We are essentially trying to implement "fire and forget" TCP with SCTP. That is, a single server port that services clients that establish separate connections. We are not broadcasting same messages over to every client. Each client can issue different sets of requests from the server and receive response as streams of messages over its own channel. Messages are time sensitive and expire fast, therefore retransmission of lost messages is not desirable. Due to large number of clients and uniqueness of clients' needs UDP is not an option. We have one-to-one SCTP connections to clients where we send unordered messages with short time to live parameters. When clients' network degrade to about 60% packet loss (which happens often in some regions), server can continue to write (sctp_sendmsg) to the clients connection but wireshark shows that message don't get to the wire (blocked at the transport layer). We enabled SCTP Kernel logging and here is what we see: We start getting the following for each write attempt (some lines are removed for brevity): May 11 09:58:43 localhost kernel: [250220.363867] sctp: sctp_outq_flush: could not transmit TSN: 0x0, status: 2 May 11 09:58:43 localhost kernel: [250220.363870] sctp: sctp_do_sm post sfx: error 0, asoc ffff88036943a000[STATE_ ESTABLISHED] After several of the above eventually we get: May 11 09:58:43 localhost kernel: [250220.363871] sctp: We sent primitively. May 11 09:58:43 localhost kernel: [250220.363933] sctp: sctp_close(sk: 0xffff880369693dc0, timeout:0) May 11 09:58:43 localhost kernel: [250220.363938] sctp: sctp_do_sm prefn: ep ffff8807d159e200, EVENT_T_PRIMITIVE, PRIMITIVE_SHUTDOWN, asoc ffff880 36943a000[STATE_ESTABLISHED], sctp_sf_do_9_2_prm_shutdown May 11 09:58:43 localhost kernel: [250220.363941] sctp: sctp_do_sm postfn: asoc ffff88036943a000, status: DISPOSITION_CONSUME May 11 09:58:43 localhost kernel: [250220.363943] sctp: sctp_cmd_new_state: asoc ffff88036943a000[STATE_SHUTDOWN_PENDING] May 11 09:58:43 localhost kernel: [250220.363945] sctp: sctp_do_sm post sfx: error 0, asoc ffff88036943a000[STATE_CLOSED] May 11 09:58:43 localhost kernel: [250220.363947] sctp: sctp_destroy_sock(sk: ffff880369693dc0) SCTP is abandoning the connection due to status = 2. I found in the kernel SCTP source that it means: SCTP_XMIT_RWND_FULL. ie. rwindow is full. Clearly SCTP reacting to what is sees as heavy congestion. We can detect congestion before connection is closed (using sctp events), my question is is there a way to reset a connection (association) without having to close it? Alternatively, is there a way to relax congestion parameters so that we can continue using the connection as we don't care about the packet loss? Thanks, Atalay -- To unsubscribe from this list: send the line "unsubscribe linux-sctp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
