These tests show the behavior of TCP round trip time (RTT) counting.

Test Topology A

  Client..................................Server

                   800kbs 100ms
   0(0)-----------------------------------1(0)
                                            |
                                          tcpdump

Test Topology B

  Client..................................Server

                   800kbs 800ms
   0(0)-----------------------------------1(0)
                                            |
                                          tcpdump

SSF Configuration Test File:

basic2a.dml

send window = receive window = 32 packets

basic2b.dml

send window = receive window = 32 packets

SSF Tcpdump File:

basic2a.tcpdump

basic2b.tcpdump

TCP Round Trip Time Counting:

For test A:

Link delay is 100 ms, round trip time is roughly 200 ms. The x-axis is marked with the TCP slow timer ticks. The RTT value is based on the RTT measured during the last round trip time. RTT value will be 500 ms multiplied by the timer ticks counted from the time packet is sent to time the ACK for this packet is received.

From this graph we find the RTT value is between 0 and 500 ms.

For test B:

Link delay is 800 ms, round trip time is roughly 1600 ms. The x-axis is marked with the TCP slow timer ticks. The RTT value is based on the RTT measured during the last round trip time. RTT value will be 500 ms multiplied by the timer ticks counted from the time packet is sent to time the ACK for this packet is received.

From this graph we find the RTT value is between 1500 ms and 2000 ms.