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net: Update TCP congestion control documentation 

Update tcp.txt to fix mandatory congestion control ops and default
CCA selection. Also, fix comment in tcp.h for undo_cwnd.

Signed-off-by: Anmol Sarma <me@anmolsarma.in>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Anmol Sarma 2017-06-03 17:40:54 +05:30 committed by David S. Miller
parent 6dc06c08be
commit 1e0ce2a1ee
2 changed files with 14 additions and 19 deletions
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31
Documentation/networking/tcp.txt
View File

@ -1,7 +1,7 @@
TCP protocol
============
Last updated: 9 February 2008
Last updated: 3 June 2017
Contents
========
@ -29,18 +29,19 @@ 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 must be valid.
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
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
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+.
@ -49,21 +50,15 @@ 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.
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 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
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.
removed, it will always be available.
How the new TCP output machine [nyi] works.
===========================================

2
include/net/tcp.h
View File

@ -924,7 +924,7 @@ struct tcp_congestion_ops {
void (*cwnd_event)(struct sock *sk, enum tcp_ca_event ev);
/* call when ack arrives (optional) */
void (*in_ack_event)(struct sock *sk, u32 flags);
/* new value of cwnd after loss (optional) */
/* new value of cwnd after loss (required) */
u32 (*undo_cwnd)(struct sock *sk);
/* hook for packet ack accounting (optional) */
void (*pkts_acked)(struct sock *sk, const struct ack_sample *sample);