test-suite-tcp.txt

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This is a brief description of the tests in the file
test-suite-tcp.tcl.  These tests can be run with the
command "./test-all-tcp".

ECN:
The "ecn" test shows two TCPs, where one is ECN-capable and the
other is not.

TIMERS:
The "timers" test is the first of a series of tests that test the
behavior of TCP's retransmit timers.  This test shows the retransmit
timeout value when the first packet of a connection is dropped,
and the backoff of the retransmit timer as a subsequent packet is
dropped.  This test also shows that once the retransmit timer is
backed off, it is later un-backed.

For the connection in the top row, the first packet sent by the
connection is dropped, and the retransmitted packet is itself
dropped.  The connection has a maximum window of three packets, so
the TCP sender has to wait for a retransmit timer to recover from
any packet drop.  When another packet from the top connection is
dropped at time 21, the test shows that the retransmit timer is no
longer backed off.  Further, by this time the estimated average
roundtrip time is close to its true value, and the retransmit timer
expires relatively quickly.

The bottom connection has an intermittent demand, constructed to
give the desired behavior for the top connection.  The bottom
connection has a maximum window of five packets, and generally
recovers from a packet drop with a Fast Retransmit.

The reported statistics show the values of the
retransmit timer variables "srtt_", "rttvar_", and "backoff_" 
for the top connection. 

TIMERSA:
The "timersA" test shows the retransmit timer when the third packet
of the top connection is dropped.  Because the top connection has
a maximum window of three packets, the TCP sender always has to
wait for a retransmit timer to recover from a packet drop.

The behavior of the "backoff_" variable in this test requires some
explanation.  After packet 2 of the top connection is dropped, the
first retransmit timer expires, packet 2 is retransmitted, and
the value for "backoff_" is increased to 2.  When the ACK for the
retransmitted packet 2 is received, Karn's algorithm is invoked,
and that ACK is not used in updating the retransmit timer variables.
When packet 5 is sent, "backoff_" is increased to 2, and a large
value is used in setting the retransmit timer.  When the ACK for
packet 5 is received, "backoff_" is reset to 1, so the retransmit
timer is no longer backed off.  When the outstanding retransmit
timer expires (because packet 6 is outstanding), packet 6 is
transmitted, "backoff_" is again increased to 2, and a backed-off
retransmit timer is again set.

TIMERS1, TIMERS2, and TIMERS3:
The test "timers1" uses the default value in the simulator of 0.1
seconds for "tcpTick_", the TCP clock granularity.  "Timers2" uses
"tcpTick_" set to 0.5 seconds, and "timers3" uses "tcpTick_" set
to 0.001 seconds.  These tests use Reno TCP, and in each test the
second packet dropped has to wait for a retransmit timer to expire
before it is retransmitted.

TIMERS4 and TIMERS5:
This test shows that the estimated round-trip time for the retransmit
timers is updated once a roundtrip time, and not once for every
arriving ACK packet.

There are no packets dropped in either of these two tests.  The
two connection differ only in their maximum window.  The reported
values for "srtt_" show that the value for "srtt_" converges at a
similar fashion for the two connections, even though the rate of
arriving ACK packets is very different in the two connections.

STATS1:
This test shows some of the available statistics.
At time 10.0, for each connection the statistics show 
the number of packets sent by the sender (in packets and in 
bytes), the number of ACK packets received by the sender,
the number of retransmitted packets (in packets and in 
bytes), and the number of retransmit timeouts.