Figure2 illustrates the relationship between ordinary hosts super nodes and

Figure2 illustrates the relationship between ordinary

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across the Skype name space. Figure2 illustrates the relationship between ordinary hosts, super nodes and login server. Apart from the login server, there is no central server in the Skype network. Online and offline user information is stored and propagated in a decentralized fashion and so are the user search queries. 4. Comparison of different scenarios between Skype and VoIP In order to demonstrate the differences between Skype and VoIP, six scenarios are chosen to be compared in detail. 4.1 Login In VoIP a client may only have to authenticate its user name and password with login server. While in Skype Login is perhaps the most critical function to the Skype operation. It is during this process 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, and discovers online Skype nodes with public IP addresses. Below is further study of Skype login process [3].
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INFOTECH Seminar Advanced Communication Services (ACS), 2005 5 Figure 2. Skype Network. Super nodes, ordinary nodes, and the login server. Login Process When SC was run for the first time after installation, it sent a HTTP 1.1 GET request to the Skype server (skype.com). The first line of this request contains the keyword ‘installed’ .During subsequent startups, a SC only sent a HTTP 1.1 GET request to the Skype server (skype.com) to determine if a new version is available. The host cache (HC) is a list of super node IP address and port pairs that SC builds and refreshes regularly. The HC must contain a valid entry for a SC to be able to connect to the Skype network. If the HC was filled with only one invalid entry, SC could not connect to the Skype network and reported a login failure. SC first sent a UDP packet to this entry. If there was no response after roughly five seconds, SC tried to establish a TCP connection with this entry. It then tried to establish a TCP connection to the HC IP address and port 80 (HTTP port). If still unsuccessful, it tried to connect to HC IP address and port 443 (HTTPS port). SC then waited for roughly 6 seconds. It repeated the whole process four more times after which it reported a login failure. It is observed that a SC must establish a TCP connection with a SN in order to connect to the Skype network. If it cannot connect to a super node, it will report a login failure. Most firewalls are configured to allow outgoing TCP traffic to port 80 (HTTP port) and port 443 (HTTPS port). A SC behind a firewall, which blocks UDP traffic and permits selective TCP traffic, takes advantage of this fact. At login, it establishes a TCP connection with another Skype node with a public IP address and port 80 or port 443.After a SC is connected to a SN, the SC must authenticate the user name and password with the Skype login server.
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