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From: darrell@sdcsvax.UUCP
Newsgroups: comp.os.research
Subject: Re: How do you tell if a remote site is alive?
Message-ID: <3302@sdcsvax.UCSD.EDU>
Date: Thu, 11-Jun-87 21:47:42 EDT
Article-I.D.: sdcsvax.3302
Posted: Thu Jun 11 21:47:42 1987
Date-Received: Sat, 13-Jun-87 11:01:16 EDT
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Organization: Hewlett Packard, Colorado Networks Division
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> How is it done in practice?  For us university-types, time-out is the usual
> approximation to a solution since we're usually just out to prove a concept
> and not to build a product.
> 
> How do the folks in industry do it?  Performance is critical there, unlike a
> university prototype.

In much (user-level) software, it's still done by timeouts.  Even in the Sun
NFS kernel there are layers of timeouts on top of the UDP protocol (they use
UDP because it's fast on a small LAN, and then put timeouts of 1 second on
top of it ... strange, huh?).

In one distributed application I read about, hosts periodically send out
sanity checks which have (host#,state) pairs, either just for themselves or
for all hosts they know about.  When a host discovers that another host is
down, it modifies its state information for that host; in the next broadcast,
all other hosts learn of the dead host.  When it comes back up it tells
everyone "I'm alive again".  This gets very expensive when there are a lot of
hosts on the network, but it does keep other hosts from having to timeout.

Not sure of any other methods, you might want to check to see what LOCUS did
-- I know they're not a commercial product (or are they?), but it seemed like
a well done system to me.

I wish I knew of more better methods (than timeouts), but I don't.  I'd be
very interested in hearing what you discover in this area.  Thanks, and happy
researching (from the land of development!).

			      --      jad      --
				 John A Dilley
			      Hewlett Packard Co.
			  Colorado Networks Division
			Fort Collins, COlorado	 80525

ARPA:			    jad%hpcndm@hplabs.HP.COM
UUCP:		           {ihnp4,hplabs} !hpfcla!jad