When a client connects to my peripheral, the output of btmon shows that the peripheral's MTU is 517 bytes, and the client's MTU is 512. So the resulting MTU should be 512 bytes. However, SimpleBluez::Characteristic::ValueOptions::mtu in the callback is 517 whenever the client sends something to my characteristic. SimpleBluez::Characteristic::mtu() of any of my characteristics always returns zero. If I split up my message assuming the MTU of 512, everything works fine, while 517 doesn't work.

On a side note: being a peripheral, how can I determine the MTU to initiate an exchange, i.e. there hasn't been anything sent to me by the client, so no callback has been called in which I could see the MTU in ValueOptions?