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- 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.
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