A BitTorrent Module for the OMNeT++ Simulator MASCOTS 2009, London, UK G. Xylomenos (with K. Katsaros, V.P. Kemerlis and C. Stais)

Outline Context Motivation Advantages Features Structure Extras Performance Conclusion

Context The ICT PSIRP Project –The Internet mostly disseminates data –Publish-Subscribe Internet Routing Paradigm –Clean slate approach to Future Internet –Pub-Sub at application and network levels What does that have to do with BitTorrent? –PSIRP was motivated by content distribution –BitTorrent is the perfect benchmark for this! –Now if we only had a good BitTorrent simulator…

Motivation Why a BitTorrent simulator? –BitTorrent swarms exhibit very complex behavior Many mechanisms and strategies are at play –Traces are hard to gather and understand Fully distributed systems are hard to monitor –Performance predictions are simply guesses What happens when we modify a strategy? –Existing simulators were not detailed enough Some omit large parts of the protocol Others only work over abstract networks

Advantages Why bother with our simulator? –It operates over the OMNeT++ platform You can use it with everything available for OMNeT++ Example: OMNeT++ supports two types of network –InetUnderlay: hosts with full TCP/IP stacks –SimpleUnderlay: simple and fast abstract hosts –It incorporates nearly all BitTorrent details All policies and options are present and tunable If something is missing, feel free to add it! –Extra features that simplify simulations Asymmetric links, churn model, GT-ITM topologies

Features Full implementation of tracker and peer-wire protocols –All messages are included Arbitrary file, piece and block sizes Variable number of peer connections (min/max) Rarest first and random first download strategies Data request pipelining Tunable choking algorithm (intervals, peers) Super seeding and end game modes Not implemented (yet!): trackerless mode (DHT based)

Structure Fully modularized code –Extends existing class hierarchy –Easier to modify and extend –Possibility of parallel simulation Tracker module –Models a multi-threaded tracker Tracker client module –Connects client with tracker Peer-wire module –Models a multi-threaded peer

Extras BitTorrent simulation scenarios are complex –Numerous widely dispersed nodes are needed –Hard to manually place and start them Topology creation: GT-ITM import filter –Extension of the BRITE import tool Supports hierarchical networks Supports asymmetric (ADSL) access links Host deployment: BitTorrent churn generator –Empirical distribution of host interarrival times –Based on actual BitTorrent trace data

Performance CPU time measurements (Intel E GHz) –200 MByte file –Mixed ADSL GT-ITM –Full stack –1400 routers Skitter –Delays only –No queueing

Performance Memory footprint measurements (Ubuntu, 4 GB RAM) –Same scenario

Performance Question: which piece size minimizes download time? –Same scenario –60 peers Any differences? –Variance –Slope –Knee at 512

Conclusion OMNeT++ based BitTorrent module –Comprehensive protocol model –Works with detailed and simplified stacks Space/time vs. accuracy tradeoff The difference is important! –Extra tools to help your simulations Churn generator, topologies, mix of ADSL links –Code available under the GPL –New version for OMNeT++ 4 coming real soon now!