• Client applications and devices
  • System and software
  • Protocol detection and control
  • Security and privacy
  • Service in the Peoples Republic of China
  • Outages and downtime
  • Timofeev Kirill Group 205

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    Moscow State University
    The Department of Computational Mathematics and Cybernetics


    Timofeev Kirill

    Group 205

    Moscow 2011

    Skype (pronounced /ˈskaɪp/) is a software application that allows users to make voice calls and chats over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phone can be made for a fee using a debit-based user account system. Skype has also become popular for its additional features which include instant messaging, file transfer, and video conferencing. Skype has 663 million registered users as of 2010. The network is operated by Skype Limited, which has its headquarters in Luxembourg and is minority owned by eBay. Most of the development team of Skype is situated in Tallinn, Estonia.

    Unlike other VoIP services, Skype is a peer-to-peer system rather than a client–server system, and makes use of background processing on computers running Skype software; the original name proposed – Sky peer-to-peer – reflects this.

    Some network administrators have banned Skype on corporate, government, home, and education networks, citing reasons such as inappropriate usage of resources, excessive bandwidth usage, and security concerns.


    Registered users of Skype are identified by a unique Skype Name, and may be listed in the Skype directory. Skype allows these registered users to communicate through both instant messaging and voice chat. Voice chat allows calls between pairs of users and conference calling, and uses a proprietary audio codec. Skype's text chat client allows group chats, emoticons, storing chat history, offline messaging (since version 5) and editing of previous messages. The usual features familiar to instant messaging users—user profiles, online status indicators, and so on—are also included.

    The Online Number (aka SkypeIn) service allows Skype users to receive calls on their computers dialed by regular phone subscribers to a local Skype phone number; local numbers are available for Australia, Belgium, Brazil, Chile, Colombia, Denmark, the Dominican Republic, Estonia, Finland, France, Germany, Hong Kong, Hungary, Ireland, Italy, Japan, Mexico, New Zealand, Poland, Romania, South Africa, South Korea, Sweden, Switzerland, the Netherlands, the United Kingdom, and the United States. A Skype user can have local numbers in any of these countries, with calls to the number charged at the same rate as calls to fixed lines in the country.

    Video conferencing between two users was introduced in January 2006 for the Windows and Mac OS X platform clients. Skype 2.0 for Linux, released on 13 March 2008, also features support for video conferencing. Version 5 beta 1 for Windows, released 13 May 2010, offers free video conferencing with up to 5 people.

    Skype for Windows, starting with version, supports "High Quality Video" with quality and features, e.g., full-screen and screen-in-screen modes, similar to those of mid-range videoconferencing systems. Skype audio conferences currently support up to 25 people at a time, including the host.

    Skype does not provide the ability to call emergency numbers such as 911 in the United States and Canada, 111 in New Zealand, 000 in Australia, or 112 in Europe. The U.S. Federal Communications Commission (FCC) has ruled that, for the purposes of section 255 of the Telecommunications Act, Skype is not an "interconnected VoIP provider". As a result, the U.S. National Emergency Number Association recommends that all VoIP users have an analog line available as a backup.


    Skype was founded in 2003 by the Swedish entrepreneur Niklas Zennström and Dane Janus Friis. The Skype software was developed by Estonian developers Ahti Heinla, Priit Kasesalu and Jaan Tallinn, the same individuals who together with Niklas and Janus were also originally behind the peer-to-peer file sharing software Kazaa. In April 2003, Skype.com and Skype.net domain names were registered. In August 2003, the first public beta version was released.

    One of the initial names for the project was "Sky peer-to-peer", which was then abbreviated to "Skyper". However, some of the domain names associated with "Skyper" were already taken. Dropping the final "r" left the current title "Skype", for which domain names were available.

    In September 2005, SkypeOut was banned in South China. In October of the same year, eBay purchased Skype and in fall of 2009 sold a majority stake to an investor group. In December 2005, videotelephony was introduced.

    In April 2006, the number of registered users reached 100 million. In October, Skype 2.0 for Mac was released, the first full release of Skype with video for Macintosh, and in December, Skype announced a new pricing structure, with connection fees for all SkypeOut calls. Skype 3.0 for Windows was released.

    In 2006, a now-discontinued feature called "Skypecasting" was introduced. It allowed recordings of Skype voice over IP voice calls and teleconferences to be used as podcasts. Skypecasts remained in beta until its end. Skypecasts hosted public conference calls, up to 100 people at a time. Unlike ordinary Skype p2p conference calls, Skypecasts supported moderation features suitable for panel discussions, lectures, and town hall forums. Skype operated a directory of public Skypecasts. Skypecasts was discontinued as of 1 September 2008.

    Throughout 2007 updates (3.1, 3.2 and 3.5) added new features including Skype Find, Skype Prime, Send Money (which allowed users to send money via PayPal from one Skype user to another), video in mood, inclusion of video content in chat, call transfer to another person or a group, and auto-redial. Skype (beta) for Mac OS X released adding availability of contacts in the Mac Address Book to the Skype contact list, auto redial, contact groups, public chat creation, and an in-window volume slider in the call window. During several days in August, Skype users were unable to connect to full Skype network in many countries because of a Skype system-wide crash which was the result of exceptional number of logins after a Windows patch reboot ("Patch Tuesday"). And in November, there was controversy when it was announced that users having London (020) 7 numbers would lose them.

    In 2008, Skype released various updates including versions for the Sony PSP hand-held gaming system, version 2.0 for Linux with support for video-conferencing, and version 4 for Windows (with both a full screen and a compact mode). This version dropped support for the "Skype Me" presence indicator, which meant that a user was interested in receiving Skype calls from a non-contact. Skype also discontinued its SkypeCast service without explanation and added internal monthly and daily usage caps on their SkypeOut subscriptions, which had been advertised as "Unlimited". Many users and observers had commented on the high rate of dropped calls and the difficulty reconnecting dropped calls. Skype was used in the seventh season of the U.S. syndicated version of the British game show Who Wants To Be a Millionaire in a new Ask the Expert video chat lifeline. In October, analysis revealed TOM-skype—the Chinese version of Skype run by TOM Online—sends content of text messages and encryption keys to monitoring servers.

    In 2009, Skype 4 was released, the Linux client was updated, and Skype for SIP, a service aimed at business users, was launched. At that time around 35% of Skype's users were business users. In April 2009, eBay announced plans to spin off Skype through an initial public offering in 2010. In August, Joltid filed a motion with the U.S. Securities and Exchange Commission, seeking to terminate a licensing agreement with eBay which allows eBay (and therefore Skype) to use the peer-to-peer communications technology on which Skype is based. If successful, this may have caused a shutdown of Skype in its current form. In September, eBay announced the sale of 65% of Skype to a consortium of Index Ventures and Silver Lake Partners. Early in September, Skype had shut down the Extras developer program. In November, eBay completed the sale of 70% of Skype to a consortium comprising Silver Lake Partners, CPPIB, Andreessen Horowitz, and the original founders valuing the business at USD2.75 billion.

    In 2010, a report by TeleGeography Research stated that Skype-to-Skype calls accounted for 13% of all international call minutes in 2009; out of the 406 billion international call minutes a total of 54 billion were used by Skype calls. In May, Skype 5.0 beta was released, with support of group video calls with up to four participants. Also in May, Skype released an updated client for the Apple iPhone which allowed Skype calls to be made over a 3G network. Originally, a 3G call subscription plan was to be instituted in 2011, but the plan was eventually dropped by Skype.

    On 9 August 2010, Skype filed with the SEC to raise up to $100 million in an initial public offering. In October 2010, Skype announced it had named Tony Bates as their CEO; Bates has been a senior VP at Cisco and responsible for its multi-billion-dollar enterprise, commercial and small business division.

    On 14 October 2010, Skype 5.0 for Windows was released with a number of improvements and feature additions, including a Facebook tab to allow users to SMS, chat with, or call their Facebook friends via Skype from the News Feed. This version dropped support for the "Search for Skype Users" option.

    On 14 January 2011, Skype announced the acquisition of Qik which is a mobile video sharing platform.

    Client applications and devices

    Skype runs on a number of platforms, and on 29 October 2007, Skype launched its own mobile phone under the brand name 3 Skypephone, which runs a BREW OS. Other platforms officially supported include:

    • Around 50 mobile phones, as of 2008

    • The Nokia N800, N810 and N900 Internet Tablets, which use the Maemo environment

    • Both the Sony mylo COM-1 and COM-2 models

    • The PlayStation Portable Slim and Lite series, though the user needs to purchase a specially-designed microphone peripheral. The PSP-3000 has a built in microphone which allows communication without the Skype peripheral. PSP Go has the ability to use Bluetooth connections with the Skype application, in addition to its built-in microphone.

    • Mobile devices running Windows Mobile; in February 2010, Skype announced its decision to discontinue development Skype for Windows Mobile. In May 2010, Skype announced it would not develop a version of Skype for Windows Phone 7.

    • Symbian; Skype version 5.0 is the latest version for the Symbian platform

    • The X-Series together with mobile operator 3. However this uses a regular mobile phone call and iSkoot to a Skype gateway, rather than mobile Internet. Other companies produce dedicated Skype phones which connect via WiFi

    • iOS, using an official application released on 31 March 2009. As of version 2.x, the iPhone application is capable of placing voice calls over 3G and EDGE networks. Skype originally announced it will charge a monthly fee for this feature at the start of 2011, but that was eventually dropped. On December 29, Skype updated their iOS app to version 3.0, allowing mobile video calling.

    • A variety of BlackBerry and Android 3G Smartphones, through Verizon Wireless' Skype mobile service. Customers can receive Skype calls, instant messages and see friends' presence any time the phone is on. Skype usage isn't charged against customers’ monthly Verizon Wireless minute allowance when calling another Skype account (Skype-to-Skype). Customers in the U.S. use minutes from their calling plan when calling U.S. land lines or cell phones.

    Some devices are made to work with Skype by talking to a desktop Skype client or by embedding Skype software into the device. These are usually either tethered to a PC, or have a built-in Wi-Fi client to allow calling from Wi-Fi hotspots like the Netgear SPH101 Skype Wi-Fi Phone, the SMC WSKP100 Skype Wi-Fi Phone, the Belkin F1PP000GN-SK Wi-Fi Skype Phone, the Panasonic KX-WP1050 Wi-Fi Phone for Skype Executive Travel Set, the IPEVO So-20 Wi-Fi Phone for Skype and the Linksys CIT200 Wi-Fi Phone. There are also embedded cordless Skype phones based on DECT, which do not need a PC either, like the DUALphone 3088 from RTX. Netgear and SMC's Wi-Fi phones are considered the two major competitors in the market, with reviews of the Wi-Fi phones giving them equal coverage.

    The Skype Wi-Fi Phone is a wireless mobile phone that allows users to make Skype calls, using a wireless Internet connection. The Skype Wi-Fi Phone has an on-screen menu that lets Skype users see who is online and available to talk, similar to what is seen on a PC. It can also be used to talk with non-Skype users. SkypeOut minutes can be used to call any phone for a low price and no monthly fee. The Skype Wi-Fi phone does not contain a web browser and so can not access hotspots, which require web-based login or authentication.

    Third-party developers, such as Truphone, Nimbuzz and Fring, have allowed Skype to run in parallel with several other competing VoIP/IM networks (Truphone and Nimbuzz provide truphoneOut and NimbuzzOut as a competing paid service) in any Symbian or Java environment. Nimbuzz has made Skype available to BlackBerry users. In July 2010, however, Fring disabled users from accessing Skype despite claiming that Skype themselves disabled the access; Fring had provided mobile video calling over Skype as well as support for the Android platform. Nimbuzz discontinued support on request of Skype in October 2010.

    Until November 2010, only certain Samsung and Panasonic TVs offered Skype video service.

    System and software


    Skype uses a proprietary Internet telephony (VoIP) network called the Skype protocol. The protocol has not been made publicly available by Skype and official applications using the protocol are closed-source. Part of the Skype technology relies on the Global Index P2P protocol belonging to the Joltid Ltd. corporation. The main difference between Skype and standard VoIP clients is that Skype operates on a peer-to-peer model (originally based on the Kazaa software), rather than the more usual client–server model (note that the very popular SIP model of VoIP is also peer to peer, but implementation generally requires registration with a server, as does Skype).

    Protocol detection and control

    Many networking and security companies claim to detect and control Skype's protocol for enterprise and carrier applications. While the specific detection methods used by these companies are often private, Pearson's chi-square test and Naive Bayes classification are two approaches that were published in 2007. Combining statistical measurements of payload properties (such as byte frequencies and initial byte sequences) as well as flow properties (like packet sizes and packet directions) has also shown to be an effective method for identifying Skype's TCP- and UDP-based protocols.

    User directory

    The Skype user directory is sometimes claimed to be entirely decentralized and distributed among the nodes.

    Audio codecs

    Skype uses an array of different audio compression methods including G.729 and SVOPC. Skype added a Skype-created codec called SILK to Skype for Windows 4 and other Skype clients. SILK is intended to be "lightweight and embeddable".


    There are versions for Linux, Linux-based Maemo, Symbian S60, Mac OS X (Intel and PPC), iOS (iPhone and iPod Touch), Android, Microsoft Windows (2000, XP, Vista, 7, Mobile), Sony PSP and also in 2010 TV models from Panasonic, Samsung and LG, but requires a special webcam built for those TVs.

    The latest vesions of Skype:

    1. Skype - for Linux

    2. Skype - for Windows

    3. Skype - for Mac OS X

    4. Skype - for Android

    5. Skype - for Symbian^3

    6. Skype - for Windows Mobile

    7. Skype 3.90 - for PSP

    Security and privacy

    Skype is a secure communication; encryption cannot be disabled, and is invisible to the user. Skype reportedly uses non-proprietary, widely trusted encryption techniques: RSA for key negotiation and the Advanced Encryption Standard to encrypt conversations. Skype provides an uncontrolled registration system for users with no proof of identity. Instead, a free choice of nicknames permits users to use the system without revealing their identity to other users. It is trivial to set up an account using any name; the displayed caller's name is no guarantee of authenticity. A third party paper analyzing the security and methodology of Skype was presented at Black Hat Europe 2006. It analyzed Skype and found a number of security issues with the current security model.

    Skype incorporates some features which tend to hide its traffic, but it is not specifically designed to thwart traffic analysis and therefore does not provide anonymous communication. Some researchers have been able to watermark the traffic so that it is identifiable even after passing through an anonymizing network.

    In an interview Kurt Sauer, the Chief Security Officer of Skype, said, "We provide a safe communication option. I will not tell you whether we can listen or not." Skype's client uses an undocumented and proprietary protocol. The Free Software Foundation (FSF) is concerned by user privacy issues arising from using proprietary software and protocols and has made a replacement for Skype one of their high priority projects. Security researchers Biondi and Desclaux have speculated that Skype may have a back door, since Skype sends traffic even when it is turned off and because Skype has taken extreme measures to obfuscate their traffic and functioning of their program. Several media sources have reported that at a meeting about the "Lawful interception of IP based services" held on 25 June 2008, high-ranking but not named officials at the Austrian interior ministry said that they could listen in on Skype conversations without problems. Austrian public broadcasting service ORF, citing minutes from the meeting, have reported that "the Austrian police are able to listen in on Skype connections". Skype declined to comment on the reports.

    The United States Federal Communications Commission (FCC) has interpreted the Communications Assistance for Law Enforcement Act (CALEA) as requiring digital phone networks to allow wiretapping if authorized by an FBI warrant, in the same way as other phone services. In February 2009 Skype said that, not being a telephone company owning phone lines, it is exempt from CALEA and similar laws which regulate US phone companies, and in fact it is not clear whether Skype could support wiretapping even if it wanted to. According to the ACLU, the Act is inconsistent with the original intent of the Fourth Amendment to the U.S. Constitution; more recently, the ACLU has expressed the concern that the FCC interpretation of the Act is incorrect.

    On 20 February 2009 the European Union's Eurojust agency announced that the Italian Desk at Eurojust would "play a key role in the coordination and cooperation of the investigations on the use of internet telephony systems (VoIP), such as 'Skype'. ... The purpose of Eurojust’s coordination role is to overcome the technical and judicial obstacles to the interception of internet telephony systems, taking into account the various data protection rules and civil rights"

    Service in the People's Republic of China

    Since September 2007, users in China trying to download the Skype software client have been redirected to the site of TOM, a joint venture between a Chinese wireless operator and Skype, from which a modified Chinese version can be downloaded. The TOM client participates in China's system of Internet censorship, monitoring text messages between Skype users in China as well as messages exchanged with users outside the country. Niklas Zennström, then chief executive of Skype, told reporters that TOM "had implemented a text filter, which is what everyone else in that market is doing. Those are the regulations." He also stated: "One thing that’s certain is that those things are in no way jeopardising the privacy or the security of any of the users." In October 2008, it was reported that TOM had been saving the full message contents of some Skype text conversations on its servers, apparently focusing on conversations containing political issues such as Tibet, Falun Gong, Taiwan independence, and the Chinese Communist Party. The saved messages contain personally identifiable information about the messages' senders and recipients, including IP addresses, usernames, land line phone numbers, and the entire content of the text messages, including the time and date of each message. Information about Skype users outside China who were communicating with a TOM-Skype user was also saved. A server misconfiguration made these log files accessible to the public for a time.

    Customer service

    There have been complaints about Skype's customer support. As of May 2010, Skype does not provide any official means to contact customer support, apart from indirect assistance through its web portal only and contact email addresses.

    There is no e-mail or phone number for complaints about billing errors. It is not clear, in case a dispute occurs, which country would be the jurisdiction for conflict resolution.

    In January 2010 Skype rescinded their policy of seizing funds in Skype accounts that have been inactive (no paid call) for 180 days. This was in settlement to a class action lawsuit. A settlement of up to US$4 was paid to persons who opted in to the action.

    Skype's refund policy states that they will provide refunds in full if a client has used less than 1 euro of their Skype Credit. "Upon a duly submitted request, Skype will refund you on a pro rata basis for the unused period of a Product".

    Outages and downtime

    On 16 August 2007, Skype became unavailable to a majority of its users. Millions of users were requesting to log-in at the same time following a routine Windows update and this flooded the peer-to-peer system. The event lasted for about two days.

    On 22 December 2010, it was reported that Skype experienced an outage estimated to represent 8 million foregone calls. Skype administrators placed the following message on their Twitter page: "Some of you may have problems signing in to Skype -- we're investigating, and we're sorry for the disruption to your conversations...engineers and site operations are working non-stop to get things back to normal." The problem came down to the limited availability of "supernodes", affecting some versions of Skype. Later in the day, Skype's CEO, Tony Bates, issued an apology. He said that the matter was being taken very seriously and was being thoroughly investigated. He had said that malicious attack was not being ruled out as a problem cause and he put his estimate of the foregone calls at about 10 million. On 23 December 2010, Skype's blog with Tony Bates issued another apology through a video posted on YouTube for the Skype outage. The problem persisted across the North America, Latin America, Europe and Asia. It had started to subside by 8:30 pm UTC; however users were warned it might take several hours for the program to be fully operative again. It was reported at 9:27 am 23 December UTC that most users were able to log in again although some still could not. On the other hand into Thursday only about a third of the expected traffic was actually using Skype, relative to the time-period. On 23 December 2010, Skype said on its blog that the system had stabilized, and a detailed explanation of the incident was published six days later. Bates offered all Pay As You Go and Pre-Pay customers a free call to any landline as compensation. Subscribers had their subscription extended by one week.

    Now Skype is one of the most popular applications, which allows users to make calls over the internet, being used in possibly all types of operating systems. The development of this application is continuing and soon may be we will see Skype 6.0 or Skype will 'dead' and companies will develop other applications, which are more convenient for using these functions.




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    Timofeev Kirill Group 205

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