The RTCIceCandidate interface's read-only relatedPort property indicates the port number of reflexive or relay candidates.

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

The relatedPort field's value is set from the candidateInfo options object passed to the RTCIceCandidate() constructor. You can't specify the value of relatedPort 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-port field).

The related address (relatedAddress) and port 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.

An unsigned 16-bit value containing the candidate's related port number, if any. For both peer and server reflexive candidates, the related address and port describe the base for that candidate. For relay candidates, the related address and port provide the mapped address selected by the TURN server.

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

The related address and port are not used by ICE itself, and are only present for diagnostic and Quality-of-Service purposes. They may in fact be omitted for security reasons, but if present can be a useful tool during debugging. See the Example, which shows a bit of this.

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 port, relatedPort, is the number immediately following the "rport" label on the a-line, or 32768.

In this example, the candidate's type is checked, and then debugging output is presented, based on the candidate type, including the candidate's type, address (ip and port), and related address (relatedAddress and relatedPort).

const ip = candidate.ip;
const port = candidate.port;
const relIP = candidate.relatedAddress;
const relPort = candidate.relatedPort;

if (relIP && relPort) {
  console.log(
    `Candidate type '${type}' — contact address: ${ip}${port}, related address: ${relIP}${relPort}`,
  );
} else {
  console.log(`Host candidate address is ${ip}${port}`);
}

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