Add skeleton for the radio mesh simulator

test/mesh/Makefile

@@ -0,0 +1,32 @@
+# Mesh simulator. `make` builds and runs an example
+ROOT = ../..
+SRCS = simulator.c l2_mesh.c l2_802154.c star.c $(ROOT)/mongoose.c
+HDRS = simulator.h l2_mesh.h l2_802154.h star.h $(ROOT)/mongoose.h
+WARN = -pedantic -W -Wall -Werror -Wshadow -Wdouble-promotion -fno-common \
+       -Wconversion -Wundef -Wmissing-prototypes -Wstrict-prototypes
+DEFS = -DMG_ENABLE_TCPIP=1 -DMG_ENABLE_TCPIP_DRIVER_INIT=0 -DMG_ENABLE_IPV6=0 \
+       -DMG_ENABLE_CUSTOM_MILLIS=1
+CFLAGS ?= -O2 -g0 $(WARN)
+X ?= 100
+Y ?= 100
+ROWS ?= 10
+COLS ?= 10
+
+# Nodes at every corner of a ROWS x COLS grid over X,Y, edges included
+GRID = $(shell for r in $$(seq 0 $(ROWS)); do \
+         for c in $$(seq 0 $(COLS)); do \
+         echo -node $$((c * $(X) / $(COLS))),$$((r * $(Y) / $(ROWS))); \
+         done; done)
+
+all: mesh_sim
+	./mesh_sim -area $(X),$(Y) \
+	  -bridge $$(($(X) / 2)),$$(($(Y) / 2)) \
+		$(GRID)
+
+mesh_sim: $(SRCS) $(HDRS) Makefile
+	$(CC) $(SRCS) $(CFLAGS) $(DEFS) -I. -I$(ROOT) -o $@
+
+clean:
+	rm -rf mesh_sim mesh_sim.dSYM
+
+.PHONY: all clean

test/mesh/README.md

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+# Ethernet over a radio mesh
+
+## The idea
+
+Small radio devices, e.g. IEEE 802.15.4, sub-GHz or LoRa boards, should run
+Mongoose as if they were on an ordinary Ethernet LAN: get an IP address over
+DHCP, answer HTTP, talk MQTT. No 6LoWPAN, no Thread, no special stack.
+
+To do that, the radio interface looks like Ethernet to Mongoose
+(`MG_TCPIP_L2_ETH`). Its driver splits each Ethernet frame into radio
+frames, sends them over the air, and the receiver puts them back together.
+One or more bridges connect the radio network to the wired LAN: a bridge has
+an Ethernet port and a radio. A node in range of a bridge talks to it
+directly; a node out of range reaches a bridge through other nodes, which
+forward its frames. That makes the radio network a mesh. With several
+bridges, each node uses one of them, e.g. the nearest; the mesh covers a
+bigger area, and keeps working if a bridge goes down. A PC on the LAN sees
+the radio nodes as regular hosts on the same LAN.
+
+```
+             +---- bridge 1 ~~~ node ~~~ node
+             |                    ~
+  PC ---- LAN                    node
+             |                    ~
+             +---- bridge 2 ~~~ node ~~~ node ~~~ node
+```
+
+`~~~` and `~` are radio hops. Nodes far from a bridge send their frames
+through nearer nodes. A node in the middle can reach either bridge.
+
+## Two parts plug in
+
+- `struct mg_radio`, a radio standard: how big a radio frame is, how
+  addresses are written into it. `mg_radio_802154` is the first one.
+- `struct mg_mesh`, a mesh strategy: how nodes find a bridge, and where each
+  frame goes next. `mg_mesh_star` is the first and simplest one: every node
+  talks to a bridge directly, one hop.
+
+Everything else, splitting and reassembling frames, sending again when a
+frame is lost, is common code that works the same for every radio and every
+mesh. The goal is to swap radios and mesh strategies without touching
+anything else.
+
+## Why a simulator
+
+Real boards are slow to test with, and you can't put 500 of them on a desk.
+The simulator runs a whole network in one process: every node is a real
+Mongoose instance, only the radio and the air between nodes are emulated.
+That lets us try mesh strategies with many nodes, and measure how long it
+takes nodes to join, get an address, and answer a request.
+
+## Virtual time
+
+The simulator has its own clock, `simulator::now`, in microseconds, and
+Mongoose reads it through `mg_millis()`. Nothing waits for real time: the
+simulator moves the clock forward itself, 250 us per step, and polls every
+station once per step. A minute of network traffic takes about a second to
+simulate. `-t SEC` sets how long to run, 10 minutes by default.
+
+The step is a trade-off. Each step polls every station's Mongoose, and even
+an idle poll costs something, so a smaller step means a slower run. A
+bigger step makes frames wait longer in queues, see below. Measured with
+121 nodes and 10 simulated minutes:
+
+| Step | Request time, mean | Run time |
+|------|--------------------|----------|
+| 1 ms | 6.1 ms | 0.9 s |
+| 250 us | 1.5 ms | 3.0 s |
+| 100 us | 0.6 ms | 7.2 s |
+| 10 us | 0.06 ms | 72 s |
+
+## The setup
+
+```
+  DHCP server ---------+
+   10.0.0.1            |
+   02:01:00:00:00:00   +-- LAN ---- bridge ~~~ nodes
+                       |
+  test station --------+
+   10.0.0.2
+   02:02:00:00:00:00
+```
+
+The LAN is assumed infinitely fast, see "How frames move on the LAN" below.
+Two stations are on it:
+
+- The DHCP server, 10.0.0.1, MAC 02:01:00:00:00:00, gives each node an
+  address made from its MAC: 02:00:00:X:Y:Z gets 10.X+1.Y.Z, so node
+  addresses never clash with the LAN stations' 10.0.0.x.
+- The test station, 10.0.0.2, MAC 02:02:00:00:00:00, is a Mongoose
+  instance. It waits for the mesh to settle, 60 seconds by default
+  (`-w SEC`), then sends `GET /` to each node in turn, one at a time, at the
+  node's own IP address. A node with no address, or one that doesn't answer
+  within 10 seconds, counts as failed. At the end it prints how many
+  requests succeeded and how long they took.
+
+Bridges are on the LAN too, and get every frame, as if in promiscuous mode:
+a frame for one of their radio nodes isn't addressed to the bridge itself.
+A bridge forwards every frame it gets from the LAN to the radio, and every
+frame from the radio to the LAN, with no filtering yet.
+
+Bridges and radio nodes get MACs 02:00:00:xx:xx:xx, numbered from 1 in
+command-line order: 02:00:00:00:00:01, 02:00:00:00:00:02, and so on.
+
+### How frames move on the LAN
+
+Every LAN station, the DHCP server, the test station and each bridge, has a
+LAN interface, `lif`: a `struct mg_tcpip_if` with a receive queue. All of
+them use the same driver, `s_lan_driver`. There are no cables and no switch:
+
+- **Sending (tx).** A station sends a frame with its driver's `tx`, which is
+  `lan_tx()`. It looks at the destination MAC and copies the frame into the
+  receive queues of the stations it is for:
+  - a broadcast or multicast goes to every station but the sender;
+  - a unicast goes to the station with that MAC;
+  - bridges get every frame, as a real bridge in promiscuous mode would:
+    a frame for a radio node is addressed to the node, not to its bridge.
+
+  That's all `tx` does. Nobody's code runs on the receiving side yet.
+- **Receiving (rx).** Each station takes the frames from its own queue when
+  the main loop polls it:
+  - the test station runs Mongoose, which reads the queue by itself, as it
+    would with a real Ethernet driver that queues frames from an interrupt;
+  - the DHCP server reads it in `dhcp_poll()`, and replies by sending
+    through its own `lif`;
+  - bridges read it in `bridges_poll()`, and forward it to the radio.
+
+So a frame waits in a queue until its receiver's next poll, at most one
+step of virtual time, 250 us. Ethernet is hundreds of times faster than the radio, so that
+doesn't matter for what we measure. And since sending never calls the
+receiver's code, a reply is never sent from inside another station's send.
+
+For example, a node asking for an address:
+
+1. The node's DHCP DISCOVER crosses the radio to a bridge, which sends it
+   out of its `lif`. It is a broadcast, so `lan_tx()` puts it into the
+   queues of the DHCP server, the test station and the other bridges.
+2. On its next poll, the DHCP server reads it and sends an OFFER, also a
+   broadcast, which lands in the test station's and the bridges' queues.
+3. On their next poll, the bridges read the OFFER and send it over the
+   radio to the node. The test station's Mongoose drops it: the test station
+   has a fixed address and no DHCP client.
+
+## Files
+
+| File | What |
+|------|------|
+| `l2_mesh.h`, `l2_mesh.c` | `struct mg_radio`, `struct mg_mesh`, common code |
+| `l2_802154.h`, `l2_802154.c` | `mg_radio_802154`, IEEE 802.15.4 |
+| `star.h`, `star.c` | `mg_mesh_star`, the star mesh |
+| `simulator.h`, `simulator.c` | The simulator |
+| `Makefile` | Builds and runs the simulator |
+
+## Where we are
+
+It works end to end: nodes get addresses over DHCP, and the test station's
+requests reach them through the bridges. But the radio is ideal for now:
+
+- The radio driver works like the LAN's: it puts whole Ethernet frames
+  straight into receive queues, all of them for a broadcast, else the one
+  with the destination MAC, and bridges get every frame. No range, no
+  loss, no airtime.
+- Bridges forward everything, with no filtering.
+- So use one bridge for now. With two, a broadcast from the LAN goes to the
+  radio through both, each hears the other's copy and sends it back to the
+  LAN, and that never ends. Deciding who forwards what, without loops, is
+  the job of `mg_mesh`.
+
+`mg_radio` and `mg_mesh` are still stubs. Next steps, one at a time: the
+air (range, loss, airtime), fragmenting frames into radio frames with
+`mg_radio`, choosing a bridge and forwarding through nodes with `mg_mesh`,
+and filtering at the bridges.
+
+## Running
+
+```sh
+make                          # 100x100 m, a bridge, 11x11 nodes
+make X=200 Y=50 ROWS=2 COLS=8 # Another area and grid
+```
+
+`make` puts a bridge in the middle of the area, and a node at every corner
+of a `ROWS` x `COLS` grid over it. The simulator itself takes:
+
+```
+./mesh_sim -area X,Y,Z -bridge X,Y,Z -node X,Y,Z ... -t SEC -w SEC
+```
+
+Positions are in metres; `X,Y` means `Z` is 0. Repeat `-bridge` and `-node`
+for more.

test/mesh/l2_802154.c

@@ -0,0 +1,6 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+
+#include "l2_802154.h"
+
+const struct mg_radio mg_radio_802154 = {"802.15.4"};

test/mesh/l2_802154.h

@@ -0,0 +1,10 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+//
+// IEEE 802.15.4 radio
+
+#pragma once
+
+#include "l2_mesh.h"
+
+extern const struct mg_radio mg_radio_802154;

test/mesh/l2_mesh.c

@@ -0,0 +1,4 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+
+#include "l2_mesh.h"

test/mesh/l2_mesh.h

@@ -0,0 +1,21 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+//
+// Ethernet over radio, through a mesh. Mongoose sees an Ethernet interface,
+// its driver splits Ethernet frames into radio frames. Two parts plug in:
+// - struct mg_radio: a radio standard, e.g. mg_radio_802154
+// - struct mg_mesh: how the mesh is built and routed, e.g. mg_mesh_star
+
+#pragma once
+
+#include "mongoose.h"
+
+// A radio standard
+struct mg_radio {
+  const char *name;  // For logs
+};
+
+// A mesh strategy
+struct mg_mesh {
+  const char *name;  // For logs
+};

test/mesh/simulator.c

@@ -0,0 +1,433 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+//
+// Radio mesh simulator. Bridges and nodes are stations at positions in an
+// area. Time is virtual: see mg_millis(). On the LAN: a DHCP server, and a
+// test station that sends HTTP requests to the nodes
+
+#include "simulator.h"
+
+#define STEP_MICROSECONDS 250  // Virtual time per main loop step, see README.md
+
+static struct simulator s_sim;  // The simulator
+
+// Mongoose's clock: virtual time. A minute of network traffic takes far less
+// than a minute to simulate
+uint64_t mg_millis(void) {
+  return s_sim.now / 1000;
+}
+
+// Parse "X,Y,Z", or "X,Y" with Z 0. Return false if it is not that
+static bool parse_pos(const char *s, struct pos *p) {
+  int n = 0;  // Characters parsed
+  p->x = p->y = p->z = 0;
+  return (sscanf(s, "%lf,%lf,%lf%n", &p->x, &p->y, &p->z, &n) == 3 ||
+          sscanf(s, "%lf,%lf%n", &p->x, &p->y, &n) == 2) &&
+         s[n] == 0;
+}
+
+// Is position p inside the area of sim
+static bool inside(const struct simulator *sim, const struct pos *p) {
+  return p->x >= 0 && p->x <= sim->area.x && p->y >= 0 && p->y <= sim->area.y &&
+         p->z >= 0 && p->z <= sim->area.z;
+}
+
+static void usage(const struct simulator *sim, const char *prog) {
+  fprintf(stderr,
+          "Usage: %s OPTIONS\n"
+          "Positions: X,Y,Z in metres, or X,Y with Z 0\n"
+          "  -area X,Y,Z    - area, a box, default: %g,%g,%g\n"
+          "  -bridge X,Y,Z  - a bridge; repeat for more\n"
+          "  -node X,Y,Z    - a radio node; repeat for more\n"
+          "  -t SEC         - simulated time, seconds, default: %g\n"
+          "  -w SEC         - mesh settle time before requests, default: %g\n",
+          prog, sim->area.x, sim->area.y, sim->area.z,
+          (double) sim->duration / 1e6, (double) sim->tester.wait / 1e6);
+  exit(EXIT_FAILURE);
+}
+
+// The next radio MAC, to mac: 02:00:00:xx:xx:xx, numbered from 1
+static void next_mac(struct simulator *sim, uint8_t *mac) {
+  uint32_t n = ++sim->last_mac;
+  mac[0] = 2, mac[1] = mac[2] = 0, mac[3] = (uint8_t) (n >> 16);
+  mac[4] = (uint8_t) (n >> 8), mac[5] = (uint8_t) n;
+}
+
+// Add a bridge at position s. Return false if s is not a position
+static bool add_bridge(struct simulator *sim, const char *s) {
+  struct bridge *b = (struct bridge *) calloc(1, sizeof(*b));
+  if (b == NULL || !parse_pos(s, &b->pos)) return free(b), false;
+  next_mac(sim, b->rif.mac);
+  LIST_ADD_TAIL(struct bridge, &sim->bridges, b);
+  sim->num_bridges++;
+  return true;
+}
+
+// Add a radio node at position s. Return false if s is not a position
+static bool add_node(struct simulator *sim, const char *s) {
+  struct radio_node *n = (struct radio_node *) calloc(1, sizeof(*n));
+  if (n == NULL || !parse_pos(s, &n->pos)) return free(n), false;
+  next_mac(sim, n->rif.mac);
+  LIST_ADD_TAIL(struct radio_node, &sim->nodes, n);
+  sim->num_nodes++;
+  return true;
+}
+
+// Exit if position x is outside the area of sim
+static void check_inside(const struct simulator *sim, const struct pos *x) {
+  if (!inside(sim, x)) {
+    fprintf(stderr, "%g,%g,%g is outside the area\n", x->x, x->y, x->z);
+    exit(EXIT_FAILURE);
+  }
+}
+
+// Check that all bridges and nodes are inside the area of sim
+static void check_area(const struct simulator *sim) {
+  struct bridge *b;
+  struct radio_node *n;
+  for (b = sim->bridges; b != NULL; b = b->next) check_inside(sim, &b->pos);
+  for (n = sim->nodes; n != NULL; n = n->next) check_inside(sim, &n->pos);
+}
+
+#define LAN_QUEUE_SIZE (64 * 1024)  // Receive queue of a LAN interface
+
+// Put frame buf, len bytes, into the receive queue of LAN interface to, if
+// it is not the sender, from
+static void lan_put(struct mg_tcpip_if *to, struct mg_tcpip_if *from,
+                    const void *buf, size_t len) {
+  if (to != from) mg_tcpip_qwrite((void *) buf, len, to);
+}
+
+// LAN driver, all LAN interfaces share it. The LAN is infinitely fast: tx
+// puts the frame straight into the receive queues of the interfaces it is
+// for: all, if broadcast; else the one with the destination MAC. Bridges get
+// every frame: some are for their radio nodes. Each station takes frames
+// from its queue when polled, see dhcp_poll(), bridges_poll(); Mongoose does
+// it for the test station
+static size_t lan_tx(const void *buf, size_t len, struct mg_tcpip_if *ifp) {
+  const uint8_t *dst = (const uint8_t *) buf;
+  bool all = dst[0] & 1;  // Broadcast, multicast
+  struct bridge *b;
+  if (all || memcmp(dst, s_sim.dhcp.lif.mac, 6) == 0) {
+    lan_put(&s_sim.dhcp.lif, ifp, buf, len);
+  }
+  if (all || memcmp(dst, s_sim.tester.lif.mac, 6) == 0) {
+    lan_put(&s_sim.tester.lif, ifp, buf, len);
+  }
+  for (b = s_sim.bridges; b != NULL; b = b->next) {
+    lan_put(&b->lif, ifp, buf, len);
+  }
+  return len;
+}
+
+static bool lan_init(struct mg_tcpip_if *ifp) {
+  (void) ifp;
+  return true;
+}
+
+static bool lan_link(struct mg_tcpip_if *ifp, bool s1) {
+  (void) ifp, (void) s1;
+  return true;
+}
+
+static struct mg_tcpip_driver s_lan_driver = {lan_init, lan_tx, NULL, lan_link};
+
+// Internet checksum of len bytes
+static uint16_t ip_csum(const uint8_t *buf, size_t len) {
+  uint32_t sum = 0;
+  size_t i;
+  for (i = 0; i + 1 < len; i += 2) sum += (uint32_t) (buf[i] << 8 | buf[i + 1]);
+  if (len & 1) sum += (uint32_t) (buf[len - 1] << 8);
+  while (sum >> 16) sum = (sum & 0xffff) + (sum >> 16);
+  return (uint16_t) ~sum;
+}
+
+// A node's address, from its MAC 02:00:00:X:Y:Z: 10.X+1.Y.Z, so it never
+// clashes with the LAN stations' 10.0.0.x
+static void node_ip(const uint8_t *mac, uint8_t *ip) {
+  ip[0] = 10, ip[1] = (uint8_t) (mac[3] + 1), ip[2] = mac[4], ip[3] = mac[5];
+}
+
+// DHCP server, a frame from its queue: a DISCOVER gets an OFFER, a REQUEST
+// an ACK. Replies are broadcast. The address comes from the client's MAC
+static void dhcp_rx(struct simulator *sim, const uint8_t *buf, size_t len) {
+  static const uint8_t opts[] = {53, 1, 0,  // Message type, set below
+                                 1,  4, 255, 0, 0,  0,   // Mask
+                                 3,  4, 10,  0, 0,  1,   // Router
+                                 54, 4, 10,  0, 0,  1,   // Server
+                                 51, 4, 0,   0, 14, 16,  // Lease, 3600 s
+                                 255};
+  uint8_t r[14 + 20 + 8 + 240 + sizeof(opts)], type = 0;
+  const uint8_t *dh, *p, *end = buf + len;
+  size_t ihl, rlen = sizeof(r);
+  if ((buf[0] & 1) == 0) return;  // Bcast
+  if (len < 34 || buf[12] != 8 || buf[13] != 0 || buf[23] != 17) return;  // UDP
+  ihl = (size_t) (buf[14] & 15) * 4, dh = buf + 14 + ihl + 8;  // DHCP
+  if (dh + 240 > end || buf[14 + ihl + 2] != 0 || buf[14 + ihl + 3] != 67) {
+    return;
+  }
+  for (p = dh + 240; p + 2 <= end && p[0] != 255; p += p[0] ? p[1] + 2 : 1) {
+    if (p[0] == 53 && p[1] == 1 && p + 3 <= end) type = p[2];
+  }
+  if (type != 1 && type != 3) return;  // DISCOVER, REQUEST
+  memset(r, 0, sizeof(r));
+  memset(r, 0xff, 6), memcpy(r + 6, sim->dhcp.lif.mac, 6),
+      r[12] = 8;                                       // Ethernet
+  r[14] = 0x45, r[16] = (uint8_t) ((rlen - 14) >> 8);  // IPv4
+  r[17] = (uint8_t) (rlen - 14), r[22] = 64, r[23] = 17;
+  r[26] = 10, r[29] = 1, memset(r + 30, 0xff, 4);  // 10.0.0.1 -> bcast
+  r[24] = (uint8_t) (ip_csum(r + 14, 20) >> 8);
+  r[25] = (uint8_t) ip_csum(r + 14, 20);
+  r[35] = 67, r[37] = 68;  // UDP, csum 0: none
+  r[38] = (uint8_t) ((rlen - 34) >> 8), r[39] = (uint8_t) (rlen - 34);
+  r[42] = 2, r[43] = 1, r[44] = 6;                      // DHCP: reply, Ethernet
+  memcpy(r + 46, dh + 4, 4);                            // xid
+  memcpy(r + 52, dh + 10, 2);                           // flags
+  node_ip(dh + 28, r + 58);                             // yiaddr
+  r[62] = 10, r[65] = 1;                                // siaddr
+  memcpy(r + 70, dh + 28, 16);                          // chaddr
+  r[278] = 99, r[279] = 130, r[280] = 83, r[281] = 99;  // Magic
+  memcpy(r + 282, opts, sizeof(opts));
+  r[284] = type == 1 ? 2 : 5;  // OFFER, ACK
+  sim->dhcp.lif.driver->tx(r, rlen, &sim->dhcp.lif);
+}
+
+// DHCP server's work: take the frames from its queue
+static void dhcp_poll(struct simulator *sim) {
+  char *buf;
+  size_t len;
+  while ((len = mg_queue_next(&sim->dhcp.lif.recv_queue, &buf)) > 0) {
+    dhcp_rx(sim, (const uint8_t *) buf, len);
+    mg_queue_del(&sim->dhcp.lif.recv_queue, len);
+  }
+}
+
+// The request on the way finished: ok, or failed
+static void tester_done(struct lan_tester *t, uint64_t now, bool ok) {
+  double ms = (double) (now - t->start) / 1000;
+  if (ok) {
+    t->ok++, t->sum += ms;
+    if (t->ok == 1 || ms < t->min) t->min = ms;
+    if (ms > t->max) t->max = ms;
+  } else {
+    t->failed++;
+  }
+  if (t->c != NULL) t->c->is_closing = 1, t->c->fn_data = NULL;
+  t->c = NULL;
+}
+
+// HTTP client: GET /, the reply ends the request
+static void tester_fn(struct mg_connection *c, int ev, void *ev_data) {
+  struct lan_tester *t = (struct lan_tester *) c->fn_data;
+  if (t == NULL) return;  // Done with
+  if (ev == MG_EV_CONNECT) {
+    mg_printf(c, "GET / HTTP/1.0\r\n\r\n");
+  } else if (ev == MG_EV_HTTP_MSG) {
+    tester_done(t, s_sim.now, true);
+  } else if (ev == MG_EV_ERROR || ev == MG_EV_CLOSE) {
+    tester_done(t, s_sim.now, false);
+  }
+  (void) ev_data;
+}
+
+// Test station's work: Mongoose, then requests, one at a time, once the mesh
+// had time to settle. A node without an IP address fails at once
+static void tester_poll(struct simulator *sim) {
+  struct radio_node *n;
+  char url[64];
+  mg_mgr_poll(&sim->tester.mgr, 0);
+  if (sim->tester.c != NULL && sim->now - sim->tester.start >
+                                   (uint64_t) sim->request_timeout_ms * 1000) {
+    tester_done(&sim->tester, sim->now, false);
+  }
+  if (sim->now < sim->tester.wait || sim->tester.c != NULL ||
+      sim->tester.next == NULL)
+    return;
+  n = sim->tester.next;
+  sim->tester.next = n->next, sim->tester.start = sim->now;
+  if (n->rif.ip == 0) {
+    tester_done(&sim->tester, sim->now, false);
+    return;
+  }
+  mg_snprintf(url, sizeof(url), "http://%M/", mg_print_ip4, &n->rif.ip);
+  if ((sim->tester.c = mg_http_connect(&sim->tester.mgr, url, tester_fn,
+                                       &sim->tester)) == NULL) {
+    tester_done(&sim->tester, sim->now, false);
+  }
+}
+
+// Set up sim: defaults, LAN stations
+static void init(struct simulator *sim) {
+  static const uint8_t dmac[6] = {2, 1, 0, 0, 0, 0},  // DHCP server MAC
+      tmac[6] = {2, 2, 0, 0, 0, 0};                   // Test station MAC
+  memset(sim, 0, sizeof(*sim));
+  sim->area.x = sim->area.y = 100;
+  sim->duration = 600 * 1000000ULL, sim->tester.wait = 60 * 1000000ULL;
+  sim->request_timeout_ms = 10000;
+  memcpy(sim->dhcp.lif.mac, dmac, 6), sim->dhcp.lif.driver = &s_lan_driver;
+  mg_queue_init(&sim->dhcp.lif.recv_queue, (char *) calloc(1, LAN_QUEUE_SIZE),
+                LAN_QUEUE_SIZE);
+  memcpy(sim->tester.lif.mac, tmac, 6);
+  sim->tester.lif.ip = MG_IPV4(10, 0, 0, 2),
+  sim->tester.lif.mask = MG_IPV4(255, 0, 0, 0);
+  sim->tester.lif.driver = &s_lan_driver,
+  sim->tester.lif.enable_mac_check = true;
+  sim->tester.lif.recv_queue.size = 128 * 1024;
+}
+
+// Radio driver, all radio interfaces share it. An ideal radio for now:
+// whole Ethernet frames, no range, no loss, no airtime. It works like the
+// LAN, see lan_tx(): a frame goes into the receive queues of all other
+// radio interfaces if broadcast, else of the one with the destination MAC.
+// Bridges get every frame: some are for the LAN. Who forwards what, and
+// loops with several bridges, are for mg_mesh
+static size_t radio_tx(const void *buf, size_t len, struct mg_tcpip_if *ifp) {
+  const uint8_t *dst = (const uint8_t *) buf;
+  bool all = dst[0] & 1;  // Broadcast, multicast
+  struct bridge *b;
+  struct radio_node *n;
+  for (b = s_sim.bridges; b != NULL; b = b->next) {
+    lan_put(&b->rif, ifp, buf, len);
+  }
+  for (n = s_sim.nodes; n != NULL; n = n->next) {
+    if (all || memcmp(dst, n->rif.mac, 6) == 0) {
+      lan_put(&n->rif, ifp, buf, len);
+    }
+  }
+  return len;
+}
+
+static bool radio_init(struct mg_tcpip_if *ifp) {
+  (void) ifp;
+  return true;
+}
+
+static bool radio_link(struct mg_tcpip_if *ifp, bool s1) {
+  (void) ifp, (void) s1;
+  return true;
+}
+
+static struct mg_tcpip_driver s_radio_driver = {radio_init, radio_tx, NULL,
+                                                radio_link};
+
+// A node's HTTP server: the test station requests GET /
+static void node_http_ev_handler(struct mg_connection *c, int ev,
+                                 void *ev_data) {
+  if (ev == MG_EV_HTTP_MSG) {
+    mg_http_reply(c, 200, "", "Hi from Mongoose, my MAC is %M\n", mg_print_mac,
+                  c->fn_data);
+  }
+  (void) ev_data;
+}
+
+// Start each node's Mongoose: its radio interface, no IP address, so DHCP,
+// and an HTTP server
+static void nodes_init(struct simulator *sim) {
+  struct radio_node *n;
+  for (n = sim->nodes; n != NULL; n = n->next) {
+    n->rif.driver = &s_radio_driver;
+    n->rif.recv_queue.size = LAN_QUEUE_SIZE;
+    mg_mgr_init(&n->mgr);
+    mg_tcpip_init(&n->mgr, &n->rif);
+    mg_http_listen(&n->mgr, "http://0.0.0.0:80", node_http_ev_handler,
+                   n->rif.mac);
+  }
+}
+
+// Start each bridge: its LAN and radio drivers. No Mongoose, no IP
+static void bridges_init(struct simulator *sim) {
+  struct bridge *b;
+  for (b = sim->bridges; b != NULL; b = b->next) {
+    b->lif.driver = &s_lan_driver, b->rif.driver = &s_radio_driver;
+    mg_queue_init(&b->lif.recv_queue, (char *) calloc(1, LAN_QUEUE_SIZE),
+                  LAN_QUEUE_SIZE);
+    mg_queue_init(&b->rif.recv_queue, (char *) calloc(1, LAN_QUEUE_SIZE),
+                  LAN_QUEUE_SIZE);
+    b->lif.driver->init(&b->lif), b->rif.driver->init(&b->rif);
+  }
+}
+
+// Move the frames waiting in the receive queue of interface from out of
+// interface to
+static void bridge_forward(struct mg_tcpip_if *from, struct mg_tcpip_if *to) {
+  char *buf;
+  size_t len;
+  while ((len = mg_queue_next(&from->recv_queue, &buf)) > 0) {
+    // MG_DEBUG(("Bridging ETH frame, size %zu", len));
+    to->driver->tx(buf, len, to);
+    mg_queue_del(&from->recv_queue, len);
+  }
+}
+
+// Bridges' work: forward every frame, LAN to radio and radio to LAN. No
+// filtering yet
+static void bridges_poll(struct simulator *sim) {
+  struct bridge *b;
+  for (b = sim->bridges; b != NULL; b = b->next) {
+    bridge_forward(&b->lif, &b->rif);
+    bridge_forward(&b->rif, &b->lif);
+  }
+}
+
+// Poll each node's Mongoose
+static void nodes_poll(struct simulator *sim) {
+  struct radio_node *n;
+  for (n = sim->nodes; n != NULL; n = n->next) mg_mgr_poll(&n->mgr, 0);
+}
+
+int main(int argc, char *argv[]) {
+  clock_t started;  // CPU time, to show how fast the simulation runs
+  int i;
+
+  init(&s_sim);
+  for (i = 1; i < argc; i++) {
+    if (i + 1 >= argc) {
+      usage(&s_sim, argv[0]);
+    } else if (strcmp(argv[i], "-area") == 0) {
+      if (!parse_pos(argv[++i], &s_sim.area)) usage(&s_sim, argv[0]);
+    } else if (strcmp(argv[i], "-bridge") == 0) {
+      if (!add_bridge(&s_sim, argv[++i])) usage(&s_sim, argv[0]);
+    } else if (strcmp(argv[i], "-node") == 0) {
+      if (!add_node(&s_sim, argv[++i])) usage(&s_sim, argv[0]);
+    } else if (strcmp(argv[i], "-t") == 0) {
+      s_sim.duration = (uint64_t) (atof(argv[++i]) * 1e6);
+    } else if (strcmp(argv[i], "-w") == 0) {
+      s_sim.tester.wait = (uint64_t) (atof(argv[++i]) * 1e6);
+    } else {
+      usage(&s_sim, argv[0]);
+    }
+  }
+
+  printf("Area %gx%gx%g m, radio %s, mesh %s, bridges: %d, nodes: %d\n",
+         s_sim.area.x, s_sim.area.y, s_sim.area.z, mg_radio_802154.name,
+         mg_mesh_star.name, s_sim.num_bridges, s_sim.num_nodes);
+  check_area(&s_sim);
+
+  // mg_log_set(MG_LL_INFO);
+
+  mg_mgr_init(&s_sim.tester.mgr);
+  mg_tcpip_init(&s_sim.tester.mgr, &s_sim.tester.lif);
+  s_sim.tester.next = s_sim.nodes;
+  bridges_init(&s_sim);
+  nodes_init(&s_sim);
+
+  // Main loop: STEP_MICROSECONDS of virtual time per step
+  started = clock();
+  while (s_sim.now < s_sim.duration) {
+    dhcp_poll(&s_sim);
+    tester_poll(&s_sim);
+    bridges_poll(&s_sim);
+    nodes_poll(&s_sim);
+    s_sim.now += STEP_MICROSECONDS;
+  }
+  printf("Requests: %d ok, %d failed, of %d nodes", s_sim.tester.ok,
+         s_sim.tester.failed, s_sim.num_nodes);
+  if (s_sim.tester.ok > 0) {
+    printf("; ms: min %.1f, mean %.1f, max %.1f", s_sim.tester.min,
+           s_sim.tester.sum / s_sim.tester.ok, s_sim.tester.max);
+  }
+  printf("\nSimulated %g s in %.3f s\n", (double) s_sim.now / 1e6,
+         (double) (clock() - started) / CLOCKS_PER_SEC);
+
+  return 0;
+}

test/mesh/simulator.h

@@ -0,0 +1,67 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+
+#pragma once
+
+#define MG_ENABLE_TCPIP 1
+
+#include "l2_802154.h"
+#include "l2_mesh.h"
+#include "mongoose.h"
+#include "star.h"
+
+// A position in metres
+struct pos {
+  double x, y, z;  // Coordinates
+};
+
+// A bridge: forwards frames between the LAN and the radio, no IP stack
+struct bridge {
+  struct bridge *next;     // Next in simulator::bridges
+  struct pos pos;          // Where it is
+  struct mg_tcpip_if lif;  // LAN side: LAN driver, gets every LAN frame
+  struct mg_tcpip_if rif;  // Radio side: radio driver, gets every frame
+};
+
+// A radio node: Mongoose on a radio interface
+struct radio_node {
+  struct radio_node *next;  // Next in simulator::nodes
+  struct pos pos;           // Where it is
+  struct mg_mgr mgr;        // Mongoose event manager
+  struct mg_tcpip_if rif;   // Radio interface. rif.ip from DHCP, 0: none yet
+};
+
+struct simulator;
+
+// LAN station: a DHCP server, 10.0.0.1. Gives each client an address made
+// from its MAC, see node_ip()
+struct lan_dhcp_server {
+  struct mg_tcpip_if lif;  // Its LAN interface, no Mongoose on it
+};
+
+// LAN station: the test station, 10.0.0.2, a Mongoose instance. Waits for
+// the mesh to settle, then sends GET / to each node
+struct lan_tester {
+  struct mg_mgr mgr;        // Its Mongoose
+  struct mg_tcpip_if lif;   // Its LAN interface
+  uint64_t wait;            // Mesh settle time: start requests then, us
+  struct radio_node *next;  // Node to request next, NULL: all done
+  struct mg_connection *c;  // Request on the way, NULL: none
+  uint64_t start;           // When it started, us
+  int ok, failed;           // Requests done, by result
+  double min, max, sum;     // Times of successful requests, ms
+};
+
+// Simulator state
+struct simulator {
+  struct pos area;              // A box: all positions inside
+  struct bridge *bridges;       // Bridges, a list
+  struct radio_node *nodes;     // Radio nodes, a list
+  int num_bridges, num_nodes;   // How many
+  uint64_t now;                 // Virtual time, us since the start
+  uint64_t duration;            // Run this long, us
+  uint32_t last_mac;            // Low 3 bytes of the last MAC given
+  int request_timeout_ms;       // A request that takes longer failed
+  struct lan_dhcp_server dhcp;  // DHCP server on the LAN
+  struct lan_tester tester;     // Test station on the LAN
+};

test/mesh/star.c

@@ -0,0 +1,6 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+
+#include "star.h"
+
+const struct mg_mesh mg_mesh_star = {"star"};

test/mesh/star.h

@@ -0,0 +1,10 @@
+// Copyright (c) 2026 Cesanta Software Limited
+// All rights reserved
+//
+// Star mesh: one hop, nodes talk to a bridge
+
+#pragma once
+
+#include "l2_mesh.h"
+
+extern const struct mg_mesh mg_mesh_star;