HTTP Working Group                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                               P-H. Kamp
Expires: May 31, August 5, 2018                        The Varnish Cache Project
                                                       November 27, 2017
                                                        February 1, 2018

                      Structured Headers for HTTP
                 draft-ietf-httpbis-header-structure-02
                 draft-ietf-httpbis-header-structure-03

Abstract

   This document describes Structured Headers, a way set of simplifying data types and parsing algorithms
   associated with them that are intended to make it easier and safer to
   define and handle HTTP header field definition and parsing. fields.  It is intended for use by new
   specifications of HTTP header fields.  This includes 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].

   _RFC EDITOR: please remove this section before publication_

   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
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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 May 31, August 5, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   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 Labels  . . . . . . . . . . . . . . . . . .   9
     4.4.  Items . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.5.  Integers  . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.6.  Floats  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.7.  Strings . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.8.  Labels  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.9.  Binary Content  . . . . . . . . . . . . . . . . . . . . .  15
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  17
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .  17
     A.1.  Since draft-ietf-httpbis-header-structure-02  . . . . . .  17
     A.2.  Since draft-ietf-httpbis-header-structure-01  . . . . . .  18
     A.3.  Since draft-ietf-httpbis-header-structure-00  . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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.

   Likewise,

   Once a header field is defined, bespoke parsers for it often need to
   be written for specific HTTP
   headers, 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 data, 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, of which only three Headers.  Dictionaries and lists are allowed only usable at the
   "top"
   level: lists, dictionaries, level, while the remaining types can be specified appear at the
   top level or items. 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.

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 headers header that use 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.

   Field definitions MUST NOT

   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 definitions 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:

# FooExample Header

The FooExample HTTP header field conveys a list of numbers 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:

* A member whose key is "foo", and whose value is an integer
  ([RFCxxxx], Section Y.Y), indicating the number of foos in
  the message.
* A member whose key is "bar", "barUrls", and whose value is a string
  ([RFCxxxx], Section Y.Y), conveying the characteristic bar-ness
     of Bar URLs for the message.
  See below for processing requirements.

If the parsed header field does not contain both, it 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 will be considered errors.

3.  Parsing Requirements for Textual 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.  Note that
   input_string may incorporate multiple header lines combined into one
   comma-separated field-value, as per [RFC7230], Section 3.2.2.

   1.  Discard any leading OWS from the beginning of input_string.

   2.  If the field-value is defined to be a dictionary, return let output be
       the result of Parsing a Dictionary from Textual headers
       (Section 4.7). 4.1.1).

   3.  If the field-value is defined to be a list, return let output be the
       result of Parsing a List from Textual Headers Text (Section 4.8). 4.2.1).

   4.  If the field-value is defined to be a parameterised label, return
       the let
       output be the result of Parsing a Parameterised Label from
       Textual headers (Section 4.4). 4.3.1).

   5.  Otherwise, return let output be the result of Parsing an Item from Textual
       Headers Text
       (Section 4.6). 4.4.1).

   6.  Discard any leading OWS from input_string.

   7.  If input_string is not empty, throw an error.

   8.  Otherwise, return output.

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

   Note that in the case of lists and dictionaries, this has the effect
   of combining multiple instances coalescing all of the header field into one. values for that field.  However, for
   singular items and parameterised labels, it has the
   effect of selecting the first value and ignoring any subsequent
   instances of the field, as well as extraneous text afterwards. will result in an error
   being thrown.

   Additionally, note that the effect of the parsing algorithms as
   specified is generally intolerant of syntax errors; if one is
   encountered, the typical response is to throw an error, thereby
   discarding the entire header field value.  This includes any 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.  Numbers

   Abstractly, numbers  Dictionaries

   Dictionaries are integers with an optional fractional part.
   They have a maximum of fifteen digits available to be used in one or
   both unordered maps of key-value pairs, where the parts, as reflected in keys
   are labels (Section 4.8) and the ABNF below; this allows them values are items (Section 4.4).
   There can be between 1 and 1024 members, and keys are required to be stored as IEEE 754 double precision numbers (binary64)
   ([IEEE754]).

   The
   unique.

   In the textual HTTP serialisation of numbers allows a maximum of fifteen
   digits between the integer serialisation, keys and fractional part, along values are separated by
   "=" (without whitespace), and key/value pairs are separated by a
   comma with an optional "-" indicating negative numbers.

   number whitespace.  Duplicate keys MUST be considered an
   error.

   dictionary = ["-"] ( "." 1*15DIGIT /
                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 /
              15DIGIT label "=" item *1023( OWS "," OWS label "=" item )

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

   integer and unsigned are defined as conveniences to specification
   authors; if their use is specified and their ABNF is not matched, a
   parser MUST consider it to be invalid.

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

   ExampleNumberHeader: 4.5

   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 Numbers a Dictionary from Textual Headers

   TBD

4.2.  Strings

   Abstractly, strings are Text

   Given an ASCII strings [RFC0020], excluding control
   characters (i.e., the range 0x20 to 0x7E).  Note that this excludes
   tabs, newlines and carriage returns.  They may be at most 1024
   characters long.

   The textual HTTP serialisation of strings uses a backslash ("") to
   escape double quotes and backslashes in strings. string    = DQUOTE 1*1024(char) DQUOTE
   char      = unescaped / escape ( DQUOTE / "\" )
   unescaped = %x20-21 / %x23-5B / %x5D-7E
   escape    = "\"
   For example, input_string, return a header whose value mapping of (label,
   item). input_string 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 modified to remove the parsed value.

   1.  Let dictionary be escaped;
   other sequences MUST generate an error.

   Unicode empty, unordered mapping.

   2.  While input_string is not directly supported in Structured Headers, because it
   causes a number of interoperability issues, and - empty:

       1.   Let this_key be the result of running Parse Label from Text
            (Section 4.8.1) with few exceptions
   - header values do not require it.

   When it input_string.  If an error is necessary for
            encountered, throw it.

       2.   If dictionary already contains this_key, throw an error.

       3.   Consume a field "=" from input_string; if none is present, throw
            an error.

       4.   Let this_value be the result of running Parse Item from Text
            (Section 4.4.1) with input_string.  If an error is
            encountered, throw it.

       5.   Add key this_key with value this_value to convey non-ASCII string
   content, binary content dictionary.

       6.   If dictionary has more than 1024 members, throw an error.

       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,
            throw an error.

       10.  Discard any leading OWS from input_string.

   3.  Return dictionary.

4.2.  Lists

   Lists are arrays of items (Section 4.5) SHOULD be specified, along 4.4) or parameterised labels
   (Section 4.3), with one to 1024 members.

   In the textual HTTP serialisation, each member is separated by a character encoding (most likely, UTF-8).
   comma and optional whitespace.

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

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

   ExampleLabelListHeader: foo, bar, baz_45

   and a header field whose value is defined as a list of parameterised
   labels could look like:

   ExampleParamListHeader: abc/def; g="hi";j, klm/nop

4.2.1.  Parsing a String List from Textual Headers Text

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

   1.  Let output_string items be an empty string. array.

   2.  If the first character of input_string is not DQUOTE, throw an
       error.

   3.  Discard the first character of input_string.

   4.  If input_string contains more than 1025 characters, throw an
       error.

   5.  While  While input_string is not empty:

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

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

           1.  If input_string an error is now empty,
           encountered, throw an error.

           2.  Else:

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

       2.  Append item to items.

       3.  If next_char is not DQUOTE or "\", items has more than 1024 members, throw an error.

               3.  Append next_char to output_string.

       3.  Else, if char is DQUOTE, remove the first character of

       4.  Discard any leading OWS from input_string.

       5.  If input_string and is empty, return output_string.

       4.  Else, append char to output_string. items.

       6.  Otherwise,  Consume a COMMA from input_string; if no comma is present,
           throw an error.

       7.  Discard any leading OWS from input_string.

   3.  Return items.

4.3.  Parameterised Labels

   Parameterised Labels are short (up labels (Section 4.8) with up to 256 characters) textual identifiers; their
   abstract model
   parameters; each parameter has a label and an optional value that is identical to their expression in the
   an item (Section 4.4).  Ordering between parameters is not
   significant, and duplicate parameters MUST be considered an error.

   The textual HTTP
   serialisation.

   label serialisation uses semicolons (";") to delimit the
   parameters from each other, and equals ("=") to delimit the parameter
   name from its value.

   parameterised = lcalpha *255( lcalpha / DIGIT / "_" / "-"/ "*" / "/" label *256( OWS ";" OWS label [ "=" item ] )
   lcalpha = %x61-7A ; a-z

   Note that labels can only contain lowercase letters.

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

   ExampleLabelHeader: foo/bar

4.3.1.  Parsing a Label from Textual Headers

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

   1.  If input_string contains more than 256 characters, throw an
       error.

   2.  If the first character of input_string is not lcalpha, throw an
       error.

   3.  Let output_string be an empty 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 not one of lcalpha, DIGIT, "_", "-", "*" or "/":

           1.  Prepend char to input_string.

           2.  Return output_string.

       3.  Append char to output_string.

   5.  Return output_string.

4.4.  Parameterised Labels

   Parameterised Labels are labels (Section 4.3) with up to 256
   parameters; each parameter has a label and an optional value that is
   an item (Section 4.6).  Ordering between parameters is not
   significant, and duplicate parameters MUST be considered an error.

   The textual HTTP serialisation uses semicolons (";") to delimit the
   parameters from each other, and equals ("=") to delimit the parameter
   name from its value.

   parameterised = label *256( OWS ";" OWS label [ "=" item ] )

   For example,

   ExampleParamHeader: abc; a=1; b=2; c

4.4.1.  Parsing

   ExampleParamHeader: abc_123;a=1;b=2; c

4.3.1.  Parsing a Parameterised Label from Textual Headers Text

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

   1.  Let primary_label be the result of Parsing a Label from Textual
       Headers Text
       (Section 4.3) 4.8.1) from input_string.

   2.  Let parameters be an empty empty, unordered mapping.

   3.  In a loop:

       1.   Consume   Discard any leading OWS from the beginning of 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.   Consume   Discard any leading OWS from the beginning of input_string.

       5.   let param_name be the result of Parsing a Label from Textual
            Headers Text
            (Section 4.3) 4.8.1) from input_string.

       6.   If param_name is already present in parameters, throw an
            error.

       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
                Textual Headers
                Text (Section 4.6) 4.4.1) from input_string.

       9.   If parameters has more than 255 members, throw an error.

       10.  Add param_name to parameters with the value param_value.

   4.  Return the tuple (primary_label, parameters).

4.5.  Binary Content

   Arbitrary binary content up to 16K in size

4.4.  Items

   An item is can be conveyed in
   Structured Headers.

   The textual HTTP serialisation indicates their presence by a leading
   "*", with the data encoded using Base 64 Encoding [RFC4648], without
   padding (as "=" might integer (Section 4.5), float (Section 4.6),
   string (Section 4.7), label (Section 4.8) or binary content
   (Section 4.9).

   item = integer / float / string / label / 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, throwing any errors encountered.

   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,
       throwing any errors encountered.

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

   5.  If the first character of input_string is an lcalpha, process
       input_string as a label (Section 4.8.1) and return the result,
       throwing any errors encountered.

   6.  Otherwise, throw an error.

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 throw an error.  Therefore, the value "9223372036854775809"
   would be invalid.  Likewise, values that do not conform to the ABNF
   above are invalid, and MUST throw an error.

   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,
       throw an error.

   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 confused 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.

   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 use ABNF above are invalid, and MUST
   throw an error.

   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 ASCII strings [RFC0020], excluding control
   characters (i.e., the range 0x20 to 0x7E).  Note that this excludes
   tabs, newlines and carriage returns.  They may be at most 1024
   characters long.

   The textual HTTP serialisation of dictionaries).

   binary strings uses a backslash ("") to
   escape double quotes and backslashes in strings.

   string    = "*" 1*21846(base64)
   base64 DQUOTE 0*1024(char) DQUOTE
   char      = ALPHA unescaped / DIGIT escape ( DQUOTE / "+" "\" )
   unescaped = %x20-21 / "/" %x23-5B / %x5D-7E
   escape    = "\"

   For example, a header whose value 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 generate an error.

   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 defined as necessary for a field value to convey non-ASCII string
   content, binary content could
   look like:

   ExampleBinaryHeader: *cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg

4.5.1. (Section 4.9) SHOULD be specified, along with
   a character encoding (most likely, UTF-8).

4.7.1.  Parsing Binary Content a String from Textual Headers Text

   Given an ASCII string input_string, return binary content. 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, throw an
       error.

   2.

   3.  Discard the first character of input_string.

   3.  Let b64_content be the result of removing content of

   4.  While input_string
       up to but not including the first character that is not in ALPHA,
       DIGIT, "+" or "/".

   4. empty:

       1.  Let binary_content char be the result of Base 64 Decoding [RFC4648]
       b64_content, synthesising padding if necessary.  If an error is
       encountered, throw it.

   5.  Return binary_content.

4.6.  Items

   An item is can be a number (Section 4.1), string (Section 4.2), label
   (Section 4.3) or binary content (Section 4.5).

   item = number / string / label / binary

4.6.1.  Parsing an Item from Textual Headers

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

   1.  Discard any OWS from removing the beginning first character of
           input_string.

       2.  If the first character of input_string char is a "-" or a DIGIT,
       process input_string as a number (Section 4.1) and return the
       result, throwing any errors encountered.

   3. backslash ("\"):

           1.  If the first character of input_string is a DQUOTE, process
       input_string as a string (Section 4.2) and return now empty, throw an error.

           2.  Else:

               1.  Let next_char be the result,
       throwing any errors encountered.

   4.  If result of removing the first
                   character of input_string input_string.

               2.  If next_char is "*", process
       input_string as binary content (Section 4.5) and not DQUOTE or "\", throw an error.

               3.  Append next_char to output_string.

       3.  Else, if char is DQUOTE, return the
       result, throwing any errors encountered. output_string.

       4.  Else, append char to output_string.

       5.  If the first character of input_string is output_string contains more than 1024 characters, throw an lcalpha, process
       input_string as a label (Section 4.3) and return the result,
       throwing any errors encountered.

   6.
           error.

   5.  Otherwise, throw an error.

4.7.  Dictionaries

   Dictionaries are unordered maps of key-value pairs, where the keys
   are labels (Section 4.3) and the values are items (Section 4.6).
   There can be between 1 and 1024 members, and keys

4.8.  Labels

   Labels are required short (up to be
   unique.

   In 256 characters) textual identifiers; their
   abstract model is identical to their expression 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.

   dictionary = label "=" item *1023( OWS "," OWS
   serialisation.

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

   Note that labels can only contain lowercase letters.

   For example, a header field whose value is defined as a dictionary label 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.

4.7.1.

   ExampleLabelHeader: foo/bar

4.8.1.  Parsing a Dictionary Label from Textual Headers Text

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

   1.  If the first character of input_string is not lcalpha, throw an
       error.

   2.  Let dictionary output_string be an empty mapping.

   2. string.

   3.  While input_string is not empty:

       1.  Let this_key char be the result of running Parse Label from
           Textual Headers (Section 4.3) with removing the first character of
           input_string.

       2.  If an error char is encountered, throw it. not one of lcalpha, DIGIT, "_", "-", "*" or "/":

           1.  Prepend char to input_string.

           2.  Return output_string.

       3.  Append char to output_string.

       4.  If dictionary already output_string contains this_key, raise more than 256 characters, throw an
           error.

       3.  Consume

   4.  Return output_string.

4.9.  Binary Content

   Arbitrary binary content up to 16K in size can be conveyed in
   Structured Headers.

   The textual HTTP serialisation indicates their presence by a "=" from input_string; if none leading
   "*", with the data encoded using Base 64 Encoding [RFC4648],
   Section 4.

   Parsers MUST consider encoded data that is present, raise padded an
           error.

       4.  Let this_value error, as "="
   might be confused with the result use of running Parse Item from
           Textual Headers (Section 4.6) with input_string.  If an error
           is encountered, throw it.

       5.  Add key this_key with value this_value to dictionary.

       6.  Discard any leading OWS from input_string.

       7.  If input_string is empty, return dictionary.

       8.  Consume a COMMA from input_string; if no comma is present,
           raise dictionaries).  See [RFC4648],
   Section 3.2.

   Likewise, parsers MUST consider encoded data that has non-zero pad
   bits an error.

       9.  Discard any leading OWS from input_string.

   3.  Return dictionary.

4.8.  Lists

   Lists are arrays of items (Section 4.6) or parameterised labels
   (Section 4.4, with one to 1024 members.

   In  See [RFC4648], Section 3.5.

   This specification does not relax the textual HTTP serialisation, each member is separated by a
   comma requirements in [RFC4648],
   Section 3.1 and optional whitespace.

   list 3.3; therefore, parsers MUST consider characters
   outside the base64 alphabet and line feeds in encoded data as errors.

   binary = list_member 1*1024( OWS "," OWS list_member )
   list_member "*" 0*21846(base64) "*"
   base64 = item ALPHA / parameterised DIGIT / "+" / "/"

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

   ExampleLabelListHeader: foo, bar, baz_45

   and a header field whose value is defined as a list of parameterised
   labels binary content could
   look like:

   ExampleParamListHeader: abc/def; g="hi";j, klm/nop

4.8.1.

   ExampleBinaryHeader: *cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg*

4.9.1.  Parsing a List Binary Content from Textual Headers Text

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

   1.  Let items be an empty array.

   2.  While  If the first character of input_string is not empty:

       1. "*", throw an
       error.

   2.  Discard the first character of input_string.

   3.  Let item b64_content be the result of running Parse Item from Textual
           Headers (Section 4.6) with input_string. removing content of input_string
       up to but not including the first instance of the character "_".
       If an error there is
           encountered, not a "_" character before the end of input_string,
       throw it.

       2.  Append item to items.

       3.  Discard any leading OWS from input_string. an error.

   4.  If input_string is empty, return items.

       5.  Consume a COMMA from input_string; if no comma the "*" character at the beginning of input_string.

   5.  If b64_content is present,
           raise has more than 21846 characters, throw an error.

   6.  Discard any leading OWS from input_string.

   3.  Let binary_content be the result of Base 64 Decoding [RFC4648]
       b64_content, synthesising padding if necessary.  If an error is
       encountered, throw it (note the requirements about recipient
       behaviour in Section 4.9).

   7.  Return items. 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>.

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-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.

A.2.  Since draft-ietf-httpbis-header-structure-01

   o  Replaced with draft-nottingham-structured-headers.

A.2.

A.3.  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. 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