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Segment length Throughput


We have a network spanning application when running on one AWS instance type caused network file copies to be really slow throughput around 30MB/s. the Avg packet size was around 3kB segment length avg about 8KB and there were many retransmisions and dup acks . I put the slowness down to the app somehow chopping up the data into loads more packets causing many more ACKS and poor performance. When the instance type was changed for one with better network performance we saw the throughput practically double now at 75MB/s but now the Avg packet size was 18KB so this made some sense as to why it was quicker as a lot more data was going on the wire requiring less ACk's. Although confused by why the segment length AVG was now only 170B

What I'm confused me even more was I changed the AWS instance type again for one with even better network performance using AWS propitiatory driver and the throughput increased again, but I cant see how as the AVG packet size is now 14KB, segment Length was about 1350

How is it possible that a smaller AVG packet size can now be quicker than larger AVG sizer? Any help as to what I can look at in terms of bottleneck? I know AWS imposes some kind of network bandwidth throttling but I cant see it reflected in the captures.

Any help making sense of this much appreciated