I'm using the Network framework to perform SSDP discovery for UPnP devices. The multicast entitlement is set, and discovery is working fine, except when another application is also using port 1900. In that case the following errors are given (this is in a catalyst app on macOS, the behavior on a physical iPhone is the same): [] nw_path_evaluator_evaluate NECP_CLIENT_ACTION_ADD error [48: Address already in use] [] nw_path_create_evaluator_for_listener nw_path_evaluator_evaluate failed [listener] nw_listener_start_on_queue [L1] nw_path_create_evaluator_for_listener failed [connection_group] nw_connection_group_handle_listener_state_change [G1] listener failed with error Is it possible that multiple applications connect on the same port? Here's the relevant code from the discovery class I'm using: public class SSDPDiscovery {     enum SSDPMessageType {         case searchResponse         case availableNotification         case updateNotification         case unavailableNotification     }     private let multicastGroupAddress = "239.255.255.250"     private let multicastUDPPort: UInt16 = 1900     private var multicastGroup: NWMulticastGroup?     private var connectionGroup: NWConnectionGroup?     private var types = [String]()          func startDiscovery(forTypes types: [String]) throws {         guard multicastGroup == nil else { throw UPnPError.alreadyConnected }         let multicastGroup = try NWMulticastGroup(for:[.hostPort(host: .init(multicastGroupAddress), port: .init(integerLiteral: multicastUDPPort))]) let params = NWParameters.udp params.allowLocalEndpointReuse = true         let connectionGroup = NWConnectionGroup(with: multicastGroup, using: params)                  connectionGroup.stateUpdateHandler = { [weak self] (newState) in             Logger.swiftUPnP.debug("Connection group entered state \(String(describing: newState))")                          switch newState {             case let .failed(error):                 Logger.swiftUPnP.error("\(error.localizedDescription)")             case .cancelled:                 self?.multicastGroup = nil                 self?.connectionGroup = nil             default:                 break             }         }         connectionGroup.setReceiveHandler(maximumMessageSize: 65535, rejectOversizedMessages: true) { (message, content, isComplete) in             if let content = content {                 self.processData(content)             }         }                  connectionGroup.start(queue: .main)                  self.types = types         self.multicastGroup = multicastGroup         self.connectionGroup = connectionGroup     }          func stopDiscovery() {         guard let connectionGroup = connectionGroup else { return }         connectionGroup.cancel()         multicastGroup = nil         self.connectionGroup = nil         types = []     } }

I have an app for which Multicast Networking is enabled, so it can find UPnP players on the network. This is working fine on iOS devices, but when I compile for Catalyst and run on a Mac, the discovery fails with the following error: SSDP discovery did fail with error: Error Domain=UPnAtom Code=0 "Could not bind socket to port" UserInfo={NSLocalizedDescription=Could not bind socket to port} The error occurs at the stage below, both binding attempts will gave the same error. // Configure unicast socket: Bind to address on the specified interface (en0) to a random port to receive unicast datagrams // Configure multicast socket: Bind to port without defining the interface to bind to the address INADDR_ANY (0.0.0.0). This prevents any address filtering which allows datagrams sent to the multicast group to be receives I validated everything mentioned on this page (https://developer.apple.com/forums/thread/663271) for the Catalyst built app, and it looks fine. Is this a known incompatibility, or am I missing something?