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Re: NEW ISSUE: content sniffing

From: Roy T. Fielding <fielding@gbiv.com>
Date: Tue, 31 Mar 2009 19:44:57 -0700
Message-Id: <E410960C-2741-4C95-A6FF-FB7E487D841D@gbiv.com>
To: HTTP Working Group <ietf-http-wg@w3.org>
It is impossible to determine a media-type (how a recipient should
process a given representation) by sniffing the content (the data  
Many media-types overlap the same data formats.  Many media types are
only distinguished from other types by what the user agent should do
when processing them, not by the bits on the wire.  When a media type
is not present (or is detectably incorrect), only the implementation
doing the processing can determine an appropriate guess because that
guess is almost always determined by the context in which the
reference was made (not by the content).  Since the context is
deliberately not sent on the wire, there is absolutely no way that
accurate sniffing can be defined by HTTP.  We aren't talking about
a protocol decision regarding communication; we are talking about
an operating default that is specific to the purpose of a given
client and will likely be different for each one.

In any case, there is no algorithm for sniffing that is anywhere
near the same level of standardization as HTTP.  The one that HTML5
is working on would barely qualify as Experimental.  If the folks
promoting such software can successfully deploy it across all HTTP
clients, then it should be referenced.  Until then, it remains an
unproven and, IMO, mistaken idea which is far more likely to
be overcome by events than become a standard way to handle HTTP.

Received on Wednesday, 1 April 2009 02:45:35 UTC

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