diff --git a/game/scripts/net_link.gd b/game/scripts/net_link.gd
new file mode 100644
index 0000000..1e5ed2c
--- /dev/null
+++ b/game/scripts/net_link.gd
@@ -0,0 +1,154 @@
+class_name NetLink
+extends RefCounted
+## The network connection of a session, over ENet. A host listens for guests;
+## a guest connects to its host. Messages of the core travel on a reliable,
+## ordered channel. Positions of figures travel on a second, unreliable
+## channel, because a lost one is soon replaced by the next.
+
+## Port a host listens on unless told otherwise.
+const DEFAULT_PORT := 24680
+## Most guests that may connect to a host. More than can play, so that the
+## core can tell the extra ones why it turns them away.
+const MAX_CONNECTIONS := 8
+const CHANNEL_CORE := 0
+const CHANNEL_FIGURES := 1
+## Seconds a guest waits for its host to answer before it gives up.
+const CONNECT_SECONDS := 8.0
+
+## Seconds this guest waits for its host. Tests shorten it.
+var connect_seconds := CONNECT_SECONDS
+
+var _peer: ENetMultiplayerPeer
+var _peers: Dictionary[int, bool] = {}
+var _events: Array[Dictionary] = []
+var _connecting := false
+var _waited := 0.0
+
+
+## Splits "host" or "host:port" into [host, port], with DEFAULT_PORT if the
+## text names none. Returns an empty array if `text` is no such address.
+static func split_address(text: String) -> Array:
+	var parts := text.strip_edges().split(":")
+	if parts[0].is_empty() or parts.size() > 2:
+		return []
+	if parts.size() == 1:
+		return [parts[0], DEFAULT_PORT]
+	if not parts[1].is_valid_int() or parts[1].to_int() < 1 or parts[1].to_int() > 65535:
+		return []
+	return [parts[0], parts[1].to_int()]
+
+
+## Starts listening for guests on `port`, or on a free port that the system
+## chooses if `port` is 0; see local_port(). `address` limits listening to
+## one network interface, for example "127.0.0.1"; "*" listens on all.
+func host(port: int, address := "*") -> Error:
+	_start()
+	_peer.set_bind_ip(address)
+	return _peer.create_server(port, MAX_CONNECTIONS, 2)
+
+
+## The port a host listens on, or 0.
+func local_port() -> int:
+	if not is_open() or _peer.host == null:
+		return 0
+	return _peer.host.get_local_port()
+
+
+## Starts connecting to the host at `address` and `port`.
+func join(address: String, port: int) -> Error:
+	_start()
+	var error := _peer.create_client(address, port, 2)
+	_connecting = error == OK
+	_waited = 0.0
+	return error
+
+
+func _start() -> void:
+	close()
+	_peer = ENetMultiplayerPeer.new()
+	_peers.clear()
+	_peer.peer_connected.connect(_on_connected)
+	_peer.peer_disconnected.connect(_on_disconnected)
+
+
+func _on_connected(peer: int) -> void:
+	_peers[peer] = true
+	_events.append({"type": "connected", "peer": peer})
+
+
+func _on_disconnected(peer: int) -> void:
+	_peers.erase(peer)
+	_events.append({"type": "disconnected", "peer": peer})
+
+
+## True while the connection is up or being made.
+func is_open() -> bool:
+	return _peer != null and _peer.get_connection_status() != MultiplayerPeer.CONNECTION_DISCONNECTED
+
+
+## True when a guest has reached its host, or a host is listening.
+func is_connected_to_peers() -> bool:
+	return _peer != null and _peer.get_connection_status() == MultiplayerPeer.CONNECTION_CONNECTED
+
+
+## Returns what happened since the last call, oldest first, as dictionaries
+## with a `type` entry: "connected" and "disconnected" (entry `peer`),
+## "message" and "figure" (entries `peer` and `bytes`), and "failed" when a
+## guest could not reach its host. `delta` is the time since the last call.
+func poll(delta: float) -> Array[Dictionary]:
+	if is_open():
+		_peer.poll()
+		while _peer.get_available_packet_count() > 0:
+			var from := _peer.get_packet_peer()
+			var channel := _peer.get_packet_channel()
+			var bytes := _peer.get_packet()
+			var type := "message" if channel == CHANNEL_CORE else "figure"
+			_events.append({"type": type, "peer": from, "bytes": bytes})
+	if _connecting:
+		_waited += delta
+		if is_connected_to_peers():
+			_connecting = false
+		elif not is_open() or _waited > connect_seconds:
+			_connecting = false
+			close()
+			_events.append({"type": "failed"})
+	var events := _events
+	_events = []
+	return events
+
+
+## Sends a message of the core to `peer`, reliably and in order. A message
+## to a connection that is gone is dropped.
+func send(peer: int, bytes: PackedByteArray) -> void:
+	_put(peer, bytes, CHANNEL_CORE, MultiplayerPeer.TRANSFER_MODE_RELIABLE)
+
+
+## Sends the position of a figure to `peer`, to every connection if `peer`
+## is 0, or to every connection but one if `peer` is that one negated.
+func send_figure(peer: int, bytes: PackedByteArray) -> void:
+	_put(peer, bytes, CHANNEL_FIGURES, MultiplayerPeer.TRANSFER_MODE_UNRELIABLE)
+
+
+func _put(peer: int, bytes: PackedByteArray, channel: int, mode: MultiplayerPeer.TransferMode) -> void:
+	if not is_connected_to_peers() or _peers.is_empty():
+		return
+	if peer > 0 and not _peers.has(peer):
+		return
+	_peer.set_target_peer(peer)
+	_peer.set_transfer_channel(channel)
+	_peer.set_transfer_mode(mode)
+	_peer.put_packet(bytes)
+
+
+## Closes the connection to `peer` once the messages waiting for it are out.
+func close_peer(peer: int) -> void:
+	if _peers.has(peer):
+		_peer.get_peer(peer).peer_disconnect_later()
+
+
+## Closes everything at once.
+func close() -> void:
+	if is_open():
+		_peer.close()
+	_peers.clear()
+	_connecting = false
diff --git a/game/scripts/net_link.gd.uid b/game/scripts/net_link.gd.uid
new file mode 100644
index 0000000..4aac885
--- /dev/null
+++ b/game/scripts/net_link.gd.uid
@@ -0,0 +1 @@
+uid://dmyy8qbtuhhdh
diff --git a/game/scripts/session_link.gd b/game/scripts/session_link.gd
new file mode 100644
index 0000000..474d854
--- /dev/null
+++ b/game/scripts/session_link.gd
@@ -0,0 +1,103 @@
+class_name SessionLink
+extends RefCounted
+## Carries the messages of a GwSession over a NetLink, and the positions of
+## the figures, which are not part of the shared world. Tells the game what
+## happened on the way.
+
+## Bytes of a position: x and y as 32-bit floats.
+const POSITION_BYTES := 8
+
+var session: GwSession
+var link: NetLink
+
+
+func _init(game_session: GwSession, net_link: NetLink) -> void:
+	session = game_session
+	link = net_link
+
+
+## Moves messages in both directions and returns notices, oldest first, as
+## dictionaries with a `type` entry:
+## - "guest_arrived" and "guest_left" (entry `peer`), at the host
+## - "world_received" (entry `resync`), "refused" (entry `reason`),
+##   "protocol_error", "host_lost" and "failed", at a guest
+## - "divergence" (entries `peer`, `tick`, `hash`), at the host
+## - "figure" (entries `player`, `position`), everywhere
+## `delta` is the time since the last call.
+func pump(delta: float) -> Array[Dictionary]:
+	var notices: Array[Dictionary] = []
+	for event in link.poll(delta):
+		match event["type"]:
+			"connected":
+				if session.is_host():
+					session.connect_peer(event["peer"])
+					notices.append({"type": "guest_arrived", "peer": event["peer"]})
+			"disconnected":
+				if session.is_host():
+					session.disconnect_peer(event["peer"])
+					notices.append({"type": "guest_left", "peer": event["peer"]})
+				else:
+					notices.append({"type": "host_lost"})
+			"failed":
+				notices.append({"type": "failed"})
+			"message":
+				notices.append_array(session.receive(event["peer"], event["bytes"]))
+			"figure":
+				_receive_figure(event["peer"], event["bytes"], notices)
+	for divergence in session.take_divergences():
+		divergence["type"] = "divergence"
+		notices.append(divergence)
+	flush()
+	return notices
+
+
+## Sends what the session has for the network and closes the connections
+## the host gave up. A guest's messages wait until its host is reached.
+func flush() -> void:
+	if not link.is_connected_to_peers():
+		return
+	for message in session.take_messages():
+		link.send(message["peer"], message["bytes"])
+	for peer in session.take_closing():
+		link.close_peer(peer)
+
+
+## Tells the other machines where this machine's figure stands.
+func send_figure(position: Vector2) -> void:
+	if session.is_host():
+		var player := session.local_player()
+		if player >= 0:
+			link.send_figure(0, _figure_bytes(player, position))
+	elif session.local_player() >= 0:
+		link.send_figure(GwSession.HOST_PEER, _position_bytes(position))
+
+
+func _receive_figure(peer: int, bytes: PackedByteArray, notices: Array[Dictionary]) -> void:
+	if session.is_host():
+		# A guest sends only a position; the host knows whose it is and
+		# passes it on to every other guest.
+		var player := session.player_of_peer(peer)
+		if player < 0 or bytes.size() != POSITION_BYTES:
+			return
+		var position := Vector2(bytes.decode_float(0), bytes.decode_float(4))
+		notices.append({"type": "figure", "player": player, "position": position})
+		link.send_figure(-peer, _figure_bytes(player, position))
+	elif bytes.size() == 1 + POSITION_BYTES:
+		var player := bytes.decode_u8(0)
+		if player != session.local_player():
+			var position := Vector2(bytes.decode_float(1), bytes.decode_float(5))
+			notices.append({"type": "figure", "player": player, "position": position})
+
+
+func _position_bytes(position: Vector2) -> PackedByteArray:
+	var bytes := PackedByteArray()
+	bytes.resize(POSITION_BYTES)
+	bytes.encode_float(0, position.x)
+	bytes.encode_float(4, position.y)
+	return bytes
+
+
+func _figure_bytes(player: int, position: Vector2) -> PackedByteArray:
+	var bytes := PackedByteArray([player])
+	bytes.append_array(_position_bytes(position))
+	return bytes
diff --git a/game/scripts/session_link.gd.uid b/game/scripts/session_link.gd.uid
new file mode 100644
index 0000000..32aac12
--- /dev/null
+++ b/game/scripts/session_link.gd.uid
@@ -0,0 +1 @@
+uid://dpau4ls365tsj
diff --git a/game/tests/test_net_link.gd b/game/tests/test_net_link.gd
new file mode 100644
index 0000000..4cf5a85
--- /dev/null
+++ b/game/tests/test_net_link.gd
@@ -0,0 +1,131 @@
+extends TestCase
+## Two NetLinks in one process, connected over the loopback interface.
+
+## Events per link, collected while frames pass.
+var _events: Dictionary[NetLink, Array] = {}
+
+
+## Polls `links` for `frames` frames and collects their events.
+func _pump(links: Array[NetLink], frames: int) -> void:
+	for frame in frames:
+		for link in links:
+			if not _events.has(link):
+				_events[link] = []
+			_events[link].append_array(link.poll(1.0 / 60.0))
+		await tree.process_frame
+
+
+func _types(link: NetLink) -> Array:
+	return _events.get(link, []).map(func(event: Dictionary) -> String: return event["type"])
+
+
+func _first(link: NetLink, type: String) -> Dictionary:
+	for event: Dictionary in _events.get(link, []):
+		if event["type"] == type:
+			return event
+	return {}
+
+
+## A host and a guest that have connected.
+func _connected_pair() -> Array[NetLink]:
+	var host := NetLink.new()
+	check_eq(host.host(0, "127.0.0.1"), OK, "the host listens")
+	check(host.local_port() > 0, "on a port the system chose")
+	var guest := NetLink.new()
+	check_eq(guest.join("127.0.0.1", host.local_port()), OK, "the guest starts connecting")
+	await _pump([host, guest], 10)
+	return [host, guest]
+
+
+func test_a_guest_connects_and_both_sides_hear_of_it() -> void:
+	var links := await _connected_pair()
+	var host := links[0]
+	var guest := links[1]
+	check(guest.is_connected_to_peers(), "the guest is connected")
+	check_eq(_first(guest, "connected").get("peer"), GwSession.HOST_PEER, "the guest sees the host as peer 1")
+	check(_first(host, "connected").get("peer", 0) > 1, "the host sees the guest")
+	host.close()
+	guest.close()
+
+
+func test_messages_arrive_in_order_on_both_channels() -> void:
+	var links := await _connected_pair()
+	var host := links[0]
+	var guest := links[1]
+	var guest_peer: int = _first(host, "connected")["peer"]
+	_events.clear()
+	var big := PackedByteArray()
+	big.resize(50000)
+	big.fill(7)
+	host.send(guest_peer, big)
+	host.send(guest_peer, PackedByteArray([1]))
+	guest.send(GwSession.HOST_PEER, PackedByteArray([2, 3]))
+	guest.send_figure(GwSession.HOST_PEER, PackedByteArray([4]))
+	await _pump([host, guest], 20)
+
+	var received := _events[guest].filter(func(event: Dictionary) -> bool: return event["type"] == "message")
+	check_eq(received.size(), 2, "two messages for the guest")
+	if received.size() == 2:
+		check_eq(received[0]["bytes"].size(), 50000, "the big one first")
+		check_eq(received[1]["bytes"], PackedByteArray([1]), "then the small one")
+	check_eq(_first(host, "message").get("bytes"), PackedByteArray([2, 3]), "the host got the guest's message")
+	check_eq(_first(host, "figure").get("bytes"), PackedByteArray([4]), "and the position on the other channel")
+	host.close()
+	guest.close()
+
+
+func test_closing_a_guest_delivers_what_was_sent_before() -> void:
+	var links := await _connected_pair()
+	var host := links[0]
+	var guest := links[1]
+	var guest_peer: int = _first(host, "connected")["peer"]
+	_events.clear()
+	host.send(guest_peer, PackedByteArray([9, 9]))
+	host.close_peer(guest_peer)
+	await _pump([host, guest], 30)
+	check_eq(_types(guest), ["message", "disconnected"], "the message comes before the end")
+	check_eq(_types(host), ["disconnected"], "the host sees the guest go")
+	check(not guest.is_open(), "the guest's connection is closed")
+	host.close()
+
+
+func test_a_guest_that_quits_is_seen_leaving() -> void:
+	var links := await _connected_pair()
+	var host := links[0]
+	var guest := links[1]
+	_events.clear()
+	guest.close()
+	await _pump([host, guest], 10)
+	check_eq(_types(host), ["disconnected"], "the host sees the guest go")
+	host.close()
+
+
+func test_messages_to_a_connection_that_is_gone_are_dropped() -> void:
+	var host := NetLink.new()
+	check_eq(host.host(0, "127.0.0.1"), OK, "the host listens")
+	host.send(12345, PackedByteArray([1]))
+	host.send_figure(12345, PackedByteArray([1]))
+	host.close_peer(12345)
+	check_eq(host.poll(0.0), [], "nothing happened")
+	host.close()
+
+
+func test_a_guest_gives_up_when_nobody_answers() -> void:
+	# A port that was free a moment ago and has nobody listening now.
+	var probe := NetLink.new()
+	probe.host(0, "127.0.0.1")
+	var port := probe.local_port()
+	probe.close()
+	var guest := NetLink.new()
+	guest.connect_seconds = 0.2
+	check_eq(guest.join("127.0.0.1", port), OK, "connecting starts")
+	await _pump([guest], 30)
+	check_eq(_types(guest), ["failed"], "the guest gives up")
+	check(not guest.is_open(), "and closes")
+
+
+func test_addresses_are_split_into_host_and_port() -> void:
+	check_eq(NetLink.split_address("192.168.1.20"), ["192.168.1.20", NetLink.DEFAULT_PORT], "default port")
+	check_eq(NetLink.split_address(" example.net:25000 "), ["example.net", 25000], "with port")
+	for text in ["", ":25000", "host:", "host:0", "host:70000", "host:port", "a:1:2"]:
+		check_eq(NetLink.split_address(text), [], "refused: '%s'" % text)
diff --git a/game/tests/test_net_link.gd.uid b/game/tests/test_net_link.gd.uid
new file mode 100644
index 0000000..c99d6ee
--- /dev/null
+++ b/game/tests/test_net_link.gd.uid
@@ -0,0 +1 @@
+uid://bmma6gn72s36v
diff --git a/game/tests/test_session_link.gd b/game/tests/test_session_link.gd
new file mode 100644
index 0000000..90ca2d9
--- /dev/null
+++ b/game/tests/test_session_link.gd
@@ -0,0 +1,141 @@
+extends TestCase
+## A host and guests in one process, each with its own GwSession and
+## NetLink, connected over the loopback interface.
+
+var host_key := PackedByteArray([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1])
+var guest_key := PackedByteArray([2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2])
+
+var host: SessionLink
+## Notices per SessionLink, collected while frames pass.
+var notices: Dictionary[SessionLink, Array] = {}
+
+
+func _start_host() -> void:
+	var session := GwSession.new()
+	session.start_new(7)
+	session.join(host_key, "Host")
+	session.step()
+	var link := NetLink.new()
+	check_eq(link.host(0, "127.0.0.1"), OK, "the host listens")
+	host = SessionLink.new(session, link)
+	notices[host] = []
+
+
+func _start_guest(key: PackedByteArray, name: String) -> SessionLink:
+	var session := GwSession.new()
+	session.start_guest(key, name)
+	var link := NetLink.new()
+	link.join("127.0.0.1", host.link.local_port())
+	var guest := SessionLink.new(session, link)
+	notices[guest] = []
+	return guest
+
+
+## Lets `frames` frames pass. The host runs a tick per frame if `ticking`;
+## every guest runs what arrived.
+func _play(guests: Array[SessionLink], frames: int, ticking := true) -> void:
+	for frame in frames:
+		notices[host].append_array(host.pump(1.0 / 60.0))
+		if ticking:
+			host.session.step()
+			host.flush()
+		for guest in guests:
+			notices[guest].append_array(guest.pump(1.0 / 60.0))
+			for i in guest.session.pending_ticks():
+				guest.session.step()
+		await tree.process_frame
+
+
+func _types(link: SessionLink) -> Array:
+	return notices[link].map(func(notice: Dictionary) -> String: return notice["type"])
+
+
+func _first(link: SessionLink, type: String) -> Dictionary:
+	for notice: Dictionary in notices[link]:
+		if notice["type"] == type:
+			return notice
+	return {}
+
+
+func _close(links: Array[SessionLink]) -> void:
+	for link in links:
+		link.link.close()
+
+
+func test_a_guest_joins_over_the_network_and_follows_the_host() -> void:
+	_start_host()
+	var guest := _start_guest(guest_key, "Grace")
+	await _play([guest], 40)
+	await _play([guest], 10, false)
+
+	check(_types(host).has("guest_arrived"), "the host sees the guest arrive")
+	check_eq(_first(guest, "world_received").get("resync"), false, "the guest got the host's world")
+	check_eq(guest.session.local_player(), 1, "the guest is player 1")
+	check_eq(guest.session.player_name(0), "Host", "the guest knows the host's name")
+	check_eq(guest.session.tick(), host.session.tick(), "the guest is at the host's tick")
+	check_eq(guest.session.state_hash_hex(), host.session.state_hash_hex(), "both worlds are the same")
+	check_eq(guest.session.save().size(), 0, "a guest does not save the host's world")
+	_close([host, guest])
+
+
+func test_a_hit_by_the_guest_reaches_the_host() -> void:
+	_start_host()
+	var guest := _start_guest(guest_key, "Grace")
+	await _play([guest], 30)
+	var rock := host.session.spawn_point() + Vector2i(7, -2)
+	guest.session.dig(rock.x, rock.y)
+	await _play([guest], 10)
+	await _play([guest], 10, false)
+	var index := rock.y * host.session.map_width() + rock.x
+	check_eq(host.session.damage_codes()[index], 1, "the host's rock took the hit")
+	check_eq(guest.session.state_hash_hex(), host.session.state_hash_hex(), "both worlds are the same")
+	_close([host, guest])
+
+
+func test_figures_travel_to_the_other_machines() -> void:
+	_start_host()
+	var guest := _start_guest(guest_key, "Grace")
+	var other := _start_guest(PackedByteArray([3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3]), "Linus")
+	await _play([guest, other], 30)
+	host.send_figure(Vector2(10.0, 20.0))
+	guest.send_figure(Vector2(5.5, 6.5))
+	await _play([guest, other], 10)
+
+	check_eq(_first(host, "figure"), {"type": "figure", "player": 1, "position": Vector2(5.5, 6.5)}, "the host sees the guest")
+	var seen := notices[other].filter(func(notice: Dictionary) -> bool: return notice["type"] == "figure")
+	check(seen.has({"type": "figure", "player": 0, "position": Vector2(10.0, 20.0)}), "the other guest sees the host")
+	check(seen.has({"type": "figure", "player": 1, "position": Vector2(5.5, 6.5)}), "and the first guest, passed on by the host")
+	check(not notices[guest].any(func(notice: Dictionary) -> bool: return notice["type"] == "figure" and notice["player"] == 1), "nobody gets their own figure back")
+	_close([host, guest, other])
+
+
+func test_a_guest_with_the_key_of_a_player_in_the_world_is_turned_away() -> void:
+	_start_host()
+	var twin := _start_guest(host_key, "Host")
+	await _play([twin], 30)
+	check_eq(_first(twin, "refused").get("reason"), GwSession.REFUSED_ALREADY_PLAYING, "the reason is given")
+	check(_types(twin).has("host_lost"), "then the host closes the connection")
+	check(not twin.session.is_running(), "the guest never got a world")
+	_close([host, twin])
+
+
+func test_a_guest_that_quits_leaves_the_world() -> void:
+	_start_host()
+	var guest := _start_guest(guest_key, "Grace")
+	await _play([guest], 30)
+	check_eq(host.session.online_players(), PackedInt64Array([0, 1]), "two players online")
+	guest.link.close()
+	await _play([], 10)
+	check(_types(host).has("guest_left"), "the host sees the guest leave")
+	check_eq(host.session.online_players(), PackedInt64Array([0]), "only the host is online")
+	_close([host])
+
+
+func test_a_guest_learns_when_the_host_is_gone() -> void:
+	_start_host()
+	var guest := _start_guest(guest_key, "Grace")
+	await _play([guest], 30)
+	host.link.close()
+	await _play([guest], 10, false)
+	check(_types(guest).has("host_lost"), "the guest notices")
+	_close([guest])
diff --git a/game/tests/test_session_link.gd.uid b/game/tests/test_session_link.gd.uid
new file mode 100644
index 0000000..1ccdbae
--- /dev/null
+++ b/game/tests/test_session_link.gd.uid
@@ -0,0 +1 @@
+uid://c6tn270be6rbu