Skype

1
VoIP Case Study Skype
Dr. Danny TsangDepartment of Electrical
Electronic EngineeringHong Kong University of
Science and Technology
2
Phone becomes the Portal ...

through which a whole new world of services are
delivered
3
What Is Skype?

• It is a peer-to-peer (P2P) overlay network for
  VoIP and other applications, developed by the
  people who did KaZaA
• To a user, it is an Instant Messaging (IM) system
  that supports P2P VoIP and other applications
• The fee-based SkypeOut service allows calls to
  regular phone numbers

4
Skype - a raving success

• Skype - computer lt-gt computer telephony
• You can talk to other computer based phones free
• Skype Out - computer -gt PSTN telephony
• You can call normal PSTN telephones - and pay for
  it
• Skype In - PSTN telephony -gt computer telephony
• You can be called from PTSN telephones
• You get a normal phone number
• Currently on-line 1.8 million users
• 150 million skype software download
  (www.skype.com)
• Skype hardware phones available

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Overview of Skype P2P Network
Major Entities

• Login Server
• Single central server
• User authentication at login (private/public
  keys)
• Ensures uniqueness of Skype user names
• Super nodes
• Nodes with public IP address, enough CPU, memory,
  and network bandwidth
• Registrar functionality (distributed directory,
  presence service)
• Signaling/media proxy (reachability of ordinary
  hosts)
• Ordinary nodes
• Hosts behind FW/NAT

Skype login Server
Message exchange with The login server during
login
Ordinary Host
Super node
Neighbors relationships in the Skype network
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Highlights of Skype

• P2P-based signaling
• to find and locate users, Skype uses "supernodes"
  that are running on peer machines (In contrast,
  traditional VoIP systems use fixed central
  servers)
• little management structure and overhead
• highly scalable
• Strong NAT/firewall traversal capability
• e.g., use a peer relay to connect clients behind
  NATs, or use a TCP tunnel to a peer relay to
  bypass a UDP blocking firewall
• Better voice quality than the MSN and Yahoo IM
  applications
• Wideband voice codec allow frequency between
  50-8000Hz to pass through
• Heavy software-based DSP operations at clients,
  including codecs, mixer and fancy echo
  cancellation, e.g., Thinkpad X31, an active Skype
  session consumes around 10-20 of CPU
• Security feature
• encrypts calls end-to-end, and stores user
  information in a decentralized fashion
• Integrated buddy list, presence information,
  chat, and audio conferencing

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Reverse Engineering of Skype

• Based on An Analysis of the Skype Peer-to- Peer
  Internet Telephony Protocol by S. Baset and H.
  Schulzrinne at Columbia, September 15, 2004
• We observe, , we think, , we conjecture,
  etc.
• Terminologies
• Skype client (SC)
• Super node (SN)
• Host cache (HC) list of IP addresses and port
  number of some super nodes

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Some Observed Facts

• For NAT and firewall traversal, SC uses a
  variation of typical schemes such as STUN to
  determine the type of NAT and firewall it is
  behind. SC refreshes this information
  periodically. This information is stored, e.g.,
  in the Windows registry
• Use TCP to bypass UDP-restricted NAT/firewall
• Unlike its file sharing counter part KaZaa, a SC
  CANNOT prevent itself from becoming a SC

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Skype Functions

• Startup
• After installation, SC sends a HTTP 1.1 GET to
  skype.com
• Login
• User search
• Call establishment and tear down
• Media transfer
• Presence messages

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Skype Login

• Advertises its presence to other peers and its
  buddies.
• Determines the type of NAT and firewall it is
  behind.
• Discover online Skype nodes with public IP
  addresses.

• Stores Skype user names and passwords and ensures
  that Skype user names are unique across the Skype
  name space.
• Login server ns14.inet.telt.dk(Denmark)

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Skype Login algorithm
authentication with login server is not shown
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Skype Login in details

• A SC authenticates its user name and password
  with the login server, advertises its presence to
  other peers and its buddies, determines the type
  of NAT and firewall it is behind, and discovers
  online Skype nodes with public IP addresses
• These newly discovered nodes were used to
  maintain connection with the Skype network should
  the SN to which SC was connected became
  unavailable
• SC must establish a TCP connection to a SN in
  order to connect to the Skype network
• For a given entry in HC, SC tries to make the
  connection in the following order TCP with the
  IP address and the port listed, the IP address
  and HTTP port (80), the IP address and SHTTP port
  (443)
• After SC is connected to a SN, the SC gets the IP
  address of the Skype login server, and
  authenticates the username and password with the
  login server
• SC sends UDP packets to 22 distinct nodes at the
  end of login process
• Advertise its arrival
• Build an alternative node tables that provide a
  list of available nodes SC can use should its
  current SN becomes unavailable

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User Search

• Perhaps use Chord-like hash-based methods,
  combined with traditional blind searching
  techniques
• SN acts like a search proxy for CN and caches
  searched results

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Use of An Immediate Node

• Two reasons
• Passing NAT and firewall
• Acting as a mixer for conferencing traffic

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Conferencing
A
AC
C
AB
B
B
C

• CPU and bandwidth usages at the bridging point
  (e.g., the A node above) increase with
  conference participants

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Conferencing
A

• A call was established between A (the most
  powerful one) and B. Then B decided to include C
  in the conference.
• It mixed its own packets with those of B and sent
  them to C over UDP and vice versa
• Even if user B or C started a conference, A,
  which was the most powerful amongst the three,
  always got elected as conference host and mixer.
• If iLBC codec is used, the total call 36 KB/s for
  a two-way call. For three-user conference, it
  jumps to 54 kb/s for the machine hosting the
  conference.
• For a three party conference, Skype does not do
  full mesh conferencing.
• To host a conference with 5 parties you need a
  big PC, a Pentium 4 or thereabouts. With a PIII
  CPU of 450 MHz you will be limited to hosting 3
  parties.

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Reflection of Skype

• Pros
• scales as it grows
• Ease of use (self configuration)
• Intelligence at the edge
• Transparent FW/NAT traversal
• Integration of IM, presence, voice
• Encrypted communication

• Cons
• Proprietary protocols
• Closed world, no SIP/H.323 GWs
• No bandwidth control at super nodes
• Reliability? Are there enough super nodes?
• Not suitable for business usage, privacy?

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Broad Impact

• Great application for WLAN-enabled handsets
• A new opportunity for fixed-line operators
• New opportunity and challenge to mobile phone
  operators and manufacturers (think when WLAN
  infrastructure and handsets become widespread)
• New challenges to SIP/ENUM-based approaches
• Since naming on overlay networks could work well,
  DNS may not need to expand its scope
• VoIP is a new must-to-have feature for IM systems
• A new view on P2P networks, which previously
  focused on content rather than communications
• A large number of online Skype VoIP clients
  distributed throughout the world can provide
  valuable network performance measurements and
  other information
• Other issues, including anonymity vs. law
  enforcement

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Profound Impact

• Skype has shown, or at least has suggested, the
  following
• Signaling, the most unique property of
  traditional phone systems, can now be
  accomplished effortlessly with self-organizing
  P2P networks
• P2P overlay networks can scale up to handle
  large-scale connection-oriented real-time
  services such as voice
• For Skype, the Internet is just a dumb "bit
  pipes
• We should focus on applications and services that
  run on layers above pipes, since pipes or roads
  always only have relatively small business value
  themselves

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Reference

• B. Goode, Voice over Internet protocol (VoIP),
  Proceedings of the IEEE, vol. 90, no. 9, pp. 1495
  1517, Sep. 2002.
• M. Maresca, N. Zingirian, and P. Baglietto,
  Internet protocol support for telephony,
  Proceedings of the IEEE, vol. 92, no. 9, pp. 1463
  1477, Sep. 2002.
• S. A. Baset and H. Schulzrinne, An analysis of
  the Skype peer-to-peer internet telephony
  protocol, Computer Science Department, Columbia
  University, Tech. Rep. CUCS-039-04, 2004.
  Online. Available http//arxiv.org/pdf/cs.NI/04
  12017
• C. Schlatter, Why SIP? 2005, Swiss Education
  and Research Network, SWITCH. Online.
  Available http//vconf.switch.ch/econf_docs/sip_h
  323_skype.pdf
• H. T. Kung, P2P VoIP, 2004, lecture notes on
  CS-143/CSCI-E135 Computer Networks. Online.
  Available http//www.eecs.harvard.edu/cs143/