Skype Network

Skype is a peer-to-peer (p2p) VoIP client developed by the organization that created Kazaa. Skype allows its users to place voice calls and send text messages to other users of Skype clients. In essence, it is very similar to the MSN and Yahoo IM applications, as it has capabilities for voice-calls, instant messaging, audio conferencing, and buddy lists. However, the underlying protocols and techniques it employs are quite different.

Like its file sharing predecessor Kazaa, Skype uses an overlay peer-to-peer network. There are two types of nodes in this overlay network, ordinary hosts and super nodes (SN). An ordinary host is a Skype application that can be used to place voice calls and send text messages. A super node is an ordinary host’s end-point on the Skype network. Any node with a public IP address having sufficient CPU, memory, and network bandwidth is a candidate to become a super node. An ordinary host must connect to a super node and must authenticate itself with the Skype login server. Although not a Skype node itself, the Skype login server is an important entity in the Skype network as user names and passwords are stored at the login server. This server ensures that Skype login names are unique across the Skype name space. Starting with Skype version 1.2, the buddy list is also stored on the login server.

Apart from the login server, there are SkypeOut and SkypeIn servers which provide PC-to-PSTN and PSTN-to-PC bridging. SkypeOut and SkypeIn servers do not play a role in PC-to-PC call establishment and hence we do not consider them to be a part of the Skype peer-to-peer network. Thus, we consider the login server to be the only central component in the Skype p2p network. Online and offline user information is
stored and propagated in a decentralized fashion.

We believe that each Skype node uses a variant of the STUN protocol to determine the type of NAT and firewall it is behind. We also believe that there is no global NAT and
firewall traversal server because if there was one, the Skype node would have exchanged traffic with it during the login and call establishment phases in the many experiments we performed.

The Skype network is an overlay network and thus each Skype client (SC) needs to build and refresh a table of reachable nodes. In Skype, this table is called host cache (HC)
and it contains IP address and port number of super nodes. Starting with Skype v1.0, the HC is stored in an XML file.
Skype claims to have implemented a ‘3G P2P’ or ‘Global Index’ technology, which is guaranteed to find a user if that user has logged in the Skype network in the last 72 hours.

Skype uses wideband codecs which allows it to maintain reasonable call quality at an available bandwidth of 32 kb/s. It uses TCP for signaling, and both UDP and TCP for
transporting media traffic.