Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 101 insertions, 0 deletions

diff --git a/Documentation/networking/tcp.txt b/Documentation/networking/tcp.txt
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+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. 
+