”, "*", return output_string.</t>
      <t>Let
		  output_string.</li>
                  <li>Let char be the result of consuming the first character of input_string.</t>
      <t>Append input_string.</li>
                  <li>Append char to output_string.</t>
    </list></t>
  <t>Return output_string.</t>
</list></t> output_string.</li>
                </ol>
              </li>
              <li>Return output_string.</li>
            </ol>
          </section>
        </section>
        <section anchor="parse-number" title="Parsing anchor="parse-number">
          <name>Parsing an Integer or Decimal"> Decimal</name>
          <t>Given an ASCII string as input_string, return an Integer or
	  Decimal. input_string is modified to remove the parsed value.</t>
          <t>NOTE: This algorithm parses both Integers (<xref target="integer"/>) and Decimals (<xref target="decimal"/>), and returns the corresponding structure.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let type be “integer”.</t>
  <t>Let "integer".</li>
            <li>Let sign be 1.</t>
  <t>Let 1.</li>
            <li>Let input_number be an empty string.</t>
  <t>If string.</li>
            <li>If the first character of input_string is “-“, "-", consume it and
	    set sign to -1.</t>
  <t>If -1.</li>
            <li>If input_string is empty, there is an empty integer; fail parsing.</t>
  <t>If
	    parsing.</li>
            <li>If the first character of input_string is not a DIGIT, fail parsing.</t>
	    parsing.</li>
            <li>
              <t>While input_string is not empty:
  <list style="numbers">
      <t>Let
              </t>
              <ol>
                <li>Let char be the result of consuming the first character of input_string.</t>
      <t>If
		input_string.</li>
                <li>If char is a DIGIT, append it to input_number.</t> input_number.</li>
                <li>
                  <t>Else, if type is “integer” "integer" and char is “.”:
      <list style="numbers">
          <t>If ".":
                  </t>
                  <ol>
                    <li>If input_number contains more than 12 characters, fail parsing.</t>
          <t>Otherwise, parsing.</li>
                    <li>Otherwise, append char to input_number and set type to “decimal”.</t>
        </list></t>
      <t>Otherwise, "decimal".</li>
                  </ol>
                </li>
                <li>Otherwise, prepend char to input_string, and exit the loop.</t>
      <t>If loop.</li>
                <li>If type is “integer” "integer" and input_number contains more than
		15 characters, fail parsing.</t>
      <t>If parsing.</li>
                <li>If type is “decimal” "decimal" and input_number contains more than
		16 characters, fail parsing.</t>
    </list></t> parsing.</li>
              </ol>
            </li>
            <li>
              <t>If type is “integer”:
  <list style="numbers">
      <t>Parse "integer":
              </t>
              <ol>
                <li>Parse input_number as an integer and let output_number be
		the product of the result and sign.</t>
      <t>If output_number is outside the range -999,999,999,999,999 to 999,999,999,999,999 inclusive, fail parsing.</t>
    </list></t> sign.</li>
              </ol>
            </li>
            <li>
              <t>Otherwise:
  <list style="numbers">
      <t>If
              </t>
              <ol>
                <li>If the final character of input_number is “.”, ".", fail parsing.</t>
      <t>If parsing.</li>
                <li>If the number of characters after “.” "." in input_number is
		greater than three, fail parsing.</t>
      <t>Parse parsing.</li>
                <li>Parse input_number as a decimal number and let
		output_number be the product of the result and sign.</t>
    </list></t>
  <t>Return output_number.</t>
</list></t> sign.</li>
              </ol>
            </li>
            <li>Return output_number.</li>
          </ol>
        </section>
        <section anchor="parse-string" title="Parsing anchor="parse-string">
          <name>Parsing a String"> String</name>
          <t>Given an ASCII string as input_string, return an unquoted
	  String. input_string is modified to remove the parsed value.</t>

<t><list style="numbers">
  <t>Let
          <ol>
            <li>Let output_string be an empty string.</t>
  <t>If string.</li>
            <li>If the first character of input_string is not DQUOTE, fail parsing.</t>
  <t>Discard parsing.</li>
            <li>Discard the first character of input_string.</t> input_string.</li>
            <li>
              <t>While input_string is not empty:
  <list style="numbers">
      <t>Let
              </t>
              <ol>
                <li>Let char be the result of consuming the first character of input_string.</t> input_string.</li>
                <li>
                  <t>If char is a backslash (“\”):
      <list style="numbers">
          <t>If ("\"):
                  </t>
                  <ol>
                    <li>If input_string is now empty, fail parsing.</t>
          <t>Let parsing.</li>
                    <li>Let next_char be the result of consuming the first
		    character of input_string.</t>
          <t>If input_string.</li>
                    <li>If next_char is not DQUOTE or “\”, "\", fail parsing.</t>
          <t>Append parsing.</li>
                    <li>Append next_char to output_string.</t>
        </list></t>
      <t>Else, output_string.</li>
                  </ol>
                </li>
                <li>Else, if char is DQUOTE, return output_string.</t>
      <t>Else, output_string.</li>
                <li>Else, if char is in the range %x00-1f or %x7f %x7f-ff (i.e., it is
		not in VCHAR or SP), fail parsing.</t>
      <t>Else, parsing.</li>
                <li>Else, append char to output_string.</t>
    </list></t>
  <t>Reached output_string.</li>
              </ol>
            </li>
            <li>Reached the end of input_string without finding a closing
	    DQUOTE; fail parsing.</t>
</list></t> parsing.</li>
          </ol>
        </section>
        <section anchor="parse-token" title="Parsing anchor="parse-token">
          <name>Parsing a Token"> Token</name>
          <t>Given an ASCII string as input_string, return a
	  Token. input_string is modified to remove the parsed value.</t>

<t><list style="numbers">
  <t>If
          <ol>
            <li>If the first character of input_string is not ALPHA or “*”, "*",
	    fail parsing.</t>
  <t>Let parsing.</li>
            <li>Let output_string be an empty string.</t> string.</li>
            <li>
              <t>While input_string is not empty:
  <list style="numbers">
      <t>If
              </t>
              <ol>
                <li>If the first character of input_string is not in tchar, “:”
		":", or “/”, "/", return output_string.</t>
      <t>Let output_string.</li>
                <li>Let char be the result of consuming the first character of input_string.</t>
      <t>Append
		input_string.</li>
                <li>Append char to output_string.</t>
    </list></t>
  <t>Return output_string.</t>
</list></t> output_string.</li>
              </ol>
            </li>
            <li>Return output_string.</li>
          </ol>
        </section>
        <section anchor="parse-binary" title="Parsing anchor="parse-binary">
          <name>Parsing a Byte Sequence"> Sequence</name>
          <t>Given an ASCII string as input_string, return a Byte
	  Sequence. input_string is modified to remove the parsed value.</t>

<t><list style="numbers">
  <t>If
          <ol>
            <li>If the first character of input_string is not “:”, ":", fail parsing.</t>
  <t>Discard parsing.</li>
            <li>Discard the first character of input_string.</t>
  <t>If input_string.</li>
            <li>If there is not a “:” ":" character before the end of input_string, fail parsing.</t>
  <t>Let parsing.</li>
            <li>Let b64_content be the result of consuming content of
	    input_string up to but not including the first instance of the
	    character “:”.</t>
  <t>Consume ":".</li>
            <li>Consume the “:” ":" character at the beginning of input_string.</t>
  <t>If input_string.</li>
            <li>If b64_content contains a character not included in ALPHA,
	    DIGIT, “+”, “/” "+", "/", and “=”, "=", fail parsing.</t>
  <t>Let parsing.</li>
            <li>Let binary_content be the result of Base 64 Decoding base64-decoding <xref target="RFC4648"/> b64_content, synthesizing
	    padding if necessary (note the requirements about recipient
	    behavior below).</t>
  <t>Return binary_content.</t>
</list></t> below). If base64 decoding fails, parsing fails.</li>
            <li>Return binary_content.</li>
          </ol>
          <t>Because some implementations of base64 do not allow rejection of
	  encoded data that is not properly “=” "=" padded (see <xref target="RFC4648"/>, Section 3.2), target="RFC4648" sectionFormat="comma" section="3.2"/>), parsers SHOULD NOT <bcp14>SHOULD
	  NOT</bcp14> fail when “=” "=" padding is not present, unless they cannot be
	  configured to do so.</t>
          <t>Because some implementations of base64 do not allow rejection of
	  encoded data that has non-zero pad bits (see <xref target="RFC4648"/>, Section 3.5), target="RFC4648" sectionFormat="comma" section="3.5"/>), parsers SHOULD NOT <bcp14>SHOULD NOT</bcp14> fail when
	  non-zero pad bits are present, unless they cannot be configured to
	  do so.</t>
          <t>This specification does not relax the requirements in <xref target="RFC4648"/>, Section 3.1 Sections <xref target="RFC4648" sectionFormat="bare" section="3.1"/> and 3.3; <xref target="RFC4648" sectionFormat="bare" section="3.3"/>; therefore,
	  parsers MUST <bcp14>MUST</bcp14> fail on characters outside the base64 alphabet, alphabet and on line
	  feeds in encoded data.</t>
        </section>
        <section anchor="parse-boolean" title="Parsing anchor="parse-boolean">
          <name>Parsing a Boolean"> Boolean</name>
          <t>Given an ASCII string as input_string, return a
	  Boolean. input_string is modified to remove the parsed value.</t>

<t><list style="numbers">
  <t>If
          <ol>
            <li>If the first character of input_string is not “?”, "?", fail parsing.</t>
  <t>Discard parsing.</li>
            <li>Discard the first character of input_string.</t>
  <t>If input_string.</li>
            <li>If the first character of input_string matches “1”, "1", discard
	    the first character, and return true.</t>
  <t>If true.</li>
            <li>If the first character of input_string matches “0”, "0", discard
	    the first character, and return false.</t>
  <t>No false.</li>
            <li>No value has matched; fail parsing.</t>
</list></t> parsing.</li>
          </ol>
        </section>
      </section>
    </section>
    <section anchor="iana-considerations" title="IANA Considerations"> anchor="iana-considerations">
      <name>IANA Considerations</name>
      <t>This document has no actions for IANA.</t> IANA actions.</t>
    </section>
    <section anchor="security-considerations" title="Security Considerations"> anchor="security-considerations">
      <name>Security Considerations</name>
      <t>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.</t>
      <t>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.</t>
    </section>
  </middle>
  <back>

    <references title='Normative References'>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7230.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5234.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.0020.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4648.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <reference  anchor="RFC7230" target='https://www.rfc-editor.org/info/rfc7230'> anchor="IEEE754" target="https://ieeexplore.ieee.org/document/8766229">
          <front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
<author initials='R.' surname='Fielding' fullname='R. Fielding' role='editor'><organization /></author>
<author initials='J.' surname='Reschke' fullname='J. Reschke' role='editor'><organization /></author>
<date year='2014' month='June' />
<abstract><t>The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems.  This document provides an overview of HTTP architecture and its associated terminology, defines the &quot;http&quot; and &quot;https&quot; Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns
            <title>IEEE Standard for implementations.</t></abstract>
</front> Floating-Point Arithmetic</title>
            <seriesInfo name='RFC' value='7230'/> name="DOI" value="10.1109/IEEESTD.2019.8766229"/>
            <seriesInfo name='DOI' value='10.17487/RFC7230'/>
</reference>

<reference  anchor="RFC2119" target='https://www.rfc-editor.org/info/rfc2119'>
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='S. Bradner'><organization /></author> name="IEEE" value="754-2019"/>
            <author>
              <organization>IEEE</organization>
            </author>
            <date year='1997' month='March' />
<abstract><t>In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract> year="2019" month="July"/>
          </front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='2119'/>
<seriesInfo name='DOI' value='10.17487/RFC2119'/>
        </reference>

<reference  anchor="RFC8174" target='https://www.rfc-editor.org/info/rfc8174'>  anchor='STD63' target='https://www.rfc-editor.org/info/std63'>
<front>
<title>Ambiguity
<title>UTF-8, a transformation format of Uppercase vs Lowercase in RFC 2119 Key Words</title> ISO 10646</title>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization initials='F.' surname='Yergeau' fullname='F. Yergeau'><organization /></author>
<date year='2017' month='May' year='2003' month='November' />
<abstract><t>RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/> name='STD' value='63'/>
<seriesInfo name='RFC' value='8174'/>
<seriesInfo name='DOI' value='10.17487/RFC8174'/> value='3629'/>
</reference>

<reference  anchor="RFC5234" target='https://www.rfc-editor.org/info/rfc5234'>
<front>
<title>Augmented BNF

        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7231.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7540.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7541.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8259.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7493.xml"/>
      </references>
    </references>
    <section anchor="faq">
      <name>Frequently Asked Questions</name>
      <section anchor="why-not-json">
        <name>Why Not JSON?</name>
        <t>Earlier proposals for Syntax Specifications: ABNF</title>
<author initials='D.' surname='Crocker' fullname='D. Crocker' role='editor'><organization /></author>
<author initials='P.' surname='Overell' fullname='P. Overell'><organization /></author>
<date year='2008' month='January' />
<abstract><t>Internet technical specifications often need Structured Fields were based upon JSON <xref target="RFC8259"/>. However, constraining its use to define a formal syntax.  Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF),
	make it suitable for HTTP header fields required senders and
	recipients to implement specific additional handling.</t>
        <t>For example, JSON has been popular among many Internet specifications.  The current specification documents ABNF. It balances compactness issues around large numbers and simplicity
	objects with reasonable representational power.  The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges.  This specification also supplies additional rule definitions and encoding duplicate members. Although advice for a core lexical analyzer of the type common to several Internet specifications.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='STD' value='68'/>
<seriesInfo name='RFC' value='5234'/>
<seriesInfo name='DOI' value='10.17487/RFC5234'/>
</reference>

<reference  anchor="RFC0020" target='https://www.rfc-editor.org/info/rfc20'>
<front>
<title>ASCII format for network interchange</title>
<author initials='V.G.' surname='Cerf' fullname='V.G. Cerf'><organization /></author>
<date year='1969' month='October' />
</front>
<seriesInfo name='STD' value='80'/>
<seriesInfo name='RFC' value='20'/>
<seriesInfo name='DOI' value='10.17487/RFC0020'/>
</reference>

<reference  anchor="RFC4648" target='https://www.rfc-editor.org/info/rfc4648'>
<front>
<title>The Base16, Base32, and Base64 Data Encodings</title>
<author initials='S.' surname='Josefsson' fullname='S. Josefsson'><organization /></author>
<date year='2006' month='October' />
<abstract><t>This document describes the commonly used base 64, base 32, and base 16 encoding schemes.  It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='4648'/>
<seriesInfo name='DOI' value='10.17487/RFC4648'/>
</reference>

    </references>

    <references title='Informative References'>

<reference anchor="IEEE754" target="https://ieeexplore.ieee.org/document/8766229">
  <front>
    <title>IEEE Standard for Floating-Point Arithmetic</title>
    <author >
      <organization>IEEE</organization>
    </author>
    <date year="2019" month="July"/>
  </front>
  <seriesInfo name="IEEE" value="754-2019"/>
  <seriesInfo name="DOI" value="10.1109/IEEESTD.2019.8766229"/>
  <seriesInfo name="ISBN" value="978-1-5044-5924-2"/>
</reference>
<reference anchor="UTF-8" target="http://www.rfc-editor.org/info/std63">
  <front>
    <title>UTF-8, a transformation format of ISO 10646</title>
    <author initials="F." surname="Yergeau" fullname="F. Yergeau">
      <organization></organization>
    </author>
    <date year="2003" month="November"/>
  </front>
  <seriesInfo name="STD" value="63"/>
  <seriesInfo name="RFC" value="3629"/>
  <seriesInfo name="DOI" value="10.17487/RFC3629"/>
</reference>

<reference  anchor="RFC7231" target='https://www.rfc-editor.org/info/rfc7231'>
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
<author initials='R.' surname='Fielding' fullname='R. Fielding' role='editor'><organization /></author>
<author initials='J.' surname='Reschke' fullname='J. Reschke' role='editor'><organization /></author>
<date year='2014' month='June' />
<abstract><t>The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems.  This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation.</t></abstract>
</front>
<seriesInfo name='RFC' value='7231'/>
<seriesInfo name='DOI' value='10.17487/RFC7231'/>
</reference>

<reference  anchor="RFC7540" target='https://www.rfc-editor.org/info/rfc7540'>
<front>
<title>Hypertext Transfer Protocol Version 2 (HTTP/2)</title>
<author initials='M.' surname='Belshe' fullname='M. Belshe'><organization /></author>
<author initials='R.' surname='Peon' fullname='R. Peon'><organization /></author>
<author initials='M.' surname='Thomson' fullname='M. Thomson' role='editor'><organization /></author>
<date year='2015' month='May' />
<abstract><t>This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2).  HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection.  It also introduces unsolicited push of representations from servers to clients.</t><t>This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax.  HTTP's existing semantics remain unchanged.</t></abstract>
</front>
<seriesInfo name='RFC' value='7540'/>
<seriesInfo name='DOI' value='10.17487/RFC7540'/>
</reference>

<reference  anchor="RFC7541" target='https://www.rfc-editor.org/info/rfc7541'>
<front>
<title>HPACK: Header Compression for HTTP/2</title>
<author initials='R.' surname='Peon' fullname='R. Peon'><organization /></author>
<author initials='H.' surname='Ruellan' fullname='H. Ruellan'><organization /></author>
<date year='2015' month='May' />
<abstract><t>This specification defines HPACK, a compression format for efficiently representing HTTP header fields, to be used in HTTP/2.</t></abstract>
</front>
<seriesInfo name='RFC' value='7541'/>
<seriesInfo name='DOI' value='10.17487/RFC7541'/>
</reference>

<reference  anchor="RFC8259" target='https://www.rfc-editor.org/info/rfc8259'>
<front>
<title>The JavaScript Object Notation (JSON) Data Interchange Format</title>
<author initials='T.' surname='Bray' fullname='T. Bray' role='editor'><organization /></author>
<date year='2017' month='December' />
<abstract><t>JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format.  It was derived from the ECMAScript Programming Language Standard.  JSON defines a small set of formatting rules for the portable representation of structured data.</t><t>This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance.</t></abstract>
</front>
<seriesInfo name='STD' value='90'/>
<seriesInfo name='RFC' value='8259'/>
<seriesInfo name='DOI' value='10.17487/RFC8259'/>
</reference>

<reference  anchor="RFC7493" target='https://www.rfc-editor.org/info/rfc7493'>
<front>
<title>The I-JSON Message Format</title>
<author initials='T.' surname='Bray' fullname='T. Bray' role='editor'><organization /></author>
<date year='2015' month='March' />
<abstract><t>I-JSON (short for &quot;Internet JSON&quot;) is a restricted profile of JSON designed to maximize interoperability and increase confidence that software can process it successfully with predictable results.</t></abstract>
</front>
<seriesInfo name='RFC' value='7493'/>
<seriesInfo name='DOI' value='10.17487/RFC7493'/>
</reference>

    </references>

<section anchor="faq" title="Frequently Asked Questions">

<section anchor="why-not-json" title="Why not JSON?">

<t>Earlier proposals for Structured Fields were based upon JSON <xref target="RFC8259"/>. However, constraining its use to make it suitable for HTTP header fields required senders and recipients to implement specific additional handling.</t>

<t>For example, JSON has specification issues around large numbers and objects with duplicate members. Although advice for avoiding these issues is available (e.g., <xref target="RFC7493"/>), it cannot be relied upon.</t>

<t>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.</t>

<t>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.</t>

<t>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.</t>

<t>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.</t>

<t>Additionally, there were widely shared feelings that JSON doesn’t “look right” in HTTP fields.</t>

</section>
</section>
<section anchor="implementation-notes" title="Implementation Notes">

<t>A generic implementation of this specification should expose the top-level serialize (<xref target="text-serialize"/>) and parse (<xref target="text-parse"/>) 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.</t>

<t>For interoperability, it’s important that generic implementations be complete and follow the algorithms closely; see <xref target="strict"/>. To aid this, a common test suite is being maintained by the community at <eref target="https://github.com/httpwg/structured-field-tests">https://github.com/httpwg/structured-field-tests</eref>.</t>

<t>Implementers should note that Dictionaries and Parameters are order-preserving maps. Some fields may not convey meaning in the ordering of avoiding these data types, but it should still be exposed so that applications which need to use it will have it available.</t>

<t>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,
	issues is available (e.g., <xref target="RFC7493"/>),
	it may cannot be necessary to create a wrapper “token” object or use relied upon.</t>
        <t>Likewise, JSON strings are by default Unicode strings, which have a parameter on functions
	number of potential interoperability issues (e.g., in
	comparison). Although implementers can be advised to assure that these types remain separate.</t>

<t>The serialization algorithm avoid non-ASCII
	content where unnecessary, this is defined in a way that it difficult to enforce.</t>
        <t>Another example is not strictly limited JSON's ability to nest content to arbitrary
	depths. Since the data types defined in <xref target="types"/> resulting memory commitment might be unsuitable
	(e.g., in every case. For example, Decimals are designed to take broader input embedded and round to allowed values.</t>

<t>Implementations are allowed other limited server deployments), it's
	necessary to limit the allowed size of different structures, subject to the minimums defined for each type. When a structure exceeds an implementation limit, that structure fails parsing or serialisation.</t>

</section>
<section anchor="changes" title="Changes">

<t><spanx style="emph">RFC Editor: Please remove this section before publication.</spanx></t>

<section anchor="since-draft-ietf-httpbis-header-structure-18" title="Since draft-ietf-httpbis-header-structure-18">

<t><list style="symbols">
  <t>Use “sf-“ prefix for ABNF, not “sh-“.</t>
  <t>Fix indentation it in Dictionary serialisation (#1164).</t>
  <t>Add example for Token; tweak example field names (#1147).</t>
  <t>Editorial improvements.</t>
  <t>Note that exceeding implementation limits implies failure.</t>
  <t>Talk about specifying order of Dictionary members some fashion; however, existing JSON
	implementations have no such limits, and Parameters, not cardinality.</t>
  <t>Allow (but don’t require) parsers to fail when even if a single field line isn’t valid.</t>
  <t>Note limit is specified,
	it's likely that some aspects of Integers and Decimals are not necessarily preserved.</t>
  <t>Allow Lists and Dictionaries to be delimited by OWS, rather than *SP, to make parsing more robust.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-17" title="Since draft-ietf-httpbis-header-structure-17">

<t><list style="symbols">
  <t>Editorial improvements.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-16" title="Since draft-ietf-httpbis-header-structure-16">

<t><list style="symbols">
  <t>Editorial improvements.</t>
  <t>Discussion on forwards compatibility.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-15" title="Since draft-ietf-httpbis-header-structure-15">

<t><list style="symbols">
  <t>Editorial improvements.</t>
  <t>Use HTTP field terminology more consistently, in line with recent changes to HTTP-core.</t>
  <t>String length requirements apply to decoded strings (#1051).</t>
  <t>Correctly round decimals in serialisation (#1043).</t>
  <t>Clarify input to serialisation algorithms (#1055).</t>
  <t>Omitted True dictionary value can have parameters (#1083).</t>
  <t>Keys can now start with ‘*’ (#1068).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-14" title="Since draft-ietf-httpbis-header-structure-14">

<t><list style="symbols">
  <t>Editorial improvements.</t>
  <t>Allow empty dictionary values (#992).</t>
  <t>Change value of omitted parameter value definition will find a need to True (#995).</t>
  <t>Explain more about splitting dictionaries violate
	it.</t>
        <t>Because of JSON's broad adoption and lists implementation, it is
	difficult to impose such additional constraints across header instances (#997).</t>
  <t>Disallow HTAB, replace OWS with spaces (#998).</t>
  <t>Change byte sequence delimiters from “*” all
	implementations; some deployments would fail to “:” (#991).</t>
  <t>Allow tokens enforce them, thereby
	harming interoperability. In short, if it looks like JSON, people will
	be tempted to start with “*” (#991).</t>
  <t>Change Floats use a JSON parser/serializer on field values.</t>
        <t>Since a major goal for Structured Fields is to fixed-precision Decimals (#982).</t>
  <t>Round the fractional component of decimal, rather than truncating it (#982).</t>
  <t>Handle duplicate dictionary improve
	interoperability and parameter keys by overwriting their values, rather than failing (#997).</t>
  <t>Allow “.” in key (#1027).</t>
  <t>Check first character of key simplify implementation, these concerns led to a
	format that requires a dedicated parser and serializer.</t>
        <t>Additionally, there were widely shared feelings that JSON doesn't
	"look right" in serialisation (#1037).</t>
  <t>Talk about greasing headers (#1015).</t>
</list></t> HTTP fields.</t>
      </section>
    </section>
    <section anchor="since-draft-ietf-httpbis-header-structure-13" title="Since draft-ietf-httpbis-header-structure-13">

<t><list style="symbols">
  <t>Editorial improvements.</t>
  <t>Define “structured header name” and “structured header value” terms (#908).</t>
  <t>Corrected text about valid characters in strings (#931).</t>
  <t>Removed most instances anchor="implementation-notes">
      <name>Implementation Notes</name>
      <t>A generic implementation of this specification should expose the word “textual”, as
      top-level serialize (<xref target="text-serialize"/>)
      and parse (<xref target="text-parse"/>) functions. They
      need not be functions; for example, it was redundant (#915).</t>
  <t>Allowed parameters on Items could be implemented as an
      object, with methods for each of the different top-level types.</t>
      <t>For interoperability, it's important that generic implementations be
      complete and Inner Lists (#907).</t>
  <t>Expand follow the range of characters in token (#961).</t>
  <t>Disallow OWS before “;” delimiter in parameters (#961).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-12" title="Since draft-ietf-httpbis-header-structure-12">

<t><list style="symbols">
  <t>Editorial improvements.</t>
  <t>Reworked float serialisation (#896).</t>
  <t>Don’t add algorithms closely; see <xref target="strict"/>. To aid this, a trailing space in inner-list (#904).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-11" title="Since draft-ietf-httpbis-header-structure-11">

<t><list style="symbols">
  <t>Allow * in key (#844).</t>
  <t>Constrain floats to six digits of precision (#848).</t>
  <t>Allow dictionary members to have parameters (#842).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-10" title="Since draft-ietf-httpbis-header-structure-10">

<t><list style="symbols">
  <t>Update abstract (#799).</t>
  <t>Input common test suite is being maintained
      by the community at <eref brackets="angle" target="https://github.com/httpwg/structured-field-tests"/>.</t>
      <t>Implementers should note that Dictionaries and output Parameters are now arrays
      order-preserving maps. Some fields may not convey meaning in the
      ordering of bytes (#662).</t>
  <t>Implementations these data types, but it should still be exposed so
      that it will be available to applications that need to use it.</t>
      <t>Likewise, implementations should note that it's important to preserve difference
      the distinction between token and string (#790).</t>
  <t>Allow empty dictionaries Tokens and lists (#781).</t>
  <t>Change parameterized lists to Strings. While most programming
      languages have primary items (#797).</t>
  <t>Allow inner lists in both dictionaries and lists; removes lists of lists (#816).</t>
  <t>Subsume Parameterised Lists into Lists (#839).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-09" title="Since draft-ietf-httpbis-header-structure-09">

<t><list style="symbols">
  <t>Changed Boolean from T/F native types that map to 1/0 (#784).</t>
  <t>Parameters are now ordered maps (#765).</t>
  <t>Clamp integers the other types well, it may be
      necessary to 15 digits (#737).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-08" title="Since draft-ietf-httpbis-header-structure-08">

<t><list style="symbols">
  <t>Disallow whitespace before items properly (#703).</t>
  <t>Created “key” for create a wrapper "token" object or use in dictionaries and parameters, rather than relying a parameter on identifier (#702). Identifiers have
      functions to assure that these types remain separate.</t>
      <t>The serialization algorithm is defined in a separate minimum supported size.</t>
  <t>Expanded way that it is not
      strictly limited to the range of special characters allowed data types defined in <xref target="types"/> in identifier every case. For example, Decimals are designed to include all of ALPHA, “.”, “:”,
      take broader input and “%” (#702).</t>
  <t>Use “?” instead of “!” round to indicate a Boolean (#719).</t>
  <t>Added “Intentionally Strict Processing” (#684).</t>
  <t>Gave better names for referring specs allowed values.</t>
      <t>Implementations are allowed to use in Parameterised Lists (#720).</t>
  <t>Added Lists limit the size of Lists (#721).</t>
  <t>Rename Identifier to Token (#725).</t>
  <t>Add implementation guidance (#727).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-07" title="Since draft-ietf-httpbis-header-structure-07">

<t><list style="symbols">
  <t>Make Dictionaries ordered mappings (#659).</t>
  <t>Changed “binary content” to “byte sequence” to align with Infra specification (#671).</t>
  <t>Changed “mapping” different
      structures, subject to “map” for #671.</t>
  <t>Don’t fail if byte sequences aren’t “=” padded (#658).</t>
  <t>Add Booleans (#683).</t>
  <t>Allow identifiers in items again (#629).</t>
  <t>Disallowed whitespace before items (#703).</t>
  <t>Explain the consequences of splitting a string across multiple headers (#686).</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-06" title="Since draft-ietf-httpbis-header-structure-06">

<t><list style="symbols">
  <t>Add a FAQ.</t>
  <t>Allow non-zero pad bits.</t>
  <t>Explicitly check minimums defined for integers that violate constraints.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-05" title="Since draft-ietf-httpbis-header-structure-05">

<t><list style="symbols">
  <t>Reorganise specification to separate parsing out.</t>
  <t>Allow referencing specs to use ABNF.</t>
  <t>Define serialisation algorithms.</t>
  <t>Refine relationship between ABNF, parsing and serialisation algorithms.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-04" title="Since draft-ietf-httpbis-header-structure-04">

<t><list style="symbols">
  <t>Remove identifiers from item.</t>
  <t>Remove most limits on sizes.</t>
  <t>Refine number parsing.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-03" title="Since draft-ietf-httpbis-header-structure-03">

<t><list style="symbols">
  <t>Strengthen language around failure handling.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-02" title="Since draft-ietf-httpbis-header-structure-02">

<t><list style="symbols">
  <t>Split Numbers into Integers and Floats.</t>
  <t>Define number parsing.</t>
  <t>Tighten up binary parsing and give it each type. When a
      structure exceeds an explicit end delimiter.</t>
  <t>Clarify implementation limit, that mappings are unordered.</t>
  <t>Allow zero-length strings.</t>
  <t>Improve string structure fails parsing algorithm.</t>
  <t>Improve limits in algorithms.</t>
  <t>Require parsers to combine header fields before processing.</t>
  <t>Throw an error on trailing garbage.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-01" title="Since draft-ietf-httpbis-header-structure-01">

<t><list style="symbols">
  <t>Replaced with draft-nottingham-structured-headers.</t>
</list></t>

</section>
<section anchor="since-draft-ietf-httpbis-header-structure-00" title="Since draft-ietf-httpbis-header-structure-00">

<t><list style="symbols">
  <t>Added signed 64bit integer type.</t>
  <t>Drop UTF8, and settle on BCP137 ::EmbeddedUnicodeChar for h1-unicode-string.</t>
  <t>Change h1_blob delimiter to “:” since “’” is valid t_char</t>
</list></t>

</section>
      or serialization.</t>
    </section>
    <section numbered="false" anchor="acknowledgements" title="Acknowledgements"> anchor="acknowledgements">
      <name>Acknowledgements</name>
      <t>Many thanks to Matthew Kerwin <contact fullname="Matthew Kerwin"/> for his detailed feedback and careful
      consideration during the development of this specification.</t>
      <t>Thanks also to Ian Clelland, Roy Fielding, Anne <contact fullname="Ian Clelland"/>, <contact fullname="Roy Fielding"/>, <contact fullname="Anne van Kesteren, Kazuho Oku, Evert Pot, Julian Reschke, Martin Thomson, Mike West, and Jeffrey Yasskin Kesteren"/>,
      <contact fullname="Kazuho Oku"/>, <contact fullname="Evert Pot"/>,
      <contact fullname="Julian Reschke"/>, <contact fullname="Martin       Thomson"/>, <contact fullname="Mike West"/>, and <contact fullname="Jeffrey Yasskin"/> for their contributions.</t>
    </section>
  </back>

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