Broadcast

The Broadcast screen (Broadcast & discovery) sends one UDP payload to many destinations at once, and listens on the other side to see who answers. Use it to find devices on a network, to check that a multicast stream reaches a receiver, or to play a device that answers discovery probes.

• The Emitter sends to a list of hosts, a broadcast address, a multicast group, or every host of a subnet — once, or again and again as a beacon.
• The Discovery listener listens on a port, joins multicast groups, lists every peer that talks to it, and can answer probes.

DANGER

Broadcast, multicast and a sweep reach every device on the network segment, not only the one you have in mind, and a beacon keeps doing it. Check which network you are on first, and only send to networks you own or are allowed to test. The limits below are guard rails, not permission.

Sending

1. Choose the Mode (below).
2. Enter the destination: the field's name changes with the mode. On a network with an IPv4 address, use my subnet fills it from this machine's address.
3. Choose the Payload and write it.
4. Press Send once: one datagram goes to each destination.

Modes

Mode	Destination	What happens	Default
List	Targets: IP:port or host:port entries separated by commas, semicolons or new lines — a space does not separate them. A name is looked up, its IPv4 address taken when it has one.	One datagram to each	127.0.0.1:9000, 127.0.0.1:9001
Broadcast	Address: 255.255.255.255:port, or an address ending in .255	One datagram that every host on the local network receives. Routers do not pass it on.	255.255.255.255:9000
Multicast	Address: a group from 224.0.0.0 to 239.255.255.255, with a port	One datagram to the group; only listeners that joined it receive it	239.1.1.1:9000
Sweep	CIDR block, a.b.c.d/nn, and a Port	One datagram to every usable host of the block, as unicast — for devices that ignore broadcast	192.168.1.0/24, port 9000

In a sweep the network and broadcast addresses are skipped (except in a /31 or /32), and a base that is not the network's own is rounded down to it: 192.0.2.77/30 sweeps 192.0.2.77 and 192.0.2.78. A sweep reaches at most 1024 hosts, so the widest block is a /22 (1022 hosts); a wider one is refused with the prefix to narrow it to.

Broadcast, multicast groups, sweeps and the discovery listener's joins are IPv4 only: IPv6 has no broadcast. A List may also name IPv6 hosts (see Socket options).

use my subnet fills x.y.z.10:9000, x.y.z.11:9000 in List, x.y.z.255:9000 in Broadcast and x.y.z.0/24 in Sweep, from this machine's address x.y.z.w. It assumes a /24 network.

Payload

Payload	What is sent
OSC	An OSC message: OSC address and typed Arguments, as on the OSC screen. Default /hello/discover with the text who-is-there.
text	The Text as UTF-8, exactly as typed, with no terminator. Default HELLO-PROBE.
raw hex	The Hex bytes byte for byte — to replay a captured frame or speak a binary discovery protocol. Pairs of hex digits; anything else between them is ignored. Default 48 45 4c 4c 4f.

Socket options

socket options opens three more settings:

Option	What	Default
Bind (source)	The local IP:port the datagrams go out from. Pin it to choose the network card, or a source port a device answers to. 0.0.0.0:0: any.	0.0.0.0:0
TTL / hops	How many routers a datagram may cross, 1–255. For multicast this is the multicast hop limit: 1 keeps it on this network.	1
loop back to this host	Multicast only: deliver the group's datagrams to this machine too, so a listener here hears them	on

With the default Bind (source) the datagrams go out from an IPv4 socket, or from an IPv6 one when every destination is IPv6. A list that mixes the two is sent from the IPv4 socket, and its IPv6 destinations fail; send them as a list of their own, or pin Bind (source) to an IPv6 address.

The result

Last emit shows what went out: Targets, Packets, Volume and Errors, and the first eight destinations it reached ("… +n more" for the rest). An error to one destination does not stop the others; the console line names how many failed.

Repeating as a beacon

A beacon sends the same round — one datagram to each destination — on a schedule, until you stop it. Devices that listen for a periodic announcement need one.

1. Set up the mode, destination and payload as for a single send.
2. Under Repeat as a beacon, set Rate (/s), and if it should stop by itself, Rounds or Seconds.
3. Press Start beacon. Stop beacon — or stopping its job in the console strip — ends it.

Field	What	Default
Rate (/s)	Rounds per second; must be above 0	2
Rounds	Stop after this many rounds; 0 — no limit	0
Seconds	Stop after this many seconds; 0 — no limit	0

The rate times the number of destinations may be at most 50 000 datagrams a second. A sweep of a /24 (254 hosts) can therefore repeat about 196 times a second at most. While the beacon runs, Last emit shows Targets (the destinations in each round, from the first report on), the totals, Rounds and PPS (datagrams per second), updated four times a second, and Send once is unavailable. A beacon that has had more than 32 failed datagrams and not one sent — no route, broadcast not allowed — stops by itself and says why.

Listening for devices

The Discovery listener binds a UDP port and records every peer that sends to it: what answers a probe, or what a device announces on its own.

1. Set Bind address, the port the devices send to.
2. For multicast, list the groups in Join multicast groups.
3. Press Listen. The settings lock until you press Stop listening.

Field	What	Default
Bind address	Where to listen, IP:port. 0.0.0.0 listens on every network card.	0.0.0.0:9000
Join multicast groups	IPv4 multicast groups to join, comma separated; empty — unicast and broadcast only	239.1.1.1
Interface	The IPv4 address of the network card to join the groups on; empty — the system chooses	empty
share the port	Listen on a port another program also uses (SO_REUSEADDR). It works only if that program allows sharing too.	on
answer probes	Answer probes, as a device would (below)	off

Peers

Peers lists who has sent something, the most recent first, with the number of peers seen, the packets heard and the replies sent above it:

Column	What
Peer	The sender's IP:port; its dot shows whether it was heard in the last 3 seconds
Proto	osc when its last datagram decoded as OSC, else udp
Packets	How many it sent
Age	Seconds since its last datagram
Last message	Its last datagram: the OSC address and arguments, or the start of the text

The list is refreshed a few times a second and holds up to 512 peers; past that, packets are still counted but new peers get no row.

Answering probes

With answer probes, the listener plays a device: it answers each datagram it receives, from the listening port, back to the sender's address and port.

Field	What	Default
Payload	The reply: OSC, text or hex, as for sending	OSC /hello/here with the text signal-lab
Reply delay (ms)	Wait this long before answering, as a slow device would	0
Only if it contains	Answer only datagrams whose decoded text contains this — the OSC address and arguments, or the start of the text; empty — every one	empty

The listener never answers a datagram identical to its own reply, so two listeners pointed at each other do not answer back and forth forever.

Firewalls and shared ports

Broadcast and multicast traffic from other machines is blocked by default by most Windows firewalls: allow Signal Lab on private networks when the app offers it. Broadcast never crosses a router. To listen on a port the real service already has, both sides must allow sharing (share the port here); without it, a taken port is refused with a hint to turn it on. See Troubleshooting.

In the Inspector

With capture armed, the screen's traffic appears with the protocol osc or udp, by its payload:

Source	What	How many
broadcast	Send once: each datagram, with the verdict fan-out, broadcast, multicast or sweep; a failed one with error: …	every one
beacon	A beacon's rounds	at most one round every 50 ms
discovery	Datagrams the listener receives	at most one every 40 ms
discovery	Its answers, with the verdict auto-reply	every one

What the Inspector leaves out of a beacon or the listener is counted: the next frame it draws carries the number in its verdict, +n not shown (for a beacon, one frame for each destination of each round left out). See Inspector.

On a server or in Docker

On a server, the screen sends and listens on the server's network. In Docker, broadcast, multicast and discovery reach the local network only when the container uses the host's network (--network host) on a Linux host. With Docker's default bridge network, or Docker Desktop, only unicast to hosts the container can reach works.

Elsewhere

• signallab send udp sends one datagram to one host; there is no command-line broadcast, multicast or sweep.
• A UDP datagram step sends a text datagram to one or more hosts, and a Wait for UDP step waits for one. See UDP and TCP.
• To play a device that answers by rules — several rules, replies built from what arrived — use a UDP device or OSC device emulator. See Emulators.

Problems

What you see	Usual cause
… is not a broadcast address	Broadcast takes 255.255.255.255:port or an address ending in .255. For another subnet mask, use Sweep.
… is not a multicast group	The address is outside 224.0.0.0–239.255.255.255.
… spans … addresses, and a sweep reaches at most 1024 hosts	The block is wider than a /22; narrow it.
Set the port to sweep	Port is 0.
… is over the … pps limit	The rate times the number of destinations is over 50 000 a second: lower Rate (/s), or narrow the destination.
… is already in use — turn on “share the port” to listen alongside it	Another program has the port; tick share the port.
Cannot join the multicast group …	The group or Interface is not usable on this machine — no network card with that address, or no multicast route.
Sent, but nobody answers	The devices listen on another port; the firewall here keeps their answers out; a router lies between you; or, in Docker, the container is not on the host network.
Probes go out, but the discovery listener hears no answers	Many devices answer to the address and port a probe came from — the emitter's own socket, which the screen does not read. Listen on the port the devices answer to, or send the probe from an experiment: a UDP datagram step with wait for a reply sends and listens on the same port.

Every error message is listed in Error messages.