HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                               P-H. Kamp
Expires: July 31, September 10, 2020                    The Varnish Cache Project
                                                        January 28,
                                                           March 9, 2020

                    Structured Headers Field Values for HTTP
                 draft-ietf-httpbis-header-structure-15
                 draft-ietf-httpbis-header-structure-16

Abstract

   This document describes a set of data types and associated algorithms
   that are intended to make it easier and safer to define and handle
   HTTP header fields. and trailer fields, known as "Structured Fields", or
   "Structured Headers".  It is intended for use by specifications of
   new HTTP header fields that wish to use a common syntax that is more
   restrictive than traditional HTTP field values.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].

   Working Group information can be found at https://httpwg.github.io/
   [2]; source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/header-structure
   [3].

   Tests for implementations are collected at https://github.com/httpwg/
   structured-header-tests [4].

   Implementations are tracked at https://github.com/httpwg/wiki/wiki/
   Structured-Headers [5].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on July 31, September 10, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Intentionally Strict Processing . . . . . . . . . . . . .   4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   2.  Defining New Structured Headers Fields  . . . . . . . . . . . . . . .   5
   3.  Structured Data Types . . . . . . . . . . . . . . . . . . . .   7   8
     3.1.  Lists . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.1.1.  Inner Lists . . . . . . . . . . . . . . . . . . . . .   8   9
       3.1.2.  Parameters  . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Dictionaries  . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Items . . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
       3.3.1.  Integers  . . . . . . . . . . . . . . . . . . . . . .  12
       3.3.2.  Decimals  . . . . . . . . . . . . . . . . . . . . . .  12
       3.3.3.  Strings . . . . . . . . . . . . . . . . . . . . . . .  12  13
       3.3.4.  Tokens  . . . . . . . . . . . . . . . . . . . . . . .  13  14
       3.3.5.  Byte Sequences  . . . . . . . . . . . . . . . . . . .  13  14
       3.3.6.  Booleans  . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Working With Structured Headers Fields in HTTP Headers  . . . . . . .  14 . . . .  15
     4.1.  Serializing Structured Headers Fields . . . . . . . . . . . . .  14 .  15
       4.1.1.  Serializing a List  . . . . . . . . . . . . . . . . .  15
       4.1.2.  Serializing a Dictionary  . . . . . . . . . . . . . .  17
       4.1.3.  Serializing an Item . . . . . . . . . . . . . . . . .  17  18
       4.1.4.  Serializing an Integer  . . . . . . . . . . . . . . .  18  19
       4.1.5.  Serializing a Decimal . . . . . . . . . . . . . . . .  19
       4.1.6.  Serializing a String  . . . . . . . . . . . . . . . .  19  20
       4.1.7.  Serializing a Token . . . . . . . . . . . . . . . . .  20
       4.1.8.  Serializing a Byte Sequence . . . . . . . . . . . . .  20  21
       4.1.9.  Serializing a Boolean . . . . . . . . . . . . . . . .  21
     4.2.  Parsing Header Fields into Structured Headers Fields . . . . . .  21 . . . . . . . . . .  22
       4.2.1.  Parsing a List  . . . . . . . . . . . . . . . . . . .  22  23
       4.2.2.  Parsing a Dictionary  . . . . . . . . . . . . . . . .  24  25
       4.2.3.  Parsing an Item . . . . . . . . . . . . . . . . . . .  25  26
       4.2.4.  Parsing a Number  . . . . . . an Integer or Decimal . . . . . . . . . . . .  27  28
       4.2.5.  Parsing a String  . . . . . . . . . . . . . . . . . .  28  29
       4.2.6.  Parsing a Token . . . . . . . . . . . . . . . . . . .  29  30
       4.2.7.  Parsing a Byte Sequence . . . . . . . . . . . . . . .  29  30
       4.2.8.  Parsing a Boolean . . . . . . . . . . . . . . . . . .  30  31
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31  32
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  31  32
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31  32
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  31  32
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  32  33
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  33
   Appendix B. A.  Frequently Asked Questions . . . . . . . . . . . . .  33
     B.1.  34
     A.1.  Why not JSON? . . . . . . . . . . . . . . . . . . . . . .  33
     B.2.  Structured Headers don't "fit" my data. . . . . . . . . .  34
   Appendix C. B.  Implementation Notes . . . . . . . . . . . . . . . .  34
   Appendix D. C.  Changes  . . . . . . . . . . . . . . . . . . . . . .  35
     D.1.
     C.1.  Since draft-ietf-httpbis-header-structure-15  . . . . . .  35
     C.2.  Since draft-ietf-httpbis-header-structure-14  . . . . . .  35
     D.2.
     C.3.  Since draft-ietf-httpbis-header-structure-13  . . . . . .  35
     D.3.  36
     C.4.  Since draft-ietf-httpbis-header-structure-12  . . . . . .  36
     D.4.
     C.5.  Since draft-ietf-httpbis-header-structure-11  . . . . . .  36
     D.5.  37
     C.6.  Since draft-ietf-httpbis-header-structure-10  . . . . . .  36
     D.6.  37
     C.7.  Since draft-ietf-httpbis-header-structure-09  . . . . . .  36
     D.7.  37
     C.8.  Since draft-ietf-httpbis-header-structure-08  . . . . . .  37
     D.8.
     C.9.  Since draft-ietf-httpbis-header-structure-07  . . . . . .  37
     D.9.  38
     C.10. Since draft-ietf-httpbis-header-structure-06  . . . . . .  38
     D.10.
     C.11. Since draft-ietf-httpbis-header-structure-05  . . . . . .  38
     D.11.
     C.12. Since draft-ietf-httpbis-header-structure-04  . . . . . .  38
     D.12.  39
     C.13. Since draft-ietf-httpbis-header-structure-03  . . . . . .  38
     D.13.  39
     C.14. Since draft-ietf-httpbis-header-structure-02  . . . . . .  38
     D.14.  39
     C.15. Since draft-ietf-httpbis-header-structure-01  . . . . . .  39
     D.15.
     C.16. Since draft-ietf-httpbis-header-structure-00  . . . . . .  39
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39  40

1.  Introduction

   Specifying the syntax of new HTTP header (and trailer) fields is an
   onerous task; even with the guidance in Section 8.3.1 of [RFC7231],
   there are many decisions - and pitfalls - for a prospective HTTP header
   field author.

   Once a header field is defined, bespoke parsers and serializers often need
   to be written, because each header field value has slightly different
   handling of what looks like common syntax.

   This document introduces a set of common data structures for use in
   definitions of new HTTP header field values to address these problems.  In
   particular, it defines a generic, abstract model for
   header field values, them, along with
   a concrete serialisation serialization for expressing that model in HTTP [RFC7230]
   header and trailer fields.

   A HTTP headers field that are is defined as a "Structured Headers" use Header" (or "Structured
   Trailer", respectively; if the field can be either, it is a
   "Structured Field") uses the types defined in this specification to
   define their its syntax and basic handling rules, thereby simplifying both their
   its definition by specification writers and handling by
   implementations.

   Additionally, future versions of HTTP can define alternative
   serialisations
   serializations of the abstract model of these structures, allowing
   headers
   fields that use it to be transmitted more efficiently without being
   redefined.

   Note that it is not a goal of this document to redefine the syntax of
   existing HTTP headers; fields; the mechanisms described herein are only
   intended to be used with headers those that explicitly opt into them.

   Section 2 describes how to specify a Structured Header. Field.

   Section 3 defines a number of abstract data types that can be used in
   Structured Headers. Fields.

   Those abstract types can be serialized into and parsed from HTTP headers
   field values using the algorithms described in Section 4.

1.1.  Intentionally Strict Processing

   This specification intentionally defines strict parsing and
   serialisation behaviours
   serialization behaviors using step-by-step algorithms; the only error
   handling defined is to fail the operation altogether.

   It is designed to encourage faithful implementation and therefore
   good interoperability.  Therefore, an implementation that tried to be
   "helpful" by being more tolerant of input would make interoperability
   worse, since that would create pressure on other implementations to
   implement similar (but likely subtly different) workarounds.

   In other words, strict processing is an intentional feature of this
   specification; it allows non-conformant input to be discovered and
   corrected by the producer early, and avoids both interoperability and
   security issues that might otherwise result.

   Note that as a result of this strictness, if a header field is appended to
   by multiple parties (e.g., intermediaries, or different components in
   the sender), an error in one party's value is likely to cause the
   entire header field value to fail parsing.

1.2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses algorithms to specify parsing and serialisation
   behaviours, serialization
   behaviors, and the Augmented Backus-Naur Form (ABNF) notation of
   [RFC5234] to illustrate expected syntax in HTTP header fields.  In
   doing so, it uses the VCHAR, SP, DIGIT, ALPHA and DQUOTE rules from
   [RFC5234].  It also includes the tchar rule from [RFC7230].

   When parsing from HTTP header fields, implementations MUST follow the
   algorithms, but MAY vary in implementation so as the behaviours behaviors are
   indistinguishable from specified behaviour. behavior.  If there is disagreement
   between the parsing algorithms and ABNF, the specified algorithms
   take precedence.  In some places, the algorithms are "greedy" with
   whitespace, but this should not affect conformance.

   For serialisation serialization to header HTTP fields, the ABNF illustrates the range of
   acceptable wire representations with as much fidelity as possible,
   and the algorithms define the recommended way to produce them.
   Implementations MAY vary from the specified behaviour behavior so long as the
   output still matches the ABNF.

2.  Defining New Structured Headers Fields

   To specify a HTTP header field as a structured header, Structured Field, its authors needs to:

   o  Reference this specification.  Recipients and generators of the
      header
      field need to know that the requirements of this document are in
      effect.

   o  Specify the type of  Identify whether the header field itself; either Dictionary
      (Section 3.2), is a Structured Header (i.e., it can
      only be used in the header section - the common case), a
      Structured Field (only in the trailer section), or a Structured
      Field (both).

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

   o  Define the semantics of those structures.

   o  Specify any additional constraints upon the structures used, as
      well as the consequences when those constraints are violated.

   Typically, this means that a header field definition will specify the top-
   level type - Dictionary, List List, Dictionary or Item - and then define its allowable
   types, and constraints upon them.  For example, a header defined as a
   List might have all Integer members, or a mix of types; a header
   defined as an Item might allow only Strings, and additionally only
   strings beginning with the letter "Q".  Likewise, inner lists Inner Lists are
   only valid when a header field definition explicitly allows them.

   When Structured Headers parsing fails, the header field is ignored (see Section 4.2); in most
   situations, violating header-specific 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, it will by default be ignored.
   If the header field requires different error handling, this should be
   explicitly specified.

   However, both items Items and inner lists Inner Lists allow parameters as an
   extensibility mechanism; this means that values can later be extended
   to accommodate more information, if need be.  As a result, header field
   specifications are discouraged from defining the presence of an
   unrecognised parameter
   unrecognized Parameter as an error condition.

   To help assure that this extensibility is available in the future,
   and to encourage consumers to use a fully capable Structured Headers
   parser, complete parser implementation, a header
   field definition can specify that "grease" parameters Parameters be added by
   senders.  For example, a specification could stipulate that all parameters
   Parameters beginning with the letter 'q' "h" are reserved for this
   use. 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.

   Note that a header field definition cannot relax the requirements of this
   specification because doing so would preclude handling by generic
   software; they can only add additional constraints (for example, on
   the numeric range of integers Integers and decimals, Decimals, the format of
   strings Strings and tokens,
   Tokens, the types allowed in a dictionary's Dictionary's values, or the number of items
   Items in a list). List).  Likewise, header field definitions can only use Structured Headers this
   specification for the entire header field value, not a portion thereof.

   This specification defines minimums for the length or number of
   various structures supported by Structured Headers implementations.  It does not specify
   maximum sizes in most cases, but header authors should be aware that HTTP
   implementations do impose various limits on the size of individual header
   fields, the total number of fields, and/or the size of the entire
   header block. or trailer section.

   Specifications can refer to a Structured Header's field-name field name as a "structured header
   name", "structured trailer name" and or "structured field name" as
   appropriate.  Likewise, they can refer its field-value field value as a
   "structured header value", "structured trailer value" or "structured
   field value" as necessary.  Header  Field definitions are encouraged to use
   the ABNF rules beginning with "sh-" defined in this specification;
   other rules in this specification are not intended for their use.

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

   42. Foo-Example Header

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

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

     Foo-Example = sh-integer

   Its value indicates the amount of Foo in the message, and MUST
   be between 0 and 10, inclusive; other values MUST cause
   the entire header to be ignored.

   The following parameters are defined:
   * A parameter Parameter whose name is "fooUrl", "foourl", and whose value is a string String
     (Section Y.Y of [RFCxxxx]), conveying the Foo URLs URL
     for the message. See below for processing requirements.

   "fooUrl"

   "foourl" contains a URI-reference (Section 4.1 of
   [RFC3986], Section 4.1).
   [RFC3986]). If its value is not a valid URI-reference,
   that URL
   it MUST be ignored. If its value is a relative reference
   (Section 4.2 of [RFC3986]), it MUST be resolved (Section 5 of
   [RFC3986]) before being used.

   For example:

     Foo-Example: 2; foourl="https://foo.example.com/"

3.  Structured Data Types

   This section defines the abstract value types that can be composed
   into Structured Headers. Fields.  The ABNF provided represents the on-wire
   format in HTTP headers. field values.

   In summary:

   o  There are three top-level types that a HTTP header field can be defined
      as; Lists, Dictionaries, and Items.

   o  Lists and Dictionaries are containers; their members can be Items
      or Inner Lists (which are themselves lists of items).

   o  Both Items and Inner Lists can be parameterised parameterized with key/value
      pairs.

3.1.  Lists

   Lists are arrays of zero or more members, each of which can be an
   item
   Item (Section 3.3) or an inner list Inner List (Section 3.1.1), both of which
   can be parameterised Parameterized (Section 3.1.2).

   The ABNF for lists Lists in HTTP headers fields is:

   sh-list       = list-member *( *SP "," *SP list-member )
   list-member   = sh-item / inner-list

   In HTTP headers, each

   Each member is separated by a comma and optional whitespace.  For
   example, a header field whose value is defined as a
   list List of strings Strings could
   look like:

   Example-StrListHeader: "foo", "bar", "It was the best of times."

   In HTTP headers, an

   An empty list List is denoted by not serialising serializing the
   header field at all.

   Note that lists Lists can have their members split across multiple
   instances lines
   inside a block header or trailer section, as per Section 3.2.2 of fields;
   [RFC7230]; for example, the following are equivalent:

   Example-Hdr: foo, bar

   and

   Example-Hdr: foo
   Example-Hdr: bar
   However, individual members of a list List cannot be safely split between instances;
   across lines; see Section 4.2 for details.

   Parsers MUST support lists Lists containing at least 1024 members.  Header  Field
   specifications can constrain the types and cardinality of individual
   list
   List values as they require.

3.1.1.  Inner Lists

   An inner list Inner List is an array of zero or more items Items (Section 3.3).  Both
   the individual items Items and the inner-list Inner List itself can be parameterised Parameterized
   (Section 3.1.2).

   The ABNF for inner-lists in HTTP headers Inner Lists is:

   inner-list    = "(" *SP [ sh-item *( 1*SP sh-item ) *SP ] ")"
                   *parameter

   In HTTP headers, inner lists
                   parameters

   Inner Lists are denoted by surrounding parenthesis, and have their
   values delimited by a single space.  A header field whose value is defined
   as a list of inner-lists Inner Lists of strings Strings could look like:

   Example-StrListListHeader: ("foo" "bar"), ("baz"), ("bat" "one"), ()

   Note that the last member in this example is an empty inner list. Inner List.

   A header field whose value is defined as a list of inner-lists Inner Lists with
   parameters
   Parameters at both levels could look like:

   Example-ListListParam: ("foo"; a=1;b=2);lvl=5, ("bar" "baz");lvl=1

   Parsers MUST support inner-lists Inner Lists containing at least 256 members.
   Header
   Field specifications can constrain the types and cardinality of
   individual inner-list Inner List members as they require.

3.1.2.  Parameters

   Parameters are an ordered map of key-values pairs that are associated
   with an item Item (Section 3.3) or inner-list Inner List (Section 3.1.1).  The keys
   are unique within the scope of a map of parameters, the Parameters they occur within, and the
   values are bare items (i.e., they themselves cannot be parameterised; parameterized;
   see Section 3.3).

   The ABNF for parameters in HTTP headers Parameters is:

   parameter

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

   In HTTP headers, parameters are

   A parameter is separated from their item its Item or inner-
   list Inner List and each other
   parameters by semicolons. a semicolon.  For example:

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

   Parameters whose value is Boolean true MUST omit that value when
   serialised.
   serialized.  For example:

   Example-IntHeader: 1; a; b=?0

   Note that this requirement is only on serialisation; serialization; parsers are
   still required to correctly handle the true value when it appears in
   parameters.
   a parameter.

   Parsers MUST support at least 256 parameters on an item Item or inner-
   list, Inner
   List, and support parameter keys with at least 64 characters.  Header  Field
   specifications can constrain the types and cardinality of individual
   parameter names and values as they require.

3.2.  Dictionaries

   Dictionaries are ordered maps of name-value pairs, where the names
   are short, textual strings and the values are items (Section 3.3) or
   arrays of items, both of which can be parameterised Parameterized (Section 3.1.2).
   There can be zero or more members, and their names are unique in the
   scope of the dictionary Dictionary they occur within.

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

   The ABNF for dictionaries in HTTP headers Dictionaries is:

   sh-dictionary  = dict-member *( *SP "," *SP dict-member )
   dict-member    = member-name [ "=" member-value ]
   member-name    = key
   member-value   = sh-item / inner-list

   In HTTP headers, members
   Members are separated by a comma with optional whitespace, while
   names and values are separated by "=" (without whitespace).  For
   example:

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

   Members whose value is Boolean true MUST omit that value when
   serialised, unless it has parameters.
   serialized.  For example, here both "b" and "c" are true, but "c"'s value is serialised because it has
   parameters: true:

   Example-DictHeader: a=?0, b, c=?1; c; foo=bar

   Note that this requirement is only on serialisation; serialization; parsers are
   still required to correctly handle the true Boolean value when it
   appears in
   dictionary Dictionary values.

   A dictionary Dictionary with a member whose value is an inner-list Inner List of tokens:

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

   A dictionary Dictionary with a mix of singular and list values, some with
   parameters:
   Parameters:

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

   As with lists, an empty dictionary Dictionary is represented in HTTP headers by omitting the
   entire header field.

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

   Note that dictionaries can have their members split across multiple
   instances
   lines inside a block of fields; header or trailer section; for example, the following
   are equivalent:

   Example-Hdr: foo=1, bar=2

   and

   Example-Hdr: foo=1
   Example-Hdr: bar=2

   However, individual members of a dictionary Dictionary cannot be safely split
   between
   instances; lines; see Section 4.2 for details.

   Parsers MUST support dictionaries Dictionaries containing at least 1024 name/value
   pairs, and names with at least 64 characters.

3.3.  Items

   An item is Item can be a integer Integer (Section 3.3.1), decimal Decimal (Section 3.3.2),
   string
   String (Section 3.3.3), token Token (Section 3.3.4), byte sequence Byte Sequence
   (Section 3.3.5), or Boolean (Section 3.3.6).  It can have associated
   parameters
   Parameters (Section 3.1.2).

   The ABNF for items in HTTP headers Items is:

   sh-item   = bare-item *parameter parameters
   bare-item = sh-integer / sh-decimal / sh-string / sh-token
               / sh-binary / sh-boolean

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

   Example-IntItemHeader: 5

   or with parameters: Parameters:

   Example-IntItemHeader: 5; foo=bar

3.3.1.  Integers

   Integers have a range of -999,999,999,999,999 to 999,999,999,999,999
   inclusive (i.e., up to fifteen digits, signed), for IEEE 754
   compatibility ([IEEE754]).

   The ABNF for integers in HTTP headers Integers is:

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

   For example:

   Example-IntegerHeader: 42

   Note that commas in integers Integers are used in this section's prose only
   for readability; they are not valid in the wire format.

3.3.2.  Decimals

   Decimals are numbers with an integer and a fractional component.  The
   Integer
   integer component has at most 12 digits; the fractional component has
   at most three digits.

   The ABNF for decimals in HTTP headers is:

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

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

   Example-DecimalHeader: 4.5

   Note that the serialisation algorithm (Section 4.1.5) rounds input
   with more than three digits of precision in the fractional component.
   If an alternative rounding strategy is desired, this should be
   specified by the header definition to occur before serialisation.

3.3.3.  Strings

   Strings are zero or more printable ASCII [RFC0020] characters (i.e.,
   the range %x20 to %x7E).  Note that this excludes tabs, newlines,
   carriage returns, etc.

   The ABNF for strings in HTTP headers Strings is:

   sh-string = DQUOTE *(chr) DQUOTE
   chr       = unescaped / escaped
   unescaped = %x20-21 / %x23-5B / %x5D-7E
   escaped   = "\" ( DQUOTE / "\" )
   In HTTP headers, strings

   Strings are delimited with double quotes, using a backslash ("\") to
   escape double quotes and backslashes.  For example:

   Example-StringHeader: "hello world"

   Note that strings Strings only use DQUOTE as a delimiter; single quotes do
   not delimit strings. Strings.  Furthermore, only DQUOTE and "\" can be
   escaped; other characters after "\" MUST cause parsing to fail.

   Unicode is not directly supported in strings, Strings, because it causes a
   number of interoperability issues, and - with few exceptions - header field
   values do not require it.

   When it is necessary for a field value to convey non-ASCII content, a
   byte sequence
   Byte Sequence (Section 3.3.5) SHOULD can be specified, along with a
   character encoding (preferably [UTF-8]).

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

3.3.4.  Tokens

   Tokens are short textual words; their abstract model is identical to
   their expression in the HTTP header serialisation. field value serialization.

   The ABNF for tokens in HTTP headers Tokens is:

   sh-token = ( ALPHA / "\*" "*" ) *( tchar / ":" / "/" )

   Parsers MUST support tokens Tokens with at least 512 characters.

   Note that a Structured Header token Token allows the characters as the "token" ABNF rule
   defined in [RFC7230], with the exceptions that the first character is
   required to be either ALPHA or "*", and ":" and "/" are also allowed
   in subsequent characters.

3.3.5.  Byte Sequences

   Byte sequences Sequences can be conveyed in Structured Headers. Fields.

   The ABNF for a byte sequence in HTTP headers Byte Sequence is:

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

   In HTTP headers, a byte sequence

   A Byte Sequence is delimited with colons and encoded using base64
   ([RFC4648], Section 4).  For example:

   Example-BinaryHdr: :cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg==:

   Parsers MUST support byte sequences Byte Sequences with at least 16384 octets after
   decoding.

3.3.6.  Booleans

   Boolean values can be conveyed in Structured Headers. Fields.

   The ABNF for a Boolean in HTTP headers is:

   sh-boolean = "?" boolean
   boolean    = "0" / "1"

   In HTTP headers, a boolean

   A Boolean is indicated with a leading "?" character followed by a "1"
   for a true value or "0" for false.  For example:

   Example-BoolHdr: ?1

4.  Working With Structured Headers Fields in HTTP Headers

   This section defines how to serialize and parse Structured Headers Fields in
   header fields,
   field values, and protocols compatible with them (e.g., in HTTP/2
   [RFC7540] before HPACK [RFC7541] is applied).

4.1.  Serializing Structured Headers Fields

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

   1.  If the structure is a Dictionary or List and its value is empty
       (i.e., it has no members), do not serialize the field at all
       (i.e., omit both the field-name and field-value).

   2.  If the structure is a Dictionary, List, let output_string be the result of
       running Serializing a Dictionary List (Section 4.1.2) 4.1.1) with the structure.

   3.  Else if the structure is a List, Dictionary, let output_string be the
       result of running Serializing a List Dictionary (Section 4.1.1) 4.1.2) with
       the structure.

   4.  Else if the structure is an Item, let output_string be the result
       of running Serializing an Item (Section 4.1.3) with the
       structure.

   5.  Else, fail serialisation. serialization.

   6.  Return output_string converted into an array of bytes, using
       ASCII encoding [RFC0020].

4.1.1.  Serializing a List

   Given an array of (member_value, parameters) tuples as input_list,
   return an ASCII string suitable for use in a HTTP header field value.

   1.  Let output be an empty string.

   2.  For each (member_value, parameters) of input_list:

       1.  If member_value is an array, append the result of running
           Serialising
           Serializing an Inner List (Section 4.1.1.1) with
           (member_value, parameters) to output.

       2.  Otherwise, append the result of running Serializing an Item
           (Section 4.1.3) with (member_value, parameters) to output.

       3.  If more member_values remain in input_list:

           1.  Append a COMMA "," to output.

           2.  Append a single SP to output.

   3.  Return output.

4.1.1.1.  Serialising  Serializing an Inner List

   Given an array of (member_value, parameters) tuples as inner_list,
   and parameters as list_parameters, return an ASCII string suitable
   for use in a HTTP header field value.

   1.  Let output be the string "(".

   2.  For each (member_value, parameters) of inner_list:

       1.  Append the result of running Serializing an Item
           (Section 4.1.3) with (member_value, parameters) to output.

       2.  If more values remain in inner_list, append a single SP to
           output.

   3.  Append ")" to output.

   4.  Append the result of running Serializing Parameters
       Section 4.1.1.2
       (Section 4.1.1.2) with list_parameters to output.

   5.  Return output.

4.1.1.2.  Serializing Parameters

   Given an ordered dictionary Dictionary as input_parameters (each member having a
   param_name and a param_value), return an ASCII string suitable for
   use in a HTTP header field value.

   1.  Let output be an empty string.

   2.  For each parameter-name param_name with a value of param_value in
       input_parameters:

       1.  Append ";" to output.

       2.  Append the result of running Serializing a Key
           (Section 4.1.1.3) with param_name to output.

       3.  If param_value is not Boolean true:

           1.  Append "=" to output.

           2.  Append the result of running Serializing a bare Item
               (Section 4.1.3.1) with param_value to output.

   3.  Return output.

4.1.1.3.  Serializing a Key

   Given a key as input_key, return an ASCII string suitable for use in
   a HTTP header field value.

   1.  If  Convert input_key is not into a sequence of characters, or ASCII characters; if
       conversion fails, fail serialization.

   2.  If input_key contains characters not in lcalpha, DIGIT, "_", "-",
       ".", or "*" fail
       serialisation.

   2. serialization.

   3.  If the first character of input_key is not lcalpha, lcalpha or "*", fail parsing.

   3.
       serialization.

   4.  Let output be an empty string.

   4.

   5.  Append input_key to output.

   5.

   6.  Return output.

4.1.2.  Serializing a Dictionary

   Given an ordered dictionary Dictionary as input_dictionary (each member having a
   member_name and a tuple value of (member_value, parameters)), return
   an ASCII string suitable for use in a HTTP header field value.

   1.  Let output be an empty string.

   2.  For each member_name with a value of (member_value, parameters)
       in input_dictionary:

       1.  Append the result of running Serializing a Key
           (Section 4.1.1.3) with member's member_name to output.

   3.  If member_value is not Boolean true or true:

       1.  Append the result of running Serializing Parameters
           (Section 4.1.1.2) with parameters is not empty: to output.

   4.  Otherwise:

       1.  Append "=" to output.

           1.

       2.  If member_value is an array, append the result of running
               Serialising
           Serializing an Inner List (Section 4.1.1.1) with
           (member_value, parameters) to output.

           2.

       3.  Otherwise, append the result of running Serializing an Item
           (Section 4.1.3) with (member_value, parameters) to output.

   4.

   5.  If more members remain in input_dictionary:

       1.  Append a COMMA "," to output.

       2.  Append a single SP to output.

   5.

   6.  Return output.

4.1.3.  Serializing an Item

   Given an item Item as bare_item and parameters item_parameters Parameters as input, item_parameters, return
   an ASCII string suitable for use in a HTTP header field value.

   1.  Let output be an empty string.

   2.  Append the result of running Serializing a Bare Item
       Section 4.1.3.1 with bare_item to output.

   3.  Append the result of running Serializing Parameters
       Section 4.1.1.2 with item_parameters to output.

   4.  Return output.

4.1.3.1.  Serialising  Serializing a Bare Item

   Given an item Item as input_item, return an ASCII string suitable for use
   in a HTTP header field value.

   1.  If input_item is an integer, Integer, return the result of running
       Serializing an Integer (Section 4.1.4) with input_item.

   2.  If input_item is a decimal, Decimal, return the result of running
       Serializing a Decimal (Section 4.1.5) with input_item.

   3.  If input_item is a string, String, return the result of running
       Serializing a String (Section 4.1.6) with input_item.

   4.  If input_item is a token, Token, return the result of running
       Serializing a Token (Section 4.1.7) with input_item.

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

   6.  If input_item is a byte sequence, Byte Sequence, return the result of running
       Serializing a Byte Sequence (Section 4.1.8) with input_item.

   7.  Otherwise, fail serialisation. serialization.

4.1.4.  Serializing an Integer

   Given an integer Integer as input_integer, return an ASCII string suitable
   for use in a HTTP header field value.

   1.  If input_integer is not an integer in the range of
       -999,999,999,999,999 to 999,999,999,999,999 inclusive, fail
       serialisation.
       serialization.

   2.  Let output be an empty string.

   3.  If input_integer is less than (but not equal to) 0, append "-" to
       output.

   4.  Append input_integer's numeric value represented in base 10 using
       only decimal digits to output.

   5.  Return output.

4.1.5.  Serializing a Decimal

   Given a decimal_number decimal number as input_decimal, return an ASCII string
   suitable for use in a HTTP header field value.

   1.   If input_decimal is not a decimal number, fail serialization.

   2.   If input_decimal has more than three significant digits to the
        right of the decimal point, round it to three decimal places,
        rounding the final digit to the nearest value, or to the even
        value if it is equidistant.

   3.   If input_decimal has more than 12 significant digits to the left
        of the decimal point after rounding, fail serialization.

   4.   Let output be an empty string.

   2.

   5.   If input_decimal is less than (but not equal to) 0, append "-"
        to output.

   3.

   6.   Append input_decimal's integer component represented in base 10
        (using only decimal digits) to output; if it is zero, append
        "0".

   4.  If the number of characters appended in the previous step is
       greater than 12, fail serialisation.

   5.

   7.   Append "." to output.

   6.

   8.   If input_decimal's fractional component is zero, append "0" to
        output.

   7.  Else if input_decimal's fractional component has up to three
       digits, append them represented in base 10 (using only decimal
       digits) to output.

   8.

   9.   Otherwise, append the first three significant digits of input_decimal's
        fractional component (represented represented in base 10, using 10 (using only decimal
        digits) to output, rounding the final digit to the nearest value,
       or to the even value if it is equidistant.

   9. output.

   10.  Return output.

4.1.6.  Serializing a String

   Given a string String as input_string, return an ASCII string suitable for
   use in a HTTP header field value.

   1.  If  Convert input_string is not into a sequence of characters, or ASCII characters; if
       conversion fails, fail serialization.

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

   2. serialization.

   3.  Let output be an empty string.

   3.

   4.  Append DQUOTE to output.

   4.

   5.  For each character char in input_string:

       1.  If char is "\" or DQUOTE:

           1.  Append "\" to output.

       2.  Append char to output.

   5.

   6.  Append DQUOTE to output.

   6.

   7.  Return output.

4.1.7.  Serializing a Token

   Given a token Token as input_token, return an ASCII string suitable for use
   in a HTTP header field value.

   1.  If  Convert input_token is not into a sequence of characters, ASCII characters; if
       conversion fails, fail serialization.

   2.  If the first character of input_token is not ALPHA or "*", or the
       remaining contain portion contains a character not in tchar, ":" or "/",
       fail serialisation.

   2. serialization.

   3.  Let output be an empty string.

   3.

   4.  Append input_token to output.

   4.

   5.  Return output.

4.1.8.  Serializing a Byte Sequence

   Given a byte sequence Byte Sequence as input_bytes, return an ASCII string suitable
   for use in a HTTP header field value.

   1.  If input_bytes is not a sequence of bytes, fail serialisation. serialization.

   2.  Let output be an empty string.

   3.  Append ":" to output.

   4.  Append the result of base64-encoding input_bytes as per
       [RFC4648], Section 4, taking account of the requirements below.

   5.  Append ":" to output.

   6.  Return output.

   The encoded data is required to be padded with "=", as per [RFC4648],
   Section 3.2.

   Likewise, encoded data SHOULD have pad bits set to zero, as per
   [RFC4648], Section 3.5, unless it is not possible to do so due to
   implementation constraints.

4.1.9.  Serializing a Boolean

   Given a Boolean as input_boolean, return an ASCII string suitable for
   use in a HTTP header field value.

   1.  If input_boolean is not a boolean, fail serialisation. serialization.

   2.  Let output be an empty string.

   3.  Append "?" to output.

   4.  If input_boolean is true, append "1" to output.

   5.  If input_boolean is false, append "0" to output.

   6.  Return output.

4.2.  Parsing Header Fields into Structured Headers Fields

   When a receiving implementation parses HTTP header fields that are known to
   be Structured Headers, Fields, it is important that care be taken, as there
   are a number of edge cases that can cause interoperability or even
   security problems.  This section specifies the algorithm for doing
   so.

   Given an array of bytes input_bytes that represents the chosen
   header's
   field's field-value (which is empty if that header field is not present),
   and header_type field_type (one of "dictionary", "list", or "item"), return the
   parsed header value.

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

   2.  Discard any leading SP characters from input_string.

   3.  If header_type field_type is "list", let output be the result of running
       Parsing a List (Section 4.2.1) with input_string.

   4.  If header_type field_type is "dictionary", let output be the result of
       running Parsing a Dictionary (Section 4.2.2) with input_string.

   5.  If header_type field_type is "item", let output be the result of running
       Parsing an Item (Section 4.2.3) with input_string.

   6.  Discard any leading SP characters from input_string.

   7.  If input_string is not empty, fail parsing.

   8.  Otherwise, return output.

   When generating input_bytes, parsers MUST combine all instances of lines in the
   same section (header or trailer) that case-insensitively match the target header
   field name into one comma-separated field-value, as per [RFC7230],
   Section 3.2.2; this assures that the header entire field value is processed
   correctly.

   For Lists and Dictionaries, this has the effect of correctly
   concatenating all instances of the header field, field's lines, as long as individual individual members
   of the top-level data structure are not split across multiple header
   instances.

   Strings split across multiple header instances field lines will have unpredictable
   results, because comma(s) and whitespace inserted upon combination
   will become part of the string output by the parser.  Since
   concatenation might be done by an upstream intermediary, the results
   are not under the control of the serializer or the parser.

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

   If parsing fails - including when calling another algorithm - the
   entire header field's field value MUST be ignored (i.e., treated as if the
   header field
   were not present in the message). section).  This is intentionally strict, to
   improve interoperability and safety, and specifications referencing
   this document are not allowed to loosen this requirement.

   Note that this requirement does not apply to an implementation that
   is not parsing the header field; for example, an intermediary is not
   required to strip a failing header field from a message before
   forwarding it.

4.2.1.  Parsing a List

   Given an ASCII string as input_string, return an array of
   (item_or_inner_list, parameters) tuples. input_string is modified to
   remove the parsed value.

   1.  Let members be an empty array.

   2.  While input_string is not empty:

       1.  Append the result of running Parsing an Item or Inner List
           (Section 4.2.1.1) with input_string to members.

       2.  Discard any leading SP characters from input_string.

       3.  If input_string is empty, return members.

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

       5.  Discard any leading SP characters from input_string.

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

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

4.2.1.1.  Parsing an Item or Inner List

   Given an ASCII string as input_string, return the tuple
   (item_or_inner_list, parameters), where item_or_inner_list can be
   either a single bare item, or an array of (bare_item, parameters)
   tuples. input_string is modified to remove the parsed value.

   1.  If the first character of input_string is "(", return the result
       of running Parsing an Inner List (Section 4.2.1.2) with
       input_string.

   2.  Return the result of running Parsing an Item (Section 4.2.3) with
       input_string.

4.2.1.2.  Parsing an Inner List

   Given an ASCII string as input_string, return the tuple (inner_list,
   parameters), where inner_list is an array of (bare_item, parameters)
   tuples. input_string is modified to remove the parsed value.

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

   2.  Let inner_list be an empty array.

   3.  While input_string is not empty:

       1.  Discard any leading SP characters from input_string.

       2.  If the first character of input_string is ")":

           1.  Consume the first character of input_string.

           2.  Let parameters be the result of running Parsing
               Parameters (Section 4.2.3.2) with input_string.

           3.  Return the tuple (inner_list, parameters).

       3.  Let item be the result of running Parsing an Item
           (Section 4.2.3) with input_string.

       4.  Append item to inner_list.

       5.  If the first character of input_string is not SP or ")", fail
           parsing.

   4.  The end of the inner list was not found; fail parsing.

4.2.2.  Parsing a Dictionary

   Given an ASCII string as input_string, return an ordered map whose
   values are (item_or_inner_list, parameters) tuples. input_string is
   modified to remove the parsed value.

   1.  Let dictionary be an empty, ordered map.

   2.  While input_string is not empty:

       1.  Let this_key be the result of running Parsing a Key
           (Section 4.2.3.3) with input_string.

       2.  If the first character of input_string is "=":

           1.  Consume the first character of input_string.

           2.  Let member be the result of running Parsing an Item or
               Inner List (Section 4.2.1.1) with input_string.

       3.  Otherwise:

           1.  Let value be Boolean true.

           2.  Let parameters be an empty, ordered map. the result of running Parsing
               Parameters Section 4.2.3.2 with input_string.

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

       4.  Add name this_key with value member to dictionary.  If
           dictionary already contains a name this_key (comparing
           character-for-character), overwrite its value.

       5.  Discard any leading SP characters from input_string.

       6.  If input_string is empty, return dictionary.

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

       8.  Discard any leading SP characters from input_string.

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

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

4.2.3.  Parsing an Item

   Given an ASCII string as input_string, return a (bare_item,
   parameters) tuple. input_string is modified to remove the parsed
   value.

   1.  Let bare_item be the result of running Parsing a Bare Item
       (Section 4.2.3.1) with input_string.

   2.  Let parameters be the result of running Parsing Parameters
       (Section 4.2.3.2) with input_string.

   3.  Return the tuple (bare_item, parameters).

4.2.3.1.  Parsing a Bare Item

   Given an ASCII string as input_string, return a bare item. Item.
   input_string is modified to remove the parsed value.

   1.  If the first character of input_string is a "-" or a DIGIT,
       return the result of running Parsing a Number an Integer or Decimal
       (Section 4.2.4) with input_string.

   2.  If the first character of input_string is a DQUOTE, return the
       result of running Parsing a String (Section 4.2.5) with
       input_string.

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

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

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

   6.  Otherwise, the item type is unrecognized; fail parsing.

4.2.3.2.  Parsing Parameters

   Given an ASCII string as input_string, return an ordered map whose
   values are bare items. Items. input_string is modified to remove the parsed
   value.

   1.  Let parameters be an empty, ordered map.

   2.  While input_string is not empty:

       1.  If the first character of input_string is not ";", exit the
           loop.

       2.  Consume a ";" character from the beginning of input_string.

       3.  Discard any leading SP characters from input_string.

       4.  let param_name be the result of running Parsing a Key
           (Section 4.2.3.3) with input_string.

       5.  Let param_value be Boolean true.

       6.  If the first character of input_string is "=":

           1.  Consume the "=" character at the beginning of
               input_string.

           2.  Let param_value be the result of running Parsing a Bare
               Item (Section 4.2.3.1) with input_string.

       7.  Append key param_name with value param_value to parameters.
           If parameters already contains a name param_name (comparing
           character-for-character), overwrite its value.

   3.  Return parameters.

4.2.3.3.  Parsing a Key

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

   1.  If the first character of input_string is not lcalpha, lcalpha or "*",
       fail parsing.

   2.  Let output_string be an empty string.

   3.  While input_string is not empty:

       1.  If the first character of input_string is not one of lcalpha,
           DIGIT, "_", "-", ".", or "*", return output_string.

       2.  Let char be the result of removing consuming the first character of
           input_string.

       3.  Append char to output_string.

   4.  Return output_string.

4.2.4.  Parsing a Number an Integer or Decimal

   Given an ASCII string as input_string, return a number. an Integer or Decimal.
   input_string is modified to remove the parsed value.

   NOTE: This algorithm parses both Integers (Section 3.3.1) and
   Decimals (Section 3.3.2), and returns the corresponding structure.

   1.   Let type be "integer".

   2.   Let sign be 1.

   3.   Let input_number be an empty string.

   4.   If the first character of input_string is "-", consume it and
        set sign to -1.

   5.   If input_string is empty, there is an empty integer; fail
        parsing.

   6.   If the first character of input_string is not a DIGIT, fail
        parsing.

   7.   While input_string is not empty:

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

        2.  If char is a DIGIT, append it to input_number.

        3.  Else, if type is "integer" and char is ".":

            1.  If input_number contains more than 12 characters, fail
                parsing.

            2.  Otherwise, append char to input_number and set type to
                "decimal".

        4.  Otherwise, prepend char to input_string, and exit the loop.

        5.  If type is "integer" and input_number contains more than 15
            characters, fail parsing.

        6.  If type is "decimal" and input_number contains more than 16
            characters, fail parsing.

   8.   If type is "integer":

        1.  Parse input_number as an integer and let output_number be
            the product of the result and sign.

        2.  If output_number is outside the range -999,999,999,999,999
            to 999,999,999,999,999 inclusive, fail parsing.

   9.   Otherwise:

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

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

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

   10.  Return output_number.

4.2.5.  Parsing a String

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

   1.  Let output_string be an empty string.

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

   3.  Discard the first character of input_string.

   4.  While input_string is not empty:

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

       2.  If char is a backslash ("\"):

           1.  If input_string is now empty, fail parsing.

           2.  Let next_char be the result of consuming the first
               character of input_string.

           3.  If next_char is not DQUOTE or "\", fail parsing.

           4.  Append next_char to output_string.

       3.  Else, if char is DQUOTE, return output_string.

       4.  Else, if char is in the range %x00-1f or %x7f (i.e., is not
           in VCHAR or SP), fail parsing.

       5.  Else, append char to output_string.

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

4.2.6.  Parsing a Token

   Given an ASCII string as input_string, return a token. Token. input_string
   is modified to remove the parsed value.

   1.  If the first character of input_string is not ALPHA or "*", fail
       parsing.

   2.  Let output_string be an empty string.

   3.  While input_string is not empty:

       1.  If the first character of input_string is not in tchar, ":"
           or "/", return output_string.

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

       3.  Append char to output_string.

   4.  Return output_string.

4.2.7.  Parsing a Byte Sequence

   Given an ASCII string as input_string, return a byte sequence. Byte Sequence.
   input_string is modified to remove the parsed value.

   1.  If the first character of input_string is not ":", fail parsing.

   2.  Discard the first character of input_string.

   3.  If there is not a ":" character before the end of input_string,
       fail parsing.

   4.  Let b64_content be the result of consuming content of
       input_string up to but not including the first instance of the
       character ":".

   5.  Consume the ":" character at the beginning of input_string.

   6.  If b64_content contains a character not included in ALPHA, DIGIT,
       "+", "/" and "=", fail parsing.

   7.  Let binary_content be the result of Base 64 Decoding [RFC4648]
       b64_content, synthesizing padding if necessary (note the
       requirements about recipient behaviour behavior below).

   8.  Return binary_content.

   Because some implementations of base64 do not allow reject of encoded
   data that is not properly "=" padded (see [RFC4648], Section 3.2),
   parsers SHOULD NOT fail when it is not present, unless they cannot be
   configured to do so.

   Because some implementations of base64 do not allow rejection of
   encoded data that has non-zero pad bits (see [RFC4648], Section 3.5),
   parsers SHOULD NOT fail when it is present, unless they cannot be
   configured to do so.

   This specification does not relax the requirements in [RFC4648],
   Section 3.1 and 3.3; therefore, parsers MUST fail on characters
   outside the base64 alphabet, and on line feeds in encoded data.

4.2.8.  Parsing a Boolean

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

   1.  If the first character of input_string is not "?", fail parsing.

   2.  Discard the first character of input_string.

   3.  If the first character of input_string matches "1", discard the
       first character, and return true.

   4.  If the first character of input_string matches "0", discard the
       first character, and return false.

   5.  No value has matched; fail parsing.

5.  IANA Considerations

   This draft document has no actions for IANA.

6.  Security Considerations

   The size of most types defined by Structured Headers Fields is not limited;
   as a result, extremely large header fields could be an attack vector (e.g.,
   for resource consumption).  Most HTTP implementations limit the sizes
   of individual header fields as well as the overall header
   block or trailer section
   size to mitigate such attacks.

   It is possible for parties with the ability to inject new HTTP header fields
   to change the meaning of a Structured Header. Field.  In some circumstances,
   this will cause parsing to fail, but it is not possible to reliably
   fail in all such circumstances.

7.  References

7.1.  Normative References

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

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

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

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

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

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

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

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/std63>.

7.3.  URIs

   [1] https://lists.w3.org/Archives/Public/ietf-http-wg/

   [2] https://httpwg.github.io/

   [3] https://github.com/httpwg/http-extensions/labels/header-structure

   [4] https://github.com/httpwg/structured-header-tests

   [5] https://github.com/httpwg/wiki/wiki/Structured-Headers

   [6] https://github.com/httpwg/structured-header-tests

Appendix A.  Acknowledgements

   Many thanks to Matthew Kerwin for his detailed feedback and careful
   consideration during the development of this specification. https://httpwg.github.io/

   [3] https://github.com/httpwg/http-extensions/labels/header-structure

   [4] https://github.com/httpwg/structured-header-tests

   [5] https://github.com/httpwg/wiki/wiki/Structured-Headers

   [6] https://github.com/httpwg/structured-header-tests

Appendix B. A.  Frequently Asked Questions

B.1.

A.1.  Why not JSON?

   Earlier proposals for structured headers Structured Fields were based upon JSON
   [RFC8259].  However, constraining its use to make it suitable for
   HTTP header fields required senders and recipients to implement
   specific additional handling.

   For example, JSON has specification issues around large numbers and
   objects with duplicate members.  Although advice for avoiding these
   issues is available (e.g., [RFC7493]), it cannot be relied upon.

   Likewise, JSON strings are by default Unicode strings, which have a
   number of potential interoperability issues (e.g., in comparison).
   Although implementers can be advised to avoid non-ASCII content where
   unnecessary, this is difficult to enforce.

   Another example is JSON's ability to nest content to arbitrary
   depths.  Since the resulting memory commitment might be unsuitable
   (e.g., in embedded and other limited server deployments), it's
   necessary to limit it in some fashion; however, existing JSON
   implementations have no such limits, and even if a limit is
   specified, it's likely that some header field definition will find a need to
   violate it.

   Because of JSON's broad adoption and implementation, it is difficult
   to impose such additional constraints across all implementations;
   some deployments would fail to enforce them, thereby harming
   interoperability.  In short, if it looks like JSON, people will be
   tempted to use a JSON parser / serialiser serializer on header fields. field values.

   Since a major goal for Structured Headers Fields is to improve
   interoperability and simplify implementation, these concerns led to a
   format that requires a dedicated parser and serializer.

   Additionally, there were widely shared feelings that JSON doesn't
   "look right" in HTTP headers.

B.2.  Structured Headers don't "fit" my data.

   Structured headers intentionally limits the complexity of data
   structures, to assure that it can be processed in a performant manner
   with little overhead.  This means that work is necessary to fit some
   data types into them.

   Sometimes, this can be achieved by creating limited substructures in
   values, and/or using more than one header.  For example, consider:

   Example-Thing: name="Widget", cost=89.2, descriptions=(foo bar)
   Example-Description: foo; url="https://example.net"; context=123,
                        bar; url="https://example.org"; context=456

   Since the description contains an array of key/value pairs, we use a
   List to represent them, with the token for each item in the array
   used to identify it in the "descriptions" member of the Example-Thing
   dictionary header.

   When specifying more than one header, it's important to remember to
   describe what a processor's behaviour should be when one of the
   headers is missing.

   If you need to fit arbitrarily complex data into a header, Structured
   Headers is probably a poor fit for your use case. fields.

Appendix C. B.  Implementation Notes

   A generic implementation of this specification should expose the top-
   level parse (Section 4.2) and serialize (Section 4.1) and parse (Section 4.2) functions.
   They need not be functions; for example, it could be implemented as
   an object, with methods for each of the different top-level types.

   For interoperability, it's important that generic implementations be
   complete and follow the algorithms closely; see Section 1.1.  To aid
   this, a common test suite is being maintained by the community at
   https://github.com/httpwg/structured-header-tests [6].

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

   Likewise, implementations should note that it's important to preserve
   the distinction between tokens Tokens and strings. Strings.  While most programming
   languages have native types that map to the other types well, it may
   be necessary to create a wrapper "token" object or use a parameter on
   functions to assure that these types remain separate.

   The serialization algorithm is defined in a way that it is not
   strictly limited to the data types defined in Section 3 in every
   case.  For example, Decimals are designed to take broader input and
   round to allowed values.

Appendix D. C.  Changes

   _RFC Editor: Please remove this section before publication._

D.1.

C.1.  Since draft-ietf-httpbis-header-structure-15

   o  Editorial improvements.

   o  Use HTTP field terminology more consistently, in line with recent
      changes to HTTP-core.

   o  String length requirements apply to decoded strings (#1051).

   o  Correctly round decimals in serialisation (#1043).

   o  Clarify input to serialisation algorithms (#1055).

   o  Omitted True dictionary value can have parameters (#1083).

   o  Keys can now start with '*' (#1068).

C.2.  Since draft-ietf-httpbis-header-structure-14

   o  Editorial improvements.

   o  Allow empty dictionary values (#992).

   o  Change value of omitted parameter value to True (#995).

   o  Explain more about splitting dictionaries and lists across header
      instances (#997).

   o  Disallow HTAB, replace OWS with spaces (#998).

   o  Change byte sequence delimiters from "*" to ":" (#991).

   o  Allow tokens to start with "*" (#991).

   o  Change Floats to fixed-precision Decimals (#982).

   o  Round the fractional component of decimal, rather than truncating
      it (#982).

   o  Handle duplicate dictionary and parameter keys by overwriting
      their values, rather than failing (#997).

   o  Allow "." in key (#1027).

   o  Check first character of key in serialisation (#1037).

   o  Talk about greasing headers (#1015).

D.2.

C.3.  Since draft-ietf-httpbis-header-structure-13

   o  Editorial improvements.

   o  Define "structured header name" and "structured header value"
      terms (#908).

   o  Corrected text about valid characters in strings (#931).

   o  Removed most instances of the word "textual", as it was redundant
      (#915).

   o  Allowed parameters on Items and Inner Lists (#907).

   o  Expand the range of characters in token (#961).

   o  Disallow OWS before ";" delimiter in parameters (#961).

D.3.

C.4.  Since draft-ietf-httpbis-header-structure-12

   o  Editorial improvements.

   o  Reworked float serialisation (#896).

   o  Don't add a trailing space in inner-list (#904).

D.4.

C.5.  Since draft-ietf-httpbis-header-structure-11

   o  Allow * in key (#844).

   o  Constrain floats to six digits of precision (#848).

   o  Allow dictionary members to have parameters (#842).

D.5.

C.6.  Since draft-ietf-httpbis-header-structure-10

   o  Update abstract (#799).

   o  Input and output are now arrays of bytes (#662).

   o  Implementations need to preserve difference between token and
      string (#790).

   o  Allow empty dictionaries and lists (#781).

   o  Change parameterized lists to have primary items (#797).

   o  Allow inner lists in both dictionaries and lists; removes lists of
      lists (#816).

   o  Subsume Parameterised Lists into Lists (#839).

D.6.

C.7.  Since draft-ietf-httpbis-header-structure-09

   o  Changed Boolean from T/F to 1/0 (#784).

   o  Parameters are now ordered maps (#765).

   o  Clamp integers to 15 digits (#737).

D.7.

C.8.  Since draft-ietf-httpbis-header-structure-08

   o  Disallow whitespace before items properly (#703).

   o  Created "key" for use in dictionaries and parameters, rather than
      relying on identifier (#702).  Identifiers have a separate minimum
      supported size.

   o  Expanded the range of special characters allowed in identifier to
      include all of ALPHA, ".", ":", and "%" (#702).

   o  Use "?" instead of "!" to indicate a Boolean (#719).

   o  Added "Intentionally Strict Processing" (#684).

   o  Gave better names for referring specs to use in Parameterised
      Lists (#720).

   o  Added Lists of Lists (#721).

   o  Rename Identifier to Token (#725).

   o  Add implementation guidance (#727).

D.8.

C.9.  Since draft-ietf-httpbis-header-structure-07

   o  Make Dictionaries ordered mappings (#659).

   o  Changed "binary content" to "byte sequence" to align with Infra
      specification (#671).

   o  Changed "mapping" to "map" for #671.

   o  Don't fail if byte sequences aren't "=" padded (#658).

   o  Add Booleans (#683).

   o  Allow identifiers in items again (#629).

   o  Disallowed whitespace before items (#703).

   o  Explain the consequences of splitting a string across multiple
      headers (#686).

D.9.

C.10.  Since draft-ietf-httpbis-header-structure-06

   o  Add a FAQ.

   o  Allow non-zero pad bits.

   o  Explicitly check for integers that violate constraints.

D.10.

C.11.  Since draft-ietf-httpbis-header-structure-05

   o  Reorganise specification to separate parsing out.

   o  Allow referencing specs to use ABNF.

   o  Define serialisation algorithms.

   o  Refine relationship between ABNF, parsing and serialisation
      algorithms.

D.11.

C.12.  Since draft-ietf-httpbis-header-structure-04

   o  Remove identifiers from item.

   o  Remove most limits on sizes.

   o  Refine number parsing.

D.12.

C.13.  Since draft-ietf-httpbis-header-structure-03

   o  Strengthen language around failure handling.

D.13.

C.14.  Since draft-ietf-httpbis-header-structure-02

   o  Split Numbers into Integers and Floats.

   o  Define number parsing.

   o  Tighten up binary parsing and give it an explicit end delimiter.

   o  Clarify that mappings are unordered.

   o  Allow zero-length strings.

   o  Improve string parsing algorithm.

   o  Improve limits in algorithms.

   o  Require parsers to combine header fields before processing.

   o  Throw an error on trailing garbage.

D.14.

C.15.  Since draft-ietf-httpbis-header-structure-01

   o  Replaced with draft-nottingham-structured-headers.

D.15.

C.16.  Since draft-ietf-httpbis-header-structure-00

   o  Added signed 64bit integer type.

   o  Drop UTF8, and settle on BCP137 ::EmbeddedUnicodeChar for h1-
      unicode-string.

   o  Change h1_blob delimiter to ":" since "'" is valid t_char

Acknowledgements

   Many thanks to Matthew Kerwin for his detailed feedback and careful
   consideration during the development of this specification.

   Thanks also to Ian Clelland, Roy Fielding, Anne van Kesteren, Kazuho
   Oku, Evert Pot, Julian Reschke, Martin Thomson, Mike West, and
   Jeffrey Yasskin for their contributions.

Authors' Addresses

   Mark Nottingham
   Fastly

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

   Poul-Henning Kamp
   The Varnish Cache Project

   Email: phk@varnish-cache.org