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 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 the RFC referenced in the
SEE ALSO section below.
The algorithm exposes the following tunable variables in the
net.inet.tcp.cc.dctcp branch of the
sysctl(3) MIB:
- alpha
- The initial value to estimate the congestion on the link.
The valid range is from 0 to 1024, where 1024 reduces the congestion
window to half, if a CE is observed in the first window and
alpha could not yet adjust to the
congestion level on that path. Default is 1024.
- shift_g
- An estimation gain in the
alpha calculation. This influences the
responsiveness when adjusting alpha to the most recent observed window.
Valid range from 0 to 10, the default is 4, resulting in an effective gain
of 1 / ( 2 ^ shift_g ), or 1/16th.
- slowstart
- A flag if the congestion window should be reduced by one
half after slow start. Valid settings 0 and 1, default 0.
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,
Dave Thaler, Praveen
Balasubramanian, Lars Eggert, and
Glenn Judd, Data Center TCP
(DCTCP): TCP Congestion Control for Data Centers,
https://tools.ietf.org/html/rfc8257.
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
[email protected]
and
Lars Eggert
[email protected]
with help and modifications from
Hiren
Panchasara
[email protected]