How is the MTU (Maximum Transmission Unit) set for NymVPN?

What is the MTU and why does it matter?

The MTU (Maximum Transmission Unit) is the largest packet size a network layer can transmit in a single operation without needing to be fragmented. The "best" MTU is the one that fits with the underlying network, which depends on its capabilities.

If a packet is too large, it may get dropped or fragmented, which can cause performance issues or even break connections in some network setups. If a packet is too small, network capacity is wasted, impacting performance.

What MTU does NymVPN use in the "Fast" mode?

In the Fast mode, NymVPN uses a 2-hop “onion” WireGuard setup, where one WireGuard tunnel is wrapped inside another. Each layer of encryption adds metadata and headers, increasing the size of each packet. This means we must subtract twice the WireGuard overhead. Moreover, we have to adapt to the constraints of poorly-implemented IPv6 networks, as well as cellular networks. Otherwise packets could exceed the networks limits and be dropped, affecting the connection reliability and performance.

That’s why we set the exit MTU between 1280 (mobile) and 1340 (desktop) bytes.

What MTU does NymVPN use in the "Anonymous" mode?

In the mixnet mode, NymVPN relies on the "Sphinx" packet format, which standard size is 2413 bytes (2048 bytes of data and 365 bytes of headers).

We've set the MTU to 1280 (mobile) and 1500 (desktop) bytes, for maximum compatibility and performance.