HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                               P-H. Kamp
Expires: October 19, 2019 January 9, 2020                       The Varnish Cache Project
                                                          April 17,
                                                            July 8, 2019

                      Structured Headers for HTTP
                 draft-ietf-httpbis-header-structure-10
                 draft-ietf-httpbis-header-structure-11

Abstract

   This document describes a set of data types and algorithms associated
   with them algorithms
   that are intended to make it easier and safer to define and handle
   HTTP header fields.  It is intended for use by new specifications of new
   HTTP header fields as well as revisions of existing
   header that wish to use a common syntax that is more
   restrictive than traditional HTTP field specifications when doing so does not cause
   interoperability issues. 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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on October 19, 2019. January 9, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Intentionally Strict Processing . . . . . . . . . . . . .   4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Defining New Structured Headers . . . . . . . . . . . . . . .   5
   3.  Structured Header Data Types  . . . . . . . . . . . . . . . .   7   6
     3.1.  Dictionaries  Lists . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Lists . . . .   6
     3.2.  Dictionaries  . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Lists of Lists  . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Parameterised Lists . . . . . . . . . . . . . . . . . . .   8
     3.5.  Items . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.6.   8
     3.4.  Integers  . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.7.   8
     3.5.  Floats  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.8.
     3.6.  Strings . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.9.   9
     3.7.  Tokens  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.10.  10
     3.8.  Byte Sequences  . . . . . . . . . . . . . . . . . . . . .  11
     3.11.  10
     3.9.  Booleans  . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  Working With Structured Headers in HTTP/1  . . . . . . . Textual HTTP Headers . . .  11
     4.1.  Serializing Structured Headers  . . . . . .  12
     4.1.  Serialising Structured Headers into HTTP/1 . . . . . . .  12  11
     4.2.  Parsing HTTP/1 Header Fields into Structured Headers . .  18 . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27  26
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  28  26
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28  26
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  28  26
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  29  27
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  29  28
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  30  28
   Appendix B.  Frequently Asked Questions . . . . . . . . . . . . .  30  28
     B.1.  Why not JSON? . . . . . . . . . . . . . . . . . . . . . .  30  28
     B.2.  Structured Headers don't "fit" my data. . . . . . . . . .  30
     B.3.  What should generic Structured Headers implementations
           expose? . . .  29
   Appendix C.  Implementation Notes . . . . . . . . . . . . . . . .  29
   Appendix D.  Changes  . . . . . .  31
   Appendix C.  Changes . . . . . . . . . . . . . . . .  30
     D.1.  Since draft-ietf-httpbis-header-structure-10  . . . . . .  31
     C.1.  30
     D.2.  Since draft-ietf-httpbis-header-structure-09  . . . . . .  31
     C.2.  30
     D.3.  Since draft-ietf-httpbis-header-structure-08  . . . . . .  32
     C.3.  31
     D.4.  Since draft-ietf-httpbis-header-structure-07  . . . . . .  32
     C.4.  31
     D.5.  Since draft-ietf-httpbis-header-structure-06  . . . . . .  33
     C.5.  32
     D.6.  Since draft-ietf-httpbis-header-structure-05  . . . . . .  33
     C.6.  32
     D.7.  Since draft-ietf-httpbis-header-structure-04  . . . . . .  33
     C.7.  32
     D.8.  Since draft-ietf-httpbis-header-structure-03  . . . . . .  33
     C.8.  32
     D.9.  Since draft-ietf-httpbis-header-structure-02  . . . . . .  33
     C.9.  32
     D.10. Since draft-ietf-httpbis-header-structure-01  . . . . . .  34
     C.10.  33
     D.11. Since draft-ietf-httpbis-header-structure-00  . . . . . .  34  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34  33

1.  Introduction

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

   Once a header field is defined, bespoke parsers and serialisers serializers often
   need to be written, because each header 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, along with a concrete serialisation for
   expressing that model in HTTP/1 textual HTTP [RFC7230] header fields.

   HTTP headers that are defined as "Structured Headers" use the types
   defined in this specification to define their syntax and basic
   handling rules, thereby simplifying both their definition by
   specification writers and handling by implementations.

   Additionally, future versions of HTTP can define alternative
   serialisations of the abstract model of these structures, allowing
   headers 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; the mechanisms described herein are only
   intended to be used with headers that explicitly opt into them.

   To

   Section 2 describes how to specify a header field that is a Structured Header, see Section 2. Header.

   Section 3 defines a number of abstract data types that can be used in
   Structured Headers.  Those abstract types can be serialised serialized into and
   parsed from textual HTTP headers - such as those used in HTTP/1 - using the algorithms described in
   Section 4.

1.1.  Intentionally Strict Processing

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

   This

   It is designed to encourage faithful implementation and therefore
   good interoperability.  Therefore, implementations an implementation that try tried to be
   "helpful" by being more tolerant of input are doing a disservice to
   the overall community, would make interoperability
   worse, since it will encourage 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), it could be that an error in one party's value causes is likely to
   cause the entire header field to fail parsing.

1.2.  Notational Conventions

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

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

   This document uses algorithms to specify parsing and serialisation
   behaviours, and ABNF to illustrate expected syntax in HTTP/1-style
   header fields.

   For

   When parsing from HTTP/1 textual HTTP header fields, implementations MUST
   follow the algorithms, but MAY vary in implementation so as the
   behaviours are indistinguishable from specified behaviour.  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 to HTTP/1 textual header 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 so long
   as the output still matches the ABNF.

2.  Defining New Structured Headers

   To define a HTTP header as a structured header, its specification
   needs to:

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

   o  Specify the header field's allowed syntax for values, in terms of
      the types described in Section 3, along with their associated
      semantics.  Syntax definitions are encouraged to use the ABNF
      rules beginning with "sh-" defined in this specification.

   o  Specify any additional constraints upon the syntax of the
      structured used, as well as the consequences when those
      constraints are violated.  When Structured Headers parsing fails,
      the header is discarded (see Section 4.2); in most situations,
      header-specific constraints should do likewise.

   Note that a header field definition cannot relax the requirements of
   a structure or its processing
   this specification because doing so would preclude handling by
   generic software; they can only add additional
   constraints. constraints (for
   example, on the numeric range of integers and floats, the format of
   strings and tokens, or the number of items in a list).  Likewise,
   header field definitions should use Structured Headers 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.

   For example:

   # example,
   42. Foo-Example Header

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

   Foo-Example is a Structured Header [RFCxxxx]. Its value MUST be a
   dictionary ([RFCxxxx], Section Y.Y). Its ABNF is:

     Foo-Example = sh-dictionary

   The dictionary MUST contain:

   * Exactly one member whose key name is "foo", and whose value is an
     integer ([RFCxxxx], Section Y.Y), indicating the number of foos
     in the message.
   * Exactly one member whose key name is "barUrls", "barUrl", and whose value is a
     string ([RFCxxxx], Section Y.Y), conveying the Bar URLs URL for the
     message. See below for processing requirements.

   If the parsed header field does not contain both, it MUST be
   ignored.

   "foo" MUST be between 0 and 10, inclusive; other values MUST cause
   the header to be ignored.

   "barUrls"

   "barUrl" contains a space-separated list of URI-references URI-reference ([RFC3986], Section 4.1):

      barURLs = URI-reference *( 1*SP URI-reference ) 4.1).

   If a member of barURLs barURL is not a valid URI-reference, it MUST cause
   that value to be ignored.
   If a member of barURLs barURL is a relative reference ([RFC3986], Section 4.2),
   it MUST be resolved ([RFC3986], Section 5) before being used.

   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.

3.  Structured Header Data Types

   This section defines the abstract value types that can be composed
   into Structured Headers.  The ABNF provided represents the on-wire
   format in HTTP/1. HTTP.

3.1.  Dictionaries

   Dictionaries  Lists

   Lists are ordered maps arrays of key-value pairs, where the keys are
   short, textual strings and the values are items (Section 3.5).  There
   can be one zero or more members, and keys are required to be unique.

   Implementations MUST provide access to dictionaries both by index and
   by key.  Specifications MAY use either means each of which can be an
   item (Section 3.3) or an inner list (an array of zero or more items).

   Each member of accessing the
   members. top-level list can also have associated parameters
   - an ordered map of key-value pairs where the keys are short, textual
   strings and the values are items (Section 3.3).  There can be zero or
   more parameters on a member, and their keys are required to be unique
   within that scope.

   The ABNF for dictionaries in HTTP/1 headers lists is:

   sh-dictionary

   sh-list       = dict-member list-member *( OWS "," OWS dict-member list-member )
   dict-member
   list-member   = member-name ( sh-item / inner-list ) *parameter
   inner-list    = "(" OWS [ sh-item *( SP sh-item ) OWS ] ")"
   parameter     = OWS ";" OWS param-name [ "=" member-value
   member-name param-value ]
   param-name    = key
   member-value   = sh-item
   key           = lcalpha *( lcalpha / DIGIT / "_" / "-" )
   lcalpha       = %x61-7A ; a-z

   In HTTP/1, keys and values are separated by "=" (without whitespace),
   and key/value pairs are separated by a comma with optional
   whitespace.  For example:

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

   Typically, a header field specification will define the semantics of
   individual keys, as well as whether their presence is required or
   optional.  Recipients MUST ignore keys that are undefined or unknown,
   unless the header field's specification specifically disallows them.

   Parsers MUST support dictionaries containing at least 1024 key/value
   pairs, and dictionary keys with at least 64 characters.

3.2.  Lists

   Lists are arrays of items (Section 3.5) with one or more members.

   The ABNF for lists in HTTP/1 headers is:

   sh-list     = list-member *( OWS "," OWS list-member )
   list-member
   param-value   = sh-item

   In HTTP/1, textual HTTP headers, each member is separated by a comma and
   optional whitespace.  For example, a header field whose value is
   defined as a list of strings could look like:

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

   Header specifications can constrain the types of individual values if
   necessary.

   Parsers MUST support

   In textual HTTP headers, inner lists containing at least 1024 members.

3.3.  Lists of Lists

   Lists of Lists are arrays of arrays containing items (Section 3.5).

   The ABNF for lists of lists in HTTP/1 headers is:

   sh-listlist = inner-list *( OWS "," OWS inner-list )
   inner-list  = list-member *( OWS ";" OWS list-member )

   In HTTP/1, each inner-list is separated denoted by a comma surrounding
   parenthesis, and optional
   whitespace, and members of the inner-list are separated have their values delimited by semicolons
   and optional whitespace.  For example, a single space.  A
   header field whose value is defined as a list of lists of strings
   could look like:

   Example-StrListListHeader: "foo";"bar", "baz", "bat"; "one"

   Header specifications can constrain ("foo" "bar"), ("baz"), ("bat" "one"), ()

   Note that the types of individual inner-
   list values if necessary. last member in this example is an empty inner list.

   In textual HTTP headers, members' parameters are separated from the
   member and each other by semicolons.  For example:

Example-ParamListHeader: abc_123;a=1;b=2; cdef_456, (ghi jkl);q="9";r="w"

   Parsers MUST support lists of lists containing at least 1024 members,
   and support
   members with at least 256 parameters, support inner-lists containing
   at least 256 members.

3.4.  Parameterised Lists

   Parameterised Lists are arrays of parameterised identifiers, with one
   or more members.

   A parameterised identifier is a primary identifier (a Section 3.9}) members, and support parameter keys with associated parameters, an at least 64
   characters.

   Header specifications can constrain the types of individual list
   values (including that of individual inner-list members and
   parameters) if necessary.

3.2.  Dictionaries

   Dictionaries are ordered map maps of key-value pairs name-value pairs, where the keys names
   are short, textual strings and the values are items (Section 3.5). 3.3) or
   arrays of items.  There can be zero or more parameters, members, and keys their names
   are required to be unique. unique within the scope of the dictionary they
   occur within.

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

   The ABNF for parameterised lists dictionaries in HTTP/1 textual HTTP headers is:

   sh-param-list

   sh-dictionary  = param-item dict-member *( OWS "," OWS param-item dict-member )
   param-item    = primary-id *parameter
   primary-id    = sh-token
   parameter
   dict-member    = OWS ";" OWS param-name [ member-name "=" param-value ]
   param-name member-value
   member-name    = key
   param-value
   member-value   = sh-item / inner-list

   In HTTP/1, each param-id is textual HTTP headers, members are separated by a comma and with
   optional
   whitespace (as in Lists), whitespace, while names and the parameters values are separated by
   semicolons. "="
   (without whitespace).  For example:

   Example-ParamListHeader: abc_123;a=1;b=2; cdef_456, ghi;q="9";r="w"

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

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

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

   Typically, a header field specification will define the semantics of
   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.

   Parsers MUST support parameterised lists dictionaries containing at least 1024
   members, support members with at least 256 parameters, name/value
   pairs, and support
   parameter keys names with at least 64 characters.

3.5.

3.3.  Items

   An item is can be a integer (Section 3.6), 3.4), float (Section 3.7), 3.5),
   string (Section 3.8), 3.6), token (Section 3.9), 3.7), byte sequence
   (Section 3.10), 3.8), or Boolean (Section 3.11). 3.9).

   The ABNF for items in HTTP/1 textual HTTP headers is:

   sh-item = sh-integer / sh-float / sh-string / sh-token / sh-binary
             / sh-boolean

3.6.

3.4.  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). signed), for IEEE 754
   compatibility ([IEEE754]).

   The ABNF for integers in HTTP/1 textual HTTP headers is:

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

   For example:

   Example-IntegerHeader: 42

3.7.

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

3.5.  Floats

   Floats are integers with a fractional part, that can be stored as
   IEEE 754 double precision numbers (binary64) ([IEEE754]).

   The ABNF for floats in HTTP/1 textual HTTP headers is:

   sh-float    = ["-"] (
                DIGIT "." 1*14DIGIT /
               2DIGIT "." 1*13DIGIT /
               3DIGIT "." 1*12DIGIT /
               4DIGIT "." 1*11DIGIT /
               5DIGIT "." 1*10DIGIT /
               6DIGIT "." 1*9DIGIT /
               7DIGIT "." 1*8DIGIT /
               8DIGIT "." 1*7DIGIT /
               9DIGIT "." 1*6DIGIT /
              10DIGIT "." 1*5DIGIT /
              11DIGIT "." 1*4DIGIT /
              12DIGIT "." 1*3DIGIT /
              13DIGIT "." 1*2DIGIT /
              14DIGIT "." 1DIGIT )

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

   Example-FloatHeader: 4.5

3.8.

3.6.  Strings

   Strings are zero or more printable ASCII [RFC0020] characters (i.e.,
   the range 0x20 to 0x7E).  Note that this excludes tabs, newlines,
   carriage returns, etc.

   The ABNF for strings in HTTP/1 textual HTTP headers is:

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

   In HTTP/1 textual HTTP headers, strings are delimited with double quotes,
   using a backslash ("\") to escape double quotes and backslashes.  For
   example:

   Example-StringHeader: "hello world"

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

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

   When it is necessary for a field value to convey non-ASCII string
   content, a byte sequence (Section 3.10) 3.8) SHOULD be specified, along
   with a character encoding (preferably UTF-8).

   Parsers MUST support strings with at least 1024 characters.

3.9.

3.7.  Tokens

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

   The ABNF for tokens in HTTP/1 textual HTTP headers is:

   sh-token = ALPHA
              *( ALPHA / DIGIT / "_" / "-" / "." / ":" / "%"
                 / "*" / "/" )

   Parsers MUST support tokens with at least 512 characters.

   Note that a Structured Header token is not the same as the "token"
   ABNF rule defined in [RFC7230].

3.10.

3.8.  Byte Sequences

   Byte sequences can be conveyed in Structured Headers.

   The ABNF for a byte sequence in HTTP/1 textual HTTP headers is:

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

   In HTTP/1 textual HTTP headers, a byte sequence is delimited with asterisks
   and encoded using base64 ([RFC4648], Section 4).  For example:

   Example-BinaryHdr: *cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg==*

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

3.11.

3.9.  Booleans

   Boolean values can be conveyed in Structured Headers.

   The ABNF for a Boolean in HTTP/1 textual HTTP headers is:

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

   In HTTP/1 textual HTTP headers, a boolean is indicated with a leading "?"
   character.  For example:

   Example-BoolHdr: ?1

4.  Working With Structured Headers in HTTP/1 Textual HTTP Headers

   This section defines how to serialise serialize and parse Structured Headers in
   HTTP/1
   textual header fields, and protocols compatible with them (e.g., in
   HTTP/2 [RFC7540] before HPACK [RFC7541] is applied).

4.1.  Serialising  Serializing Structured Headers into HTTP/1

   Given a structured structure defined in this specification:

   1.  If the structure is a dictionary, return dictionary or list and its value is empty
       (i.e., it has no members), do not send the result of
       Serialising a Dictionary (Section 4.1.1). serialize field at all
       (i.e., omit both the field-name and field-value).

   2.  If the structure is a parameterised list, return dictionary, let output_string be the result
       of
       Serialising Serializing a Parameterised List Dictionary (Section 4.1.4). 4.1.2).

   3.  If the structure is a list of lists, return the result of
       Serialising a List of Lists ({ser-listlist}).

   4.  If  Else if the structure is a list, return let output_string be the result
       of Serialising Serializing a List Section 4.1.2.

   5.  If 4.1.1.

   4.  Else if the structure is an item, return let output_string be the result
       of Serialising Serializing an Item (Section 4.1.5).

   6.  Otherwise, 4.1.3).

   5.  Else, fail serialisation.

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

4.1.1.  Serialising  Serializing a Dictionary List

   Given a dictionary list of (member, parameters) as input_dictionary: input_list:

   1.  Let output be an empty string.

   2.  For each member mem (member, parameters) of input_dictionary: input_list:

       1.  Let name  If member is an array, let mem_value be the result of
           applying Serialising an Key Inner List (Section 4.1.1.1) to mem's member-name.
           member.

       2.  Append name to output.

       3.  Append "=" to output.

       4.  Let value  Otherwise, let mem_value be the result of applying Serialising
           Serializing an Item (Section 4.1.5) 4.1.3) to mem's member-value.

       5. member.

       3.  Append value mem_value to output.

       6.  If more members remain

       4.  For each parameter in input_dictionary: parameters:

           1.  Append a COMMA to output.

           2.  Append a single WS ";" to output.

   3.  Return output.

4.1.1.1.  Serialising a Key

   Given a key as input_key:

   1.  If input_key is not a sequence of characters, or contains
       characters not allowed in the ABNF for key, fail serialisation.

           2.  Let output name be an empty string. the result of applying Serializing a Key
               (Section 4.1.1.2) to parameter's param-name.

           3.  Append input_key name to output, using ASCII encoding [RFC0020].

   4.  Return output.

4.1.2.  Serialising a List

   Given

           4.  If parameter has a list as input_list:

   1.  Let output be an empty string.

   2.  For each member mem of input_list: param-value:

               1.  Let value be the result of applying Serialising Serializing an
                   Item (Section 4.1.5) 4.1.3) to mem. parameter's param-value.

               2.  Append value "=" to output.

               3.  Append value to output.

       5.  If more members remain in input_list: input_plist:

           1.  Append a COMMA to output.

           2.  Append a single WS to output.

   3.  Return output.

4.1.3.

4.1.1.1.  Serialising a an Inner List of Lists

   Given a list of lists of items as input_list: an array inner_list:

   1.  Let output be an empty string. the string "(".

   2.  For each member inner_list of input_list:

       1.  If inner_list is not a list, fail serialisation.

       2.  If inner_list is empty, fail serialisation.

       3.  For each inner_mem mem of inner_list:

       1.  Let value be the result of applying Serialising Serializing an Item
           (Section 4.1.5) to inner_mem.

           2.  Append value to output.

           3.  If more members remain in inner_list:

               1.  Append a ";" 4.1.3) to output. mem.

       2.  Append value to output.

       3.  If inner_list is not empty, append a single WS to output.

       4.  If more members remain in input_list:

           1.

   3.  Append a COMMA ")" to output.

   4.  Return output.

4.1.1.2.  Serializing a Key

   Given a key as input_key:

   1.  If input_key is not a sequence of characters, or contains
       characters not allowed in the ABNF for key, fail serialisation.

   2.  Let output be an empty string.

   3.  Append a single WS input_key to output.

   3.

   4.  Return output.

4.1.4.  Serialising

4.1.2.  Serializing a Parameterised List Dictionary

   Given a parameterised list dictionary as input_plist: input_dictionary:

   1.  Let output be an empty string.

   2.  For each member mem of input_plist: input_dictionary:

       1.  Let id name be the result of applying Serialising Serializing a Token Key
           (Section 4.1.9) 4.1.1.2) to mem's token. member-name.

       2.  Append id name to output.

       3.  For each parameter in mem's parameters:

           1.  Append ";" "=" to output.

           2.  Let name

       4.  If mem is an array, let value be the result of applying
           Serialising a Key an Inner List (Section 4.1.1.1) to parameter's param-name.

           3.  Append name to output.

           4.  If parameter has a param-value:

               1.  Let mem.

       5.  Otherwise, let value be the result of applying Serialising Serializing an
           Item (Section 4.1.5) to parameter's param-value.

               2.  Append "=" 4.1.3) to output.

               3. mem.

       6.  Append value to output.

       4.

       7.  If more members remain in input_plist: input_dictionary:

           1.  Append a COMMA to output.

           2.  Append a single WS to output.

   3.  Return output.

4.1.5.  Serialising

4.1.3.  Serializing an Item

   Given an item as input_item:

   1.  If input_item is an integer, return the result of applying
       Serialising
       Serializing an Integer (Section 4.1.6) 4.1.4) to input_item.

   2.  If input_item is a float, return the result of applying
       Serialising
       Serializing a Float (Section 4.1.7) 4.1.5) to input_item.

   3.  If input_item is a string, return the result of applying
       Serialising
       Serializing a String (Section 4.1.8) 4.1.6) to input_item.

   4.  If input_item is a token, return the result of Serialising Serializing a
       Token (Section 4.1.9) 4.1.7) to input_item.

   5.  If input_item is a Boolean, return the result of applying
       Serialising
       Serializing a Boolean (Section 4.1.11) 4.1.9) to input_item.

   6.  If input_item is a byte sequence, return the result of applying
       Serialising
       Serializing a Byte Sequence (Section 4.1.10) 4.1.8) to input_item.

   7.  Otherwise, fail serialisation.

4.1.6.  Serialising

4.1.4.  Serializing an Integer

   Given an integer as input_integer:

   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.

   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.

4.1.7.  Serialising

4.1.5.  Serializing a Float

   Given a float as input_float:

   1.  If input_float is not a IEEE 754 double precision number, fail
       serialisation.

   2.  Let output be an empty string.

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

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

   5.  Append "." to output.

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

   7.  Return output.

4.1.8.  Serialising

4.1.6.  Serializing a String

   Given a string as input_string:

   1.  If input_string is not a sequence of characters, or contains
       characters outside the range allowed by VCHAR or SP, fail
       serialisation.

   2.  Let output be an empty string.

   3.  Append DQUOTE to output.

   4.  For each character char in input_string:

       1.  If char is "\" or DQUOTE:

           1.  Append "\" to output.

       2.  Append char to output, using ASCII encoding [RFC0020]. output.

   5.  Append DQUOTE to output.

   6.  Return output.

4.1.9.  Serialising

4.1.7.  Serializing a Token

   Given a token as input_token:

   1.  If input_token is not a sequence of characters, or contains
       characters not allowed in Section 3.9}, 3.7}, fail serialisation.

   2.  Let output be an empty string.

   3.  Append input_token to output, using ASCII encoding [RFC0020]. output.

   4.  Return output.

4.1.10.  Serialising

4.1.8.  Serializing a Byte Sequence

   Given a byte sequence as input_bytes:

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

   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.

4.1.11.  Serialising

4.1.9.  Serializing a Boolean

   Given a Boolean as input_boolean:

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

   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 HTTP/1 Header Fields into Structured Headers

   When a receiving implementation parses textual HTTP header fields
   (e.g., in HTTP/1 or HTTP/2)
   that are known to be Structured Headers, 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 ASCII string input_string array of bytes input_bytes that represents the chosen
   header's field-value, field-value (which is an empty string if that header is not
   present), and header_type, one header_type (one of "dictionary", "list",
   "list-list", "param-list", or "item", "item"),
   return the parsed header value.

   1.  Discard any leading OWS from input_string.

   2.  If header_type is "dictionary", let output be the result of
       Parsing a Dictionary from Text (Section 4.2.1).

   3.  If header_type is "list", let output be the result of Parsing a
       List  Convert input_bytes into an ASCII string input_string; if
       conversion fails, fail parsing.

   2.  Discard any leading OWS from Text (Section 4.2.3).

   4. input_string.

   3.  If header_type is "list-list", "list", let output be the result of Parsing a
       List of Lists from Text (Section 4.2.4).

   5. 4.2.1).

   4.  If header_type is "param-list", "dictionary", let output be the result of
       Parsing a Parameterised List Dictionary from Text (Section 4.2.5).

   6. 4.2.2).

   5.  If header_type is "item", let output be the result of Parsing an
       Item from Text (Section 4.2.7).

   7. 4.2.4).

   6.  Discard any leading OWS from input_string.

   8.

   7.  If input_string is not empty, fail parsing.

   9.

   8.  Otherwise, return output.

   When generating input_string, input_bytes, parsers MUST combine all instances of
   the target header field into one comma-separated field-value, as per
   [RFC7230], Section 3.2.2; this assures that the header is processed
   correctly.

   For Lists, Lists of Lists, Parameterised Lists and Dictionaries, this has the effect of correctly
   concatenating all instances of the header field, 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 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 serialiser serializer or the parser.

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

   If parsing fails - including when calling another algorithm - the
   entire header field's value MUST be discarded.  This is intentionally
   strict, to improve interoperability and safety, and specifications
   referencing this document cannot are not allowed to loosen this requirement.

   Note that this has the effect of discarding any header field with
   non-ASCII characters in input_string.

4.2.1.  Parsing a Dictionary List from Text

   Given an ASCII string input_string, return an ordered map array of (key,
   item). (member,
   parameters). input_string is modified to remove the parsed value.

   1.  Let dictionary members be an empty, ordered map. empty array.

   2.  While input_string is not empty:

       1.  Let this_key member be the result of running Parse Parsing a Key Parameterized
           Member from Text (Section 4.2.2) 4.2.1.1) with input_string.

       2.   If dictionary already contains this_key, fail parsing.

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

       4.   Let this_value be the result of running Parse Item from Text
            (Section 4.2.7) with input_string.

       5.   Add key this_key with value this_value  Append member to dictionary.

       6. members.

       3.  Discard any leading OWS from input_string.

       7.

       4.  If input_string is empty, return dictionary.

       8. members.

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

       9.

       6.  Discard any leading OWS from input_string.

       10.

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

   3.  No structured data has been found; fail parsing.

4.2.2. return members (which is
       empty).

4.2.1.1.  Parsing a Key Parameterized Member from Text

   Given an ASCII string input_string, return a key. an token with an ordered
   map of parameters. input_string is modified to remove the parsed
   value.

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

   2.  Let output_string be an empty string.

   3.  While input_string is not empty:

       1.  Let char "(", let member be the
       result of removing the first character of
           input_string.

       2.  If char is not one of lcalpha, DIGIT, "_", or "-":

           1.  Prepend char to input_string.

           2.  Return output_string.

       3.  Append char to output_string.

   4.  Return output_string.

4.2.3. running Parsing a List from Text

   Given an ASCII string input_string, return a list of items.
   input_string is modified to remove the parsed value.

   1.  Let items be an empty array. Inner List (Section 4.2.1.2) with
       input_string.

   2.  While input_string is not empty:

       1.  Let item  Else, let member be the result of running Parse Parsing an Item from Text
       (Section 4.2.7) 4.2.4) with input_string.

       2.  Append item to items.

   3.  Let parameters be an empty, ordered map.

   4.  In a loop:

       1.  Discard any leading OWS from input_string.

       4.

       2.  If input_string is empty, return items.

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

       6. ";", exit the
           loop.

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

       4.  Discard any leading OWS from input_string.

       7.

       5.  let param_name be the result of Parsing a key from Text
           (Section 4.2.3) from input_string.

       6.  If input_string param_name is empty, fail parsing.

   3.  No structured data has been found; already present in parameters, there is a
           duplicate; fail parsing.

4.2.4.  Parsing

       7.  Let param_value be a List null value.

       8.  If the first character of Lists input_string is "=":

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

           2.  Let param_value be the result of Parsing an Item from
               Text (Section 4.2.4) from input_string.

       9.  Append key param_name with value param_value to parameters.

   5.  Return the tuple (member, parameters).

4.2.1.2.  Parsing an Inner List

   Given an ASCII string input_string, return a list of lists an array of items.
   input_string is modified to remove the parsed value.

   1.  let top_list be an empty array.  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.  Let item be the result of running Parse Item from Text
           (Section 4.2.7) with input_string.

       2.  Append item to inner_list.

       3.  Discard any leading OWS from input_string.

       4.

       2.  If input_string is empty, append inner_list to top_list and
           return top_list.

       5.  Let char be the result first character of consuming input_string is ")":

           1.  Consume the first character of input_string.

       6.  If char is COMMA:

           1.  Append inner_list to top_list.

           2.  Return inner_list.

       3.  Let inner_list item be the result of running Parsing an empty array.

       7.  Else if char is not ";", fail parsing.

       8.  Discard any leading OWS Item from Text
           (Section 4.2.4) with input_string.

       9.

       4.  Append item to inner_list.

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

   4.  No structured data has been  The end of the inner list was not found; fail parsing.

4.2.5.

4.2.2.  Parsing a Parameterised List Dictionary from Text

   Given an ASCII string input_string, return a list an ordered map of parameterised
   identifiers. (key,
   item). input_string is modified to remove the parsed value.

   1.  Let items dictionary be an empty array. empty, ordered map.

   2.  While input_string is not empty:

       1.   Let item this_key be the result of running Parse Parameterised
           Identifier Parsing a Key from
            Text (Section 4.2.6) 4.2.3) with input_string.

       2.  Append item to items.

       3.  Discard any leading OWS from input_string.

       4.   If input_string dictionary already contains the name this_key, there is empty, return items.

       5. a
            duplicate; fail parsing.

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

       6.  Discard any leading OWS from input_string.

       7.

       4.   If input_string is empty, fail parsing.

   3.  No structured data has been found; fail parsing.

4.2.6.  Parsing a Parameterised Identifier from Text

   Given an ASCII string input_string, return an token with an unordered
   map the first character of parameters. input_string is modified to remove the parsed
   value.

   1.  Let primary_identifier "(", let
            this_value be the result of running Parsing a Token from Text an Inner List
            (Section 4.2.10) from 4.2.1.2) with input_string.

   2.  Let parameters

       5.   Else, let this_value be the result of running Parsing an empty, ordered map.

   3.  In a loop:

       1.
            Item (Section 4.2.4) with input_string.

       6.   Add name this_key with value this_value to dictionary.

       7.   Discard any leading OWS from input_string.

       2.

       8.   If input_string is empty, return dictionary.

       9.   Consume the first character of input_string input_string; if it is not ";", exit the
           loop.

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

       4.
            COMMA, fail parsing.

       10.  Discard any leading OWS from input_string.

       5.  let param_name be the result of Parsing a key from Text
           (Section 4.2.2) from input_string.

       6.

       11.  If param_name input_string is already present in parameters, empty, there is a trailing comma; fail
            parsing.

       7.  Let param_value be

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

4.2.3.  Parsing a Key from Text

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

       8.

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

   2.  Let output_string be an empty string.

   3.  While input_string is not empty:

       1.  Consume  Let char be the "=" character at result of removing the beginning first character of
           input_string.

       2.  Let param_value be the result  If char is not one of Parsing an Item from
               Text (Section 4.2.7) from lcalpha, DIGIT, "_", or "-":

           1.  Prepend char to input_string.

       9.  Add key param_name with value param_value

           2.  Return output_string.

       3.  Append char to parameters. output_string.

   4.  Return the tuple (primary_identifier, parameters).

4.2.7. output_string.

4.2.4.  Parsing an Item from Text

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

   1.  If the first character of input_string is a "-" or a DIGIT,
       process input_string as a number (Section 4.2.8) 4.2.5) and return the
       result.

   2.  If the first character of input_string is a DQUOTE, process
       input_string as a string (Section 4.2.9) 4.2.6) and return the result.

   3.  If the first character of input_string is "*", process
       input_string as a byte sequence (Section 4.2.11) 4.2.8) and return the
       result.

   4.  If the first character of input_string is "?", process
       input_string as a Boolean (Section 4.2.12) 4.2.9) and return the result.

   5.  If the first character of input_string is an ALPHA, process
       input_string as a token (Section 4.2.10) 4.2.7) and return the result.

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

4.2.8.

4.2.5.  Parsing a Number from Text

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

   NOTE: This algorithm parses both Integers Section 3.6 3.4 and Floats
   Section 3.7, 3.5, 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 "-", remove consume it from
        input_string 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 removing 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 ".", append char to
            input_number and set type to "float".

        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 "float" 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 defined in
            Section 3.6, 3.4, fail parsing.

   9.   Otherwise:

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

        2.  Parse input_number as a float and let output_number be the
            product of the result and sign.

   10.  Return output_number.

4.2.9.

4.2.6.  Parsing a String from Text

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

   1.  Let output_string be an empty string.

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

   3.  Discard the first character of input_string.

   4.  While input_string is not empty:

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

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

           1.  If input_string is now empty, fail parsing.

           2.  Else:

               1.  Let next_char be the result of removing consuming the first
                   character of input_string.

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

               3.  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.

4.2.10.

4.2.7.  Parsing a Token from Text

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

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

   2.  Let output_string be an empty string.

   3.  While input_string is not empty:

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

       2.  If char is not one of ALPHA, DIGIT, "_", "-", ".", ":", "%",
           "*" or "/":

           1.  Prepend char to input_string.

           2.  Return output_string.

       3.  Append char to output_string.

   4.  Return output_string.

4.2.11.

4.2.8.  Parsing a Byte Sequence from Text

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

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

   2.  Discard the first character of input_string.

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

   4.  Let b64_content be the result of removing 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, synthesising synthesizing padding if necessary (note the
       requirements about recipient behaviour 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.

4.2.12.

4.2.9.  Parsing a Boolean from Text

   Given an ASCII string 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 has no actions for IANA.

6.  Security Considerations

   The size of most types defined by Structured Headers 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 size of individual header fields as well as the overall header
   block 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.  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 754-2008, DOI 10.1109/IEEESTD.2008.4610935,
              ISBN 978-0-7381-5752-8, August 2008,
              <http://ieeexplore.ieee.org/document/4610935/>.

              See also http://grouper.ieee.org/groups/754/ [6].

   [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>.

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.

Appendix B.  Frequently Asked Questions

B.1.  Why not JSON?

   Earlier proposals for structured headers 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 on header fields.

   Since a major goal for Structured Headers is to improve
   interoperability and simplify implementation, these concerns led to a
   format that requires a dedicated parser and serialiser. 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" descriptions=(foo bar)
   Example-Description: foo; url="https://example.net"; context=123,
                        bar; url="https://example.org"; context=456

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

B.3.  What should generic Structured Headers implementations expose?

Appendix C.  Implementation Notes

   A generic implementation of this specification should expose the top-level top-
   level parse (Section 4.2) and serialise serialize (Section 4.1) 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; see
   https://github.com/httpwg/structured-header-tests [7].

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

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

Appendix C. D.  Changes

   _RFC Editor: Please remove this section before publication._

C.1.

D.1.  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.2.  Since draft-ietf-httpbis-header-structure-09

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

   o  Parameters are now ordered maps (#765).

   o  Clamp integers to 15 digits (#737).

C.2.

D.3.  Since draft-ietf-httpbis-header-structure-08

   o  Disallow whitespace before items properly (#703).

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

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

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

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

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

   o  Added Lists of Lists (#721).

   o  Rename Identifier to Token (#725).

   o  Add implementation guidance (#727).

C.3.

D.4.  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).

C.4.

D.5.  Since draft-ietf-httpbis-header-structure-06

   o  Add a FAQ.

   o  Allow non-zero pad bits.

   o  Explicitly check for integers that violate constraints.

C.5.

D.6.  Since draft-ietf-httpbis-header-structure-05

   o  Reorganise specification to separate parsing out.

   o  Allow referencing specs to use ABNF.

   o  Define serialisation algorithms.

   o  Refine relationship between ABNF, parsing and serialisation
      algorithms.

C.6.

D.7.  Since draft-ietf-httpbis-header-structure-04

   o  Remove identifiers from item.

   o  Remove most limits on sizes.

   o  Refine number parsing.

C.7.

D.8.  Since draft-ietf-httpbis-header-structure-03

   o  Strengthen language around failure handling.

C.8.

D.9.  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.

C.9.

D.10.  Since draft-ietf-httpbis-header-structure-01

   o  Replaced with draft-nottingham-structured-headers.

C.10.

D.11.  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

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