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Network Working Group A. McKenzie RFC # 241 BBN NIC # 7671 29 September 1971 Categories: B.1, C.1, I.1 Updates: none Obsoletes: Our Previous Verbal Comments

      Several times we have been asked if computers can be con- nected
 through serial communication lines to ports on the Terminal IMP's
 Multi-Line Controller (MLC) [related questions about the level of
 software support provided by the Terminal IMP to such a connection,
 have also been raised].  In the past we have said, "Please don't!" We
 now say, "Sure, but will that really help you the way you think it
      (1) Connections between computers and IMPs (i.e., the Host
 interfaces) have been assumed to be error-free.  This assumption is
 justifiable on the basis that the IMP and Host computers were
 expected to be either in the same room (up to 30 feet of cable) or,
 via the Distant Host option, within 2000 feet on well- controlled,
 shielded cables.  A connection through common carrier facilities is
 not comparably free of errors.  Usage of common- carrier lines for
 connecting a terminal to an IMP, including the assumption of a human
 at the terminal, is a situation in which the typical errors which do
 occur can be accommodated.  Usage of the same wire, with the same
 typical errors, for a computer-to- computer connection is likely to
 be a situation in which the errors are unacceptable.  The present
 version of the Terminal IMP does not provide error control either
 within its hardware or within its software on any ports of the
 Multi-Line Controller.  Further, we feel that computer-to-computer
 connections over common carrier circuits should employ strong error
 control, such as that
                                                              [Page 1]

RFC # 241

 used on the IMP/IMP circuits, and that attempts to use minimal error
 control (e.g., character parity) is an undesirable technical choice.
 Strong error control, with its retransmission scheme, not only would
 imply significant changes in the Terminal IMP, but a non-trivial
 hardware/software implementation at the remote computer end of the
      (2) Because the Terminal IMP has many obligations, the share of
 its bandwidth which can be given to a Host coming in over the MLC
 will be small.
      (3) The command language provided at a port of the Multi- Line
 Controller was designed with terminals and people in mind.  It
 provides very few of the capabilities which a computer requires in
 order to effectively utilize the communication network.  For example,
 only a single pair of connections can be made from a given Terminal
 TMP port; Host computers generally desire a larger number of
 simultaneous connections to other Hosts on the network.  Assuming the
 present Host/Host protocols, such a Host could not conveniently act
 as a server.
      If, despite these potential difficulties, connection of a
 computer to the network through an MLC port appears to be useful, BBN
 has no objection.  In fact, we would be extremely interested in
 hearing about actual experience with this type of network connection.
       [ This RFC was put into machine readable form for entry ]
       [ into the online RFC archives by BBN Corp. under the   ]
       [ direction of Alex McKenzie.                   12/96   ]
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