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81f7e3824   Eric Lee   Initial Release, ... 
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  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.