Analysis of the Scalability of the Overlay Skype System

In this paper, we study the Skype system in order to determine the presence of bottleneck for its scalability. In fact, as the Internet grows and the number of Skype users increases constantly, the Skype traffic carried by network links grows and also the load of Skype supernodes increases, mainly due to the need of relaying calls for users behind NAT restrictions. We model both the underlay Internet at the inter-autonomous system level and the overlay Skype system. We let the two systems interact through a computer implementation of their respective models and, by varying critical parameters such as the size of the underlay Internet, the number of Skype users, the per-user traffic and the percentage of users behind NAT restrictions, we calculate the sensitivity of link load and of the supernode load to these parameters. In this way, we are able to determine in which conditions link load and supernode load may become a bottleneck for the scalability of the Skype system.