Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!arisia!jlevy
From: jlevy@arisia.Xerox.COM (Jacob Levy)
Newsgroups: comp.sys.isis
Subject: Re: Another proposed interface, more comments solicited
Message-ID: <1988@arisia.Xerox.COM>
Date: 27 Jul 89 20:55:00 GMT
References: <30421@cornell.UUCP>
Reply-To: jlevy@arisia.xerox.com (Jacob Levy)
Organization: Xerox Palo Alto Research Center
Lines: 33

Do you have any delivery guarantees on the messages going through
this dedicated channel (or only those (non)guarantees provided by
TCP)? If there are delivery guarantees which mimick, say, the
ordering semantics of cbcast, what is the relative merit of using
a dedicated channel over using cbcast with a process group
containing only the destination process?

The following comments assume that strict adherence to the
semantics of TPC transport are imposed by the referenced
suggestion, with no additional ordering guarantees. Specifically,
assume that we have three processs, A, B and C. A and C both send
to B via a dedicated channel, but A and C communicate via some
ISIS primitive. Clearly, it is possible for A to send to C, then
to B via the dedicated channel. C reacts to A's message by
sending to B on its dedicated channel. If there are no ordering
constraints, B may see C's message before A's. Thus, having
included a mechanism for communication which fails to obey ISIS
conventions but which can be used in conjunction with ISIS (e.g.
it can be monitored with isis_input), you've opened up a whole
can of worms.

In summary, the suggestion Ken made, if my assumptions about lack
of ordering guarantees are correct, is NOT a solution to the
efficiency problem encountered when you want to communicate with
only one process instead of with a group. We need a mechanism
whereby you can use the process group semantics (perhaps you may
want to include some declaration about the location and
cardinality of the process group when you create the group) but
which will be as efficient as direct communication. The proposed
solution achieves the desired efficiency but discards all that is
desirable and nice about ISIS.

--Jacob