docs: net: Remove TCP congestion document

Document is stale, let's remove it.

Remove TCP congestion document.
Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/00-INDEX

@@ -198,8 +198,6 @@ tc-actions-env-rules.txt
 	- rules for traffic control (tc) actions.
 timestamping.txt
 	- overview of network packet timestamping variants.
-tcp.txt
-	- short blurb on how TCP output takes place.
 tcp-thin.txt
 	- kernel tuning options for low rate 'thin' TCP streams.
 team.txt

Documentation/networking/tcp.txt

-TCP protocol
-============
-
-Last updated: 3 June 2017
-
-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, the congestion control
-mechanism must provide a valid name and must implement either ssthresh,
-cong_avoid and undo_cwnd hooks or the "omnipotent" cong_control hook.
-
-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 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 default congestion control mechanism is chosen based on the
-DEFAULT_TCP_CONG Kconfig parameter. If you really want a particular default
-value then you can set it using sysctl net.ipv4.tcp_congestion_control. 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 cannot be built as a module, and cannot be
-removed, 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
-walks down the list from the start.
-
-We keep a set of control flags
-
-
-	sk->tcp_pend_event
-
-		TCP_PEND_ACK			Ack needed
-		TCP_ACK_NOW			Needed now
-		TCP_WINDOW			Window update check
-		TCP_WINZERO			Zero probing
-
-
-	sk->transmit_queue		The transmission frame begin
-	sk->transmit_new		First new frame pointer
-	sk->transmit_end		Where to add frames
-
-	sk->tcp_last_tx_ack		Last ack seen
-	sk->tcp_dup_ack			Dup ack count for fast retransmit
-
-
-Frames are queued for output by tcp_write. We do our best to send the frames
-off immediately if possible, but otherwise queue and compute the body
-checksum in the copy. 
-
-When a write is done we try to clear any pending events and piggy back them.
-If the window is full we queue full sized frames. On the first timeout in
-zero window we split this.
-
-On a timer we walk the retransmit list to send any retransmits, update the
-backoff timers etc. A change of route table stamp causes a change of header
-and recompute. We add any new tcp level headers and refinish the checksum
-before sending. 
-