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A couple of our users reported problems downloading zip files from our server. Downloads start but they dry out soon. They basically stop after short period of time.

I wasn't able to reproduce this from work where everything works as expected. I do experience the same issue from home though.

Path MTU Discovery

Here's our setup:

  • Amazon EC2, m1.large
  • Windows 2012
  • IIS serves files

Curious thing is, we have another EC2 with similar configuration in the same availability zone where everything works well.

At this point I don't know where to start looking. It seems like a bizarre combination.

I'm able to download said file in multiple steps (resuming where it stopped) using http range queries.

This looks like Path MTU discovery blackhole. What are my options here?

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  • Start with tcpdump or wireshark on the client. What happens when download speeds decrease?
    – devicenull
    Mar 24, 2014 at 3:14
  • Sounds like a path MTU problem.
    – Michael Hampton
    Mar 24, 2014 at 3:33
  • @MichaelHampton can you elaborate? I had some remote desktop issues with the server which may have been MTU related.
    – Kugel
    Mar 24, 2014 at 3:36

1 Answer 1

Reset to default
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I contacted Amazon and apparently it's known issue with some m1 EC2 instances. I lowered MTU in Windows using netsh command for now.

They recommended upgrade to m3.

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