Why does the Internet drop when downloading custom maps?

Question

This is something I've fixed recently and thought I'd document for benefit of others, as it's so obscure.

When anyone on my network downloads a custom StarCraft 2 map, the map download will fail (or appear to fail and later work) and all connections (even on other computers) will be dropped; they'll sign out of battlenet, skype, msn, and pings will time out for a brief moment.

Answer 1

The solution is to fire up cmd and reduce MTU size;

 netsh interface ipv4 set subinterface "Local Area Connection" mtu=1432 store=persistent

Where "Local Area Connection" is the device name for the adapter you connect to bnet through.

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The problem was the router; Belkin F5D7633-4. Even the latest firmware only allows TCP/IP packets to be ~1492 bytes, despite 1500 being ethernet max. My theory is that the router was fragmenting the packets, and then either this process or the response from battlenet to these router-fragmented packets caused the router to crash.

The MTU value I determined following this process;

• Open an administrative MSDOS command prompt
• ping -l 1472 -f google.com
• Reduce 1472 until you no longer get the "packet needs to be fragmented" error message
• Add 28 more to this (since you specified ping packet data size, not including a packet header of 28 bytes)
• This value is your max MTU