iPerf speed test

[cisco8887]

Hi All,

I am using iPerf to do a speed test and using a window size of 1Mb .

I can see that iPerf makes an initial connection without using the scaling flag and that is then finished.

It makes a second connection this time with scaling flag set which is ackd by responder and then it starts to use that window size.

Issue I have is , using 1MB of window size, I expect a throughput of 200 Mb with my current latency .

I get a varying speed of 40-120 Mb but what is intersting is wireshark shows the window size fixed at 1Mb after looking at its calculated window size which I think essentially times the scaling window number to the maximum window size.

how can that be? I only  few duplicate packets so what am I missing here?

[Joseph W. Doherty]

how can that be?

Unclear, at least to me, what you're asking about.  Are you asking a question about TCP scaling and window sizes or about the lower than expected throughput?

Assuming your question is about lower than expected throughput, there are many possible reasons.  First, you mention a "few duplicate packets".  Even a few can impact dramatically impact throughput.

Second, you didn't detail your environment or for how long a test you're running.  TCP takes time to "ramp-up" its "speed", especially as latency increases.

How are you measuring throughput vs. what's expected?  I.e. TCP, due to L2 and L3 overhead, and packet sizes, will have lower bps than what the "wire" supports.