1. Skype P2P communication
Group Members: Andre Tran, Balta Infante,
Priyanka Gangishetty, Irena Mao

2. In this presentation... 1) Introduction to P2P 2) Skype Overview
3) Architecture
4) Key components of Skype software
5) Functions of Skype

3. P2P Architecture No always-on server
Peers are both clients and servers
Any peer can initiate a connection and change IP address

4. P2P Characteristics Clients are also servers and routers
Nodes are autonomous
Network is dynamic
Nodes collaborate directly with each other

5. P2P Benefits Efficient use of resources Scalability Reliability
Ease of administration

6. Skype VoIP application services provided: host-to-host VoIP
host-to-phone
phone-to-host
multi-party host-to-host video conferencing

7. Skype P2P Components Organization: hierarchical overlay network
clients: skype peers connect directly to each other for VoIP call
super nodes (SN): skype peers with special functions
overlay network: among SNs to locate clients
login server

8. How Skype Works skype client operation:
1. joins skype network by contacting SN (IP address cached) using TCP
2. logs-in (usename, password) to centralized skype login server
3. obtains IP address for callee from SN, SN overlay or client buddy list
4. initiate call directly to callee

9. Multi-party Conference Calls
audio calls (N >2) :
instead of each peer send audio to all other peers
number of streams sent: N(N-1)
reduce bandwidth by:
all peers send audio to conference initiator
conference initiator combine all audio into one stream
sends combined steam to all other peers
number of stream sent: 2(N -1)

10. Multi-party Conference Calls
video call (N > 2):
each peer's video stream is routed to a server cluster
server cluster relays to each peer the N-1 streams of other peers

11. Skype Privacy Problems
A peer to sniff IP address of another peer
After obtaining IP address:
use geo-location services to track location of another peer
correlate to BitTorrent
partially decrypt a Skype call

12. Architecture Overlay network Can become super node if:
Sufficient bandwidth
Sufficient processing power
Won’t be a super node if:
Behind NAT
Restrictive firewall
Behind HTTP or SOCKS5 proxy

13. Architecture Super nodes replaced by linux boxes
~10,000 boxes using grsecurity
More resilient to attacks
Accommodates more users
More centralized and more secure

14. Architecture Skype client keeps table of reachable nodes
Buddy lists and host cache
Shared over the super nodes
Contains IP addresses and port numbers
Stored in XML format

15. Codecs SILK codec replaces SVOPC (Sinusoidal Voice Over Packet Coder)
Sampling frequency: 8, 12, 16, 24 kHz
Bit rate from 6 to 40 kbits/s
IETF currently developing Opus
based off the speech-oriented SILK and low latency CELT

16. Skype Protocol Protocol is proprietary
It is unknown how index mappings are organized across super nodes
Claimed to have 3G P2P or Global Index technology
Skype doesn’t support use of IPV6 protocol

17. Skype Protocol By default audio and video sent over UDP
Control packets are sent over TCP
Media packets over TCP when firewall blocks UDP
FEC (Forward Error Correction) used for loss recovery of voice and video over UDP
Skype client adapts by changing video quality and FEC overhead

18. Skype Protocol Signaling is encrypted with RC4
obscures the traffic, but key can still be recovered
Voice data encrypted by AES
Skype client API is open to developers

19. Relays
Super nodes relay communications for clients behind firewalls or NAT
NAT and firewalls may prevent client from receiving
UDP is not usable with NAT
Firewalls block many ports
TCP through many-to-one NAT is outward only
Three super nodes are needed, one for each client and one to serve as relay

21. Key Components Ports Host Cache Codecs Buddy List Encryption
NAT and Firewall

22. Ports
A SC opens TCP and UDP listening ports at the number configured in its connection dialog box
In addition, SC also opens port number 80 and 443 as alternatives
There is no default TCP or UDP listening port

23. Port’s snapshot

24. Host Cache
List of super node IP address and port pairs that SC builds and refreshes regularly
v0.97 stops and reports login failure if unable
v1.2 and onwards, tries to establish a TCP connection with one of the seven bootstrap (hardcoded)
Stored as XML files
Maximum of 200 entries

25. Codecs iLBC, iSAC, iPCM Developed by GlobalIPSound
For SC v1.4 allows frequencies from 50 to Hz
Thus, frequency range of a wideband codec

26. Buddy List Stored in config.xml
v1.2 and onwards for XP also stores it on a central Skype server:
Stored unencrypted on a computer

27. Buddy List’s snapshot

28. Encryption Skype uses AES
256-bit encryption with a total of 1.1 x 10^77
Uses 1024 bit RSA to negotiate symmetric AES keys
User public keys certified by the Skype server at login using 1536 or 2048-bit RSA certificates

29. NAT and Firewall
SC uses a variation of STUN and TURN protocols to determine the type of NAT and firewall it is behind
SC refreshes info periodically
Info also stored in shared.xml file
Unlike Kaaza, a SC cannot prevent itself from becoming a super node

30. Functions of Skype 1) Login
SC sends a UDP packet to the bootstrap super nodes
If no response, it tries to establish a TCP connection, repeats the whole process after 6 seconds if this fails too
Next step is to authenticate itself to a server which is done over a TCP connection
2) NAT traversal
Skype uses STUN (Session traversal utilities for NAT) to detect and traverse NATs
3) User search
Skype uses a distributed search on a Global index

31. contd... 4) Call establishment and teardown
For a user not on the friends list, its a user search plus call signalling
For a person on the friend’s list, the caller SC establishes a TCP connection with the callee via a challenge-response mechanism
For a machine behind a NAT, caller establishes a connection with an online Skype node
5) Media transfer
If both machines have public IP addresses, media is exchanged over UDP between caller and callee SC
Otherwise, a TCP connection is used via an online node

32. References
56/slides/p2p-mie.pdf
Computer Networking: A Top Down Approach
ations/skype1_4.pdf