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Individual submission RFC 1123 is ambiguous regarding the specification for top level domain (TLD) labels used in the domain name system. This document clarifies the specification, and aligns it with current praxis, including the use of Internationalized Domain Name (IDN) Labels in TLD names.

RFC 1123 is ambiguous regarding the specification for top level domain (TLD) labels used in the domain name system. There are different interpretations which leads to problems when deciding on new TLD names. One important aspect of this is the wish to use internationalized domain name (IDN, RFC 3490 et. al.) labels in top level domains to facilitate use of local scripts. This document strives to make a new and clear definition of which character sequences are allowed in TLD labels. This document is a proposal and a work in progress.

The terminology used in this document is as defined in RFC 0952 and RFC 1035.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

RFC 952 gives the specification for names that may be used for host names on the Internet. It is updated by RFC 1123, which relaxes the specification somewhat. It is somewhat unclear wheter the relaxation is per label or per entire host name. In order to avoid problems based on lack of clarity, the specification for a TLD name is hereby stated to be as follows: A TLD label MUST be at least two characters long and MAY be as long as 63 characters - not counting any leading or trailing periods (.). It MUST consist of only ASCII characters from the groups "letters" (A-Z), "digits" (0-9) and "hyphen" (-), and it MUST start with an ASCII "letter", and it MUST NOT end with a "hyphen". Upper and lower case MAY be mixed at random, since DNS lookups are case-insensitive.

Hence, the ABNF expression that matches a valid TLD label is as follows: tldlabel = ALPHA *61(ldh) ld ldh = ld / "-" ld = ALPHA / DIGIT ALPHA = %x41-5A / %x61-7A ; A-Z / a-z DIGIT = %x30-39 ; 0-9

This new specification reflects current praxis in registration of TLD names by the IANA, and allows for IDNs.

It is carefully noted that the above specification is not the only limiting factor on TLD labels. There may be other entities than the IETF that have influence over TLD names, and which may decide to restrict the names further. The above technical specification is just one limiting factor.

This memo changes the specifications for TLD names registered by the IANA, and the IANA is requested to change its registration process to use the above specification.

This document does not introduce any new security issues. It may introduce stability issues where names registered under this new specification may interoperate badly with old software written to enforce a strict interpretation of the old specification. However, it is belived that such software is scarce on the Internet, and since TLD names that do not adhere to a strict interpretation of the old specification are already used (including IDNs) without apparent problems, it is believed that this change of the specicification will not create major stability problems on the Internet.