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On the Stability of Skype Super Nodes

On the Stability of Skype Super Nodes. Anat Bremler-Barr Ran Goldschmidt Interdisciplinary Center Herzliya Haifa University bremler@idc.ac.il ran.goldschmidt@gmail.com. Background:. VoIP P2P Technology Proprietary signaling and media protocol. Architecture:.

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On the Stability of Skype Super Nodes

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  1. On the Stability of Skype Super Nodes Anat Bremler-Barr Ran Goldschmidt Interdisciplinary Center Herzliya Haifa University bremler@idc.ac.ilran.goldschmidt@gmail.com

  2. Background: • VoIP • P2P Technology • Proprietary signaling and media protocol

  3. Architecture: • Based on partially centralized P2P networks • Two types of peer nodes: • Regular Clients and Super Nodes • Super Nodes (SNs): • Control level – heart of Skype • Super Nodes = Skype Clients with good Bandwidth, CPU usage ...

  4. The Role of a Super Node (SN) • Maintains control information: the IPs of the Skype users • Each client maintains an SN list  subset of SNs • SN list is constantly updated • Client that wishes to use Skype (to call) picks one SN from the SN list • Querying the IP of the callee • SN is defined by (IP,Port) Call Bob IP=12.3.2.4, Port=3

  5. Main goal: Understanding the stability of SNs • P2P networks have an inherent dynamic nature • Clients and computers join and leave the network constantly • Choosing stable SNs is an important task: • Improve the performance and quality of the P2P network • Our goal: measure the stability of SNs and understand how to choose stable SNs

  6. Experiment Methodology Stage 2: Ping each SN every 15 minutes for 3 months Stage 1: Collecting 10,000 SNs in 15 minutes UDP packet of Skype login Harvesting of SNs: Using the SN list - version 2.5 holds a list of 200 SNs

  7. Absolute Availability • Absolute Availability = the percentage of time the SN is up during the test 18% -16 days • Low: 50% of the SNs are available less than 18% of the test time, < 16 days

  8. Number of Sessions of SN • A session is the continuous period of time the node is up. • 40% of the SNs have one session!

  9. MSLT vs SSLT • We define two group of SNs: • Single Session in Life Time (SSLT) • Multiple Sessions in Life Time (MSLT) • We analyze the different characteristics of the two groups • We play “what if” game and show the stability of the system if we take SNs only from one group (SSLT or MSLT)

  10. Residual life Residual life time - time elapsed between the harvesting of the SN until the end of its last session 67.5 1.75 • SSLT (one session) : median of residual life is 1.75 days • MSLT (multi session) : median of residual life is 67.5days

  11. First Session Length • SSLT (one session) : median of session is 1.75 • MSLT (multi session) : median of session is 4.35 Surprise ! Super Nodes in SSLT have only one session and a shorter one…

  12. The impact on the System: Churn • Churn measures the number of times an SN goes down and we need to replace the SN. • Assumption: system maintains a fixed number of SNs • When an SN fails, the system picks another one to replace it. • When an SN fails and then recovers, the SN is like a new SN. Result: SSLT: 0.35 turnovers per day MSLT: 0.22 turnovers per day

  13. The impact on the System: Accessibility • Accessibility: the probability that an SN is alive as a function of time • If an SN fails and then recovers, the SN is viewed as a node that never failed. • Impact: The lower accessibility value the higher the update rate of the SN list

  14. Skype Accessibility • The SNs are distributed over the world  good accessibility. • The SNs in the SN list arealso distributed over the world.

  15. SSLT and MSLT Summery • A huge difference between the characteristics of SSLT and MSLT SNs • Choosing SNs from the MSLT group would improve the churn and accessibility of the P2P system dramatically

  16. Our Answer: Static vs. Dynamic IPs • SSLT = dynamic IPs • Residential users: Cable, xDSL… • The address is changed from time to time (or between sessions) • SN died since the IP address of the SN was replaced • Good chance that this SN is alive but with a different IP address • MSLT = static IPs • SN can leave and return with the same IP • Servers or academic networks  high availability  good infrastructure  longer session

  17. Classifying SNs according to the address type Classifying using DNS information • The vast majority 84.92% of the static IP SNs belongs to the MSLT group • The dynamic IPs group shows weaker correlation with only 61.45% of the SNs belongs to SSLT group • We think that this is due to “Sticky Dynamic IPs” – DHCP technology • Client that is disconnected from the network returns to its old IP if the break is short

  18. Related Work Check: correlation between ICMP and Skype Ping • Our work: Skype Super Nodes • Most previous work: on file transfer • Skype: application is on all the time the computer is on • File transfer: application is on only when performing file transfer

  19. Summary & Practical Implication • Practical implication: choosing SNs with static IPs  stable P2P network • Note: Classifying the IP is an easy task for the P2P application. • The high stability of static IPs is due to two reasons: • 1. Static IPs do not change the IP address - Impact the Accessibility • Computers with static IPs are more stable computers - Impact the Churn

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