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 0: Initiator NAT ’ ed Level 1: Recipient NAT'ed Level 2: Both NAT'ed (well-behaved NATs) Level 3: 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 A list of SN Connect to a 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 diurnal behavior of SN Weekend

Expt. 5: Supernode presence - Cont  Geographic Distribution of Active SuperNodes % peak at 11am UTC (Europe mid-day) 20-25% 15-25%

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 Node departure concentrated toward evening Skype usage is correlated with working hours Different from P2P file-sharing

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

My Opinion  Pros