| 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 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. | |