Documentation update

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 16 insertions, 5 deletions

diff --git a/docs/tcp.txt b/docs/tcp.txt
index 766766387..2000f3110 100644
--- a/docs/tcp.txt
+++ b/docs/tcp.txt
@@ -18,8 +18,11 @@ What will happen if we close the socket?
  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.
+IOW: if we just close(sock) now, kernel can reset the TCP connection
+(send RST packet).
+
+This is problematic for two reasons: it discards some not-yet sent
+data, and it may be reported as error, not EOF, on peer's side.
 
 What can be done about it?
 
@@ -46,14 +49,14 @@ This makes kernel send FIN after all data is written:
 
 However, experiments on Linux 3.9.4 show that kernel can return from
 shutdown() and from close() before all data is sent,
-and if peer sends any data to us after this, kernel stll responds with
+and if peer sends any data to us after this, kernel still responds with
 RST before all our data is sent.
 
 In practice the protocol in use often does not allow peer to send
 such data to us, in which case this solution is acceptable.
 
-If you know that peer is going to close its end after it sees our FIN
-(as EOF), it might be a good idea to perform a read after shutdown().
+Solution #3: if you know that peer is going to close its end after it sees
+our FIN (as EOF), it might be a good idea to perform a read after shutdown().
 When read finishes with 0-sized result, we conclude that peer received all
 the data, saw EOF, and closed its end.
 
@@ -61,6 +64,14 @@ However, this incurs small performance penalty (we run for a longer time)
 and requires safeguards (nonblocking reads, timeouts etc) against
 malicious peers which don't close the connection.
 
+Solutions #1 and #2 can be combined:
+
+    /* ...set up struct linger... then: */
+    setsockopt(sock, SOL_SOCKET, SO_LINGER, &linger, sizeof(linger));
+    shutdown(sock, SHUT_WR);
+    /* At this point, kernel sent FIN packet, not RST, to the peer, */
+    /* even if there is buffered read data from the peer. */
+    close(sock);
 
 	Defeating Nagle.