How does sctp identify which association will receive the data

Question

Suppose I have a socket which contain several associations in it. that means there are several clients connect with server.

If I use sendmsg(...) function to send msg from server, How does sctp identify which association will receive the data? Does all clients will receive the msg?

And, Does sctp provide a way for server to send msg to all associations in one syscall ?

Answer 1

The receiving association is specified in either the msg_name field of the message header or the association id field of the sndrcv_info cmsg header in the datagram. IIRC there is no was to send to all associations at once in a single syscall, unless you want to marshall up all the data and pass it down via the sendmmsg syscall, which I don't think lksctp-tools supports yet (though that would be a nice feature).

Answer 2

You can use the deprecated cmsg SCTP_SNDRCV and specify the association id.
See http://tools.ietf.org/html/rfc6458#section-5.3.2
I'm not sure if Linux supports the newer cmsg SCTP_SNDINFO yet.
See http://tools.ietf.org/html/rfc6458#section-5.3.4