mirror of
https://github.com/torvalds/linux.git
synced 2024-10-31 01:01:52 +00:00
45f7435968
Commit 684bad1107 "tcp: use PRR to reduce cwin in CWR state" removed all
calls to min_cwnd, so we can safely remove it.
Also, remove tcp_reno_min_cwnd because it was only used for min_cwnd.
Signed-off-by: Stanislav Fomichev <stfomichev@yandex-team.ru>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
107 lines
4.1 KiB
Plaintext
107 lines
4.1 KiB
Plaintext
TCP protocol
|
|
============
|
|
|
|
Last updated: 9 February 2008
|
|
|
|
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 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.
|
|
|