1. Managing Inter-domain Traffic in the Presence of BitTorrent File-Sharing Srinivasan Seetharaman and Mostafa Ammar
School of
Computer Science
Objective: i) Analyze impact of BitTorrent file-sharing on the load balance across inter-domain access links, ii) Investigate existing Traffic management schemes, and iii) Propose strategies for better coexistence
Typically in BitTorrent swarms…
Main characteristics
Centralized tracker regulating neighborhood, with no knowledge of peer location or piece distribution
Dynamically swap peers by choke/unchoke protocol, every 30 seconds!
Tit-for-tat incentive for uploading decisions
Leecher: Unchoke the fastest uploaders
Seed: Unchoke the fastest downloaders
Popular strategy to improve performance
Optimistic unchoke: Periodically look for faster peers
Objective: Minimize (Download finish time)
Similarity to Overlay Routing
Selfish protocol, with no concern for native layer preferences
Peer and piece selection is similar to picking multi-hop overlay path
Simulating BitTorrent swarms
Pick 100 ASes with 60% being non-stub ASes.
Generate 1-50 peers per AS.
Each associates with 1-3 torrents
Torrent birth time is uniformly distributed between [-6 hrs, 18 hrs] L1
Data2
Each torrent has between [100, 10000] pieces with each piece being 256KB
Data1 D B F F
Arrival rate decreases exponentially: (t) = 0 e-t/
0 = initial arrival rate  = attentuation factor
0 is uniformly distributed between 1-10 and  is set at 5000 for all torrents B1 B2 C A3 A2 A A1 E L2 BY
50 peers randomly assigned by tracker AX
Observations
We tried the following changes to the BitTorrent protocol, neither of reduced impact satisfactorily
Make peer selection random
Make piece selection random
Reduce duration of optimistic unchoking
Freeze list of unchoked peers after 10 mins
Tune the unchoking timers
Native layer Traffic Management
 Inter-domain Traffic Engg  Rate Throttling at the border
 Locality-based traffic management strategies
Summary: They are effective. But, incur drop in user experience or infrastructure costs
Friendly BitTorrent Strategies
…to resolve cross-layer conflict
Observation 1: Several peers simultaneously unchoke same peer, causing large # of parallel downloads
Strategy 1: Limit number of parallel downloads
Observation 2: Often, two peers in the same AS contact the same peer outside the AS
Strategy 2: Stop this synchronization by avoiding common neighbors (Requires bilateral exchange of neighbor info.)
Summary: Reduces maximum utilization effecitvely, without incurring inflation of client finish times
Aggressive and performance-aware causing high max. util
Applying TE does not make much difference
Disproportionate load on inter-domain access links