From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Subject: Re: Future of IP routers
Organization: Silicon Graphics, Inc.  Mountain View, CA
Date: Sat, 29 Aug 1992 16:24:49 GMT

In article <1992Aug28.151702.12575@advtech.uswest.com>, jsims@uswest.com (John Sims) writes:

> Could you be more specific about the environment in which these results
> were acheived and exactly what IP over ATM results  you expect?
> I have one of the early implementations of ATM for Suns and I see
>throughput (Sparc2 to Sparc2) of 19.5Mbps TCP/IP over a 100 Mbps ATM interface.
> So  the results reported by M. Burak seem in-line with my observations.
> Not that someone couldn't produce a better implementation, so I'm asking if
> anyone out there knows of one?

At least some of the current crop of ATM cards for workstations
move all data with "programmed I/O".  In other words, the card
is not a DMA master.  That they do better than ethernet amazes me.
Their speed has nothing to do with whatever ATM will eventually provide.

 
> It seems obvious to me that (all other factors being equal) TCP/IP throughput
> on an FDDI would be better than throughput on a 100Mbps ATM.   And of course
> I'm talking about just having 2 stations on the FDDI, etc...  The overhead
> that ATM adds to be able to support voice and video would make it less
> efficient at carrying TCP/IP than an  FDDI.  Anyone disagree?

In the simplest, unintended sense, an ATM switch can be many times
faster than an FDDI ring.  Each pair of ATM ports can be running at
155Mb/s (or whatever), but you get only 100Mb/s (or 200Mb/s if you
believe in dual-MAC) to divide among all stations on an FDDI ring.

I disagree with the intended point in the weak sense of being unwilling
to entertain an opinion yet.  Few of the few people who have ever moved
TCP data faster than 50Mbit/sec over any medium say much of anything to
anyone.  Many people who speculate about commercially available network
speeds above 50Mbit/sec would be surprised by what is really involved,
by the real nature of the bottlenecks.

ATM, as I understand it, can be more full-duplex than FDDI.  So you
might argue that 100 Mb ATM would be faster.  Then again, those 53 byte
cells waste a lot of bandwidth.  On the other hand, you don't hear so
much about 100Mb/s ATM these days compared to 155 or 165 or other
rates, and 155Mb ATM sounds faster than 100Mb FDDI.  However,
assempling cells into IP packets is not going to be cheap and could
introduce significant latency, and latency is Not Your Friend when
you're trying to push a single virtual circut.  On the other hand, FDDI
latencies can be amazing.

I have spent some time trying to make TCP run fast over FDDI.  I have
learned just enough to know that "it just aint that simple," at least
for me.

For example, if your FDDI station uses medium sized TCP windows
[32K,64K) and can easily transmit at > 100 Mb/s, then you might find it
difficult to reach 98Mb/s because of the half-duplex, release the token
only nothing more is available to transmit nature of FDDI.  (Consider
when the ACKs from the receiver are seen by the transmitter, and the
potential idle time on the ring.)


Vernon Schryver,  vjs@sgi.com
Newsgroups: comp.dcom.cell-relay