I have two tunnels over satellite and cellular and am wondering what experiences are with cellular mtu.
I had setup the wireguard over each with the default 1420mtu, but was my experience on cellular was not optimal. I found some commercial lte wan documentation that suggests cellular mtu is 1420, so after setting wireguard mtu to 1340 performance is improved. Since i fail between both, the tunnel for satellite also needs to be 1420.
Is this generally the best performance over cellular? Are there any experiences with slightly higher mtu and LTE?
Generally when I cant use MTU 1500 bytes I tend to select 1280 bytes which is the lowest allowed MTU when it comes to IPv6 according to RFC 2460 and RFC 8200.
If thats not enough (due to tunnel within tunnel within tunnel etc) I go for MTU 576 bytes which is the lowest allowable MTU for IPv4 according to RFC 791.
Other than that if the equipment supports virtual reassemble thats a nice way to overcome shortcomings of small MTU’s at links in between.
With virtual reassemble if the downstream is for example MTU 1500 bytes which when sent over the uplink is being fragmented but once received on the other tunnel end those fragments are reassembled to a full packet before forwarded to the destination client (normally firewalls doesnt care about that so the client will receive the fragments instead of full packets but with virtual reassemble the clients will only see full packets in both directions).
When using shaky transmissions other option is to implement a tunneling that involves FEC (Forward Error Correction). This way a broken packet that arrives at destination can thanks to the extra FEC information be reconstructed to its original form instead of being dropped due to bad checksum.