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