MTU / MSS Calculator
Find the right MTU and matching TCP MSS for your network. Includes VPN tunnel overhead and PPPoE.
MTU vs MSS — what the difference is
Two related sizing values that often get confused:
- MTU (Maximum Transmission Unit) is the largest single packet your link will carry, including IP and transport headers. Standard Ethernet is 1500 bytes.
- MSS (Maximum Segment Size) is what's left for actual TCP payload after subtracting IP and TCP headers. For standard IPv4 TCP, MSS = MTU − 40.
If MTU is too high for the path (some link in the middle drops packets bigger than 1492, say), you get black-hole behavior: short connections work, long ones stall. Symptoms: pages load partially, SSH hangs after auth, file transfers freeze. Lowering MSS to match path-MTU fixes it.
Where MTU goes wrong
- PPPoE (DSL service) eats 8 bytes for its own header. ISP-provided routers usually set MTU to 1492 automatically; consumer routers often don't.
- VPN tunnels add 50–80 bytes of encryption overhead. Every IP packet through the tunnel is wrapped in another IP packet. If MTU isn't lowered, you get fragmentation or drops.
- Jumbo frames (9000 MTU) only work end-to-end if every device in the path supports them. Mostly used inside data centers, not across the internet.