# Signalhouse MTU, Fragmentation & TCP Throughput Calculator

> Tunnel overhead, inner MTU and MSS, IPv4/IPv6 fragments and a TCP throughput model. Stack encapsulations on a link — PPPoE, MPLS, GRE, IP-in-IP, 6in4, DS-Lite, VXLAN, Geneve, WireGuard and IPsec ESP (approximate) — and get the inner MTU, the TCP MSS for IPv4 and IPv6 and the wire efficiency. Send a packet through it to see the exact IPv4 fragments (offsets, lengths, MF flag), what happens with “Don’t Fragment” and why IPv6 routers answer “Packet Too Big”. A TCP model adds the bandwidth-delay product, the window scale you need and the Mathis loss limit. It is the MTU panel of the Signalhouse Network lab.

Part of usefulweb.site (https://usefulweb.site/). Part of [Signalhouse](https://signalhouse.usefulweb.site/) (ownerApp: signalhouse). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://signalhousemtucalculator.usefulweb.site/ (interactive tool at https://signalhousemtucalculator.usefulweb.site/tool)
- JSON API: POST https://signalhousemtucalculator.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://signalhousemtucalculator.usefulweb.site/api/v1/tool)
- OpenAPI: https://signalhousemtucalculator.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://signalhousemtucalculator.usefulweb.site/mcp - tool `signalhouse_mtu_calculator`

## Inputs
- linkMtu (integer): Link MTU - MTU of the physical link (Ethernet 1500, jumbo frames 9000).
- layers (table): Encapsulations, from the wire inwards
- ipVersion (select): Inner IP version
- packetSize (integer): Packet to send (total length) - Optional: an IP packet of this size is checked against the inner MTU and fragmented if needed.
- df (boolean): “Don’t Fragment” set (IPv4)
- ipv4HeaderLength (integer): IPv4 header length
- ipv6ExtensionBytes (integer): IPv6 extension headers before the Fragment header
- bandwidthMbps (number): TCP model: bandwidth - Fill in bandwidth and round-trip time to add the TCP throughput model.
- rttMs (number): TCP model: round-trip time
- lossPercent (number): TCP model: packet loss
- windowBytes (integer): TCP model: receive window - Empty: as large as the path needs (with window scaling) or 65,535 bytes (without).
- windowScaling (boolean): TCP model: window scaling (RFC 7323) enabled
- transferMB (number): TCP model: file size to transfer

## Method
- Each encapsulation subtracts its header bytes from the MTU of the layer below it, from the wire inwards: PPPoE 8, MPLS 4 per label, GRE over IPv4 24 (28 with a key), IPIP and 6in4 20, IP-in-IPv6 40, VXLAN and Geneve 50 over IPv4 or 70 over IPv6 (they carry an inner Ethernet header), WireGuard 60 over IPv4 or 80 over IPv6.
- IPsec ESP in tunnel mode is computed from the cipher: outer IPv4 20, ESP header 8, IV, padding of the encrypted part to the cipher block (16 bytes for AES-CBC, 4 for AES-GCM) including the 2-byte trailer, and the ICV (12 for HMAC-SHA1-96, 16 for GCM); NAT-T adds a UDP header. Real overhead varies with options, so the figure is labelled approximate.
- IPv4 fragments carry data in multiples of 8 bytes (the offset field counts 8-byte units); every fragment but the last has MF set (RFC 791). IPv6 packets are only fragmented by the source, each fragment repeats the unfragmentable headers and adds an 8-byte Fragment header (RFC 8200); routers drop oversize packets with “Packet Too Big”.
- TCP MSS = MTU − 40 (IPv4) or − 60 (IPv6), without TCP options. The TCP estimate is the minimum of the bandwidth, window ÷ RTT and the Mathis et al. model MSS ÷ RTT × √(3/2) ÷ √loss. It models steady-state Reno-style congestion avoidance; CUBIC, BBR, slow start and queueing behave differently.