HTTP Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track July 8, 2016
Expires: January 9, 2017
A JSON Encoding for HTTP Header Field Values
draft-ietf-httpbis-jfv-01
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)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
Working Group information can be found at ;
source code and issues list for this draft can be found at
.
The changes in this draft are summarized in Appendix A.
Status of This Memo
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Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
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
6.4. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 8
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Deployment Considerations . . . . . . . . . . . . . . . . . . 9
9. Internationalization Considerations . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 11
A.1. Since draft-reschke-http-jfv-00 . . . . . . . . . . . . . 11
A.2. Since draft-reschke-http-jfv-01 . . . . . . . . . . . . . 12
A.3. Since draft-reschke-http-jfv-02 . . . . . . . . . . . . . 12
A.4. Since draft-reschke-http-jfv-03 . . . . . . . . . . . . . 12
A.5. Since draft-reschke-http-jfv-04 . . . . . . . . . . . . . 12
A.6. Since draft-ietf-httpbis-jfv-00 . . . . . . . . . . . . . 12
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 12
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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 reject 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
([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, [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, where
the goals were:
o to be compatible with header field recombination when fields occur
multiple times in a single message (Section 3.2.2 of [RFC7230]),
and
o not to use any problematic characters in the field value (non-
ASCII characters and certain whitespace characters).
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
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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").
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.
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 need to 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
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,
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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
US-ASCII character encoding scheme ([RFC0020]).
4. Recipient Requirements
To map a set of HTTP header field instances to a JSON array:
1. combine all header field instances into a single field as per
Section 3.2.2 of [RFC7230],
2. add a leading begin-array ("[") octet and a trailing end-array
("]") octet, then
3. 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
[[anchor5: Explain what a definition of a new header field needs to
do precisely to use this format, mention must-ignore extensibility]]
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 ([RFC7159], 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.
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If Content-Length was defined using the JSON format discussed here,
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
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been needed at all.
The JSON representation of this value could then be:
{ "attachment": { "filename" : "\u20AC rates" } }
6.3. WWW-Authenticate
The WWW-Authenticate header field value 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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6.4. Accept-Encoding
The Accept-Encoding header field value is defined in Section 5.3.4 of
[RFC7231] as a list of codings, each of which allowing a weight
parameter 'q':
Accept-Encoding = #( codings [ weight ] )
codings = content-coding / "identity" / "*"
weight = OWS ";" OWS "q=" qvalue
qvalue = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] )
An example for a complex header field value given in the definition
of the header field is:
gzip;q=1.0, identity; q=0.5, *;q=0
Due to the defaulting rules for the quality value ([RFC7231], Section
5.3.1), this could also be written as:
gzip, identity; q=0.5, *; q=0
A JSON representation could be:
[
{
"gzip" : {
}
},
{
"identity" : {
"q": 0.5
}
},
{
"*" : {
"q": 0
}
}
]
...which would translate to a header field value of:
{"gzip": {}}, {"identity": {"q": 0.5}}, {"*": {"q": 0}}
In this example, the part about "gzip" appears unnecessarily verbose,
as the value is just an empty object. A simpler notation would
collapse members like these to string literals:
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"gzip", {"identity": {"q": 0.5}}, {"*": {"q": 0}}
If this is desirable, the header field definition could allow both
string literals and objects, and define that a mere string literal
would be mapped to a member whose name is given by the string
literal, and the value is an empty object.
For what it's worth, one of the most common cases for 'Accept-
Encoding' would become:
"gzip", "deflate"
which would be only a small overhead over the original format.
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 header fields, 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.
Note: a concrete proposal was made by Kazuho Oku in .
[[anchor7: Use of generic libs vs compactness of field values..]]
[[anchor8: Mention potential "Key" header field extension ([KEY]).]]
8. Deployment Considerations
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 [RFC7230]).
9. Internationalization Considerations
In HTTP/1.1, header field values are represented by octet sequences,
usually used to transmit ASCII characters, with restrictions on the
use of certain control characters, and no associated default
character encoding, nor a way to describe it ([RFC7230], Section
3.2). HTTP/2 does not change this.
This specification maps all characters which can cause problems to
JSON escape sequences, thereby solving the HTTP header field
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internationalization problem.
Future specifications of HTTP might change to allow non-ASCII
characters natively. In that case, header fields using the syntax
defined by this specification would have a simple migration path (by
just stopping to require escaping of non-ASCII characters).
10. Security Considerations
Using JSON-shaped field values is believed to not introduce any new
threads beyond those described in Section 12 of [RFC7159], namely the
risk of recipients using the wrong tools to parse them.
Other than that, any syntax that makes extensions easy can be used to
smuggle information through field values; however, this concern is
shared with other widely used formats, such as those using parameters
in the form of name/value pairs.
11. References
11.1. Normative References
[RFC0020] Cerf, V., "ASCII format for network interchange",
STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969,
.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON)
Data Interchange Format", RFC 7159, DOI 10.17487/
RFC7159, March 2014,
.
[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,
.
[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,
.
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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.
[KEY] Fielding, R. and M. Nottingham, "The Key HTTP
Response Header Field", draft-ietf-httpbis-key-01
(work in progress), March 2016.
[RFC5987] Reschke, J., "Character Set and Language Encoding
for Hypertext Transfer Protocol (HTTP) Header Field
Parameters", RFC 5987, DOI 10.17487/RFC5987,
August 2010,
.
[RFC6266] Reschke, J., "Use of the Content-Disposition Header
Field in the Hypertext Transfer Protocol (HTTP)",
RFC 6266, DOI 10.17487/RFC6266, June 2011,
.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166,
RFC 6365, DOI 10.17487/RFC6365, September 2011,
.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Authentication",
RFC 7235, DOI 10.17487/RFC7235, 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
.
Appendix A. Change Log (to be removed by RFC Editor before publication)
A.1. Since draft-reschke-http-jfv-00
Editorial fixes + working on the TODOs.
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A.2. Since draft-reschke-http-jfv-01
Mention slightly increased risk of smuggling information in header
field values.
A.3. Since draft-reschke-http-jfv-02
Mention Kazuho Oku's proposal for abbreviated forms.
Added a bit of text about the motivation for a concrete JSON subset
(ack Cory Benfield).
Expand I18N section.
A.4. Since draft-reschke-http-jfv-03
Mention relation to KEY header field.
A.5. Since draft-reschke-http-jfv-04
Change to HTTP Working Group draft.
A.6. Since draft-ietf-httpbis-jfv-00
Added example for "Accept-Encoding" (inspired by Kazuho's feedback),
showing a potential way to optimize the format when default values
apply.
Appendix B. Acknowledgements
Thanks go to the Hypertext Transfer Protocol Working Group
participants.
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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