CC_DCTCP(4FREEBSD) - Linux man page online | Special files

DCTCP Congestion Control Algorithm.

January 12, 2015
CC_DCTCP(4) BSD Kernel Interfaces Manual CC_DCTCP(4)


cc_dctcp — DCTCP Congestion Control Algorithm


The DCTCP (data center TCP) congestion control algorithm aims to maximise throughput and minimise latency in data center networks by utilising 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 clearing) ECE marks. This feedback methodol‐ ogy is also adopted when the receiver uses delayed ACK. The FreeBSD DCTCP implementation includes two minor modifications for the one-sided deploy‐ ment. 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 clas‐ sic 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 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.


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,, 63-74, July 2010. Stephen Bensley, Lars Eggert, and Dave Thaler, Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters,


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.


The cc_dctcp congestion control module and this manual page were written by Midori Kato and Lars Eggert with help and modifications from Hiren Panchasara
BSD January 12, 2015 BSD
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cc_dctcp(4freebsd) referred by mod_cc(4freebsd)
refer to cc_chd(4freebsd) | cc_cubic(4freebsd) | cc_hd(4freebsd) | cc_htcp(4freebsd) | cc_newreno(4freebsd) | cc_vegas(4freebsd) | mod_cc(4freebsd) | mod_cc(9freebsd) | sysctl(3freebsd) | tcp(4freebsd)
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