Calculator Inputs
Use common MTU values like 1500, 1492, 1460, or tunnel-adjusted values. MSS equals payload bytes available to TCP after lower overhead is removed.
Visualization
The chart compares effective MTU, total headers, actual MSS, and the recommended clamp value for the entered network profile.
Example Data Table
| Scenario | MTU | IP | TCP Options | Tunnel | PPPoE | VLAN Tags | Estimated MSS |
|---|---|---|---|---|---|---|---|
| Standard Ethernet | 1500 | IPv4 | 20 | 0 | 0 | 0 | 1440 |
| PPPoE Broadband | 1500 | IPv4 | 12 | 0 | 8 | 0 | 1440 |
| IPv6 with Options | 1500 | IPv6 | 20 | 0 | 0 | 0 | 1420 |
| VXLAN Overlay | 1500 | IPv4 | 12 | 50 | 0 | 1 | 1394 |
Formula Used
TCP MSS is the maximum TCP payload that fits inside one segment without fragmentation.
Effective Path MTU = Base MTU − PPPoE − VLAN Overhead − Tunnel Overhead − Custom Overhead
TCP MSS = Effective Path MTU − IP Header − TCP Header − TCP Options − IP Extra Overhead
Payload Efficiency % = (MSS ÷ Effective Path MTU) × 100
Default IP header sizes are 20 bytes for IPv4 and 40 bytes for IPv6. Base TCP header size is 20 bytes before options.
How to Use This Calculator
- Enter the base MTU used by your interface, WAN link, or expected path.
- Select IPv4 or IPv6 because header size changes the final MSS value.
- Add TCP option bytes such as timestamps or SACK-related overhead.
- Include PPPoE, VLAN, tunnel, or custom overhead if encapsulation reduces usable MTU.
- Press the calculate button to see MSS, clamp value, efficiency, and payload estimates.
- Use the chart and export buttons to document results for network tuning or firewall MSS clamping.
Common Networking Notes
MSS clamping is often configured on firewalls, routers, and VPN gateways to avoid fragmentation, black-hole paths, and slow transfers across encapsulated links.
Frequently Asked Questions
1. What is TCP MSS?
TCP MSS is the maximum application payload carried in one TCP segment. It excludes IP and TCP headers, helping devices choose packet sizes that avoid fragmentation.
2. How is MSS different from MTU?
MTU is the full IP packet size allowed on a link. MSS is only the TCP payload inside that packet after subtracting IP and TCP overhead.
3. Why does PPPoE reduce MSS?
PPPoE adds encapsulation bytes. If the physical frame size stays fixed, less space remains for the IP packet, so the final TCP MSS must be smaller.
4. Why does IPv6 usually have a lower MSS?
IPv6 uses a larger base header than IPv4. That extra header space reduces the remaining bytes available for TCP payload inside the same MTU.
5. What is a recommended MSS clamp?
A clamp is a safe advertised MSS value set by a firewall or router. It is often rounded down to a practical value to reduce mismatch risks.
6. Do VLAN tags always change MSS?
They can reduce effective payload if the underlying service has a fixed maximum frame budget. In some networks, hardware already accounts for tags separately.
7. When should I lower MSS manually?
Lower it when tunnels, VPNs, PPPoE, or overlays cause fragmentation, retransmissions, or stalled sessions. MSS clamping is common at WAN and edge devices.
8. Can this calculator help troubleshoot slow transfers?
Yes. It helps estimate whether packet overhead leaves too little payload or causes fragmentation. That can explain unstable sessions, poor throughput, and broken large transfers.