MTU (Maximum Transmission Unit)

The Maximum Transmission Unit (MTU) is the biggest packet a network link will carry in one piece. On typical Ethernet it is 1500 bytes. When a packet exceeds the MTU of a link along its path, it must be fragmented into smaller pieces or dropped, either of which hurts performance. MTU matters especially for VPN tunnels, because wrapping your traffic in an encrypted tunnel adds header overhead, leaving less room for your actual data inside each packet — so the effective MTU inside the tunnel is smaller than on the raw link.

When a tunneling protocol like WireGuard or OpenVPN adds its encapsulation, the inner packets must be small enough that the outer, wrapped packet still fits the path MTU. If the MTU is set too high, packets get fragmented or silently dropped, which shows up as stalls, slow transfers or connections that hang on large responses while small requests work fine. The fix is to lower the tunnel's MTU (WireGuard commonly uses around 1420) so wrapped packets fit without fragmentation, trading a little per-packet efficiency for reliable delivery.

If a connection through the s4m VPN behaves oddly — pages that hang, large downloads that stall while small requests succeed — an MTU mismatch is a likely cause, since the encrypted tunnel adds overhead. Lowering the MTU in your WireGuard or OpenVPN client usually resolves it. This is a client-side setting rather than something the server dictates. For general use the defaults are fine; MTU tuning is a targeted fix for fragmentation symptoms on specific links.