Structured Headers for HTTP
draft-ietf-httpbis-header-structure-04

HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                               P-H. Kamp
Expires: September 5, 2018                     The Varnish Cache Project
                                                           March 4, 2018

                      Structured Headers for HTTP
                 draft-ietf-httpbis-header-structure-04

Abstract

   This document describes a set of data types and parsing algorithms
   associated with them 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 HTTP header fields as well as revisions of existing
   header field specifications when doing so does not cause
   interoperability issues.

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].

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 5, 2018.

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Copyright Notice

   Copyright (c) 2018 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.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Specifying Structured Headers . . . . . . . . . . . . . . . .   4
   3.  Parsing Text into Structured Headers  . . . . . . . . . . . .   5
   4.  Structured Header Data Types  . . . . . . . . . . . . . . . .   6
     4.1.  Dictionaries  . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Lists . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Parameterised Lists . . . . . . . . . . . . . . . . . . .   9
     4.4.  Items . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.5.  Integers  . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.6.  Floats  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.7.  Strings . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.8.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .  15
     4.9.  Binary Content  . . . . . . . . . . . . . . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  18
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .  18
     A.1.  Since draft-ietf-httpbis-header-structure-03  . . . . . .  18
     A.2.  Since draft-ietf-httpbis-header-structure-02  . . . . . .  18
     A.3.  Since draft-ietf-httpbis-header-structure-01  . . . . . .  19
     A.4.  Since draft-ietf-httpbis-header-structure-00  . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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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 for it often need to
   be written, because each header has slightly different handling of
   what looks like common syntax.

   This document introduces structured HTTP header field values
   (hereafter, Structured Headers) to address these problems.
   Structured Headers define a generic, abstract model for header field
   values, along with a concrete serialisation for expressing that model
   in textual HTTP headers, as used by HTTP/1 [RFC7230] and HTTP/2
   [RFC7540].

   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 and
   parsing.

   Additionally, future versions of HTTP can define alternative
   serialisations of the abstract model of Structured Headers, 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 specify a header field that uses Structured Headers, see
   Section 2.

   Section 4 defines a number of abstract data types that can be used in
   Structured Headers.  Dictionaries and lists are only usable at the
   "top" level, while the remaining types can be specified appear at the
   top level or inside those structures.

   Those abstract types can be serialised into textual headers - such as
   those used in HTTP/1 and HTTP/2 - using the algorithms described in
   Section 3.

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1.1.  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 Backus-Naur Form (ABNF) notation of
   [RFC5234], including the DIGIT, ALPHA and DQUOTE rules from that
   document.  It also includes the OWS rule from [RFC7230].

2.  Specifying Structured Headers

   A HTTP header that uses Structured Headers need to be defined to do
   so explicitly; recipients and generators need to know that the
   requirements of this document are in effect.  The simplest way to do
   that is by referencing this document in its definition.

   The field's definition will also need to specify the field-value's
   allowed syntax, in terms of the types described in Section 4, along
   with their associated semantics.

   A header field definition cannot relax or otherwise modify the
   requirements of this specification; doing so would preclude handling
   by generic software.

   However, header field authors are encouraged to clearly state
   additional constraints upon the syntax, as well as the consequences
   when those constraints are violated.  Such additional constraints
   could include additional structure (e.g., a list of URLs [RFC3986]
   inside a string) that cannot be expressed using the primitives
   defined here.

   For example:

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# FooExample Header

The FooExample HTTP header field conveys a list of integers about how
much Foo the sender has.

FooExample is a Structured header [RFCxxxx]. Its value MUST be a
dictionary ([RFCxxxx], Section Y.Y).

The dictionary MUST contain:

* Exactly one member whose key 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 is "barUrls", and whose value is a string
  ([RFCxxxx], Section Y.Y), conveying the Bar URLs 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 be ignored.

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

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

If a member of barURLs is not a valid URI-reference, it MUST be ignored.

If a member of barURLs is a relative reference ([RFC3986], Section 4.2),
it MUST be resolved ([RFC3986], Section 5) before being used.

   Note that empty header field values are not allowed by the syntax,
   and therefore parsing for them will fail.

3.  Parsing Text 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 that represents the chosen
   header's field-value, return the parsed header value.  When
   generating input_string, 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.

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   1.  Discard any leading OWS from input_string.

   2.  If the field-value is defined to be a dictionary, let output be
       the result of Parsing a Dictionary from Text (Section 4.1.1).

   3.  If the field-value is defined to be a list, let output be the
       result of Parsing a List from Text (Section 4.2.1).

   4.  If the field-value is defined to be a parameterised list, let
       output be the result of Parsing a Parameterised List from Text
       (Section 4.3.1).

   5.  Otherwise, let output be the result of Parsing an Item from Text
       (Section 4.4.1).

   6.  Discard any leading OWS from input_string.

   7.  If input_string is not empty, fail parsing.

   8.  Otherwise, return output.

   Note that in the case of lists, parameterised lists and dictionaries,
   this has the effect of coalescing all of the values for that field.
   However, for singular items, parsing will fail if more than instance
   of that header field is present.

   If parsing fails, the entire header field's value MUST be discarded.
   This is intentionally strict, to improve interoperability and safety,
   and specifications referencing this document MUST NOT loosen this
   requirement.

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

4.  Structured Header Data Types

   This section defines the abstract value types that can be composed
   into Structured Headers, along with the textual HTTP serialisations
   of them.

4.1.  Dictionaries

   Dictionaries are unordered maps of key-value pairs, where the keys
   are identifiers (Section 4.8) and the values are items (Section 4.4).
   There can be between 1 and 1024 members, and keys are required to be
   unique.

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   In the textual HTTP serialisation, keys and values are separated by
   "=" (without whitespace), and key/value pairs are separated by a
   comma with optional whitespace.  Duplicate keys MUST cause parsing to
   fail.

dictionary        = dictionary_member *1023( OWS "," OWS dictionary_member )
dictionary_member = identifier "=" item

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

   ExampleDictHeader: foo=1.23, en="Applepie", da=*w4ZibGV0w6ZydGUK

   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.

4.1.1.  Parsing a Dictionary from Text

   Given an ASCII string input_string, return a mapping of (identifier,
   item). input_string is modified to remove the parsed value.

   1.  Let dictionary be an empty, unordered mapping.

   2.  While input_string is not empty:

       1.   Let this_key be the result of running Parse Identifier from
            Text (Section 4.8.1) with input_string.

       2.   If dictionary already contains this_key, fail parsing.

       3.   Consume a "=" from input_string; if none is present, fail
            parsing.

       4.   Let this_value be the result of running Parse Item from Text
            (Section 4.4.1) with input_string.

       5.   Add key this_key with value this_value to dictionary.

       6.   If dictionary has more than 1024 members, fail parsing.

       7.   Discard any leading OWS from input_string.

       8.   If input_string is empty, return dictionary.

       9.   Consume a COMMA from input_string; if no comma is present,
            fail parsing.

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       10.  Discard any leading OWS from input_string.

       11.  If input_string is empty, fail parsing.

   3.  If dictionary is empty, fail parsing.

   4.  Return dictionary.

4.2.  Lists

   Lists are arrays of items (Section 4.4) with one to 1024 members.

   In the textual HTTP serialisation, each member is separated by a
   comma and optional whitespace.

   list = list_member 0*1023( OWS "," OWS list_member )
   list_member = item

   For example, a header field whose value is defined as a list of
   identifiers could look like:

   ExampleIdListHeader: foo, bar, baz_45

4.2.1.  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.

   2.  While input_string is not empty:

       1.  Let item be the result of running Parse Item from Text
           (Section 4.4.1) with input_string.

       2.  Append item to items.

       3.  If items has more than 1024 members, fail parsing.

       4.  Discard any leading OWS from input_string.

       5.  If input_string is empty, return items.

       6.  Consume a COMMA from input_string; if no comma is present,
           fail parsing.

       7.  Discard any leading OWS from input_string.

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       8.  If input_string is empty, fail parsing.

   3.  If items is empty, fail parsing.

   4.  Return items.

4.3.  Parameterised Lists

   Parameterised Lists are arrays of a parameterised identifiers with 1
   to 256 members.

   A parameterised identifier is an identifier (Section 4.8) with up to
   256 parameters, each parameter having a identifier and an optional
   value that is an item (Section 4.4).  Ordering between parameters is
   not significant, and duplicate parameters MUST cause parsing to fail.

   In the textual HTTP serialisation, each parameterised identifier is
   separated by a comma and optional whitespace.  Parameters are
   delimited from each other using semicolons (";"), and equals ("=")
   delimits the parameter name from its value.

   param_list = param_id 0*255( OWS "," OWS param_id )
   param_id   = identifier 0*256( OWS ";" OWS identifier [ "=" item ] )

   For example,

   ExampleParamListHeader: abc_123;a=1;b=2; c, def_456, ghi;q="19";r=foo

4.3.1.  Parsing a Parameterised List from Text

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

   1.  Let items be an empty array.

   2.  While input_string is not empty:

       1.  Let item be the result of running Parse Parameterised
           Identifier from Text (Section 4.3.2) with input_string.

       2.  Append item to items.

       3.  If items has more than 256 members, fail parsing.

       4.  Discard any leading OWS from input_string.

       5.  If input_string is empty, return items.

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       6.  Consume a COMMA from input_string; if no comma is present,
           fail parsing.

       7.  Discard any leading OWS from input_string.

       8.  If input_string is empty, fail parsing.

   3.  If items is empty, fail parsing.

   4.  Return items.

4.3.2.  Parsing a Parameterised Identifier from Text

   Given an ASCII string input_string, return a identifier with an
   mapping of parameters. input_string is modified to remove the parsed
   value.

   1.  Let primary_identifier be the result of Parsing a Identifier from
       Text (Section 4.8.1) from input_string.

   2.  Let parameters be an empty, unordered mapping.

   3.  In a loop:

       1.   Discard any leading OWS from input_string.

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

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

       4.   Discard any leading OWS from input_string.

       5.   let param_name be the result of Parsing a Identifier from
            Text (Section 4.8.1) from input_string.

       6.   If param_name is already present in parameters, fail
            parsing.

       7.   Let param_value be a null value.

       8.   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 Parsing an Item from
                Text (Section 4.4.1) from input_string.

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       9.   If parameters has more than 255 members, fail parsing.

       10.  Add param_name to parameters with the value param_value.

   4.  Return the tuple (primary_identifier, parameters).

4.4.  Items

   An item is can be a integer (Section 4.5), float (Section 4.6),
   string (Section 4.7), identifier (Section 4.8) or binary content
   (Section 4.9).

   item = integer / float / string / identifier / binary

4.4.1.  Parsing an Item from Text

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

   1.  Discard any leading OWS from input_string.

   2.  If the first character of input_string is a "-" or a DIGIT,
       process input_string as a number (Section 4.5.1) and return the
       result.

   3.  If the first character of input_string is a DQUOTE, process
       input_string as a string (Section 4.7.1) and return the result.

   4.  If the first character of input_string is "*", process
       input_string as binary content (Section 4.9.1) and return the
       result.

   5.  If the first character of input_string is an lcalpha, process
       input_string as a identifier (Section 4.8.1) and return the
       result.

   6.  Otherwise, fail parsing.

4.5.  Integers

   Abstractly, integers have a range of -9,223,372,036,854,775,808 to
   9,223,372,036,854,775,807 inclusive (i.e., a 64-bit signed integer).

   integer   = ["-"] 1*19DIGIT

   Parsers that encounter an integer outside the range defined above
   MUST fail parsing.  Therefore, the value "9223372036854775808" would

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   be invalid.  Likewise, values that do not conform to the ABNF above
   are invalid, and MUST fail parsing.

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

   ExampleIntegerHeader: 42

4.5.1.  Parsing a Number from Text

   NOTE: This algorithm parses both Integers and Floats Section 4.6, and
   returns the corresponding structure.

   1.  If the first character of input_string is not "-" or a DIGIT,
       fail parsing.

   2.  Let input_number be the result of consuming input_string up to
       (but not including) the first character that is not in DIGIT,
       "-", and ".".

   3.  If input_number contains ".", parse it as a floating point number
       and let output_number be the result.

   4.  Otherwise, parse input_number as an integer and let output_number
       be the result.

   5.  Return output_number.

4.6.  Floats

   Abstractly, floats are integers with a fractional part.  They have a
   maximum of fifteen digits available to be used in both of the parts,
   as reflected in the ABNF below; this allows them to be stored as IEEE
   754 double precision numbers (binary64) ([IEEE754]).

   The textual HTTP serialisation of floats allows a maximum of fifteen
   digits between the integer and fractional part, with at least one
   required on each side, along with an optional "-" indicating negative
   numbers.

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   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 )

   Values that do not conform to the ABNF above are invalid, and MUST
   fail parsing.

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

   ExampleFloatHeader: 4.5

   See Section 4.5.1 for the parsing algorithm for floats.

4.7.  Strings

   Abstractly, strings are up to 1024 printable ASCII [RFC0020]
   characters (i.e., the range 0x20 to 0x7E).  Note that this excludes
   tabs, newlines and carriage returns.

   The textual HTTP serialisation of strings uses a backslash ("\") to
   escape double quotes and backslashes in strings.

   string    = DQUOTE 0*1024(char) DQUOTE
   char      = unescaped / escape ( DQUOTE / "\" )
   unescaped = %x20-21 / %x23-5B / %x5D-7E
   escape    = "\"

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

   ExampleStringHeader: "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.

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   Unicode is not directly supported in Structured Headers, 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, binary content (Section 4.9) SHOULD be specified, along with
   a character encoding (preferably, UTF-8).

4.7.1.  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 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 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, append char to output_string.

       5.  If output_string contains more than 1024 characters, fail
           parsing.

   5.  Otherwise, fail parsing.

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4.8.  Identifiers

   Identifiers are short (up to 256 characters) textual identifiers;
   their abstract model is identical to their expression in the textual
   HTTP serialisation.

   identifier = lcalpha *255( lcalpha / DIGIT / "_" / "-"/ "*" / "/" )
   lcalpha    = %x61-7A ; a-z

   Note that identifiers can only contain lowercase letters.

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

   ExampleIdHeader: foo/bar

4.8.1.  Parsing a Identifier from Text

   Given an ASCII string input_string, return a identifier. 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 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.  If output_string contains more than 256 characters, fail
           parsing.

   4.  Return output_string.

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4.9.  Binary Content

   Arbitrary binary content up to 16384 bytes in size can be conveyed in
   Structured Headers.

   The textual HTTP serialisation encodes the data using Base 64
   Encoding [RFC4648], Section 4, and surrounds it with a pair of
   asterisks ("*") to delimit from other content.

   The encoded data is required to be padded with "=", as per [RFC4648],
   Section 3.2.  It is RECOMMENDED that parsers reject encoded data that
   is not properly padded, although this might not be possible with some
   base64 implementations.

   Likewise, encoded data is required to have pad bits set to zero, as
   per [RFC4648], Section 3.5.  It is RECOMMENDED that parsers fail on
   encoded data that has non-zero pad bits, although this might not be
   possible with some base64 implementations.

   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.

   binary = "*" 0*21846(base64) "*"
   base64 = ALPHA / DIGIT / "+" / "/" / "="

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

   ExampleBinaryHeader: *cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg*

4.9.1.  Parsing Binary Content from Text

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

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

   2.  Discard the first character of input_string.

   3.  Let b64_content be the result of removing content of input_string
       up to but not including the first instance of the character "*".
       If there is not a "*" character before the end of input_string,
       fail parsing.

   4.  Consume the "*" character at the beginning of input_string.

   5.  If b64_content is has more than 21846 characters, fail parsing.

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   6.  Let binary_content be the result of Base 64 Decoding [RFC4648]
       b64_content, synthesising padding if necessary (note the
       requirements about recipient behaviour in Section 4.9).

   7.  Return binary_content.

5.  IANA Considerations

   This draft has no actions for IANA.

6.  Security Considerations

   TBD

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>.

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7.2.  Informative References

   [IEEE754]  IEEE, "IEEE Standard for Floating-Point Arithmetic", 2008,
              <http://grouper.ieee.org/groups/754/>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [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>.

   [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>.

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

Appendix A.  Changes

A.1.  Since draft-ietf-httpbis-header-structure-03

   o  Strengthen language around failure handling.

A.2.  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.

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   o  Improve limits in algorithms.

   o  Require parsers to combine header fields before processing.

   o  Throw an error on trailing garbage.

A.3.  Since draft-ietf-httpbis-header-structure-01

   o  Replaced with draft-nottingham-structured-headers.

A.4.  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

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