Network Working Group
Request for Comments: 660
NIC #31202
D. Walden (BBN-NET)
Oct 1974


In the next few weeks several changes will be made to the IMP software including changes to the IMP/Host software interface
as specified in BBN Report No. 1822, Specifications for the Interconnection of a Host and an IMP. These changes come in
four areas: a) decoupling of the message number sequences of
Hosts; b) Host/Host access control; c) expansion of the
message number window from four to eight; and d) provision for messages outside the normal message number mechanism. All changes are backward compatible with possible minor exceptions in timing.

a. Decoupling of the Host/Host message number sequences:

Since 1972 the IMP system has provided for exactly four
messages to be outstanding at a time between any pair of
IMPs, and thus, a total of only four messages between
all the possible pairs of Hosts on the two IMPs. Because
all the pairs of Hosts on the two IMPs have had to share
the four outstanding messages, it has been quite possible
for the various Hosts to interfere with each other. To
remove this possibility of interference, the IMP's
message number logic will soon be changed to allow a
separate message number sequence between each pair of Hosts.

To keep manageable the space required to maintain the
Host/Host message sequences above that presently are required
for the IMP/IMP message sequences, the Host/Host sequences
will be taken dynamically from a limited pool of possible sequences. The pool will be sufficiently large to seldom
interfere with a pair of Hosts wishing to communicate. In
no case will Hosts be prevented from communicating. In
the event that the Hosts on an IMP desire to simultaneously communicate with so many other Hosts that the pool would
be exhausted, the space in the pool is quickly multiplexed
in time among all the desired Host/Host conversations
so that none is stopped although all are possibly slowed.

b. Host/Host access control:

Upon instructions from ARPA, we will soon add a Host/Host access control mechanism to the IMPs. Any pair of Hosts
wishing to communicate is checked (via bits in the IMP)
to verify that they have administrative permission to
communicate. This check normally is made whenever a pair
of Hosts attempts to communicate after not having
communicated for two minutes. If the pair of Hosts is
not allowed to communicate, a special type of Destination
Dead Message (sub-code 3) is returned to the source
Host. The default case initially will be to allow all
Hosts to communicate with each other.

c. Message number window.:

Once the message number sequences are on a Host/Host
rather than IMP/IMP basis, the number of messages that
will be permitted to be outstanding at a time between
a pair of Hosts will be expanded from four to eight, permitting increased Host/Host throughput in some cases.

d. Message outside the normal mechanism:.

For certain limited experiments which are being carried on
using the network, it is thought to be desirable
for specified Hosts to be able to communicate outside the normal ordered, error controlled message sequences.
Thus, the following expansion to the IMP/Host protocol is being provided.

i. a single packet message coming from the source Host

to the source IMP with a (new) special message type,
3, will be put directly into the IMP store-and-forward
logic with a mark saying the packet is this special
kind of message. A multi-packet message of type 3
will be discarded.

ii. such messages (packets) are routed normally to the

destination IMP, possibly arriving out of order.

iii. at the destination IMP, messages of the special

type will be put directly on the destination Host
output queue skipping the reassembly logic and marked
with a special (new) IMP to Host message type, also 3.

iv. there is no source-to-destination retransmission
   logic, no reassembly, no RFNMs, no incomplete
   transmissions, etc.

v. if at any time there are insufficient resources in the

network to handle one of these special messages
(e.g., the destination Host won't take it), the
message will be discarded.

vi. by using the special message type between the Host

and the IMP, the normal message number mechanism is
preserved for all the Host/Host transmissions which
presently depend on it.

Because the uncontrolled use of this mechanism will degrade the performance of the network for all users, the set of Hosts permitted to use this mechanism will be regulated by the Network Control

Please file this note with your copy of BBN Report 1822 until

that document is updated.