RFC 642 (rfc642) - Page 1 of 4

Ready line philosophy and implementation

Network Working Group                                    Jerry Burchfiel
Request for Comments: 642                                            BBN
NIC: 30872                                                  July 5, 1974


                Ready Line Philosophy and Implementation

I.  Introduction

   BBN Report #1822, Specifications for the Interconnection of a Host
   and an IMP, gives a complete specification of the Host-IMP interface.
   However, the authors of this document bent over backward to avoid
   issuing arbitrary dictatorial directives to host interface
   implementors.  They succeeded admirably in this goal by describing
   the IMP implementation, and suggesting similar behavior on the part
   of the host.

   ARPA has appointed a PDP-11 local host interface standardization
   committee composed of myself, Dave Retz of SCRL, and Yuval Peduel of
   MIT Lincoln Labs.  During our review of various interfaces designed
   by the ARPA community, we have found total chaos, confusion and
   misunderstanding about the recommended host interface implementation.

   This note is an attempt to make explicit the recommendations which
   are implicit in Report #1822.  It provides a cookbook for interface
   implementors, including a set of recommended do's and don't's in the
   common problem areas.  This document has been reviewed and approved
   by the BBN IMP group.

II.  Ready-line Philosophy

   The following is an attempt to spell out in detail a consistent plan
   for operation of the IMP ready line and host ready line with the
   following objectives:

      1.  Reliably resynchronize and resume transmission after a
          temporary lapse of service and possible loss of state
          information by either system.

      2.  Make the programming of the host interface as simple as
          possible.  This will minimize bugs, and make it possible to
          create a small ROM network-bootstrap loader.

   First, consider the IMP ready line.  When it drops, the IMP has
   suffered a possible loss of state, so the message in transit from the
   IMP to the host (if any) is likely to be incomplete.  Similarly, the
   message in transit from the host to the IMP (if any) is likely to be
   incomplete.  Both the host and the IMP must recognize this explicitly



Burchfiel