5. Extended Label Types

5. Extended Label Types

The first octet in the on-the-wire representation of a DNS label specifies the label type; the basic DNS specification [RFC1035] dedicates the 2 most significant bits of that octet for this purpose.

[RFC2671] defined DNS label type 0b01 for use as an indication for extended label types. A specific extended label type was selected by the 6 least significant bits of the first octet. Thus, extended label types were indicated by the values 64-127 in the first octet of the label.

Extended label types are difficult to use due to lack of support in clients and intermediate gateways, as described in [RFC3363] and [RFC3364], which moved [RFC2673] to Experimental status; and [RFC3363], which describes the pros and cons. As such, proposals that contemplate extended labels SHOULD weigh this deployment concern against the perceived advantages of deployment of a new extended label type.

This document deprecates the use of the extended label types; implementations SHOULD NOT produce or pass binary labels in DNS traffic. Implementations MUST continue to correctly parse the label type field and handle its value even if deprecated. Specifications that wish to use DNS names with binary labels SHOULD instead consider an alternate transformation of the binary data to allow for its use with normal DNS names.