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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
+
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+                   CONNECTING COMPUTERS TO MLC PORTS
+                   ---------------------------------
+
+        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
+   will?"
+
+
+        (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
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+                                                                [Page 1]
+
+RFC # 241
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+   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
+   circuit.
+
+
+        (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.
+
+
+
+   AMcK:jm
+
+         [ 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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+                                                                [Page 2]
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