A study on P2P VoIP applications focusing on Skype for the 1st year EMDC course on Decentralized Systems.

1. P2P VoIP Applications:P2P VoIP Applications:
“A Skype case-study”“A Skype case-study”
Vasia Kalavri (vasia@ac.upc.edu)
EMDC 2011 – Decentralized Systems
Universitat Politècnica de Catalunya

2. 2
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

3. 3
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

4. 4
P2P VoIP OverviewP2P VoIP Overview
● Communication Over IP Networks
● Free calls among Internet users
● Usually mixed centralized and P2P scheme
● Requirements
● Resource location
● Session establishment
● Session management
● Presence
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

5. 5
P2P VoIP ChallengesP2P VoIP Challenges
● Best-effort Internet
● Transmission delays
● Delay variations
● Packet loss
● Network failures
● NATs and Firewalls
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

6. 6
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

7. 7
Skype: why so popular?Skype: why so popular?
● Easy-to-use
● Works behind NATs and Firewalls (always?)
● More than 50 million users!
● Offers three services
● VoIP
● IM
● File transfer
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

8. 8
System Design: A KaZaASystem Design: A KaZaA
extension?extension?
● Several studies conclude that Skype is related
to Kazaa
● Founded by the same individuals
● Overlap of technical staff
● Use of the “supernode” hierarchical P2P
network scheme
● KaZaA V 3.0 included a Skype client
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

9. 9
Skype Peer-to-Peer NetworkSkype Peer-to-Peer Network
● Supernodes
● Must be publicly reachable
● Run the regular Skype software
● Form an overlay among themselves
● Also act as ordinary nodes
● Ordinary Nodes
● Connect to one or more supernodes
● Issue queries through supernodes
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

10. 10
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

11. 11
Explaining SupernodesExplaining Supernodes
Image Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.htmlImage Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.html
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

12. 12
The complete imageThe complete image
Image Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.htmlImage Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.html
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

13. 13
System Design AdvantagesSystem Design Advantages
● High scalability
● Use of processing and networking power of the
end-users
● Reduce costs
● Reliability by self-healing
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

14. 14
Self-healingSelf-healing
Image Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.htmlImage Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.html
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

15. 15
Becoming a Supernode: How will itBecoming a Supernode: How will it
affect my connection?affect my connection?
● Negligible additional
processing power,
memory and
storage compared
to an ordinary node
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

16. 16
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

17. 17
NAT TraversalNAT Traversal
● If one client is behind NAT
● Connection Reversal
● Hole Punching
● If both clients are behind NATs
● STUN
● If STUN fails
● TURN
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

18. 18
NAT Traversal: Connection ReversalNAT Traversal: Connection Reversal
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

19. 19
NAT Traversal: Hole PunchingNAT Traversal: Hole Punching
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

20. 20
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

21. 21
Self-healing - revisitedSelf-healing - revisited
Image Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.htmlImage Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.html
What could possibly go wrong?
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

22. 22
11stst
major outage: August 2007major outage: August 2007
● The number of on-line users started falling
unexpectedly
● Users faced login problems or slow services
● Problems lasted for more than 48h
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

23. 23
11stst
major outage:major outage:
● Skype's Official Explanation
● A Microsoft Windows Update caused Supernodes
to restart all at once
● Self-healing failed because too many users were
chasing too few Supernodes
““Supernodes not so super”Supernodes not so super”
● Skype's Solution
● The system self-recovered completely after almost
48 hours.
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

24. 24
11stst
major outagemajor outage
Image Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.htmlImage Source: http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.html
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

25. 25
11stst
major outage:major outage:
● Questions raised
““Blame it on Microsoft!”Blame it on Microsoft!”
● Why this particular Tuesday?
– Microsoft has been releasing security fixes on the 2nd
Tuesday of each month since October 2003
● Why at the same time?
– Updates are downloaded and installed at 3am in the PC's
local time
– At the very least, systems would have rebooted time
zone by time zone!
● Microsoft: “The disruption was caused by a bug in
their software.”
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

26. 26
22ndnd
major outage: December 2010major outage: December 2010
● The network became unstable
● Suffered a critical failure
● The outage lasted for 24h approx.
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

27. 27
22ndnd
major outage:major outage:
““Supernodes fail again”Supernodes fail again”
● A cluster of servers became overloaded...
● In version 5.0.0152 of Skype Windows client, the
delayed responses caused crashes...
● 50% of all Skype users globally were running this
version...
● Including 25–30% of the publicly available
Supernodes!
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

28. 28
22ndnd
major outage:major outage:
Permanent Solutions?Permanent Solutions?
● Fixing software bugs
● Building a more resistant core-system
● Suggestions?
● Can we rely on user-Supernodes only?
● How can we recover faster next time?
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

29. 29
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

30. 30
Skype SecuritySkype Security
● Privacy: Does the Skype system allow an
outsider to eavesdrop on a conversation?
I don't mean a person in
the next room!
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

31. 31
Skype Security: PrivacySkype Security: Privacy
Login behind the scenes
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

32. 32
Skype Security: PrivacySkype Security: Privacy
● Skype uses encryption
● Specific algorithms not publicly available
● Key-exchange mechanisms not known
● Secure against casual snooping
● Packets can be observed but are undecipherable
● Searches are observable
● Unknown security level against sophisticated
attacks
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

33. 33
Skype Security: PrivacySkype Security: Privacy
Security
PSTN
VoIP
Skype
VoIP over VPN
Silence ;)
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

34. 34
Skype Security: AuthenticitySkype Security: Authenticity
● Email-based Identification and Authentication
● username/password
● Registered e-mail address
● The network is involved → attacks are possible!
Authentication security level: AOL, Webmail
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

35. 35
OutlineOutline
● Introduction
● The Skype P2P System
● Explaining Supernodes
● NAT Traversal
● Discussing outages
● Skype Security
● Conclusions

36. 36
Conclusions on SkypeConclusions on Skype
● Gained popularity and offers good quality
despite Internet limitations
● Good performance and user satisfaction
● Supernodes increase availability but also fail
● Not clear Security level
● Encryption, network involvement
● As secure as webmail
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

37. 37
General ConclusionsGeneral Conclusions
● VoIP over P2P is challenging but works
● Cost reduction sharing resources
● Fault Tolerance
● Increase call quality over best-effort Internet
Introduction - Supernodes - NAT Traversal - Outages - Security - Conclusions

38. 38
ReferencesReferences
● [1] Barbosa R, Kamienski C., Mariz D., Callado A., Fernandes S. Sadok D., “Performance
evaluation os P2P VoIP applications”
● [2] Garfinkel, S.L. “VoIP and Skype Security”, Jan. 2005
● [3] Saikat G., Daswani N., Jain R., “An Experimental study of the Skype Peer-to-Peer VoIP
System”, February 2006
● [4] Baset, S. A., And Schrulzrinne, H. “An Analysis of the Skype Peer-to-Peer Internet Telephony
Protocol”, April 2006
● [5] Liang, J., Kumar, R. and Ross, K. W., “ The kazaa overlay: A measurement study”
● [6] Haiyong Xie, Yang R. Yang, “A Measurement-based Study of the Skype Peer-to-Peer VoIP
Performance”.
● [7] David A. Bryan, Bruce B. Lowerkamp, “SOSIMPLE: A SIP/SIMPLE Based P2P VoIP and IM
System”
● [8] Ford B., Srisuresh P., Kegel D., “Peer-to-Peer Communication Across Network Address
Translators”
● [9] http://blogs.skype.com/en/2010/12/cio_update.html
● [10] http://www.disruptivetelephony.com
● [11] http://news.techworld.com