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NCP, ICP, and Telnet: The Terminal IMP implementation (RFC0215) Disclosure Number: IPCOM000002706D
Original Publication Date: 1971-Aug-30
Included in the Prior Art Database: 2000-Sep-13
Document File: 6 page(s) / 16K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

A.M. McKenzie: AUTHOR


The Terminal IMP Implementation

This text was extracted from a ASCII document.
This is the abbreviated version, containing approximately 17% of the total text.

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


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


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


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.

The Terminal IMP program currently implements only the NCP, the

ICP, and the TELNET protocol since these are of immediate interest to

the sites with Terminal IMPs. It is anticipated that portions of the

data transfer protocol will be implemented in the future; the portions

to be implemented are not yet clearly defined, but will probably include

the infinite bit stream (first) and the "transparent" mode (later).

Developments in the area of data transmission protocol will be

documented in the future.

The remainder of this note describes, and attempts to justify,

deviations from the official protocols...