Internet Protocol - Research by Kirils Solovjovs

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Skype protocol 76 • The external port numbers or IP address are not derivable, because NAT rewrites them, • The firewall and NAT in use prevents the session being received • UDP is not usable due to NAT issues, such as timeout • firewalls block many ports • TCP through many to one NAT is always "outward only" by default - Adding Port Forwarding settings to the NAT router can allow receiving TCP sessions Supernodes are grouped into slots (9-10 supernodes), and slots are grouped into blocks (8 slots). Protocol Signaling is encrypted using RC4; however, the method only obfuscates the traffic as the key can be recovered from the packet. Voice data is encrypted with AES. [4] The Skype client's application programming interface (API) opens 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 Skype code is closed source, and the protocol is not standardized. [5] Parts of the client use Internet Direct (Indy), an open source socket communication library. 8 July 2012, a searcher from Benin, Ouanilo Medegan, released articles and a proof of concept client source code, results of reverse engineering on the Skype client [6] . Protocol detection 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 proprietary, Pearson's chi-squared test and stochastic characterization with Naive Bayes classifiers are two approaches that were published in 2007. [7] Preliminaries Abbreviations that are used: • SN: Skype network • SC: Skype client • HC: host cache Skype client The main functions of a Skype client are: • login • user search • start and end calls • media transfer • presence messages • video conference
Skype protocol 77 Login A Skype client authenticates the user with the login server, advertises its presence to other peers, determines the type of NAT and firewall it is behind and discovers nodes that have public IP addresses. To connect to the Skype network, the host cache must contain a valid entry. A TCP connection must be established (i.e. to a supernode) otherwise the login will fail. 1. start 2. send UDP packet(s) to HC 3. if no response within 5 seconds then 4. attempt TCP connection with HC 5. if not connected then 6. attempt TCP connection with HC on port 80 (HTTP) 7. if not connected then 8. attempt TCP connection with HC on port 443 (HTTPS) 9. if not connected then 10. attempts++ 11. if attempts==5 then 12. fail 13. else 14. wait 6 seconds 15. goto step 2 16. Success After a Skype client is connected it must authenticate the username and password with the Skype login server. There are many different Skype login servers using different ports. An obfuscated list of servers is hardcoded in the Skype executable. Skype servers are: • dir1.sd.skype.net:9010 • dir2.sd.skype.net:9010 • dir3.sd.skype.net:9010 • dir4.sd.skype.net:9010 • dir5.sd.skype.net:9010 • dir6.sd.skype.net:9010 • dir7.sd.skype.net:9010 • dir8.sd.skype.net:9010 • http1.sd.skype.net:80 • http2.sd.skype.net:80 • http3.sd.skype.net:80 • http4.sd.skype.net:80 • http5.sd.skype.net:80 • http6.sd.skype.net:80 • http7.sd.skype.net:80 • http8.sd.skype.net:80 Skype-SW connects randomly to 1-8. On each login session, Skype generates a session key from 192 random bits. The session key is encrypted with the hard-coded login server's 1536-bit RSA key to form an encrypted session key. Skype also generates a 1024-bit private/public RSA key pair. An MD5 hash of a concatenation of the user name, constant string ("\nSkyper\n") and
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Internet 7 Internet, though they ma
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Internet 9 Services World Wide Web
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Internet 11 Digital media streaming
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Internet 13 In an American study in
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Internet 15 Internet, Berdal also n
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Internet 17 [32] Walter Willinger,
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Internet 19 • "The Internet sprea
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Internet protocol suite 21 The netw
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Internet protocol suite 23 Layers i
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Page 35 and 36: OSI model 33 Parallel SCSI buses op
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Page 39 and 40: OSI model 37 5 Session ISO/IEC 8327
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Page 83 and 84: Skype protocol 81 • L. De Cicco,
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Page 103 and 104: Hostname 101 rr.pmtpa.wikimedia.org
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Page 107 and 108: End-to-end principle 105 [5] Saltze
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Internet Protocol - Research by Kirils Solovjovs

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