Default MTU 1440 may not work in cellular networks

[paul356]

It may be not a bug, but it may cause troubles in celluar networks. It takes quite some time for me to get to the bottom of this issue. I have a kcptun-raw tunnel which works fine through a cable connection, but it won't work through the cellular network of my phone. The cellular network is the 5G network run by China Telcom. After many failures I was determined to find out the cause. Using gdb I found some segments were always stuck in kcp->snd_buf of the client, and the size of segment is quite large, for example 1428. With a speculation that the stuck segments have sizes larger than MTU of the cellular network I tried to change MTU to 1200, then the stuck issue was fixed. It took me quite some time to get this issue fixed. I assume it may also happen to other users. So it may be a good idea to add a note explaining the MTU and lower the default value.