view Documentation/networking/tcp.txt @ 912:dd42cdb0ab89

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Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
author Isaku Yamahata <yamahata@valinux.co.jp>
date Mon Jun 29 12:09:16 2009 +0900 (2009-06-29)
parents 831230e53067
line source
1 TCP protocol
2 ============
4 Last updated: 21 June 2005
6 Contents
7 ========
9 - Congestion control
10 - How the new TCP output machine [nyi] works
12 Congestion control
13 ==================
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.
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 name, ssthresh,
33 cong_avoid, min_cwnd must be valid.
35 Private data for a congestion control mechanism is stored in tp->ca_priv.
36 tcp_ca(tp) returns a pointer to this space. This is preallocated space - it
37 is important to check the size of your private data will fit this space, or
38 alternatively space could be allocated elsewhere and a pointer to it could
39 be stored here.
41 There are three kinds of congestion control algorithms currently: The
42 simplest ones are derived from TCP reno (highspeed, scalable) and just
43 provide an alternative the congestion window calculation. More complex
44 ones like BIC try to look at other events to provide better
45 heuristics. There are also round trip time based algorithms like
46 Vegas and Westwood+.
48 Good TCP congestion control is a complex problem because the algorithm
49 needs to maintain fairness and performance. Please review current
50 research and RFC's before developing new modules.
52 The method that is used to determine which congestion control mechanism is
53 determined by the setting of the sysctl net.ipv4.tcp_congestion_control.
54 The default congestion control will be the last one registered (LIFO);
55 so if you built everything as modules. the default will be reno. If you
56 build with the default's from Kconfig, then BIC will be builtin (not a module)
57 and it will end up the default.
59 If you really want a particular default value then you will need
60 to set it with the sysctl. If you use a sysctl, the module will be autoloaded
61 if needed and you will get the expected protocol. If you ask for an
62 unknown congestion method, then the sysctl attempt will fail.
64 If you remove a tcp congestion control module, then you will get the next
65 available one. Since reno can not be built as a module, and can not be
66 deleted, it will always be available.
68 How the new TCP output machine [nyi] works.
69 ===========================================
71 Data is kept on a single queue. The skb->users flag tells us if the frame is
72 one that has been queued already. To add a frame we throw it on the end. Ack
73 walks down the list from the start.
75 We keep a set of control flags
78 sk->tcp_pend_event
80 TCP_PEND_ACK Ack needed
81 TCP_ACK_NOW Needed now
82 TCP_WINDOW Window update check
83 TCP_WINZERO Zero probing
86 sk->transmit_queue The transmission frame begin
87 sk->transmit_new First new frame pointer
88 sk->transmit_end Where to add frames
90 sk->tcp_last_tx_ack Last ack seen
91 sk->tcp_dup_ack Dup ack count for fast retransmit
94 Frames are queued for output by tcp_write. We do our best to send the frames
95 off immediately if possible, but otherwise queue and compute the body
96 checksum in the copy.
98 When a write is done we try to clear any pending events and piggy back them.
99 If the window is full we queue full sized frames. On the first timeout in
100 zero window we split this.
102 On a timer we walk the retransmit list to send any retransmits, update the
103 backoff timers etc. A change of route table stamp causes a change of header
104 and recompute. We add any new tcp level headers and refinish the checksum
105 before sending.