[TCP]: Update sysctl and congestion control documentation.

Update the documentation to remove the old sysctl values and
include the new congestion control infrastructure. Includes
changes to tcp.txt by Ian McDonald.
Signed-off-by: NStephen Hemminger <shemminger@osdl.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -304,57 +304,6 @@ tcp_low_latency - BOOLEAN
 	changed would be a Beowulf compute cluster.
 	Default: 0
 
-tcp_westwood - BOOLEAN
-        Enable TCP Westwood+ congestion control algorithm.
-	TCP Westwood+ is a sender-side only modification of the TCP Reno 
-	protocol stack that optimizes the performance of TCP congestion 
-	control. It is based on end-to-end bandwidth estimation to set 
-	congestion window and slow start threshold after a congestion 
-	episode. Using this estimation, TCP Westwood+ adaptively sets a 
-	slow start threshold and a congestion window which takes into 
-	account the bandwidth used  at the time congestion is experienced. 
-	TCP Westwood+ significantly increases fairness wrt TCP Reno in 
-	wired networks and throughput over wireless links.   
-        Default: 0
-
-tcp_vegas_cong_avoid - BOOLEAN
-	Enable TCP Vegas congestion avoidance algorithm.
-	TCP Vegas is a sender-side only change to TCP that anticipates
-	the onset of congestion by estimating the bandwidth. TCP Vegas
-	adjusts the sending rate by modifying the congestion
-	window. TCP Vegas should provide less packet loss, but it is
-	not as aggressive as TCP Reno.
-	Default:0
-
-tcp_bic - BOOLEAN
-	Enable BIC TCP congestion control algorithm.
-	BIC-TCP is a sender-side only change that ensures a linear RTT
-	fairness under large windows while offering both scalability and
-	bounded TCP-friendliness. The protocol combines two schemes
-	called additive increase and binary search increase. When the
-	congestion window is large, additive increase with a large
-	increment ensures linear RTT fairness as well as good
-	scalability. Under small congestion windows, binary search
-	increase provides TCP friendliness.
-	Default: 0
-
-tcp_bic_low_window - INTEGER
-	Sets the threshold window (in packets) where BIC TCP starts to
-	adjust the congestion window. Below this threshold BIC TCP behaves
-	the same as the default TCP Reno. 
-	Default: 14
-
-tcp_bic_fast_convergence - BOOLEAN
-	Forces BIC TCP to more quickly respond to changes in congestion
-	window. Allows two flows sharing the same connection to converge
-	more rapidly.
-	Default: 1
-
-tcp_default_win_scale - INTEGER
-	Sets the minimum window scale TCP will negotiate for on all
-	conections.
-	Default: 7
-
 tcp_tso_win_divisor - INTEGER
        This allows control over what percentage of the congestion window
        can be consumed by a single TSO frame.
@@ -368,6 +317,11 @@ tcp_frto - BOOLEAN
 	where packet loss is typically due to random radio interference
 	rather than intermediate router congestion.
 
+tcp_congestion_control - STRING
+	Set the congestion control algorithm to be used for new
+	connections. The algorithm "reno" is always available, but
+	additional choices may be available based on kernel configuration.
+
 somaxconn - INTEGER
 	Limit of socket listen() backlog, known in userspace as SOMAXCONN.
 	Defaults to 128.  See also tcp_max_syn_backlog for additional tuning

Documentation/networking/tcp.txt

-How the new TCP output machine [nyi] works.
+TCP protocol
+============
+
+Last updated: 21 June 2005
+
+Contents
+========
+
+- Congestion control
+- How the new TCP output machine [nyi] works
+
+Congestion control
+==================
+
+The following variables are used in the tcp_sock for congestion control:
+snd_cwnd		The size of the congestion window
+snd_ssthresh		Slow start threshold. We are in slow start if
+			snd_cwnd is less than this.
+snd_cwnd_cnt		A counter used to slow down the rate of increase
+			once we exceed slow start threshold.
+snd_cwnd_clamp		This is the maximum size that snd_cwnd can grow to.
+snd_cwnd_stamp		Timestamp for when congestion window last validated.
+snd_cwnd_used		Used as a highwater mark for how much of the
+			congestion window is in use. It is used to adjust
+			snd_cwnd down when the link is limited by the
+			application rather than the network.
+
+As of 2.6.13, Linux supports pluggable congestion control algorithms.
+A congestion control mechanism can be registered through functions in
+tcp_cong.c. The functions used by the congestion control mechanism are
+registered via passing a tcp_congestion_ops struct to
+tcp_register_congestion_control. As a minimum name, ssthresh,
+cong_avoid, min_cwnd must be valid.
 
+Private data for a congestion control mechanism is stored in tp->ca_priv.
+tcp_ca(tp) returns a pointer to this space.  This is preallocated space - it
+is important to check the size of your private data will fit this space, or
+alternatively space could be allocated elsewhere and a pointer to it could
+be stored here.
+
+There are three kinds of congestion control algorithms currently: The
+simplest ones are derived from TCP reno (highspeed, scalable) and just
+provide an alternative the congestion window calculation. More complex
+ones like BIC try to look at other events to provide better
+heuristics.  There are also round trip time based algorithms like
+Vegas and Westwood+.
+
+Good TCP congestion control is a complex problem because the algorithm
+needs to maintain fairness and performance. Please review current
+research and RFC's before developing new modules.
+
+The method that is used to determine which congestion control mechanism is
+determined by the setting of the sysctl net.ipv4.tcp_congestion_control.
+The default congestion control will be the last one registered (LIFO);
+so if you built everything as modules. the default will be reno. If you
+build with the default's from Kconfig, then BIC will be builtin (not a module)
+and it will end up the default.
+
+If you really want a particular default value then you will need
+to set it with the sysctl.  If you use a sysctl, the module will be autoloaded
+if needed and you will get the expected protocol. If you ask for an
+unknown congestion method, then the sysctl attempt will fail.
+
+If you remove a tcp congestion control module, then you will get the next
+available one. Since reno can not be built as a module, and can not be
+deleted, it will always be available.
+
+How the new TCP output machine [nyi] works.
+===========================================
 
 Data is kept on a single queue. The skb->users flag tells us if the frame is
 one that has been queued already. To add a frame we throw it on the end. Ack