diff --git a/.gitignore b/.gitignore
index 8c2b884..da79429 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,14 +1,7 @@
-# ---> VisualStudioCode
+node_modules/
+dist/
+coverage/
+*.tsbuildinfo
 .vscode/*
 !.vscode/settings.json
-!.vscode/tasks.json
-!.vscode/launch.json
 !.vscode/extensions.json
-!.vscode/*.code-snippets
-
-# Local History for Visual Studio Code
-.history/
-
-# Built Visual Studio Code Extensions
-*.vsix
-
diff --git a/CHANGES.md b/CHANGES.md
new file mode 100644
index 0000000..9e27ee5
--- /dev/null
+++ b/CHANGES.md
@@ -0,0 +1,24 @@
+# Herkunft und Änderungen
+
+Nach GPL §5a: was aus welcher Quelle übertragen und wie es verändert wurde.
+Swift-Pfade relativ zu `filius-ipad/ios/FiliusPad/TopologyEditor/` (Stand
+`3b5ded0`, 27.08.2026), Tests relativ zu `filius-ipad/ios/FiliusPadTests/`.
+
+## packages/core
+
+| Datei | Quelle | Art der Übertragung |
+| --- | --- | --- |
+| `src/ids.ts` | Swift-Sortierung nach `UUID.uuidString` | neu, bildet die Swift-Ordnung nach |
+| `src/model/node.ts` | `Model/TopologyNode.swift` | übertragen; Standard-Ports und -Namen wie Java |
+| `src/runtime/config.ts` | `State/TopologyEditorState.swift` (Laufzeit-Konfigurationen, Firewall) | übertragen; Java-Beschriftungen der Oberfläche weggelassen |
+| `src/runtime/engine.ts` | `State/TopologyNetworkRuntimeEngine.swift` Z. ~313–3585, ~4753–4850 | übertragen: Warteschlange, Uhr, Ethernet, Switch, ARP, IPv4, ICMP, Routing, Firewall. Klasse statt Werttyp. Noch nicht übertragen (wirft `NotPortedError`): Gateway-NAT, UDP, TCP, DHCP, DNS, RIP, Remote Link, WLAN |
+| `src/runtime/ipv4.ts` | Engine `parseIPv4`/`sameSubnet` | übertragen |
+| `src/runtime/packets.ts` | Engine, Paketmodelle | übertragen; `Data` → `Uint8Array` |
+| `src/runtime/random.ts` | Engine, `TopologyJavaCompatibleRandom` | übertragen; BigInt für die 48-Bit-Arithmetik |
+| `src/runtime/routeTable.ts` | `State/TopologyJavaRouteTable.swift` | übertragen ohne Tabellengeometrie (gehört in den Editor) |
+| `src/runtime/snapshot.ts` | Engine, `TopologyNetworkRuntimeTopologySnapshot` | übertragen, bisher mit den Feldern der übertragenen Schichten |
+| `src/runtime/speed.ts` | Engine, `TopologyNetworkRuntimeSpeed` | übertragen |
+| `src/runtime/trace.ts` | Engine, Paketspur-Modell | übertragen |
+| `src/util/equal.ts` | — | neu (Ersatz für Swifts `Equatable`) |
+| `test/engine.test.ts` | `TopologyNetworkRuntimeEngineTests.swift` (21 von 72 Tests) | übertragen; `nextInt`-Referenzwerte an OpenJDK 21 gemessen |
+| `test/routeTable.test.ts` | `TopologyJavaRouteTableTests.swift` (8 von 12 Tests) | übertragen; Geometrie- und Reducer-Tests folgen mit Editor und Dokument |
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..94a9ed0
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,674 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
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+ Everyone is permitted to copy and distribute verbatim copies
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+
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+to the third party based on the extent of your activity of conveying
+the work, and under which the third party grants, to any of the
+parties who would receive the covered work from you, a discriminatory
+patent license (a) in connection with copies of the covered work
+conveyed by you (or copies made from those copies), or (b) primarily
+for and in connection with specific products or compilations that
+contain the covered work, unless you entered into that arrangement,
+or that patent license was granted, prior to 28 March 2007.
+
+  Nothing in this License shall be construed as excluding or limiting
+any implied license or other defenses to infringement that may
+otherwise be available to you under applicable patent law.
+
+  12. No Surrender of Others' Freedom.
+
+  If conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot convey a
+covered work so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you may
+not convey it at all.  For example, if you agree to terms that obligate you
+to collect a royalty for further conveying from those to whom you convey
+the Program, the only way you could satisfy both those terms and this
+License would be to refrain entirely from conveying the Program.
+
+  13. Use with the GNU Affero General Public License.
+
+  Notwithstanding any other provision of this License, you have
+permission to link or combine any covered work with a work licensed
+under version 3 of the GNU Affero General Public License into a single
+combined work, and to convey the resulting work.  The terms of this
+License will continue to apply to the part which is the covered work,
+but the special requirements of the GNU Affero General Public License,
+section 13, concerning interaction through a network will apply to the
+combination as such.
+
+  14. Revised Versions of this License.
+
+  The Free Software Foundation may publish revised and/or new versions of
+the GNU General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+  Each version is given a distinguishing version number.  If the
+Program specifies that a certain numbered version of the GNU General
+Public License "or any later version" applies to it, you have the
+option of following the terms and conditions either of that numbered
+version or of any later version published by the Free Software
+Foundation.  If the Program does not specify a version number of the
+GNU General Public License, you may choose any version ever published
+by the Free Software Foundation.
+
+  If the Program specifies that a proxy can decide which future
+versions of the GNU General Public License can be used, that proxy's
+public statement of acceptance of a version permanently authorizes you
+to choose that version for the Program.
+
+  Later license versions may give you additional or different
+permissions.  However, no additional obligations are imposed on any
+author or copyright holder as a result of your choosing to follow a
+later version.
+
+  15. Disclaimer of Warranty.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<http://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<http://www.gnu.org/philosophy/why-not-lgpl.html>.
diff --git a/README.md b/README.md
index fb992d0..cbdffaa 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,45 @@
 # mdg-filius
 
+Netzwerksimulation nach [FILIUS](https://www.lernsoftware-filius.de) für den
+Browser — als App „Filius" im MDG-Dashboard, gemeinsam bearbeitbar.
+
+> Based on Filius by Dr. Stefan Freischlad and the Filius project.
+>
+> Dieses Projekt ist unabhängig. Es wird weder vom ursprünglichen
+> Filius-Projekt (Universität Siegen) noch von `filius-project`
+> (Filius on iPad) herausgegeben, betrieben oder unterstützt.
+
+## Herkunft
+
+Der Simulationskern ist eine Übertragung nach TypeScript aus
+
+* [`filius-project/filius-ipad`](https://github.com/filius-project/filius-ipad)
+  (Swift, GPLv2 oder GPLv3) — Vorlage für Kern, `.fls`-Codec und Tests,
+* [`filius1/filius`](https://gitlab.com/filius1/filius) (Java, GPLv2 oder
+  GPLv3) — Referenz für Dateiformat und Verhalten.
+
+Jede übertragene Datei nennt in ihrem Kopf die Quelldatei. `CHANGES.md` führt
+die Herkunft je Paket. Lizenz dieses Repos: **GPL-3.0** (`LICENSE`).
+
+## Aufbau
+
+| Pfad | Inhalt |
+| --- | --- |
+| `packages/core` | Simulationskern: Modell, Ereignis-Warteschlange, Protokolle, Anwendungen — ohne DOM, ohne Node |
+| `packages/fls` | `.fls` lesen, schreiben, Unbekanntes bewahren *(folgt)* |
+| `packages/doc` | gemeinsames Dokument (Yjs) *(folgt)* |
+| `packages/editor` | Oberfläche im Iframe des Dashboards *(folgt)* |
+| `server` | Filius-Dienst auf Server F *(folgt)* |
+
+Plan und Entscheidungen: `knowledge/plans/FILIUS_WEB_PLAN.md` im Dashboard.
+
+## Entwickeln
+
+```bash
+npm install
+npm run hooks      # Sprachprüfung vor jedem Commit (braucht das Dashboard drumherum)
+npm run check      # Typprüfung + Tests
+```
+
+Code, Kommentare und Testnamen sind englisch; sichtbare Texte stehen hinter
+i18n-Schlüsseln (Deutsch/Englisch).
diff --git a/package-lock.json b/package-lock.json
new file mode 100644
index 0000000..e3e37fa
--- /dev/null
+++ b/package-lock.json
@@ -0,0 +1,1650 @@
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+      "dev": true,
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+}
diff --git a/package.json b/package.json
new file mode 100644
index 0000000..b853742
--- /dev/null
+++ b/package.json
@@ -0,0 +1,23 @@
+{
+  "name": "mdg-filius",
+  "private": true,
+  "license": "GPL-3.0-only",
+  "type": "module",
+  "workspaces": [
+    "packages/*"
+  ],
+  "engines": {
+    "node": ">=20"
+  },
+  "scripts": {
+    "test": "vitest run",
+    "typecheck": "tsc -b",
+    "check": "npm run typecheck && npm run test",
+    "hooks": "git config core.hooksPath scripts/hooks"
+  },
+  "devDependencies": {
+    "@types/node": "^20.19.0",
+    "typescript": "^5.9.0",
+    "vitest": "^3.2.0"
+  }
+}
diff --git a/packages/core/package.json b/packages/core/package.json
new file mode 100644
index 0000000..55fd066
--- /dev/null
+++ b/packages/core/package.json
@@ -0,0 +1,10 @@
+{
+  "name": "@mdg-filius/core",
+  "version": "0.0.0",
+  "private": true,
+  "license": "GPL-3.0-only",
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts"
+  }
+}
diff --git a/packages/core/src/ids.ts b/packages/core/src/ids.ts
new file mode 100644
index 0000000..efc9d24
--- /dev/null
+++ b/packages/core/src/ids.ts
@@ -0,0 +1,19 @@
+// Ported from filius-ipad (GPLv2/GPLv3), ios/FiliusPad/TopologyEditor.
+// Swift orders identifiers by `UUID.uuidString`, which is upper case. Every
+// deterministic ordering in the runtime depends on it, so identifiers are
+// compared the same way here whatever case they were written in.
+
+/** A node, port, link or socket identifier: a UUID string. */
+export type Id = string;
+
+/** `lhs.uuidString < rhs.uuidString` in Swift. */
+export function idLess(lhs: Id, rhs: Id): boolean {
+  return lhs.toUpperCase() < rhs.toUpperCase();
+}
+
+/** Comparator for `Array.prototype.sort` with the Swift ordering. */
+export function compareIds(lhs: Id, rhs: Id): number {
+  const left = lhs.toUpperCase();
+  const right = rhs.toUpperCase();
+  return left < right ? -1 : left > right ? 1 : 0;
+}
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
new file mode 100644
index 0000000..3232dc4
--- /dev/null
+++ b/packages/core/src/index.ts
@@ -0,0 +1,13 @@
+// Public surface of the simulation core.
+export * from './ids.js';
+export * from './model/node.js';
+export * from './runtime/config.js';
+export * from './runtime/engine.js';
+export * from './runtime/ipv4.js';
+export * from './runtime/packets.js';
+export * from './runtime/random.js';
+export * from './runtime/routeTable.js';
+export * from './runtime/snapshot.js';
+export * from './runtime/speed.js';
+export * from './runtime/trace.js';
+export * from './util/equal.js';
diff --git a/packages/core/src/model/node.ts b/packages/core/src/model/node.ts
new file mode 100644
index 0000000..57024f6
--- /dev/null
+++ b/packages/core/src/model/node.ts
@@ -0,0 +1,102 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   ios/FiliusPad/TopologyEditor/Model/TopologyNode.swift
+// Java counterpart: filius.hardware.knoten.* (port counts per device kind).
+
+import type { Id } from '../ids.js';
+
+export type NodeKind =
+  | 'pc'
+  | 'notebook'
+  | 'networkSwitch'
+  | 'router'
+  | 'gateway'
+  | 'remoteLink'
+  | 'unsupported';
+
+export function isPcClassEndpoint(kind: NodeKind): boolean {
+  return kind === 'pc' || kind === 'notebook';
+}
+
+export type HostLabelPolicy = 'manual' | 'ipAddress' | 'macAddress' | 'ipAndMAC';
+
+export interface PortMetadata {
+  readonly id: Id;
+  label: string;
+  isOccupied: boolean;
+  /** Canonical (upper case, colon separated) or undefined. */
+  readonly importedMacAddress?: string;
+}
+
+/**
+ * The MAC address of a port nobody gave one: `02:` followed by the last ten
+ * hex digits of the port's UUID — stable across saves, locally administered.
+ */
+export function stableGeneratedMacAddress(portId: Id): string {
+  const compact = portId.toUpperCase().replaceAll('-', '');
+  const suffix = compact.slice(-10);
+  const octets = ['02'];
+  for (let offset = 0; offset < suffix.length; offset += 2) {
+    octets.push(suffix.slice(offset, offset + 2).toUpperCase());
+  }
+  return octets.join(':');
+}
+
+/** Six colon-separated pairs of hex digits, upper-cased; anything else is undefined. */
+export function canonicalMacAddress(value: string | undefined | null): string | undefined {
+  if (value === undefined || value === null) return undefined;
+  const segments = value.trim().split(':');
+  if (segments.length !== 6) return undefined;
+  if (!segments.every((segment) => /^[0-9A-Fa-f]{2}$/.test(segment))) return undefined;
+  return segments.map((segment) => segment.toUpperCase()).join(':');
+}
+
+export function effectiveMacAddress(port: PortMetadata): string {
+  return port.importedMacAddress ?? stableGeneratedMacAddress(port.id);
+}
+
+/**
+ * The name a new device gets. These are the Java FILIUS device names; the
+ * editor shows them translated (packages/editor i18n), the model keeps them
+ * as they appear in a Java .fls file.
+ */
+export function defaultDisplayName(kind: NodeKind): string {
+  switch (kind) {
+    case 'pc':
+      return 'Rechner';
+    case 'notebook':
+      return 'Notebook';
+    case 'networkSwitch':
+      return 'Switch';
+    case 'router':
+      return 'Vermittlungsrechner';
+    case 'gateway':
+      return 'Gateway';
+    case 'remoteLink':
+      return 'Remote Link';
+    case 'unsupported':
+      return 'Unbekanntes Gerät';
+  }
+}
+
+/** Labels of the ports a new device of this kind has, in order. */
+export function defaultPortLabels(kind: NodeKind): string[] {
+  switch (kind) {
+    case 'pc':
+    case 'notebook':
+      return ['eth0'];
+    case 'networkSwitch':
+      // Java Switch constructs exactly 24 physical ports.
+      return Array.from({ length: 24 }, (_, index) => `sw${index + 1}`);
+    case 'router':
+      // Java Vermittlungsrechner starts with one configurable network interface.
+      return ['rt1'];
+    case 'gateway':
+      // Java Gateway has fixed WAN (index 0) and LAN (index 1) interfaces.
+      return ['wan0', 'lan0'];
+    case 'remoteLink':
+      // One local physical attachment, matching the Java Modem.
+      return ['remote0'];
+    case 'unsupported':
+      return [];
+  }
+}
diff --git a/packages/core/src/runtime/config.ts b/packages/core/src/runtime/config.ts
new file mode 100644
index 0000000..6b6972e
--- /dev/null
+++ b/packages/core/src/runtime/config.ts
@@ -0,0 +1,150 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyEditorState.swift (runtime configuration value types)
+
+import type { Id } from '../ids.js';
+import { maskAddress, parseIPv4 } from './ipv4.js';
+
+/** The IP settings of a PC or notebook (its single interface) — and a router's gateway. */
+export interface DeviceConfiguration {
+  readonly ipAddress: string;
+  readonly subnetMask: string;
+  readonly defaultGateway: string;
+  readonly dnsServer: string;
+}
+
+export function deviceConfiguration(
+  ipAddress: string,
+  subnetMask: string,
+  defaultGateway = '',
+  dnsServer = '',
+): DeviceConfiguration {
+  return { ipAddress, subnetMask, defaultGateway, dnsServer };
+}
+
+/** One interface of a router or gateway. */
+export interface InterfaceConfiguration {
+  readonly ipAddress: string;
+  readonly subnetMask: string;
+}
+
+/** Map key for an interface: node and port. */
+export type InterfaceKey = string;
+
+export function interfaceKey(nodeId: Id, portId: Id): InterfaceKey {
+  return `${nodeId}/${portId}`;
+}
+
+/** A row of the manual routing table (Java `Weiterleitungstabelle`). */
+export interface ManualRoute {
+  readonly destinationNetwork: string;
+  readonly subnetMask: string;
+  readonly gateway: string;
+  readonly interfaceIPAddress: string;
+}
+
+/** Longest numeric mask wins; on a tie the first row stays. */
+export function bestMatchingManualRoute(
+  targetIPAddress: string,
+  routes: readonly ManualRoute[],
+): ManualRoute | undefined {
+  const target = parseIPv4(targetIPAddress);
+  if (target === undefined) return undefined;
+  let best: ManualRoute | undefined;
+  let bestMask: number | undefined;
+  for (const route of routes) {
+    const destination = parseIPv4(route.destinationNetwork);
+    const mask = parseIPv4(route.subnetMask);
+    if (destination === undefined || mask === undefined) continue;
+    if (destination !== maskAddress(target, mask)) continue;
+    if (bestMask !== undefined && mask <= bestMask) continue;
+    bestMask = mask;
+    best = route;
+  }
+  return best;
+}
+
+export interface SwitchConfiguration {
+  readonly ssid: string;
+  readonly retentionTimeMilliseconds: number;
+}
+
+/** Java SwitchFirmware keeps a learned MAC address for five minutes. */
+export const DEFAULT_SWITCH_RETENTION_MILLISECONDS = 300_000;
+
+export function switchConfiguration(
+  ssid: string,
+  retentionTimeMilliseconds = DEFAULT_SWITCH_RETENTION_MILLISECONDS,
+): SwitchConfiguration {
+  return { ssid, retentionTimeMilliseconds };
+}
+
+/** The first six hex digits of the node's UUID, lower case. */
+export function defaultSwitchConfiguration(nodeId: Id): SwitchConfiguration {
+  const compact = nodeId.replaceAll('-', '').toLowerCase();
+  return switchConfiguration(compact.slice(0, 6));
+}
+
+// ── Firewall (Java filius.software.firewall.Firewall, FirewallRule) ─────────
+
+export const FIREWALL_PROTOCOL = { all: -1, icmp: 1, tcp: 6, udp: 17 } as const;
+export type FirewallProtocol = keyof typeof FIREWALL_PROTOCOL;
+
+export type FirewallAction = 'drop' | 'accept';
+
+/** Java writes this as the source of a rule that means "any directly connected network". */
+export const DIRECTLY_CONNECTED_SOURCE_MARKER = '999.999.999.999';
+export const ALL_PORTS = -1;
+
+export interface FirewallRule {
+  readonly sourceIPAddress: string;
+  readonly sourceSubnetMask: string;
+  readonly destinationIPAddress: string;
+  readonly destinationSubnetMask: string;
+  /** A port number or ALL_PORTS. */
+  readonly port: number;
+  readonly protocol: FirewallProtocol;
+  readonly action: FirewallAction;
+}
+
+export function firewallRule(fields: Partial<FirewallRule> = {}): FirewallRule {
+  return {
+    sourceIPAddress: fields.sourceIPAddress ?? '',
+    sourceSubnetMask: fields.sourceSubnetMask ?? '',
+    destinationIPAddress: fields.destinationIPAddress ?? '',
+    destinationSubnetMask: fields.destinationSubnetMask ?? '',
+    port: fields.port ?? ALL_PORTS,
+    protocol: fields.protocol ?? 'tcp',
+    action: fields.action ?? 'drop',
+  };
+}
+
+export interface FirewallConfiguration {
+  readonly isActive: boolean;
+  readonly defaultPolicy: FirewallAction;
+  readonly dropIcmp: boolean;
+  readonly filterSynSegmentsOnly: boolean;
+  readonly filterUdp: boolean;
+  readonly rules: readonly FirewallRule[];
+}
+
+/** What a router or gateway has without a firewall configured: inactive. */
+export function firewallConfiguration(fields: Partial<FirewallConfiguration> = {}): FirewallConfiguration {
+  return {
+    isActive: fields.isActive ?? false,
+    defaultPolicy: fields.defaultPolicy ?? 'drop',
+    dropIcmp: fields.dropIcmp ?? false,
+    filterSynSegmentsOnly: fields.filterSynSegmentsOnly ?? true,
+    filterUdp: fields.filterUdp ?? true,
+    rules: fields.rules ?? [],
+  };
+}
+
+/** What Java installs as the personal firewall of a PC: active, drop by default. */
+export const JAVA_PERSONAL_FIREWALL_DEFAULTS: FirewallConfiguration = firewallConfiguration({
+  isActive: true,
+  defaultPolicy: 'drop',
+  dropIcmp: false,
+  filterSynSegmentsOnly: true,
+  filterUdp: true,
+  rules: [],
+});
diff --git a/packages/core/src/runtime/engine.ts b/packages/core/src/runtime/engine.ts
new file mode 100644
index 0000000..e283f52
--- /dev/null
+++ b/packages/core/src/runtime/engine.ts
@@ -0,0 +1,1939 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift
+//     "Runtime snapshots and transient state"         (lines ~313–938)
+//     "Deterministic runtime engine"                   (lines ~984–1284)
+//     "Ethernet, ARP, IPv4, and ICMP packet runtime"   (lines ~1284–3585,
+//       without remote link, wireless and gateway NAT; with the firewall)
+//     processScheduledRuntimeEvent, dropForGlobalPacketLoss (lines ~4753–4850)
+// Java counterparts: filius.hardware.Verbindung, filius.software.netzzugangsschicht,
+//   filius.software.vermittlungsschicht (ARP, IP, ICMP), system.SwitchFirmware.
+//
+// The Swift engine is a value type driven by a reducer; here it is a class
+// with the same inputs and outputs. Time is virtual: nothing sleeps, every
+// delay is an event in the queue, ordered by deadline and then by insertion.
+// Calls that block in Java (resolving an address, waiting for an echo reply)
+// advance the virtual clock inside the call, exactly as the Swift port does.
+//
+// Not ported yet — each place throws NotPortedError instead of guessing:
+// gateway NAT, UDP and TCP delivery, RIP routing, remote links.
+
+import { compareIds, type Id } from '../ids.js';
+import { isPcClassEndpoint, stableGeneratedMacAddress } from '../model/node.js';
+import type {
+  DeviceConfiguration,
+  FirewallConfiguration,
+  FirewallRule,
+  InterfaceConfiguration,
+  ManualRoute,
+} from './config.js';
+import {
+  ALL_PORTS,
+  DEFAULT_SWITCH_RETENTION_MILLISECONDS,
+  DIRECTLY_CONNECTED_SOURCE_MARKER,
+  FIREWALL_PROTOCOL,
+  firewallConfiguration,
+  interfaceKey,
+} from './config.js';
+import { deepEqual } from '../util/equal.js';
+import { maskAddress, parseIPv4, sameSubnet } from './ipv4.js';
+import {
+  forwardingClone,
+  icmpMessage,
+  IPV4_PROTOCOL,
+  packetIdentityInFrame,
+  TCP_FLAG,
+  type ArpPacket,
+  type EthernetFrame,
+  type IcmpMessage,
+  type IcmpMessageKind,
+  type IPv4Packet,
+} from './packets.js';
+import { JavaCompatibleRandom } from './random.js';
+import { routeTableRows, type RouteRow } from './routeTable.js';
+import { EMPTY_TOPOLOGY_SNAPSHOT, withSnapshot, type NodeSnapshot, type TopologySnapshot } from './snapshot.js';
+import { RuntimeSpeed } from './speed.js';
+import {
+  PRODUCTION_TRACE_RETENTION,
+  type HeaderField,
+  type TraceDirection,
+  type TraceEvent,
+  type TraceLayer,
+  type TraceOperation,
+  type TraceRetentionPolicy,
+} from './trace.js';
+
+export const ETHERNET_BROADCAST_MAC_ADDRESS = 'FF:FF:FF:FF:FF:FF';
+export const UNSPECIFIED_MAC_ADDRESS = '00:00:00:00:00:00';
+export const LIMITED_BROADCAST_IP_ADDRESS = '255.255.255.255';
+export const UNSPECIFIED_IP_ADDRESS = '0.0.0.0';
+export const LOCALHOST_IP_ADDRESS = '127.0.0.1';
+
+/** The seed a simulation uses unless told otherwise. */
+export const DEFAULT_SEED = 0xf11a5n;
+
+/** A part of the Swift engine that has not been carried over yet. */
+export class NotPortedError extends Error {
+  constructor(feature: string) {
+    super(`Not ported yet: ${feature}`);
+    this.name = 'NotPortedError';
+  }
+}
+
+// ── Events, inputs, outputs ────────────────────────────────────────────────
+
+export type ScheduledEventKind =
+  | { readonly kind: 'parityMarker'; readonly label: string }
+  | { readonly kind: 'ripBeacon'; readonly nodeId: Id }
+  | { readonly kind: 'dhcpClientStart'; readonly nodeId: Id }
+  | { readonly kind: 'dhcpTimeout'; readonly nodeId: Id }
+  | { readonly kind: 'tcpTimeout'; readonly sessionId: Id }
+  | {
+      readonly kind: 'ethernetLinkDelivery';
+      readonly sourceNodeId: Id;
+      readonly targetNodeId: Id;
+      readonly targetPortId: Id;
+      readonly latencyMilliseconds: number;
+      readonly frame: EthernetFrame;
+    }
+  | {
+      readonly kind: 'remoteLinkDelivery';
+      readonly sourceNodeId: Id;
+      readonly partnerNodeId: Id;
+      readonly partnerPortId: Id;
+      readonly pairIdentifier: string;
+      readonly latencyMilliseconds: number;
+      readonly frame: EthernetFrame;
+    }
+  | {
+      readonly kind: 'lanRemoteLinkTransmission';
+      readonly nodeId: Id;
+      readonly pairIdentifier: string;
+      readonly latencyMilliseconds: number;
+      readonly frame: EthernetFrame;
+    }
+  | { readonly kind: 'natExpirySweep' };
+
+export interface ScheduledEvent {
+  readonly id: number;
+  readonly deadlineMilliseconds: number;
+  readonly sequenceNumber: number;
+  readonly kind: ScheduledEventKind;
+}
+
+export type CompatibilityOperationKind = 'terminalCommand' | 'applicationOperation' | 'socketOperation';
+
+export interface CompatibilityOperation {
+  readonly kind: CompatibilityOperationKind;
+  readonly nodeId: Id;
+  readonly interfaceId?: Id;
+  readonly detail: string;
+}
+
+export type RuntimeInput =
+  | {
+      readonly type: 'start';
+      readonly snapshot: TopologySnapshot;
+      readonly seed: bigint | number;
+      readonly initialTimeMilliseconds: number;
+    }
+  | { readonly type: 'stop' }
+  | { readonly type: 'advance'; readonly toMilliseconds: number }
+  | { readonly type: 'schedule'; readonly deadlineMilliseconds: number; readonly kind: ScheduledEventKind }
+  | { readonly type: 'compatibilityOperation'; readonly operation: CompatibilityOperation };
+
+export type RuntimeOutputKind =
+  | { readonly type: 'started' }
+  | { readonly type: 'startIgnoredAlreadyRunning' }
+  | { readonly type: 'stopped' }
+  | { readonly type: 'stopIgnoredAlreadyStopped' }
+  | { readonly type: 'advanced'; readonly fromMilliseconds: number; readonly toMilliseconds: number }
+  | { readonly type: 'advanceIgnoredWhileStopped' }
+  | { readonly type: 'advanceRejectedPastTime' }
+  | { readonly type: 'scheduleRejectedPastTime'; readonly deadlineMilliseconds: number }
+  | { readonly type: 'scheduled'; readonly event: ScheduledEvent }
+  | { readonly type: 'fired'; readonly event: ScheduledEvent }
+  | { readonly type: 'compatibilityOperationRecorded'; readonly operation: CompatibilityOperation };
+
+export interface RuntimeOutput {
+  readonly timeMilliseconds: number;
+  readonly kind: RuntimeOutputKind;
+}
+
+/** A named, seeded list of events to replay without any user interface. */
+export interface RuntimeScenario {
+  readonly name: string;
+  readonly seed: bigint | number;
+  readonly advanceToMilliseconds: number;
+  readonly scheduledEvents: readonly { deadlineMilliseconds: number; kind: ScheduledEventKind }[];
+}
+
+// ── Runtime state ──────────────────────────────────────────────────────────
+
+export interface ArpCacheEntry {
+  readonly ipAddress: string;
+  readonly macAddress: string;
+  readonly updatedAtMilliseconds: number;
+}
+
+export interface SwitchSatEntry {
+  readonly macAddress: string;
+  readonly portId: Id;
+  readonly portLabel: string;
+  readonly updatedAtMilliseconds: number;
+}
+
+/** An interface of a node that has an IP configuration. */
+export interface NetworkInterface {
+  readonly nodeId: Id;
+  readonly portId: Id;
+  readonly index: number;
+  readonly ipAddress: string;
+  readonly subnetMask: string;
+  readonly defaultGateway: string;
+  readonly macAddress: string;
+}
+
+export interface DeliveredIPv4Packet {
+  readonly nodeId: Id;
+  readonly interfaceId: Id;
+  readonly packet: IPv4Packet;
+}
+
+export interface IcmpObservation {
+  readonly nodeId: Id;
+  readonly interfaceId?: Id;
+  readonly packet: IPv4Packet;
+  readonly message: IcmpMessage;
+}
+
+export type IPv4DeliveryResult =
+  | { readonly type: 'delivered'; readonly packetIdentity: number; readonly nodeId: Id }
+  | {
+      readonly type: 'icmpError';
+      readonly packetIdentity: number;
+      readonly nodeId: Id;
+      readonly kind: IcmpMessageKind;
+    }
+  | { readonly type: 'dropped'; readonly packetIdentity: number; readonly nodeId?: Id };
+
+export interface FirewallDecision {
+  readonly packetIdentity: number;
+  readonly nodeId: Id;
+  readonly accepted: boolean;
+  readonly ruleIndex?: number;
+}
+
+export type RuntimePhase = 'stopped' | 'running';
+
+export interface RuntimeState {
+  phase: RuntimePhase;
+  currentTimeMilliseconds: number;
+  simulationSpeedPercent: number;
+  globalPacketLossEnabled: boolean;
+  seed: bigint;
+  topologySnapshot: TopologySnapshot;
+  pendingEvents: ScheduledEvent[];
+  packetTraces: TraceEvent[];
+  discardedPacketTraceCount: number;
+  arpCachesByNodeId: Map<Id, Map<string, ArpCacheEntry>>;
+  switchForwardingTablesByNodeId: Map<Id, Map<string, Id>>;
+  switchForwardingUpdatedAtMillisecondsByNodeId: Map<Id, Map<string, number>>;
+  switchSeenFrameIdentitiesByNodeId: Map<Id, Set<number>>;
+  deliveredIPv4PacketsByNodeId: Map<Id, DeliveredIPv4Packet[]>;
+  icmpObservationsByNodeId: Map<Id, IcmpObservation[]>;
+  firewallDecisions: FirewallDecision[];
+  nextEventSequenceNumber: number;
+  nextFrameIdentity: number;
+  nextPacketIdentity: number;
+  nextTraceIdentity: number;
+}
+
+function initialState(seed: bigint): RuntimeState {
+  return {
+    phase: 'stopped',
+    currentTimeMilliseconds: 0,
+    simulationSpeedPercent: new RuntimeSpeed().percent,
+    globalPacketLossEnabled: false,
+    seed,
+    topologySnapshot: EMPTY_TOPOLOGY_SNAPSHOT,
+    pendingEvents: [],
+    packetTraces: [],
+    discardedPacketTraceCount: 0,
+    arpCachesByNodeId: new Map(),
+    switchForwardingTablesByNodeId: new Map(),
+    switchForwardingUpdatedAtMillisecondsByNodeId: new Map(),
+    switchSeenFrameIdentitiesByNodeId: new Map(),
+    deliveredIPv4PacketsByNodeId: new Map(),
+    icmpObservationsByNodeId: new Map(),
+    firewallDecisions: [],
+    nextEventSequenceNumber: 0,
+    nextFrameIdentity: 1,
+    nextPacketIdentity: 1,
+    nextTraceIdentity: 1,
+  };
+}
+
+export interface TraceInput {
+  frameIdentity?: number | undefined;
+  packetIdentity?: number | undefined;
+  nodeId: Id;
+  interfaceId?: Id | undefined;
+  direction: TraceDirection;
+  layer: TraceLayer;
+  operation: TraceOperation;
+  beforeHeaders?: readonly HeaderField[];
+  afterHeaders?: readonly HeaderField[];
+  detail?: string | undefined;
+}
+
+type PacketRoute =
+  | { readonly type: 'local'; readonly interface: NetworkInterface | undefined }
+  | { readonly type: 'broadcast' }
+  | { readonly type: 'egress'; readonly interface: NetworkInterface; readonly nextHop: string }
+  | { readonly type: 'unavailable' };
+
+interface PendingIPv4Transmission {
+  readonly packet: IPv4Packet;
+  readonly interface: NetworkInterface;
+  readonly operation: TraceOperation;
+}
+
+function toSeed(seed: bigint | number): bigint {
+  return BigInt.asUintN(64, BigInt(seed));
+}
+
+function isNetworkDeliveryEvent(kind: ScheduledEventKind): boolean {
+  return kind.kind === 'ethernetLinkDelivery' || kind.kind === 'remoteLinkDelivery';
+}
+
+function isRemoteLinkWaitEvent(kind: ScheduledEventKind): boolean {
+  return isNetworkDeliveryEvent(kind) || kind.kind === 'lanRemoteLinkTransmission';
+}
+
+function getOrCreate<K, V>(map: Map<K, V>, key: K, create: () => V): V {
+  let value = map.get(key);
+  if (value === undefined) {
+    value = create();
+    map.set(key, value);
+  }
+  return value;
+}
+
+// ── Engine ─────────────────────────────────────────────────────────────────
+
+export class NetworkRuntimeEngine {
+  private stateValue: RuntimeState;
+  private random: JavaCompatibleRandom;
+  private udpPortRandom: JavaCompatibleRandom;
+  private tcpPortRandom: JavaCompatibleRandom;
+  private runtimeSpeed: RuntimeSpeed;
+  private readonly packetTraceRetentionPolicy: TraceRetentionPolicy;
+  private isProcessingScheduledEvents = false;
+  private pendingIPv4TransmissionsByArpKey = new Map<string, PendingIPv4Transmission[]>();
+
+  constructor(
+    seed: bigint | number = DEFAULT_SEED,
+    packetTraceRetentionPolicy: TraceRetentionPolicy = PRODUCTION_TRACE_RETENTION,
+  ) {
+    const normalizedSeed = toSeed(seed);
+    this.stateValue = initialState(normalizedSeed);
+    this.random = new JavaCompatibleRandom(normalizedSeed);
+    this.udpPortRandom = new JavaCompatibleRandom(normalizedSeed ^ 0x5544_5050n);
+    this.tcpPortRandom = new JavaCompatibleRandom(normalizedSeed ^ 0x5443_5050n);
+    this.runtimeSpeed = new RuntimeSpeed();
+    this.packetTraceRetentionPolicy = packetTraceRetentionPolicy;
+    this.stateValue.simulationSpeedPercent = this.runtimeSpeed.percent;
+  }
+
+  /** The runtime state. Read it; change it only through the engine's methods. */
+  get state(): RuntimeState {
+    return this.stateValue;
+  }
+
+  get simulationSpeed(): RuntimeSpeed {
+    return this.runtimeSpeed;
+  }
+
+  setSimulationSpeed(percent: number): void {
+    this.runtimeSpeed = new RuntimeSpeed(percent);
+    this.stateValue.simulationSpeedPercent = this.runtimeSpeed.percent;
+  }
+
+  setGlobalPacketLossEnabled(enabled: boolean): void {
+    this.stateValue.globalPacketLossEnabled = this.stateValue.phase === 'running' && enabled;
+  }
+
+  handle(input: RuntimeInput): RuntimeOutput[] {
+    switch (input.type) {
+      case 'start': {
+        if (this.stateValue.phase !== 'stopped') return [this.output({ type: 'startIgnoredAlreadyRunning' })];
+        const seed = toSeed(input.seed);
+        this.stateValue = initialState(seed);
+        this.stateValue.phase = 'running';
+        this.stateValue.currentTimeMilliseconds = input.initialTimeMilliseconds;
+        this.stateValue.simulationSpeedPercent = this.runtimeSpeed.percent;
+        this.stateValue.topologySnapshot = input.snapshot;
+        this.random = new JavaCompatibleRandom(seed);
+        this.udpPortRandom = new JavaCompatibleRandom(seed ^ 0x5544_5050n);
+        this.tcpPortRandom = new JavaCompatibleRandom(seed ^ 0x5443_5050n);
+        this.isProcessingScheduledEvents = false;
+        this.pendingIPv4TransmissionsByArpKey.clear();
+        this.initializeRipState();
+        this.initializeDhcpState();
+        this.initializeNatState();
+        return [this.output({ type: 'started' })];
+      }
+
+      case 'stop': {
+        if (this.stateValue.phase !== 'running') return [this.output({ type: 'stopIgnoredAlreadyStopped' })];
+        const state = this.stateValue;
+        state.phase = 'stopped';
+        state.globalPacketLossEnabled = false;
+        this.isProcessingScheduledEvents = false;
+        this.pendingIPv4TransmissionsByArpKey.clear();
+        state.pendingEvents = [];
+        state.arpCachesByNodeId.clear();
+        state.switchForwardingTablesByNodeId.clear();
+        state.switchForwardingUpdatedAtMillisecondsByNodeId.clear();
+        state.switchSeenFrameIdentitiesByNodeId.clear();
+        state.deliveredIPv4PacketsByNodeId.clear();
+        state.icmpObservationsByNodeId.clear();
+        state.firewallDecisions = [];
+        return [this.output({ type: 'stopped' })];
+      }
+
+      case 'advance':
+        return this.advance(input.toMilliseconds);
+
+      case 'schedule': {
+        if (this.stateValue.phase !== 'running') return [this.output({ type: 'advanceIgnoredWhileStopped' })];
+        if (input.deadlineMilliseconds < this.stateValue.currentTimeMilliseconds) {
+          return [
+            this.output({ type: 'scheduleRejectedPastTime', deadlineMilliseconds: input.deadlineMilliseconds }),
+          ];
+        }
+        const event = this.schedule(input.deadlineMilliseconds, input.kind);
+        return [this.output({ type: 'scheduled', event })];
+      }
+
+      case 'compatibilityOperation':
+        if (this.stateValue.phase !== 'running') return [this.output({ type: 'advanceIgnoredWhileStopped' })];
+        this.recordCompatibilityTrace(input.operation);
+        return [this.output({ type: 'compatibilityOperationRecorded', operation: input.operation })];
+    }
+  }
+
+  nextRipJitterMilliseconds(): number {
+    return this.random.nextRipJitterMilliseconds();
+  }
+
+  allocateFrameIdentity(): number {
+    return this.stateValue.nextFrameIdentity++;
+  }
+
+  allocatePacketIdentity(): number {
+    return this.stateValue.nextPacketIdentity++;
+  }
+
+  recordTrace(input: TraceInput): void {
+    const state = this.stateValue;
+    const trace: TraceEvent = {
+      id: state.nextTraceIdentity,
+      timeMilliseconds: state.currentTimeMilliseconds,
+      ...(input.frameIdentity === undefined ? {} : { frameIdentity: input.frameIdentity }),
+      ...(input.packetIdentity === undefined ? {} : { packetIdentity: input.packetIdentity }),
+      nodeId: input.nodeId,
+      ...(input.interfaceId === undefined ? {} : { interfaceId: input.interfaceId }),
+      direction: input.direction,
+      layer: input.layer,
+      operation: input.operation,
+      beforeHeaders: input.beforeHeaders ?? [],
+      afterHeaders: input.afterHeaders ?? [],
+      ...(input.detail === undefined ? {} : { detail: input.detail }),
+    };
+    state.nextTraceIdentity += 1;
+    if (state.packetTraces.length >= this.packetTraceRetentionPolicy.maximumEventCount) {
+      const evictionCount = Math.min(this.packetTraceRetentionPolicy.evictionBatchSize, state.packetTraces.length);
+      state.packetTraces.splice(0, evictionCount);
+      state.discardedPacketTraceCount += evictionCount;
+    }
+    state.packetTraces.push(trace);
+  }
+
+  static run(scenario: RuntimeScenario, snapshot: TopologySnapshot = EMPTY_TOPOLOGY_SNAPSHOT): RuntimeState {
+    const engine = new NetworkRuntimeEngine(scenario.seed);
+    engine.handle({ type: 'start', snapshot, seed: scenario.seed, initialTimeMilliseconds: 0 });
+    for (const scheduled of scenario.scheduledEvents) {
+      engine.handle({ type: 'schedule', deadlineMilliseconds: scheduled.deadlineMilliseconds, kind: scheduled.kind });
+    }
+    engine.handle({ type: 'advance', toMilliseconds: scenario.advanceToMilliseconds });
+    return engine.state;
+  }
+
+  private schedule(deadlineMilliseconds: number, kind: ScheduledEventKind): ScheduledEvent {
+    const state = this.stateValue;
+    const event: ScheduledEvent = {
+      id: state.nextEventSequenceNumber,
+      deadlineMilliseconds,
+      sequenceNumber: state.nextEventSequenceNumber,
+      kind,
+    };
+    state.nextEventSequenceNumber += 1;
+    state.pendingEvents.push(event);
+    return event;
+  }
+
+  private advance(targetTimeMilliseconds: number): RuntimeOutput[] {
+    const state = this.stateValue;
+    if (state.phase !== 'running') return [this.output({ type: 'advanceIgnoredWhileStopped' })];
+    if (targetTimeMilliseconds < state.currentTimeMilliseconds) {
+      return [this.output({ type: 'advanceRejectedPastTime' })];
+    }
+
+    const startTime = state.currentTimeMilliseconds;
+    const outputs: RuntimeOutput[] = [];
+    const wasProcessingScheduledEvents = this.isProcessingScheduledEvents;
+    this.isProcessingScheduledEvents = true;
+    try {
+      for (;;) {
+        const nextEventIndex = this.nextDueEventIndex(targetTimeMilliseconds);
+        if (nextEventIndex === undefined) break;
+        const [event] = this.stateValue.pendingEvents.splice(nextEventIndex, 1) as [ScheduledEvent];
+        this.stateValue.currentTimeMilliseconds = event.deadlineMilliseconds;
+        this.ageAllSwitchSatEntries();
+        this.expireRipRoutes();
+        this.processScheduledRuntimeEvent(event.kind);
+        outputs.push(this.output({ type: 'fired', event }));
+      }
+
+      this.stateValue.currentTimeMilliseconds = targetTimeMilliseconds;
+      this.ageAllSwitchSatEntries();
+      this.expireRipRoutes();
+      outputs.push(
+        this.output({ type: 'advanced', fromMilliseconds: startTime, toMilliseconds: targetTimeMilliseconds }),
+      );
+      return outputs;
+    } finally {
+      this.isProcessingScheduledEvents = wasProcessingScheduledEvents;
+    }
+  }
+
+  private nextDueEventIndex(targetTimeMilliseconds: number): number | undefined {
+    let best: number | undefined;
+    const events = this.stateValue.pendingEvents;
+    for (let index = 0; index < events.length; index += 1) {
+      const candidate = events[index]!;
+      if (candidate.deadlineMilliseconds > targetTimeMilliseconds) continue;
+      if (best === undefined) {
+        best = index;
+        continue;
+      }
+      const current = events[best]!;
+      if (
+        candidate.deadlineMilliseconds < current.deadlineMilliseconds ||
+        (candidate.deadlineMilliseconds === current.deadlineMilliseconds &&
+          candidate.sequenceNumber < current.sequenceNumber)
+      ) {
+        best = index;
+      }
+    }
+    return best;
+  }
+
+  private recordCompatibilityTrace(operation: CompatibilityOperation): void {
+    this.recordTrace({
+      nodeId: operation.nodeId,
+      interfaceId: operation.interfaceId,
+      direction: 'local',
+      layer: 'application',
+      operation: 'compatibilityAdapter',
+      afterHeaders: [{ name: 'kind', value: operation.kind }],
+      detail: operation.detail,
+    });
+  }
+
+  private output(kind: RuntimeOutputKind): RuntimeOutput {
+    return { timeMilliseconds: this.stateValue.currentTimeMilliseconds, kind };
+  }
+
+  private processScheduledRuntimeEvent(kind: ScheduledEventKind): void {
+    switch (kind.kind) {
+      case 'ethernetLinkDelivery':
+        if (this.dropForGlobalPacketLoss(kind.frame, kind.targetNodeId, kind.targetPortId, 'inbound', 'physical')) {
+          return;
+        }
+        this.receiveEthernetFrame(kind.frame, kind.targetNodeId, kind.targetPortId);
+        return;
+      case 'parityMarker':
+        return;
+      case 'ripBeacon':
+        throw new NotPortedError('RIP beacons');
+      case 'dhcpClientStart':
+      case 'dhcpTimeout':
+        throw new NotPortedError('DHCP client');
+      case 'tcpTimeout':
+        throw new NotPortedError('TCP timeouts');
+      case 'remoteLinkDelivery':
+      case 'lanRemoteLinkTransmission':
+        throw new NotPortedError('remote links');
+      case 'natExpirySweep':
+        throw new NotPortedError('gateway NAT');
+    }
+  }
+
+  private dropForGlobalPacketLoss(
+    frame: EthernetFrame,
+    nodeId: Id,
+    interfaceId: Id | undefined,
+    direction: TraceDirection,
+    layer: TraceLayer,
+  ): boolean {
+    if (!this.stateValue.globalPacketLossEnabled) return false;
+    this.recordTrace({
+      frameIdentity: frame.identity,
+      packetIdentity: packetIdentityInFrame(frame),
+      nodeId,
+      interfaceId,
+      direction,
+      layer,
+      operation: 'dropped',
+      afterHeaders: ethernetHeaders(frame),
+      detail: 'global packet-loss simulation',
+    });
+    return true;
+  }
+
+  // ── Later slices hook in here ────────────────────────────────────────────
+
+  private initializeRipState(): void {
+    // RIP (F2): builds the per-router tables and schedules the first beacons.
+    for (const node of this.stateValue.topologySnapshot.nodes) {
+      if (this.isRipEnabledRouter(node.id)) throw new NotPortedError('RIP');
+    }
+  }
+
+  private initializeDhcpState(): void {
+    // DHCP (F2): no DHCP configuration exists in the snapshot yet.
+  }
+
+  private initializeNatState(): void {
+    // Gateway NAT (F2): schedules the expiry sweep when a gateway exists.
+    if (this.stateValue.topologySnapshot.nodes.some((node) => node.kind === 'gateway')) {
+      throw new NotPortedError('gateway NAT');
+    }
+  }
+
+  private expireRipRoutes(): void {
+    // RIP (F2): marks timed-out learned routes with metric infinity.
+  }
+
+  private isRipEnabledRouter(nodeId: Id): boolean {
+    return (
+      this.nodeSnapshot(nodeId)?.kind === 'router' &&
+      this.stateValue.topologySnapshot.ripEnabledByNodeId.get(nodeId) === true
+    );
+  }
+
+  // ── Interfaces and addresses ─────────────────────────────────────────────
+
+  networkInterfaces(nodeId: Id): NetworkInterface[] {
+    const snapshot = this.stateValue.topologySnapshot;
+    const node = snapshot.nodes.find((candidate) => candidate.id === nodeId);
+    if (node === undefined) return [];
+    const deviceConfiguration = snapshot.deviceConfigurations.get(nodeId);
+    const result: NetworkInterface[] = [];
+    node.ports.forEach((port, index) => {
+      let configuration: InterfaceConfiguration | undefined;
+      switch (node.kind) {
+        case 'pc':
+        case 'notebook':
+          if (index !== 0 || deviceConfiguration === undefined) return;
+          configuration = { ipAddress: deviceConfiguration.ipAddress, subnetMask: deviceConfiguration.subnetMask };
+          break;
+        case 'router':
+        case 'gateway':
+          configuration = snapshot.interfaceConfigurations.get(interfaceKey(nodeId, port.id));
+          break;
+        case 'networkSwitch':
+        case 'remoteLink':
+        case 'unsupported':
+          configuration = undefined;
+          break;
+      }
+      if (configuration === undefined) return;
+      result.push({
+        nodeId,
+        portId: port.id,
+        index,
+        ipAddress: configuration.ipAddress,
+        subnetMask: configuration.subnetMask,
+        defaultGateway: deviceConfiguration?.defaultGateway ?? '',
+        macAddress: port.macAddress ?? stableMacAddress(port.id),
+      });
+    });
+    return result;
+  }
+
+  clearArpCache(nodeId?: Id): void {
+    if (nodeId === undefined) {
+      this.stateValue.arpCachesByNodeId.clear();
+    } else {
+      this.stateValue.arpCachesByNodeId.set(nodeId, new Map());
+    }
+  }
+
+  /**
+   * The MAC address for `targetIPAddress`, asking by ARP broadcast when it is
+   * not cached. Blocks like Java: the frames travel (virtual time passes)
+   * before the call returns.
+   */
+  resolveMacAddress(
+    nodeId: Id,
+    targetIPAddress: string,
+    options: { preferredInterfaceId?: Id | undefined; maxRetries?: number; selectInterfaceByAddress?: boolean } = {},
+  ): string | undefined {
+    const maxRetries = options.maxRetries ?? 2;
+    const selectInterfaceByAddress = options.selectInterfaceByAddress ?? true;
+    const interfaces = this.networkInterfaces(nodeId);
+    const primaryInterface = interfaces[0];
+    if (primaryInterface === undefined) return undefined;
+    if (targetIPAddress === LOCALHOST_IP_ADDRESS) return primaryInterface.macAddress;
+    const localInterface = interfaces.find((candidate) => candidate.ipAddress === targetIPAddress);
+    if (localInterface !== undefined) return localInterface.macAddress;
+    const cached = this.stateValue.arpCachesByNodeId.get(nodeId)?.get(targetIPAddress);
+    if (cached !== undefined) return cached.macAddress;
+
+    let selectedInterface: NetworkInterface | undefined;
+    if (options.preferredInterfaceId !== undefined) {
+      selectedInterface = interfaces.find((candidate) => candidate.portId === options.preferredInterfaceId);
+    } else if (selectInterfaceByAddress) {
+      selectedInterface = bestInterface(targetIPAddress, interfaces);
+    } else {
+      selectedInterface = interfaces[0];
+    }
+    if (selectedInterface === undefined) return undefined;
+
+    for (let attempt = 0; attempt < Math.max(0, maxRetries); attempt += 1) {
+      const request: ArpPacket = {
+        operation: 'request',
+        senderMacAddress: selectedInterface.macAddress,
+        senderIPAddress: selectedInterface.ipAddress,
+        targetMacAddress: UNSPECIFIED_MAC_ADDRESS,
+        targetIPAddress,
+      };
+      const frame: EthernetFrame = {
+        identity: this.allocateFrameIdentity(),
+        sourceMacAddress: selectedInterface.macAddress,
+        destinationMacAddress: ETHERNET_BROADCAST_MAC_ADDRESS,
+        payload: { kind: 'arp', packet: request },
+      };
+      this.recordTrace({
+        frameIdentity: frame.identity,
+        nodeId,
+        interfaceId: selectedInterface.portId,
+        direction: 'outbound',
+        layer: 'dataLink',
+        operation: 'created',
+        afterHeaders: ethernetHeaders(frame),
+        detail: `ARP request for ${targetIPAddress}`,
+      });
+      this.sendEthernetFrame(nodeId, selectedInterface.portId, frame);
+      this.advanceRemoteLinkEventsUntilArpCacheResolves(nodeId, targetIPAddress);
+      const resolved = this.stateValue.arpCachesByNodeId.get(nodeId)?.get(targetIPAddress);
+      if (resolved !== undefined) return resolved.macAddress;
+    }
+    return undefined;
+  }
+
+  private advanceRemoteLinkEventsUntilArpCacheResolves(nodeId: Id, targetIPAddress: string): void {
+    if (this.isProcessingScheduledEvents) return;
+    this.advancePendingRemoteLinkEvents(
+      () => this.stateValue.arpCachesByNodeId.get(nodeId)?.get(targetIPAddress) !== undefined,
+    );
+  }
+
+  private advanceRemoteLinkEventsUntilIcmpObservation(
+    nodeId: Id,
+    observationCount: number,
+    identifier: number,
+    sequenceNumber: number,
+  ): void {
+    if (this.isProcessingScheduledEvents) return;
+    this.advancePendingRemoteLinkEvents(() => {
+      const newObservations = (this.stateValue.icmpObservationsByNodeId.get(nodeId) ?? []).slice(observationCount);
+      return newObservations.some(
+        (observation) =>
+          observation.message.identifier === identifier && observation.message.sequenceNumber === sequenceNumber,
+      );
+    });
+  }
+
+  private advancePendingRemoteLinkEvents(condition: () => boolean): void {
+    const maximumEvents = 1_024;
+    let processedEventCount = 0;
+    while (!condition() && processedEventCount < maximumEvents) {
+      const nextDeadline = this.earliestDeadline(isRemoteLinkWaitEvent);
+      if (nextDeadline === undefined) return;
+      this.advance(nextDeadline);
+      processedEventCount += 1;
+    }
+  }
+
+  private earliestDeadline(filter: (kind: ScheduledEventKind) => boolean): number | undefined {
+    let earliest: number | undefined;
+    for (const event of this.stateValue.pendingEvents) {
+      if (!filter(event.kind)) continue;
+      if (earliest === undefined || event.deadlineMilliseconds < earliest) earliest = event.deadlineMilliseconds;
+    }
+    return earliest;
+  }
+
+  // ── Ethernet ─────────────────────────────────────────────────────────────
+
+  sendEthernetFrame(fromNodeId: Id, outgoingPortId: Id, frame: EthernetFrame): void {
+    this.recordTrace({
+      frameIdentity: frame.identity,
+      packetIdentity: packetIdentityInFrame(frame),
+      nodeId: fromNodeId,
+      interfaceId: outgoingPortId,
+      direction: 'outbound',
+      layer: 'physical',
+      operation: 'sent',
+      afterHeaders: ethernetHeaders(frame),
+    });
+    const endpoint = this.attachedEndpoint(fromNodeId, outgoingPortId);
+    if (endpoint === undefined) return;
+    const latencyMilliseconds = this.runtimeSpeed.linkTransmissionDelayMilliseconds;
+    const deadlineMilliseconds = this.stateValue.currentTimeMilliseconds + latencyMilliseconds;
+    if (!Number.isSafeInteger(deadlineMilliseconds)) {
+      this.recordTrace({
+        frameIdentity: frame.identity,
+        packetIdentity: packetIdentityInFrame(frame),
+        nodeId: fromNodeId,
+        interfaceId: outgoingPortId,
+        direction: 'outbound',
+        layer: 'physical',
+        operation: 'dropped',
+        afterHeaders: ethernetHeaders(frame),
+        detail: 'link transmission deadline overflow',
+      });
+      return;
+    }
+
+    this.schedule(deadlineMilliseconds, {
+      kind: 'ethernetLinkDelivery',
+      sourceNodeId: fromNodeId,
+      targetNodeId: endpoint.nodeId,
+      targetPortId: endpoint.portId,
+      latencyMilliseconds,
+      frame,
+    });
+    this.drainPendingNetworkDeliveryEventsIfNeeded();
+  }
+
+  private drainPendingNetworkDeliveryEventsIfNeeded(): void {
+    if (this.isProcessingScheduledEvents) return;
+    const maximumEvents = 16_384;
+    let processedEventCount = 0;
+    while (processedEventCount < maximumEvents) {
+      const nextDeadline = this.earliestDeadline(isNetworkDeliveryEvent);
+      if (nextDeadline === undefined) return;
+      this.advance(nextDeadline);
+      processedEventCount += 1;
+    }
+  }
+
+  private attachedEndpoint(nodeId: Id, portId: Id): { nodeId: Id; portId: Id } | undefined {
+    for (const link of this.stateValue.topologySnapshot.links) {
+      if (link.sourceNodeId === nodeId && link.sourcePortId === portId) {
+        return { nodeId: link.targetNodeId, portId: link.targetPortId };
+      }
+      if (link.targetNodeId === nodeId && link.targetPortId === portId) {
+        return { nodeId: link.sourceNodeId, portId: link.sourcePortId };
+      }
+    }
+    return undefined;
+  }
+
+  private nodeSnapshot(nodeId: Id): NodeSnapshot | undefined {
+    return this.stateValue.topologySnapshot.nodes.find((node) => node.id === nodeId);
+  }
+
+  private receiveEthernetFrame(frame: EthernetFrame, nodeId: Id, incomingPortId: Id): void {
+    this.recordTrace({
+      frameIdentity: frame.identity,
+      packetIdentity: packetIdentityInFrame(frame),
+      nodeId,
+      interfaceId: incomingPortId,
+      direction: 'inbound',
+      layer: 'physical',
+      operation: 'received',
+      beforeHeaders: ethernetHeaders(frame),
+    });
+    const node = this.nodeSnapshot(nodeId);
+    if (node === undefined) return;
+    if (node.kind === 'networkSwitch') {
+      this.forwardFrameThroughSwitch(frame, node, incomingPortId);
+      return;
+    }
+    if (node.kind === 'remoteLink') throw new NotPortedError('remote links');
+
+    this.recordTrace({
+      frameIdentity: frame.identity,
+      packetIdentity: packetIdentityInFrame(frame),
+      nodeId,
+      interfaceId: incomingPortId,
+      direction: 'inbound',
+      layer: 'dataLink',
+      operation: 'received',
+      beforeHeaders: ethernetHeaders(frame),
+    });
+    const networkInterface = this.networkInterfaces(nodeId).find((candidate) => candidate.portId === incomingPortId);
+    if (networkInterface === undefined) return;
+    const destination = frame.destinationMacAddress.toUpperCase();
+    if (destination !== networkInterface.macAddress.toUpperCase() && destination !== ETHERNET_BROADCAST_MAC_ADDRESS) {
+      this.recordTrace({
+        frameIdentity: frame.identity,
+        packetIdentity: packetIdentityInFrame(frame),
+        nodeId,
+        interfaceId: incomingPortId,
+        direction: 'inbound',
+        layer: 'dataLink',
+        operation: 'dropped',
+        beforeHeaders: ethernetHeaders(frame),
+        detail: 'destination MAC does not match interface',
+      });
+      return;
+    }
+    switch (frame.payload.kind) {
+      case 'arp':
+        this.receiveArpPacket(frame.payload.packet, frame.identity, nodeId, networkInterface);
+        return;
+      case 'ipv4':
+        this.receiveIPv4Packet(frame.payload.packet, frame.identity, nodeId, networkInterface);
+        return;
+    }
+  }
+
+  // ── Switch ───────────────────────────────────────────────────────────────
+
+  private forwardFrameThroughSwitch(frame: EthernetFrame, switchNode: NodeSnapshot, incomingPortId: Id): void {
+    const state = this.stateValue;
+    const seen = state.switchSeenFrameIdentitiesByNodeId.get(switchNode.id);
+    if (seen?.has(frame.identity) === true) {
+      this.recordTrace({
+        frameIdentity: frame.identity,
+        packetIdentity: packetIdentityInFrame(frame),
+        nodeId: switchNode.id,
+        interfaceId: incomingPortId,
+        direction: 'inbound',
+        layer: 'dataLink',
+        operation: 'dropped',
+        beforeHeaders: ethernetHeaders(frame),
+        detail: 'switch cycle suppression',
+      });
+      return;
+    }
+    getOrCreate(state.switchSeenFrameIdentitiesByNodeId, switchNode.id, () => new Set()).add(frame.identity);
+    this.ageSwitchSatEntries(switchNode.id);
+
+    const source = frame.sourceMacAddress.toUpperCase();
+    getOrCreate(state.switchForwardingTablesByNodeId, switchNode.id, () => new Map()).set(source, incomingPortId);
+    getOrCreate(state.switchForwardingUpdatedAtMillisecondsByNodeId, switchNode.id, () => new Map()).set(
+      source,
+      state.currentTimeMilliseconds,
+    );
+    this.recordTrace({
+      frameIdentity: frame.identity,
+      packetIdentity: packetIdentityInFrame(frame),
+      nodeId: switchNode.id,
+      interfaceId: incomingPortId,
+      direction: 'inbound',
+      layer: 'dataLink',
+      operation: 'accepted',
+      beforeHeaders: ethernetHeaders(frame),
+      detail: 'switch learned source MAC',
+    });
+
+    const destination = frame.destinationMacAddress.toUpperCase();
+    const connectedPorts = switchNode.ports
+      .filter((port) => port.id !== incomingPortId && this.attachedEndpoint(switchNode.id, port.id) !== undefined)
+      .map((port) => port.id);
+
+    let outgoingPorts: Id[];
+    let forwardingDetail: string;
+    const learnedPort = state.switchForwardingTablesByNodeId.get(switchNode.id)?.get(destination);
+    if (destination === ETHERNET_BROADCAST_MAC_ADDRESS) {
+      outgoingPorts = connectedPorts;
+      forwardingDetail = 'switch broadcast flooding';
+    } else if (learnedPort !== undefined) {
+      if (learnedPort === incomingPortId) {
+        this.recordTrace({
+          frameIdentity: frame.identity,
+          packetIdentity: packetIdentityInFrame(frame),
+          nodeId: switchNode.id,
+          interfaceId: incomingPortId,
+          direction: 'inbound',
+          layer: 'dataLink',
+          operation: 'dropped',
+          beforeHeaders: ethernetHeaders(frame),
+          detail: 'switch filtered destination on incoming port',
+        });
+        return;
+      }
+      if (this.attachedEndpoint(switchNode.id, learnedPort) !== undefined) {
+        outgoingPorts = [learnedPort];
+        forwardingDetail = 'switch learned unicast forwarding';
+      } else {
+        state.switchForwardingTablesByNodeId.get(switchNode.id)?.delete(destination);
+        state.switchForwardingUpdatedAtMillisecondsByNodeId.get(switchNode.id)?.delete(destination);
+        outgoingPorts = connectedPorts;
+        forwardingDetail = 'switch unknown unicast flooding';
+      }
+    } else {
+      outgoingPorts = connectedPorts;
+      forwardingDetail = 'switch unknown unicast flooding';
+    }
+
+    for (const portId of outgoingPorts) {
+      this.recordTrace({
+        frameIdentity: frame.identity,
+        packetIdentity: packetIdentityInFrame(frame),
+        nodeId: switchNode.id,
+        interfaceId: portId,
+        direction: 'outbound',
+        layer: 'dataLink',
+        operation: 'forwarded',
+        afterHeaders: ethernetHeaders(frame),
+        detail: forwardingDetail,
+      });
+      this.sendEthernetFrame(switchNode.id, portId, frame);
+    }
+  }
+
+  clearSwitchSat(nodeId: Id): void {
+    this.stateValue.switchForwardingTablesByNodeId.delete(nodeId);
+    this.stateValue.switchForwardingUpdatedAtMillisecondsByNodeId.delete(nodeId);
+  }
+
+  /** The switch's source address table (Java "SAT"), sorted by MAC address. */
+  switchSatEntries(nodeId: Id): SwitchSatEntry[] {
+    const node = this.nodeSnapshot(nodeId);
+    if (node === undefined || node.kind !== 'networkSwitch') return [];
+    const labelsByPortId = new Map(node.ports.map((port) => [port.id, port.label]));
+    const updatedAtByMac = this.stateValue.switchForwardingUpdatedAtMillisecondsByNodeId.get(nodeId) ?? new Map();
+    const table = this.stateValue.switchForwardingTablesByNodeId.get(nodeId) ?? new Map<string, Id>();
+    return [...table]
+      .map(([macAddress, portId]) => ({
+        macAddress,
+        portId,
+        portLabel: labelsByPortId.get(portId) ?? '?',
+        updatedAtMilliseconds: updatedAtByMac.get(macAddress) ?? 0,
+      }))
+      .sort((lhs, rhs) =>
+        lhs.macAddress === rhs.macAddress
+          ? compareIds(lhs.portId, rhs.portId)
+          : lhs.macAddress < rhs.macAddress
+            ? -1
+            : 1,
+      );
+  }
+
+  private ageAllSwitchSatEntries(): void {
+    for (const nodeId of [...this.stateValue.switchForwardingTablesByNodeId.keys()]) {
+      this.ageSwitchSatEntries(nodeId);
+    }
+  }
+
+  private ageSwitchSatEntries(nodeId: Id): void {
+    const state = this.stateValue;
+    const retention =
+      state.topologySnapshot.switchConfigurationsByNodeId.get(nodeId)?.retentionTimeMilliseconds ??
+      DEFAULT_SWITCH_RETENTION_MILLISECONDS;
+    const updatedAtByMac = state.switchForwardingUpdatedAtMillisecondsByNodeId.get(nodeId);
+    if (updatedAtByMac === undefined) return;
+    const expired: string[] = [];
+    for (const [macAddress, updatedAt] of updatedAtByMac) {
+      if (state.currentTimeMilliseconds >= updatedAt && state.currentTimeMilliseconds - updatedAt >= retention) {
+        expired.push(macAddress);
+      }
+    }
+    if (expired.length === 0) return;
+    for (const macAddress of expired) {
+      state.switchForwardingTablesByNodeId.get(nodeId)?.delete(macAddress);
+      state.switchForwardingUpdatedAtMillisecondsByNodeId.get(nodeId)?.delete(macAddress);
+    }
+    if (state.switchForwardingTablesByNodeId.get(nodeId)?.size === 0) {
+      state.switchForwardingTablesByNodeId.delete(nodeId);
+    }
+    if (state.switchForwardingUpdatedAtMillisecondsByNodeId.get(nodeId)?.size === 0) {
+      state.switchForwardingUpdatedAtMillisecondsByNodeId.delete(nodeId);
+    }
+  }
+
+  // ── ARP ──────────────────────────────────────────────────────────────────
+
+  private receiveArpPacket(
+    packet: ArpPacket,
+    frameIdentity: number,
+    nodeId: Id,
+    networkInterface: NetworkInterface,
+  ): void {
+    if (packet.senderIPAddress !== UNSPECIFIED_IP_ADDRESS) {
+      getOrCreate(this.stateValue.arpCachesByNodeId, nodeId, () => new Map()).set(packet.senderIPAddress, {
+        ipAddress: packet.senderIPAddress,
+        macAddress: packet.senderMacAddress,
+        updatedAtMilliseconds: this.stateValue.currentTimeMilliseconds,
+      });
+      this.flushIPv4TransmissionsAwaitingArp(nodeId, packet.senderIPAddress, packet.senderMacAddress);
+    }
+    this.recordTrace({
+      frameIdentity,
+      nodeId,
+      interfaceId: networkInterface.portId,
+      direction: 'inbound',
+      layer: 'network',
+      operation: 'accepted',
+      beforeHeaders: arpHeaders(packet),
+    });
+    if (packet.operation !== 'request' || packet.targetIPAddress !== networkInterface.ipAddress) return;
+
+    const unspecifiedSender = packet.senderIPAddress === UNSPECIFIED_IP_ADDRESS;
+    const replyTargetIPAddress = unspecifiedSender ? LIMITED_BROADCAST_IP_ADDRESS : packet.senderIPAddress;
+    const replyTargetMacAddress = unspecifiedSender ? ETHERNET_BROADCAST_MAC_ADDRESS : packet.senderMacAddress;
+    const reply: ArpPacket = {
+      operation: 'reply',
+      senderMacAddress: networkInterface.macAddress,
+      senderIPAddress: networkInterface.ipAddress,
+      targetMacAddress: replyTargetMacAddress,
+      targetIPAddress: replyTargetIPAddress,
+    };
+    const replyFrame: EthernetFrame = {
+      identity: this.allocateFrameIdentity(),
+      sourceMacAddress: networkInterface.macAddress,
+      destinationMacAddress: replyTargetMacAddress,
+      payload: { kind: 'arp', packet: reply },
+    };
+    this.recordTrace({
+      frameIdentity: replyFrame.identity,
+      nodeId,
+      interfaceId: networkInterface.portId,
+      direction: 'outbound',
+      layer: 'dataLink',
+      operation: 'created',
+      afterHeaders: ethernetHeaders(replyFrame),
+      detail: `ARP reply for ${networkInterface.ipAddress}`,
+    });
+    this.sendEthernetFrame(nodeId, networkInterface.portId, replyFrame);
+  }
+
+  // ── IPv4 ─────────────────────────────────────────────────────────────────
+
+  sendIPv4Packet(fromNodeId: Id, packet: IPv4Packet, preferredInterfaceId?: Id): IPv4DeliveryResult {
+    return this.sendIPv4PacketWithOptions(fromNodeId, packet, preferredInterfaceId, true, 'sent');
+  }
+
+  updateDeviceConfiguration(nodeId: Id, configuration: DeviceConfiguration): void {
+    const state = this.stateValue;
+    if (state.phase !== 'running' || !state.topologySnapshot.nodes.some((node) => node.id === nodeId)) return;
+    const deviceConfigurations = new Map(state.topologySnapshot.deviceConfigurations);
+    deviceConfigurations.set(nodeId, configuration);
+    state.topologySnapshot = withSnapshot(state.topologySnapshot, { deviceConfigurations });
+  }
+
+  /** Send one ICMP echo request and wait (virtually) for its answer. */
+  sendIcmpEcho(
+    fromNodeId: Id,
+    targetIPAddress: string,
+    options: { timeToLive?: number; identifier?: number; sequenceNumber?: number; data?: Uint8Array } = {},
+  ): IPv4DeliveryResult {
+    const timeToLive = options.timeToLive ?? 64;
+    const identifier = options.identifier ?? 1;
+    const sequenceNumber = options.sequenceNumber ?? 1;
+    const interfaces = this.networkInterfaces(fromNodeId);
+    const firstInterface = interfaces[0];
+    if (firstInterface === undefined) return { type: 'dropped', packetIdentity: 0, nodeId: fromNodeId };
+    const route = this.routeForPacket(fromNodeId, targetIPAddress);
+    let sourceInterface: NetworkInterface;
+    switch (route.type) {
+      case 'local':
+        sourceInterface = route.interface ?? firstInterface;
+        break;
+      case 'egress':
+        sourceInterface = route.interface;
+        break;
+      case 'broadcast':
+      case 'unavailable':
+        sourceInterface = firstInterface;
+        break;
+    }
+    const packet: IPv4Packet = {
+      identity: this.allocatePacketIdentity(),
+      senderIPAddress: sourceInterface.ipAddress,
+      receiverIPAddress: targetIPAddress,
+      timeToLive,
+      protocol: 'icmp',
+      payload: {
+        kind: 'icmp',
+        message: icmpMessage('echoRequest', { identifier, sequenceNumber, data: options.data ?? new Uint8Array() }),
+      },
+    };
+    this.recordTrace({
+      packetIdentity: packet.identity,
+      nodeId: fromNodeId,
+      interfaceId: sourceInterface.portId,
+      direction: 'outbound',
+      layer: 'network',
+      operation: 'created',
+      afterHeaders: ipv4Headers(packet),
+      detail: 'ICMP echo request',
+    });
+    const observationCount = this.stateValue.icmpObservationsByNodeId.get(fromNodeId)?.length ?? 0;
+    const result = this.sendIPv4Packet(fromNodeId, packet, sourceInterface.portId);
+    this.advanceRemoteLinkEventsUntilIcmpObservation(fromNodeId, observationCount, identifier, sequenceNumber);
+    const newObservations = (this.stateValue.icmpObservationsByNodeId.get(fromNodeId) ?? []).slice(observationCount);
+    const observation = newObservations.findLast(
+      (candidate) => candidate.message.identifier === identifier && candidate.message.sequenceNumber === sequenceNumber,
+    );
+    if (observation !== undefined) {
+      switch (observation.message.kind) {
+        case 'echoReply':
+          return { type: 'delivered', packetIdentity: packet.identity, nodeId: observation.nodeId };
+        case 'destinationNetworkUnreachable':
+        case 'destinationHostUnreachable':
+        case 'timeExceeded':
+          return {
+            type: 'icmpError',
+            packetIdentity: packet.identity,
+            nodeId: observation.nodeId,
+            kind: observation.message.kind,
+          };
+        case 'echoRequest':
+          break;
+      }
+    }
+    return result;
+  }
+
+  /** `traceroute`: echoes with TTL 1, 2, … until one is answered. */
+  traceIcmpEcho(
+    fromNodeId: Id,
+    targetIPAddress: string,
+    options: { maximumTimeToLive?: number; identifier?: number } = {},
+  ): IcmpObservation[] {
+    const maximumTimeToLive = options.maximumTimeToLive ?? 64;
+    const identifier = options.identifier ?? 1;
+    const observations: IcmpObservation[] = [];
+    for (let ttl = 1; ttl <= maximumTimeToLive; ttl += 1) {
+      const previousCount = this.stateValue.icmpObservationsByNodeId.get(fromNodeId)?.length ?? 0;
+      this.sendIcmpEcho(fromNodeId, targetIPAddress, { timeToLive: ttl, identifier, sequenceNumber: ttl });
+      const newItems = (this.stateValue.icmpObservationsByNodeId.get(fromNodeId) ?? []).slice(previousCount);
+      const observation = newItems.at(-1);
+      if (observation === undefined) continue;
+      observations.push(observation);
+      if (observation.message.kind === 'echoReply') break;
+    }
+    return observations;
+  }
+
+  /** The nodes a packet passed, in the order it first reached them. */
+  tracedNodePath(packetIdentity: number): Id[] {
+    const result: Id[] = [];
+    for (const trace of this.stateValue.packetTraces) {
+      if (trace.packetIdentity !== packetIdentity || result.includes(trace.nodeId)) continue;
+      result.push(trace.nodeId);
+    }
+    return result;
+  }
+
+  private sendIPv4PacketWithOptions(
+    fromNodeId: Id,
+    packet: IPv4Packet,
+    preferredInterfaceId: Id | undefined,
+    allowIcmpErrorGeneration: boolean,
+    operation: TraceOperation,
+  ): IPv4DeliveryResult {
+    const route = this.routeForPacket(fromNodeId, packet.receiverIPAddress, preferredInterfaceId);
+    switch (route.type) {
+      case 'local': {
+        const networkInterface = route.interface ?? this.networkInterfaces(fromNodeId)[0];
+        if (networkInterface === undefined) return { type: 'dropped', packetIdentity: packet.identity, nodeId: fromNodeId };
+        this.receiveIPv4Packet(packet, undefined, fromNodeId, networkInterface);
+        return { type: 'delivered', packetIdentity: packet.identity, nodeId: fromNodeId };
+      }
+
+      case 'broadcast': {
+        const interfaces = this.networkInterfaces(fromNodeId);
+        if (interfaces.length === 0) return { type: 'dropped', packetIdentity: packet.identity, nodeId: fromNodeId };
+        const broadcastPacket: IPv4Packet = { ...packet, timeToLive: 1 };
+        for (const networkInterface of interfaces) {
+          const frame: EthernetFrame = {
+            identity: this.allocateFrameIdentity(),
+            sourceMacAddress: networkInterface.macAddress,
+            destinationMacAddress: ETHERNET_BROADCAST_MAC_ADDRESS,
+            payload: { kind: 'ipv4', packet: broadcastPacket },
+          };
+          this.recordIPv4Send(broadcastPacket, frame, fromNodeId, networkInterface.portId, operation);
+          this.sendEthernetFrame(fromNodeId, networkInterface.portId, frame);
+        }
+        return { type: 'delivered', packetIdentity: packet.identity, nodeId: fromNodeId };
+      }
+
+      case 'egress': {
+        const destinationMacAddress = this.resolveMacAddress(fromNodeId, route.nextHop, {
+          preferredInterfaceId: route.interface.portId,
+          maxRetries: 2,
+        });
+        if (destinationMacAddress === undefined) {
+          if (this.isProcessingScheduledEvents && this.hasConfiguredInterface(route.nextHop)) {
+            this.enqueueIPv4TransmissionAwaitingArp(fromNodeId, route.nextHop, packet, route.interface, operation);
+            return { type: 'delivered', packetIdentity: packet.identity, nodeId: fromNodeId };
+          }
+          if (allowIcmpErrorGeneration) {
+            this.observeLocalIcmpError(fromNodeId, route.interface.portId, 'destinationHostUnreachable', packet);
+            return {
+              type: 'icmpError',
+              packetIdentity: packet.identity,
+              nodeId: fromNodeId,
+              kind: 'destinationHostUnreachable',
+            };
+          }
+          return { type: 'dropped', packetIdentity: packet.identity, nodeId: fromNodeId };
+        }
+        this.transmitIPv4Packet(fromNodeId, packet, route.interface, destinationMacAddress, operation);
+        return { type: 'delivered', packetIdentity: packet.identity, nodeId: fromNodeId };
+      }
+
+      case 'unavailable':
+        if (allowIcmpErrorGeneration) {
+          this.observeLocalIcmpError(fromNodeId, preferredInterfaceId, 'destinationNetworkUnreachable', packet);
+          return {
+            type: 'icmpError',
+            packetIdentity: packet.identity,
+            nodeId: fromNodeId,
+            kind: 'destinationNetworkUnreachable',
+          };
+        }
+        return { type: 'dropped', packetIdentity: packet.identity, nodeId: fromNodeId };
+    }
+  }
+
+  private hasConfiguredInterface(ipAddress: string): boolean {
+    return this.stateValue.topologySnapshot.nodes.some((node) =>
+      this.networkInterfaces(node.id).some((candidate) => candidate.ipAddress === ipAddress),
+    );
+  }
+
+  private enqueueIPv4TransmissionAwaitingArp(
+    fromNodeId: Id,
+    nextHop: string,
+    packet: IPv4Packet,
+    networkInterface: NetworkInterface,
+    operation: TraceOperation,
+  ): void {
+    const key = arpKey(fromNodeId, nextHop);
+    const transmission: PendingIPv4Transmission = { packet, interface: networkInterface, operation };
+    const pending = this.pendingIPv4TransmissionsByArpKey.get(key);
+    if (pending?.some((candidate) => deepEqual(candidate, transmission)) === true) return;
+    getOrCreate(this.pendingIPv4TransmissionsByArpKey, key, () => []).push(transmission);
+    this.recordTrace({
+      packetIdentity: packet.identity,
+      nodeId: fromNodeId,
+      interfaceId: networkInterface.portId,
+      direction: 'outbound',
+      layer: 'network',
+      operation: 'created',
+      beforeHeaders: ipv4Headers(packet),
+      detail: `waiting for ARP resolution of ${nextHop}`,
+    });
+  }
+
+  private flushIPv4TransmissionsAwaitingArp(nodeId: Id, ipAddress: string, macAddress: string): void {
+    const key = arpKey(nodeId, ipAddress);
+    const transmissions = this.pendingIPv4TransmissionsByArpKey.get(key);
+    if (transmissions === undefined) return;
+    this.pendingIPv4TransmissionsByArpKey.delete(key);
+    for (const transmission of transmissions) {
+      this.transmitIPv4Packet(nodeId, transmission.packet, transmission.interface, macAddress, transmission.operation);
+    }
+  }
+
+  private transmitIPv4Packet(
+    fromNodeId: Id,
+    packet: IPv4Packet,
+    networkInterface: NetworkInterface,
+    destinationMacAddress: string,
+    operation: TraceOperation,
+  ): void {
+    const frame: EthernetFrame = {
+      identity: this.allocateFrameIdentity(),
+      sourceMacAddress: networkInterface.macAddress,
+      destinationMacAddress,
+      payload: { kind: 'ipv4', packet },
+    };
+    this.recordIPv4Send(packet, frame, fromNodeId, networkInterface.portId, operation);
+    this.sendEthernetFrame(fromNodeId, networkInterface.portId, frame);
+  }
+
+  private receiveIPv4Packet(
+    packet: IPv4Packet,
+    frameIdentity: number | undefined,
+    nodeId: Id,
+    networkInterface: NetworkInterface,
+  ): void {
+    this.recordTrace({
+      frameIdentity,
+      packetIdentity: packet.identity,
+      nodeId,
+      interfaceId: networkInterface.portId,
+      direction: 'inbound',
+      layer: 'network',
+      operation: 'received',
+      beforeHeaders: ipv4Headers(packet),
+    });
+    const node = this.nodeSnapshot(nodeId);
+    if (node === undefined) return;
+
+    // Swift: the gateway branch (WAN/LAN, firewall, NAT rewrite) comes here.
+    if (node.kind === 'gateway') throw new NotPortedError('gateway NAT');
+    if (node.kind === 'router') {
+      const decision = this.evaluateFirewall(packet, nodeId, networkInterface.portId, frameIdentity);
+      if (!decision.accepted) return;
+    }
+    const packetForProcessing = packet;
+
+    const localAddresses = new Set(this.networkInterfaces(nodeId).map((candidate) => candidate.ipAddress));
+    if (
+      localAddresses.has(packetForProcessing.receiverIPAddress) ||
+      packetForProcessing.receiverIPAddress === LIMITED_BROADCAST_IP_ADDRESS
+    ) {
+      if (
+        isPcClassEndpoint(node.kind) &&
+        this.stateValue.topologySnapshot.firewallConfigurationsByNodeId.has(nodeId)
+      ) {
+        const decision = this.evaluateFirewall(packetForProcessing, nodeId, networkInterface.portId, frameIdentity);
+        if (!decision.accepted) return;
+      }
+      this.deliverIPv4Locally(packetForProcessing, nodeId, networkInterface);
+      return;
+    }
+    if (node.kind !== 'router') {
+      this.recordTrace({
+        frameIdentity,
+        packetIdentity: packetForProcessing.identity,
+        nodeId,
+        interfaceId: networkInterface.portId,
+        direction: 'inbound',
+        layer: 'network',
+        operation: 'dropped',
+        beforeHeaders: ipv4Headers(packetForProcessing),
+        detail: 'IPv4 forwarding disabled',
+      });
+      return;
+    }
+    if (packetForProcessing.timeToLive <= 1) {
+      this.emitIcmpError(nodeId, networkInterface, 'timeExceeded', packet);
+      return;
+    }
+
+    const forwarded = forwardingClone(packetForProcessing);
+    this.recordTrace({
+      frameIdentity,
+      packetIdentity: forwarded.identity,
+      nodeId,
+      interfaceId: networkInterface.portId,
+      direction: 'outbound',
+      layer: 'network',
+      operation: 'forwarded',
+      beforeHeaders: ipv4Headers(packetForProcessing),
+      afterHeaders: ipv4Headers(forwarded),
+      detail: 'TTL decremented',
+    });
+    const result = this.sendIPv4PacketWithOptions(nodeId, forwarded, undefined, false, 'forwarded');
+    if (result.type === 'dropped') {
+      const route = this.routeForPacket(nodeId, packetForProcessing.receiverIPAddress);
+      this.emitIcmpError(
+        nodeId,
+        networkInterface,
+        route.type === 'unavailable' ? 'destinationNetworkUnreachable' : 'destinationHostUnreachable',
+        packet,
+      );
+    }
+  }
+
+  private deliverIPv4Locally(packet: IPv4Packet, nodeId: Id, networkInterface: NetworkInterface): void {
+    const payload = packet.payload;
+    switch (payload.kind) {
+      case 'icmp': {
+        const message = payload.message;
+        switch (message.kind) {
+          case 'echoRequest': {
+            if (packet.receiverIPAddress === LIMITED_BROADCAST_IP_ADDRESS && packet.timeToLive !== 1) return;
+            const reply: IPv4Packet = {
+              identity: this.allocatePacketIdentity(),
+              senderIPAddress: networkInterface.ipAddress,
+              receiverIPAddress: packet.senderIPAddress,
+              timeToLive: 64,
+              protocol: 'icmp',
+              payload: {
+                kind: 'icmp',
+                message: icmpMessage('echoReply', {
+                  identifier: message.identifier,
+                  sequenceNumber: message.sequenceNumber,
+                  data: message.data,
+                }),
+              },
+            };
+            this.recordTrace({
+              packetIdentity: reply.identity,
+              nodeId,
+              interfaceId: networkInterface.portId,
+              direction: 'outbound',
+              layer: 'network',
+              operation: 'created',
+              afterHeaders: ipv4Headers(reply),
+              detail: 'ICMP echo reply',
+            });
+            this.sendIPv4PacketWithOptions(nodeId, reply, undefined, false, 'sent');
+            return;
+          }
+          case 'echoReply':
+          case 'destinationNetworkUnreachable':
+          case 'destinationHostUnreachable':
+          case 'timeExceeded':
+            getOrCreate(this.stateValue.icmpObservationsByNodeId, nodeId, () => []).push({
+              nodeId,
+              interfaceId: networkInterface.portId,
+              packet,
+              message,
+            });
+            this.recordTrace({
+              packetIdentity: packet.identity,
+              nodeId,
+              interfaceId: networkInterface.portId,
+              direction: 'local',
+              layer: 'transport',
+              operation: 'accepted',
+              beforeHeaders: icmpHeaders(message),
+              detail: 'ICMP observation queued',
+            });
+            return;
+        }
+        return;
+      }
+      case 'udp':
+        getOrCreate(this.stateValue.deliveredIPv4PacketsByNodeId, nodeId, () => []).push({
+          nodeId,
+          interfaceId: networkInterface.portId,
+          packet,
+        });
+        throw new NotPortedError('UDP delivery');
+      case 'tcp':
+        getOrCreate(this.stateValue.deliveredIPv4PacketsByNodeId, nodeId, () => []).push({
+          nodeId,
+          interfaceId: networkInterface.portId,
+          packet,
+        });
+        throw new NotPortedError('TCP delivery');
+    }
+  }
+
+  private emitIcmpError(
+    fromNodeId: Id,
+    incomingInterface: NetworkInterface,
+    kind: IcmpMessageKind,
+    originalPacket: IPv4Packet,
+  ): void {
+    if (
+      originalPacket.senderIPAddress === UNSPECIFIED_IP_ADDRESS ||
+      originalPacket.senderIPAddress === LIMITED_BROADCAST_IP_ADDRESS
+    ) {
+      return;
+    }
+    const errorPacket: IPv4Packet = {
+      identity: this.allocatePacketIdentity(),
+      senderIPAddress: incomingInterface.ipAddress,
+      receiverIPAddress: originalPacket.senderIPAddress,
+      timeToLive: 64,
+      protocol: 'icmp',
+      payload: {
+        kind: 'icmp',
+        message: icmpMessage(kind, {
+          identifier: icmpIdentifier(originalPacket),
+          sequenceNumber: icmpSequenceNumber(originalPacket),
+          embeddedOriginalPacket: embeddedOriginal(originalPacket),
+        }),
+      },
+    };
+    this.recordTrace({
+      packetIdentity: errorPacket.identity,
+      nodeId: fromNodeId,
+      interfaceId: incomingInterface.portId,
+      direction: 'outbound',
+      layer: 'network',
+      operation: 'created',
+      afterHeaders: ipv4Headers(errorPacket),
+      detail: `ICMP ${kind}`,
+    });
+    this.sendIPv4PacketWithOptions(fromNodeId, errorPacket, undefined, false, 'sent');
+  }
+
+  private observeLocalIcmpError(
+    nodeId: Id,
+    interfaceId: Id | undefined,
+    kind: IcmpMessageKind,
+    originalPacket: IPv4Packet,
+  ): void {
+    const message = icmpMessage(kind, {
+      identifier: icmpIdentifier(originalPacket),
+      sequenceNumber: icmpSequenceNumber(originalPacket),
+      embeddedOriginalPacket: embeddedOriginal(originalPacket),
+    });
+    const packet: IPv4Packet = {
+      identity: this.allocatePacketIdentity(),
+      senderIPAddress: this.networkInterfaces(nodeId)[0]?.ipAddress ?? UNSPECIFIED_IP_ADDRESS,
+      receiverIPAddress: originalPacket.senderIPAddress,
+      timeToLive: 64,
+      protocol: 'icmp',
+      payload: { kind: 'icmp', message },
+    };
+    getOrCreate(this.stateValue.icmpObservationsByNodeId, nodeId, () => []).push({
+      nodeId,
+      ...(interfaceId === undefined ? {} : { interfaceId }),
+      packet,
+      message,
+    });
+    this.recordTrace({
+      packetIdentity: packet.identity,
+      nodeId,
+      interfaceId,
+      direction: 'local',
+      layer: 'network',
+      operation: 'created',
+      afterHeaders: ipv4Headers(packet),
+      detail: `ICMP ${kind}`,
+    });
+  }
+
+  // ── Routing ──────────────────────────────────────────────────────────────
+
+  private routeForPacket(nodeId: Id, targetIPAddress: string, preferredInterfaceId?: Id): PacketRoute {
+    const interfaces = this.networkInterfaces(nodeId);
+    if (targetIPAddress === LIMITED_BROADCAST_IP_ADDRESS) return { type: 'broadcast' };
+    if (targetIPAddress === LOCALHOST_IP_ADDRESS) return { type: 'local', interface: interfaces[0] };
+    const local = interfaces.find((candidate) => candidate.ipAddress === targetIPAddress);
+    if (local !== undefined) return { type: 'local', interface: local };
+    if (this.isRipEnabledRouter(nodeId)) throw new NotPortedError('RIP routing');
+    if (preferredInterfaceId !== undefined) {
+      const preferred = interfaces.find((candidate) => candidate.portId === preferredInterfaceId);
+      if (preferred !== undefined && sameSubnet(preferred.ipAddress, targetIPAddress, preferred.subnetMask)) {
+        return { type: 'egress', interface: preferred, nextHop: targetIPAddress };
+      }
+    }
+
+    const snapshot = this.stateValue.topologySnapshot;
+    const rows = routeTableRows(
+      interfaces.map((candidate) => ({ ipAddress: candidate.ipAddress, subnetMask: candidate.subnetMask })),
+      snapshot.manualRoutesByNodeId.get(nodeId) ?? [],
+      snapshot.deviceConfigurations.get(nodeId)?.defaultGateway,
+    );
+    const target = parseIPv4(targetIPAddress);
+    if (target === undefined) return { type: 'unavailable' };
+    let selected: RouteRow | undefined;
+    let selectedMask: number | undefined;
+    for (const row of rows) {
+      const destination = parseIPv4(row.destinationNetwork);
+      const mask = parseIPv4(row.subnetMask);
+      if (destination === undefined || mask === undefined || destination !== maskAddress(target, mask)) continue;
+      if (selectedMask !== undefined && mask <= selectedMask) continue;
+      selected = row;
+      selectedMask = mask;
+    }
+    if (selected === undefined) return { type: 'unavailable' };
+    if (selected.interfaceIPAddress === LOCALHOST_IP_ADDRESS) {
+      return {
+        type: 'local',
+        interface: interfaces.find((candidate) => candidate.ipAddress === targetIPAddress) ?? interfaces[0],
+      };
+    }
+    const egress = interfaces.find((candidate) => candidate.ipAddress === selected.interfaceIPAddress);
+    if (egress === undefined) return { type: 'unavailable' };
+    const nextHop = sameSubnet(egress.ipAddress, targetIPAddress, egress.subnetMask)
+      ? targetIPAddress
+      : selected.nextHop;
+    return { type: 'egress', interface: egress, nextHop };
+  }
+
+  private recordIPv4Send(
+    packet: IPv4Packet,
+    frame: EthernetFrame,
+    nodeId: Id,
+    interfaceId: Id,
+    operation: TraceOperation,
+  ): void {
+    this.recordTrace({
+      frameIdentity: frame.identity,
+      packetIdentity: packet.identity,
+      nodeId,
+      interfaceId,
+      direction: 'outbound',
+      layer: 'network',
+      operation,
+      afterHeaders: ipv4Headers(packet),
+    });
+    this.recordTrace({
+      frameIdentity: frame.identity,
+      packetIdentity: packet.identity,
+      nodeId,
+      interfaceId,
+      direction: 'outbound',
+      layer: 'dataLink',
+      operation: 'created',
+      afterHeaders: ethernetHeaders(frame),
+    });
+  }
+
+  // ── Firewall ─────────────────────────────────────────────────────────────
+
+  setFirewallConfiguration(nodeId: Id, configuration: FirewallConfiguration): void {
+    const configurations = new Map(this.stateValue.topologySnapshot.firewallConfigurationsByNodeId);
+    configurations.set(nodeId, configuration);
+    this.stateValue.topologySnapshot = withSnapshot(this.stateValue.topologySnapshot, {
+      firewallConfigurationsByNodeId: configurations,
+    });
+  }
+
+  /**
+   * Java Firewall: inactive accepts; ICMP obeys only the general setting;
+   * TCP (optionally only opening SYNs) and UDP go through the ordered rules,
+   * first match wins; otherwise the default policy. Every decision is
+   * recorded, including "firewall inactive".
+   */
+  evaluateFirewall(packet: IPv4Packet, nodeId: Id, incomingInterfaceId?: Id, frameIdentity?: number): FirewallDecision {
+    const configuration =
+      this.stateValue.topologySnapshot.firewallConfigurationsByNodeId.get(nodeId) ?? firewallConfiguration();
+    const decide = (accepted: boolean, ruleIndex: number | undefined, detail: string): FirewallDecision =>
+      this.recordFirewallDecision(packet, frameIdentity, nodeId, incomingInterfaceId, accepted, ruleIndex, detail);
+
+    if (!configuration.isActive) return decide(true, undefined, 'firewall inactive');
+
+    if (packet.protocol === 'icmp') {
+      return decide(
+        !configuration.dropIcmp,
+        undefined,
+        configuration.dropIcmp ? 'ICMP blocked by general setting' : 'ICMP bypasses ordered rules',
+      );
+    }
+
+    const payload = packet.payload;
+    switch (payload.kind) {
+      case 'tcp': {
+        const flags = payload.segment.flags;
+        const isOpeningSyn = (flags & TCP_FLAG.synchronize) !== 0 && (flags & TCP_FLAG.acknowledgement) === 0;
+        if (configuration.filterSynSegmentsOnly && !isOpeningSyn) {
+          return decide(true, undefined, 'established TCP traffic bypasses SYN-only filtering');
+        }
+        for (const [index, rule] of configuration.rules.entries()) {
+          if (!firewallProtocolMatches(packet.protocol, rule)) continue;
+          if (
+            !this.firewallEndpointsMatch(
+              packet.senderIPAddress,
+              packet.receiverIPAddress,
+              payload.segment.destinationPort,
+              rule,
+              nodeId,
+            )
+          ) {
+            continue;
+          }
+          return decide(rule.action === 'accept', index, `TCP matched rule #${index + 1}`);
+        }
+        break;
+      }
+      case 'udp': {
+        if (!configuration.filterUdp) return decide(true, undefined, 'UDP filtering disabled');
+        const datagram = payload.datagram;
+        for (const [index, rule] of configuration.rules.entries()) {
+          if (!firewallProtocolMatches(packet.protocol, rule)) continue;
+          const requestMatches = this.firewallEndpointsMatch(
+            packet.senderIPAddress,
+            packet.receiverIPAddress,
+            datagram.destinationPort,
+            rule,
+            nodeId,
+          );
+          const responseMatches =
+            rule.port !== ALL_PORTS &&
+            this.firewallEndpointsMatch(
+              packet.receiverIPAddress,
+              packet.senderIPAddress,
+              datagram.sourcePort,
+              rule,
+              nodeId,
+            );
+          if (!requestMatches && !responseMatches) continue;
+          return decide(rule.action === 'accept', index, `UDP matched rule #${index + 1}`);
+        }
+        break;
+      }
+      case 'icmp':
+        break;
+    }
+
+    return decide(configuration.defaultPolicy === 'accept', undefined, 'firewall default policy');
+  }
+
+  private recordFirewallDecision(
+    packet: IPv4Packet,
+    frameIdentity: number | undefined,
+    nodeId: Id,
+    interfaceId: Id | undefined,
+    accepted: boolean,
+    ruleIndex: number | undefined,
+    detail: string,
+  ): FirewallDecision {
+    const decision: FirewallDecision = {
+      packetIdentity: packet.identity,
+      nodeId,
+      accepted,
+      ...(ruleIndex === undefined ? {} : { ruleIndex }),
+    };
+    this.stateValue.firewallDecisions.push(decision);
+    this.recordTrace({
+      frameIdentity,
+      packetIdentity: packet.identity,
+      nodeId,
+      interfaceId,
+      direction: 'inbound',
+      layer: 'network',
+      operation: accepted ? 'accepted' : 'dropped',
+      beforeHeaders: ipv4Headers(packet),
+      detail,
+    });
+    return decision;
+  }
+
+  private firewallEndpointsMatch(
+    sourceIPAddress: string,
+    destinationIPAddress: string,
+    port: number,
+    rule: FirewallRule,
+    nodeId: Id,
+  ): boolean {
+    return (
+      this.firewallSourceMatches(sourceIPAddress, rule, nodeId) &&
+      firewallAddressMatches(destinationIPAddress, rule.destinationIPAddress, rule.destinationSubnetMask) &&
+      (rule.port === ALL_PORTS || rule.port === port)
+    );
+  }
+
+  private firewallSourceMatches(sourceIPAddress: string, rule: FirewallRule, nodeId: Id): boolean {
+    if (rule.sourceIPAddress.length === 0) return true;
+    if (rule.sourceIPAddress === DIRECTLY_CONNECTED_SOURCE_MARKER) {
+      return this.networkInterfaces(nodeId).some((candidate) =>
+        sameSubnet(sourceIPAddress, candidate.ipAddress, candidate.subnetMask),
+      );
+    }
+    return firewallAddressMatches(sourceIPAddress, rule.sourceIPAddress, rule.sourceSubnetMask);
+  }
+
+  /** Replace the manual routes of a router (or gateway) while the simulation runs. */
+  replaceManualRoutes(nodeId: Id, routes: readonly ManualRoute[]): boolean {
+    const state = this.stateValue;
+    const node = state.topologySnapshot.nodes.find((candidate) => candidate.id === nodeId);
+    if (state.phase !== 'running' || node === undefined || (node.kind !== 'router' && node.kind !== 'gateway')) {
+      return false;
+    }
+    const routesByNodeId = new Map(state.topologySnapshot.manualRoutesByNodeId);
+    if (routes.length === 0) {
+      routesByNodeId.delete(nodeId);
+    } else {
+      routesByNodeId.set(nodeId, [...routes]);
+    }
+    state.topologySnapshot = withSnapshot(state.topologySnapshot, { manualRoutesByNodeId: routesByNodeId });
+    return true;
+  }
+}
+
+// ── Helpers ────────────────────────────────────────────────────────────────
+
+export function stableMacAddress(portId: Id): string {
+  return stableGeneratedMacAddress(portId);
+}
+
+function firewallProtocolMatches(protocol: IPv4Packet['protocol'], rule: FirewallRule): boolean {
+  return rule.protocol === 'all' || FIREWALL_PROTOCOL[rule.protocol] === IPV4_PROTOCOL[protocol];
+}
+
+function firewallAddressMatches(packetIPAddress: string, configuredIPAddress: string, subnetMask: string): boolean {
+  if (configuredIPAddress.length === 0) return true;
+  if (subnetMask.length === 0) return false;
+  return sameSubnet(packetIPAddress, configuredIPAddress, subnetMask);
+}
+
+function arpKey(nodeId: Id, ipAddress: string): string {
+  return `${nodeId}|${ipAddress}`;
+}
+
+function bestInterface(targetIPAddress: string, interfaces: readonly NetworkInterface[]): NetworkInterface | undefined {
+  let selected: NetworkInterface | undefined;
+  let selectedMask: number | undefined;
+  for (const candidate of interfaces) {
+    const mask = parseIPv4(candidate.subnetMask);
+    if (!sameSubnet(candidate.ipAddress, targetIPAddress, candidate.subnetMask) || mask === undefined) continue;
+    if (selectedMask !== undefined && mask <= selectedMask) continue;
+    selected = candidate;
+    selectedMask = mask;
+  }
+  return selected;
+}
+
+/** An ICMP error about an ICMP error carries the packet the first one was about. */
+function embeddedOriginal(packet: IPv4Packet): IPv4Packet {
+  if (packet.payload.kind === 'icmp' && packet.payload.message.embeddedOriginalPacket !== undefined) {
+    return packet.payload.message.embeddedOriginalPacket;
+  }
+  return packet;
+}
+
+function icmpIdentifier(packet: IPv4Packet): number {
+  return packet.payload.kind === 'icmp' ? packet.payload.message.identifier : 0;
+}
+
+function icmpSequenceNumber(packet: IPv4Packet): number {
+  return packet.payload.kind === 'icmp' ? packet.payload.message.sequenceNumber : 0;
+}
+
+export function ethernetHeaders(frame: EthernetFrame): HeaderField[] {
+  return [
+    { name: 'sourceMAC', value: frame.sourceMacAddress },
+    { name: 'destinationMAC', value: frame.destinationMacAddress },
+  ];
+}
+
+export function arpHeaders(packet: ArpPacket): HeaderField[] {
+  return [
+    { name: 'operation', value: packet.operation },
+    { name: 'senderMAC', value: packet.senderMacAddress },
+    { name: 'senderIP', value: packet.senderIPAddress },
+    { name: 'targetMAC', value: packet.targetMacAddress },
+    { name: 'targetIP', value: packet.targetIPAddress },
+  ];
+}
+
+export function ipv4Headers(packet: IPv4Packet): HeaderField[] {
+  return [
+    { name: 'senderIP', value: packet.senderIPAddress },
+    { name: 'receiverIP', value: packet.receiverIPAddress },
+    { name: 'ttl', value: String(packet.timeToLive) },
+    { name: 'protocol', value: String(IPV4_PROTOCOL[packet.protocol]) },
+  ];
+}
+
+export function icmpHeaders(message: IcmpMessage): HeaderField[] {
+  return [
+    { name: 'kind', value: message.kind },
+    { name: 'identifier', value: String(message.identifier) },
+    { name: 'sequence', value: String(message.sequenceNumber) },
+  ];
+}
diff --git a/packages/core/src/runtime/ipv4.ts b/packages/core/src/runtime/ipv4.ts
new file mode 100644
index 0000000..9395efe
--- /dev/null
+++ b/packages/core/src/runtime/ipv4.ts
@@ -0,0 +1,48 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift (parseIPv4, sameSubnet)
+//   State/TopologyEditorState.swift (TopologyRuntimeManualRoute.parseIPv4)
+//
+// Two parsers exist on purpose, as in the Swift source: the runtime reads an
+// octet the way Swift's `UInt8(String)` does, the route editor the way Java
+// FILIUS validates its table cells (see routeTable.ts).
+
+/**
+ * Swift `UInt8(segment)`: an optional sign, then decimal digits, value 0…255.
+ * Leading zeros are allowed ("007" is 7), whitespace is not.
+ */
+function parseSwiftOctet(segment: string): number | undefined {
+  if (!/^[+-]?[0-9]+$/.test(segment)) return undefined;
+  const value = Number(segment);
+  if (!Number.isInteger(value) || value < 0 || value > 255) return undefined;
+  return Object.is(value, -0) ? 0 : value;
+}
+
+/** Dotted quad to an unsigned 32-bit number, or undefined. */
+export function parseIPv4(value: string): number | undefined {
+  const segments = value.split('.');
+  if (segments.length !== 4) return undefined;
+  let address = 0;
+  for (const segment of segments) {
+    const octet = parseSwiftOctet(segment);
+    if (octet === undefined) return undefined;
+    address = ((address << 8) | octet) >>> 0;
+  }
+  return address;
+}
+
+export function formatIPv4(address: number): string {
+  return [24, 16, 8, 0].map((shift) => String((address >>> shift) & 0xff)).join('.');
+}
+
+export function sameSubnet(lhs: string, rhs: string, mask: string): boolean {
+  const left = parseIPv4(lhs);
+  const right = parseIPv4(rhs);
+  const netmask = parseIPv4(mask);
+  if (left === undefined || right === undefined || netmask === undefined) return false;
+  return ((left & netmask) >>> 0) === ((right & netmask) >>> 0);
+}
+
+/** `address & mask` as an unsigned number. */
+export function maskAddress(address: number, mask: number): number {
+  return (address & mask) >>> 0;
+}
diff --git a/packages/core/src/runtime/packets.ts b/packages/core/src/runtime/packets.ts
new file mode 100644
index 0000000..9c17509
--- /dev/null
+++ b/packages/core/src/runtime/packets.ts
@@ -0,0 +1,124 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift ("Java-observable packet and route models")
+// Frames and packets are typed values, not bytes — what the Java packet viewer
+// shows, not what a real wire would carry.
+
+export interface EthernetFrame {
+  readonly identity: number;
+  readonly sourceMacAddress: string;
+  readonly destinationMacAddress: string;
+  readonly payload: EthernetPayload;
+}
+
+export type EthernetPayload =
+  | { readonly kind: 'arp'; readonly packet: ArpPacket }
+  | { readonly kind: 'ipv4'; readonly packet: IPv4Packet };
+
+export type ArpOperation = 'request' | 'reply';
+
+export interface ArpPacket {
+  readonly operation: ArpOperation;
+  readonly senderMacAddress: string;
+  readonly senderIPAddress: string;
+  readonly targetMacAddress: string;
+  readonly targetIPAddress: string;
+}
+
+export const IPV4_PROTOCOL = { icmp: 1, tcp: 6, udp: 17 } as const;
+export type IPv4Protocol = keyof typeof IPV4_PROTOCOL;
+
+export interface IPv4Packet {
+  readonly identity: number;
+  readonly senderIPAddress: string;
+  readonly receiverIPAddress: string;
+  /** 0…255 */
+  readonly timeToLive: number;
+  readonly protocol: IPv4Protocol;
+  readonly payload: IPv4Payload;
+}
+
+export type IPv4Payload =
+  | { readonly kind: 'icmp'; readonly message: IcmpMessage }
+  | { readonly kind: 'tcp'; readonly segment: TcpSegment }
+  | { readonly kind: 'udp'; readonly datagram: UdpDatagram };
+
+/** The same packet one hop further: identity kept, TTL lowered, never below 0. */
+export function forwardingClone(packet: IPv4Packet, decrementingTtlBy = 1): IPv4Packet {
+  return {
+    ...packet,
+    timeToLive: decrementingTtlBy >= packet.timeToLive ? 0 : packet.timeToLive - decrementingTtlBy,
+  };
+}
+
+export type IcmpMessageKind =
+  | 'echoRequest'
+  | 'echoReply'
+  | 'destinationNetworkUnreachable'
+  | 'destinationHostUnreachable'
+  | 'timeExceeded';
+
+export interface IcmpMessage {
+  readonly kind: IcmpMessageKind;
+  /** 0…65535 */
+  readonly identifier: number;
+  /** 0…65535 */
+  readonly sequenceNumber: number;
+  readonly data: Uint8Array;
+  readonly embeddedOriginalPacket?: IPv4Packet;
+}
+
+export function icmpMessage(
+  kind: IcmpMessageKind,
+  fields: Partial<Omit<IcmpMessage, 'kind'>> = {},
+): IcmpMessage {
+  const message: IcmpMessage = {
+    kind,
+    identifier: fields.identifier ?? 0,
+    sequenceNumber: fields.sequenceNumber ?? 0,
+    data: fields.data ?? new Uint8Array(),
+  };
+  return fields.embeddedOriginalPacket === undefined
+    ? message
+    : { ...message, embeddedOriginalPacket: fields.embeddedOriginalPacket };
+}
+
+export const TCP_FLAG = {
+  finish: 1 << 0,
+  synchronize: 1 << 1,
+  reset: 1 << 2,
+  push: 1 << 3,
+  acknowledgement: 1 << 4,
+} as const;
+
+export type TcpSocketState =
+  | 'CLOSED'
+  | 'LISTEN'
+  | 'SYN_RCVD'
+  | 'SYN_SENT'
+  | 'ESTABLISHED'
+  | 'CLOSE_WAIT'
+  | 'LAST_ACK'
+  | 'FIN_WAIT_1'
+  | 'FIN_WAIT_2'
+  | 'CLOSING'
+  | 'TIME_WAIT';
+
+export interface TcpSegment {
+  readonly sourcePort: number;
+  readonly destinationPort: number;
+  readonly sequenceNumber: number;
+  readonly acknowledgementNumber: number;
+  /** Bit set of TCP_FLAG values. */
+  readonly flags: number;
+  readonly payload: Uint8Array;
+}
+
+export interface UdpDatagram {
+  readonly sourcePort: number;
+  readonly destinationPort: number;
+  readonly payload: Uint8Array;
+}
+
+export function packetIdentityInFrame(frame: EthernetFrame): number | undefined {
+  return frame.payload.kind === 'ipv4' ? frame.payload.packet.identity : undefined;
+}
diff --git a/packages/core/src/runtime/random.ts b/packages/core/src/runtime/random.ts
new file mode 100644
index 0000000..cd69fc4
--- /dev/null
+++ b/packages/core/src/runtime/random.ts
@@ -0,0 +1,58 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift (TopologyJavaCompatibleRandom)
+// A bit-exact java.util.Random: the 48-bit linear congruential generator
+// Java FILIUS draws its RIP beacon jitter and ephemeral ports from. BigInt
+// keeps the 48-bit arithmetic exact; a double would lose the low bits.
+
+const MULTIPLIER = 0x5deece66dn;
+const ADDEND = 0xbn;
+const MASK = (1n << 48n) - 1n;
+const INT32_MAX = 2147483647n;
+
+export class JavaCompatibleRandom {
+  private state: bigint;
+
+  constructor(seed: bigint | number) {
+    this.state = (BigInt.asUintN(64, BigInt(seed)) ^ MULTIPLIER) & MASK;
+  }
+
+  /** The internal 48-bit state, for tests and for copying a generator. */
+  get seed(): bigint {
+    return this.state;
+  }
+
+  clone(): JavaCompatibleRandom {
+    const copy = new JavaCompatibleRandom(0);
+    copy.state = this.state;
+    return copy;
+  }
+
+  next(bits: number): number {
+    if (!(bits > 0 && bits <= 32)) throw new RangeError(`bits out of range: ${bits}`);
+    this.state = (this.state * MULTIPLIER + ADDEND) & MASK;
+    return Number(this.state >> BigInt(48 - bits));
+  }
+
+  nextInt(upperBound: number): number {
+    if (!(upperBound > 0)) throw new RangeError(`upperBound must be positive: ${upperBound}`);
+    const bound = BigInt(upperBound);
+    if ((upperBound & -upperBound) === upperBound) {
+      return Number((bound * BigInt(this.next(31))) >> 31n);
+    }
+    for (;;) {
+      const bits = BigInt(this.next(31));
+      const value = bits % bound;
+      if (bits - value + (bound - 1n) <= INT32_MAX) return Number(value);
+    }
+  }
+
+  nextFloat(): number {
+    return Math.fround(this.next(24) / (1 << 24));
+  }
+
+  /** Java RIP: 30 s · (f/3 + 0.84), truncated — somewhere between 25.2 s and 35.2 s. */
+  nextRipJitterMilliseconds(): number {
+    const javaFloat = this.nextFloat();
+    return Math.trunc(30_000.0 * (javaFloat / 3.0 + 0.84));
+  }
+}
diff --git a/packages/core/src/runtime/routeTable.ts b/packages/core/src/runtime/routeTable.ts
new file mode 100644
index 0000000..ee6be1c
--- /dev/null
+++ b/packages/core/src/runtime/routeTable.ts
@@ -0,0 +1,222 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyJavaRouteTable.swift (row projection, validation, drafts)
+// Java counterpart: filius.software.vermittlungsschicht.Weiterleitungstabelle
+//   (holeTabelle: the order of the generated rows).
+// The Swift file also carries table geometry for SwiftUI; that belongs to the
+// editor and is not ported here.
+
+import { formatIPv4, maskAddress } from './ipv4.js';
+import type { InterfaceConfiguration, ManualRoute } from './config.js';
+
+export type RouteOrigin = 'localHost' | 'connected' | 'localhost' | 'manual' | 'defaultGateway' | 'rip';
+
+export interface RouteRow {
+  readonly destinationNetwork: string;
+  readonly subnetMask: string;
+  readonly nextHop: string;
+  readonly interfaceIPAddress: string;
+  readonly origin: RouteOrigin;
+  readonly isEditable: boolean;
+  readonly metric?: number;
+  readonly expiresAtMilliseconds?: number;
+}
+
+export type RouteTableColumn = 'destination' | 'subnetMask' | 'nextGateway' | 'interface';
+
+/**
+ * A Java table octet: digits only, leading zeros allowed, at most three
+ * significant digits, 0…255 ("001" is valid, " 1" and "+1" are not).
+ */
+function parseJavaOctet(segment: string): number | undefined {
+  if (segment.length === 0 || !/^[0-9]+$/.test(segment)) return undefined;
+  const significant = segment.replace(/^0+/, '');
+  if (significant.length === 0) return 0;
+  if (significant.length > 3) return undefined;
+  const value = Number(significant);
+  return value <= 255 ? value : undefined;
+}
+
+function parseJavaIPv4(value: string): number | undefined {
+  const segments = value.split('.');
+  if (segments.length !== 4) return undefined;
+  let address = 0;
+  for (const segment of segments) {
+    const octet = parseJavaOctet(segment);
+    if (octet === undefined) return undefined;
+    address = ((address << 8) | octet) >>> 0;
+  }
+  return address;
+}
+
+export function isValidJavaIPAddress(value: string): boolean {
+  return parseJavaIPv4(value) !== undefined;
+}
+
+/**
+ * FILIUS uses `musterSubNetz`, a dotted-decimal syntax check. It deliberately
+ * accepts non-contiguous masks in the route editor.
+ */
+export function isValidJavaSubnetMask(value: string): boolean {
+  return parseJavaIPv4(value) !== undefined;
+}
+
+export function isValidRouteCell(value: string, column: RouteTableColumn): boolean {
+  return column === 'subnetMask' ? isValidJavaSubnetMask(value) : isValidJavaIPAddress(value);
+}
+
+function sameNetwork(lhs: string, rhs: string, mask: string): boolean {
+  const left = parseJavaIPv4(lhs);
+  const right = parseJavaIPv4(rhs);
+  const netmask = parseJavaIPv4(mask);
+  if (left === undefined || right === undefined || netmask === undefined) return false;
+  return maskAddress(left, netmask) === maskAddress(right, netmask);
+}
+
+function networkAddress(ipAddress: string, subnetMask: string): string | undefined {
+  const address = parseJavaIPv4(ipAddress);
+  const mask = parseJavaIPv4(subnetMask);
+  if (address === undefined || mask === undefined) return undefined;
+  return formatIPv4(maskAddress(address, mask));
+}
+
+/**
+ * The static table as Java shows it: the interfaces' own addresses (newest
+ * first), their networks, loopback, the manual rows, then the default route.
+ */
+export function routeTableRows(
+  interfaceConfigurations: readonly InterfaceConfiguration[],
+  manualRoutes: readonly ManualRoute[],
+  defaultGateway: string | undefined,
+): RouteRow[] {
+  const validInterfaces = interfaceConfigurations.filter(
+    (configuration) =>
+      parseJavaIPv4(configuration.ipAddress) !== undefined &&
+      parseJavaIPv4(configuration.subnetMask) !== undefined,
+  );
+
+  const result: RouteRow[] = manualRoutes.map((route) => ({
+    destinationNetwork: route.destinationNetwork,
+    subnetMask: route.subnetMask,
+    nextHop: route.gateway,
+    interfaceIPAddress: route.interfaceIPAddress,
+    origin: 'manual',
+    isEditable: true,
+  }));
+
+  result.unshift({
+    destinationNetwork: '127.0.0.0',
+    subnetMask: '255.0.0.0',
+    nextHop: '127.0.0.1',
+    interfaceIPAddress: '127.0.0.1',
+    origin: 'localhost',
+    isEditable: false,
+  });
+
+  for (const configuration of validInterfaces) {
+    const network = networkAddress(configuration.ipAddress, configuration.subnetMask);
+    if (network === undefined) continue;
+    result.unshift({
+      destinationNetwork: network,
+      subnetMask: configuration.subnetMask,
+      nextHop: configuration.ipAddress,
+      interfaceIPAddress: configuration.ipAddress,
+      origin: 'connected',
+      isEditable: false,
+    });
+  }
+
+  for (const configuration of validInterfaces) {
+    result.unshift({
+      destinationNetwork: configuration.ipAddress,
+      subnetMask: '255.255.255.255',
+      nextHop: '127.0.0.1',
+      interfaceIPAddress: '127.0.0.1',
+      origin: 'localHost',
+      isEditable: false,
+    });
+  }
+
+  const trimmedGateway = defaultGateway?.trim() ?? '';
+  const primaryInterface = validInterfaces[0];
+  if (parseJavaIPv4(trimmedGateway) !== undefined && primaryInterface !== undefined) {
+    let selectedInterface = primaryInterface.ipAddress;
+    for (const configuration of validInterfaces) {
+      if (sameNetwork(trimmedGateway, configuration.ipAddress, configuration.subnetMask)) {
+        selectedInterface = configuration.ipAddress;
+      }
+    }
+    result.push({
+      destinationNetwork: '0.0.0.0',
+      subnetMask: '0.0.0.0',
+      nextHop: trimmedGateway,
+      interfaceIPAddress: selectedInterface,
+      origin: 'defaultGateway',
+      isEditable: false,
+    });
+  }
+
+  return result;
+}
+
+export function visibleRouteRows(rows: readonly RouteRow[], showAllEntries: boolean): RouteRow[] {
+  return showAllEntries ? [...rows] : rows.filter((row) => row.isEditable);
+}
+
+/** One row being edited in the route editor. */
+export interface RouteDraft {
+  readonly id: string;
+  destinationNetwork: string;
+  subnetMask: string;
+  nextGateway: string;
+  interfaceIPAddress: string;
+  isPendingInsertion: boolean;
+}
+
+export function blankRouteDraft(id: string): RouteDraft {
+  return {
+    id,
+    destinationNetwork: '',
+    subnetMask: '',
+    nextGateway: '',
+    interfaceIPAddress: '',
+    isPendingInsertion: true,
+  };
+}
+
+export function routeDraftFromRoute(id: string, route: ManualRoute): RouteDraft {
+  return {
+    id,
+    destinationNetwork: route.destinationNetwork,
+    subnetMask: route.subnetMask,
+    nextGateway: route.gateway,
+    interfaceIPAddress: route.interfaceIPAddress,
+    isPendingInsertion: false,
+  };
+}
+
+/** The route a draft stands for — only once all four cells are valid. */
+export function manualRouteFromDraft(draft: RouteDraft): ManualRoute | undefined {
+  if (
+    !isValidRouteCell(draft.destinationNetwork, 'destination') ||
+    !isValidRouteCell(draft.subnetMask, 'subnetMask') ||
+    !isValidRouteCell(draft.nextGateway, 'nextGateway') ||
+    !isValidRouteCell(draft.interfaceIPAddress, 'interface')
+  ) {
+    return undefined;
+  }
+  return {
+    destinationNetwork: draft.destinationNetwork,
+    subnetMask: draft.subnetMask,
+    gateway: draft.nextGateway,
+    interfaceIPAddress: draft.interfaceIPAddress,
+  };
+}
+
+/** At most one blank row waits at the end of the editor. */
+export function appendingBlankDraftIfNeeded(
+  drafts: readonly RouteDraft[],
+  newId: () => string,
+): RouteDraft[] {
+  if (drafts.some((draft) => draft.isPendingInsertion)) return [...drafts];
+  return [...drafts, blankRouteDraft(newId())];
+}
diff --git a/packages/core/src/runtime/snapshot.ts b/packages/core/src/runtime/snapshot.ts
new file mode 100644
index 0000000..7edb555
--- /dev/null
+++ b/packages/core/src/runtime/snapshot.ts
@@ -0,0 +1,90 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift (TopologyNetworkRuntimeTopologySnapshot)
+// What the runtime starts from: the topology and every configuration, frozen
+// at the moment the simulation starts. Later protocol slices add their
+// configuration maps here (DHCP, NAT, remote link, wireless).
+
+import { compareIds, type Id } from '../ids.js';
+import type { NodeKind } from '../model/node.js';
+import { canonicalMacAddress } from '../model/node.js';
+import type {
+  DeviceConfiguration,
+  FirewallConfiguration,
+  InterfaceConfiguration,
+  InterfaceKey,
+  ManualRoute,
+  SwitchConfiguration,
+} from './config.js';
+
+export interface PortSnapshot {
+  readonly id: Id;
+  readonly label: string;
+  /** Canonical, or undefined for a port whose MAC address is derived from its id. */
+  readonly macAddress?: string;
+}
+
+export function portSnapshot(id: Id, label: string, macAddress?: string): PortSnapshot {
+  const canonical = canonicalMacAddress(macAddress);
+  return canonical === undefined ? { id, label } : { id, label, macAddress: canonical };
+}
+
+export interface NodeSnapshot {
+  readonly id: Id;
+  readonly kind: NodeKind;
+  readonly ports: readonly PortSnapshot[];
+}
+
+export interface LinkSnapshot {
+  readonly id: Id;
+  readonly sourceNodeId: Id;
+  readonly sourcePortId: Id;
+  readonly targetNodeId: Id;
+  readonly targetPortId: Id;
+}
+
+export interface TopologySnapshot {
+  /** Sorted by id (Swift `uuidString` order). */
+  readonly nodes: readonly NodeSnapshot[];
+  /** Sorted by id (Swift `uuidString` order). */
+  readonly links: readonly LinkSnapshot[];
+  readonly deviceConfigurations: ReadonlyMap<Id, DeviceConfiguration>;
+  readonly interfaceConfigurations: ReadonlyMap<InterfaceKey, InterfaceConfiguration>;
+  readonly manualRoutesByNodeId: ReadonlyMap<Id, readonly ManualRoute[]>;
+  readonly ripEnabledByNodeId: ReadonlyMap<Id, boolean>;
+  readonly firewallConfigurationsByNodeId: ReadonlyMap<Id, FirewallConfiguration>;
+  readonly switchConfigurationsByNodeId: ReadonlyMap<Id, SwitchConfiguration>;
+}
+
+export interface TopologySnapshotInput {
+  nodes: readonly NodeSnapshot[];
+  links: readonly LinkSnapshot[];
+  deviceConfigurations?: ReadonlyMap<Id, DeviceConfiguration>;
+  interfaceConfigurations?: ReadonlyMap<InterfaceKey, InterfaceConfiguration>;
+  manualRoutesByNodeId?: ReadonlyMap<Id, readonly ManualRoute[]>;
+  ripEnabledByNodeId?: ReadonlyMap<Id, boolean>;
+  firewallConfigurationsByNodeId?: ReadonlyMap<Id, FirewallConfiguration>;
+  switchConfigurationsByNodeId?: ReadonlyMap<Id, SwitchConfiguration>;
+}
+
+export function topologySnapshot(input: TopologySnapshotInput): TopologySnapshot {
+  return {
+    nodes: [...input.nodes].sort((lhs, rhs) => compareIds(lhs.id, rhs.id)),
+    links: [...input.links].sort((lhs, rhs) => compareIds(lhs.id, rhs.id)),
+    deviceConfigurations: input.deviceConfigurations ?? new Map(),
+    interfaceConfigurations: input.interfaceConfigurations ?? new Map(),
+    manualRoutesByNodeId: input.manualRoutesByNodeId ?? new Map(),
+    ripEnabledByNodeId: input.ripEnabledByNodeId ?? new Map(),
+    firewallConfigurationsByNodeId: input.firewallConfigurationsByNodeId ?? new Map(),
+    switchConfigurationsByNodeId: input.switchConfigurationsByNodeId ?? new Map(),
+  };
+}
+
+/** The same snapshot with some fields replaced (Swift rebuilds it field by field). */
+export function withSnapshot(
+  snapshot: TopologySnapshot,
+  changes: Partial<TopologySnapshotInput>,
+): TopologySnapshot {
+  return topologySnapshot({ ...snapshot, ...changes });
+}
+
+export const EMPTY_TOPOLOGY_SNAPSHOT: TopologySnapshot = topologySnapshot({ nodes: [], links: [] });
diff --git a/packages/core/src/runtime/speed.ts b/packages/core/src/runtime/speed.ts
new file mode 100644
index 0000000..483359c
--- /dev/null
+++ b/packages/core/src/runtime/speed.ts
@@ -0,0 +1,33 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift (TopologyNetworkRuntimeSpeed)
+// Java counterpart: filius.hardware.Verbindung (MIN_VERZOEGERUNG_IN_MILLIS,
+//   verzoegerungsFaktor) and GUIMainMenu.updateLatency.
+
+export const MINIMUM_SPEED_PERCENT = 1;
+export const MAXIMUM_SPEED_PERCENT = 100;
+export const DEFAULT_SPEED_PERCENT = 90;
+export const JAVA_MINIMUM_LINK_DELAY_MILLISECONDS = 5;
+
+export class RuntimeSpeed {
+  readonly percent: number;
+
+  constructor(percent: number = DEFAULT_SPEED_PERCENT) {
+    this.percent = Math.min(MAXIMUM_SPEED_PERCENT, Math.max(MINIMUM_SPEED_PERCENT, Math.trunc(percent)));
+  }
+
+  /** Java's delay factor: 1 at full speed, 100 at the slowest. */
+  get javaDelayFactor(): number {
+    return MAXIMUM_SPEED_PERCENT - this.percent + 1;
+  }
+
+  /** How long a frame travels over one cable. */
+  get linkTransmissionDelayMilliseconds(): number {
+    return JAVA_MINIMUM_LINK_DELAY_MILLISECONDS * this.javaDelayFactor;
+  }
+
+  /** A base delay scaled like a cable, or undefined past the safe integer range. */
+  scaledLinkDelay(baseMilliseconds: number): number | undefined {
+    const delay = baseMilliseconds * this.javaDelayFactor;
+    return Number.isSafeInteger(delay) ? delay : undefined;
+  }
+}
diff --git a/packages/core/src/runtime/trace.ts b/packages/core/src/runtime/trace.ts
new file mode 100644
index 0000000..8cc42f3
--- /dev/null
+++ b/packages/core/src/runtime/trace.ts
@@ -0,0 +1,58 @@
+// Ported from filius-ipad (GPLv2/GPLv3):
+//   State/TopologyNetworkRuntimeEngine.swift (packet trace model)
+// Java counterpart: filius.rahmenprogramm.nachrichten.Lauscher — here every
+// step a frame or packet takes is one trace event; the packet viewer groups them.
+
+import type { Id } from '../ids.js';
+
+export type TraceDirection = 'inbound' | 'outbound' | 'local';
+
+export type TraceLayer = 'application' | 'transport' | 'network' | 'dataLink' | 'physical';
+
+export type TraceOperation =
+  | 'created'
+  | 'sent'
+  | 'received'
+  | 'forwarded'
+  | 'accepted'
+  | 'dropped'
+  | 'rewritten'
+  | 'retransmitted'
+  | 'compatibilityAdapter';
+
+export interface HeaderField {
+  readonly name: string;
+  readonly value: string;
+}
+
+export interface TraceEvent {
+  readonly id: number;
+  readonly timeMilliseconds: number;
+  readonly frameIdentity?: number;
+  readonly packetIdentity?: number;
+  readonly nodeId: Id;
+  readonly interfaceId?: Id;
+  readonly direction: TraceDirection;
+  readonly layer: TraceLayer;
+  readonly operation: TraceOperation;
+  readonly beforeHeaders: readonly HeaderField[];
+  readonly afterHeaders: readonly HeaderField[];
+  readonly detail?: string;
+}
+
+/** How many trace events the runtime keeps, and how many it drops at once when full. */
+export interface TraceRetentionPolicy {
+  readonly maximumEventCount: number;
+  readonly evictionBatchSize: number;
+}
+
+export function traceRetentionPolicy(maximumEventCount: number, evictionBatchSize: number): TraceRetentionPolicy {
+  if (!(maximumEventCount > 0)) throw new RangeError('Packet-trace retention must keep at least one event');
+  if (!(evictionBatchSize > 0)) throw new RangeError('Packet-trace eviction must remove at least one event');
+  if (evictionBatchSize > maximumEventCount) {
+    throw new RangeError('Packet-trace eviction cannot exceed the retention limit');
+  }
+  return { maximumEventCount, evictionBatchSize };
+}
+
+export const PRODUCTION_TRACE_RETENTION = traceRetentionPolicy(10_000, 1_000);
diff --git a/packages/core/src/util/equal.ts b/packages/core/src/util/equal.ts
new file mode 100644
index 0000000..60d6f53
--- /dev/null
+++ b/packages/core/src/util/equal.ts
@@ -0,0 +1,37 @@
+/**
+ * Structural equality for the runtime's value types — what Swift's synthesized
+ * `Equatable` gives a struct or enum for free. Handles plain objects, arrays,
+ * `Uint8Array`, `Map` and `Set`; everything else is compared with `Object.is`.
+ */
+export function deepEqual(lhs: unknown, rhs: unknown): boolean {
+  if (Object.is(lhs, rhs)) return true;
+  if (typeof lhs !== 'object' || typeof rhs !== 'object' || lhs === null || rhs === null) {
+    return false;
+  }
+  if (lhs instanceof Uint8Array || rhs instanceof Uint8Array) {
+    if (!(lhs instanceof Uint8Array && rhs instanceof Uint8Array)) return false;
+    return lhs.length === rhs.length && lhs.every((byte, index) => byte === rhs[index]);
+  }
+  if (Array.isArray(lhs) || Array.isArray(rhs)) {
+    if (!(Array.isArray(lhs) && Array.isArray(rhs)) || lhs.length !== rhs.length) return false;
+    return lhs.every((item, index) => deepEqual(item, rhs[index]));
+  }
+  if (lhs instanceof Map || rhs instanceof Map) {
+    if (!(lhs instanceof Map && rhs instanceof Map) || lhs.size !== rhs.size) return false;
+    for (const [key, value] of lhs) {
+      if (!rhs.has(key) || !deepEqual(value, rhs.get(key))) return false;
+    }
+    return true;
+  }
+  if (lhs instanceof Set || rhs instanceof Set) {
+    if (!(lhs instanceof Set && rhs instanceof Set) || lhs.size !== rhs.size) return false;
+    for (const value of lhs) if (!rhs.has(value)) return false;
+    return true;
+  }
+  const leftKeys = Object.keys(lhs).filter((key) => (lhs as Record<string, unknown>)[key] !== undefined);
+  const rightKeys = Object.keys(rhs).filter((key) => (rhs as Record<string, unknown>)[key] !== undefined);
+  if (leftKeys.length !== rightKeys.length) return false;
+  return leftKeys.every((key) =>
+    deepEqual((lhs as Record<string, unknown>)[key], (rhs as Record<string, unknown>)[key]),
+  );
+}
diff --git a/packages/core/test/engine.test.ts b/packages/core/test/engine.test.ts
new file mode 100644
index 0000000..e60489a
--- /dev/null
+++ b/packages/core/test/engine.test.ts
@@ -0,0 +1,631 @@
+// Ported from filius-ipad ios/FiliusPadTests/TopologyNetworkRuntimeEngineTests.swift:
+// the event queue, the virtual clock, Java-compatible randomness, restart,
+// Ethernet/switch/ARP/IPv4/ICMP, routing errors, the firewall, packet loss
+// and trace retention. The remaining Swift tests (UDP, TCP, DHCP, DNS, RIP,
+// NAT, remote link, packet viewer) follow with their protocol slices.
+
+import { describe, expect, it } from 'vitest';
+import {
+  EMPTY_TOPOLOGY_SNAPSHOT,
+  ETHERNET_BROADCAST_MAC_ADDRESS,
+  firewallConfiguration,
+  firewallRule,
+  DIRECTLY_CONNECTED_SOURCE_MARKER,
+  forwardingClone,
+  icmpMessage,
+  interfaceKey,
+  JavaCompatibleRandom,
+  NetworkRuntimeEngine,
+  portSnapshot,
+  stableMacAddress,
+  switchConfiguration,
+  TCP_FLAG,
+  topologySnapshot,
+  traceRetentionPolicy,
+  UNSPECIFIED_MAC_ADDRESS,
+  type EthernetFrame,
+  type EthernetPayload,
+  type FirewallConfiguration,
+  type IPv4Packet,
+  type NodeSnapshot,
+  type RuntimeOutput,
+  type ScheduledEventKind,
+} from '../src/index.js';
+import { device, link, pc, startedEngine, uuid } from './fixtures.js';
+
+const marker = (label: string): ScheduledEventKind => ({ kind: 'parityMarker', label });
+
+function firedKinds(outputs: RuntimeOutput[]): ScheduledEventKind[] {
+  return outputs.flatMap((output) => (output.kind.type === 'fired' ? [output.kind.event.kind] : []));
+}
+
+function routedTopology() {
+  const sourceNodeId = uuid(0x1451);
+  const sourcePortId = uuid(0x1461);
+  const routerNodeId = uuid(0x1452);
+  const routerPortA = uuid(0x1462);
+  const routerPortB = uuid(0x1463);
+  const targetNodeId = uuid(0x1453);
+  const targetPortId = uuid(0x1464);
+  const router: NodeSnapshot = {
+    id: routerNodeId,
+    kind: 'router',
+    ports: [portSnapshot(routerPortA, 'rt1'), portSnapshot(routerPortB, 'rt2')],
+  };
+  const snapshot = topologySnapshot({
+    nodes: [pc(sourceNodeId, sourcePortId), router, pc(targetNodeId, targetPortId)],
+    links: [
+      link(sourceNodeId, sourcePortId, routerNodeId, routerPortA),
+      link(routerNodeId, routerPortB, targetNodeId, targetPortId),
+    ],
+    deviceConfigurations: new Map([
+      [sourceNodeId, device('10.0.0.2', { gateway: '10.0.0.1' })],
+      [routerNodeId, device('10.0.0.1')],
+      [targetNodeId, device('20.0.0.2', { gateway: '20.0.0.1' })],
+    ]),
+    interfaceConfigurations: new Map([
+      [interfaceKey(routerNodeId, routerPortA), { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
+      [interfaceKey(routerNodeId, routerPortB), { ipAddress: '20.0.0.1', subnetMask: '255.255.255.0' }],
+    ]),
+  });
+  const engine = new NetworkRuntimeEngine(14);
+  engine.handle({ type: 'start', snapshot, seed: 14, initialTimeMilliseconds: 0 });
+  return { engine, sourceNodeId, routerNodeId };
+}
+
+function firewallEngine(configuration: FirewallConfiguration) {
+  const routerId = uuid(0x1802);
+  const portId = uuid(0x1812);
+  const engine = startedEngine(
+    {
+      nodes: [{ id: routerId, kind: 'router', ports: [portSnapshot(portId, 'rt1')] }],
+      links: [],
+      interfaceConfigurations: new Map([
+        [interfaceKey(routerId, portId), { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
+      ]),
+      firewallConfigurationsByNodeId: new Map([[routerId, configuration]]),
+    },
+    18,
+  );
+  return { engine, routerId };
+}
+
+function tcpPacket(identity: number, destinationPort: number, flags: number): IPv4Packet {
+  return {
+    identity,
+    senderIPAddress: '203.0.113.10',
+    receiverIPAddress: '10.0.0.1',
+    timeToLive: 64,
+    protocol: 'tcp',
+    payload: {
+      kind: 'tcp',
+      segment: {
+        sourcePort: 50_000,
+        destinationPort,
+        sequenceNumber: identity,
+        acknowledgementNumber: 0,
+        flags,
+        payload: new Uint8Array(),
+      },
+    },
+  };
+}
+
+describe('event queue and virtual clock', () => {
+  it('orders events by deadline, then by insertion sequence', () => {
+    const engine = new NetworkRuntimeEngine(42);
+    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 42, initialTimeMilliseconds: 0 });
+    engine.handle({ type: 'schedule', deadlineMilliseconds: 100, kind: marker('first-at-100') });
+    engine.handle({ type: 'schedule', deadlineMilliseconds: 50, kind: marker('at-50') });
+    engine.handle({ type: 'schedule', deadlineMilliseconds: 100, kind: marker('second-at-100') });
+
+    const outputs = engine.handle({ type: 'advance', toMilliseconds: 100 });
+
+    expect(firedKinds(outputs)).toEqual([marker('at-50'), marker('first-at-100'), marker('second-at-100')]);
+    expect(engine.state.currentTimeMilliseconds).toBe(100);
+    expect(engine.state.pendingEvents).toEqual([]);
+  });
+
+  it('rejects a past time without changing the clock', () => {
+    const engine = new NetworkRuntimeEngine(7);
+    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 7, initialTimeMilliseconds: 10 });
+    engine.handle({ type: 'advance', toMilliseconds: 25 });
+
+    const outputs = engine.handle({ type: 'advance', toMilliseconds: 24 });
+
+    expect(outputs.map((output) => output.kind)).toEqual([{ type: 'advanceRejectedPastTime' }]);
+    expect(engine.state.currentTimeMilliseconds).toBe(25);
+  });
+
+  it('rejects a past deadline without using up a sequence number', () => {
+    const engine = new NetworkRuntimeEngine(8);
+    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 8, initialTimeMilliseconds: 25 });
+
+    const outputs = engine.handle({ type: 'schedule', deadlineMilliseconds: 24, kind: marker('past') });
+
+    expect(outputs.map((output) => output.kind)).toEqual([
+      { type: 'scheduleRejectedPastTime', deadlineMilliseconds: 24 },
+    ]);
+    expect(engine.state.pendingEvents).toEqual([]);
+    expect(engine.state.nextEventSequenceNumber).toBe(0);
+    expect(engine.state.currentTimeMilliseconds).toBe(25);
+  });
+
+  it('replays a named scenario straight to a stable state', () => {
+    const finalState = NetworkRuntimeEngine.run({
+      name: 'equal-deadline-order',
+      seed: 1,
+      advanceToMilliseconds: 1_000,
+      scheduledEvents: [
+        { deadlineMilliseconds: 500, kind: marker('a') },
+        { deadlineMilliseconds: 500, kind: marker('b') },
+      ],
+    });
+
+    expect(finalState.currentTimeMilliseconds).toBe(1_000);
+    expect(finalState.pendingEvents).toEqual([]);
+    expect(finalState.seed).toBe(1n);
+  });
+
+  it('clears transient state on restart and numbers packets from one again', () => {
+    const nodeId = uuid(0x12);
+    const engine = new NetworkRuntimeEngine(12);
+    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 12, initialTimeMilliseconds: 0 });
+    engine.handle({ type: 'schedule', deadlineMilliseconds: 40, kind: marker('stale') });
+    expect(engine.allocateFrameIdentity()).toBe(1);
+    expect(engine.allocatePacketIdentity()).toBe(1);
+    engine.recordTrace({ packetIdentity: 1, nodeId, direction: 'local', layer: 'application', operation: 'created' });
+    engine.handle({ type: 'stop' });
+
+    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 12, initialTimeMilliseconds: 0 });
+
+    const state = engine.state;
+    expect(state.pendingEvents).toEqual([]);
+    expect(state.packetTraces).toEqual([]);
+    expect(state.arpCachesByNodeId.size).toBe(0);
+    expect(state.switchForwardingTablesByNodeId.size).toBe(0);
+    expect(state.switchSeenFrameIdentitiesByNodeId.size).toBe(0);
+    expect(state.deliveredIPv4PacketsByNodeId.size).toBe(0);
+    expect(state.icmpObservationsByNodeId.size).toBe(0);
+    expect(state.firewallDecisions).toEqual([]);
+    expect(engine.allocateFrameIdentity()).toBe(1);
+    expect(engine.allocatePacketIdentity()).toBe(1);
+  });
+});
+
+describe('Java-compatible randomness', () => {
+  it('draws the same RIP jitter as java.util.Random', () => {
+    const random = new JavaCompatibleRandom(42);
+
+    expect(Array.from({ length: 5 }, () => random.nextRipJitterMilliseconds())).toEqual([
+      32_475, 25_746, 32_032, 25_679, 28_287,
+    ]);
+  });
+
+  it('matches java.util.Random.nextInt for bounded and power-of-two ranges', () => {
+    // Measured on OpenJDK 21 (29.09.2026): new java.util.Random(42),
+    // nextInt(100) three times, then nextInt(16) three times.
+    const random = new JavaCompatibleRandom(42);
+
+    expect([random.nextInt(100), random.nextInt(100), random.nextInt(100)]).toEqual([30, 63, 48]);
+    expect([random.nextInt(16), random.nextInt(16), random.nextInt(16)]).toEqual([0, 4, 15]);
+  });
+});
+
+describe('IPv4 packets', () => {
+  it('keeps the identity and lowers the TTL when forwarding', () => {
+    const packet: IPv4Packet = {
+      identity: 99,
+      senderIPAddress: '192.168.0.10',
+      receiverIPAddress: '192.168.1.20',
+      timeToLive: 2,
+      protocol: 'icmp',
+      payload: { kind: 'icmp', message: icmpMessage('echoRequest', { identifier: 3, sequenceNumber: 4 }) },
+    };
+
+    const forwarded = forwardingClone(packet);
+    const expired = forwardingClone(forwarded);
+
+    expect(forwarded).toMatchObject({ identity: 99, timeToLive: 1 });
+    expect(expired).toMatchObject({ identity: 99, timeToLive: 0 });
+  });
+});
+
+describe('Ethernet, ARP and ICMP', () => {
+  it('fills both ARP caches with one request and reply over a direct cable', () => {
+    const sourceNodeId = uuid(0x1401);
+    const sourcePortId = uuid(0x1411);
+    const targetNodeId = uuid(0x1402);
+    const targetPortId = uuid(0x1412);
+    const engine = startedEngine({
+      nodes: [pc(sourceNodeId, sourcePortId), pc(targetNodeId, targetPortId)],
+      links: [link(sourceNodeId, sourcePortId, targetNodeId, targetPortId)],
+      deviceConfigurations: new Map([
+        [sourceNodeId, device('192.168.0.10')],
+        [targetNodeId, device('192.168.0.20')],
+      ]),
+    });
+
+    const resolved = engine.resolveMacAddress(sourceNodeId, '192.168.0.20');
+
+    expect(resolved).toBe(stableMacAddress(targetPortId));
+    expect(engine.state.arpCachesByNodeId.get(sourceNodeId)?.get('192.168.0.20')?.macAddress).toBe(resolved);
+    expect(engine.state.arpCachesByNodeId.get(targetNodeId)?.get('192.168.0.10')?.macAddress).toBe(
+      stableMacAddress(sourcePortId),
+    );
+  });
+
+  it('pings across a switch with Ethernet, ARP and ICMP packets', () => {
+    const sourceNodeId = uuid(0x1421);
+    const sourcePortId = uuid(0x1431);
+    const switchNodeId = uuid(0x1422);
+    const switchPortA = uuid(0x1432);
+    const switchPortB = uuid(0x1433);
+    const targetNodeId = uuid(0x1423);
+    const targetPortId = uuid(0x1434);
+    const switchNode: NodeSnapshot = {
+      id: switchNodeId,
+      kind: 'networkSwitch',
+      ports: [portSnapshot(switchPortA, 'sw1'), portSnapshot(switchPortB, 'sw2')],
+    };
+    const engine = startedEngine({
+      nodes: [pc(sourceNodeId, sourcePortId), switchNode, pc(targetNodeId, targetPortId)],
+      links: [
+        link(sourceNodeId, sourcePortId, switchNodeId, switchPortA),
+        link(switchNodeId, switchPortB, targetNodeId, targetPortId),
+      ],
+      deviceConfigurations: new Map([
+        [sourceNodeId, device('192.168.0.10')],
+        [targetNodeId, device('192.168.0.20')],
+      ]),
+    });
+
+    const result = engine.sendIcmpEcho(sourceNodeId, '192.168.0.20', { identifier: 14, sequenceNumber: 1 });
+
+    expect(result.type).toBe('delivered');
+    expect(engine.state.icmpObservationsByNodeId.get(sourceNodeId)?.at(-1)?.message.kind).toBe('echoReply');
+    expect(engine.state.switchForwardingTablesByNodeId.get(switchNodeId)?.size).toBe(2);
+    expect(engine.state.packetTraces.some((trace) => trace.detail === 'ARP request for 192.168.0.20')).toBe(true);
+    expect(
+      engine.state.packetTraces.some((trace) => trace.packetIdentity !== undefined && trace.layer === 'network'),
+    ).toBe(true);
+  });
+
+  it('answers an expired TTL at the router with time exceeded and the original packet', () => {
+    const topology = routedTopology();
+
+    const result = topology.engine.sendIcmpEcho(topology.sourceNodeId, '20.0.0.2', {
+      timeToLive: 1,
+      identifier: 140,
+      sequenceNumber: 1,
+    });
+
+    expect(result).toMatchObject({ type: 'icmpError', kind: 'timeExceeded' });
+    const observation = topology.engine.state.icmpObservationsByNodeId.get(topology.sourceNodeId)?.at(-1);
+    expect(observation?.message.kind).toBe('timeExceeded');
+    expect(observation?.message.embeddedOriginalPacket?.timeToLive).toBe(1);
+    expect(observation?.message.embeddedOriginalPacket?.receiverIPAddress).toBe('20.0.0.2');
+  });
+
+  it('keeps the packet identity and lowers the TTL when a router forwards', () => {
+    const topology = routedTopology();
+
+    const result = topology.engine.sendIcmpEcho(topology.sourceNodeId, '20.0.0.2', {
+      timeToLive: 8,
+      identifier: 141,
+      sequenceNumber: 1,
+    });
+
+    expect(result.type).toBe('delivered');
+    const forwardingTrace = topology.engine.state.packetTraces.find(
+      (trace) =>
+        trace.nodeId === topology.routerNodeId && trace.operation === 'forwarded' && trace.detail === 'TTL decremented',
+    );
+    expect(forwardingTrace?.packetIdentity).toBeDefined();
+    expect(forwardingTrace?.beforeHeaders.find((field) => field.name === 'ttl')?.value).toBe('8');
+    expect(forwardingTrace?.afterHeaders.find((field) => field.name === 'ttl')?.value).toBe('7');
+  });
+
+  it('reports network unreachable when the router has no route', () => {
+    const topology = routedTopology();
+    const snapshot = topology.engine.state.topologySnapshot;
+    const sourceToRouter = snapshot.links.find(
+      (candidate) => candidate.sourceNodeId === topology.sourceNodeId || candidate.targetNodeId === topology.sourceNodeId,
+    );
+    const engine = new NetworkRuntimeEngine(14);
+    engine.handle({
+      type: 'start',
+      snapshot: topologySnapshot({
+        nodes: snapshot.nodes,
+        links: sourceToRouter === undefined ? [] : [sourceToRouter],
+        deviceConfigurations: snapshot.deviceConfigurations,
+        interfaceConfigurations: snapshot.interfaceConfigurations,
+        manualRoutesByNodeId: snapshot.manualRoutesByNodeId,
+      }),
+      seed: 14,
+      initialTimeMilliseconds: 0,
+    });
+
+    const result = engine.sendIcmpEcho(topology.sourceNodeId, '30.0.0.2', { identifier: 142, sequenceNumber: 1 });
+
+    expect(result).toMatchObject({ type: 'icmpError', kind: 'destinationNetworkUnreachable' });
+  });
+
+  it('reports host unreachable when a connected target never answers ARP', () => {
+    const sourceNodeId = uuid(0x1481);
+    const sourcePortId = uuid(0x1491);
+    const engine = startedEngine({
+      nodes: [pc(sourceNodeId, sourcePortId)],
+      links: [],
+      deviceConfigurations: new Map([[sourceNodeId, device('192.168.50.10')]]),
+    });
+
+    const result = engine.sendIcmpEcho(sourceNodeId, '192.168.50.99', { identifier: 143, sequenceNumber: 1 });
+
+    expect(result).toMatchObject({ type: 'icmpError', kind: 'destinationHostUnreachable' });
+  });
+});
+
+describe('switch', () => {
+  it('learns, floods, forwards, filters and ages its address table', () => {
+    const hostAId = uuid(0x1a01);
+    const hostAPortId = uuid(0x1a11);
+    const hostBId = uuid(0x1a02);
+    const hostBPortId = uuid(0x1a12);
+    const hostCId = uuid(0x1a03);
+    const hostCPortId = uuid(0x1a13);
+    const switchId = uuid(0x1a04);
+    const switchPorts = [1, 2, 3].map((index) => portSnapshot(uuid(0x1a20 + index), `sw${index}`));
+    const engine = startedEngine(
+      {
+        nodes: [
+          pc(hostAId, hostAPortId),
+          pc(hostBId, hostBPortId),
+          pc(hostCId, hostCPortId),
+          { id: switchId, kind: 'networkSwitch', ports: switchPorts },
+        ],
+        links: [
+          link(hostAId, hostAPortId, switchId, switchPorts[0]!.id),
+          link(hostBId, hostBPortId, switchId, switchPorts[1]!.id),
+          link(hostCId, hostCPortId, switchId, switchPorts[2]!.id),
+        ],
+        switchConfigurationsByNodeId: new Map([[switchId, switchConfiguration('Classroom', 1_000)]]),
+      },
+      20,
+    );
+    const macA = stableMacAddress(hostAPortId);
+    const macB = stableMacAddress(hostBPortId);
+    const payload: EthernetPayload = {
+      kind: 'arp',
+      packet: {
+        operation: 'request',
+        senderMacAddress: macA,
+        senderIPAddress: '192.0.2.1',
+        targetMacAddress: UNSPECIFIED_MAC_ADDRESS,
+        targetIPAddress: '192.0.2.200',
+      },
+    };
+    const frame = (sourceMacAddress: string, destinationMacAddress: string): EthernetFrame => ({
+      identity: engine.allocateFrameIdentity(),
+      sourceMacAddress,
+      destinationMacAddress,
+      payload,
+    });
+    const switchTraces = (detail: string) =>
+      engine.state.packetTraces.filter((trace) => trace.nodeId === switchId && trace.detail === detail);
+
+    engine.sendEthernetFrame(hostAId, hostAPortId, frame(macA, '02:00:00:00:00:FE'));
+    expect(switchTraces('switch unknown unicast flooding')).toHaveLength(2);
+
+    engine.sendEthernetFrame(hostBId, hostBPortId, frame(macB, macA));
+    expect(switchTraces('switch learned unicast forwarding')).toHaveLength(1);
+
+    engine.sendEthernetFrame(hostAId, hostAPortId, frame(macA, macA));
+    expect(switchTraces('switch filtered destination on incoming port').length).toBeGreaterThan(0);
+    expect(engine.switchSatEntries(switchId)).toHaveLength(2);
+
+    const retention = 1_000;
+    const updateTimes = [...engine.state.switchForwardingUpdatedAtMillisecondsByNodeId.get(switchId)!.values()];
+    const expirationDeadlines = updateTimes.map((time) => time + retention);
+    const firstExpiration = Math.min(...expirationDeadlines);
+    const lastExpiration = Math.max(...expirationDeadlines);
+
+    engine.handle({ type: 'advance', toMilliseconds: firstExpiration - 1 });
+    expect(engine.switchSatEntries(switchId)).toHaveLength(2);
+    engine.handle({ type: 'advance', toMilliseconds: lastExpiration });
+    expect(engine.switchSatEntries(switchId)).toEqual([]);
+  });
+});
+
+describe('firewall', () => {
+  it('accepts when inactive, and an active ICMP drop bypasses the ordered rules', () => {
+    const routerId = uuid(0x1801);
+    const portId = uuid(0x1811);
+    const router: NodeSnapshot = { id: routerId, kind: 'router', ports: [portSnapshot(portId, 'rt1')] };
+    const packet: IPv4Packet = {
+      identity: 18,
+      senderIPAddress: '203.0.113.10',
+      receiverIPAddress: '10.0.0.1',
+      timeToLive: 64,
+      protocol: 'icmp',
+      payload: { kind: 'icmp', message: icmpMessage('echoRequest') },
+    };
+    const interfaces = new Map([
+      [interfaceKey(routerId, portId), { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
+    ]);
+
+    const inactive = startedEngine(
+      {
+        nodes: [router],
+        links: [],
+        interfaceConfigurations: interfaces,
+        firewallConfigurationsByNodeId: new Map([[routerId, firewallConfiguration()]]),
+      },
+      18,
+    );
+    expect(inactive.evaluateFirewall(packet, routerId).accepted).toBe(true);
+
+    const active = startedEngine(
+      {
+        nodes: [router],
+        links: [],
+        interfaceConfigurations: interfaces,
+        firewallConfigurationsByNodeId: new Map([
+          [
+            routerId,
+            firewallConfiguration({
+              isActive: true,
+              defaultPolicy: 'accept',
+              dropIcmp: true,
+              rules: [firewallRule({ protocol: 'icmp', action: 'accept' })],
+            }),
+          ],
+        ]),
+      },
+      18,
+    );
+    expect(active.evaluateFirewall(packet, routerId).accepted).toBe(false);
+    expect(active.state.firewallDecisions.at(-1)?.ruleIndex).toBeUndefined();
+    expect(active.state.packetTraces.at(-1)?.operation).toBe('dropped');
+  });
+
+  it('lets the first matching TCP rule win, and established traffic bypasses SYN-only rules', () => {
+    const { engine, routerId } = firewallEngine(
+      firewallConfiguration({
+        isActive: true,
+        defaultPolicy: 'drop',
+        filterSynSegmentsOnly: true,
+        rules: [
+          firewallRule({ port: 443, protocol: 'tcp', action: 'accept' }),
+          firewallRule({ port: 443, protocol: 'tcp', action: 'drop' }),
+        ],
+      }),
+    );
+
+    const synDecision = engine.evaluateFirewall(tcpPacket(181, 443, TCP_FLAG.synchronize), routerId);
+    expect(synDecision.accepted).toBe(true);
+    expect(synDecision.ruleIndex).toBe(0);
+    expect(engine.evaluateFirewall(tcpPacket(182, 22, TCP_FLAG.acknowledgement), routerId).accepted).toBe(true);
+  });
+
+  it('matches a UDP response by its explicit source port and the direct-network marker', () => {
+    const { engine, routerId } = firewallEngine(
+      firewallConfiguration({
+        isActive: true,
+        defaultPolicy: 'drop',
+        rules: [
+          firewallRule({
+            sourceIPAddress: DIRECTLY_CONNECTED_SOURCE_MARKER,
+            destinationIPAddress: '8.8.8.8',
+            destinationSubnetMask: '255.255.255.255',
+            port: 53,
+            protocol: 'udp',
+            action: 'accept',
+          }),
+        ],
+      }),
+    );
+    const udp = (identity: number, sender: string, receiver: string, sourcePort: number, destinationPort: number): IPv4Packet => ({
+      identity,
+      senderIPAddress: sender,
+      receiverIPAddress: receiver,
+      timeToLive: 64,
+      protocol: 'udp',
+      payload: { kind: 'udp', datagram: { sourcePort, destinationPort, payload: new Uint8Array() } },
+    });
+
+    expect(engine.evaluateFirewall(udp(183, '10.0.0.20', '8.8.8.8', 50_001, 53), routerId).accepted).toBe(true);
+    expect(engine.evaluateFirewall(udp(184, '8.8.8.8', '10.0.0.20', 53, 50_001), routerId).accepted).toBe(true);
+  });
+});
+
+describe('packet loss and trace retention', () => {
+  it('drops every physical delivery while global packet loss is on', () => {
+    const firstNodeId = uuid(0x101);
+    const firstPortId = uuid(0x102);
+    const secondNodeId = uuid(0x103);
+    const secondPortId = uuid(0x104);
+    const engine = startedEngine({
+      nodes: [pc(firstNodeId, firstPortId), pc(secondNodeId, secondPortId)],
+      links: [link(firstNodeId, firstPortId, secondNodeId, secondPortId)],
+      deviceConfigurations: new Map([
+        [firstNodeId, device('10.0.0.1')],
+        [secondNodeId, device('10.0.0.2')],
+      ]),
+    });
+    const frame: EthernetFrame = {
+      identity: engine.allocateFrameIdentity(),
+      sourceMacAddress: stableMacAddress(firstPortId),
+      destinationMacAddress: ETHERNET_BROADCAST_MAC_ADDRESS,
+      payload: {
+        kind: 'arp',
+        packet: {
+          operation: 'request',
+          senderMacAddress: stableMacAddress(firstPortId),
+          senderIPAddress: '10.0.0.1',
+          targetMacAddress: UNSPECIFIED_MAC_ADDRESS,
+          targetIPAddress: '10.0.0.2',
+        },
+      },
+    };
+
+    engine.setGlobalPacketLossEnabled(true);
+    engine.sendEthernetFrame(firstNodeId, firstPortId, frame);
+
+    expect(engine.state.globalPacketLossEnabled).toBe(true);
+    expect(engine.state.arpCachesByNodeId.get(secondNodeId)?.size ?? 0).toBe(0);
+    expect(
+      engine.state.packetTraces.some(
+        (trace) =>
+          trace.frameIdentity === frame.identity &&
+          trace.nodeId === secondNodeId &&
+          trace.operation === 'dropped' &&
+          trace.detail === 'global packet-loss simulation',
+      ),
+    ).toBe(true);
+
+    engine.setGlobalPacketLossEnabled(false);
+    engine.sendEthernetFrame(firstNodeId, firstPortId, frame);
+
+    expect(engine.state.globalPacketLossEnabled).toBe(false);
+    expect(engine.state.arpCachesByNodeId.get(secondNodeId)?.get('10.0.0.1')?.macAddress).toBe(
+      stableMacAddress(firstPortId),
+    );
+  });
+
+  it('resets packet loss across stop and restart', () => {
+    const engine = new NetworkRuntimeEngine(42);
+    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 42, initialTimeMilliseconds: 0 });
+    engine.setGlobalPacketLossEnabled(true);
+    expect(engine.state.globalPacketLossEnabled).toBe(true);
+
+    engine.handle({ type: 'stop' });
+    expect(engine.state.globalPacketLossEnabled).toBe(false);
+
+    engine.handle({ type: 'start', snapshot: EMPTY_TOPOLOGY_SNAPSHOT, seed: 42, initialTimeMilliseconds: 0 });
+    expect(engine.state.globalPacketLossEnabled).toBe(false);
+  });
+
+  it('evicts the oldest trace events in configured batches', () => {
+    const nodeId = uuid(0x2001);
+    const interfaceId = uuid(0x2002);
+    const engine = new NetworkRuntimeEngine(20, traceRetentionPolicy(3, 2));
+
+    for (let identity = 1; identity <= 6; identity += 1) {
+      engine.recordTrace({
+        frameIdentity: identity,
+        nodeId,
+        interfaceId,
+        direction: 'outbound',
+        layer: 'dataLink',
+        operation: 'sent',
+        detail: `trace-${identity}`,
+      });
+    }
+
+    expect(engine.state.packetTraces.map((trace) => trace.id)).toEqual([5, 6]);
+    expect(engine.state.packetTraces.map((trace) => trace.detail)).toEqual(['trace-5', 'trace-6']);
+    expect(engine.state.discardedPacketTraceCount).toBe(4);
+    expect(engine.state.nextTraceIdentity).toBe(7);
+  });
+});
diff --git a/packages/core/test/fixtures.ts b/packages/core/test/fixtures.ts
new file mode 100644
index 0000000..344e90d
--- /dev/null
+++ b/packages/core/test/fixtures.ts
@@ -0,0 +1,50 @@
+// Test fixtures, ported from the private helpers of
+// filius-ipad ios/FiliusPadTests/TopologyNetworkRuntimeEngineTests.swift.
+
+import {
+  deviceConfiguration,
+  NetworkRuntimeEngine,
+  portSnapshot,
+  topologySnapshot,
+  type DeviceConfiguration,
+  type Id,
+  type LinkSnapshot,
+  type NodeSnapshot,
+  type TopologySnapshotInput,
+} from '../src/index.js';
+
+let linkCounter = 0;
+
+/** Swift uses a fresh `UUID()` per link; a counter keeps runs reproducible. */
+export function link(sourceNodeId: Id, sourcePortId: Id, targetNodeId: Id, targetPortId: Id): LinkSnapshot {
+  linkCounter += 1;
+  return {
+    id: `ffffffff-0000-0000-0000-${linkCounter.toString(16).padStart(12, '0')}`,
+    sourceNodeId,
+    sourcePortId,
+    targetNodeId,
+    targetPortId,
+  };
+}
+
+export function pc(nodeId: Id, portId: Id): NodeSnapshot {
+  return { id: nodeId, kind: 'pc', ports: [portSnapshot(portId, 'eth0')] };
+}
+
+export function device(ipAddress: string, options: { mask?: string; gateway?: string } = {}): DeviceConfiguration {
+  return deviceConfiguration(ipAddress, options.mask ?? '255.255.255.0', options.gateway ?? '');
+}
+
+export function startedEngine(
+  input: TopologySnapshotInput,
+  seed: number = 14,
+): NetworkRuntimeEngine {
+  const engine = new NetworkRuntimeEngine(seed);
+  engine.handle({ type: 'start', snapshot: topologySnapshot(input), seed, initialTimeMilliseconds: 0 });
+  return engine;
+}
+
+/** `00000000-0000-0000-0000-` followed by twelve hex digits. */
+export function uuid(suffix: number): Id {
+  return `00000000-0000-0000-0000-${suffix.toString(16).toUpperCase().padStart(12, '0')}`;
+}
diff --git a/packages/core/test/routeTable.test.ts b/packages/core/test/routeTable.test.ts
new file mode 100644
index 0000000..5502914
--- /dev/null
+++ b/packages/core/test/routeTable.test.ts
@@ -0,0 +1,121 @@
+// Ported from filius-ipad ios/FiliusPadTests/TopologyJavaRouteTableTests.swift.
+// Left out: column titles and geometry (editor), and the reducer tests
+// (they move with the document layer, packages/doc).
+
+import { describe, expect, it } from 'vitest';
+import {
+  appendingBlankDraftIfNeeded,
+  bestMatchingManualRoute,
+  blankRouteDraft,
+  isValidJavaIPAddress,
+  isValidJavaSubnetMask,
+  manualRouteFromDraft,
+  routeTableRows,
+  visibleRouteRows,
+  type ManualRoute,
+} from '../src/index.js';
+
+function route(destinationNetwork: string, subnetMask: string, gateway: string, interfaceIPAddress: string): ManualRoute {
+  return { destinationNetwork, subnetMask, gateway, interfaceIPAddress };
+}
+
+describe('Java route table', () => {
+  it('projects multiple interfaces in Java order with Java values', () => {
+    const rows = routeTableRows(
+      [
+        { ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' },
+        { ipAddress: '192.168.0.1', subnetMask: '255.255.255.0' },
+      ],
+      [
+        route('172.16.0.0', '255.255.0.0', '10.0.0.2', '10.0.0.1'),
+        route('203.0.113.0', '255.255.255.0', '192.168.0.2', '192.168.0.1'),
+      ],
+      '10.0.0.254',
+    );
+
+    expect(rows.map((row) => row.destinationNetwork)).toEqual([
+      '192.168.0.1', '10.0.0.1',
+      '192.168.0.0', '10.0.0.0',
+      '127.0.0.0',
+      '172.16.0.0', '203.0.113.0',
+      '0.0.0.0',
+    ]);
+    expect(rows.map((row) => row.origin)).toEqual([
+      'localHost', 'localHost', 'connected', 'connected', 'localhost', 'manual', 'manual', 'defaultGateway',
+    ]);
+    expect(rows.map((row) => row.isEditable)).toEqual([false, false, false, false, false, true, true, false]);
+    expect(rows[0]).toMatchObject({ subnetMask: '255.255.255.255', nextHop: '127.0.0.1', interfaceIPAddress: '127.0.0.1' });
+    expect(rows[2]?.nextHop).toBe('192.168.0.1');
+    expect(rows[4]?.nextHop).toBe('127.0.0.1');
+    expect(rows.at(-1)).toMatchObject({ nextHop: '10.0.0.254', interfaceIPAddress: '10.0.0.1' });
+  });
+
+  it('puts the default route on the last matching interface and falls back to the primary one', () => {
+    const interfaces = [
+      { ipAddress: '10.0.0.1', subnetMask: '255.0.0.0' },
+      { ipAddress: '10.1.0.1', subnetMask: '255.0.0.0' },
+    ];
+
+    const matching = routeTableRows(interfaces, [], '10.2.0.1');
+    expect(matching.at(-1)).toMatchObject({ origin: 'defaultGateway', interfaceIPAddress: '10.1.0.1' });
+
+    const fallback = routeTableRows(interfaces, [], '192.168.0.1');
+    expect(fallback.at(-1)?.interfaceIPAddress).toBe('10.0.0.1');
+  });
+
+  it('shows only manual rows when generated rows are hidden', () => {
+    const rows = routeTableRows(
+      [{ ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }],
+      [route('172.16.0.0', '255.255.0.0', '10.0.0.2', '10.0.0.1')],
+      '10.0.0.254',
+    );
+
+    expect(visibleRouteRows(rows, false).map((row) => row.origin)).toEqual(['manual']);
+    expect(visibleRouteRows(rows, true)).toEqual(rows);
+  });
+
+  it('accepts leading zeros and non-contiguous masks like Java', () => {
+    expect(isValidJavaIPAddress('001.002.003.004')).toBe(true);
+    expect(isValidJavaSubnetMask('255.0.255.0')).toBe(true);
+    expect(isValidJavaIPAddress('256.0.0.1')).toBe(false);
+    expect(isValidJavaSubnetMask('255.255.255')).toBe(false);
+    expect(isValidJavaIPAddress(' 10.0.0.1')).toBe(false);
+  });
+
+  it('commits a draft only when all four cells are valid', () => {
+    const draft = blankRouteDraft('00000000-0000-0000-0000-000000000013');
+    draft.destinationNetwork = '10.0.0.0';
+    draft.subnetMask = '255.0.255.0';
+    draft.nextGateway = '10.0.0.2';
+
+    expect(manualRouteFromDraft(draft)).toBeUndefined();
+
+    draft.interfaceIPAddress = '10.0.0.1';
+    expect(manualRouteFromDraft(draft)).toEqual(route('10.0.0.0', '255.0.255.0', '10.0.0.2', '10.0.0.1'));
+  });
+
+  it('keeps at most one pending blank draft', () => {
+    let counter = 0;
+    const newId = () => `draft-${(counter += 1)}`;
+    const first = appendingBlankDraftIfNeeded([], newId);
+    const second = appendingBlankDraftIfNeeded(first, newId);
+
+    expect(first).toHaveLength(1);
+    expect(second).toEqual(first);
+    expect(second[0]?.isPendingInsertion).toBe(true);
+  });
+
+  it('picks the greatest numeric mask and keeps the first row on a tie', () => {
+    const broad = route('10.0.0.0', '255.0.0.0', '1.1.1.1', '10.0.0.1');
+    const firstSpecific = route('10.1.0.0', '255.255.0.0', '2.2.2.2', '10.0.0.1');
+    const tiedSpecific = route('10.1.0.0', '255.255.0.0', '3.3.3.3', '10.0.0.1');
+
+    expect(bestMatchingManualRoute('10.1.2.3', [broad, firstSpecific, tiedSpecific])).toBe(firstSpecific);
+  });
+
+  it('never includes learned RIP rows in the static projection', () => {
+    const rows = routeTableRows([{ ipAddress: '10.0.0.1', subnetMask: '255.255.255.0' }], [], undefined);
+
+    expect(rows.some((row) => row.origin === 'rip')).toBe(false);
+  });
+});
diff --git a/packages/core/tsconfig.json b/packages/core/tsconfig.json
new file mode 100644
index 0000000..61638e5
--- /dev/null
+++ b/packages/core/tsconfig.json
@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "rootDir": ".",
+    "outDir": "dist",
+    "types": []
+  },
+  "include": ["src", "test"]
+}
diff --git a/scripts/code_language_allow.txt b/scripts/code_language_allow.txt
new file mode 100644
index 0000000..6eb6b2a
--- /dev/null
+++ b/scripts/code_language_allow.txt
@@ -0,0 +1,15 @@
+# Words only this repository may use, in addition to the dashboard's
+# scripts/code_language_allow.txt. Format: `path::word` (bound to one path or
+# path prefix) or a bare word (anywhere). Each entry says why.
+#
+# The .fls format is Java's XMLEncoder output: its class and property names
+# (filius.hardware.knoten.Vermittlungsrechner, GUIKnotenItem, ipAdresse …)
+# are fields of a foreign API and keep their German names — in packages/fls
+# only.
+#
+# Java FILIUS class and package names, cited in the provenance header of each
+# ported file and in comments that point at the Java behaviour they copy.
+vermittlungsrechner
+verbindung
+netzzugangsschicht
+vermittlungsschicht
diff --git a/scripts/hooks/pre-commit b/scripts/hooks/pre-commit
new file mode 100755
index 0000000..98385ec
--- /dev/null
+++ b/scripts/hooks/pre-commit
@@ -0,0 +1,22 @@
+#!/bin/sh
+# Keeps German code out of this repository — the same rule as in the dashboard
+# (its CLAUDE.md). The checker is borrowed from the dashboard this repository is
+# a submodule of; CODE_LANGUAGE_ROOT points it at our files, and words only we
+# may use (the Java class names of the .fls format) stand in
+# scripts/code_language_allow.txt.
+#
+# Enable:  npm run hooks
+# Bypass:  git commit --no-verify
+top="$(git rev-parse --show-toplevel)"
+super="$(git rev-parse --show-superproject-working-tree 2>/dev/null)"
+checker="$super/scripts/check_code_language.py"
+if [ -z "$super" ] || [ ! -f "$checker" ]; then
+    echo "pre-commit: not inside the dashboard checkout, language check skipped"
+    exit 0
+fi
+CODE_LANGUAGE_ROOT="$top" python3 "$checker" --changed HEAD || {
+    echo
+    echo "German code in the commit. Glossary: knowledge/plans/CODE_ENGLISH_REFACTOR.md §3 (dashboard)"
+    echo "Commit anyway: git commit --no-verify"
+    exit 1
+}
diff --git a/tsconfig.base.json b/tsconfig.base.json
new file mode 100644
index 0000000..169e476
--- /dev/null
+++ b/tsconfig.base.json
@@ -0,0 +1,19 @@
+{
+  "compilerOptions": {
+    "target": "ES2023",
+    "lib": ["ES2023"],
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "strict": true,
+    "noUncheckedIndexedAccess": true,
+    "noImplicitOverride": true,
+    "noFallthroughCasesInSwitch": true,
+    "exactOptionalPropertyTypes": true,
+    "isolatedModules": true,
+    "verbatimModuleSyntax": true,
+    "declaration": true,
+    "sourceMap": true,
+    "composite": true,
+    "skipLibCheck": true
+  }
+}
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..0bc8463
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,4 @@
+{
+  "files": [],
+  "references": [{ "path": "packages/core" }]
+}
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 0000000..6e6e3bc
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,7 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    include: ['packages/*/test/**/*.test.ts'],
+  },
+});