Historical note... --On Sunday, 06 July, 2008 09:34 +1200 Brian E Carpenter <brian.e.carpenter@xxxxxxxxx> wrote: >> Beats me, but since there are several hundred TLDs, it seems >> to me that the chances are pretty low that everyone in the >> world has managed to avoid using them as host names. > > Back in the early days, when Czechoslovakia existed, and cs. > was active, I recall cs.ucl.ac.uk causing great difficulty. Of > course, the difficulty was due to email systems doing > pragmatic things about JANET formats, before MX records were > in use. We also had great difficulty, I remember, with > uk.oracle.com. Part of the problem in that case was that, because JANET used little-endian names internally, the big-endian foo.ucl.ac.uk (in DNS order) had to be be mapped into uk.ac.uck.foo (in JANET order) and vice versa. That mapping was trivial as long as one could run a simplistic "whichever end the TLD was on had to be the big side" test. When "CS" was introduced, blew up that simple test. In the JANET case, it failed since there were strings that could be TLDs at both ends of the string, i.e., in principle, cs.ucl.ac.uk could have been a string that was already in JANET order and that would appear in the DNS order as uk.ac.ucl.cs. While other heuristics could disambiguate that one (as long as the Czech TLD didn't do deliberately pathological things), uk.oracle.com illustrates a case in which none of the heuristics worked -- the string was simply ambiguous and one had to know, via out-of-band info, whether it was a JANET or DNS name. The introduction of "cs" caused more general problems, unrelated to name ordering, because there were systems all over the network in computer science departments with FQDNs like host.cs.someuniversity.edu. It was common in many of those institutions to set up university-wide search rules so that a reference to host.cs would do the right thing, just like host.physics, host.philosophy, and so on. When "CS" was introduced as a TLD, "host.cs" suddenly became ambiguous (or at least dependent on exactly how the search rules were set up) as to whether it represented "host.cs." or "host.cs.someuniversity.edu.". And, that, if my memory is correct, was the beginning of our understanding that search rules needed to be used with great care, if at all, and that incomplete domain names should not be sent on the wire as part of protocols. Note that MX had little or nothing to do with any of this unless MX records were probed as part of trying to disambiguate the names. As an aside, we are at some risk of reinventing the "can't quite figure out which order the label of this domain name are in" as we introduce IDNs that contain labels in right to left scripts. Those who are interested in that issue should pay very careful attention to the IRI spec and to draft-ietf-idnabis-bidi. john _______________________________________________ Ietf@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf