Nonce difference

GoVPN prevents replay attacks by remembering the last used nonce in messages from the remote peer. All incoming messages must have higher nonce number (technically it is counter), otherwise they are dropped.

Because of UDP nature that does not guarantee packet ordering during transmission, GoVPN will drop valid non-replayed UDP packets. That leads to performance decrease.

In most cases there is no need in so strict nonce boundaries and -noncediff command line option allows to create the window of allowable nonce differences. This is trade-off between highest security and possible performance degradation. For example -noncediff 128 works rather well (no packet drops) with 1 Gbps link with two switches. By default no nonce differences are allowed (highest security).