PACKET SIZING

MTU & MSS Calculator | TCP MSS

06/ 06

Calculate TCP MSS and ICMP payload limits for IPv4, IPv6, MTU and encapsulation overhead.

Calculator inputs

IPv4 / IPv6

Calculation results

TCP MSS · FIXED HEADERS

1,460 bytes

IPv4protocol

Effective MTU: 1,500 bytes
ICMP payload: 1,472 bytes
Fixed IP header: 20 bytes
Fixed TCP header: 20 bytes

1500 − 0 − 20 − 20 = 1460

Fixed-header calculation. IP/TCP options and IPv6 extension headers affect actual sent data sizes.

HOW IT WORKS

Calculate TCP MSS with MTU and overhead

Effective MTU is the input MTU minus additional encapsulation overhead. Base TCP MSS subtracts the fixed IP header and the 20-byte fixed TCP header from that effective MTU.

With MTU 1500 and no extra overhead, IPv4 MSS is 1460 bytes and IPv6 MSS is 1440 bytes. If you enter an already-adjusted tunnel MTU, do not subtract the tunnel overhead again.

This is a fixed-header MSS reference. Per RFC 6691, extra space for IP/TCP options is accounted for in actual sent segments. Verify IPv6 extension headers, path MTU and variable IPsec padding in the actual environment.

RFC 6691 · TCP MSS ↗RFC 8200 · IPv6 ↗

FAQ

Frequently asked questions

Why do IPv4 and IPv6 differ?

The fixed IPv4 header is 20 bytes and the IPv6 header is 40 bytes, making the IPv6 MSS 20 bytes smaller at the same MTU.

What is the ICMP payload value?

It is the data length after subtracting the fixed IP header and eight-byte ICMP header, without IP options or IPv6 extension headers.

Is IPsec overhead fixed?

No. It depends on mode, algorithms, NAT-T and padding. Enter the extra bytes established for your configuration.