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Skype = internet telephony solution that is based on P2P principles



"Skype is a P2P internet telephony network that allows users to talk to other Skype users free. Unlike open VoIP protocols like SIP, IAX, or H.323 the Skype protocol requires no servers and has been found to work across different types of network connections including firewalls and NAT. In addition, Skype also allows users to call traditional telephone numbers or receive calls from traditional phones, and receive voicemail messages." (


Skype's encryption

"When you call another Skype user your call is encrypted with strong encryption algorithms ensuring you privacy. In some cases your Skype communication may be routed via other users in the peer-to-peer network. Skype encryption protects you from potential eavesdropping from malicious users... Skype encryption ensures that no other party can eavesdrop on your call or read your instant messages. Skype uses AES (Advanced Encryption Standard) – also known as Rijndel – which is also used by U.S. Government organizations to protect sensitive, information. Skype uses 256-bit encryption, which has a total of 1.1 x 1077 possible keys, in order to actively encrypt the data in each Skype call or instant message. Skype uses 1536 to 2048 bit RSA to negotiate symmetric AES keys. User public keys are certified by Skype server at login." (

See: Taliban Using Skype to avoid detection


Skype's use of proprietary P2P technology

From :

""Skype uses a different type of Peer-To-Peer network than most companies -- a proprietary form based on SuperNodes. A SuperNode P2P System is one in which you rely on your customers rather than your own servers to handle the majority of your traffic. SuperNodes are just normal computers which get promoted by the Skype software to serve as the traffic cops for their entire network. In theory this is a good idea, but it does have unique vulnerabilities that have been exposed during the past week. Skype, as a company, has no physical or programmatic control over the most vital piece of its product when the network destabilizes for any reason.

Another issue with SuperNode models concerns system recovery after a crash. A SuperNode-based network can only recover as fast as new SuperNodes can be identified." (

Skype is a closed calling network

Letter by Michael Robertson, the founder of SipPhone/GizmoProject to Skype:

"I recently saw your letter to FCC Chairman Kevin J. Martin demanding that wireless companies open their networks. While I concur this would be beneficial for consumers, Skype's actions do not mirror their words to the commission which diminishes credibility for Skype to demand openness.

I am CEO of Gizmo5, a standards based VOIP and IM network. We invite other networks to connect to our users and currently route calls to and from more than 500 networks big and small on the Internet. We cannot route calls to Skype users in spite of the fact that consumers would like to do this because Skype has a closed network.

The wireless companies you chastise in your letter to commissioner Martin allow me to send and receive calls to the networks they operate but Skype does not. Skype operates the largest closed calling network on the planet. We have requested peering information from Skype in the past on multiple occasions and our requests have been ignored. Skype continues to deny Gizmo5 and others in the internet calling world the information and access to allow calls to flow to and from your network.

It appears that when it is convenient for Skype's business objectives Skype waves the flag of openness, at the same time conveniently ignoring competitors requests for openness. It is disingenuous for Skype to demand mobile operators open their networks so that Skype can infiltrate their systems with their proprietary, closed calling scheme which locks out all others.

If Skype truly believes there should be open competition then they should start by enabling other networks such as Gizmo5 to call Skype users in an official and supported capacity. Until you remove the padlock from your own front door you would seem to have no right to demand that others adopt an open door strategy.

I'm open to discuss and implement an open peering relationship which will demonstrates Skype true commitment to open networks and make your actions match your words. Contact me anytime at [email protected]" (

Commentary on the August 2007 Skype outage

... and what it means for P2P Computing.

By Gwendal Simon:

"Skype is undoubtedly one of the most famous peer-to-peer systems over Internet. Skype sofwtare is so efficient that many users consider Skype as a casual phone company, although the system is actually based on mechanisms that were initially designed for file sharing systems. This small start-up leverages on known advantages of such architectures: scalability, self-healing ability, no (or few) required infrastructure...

Users were quite disappointed when the system experienced a major crash during two days in mid-August. Reported in the official Skype blog (, this bug emphasized the deficiency of Skype's self-healing property. Many experts have debated about this crash, however very few facts have been made public by the company which uses a close proprietary network protocol.

A nice reaction to this crash comes from Alen Peacock. He claims ( that the close proprietary model of Skype is the main cause of this crash. He convincingly argues that if Skype were open, many different programs would form an unique Skype system and this system would be far more reliable because one bug in one program could not affect the whole system. In other words, the self-healing property of a peer-to-peer system does not depend only on the decentralized architecture, but should also rely on a full distribution of the system.

We make here the parallel with a previous study on the Gnutella system where the diversity of actors was also shown as a positive characteristic ( In a classic market view, Gwendal Simon showed that the competition between several actors united in the same system is good for the system itself as it benefits from innovations from several actors. These competitors try to differentiate but they also try to make the system more attractive. This dilemma results, for Gnutella at least, in a positive evolution. As an example, the seminal mail in the Gnutella Developers Forum said : « The network effect implies that the more the network grows, the more powerful and valuable it will be. We have more to gain in cooperation than in competition. Lets try to keep this in mind as we have technical discussions. »

In both cases (self-healing and innovation), the diversity of actors appears to be positive. The competition between actors reaches a natural collaborative state as actors know that their main value comes from the network. The recent Skype outages emphasize another positive aspect of the diversity generated by open common system."


Skype, using P2P filesharing principles for telephony:

Zennström and Friis, the creators of KaZaa, one of the early and popular P2P filesharing systems, came up with the idea of using P2P to enable free phone calls on the internet, and Skype was born, poised for an extraordinary rapid update. Beyond phone calls, users have been creatively tinkering with it to enable audio and video broadcasts (i.e. Skypecasting). Excerpts from an interview in

Business Week:

"Q: Where else could this go, beyond files and people?

A: It could be other resources -- you know, storage, video streams. But this really works on two levels. First there's the peer network, and I've been stressing that because it's the enabler for everything. But then there are the applications. We could not have foreseen -- wow! -- all the things that could be developed on top of P2P. For instance, when we first used peer-to-peer technology, we didn't foresee that we could do voice. It became obvious to us after some time, but when we started we didn't know what the applications would be. But when we applied the technology to various industries, we realized we could create a sustainable competitive advantage. That's because, in the normal system you have a marginal cost for every unit you add. If your network is client/server-based, you have to add a new network card for each new Web server, central switch, and so on. But in a peer-to-peer network, you're reusing the system resources in the network, so the marginal cost of producing a phone call or a file transfer or something else is zero. " (;

More Information

An article explaining the rapid diffusion of Skype, at

Open Source telephony developments for the enterprise market are summarized here at,1759,,00.asp

PC to Phone services such as SkypeOut are reviewed by the New York Times, at; Among the competing services it mentions are :,

Google Talk vs. Skype VoIP, a comparison by Red Herring: