XDAP header

A DAP server should use the XDAP header to announce the lowest version of the needed to understand the current response. Each time a server builds/returns a response, it should try to set the protocol version in the XDAP to the lowest needed to read the response.

Higher versions of DAP should be compatible with the features in older versions of the protocol. For example, this means that a 3.3 client must be able to read a response that conforms to 2.0, 3.0, ..., 3.3.

DAP 3.1 behavior

A server conforming to DAP 3.1 or greater MUST always return the XDAP header and it MUST always contain the version number of the DAP such that a client implementing that version can understand the response. The server SHOULD return the lowest protocol version number needed to read the response, but MAY return a higher number if appropriate.