The referee box sends information about the current state of the game to teams over Ethernet on the field network. Packets are encoded using Google Protocol Buffers, just like SSL-Vision. Each referee box packet is a message of type SSL_Referee, as defined in the Protocol Buffers file. For details about the fields in the message, please see the comments in that file. Each packet is a UDP packet containing exactly the binary encoded form of the SSL_Referee message; packets are sent to UDP port 10003 at IP multicast group 224.5.23.1 (this can be changed in the configuration file for local testing if necessary).
The old legacy protocol provides much less information than the Protobuf protocol and is therefore not recommended. For compatibility with old team software, it is still transmitted and is documented here.