On Thursday, November 18, 2010, 8:19:20 PM, Julian wrote:
JR> On 18.11.2010 19:02, Chris Lilley wrote:
...
Security considerations:
...
SVG documents may be transmitted in compressed form using gzip
compression. For systems which employ MIME-like mechanisms, such
as HTTP, this is indicated by the Content-Encoding or
Transfer-Encoding header, as appropriate; for systems which do
not, such as direct filesystem access, this is indicated by the
filename extension and by the Macintosh File Type Codes. In
addition, gzip compressed content is readily recognised by the
initial byte sequence as described in [RFC1952] section 2.3.1.
...
JR> 1) What does this have to do with "Security Considerations"?
Please read BCP 13, RFC 4288 section 4.6 "Security requirements" where you will
find
A media type that employs compression may provide an opportunity
for sending a small amount of data that, when received and
evaluated, expands enormously to consume all of the recipient's
resources. All media types SHOULD state whether or not they
employ compression, and if they do they should discuss
what steps need to be taken to avoid such attacks.
JR> 2) I find the whole paragraph misleading; I'd like to see a clear
JR> statement about whether the stream of octets resulting from gzipping SVG
JR> can be labeled as "image/svg+xml" or not
Not by itself, no. In a MIME context, it must be labelled as Content-type:
image/svg+xml **AND** Transfer-Encoding: gzip. Please note the AND.
This is not the same thing as Content-type: application/octet-stream and
Transfer-Encoding: gzip - because that conveys the encoding, but omits the
content type.
In other words the encoding label ADDS TO the media type; it does not remove
the type.
Indeed, this is why separate labelling of encoding was added. Back in the early
days people would use gzipped VRML or gzipped PostScript, and attempted to
register application/gzip; but since they were using the Media Type to hold the
encoding information they had lost important information, so VRML viewers were
sent PostScript and so on. Some people said this was okay, unzip and then look
at the filename extension. But a much better way was to add the encoding
headers.
JR> (please consider transports
JR> other than HTTP, such as a file system that actually supports typing by
JR> Internet media types).
Please feel free to file a bug report for the BeOS filesystem saying that it
should support labelling of encodings in addition to media types.
Speaking as a former BeOS user myself, I still consider modern SVG
implementations (of which there are many) to be a rather more numerous and
relevant consideration than a promising, but obsolete and abandoned, operating
system from 15 years ago.
JR> If yes, that's a violation of "+xml" (and the last sentence points into
JR> this direction). If not, please remove the paragraph above.
JR> 3) If the intent is to say that "svgz" acts as file extension for
JR> gzipped SVG, and *that* content can be served over HTTP as-is with
JR> Content-Type: image/svg+xml
JR> Content-Encoding: gzip
That is exactly what it says, yes
JR> than this is obviously ok
I'm glad its obviously OK.
JR> because it follows from RFC 2616, and has
JR> *nothing* to do with the media type (except for the extension
JR> recommendation).
So you oppose reminding people how to detect such gzipped content?
Why would you want to do that?
--
Chris Lilley Technical Director, Interaction Domain
W3C Graphics Activity Lead, Fonts Activity Lead
Co-Chair, W3C Hypertext CG
Member, CSS, WebFonts, SVG Working Groups