[ns] Question about warning in Full-TCP agent

[Pregler Peter <peter.pregler@spl.siemens.at>]

Hi,

during simulations with an underdimensioned link I got the following kind of
error-messages:

22.936114: FullTcpAgent::recv(_o318) got packet lacking ACK (seq 1)

The setup is a tree-like setup where the trunk (i.e. the backbone-link) is
totally overloaded. But since it appears in any simulation I have done so
far if congestion occurs I think it is independant of the actual topology.
The source/sink-agents are of type FullTCP. I have attached the relevant
trace. I have cut the trace after the receipt of the packet that caused the
warning listed above. A first SYN is generated at time 6.35. As one can see
due to congestion of the link 11-10 a second SYN-packet is generated at time
12.35. However due to the buffer size the original SYN is finally
transmitted at 13.60. The second SYN is transmitted at 19.69 and causes the
error message given above as soon as it is received at 23.93.

My question: why this error-message? AFAIS, this is totally normal behavior
of an overloaded TCP/IP link. Or did I oversee something?

-Peter

----
Peter.Pregler@spl.siemens.at
=================
+ 6.352436 11 10 tcp 40 ------- 0 11.7 0.7 0 9
+ 12.352436 11 10 tcp 40 ---A--- 0 11.7 0.7 0 18
- 13.601114 11 10 tcp 40 ------- 0 11.7 0.7 0 9
r 13.686114 11 10 tcp 40 ------- 0 11.7 0.7 0 9
+ 13.686114 10 0 tcp 40 ------- 0 11.7 0.7 0 9
- 13.686114 10 0 tcp 40 ------- 0 11.7 0.7 0 9
r 13.696114 10 0 tcp 40 ------- 0 11.7 0.7 0 9
+ 13.696114 0 10 ack 40 ------- 0 0.7 11.7 0 21
- 13.696114 0 10 ack 40 ------- 0 0.7 11.7 0 21
r 13.706114 0 10 ack 40 ------- 0 0.7 11.7 0 21
+ 13.706114 10 11 ack 40 ------- 0 0.7 11.7 0 21
- 13.706114 10 11 ack 40 ------- 0 0.7 11.7 0 21
r 13.791114 10 11 ack 40 ------- 0 0.7 11.7 0 21
+ 13.791114 11 10 tcp 40 ------- 0 11.7 0.7 1 24
+ 13.791114 11 10 tcp 1500 ------- 0 11.7 0.7 1 25
+ 19.696114 0 10 ack 40 ---A--- 0 0.7 11.7 0 31
- 19.696114 0 10 ack 40 ---A--- 0 0.7 11.7 0 31
r 19.706114 0 10 ack 40 ---A--- 0 0.7 11.7 0 31
+ 19.706114 10 11 ack 40 ---A--- 0 0.7 11.7 0 31
- 19.706114 10 11 ack 40 ---A--- 0 0.7 11.7 0 31
r 19.791114 10 11 ack 40 ---A--- 0 0.7 11.7 0 31
+ 19.791114 11 10 tcp 40 ------- 0 11.7 0.7 1461 33
- 22.841114 11 10 tcp 40 ---A--- 0 11.7 0.7 0 18
r 22.926114 11 10 tcp 40 ---A--- 0 11.7 0.7 0 18
+ 22.926114 10 0 tcp 40 ---A--- 0 11.7 0.7 0 18
- 22.926114 10 0 tcp 40 ---A--- 0 11.7 0.7 0 18
r 22.936114 10 0 tcp 40 ---A--- 0 11.7 0.7 0 18