Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!tut.cis.ohio-state.edu!bloom-beacon!bu-cs!purdue!decwrl!vixie
From: vixie@decwrl.dec.com (Paul Vixie)
Newsgroups: comp.mail.uucp
Subject: the crucial distinction
Message-ID: <804@bacchus.dec.com>
Date: 25 Sep 88 13:48:53 GMT
References: <8809212215.AA21035@naggum.se> <2540@sultra.UUCP>
Sender: vixie@decwrl.dec.com
Organization: DEC Western Research Lab
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I'm still working on my reply to the original "considered GOOD" article, and
so far it looks long and hot.  Stay tuned.  But meanwhile, Der Tynan says:

# a lot of times I haven't got the foggiest idea how to get mail to a given
# site, and would appreciate a relay along the way cleaning up my routing

I think I'll build a list of frequently said things so I can quote myself.
I am probably Usenet's most voiciferous opponent of what I've been calling
"active rerouting", yet you must believe me when I say (again) that I see a
very definite need for the service you describe.

I call it "passive rerouting".  "Active" vs. "passive": the distinction is
very important.  Sorry to burden you all with another set of terms, but I
had to describe this crucial distinction somehow.

Passive routing means that you have to know only two things:

	1. the user name and the name of the host/domain where she receives
	  her mail.  this is usually on a business card or a .signature file,
	  and if various sites on the net would fix their header rewriting
	  code, it would be in the headers of messages you receive from her.

	2. a path from your host to a host that has two attributes:
		-- practices "passive rerouting"
		-- is likely to know how to reach the "final host"

Knowing this, you can send the mail with an explicit route from where you
are to the passive rerouter, and then to the final destination AS THOUGH THE
FINAL DESTINATION WERE A DIRECT NEIGHBOR OF THE PASSIVE REROUTER.

So, for example, if you are "ireland!der", and you UUCP to "france" who UUCPs
to "england" who runs a passive rerouter; if you know that "england" will
know how to reach "california" even though you have no idea what path (or
mechanism; carrier pigeons aren't impossible) will be used; given these facts,
you can send mail to

	france!england!california!fresno!friend

You are actually _counting_ on "england" to reroute (or just "route", really)
the message.  You don't know and probably don't care what route will be used.
You are also counting on "california" knowing how to reach "fresno" if it is
not a direct UUCP neighbor (or within carrier pigeon range, or whatever).

So what's the ever-so-crucial distinction?  Why call it "passive" and praise
it?  Because of something "england" will not do:

	IT WILL NOT TRY TO FIND A ROUTE TO "fresno"

"england" doesn't know about "fresno".  There may be a machine on "england"'s
ethernet called "fresno" which it is willing to reach via "!"-notation out of
convenience; there may be a machine at a lab member's home called "fresno"
which noone wants to map because it will be back in the lab next week; there
are many good reasons to only do what you have been asked to do, which in
this case, means: send it to "california" and let it puzzle out the rest.

So you see, hopefully, that I am much in favor of _PASSIVE_ (re-)routing.  I
use this kind of thing constantly; I have passive routers one hop away from
decwrl in each of several directions.  (Brian Reid won't let me run a router
here, not yet anyway.)

I will be repeating this again in my reply to the original "considered GOOD"
article, but here's my small list of useful and unrefuted assertions:

	the map is not the territory.

	there is no problem solved by active rerouting which cannot be solved
	another way; there are problems _caused_ by active rerouting which
	cannot be solved at all.

Good day, all.
-- 
Paul Vixie
Work:    vixie@decwrl.dec.com    decwrl!vixie    +1 415 853 6600
Play:    paul@vixie.sf.ca.us     vixie!paul      +1 415 864 7013