path: root/Documentation/networking/tcp.txt
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authorFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
committerFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
commit3b4bd47f8f4ed3aaf7c81c9b5d2d37ad79fadf4a (patch)
treeb9996006addfd7ae70a39672b76843b49aebc189 /Documentation/networking/tcp.txt
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+TCP protocol
+Last updated: 9 February 2008
+- 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, min_cwnd 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.