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Routing Dynamics in Simultaneous Overlay
Networks
Mukund Seshadri Randy Katz([email protected]
Berkeley-Helsinki Short Course Aug. 2003
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Problem Consider overlay routing when multiple
independent overlay networks/flows interact: Can this be unstable/inefficient?
Identify such scenarios. Suggest improvements.
Identify scope for reduction of measurement overhead.
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General Motivation End-host controlled routing can become significant
Pure Overlay Network protocols (RON[3], Detour[4], ESM[5]) Overlay primitives (“Path reflection”[1], i3-based [2]) Better routing than Internet/BGP (resilience/performance/multicast/etc.)
What if several entities set up their own overlays? Companies setting up distribution overlay networks… Or, more ad-hoc users setting up overlay networks… Flows within a single overlay…
Consider overlay networks/flows which have some physical links in common, but don’t explicitly coordinate with each other.
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Unstable Routing Example
L1 failure can cause synchronized oscillation of both flows between the two alternate paths
Primary Paths
Alternate Paths
BottleneckPhy. Link
1+ Mbps(L2)
2 MbpsL1
1 Mbps(L3) Ov.Nw. Nodes
(2 Ovns)
Sources
Destinations
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Focus Main application – multimedia streams
Long-lived (medium) flows : ~ 1hr (5min) . Flows require specified bandwidth levels Flows require route stability (Packet-reordering, jitter undesirable) Secondary app – long high volume transfers/sessions Problem considered: selection of best routes (not location/DHTs)
Size: 50-500 overlay flows; 10-50 nodes each.
Independent decision makers - no explicit info. sharing
Unlike PlanetLab[6], underlay[7] model, i3-based soln.[2] Independent administration might be desirable. Don’t have to wait for infrastructure nodes to come up. Most protocols like ESM can’t scale to thousands of nodes.
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Overlay Network Model Given M overlay networks/flows with N nodes each
Probing of all potential paths is done (O(N) cost). Path characteristics are inferred from probes in some time
window With some error factor We consider only bandwidth
Best path is selected to send traffic on (GREEDY) Route change based on bandwidth improvement threshold (H)
Path-level simulator Characterizes shared bottleneck links. The level of sharing is characterized by “path density” Unicast CBR flows with bandwidth requirement.
Metrics of interest Loss Rate (related to bandwidth) Stabilization time
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Contribution Study the need for “restraint” in
route selection Randomness in selection selection Hysteresis Time between re-route decisions
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Hysteresis Required No hysteresis threshold (H) for route change =>
unstable. We will use 99% stabilization time.
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H affects loss rate…
Will explore more later in the talk…
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When does Greedy “fail”?
Large flows => more effect when re-routed => lower stability
Defaults: 500 overlay flows, 50 bottleneck links link capacities ~ flow
requirements ~50% cross-traffic 10% measurement error. 4x variation in link b/w. ~25 links/flow (density) Optimal Threshold
Assumed
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When does Greedy “fail”?
High sharing=>many route-changes Flows within a single overlay. when overlay nodes are skewed towards certain ASes, like univ.s. if several overlay flows independently use a medium size shared
infrastructure.
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Cross-Traffic
High Cross-Traffic causes the effect of overlay flows on available bandwidths to be lower, so greedy is more stable.
Other factors investigated: routing window variation, measurement error, excess capacity, bandwidth distribution.
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Summary of “Greedy” The following factors contribute to poor
stability and performance of “Greedy” overlay path selection
Several flows’ paths share a large number of bottleneck links.
There is not much spare capacity in paths used. There is a large variation in link and flow bandwidths. The overlay traffic is a high fraction of traffic on the
bottleneck links Each flow’s bandwidth is significant compared to
bottleneck link bandwidth.
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Improvements to Greedy Randomly select path to be chosen
ARAND: In proportion to available bandwidths SRAND: Best of randomly selected subset of
size S …in proportion to capacity Reduces measurement overhead Works well for server load balancing [8]
(but different work model: jobs arrive and leave, and are assigned to only one server for their lifetime)
GRAND: Randomly select from the best S paths
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Does Randomizing Help?
Randomization more useful at high densities. More stable, lower loss, less sensitive to threshold
setting.
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Hysteresis Threshold Optimal value of H very sensitive to
parameters. Flows can automatically discover the values of
H. Flows can independently “probe” values of H
No route change => decrease H Route change => increase H
Try AIAD, MIMD, etc. Can perform even better than with fixed H…
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Exploring “H”
Very similar, MIMD stabilizes slightly quicker… I/D pmtrs. not as sensitive to simulated network pmtrs.
as H.
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Exploring “H” (Contd.)
Performs much better than with fixed threshold, loss rates close to 0
Stabilization times similar to fixed case.
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Summary SRAND is as good as or better than
GREEDY in most cases Measurement costs lowered, with performance
similar to the proportional randomization method. Automatic discovery of H works better than
fixed H (and is more feasible). Increasing time windows can help, particularly
when flows arrive/depart.
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Future Work Define a general method that combines
randomization, hysteresis estimation, and time variation (like simulated annealing)
Explore dynamic scenarios (flows arrive/depart). Explore 2nd level control loop for MIMD pmtrs. Implement/simulate using real topologies.
Can we define a general notion of “friendliness” pertaining to both route selection and traffic distribution over different routes?
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References1. Network layer Support for Overlay Networks – John Jannotti –
OpenArch 2002.2. Infrastructure Primitives for Overlay Networks – Karthik
Lakshminarayanan et al. – under submission.3. Resilient Overlay Networks – Andersen et al – SOSP 20014. Detour: a Case for Informed Routing and Transport – Savage
et al. – IEEE Micro Jan 1999.5. A Case for End System Multicast – Yang-hua Chu et al. – JSAC
2002.6. PlanetLab – http://www.planet-lab.org7. A Routing Underlay for Overlay Networks – Nakao et al. –
Sigcomm 2003.8. How Useful is Old Information – M.Mitzenmacher – PODC 19979. An Analysis of Internet Content Delivery Systems – Saroiu et
al. – OSDI 2002.
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…Backup Slides…
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Stabilization Times of the *RANDs
Generally SRAND and ARAND stabilize quickly and have a very low loss rate.
Also investigated the effect of subset size on SRAND
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Other Factors Small amount of cheating doesn’t hurt the
good flows, large amount does.
If link bandwidths are much higher than flow bandwidths, Greedy is more stable and performs better.
If link and flow BW are similar, then a high variation in the same causes Greedy to be fairly unstable.
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Extra Slide
2-Flow Illustration
We can randomize Route selection
Proportional to Available BW
Time intervals Of assessment and
rerouting.
2-Flow StrawMan
0
20
40
60
80
100
120
Synced Oscillations Desynced Rerouting
ScenarioSt
abiliz
ation
Tim
e (se
cond
s)
Greedy
Proportional
