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An Experimental Study of the Skype Peer-to-Peer VoIP System Saikat Guha, Cornell University Neil DasWani, Google Ravi Jain, Google IPTPS’06 Presenter: Te-Yuan What do they want to know? What makes Skype so successful? Compare with File-sharing P2P network By Observing Skype’s User behavior Node Session Time Overlay Network Traffic SuperNode overlay network Overall utilization & resource consumption Skype Three Services two-way audio streams & conference call up to 4 users Instant Message file-transfer Structure Alike KaZaA – SuperNode-based Ordinary Node (ON) Super Node (SN) Outline - Experiments Expt. 1: Basic operation Expt. 2: Promotion to supernode Expt. 3: Supernode network activity Expt. 4: Supernode and client population Expt. 5: Supernode presence Expt. 1: Basic operation To Answer: How do two Skype clients connect to each other? Normally, ON send control traffic through SN-p2p Including Availability information Instant messages Request for VoIP & File-transfer What if ON is behind NAT/Firewall? Expt. 1: Basic operation – Cont. NAT Traversal in Skype: Level Level Level Level 0: 1: 2: 3: Initiator NAT’ed Recipient NAT'ed Both NAT'ed (well-behaved NATs) Both NAT'ed Expt. 1: Basic operation – Cont. Level 0: Initiator NAT’ed Expt. 1: Basic operation – Cont. Level 1: Recipient NAT’ed Expt. 1: Basic operation – Cont. Level 2: Both NAT'ed (well-behaved NATs) Expt. 1: Basic operation – Cont. Level 3: Both NAT'ed Expt. 1: Basic operation – Cont. Level 0 Level 1 Level 2 Level 3 Outline - Experiments Expt. 1: Basic operation Expt. 2: Promotion to supernode Expt. 3: Supernode network activity Expt. 4: Supernode and client population Expt. 5: Supernode presence Expt. 2: Promotion to supernode To Answer: What kind of node will be promote to SN? Setup several Skype clients One behind a saturated network uplink One behind a NAT One with a 10 Mbps connection & public IP Key to be SN plenty of spare bandwidth publicly reachable Outline - Experiments Expt. 1: Basic operation Expt. 2: Promotion to supernode Expt. 3: Supernode network activity Expt. 4: Supernode and client population Expt. 5: Supernode presence Expt. 3: Supernode network activity Goal: To observe the network traffic of a Skype supernode Duration: 135 days (Sep. 1, 2005 to Jan. 14, 2006) Data captured: 13GB with ethereal Expt. 4: Supernode and client population Goal: Collect SN & client IP/port Duration:2005/7/25 – 2005/10/12 Result: Crawl 150K SN Collect 250K SN info Expt. 4: Supernode and client population Connect to a SN A list of SN Save the list Connect to a SN from the list A list of SN Expt. 4: Supernode and client population Collect client info Collect the number reported by skype client Expt. 5: Supernode presence Goal: how many SN online at a give time Flow Randomly Select 6000 SN - from the list collected by expt. 4 Send “application-layer Ping” Repeat every 30 mins for a month Expt. 5: Supernode presence Cont Num. of SuperNode is more Stable Weekend diurnal behavior of SN Expt. 5: Supernode presence Cont Geographic Distribution of Active SuperNodes 15-25% 20-25% 45--60% peak at 11am UTC (Europe mid-day) Expt. 5: Supernode presence Cont SuperNode Session Time Median is 5.5h Expt. 5: Supernode presence Cont Fraction of supernodes joining or departing Node arrival concentrated toward morning Skype usage is correlated with working hours Different from P2P file-sharing Node departure concentrated toward evening Expt. 5: Supernode presence Cont Node Arrival dependent on Time Not Poisson or Uniform process Poisson process with varying hourly rate Node arrival concentrated toward morning Node departure concentrated toward evening VoIP in Skype: Preliminary Observation SuperNode Traffic 90.4%SN no need to relay VoIP traffic VoIP in Skype: Preliminary Observation VoIP Relayed Session Arrival Behavior Inter arrival time of Relayed VoIP/File sessions may be Poisson VoIP in Skype: Preliminary Observation VoIP Session Length Behavior Skype: Median: 2m50s Average: 12m53s Longest: 3h 26s Traditional: Average: 3m Fraudulent: Average: 9m VoIP in Skype: Preliminary Observation File-transfer sizes File size: Median: 346kB Conclusion First measurement study of Skype VoIP system Skype differs significantly from file-sharing P2P User Behavior Diurnal & Work-week Calls are significantly longer File transferred are significantly smaller SuperNode of Skype Consume little bandwidth Relatively stable