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Any objections to "Accept-encoding: gzip, *;q=0"?

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Mon, 21 Jul 97 15:59:25 MDT
Message-Id: <9707212259.AA28854@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com

has been assigned to myself and Henrik for resolution.  We're pretty
close to solving most of it, except for a seemingly minor concern:
How does a client say "don't send me the 'identity' encoding"?
I.e., "please send me a compressed form or send me nothing."

(Why would a client want to say "don't send 'identity'"?  Well,
perhaps the bandwidth costs or latency costs are too high.)

A month or two ago, I proposed adding an "identity" content-coding
so that we could define a way for the client to say that
it does NOT want the identity encoding.  I had originally
thought "if the client sends a list of explicitly acceptable
encodings, but 'identity' isn't on this list, then this
means that it doesn't want 'identity'".  But that is obviously
not going to work.  For example, existing clients (apparently
including Lynx) send
	Accept-encodings: gzip, compress
even though they are presumably willing to take "identity."  If
we were to adopt the rule that I had originally proposed, Lynx
clients would suddenly get error returns for most resources!

Since we certainly do not want break existing clients, the
other alternative that I thought of seems simpler, if somewhat
odd at first glance: define an explicit way for the client to say
"I would rather have 406 than the identity-encoding".  Here are
some ways to do this:

  (a)	Accept-Encoding: gzip, compress, no-identity
		/* an explicit "no identity-encoding wanted" token */

  (b)	Accept-Encoding: gzip, compress, strict
		/* "strict" means "this set, or nothing" */

  (c)	Accept-Encoding: gzip, compress, identity;q=0.0
		/* allow qvalues here */

  (d)	Accept-Encoding-Strict: gzip, compress
		/* define new header to avoid compatibility questions */

Which one to choose?  (d) is least likely to cause trouble, but
it also means that the client has to guess the origin-server version.
We have no reliable way to do this, so in practice (d) would not
really encourage the use of compression.

(c) seems closest to existing practice, although it's possible
that some existing servers might choke on the qvalue.

(a) and (b) mean roughly the same thing, and are certainly not
going to cause compatibility problems, but they are a little kludgey.

We're already likely to propose that Accept-Encoding allow
the use of "*" to mean "whatever you want to send."  So a slight
variation on (c) would be

  (e)	Accept-Encoding: gzip, compress, *;q=0

I.e., the "coding"
is semantically equivalent to the "strict" token in (b) above,
but somewhat closer to the form of the other Accept-* headers.

My own preference is for one of these two approaches:

  (c)	Accept-Encoding: gzip, compress, identity;q=0.0
  (e)	Accept-Encoding: gzip, compress, *;q=0

However, neither of these will work if any existing servers
or proxies would choke on a qvalue in an Accept-Encoding header.
Also, while (e) is cleaner in some ways, it also is infeasible
if any existing servers or proxies would choke on a "*" here.

Comments?  As soon as possible, please!


P.S.: We may have to include a Note to client implementors that
sending a qvalue for a content-coding from the set {"compress",
"gzip", "x-compress", "x-gzip"} might be misinterpreted by older
servers, and so is not recommended.  At least, the one older server
source that I looked at (Apache_1.1b3) looks like it will treat
	Accept-encoding: gzip;q=1.0
as a request for a content-coding that it doesn't know about.
Received on Monday, 21 July 1997 16:05:49 EDT

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