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From: mcgregor@hpsmtc1.HP.COM (Scott McGregor)
Newsgroups: comp.mail.misc
Subject: Re: address writing by gateways (was: NO NO NO NO NO)
Message-ID: <11610003@hpsmtc1.HP.COM>
Date: Mon, 27-Jul-87 14:36:36 EDT
Article-I.D.: hpsmtc1.11610003
Posted: Mon Jul 27 14:36:36 1987
Date-Received: Wed, 29-Jul-87 04:34:18 EDT
References: <684@vixie.UUCP>
Organization: Hewlett Packard, Cupertino
Lines: 78

>(This makes sense.  An example:
>
>	<@athena.mit.edu,@hiddengate,user@hiddenhost>
>
>The sending mailer need only be able to figure out the part between `<'
>and `,'; the fact that `hiddengate' and `hiddenhost' are not registered
>does not have to matter.  Despite this, RFCnnn [fill in the n's] says
>it does.)

That's correct.  But now consider what athena.mit.edu needs to do.
(The following does not represent opinion as to what *should* be done
only an analysis of what *is* done in the Internet Community.)
Athena must send to hiddengate.  This is fine if we insist that hiddengate
is a neighbor machine to athena.  But the @ source routing in RFC 822
doesn't require that.  It only says that the mail should travel through
hiddengate on its way to hiddenhost.  There may be other not specified
hosts along the way between athena and hiddengate, and there may be
multiple hosts between hiddengate and hiddenhost too.

What if there are TWO machines called "hiddengate"?  How does athena
know which one to send to?  To avoid this ambiguous possibility (which Unix
users accept but which the Internet has constantly fought), Crocker
"solved" it by defining which one to send to: namely the registered one.
Since the Internet has a managed namespace there can be one and only one
"hiddengate.???.???" site and the possibility of ambiguity is eliminated.

Also note, that if hiddengate and hiddenhost are registered and for
some strange reason the path to  hiddengate is down, but there is
an alternate path to hiddenhost, then some helpful mail administrator
can clear their mail queue by rerouting your mail directly to hiddenhost.
If you don't really care how the mail gets there then this will be a
win for you.  If you really did care that it went through hiddengate
you also wouldn't care because of the lack of a path from athena to
hiddengate  (i.e. you'd recognize that you had asked for the impossible).
If hiddengate and hiddenhost were NOT registered such a "helpful"
action on the behalf of some intermediate mail administrator might
actually cause MISDELIVERY of your mail.  So having registered hosts
allows more robust recovery from network mail problems, whereas
duplicate unregistered mail can lead to undeliverable or misdelivered
mail.

>If you were sending to an unregistered host,
>
>	<@berkeley.edu:him@bouncy.ucb-test>
>
>would be officially illegal.  Nonetheless it would probably work.

Yes, you are right, it would probably work (unless your mail
happened to travel through some host that explictly filters out
mail with unregistered hosts in it.  )

Is there a chance that your mail might be mis-delivered? Sure, especially
if rather than bouncy.ucb-test you have a name like "bilbo" that is more
likely to have name conflicts. 

>The string
>
>	him%bouncy.ucb-test@berkeley.edu
>
>would be legal.  Why, then, is the  version illegal?

If you consider what the above discussion you can see that the
first one would be subject to possible misdelivery whereas the latter
is not.  This is because the latter absolutely insists that the mail
be given to berkeley and no helpful mail administrator should redirect
it any otherway, while the former represents itself as being a registered
address that is capable of redirected delivery if the path to berekeley.edu
is blocked.

> Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)

Everyone reading this note can decide on their own whether the
benefits of enforced registration outweigh the benefits of avoiding
%-style gateways.  In fact each reader WILL decide on their own,
and it has been my experience that there is little in terms of
argument that can be done to bring all into agreement.

Scott McGregor
Hewlett-Packard Company