MESSAGE-ID NUMBERS

It has been noted that the IMP message format (specified in BBN

Report 1822) constrains network users to low throughputs if messages are

to be uniquely identified via the link number. We have recently

implemented a change in the IMP/Host protocol which alleviates this

difficulty. The link field in the IMP/Host and Host/IMP leaders has

been extended to the right by four bits (in other words, it now uses

bits 17 to 28 of the leader), and the name of the link field has been

changed to the _message-id field_. We expect this field to be used to

uniquely identify messages between the IMPs and the Hosts. All 12 bits

in the message-id will be returned to the Host in RFNMs, INCOMPLETE

TRANSMISSIONs, etc. Note that no ordering is implied by the use of the

message-id field.

Given that the Host/Host protocol uses the old link field to

identify connections, the additional four bits in the new message-id

field might well be used to uniquely identify messages on a given

connection (i.e., over a given link). Of course, there is no obligation

for Hosts to uniquely identify their messages (the IMP subnetwork

certainly doesn't care), so this change in the IMP/Host message format

is completely backward compatible. In fact, since the receiving Host

does not have to look at these extra four bits on arriving messages, the

change is completely backward compatible even when senders of messages

do use the extra four bits to uniquely identify messages.

Please store this RFC with your copy of BBN Report 1822 until 1822

is updated.

[ This RFC was put into machine readable form for entry ]

       [ into the online RFC archives by Alex McKenzie with    ]
       [ support from GTE, formerly BBN Corp.            10/99 ]

---