1. 指定在某個stream上發(fā)送消息

在調(diào)用sctp_sendmsg函數(shù)發(fā)送消息的時候，我們可以設(shè)置stream ID，

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

倒數(shù)第三個參數(shù)就是stream，我們可以指定在某個stream發(fā)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(xiàn)阻塞了，不會影響stream 1上的消息傳輸
• 每個stream上傳輸?shù)臄?shù)據(jù)包都有序列號

2. 設(shè)置最大最小stream數(shù)

可以通過設(shè)置socket的屬性來設(shè)置最大的stream數(shù)，

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(shù)據(jù)包的序列

發(fā)送端每一個消息發(fā)送五次，都在同一個stream 1上發(fā)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(fā)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(xù)再往server端發(fā)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(xiàn)丟包的情況

這里我們沒法兒模擬丟包的場景，unix網(wǎng)絡(luò)編程這本書中描述的，如果一個數(shù)據(jù)包丟失，也就是如果出現(xiàn)NACK

• 這個stream上的后續(xù)的數(shù)據(jù)包會保存到隊列中；
• 其他的stream不受影響；
• 丟失的數(shù)據(jù)包會進行重傳，重傳成功后，后續(xù)隊列中的數(shù)據(jù)包繼續(xù)按順序發(fā)送；