[Live-devel] Unicast, Socket Bind Issues on localhost?

[Hubert Pham]

Hi,

I'm trying to use testMP3Sender and testMP3Receiver to stream an MP3 file 
via unicast. I've modified the examples appropriately for unicast 
streaming: testMP3Sender sends to a target unicast address, while 
testMP3Receiver listens on 0.0.0.0.

This works fine---except when a) the sender is sending to a receiver on the 
same host, and b) the receiver starts up before the sender. Under this 
scenario, the receiver never receives any packets; more precisely, the 
receiver's select() never indicates that the socket is readable. If instead 
the sender starts before the receiver, then the setup works fine (except 
the receiver obviously doesn't receive the first few initial packets). 
Additionally, if using multicast instead of unicast, then everything also 
works fine regardless of startup order.

My guess is that GroupsockHelper.cpp::setupDatagramSocket (ultimately 
called when constructing a Groupsock) attempts to bind the socket to the 
user-specified port if the port is non-zero, disregarding whether or not 
the socket is intended as send-only to a unicast address:

   if (port.num() != 0 || ReceivingInterfaceAddr != INADDR_ANY) {
   ...
     MAKE_SOCKADDR_IN(name, ReceivingInterfaceAddr, port.num());
     if (bind(newSocket, (struct sockaddr*)&name, sizeof name) != 0) {
   ...

I imagine what is happening is that when testMP3Receiver and testMP3Sender 
run on the same host, they both try to bind to port 6666 and 6667 on the 
same interface. This might make sense when using multicast, but my guess is 
that under unicast---in which not all the multicast socket options are 
set---one process will lose on the bind and not receive packets.

While streaming data between two processes on the same host might seem like 
a corner case, are there any suggestions for doing so? (e.g., a different 
available Groupsock interface or constructor parameter?)

Thanks for your time,
hubert

PS I noticed that someone did bring up this issue on March 5, 2002, but I 
didn't see a resolution, nor did the post mention anything about the sender 
and receiver being on the same host. Apologies if I missed the resolution...