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Re: Effects of Persistent Connections

From: Simon Spero <ses@tipper.oit.unc.edu>
Date: Mon, 13 Nov 1995 10:58:11 -0800 (PST)
To: Mike Cowlishaw <mfc@vnet.ibm.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <Pine.SOL.3.91.951113103124.16928B-100000@chivalry>
On Sun, 12 Nov 1995, Mike Cowlishaw wrote:
> 
> It's interesting to note that the number of packets transmitted has
> increased slightly (per request) from before; doubtless because
> persistent connections require that TCPIP_NODELAY be set.  Has anyone
> done any simulation of this side-effect?

[So how long till you announce HotREXX? :-)]

I don't really understand why TCPIP_NODELAY would need to be set for P-HTTP;
although I haven't implemented the P-HTTP specifications, I have taken 
measurements for HTTP-NG operating in the closest available mode, using 
the HTTP-TOS relay facility. Even though the implementation wasn't fully 
tuned to take best advantage, we found very little negative interaction
with the Nagle algorithm, and very few short packets. Indeed, even with 
the simplest possible scheduling, we generally found that multiple 
requests would be fitted into a single packet, *reducing* the average 
packet count per request. This effect was seen even without 
header-caching enabled; header-caching gives better interactions.

[details: The test was run using netscape browsers talking to the 
HP-proxy, which relayed HTTP/1.0 requests over HTTP-NG to other 
HP-proxies, which then connected to other HTTP/1.0 servers to perform the 
actual requests. Requests were processed in parallel - i.e. data from 
multiple requests was interleaved over the HTTP-NG channel].


The pure-NG approach hasn't been tested yet, but hand coding gave some 
absolutely ridiculous results :-) For example, cache-checking with  1.0
If-Modified for  a page with 20 images takes 6 packets per request 
(SYN->, <-SYN-ACK, REQ->, <-RESP+FIN, FIN+ACK->,<-ACK). 120 packets total.

HTTP-NG can fit all the requests and responses in a single packet; if the 
connection is closed, this takes 6 packets - a 20 fold saving. If the 
connection is warm (as it would be if we're fetchig inline images), the 
total packet could would be 2, a 60 fold saving. 

In the typical case, the packet savings are less- requests are co-aleced, 
but responses typically need a whole packet or more. There are some extra
savings available; typically 1/2 MSS per response. These savings are in 
addition to the savings that come from avoiding the SYN/FIN/LAST-ACK 
transactions documented in Jeff Mogul's SIGCOMM 95 paper.

Simon
Received on Monday, 13 November 1995 10:59:39 EST

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