A method, apparatus, system, and signal-bearing medium that in an embodiment generate keys based on a number of commands that can be outstanding at a device under test at any one time, embed the keys into respective data associated with respective commands, and send the commands to a device under test. When output is received from the device under test, the key embedded in the output may be used to determine the command associated with the output. In this way, the data may remain random while the test case command associated with the output may be determined based on the data.