Network Working Group
Request for Comments: 687
NIC #32654
David C. Walden (WALDEN@BBN)
Jun 1975
IMP/Host and Host/IMP Protocol Change

This note sketches the design of an expansion to the
IMP/host and host/IMP protocol which will include among other
things the possibility of addressing hosts on more than 63 IMPs. Our intention in this expansion is to correct certain existing limits without fundamental changes in the philosophy of the IMP/host protocol; i.e., while many issues which would represent fundamental changes to the IMP/host protocol are presently under discussion in the world-wide packet-switching com unity, we are
not able to undertake massive fundamental changes on a time scale compatible with the short term needs for network improvement (e.g., already there are almost 60 IMPs).

The following paragraphs cover each of the major
characteristics of the expanded protocol. A knowledge of Section
3 of BBN Report 1822 is assumed. As is discussed below, the expanded protocol is backwards compatible.

1. Expanded Leader Size. The leader will be expanded from two
to five 16-bit words. This will provide space for necessary field expansions and additions.

2. Expanded Address Field. The address field will be expanded
to 24 bits, 16 bits of IMP address and 8 bits of host address. This expansion is more than adequate for any foreseeable ARPA Network growth.

3. New Message Length Field. A new field will be added which
will allow the source host to optionally specify the message
length (in bits) to the IMP subnetwork. The IMP subnetwork may
be able to use this information (when available) to better
utilize network buffer storage. The destination host may also be
able to use this information to better utilize its buffer
storage. This field will be 13 bits wide.

4. Expanded Handling Type Field. The handling type field which
now is used to distinguish between priority and non-priority message streams, etc., will be expanded to eight bits. This
expanded field will provide for the possibility of a number of parallel message streams having different handling
characteristics between pairs of hosts; e.g., priority, non-priority, varying numbers of packets per message (see below), unordered messages (i.e., the present type-3 messages), a message stream requiring guaranteed capacity, etc. Note that only some
of these facilities will be available in the near term.

5. Source Host Control of Packets per Message. The possibility

will exist for the source host to specify a message stream which

will use a given number of packets per multi-packet message (e.g, two packets per message or five packets per message). Since the IMP network will not have to use eight packet-buffers for reassembly purposes, as at present, this may result in better
services for such messages. This will help users who need both
low delay and high throughput.

6.    Unordered (type-3) Message Change. Unordered messages will
be indicated by a handling type rather than by a message type as
at present. This is compatible with the need to check the host
access control capabilities of all messages. This will provide a
slight backward incompatibility for the three or so hosts which
presently use type-3 messages in their research.

7.    Change in Format of Fake Host Addresses. The For/From IMP
bit will be eliminated. The fake host addresses will be the four
highest host numbers (e.g., IMP Teletype will be host 252).

8.    Addition of a Parameter to the IMP to Host NOP. The IMP to
host NOP will have added to it a parameter specifying the address
(IMP and host number) of the host.

9.    Backward Compatibility. The old and new formats will be
supported in parallel in the IMPs for the foreseeable future to
allow gradual phaseover of host software. A host will be able to
specify to its IMP whether the old or new formats are to be used;
thus, it will be possible for the host to specify switching back
and forth between the two modes for debugging purposes. The
specification of the mode to be used will be possible via a
proper choice of format in the host to IMP NOP message; the IMP
will use the mode of the host to IMP NOP message the IMP has
received. Further, a host may select to use either the old or
new format without needing to know more about the other format
messages than to discard them should they arrive. The IMP will
initialize by sending several NOP messages of each type to give
the hosts its choice. Although a host not implementing the new
format will not be able to address hosts on IMPs with IMP-number
greater than 63, the IMPs will wherever possible do the
conversion necessary to permit hosts using the old format to
com unicate with hosts using the new format and the reverse.
Finally, it will be possible to convert the leader format from
old to new or the reverse without knowledge of the message type.

11.    Non-blocking Host Interface. A mechanism will be provided
which allows the IMP to refuse a message from a host without
blocking the host interface. This mechanism will permit the IMP
to gather the necessary resources to send the refused message and
then ask the host to resend the message. Finally, the host will
be permitted to ask to be able to send a message and be notified
when it is possible without requiring the message to actually be
sent and refused.

12. Maximum Message Length. The maximum number of bits of data

in a message may be reduced by a few bits.

     We are presently working out the details of an
implementation plan for making the above changes to the IMP
software. We will distribute an implementation schedule and
other necessary information (e.g., format details) in plenty of
time for hosts desiring to use the new protocol as soon as it is
available to implement in time.