Skype 101


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Skype 101

  1. 1. Skype - 101 Ahmed Ibrahim October 25, 2013 Abstract - This report studies the architecture of the commonly used communication program Skype. It involve the identication of use-cases for this program and the corresponding use case mode. Furthermore, it describes the applied architecture model and its key concers/ drivers. Finally, it illustrates the scenario of user authentication using collaboration diagram. 1 Skype Skype is a commonly used communication program. It was launched in August 2003. And, it has largely dominated the free Internet telephony application space. As of January 2008, Skype claim 276 million users accounts. Many of these can be expected to be inactive, and typically users may have more than one account on this (free) service. With Skype, the users were able to have phone conversations over the Internet with Skype members, and all of this communication was free over the internet. Figure 1 illustrates the use case model of Skype. Figure 2 illustrates the log-in use case scenario for Skype. First, Skype client (SC) starts and a log-in attempt by sending a message with UDP packet to one of the SN buddies list. If there was no response after roughly ve seconds, SC tried to establish a TCP connection with this SN. It then tried to establish a TCP connection to the SN IP address and port 80 (HTTP port). If still unsuccessful, it tried to connect to SN 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 log-in failure. 2 Skype architecture The Skype network consists of conventional nodes and Super-nodes (SN), which are themselves Skype endpoints, as well as providing signaling and media proxy services to conventional nodes. Skype applications are provided with a basic set of SN addresses when they rst register, and maintain a cache of these and additional SN long-term. Also, Skype network has a central registration server to which all clients register. 1
  2. 2. Skype User login user authentication «in clud e» «in clud e» pla ce c alls This is part of my participation in ECE750 course. The following diagram (use-case model) has been produced after identifying key use cases for Sky program. Ref.: Skype 101 - Skype architecture - work in progress, [online] p=10 Sk ype Client (SC) show genera tic e rror me ssage se nd m essa ges «in clud e» Super Node (SN) Sk ype a uthe ntica tion se rv er loc k ac count afte r N failed login attempts signalling and media proxy s erv ic es Figure 1: Skype use-case model :Skype Client (SC) :Su per Node (SN) loop Login2S kype Serv ices [Un till c onnec ted] [wa it for reply in 5 sec]:sent_ UDP_ packet(SN_IP) [wa it for reply in 5 sec]: est ablish _TCP_co nnect ion(S N_T P) [wa it for reply in 5 sec]:e stab lish_T CP_ conn ection (SN_ IP, Po rt80) [wa it for reply in 5 sec]:e stab lish_T CP_ conn ection (SN_ IP, Po rt 433 ) Wa it6Te llRep ortFa il() Figure 2: Skype log-in scenario 2
  3. 3. Figure 3: Skype Architecture Layering is an architectural style which is suitable for applications that involve distinct classes of services that can be arranged hierarchically. In this style, there are layers for basic system-level services, for utilities appropriate to many applications, and for specic tasks of the application. Generally, the layered architectural pattern has been used mostly to model message-passing situations. So obviously the Skype system can be modeled as a layered architecture. Figure 3 illustrates the Skype architectures from our point of view. Skype system can be decomposed into two main packages: Skype client and authentication server (gure 4). The Skype client contains two components. The Skype client components that holds all the functions needed to place calls and send messages. And, the second components holds all the functionality of routing the calls and messages between Skype users. 3 conclusion Skype is denitely one of the better entry-level, low-cost competitors in our VoIP services lineup. The VoIP provider oers well-rounded service plans, and it is easy to install, even for less tech-savvy individuals. While its feature set is limited, Skype is an excellent alternative if your VoIP needs are minimal. 3
  4. 4. Authentication Authentication Se rv er Client Super Node Sk ype Clinet (SC) Figure 4: Skype package diagram 4