Described is a data communication protocol, in which a client and server negotiate in a manner that does not require the client to retry negotiation when servers are not capable of the client-desired protocol. In one example implementation, the desired protocol is SMB 2.0 or greater. The protocol describes a create command with possibly additional context data attached for built-in extensibility, and a compound command comprising a plurality of related commands or unrelated commands. A multi-channel command requests data transfer on a separate data channel, a signed capability verification may be used to ensure that a secure connection is established, and the protocol provides the ability to transfer extended error data from the server in response to a request.