The RTCIceCandidate interface's read-only relatedAddress property is a string indicating the related address of a relay or reflexive candidate.

If the candidate is a host candidate (that is, its address is in fact the real IP address of the remote peer), relatedAddress is null.

The relatedAddress field's value is set from the candidateInfo options object passed to the RTCIceCandidate() constructor. You can't specify the value of relatedAddress directly in the options object, but its value is automatically extracted from the object's candidate a-line if it's formatted properly(the rel-address field).

The related address and port (relatedPort) are not used at all by ICE itself; they are provided for analysis and diagnostic purposes only, and their inclusion may be blocked by security systems, so do not rely on them having non-null values.

A string which contains the candidate's related address. For both peer and server reflexive candidates, the related address (and related port) are the base for that server or peer reflexive candidate. For relay candidates, the related address and port are set to the mapped address selected by the TURN server.

For host candidates, relatedAddress is null, meaning the field is not included in the candidate's a-line.

The related address is included in ICE candidates despite not being used by ICE itself. relatedAddress can be used for diagnostic purposes; by observing the relationships between the various types of candidates and their addresses and related addresses. relatedAddress can also be used by Quality-of-Service (QoS) mechanisms.

Here's an SDP attribute line (a-line) describing an ICE candidate discovered by the STUN server:

a=candidate:4234997325 1 udp 2043278322 192.0.2.172 6502 typ srflx raddr 198.51.100.45 rport 32768 generation 0

The remote address, relatedAddress, is the dotted quad (for IPv4) or colon-delineated 64-bit address (for IPv6) immediately following the text "raddr", or "198.51.100.45".

In this example, the candidate's type is checked, and then debugging output is presented, based on the candidate type, including the candidate's ip and relatedAddress.

switch (candidate.type) {
  case "host":
    console.log(`Host candidate's IP address is ${candidate.ip}`);
    break;
  case "srflx":
    console.log(
      `Server reflexive candidate's base address is ${candidate.relatedAddress}; reachable at ${candidate.ip}`,
    );
    break;
  case "prflx":
    console.log(
      `Peer reflexive candidate's base address is ${candidate.relatedAddress}; reachable at ${candidate.ip}`,
    );
    break;
  case "relay":
    console.log(
      `Relay candidate's address assigned by the TURN server is ${candidate.relatedAddress}; reachable at ${candidate.ip}`,
    );
    break;
}

• WebRTC API
• Introduction to WebRTC protocols
• WebRTC connectivity
• Lifetime of a WebRTC session
• RTCIceCandidate.relatedPort
• RTCIceCandidate.address and RTCIceCandidate.port