``A Performance Study of BitTorrent-like Peer-to-Peer Systems" 
Lei Guo, Songqing Chen, Zhen Xiao, Enhua Tan, Xiaoning Ding, and Xiaodong Zhang

IEEE Journal on Selected Areas in Communications, Vol. 25, No. 1, 2007, 
pp. 155-169.  


This paper presents a performance study of BitTorrent-like P2P systems by 
modeling, based on extensive measurements and trace analysis.  Existing 
studies on BitTorrent systems are single-torrent based and usually assume 
the process of request arrivals to a torrent is Poisson-like.  However, in 
reality, most BitTorrent peers participate in multiple torrents and file 
popularity changes over time.  

Our study of representative BitTorrent traffic provides insights into the 
evolution of single-torrent systems and several new findings regarding the 
limitations of BitTorrent systems:  (1) Due to the exponentially decreasing 
peer arrival rate in a torrent, the service availability of the corresponding 
file becomes poor quickly, and eventually it is hard to locate and download 
this file.  (2) Client performance in the BitTorrent-like system is unstable, 
and fluctuates significantly with the changes of the number of online peers.  
(3) Existing systems could provide unfair services to peers, where a peer 
with a higher downloading speed tends to download more and upload less.  
Motivated by the analysis and modeling results, we have further proposed a 
graph based model to study interactions among multiple torrents.  Our model 
quantitatively demonstrates that inter-torrent collaboration is much more 
effective than stimulating seeds to serve longer for addressing the service 
unavailability in BitTorrent systems.  An architecture for inter-torrent 
collaboration under an exchange based instant incentive mechanism is also 
discussed and evaluated by simulations.