TCP_RACK(4)

💅 TCP_RACK(4) — RACK-TLP but make it make sense ✨

okay so bestie, this is the manual page for tcp_rack(4): the TCP RACK-TLP Loss Detection Algorithm for TCP. Yes, manual name is right there near the top, we're doing it cute today. 🫶

💖 NAME 💖

tcp_rack — TCP RACK-TLP Loss Detection Algorithm for TCP.

✨ SYNOPSIS ✨

Need to load the TCP stack as a module at boot time? Put this line in loader.conf(5):

tcp_rack_load="YES"

Need to enable the TCP stack? Put this line in sysctl.conf(5):

net.inet.tcp.functions_default=rack

🦋 DESCRIPTION 🦋

RACK-TLP uses per-segment transmit timestamps and selective acknowledgments (SACKs), and it has two parts. Recent Acknowledgment (RACK) starts fast recovery quickly using time-based inferences derived from acknowledgment (ACK) feedback. 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. No cap.

🧚 MIB Variables 🧚

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, aka the little random-knob corner.
• 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.

💅 SEE ALSO 💅

• 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) — go read these too, bestie.
• 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.

🌈 HISTORY 🌈

The tcp_rack congestion control module first appeared in FreeBSD 13.0. ✨

👑 AUTHORS 👑

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>. Give them flowers. 💐

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Debian, March 18, 2024 — that's the source/version and date, bestie. Bye! 💖