One document matched: draft-reschke-http-jfv-01.xml

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<rfc ipr="trust200902" docName="draft-reschke-http-jfv-01" category="std">

  

	<front>
  <title abbrev="JSON Encoding for HTTP Field Values">A JSON Encoding for HTTP Header Field Values</title>
  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
    <organization abbrev="greenbytes">greenbytes GmbH</organization>
    <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <email>julian.reschke@greenbytes.de</email>	
      <uri>http://greenbytes.de/tech/webdav/</uri>	
    </address>
  </author>

  <date year="2015" month="March" day="9"/>
  
  <area>Applications</area>
  <keyword>HTTP</keyword>
  <keyword>JSON</keyword>
  <keyword>Header Field Value</keyword>

  <abstract>
    <t>
      This document establishes a convention for use of JSON-encoded field
      values in HTTP header fields.
    </t>
  </abstract>
  

  <note title="Editorial Note (To be removed by RFC Editor before publication)">
    <t>
      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 <eref target="mailto:ietf-http-wg@w3.org">ietf-http-wg@w3.org</eref>,
      which may be joined by sending a message with subject 
      "subscribe" to <eref target="mailto:ietf-http-wg-request@w3.org?subject=subscribe">ietf-http-wg-request@w3.org</eref>.
    </t>
    <t>
      Discussions of the HTTPbis Working Group are archived at
      <eref target="http://lists.w3.org/Archives/Public/ietf-http-wg/"/>.               
    </t> 
    <t>
      XML versions and latest edits for this document
      are available from <eref target="http://greenbytes.de/tech/webdav/#draft-reschke-http-jfv"/>.
    </t>
  </note>


  </front>

  <middle>

<section anchor="introduction" title="Introduction">
<t>
  Defining syntax for new HTTP header fields (<xref target="RFC7230"/>, Section 3.2) is non-trivial. Among the commonly encountered
  problems are:
  <list style="symbols">
    <t>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 reject invalid ones.
    </t>
    <t>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.
    </t>
    <t>HTTP/1.1 does not define a character encoding scheme (<xref target="RFC6365"/>, Section 2), so header fields are either stuck with US-ASCII
    (<xref target="RFC0020"/>), 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, <xref target="XMLHttpRequest"/>
    uses the IDL type "ByteString", effectively resulting in the
    ISO-8859-1 character encoding scheme <xref target="ISO-8859-1"/> being used).
    </t>
  </list>
</t>
<t>
  (See Section 8.3.1 of <xref target="RFC7231"/>
  for a summary of considerations for new header fields.)
</t>
<t>
  This specification addresses the issues listed above by defining both a generic
  JSON-based (<xref target="RFC7159"/>) data model and a concrete
  wire format that can be used in definitions of new header fields.
</t>
</section>  

<!--<section title="Notational Conventions" anchor="notational.conventions">
<t>
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
   This document reuses terminology used in the base HTTP specifications,
   namely <xref target="RFC7230" x:fmt="of" x:rel="#header.fields"/>.
</t>
</section>-->

<section title="Data Model and Format">
<t>
  In HTTP, header fields with the same field name can occur multiple times
  within a single message (Section 3.2.2 of <xref target="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 <xref target="RFC7159"/>). Thus, the basic data model
  used here is the JSON array.
</t>
<t>
  Header field definitions that need only a single value can restrict
  themselves to arrays of length 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").
</t>
<t>
  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" (']') delimiters.
</t>
<figure>
<preamble>
  The ABNF character names and classes below are used (copied
  from <xref target="RFC5234"/>, Appendix B.1):
</preamble>
<artwork type="abnf2616"><![CDATA[
  CR               = %x0D    ; carriage return
  HTAB             = %x09    ; horizontal tab
  LF               = %x0A    ; line feed
  SP               = %x20    ; space
  VCHAR            = %x21-7E ; visible (printing) characters
]]></artwork></figure>
<t>
  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
  (<xref target="RFC7159"/>, Section 7).
</t>
<t>
  The control characters CR, LF, and HTAB do not appear inside JSON
  strings, but can be used outside (line breaks, indentation etc). These characters
  need to be either stripped or replaced by space characters (ABNF "SP").
</t>
<t>
  Formally, using the HTTP specification's ABNF extensions defined in
  Section 7 of <xref target="RFC7230"/>:
</t>
<figure><artwork type="abnf2616"><![CDATA[
  json-field-value = #json-field-item
  json-field-item  = JSON-Text
                   ; see [RFC7159], Section 2,
                   ; post-processed so that only VCHAR characters
                   ; are used
]]></artwork></figure>
</section>

<section title="Sender Requirements">
<t>
  To map a JSON array to an HTTP header field value, process each array
  element separately by:
  <list style="numbers">
    <t>generating the JSON representation,</t>
    <t>stripping all JSON control characters (CR, HTAB, LF), or replacing
      them by space ("SP") characters,</t>
    <t>replacing all remaining non-VSPACE characters by the equivalent
    backslash-escape sequence (<xref target="RFC7159"/>, Section 7).</t>
  </list>
</t>
<t>
  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 
  US-ASCII character encoding scheme (<xref target="RFC0020"/>).
</t>
</section>

<section title="Recipient Requirements">
<t>
  To map a set of HTTP header field instances to a JSON array:
  <list style="numbers">
    <t>combine all header field instances into a single field as per
    Section 3.2.2 of <xref target="RFC7230"/>,</t>
    <t>add a leading begin-array ("[") octet and a trailing end-array ("]") octet, then</t>
    <t>run the resulting octet sequence through a JSON parser.
    </t>
  </list>
</t>
<t>
  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.
</t>
</section>

<section title="Using this Format in Header Field Definitions">
<t>
  <cref>Explain what a definition of a new header field needs to do precisely
  to use this format</cref>
</t>
</section>

<section anchor="examples" title="Examples">
<t>
  This section shows how some of the existing HTTP header fields would look
  like if they would use the format defined by this specification.
</t>
<section anchor="example.content-length" title="Content-Length">
<t>
  "Content-Length" is defined in Section 3.3.2 of <xref target="RFC7230"/>, with the field value's ABNF being:
</t>
<figure><artwork type="abnf2616"><![CDATA[
  Content-Length = 1*DIGIT
]]></artwork></figure>
<t>
  So the field value is similar to a JSON number (<xref target="RFC7230"/>, Section 6).
</t>
<t>
  Content-Length is restricted to a single field instance, as it doesn't use
  the list production (as per Section 3.2.2 of <xref target="RFC7230"/>).
  However, in practice multiple instances do occur, and the definition of
  the header field does indeed discuss how to handle these cases.
</t>
<t>
  If Content-Length was defined using the JSON format discussed here, the
  ABNF would be something like:
</t>
<figure><artwork type="abnf2616"><![CDATA[
  Content-Length = #number
                 ; number: [RFC7159], Section 6
]]></artwork></figure>
<t>
  ...and the prose definition would:
  <list style="symbols">
    <t>restrict all numbers to be non-negative integers without fractions, and</t>
    <t>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)</t>
  </list>
</t>
</section>
<section anchor="example.content-disposition" title="Content-Disposition">
<t>
  Content-Disposition field values, defined in <xref target="RFC6266"/>, consist of
  a "disposition type" (a string), plus multiple parameters, of which at least
  one ("filename") sometime needs to carry non-ASCII characters.
</t>
<t>
  For instance, the first example in Section 5 of <xref target="RFC6266"/>:
</t>
<figure><artwork type="example"><![CDATA[
  Attachment; filename=example.html
]]></artwork></figure>
<t>
  has a disposition type of "Attachment", with filename parameter value
  "example.html". A JSON representation of this information might be:
</t>
<figure><artwork type="application/json"><![CDATA[
  {
    "Attachment": {
      "filename" : "example.html"
    }
  }
]]></artwork></figure>
<t>
  which would translate to a header field value of:
</t>
<figure><artwork type="application/json"><![CDATA[
  { "Attachment": { "filename" : "example.html" } }
]]></artwork></figure>
<t>
  The third example in Section 5 of <xref target="RFC6266"/>
  uses a filename parameter containing non-US-ASCII characters:
</t>
<figure><artwork type="example"><![CDATA[
  attachment; filename*=UTF-8''%e2%82%ac%20rates
]]></artwork></figure>
<t>
  Note that in this case, the "filename*" parameter uses the encoding defined in
  <xref target="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.
</t>
<t>
  The JSON representation of this value could then be:
</t>
<figure><artwork type="application/json"><![CDATA[
  { "attachment": { "filename" : "\u20AC rates" } }
]]></artwork></figure>
</section>
<section anchor="example.www-authenticate" title="WWW-Authenticate">
<t>
  The WWW-Authenticate is defined in Section 4.1 of <xref target="RFC7235"/> as a list of "challenges":
</t>
<figure><artwork type="abnf2616"><![CDATA[
   WWW-Authenticate = 1#challenge
]]></artwork></figure>
<t>
  ...where a challenge consists of a scheme with optional parameters:
</t>
<figure><artwork type="abnf2616"><![CDATA[
  challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
]]></artwork></figure>
<t>
  An example for a complex header field value given in the definition of
  the header field is:
</t>
<figure><artwork type="example"><![CDATA[
  Newauth realm="apps", type=1, title="Login to \"apps\"",
  Basic realm="simple"
]]></artwork>
<postamble>(line break added for readability)</postamble>
</figure>
<t>
  A possible JSON representation of this field value would be the array below:
</t>
<figure><artwork type="application/json"><![CDATA[
  [
    {
      "Newauth" : {
        "realm": "apps",
        "type" : 1,
        "title" : "Login to \"apps\""
      }
    },
    {
      "Basic" : {
        "realm": "simple"
      }
    } 
  ]
]]></artwork></figure>
<t>
  ...which would translate to a header field value of:
</t>
<figure><artwork type="example"><![CDATA[
  { "Newauth" : { "realm": "apps", "type" : 1,
                  "title": "Login to \"apps\"" }},
  { "Basic" : { "realm": "simple"}}
]]></artwork></figure>
</section>
</section>

<section title="Discussion">
<t>
  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.
</t>
<t>
  <cref>Use of generic libs vs compactness of field values..</cref>
</t>
</section>

<section title="Deployment Considerations">
<t>
  This JSON-based syntax will only apply to newly introduced
  header fields, thus backwards compatibility is not a problem. That being
  said, it is conceivable that there is existing code that might trip over
  double quotes not being used for HTTP's quoted-string syntax (Section 3.2.6 of <xref target="RFC7230"/>).
</t>
</section>

<section title="Internationalization Considerations">
<t>
  <cref>TBD, mention migration path to message format that is robust wrt UTF-8, or other binary encodings of JSON</cref>
</t>
</section>

<section title="Security Considerations">
<t>
  <cref>TBD</cref>
</t>
</section>

  </middle>
  <back>
  
<references title="Normative References">

  <reference anchor="RFC0020">
    <front>
      <title>ASCII format for network interchange</title>
      <author initials="V." surname="Cerf" fullname="Vint Cerf"/>
      <date year="1969" month="October"/>
    </front>
    <seriesInfo name="STD" value="80"/>
    <seriesInfo name="RFC" value="20"/>
  </reference>

<!--  <reference anchor="RFC2119">
    <front>
      <title>Key words for use in RFCs to Indicate Requirement Levels</title>
      <author initials="S." surname="Bradner" fullname="Scott Bradner">
        <organization>Harvard University</organization>
        <address><email>sob@harvard.edu</email></address>
      </author>
      <date month="March" year="1997"/>
    </front>
    <seriesInfo name="BCP" value="14"/>
    <seriesInfo name="RFC" value="2119"/>
  </reference>-->

  <reference anchor="RFC5234">
    <front>
      <title abbrev="ABNF for Syntax Specifications">Augmented BNF for Syntax Specifications: ABNF</title>
      <author initials="D." surname="Crocker" fullname="Dave Crocker" role="editor"/>
      <author initials="P." surname="Overell" fullname="Paul Overell"/>
      <date month="January" year="2008"/>
    </front>
    <seriesInfo name="STD" value="68"/>
    <seriesInfo name="RFC" value="5234"/>
  </reference>

  <reference anchor="RFC7159">
    <front>
      <title>The JavaScript Object Notation (JSON) Data Interchange Format</title>
      <author initials="T." surname="Bray" fullname="T. Bray"/>
      <date year="2014" month="March"/>
    </front>
    <seriesInfo name="RFC" value="7159"/>
  </reference>

  <reference anchor="RFC7230">
    <front>
      <title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
        <address><email>fielding@gbiv.com</email></address>
      </author>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
        <organization abbrev="greenbytes">greenbytes GmbH</organization>
        <address><email>julian.reschke@greenbytes.de</email></address>
      </author>
      <date month="June" year="2014"/>
    </front>
    <seriesInfo name="RFC" value="7230"/>
    
  </reference>

  <reference anchor="RFC7231">
    <front>
      <title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
        <organization abbrev="Adobe">Adobe Systems Incorporated</organization>
        <address><email>fielding@gbiv.com</email></address>
      </author>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
        <organization abbrev="greenbytes">greenbytes GmbH</organization>
        <address><email>julian.reschke@greenbytes.de</email></address>
      </author>
      <date month="June" year="2014"/>
    </front>
    <seriesInfo name="RFC" value="7231"/>
    
  </reference>
</references>

<references title="Informative References">
  <reference anchor="ISO-8859-1">
    <front>
      <title>Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1</title>
      <author>
        <organization>International Organization for Standardization</organization>
      </author>
      <date year="1998"/>
    </front>
    <seriesInfo name="ISO/IEC" value="8859-1:1998"/>
  </reference>

  <reference anchor="RFC5987">
  	<front>
      <title>Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters</title>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke"/>
      <date month="August" year="2010"/>
    </front>
    <seriesInfo name="RFC" value="5987"/>
  </reference>

  <reference anchor="RFC6266">
    <front>
      <title abbrev="Content-Disposition in HTTP">Use of the Content-Disposition Header Field
      in the Hypertext Transfer Protocol (HTTP)</title>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke"/>
      <date month="June" year="2011"/>
    </front>
    <seriesInfo name="RFC" value="6266"/>
  </reference>

  <reference anchor="RFC6365">
    <front>
      <title>Terminology Used in Internationalization in the IETF</title>
      <author initials="P." surname="Hoffman" fullname="P. Hoffman"/>
      <author initials="J." surname="Klensin" fullname="J. Klensin"/>
      <date year="2011" month="September"/>
    </front>
    <seriesInfo name="BCP" value="166"/>
    <seriesInfo name="RFC" value="6365"/>
  </reference>

  <reference anchor="RFC7235">
    <front>
      <title>Hypertext Transfer Protocol (HTTP/1.1): Authentication</title>
      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor"/>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor"/>
      <date month="June" year="2014"/>
    </front>
    <seriesInfo name="RFC" value="7235"/>
    
  </reference>

  <reference anchor="XMLHttpRequest" target="http://www.w3.org/TR/2014/WD-XMLHttpRequest-20140130/">
    <front>
      <title>XMLHttpRequest Level 1</title>
      <author fullname="Anne van Kesteren" surname="van Kesteren" initials="A."/>
      <author fullname="Julian Aubourg" surname="Aubourg" initials="J."/>
      <author fullname="Jungkee Song" surname="Song" initials="J."/>
      <author fullname="Hallvord Steen" surname="Steen" initials="H."/>
      <date year="2014" month="January" day="30"/>
    </front>
    <seriesInfo name="W3C Working Draft" value="WD-XMLHttpRequest-20140130"/>
    <annotation>
      Latest version available at
      <eref target="http://www.w3.org/TR/XMLHttpRequest/"/>.
    </annotation>
  </reference>
</references>

<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">

<section anchor="changes.since.00" title="draft-reschke-http-jfv-00">
<t>
  Editorial fixes + working on the TODOs.
</t>
</section>
</section>

  </back>

</rfc>