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iperf3: A TCP, UDP, and SCTP network bandwidth measurement tool
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Windows iPerf TCP window default #481

Closed amass87 closed 7 years ago

amass87 commented 7 years ago

It appears that iPerf3 has a default window size cap of 212992. When I packet capture iPerf traffic between 2 Windows Servers, TCP autotuning in Windows only scales to a maximum window of 53248 with window scale multiplier of 4. This issue persists across 2 separate tunnels with similar RTT and is the same whether I am using Server 2008 R2, Server 2012R2, or Windows 7. This is a problem for testing long fat networks.

I am trying to to test the limitations on an ipsec tunnel with 1Gbps Internet at both ends and a 28ms RTT. However, I appear to be hitting an artificial limit. I can use the -w flag, but it requires a lot of guess and check work. It seems the sweet spot is somewhere around a window of 524288. It might be asking a lot, but their should be a way to allow iperf to use the underlying receive window of the OS to test real world limitations.

taismi commented 7 years ago

I use 4 parallel streams with -P 4 parameter. By using multiple streams I get better throughput than using one stream.

amass87 commented 7 years ago

I do the same, and I get better throughput. Honestly though I am dealing with a customer who swears they should get better throughout with 1 TCP session instead of multiple. So I am now trying to find a tool that can test the limits of Windows TCP Receive Window Autotuning.

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On Nov 23, 2016, at 1:22 PM, taismi notifications@github.com<mailto:notifications@github.com> wrote:

I use 4 parallel streams with -P 4 parameter. By using multiple streams I get better throughput than using one stream.

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bmah888 commented 7 years ago

To address the original comment, the window size limitation you observed isn't from iperf3. If you specify a maximum window size from the command line, there's a 512MB sanity-check limitation, but if you let it autoscale (I don't know if Windows supports autoscaling in its TCP implementation) there's no theoretical limit imposed by iperf3.