Re: "Hits" pragma

Paul Burchard (
Sun, 13 Aug 95 21:53:31 -0400

Talking to several people privately, I've been convinced that
"bundled request" reporting information would most naturally be
placed into a Forwarded header, instead of a Pragma. Forwarded
headers have similar semantics to Pragmas (they accumulate and must
be passed on down the line of proxies), and already represent a
primitive form of reporting.

So, keeping in mind Brian Behlendorf's requirements list, here is a
more refined proposal containing two extensions of the Forwarded
header. I think both should be considered, since they provide
different balances between the demands of proxies and servers.

The first (proxy-biased) extension, which only requires constant
storage per cached item at the proxy, adds a "count" clause to the
Forwarded header:

Forwarded: by http://proxy/ count 34

This is equivalent to the hits pragma, except that hit counts are
assigned to specific "leaf" proxies. To be precise: The count is
reset to zero after the proxy forwards a request. Thereafter, every
request for the resource received _without_ any Forwarded headers
increases the count by one; every request _with_ one or more
Forwarded headers simply contributes its Forwarded headers to the
eventual list of headers to be submitted by the proxy.

The second extension, which supplies a more acceptable level of
information, but at the cost of proxy storage proportional to the
number of hits received, adds an "mfor" (multiple-for) clause to the
Forwarded header:

Forwarded: by http://proxy/ mfor Pr5CH77RbN7g0HTux90R7GHK

Here, the argument of the "mfor" clause is a compressed logfile,
representing those requests received by the proxy for this resource
since last forwarding, which did not contain any Forwarded headers
of their own (as before, requests forwarded from proxies upstream
simply contribute their own Forwarded headers).

What needs to be decided more precisely is (a) the compression
algorithm to be applied to the logfile, and (b) the format and
fields of the logfile. Brian Behlendorf suggested a good minimum
set of log fields: host, timestamp, referer (although some sort of
"host hiding" should probably be supported for privacy/security
reasons). Any suggestions for the compression? It would be nice to
have something that could be used incrementally by the proxy to
save space.

Paul Burchard <>
``I'm still learning how to count backwards from infinity...''