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27 .Dd September 24, 2020
32 .Nd TCP Bottleneck Bandwidth and Round-Trip Time Algorithm
34 To use this TCP stack you have to place the following line in your
35 kernel configuration file:
36 .Bd -ragged -offset indent
40 To load the driver as a module at boot time, place the following line in
42 .Bd -literal -offset indent
46 To enable the TCP stack you must place the following line in the
48 .Bd -literal -offset indent
49 net.inet.tcp.functions_default=bbr
52 Bottleneck bandwidth and round-trip time (BBR) is a congestion control algorithm which
53 seeks high throughput with a small queue by probing BW and RTT.
54 It is a round-up redesign of congestion control, which is not loss-based, delay-based,
55 ECN-based or AIMD-based.
57 The core design of BBR is about creating a model graph of the network path by
58 estimating the maximum BW and minimum RTT on each ACK.
60 The algorithm exposes the following scopes in the
65 .Bl -tag -width ".Va exp_backoff_scale"
67 Cwnd controls, for example "target cwnd rtt measurment" and "BBR initial window".
71 Connection pacing controls.
73 Policer controls, for example "false detection threshold" and "loss threshold".
84 Besides the variables within the above scopes the following
85 variables are also exposed in the
88 .Bl -tag -width ".Va exp_backoff_scale"
91 .It Va software_pacing
92 Total number of software paced flows.
94 Total number of hardware paced flows.
95 .It Va enob_no_hdwr_pacing
96 Total number of enobufs for non-hardware paced flows.
97 .It Va enob_hdwr_pacing
98 Total number of enobufs for hardware paced flows.
100 What divisor for TLP rtt/retran will be added (1=rtt, 2=1/2 rtt etc).
102 Does reorder detection fade, if so how many ms (0 means never).
103 .It Va reorder_thresh
104 What factor for rack will be added when seeing reordering (shift right).
106 Should BBR black box logging be verbose.
108 When do we start ignoring small sack blocks.
109 .It Va resend_use_tso
111 .It Va data_after_close
112 Do we hold off sending a RST until all pending data is ack'd.
114 When we hit this many errors in a row, kill the session?
116 When we hit an error what is the min to pace out in usec's?
118 Do we burst 1ms between sends on retransmissions (like rack)?
137 .%A "Soheil Hassas Yeganeh"
139 .%T "BBR: Congestion-Based Congestion Control"
140 .%J "ACM Queue, Vol. 14"
141 .%D "September / October 2016"
145 .%A "Benedikt Jaeger"
146 .%A "Lukas Schwaighofer"
150 .%T "Towards a Deeper Understanding of TCP BBR Congestion Control"
151 .%J "IFIP Networking 2018"
153 .%U "http://www.net.in.tum.de/fileadmin/bibtex/publications/papers/IFIP-Networking-2018-TCP-BBR.pdf"
158 congestion control module first appeared in
164 congestion control module was written by
165 .An Randall Stewart Aq Mt rrs@FreeBSD.org
166 and sponsored by Netflix, Inc.
167 This manual page was written by
168 .An Gordon Bergling Aq Mt gbe@FreeBSD.org .