Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!usc!snorkelwacker!think!kulla!barmar From: barmar@kulla (Barry Margolin) Newsgroups: comp.protocols.tcp-ip Subject: Re: MX-registration vs %-hack (was Re: New Host-Requirement RFCs) Message-ID: <31038@news.Think.COM> Date: 25 Oct 89 07:20:59 GMT References: <8910201839.AA29376@arcturus.mitre.org> <1989Oct23.173855.1370@utzoo.uucp> <7696@ditmela.oz> Sender: news@Think.COM Organization: Thinking Machines Corporation, Cambridge MA, USA Lines: 62 In article <7696@ditmela.oz> smart@ditmela.oz.au (Robert Smart) writes: >Perhaps the >Internet is too BIG and IMPORTANT to ask other networks to put in >gateways. In which case there seems to be a certain asymmetry in >arrangements. Are these other networks using standardized protocols? Yes, there's an asymmetry. The Internet is an order of magnitude larger than most other networks. >Actually I don't see the problem. "user%node" is a perfectly legitimate >"local part" of an RFC-822 address, and I don't see why I shouldn't >set up my RFC-822 compliant domain to have that local part mean that >the mail should be forwarded to some local host that is too insignificant >to register. Certainly we shouldn't mandate that meaning of "%". If >other RFC-822 compliant domains want to have actual users with %s in >their names then that is also ok. No one has proposed legislating away the % hack. It can't be done, for the reason you state: every host is free to interpret the local part as it sees fit. They're just trying to encourage people to switch to the better solutions that have been devised since % was invented. The % hack has several problems: 1) Users have to remember gateways. smart@ditmela.oz.au is easier to remember than smart%ditmela.oz.au@uunet.uu.net. 2) % isn't the only such hack, and it's not always clear what happens when multiple hacks are used together. What is the meaning of foo!bar%baz@quux? Is it quux -> foo -> baz -> user bar or quux -> baz -> foo -> user bar? 3) Problems occur when network topology changes. If uunet.uu.net is replaced as the Internet->UUCP gateway, all the %host@uunet.uu.net addresses are invalidated. This means that users must learn a new gateway, and mailing lists all over the place have to be updated. The domain system simply requires updating the database, and users never notice. This can be very important if the topology change is temporary (for instance, uunet goes down for a week because the company is moving to a new location, so someone else takes over gatewaying in the interim). 4) The domain facility has additional features. For instance, there can be multiple MX records for a host, so that mail can still go through when the default forwarder is down. With %, the decision of which forwarder is used is made statically by the sender. 5) You're dependent on an arcane, nonstandard feature being implemented on a host over which you have no control. Uunet may decide at any time to stop supporting %, and you'll run into problems. Besides encouraging hosts to switch away from %, I think they are also encouraging new systems NOT to implement it in the first place. More importantly, it sends a signal to implementors not to try related kludges. For instance, I've heard of systems that try to parse non-local local parts, in an attempt to optimize routes. They see foo%bar@baz, realize that they know a host named "bar", so send the message directly to bar rather than routing through baz; this loses if baz doesn't interpret % as they think it does, or if baz has a different notion of the host bar. Barry Margolin, Thinking Machines Corp. barmar@think.com {uunet,harvard}!think!barmar