RTO connection timeout: sysctl documentation update

This patch updates the sysctl documentation concerning the interpretation
of tcp_retries{1,2} and tcp_orphan_retries.

Signed-off-by: Damian Lukowski <damian@tvk.rwth-aachen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Damian Lukowski

Documentation/networking/ip-sysctl.txt

@@ -311,9 +311,12 @@ tcp_no_metrics_save - BOOLEAN
 	connections.
 
 tcp_orphan_retries - INTEGER
-	How may times to retry before killing TCP connection, closed
-	by our side. Default value 7 corresponds to ~50sec-16min
-	depending on RTO. If you machine is loaded WEB server,
+	This value influences the timeout of a locally closed TCP connection,
+	when RTO retransmissions remain unacknowledged.
+	See tcp_retries2 for more details.
+
+	The default value is 7.
+	If your machine is a loaded WEB server,
 	you should think about lowering this value, such sockets
 	may consume significant resources. Cf. tcp_max_orphans.
 
@@ -327,16 +330,28 @@ tcp_retrans_collapse - BOOLEAN
 	certain TCP stacks.
 
 tcp_retries1 - INTEGER
-	How many times to retry before deciding that something is wrong
-	and it is necessary to report this suspicion to network layer.
-	Minimal RFC value is 3, it is default, which corresponds
-	to ~3sec-8min depending on RTO.
+	This value influences the time, after which TCP decides, that
+	something is wrong due to unacknowledged RTO retransmissions,
+	and reports this suspicion to the network layer.
+	See tcp_retries2 for more details.
+
+	RFC 1122 recommends at least 3 retransmissions, which is the
+	default.
 
 tcp_retries2 - INTEGER
-	How may times to retry before killing alive TCP connection.
-	RFC1122 says that the limit should be longer than 100 sec.
-	It is too small number.	Default value 15 corresponds to ~13-30min
-	depending on RTO.
+	This value influences the timeout of an alive TCP connection,
+	when RTO retransmissions remain unacknowledged.
+	Given a value of N, a hypothetical TCP connection following
+	exponential backoff with an initial RTO of TCP_RTO_MIN would
+	retransmit N times before killing the connection at the (N+1)th RTO.
+
+	The default value of 15 yields a hypothetical timeout of 924.6
+	seconds and is a lower bound for the effective timeout.
+	TCP will effectively time out at the first RTO which exceeds the
+	hypothetical timeout.
+
+	RFC 1122 recommends at least 100 seconds for the timeout,
+	which corresponds to a value of at least 8.
 
 tcp_rfc1337 - BOOLEAN
 	If set, the TCP stack behaves conforming to RFC1337. If unset,