Network Framework Broadcast Support

Hi all,

I am overhauling code of an iPadOS app that discovers devices on a network using a custom UDP broadcast based discovery protocol.

This is how the discovery mechanism should work: The iPad sends an IPv4 broadcast message to the network's broadcast address using a fixed destination port, but a random source port (determined at bind time). The device responds with a unicast message to the source IP address and port of the discovery message.

Until now the code is based on BSD sockets using GCDAsyncUdpSocket and has been working well for around ten years with a single socket that was used to both send and receive the discovery messages and replies.

We would like to make the move to the Network Framework now and I tried to recreate this discovery mechanism with the Network Framework in Objective-C.

I am able to create an nw_connection_t with the broadcast address as hostname and the specific destination port as port. I am able to send discovery messages to the device and the device sends a reply (verified with Wireshark). But calling nw_connection_receive_message(...) never fires. Also in Wireshark the iPad replies with Destination unreachable (Port unreachable).

When I create the connection with the unicast address of the device, the reply is received.

It seems to me, that the connection doesn't accept replies from addresses / ports that don't match, what was set when the connection was created. Is there a way to also accept messages from other sources? E.g. there is nw_multicast_group_descriptor_set_disable_unicast_traffic when doing multicast. This seems to solve this problems when doing mutlicast.

This is a code excerpt of what I tried:

// Create default UDP parameters without DTLS
nw_parameters_t params = nw_parameters_create_secure_udp(NW_PARAMETERS_DISABLE_PROTOCOL, NW_PARAMETERS_DEFAULT_CONFIGURATION);
// Enable P2P (should enable broadcast and multicast)
nw_parameters_set_include_peer_to_peer(params, true);
// Require the active interface
// The active interface comes from a path monitor callback
nw_parameters_require_interface(params, self.networkUtils.activeInterface.interface);
          
// Setup the remote endpoint with the "ping" (discovery) broadcast IP address and port
const char *endpointAddress = [pingAddress.ipAddress cStringUsingEncoding:NSUTF8StringEncoding];
NSString *portString = @(pingAddress.port).stringValue;
const char *endpointPort = [portString cStringUsingEncoding:NSUTF8StringEncoding];
nw_endpoint_t broadcastEndpoint = nw_endpoint_create_host(endpointAddress, endpointPort);
            
nw_connection_t tmpConnection = nw_connection_create(broadcastEndpoint, params);
__weak __typeof(self) weakSelf = self;
nw_connection_set_state_changed_handler(tmpConnection, ^(nw_connection_state_t state, nw_error_t  _Nullable error) {
    __strong __typeof(weakSelf) strongSelf = weakSelf;
    MSLogVerbose("State changed: %d; error: %@", state, error);
    strongSelf.connectionState = state;
    if (state == nw_connection_state_ready) {
        [strongSelf receiveMessageForConnection:tmpConnection];
     }
});
nw_connection_set_queue(tmpConnection, AGGalileoBrowser.browserQueue);
nw_connection_start(tmpConnection);

Thanks for your help! Arno