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TCP_RACK(4) Device Drivers Manual TCP_RACK(4)

tcp_rack — TCP RACK-TLP Loss Detection Algorithm for TCP

To load the TCP stack as a module at boot time, place the following line in loader.conf(5):

tcp_rack_load="YES"

To enable the TCP stack, place the following line in the sysctl.conf(5):

net.inet.tcp.functions_default=rack

RACK-TLP uses per-segment transmit timestamps and selective acknowledgments (SACKs) and has two parts. Recent Acknowledgment (RACK) starts fast recovery quickly using time-based inferences derived from acknowledgment (ACK) feedback, and Tail Loss Probe (TLP) leverages RACK and sends a probe packet to trigger ACK feedback to avoid retransmission timeout (RTO) events.

Compared to the widely used duplicate acknowledgment (DupAck) threshold approach, RACK-TLP detects losses more efficiently when there are application-limited flights of data, lost retransmissions, or data packet reordering events.

It is intended to be an alternative to the DupAck threshold approach.

The algorithm exposes the following scopes in the net.inet.tcp.rack branch of the sysctl(3) MIB:

net.inet.tcp.rack.misc
Misc related controls
net.inet.tcp.rack.features
Feature controls
net.inet.tcp.rack.measure
Measure related controls
net.inet.tcp.rack.timers
Timer related controls
net.inet.tcp.rack.tlp
TLP and Rack related Controls
net.inet.tcp.rack.timely
Rack Timely RTT Controls
net.inet.tcp.rack.hdwr_pacing
Pacing related Controls
net.inet.tcp.rack.pacing
Pacing related Controls
net.inet.tcp.rack.tp
Rack tracepoint facility
net.inet.tcp.rack.probertt
ProbeRTT related Controls
net.inet.tcp.rack.stats
Rack Counters
net.inet.tcp.rack.sack_attack
Rack Sack Attack Counters and Controls

Besides the variables within the above scopes the following variables are also exposed in the net.inet.tcp.rack branch:

net.inet.tcp.rack.clear
Clear counters
net.inet.tcp.rack.opts
RACK Option Stats
net.inet.tcp.rack.outsize
MSS send sizes
net.inet.tcp.rack.req_measure_cnt
If doing dynamic pacing, how many measurements must be in before we start pacing?
net.inet.tcp.rack.use_pacing
If set we use pacing, if clear we use only the original burst mitigation
net.inet.tcp.rack.rate_sample_method
What method should we use for rate sampling 0=high, 1=low

cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), h_ertt(4), mod_cc(4), tcp(4), tcp_bbr(4), mod_cc(9)

Neal Cardwell, Yuchung Cheng, Nandita Dukkipati, and Priyaranjan Jha, The RACK-TLP Loss Detection Algorithm for TCP, February 2021, RFC 8985.

M. Allman, V. Paxson, and E. Blanton, TCP Congestion Control, September 2009, RFC 5681.

M. Mathis, Nandita Dukkipati, and Yuchung Cheng, Proportional Rate Reduction for TCP, May 2013, RFC 6937.

The tcp_rack congestion control module first appeared in FreeBSD 13.0.

The tcp_rack congestion control module was written by Randall Stewart <rrs@FreeBSD.org> and sponsored by Netflix, Inc. This manual page was written by Gordon Bergling <gbe@FreeBSD.org>.

March 18, 2024 dev