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From: spaf@gatech.UUCP (Gene Spafford)
Newsgroups: net.mail
Subject: Re: definitions for LOCAL, DEMAND, DAILY, etc
Message-ID: <11709@gatech.UUCP>
Date: Sat, 19-Jan-85 22:39:29 EST
Article-I.D.: gatech.11709
Posted: Sat Jan 19 22:39:29 1985
Date-Received: Mon, 21-Jan-85 02:16:36 EST
References: <700@cbosgd.UUCP> <2308@garfield.UUCP> <422@down.FUN>
Reply-To: spaf@gatech.UUCP (Gene Spafford)
Organization: The Clouds Project, School of ICS, Georgia Tech
Lines: 33

Let me attempt a little more explanation, and maybe it will help.

Imagine drawing a directed graph (digraph) where every edge represents
a uucp (or other) connection.  If you weight each edge with a cost, you
have a network.  Pathalias is a program which applies an algorithm to
figure least-cost routes to other sites (I dunno which algorithm -- I've
never bothered to look.  I hope it's one of the more efficient ones,
perhaps even using a heuristic lookahead, but it really doesn't matter.)

Now the problems come in when a site has a link with basically two (or
more) possible classifications.  Suppose site A calls site B on demand
whenever there is news to transfer, and after midnight otherwise.
Well, in our network, we have two (2) directed edges from node A to
node B -- one with a weight of DEMAND and one with a weight of
EVENING.  Now, if you look at the values behind DEMAND and EVENING,
you'll see that DEMAND is a smaller value than EVENING.  Since
pathalias tries to find the least cost (weight) path, it should never
traverse the EVENING path since there is a cheaper route from A to B --
namely, the DEMAND link.  Thus, there is no use in specifying the
EVENING route at all.  I believe, in fact, that if you actually include
both in your listing, as in:
a	b(DEMAND), b(EVENING)
then pathalias will complain about a redundant link and use the DEMAND
link since it is cheaper.

The underlying point is, if two specifications could be used to describe
the way your site connects to another, use the one with lowest cost
(weight).
-- 
Gene "7 months and counting" Spafford
The Clouds Project, School of ICS, Georgia Tech, Atlanta GA 30332
CSNet:	Spaf @ GATech		ARPA:	Spaf%GATech.CSNet @ CSNet-Relay.ARPA
uucp:	...!{akgua,allegra,hplabs,ihnp4,linus,seismo,ulysses}!gatech!spaf