Message-ID numbers
A wider IMP/Host field that can identify individual messages rather than only links.
Walden, D.C., "Message-ID numbers", RFC 533, DOI 10.17487/RFC0533, July 1973, <https://www.rfc-editor.org/info/rfc533>.
@misc{rfc533,
series = {Request for Comments},
number = 533,
howpublished = {RFC 533},
publisher = {RFC Editor},
doi = {10.17487/RFC0533},
url = {https://www.rfc-editor.org/info/rfc533},
author = {D.C. Walden},
title = {{Message-ID numbers}},
pagetotal = 1,
year = 1973,
month = jul,
} RFC text
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.
[ This RFC was put into machine readable form for entry ]
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[ support from GTE, formerly BBN Corp. 10/99 ]
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§1Why Change the Link Field
The IMP message format in BBN Report 1822 uses a link field. RFC 533 says that uniquely identifying messages through the link number limits throughput.
BBN reports that it has changed the IMP/Host protocol to address this limit. The change expands the field used in both IMP/Host and Host/IMP leaders.
A link number did not give hosts enough room to identify messages while sending at higher throughput. The RFC describes a wider field for message identifiers.
§2The Message-ID Field
The link field is extended to the right by four bits and renamed the message-id field. It occupies bits 17 through 28 of the leader, for a total of 12 bits. The RFC expects hosts to use it to identify messages between IMPs and hosts.
All 12 bits are returned to the host in RFNMs, incomplete transmissions and similar responses. The RFC explicitly says that message-id values do not imply an ordering.
The field is now 12 bits wide and comes back in responses. It can identify a message, but it does not tell the host which message came first.
§3Optional Use and Compatibility
Because the Host/Host protocol already uses the old link field to identify connections, hosts could use the additional four bits to identify messages within one connection. The RFC says hosts are not obligated to uniquely identify their messages, and the IMP subnetwork does not depend on that use.
A receiving host need not examine the extra bits. The RFC therefore describes the change as backward compatible even when a sending host uses those bits.
Hosts may use the extra bits or ignore them. Older receivers can still work because they do not have to inspect the new portion of the field.