Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 9b9375b5 authored by Neal Cardwell's avatar Neal Cardwell Committed by David S. Miller
Browse files

tcp_bbr: add a state transition diagram and accompanying comment 



Document the possible state transitions for a BBR flow, and also add a
prose summary of the state machine, covering the life of a typical BBR
flow.

Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent a283ad50

net/ipv4/tcp_bbr.c

+31 −1
Original line number Diff line number Diff line
@@ -14,6 +14,36 @@ 
 * observed, or adjust the sending rate if it estimates there is a

 * traffic policer, in order to keep the drop rate reasonable.

 *

 * Here is a state transition diagram for BBR:

 *

 *             |

 *             V

 *    +---> STARTUP  ----+

 *    |        |         |

 *    |        V         |

 *    |      DRAIN   ----+

 *    |        |         |

 *    |        V         |

 *    +---> PROBE_BW ----+

 *    |      ^    |      |

 *    |      |    |      |

 *    |      +----+      |

 *    |                  |

 *    +---- PROBE_RTT <--+

 *

 * A BBR flow starts in STARTUP, and ramps up its sending rate quickly.

 * When it estimates the pipe is full, it enters DRAIN to drain the queue.

 * In steady state a BBR flow only uses PROBE_BW and PROBE_RTT.

 * A long-lived BBR flow spends the vast majority of its time remaining

 * (repeatedly) in PROBE_BW, fully probing and utilizing the pipe's bandwidth

 * in a fair manner, with a small, bounded queue. *If* a flow has been

 * continuously sending for the entire min_rtt window, and hasn't seen an RTT

 * sample that matches or decreases its min_rtt estimate for 10 seconds, then

 * it briefly enters PROBE_RTT to cut inflight to a minimum value to re-probe

 * the path's two-way propagation delay (min_rtt). When exiting PROBE_RTT, if

 * we estimated that we reached the full bw of the pipe then we enter PROBE_BW;

 * otherwise we enter STARTUP to try to fill the pipe.

 *

 * BBR is described in detail in:

 *   "BBR: Congestion-Based Congestion Control",

 *   Neal Cardwell, Yuchung Cheng, C. Stephen Gunn, Soheil Hassas Yeganeh,
@@ -51,7 +81,7 @@ enum bbr_mode { 
	BBR_STARTUP,	/* ramp up sending rate rapidly to fill pipe */

	BBR_DRAIN,	/* drain any queue created during startup */

	BBR_PROBE_BW,	/* discover, share bw: pace around estimated bw */

	BBR_PROBE_RTT,	/* cut cwnd to min to probe min_rtt */

	BBR_PROBE_RTT,	/* cut inflight to min to probe min_rtt */

};



/* BBR congestion control block */