What does it mean to bind a multicast UDP socket

Question

I am using multicast UDP between hosts that have multiple network interfaces  I am using boost  asio  and am confused by the 2 operations receivers have to make  bind  then join-group   Why do you need to specify the local address of an interface  during bind  when you do that with every multicast group that you join   The sister-question regards the multicast port  Since during sending  you send to a multicast address  amp  port  why  during subscription to a multicast group  you only specify the address  not the port - the port being specified in the confusing call to bind   Note  the  join-group  is a wrapper over setsockopt IP ADD MEMBERSHIP   which as documented  may be called multiple times on the same socket to subscribe to different groups  over different networks    It would therefore make perfect sense to ditch the bind call and specify the port every time I subscribe to a group   From what I see  always binding to  0 0 0 0  and specifying the interface address when joining the group  works very well  Confused

User · Answer

It is also very important to distinguish a SENDING multicast socket from a RECEIVING multicast socket.

I agree with all the answers above regarding RECEIVING multicast sockets. The OP noted that binding a RECEIVING socket to an interface did not help. However, it is necessary to bind a multicast SENDING socket to an interface.

For a SENDING multicast socket on a multi-homed server, it is very important to create a separate socket for each interface you want to send to. A bound SENDING socket should be created for each interface.

  // This is a fix for that bug that causes Servers to pop offline/online.
  // Servers will intermittently pop offline/online for 10 seconds or so.
  // The bug only happens if the machine had a DHCP gateway, and the gateway is no longer accessible.
  // After several minutes, the route to the DHCP gateway may timeout, at which
  // point the pingponging stops.
  // You need 3 machines, Client machine, server A, and server B
  // Client has both ethernets connected, and both ethernets receiving CITP pings (machine A pinging to en0, machine B pinging to en1)
  // Now turn off the ping from machine B (en1), but leave the network connected.
  // You will notice that the machine transmitting on the interface with
  // the DHCP gateway will fail sendto() with errno 'No route to host'
  if ( theErr == 0 )
  {
     // inspired by 'ping -b' option in man page:      
     //      -b boundif
     //             Bind the socket to interface boundif for sending.
     struct sockaddr_in bindInterfaceAddr;
     bzero(&bindInterfaceAddr, sizeof(bindInterfaceAddr));
     bindInterfaceAddr.sin_len = sizeof(bindInterfaceAddr);
     bindInterfaceAddr.sin_family = AF_INET;
     bindInterfaceAddr.sin_addr.s_addr = htonl(interfaceipaddr);
     bindInterfaceAddr.sin_port = 0; // Allow the kernel to choose a random port number by passing in 0 for the port.
     theErr = bind(mSendSocketID, (struct sockaddr *)&bindInterfaceAddr, sizeof(bindInterfaceAddr));
     struct sockaddr_in serverAddress;
     int namelen = sizeof(serverAddress);  
     if (getsockname(mSendSocketID, (struct sockaddr *)&serverAddress, (socklen_t *)&namelen) < 0) {
        DLogErr(@"ERROR Publishing service... getsockname err");
     }
     else
     {
        DLog( @"socket %d bind, %@ port %d", mSendSocketID, [NSString stringFromIPAddress:htonl(serverAddress.sin_addr.s_addr)], htons(serverAddress.sin_port) );
     }

Without this fix, multicast sending will intermittently get sendto() errno 'No route to host'. If anyone can shed light on why unplugging a DHCP gateway causes Mac OS X multicast SENDING sockets to get confused, I would love to hear it.

User · Answer

The  bind  operation is basically saying   use this local UDP port for sending and receiving data   In other words  it allocates that UDP port for exclusive use for your application    Same holds true for TCP sockets    When you bind to  0 0 0 0   INADDR ANY   you are basically telling the TCP IP layer to use all available adapters for listening and to choose the best adapter for sending   This is standard practice for most socket code   The only time you wouldn t specify 0 for the IP address is when you want to send receive on a specific network adapter   Similarly if you specify a port value of 0 during bind  the OS will assign a randomly available port number for that socket   So I would expect for UDP multicast  you bind to INADDR ANY on a specific port number where multicast traffic is expected to be sent to   The  join multicast group  operation  IP ADD MEMBERSHIP  is needed because it basically tells your network adapter to listen not only for ethernet frames where the destination MAC address is your own  it also tells the ethernet adapter  NIC  to listen for IP multicast traffic as well for the corresponding multicast ethernet address   Each multicast IP maps to a multicast ethernet address   When you use a socket to send to a specific multicast IP  the destination MAC address on the ethernet frame is set to the corresponding multicast MAC address for the multicast IP   When you join a multicast group  you are configuring the NIC to listen for traffic sent to that same MAC address  in addition to its own    Without the hardware support  multicast wouldn t be any more efficient than plain broadcast IP messages   The join operation also tells your router gateway to forward multicast traffic from other networks   Anyone remember MBONE    If you join a multicast group  all the multicast traffic for all ports on that IP address will be received by the NIC   Only the traffic destined for your binded listening port will get passed up the TCP IP stack to your app   In regards to why ports are specified during a multicast subscription - it s because multicast IP is just that - IP only    ports  are a property of the upper protocols  UDP and TCP    You can read more about how multicast IP addresses map to multicast ethernet addresses at various sites   The Wikipedia article is about as good as it gets      The IANA owns the OUI MAC address 01 00 5e  therefore multicast   packets are delivered by using the Ethernet MAC address range   01 00 5e 00 00 00 - 01 00 5e 7f ff ff  This is 23 bits of available   address space  The first octet  01  includes the broadcast multicast   bit  The lower 23 bits of the 28-bit multicast IP address are mapped   into the 23 bits of available Ethernet address space

User · Answer

Correction for What does it mean to bind a multicast  udp  socket  as long as it partially true at the following quote      The  bind  operation is basically saying   use this local UDP port for sending and receiving data  In other words  it allocates that UDP port for exclusive use for your application   There is one exception  Multiple applications can share the same port for listening  usually it has practical value for multicast datagrams   if the SO REUSEADDR option applied  For example  int sock   socket AF INET  SOCK DGRAM  IPPROTO UDP      create UDP socket somehow     int set option on   1     it is important to do  reuse address  before bind  not after int res   setsockopt sock  SOL SOCKET  SO REUSEADDR   char    amp set option on       sizeof set option on    res   bind sock  src addr  len     If several processes did such  reuse binding   then every UDP datagram received on that shared port will be delivered to each of the processes  providing natural joint with multicasts traffic     Here are further details regarding what happens in a few cases    attempt of any bind   exclusive  or  reuse   to free port will be successful attempt to  exclusive binding  will fail if the port is already  reuse-binded  attempt to  reuse binding  will fail if some process keeps  exclusive binding

User · Answer

To bind a UDP socket when receiving multicast means to specify an address and port from which to receive data  NOT a local interface  as is the case for TCP acceptor bind   The address specified in this case has a filtering role  i e  the socket will only receive datagrams sent to that multicast address  amp  port  no matter what groups are subsequently joined by the socket  This explains why when binding to INADDR ANY  0 0 0 0  I received datagrams sent to my multicast group  whereas when binding to any of the local interfaces I did not receive anything  even though the datagrams were being sent on the network to which that interface corresponded   Quoting from UNIX   Network Programming Volume 1  Third Edition  The Sockets Networking API by W R Stevens  21 10  Sending and Receiving           We want the receiving socket to bind the multicast group and   port  say 239 255 1 2 port 8888   Recall that we could just bind the   wildcard IP address and port 8888  but binding the multicast address   prevents the socket from receiving any other datagrams that might   arrive destined for port 8888   We then want the receiving socket to   join the multicast group  The sending socket will send datagrams to   this same multicast address and port  say 239 255 1 2 port 8888

[sockets] What does it mean to bind a multicast (UDP) socket?

Examples related to sockets

Examples related to udp

Examples related to bind

Examples related to boost-asio

Examples related to multicast