- The mean RTT is updated at the end of each congestion epoch, but if we switch
to congestion avoidance within the first epoch (e.g. if ssthresh was primed
from the hostcache), we'll trigger a divide by zero panic in
cubic_ack_received(). Set the mean to the min in cubic_record_rtt() if the
mean is less than the min to ensure we have a sane mean for use in this
situation. This fixes the panic reported by Nick Hibma.
- Adjust conditions under which we update the mean RTT in cubic_post_recovery()
to ensure a low latency path won't yield an RTT of less than 1. This avoids
another potential divide by zero panic when running CUBIC in networks with
sub-millisecond latencies.
- Remove the "safety" assignment of min into mean when we don't update the mean
because of failed conditions. The above change to the conditions for updating
the mean ensures the safety issue is addressed and I feel it is better to keep
our previous mean estimate around if we can't update than to revert to the
min.
- Initialise the mean RTT to 1 on connection startup to act as a safety belt if
a situation we haven't considered and addressed with the above changes were to
crop up in the wild.
Sponsored by: FreeBSD Foundation
Reported and tested by: Nick Hibma
Discussed with: David Hayes <dahayes at swin edu au>
MFC after: 5 weeks
X-MFC with: r216114
algorithm based on the Internet-Draft "draft-rhee-tcpm-cubic-02.txt". It is
implemented as a kernel module compatible with the recently committed modular
congestion control framework.
CUBIC was designed for 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
paper "CUBIC: A New TCP-Friendly High-Speed TCP Variant" provides additional
detail.
In collaboration with: David Hayes <dahayes at swin edu au> and
Grenville Armitage <garmitage at swin edu au>
Sponsored by: FreeBSD Foundation
Reviewed by: rpaulo (older patch from a few weeks ago)
MFC after: 3 months
