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Diffstat (limited to 'Documentation/networking/tcp.txt')
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diff --git a/Documentation/networking/tcp.txt b/Documentation/networking/tcp.txt new file mode 100644 index 000000000..9c7139d57 --- /dev/null +++ b/Documentation/networking/tcp.txt @@ -0,0 +1,101 @@ +TCP protocol +============ + +Last updated: 3 June 2017 + +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, 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 + + + 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. + |