ICANN is in the process of hammering out its new generic top-level domain (gTLD) policy, and has begun to address technical issues that could pop up once the new round of applications is open later this year. As part of a discussion on how to manage DNS stability during this potentially huge expansion of the TLD system, an organization within ICANN has proposed a few basic rules for what can and can't be used as a domain extension. Put simply, new domains will be able to make use of nearly any string of letters—including common file extensions such as .exe, .doc, and .pdf—but there will be a number of other restrictions.

Why is this important? Currently, there is no way for organizations to apply for new TLDs whenever they want, as ICANN's Jason Keenan pointed out to us in November. In order to get approved, domains must go through a round of ICANN applications when such a round is open (you cannot apply for a new domain today, for example). The policies for 2008's round are being developed by ICANN's Generic Names Supporting Organization (GNSO) in such a way that the process for approval will be streamlined, and the GNSO expects to see a large number of new applicants.

The GNSO acknowledges that such potentially large expansion of TLDs could impact the stability of the current DNS system (which currently only needs to handle a handful of TLDs). "The addition of gTLDs to the namespace is an expansion of the DNS on a potentially large scale, to include many more names at the top level," wrote the GNSO in its paper last week. "Strings must not cause any technical instability."

As a result, the group has put together a number of recommendations for what is and isn't acceptable as a gTLD. First, gTLDs must conform to all existing syntax rules which forbid the use of blank spaces as part of a name, specify that TLDs are not case-sensitive, and stipulate that the first character must be a letter from the Latin alphabet. But the GNSO also recommends that ICANN prohibit gTLDs that are entirely numeric in order to avoid confusion with IP addresses, which are also numeric and separated by dots. Extensions beginning with "0x" and followed by another hexadecimal character should also be prohibited, GNSO says, because they could be confused for a hex string that is usually converted to an IP address.

When it comes to certain string combos, though, the organization says that it has given considerable thought to whether certain popular file extensions should be prohibited. GNSO points out that browsers would need to be able to identify the difference between a domain ending in, for example, .pdf and a link to a file that has the extension of .pdf, but says that this would need to be a function of the application and not the DNS system.

There's also a much greater danger of user confusion when it comes to using common file extensions, but GNSO says that the task of determining exactly which extensions are "too popular" would be a rather large one that can't yet be addressed. "If ICANN wished to prevent potentially significant problems in the application layer or avoid user confusion issues resulting from a string like .EXE, it would need to do so based on a defined list of extensions that were disallowed, a list maintained outside of ICANN and internationally recognized as the authoritative source for 'commonly used file extensions,'" wrote the organization. "Additionally, a mechanism for maintaining and updating such a list would need to be in place, because new file extensions could become prevalent at any time. To date, staff has not been able to locate a list of common file extensions that is generally acknowledged to be authoritative."

These guidelines are not yet set in stone, and ICANN is soliciting feedback on the proposed rules from the community. If you have comments, they can be sent to new-gtlds-dns-stability@icann.org and viewed on ICANN's forum.

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