1. 指定在某個stream上發送消息
在調用sctp_sendmsg函數發送消息的時候，我們可以設置stream ID，
sctp_sendmsg(sock_op->socket_fd(), message, MAX_BUFFER,
(sockaddr *)&servaddr, sizeof(servaddr), 0, 0, stream, 0, 0);
倒數第三個參數就是stream，我們可以指定在某個stream發送消息，
- 一個SCTP association包含多個stream，默認情況下最大有10個stream；
- 每個stream上的傳輸相互獨立，比如說stream 0的傳輸出現阻塞了，不會影響stream 1上的消息傳輸
- 每個stream上傳輸的數據包都有序列號
2. 設置最大最小stream數
可以通過設置socket的屬性來設置最大的stream數，
sctp_initmsg initmsg;
sctp_event_subscribe evnts;
// Specify that a maximum of 5 streams will be available per socket
memset( &initmsg, 0, sizeof(initmsg) );
initmsg.sinit_num_ostreams = 5;
initmsg.sinit_max_instreams = 5;
initmsg.sinit_max_attempts = 4;
if(0 != setsockopt( sock_fd, IPPROTO_SCTP, SCTP_INITMSG,
&initmsg, sizeof(initmsg) ))
3. 同一個stream上數據包的序列
發送端每一個消息發送五次，都在同一個stream 1上發送，
printf("[Client]: Input message content!\n");
std::cin >> message ;
for(int i = 0; i < 5; i++)
{
std::string newMsg = std::move(message + (std::to_string(i)));
printf("[Client]: Send message: %s\n", newMsg.c_str());
sctp_sendmsg(sock_op->socket_fd(), newMsg.c_str(), MAX_BUFFER,
(sockaddr *)&servaddr, sizeof(servaddr), 0, 0, stream, 0, 0);
}
第一次發送5個‘hello’，Client端打印如下，
[Client]: Send message: hello0
[Client]: Send message: hello1
[Client]: Send message: hello2
[Client]: Send message: hello3
[Client]: Send message: hello4
我們再來看一下Sever端的接收情況，同時把每個消息的ssn打印出來，
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('hello0') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(0)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('hello1') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(1)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('hello2') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(2)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('hello3') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(3)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('hello4') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(4)
可以看到，每個消息都有一個自己的序列號，保證消息是有序的，
我們繼續再往server端發送5個'world'，server端的接收消息依舊是有序的，
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('world0') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(5)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('world1') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(6)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('world2') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(7)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('world3') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(8)
====================================================================
[Server]: Wait for new message or command([0: Exit]; [1: NewMessage])...
[Server]: SCTP message('world4') received from IP/Port(127.0.0.1:47378) on assoc(0x1e) / stream(1), ssn(9)
4. 如果出現丟包的情況
這里我們沒法兒模擬丟包的場景，unix網絡編程這本書中描述的，如果一個數據包丟失，也就是如果出現NACK
- 這個stream上的后續的數據包會保存到隊列中；
- 其他的stream不受影響；
- 丟失的數據包會進行重傳，重傳成功后，后續隊列中的數據包繼續按順序發送；
