7801148a8
Denys Vlasenko
2013-07-25
Subject
Add notes about TCP programming quirks.
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docs/tcp.txt+65 -0
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| + | Some less-widely known details of TCP connections. |
| + | |
| + | Properly closing the connection. |
| + | |
| + | After this code sequence: |
| + | |
| + | sock = socket(AF_INET, SOCK_STREAM, 0); |
| + | connect(sock, &remote, sizeof(remote)); |
| + | write(sock, buffer, 1000000); |
| + | |
| + | a large block of data is only buffered by kernel, it can't be sent all at once. |
| + | What will happen if we close the socket? |
| + | |
| + | "A host MAY implement a 'half-duplex' TCP close sequence, so that |
| + | an application that has called close() cannot continue to read |
| + | data from the connection. If such a host issues a close() call |
| + | while received data is still pending in TCP, or if new data is |
| + | received after close() is called, its TCP SHOULD send a RST |
| + | to show that data was lost." |
| + | |
| + | IOW: if we just close(sock) now, kernel can reset the TCP connection, |
| + | discarding some not-yet sent data. |
| + | |
| + | What can be done about it? |
| + | |
| + | Solution #1: block until sending is done: |
| + | |
| + | /* When enabled, a close(2) or shutdown(2) will not return until |
| + | * all queued messages for the socket have been successfully sent |
| + | * or the linger timeout has been reached. |
| + | */ |
| + | struct linger { |
| + | int l_onoff; /* linger active */ |
| + | int l_linger; /* how many seconds to linger for */ |
| + | } linger; |
| + | linger.l_onoff = 1; |
| + | linger.l_linger = SOME_NUM; |
| + | setsockopt(sock, SOL_SOCKET, SO_LINGER, &linger, sizeof(linger)); |
| + | close(sock); |
| + | |
| + | Solution #2: tell kernel that you are done sending. |
| + | This makes kernel send FIN, not RST: |
| + | |
| + | shutdown(sock, SHUT_WR); |
| + | close(sock); |
| + | |
| + | |
| + | Defeating Nagle. |
| + | |
| + | Method #1: manually control whether partial sends are allowed: |
| + | |
| + | This prevents partially filled packets being sent: |
| + | |
| + | int state = 1; |
| + | setsockopt(fd, IPPROTO_TCP, TCP_CORK, &state, sizeof(state)); |
| + | |
| + | and this forces last, partially filled packet (if any) to be sent: |
| + | |
| + | int state = 0; |
| + | setsockopt(fd, IPPROTO_TCP, TCP_CORK, &state, sizeof(state)); |
| + | |
| + | Method #2: make any write to immediately send data, even if it's partial: |
| + | |
| + | int state = 1; |
| + | setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &state, sizeof(state)); |