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Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                            July 2, 2014
Expires: January 3, 2015


              A JSON Encoding for HTTP Header Field Values
                       draft-reschke-http-jfv-00

Abstract

   This document establishes a convention for use of JSON-encoded field
   values in HTTP header fields.

Editorial Note (To be removed by RFC Editor before publication)

   Distribution of this document is unlimited.  Although this is not a
   work item of the HTTPbis Working Group, comments should be sent to
   the Hypertext Transfer Protocol (HTTP) mailing list at
   ietf-http-wg@w3.org [1], which may be joined by sending a message
   with subject "subscribe" to ietf-http-wg-request@w3.org [2].

   Discussions of the HTTPbis Working Group are archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   XML versions and latest edits for this document are available from
   <http://greenbytes.de/tech/webdav/#draft-reschke-http-jfv>.

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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   This Internet-Draft will expire on January 3, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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
   2.  Data Model and Format . . . . . . . . . . . . . . . . . . . . . 3
   3.  Sender Requirements . . . . . . . . . . . . . . . . . . . . . . 4
   4.  Recipient Requirements  . . . . . . . . . . . . . . . . . . . . 5
   5.  Using this Format in Header Field Definitions . . . . . . . . . 5
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     6.1.  Content-Length  . . . . . . . . . . . . . . . . . . . . . . 5
     6.2.  Content-Disposition . . . . . . . . . . . . . . . . . . . . 6
     6.3.  WWW-Authenticate  . . . . . . . . . . . . . . . . . . . . . 7
   7.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  Deployment Considerations . . . . . . . . . . . . . . . . . . . 8
   9.  Internationalization Considerations . . . . . . . . . . . . . . 8
   10. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     11.1. Normative References  . . . . . . . . . . . . . . . . . . . 8
     11.2. Informative References  . . . . . . . . . . . . . . . . . . 9
   Appendix A.  Sample Code and Test Cases . . . . . . . . . . . . . . 9




















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1.  Introduction

   Defining syntax for new HTTP header fields ([RFC7230], Section 3.2)
   is non-trivial.  Among the commonly encountered problems are:

   o  There is no common syntax for complex field values.  Several well-
      known header fields do use a similarly looking syntax, but it is
      hard to write generic parsing code that will both correctly handle
      valid field values but also fail on invalid ones.

   o  The HTTP message format allows header fields to repeat, so field
      syntax needs to be designed in a way that these cases are either
      meaningful, or can be unambiguously detected and rejected.

   o  HTTP/1.1 does not define a character encoding scheme ([RFC6365],
      Section 2), so header fields are either stuck with US-ASCII
      ([USASCII]), or need out-of-band information to decide what
      encoding scheme is used.  Furthermore, APIs usually assume a
      default encoding scheme in order to map from octet sequences to
      strings (for instance, [XMLHttpRequest] uses the IDL type
      "ByteString", effectively resulting in the ISO-8859-1 character
      encoding scheme [ISO-8859-1] being used).

   (See Section 8.3.1 of [RFC7231] for a summary of considerations for
   new header fields.)

   This specification addresses the issues listed above by defining both
   a generic JSON-based ([RFC7159]) data model and a concrete wire
   format that can be used in definitions of new header fields.

2.  Data Model and Format

   In HTTP, header fields with the same field name can occur multiple
   times within a single message (Section 3.2.2 of [RFC7230]).  When
   this happens, recipients are allowed to combine the field values
   using commas as delimiter.  This rule matches nicely JSON's array
   format (Section 5 of [RFC7159]).  Thus, the basic data model used
   here is the JSON array.

   Header field definitions that need only a single value can restrict
   themselves to arrays of lenght 1, and are encouraged to define error
   handling in case more values are received (such as "first wins",
   "last wins", or "abort with fatal error message").

   JSON arrays are mapped to field values by creating a sequence of
   serialized member elements, separated by commas and optionally
   whitespace.  This is equivalent to using the full JSON array format,
   while leaving out the "begin-array" ('[') and "end-array" (']')



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   delimiters.

   The ABNF character names and classes below are used (copied from
   [RFC5234], Appendix B.1):

     CR               = %x0D    ; carriage return
     HTAB             = %x09    ; horizontal tab
     LF               = %x0A    ; line feed
     SP               = %x20    ; space
     VCHAR            = %x21-7E ; visible (printing) characters

   Characters in JSON strings that are not allowed or discouraged in
   HTTP header field values -- that is, not in the "VCHAR" definition --
   need to be represented using JSON's "backslash" escaping mechanism
   ([RFC7159], Section 7).

   The control characters CR, LF, and HTAB do not appear inside JSON
   strings, but can be used outside (line breaks, indentation etc).
   These characters can be either stripped or replaced by space
   characters (ABNF "SP").

   Formally, using the HTTP specification's ABNF extensions defined in
   Section 7 of [RFC7230]:

     json-field-value = #json-field-item
     json-field-item  = JSON-Text
                      ; see [RFC7159], Section 2,
                      ; post-processed so that only VCHAR characters
                      ; are used

3.  Sender Requirements

   [[anchor3: The text below assumes we're starting with a JSON-
   formatted sequence of characters, not octets; need to clarify.]]  To
   map a JSON array to an HTTP header field value, process each array
   element separately by:

   1.  generating the JSON representation,

   2.  stripping all JSON control characters (CR, HTAB, LF), or
       replacing them by space ("SP") characters,

   3.  replacing all remaining non-VSPACE characters by the equivalent
       backslash-escape sequence ([RFC7159], Section 7).

   The resulting list of strings is transformed into an HTTP field value
   by combining them using comma (%x2C) plus optional SP as delimiter,
   and encoding the resulting string into an octet sequence using the



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   US-ASCII character encoding scheme.

4.  Recipient Requirements

   To map a set of HTTP header field instances to a JSON array:

   1.  remove all header field instances that only contain whitespace
       (SP / HTAB) and "comma" characters [[anchor5: either drop this or
       make it more precise]],

   2.  combine all header field instances into a single field as per
       Section 3.2.2 of [RFC7230],

   3.  add a leading begin-array ("[") octet and a trailing end-array
       ("]") octet, then

   4.  run the resulting octet sequence through a JSON parser.

   The result of the parsing operation is either an error (in which case
   the header field values needs to be considered invalid), or a JSON
   array.

5.  Using this Format in Header Field Definitions

   [[anchor7: Explain what a definition of a new header field needs to
   do precisely to use this format]]

6.  Examples

   This section shows how some of the existing HTTP header fields would
   look like if they would use the format defined by this specification.

6.1.  Content-Length

   "Content-Length" is defined in Section 3.3.2 of [RFC7230], with the
   field value's ABNF being:

     Content-Length = 1*DIGIT

   So the field value is similar to a JSON number ([RFC7230], Section
   6).

   Content-Length is restricted to a single field instance, as it
   doesn't use the list production (as per Section 3.2.2 of [RFC7230]).
   However, in practice multiple instances do occur, and the definition
   of the header field does indeed discuss how to handle these cases.

   If Content-Length was defined using the JSON format discussed here,



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   the ABNF would be something like:

     Content-Length = #number
                    ; number: [RFC7159], Section 6

   ...and the prose definition would:

   o  restrict all numbers to be non-negative integers without
      fractions, and

   o  require that the array of values is of length 1 (but allow the
      case where the array is longer, but all members represent the same
      value)

6.2.  Content-Disposition

   Content-Disposition field values, defined in [RFC6266], consist of a
   "disposition type" (a string), plus multiple parameters, of which at
   least one ("filename") sometime needs to carry non-ASCII characters.

   For instance, the first example in Section 5 of [RFC6266]:

     Attachment; filename=example.html

   has a disposition type of "Attachment", with filename parameter value
   "example.html".  A JSON representation of this information might be:

     {
       "Attachment": {
         "filename" : "example.html"
       }
     }

   which would translate to a header field value of:

     { "Attachment": { "filename" : "example.html" } }

   The third example in Section 5 of [RFC6266] uses a filename parameter
   containing non-US-ASCII characters:

     attachment; filename*=UTF-8''%e2%82%ac%20rates

   Note that in this case, the "filename*" parameter uses the encoding
   defined in [RFC5987], representing a filename starting with the
   Unicode character U+20AC (EURO SIGN), followed by " rates".  If the
   definition of Content-Disposition would have used the format proposed
   here, the workaround involving the "parameter*" syntax would not have
   been needed at all.



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   The JSON representation of this value could then be:

     { "attachment": { "filename" : "\u20AC rates" } }

6.3.  WWW-Authenticate

   The WWW-Authenticate is defined in Section 4.1 of [RFC7235] as a list
   of "challenges":

      WWW-Authenticate = 1#challenge

   ...where a challenge consists of a scheme with optional parameters:

     challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

   An example for a complex header field value given in the definition
   of the header field is:

     Newauth realm="apps", type=1, title="Login to \"apps\"",
     Basic realm="simple"

   (line break added for readability)

   A possible JSON representation of this field value would be the array
   below:

     [
       {
         "Newauth" : {
           "realm": "apps",
           "type" : 1,
           "title" : "Login to \"apps\""
         }
       },
       {
         "Basic" : {
           "realm": "simple"
         }
       }
     ]

   ...which would translate to a header field value of:

     { "Newauth" : { "realm": "apps", "type" : 1,
                     "title": "Login to \"apps\"" }},
     { "Basic" : { "realm": "simple"}}





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7.  Discussion

   This approach uses a default of "JSON array", using implicit array
   markers.  An alternative would be a default of "JSON object".  This
   would simplify the syntax for non-list-typed haeders, but all the
   benefits of having the same data model for both types of header
   fields would be gone.  A hybrid approach might make sense, as long as
   it doesn't require any heuristics on the recipient's side.

   [[anchor9: Use of generic libs vs compactness of field values..]]

8.  Deployment Considerations

   [[anchor11: Mention that some code might be refused by double quotes
   not being used for quoted-string.]]

9.  Internationalization Considerations

   [[anchor13: TBD, mention migration path to message format that is
   robust wrt UTF-8, or other binary encodings of JSON]]

10.  Security Considerations

   [[anchor15: TBD]]

11.  References

11.1.  Normative References

   [RFC5234]         Crocker, D., Ed. and P. Overell, "Augmented BNF for
                     Syntax Specifications: ABNF", STD 68, RFC 5234,
                     January 2008.

   [RFC7159]         Bray, T., "The JavaScript Object Notation (JSON)
                     Data Interchange Format", RFC 7159, March 2014.

   [RFC7230]         Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                     Transfer Protocol (HTTP/1.1): Message Syntax and
                     Routing", RFC 7230, June 2014.

   [RFC7231]         Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                     Transfer Protocol (HTTP/1.1): Semantics and
                     Content", RFC 7231, June 2014.

   [USASCII]         American National Standards Institute, "Coded
                     Character Set -- 7-bit American Standard Code for
                     Information Interchange", ANSI X3.4, 1986.




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11.2.  Informative References

   [ISO-8859-1]      International Organization for Standardization,
                     "Information technology -- 8-bit single-byte coded
                     graphic character sets -- Part 1: Latin alphabet
                     No. 1", ISO/IEC 8859-1:1998, 1998.

   [RFC5987]         Reschke, J., "Character Set and Language Encoding
                     for Hypertext Transfer Protocol (HTTP) Header Field
                     Parameters", RFC 5987, August 2010.

   [RFC6266]         Reschke, J., "Use of the Content-Disposition Header
                     Field in the Hypertext Transfer Protocol (HTTP)",
                     RFC 6266, June 2011.

   [RFC6365]         Hoffman, P. and J. Klensin, "Terminology Used in
                     Internationalization in the IETF", BCP 166,
                     RFC 6365, September 2011.

   [RFC7235]         Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                     Transfer Protocol (HTTP/1.1): Authentication",
                     RFC 7235, June 2014.

   [XMLHttpRequest]  van Kesteren, A., Aubourg, J., Song, J., and H.
                     Steen, "XMLHttpRequest Level 1", W3C Working
                     Draft WD-XMLHttpRequest-20140130, January 2014, <ht
                     tp://www.w3.org/TR/2014/
                     WD-XMLHttpRequest-20140130/>.

                     Latest version available at
                     <http://www.w3.org/TR/XMLHttpRequest/>.

URIs

   [1]  <mailto:ietf-http-wg@w3.org>

   [2]  <mailto:ietf-http-wg-request@w3.org?subject=subscribe>

Appendix A.  Sample Code and Test Cases

   [[anchor19: TBD]]










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Author's Address

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/









































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