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Path: utzoo!mnetor!spectrix!clewis
From: clewis@spectrix.UUCP (Chris Lewis)
Newsgroups: net.sources.d
Subject: Re: \"FAST\" protocol proposal from Hayes
Message-ID: 
Date: Tue, 19-Aug-86 11:23:42 EDT
Article-I.D.: spectrix.120
Posted: Tue Aug 19 11:23:42 1986
Date-Received: Tue, 19-Aug-86 17:35:42 EDT
References:   
Reply-To: clewis@.UUCP (Chris Lewis)
Organization: Spectrix Microsystems Inc., Toronto, Ontario, Canada
Lines: 40
Keywords: errors reliability end-to-end CRC

I have some experience with so-called "error-free" links.

4.3 BSD UUCP contains protocol "f", which is intended for X.25 links
where the underlying network generally guarantees *NO* errors.
Protocol "f" simply blasts the file along the line, only transmitting 
a checksum at the end of a file, and if incorrect, retransmits the file.  

Sigh....

The one thing not error-free was the network to computer connection 
(eg: modem or PAD to computer) - many of the links I used protocol 
"f" on had computers on the other end that would drop characters 
when talking to their modems even with X-ON/X-OFF flow control (missed 
interrupts).  We routinely were transmitting files 500K or larger.  
Having to retransmit the durn things multiple times is extremely 
painful and expensive - especially when most of the time the effective 
thruput was around 1200 baud (inspite of 9600 baud or higher links all 
the way thru).  There were times when we had to abandon attempts to 
transmit a large file because, after 3500 miles of error-free transmission, 
the link between the modem and the computer (connected by 2 feet of 
wire) simply refused to carry our megabyte of data error-free because the 
computer missed the occasional interrupt at high baud rates.

PLEASE PLEASE PLEASE: even on *perfect* channels, packetize and retransmit
packets on errors.  If the network is reliable, make the packets *big*.
Say 16K or 64K, or better still, make it a parameter.  There were
times when 64K packets would have reduced our line charges by an order
of magnitude (no exaggeration - after 40 failures on the same file...).

Think about the poor suckers trying to transmit files so large that the 
probability of a cosmic ray particle nailing the data register in the 
UART approaches unity.  (probably an exaggeration, but you get the idea)

Never assume that anything is ever perfectly error-free - in the real world, 
nothing is (and sometimes the "perfect" ones aren't even close) - don't 
penalize the people working with imperfect equipment (or software in my case).
-- 
Chris Lewis
UUCP: {utzoo|utcs|yetti|genat|seismo}!mnetor!spectrix!clewis
Phone: (416)-474-1955