Network Working Group J. Burchfiel
Request for Comments: 594
Speedup of Host-IMP Interface
In order to make the full performance capabilities of the subnet
available for interprocess communication, the host's IMP interface
and the IMP's host interface should operate at the highest speed
First, this high throughput will minimize the latency observed when
RFNM's, control messages, and NVT (network virtual terminal)
characters are queued behind full sized messages. A full-sized
message currently ties up a 100 kb interface for almost 100 Msec.
delaying short messages behind it by 100 Msec. Speeding up the host
interface to 300 kilobaud will shrink this latency to 30 Msec.
Secondly, this high-speed operation minimizes the time that the IMP
buffer and the host core buffer are locked down during message
transfer. (One being emptied, one being filled). Being able to
dispose of buffers far faster means that many fewer of them will
suffice to carry the communications traffic; each buffer can be
reused far more often.
Third, high-speed operation makes it possible to improve error
control: currently, a destination IMP returns an RFNM after
transmitting the first packet of a multipacket message to the
destination host. If an error occurs during the transmission of the
(up to seven) other packets into the destination host, the source
host will not be informed of the error: it has already been given a
positive message acknowledgement in the RFNM. The alternative,
holding off the RFNM until all packets have been transmitted into the
destination host, would add another 80 Msec. to the round trip
message - RFNM time with the current 100 kilobaud interface. A
higher speed interface will reduce this delayed - RFNM cost to a more
acceptable value, making it practical to eliminate this source of
undetected message transmission errors.