    Network Working Group                                       J. Burchfiel
    Request for Comments: 594                                      BBN-TENEX
    NIC: 20616                                                 December 1973

                         Speedup of Host-IMP Interface

    I. Introduction

       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
       obtainable.

       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.

    Burchfiel                                                       [Page 1]

    RFC 594              Speedup of Host-IMP Interface         December 1973

       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
       would operate.

       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
       speed.

       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.

    Burchfiel                                                       [Page 2]

    RFC 594              Speedup of Host-IMP Interface         December 1973

       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
       followiing table:

       Operation  Byte Size    Type        Bandwidth       User CPU seconds/
                                                               megabit

       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
       obtainable.

              [This RFC was put into machine readable form for entry]
               [into the online RFC archives by Alan Whinery, 1/02]

    Burchfiel                                                       [Page 3]