Some changes to the IMP and the IMP/Host interface

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 …