Documentation update
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
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21
docs/tcp.txt
21
docs/tcp.txt
@ -18,8 +18,11 @@ What will happen if we close the socket?
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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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IOW: if we just close(sock) now, kernel can reset the TCP connection
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(send RST packet).
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This is problematic for two reasons: it discards some not-yet sent
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data, and it may be reported as error, not EOF, on peer's side.
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What can be done about it?
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@ -46,14 +49,14 @@ This makes kernel send FIN after all data is written:
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However, experiments on Linux 3.9.4 show that kernel can return from
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shutdown() and from close() before all data is sent,
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and if peer sends any data to us after this, kernel stll responds with
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and if peer sends any data to us after this, kernel still responds with
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RST before all our data is sent.
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In practice the protocol in use often does not allow peer to send
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such data to us, in which case this solution is acceptable.
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If you know that peer is going to close its end after it sees our FIN
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(as EOF), it might be a good idea to perform a read after shutdown().
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Solution #3: if you know that peer is going to close its end after it sees
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our FIN (as EOF), it might be a good idea to perform a read after shutdown().
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When read finishes with 0-sized result, we conclude that peer received all
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the data, saw EOF, and closed its end.
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@ -61,6 +64,14 @@ However, this incurs small performance penalty (we run for a longer time)
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and requires safeguards (nonblocking reads, timeouts etc) against
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malicious peers which don't close the connection.
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Solutions #1 and #2 can be combined:
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/* ...set up struct linger... then: */
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setsockopt(sock, SOL_SOCKET, SO_LINGER, &linger, sizeof(linger));
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shutdown(sock, SHUT_WR);
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/* At this point, kernel sent FIN packet, not RST, to the peer, */
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/* even if there is buffered read data from the peer. */
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close(sock);
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Defeating Nagle.
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