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Path: utzoo!mnetor!uunet!seismo!rutgers!topaz.rutgers.edu!ron
From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.mail.uucp,comp.mail.misc
Subject: Re: dynamic routing for UUCP mail
Message-ID: <13680@topaz.rutgers.edu>
Date: Mon, 3-Aug-87 18:03:02 EDT
Article-I.D.: topaz.13680
Posted: Mon Aug  3 18:03:02 1987
Date-Received: Tue, 4-Aug-87 05:23:20 EDT
References: <915@bsu-cs.UUCP>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 51
Xref: mnetor comp.mail.uucp:738 comp.mail.misc:455

Dynamic routing where each host runs pathalias on the last host path is
wrong.  This can only lead to problems.  Mail should work as follows:

1. The originator (or his agent) should generate the path by pathalias
   or some other reasonable method.  By the originator this can mean
   either the user composing the message or the mail system the message
   is posted from.  By the "or his agent" I mean, for example, that certain
   machines can be clustered around one mail forwarding machine. 
   They post with an address "destination-host!user" and always send
   the message to the agent machine.  The agent then substitutes the
   pathalias path.

2. Other hosts passing the message if they do not know the next hop 
   should return an error message to the sender.

3. The one optimization that might be done is path compression...
    "a!b!c!dest!user" may be sent directly to "c" if it knows
    a route.

-------------------
If you are going to have dynamic rerouting of the addresses, as you
described, you must operate in the following method...

You get a message with the address

    a!b!c!d!e!f!dest!user

You are machine "c" and you know a better way to "dest" is through
the path "x!y", then you rewrite the address...

    a!b!c!x!y!dest!user

and send the message to "x".

If you have no idea how to get to "d" and want to send the message to
"z" which you think is running pathalias, then you should rewrite to

    a!b!c!z!d!e!f!dest!user

In each case, the sender should examine the hosts preceding it in the
uucp path and make sure that none of the hosts that it is inserting
in the new route are listed (which is evidence of a loop).  Never rewrite
the information before your own host name.

To assure this will work will require specific changes to UUCP mailers
as well.  However, updating of path databases on UUCP is not timely
enough that any amount of intervention will guarantee that loops will
not form, so you need to have a procedure for bypassing them when they
might happen.

-Ron