Commit 1f8b900a authored by Pramod R Sanaga's avatar Pramod R Sanaga

First draft of the bw-bottleneck paper - includes the abstract.

parent 6cd0fa2f
Abstract:
---------
We present a new Internet path emulation model for use in the Emulab testbed.
This gives the experimenters acccess to load-free and completely controllable
host conditions and relistic network conditions. We use a
lightweight monitoring system to take periodic snapshots of the network
conditions on Planetlab and detect shared bottlenecks. Our system reliably
detects bottlenecks due to congestion and measures how the bandwidth available
to TCP transfers on Planetlab scales with the number of streams. We use a
modified FreeBSD Dummynet network configurator to faithfully reproduce these
network conditions in Emulab.Unlike the dynamic Internet model provided by
Flexlab, the static model scales reliably and the size of the experiments
possible is only limited by the number of available nodes in Emulab.
Our evaluation suggests that this new static Internet emulation model provides
an improved method for setting realistic network characteristics in Emulab
while preserving its strengths of repeatibility and host control.
Possible additions/changes to the abstract:
--------------------------------------------
- Intro about Emulab & Planetlab:
- Network conditions are artificial in Emulab.
- Planetlab is overloaded and execution environment(Eg: Kernel)
can't be modified.
- Something about Node-caps(&/HTB) and how they might affect experiments?
- Reliable experiment setup: For large experiments, some Planetlab nodes
always fail to come up. We can eliminate this problem with our static
model and Emulab.
- More emphasis on repeatability and being able to run experiments
against particular snapshots of Internet conditions, useful for debugging.
- Mention the changes to Dummynet to enable Backfill, realistic emulation
of asymmetric paths and quantifying their prescence/extent on Planetlab.
Paper Outline:
--------------
- High level goal: A better static model in Emulab to emulate the available bandwidth seen
on Planetlab paths.
- Contributions ?
- A new(simpler/less complex/?..) method for emulating Internet conditions
inside Emulab, while providing control over the host environment and non-overloaded
host conditions.
- The experiments will be repeatable using any snapshot of the static
conditions stored in our database - ability to debug the applications while
running them under realistic Internet conditions.
- Our Dummynet mods ?? - realistic emulation of available bandwidth in case
of asymmetric paths.
- Quantification of the extent and effect of Node-caps on Planetlab.
- Problems with the existing static model(s):
- The cloud model creates independant pipes between each pair
of nodes. It fails to take into account potential bottlenecks in
the internet.
- The LAN model assumes that each node -> all other nodes is a bottleneck.
Again, this is an over-simplication.
- Hybrid model assumes that only Inet -> everywhere else paths are
bottlenecks.
- Glean whatever else is possible from the Flexlab paper/Bittorrent results.
- What is the new model ?
- Detects bottlenecks using Wavelet based Correlation detection. Light-weight
UDP probes sent out on all paths of interest and their delays are correlated.
- The Wavelet method is based on a SIGCOMM '04 paper(which is an increment over
Rubenstein's SIGMETRICS '02 paper) and it has been
verified to correctly detect congestion in an Emulab experiment.
- Setup the Dummynet pipes with the detected bottlenecks
- The bandwidth for these bottlenecks is currently being set as
the maximum bandwidth (as reported by Flexmon) of the paths in
question.
- Insert about Jon's current work: Instead of doing the above for
bottleneck bandwidth, we measure the bandwidth obtained for a given
number of streams on Planetlab on these paths. While running
the static model in Emulab, we could detect the number of streams
going through the bottleneck and vary the amount of available bandwidth
to match what we saw earlier in Planetlab according to our snapshot.
We don't have the numbers on how well this performs yet.
- Issues in emulating the available bandwidth:
- ~10% of Planetlab paths have asymmetric links of > 3:1 asymmetry.
- Simply setting up dummynet pipes with the available bandwidth values
causes problems in the case of asymmetric links( with asymmetry > 3:1 ).
- We have experimental proof of this phenomenon(Jon's IPerf mesh runs in Emulab).
Its main cause is the large queuing delay caused on the reverse path and
the ACKs of the forward path transfer getting stuck there while waiting
to be transmitted on the slow link.
- This problem can be alleviated if we setup the links as high-capacity, but
providing the appropriate amount of available bandwidth. On the Internet,
this is accomplished by the cross-traffic from other sources.
- We do this by way of our modifications to Dummynet. We add a new parameter
called "Backfill" ( background fill ) to restrict the amount of available
bandwidth. This provides better performance in the case of asymmetric links
which have TCP transfers going on in both directions. ( we have the numbers for
this too).
- Our Dummynet modifications do not increase the load on the delay nodes.
( Top numbers available ).
- Evaluation(We don't have any of these numbers yet):
- IPerf mesh run in parallel on Planetlab and on Emulab.
- We need to show a contrast between how the Cloud/Hybrid models
perform against our new static model.
- We could run BitTorrent between the same nodes that we put in the
Flexlab NSDI paper and check how the new model does on that set of nodes.
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment