RFC 215 (rfc215) - Page 1 of 8


NCP, ICP, and Telnet: The Terminal IMP implementation



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Network Working Group                                      A. McKenzie
Request for Comments: 215                                          BBN
NIC #7545                                               30 August 1971
Categories: C.2, D.1, D.3, G.1
Updates: none
Obsoletes: none

                         NCP, ICP, and TELNET:

                    The Terminal IMP Implementation

       By early December there will be six Terminal IMPs incorporated
into the network, with additional Terminal IMPs scheduled for delivery
at a rate of about one per month thereafter.  For this reason the
implementation of network protocols (and deviations from them) may be of
interest to the network community.  This note describes the choices made
by the Terminal IMP system programmers where choices are permitted by
the protocols, and documents some instances of non-compliance with
protocols.

     Most of the choices made during protocol implementation on the
Terminal IMP were influenced strongly by storage limitations.  The
Terminal IMP has no bulk storage for buffering, and has only 8K of 16-
bit words available for both device I/O buffers and program.  The
program must drive up to 64 terminals which generally will include a
variety of terminal types with differing code sets and communication
protocols (e.g., the IBM 2741 terminals).  In addition, the Terminal IMP
must include a rudimentary language processor which allows a terminal
user to specify parameters affecting his network connections.  Since the
Terminal IMP exists only to provide access to the network for 64
terminals, it must be prepared to maintain 128 (simplex) network
connections at any time; thus each word stored in the NCP tables on a
per-connection basis consumes a significant portion of the Terminal IMP
memory.

     It should be remembered that the Terminal IMP is designed to
provide access to the network for its users, not to provide service to
the rest of the network.  Thus the Terminal IMP does not contain
programs to perform the "server" portion of the ICP; in fact, it does
not have a "logger" socket.


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