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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	TCP protocol
2	============
3	
4	Last updated: 3 June 2017
5	
6	Contents
7	========
8	
9	- Congestion control
10	- How the new TCP output machine [nyi] works
11	
12	Congestion control
13	==================
14	
15	The following variables are used in the tcp_sock for congestion control:
16	snd_cwnd		The size of the congestion window
17	snd_ssthresh		Slow start threshold. We are in slow start if
18				snd_cwnd is less than this.
19	snd_cwnd_cnt		A counter used to slow down the rate of increase
20				once we exceed slow start threshold.
21	snd_cwnd_clamp		This is the maximum size that snd_cwnd can grow to.
22	snd_cwnd_stamp		Timestamp for when congestion window last validated.
23	snd_cwnd_used		Used as a highwater mark for how much of the
24				congestion window is in use. It is used to adjust
25				snd_cwnd down when the link is limited by the
26				application rather than the network.
27	
28	As of 2.6.13, Linux supports pluggable congestion control algorithms.
29	A congestion control mechanism can be registered through functions in
30	tcp_cong.c. The functions used by the congestion control mechanism are
31	registered via passing a tcp_congestion_ops struct to
32	tcp_register_congestion_control. As a minimum, the congestion control
33	mechanism must provide a valid name and must implement either ssthresh,
34	cong_avoid and undo_cwnd hooks or the "omnipotent" cong_control hook.
35	
36	Private data for a congestion control mechanism is stored in tp->ca_priv.
37	tcp_ca(tp) returns a pointer to this space.  This is preallocated space - it
38	is important to check the size of your private data will fit this space, or
39	alternatively, space could be allocated elsewhere and a pointer to it could
40	be stored here.
41	
42	There are three kinds of congestion control algorithms currently: The
43	simplest ones are derived from TCP reno (highspeed, scalable) and just
44	provide an alternative congestion window calculation. More complex
45	ones like BIC try to look at other events to provide better
46	heuristics.  There are also round trip time based algorithms like
47	Vegas and Westwood+.
48	
49	Good TCP congestion control is a complex problem because the algorithm
50	needs to maintain fairness and performance. Please review current
51	research and RFC's before developing new modules.
52	
53	The default congestion control mechanism is chosen based on the
54	DEFAULT_TCP_CONG Kconfig parameter. If you really want a particular default
55	value then you can set it using sysctl net.ipv4.tcp_congestion_control. The
56	module will be autoloaded if needed and you will get the expected protocol. If
57	you ask for an unknown congestion method, then the sysctl attempt will fail.
58	
59	If you remove a TCP congestion control module, then you will get the next
60	available one. Since reno cannot be built as a module, and cannot be
61	removed, it will always be available.
62	
63	How the new TCP output machine [nyi] works.
64	===========================================
65	
66	Data is kept on a single queue. The skb->users flag tells us if the frame is
67	one that has been queued already. To add a frame we throw it on the end. Ack
68	walks down the list from the start.
69	
70	We keep a set of control flags
71	
72	
73		sk->tcp_pend_event
74	
75			TCP_PEND_ACK			Ack needed
76			TCP_ACK_NOW			Needed now
77			TCP_WINDOW			Window update check
78			TCP_WINZERO			Zero probing
79	
80	
81		sk->transmit_queue		The transmission frame begin
82		sk->transmit_new		First new frame pointer
83		sk->transmit_end		Where to add frames
84	
85		sk->tcp_last_tx_ack		Last ack seen
86		sk->tcp_dup_ack			Dup ack count for fast retransmit
87	
88	
89	Frames are queued for output by tcp_write. We do our best to send the frames
90	off immediately if possible, but otherwise queue and compute the body
91	checksum in the copy. 
92	
93	When a write is done we try to clear any pending events and piggy back them.
94	If the window is full we queue full sized frames. On the first timeout in
95	zero window we split this.
96	
97	On a timer we walk the retransmit list to send any retransmits, update the
98	backoff timers etc. A change of route table stamp causes a change of header
99	and recompute. We add any new tcp level headers and refinish the checksum
100	before sending. 
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