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Versions: (draft-nottingham-structured-headers) 00 01 02 03 04 05 06 07 08 09 10

HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                               P-H. Kamp
Expires: April 26, 2019                        The Varnish Cache Project
                                                        October 23, 2018


                      Structured Headers for HTTP
                 draft-ietf-httpbis-header-structure-08

Abstract

   This document describes a set of data types and 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].

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




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   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 April 26, 2019.

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.  Defining New Structured Headers . . . . . . . . . . . . . . .   4
   3.  Structured Header Data Types  . . . . . . . . . . . . . . . .   7
     3.1.  Dictionaries  . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Lists . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Parameterised Lists . . . . . . . . . . . . . . . . . . .   8
     3.4.  Items . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Integers  . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.6.  Floats  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.7.  Strings . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.8.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .  10
     3.9.  Byte Sequences  . . . . . . . . . . . . . . . . . . . . .  10
     3.10. Booleans  . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  Structured Headers in HTTP/1  . . . . . . . . . . . . . . . .  11
     4.1.  Serialising Structured Headers into HTTP/1  . . . . . . .  11
     4.2.  Parsing HTTP/1 Header Fields into Structured Headers  . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  25
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   Appendix A.  Frequently Asked Questions . . . . . . . . . . . . .  26



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     A.1.  Why not JSON? . . . . . . . . . . . . . . . . . . . . . .  26
     A.2.  Structured Headers don't "fit" my data. . . . . . . . . .  27
   Appendix B.  Changes  . . . . . . . . . . . . . . . . . . . . . .  28
     B.1.  Since draft-ietf-httpbis-header-structure-07  . . . . . .  28
     B.2.  Since draft-ietf-httpbis-header-structure-06  . . . . . .  28
     B.3.  Since draft-ietf-httpbis-header-structure-05  . . . . . .  28
     B.4.  Since draft-ietf-httpbis-header-structure-04  . . . . . .  28
     B.5.  Since draft-ietf-httpbis-header-structure-03  . . . . . .  29
     B.6.  Since draft-ietf-httpbis-header-structure-02  . . . . . .  29
     B.7.  Since draft-ietf-httpbis-header-structure-01  . . . . . .  29
     B.8.  Since draft-ietf-httpbis-header-structure-00  . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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 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
   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
   [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 specify a header field that is a Structured Header, see Section 2.

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



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   Those abstract types can be serialised into and parsed from textual
   headers - such as those used in HTTP/1 - using the algorithms
   described in Section 4.

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 VCHAR, SP, DIGIT, ALPHA and DQUOTE rules
   from that document.  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 parsing from HTTP/1 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 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.





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   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 because doing so would preclude
   handling by generic software; they can only add additional
   constraints.

   For example:







































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   # 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 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 cause
   the header to 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 cause
   that value to 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.

   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.








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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.

3.1.  Dictionaries

   Dictionaries are ordered maps of key-value pairs, where the keys are
   identifiers (Section 3.8) and the values are items (Section 3.4).
   There can be one 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 of accessing the
   members.

   The ABNF for dictionaries in HTTP/1 headers is:

   sh-dictionary  = dict-member *( OWS "," OWS dict-member )
   dict-member    = member-name "=" member-value
   member-name    = sh-identifier
   member-value   = sh-item

   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.

3.2.  Lists

   Lists are arrays of items (Section 3.4) with one or more members.

   The ABNF for lists in HTTP/1 headers is:

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






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   In HTTP/1, 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 lists containing at least 1024 members.

3.3.  Parameterised Lists

   Parameterised Lists are arrays of a parameterised identifiers.

   A parameterised identifier is an identifier (Section 3.8) with an
   optional set of parameters, each parameter having an identifier and
   an optional value that is an item (Section 3.4).  Ordering between
   parameters is not significant, and duplicate parameters MUST cause
   parsing to fail.

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

   sh-param-list = param-id *( OWS "," OWS param-id )
   param-id      = sh-identifier *parameter
   parameter     = OWS ";" OWS param-name [ "=" param-value ]
   param-name    = sh-identifier
   param-value   = sh-item

   In HTTP/1, each param-id is separated by a comma and optional
   whitespace (as in Lists), and the parameters are separated by
   semicolons.  For example:

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

   Parsers MUST support parameterised lists containing at least 1024
   members, and support members with at least 256 parameters.

3.4.  Items

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

   The ABNF for items in HTTP/1 headers is:

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



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3.5.  Integers

   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).

   The ABNF for integers in HTTP/1 headers is:

   sh-integer = ["-"] 1*19DIGIT

   For example:

   Example-IntegerHeader: 42

3.6.  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 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

3.7.  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 headers is:



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

   In HTTP/1 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 3.9) SHOULD be specified, along
   with a character encoding (preferably UTF-8).

   Parsers MUST support strings with at least 1024 characters.

3.8.  Identifiers

   Identifiers are short textual identifiers; their abstract model is
   identical to their expression in the textual HTTP serialisation.
   Parsers MUST support identifiers with at least 64 characters.

   The ABNF for identifiers in HTTP/1 headers is:

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

   Note that identifiers can only contain lowercase letters.

3.9.  Byte Sequences

   Byte sequences can be conveyed in Structured Headers.

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

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





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   In HTTP/1 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.

3.10.  Booleans

   Boolean values can be conveyed in Structured Headers.

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

   sh-boolean = "!" boolean
   boolean    = "T" / "F"

   In HTTP/1 headers, a byte sequence is delimited with a "!" character.
   For example:

   Example-BoolHdr: !T

4.  Structured Headers in HTTP/1

   This section defines how to serialise 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 Structured Headers into HTTP/1

   Given a structured defined in this specification:

   1.  If the structure is a dictionary, return the result of
       Serialising a Dictionary {#ser-dictionary}.

   2.  If the structure is a list, return the result of Serialising a
       List {#ser-list}.

   3.  If the structure is a parameterised list, return the result of
       Serialising a Parameterised List {#ser-param-list}.

   4.  If the structure is an item, return the result of Serialising an
       Item {#ser-item}.

   5.  Otherwise, fail serialisation.






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4.1.1.  Serialising a Dictionary

   Given a dictionary as input:

   1.  Let output be an empty string.

   2.  For each member mem of input:

       1.  Let name be the result of applying Serialising an Identifier
           Section 4.1.8 to mem's member-name.

       2.  Append name to output.

       3.  Append "=" to output.

       4.  Let value be the result of applying Serialising an Item
           Section 4.1.4 to mem's member-value.

       5.  Append value to output.

       6.  If more members remain in input:

           1.  Append a COMMA to output.

           2.  Append a single WS to output.

   3.  Return output.

4.1.2.  Serialising a List

   Given a list as input:

   1.  Let output be an empty string.

   2.  For each member mem of input:

       1.  Let value be the result of applying Serialising an Item
           Section 4.1.4 to mem.

       2.  Append value to output.

       3.  If more members remain in input:

           1.  Append a COMMA to output.

           2.  Append a single WS to output.

   3.  Return output.



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4.1.3.  Serialising a Parameterised List

   Given a parameterised list as input:

   1.  Let output be an empty string.

   2.  For each member mem of input:

       1.  Let id be the result of applying Serialising an Identifier
           Section 4.1.8 to mem's identifier.

       2.  Append id to output.

       3.  For each parameter in mem's parameters:

           1.  Append ";" to output.

           2.  Let name be the result of applying Serialising an
               Identifier Section 4.1.8 to parameter's param-name.

           3.  Append name to output.

           4.  If parameter has a param-value:

               1.  Let value be the result of applying Serialising an
                   Item Section 4.1.4 to parameter's param-value.

               2.  Append "=" to output.

               3.  Append value to output.

       4.  If more members remain in input:

           1.  Append a COMMA to output.

           2.  Append a single WS to output.

   3.  Return output.

4.1.4.  Serialising an Item

   Given an item as input:

   1.  If input is a type other than an integer, float, string,
       identifier, byte sequence, or Boolean, fail serialisation.

   2.  If input is an integer, return the result of applying Serialising
       an Integer Section 4.1.5 to input.



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   3.  If input is a float, return the result of applying Serialising a
       Float Section 4.1.6 to input.

   4.  If input is a string, return the result of applying Serialising a
       String Section 4.1.7 to input.

   5.  If input is an identifier, return the result of Serialising an
       Identifier {#ser-identifier}.

   6.  If input is a Boolean, return the result of applying Serialising
       a Boolean Section 4.1.10 to input.

   7.  Otherwise, return the result of applying Serialising a Byte
       Sequence Section 4.1.9 to input.

4.1.5.  Serialising an Integer

   Given an integer as input:

   1.  If input is not an integer in the range of
       -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
       inclusive, fail serialisation.

   2.  Let output be an empty string.

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

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

   5.  Return output.

4.1.6.  Serialising a Float

   Given a float as input:

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

   2.  Let output be an empty string.

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

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

   5.  Append "." to output.




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

   7.  Return output.

4.1.7.  Serialising a String

   Given a string as input:

   1.  If input 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:

       1.  If char is "\" or DQUOTE:

           1.  Append "\" to output.

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

   5.  Append DQUOTE to output.

   6.  Return output.

4.1.8.  Serialising an Identifier

   Given an identifier as input:

   1.  If input is not a sequence of characters, or contains characters
       not allowed in Section 3.8, fail serialisation.

   2.  Let output be an empty string.

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

   4.  Return output.

4.1.9.  Serialising a Byte Sequence

   Given a byte sequence as input:

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

   2.  Let output be an empty string.



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   3.  Append "*" to output.

   4.  Append the result of base64-encoding input 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.

4.1.10.  Serialising a Boolean

   Given a Boolean as input:

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

   2.  Let output be an empty string.

   3.  Append "!" to output.

   4.  If input is true, append "T" to output.

   5.  If input is false, append "F" 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 that represents the chosen
   header's field-value, and header_type, one of "dictionary", "list",
   "param-list", or "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).



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   3.  If header_type is "list", let output be the result of Parsing a
       List from Text (Section 4.2.2).

   4.  If header_type is "param-list", let output be the result of
       Parsing a Parameterised List from Text (Section 4.2.3).

   5.  Otherwise, let output be the result of Parsing an Item from Text
       (Section 4.2.5).

   6.  Discard any leading OWS from input_string.

   7.  If input_string is not empty, fail parsing.

   8.  Otherwise, return output.

   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.

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

   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 or the parser.

   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 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 from Text

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

   1.  Let dictionary be an empty, ordered map.




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   2.  While input_string is not empty:

       1.   Let this_key be the result of running Parse Identifier from
            Text (Section 4.2.8) 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 4.2.5) with input_string.

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

       9.   Discard any leading OWS from input_string.

       10.  If input_string is empty, fail parsing.

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

4.2.2.  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 4.2.5) with input_string.

       2.  Append item to items.

       3.  Discard any leading OWS from input_string.

       4.  If input_string is empty, return items.

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



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       6.  Discard any leading OWS from input_string.

       7.  If input_string is empty, fail parsing.

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

4.2.3.  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 4.2.4) with input_string.

       2.  Append item to items.

       3.  Discard any leading OWS from input_string.

       4.  If input_string is empty, return items.

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

       6.  Discard any leading OWS from input_string.

       7.  If input_string is empty, fail parsing.

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

4.2.4.  Parsing a Parameterised Identifier from Text

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

   1.  Let primary_identifier be the result of Parsing an Identifier
       from Text (Section 4.2.8) from input_string.

   2.  Let parameters be an empty, unordered map.

   3.  In a loop:

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



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

       3.  Discard any leading OWS from input_string.

       4.  let param_name be the result of Parsing an Identifier from
           Text (Section 4.2.8) from input_string.

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

       6.  Let param_value be a null value.

       7.  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 4.2.5) from input_string.

       8.  Insert (param_name, param_value) into parameters.

   4.  Return the tuple (primary_identifier, parameters).

4.2.5.  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 4.2.6) and return the
       result.

   3.  If the first character of input_string is a DQUOTE, process
       input_string as a string (Section 4.2.7) and return the result.

   4.  If the first character of input_string is "*", process
       input_string as a byte sequence (Section 4.2.9) and return the
       result.

   5.  If the first character of input_string is "!", process
       input_string as a Boolean (Section 4.2.10) and return the result.

   6.  If the first character of input_string is a lcalpha, process
       input_string as an identifier (Section 4.2.8) and return the
       result.




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   7.  Otherwise, fail parsing.

4.2.6.  Parsing a Number from Text

   NOTE: This algorithm parses both Integers Section 3.5 and Floats
   Section 3.6, 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 it from
        input_string and set sign to -1.

   5.   If input_string is empty, 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 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 19
            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 3.5, fail parsing.

   9.   Otherwise:



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        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.

4.2.7.  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, 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.





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4.2.8.  Parsing an Identifier from Text

   Given an ASCII string input_string, return an 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.  Return output_string.

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

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




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   7.  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.

4.2.10.  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 case-sensitively matches
       "T", discard the first character, and return true.

   4.  If the first character of input_string case-sensitively matches
       "F", 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




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   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>.




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   [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

Appendix A.  Frequently Asked Questions

A.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.




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   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.

   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.

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

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

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

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

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

   Since the description contains a list of key/value pairs, we use a
   Parameterised List to represent them, with the identifier for each
   item in the list 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.






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Appendix B.  Changes

   _RFC Editor: Please remove this section before publication._

B.1.  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).

B.2.  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.

B.3.  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.

B.4.  Since draft-ietf-httpbis-header-structure-04

   o  Remove identifiers from item.

   o  Remove most limits on sizes.



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   o  Refine number parsing.

B.5.  Since draft-ietf-httpbis-header-structure-03

   o  Strengthen language around failure handling.

B.6.  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.

B.7.  Since draft-ietf-httpbis-header-structure-01

   o  Replaced with draft-nottingham-structured-headers.

B.8.  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/





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   Poul-Henning Kamp
   The Varnish Cache Project

   Email: phk@varnish-cache.org















































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