Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!eecae!netnews.upenn.edu!linc.cis.upenn.edu!brant
From: brant@linc.cis.upenn.edu (Brant Cheikes)
Newsgroups: comp.mail.uucp
Subject: Re: some (hopefully) useful comments about mail routing
Message-ID: <5439@netnews.upenn.edu>
Date: 3 Oct 88 23:14:06 GMT
References: <433@manta.pha.pa.us> <1915@datapg.MN.ORG>
Sender: news@netnews.upenn.edu
Reply-To: brant@manta.pha.pa.us (Brant Cheikes)
Organization: University of Pennsylvania
Lines: 54

In article <1915@datapg.MN.ORG> sewilco@datapg.MN.ORG contributes a
nice generalization of two concerns I raised:

>	Smail should allow local exceptions to mapped links.

He goes on to give a list of possible actions to take when mail comes
in that is routed out over an "exceptional" link.

A subtle case is, I think, overlooked, suggesting yet further extensions
to smail (and perhaps pathalias).

Consider this scenario: on manta, I want to maintain a exceptional link
to site baz.  Only local mail or mail mail originating from sites foo
and bletch should be able to use the manta!baz link.  Sites foo and
bletch need not be neighbors of manta.  IF we allow sewilco's actions
2(a) or (b), to wit:

>	2a. Reroute all mail except that from certain sites.
>	2b. Reroute only mail from certain sites.

Here's the problem:  Since I'm allowing local mail to use the manta!baz
link, I presumably incorporate a local connectivity entry in manta's
pathalias run of the form:

manta	baz(DIRECT)

This link is not published generally, though foo and bletch have been
authorized to make similar extensions to their pathalias data.

Now manta's paths database contains paths to various machines over the
manta!baz link.  Suppose that to get to uunet from manta, the path
would be manta!baz!uunet.  So far, so good: users on manta can address
mail to user@uunet and it'll go out as manta!baz!uunet!user.  And,
presumably, paths originating at foo or bletch, destined for uunet!user,
could use the manta!baz link as well.

But say mail arrives on manta from site mumble, destined for baz!uunet!user.
Could happen, right?  Some bozo using Reply-Path to a news article maybe?
Say I want to reroute that mail.

Oops, what's this?  The only path I know to uunet is through baz...

One solution that comes immediately to mind is to maintain TWO paths
databases, one using "exceptional" links and the other not.  Is that
a solution?  Is it the best solution?  Are there other problems?

Another idea would be to somehow redesign pathalias so that paths can be
generated "on the fly" under different constraints, but I'm not sure that's
necessary, and it's certainly more expensive than the periodic database
update scheme currently in place.
Brant Cheikes
University of Pennsylvania
Department of Computer and Information Science
brant@linc.cis.upenn.edu, brant@manta.pha.pa.us, bpa!manta!brant