Described are sequence numbers for client-server communication, to control a client's use of server resources. A server grants the client credits, and the client consumes a credit for sending each command to the server. Each credit corresponds to a sequence number, with the set of sequence numbers forming a valid command window. The server enforces that for each received command, the command includes a sequence number that is within the valid command window and that the sequence number has not been used with another command. The server may also maintain a maximum window size, such that clients with credits cannot send a command with a sequence number that beyond a maximum sequence number. When incorporated into a data communication protocol, quality of service, combating denial of service, detection of message loss, division of server resources, secure message signing, and other numerous benefits result.