diff options
| author | Neal Cardwell <ncardwell@google.com> | 2016-10-27 13:26:37 -0400 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2016-10-29 17:12:43 -0400 |
| commit | 9b9375b5b7fdf75804de7df956f8e7ef0190c69b (patch) | |
| tree | d7931dbc3827821f9e1d27177ce4f7b8cfa89076 | |
| parent | a283ad5066cd63f595224c7476001cfc367fdf2e (diff) | |
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: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
| -rw-r--r-- | net/ipv4/tcp_bbr.c | 32 |
1 files changed, 31 insertions, 1 deletions
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c index 0ea66c2c9344..b89bce4c721e 100644 --- a/net/ipv4/tcp_bbr.c +++ b/net/ipv4/tcp_bbr.c @@ -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 */ |