RFC 68 (rfc68) - Page 1 of 2

Comments on Memory Allocation Control Commands: CEASE, ALL, GVB, RET, and RFNM

Network Working Group                                           M. Elie
Request for Comments #68                                        31 August 70
                                                                UCLA

             Comments on memory allocation control commands
                     CEASE, ALL, GVB, RET) and RFNM

The protocol provides a scheme for buffer allocation.  This scheme is
rather complicated because it necessitates two parallel mechanisms.
It is not obvious that both are necessary.  In fact it is suggested
that this scheme could be probably replaced by a slightly different
conception of the Request for Next Message (RFNM).  Now the RFNM is
sent back from the receiving imp after the message has been
reconstituted and the first packet transmitted to the host.  Nothing
insures that the whole message has been accepted and correctly
received by the host; also the design of the host imp interface
permits the host to stop accepting data from the imp during any length
of time; as the link has been already unblocked by sending back the
RFNM another message may be transmitted by the sending foreign host
which will congest the imp's memory.  On the other hand it is prob-
able that usually the host is able to accept data from the imp at a
higher rate than it is transmitted on the network, e.g. 200k bits/sec;
thus the time to transmit a full message from the imp to the host
would be approximately 1/20th of a second which is 10 times less than
the average delay of transmission of a message over the network.  This
indicates that to send a RFNM after the reception of a full message by
the host would not increase significantly the response time on the
network.

In this case there is no reason why the RFNM could not be initiated by
the receiving host as an acknowledgment of the correct reception of
the message (ACK), and take the form of either a host imp or a control
command message.  This RFNM could have the two forms

         ACK  (CONTINUE)
or       ACK  (CEASE)

This would permit to add to the message some error detection
redundancy, such as check sum bits as proposed in [DELO 69].  In the
present design nothing insures that one or several bits of the text
has not been altered, e.g., by an interference or a deficiency of one
of the host imp interfaces.  This could have important consequences,
e.g. if the text is used to update a centralized data base.  Also, if
the user has a way of detecting the error, but none of correcting it,
it has no way of asking for the retransmission of the message, which
has probably been discarded at the sending end upon reception of the
RFNM.  In fact it seems not up to the user to have to detect errors in