How does Chrome/webRTC select which TURN server to use if multiple options are given?

Question

When passing multiple TURN options to RTCPeerConnection, and assuming that several of them are technically usable, how is the actual server selected?

Does network performance play a role at all?

Answer 1

ICE RFC doesn't specify what happens when multiple TURN servers are available:

This specification only considers usage of a single STUN or TURN server. When there are multiple choices for that single STUN or TURN server (when, for example, they are learned through DNS records and multiple results are returned), an agent SHOULD use a single STUN or TURN server (based on its IP address) for all candidates for a particular session. This improves the performance of ICE. The result is a set of pairs of host candidates with STUN or TURN servers. The agent then chooses one pair, and sends a Binding or Allocate request to the server from that host candidate.

So any particular behavior shouldn't be expected.

Currently WebRTC chooses first server which matches the given criteria. E.g. first server which supports UDP for UDP connection.

basicportallocator.cc:

void AllocationSequence::CreateUDPPorts() {
...
  // If STUN is not disabled, setting stun server address to port.
  if (!IsFlagSet(PORTALLOCATOR_DISABLE_STUN)) {
    // If config has stun_address, use it to get server reflexive candidate
    // otherwise use first TURN server which supports UDP.
    if (config_ && !config_->stun_address.IsNil()) {
      LOG(LS_INFO) << "AllocationSequence: UDPPort will be handling the "
                   <<  "STUN candidate generation.";
      port->set_server_addr(config_->stun_address);
    } else if (config_ &&
               config_->SupportsProtocol(RELAY_TURN, PROTO_UDP)) {
      port->set_server_addr(config_->GetFirstRelayServerAddress(
          RELAY_TURN, PROTO_UDP));
      LOG(LS_INFO) << "AllocationSequence: TURN Server address will be "
                   << " used for generating STUN candidate.";
    }
  }
...