JANUS interface specifications
RFC 745

Document Type RFC - Unknown (March 1978; No errata)
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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications

Network Working Group                                     Michael Beeler
Request for Comments 745                                             BBN
NIC 43649                                                  30 March 1978
PRTN 245

                     JANUS Interface Specifications

                   (Symmetrical, 1822-like Interface)

1.  INTRODUCTION
1.1.  Motivation

A need arose in the Packet Radio project for specification of an
interface between Packet Radio units and other equipment.  This paper is
to meet BBN's responsibility to supply that specification.  It is our
hope that it will find application in other areas as well.

1.2.  Historical Relationship to 1822

The ARPANET employs a network of switching nodes, called IMPs, to
provide interconnection among user equipment, called hosts.  A uniform
means of connecting a host to an IMP is specified in BBN Report Number
1822.  Consequently, this interface has become known as an 1822
interface.

As the need to interconnect new types of devices has grown, it has
become attractive to implement an 1822-like interface on each end of
pairs of devices which are to communicate.  The devices are then
connected electrically, and communication can take place in spite of
differences in processing speed, word length, signal levels and so forth
in the two devices.  A part of Report 1822 reads as follows.

   "The technique of transferring information between the Host and the
   IMP is identical in each direction; we will, therefore, refer to the
   sender and the receiver without specifying the Host or IMP
   explicitly."
   [BBN Report Number 1822, 12/75 revision, page 4-2.]

Unfortunately, Report 1822 does not specify a completely symmetrical
interface.  Although there is a high degree of symmetry, some aspects
are peculiar to the IMP side and some to the host side.  Therefore, two
interfaces constructed to connect to IMPs may not function connected to
each other.  In what follows, the unsymmetrical aspects are respecified
in a way which will accomplish full interchangeability.

The interface specified here is called the JANUS interface, to
distinguish it from the Report 1822 interface.

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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications

1.3.  Terminology

The terms, "IMP" and "host," are not relevant in the present context.
Sections of Report 1822 such as Appendix B are conveniently
re-interpreted by substituting "foreign interface" and "home interface,"
respectively.

2.  SPECIFICATIONS

Report 1822 addresses two aspects of the connection of a host to the
ARPANET, the hardware requirements and the software protocols.  Since
the JANUS interface will typically be used in applications other than
connection to the ARPANET, the higher level software protocols are
beyond the scope of this paper. They are properly addressed by
documentation specific to each application.  Concern here is only for
electrical specification of the JANUS interface.  The various areas
which differ from Report 1822 are as follows.

2.1.  Low-level Protocol

Certain aspects of the JANUS interface and its operation may be
implemented in hardware, software of a mixture of the two.  We refer to
these aspects as "low-level protocol."  They are to be distinguished
from such "high-level protocol" aspects as header definitions and data
formats.

2.1.1.  Padding

Requirement:
Received messages are padded out to a full word (of the home device's
size), if necessary, with zeros only.

Discussion:
A one-bit to mark the end of received data, as IMPs employ, is NOT used.
The mark bit has not proved very useful, although the ARPANET IMPs do
use it to generate the message length field in the new format header.
Rather, counts at one or another level of protocol are generally used,
so the complication of a mark bit can be eliminated.  It is the author's
impression that the ARPANET will not implement this aspect of
symmetrical interfaces, so hosts communicating through the ARPANET will
continue to see the marker one-bit appended by the source IMP regardless
of whether the hosts have 1822 or JANUS interfaces.

2.1.2.  Message Length

Requirement:
A JANUS interface must accept messages up to and including 8160 bits
long.

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NWG/RFC# 745                                        MDB2 30-MAR-78 43649
JANUS Interface Specifications

Exception:
If the interface is absolutely never intended for use in
ARPANET-compatible applications, this requirement may be relaxed in any
of three ways.  A smaller maximum length may be implemented;  a larger
maximum lengthbe implemented; or the maximum length may be so large as
to be in practice infinite.

Discussion:
A JANUS interface may discard messages longer than 8160 bits when used
with the ARPANET.  This constraint can be enforced in software rather
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