1. 指定在某个stream上发送消息

在调用sctp_sendmsg函数发送消息的时候，我们可以设置stream ID，

sctp_sendmsg(sock_op->socket_fd(), message, MAX_BUFFER, 
                    (sockaddr *)&servaddr, sizeof(servaddr), 0, 0, stream, 0, 0);

倒数第三个参数就是stream，我们可以指定在某个stream发送消息，

• 一个SCTP association包含多个stream，默认情况下最大有10个stream；
• 每个stream上的传输相互独立，比如说stream 0的传输出现阻塞了，不会影响stream 1上的消息传输
• 每个stream上传输的数据包都有序列号

2. 设置最大最小stream数

可以通过设置socket的属性来设置最大的stream数，

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. 同一个stream上数据包的序列

发送端每一个消息发送五次，都在同一个stream 1上发送，

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);
}

第一次发送5个‘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

我们再来看一下Sever端的接收情况，同时把每个消息的ssn打印出来，

====================================================================
[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，每个消息都有一个自己的序列号，保证消息是有序的，
我们继续再往server端发送5个'world'，server端的接收消息依旧是有序的，

 ====================================================================
[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. 如果出现丢包的情况

这里我们没法儿模拟丢包的场景，unix网络编程这本书中描述的，如果一个数据包丢失，也就是如果出现NACK

• 这个stream上的后续的数据包会保存到队列中；
• 其他的stream不受影响；
• 丢失的数据包会进行重传，重传成功后，后续队列中的数据包继续按顺序发送；