From: gerg@netcom.com (Greg Andrews)
Subject: Re: PEP over SLIP?
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date: Mon, 23 Nov 1992 08:21:26 GMT

vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
>In article <1992Nov20.083723.11984@netcom.com>, gerg@netcom.com (Greg Andrews) writes:
>> vjs@rhyolite.wpd.sgi.com (Vernon Schryver) writes:
>> >
>> >I just don't see how SLIP/PEP can "degrade under [increased] offered
>> >load."  With increased load, the modems turn the line around less
>> >frequently, thereby wasting less bandwidth.  "Collapse" would require
>> >just the opposite, decreased effecency with increased offered load
>> >
>> 
>> If Bob's "offered load" is multiple TCP sessions, he could again be
>> describing a link that's returning multiple ACKs to the modem before
>> the line turns around.  If the SLIP packets were small, the results
>> would probably be as bad as he indicates.
>> ...
>
>I do not see this.
>
>The word "collapse" is often reserved for situations where a small
>increase in offered load results in a substantial decrease in
>throughput.  The system, whether a computer virtual memory system
>during "thrashing" or ethernet in "collision collapse" or rush-hour
>traffic in gridlock becomes all but useless in "collapse."
>
>As far as PEP modems are concerned, a high offered load is just a lot
>of bytes.  It does not matter to them whether those bytes are for 1 TCP
>stream or a thousand.  The modems will be presented with a high load in
>both directions, and will switch to large packets.  Once they have
>switched to large packets, the modem-to-modem link does not "collapse"
>any more, does it?
>

Before I try to answer your last question, let me admit that I didn't
fully understand the use of the word "collapse" in this context.  I'm
vaguely familiar with data flow patterns over SLIP, but I'm probably not
aware of the more subtle details.  My response may have been about a
different situation than the one you and Bob have in mind, confusing the
picture rather than making it clearer...

The type of session I had in mind is where there are multiple TCP sessions
passing through the same SLIP link.  With my vague understanding of how
SLIP works, I pictured each TCP session returning ACKs independently of
(and asynchronously from) the other sessions.  Perhaps I erred in this,
but if not, then the additional ACKs could conspire to increase PEP's
overhead in the reverse direction.

Now for the question you asked.  In theory, Yes, the link can "collapse"
further after the modem begins using Long packets in the reverse direction.
Remember that the more packets the modem sends in the reverse direction,
the more time is stolen away from the forward direction.  If the modem has
enough data (ACKs) to overflow one Long packet, it will send two.  If it
has more data than will fit into two, it will send three, and so on up
to the maximum of 12 Long packets in a single burst.  Generally, I would
not expect ACKs to fill the modem up enough to force additional Longs, but
it's not impossible.

Also, looking back on your first article (still quoted above), I think
I see a misconception where you say "...the modems turn the line around
less frequently, thereby wasting less bandwidth."  Bandwidth is wasted
far less by a line turnaround than it is by transmitting something back
in the reverse direction.  If you count the number of line turnarounds
per second, you'll think returning a Long packet is good, as the modems
turn the line around less often.  However, extend this concept a little
further and the problem will rear its head.  Return 12 Long packets and
you'll have enormously reduced the number of line turnarounds, but your
throughput will drop through the floor.  It's not the number of line
turnarounds that's important in PEP.  It's the proportion of forward
transmit time to reverse transmit time.

(Sorry if you're already aware of all this - I'm just trying to
clear up some things that appeared to be misleading...)


-- 
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Greg Andrews  --  gerg@netcom.com  --  ...!{claris,amdahl}!netcom!gerg

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