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Network Working Group                                       J. BurchfielRequest for Comments: 594                                      BBN-TENEXNIC: 20616                                                 December 1973Speedup of Host-IMP InterfaceI. 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]