Network Working Group J. Burchfiel
Request for Comments: 594 BBN-TENEX
NIC: 20616 December 1973
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.
Fourth, a high speed interface will permit greater host
communications bandwidth. (Currently limited to 100 kilobaud). This
increase in bandwidth will be essential for communications between
hosts at a "network-structured" site, where different hosts on the
same IMP are specialized to perform different parts of a computation.
Clearly, any new or retrofitted host interfaces should be very high
speed, and existing host interfaces should be adjusted to operate at
their maximum speed, which is in excess of 300 kilobaud.
II. Experimental Results
In support of the above predictions, the BBN TENEX staff performed an
experiment in cooperation with the BBN IMP group to determine how
fast the System A (BBN-TENEX) and System B (BBNB) distant interfaces
Results are as follows:
The Host-to-IMP connection is synchronized by a two-way handshake
which has an available burst bandwidth of 1 bit/(2225 nsec + 3
nsec/ft.*<cable length>ft) For our cable length, this results in a
bandwidth of 310 kilobaud.
The IMP-to-Host connection is synchronized by a four-way handshake
which has an available burst bandwidth of 1 bit/(1350 nsec + 6
nsec/ft.*<cable length>ft.) which results in a bandwidth of 290
kilobaud for our installation.
Both System A and System B are now operating at this higher interface
Since the propogation delay time through a distant host driver-
receiver pair amounts to 250 nsec, it is expected that local host
interfaces (<30ft) can be operated at speeds substantially faster
than our 300 kilobaud.
In addition to the above measurements of hardware speed, new results
were obtained in measurements of file transfer performance, i.e. the
CPU time and real time used per megabit of information transmitted
over the network.
This experiment involved the movement of one-megabit data files to
and from an FTP User process in System B communicating with the FTP
Server Process in System A. The results are summarized in the
Operation Byte Size Type Bandwidth User CPU seconds/
Get 8 ASCII 47Kbaud 7.9
Send 8 ASCII 50Kbaud 7.9
Get 32 LocalByte 43Kbaud 1.80
Send 32 LocalByte 38Kbaud 1.70
Get 36 Image 79Kbaud 1.85
Send 36 Image 85Kbaud .95
The 36-bit bandwidth of around 80Kbaud is a great improvement from
the (typically 25Kbaud measured before the speedup of the interface
hardware. The CPU time use has also decreased somewhat from that
reported in RFC #557 by Barry Wessler: this demonstrates continued
improvement of system efficiency between the TENEX version 1.31 and
TENEX version 1.32.
In conclusion, the BBN-TENEX staff recommends that all host-IMP
interfaces in the network be speeded up to the fastest operation
[This RFC was put into machine readable form for entry]
[into the online RFC archives by Alan Whinery, 1/02]