The RTCIceCandidate interface's read-only protocol property is a string which indicates whether the candidate uses UDP or TCP as its transport protocol.

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

protocol is null by default if not specified properly in the SDP, but this is an error condition and will result in a thrown exception when you call RTCPeerConnection.addIceCandidate().

A string that indicates what network protocol the candidate uses:

tcp

The candidate, if selected, would use TCP as the transport protocol for its data. The tcpType property provides additional information about the kind of TCP candidate represented by the object.

udp

The candidate will use the UDP transport protocol for its data. This is the preferred protocol for media interactions because of its better performance profile.

Here's an example candidate a-line from an ICE transaction:

a=candidate:4234997325 1 udp 2043278322 192.0.2.172 44323 typ host

The third field, "udp", is the protocol type, indicating that the candidate would use the UDP transport protocol.

This code snippet examines the value of protocol to decide if it should look at the value of tcpType to see if it's a simultaneous-open (S-O) candidate.

if (candidate.protocol === "tcp") {
  if (candidate.tcpType === "so") {
    adjustForSimultaneousOpen(candidate);
  }
}