{ "openapi": "3.1.1", "info": { "title": "Signalhouse MTU, Fragmentation & TCP Throughput Calculator API", "version": "1.0.0", "summary": "Tunnel overhead, inner MTU and MSS, IPv4/IPv6 fragments and a TCP throughput model.", "description": "Every endpoint performs the same calculation as the human page and returns inputs, result, units, assumptions, sources, warnings and computedAt. MCP endpoint: /mcp." }, "servers": [ { "url": "https://signalhousemtucalculator.usefulweb.app" } ], "paths": { "/api/v1/calculate": { "post": { "operationId": "calculate", "summary": "Tunnel overhead, inner MTU and MSS, IPv4/IPv6 fragments and a TCP throughput model.", "description": "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.\n\nMethodology:\n- 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.\n- 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.\n- 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”.\n- 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.", "tags": [ "apps" ], "x-mcp-tool": "signalhouse_mtu_calculator", "x-owner-app": "signalhouse", "requestBody": { "required": true, "content": { "application/json": { "schema": { "$ref": "#/components/schemas/SignalhouseMtuCalculatorInput" }, "examples": { "example1": { "summary": "Textbook: a 4000-byte IPv4 datagram over a 1500-byte MTU", "value": { "linkMtu": 1500, "packetSize": 4000, "df": false } }, "example2": { "summary": "WireGuard over a PPPoE DSL line", "value": { "linkMtu": 1500, "layers": [ { "layer": "pppoe" }, { "layer": "wireguard4" } ], "packetSize": 1500, "df": true } }, "example3": { "summary": "VXLAN over an IPv6 underlay with 1500-byte links", "value": { "linkMtu": 1500, "layers": [ { "layer": "vxlan6" } ] } }, "example4": { "summary": "IPsec ESP with NAT traversal (AES-CBC, approximate) and IPv6 inside", "value": { "linkMtu": 1500, "layers": [ { "layer": "esp-cbc-natt" } ], "ipVersion": "6", "packetSize": 1500 } }, "example5": { "summary": "Long fat network: 1 Gbit/s over 150 ms without window scaling", "value": { "linkMtu": 1500, "bandwidthMbps": 1000, "rttMs": 150, "windowScaling": false, "transferMB": 1000 } }, "example6": { "summary": "TCP with 0.1 % loss on a 100 Mbit/s, 80 ms path", "value": { "linkMtu": 1500, "bandwidthMbps": 100, "rttMs": 80, "lossPercent": 0.1 } } } } } }, "responses": { "200": { "description": "Calculation result envelope.", "content": { "application/json": { "schema": { "$ref": "#/components/schemas/SignalhouseMtuCalculatorResult" } } } }, "400": { "description": "Malformed JSON body.", "content": { "application/problem+json": { "schema": { "$ref": "#/components/schemas/Problem" } } } }, "422": { "description": "Invalid input (see errors[]).", "content": { "application/problem+json": { "schema": { "$ref": "#/components/schemas/Problem" } } } }, "429": { "description": "Rate limit exceeded (see Retry-After).", "content": { "application/problem+json": { "schema": { "$ref": "#/components/schemas/Problem" } } } } } } }, "/api/v1/tool": { "get": { "operationId": "getToolDefinition", "summary": "Inputs, output schema and examples for this tool", "responses": { "200": { "description": "Tool definition.", "content": { "application/json": { "schema": { "type": "object" } } } } } } } }, "components": { "schemas": { "Problem": { "type": "object", "description": "RFC 9457 problem details. 422 responses include per-field `errors`.", "required": [ "type", "title", "status", "detail" ], "properties": { "type": { "type": "string" }, "title": { "type": "string" }, "status": { "type": "integer" }, "detail": { "type": "string" }, "code": { "type": "string" }, "errors": { "type": "array", "items": { "type": "object", "required": [ "field", "message" ], "properties": { "field": { "type": "string" }, "message": { "type": "string" } } } } } }, "SignalhouseMtuCalculatorInput": { "type": "object", "additionalProperties": false, "properties": { "linkMtu": { "title": "Link MTU", "description": "MTU of the physical link (Ethernet 1500, jumbo frames 9000).", "type": "integer", "minimum": 68, "maximum": 65535, "default": 1500, "x-unit": "bytes" }, "layers": { "title": "Encapsulations, from the wire inwards", "type": "array", "maxItems": 8, "items": { "type": "object", "additionalProperties": false, "properties": { "layer": { "title": "Encapsulation", "type": "string", "enum": [ "pppoe", "mpls", "gre", "gre-key", "gre6", "ipip", "6in4", "ip6tnl", "vxlan4", "vxlan6", "geneve4", "wireguard4", "wireguard6", "esp-cbc", "esp-cbc-natt", "esp-gcm", "esp-gcm-natt" ], "default": "pppoe" } }, "required": [] }, "default": [] }, "ipVersion": { "title": "Inner IP version", "type": "string", "enum": [ "4", "6" ], "default": "4" }, "packetSize": { "title": "Packet to send (total length)", "description": "Optional: an IP packet of this size is checked against the inner MTU and fragmented if needed.", "type": "integer", "minimum": 20, "maximum": 65575, "x-unit": "bytes" }, "df": { "title": "“Don’t Fragment” set (IPv4)", "type": "boolean", "default": false }, "ipv4HeaderLength": { "title": "IPv4 header length", "type": "integer", "minimum": 20, "maximum": 60, "default": 20, "x-unit": "bytes" }, "ipv6ExtensionBytes": { "title": "IPv6 extension headers before the Fragment header", "type": "integer", "minimum": 0, "maximum": 2000, "default": 0, "x-unit": "bytes" }, "bandwidthMbps": { "title": "TCP model: bandwidth", "description": "Fill in bandwidth and round-trip time to add the TCP throughput model.", "type": "number", "minimum": 0, "maximum": 10000000, "x-unit": "Mbit/s" }, "rttMs": { "title": "TCP model: round-trip time", "type": "number", "minimum": 0, "maximum": 100000, "x-unit": "ms" }, "lossPercent": { "title": "TCP model: packet loss", "type": "number", "minimum": 0, "maximum": 99, "default": 0, "x-unit": "%" }, "windowBytes": { "title": "TCP model: receive window", "description": "Empty: as large as the path needs (with window scaling) or 65,535 bytes (without).", "type": "integer", "minimum": 1, "maximum": 1073725440, "x-unit": "bytes" }, "windowScaling": { "title": "TCP model: window scaling (RFC 7323) enabled", "type": "boolean", "default": true }, "transferMB": { "title": "TCP model: file size to transfer", "type": "number", "minimum": 0, "x-unit": "MB" } }, "required": [] }, "SignalhouseMtuCalculatorResult": { "type": "object", "additionalProperties": false, "required": [ "schemaVersion", "tool", "computedAt", "inputs", "summary", "result", "units", "assumptions", "sources", "warnings", "presentation", "links" ], "properties": { "schemaVersion": { "type": "string", "const": "1.0" }, "tool": { "type": "object", "additionalProperties": false, "required": [ "id", "name", "version" ], "properties": { "id": { "type": "string" }, "name": { "type": "string" }, "version": { "type": "string" } } }, "computedAt": { "type": "string", "format": "date-time" }, "inputs": { "type": "object", "description": "Normalised inputs actually used, including applied defaults." }, "summary": { "type": "string" }, "result": { "type": "object", "additionalProperties": false, "required": [ "linkMtu", "ipVersion", "layers", "innerMtu", "overheadBytes", "mssIPv4", "mssIPv6", "ethernetEfficiencyPercent", "packet", "tcp", "notes" ], "properties": { "linkMtu": { "type": "number" }, "ipVersion": { "enum": [ 4, 6 ] }, "layers": { "type": "array", "items": { "type": "object", "additionalProperties": false, "required": [ "id", "label", "overheadBytes", "mtuBefore", "mtuAfter", "approximate", "standard" ], "properties": { "id": { "type": "string" }, "label": { "type": "string" }, "overheadBytes": { "type": "number" }, "mtuBefore": { "type": "number" }, "mtuAfter": { "type": "number" }, "approximate": { "type": "boolean" }, "standard": { "type": "string" } } } }, "innerMtu": { "type": "number" }, "overheadBytes": { "type": "number" }, "mssIPv4": { "type": "number" }, "mssIPv6": { "type": "number" }, "ethernetEfficiencyPercent": { "type": "number", "description": "TCP payload share of the Ethernet wire bytes (MTU + 38 bytes framing)." }, "packet": { "type": [ "object", "null" ], "description": "The packet check: size, fits, action (fits, fragmented, dropped), message and the fragments (index, offset in 8-byte units, offset in bytes, data bytes, total length, more-fragments flag)." }, "tcp": { "type": [ "object", "null" ], "description": "TCP model: bandwidth-delay product, window, effective window, window scale needed, window limit, Mathis loss limit, estimate, limiting factor, transfer time." }, "notes": { "type": "array", "items": { "type": "string" } } } }, "units": { "type": "object", "additionalProperties": { "type": "string" } }, "assumptions": { "type": "array", "items": { "type": "string" } }, "sources": { "type": "array", "items": { "type": "object", "additionalProperties": false, "required": [ "title" ], "properties": { "title": { "type": "string" }, "url": { "type": "string" }, "publisher": { "type": "string" }, "retrievedAt": { "type": "string" }, "note": { "type": "string" } } } }, "warnings": { "type": "array", "items": { "type": "string" } }, "presentation": { "type": "array", "items": { "type": "object" }, "description": "Display blocks used by the human page (same data)." }, "links": { "type": "object", "additionalProperties": false, "required": [ "human", "api", "openapi", "methodology" ], "properties": { "human": { "type": "string", "format": "uri" }, "api": { "type": "string", "format": "uri" }, "openapi": { "type": "string", "format": "uri" }, "methodology": { "type": "string", "format": "uri" } } } } } } } }