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Re: Content-MD5 and partial responses

From: Roy T.Fielding <fielding@gbiv.com>
Date: Sat, 25 Jul 2009 23:32:33 -0700
Message-Id: <6430C948-5309-48DB-8E09-C2B1788A4A29@gbiv.com>
Cc: HTTP Working Group <ietf-http-wg@w3.org>
To: Mark Nottingham <mnot@mnot.net>
On Jun 28, 2009, at 7:00 PM, Mark Nottingham wrote:

> After a quick look, my reading is that a Content-MD5 header on a  
> partial response reflects the bytes in that message, rather than  
> the whole (non-partial) response:

That is correct.  Content-MD5 is a MIME header referring to the content
of a specific body-part (or the entire body if it is in the main  

If folks want to define a resource metadata header for a hash on
the entire representation, then I suggest one be defined or use
one of the many defined elsewhere (algorithm-independent, of course).
Just don't use the "Content-" prefix on the name.

> However, I'm wondering what a cache should do when combining  
> partial responses that include Content-MD5. This doesn't seem to be  
> addressed in 2616, nor in p5 or p6.

Toss the MD5s unless it plans to recompose them.

> It looks like there are two options here;
> a) C-MD5 applies to the bytes in the entity-body (as above), and  
> therefore we need to specify what a cache does with it when it  
> combines partial responses (throw it away?).
> b) C-MD5 applies to the *full* response body, avoiding the  
> combination issues, and allowing clients to do a MIC of the full  
> response (assuming they have it), but removing the ability to do a  
> MIC on a partial response on its own.
> Anybody aware of C-MD5 being used with partial responses in the  
> wild (I'm looking at you, Adobe)?

It is a MIME header field.  (a) is the definition.

Received on Sunday, 26 July 2009 06:32:44 UTC

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