[e2e] Historical question: Link layer flow control / silent discard

John Day jeanjour at comcast.net
Fri May 24 10:16:07 PDT 2013

The nature of the link layer protocol is mostly 
determined by the characteristics of the media. 
In the early networks, it was not uncommon for 
the link layer to be a protocol that looked 
something like what we think of HDLC, i.e. 
ack/retransmission with fixed (usually small) 
window size.  In these early fixed window 
protocols, ack and flow control were seen as all 
part of the window scheme.  The quality of the 
lines pretty much dictated the use of these sorts 
of protocols over long distances.  As the data 
rate increased, the delay imposed by this class 
of protocols made them inadequate.

With the advent of LANs, these HDLC-like 
protocols were not really required.  But to say, 
they went completely out of use by 1981 would 
probably be incorrect.  In the US, some of the 
longer runs of the ARPANET, such as Illinois to 
Utah were relatively error free.  At the same 
time, short runs like Rome, NY to Cambridge had 
much higher rates.

It would be certainly be incorrect to conclude 
that a decision was made not to use them, 
especially since the ARPANET was not turned off 
until 1990 or thereabouts.

At 5:34 PM +0200 5/24/13, Detlef Bosau wrote:
>O.k., perhaps this is for all readers with grey 
>hair (if there is still hair at all....) and 
>grey beards ;-)
>When I read the original catenet work by Cerf, 
>the Catenet employed link layer flow control.
>To my understanding, this was abandoned when the 
>ARPAnet turned into the Internet (in 1981?). 
>After this change, the link layer flow control 
>was replaced by a "silent discard" of packets 
>which cannot be accepted for delivery.
>Is this correct?
>What was the reason for this decision and have 
>there been any alternative approaches?
>Detlef Bosau
>Galileistraße 30
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