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authorStephen Hemminger <shemminger@osdl.org>2005-06-23 12:22:36 -0700
committerDavid S. Miller <davem@davemloft.net>2005-06-23 12:22:36 -0700
commit9d7bcfc6b8586ee5a52043f061e0411965e71b88 (patch)
treeef6aa8e6fd9dc0c7187b9cd1497d13e180ae36a8 /Documentation/networking/tcp.txt
parent056ede6cface66b400cd3b8e60ed077cc5b85c18 (diff)
[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>
Diffstat (limited to 'Documentation/networking/tcp.txt')
-rw-r--r--Documentation/networking/tcp.txt69
1 files changed, 68 insertions, 1 deletions
diff --git a/Documentation/networking/tcp.txt b/Documentation/networking/tcp.txt
index 71749007091..0fa30042557 100644
--- a/Documentation/networking/tcp.txt
+++ b/Documentation/networking/tcp.txt
@@ -1,5 +1,72 @@
-How the new TCP output machine [nyi] works.
+TCP protocol
+============
+
+Last updated: 21 June 2005
+
+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, 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 default's from Kconfig, then BIC 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 can not be built as a module, and can not 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