Based on kernel version 4.1. Page generated on 2015-06-28 12:13 EST.
1 TCP protocol 2 ============ 3 4 Last updated: 9 February 2008 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 name, ssthresh, 33 cong_avoid must be valid. 34 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. 40 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+. 47 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. 51 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 defaults from Kconfig, then CUBIC will be builtin (not a 57 module) and it will end up the default. 58 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. 63 64 If you remove a tcp congestion control module, then you will get the next 65 available one. Since reno cannot be built as a module, and cannot be 66 deleted, it will always be available. 67 68 How the new TCP output machine [nyi] works. 69 =========================================== 70 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. 74 75 We keep a set of control flags 76 77 78 sk->tcp_pend_event 79 80 TCP_PEND_ACK Ack needed 81 TCP_ACK_NOW Needed now 82 TCP_WINDOW Window update check 83 TCP_WINZERO Zero probing 84 85 86 sk->transmit_queue The transmission frame begin 87 sk->transmit_new First new frame pointer 88 sk->transmit_end Where to add frames 89 90 sk->tcp_last_tx_ack Last ack seen 91 sk->tcp_dup_ack Dup ack count for fast retransmit 92 93 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. 97 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. 101 102 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.