Subject: Use of the Multicast Disemmination Protocol (MDP) for Unicast

From: "Lovuola, James J CECOM RDEC CSC" <[email-masked]>
Date: Thu, 26 Jul 2001 14:59:06 -0400

Joe:  I'm currently supporting the Army Systems Engineering Office (ASEO) in
investigating which Multicast and Reliable Multicast protocols would be
suitable for inclusion into the Joint Technical Architecture-Army (JTA-A).
One of the candidates we are looking at is MDP which is being supported
within the Army by FBCB2 for use over Combat Net Radio (CNR) and other
networks within the lower and upper Tactical Internet (TI). 

At the CNR Implementation WG held last week, the issue surfaced as to
whether Segmentation/Reassembly (a CNR WG developed Application Framing
Prptocol to support Unicast) should be mandated over UDP for all large
messages (those larger than the maximum segment size of 496 octets).  At the
meeting, I pointed out that S/R was never designed to support multicast and
other protocols (e.g., MDP) are much more suited to provide a reliable
transport service for multcast applications.  My question is; Is it possible
that MDP could also provide a reliable transport service for Unicast (e.g.,
be used by different applications for both Multicast and Unicast)?  The
problem with using TCP is that it imposes to much overhead for use over CNR
networks.  S/R can also be used for Unicast but is not an open standard and
has not gained wide acceptance even within the CNR community (i.e., is only
being supported by Fire Support and a handful of synchronous users).  S/R is
also not well documented ands does not contain any guidance on use of
timers, etc.  Also I know that there are real-time protocols such a RTP that
could be used to ensure sequencing of packets for Unicast but they don't
provide a reliable transport service.

For MDP to support Unicast, it appears that addressing would be an obvious
issues; Could MDP support for instance both Class D addresses as well as
Unicast addresses?  Would all Unicast receivers have to be considered as a
separate groups?    Again, it may be more than just addressing, as I would
guess that for Unicast you would want to shut off some of the mechanisms
such as NACK supression, NAK aggregation for sending repair information,
etc.  Would this be easy to do or would it require a major SW modification
effort?  Has MDP ever been tested for Unicast?  Any insight you could give
me on this subject would be greatly appreciated.

Regards
Jim L