XTGSP, the Inter-TGS protocol for cross-realm operations in Kerberos.
The XTGSP protocol defines an extension to the Kerberos protocol. This extension allows a KDC to build a TGS-REP message for services that are not registered in the local realm. Part of the components of the TGS-REP message are obtained from the KDC where the service is registered. The communication between the local KDC and the remote KDC is encrypted using cross-realm keys maintained using the PKINIT extension.