Skype protocol

~ the obit ~

Skype's first public beta shipped on 29 August 2003, and the thing underneath it was not SIP and not anything else anybody could read. The protocol was proprietary and undocumented, encrypted end to end at a time when that was rare, and the official clients were all closed source. It did not interoperate with the rest of voice over IP, and Skype never published a specification for anyone who wanted to try.

The engineering came out of Kazaa. The same Estonian team that had built FastTrack took the peer-to-peer idea and pointed it at telephony instead of file sharing, which is why the architecture reads like a file-sharing network with voice bolted on. Ordinary nodes talked to supernodes, and behind both sat a login server that was the only centralized piece in the whole design.

Supernodes are the part worth remembering. Any client with a public IP address and enough bandwidth could be promoted into one without being asked, and once promoted it carried directory data and helped punch calls through other people's firewalls. The user directory was not stored anywhere in particular. It was distributed across those machines in slots and blocks, which meant there was no single box an operator or a government could switch off to stop Skype working.

That property is why I remember it as an engineering object rather than as an app. Around 2005 in Turkey, the gap between what a call to Germany cost on a landline and what it cost on Skype was not a discount, it was the difference between paying and not paying. The reason it was free sat in the protocol. Anybody on a decent connection noticed the upstream traffic eventually. On the asymmetric ADSL lines TTNet was selling then, upstream was the scarce half, and discovering that your machine had quietly volunteered to relay somebody else's conversation was a specific kind of annoyance.

~ the rap sheet ~

~ the reverse engineers ~

A closed protocol with that many users is an invitation. Researchers spent years pulling it apart, partly to check whether the encryption claims held up and partly to build clients Skype had not authorized. Efforts to document the wire format turned up repeatedly through the 2000s, and in 2011 a researcher published a reverse-engineered description that got legal attention almost immediately.

Third-party clients that managed to speak it did not last. In July 2010 the mobile app fring shipped iPhone video calling by going over Skype's protocol, and Skype cut off its access inside a week while accusing it of unauthorized API use. Nobody outside the company got further than fring did.

~ the move to the datacenter ~

Microsoft bought Skype in 2011, and in 2012 it quietly replaced the user-run supernodes with Linux servers in its own datacenters. The official reason was performance and scalability, and that was true as far as it went. It also turned a network with no center into a network with one, which changed what was possible to do with it and who could be asked to do it. Security researchers said so at the time.

After that the protocol was a peer-to-peer design with nothing peer to peer left in the middle of it. The clients still spoke the old wire format for another two years, talking to Microsoft hardware at the other end.

~ what replaced it ~

On 20 June 2014 Microsoft announced the old protocol was being deprecated. Anyone still on an older client had to move to the 2014 release or stop logging in, and by the second week of August the replacement was in place: Microsoft Notification Protocol 24, a cloud design with proper offline messaging and message sync across devices, which the peer-to-peer version had never handled well. Skype the product carried on for another decade and did not switch off until 5 May 2025. The protocol that made it worth using had been gone for eleven years by then.

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