cc_htcp —
H-TCP
Congestion Control Algorithm
The H-TCP congestion control algorithm was designed to provide increased
throughput in fast and long-distance networks. It attempts to maintain
fairness when competing with legacy NewReno TCP in lower speed scenarios where
NewReno is able to operate adequately.
The congestion window is increased as a function of the time elapsed since the
last congestion event. The window increase algorithm operates like NewReno for
the first second after a congestion event, and then switches to a high-speed
mode based on a quadratic increase function.
The implementation was done in a clean-room fashion, and is based on the
Internet Draft and other documents referenced in the
SEE ALSO section below.
The algorithm exposes the following tunable variables in the
net.inet.tcp.cc.htcp branch of the
sysctl(3) MIB:
- adaptive_backoff
- Controls use of the adaptive backoff algorithm, which is
designed to keep network queues non-empty during congestion recovery
episodes. Default is 0 (disabled).
- rtt_scaling
- Controls use of the RTT scaling algorithm, which is
designed to make congestion window increase during congestion avoidance
mode invariant with respect to RTT. Default is 0 (disabled).
cc_chd(4),
cc_cubic(4),
cc_hd(4),
cc_newreno(4),
cc_vegas(4),
mod_cc(4),
tcp(4),
mod_cc(9)
D. Leith and
R. Shorten, H-TCP: TCP Congestion
Control for High Bandwidth-Delay Product Paths,
http://tools.ietf.org/id/draft-leith-tcp-htcp-06.txt.
D. Leith,
R. Shorten, and T. Yee,
H-TCP: A framework for congestion control in high-speed and
long-distance networks, Proc. PFLDnet,
2005.
G. Armitage,
L. Stewart, M. Welzl, and
J. Healy, An independent H-TCP
implementation under FreeBSD 7.0: description and observed behaviour,
SIGCOMM Comput. Commun. Rev., 3,
38, 27-38,
July 2008.
Development and testing of this software were made possible in part by grants
from the FreeBSD Foundation and Cisco University Research Program Fund at
Community Foundation Silicon Valley.
The
cc_htcp congestion control module first
appeared in
FreeBSD 9.0.
The module was first released in 2007 by James Healy and Lawrence Stewart whilst
working on the NewTCP research project at Swinburne University of Technology's
Centre for Advanced Internet Architectures, Melbourne, Australia, which was
made possible in part by a grant from the Cisco University Research Program
Fund at Community Foundation Silicon Valley. More details are available at:
http://caia.swin.edu.au/urp/newtcp/
The
cc_htcp congestion control module was written
by
James Healy
<
[email protected]>
and
Lawrence Stewart
<
[email protected]>.
This manual page was written by
Lawrence
Stewart
<
[email protected]>
and
David Hayes
<
[email protected]>.