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The handshake message is used to fully establish the major and minor version of the protocol in use on the connection.  Once The first message sent from a client to a server must be a handshake. 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.

Authentication

If a server is set to require client authentication, the next message after handshake must be an authentication message. The exact requirements of this message may vary according to the security manager implemented on the server, but the message will always take the client credentials as a series of key, value string pairs.  Authentication is done at a connection level and once it succeeds, all future messages handled by that connection will be authorized against the credentials provided at authentication.

If a server does not require authentication, the client can immediately start sending region operations and is assumed to be authorized for any operation.  If the client tries to authenticate on such a server, it will respond with a AUTHENTICATION_NOT_SUPPORTED error.