net-tcp: retire TFO_SERVER_WO_SOCKOPT2 config

TFO_SERVER_WO_SOCKOPT2 was intended for debugging purposes during
Fast Open development. Remove this config option and also
update/clean-up the documentation of the Fast Open sysctl.

Reported-by: Piotr Jurkiewicz <piotr.jerzy.jurkiewicz@gmail.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -575,32 +575,33 @@ tcp_syncookies - BOOLEAN
 	unconditionally generation of syncookies.
 
 tcp_fastopen - INTEGER
-	Enable TCP Fast Open feature (draft-ietf-tcpm-fastopen) to send data
-	in the opening SYN packet. To use this feature, the client application
-	must use sendmsg() or sendto() with MSG_FASTOPEN flag rather than
-	connect() to perform a TCP handshake automatically.
+	Enable TCP Fast Open (RFC7413) to send and accept data in the opening
+	SYN packet.
+
+	The client support is enabled by flag 0x1 (on by default). The client
+	then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
+	rather than connect() to send data in SYN.
+
+	The server support is enabled by flag 0x2 (off by default). Then
+	either enable for all listeners with another flag (0x400) or
+	enable individual listeners via TCP_FASTOPEN socket option with
+	the option value being the length of the syn-data backlog.
 
 	The values (bitmap) are
-	1: Enables sending data in the opening SYN on the client w/ MSG_FASTOPEN.
-	2: Enables TCP Fast Open on the server side, i.e., allowing data in
-	   a SYN packet to be accepted and passed to the application before
-	   3-way hand shake finishes.
-	4: Send data in the opening SYN regardless of cookie availability and
-	   without a cookie option.
-	0x100: Accept SYN data w/o validating the cookie.
-	0x200: Accept data-in-SYN w/o any cookie option present.
-	0x400/0x800: Enable Fast Open on all listeners regardless of the
-	   TCP_FASTOPEN socket option. The two different flags designate two
-	   different ways of setting max_qlen without the TCP_FASTOPEN socket
-	   option.
+	  0x1: (client) enables sending data in the opening SYN on the client.
+	  0x2: (server) enables the server support, i.e., allowing data in
+			a SYN packet to be accepted and passed to the
+			application before 3-way handshake finishes.
+	  0x4: (client) send data in the opening SYN regardless of cookie
+			availability and without a cookie option.
+	0x200: (server) accept data-in-SYN w/o any cookie option present.
+	0x400: (server) enable all listeners to support Fast Open by
+			default without explicit TCP_FASTOPEN socket option.
 
-	Default: 1
+	Default: 0x1
 
-	Note that the client & server side Fast Open flags (1 and 2
-	respectively) must be also enabled before the rest of flags can take
-	effect.
-
-	See include/net/tcp.h and the code for more details.
+	Note that that additional client or server features are only
+	effective if the basic support (0x1 and 0x2) are enabled respectively.
 
 tcp_syn_retries - INTEGER
 	Number of times initial SYNs for an active TCP connection attempt