Despite its popularity, relatively little is known about the traffic characteristics of the Skype VoIP system and how they differ from other P2P systems. We describe an experimental study of Skype VoIP traffic conducted over a five month period, where over 82 million datapoints were collected regarding the population of online clients, the number of supernodes, and their traffic characteristics. This data was collected from September 1, 2005 to January 14, 2006. Experiments on this data were done in a black-box manner, i.e., without knowing the internals or specifics of the Skype system or messages, as Skype encrypts all user traffic and signaling traffic payloads. The results indicate that although the structure of the Skype system appears to be similar to other P2P systems, particularly KaZaA, there are several significant differences in traffic. The number of active clients shows diurnal and work-week behavior, correlating with normal working hours regardless of geography. The population of supernodes in the system tends to be relatively stable; thus node churn, a significant concern in other systems, seems less problematic in Skype. The typical bandwidth load on a supernode is relatively low, even if the supernode is relaying VoIP traffic.
The paper aims to aid further understanding of a significant, successful P2P VoIP system, as well as provide experimental data that may be useful for future design and modeling of such systems. These results also imply that the nature of a VoIP P2P system like Skype differs fundamentally from earlier P2P systems that are oriented toward file-sharing, and music and video download applications, and deserves more attention from the research community.
Figure 4 shows that the number of Skype supernodes is more stable than the number of online Skype users. The figure is split into two parts for clarity; the plot on the left tracks daily variations in client and supernode populations from Sep. 18 to Oct. 4, while the plot on the right zooms-in on hourly variations on Sep. 22. As mentioned in Section 3, the number of online users is reported by the official Skype client, while online supernodes are determined through periodic application-level pings.
Session times reflect this correlation with normal working hours. As has been observed widely for interactive applications like telnet, web, and email [20, 9], node arrivals in Skype are concentrated towards the morning, while departures are concentrated towards the evening (Figure 2). Figure 2 plots the fraction of supernodes joining and leaving the network in consecutive snapshots taken at 30 minute intervals. The median supernode session time from the same experiment is 5.5 hours, as shown in Figure 3. The median is higher than reported in previous studies of file-sharing networks [26, 27, 2, 13, 7]; however, these studies measure session times for all nodes and not just the supernodes that form the P2P overlay. We plot the complementary CDF in Figure 3 as it is useful for detecting power law relationships. We observe that while the supernode session time complementary CDF is not a strict straight line, i.e., a strict power law, it may be approximated as such for our data.
Supernode traffic. We separate out low-bandwidth control and IM traffic, and high-bandwidth, relayed VoIP and file-transfer traffic; due to Skype's use of encryption, however, we resort to statistical approaches for this, which may misclassify small file-transfers as control traffic. We separate control traffic from data traffic by identifying the respective connections as explained in . We further classify the data traffic as VoIP or file-transfer based on the ratio of bytes sent in the two directions. We use empirical results to conservatively classify data sessions with ~33 packets transferred per second and an overall one-way bytes-sent ratio greater than 0.2 as VoIP.
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