Skype
From Wikipedia, the free encyclopedia.
 |
|
|---|---|
 Skype on Windows XP |
|
| Maintainer: | Skype Technologies S.A. |
| Stable release: | 1.4.0.84 (October 25, 2005) [+/-] |
| Preview release: | none (?) [+/-] |
| OS: | Cross-platform |
| Genre: | P2P |
| License: | Freeware |
| Website: | www.skype.com |
Skype (IPA pronunciation: [skaɪp] to rhyme with type) is a proprietary peer-to-peer internet telephony (VoIP) network, founded by Niklas Zennström and Janus Friis, the creators of KaZaA and competing against established open VoIP protocols like SIP or H.323. The Skype Group is headquartered in Luxembourg with offices also in London and Tallinn. The system has a reputation for working across different types of network connections (including firewalls and NAT) because voice packets are routed by the combined users of the free desktop software application. Skype users can speak to other Skype users for free, call traditional telephone numbers for a fee (SkypeOut), receive calls from traditional phones for a fee (SkypeIn), and receive voicemail messages for a fee.
In a deal generally criticised by many as overpriced, eBay has recently acquired the company for $US 2.6 billion in cash and stock, plus an additional 1.5 billion in rewards if goals are met by 2008. [1]
Contents |
Features
The basic computer-to-computer service allows users to speak, to send instant messages or to send files to one another from their computers via the Internet at no cost. Conferences of up to five users are supported.
SkypeOut is a paid feature of the Skype internet telephony service, which allows Skype users to call virtually any non-computer-based landline or mobile telephone in the world.
Unlike international long-distance calls made from conventional telephones, which are rated according to the relative distance between countries, SkypeOut bills all calls according to the relative prosperity of the country, the volume of calls made from and to a given country, access charges such as those to mobile (cell) phones. This rating method is used due to the fact that calls are rated the same regardless of place of origin.
This way, a SkypeOut user will be billed the same for a call placed to a telephone number in London, whether the user is calling from his/her computer in London itself, or from a different country. The Global Rate which is the one used for many countries, and all First World countries, is currently 1.7 euro cents per minute. Credit, which is necessary to use SkypeOut, is usually purchased via credit card, personal check or money order, or online services such as PayPal or Moneybookers. The current deposit is either €10 or €25, which automatically expires after 180 days of inactivity — a timer is reset after each successful SkypeOut connect. In European countries VAT is added to the charge, which makes the actual prices closer to €0.02 per minute.
SkypeIn allows Skype users to receive calls on their computers dialed by regular phone subscribers to regular phone numbers. Beta released on March 10, 2005, SkypeIn permits users to subscribe to numbers in UK, USA, France, Hong Kong, Denmark, Finland, Norway, Sweden, Estonia and Poland. Cost is €30 for a 12-month subscription, or €10 for a three-month subscription. Users can have multiple SkypeIn phone numbers bound to the same account (a separate subscription is required for each). For example, someone who does business in both San Francisco and Helsinki could create a local telephone number in each city, and callers from those locations would pay cheap or free local rates.
Voicemail was also released on March 10, 2005. This service allows callers to leave voice-mail messages for Skype users who are not online, on another call or otherwise indisposed. This can be purchased separately and is automatically bundled in with SkypeIn.
Skype Version 1.2 for Windows was released in March 23, 2005. Its most significant new feature is the provision of centrally-stored contact lists so that a user's contact information is available from any computer that is connected to Skype (in other previous versions, contact information was stored on the local computer).
Versions now exist for Microsoft Windows (Windows 2000, Windows XP and Windows CE (Pocket PC)), Mac OS X and GNU/Linux. The Linux version runs on FreeBSD through its Linux binary compatibility.
Technology
Each Skype user must have the Skype software running on his/her computer. This software is available for free and can be downloaded from the company website.
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. The Skype user directory is entirely decentralised and distributed among the nodes in the network, which means the network can scale very easily to large sizes (currently just over forty million users) without a complex and costly centralised infrastructure.
Skype also routes calls through other Skype peers on the network, which allows it to traverse Symmetric NATs and firewalls, unlike most other VoIP programs (The two most common VoIP protocols, SIP and H323 are usually UDP and point-to-point, making NAT traversal problematic; see article). This, however, puts an extra burden on those who connect to the Internet without NAT, as their computers and network bandwidth may be used to route the calls of other users. The selection of intermediary computers is fully automatic, with individual users having no option to disable such use of their resources. This fact is not clearly communicated, however, and seems to contradict the license agreement which would limit Skype's utilisation of the user's "processor and bandwidth [to the] purpose of facilitating the communication between [the user] and other Skype Software users" (section 4.1).
The Skype code is closed and the protocol is proprietary which has raised suspicion and drawn broad criticism from software developers and the voip user communities.
The Skype client's application programming interface (API) exposes the network to software developers. The Skype API allows other programs to use the Skype network to get "white pages" information and manage calls.
The program was developed on Pascal Programming Language using Delphi and later it was ported to Linux using Qt and to Mac OS X using its native application programming interface (API). [2]
GIPS iLBC CODEC
iLBC (internet Low Bit-rate Codec) is a free speech codec suitable for most voice communication over IP. The codec is designed for narrowband speech and results in a payload bit rate of 13.33 kbit/s with an encoding frame length of 30 ms and 15.20 kbit/s with an encoding length of 20 ms. The iLBC codec enables graceful speech quality degradation in the case of lost frames, which occurs in connection with lost or delayed IP packets.
Features
- Bitrate 13.33 kbit/s (399 bits, packetised in 50 bytes) for the frame size of 30 ms and 15.2 kbit/s (303 bits, packetised in 38 bytes) for the frame size of 20 ms
- Basic quality higher than G.729A, high robustness to packet loss
- Computational complexity in a range of G.729A
- Royalty-free codec
Security
Generality
Since the Skype code is proprietary and closed source the security of the software cannot be readily established. When run on Microsoft Windows, Skype binds to three ports on the user's computer and directly manipulates Windows XP's built-in firewall to accommodate these network bindings.
Skype's file transfer function does not contain any programmatic interfaces to antivirus products, but Skype claims to have tested its product against antivirus "Shield" products. If the EICAR test file is sent over Skype's file transfer service, every major antivirus product appears to catch the virus and halt its transmission or reception via Skype.
Unlike other applications, Skype polls the hard disk several times per minute. This can be verified either by observing the HDD led or by using a file access monitor such as Filemon. Although those accesses are small, extremely fast and safe in the short term, they can be extremely harmful in the long term. In particular the continuous access pattern does not allow the disk to enter sleep or idle modes while Skype is active, even when offline. This can severely reduce the lifespan of the HDD when Skype is running for a long time. Stronger HDD caching does not seem to improve the situation.
Confidentiality of Data
Since a Skype connection may be routed through an intermediate peer, 256-bit AES encryption actively encodes the data stream of each call, file transfer or instant message. Skype uses 1536-bit RSA (2048-bit RSA for customers who have purchased any "paid services" such as voicemail) to secure the pairwise negotiation of an AES symmetric session key over an untrusted channel. The proprietary session establishment protocol is efficient and prevents both man-in-the-middle and replay attacks. The Skype server certifies each user's public key at log in.
Integrity/Authenticity of Data
The integrity of the data, i.e data modified while traveling though peers, even if encrypted, is unknown and undocumented.
Authenticity of user identity
Skype provides an uncontrolled registration system for users: registration requires no proof of the identity of the user at all. This works two ways: you can use the system without revealing your identity to other users of the system, but on the other hand you have no guarantees that the person you communicate with is the one he says he is. The down side of this is that it is easy to use the identity of a trusted person and trick a user to reveal information or execute the little program sent to him.
History
- April 23, 2003: Skype.com and Skype.net domain names registered
- August 29, 2003: First public beta version released
- June 15, 2004: Beta release of version 0.98.0.28 with first support for SkypeOut. Credits by voucher only.
- June 27, 2004: SkypeOut credits first available for purchase on Skype website.
- July 27, 2004: Release of Version 1.0 for Windows.
- October 20, 2004: First time 1 million Skype users are online at once.
- February 14, 2005: First reached 2 million online.
- March 10, 2005: SkypeIn Public Beta starts.
- March 11, 2005: Skype press release reports 1 million Skype-out users and 29 million registered users.
- March 11, 2005: Software has been downloaded 84 million times and 5.98 billion talk minutes served.
- April 15, 2005: Downloaded more than 100 million times.
- May 18, 2005: Three million online at once.
- June 19, 2005: Ten billion minutes of voice conversation served.
- August 31, 2005: Skype launches the new "1.4 beta" containing improved sound and call forwarding.
- September 2005: SkypeOut Banned in South China.
- September 12, 2005: eBay announces purchase of Skype
- October 18, 2005: eBay completes purchase of Skype (announcement)
Skype business ecology
Distribution partners
Skype has partnered with online web properties including Tom.com, PcHomeOnline, Daum, Livedoor and Onet (http://www.onet.pl/) and hardware manufacturers including Plantronics, Logitech, Motorola, VTech, RTX, Siemens and Linksys.
Third Party Software products
Festoon (official website) is a Skype add-on product for Windows computers (2000 or XP with Internet Explorer 5+) that "adds video and sharing to Skype. Built on top of the Skype messaging platform, Festoon enables Skype users to securely conduct video calls in groups from 2 to 200 and share applications, spreadsheets, presentations, or photos with others on a call" (from official website).
HansaWorld 4.3 attempts to allow users the seamless integration of Skype with ERP and CRM in business.
Usage
SR Consulting surveyed 4 million Skype user profiles in October 2005. They produced some demographic information reported by Mathaba.net and Skype Journal. Some findings:
- Average age: 29.7 years old.
- About 46% of Skypers are in Europe, but Brazil and China have the most Skype users of any country, each with 8.1% of the Skype population.
- Gender information is inconclusive so far. More than half of all users declined to state their sex.
Criticisms
As of July 2005, the line quality varies from excellent (comparable to or even better than traditional telephony) to barely usable. The main drawbacks are:
- time lag, often around 0.5 seconds, but occasionally up to two or three seconds, apparently caused by distant remote routing or the low bandwidth of either or both parties;
- dropout and fizz;
These problems can be minimised by a properly set firewall, so the user doesn't have to rely on relays to route a call. This is however sometimes impossible, especially in corporate or university environments, where users can't change firewall settings, or when the user is behind NAT router. The issues are common for all VoIP applications; in fact in restricted networks many of them don't work at all, while Skype is at least able to connect through third party relay host.
A broader criticism leveled at Skype is over its use of a proprietary protocol, instead of an open standard like H.323 or SIP, making it difficult for other providers to interact with the Skype network. There are of course clear business and technical reasons for this, such as protecting the SkypeOut revenue stream, and the poor interaction of open IP Telephony protocols with firewalls and NAT devices, increasingly common in homes (a large portion of Skype's userbase).
If given access to an unrestricted network connection, Skype clients can become supernodes. These supernodes hold together the peer-peer network and provide data routing for those behind restrictve firewalls. Unfortunately these supernodes can generate a significant amount of bandwidth -- saturating a high speed, 100Mbps connection is not unheared of. For this reason some network providers, such as universities, have banned Skype.
Prohibitions and warnings
Legal and other barriers have been erected by companies, government regulators, and school systems. Reasons given include a perceived threats to an economic interest, to national or enterprise security, to system reliability.
China 2005
SkypeOut was recently blocked in some regions of mainland China (notably Shenzhen) by the operator China Telecom for undisclosed reasons, believed to relate to SkypeOut's ability to take lucrative international and long distance business away from the People's Republic of China’s state controlled telecoms companies. Alternate theories include China's inability to monitor content and control communications over Skype.
France 2005
In September 2005, the French Ministry of Research, acting on advice from the general secretariat of national defense, disapproved the use of Skype in public research and higher education; some services are interpreting this decision as an outright ban. The exact reasons for the decision were not given, but computer security professionals point out that:
- Skype is a proprietary software program using undocumented protocols, and laws prohibit reverse-engineering it;
- Skype implements some kind of "peer-to-peer" network over client machines, with clients on fast connections becoming major exchange points; since research centers typically have very high speed connections, machines running Skype in those centers may generate very high traffic; some networks were reportedly nearly saturated by Skype traffic;
- the information flow implemented by Skype is unknown; though encryption is used, it is unknown where traffic goes.
As such, Skype is considered a security hazard for research networks, in which there may be significant intellectual property.
Germany 2005
Vodafone Germany plans to disable calls from Skype and net phone operators beginning in 2007. [3].
Skype vs Traditional phone companies
Phone companies have traditionally charged users a large amount, often proportional to the distance, for long distance calls. Skype, arguably the first major VoIP software, allowed people to talk over the internet for free. This led to many home users with broadband capability to switch to Skype for placing their calls over the internet. Skype being secure and encrypted end-to-end, has also attracted large corporations who are beginning to switch from their traditional phone companies for their internal calls. Phone companies were all of a sudden out of favor in the markets which patronized Skype.
See also
- Comparison of instant messengers
- Voice over IP
- Gizmo Project: A closed source VoIP application based on SIP open standard
- OpenWengo: An open source VoIP application based on SIP open standard
- Skype Journal - An independent online magazine about Skype
