Path: utzoo!utgpu!watserv1!watmath!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!usc!apple!agate!ucbvax!techops.cray.com!mni
From: mni@techops.cray.com (Michael Nittmann)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP Bandwidth limits (was Re: TCP window size restriction)
Message-ID: <9101161933.AA23236@techops.cray.com>
Date: 16 Jan 91 19:33:14 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 56

Mea culpa!

Please!

RTT is under no control whatsoever neither by sender 
nor by receiver.
This holds also for the trivial "net" of two hosts.
Why:

Satellite: RTT is totally determined by the signal propagation.
Stationary orbit means about 80.000 km travel. With electronic 
delays this is then about 1 sec. ONLY THEN you could (but it
makes no sense for quantification!) say: 1 max window in the
net. But even that's no limitation if the receiver acks 
early so that the window slides and not concatenates end to end
(try to explain queuing theory inplain language).

Two hosts back to back. Take Sun WS: the ethernet card
of a Sun 3 gives about 4Mbit/s throughput. BUT: you do not
control memory accessibility, processor activity etc.
of the receiver. RTT may have a very big variance, big
enough to make talking of an "average" senseless (although
you can calculate one) since the average time may by
far be NOT the most frequent timelag occurring.
Take a disk at the receiver. It might be scattered
in free sectors. Then not PING RTT but receiver's I/O
determines RTT. RTT cannot be determined by taking some
TTL counts in seconds (who brings that up, TTL in 
TCP/IP is hops and should be internet diameter times
two to account for enough hops for the packet to 
propagate through the net on a fairly direct path).
And RTT does not determine or limit throughput
(early ack e.g.).
The case of one max. window in propagation on the
net is an interesting special case. 

By the way, I had to reply to the net 
because the original author was not reachable
since his path was not listed in nodes my mail
passes. Next time I will not reply to the net
if some obvious wrong posting is there. 


Could we please take this now off the list?
Please mail to me, not to the list.


Michael




And the disclaimer!!!!


:wq