Scouting

Scouting is the mechanism by which Zenoh nodes discover each other without prior knowledge of network addresses. It operates over UDP multicast (for automatic local-network discovery) or UDP unicast (for targeted discovery across routed networks).

Scouting only discovers endpoints — it does not establish a Zenoh session. After discovery the node uses the locators advertised in the HELLO message to initiate the INIT/OPEN handshake over a transport link.

Message Flow

Node A                          Node B
  |── SCOUT (multicast/unicast)─>|
  |<──────────── HELLO ──────────|   unicast reply

Scouting Modes

Multicast: SCOUT and HELLO messages are exchanged on a well-known UDP multicast group and port. All nodes on the local network segment that match the WhatAmIMatcher respond with a HELLO.
Unicast: SCOUT messages are sent directly to one or more configured unicast endpoints. Used when IP multicast is unavailable or when discovering specific routers by address.
Passive (HELLO-only): A node may send periodic HELLO messages in multicast without a preceding SCOUT, advertising itself to any listening node.

Relationship to Session Establishment

Once a suitable node has been identified via HELLO, the discovering node opens a transport link to one of the advertised locators and performs the INIT/OPEN handshake to establish a full session.