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
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Copyright (c) 2014 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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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, .
Latest version available at
.
URIs
[1]
[2]
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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