src/kernel/linux/v4.19/Documentation/networking/tcp.txt

TCP protocol
============

Last updated: 3 June 2017

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, 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
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 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 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 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
removed, 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.