dcrocker@DECWRL.DEC.COM (Dave Crocker) (10/29/89)
I have been waiting for someone to observe that static entries in a data base do not work for routing information.. Thank you. The MX mechanism is fine for real 'off-net' mail relaying. It does NOT work as a means of communicating complex and/or dynamic and/or alternate routes. Your scenario exemplifies this limitation. The %-hack doesn't fix the problem. It does, however, let knowledgeable users source-route around some of these types of problems. That is, if you are a wizard, you can make up for a basic deficiency in the mail architecture. Note that IP-level routing can, albeit slowly, discover alternate routes. d/
rayan@cs.toronto.edu (Rayan Zachariassen) (10/30/89)
Dave writes: >Note that IP-level routing can, albeit slowly, discover alternate routes. Note also that IP-level routing is qualitatively different from Mail routing, in that the latter is single-hop Point-to-Point (on the Internet at least) which doesn't leave a lot of room for the refinement of the routing decision that takes place with IP-level routing. Another key point wrt the Australian problem (which, as Amanda points out is the same on all IP-islands about to get full connectivity -- we went through it here) is that there is no nice way to prevent the world at large from zipping right through the IP gateway and talk directly to internal hosts if they know about them (e.g. due to coherent nameservers). If one has full control of all the mailers on the island then arranging to send outbound mail through a gateway is not a (technical) problem. If one doesn't, the only solution that doesn't require cooperation is to partition both worlds' mail access (e.g. hack routers at the link to redirect smtp packets). Neither of these choices are desirable. While I'm at it, there's a third undesirable choice: have a high-priority MX pointing at the gateway, and arrange that the mail server on it rejects connections from hosts within the island, but accepts them from hosts on the outside. There are various ways of accomplishing this, but it means a LOT of "are you there?" traffic to that gateway and the obvious MX maintenance problem. Due to the G-B links in the Australia scenario, this is obviously not acceptable, but it would work if G-B was well-connected and packets were cheap. rayan (this is still more tcp-ip'ish than maildropper'ish... umm, namedropper'ish :-)