In this section, we answer the question: Under what network
conditions, can OverQoS achieve a CLVL abstraction across an overlay
link? For all the scenarios described in the section, we choose a
target loss-rate to be a small value , i.e.,,
. To
compute the available bandwidth,
, we use N-TCP with a value of
.
Simulations: We first test whether the FEC+ARQ CLVL construction
can achieve the target loss-rate across a variety of bursty loss
models. Our key conclusion from the simulations is that in all
cases, we meet the target loss rate , despite bursty losses
and the average loss-rate varying between
% and
%.
Furthermore, this conclusion is true not just for the means, but for
the tails of the distribution as well. Figure 7
shows the achieved loss rate for the FEC+ARQ based CLVL for three
different background traffic scenarios. In addition, our recovery
algorithm achieves the target loss irrespective of whether the IP
routers along the virtual link use FIFO or RED queues. These results
demonstrate that our CLVL algorithm is robust over a range of dynamic
traffic conditions and works even when the underlying loss rate is 30
times larger that the target loss rate,
.
Wide Area Evaluation: Given our specific choice of overlay
nodes, we found virtual links in our overlay testbed to be
lossy. A link is characterized as lossy if the loss-rate along the
link is at least
. Across each link, we ran a CLVL abstraction
for time-ranges varying from
minutes to
hour. In order to
measure the system under stress, the sending rate as determined by
N-TCP averaged between
Kbps (across Cable modems and DSL lines)
to
Mbps from other nodes. 2
The FEC+ARQ based CLVL achieved the target loss-rate over of the
virtual links. Our FEC+ARQ algorithm failed to achieve the target
loss rate of
only across
of the overlay links. Upon closer
investigation, we found the causes to be : short outages and
bi-modal loss distributions. A short outage refers to a period
of time when all packets transmitted along a virtual link are
lost. Within our testbed, we noticed non-recoverable losses along two
links: PDI-NBG and Unibo-Media. These non-recoverable losses lasted
for short periods of time (
s). Short outages can occur due to a
variety of problems such as routing changes or link resets.
A loss distribution is said to be bi-modal if the losses experienced
in every window is zero or very high. Links with very bursty losses
have a bi-modal distribution. An FEC+ARQ based CLVL cannot recover a
large portion of a window of packets from a bimodal loss distribution
if a long burst affects both the FEC window, and the ARQ
transmissions. During our experiments, Mazu-Cba1 experienced a
bimodal loss distribution.