ON THE STABILITY OF SKYPE SUPER NODES Anat Bremler-Barr Ran Goldschmidt Interdisciplinary Center Herzliya Haifa University

Background: VoIP P2P Technology Proprietary signaling and media protocol

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...

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= , Port=3

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

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

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

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

MSLT vs SSLT  We define two group of SNs: o Single Session in Life Time (SSLT) o 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)

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

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…

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

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

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

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 Why SSLT and MSLT are so different?

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

Classifying SNs according to the address type 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 Classifying using DNS information

Related Work 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 File TransferSkype Session Short (hours)Long (days) Dynamic Application impactComputer impact (IP address type effect) Check: correlation between ICMP and Skype Ping

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 2. Computers with static IPs are more stable computers - Impact the Churn