MTU and TCP MSS Calculator (GRE, VXLAN, GTP, IPsec)
Find the safe inner MTU and MSS for your tunnel.
Your numbers
Your results
MTU and TCP MSS Calculator (GRE, VXLAN, GTP, IPsec) by TheCalcTools
How the MTU and TCP MSS Calculator (GRE, VXLAN, GTP, IPsec) works
Enter the physical MTU (1500 on most Ethernet, 9000 with jumbo frames).
Choose the encapsulation and add any extra bytes.
Set the inner MTU on the tunnel interface and clamp the TCP MSS to MTU − 40 (IPv4) or MTU − 60 (IPv6).
The formula
Worked example
Using the default values shown in the calculator:
| Physical link MTU (bytes) | 1500 |
|---|---|
| Tunnel / encapsulation | GTP-U over IPv4 (36 bytes) |
| Extra overhead (VLAN tags, more labels, etc.) | 0 bytes |
- 1500 − 36 (GTP-U) = 1,464 bytes inner MTU.
- MSS is 1,424 for IPv4 and 1,404 for IPv6.
Result:
| Inner MTU to configure | 1,464 |
|---|---|
| TCP MSS for IPv4 (MTU − 40) | 1,424 |
| TCP MSS for IPv6 (MTU − 60) | 1,404 |
| Overhead | 2.4% |
Frequently asked questions
What is the GTP-U overhead?
IPv4 header 20 + UDP 8 + GTP-U 8 = 36 bytes, more if extension headers are used. Mobile cores commonly clamp UE MTU near 1400-1436.
IPsec overhead is not fixed. Why?
It depends on cipher, padding, NAT-T and mode, so 73 bytes is an approximation. Enter extra bytes to adjust.
Sources and further reading
- RFC 4459: MTU and Fragmentation Issues with In-the-Network Tunneling
- RFC 7348: VXLAN
- Wikipedia: Maximum transmission unit
Related calculators
More in IP & Domain Tools · All calculators
This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.