MFC 218153:

MFC 218153:

Import a clean-room implementation of the Hamilton-Delay (HD) congestion control
algorithm based on the paper "A strategy for fair coexistence of loss and
delay-based congestion control algorithms" by Budzisz, Stanojevic, Shorten and
Baker. It is implemented as a kernel module compatible with the recently
committed modular congestion control framework.

HD uses a probabilistic approach to reacting to delay-based congestion. The
probability of reducing cwnd is zero when the queuing delay is very small,
increasing to a maximum at a set threshold, then back down to zero again when
the queuing delay is high. Normal operation keeps the queuing delay below the
set threshold. However, since loss-based congestion control algorithms push the
queuing delay high when probing for bandwidth, having the probability of
reducing cwnd drop back to zero for high delays allows HD to compete with
loss-based algorithms.

In collaboration with:	David Hayes <dahayes at swin edu au> and
				Grenville Armitage <garmitage at swin edu au>
Sponsored by:	FreeBSD Foundation
Reviewed by:	bz and others along the way

git-svn-id: svn://svn.freebsd.org/base/stable/8@222412 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f