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A customer is having issue with our system and after investigating and analyzing tcp traffic with tcpdump, I noticed that fragmented packets were not received by our cloud server (OVH).

I did some tests:

  1. Changed the cloud server address to our old one (Azure), and everything was working fine!
  2. Reverted back the server to production server and again, fragmented packets are not received.
  3. Modified the MTU from 1500 to 1492 on the customer system, and now it's working perfectly!

Why MTU size is affecting communication for one server and not the other? This customer is using verizon hotspot, I saw something similar at another customer using Satellite connection (Xplornet)

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  • Did you trace the route from the client to both servers? I suspect that the path is different and that's why you're seeing this difference between the old and new servers.
    – joeqwerty
    Jan 10, 2018 at 17:18
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    A simple test would be to use the evaluation trial of PingPlotter, and use the TCP packet type (for the protocol/port you use), with the expected packet size, and uncheck the fragment box. It may show you in the graph where in the route the MTU is dropped. pingplotter.com/manual/packetoptions.html
    – Greg Askew
    Jan 10, 2018 at 17:28
  • @joeqwerty yes i did, path is different since one is on Azure and the other one OVH but doesn't help me
    – JPelletier
    Jan 10, 2018 at 17:46
  • @GregAskew I need something for Ubuntu Server, any idea ?
    – JPelletier
    Jan 10, 2018 at 17:46
  • iPerf?.........
    – joeqwerty
    Jan 10, 2018 at 21:08

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