RFC 47 (rfc47) - Page 2 of 4

BBN's Comments on NWG/RFC #33

RFC 47               BBN's Comments on NWG/RFC #33            April 1970


I found two substantial errors in the Host Protocol Paper, which was
otherwise an excellent paper.  Both concern a misunderstanding of the
nature of the IMP as a communications device, and in particular the
nature of buffering an IMP must do.  The authors consider the network as
a device into which one pushes a message which travels around some,
waits in buffers for substantial lengths of times, and then emerges at
the destination.  In fact a better model would be that the message pops
out again an instant after it is inserted.  While it is true there is a
delay, it is imposed by phone line hardware for the most part.  The IMP
buffering is minimal, and devoted to error control and momentary traffic
surges.

Since we cannot force a Host to take a message, we have built an elab-
orate RFNM mechanism to suspend new input until he does.  This mech-
anism is an imperfect attempt to solve a very hard communications
problem.  The desire is to regulate traffic in such a way that as the
Host takes its message from the IMP the next message is arriving on the
phone line, and no buffering occurs at all.

In fact we cannot achieve this, and therefore have included buffering to
handle traffic surges.  These buffers are useless for their intended
purpose unless they are empty.  Only empty buffers are available to soak
up a traffic surge.

The two specific errors occur on pages 5 and 23.  On page 5 the authors
say "Implicit in this purpose is the assumption that a user does not use
multiple links to achieve a wide band."  In fact one of the primary
purposes of links is to achieve a wider band.