Path: utzoo!utgpu!watserv1!watmath!att!tut.cis.ohio-state.edu!ucbvax!DUFF.UOREGON.EDU!jqj
From: jqj@DUFF.UOREGON.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP down scaling (linear or what) ?
Message-ID: <9012072046.AA05781@duff.uoregon.edu>
Date: 7 Dec 90 20:46:38 GMT
References: <58@nixeid.UUCP>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 37

>I am trying to determine, quantitively or qualititively, the effect of 
>reducing the bandwidth of an IP link.

One important point with slow links is cascade effects going through
multiple routers or bridges.  Since very few router technologies allow the
retransmission of a packet until reception is complete, each hop typically
adds the total transmission time for the packet on that hop to the round
trip time.  For example, a 19.2Kbit/sec serial connection with 640byte
packets contributes about 270ms each way.

Each hop of course also adds the bit propagation delay, e.g. a satellite
link may add hundreds of ms.  On the other hand, terrestrial links seem to
have propagation delays of a few 10s of ms for digital circuits and on the
order of 100ms for analog circuits with modems (speed of light Oregon to
South Dakota places a lower bound on that link of about 15ms; empirically
I measured about 150ms including modulation/demod delay in a pair of
Fujitsu 19.2Kb trellis modems).  It also adds think time in a router for
routing decisions and transfer of the packet from the input interface to
the output interface, but that's typically well under 10ms in modern
routers.

Bottom line is that if your path is through 3 19.2Kbps analog links you
should expect to see RTTs of more than 2s, and that RTT will be affine in
1/bandwidth for any given technology and topology.

For FTP with well-tuned TCP implementations, the RTT should not
dramatically effect total thruput at typical current bandwidths and
delays; only the minimum bandwidth over the various hops in the path will
have a significant (and to a first approximation linear!) effect. 
However, for any synchronous or packet exchange application, e.g. typical
telnet with remote echo or RPC (e.g. NFS!), large RTTs will kill you.  

Long RTTs can even have a big effect on FTP, since TCP allows only a
fairly small amount of unacked data.  But "small" is relative here, and it
isn't too relevant for the range of bandwidths the original poster asked
about, *unless* you have a couple of satellite links in the picture.  
The picture is very different for gigabit nets!