cc_dctcp(4) [freebsd man page]
CC_DCTCP(4) BSD Kernel Interfaces Manual CC_DCTCP(4) NAME
cc_dctcp -- DCTCP Congestion Control Algorithm DESCRIPTION
The DCTCP (data center TCP) congestion control algorithm aims to maximise throughput and minimise latency in data center networks by utilis- ing the proportion of Explicit Congestion Notification (ECN) marks received from capable hardware as a congestion signal. DCTCP uses fraction of ECN marked packets to update congestion window. The window reduction ratio is always <= 1/2. Only when all of the packets are marked, congestion window is halved. In order to keep the accuracy of the ECN marked fraction, a DCTCP receiver mirrors back incoming (or missing) CE marks by setting (or clear- ing) ECE marks. This feedback methodology is also adopted when the receiver uses delayed ACK. The FreeBSD DCTCP implementation includes two minor modifications for the one-sided deployment. Considering the situation that DCTCP is used as sender and classic ECN is used as receiver, DCTCP sets the CWR flag as the reaction to the ECE flag. In addition, when classic ECN is used as sender and DCTCP is used as receiver, DCTCP avoids to mirror back ACKs only when the CWR flag is set in the incoming packet. The other specifications are based on the paper and Internet Draft referenced in the SEE ALSO section below. MIB Variables The algorithm exposes the following tunable variables in the net.inet.tcp.cc.dctcp branch of the sysctl(3) MIB: alpha An initial estimator of the congestion on the link. Default is 0. dctcp_shift_g An estimation gain in the alpha calculation. Default is 16. slowstart A trigger to halve congestion window after slow start. Default does nothing to halve window. SEE ALSO
cc_chd(4), cc_cubic(4), cc_hd(4), cc_htcp(4), cc_newreno(4), cc_vegas(4), mod_cc(4), tcp(4), mod_cc(9) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye, Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan, "Data Center TCP (DCTCP)", ACM SIGCOMM 2010, http://research.microsoft.com/pubs/121386/dctcp-public.pdf, 63-74, July 2010. Stephen Bensley, Lars Eggert, and Dave Thaler, Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters, http://tools.ietf.org/html/draft-bensley-tcpm-dctcp-01. HISTORY
The cc_dctcp congestion control module first appeared in FreeBSD 11.0. The module was first released in 2014 by Midori Kato studying at Keio University, Japan. AUTHORS
The cc_dctcp congestion control module and this manual page were written by Midori Kato katoon@sfc.wide.ad.jp and Lars Eggert lars@netapp.com with help and modifications from Hiren Panchasara hiren@FreeBSD.org BSD
January 12, 2015 BSD
Check Out this Related Man Page
CC_CDG(4) BSD Kernel Interfaces Manual CC_CDG(4) NAME
cc_cdg -- CDG Congestion Control Algorithm DESCRIPTION
CAIA-Delay Gradient (CDG) is a hybrid congestion control algorithm which reacts to both packet loss and inferred queuing delay. It attempts to operate as a delay-based algorithm where possible, but utilises heuristics to detect loss-based TCP cross traffic and will compete effec- tively as required. CDG is therefore incrementally deployable and suitable for use on shared networks. During delay-based operation, CDG uses a delay-gradient based probabilistic backoff mechanism, and will also try to infer non congestion related packet losses and avoid backing off when they occur. During loss-based operation, CDG essentially reverts to cc_newreno(4)-like be- haviour. CDG switches to loss-based operation when it detects that a configurable number of consecutive delay-based backoffs have had no measurable effect. It periodically attempts to return to delay-based operation, but will keep switching back to loss-based operation as required. MIB Variables The algorithm exposes the following variables in the net.inet.tcp.cc.cdg branch of the sysctl(3) MIB: version Current algorithm/implementation version number. beta_delay Delay-based window decrease factor as a percentage (on delay-based backoff, w = w * beta_delay / 100). Default is 70. beta_loss Loss-based window decrease factor as a percentage (on loss-based backoff, w = w * beta_loss / 100). Default is 50. exp_backoff_scale Scaling parameter for the probabilistic exponential backoff. Default is 2. smoothing_factor Number of samples used for moving average smoothing (0 means no smoothing). Default is 8. loss_compete_consec_cong Number of consecutive delay-gradient based congestion episodes which will trigger loss-based CC compatibility. Default is 5. loss_compete_hold_backoff Number of consecutive delay-gradient based congestion episodes to hold the window backoff for loss-based CC compatibility. Default is 5. alpha_inc If non-zero, this enables an experimental mode where CDG's window increase factor (alpha) is increased by 1 MSS every alpha_inc RTTs during congestion avoidance mode. (Setting alpha_inc to 1 results in the most aggressive growth of the window increase factor over time. Use higher alpha_inc values for slower growth.) Default is 0. 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), khelp(9), mod_cc(9) D. A. Hayes and G. Armitage, "Revisiting TCP Congestion Control using Delay Gradients", Networking 2011 Proceedings, Part II, 328-341, May 2011. N. Khademi and G. Armitage, Minimising RTT across homogeneous 802.11 WLANs with CAIA Delay-Gradient TCP (v0.1), CAIA Technical Report 121113A, http://caia.swin.edu.au/reports/121113A/CAIA-TR-121113A.pdf, November 2012. ACKNOWLEDGEMENTS
Development and testing of this software were made possible in part by grants from the FreeBSD Foundation and The Cisco University Research Program Fund, a corporate advised fund of Silicon Valley Community Foundation. HISTORY
The cc_cdg congestion control module first appeared in FreeBSD 9.2. The module was first released in 2011 by David Hayes whilst working on the NewTCP research project at Swinburne University of Technology's Centre for Advanced Internet Architectures, Melbourne, Australia. More details are available at: http://caia.swin.edu.au/urp/newtcp/ AUTHORS
The cc_cdg congestion control module was written by David Hayes <david.hayes@ieee.org>. This manual page was written by Lawrence Stewart <lstewart@FreeBSD.org> and Grenville Armitage <garmitage@swin.edu.au>. BUGS
The underlying algorithm and parameter values are still a work in progress and may not be optimal for some network scenarios. BSD
July 2, 2013 BSD