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After connecting to the server (and doing any socket level security negotiation such as SSL), the client will need to send a single byte to identify the protocol they are using. For the protobuf protocol this byte should be 110. Additionally once the client has identified that they'll be communicating via protobuf, they'll need to send one more byte describing the major version of the protobuf protocol they'll be speaking. This will provide the server with enough information to exchange a protobuf handshake message with the client.
Handshake
The handshake message is used to fully establish the major and minor version of the protocol in use on the connection. Once the server accepts the client's handshake, it will guarantee that it's response messages match the client's protocol version, regardless of the version of protobuf being run on the server.