Platform Sockets and Multicast Notes

Summary
Platform Sockets and Multicast Notes
BSD Sockets interface IPv4 (including Windows)Unfortunately the behavior of the sockets interface, particularly with regard to multicast reception is very inconsistent across implementations and it is very hard to find in documentation what a particular implementation does.
General sequence to receive multicast.sock = socket(AF_INET, SOCK_DGRAM, 0);
Multiple InterfacesThis can mean multiple physical interfaces (Ethernets) or multiple IP networks running on the same physical interface.

BSD Sockets interface IPv4 (including Windows)

Unfortunately the behavior of the sockets interface, particularly with regard to multicast reception is very inconsistent across implementations and it is very hard to find in documentation what a particular implementation does.  This is particularly the case where you have more than one socket listening and more than one multicast address.

Suppose you have 3 sockets

socket-A listens on port-A and subscribed to group-A. socket-B listens on port-B and subscribed to group-B. socket-C listens on port-C but not subscibed to any group

Now what happens if a packet arrives for group-A port-B? or for group-A port-C?

In many TCP/IP implementations these packets will be received on socket-B and socket-C respectively even though they did not subscribe to group-A, but this behaviour can be dependent on the details of how the sockets are set up, and unfortunately the way socket-A is set up can influence the behaviour of socket-B or socket-C.

Linux, free-BSD, OSX (Darwin), Solaris all have subtle differences.  Windows varies wildly from version to version and often from one service pack to another.

Socan you set the socket to only allow group-A to come in on
socket-A?sometimes but details vary and there are usually restrictions, for example in Linux calling bind with the multicast address has this effect, but won’t work if you want the socket to accept multiple groups.  In Windows this works well on some versions and not at all on others!

Alternatively, can you examine the packet when it arrives to see what group address it was actually carrying?  Maybe - but often not!  To do this: Set socket option IP_PKTINFO, then call recvmsg() to get the packet coupled with ancilliary data, then use cmsg() to get at the ancilliary data.  This works with Linux but not with OSX.  Works with Windows after XP-SP1 maybe.  Don’t know about BSD or Solaris.

General sequence to receive multicast.

1. open a socket

sock = socket(AF_INET, SOCK_DGRAM, 0);

sometimes PF_INET instead of AF_INET sometimes IPPROTO_UDP instead of 0

2. set reuse-address if requiredmust be done before bind

result = setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &value, sizeof(value));

value is either 1 (on) or 0 (off).

BSD has SO_REUSEPORT as well as SO_REUSEADDR.  Linux does not.

3. bind the socket to a local port and address

result = bind(sock, struct sockaddr *localaddress, addrlength)

4. add group membership

setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, struct ip_mreq *mreq, sizeof(mreq))

the ip_mreq structure contains two addressesone to specify the interface and the other to specify the group.

Variations and puzzles

SO_REUSEADDRshould this be used?  It is necessary to allow multiple sockets to receive the same packets (multiple sockets on the same port) but this is not always as seems and you will almost certainly find that unicast packets do not get copied to all these sockets.  It can also influence whether sockets which are not subscribed to a multicast see multicast traffic.  You might also find it allows you to open multiple sockets but the packets do not actually get received by more than one.

Bind address Which address to bind to?  The local address parameter to bind() specifies a port and IP-address.  The port is always relevant, but what to use for the address?  There are three options:

INADDR_ANYallow the stack to select Unicast address of an interface (e.g.  192.168.1.23) Multicast address e.g.  239.123.45.6

Some stacks allow a multicast address, some forbid it.  In many cases more than one variant will work but the choice causes differences in multicast behaviour and filtering.

Usually INADDR_ANY and reuse-address ON will work but you may get lots of unwanted packets arriving at all sockets.

Ephemeral Ports How do you get the stack to pick an ephemeral port?  Often bind with a port of 0 will do this, but it is very hard to find this documented.  Once it has done so call getsockname() to find the one it has picked.

IP_ADD_MEMBERSHIPwhich address do you use for the interface part of the ip_mreq structure?  What if you don’t care?  If you specify INADDR_ANY
for the interface, it may mean the stack will pick a defaultit probably does not mean that packets arriving at any interface will get through to your socket.

Other options that are often relevant IP_MULTICAST_IF “Set the local device for a multicast socket” IP_MULTICAST_LOOP sets whether outgoing multicast packets are received on local sockets.  IP_MULTICAST_TTL - The old way was to set this to a low figure but SLP specifes 255 and RFC2365 recommends you use 255 and configure routers to form boundaries.

Multiple Interfaces

This can mean multiple physical interfaces (Ethernets) or multiple IP networks running on the same physical interface.  For unicast traffic, the routing tables generally determine which interface is used for outgoing, and if you bind to INADDR_ANY you will receive packets from any interface.

With multicast the stack usually selects just one physical interface for multicasting and it might not be the one you want - IP_ADD_MEMBERSHIP allows you to specify which interface you are subscribing to the multicast but specifying INADDR_ANY may mean “stack picks one” and not “all of them” as you might expect so you probably have to examine the list of interfaces and set IP_ADD_MEMBERSHIP on any which are relevant.

Useful manual pages on Linuxbut don’t expect to find all the answers. socket(2) socket(7) ip(7) udp(7) bind(2) setsockopt(2) RFC1112 specifies IP to Ethernet multicast transformation -

An IP host group address is mapped to an Ethernet multicast address by placing the low-order 23-bits of the IP address into the low-order 23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).  Because there are 28 significant bits in an IP host group address, more than one host group address may map to the same Ethernet multicast address.

Results of testing

2010-10-15 On linux amd64 2.6.34:

The address passed to bind acts as a simple filter behind the socket - if a specific address is used here then only packets with that destination address will be received.  This works for unicast or multicast and is independent of IP_ADD_MEMBERSHIP so for example, if a multicast address is used, then no unicast packets will be received, neither will any for other multicast groups.

The IP_ADD_MEMBERSHIP call sends IGMP messages and “opens the stack” to the given multicast, irrespective of which socket or even process the call is used on.  So if one socket anywhere in the system adds group X, then any socket in the system which is bound to INADDR_ANY (or to group X) will receive group X packets (subject to the port matching).  The only real link between the socket used to call IP_ADD_MEMBERSHIP and its effect is that when that socket is closed the membership is automatically revoked.  The system stacks calls to IP_ADD_MEMBERSHIP from multiple sockets and only revokes the membership when they are all closed.

SO_REUSEADDRmust be called if two sockets need to listen to the same port.  All sockets bound to the port must set this option or the bind call will fail for some of them.  Despite this flag, unicast packets for the shared port will only ever appear at one socket, whilst multicast packets are copied to all.  The choice of which socket to deliver unicast packets to is: if any sockets are bound to the specific unicast address they are selected, otherwise all sockets bound to INADDR_ANY are selected.  Within the resulting set of sockets the most recent to bind is selected.

Linux model then has IP_ADD_MEMBERSHIP operating at the bottom of the stack and determining which packets to allow into the stack, and bind() working just below the socket to determine which packets rising through the stack are allowed out of the socket.  SO_REUSEADDR allows multiple sockets to share a port but only copies packets to them all for multicast packets.

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