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Infrastructure Primitives for Overlay Networks

Discussing the shared overlay functionalities, overlay requirements, diverse overlay challenges, and implementation alternatives for network overlays. Covering path selection, packet replication, measurement metrics, and experiments.

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Infrastructure Primitives for Overlay Networks

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  1. Infrastructure Primitives forOverlay Networks Karthik Lakshminarayanan (with Ion Stoica and Scott Shenker) SAHARA/i3 Retreat – Summer, 2003

  2. Goal: Share Overlay Functionality • What do overlays share? • Underlying IP infrastructure (of course!) • Underlying hardware (maybe, e.g. PlanetLab) Why not share… • Higher level overlay functionality • Each application designs overlay routing from scratch • Lower deployment barrier: design effort & deployment expense • Network weather information • Each application performs probes to find good overlay paths • Reduce overlay maintenance overhead

  3. Diverse Overlay Requirements What are the requirements for supporting most of the overlays applications? • Routing control • Adaptive routing based on application sensitive metrics • Measurements of the virtual link characteristics • Data manipulation • Manipulate/store (e.g. transcode) data in the path to the destination

  4. Our Approach • Embed in the infrastructure: • Low-level routing mechanisms, e.g. forwarding, replication • Third-party services: • Services are implemented at end-hosts, shared using an open interface • Information for making routing decisions, e.g. measurements of path delay, loss-rate, bandwidth • At the end-hosts: • Not shared at all, e.g. policies for choosing paths

  5. Outline • Motivation and Challenges • Infrastructure Primitives • Network Measurements • System Architecture – Weather Service • Experiments • Some Applications

  6. m m m R’ R Path Selection n1 n2 • Similar to “loose source routing” • End-hosts specify points through which packet is routed • Routing between the specified points handled by IP

  7. m m m1 m R’ R Path Replication n1 n2 • End-host specify that a particular packet be replicated at a node and then sent along a path

  8. Infrastructure Primitives • Path Selection • Packet Replication Claim: This is enough to do (i) Adaptive routing (ii) Measurements (iii) Data manipulation • Why this approach? • Control path must be outside – collective knowledge to decide what to monitor • No difference between data and measurement traffic – better security, nodes have no incentive to lie

  9. Implementation alternatives • At the IP layer: • Path selection • Implemented in the form of loose source routing • Requires path in the packet header • Path replication requires a new primitive • Why we chose i3: • Implements the two primitives without any changes • Path selection: Set up routing state beforehand (instead of in the header) • Robustness to node failures • We know it well! This is one possible realization, and not the only one

  10. Outline • Motivation and Challenges • Infrastructure Primitives • Network Measurements • System Architecture – Weather Service • Experiments • Some Applications

  11. Metrics of measurement • Round-trip delay • Loss-rate • Available bandwidth • Bottleneck bandwidth … in the process, demonstrate the versatility of the primitives

  12. m1 m1 m1 m m R Round-trip Delay n1 n2 • Use path selection primitive to send packet m along R→n1→R • Use path selection in conjunction with packet replication to send packet along R→n1→n2→n1→R • Difference yields the RTT of the link (n1↔n2) To measure: RTT(n1→n2)

  13. m1 m1 m1 m R Round-trip Delay n1 n2 • Use path selection primitive to send packet m along R→n1→R • Use path selection in conjunction with packet replication to send packet along R→n1→n2→n1→R • Difference yields the RTT of the link (n1↔n2) To measure: RTT(n1→n2)

  14. m1 m1 m1 m2 m One-way Loss Rate n2 n1 • m2 used to differentiate loss on (n1→n2) from that on (n2→n1) • (m Λ ~m1 Λ ~m2)  loss on virtual link (n1→n2) • False positives • False negatives • Probability of false positives/negatives ≈ O(p2 ) R To measure l(n1→n2)

  15. Available Bandwidth • Come to the poster session.

  16. Outline • Motivation and Challenges • Infrastructure Primitives • Network Measurements • System Architecture – Weather Service • Experiments • Some Applications

  17. Client A Network measurements Query/reply routing info. Weather Service 1 Setup routes Weather Service 2 Client D Client B Client C What we envision Challenge: To make the measurements scale to an infrastructure of 1000s of nodes

  18. Outline • Motivation and Challenges • Infrastructure Primitives • Network Measurements • System Architecture – Weather Service • Experiments • Some Applications

  19. Experiments: Delay Estimation • More than 92% of the samples have error < 10% • If we consider median over 15 consecutive samples, 98.3% of the samples have error < 10%

  20. Experiments: Loss-Rate Estimation • Accuracy of 90% in over 89% of the cases (after filtering the few nodes with high losses)

  21. Experiments: Avail-BW Estimation • Within a factor of two for 70% of the pairs • Avail-BW is not static, so this is reasonable

  22. How applications can use this • Adaptive routing: • End-hosts query the WS and construct the overlay • Quality of paths depends on how sophisticated the WS is • No changes to infrastructure if metrics change • Multicast: • Union of different unicast paths that the WS returns • Number of replicas is no larger than the degree of the overlay graph • Finding closest replica: • Client queries the WS to get the best among a set of nodes • WS may export an API that allows this*

  23. Multicast experiment • Nodes at 37 sites in PlanetLab (1-3 per site). • Delay-optimized multicast tree rooted at Stanford • Union of delay-optimized unicast paths • 90% of the nodes had RDP < 1.38; 99.7% of the nodes had RDP < 2

  24. Summary of design • Minimalist infrastructure functionality • Delegate routing to applications • Applications know their requirements best • Delegate performance measurements to third-party applications • Allows this to evolve to meet changing requirement

  25. Open questions & Future work • Why minimalist design? • Why not more primitives? E.g. For supporting QoS • What if path characteristics are correlated? • Shared bottleneck • Losses at the egress/ingress link • Sub-problems • By having incomplete information about network weather, how much do we lose (if at all)? • How much does accuracy of measurements affect the final outcome? • If the underlying routing is bad, what is the diversity of such an overlay needed to do a good job? • Design API and develop applications based on it

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