TCP protocol
  ============

  Last updated: 9 February 2008

  
  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 defaults from Kconfig, then CUBIC 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 cannot be built as a module, and cannot 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
  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.