Skype VoIP service- architecture and comparison

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Skype VoIP service- architecture and comparison
Hao Wang
INFOTECH Seminar June 30, 2005
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Agenda

• 1. Background knowledge of VoIP
  Architecture and components
• 2. Skype A P2P VoIP Service
• What is peer-to-peer ?
• Skype Introduction and Overview
• 3. Comparison of different scenarios between
  Skype and VoIP
• Login, NAT and Firewall Traversal, User
  Search,
• Call Establishment and Teardown, Media
  Transfer and Codecs, Security

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VoIP Network
Signaling protocols H.323, SIP
Data transfer Real-time Transport Protocol.
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VoIP Transmission

• Step 1 The callers voice is digitized.
• Step 2 Digital voice is compressed and then
  separated into packets the packets are
  addressed and sent across the network.
• Step3 Error detection and correction systems are
  used to create sound to fill in the gaps.
• Step4 Restore the data to an approximation of
  the original form.

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Skype Peer-to-peer internet telephony (VoIP)
network
A pure peer-to-peer network only have equal peer
nodes that simultaneously function as both
"clients" and "servers" to the other nodes on
the network.
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Main Difference

• The main difference between Skype and other VoIP
  clients is that it operates on a peer-to-peer
  model rather than the more traditional
  server-client model.

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Login Process

• VoIP A client may only have to authenticate its
  user name and password with login server.
• Skype
• Connects to a Super Node
• Determines the type of NAT and firewall
• Discovers online Skype nodes with public IP
  addresses (Alternate Node Table)

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NAT and Firewall

• VoIP
• NAT port forwarding manual configuration or via
  UPnP (Universal Plug and Play)
• STUN protocol (Simple Traversal of User Datagram
  Protocol)
• ALG (Application Layer Gateway)

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NAT and Firewall

• Skype
• Variation of typical schemes such as STUN
• Natural and elegant, involving little management
  structure and overhead.

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User Search

• SIP caller will search the users by asking the
  SIP server
• Skype
• Perhaps use Chord like hash-based methods,
• combined with traditional blind searching
  techniques.
• SN acts like a search proxy for SC and caches
  searched results.

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Call Establishment and Teardown

• Process in SIP
• The SIP proxy uses
• DNS to find the SIP
• registrar of Bob .

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Call Establishment and Teardown

• Process in Skype
• Call signaling is always carried over TCP.
• Call placement is equal to user search plus call
  signaling.
• If users are behind port restricted NAT and
  UDP-restricted firewall, both caller and callee
  SC exchange signaling information over TCP with
  another online Skype node.

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Media Transfer

• VoIP
• H.323 and SIP are only the signaling protocols.
  Audio data is carried using RTP (Real-time
  Transport Protocol).
• Skype
• The Skype protocol seems to prefer the use of UDP
  for voice transmission as much as possible. If
  not possible , it will make use of TCP .

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Codecs

• VoIP
• Various standards
• Compress the data stream
• Silence suppression
• Skype
• Global IP Sound multiple description coding,
• variable bit-rate
• No silence suppression

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Security in VoIP

• IP security
• IPsec is a suite of protocols to provide
  security services on the network layer in IP
  based networks.
• Often in combination with VPN (Virtual Private
  Network )
• Firewall
• Located at border to public Internet
• Control incoming and outgoing traffic

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Security in Skype

• Skype is encrypted end-to-end
• Skype uses AES (Advanced Encryption Standard) to
  protect sensitive information.
• Skype uses 1024 bit RSA to negotiate symmetric
  AES keys.
• User public keys are certified by the Skype
  server at login.

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Conclusions

• Skype is a successful combination of several
  technologies basic on p2p overlay networks
• Much better voice quality, ability to traverse
  the NAT and firewalls, encrypted media channel
• Ability to scale up to handle large-scale
  connection-oriented real-time services
• Heavy software based DSP operations at Skype
  clients

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d

• Q A Session

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D

• Thank you for your attention!