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NETWORK PACKET ENGINEERING TOOL

MTU & MSS Calculator

Build an encapsulation stack, calculate effective inner MTU, TCP MSS, UDP payload and ping test size, then identify fragmentation and frame-size risks.

PPPoEGRE / VXLANIPsec / WireGuardMPLS
MT

Link and encapsulation parameters

Select a preset, then edit every byte value when required.

Encapsulation byte values are editable. Confirm IPsec, VPN and hardware frame behavior against the actual platform.
Underlay and target
bytes
bytes
tags
Inner protocol headers
bytes
bytes

Encapsulation stack

LayerBytesCountTotal
MSS

MTU, MSS and fragmentation result

Effective inner packet size after all configured overhead.

Effective inner MTU

1500bytes
—
Total overhead0 B
Recommended TCP MSS1460 B
Actual TCP data with options1460 B
Maximum UDP payload1472 B
Effective inner MTU1500 B outer limit
Overhead percentage0.00%

Encapsulation path

ICMP echo data1472 B
Required base MTU1500 B
Target headroom0 B
Excess / fragmentation bytes0 B
Current maximum wire frame1518 B
Required wire frame1518 B
Configured IP header20 B
Configured TCP header20 B

Ping / PMTU test commands

Windows
Linux

MTU breakdown

Engineering assessment

    METHODOLOGY

    Methodology

    TCP MSS

    RFC 6691 recommends subtracting only the fixed IP and TCP headers when advertising MSS. The tool separately calculates actual data after configured options.

    PPPoE

    RFC 2516 uses 6 bytes for the PPPoE header and 2 bytes for the PPP protocol field, producing the familiar 1492-byte MTU on a 1500-byte Ethernet payload.

    VLAN and MPLS

    A VLAN tag is 4 bytes on the wire. Each MPLS label stack entry is 4 bytes. VLAN does not normally reduce configured IP MTU when tagged-frame expansion is supported.

    Tunnels and security

    GRE, VXLAN, IPsec and WireGuard overhead depends on outer IP version and optional fields. IPsec padding is variable, so its presets are editable estimates.

    Primary references: RFC 2516 · RFC 6691 · RFC 2784 · RFC 2890 · RFC 7348 · RFC 3032 · RFC 4303 · RFC 8200 · RFC 8201 · WireGuard Protocol

    FAQ

    Frequently asked questions

    Why is PPPoE MTU usually 1492?

    A 1500-byte Ethernet payload minus 8 bytes of PPPoE and PPP protocol overhead leaves 1492 bytes.

    Should VLAN always reduce IP MTU by 4 bytes?

    No. Most capable Ethernet devices support the slightly larger tagged frame while keeping IP MTU at 1500. Use strict mode only when the physical frame limit does not expand.

    Why are IPsec values estimates?

    ESP overhead depends on outer IP, NAT-T, IV, integrity tag and variable padding. Verify the actual cipher suite and vendor implementation.

    What is the correct TCP MSS for IPv6 MTU 1500?

    With fixed headers only, 1500 - 40-byte IPv6 header - 20-byte TCP header = 1440 bytes.

    Can ping prove the complete path MTU?

    It is useful but not conclusive. ICMP filtering, ECMP, tunnels and asymmetric routing can affect the result. Combine it with PMTUD, captures and device counters.

    What does IPv6 MTU below 1280 mean?

    IPv6 defines 1280 bytes as the minimum link MTU. Designs below that require lower-layer fragmentation/reassembly or adaptation and should be reviewed.

    PROJECT RECORDS

    History

    ENGINEERING WORKFLOW

    Continue the engineering workflow

    Current workflowEnterprise LAN & IP Planning Workflow

    Plan addressing, VLAN capacity, packet size, uplinks, optics, access policy and DNS behavior as one LAN design sequence.

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