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Skype is a proprietary VoIP protocol, evolving and not fully known. Skype uses many concepts of networks PeerToPeer, since developers are the same as Kazaa.

Skype appears to have many strengths to SIP, but also some weaknesses that make little use at farm level:
-It is not necessary to have public addresses for VoIP (NATs are not a problem).
-The quality of service is not a necessary prerequisite for Skype.
-You do not need expensive infrastructure to use it.
-He has shown that a P2P network may be useful for other purposes in addition to sharing files.
-The item can also be transmitted over TCP.
Hole-firewalls. This is possible thanks to TCP and different techniques:
-The call is often directed.
-Triangulation through relay (no loss of quality).
And echo-suppression breaks.
-Excellent management of the voice.
-It is free, but with encrypted code. E 'present a mechanism for obfuscation of the code.

Skype relies on clients and supernodes. And 'present a bootstrap server, useful for first connections (when a client does not have the index of supernodes in the cache).
The supernodes contain the index of the neighboring nodes, supernodes neighbors seek and exchange information. And 'possible to disable the ability to become a supernode. In some versions is also possible to force the choice of a supernode through editing a configuration file.
Skype also has numerous gateways in many countries, in order to allow call costs reduced even at fixed numbers.

Phase Boot
The client connects to one or more supernodes, saved to a local file. In the absence of a list or in the event of a failed connection you connect to a predefined set of nodes, called bootstrap servers.
The preferred protocol is UDP, although in the event of a fallback is TCP. The ports change often (to avoid being discovered by the firewall).

Phase of Login
To start the communication phase is required to login from a specific server.
A first attempt is made by contacting the server, while in case of bankruptcy, the login request is forwarded through the supernodes.
Is checked if the version used is the latest available.
You use a STUN-like protocol to determine if you are behind a NAT.

Search for a user
The search is done through the supernode to which you linked. A user is always localizable if it is connected in the last 72 hours.

Data exchange (VoIP)
Normally the connection is direct (in the case in which the hosts are public or only one is behind NAT). In the event that this is not possible the connection step is obtained by the triangulation thanks to supernodes. In this case the traffic is encrypted.
It 's also possible to exchange files, which is obtained in the same way of VoIP.

Transmission of voice
The codecs used are standard and occupy 3-16 Kbps banda There is no silence suppression and excellent management of the delay.
Compared to other mechanisms VoIP, Skype allows, as mentioned, the TCP communication.

More Features
Multiple-conference, through the use of a client HUB. It is therefore not required the set-up of additional nodes.
Instant-messaging, non-interoperable with other networks.
File-transfer, although with poor performance.

Blocking Skype
-When the communication is very difficult in that it uses either TCP and UDP any port (sometimes even uses port 80).
-During the authentication phase is difficult because all connections should be blocked with supernodes and all packets directed to the Bootstrap Servers.
-During login is impossible because bloccale should be made to provide overlay.
-During the exchange of data is difficult because it is encrypted and often direct.

Skype Issues
Skype is a protocol widely used in private, but rarely used in business, because:
-The code is closed and proprietary protocols (impossible to know how is the data).
-Interception problems (legal).
-L'overlay is not influenced by the user (not the user control on the recipient of its information).

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