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c#multicastmulticastsocketigmp

JoinMulticastGroup with TTL

I am using C# .net framework 4.7. My application is trying to join a multicast group that's a few routers (hops) away. As a result, I am trying to send the IGMP packet with TTL > 1. Currently, I am using UdpClient.JoinMulticastGroup() with the TTL parameter set to 3, but when I examine the IGMP packet with Wireshark, the TTL stays at 1.

Here is my code

        UdpClient udpClient = new UdpClient();
        // Creates an IPAddress to use to join and drop the multicast group.
        IPAddress multicastIpAddress = IPAddress.Parse("239.192.16.107");
        udpClient.Client.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.IpTimeToLive, 10);
        udpClient.Client.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastTimeToLive, 10);
        udpClient.Ttl = 10;
        // The packet dies after 10 router hops.
        udpClient.JoinMulticastGroup(multicastIpAddress, 10);

The several posts online seems to suggest that IGMP is meant for local network, so TTL is always 1; however, if that's true, why would JoinMulticastGroup allow us to set the TTL? but on the other side, why would TTL not change when I set TTL to 3?

Could someone confirm if the multicast join packet (IGMP) can have a TTL value other than 1? If it can, then is there something missing in my code?

Update: I set TTL to 10, and Wireshark still shows TTL=1

Time to live: 1

Solution

  • The specified TTL only applies to outgoing multicast packets. From the Microsoft Docs under Remarks:

    The timeToLive parameter specifies how many router hops will be allowed for a multicasted datagram before being discarded.

    IGMP messages will always have a TTL of 1, as they only need to be received by the local router. The router will then send its own messages to other routers as necessary. See https://networkengineering.stackexchange.com/questions/9636/why-ttl-value-1-in-igmp for more details.