[TCP]: Update sysctl and congestion control documentation.
Update the documentation to remove the old sysctl values and include the new congestion control infrastructure. Includes changes to tcp.txt by Ian McDonald. Signed-off-by: Stephen Hemminger <shemminger@osdl.org> Signed-off-by: David S. Miller <davem@davemloft.net>
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Stephen Hemminger
David S. Miller
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@ -304,57 +304,6 @@ tcp_low_latency - BOOLEAN
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changed would be a Beowulf compute cluster.
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Default: 0
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tcp_westwood - BOOLEAN
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Enable TCP Westwood+ congestion control algorithm.
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TCP Westwood+ is a sender-side only modification of the TCP Reno
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protocol stack that optimizes the performance of TCP congestion
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control. It is based on end-to-end bandwidth estimation to set
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congestion window and slow start threshold after a congestion
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episode. Using this estimation, TCP Westwood+ adaptively sets a
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slow start threshold and a congestion window which takes into
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account the bandwidth used at the time congestion is experienced.
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TCP Westwood+ significantly increases fairness wrt TCP Reno in
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wired networks and throughput over wireless links.
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Default: 0
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tcp_vegas_cong_avoid - BOOLEAN
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Enable TCP Vegas congestion avoidance algorithm.
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TCP Vegas is a sender-side only change to TCP that anticipates
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the onset of congestion by estimating the bandwidth. TCP Vegas
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adjusts the sending rate by modifying the congestion
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window. TCP Vegas should provide less packet loss, but it is
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not as aggressive as TCP Reno.
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Default:0
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tcp_bic - BOOLEAN
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Enable BIC TCP congestion control algorithm.
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BIC-TCP is a sender-side only change that ensures a linear RTT
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fairness under large windows while offering both scalability and
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bounded TCP-friendliness. The protocol combines two schemes
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called additive increase and binary search increase. When the
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congestion window is large, additive increase with a large
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increment ensures linear RTT fairness as well as good
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scalability. Under small congestion windows, binary search
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increase provides TCP friendliness.
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Default: 0
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tcp_bic_low_window - INTEGER
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Sets the threshold window (in packets) where BIC TCP starts to
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adjust the congestion window. Below this threshold BIC TCP behaves
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the same as the default TCP Reno.
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Default: 14
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tcp_bic_fast_convergence - BOOLEAN
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Forces BIC TCP to more quickly respond to changes in congestion
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window. Allows two flows sharing the same connection to converge
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more rapidly.
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Default: 1
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tcp_default_win_scale - INTEGER
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Sets the minimum window scale TCP will negotiate for on all
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conections.
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Default: 7
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tcp_tso_win_divisor - INTEGER
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This allows control over what percentage of the congestion window
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can be consumed by a single TSO frame.
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@ -368,6 +317,11 @@ tcp_frto - BOOLEAN
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where packet loss is typically due to random radio interference
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rather than intermediate router congestion.
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tcp_congestion_control - STRING
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Set the congestion control algorithm to be used for new
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connections. The algorithm "reno" is always available, but
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additional choices may be available based on kernel configuration.
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somaxconn - INTEGER
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Limit of socket listen() backlog, known in userspace as SOMAXCONN.
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Defaults to 128. See also tcp_max_syn_backlog for additional tuning
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@ -1,5 +1,72 @@
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How the new TCP output machine [nyi] works.
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TCP protocol
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============
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Last updated: 21 June 2005
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Contents
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========
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- Congestion control
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- How the new TCP output machine [nyi] works
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Congestion control
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==================
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The following variables are used in the tcp_sock for congestion control:
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snd_cwnd The size of the congestion window
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snd_ssthresh Slow start threshold. We are in slow start if
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snd_cwnd is less than this.
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snd_cwnd_cnt A counter used to slow down the rate of increase
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once we exceed slow start threshold.
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snd_cwnd_clamp This is the maximum size that snd_cwnd can grow to.
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snd_cwnd_stamp Timestamp for when congestion window last validated.
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snd_cwnd_used Used as a highwater mark for how much of the
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congestion window is in use. It is used to adjust
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snd_cwnd down when the link is limited by the
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application rather than the network.
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As of 2.6.13, Linux supports pluggable congestion control algorithms.
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A congestion control mechanism can be registered through functions in
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tcp_cong.c. The functions used by the congestion control mechanism are
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registered via passing a tcp_congestion_ops struct to
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tcp_register_congestion_control. As a minimum name, ssthresh,
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cong_avoid, min_cwnd must be valid.
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Private data for a congestion control mechanism is stored in tp->ca_priv.
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tcp_ca(tp) returns a pointer to this space. This is preallocated space - it
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is important to check the size of your private data will fit this space, or
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alternatively space could be allocated elsewhere and a pointer to it could
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be stored here.
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There are three kinds of congestion control algorithms currently: The
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simplest ones are derived from TCP reno (highspeed, scalable) and just
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provide an alternative the congestion window calculation. More complex
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ones like BIC try to look at other events to provide better
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heuristics. There are also round trip time based algorithms like
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Vegas and Westwood+.
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Good TCP congestion control is a complex problem because the algorithm
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needs to maintain fairness and performance. Please review current
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research and RFC's before developing new modules.
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The method that is used to determine which congestion control mechanism is
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determined by the setting of the sysctl net.ipv4.tcp_congestion_control.
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The default congestion control will be the last one registered (LIFO);
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so if you built everything as modules. the default will be reno. If you
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build with the default's from Kconfig, then BIC will be builtin (not a module)
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and it will end up the default.
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If you really want a particular default value then you will need
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to set it with the sysctl. If you use a sysctl, the module will be autoloaded
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if needed and you will get the expected protocol. If you ask for an
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unknown congestion method, then the sysctl attempt will fail.
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If you remove a tcp congestion control module, then you will get the next
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available one. Since reno can not be built as a module, and can not be
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deleted, it will always be available.
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How the new TCP output machine [nyi] works.
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===========================================
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Data is kept on a single queue. The skb->users flag tells us if the frame is
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one that has been queued already. To add a frame we throw it on the end. Ack
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