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From: geoff@hinode.East.Sun.COM (Geoff Arnold @ Sun BOS - R.H. coast near the top)
Newsgroups: comp.protocols.tcp-ip,comp.protocols.nfs
Subject: How to get an urgent message to an arbitrary system
Message-ID: <681@east.East.Sun.COM>
Date: 8 Aug 89 21:51:17 GMT
Sender: news@east.East.Sun.COM
Reply-To: geoff@hinode.East.Sun.COM (Geoff Arnold @ Sun BOS - R.H. coast near the top)
Followup-To: comp.protocols.tcp-ip
Organization: Sun Microsystems, Billerica MA
Lines: 37

One of the questions I am often asked about PC-NFS is "how come
there's no way for me to find out when a particular file server
is going down? Unix users get notified." I point out that (at least
on SunOS) the mechanism used is "rwall", which is an RPC service, and that
for size reasons we can't afford to embed a version of rpc.rwalld in 
PC-NFS. This explanation is reasonable, but unsatisfactory. 

My reaction was to say "let's ask the NIC for a UDP port so that
we can use it to send unsolicited messages to PCs running PC-NFS."
That would certainly do the trick. However, a moment's thought
reveals that the problem is bigger than just PC-NFS. Surprisingly,
there is at present no simple ubiquitous message protocol to fulfil this
function. rwall is fine for SunOS and other ONC licensees, but
what about other systems? Do I have to rely upon SMTP? That's
incompatible with the idea of broadcasting a simple message
such as "The backbone will be down for five minutes at 12:00
to replace a bridge." 

This could be trivially simple or slightly more involved
(but still simple). The trivial approach is to dedicate
a UDP port for unsolicited system messages. Anyone could send one,
in a single datagram, and the listener process would be responsible
for delivering it as seemed appropriate for the system (dialog
box, console message, etc.) A more complete approach would be to
define a formal protocol so that it would be possible to convey
information about the coding of the message, message length (so that
TCP could be used instead) and so forth. [If the spec exceeds
one page, it's too complicated.]

Comments?

Geoff

Geoff Arnold,                              Internet: geoff@East.Sun.COM
PCDS Group, Sun Microsystems Inc.
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Just think: If Unix had been developed in England, we'd all be using BCPL...