|Last updated: 3 June 2017
|- Congestion control
|- How the new TCP output machine [nyi] works
|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, the congestion control
|mechanism must provide a valid name and must implement either ssthresh,
|cong_avoid and undo_cwnd hooks or the "omnipotent" cong_control hook.
|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 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 default congestion control mechanism is chosen based on the
|DEFAULT_TCP_CONG Kconfig parameter. If you really want a particular default
|value then you can set it using sysctl net.ipv4.tcp_congestion_control. 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
|removed, 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
| 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