RE: ETag Validiation?

From: Nottingham, Mark (Australia) <>
Date: Wed, 21 Oct 1998 16:29:41 +1000

As far as I can tell, an ETag request header is invalid; 14.9 states only
that it is a response header.

I'm particularly interested in using ETags for cache validation; i.e., with
If-None-Match (If-Match IMHO seems of limited use).

From limited testing that I've done, Apache >=1.2 and IIS >=4.0 send ETags
for all static content, and support If-None-Match request headers, by
sending a 304 when appropriate.

The immediate gain is that ETags can be a stronger validator than LM time,
and less ambiguous. Another no-so-obvious use is for dynamic apps to use
them to keep information about state, so that they can tell if a request
needs to be regenerated and resent.

AFAIK, you won't see much If-None-Match traffic at all; no clients seem to
support Etags (I may be wrong). However, the specification doesn't preclude
(and actually encourages, in my understanding) caches using ETags as
validators when they're available. Because LM headers will still be
available in most cases, 1.0 client IMS requests shouldn't be a problem.

As TODO notes, it will mean storing more metainformation.


> -----Original Message-----
> From: Alex Rousskov []
> Sent: Wednesday, October 21, 1998 4:12 PM
> To: Nottingham, Mark (Australia)
> Cc: 'Squid'
> Subject: Re: ETag Validiation?
> On Wed, 21 Oct 1998, Nottingham, Mark (Australia) wrote:
> > One of the items on TODO is cache validation based upon Entity Tags.
> > Anybody have a rough idea of when this might be done?
> RFC 2068: "HTTP/1.1 uses entity tags in
> the ETag (section 14.20),
> If-Match (section 14.25),
> If-None-Match (section 14.26), and
> If-Range (section 14.27) header fields."
> Squid already honors ETags in If-Range request headers (for
> Range requests)
> and takes a note of "ETag:" reply header.
> I read the specs for other headers; it looks like a simple
> hack may not do
> here.
> I've just added counters for If-Match and If-None-Match
> request headers. If
> those headers are present in more than 1% of requests coming
> through the
> NLANR Caches, the support will likely to be added within
> two-four weeks.
> Alex.
> P.S. Anybody knows what to do with an "ETag:" _header_ in a
> _request_ ?!
Received on Wed Oct 21 1998 - 00:41:32 MDT

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