VPN & MTU/MSS Calculator - iN2 LLC

Calculate the effective Maximum Segment Size (MSS) from your MTU, accounting for IP headers, TCP headers, and various network encapsulations.

VPN & MTU/MSS Calculator

Calculate effective Maximum Segment Size (MSS) and interface MTU after accounting for VPN, transport, and tunnel layer encapsulation overheads.

Calculation Results

Total Encapsulation Overhead-
Effective Tunnel MTU-
Calculated TCP MSS-
Header Overhead Efficiency-
Recommended MSS Clamping-

Packet Structure Visualization

Shows how headers wrap payload data inside the physical MTU frame size limit.

Path MTU Discovery & Troubleshooting

Misconfigured MTU is a primary cause of connection dropouts, slow downloads, or partial page loads over VPNs.

Test Path MTU (Windows PowerShell / CMD)

Run the following command to check if a specific payload size can pass without fragmentation. If it fails, reduce the size until it passes.

ping -f -l 1472 8.8.8.8

Test Path MTU (Linux / macOS Terminal)

Run the following command. The -M do flag sets the Don't Fragment (DF) bit. If you get "frag needed", reduce the packet size.

ping -M do -s 1472 8.8.8.8

MSS Clamping Explanation

MSS Clamping is a router-level technique that intercepts the TCP 3-way handshake SYN packets and alters the declared Maximum Segment Size (MSS) option to match the path's bottleneck MTU. Since it operates during the handshake phase, it automatically prevents clients from sending oversized packets, completely resolving "black hole" router dropouts without needing manual adjustments on client endpoints.