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We have web application that runs under Tomcat in a local network. Our customers experience strange problem using this web application. Let's say Tomcat server runs on host1 and we open webapp URL in browser on host2.

Any browser on host 2 starts opening page and downloading of content "hangs" for hours. We've made bunch of experiments and found that any content larger than 2000 bytes makes browser request hang. Tried in Internet Explorer 8, Opera 12, Firefox. At the same time if user opens website from internet, it works. Opening webapp from the same host1 where Tomcat is running works normally. Local network is organized with D-Link DGS-3120-48TC switch.

Additional info. During experiments we've noticed XP Tweaker installed on hosts. Network settings from that tool:

  • MTU is manually set to 1500
  • RWIN = 14600
  • Support of TCP frames larger than 64 KB is on
  • Time to Live = 32
  • SACK is on
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I suggest using packet captures on both ends to try and see what's happening when it hangs. –  John Gardeniers Oct 24 '12 at 9:56
    
Unfortunately there is no such possibility with this customer. –  artplastika Oct 24 '12 at 10:17
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1 Answer 1

This might be simply PMTU discovery broken somewhere along the path1 , so your server is sending larger packets than what is able to come through to the customers' hosts. Try reducing the MTU to 1300 and seeing if it is going to be of any help.

The side on which you are going to alter the MTU does not matter - reducing it on the client side should also reduce the MSS reported to your server upon TCP connection setup, so it is going to send smaller packets. Reducing it on the server side would break the data stream into smaller segments right away.

If you see this helping, by all means, do not stop there but fix PMTUD.


1 most likely you are to blame - if ICMP is filtered in the wrong way in your firewall configuration, ICMP-fragmentation-required messages are not getting through to your server's TCP stacks, effectively breaking PMTUD and resulting in packets exceeding the MTU which simply are unable to get past routers with an MTU smaller than your server's MTU.

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