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From: honey@down.FUN (code 101)
Newsgroups: net.mail
Subject: Re: definitions for LOCAL, DEMAND, DAILY, etc
Message-ID: <422@down.FUN>
Date: Thu, 17-Jan-85 13:35:17 EST
Article-I.D.: down.422
Posted: Thu Jan 17 13:35:17 1985
Date-Received: Fri, 18-Jan-85 02:45:14 EST
References: <700@cbosgd.UUCP> <2308@garfield.UUCP>
Organization: Princeton University, EECS
Lines: 63

let me respond to some of andrew droskoy's queries.  i gather this is a
confusing point, since so many people seem confused by it.  mea culpa.

in simplest terms, pathalias solves the single-source shortest paths
problem in a weighted, directed graph.  see aho, hopcroft and ullman
for details.

all pathalias wants out of life is a list of weighted edges so that it
can do its thing.  it yaccs a high-level language to make life a little
easier (for you!  for me and steve it's like pulling teeth, or
scratching fingernails on a blackboard).  LOCAL, DEMAND, etc. act like
macros for integers (cat lex.l), not "classes of service" or whatever
the common misinterpretation is.

on to the misinterpretations.  (but first, please glance at the man
page.  thank you.)

	I have found the definitions to be a bit incomplete.  For
	example what do you use if you will call someone on demand, but
	also poll them daily?  I have seen people using DAILY for this,
	but some value less than DEMAND would perhaps be more
	appropriate.

if you call someone on demand, the appropriate weight is DEMAND.  if
someone polls you every day, use DAILY.

if you call on demand *and* once a day, that's no better than calling
on demand, is it?  so use DEMAND.  DAILY+DEMAND definitely doesn't make
it, since this indicates a link that is more costly (i.e., less
useful/reliable/worthwhile) than DEMAND or even DAILY.

as a rule of thumb, do not mix the basic costs (see man page again) --
use HIGH and LOW for minor adjustments.

	Another example:  We call most of our connections after
	midnight, but only if we have something for them - and we will
	call them on demand up until 8am.  We have been using EVENING
	to express this, but I am not sure this is right.

EVENING is the appropriate cost.

	I think the definitions could stand to be made clearer.

you're probably right -- help?

	Since Mark's reason for posting the official definitions is
	presumably to get us all using the same values to describe
	similar situations, I think some discussion on the relation of
	these definitions to reality would be appropriate.

the values were pulled out of thin air, played with, adjusted,
adjusted, etc. until the routes started looking reasonable.  i don't
know a better way to describe this.  perhaps the mistake was to allow
arithmetic expressions in the weights, which has led to confusion (as
well as providing flexibility).

i abase myself for having anything to do with this and am open to
suggestion.

coming soon in a net.sources near you:  the third iteration of
pathalias.  i have sworn my honor that i'll post it before dallas.

	peter