When a scenario starts with a <nop> followed by <recvCmd> and a command is received from other SIPP instance, I get an error "Unexpected control message received ...". Simply moving the <nop> after the recvCmd allows it to work. This does not seem to me to be correct behavior. When I have a <recv> following a <nop> the scenario works as expected.
SIPp v3.1-TLS version svn570