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I am little new to MTU settings and am wondering how to deal with fragmentation seen over TCP/UDP (we are sending packets over a Cellular network via an Android application).

We are seeing a lot of packet fragmentation sending from an android device to server. I would like to get some further information on the topic:

  1. What is MTU ?
  2. What tools we can use to look or verify packet fragmentation (Wireshark, tcpdump etc.) ?
  3. How can we use standard tools like ping/traceroute from an Android device (and is this useful)?

Thanks in advance

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1 Answer 1

MTU means Maximum Transmission Unit -- it means the largest packet size the native link layer will transmit. For example, the MTU of Ethernet based networks is typically 1500 bytes. When a system wishes to send an IP datagram larger than the MTU of the network, it must fragment the packet into chunks no larger than the MTU. Note that fragmentation happens quite rarely with TCP, especially since most TCP implementations now use Path MTU Discovery. Fragmentation is typically only an issue when UDP is used with unusually large packets.

You can indeed observe packet fragmentation with all sorts of tools that monitor the raw network, including tcpdump, wireshark, etc.

If you want to learn more about low level TCP/IP mechanics like this, beyond the usual resources like Wikipedia, I suggest reading Stevens, "TCP/IP Illustrated", Volume 1.

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