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authorNeal Cardwell <ncardwell@google.com>2016-10-27 13:26:37 -0400
committerDavid S. Miller <davem@davemloft.net>2016-10-29 17:12:43 -0400
commit9b9375b5b7fdf75804de7df956f8e7ef0190c69b (patch)
treed7931dbc3827821f9e1d27177ce4f7b8cfa89076 /net/ipv4/tcp_bbr.c
parenta283ad5066cd63f595224c7476001cfc367fdf2e (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>
Diffstat (limited to 'net/ipv4/tcp_bbr.c')
-rw-r--r--net/ipv4/tcp_bbr.c32
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 */