Port units, methods, and computer readable storage media for testing a network are disclosed. A traffic generator may generate and transmit test traffic over a network under test, the test traffic including a plurality of interleaved packet streams, each traffic stream associated with one of a plurality of flow control groups. A traffic receiver may receive flow control packets from the network under test. The traffic generator may be configured to stop transmission of all packet streams associated with one or more paused flow control groups when the traffic receiver receives a flow control packet identifying the one or more paused flow control groups.