Network Working Group                                      David Walden
RFC # 533                                                  BBN-NET
NIC # 17452                                                July 17, 1973


                           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.










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