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+Network Working Group                                      David Walden
+RFC # 533                                                  BBN-NET
+NIC # 17452                                                July 17, 1973
+
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+                           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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+       [ 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 ]
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+Walden                                                          [Page 1]
+