Overview

1. From Packet-level to Flow-level Simulations of P2P Networks Kolja Eger, Ulrich KillatHamburg University of TechnologyITG-Fachgruppentreffen, Aachen4. Mai 2006

2. Overview • P2P Content Distribution • Packet-level Simulation • Flow-level Simulation • Simulation complexity & accuracy • Conclusion

3. P2P Content Distribution • Objective: Disseminate large files in minimal time to a large number of users • Swarming principle: • A file is fragmented into small pieces which can be shared before download of the whole file is completed • E.g.: BitTorrent protocol • Our research interests: • Efficiency: Peer has something of interest for at least one other peer at any point of time • Fairness: Peers which contribute much should also gain much ⇒ incentive to contribute

4. Complexity of P2P Simulation • P2P networks are complex: • Large and varying peer populations • Peer behaviour is user-driven • Peers provide and consume different services, e.g. exchange different pieces of a file with each other • Services are offered with different quality, e.g. upload bandwidth • Each peer has only local information about the network • Only simple cases can be studied analytically, e.g. flash crowd of homogeneous peers • Simulations must be based on simplifications

5. Packet-level Simulations • Assumptions: • Access line of the peers is the bottleneck in the network • No packet drops in the core network • Simplified topology: • Access link plus overlay link • Different RTTs between access routers • No. of links: Z = (NP -1)NP/2 + NP = NP/2 (NP+1) • Memory increases quadratically with NP • No. of events is decreased, because of small no. of hops

6. Event x Timer x Timer x Peer contacts tracker Timer x Event 1 Timer x Timer x Timer x Timer x Timer x Connects to other peers Timer x Timer x Timer x Timer x Timer x Timer x Inform about pieces Timer x Timer x Timer x Timer x Timer x Timer x Check interest Timer x Timer x Timer x Timer x Timer x Timer x Timer x Timer x Timer x Timer x Timer x Timer x Peer selection (Unchoke) Timer x Event 2 Timer x Timer x Timer x Timer x Timer x Request pieces Timer x Timer x Timer x Timer x Timer x Timer x Upload Timer x Timer x Timer x Timer x Timer x Timer x Timer x Download Timer x Timer x Timer x Timer x Timer x Timer x Have Timer x Timer x Timer x Timer x Timer x Timer x Timer x Check interest Timer x BitTorrent Messages Packet-level Flow-level

7. RTT RTT RTT RTT RTT Exponential increase TCP Behaviour • In BitTorrent each peer uploads to a number of other peers (default = 5) (called unchoking) • Every 10s peers are chosen based on the download rates from them TCP throughput / max. throughput • If uplink of a peer is the bottleneck, TCP reduces to exponential increase at the beginning Cup / (No. of uploads) * RTT Upload Capacity: 1*10 kbit/s to 30*10 kbit/s RTT 1*10ms to 25*10ms

8. Flow-level Simulation • In peer selection algorithm download volume is computed beforehand • If remote peer needs less, it is redistributed over the remaining connections • Thus, peer allocates its upload bandwidth max-min fair Demand Demand Demand Volume = (Upload Capacity * unchoking interval) / (No. of uploads) Surplus / 3 Surplus / 2

9. Simulation Setup • Flash crowd scenario where a single peer holds the complete file at the beginning • Time measured until all peers have finished their download • No peer leaves the network beforehand • File size: 10MB, piece size: 256KB • Homogeneous peers with upload capacity of 10KB/s and download capacity 8 times higher (asymmetric access line) • Packet-level simulation with ns-2 • Flow-level simulation uses timer functionality of ns-2 • Simulation are run on a Pentium 4: 3,2 GHz, 1 GB RAM

10. Simulation Time • approx. 11hfor 60 peers with packet-level compared to 2 sec. with flow-level simulation • Calendar queue is used

11. List insert: O(n) delete: O(1) Calendar insert: O(1) to O(n) delete: O(1) to O(n) Map insert: O(log(n)+) delete: O(log(n)+) Heap insert: O(log(n)) delete: O(log(n)) Event Scheduler Simulation Time [s] No. of peers

12. Flow-level Simulation 170 000 peers in less than 30min.

13. Simulation time for a flash crowd of 4000 peers with different upload capacities Higher capacities result in less events for the same download volume 640 B/s = 0,625 KiB/s = 5 Kib/s Standard-Upload-Kapazität 10.240 B/s = 10 KiB/s = 80 Kib/s ADSL 1000 16.384 B/s = 16 KiB/s = 128 Kib/s ADSL 2000 24.576 B/s = 24 KiB/s = 192 Kib/s ADSL 6000 76.800 B/s = 75 KiB/s = 600 Kib/s Flow-level Simulation (cont.)

14. Simulation Accuracy • Both curves have the same shape • But results differ by around 10% • Reasons: • Packet headers • TCP behaviour • Load for BitTorrent messages

15. Conclusion • Packet-level simulation does not scale for P2P networks • Flow-level simulation is inevitable to study networks of reasonable size • Results with flow-level simulator are qualitatively comparable but underestimate the true values due to the simplifications made • Flow-level simulation is a good compromise to study protocol design • But inadequate to take cross-layer interactions into account, e.g. unchoking is based on TCP throughput which depends on RTT

16. Thank you for your attention!