At the time of writing there is only a plain/text X2Go Session Broker communication protocol implemented.
The plain text protocol has been the first implementation of the X2Go Session Broker communication. The advantage of it is its simplicity. The disadvantage is that it is not very error tolerant. Neither can the plain text be used to transmit complex data structures.
The protocol's definition is explained on this separate page.
Unwritten future plans whisper about a JSON based broker protocol. No ETA for such an implementation is available, yet.