You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
106 lines
4.1 KiB
106 lines
4.1 KiB
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. |
|
|
|
|