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Re: Connection Header

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Tue, 20 Dec 94 15:05:06 PST
Message-Id: <9412202305.AA24936@acetes.pa.dec.com>
To: Henrik Frystyk Nielsen <frystyk@www0.cern.ch>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
    2) Experimental client / old proxy: This will break if the remote host
    does in fact understand the connection header. As Roy points out, this
    is a weakness of the design.

OK, I modified my simulator (described in a previous message,
http://www.ics.uci.edu/pub/ietf/http/hypermail/current/0181.html)
to use the algorithm I suggested (in http/hypermail/current/0198.html),
using an adaptive timeout on the server.

What I simulated is that the server would "learn" which clients were
reusing a connection (i.e., were not "new client/old proxy" situations)
and those clients would see a long timeout.  "New client/old proxy"
connections would be timed out after 1 second.

This has the effect of adding an extra second of connection time for
these clients, which is NOT a delay in the retrieval time (i.e., it
doesn't affect UPP).  This is because new clients and servers will be
using Content-length or boundary delimiters to mark the ends of the
data.  The client will certainly not be looking for the end of the
connection (since it is hoping that the connection will never end!)

(I'm assuming that the relay pushes the returned data to the client
without waiting for the end of the connection; is this not what relays
do?  If the relay waits, then this does add 1 second to the UPP for
each retrieval.)

It also has the effect of giving up on the opportunity to reuse
connections, if the client is relatively slow about generating a
new request (or if the network RTT > 1 second).  Of course, "1
second" is an arbitrary value, and it perhaps could be somewhat longer
(depending whether old relays behave "right").

I used the logs from 1 day's accesses (243177 retrievals) to our
election server (actually, to one of the three server machines) and
assumed that all the clients are saying "hold connection open".
I think this log includes references from 5198 distinct client IP
addresses.

I added a new parameter to the simulation: the number of clients that
the server should keep adaptive-timeout state about.  This is managed
as an LRU cache; if a client drops out of the cache and then reappears,
the server then makes the conservative assumption that this client is
behind an "old proxy" and goes back to using a 1-second timeout.

I fixed the other simulation parameters: 64 processes max, and a 120-second
idle timeout for "known reusers".

With vanilla HTTP, there were 243177 TCP connections and the maximum
PCB table size was 2496.  With persistent-connection HTTP and no
adaptive timeouts (i.e., all timeouts = 120 seconds), there were
21955 TCP connections and 440 PCB entries max.

With 100 entries in the adaptive-timeout cache, there were 74285
TCP connections and 763 PCB entries max.  So this scheme does give
up on some of the potential savings from persistent-connection HTTP,
but still reaps most of them.

With 1000 entries in the adaptive-timeout cache, there were 62679
TCP connections and 642 PCB entries max.  With 10000 entries in
the cache (i.e., enough to hold all 5198 client hosts), there were
60632 TCP connections and 643 PCB entries max.  So increasing the
cache size doesn't help much; presumably, the missed opportunities
come from clients that don't reuse the connection quickly, not
from cache-thrashing.

Comparing the adaptive-timeout scheme to a fixed-timeout scheme
with a 1-second timeout, I found that the latter resulted in
149355 connections and 1396 PCB entries max.  So, there is some
benefit to the adaptive scheme.

Finally, I simulated the adaptive-timeout scheme with a 120-second
"long" timeout and a 10-second (instead of 1-second) "short" timeout.
This would only be safe to use if we believe that "old proxy" relays
transfer all the returned data without waiting for the connection
to close.  This approach resulted in 25551 TCP connections and
447 PCB entries max, almost the same as the non-adaptive scheme.
But because it would tie up fewer server and proxy resources in
an environment full of "old proxies", it might be worth doing.

Bottom line: the "new client/old proxy/new server" problem is not
that hard to solve, if one doesn't insist on optimal performance
in every case.

-Jeff
Received on Tuesday, 20 December 1994 15:14:02 EST

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