Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!killer!jolnet!clout!mark
From: mark@clout.Jhereg.MN.ORG (Mark H. Colburn)
Newsgroups: news.sysadmin
Subject: Re: How does "routing" responsibility work? (.US Domain)
Message-ID: <309@clout.Jhereg.MN.ORG>
Date: 22 Jul 88 02:43:42 GMT
References: <8576@ihlpa.ATT.COM> <397@comdesign.UUCP>
Reply-To: mark@clout.Jhereg.MN.ORG (Mark H. Colburn)
Organization: House Jhereg, Streamwood, IL
Lines: 44

In article <397@comdesign.UUCP> pst@comdesign.uucp (Paul Traina) writes:
>I may be wrong about this,  but I was under the impression that any
>gateway into a domain is responsible for being able to connect to (or at
>least route to) any other host in that domain.
>
>How does one take (the simplest example) mail from:
>	foo.la.ca.us to bar.la.ca.us
>
>Assuming foo does not know about bar?  Do we then pass the mail back
>up to the "ca" router and it finds & computes a new path to bar?

In this particular case, foo does not need to know about bar, but the
domain naming conventions state that the site listed to the right of
a dot in a domain style adress knows how to get the the site
immediately to the left of the dot.  (This is a bit of a generalization, 
but it will work for the current discussion...) Therefore '.us' can get to 
'.ca', '.ca' can get to '.la' and '.la' can get to 'bar'.  In your example,
'.la' also knows how to get to 'foo'.  Therefore, the mail should only
have to go up to the server for '.la.ca.us' and then back down to
'bar' if 'foo' and 'bar' do not have a link.

Given the following example, 

	foo.sf.ca.us to bar.la.ca.us

The mail would have to take the following route (assuming, of course, that 
foo and bar do not have a direct connection, and that bar and sf.ca.us 
don't have a direct connection):

    foo.sf.ca.us -> sf.ca.us -> ca.us -> la.ca.us -> bar.la.ca.us

This is the beauty of hierarchical namespace, connections to lower
level domains are gaurenteed (well, assuming machines are up :-).  The 
difficulties come in when you branch across top level domains (i.e. mail 
from foo.la.ca.us to jhereg.mn.org).  In this case, all top level servers
know how to get to all other top level servers.  So in the example
above, us knows how to get mail to org, who then passes it down to
mn.org and finally to jhereg.mn.org.

-- 
Mark H. Colburn           mark@jhereg.mn.org, ..!chinet!jhereg!mark

        They didn't understand a different kind of smack was needed, 
           than the back of a hand, something was always needed.