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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: <3324@sdcsvax.UCSD.EDU>
Date: Fri, 12-Jun-87 13:37:17 EDT
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Posted: Fri Jun 12 13:37:17 1987
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In article <3302@sdcsvax.UCSD.EDU> jad@hpcndm.HP.COM (John A Dilley) writes:
>> 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?).
>
...
>				 John A Dilley

	If you are using a token-bus, as opposed to ethernet, or other similar
protocols, you should be able to know if a node is down in one circuit of the
bus.  This requires a timeout, but it is miniscule compared to things like
1 sec.  If the node's packet was somehow lost, or the node comes back up,
it notices it isn't in the ring and inserts a request to be added at a 
predefined spot of the ring.  (This is of course an over-simplified 
explanation, but I think everyone should get the idea).

	Randell Jesup
	jesup@ge-crd.arpa
	jesup@steinmetz.uucp