Skype's video-calling service is made possible by a distributed peer-to-peer (P2P) architecture, in which calls flow over the user network. Users' computers are sometimes turned into supernodes, which serve as directories and routing points. When problems happen, Skype can create mega-supernodes — which are mounted on dedicated servers — to provide a resilient point from which to resurrect the network.
For the Facebook tool, however, Skype is creating and placing supernodes in datacentres and the Amazon Web Service (AWS) Elastic Compute Cloud (EC2) cloud, in a bid to deal with the large amount of traffic Facebook is expected to throw at the service.
"It's the same underlying technology, but the supernodes are hosted on dedicated servers," Jonathan Rosenberg, Skype's chief technology strategist, told ZDNet UK on Thursday.
Previously, the company created server-backed supernodes only to deal with outages to the service, he noted. For example, when the video-calling service went down at Christmas, Skype manually created mega-supernodes to bring the network back online.
The Facebook-related set-up is "definitely the largest usage of supernodes we've had outside a recovery situation", Rosenberg said.
Building an architecture
Because Facebook's infrastructure is not based on P2P technology, the companies have had to build an architecture that allows the social network to bring its users into Skype's infrastructure without the use of a desktop client.
To do this, they are providing a 3MB Skype plug-in; this takes a video call to the edge of Facebook's infrastructure, initiates an anonymous Skype instance and then routes the call via a Skype-owned dedicated supernode.
The way it works is there is a fairly clean separation of Skype from Facebook.– Jonathan Rosenberg, Skype
"The way it works there is a fairly clean separation of Skype from Facebook," Rosenberg said.
Every Facebook video call is connected to a dedicated supernode, he explained, at which point the call is routed through the Skype global network. "There's no peer-to-peer technology in Facebook's site at all," he noted.
"Facebook's servers interact with Skype's technology through a proprietary Rest API that we have developed," he wrote. "Through that API, Facebook automatically creates an anonymous Skype account for new callers (or 'callees').
"Our Rest API also allows Facebook servers to obtain a login credential that can be used to log the user into Skype using the anonymous account," he added.
For its part, Facebook is handling the distribution and management of the Skype plug-in. In addition, the social-networking company is storing "some data" in its own databases, Rosenberg said.
He added that the greatest challenge for Skype was scaling the service to meet huge demand — a problem that Google has also experienced. "[We've] designed the system under the assumption we can double the Skype user base in a very short period of time," he said. "Even though we're still using our P2P network for voice and video traffic, we are talking about a lot of users."
Another hurdle was dealing with users who have Facebook open in multiple browser tabs or windows at once. Getting over this required some major redesigns, according to Rosenberg, as Skype found that the multiple Facebook pages and tabs were talking independently to the people on each end of the call.
However, the project saw benefits as well as challenges. Skype took the lessons it learned from squeezing the video-calling elements of its 20MB desktop client into Facebook's 3MB plug-in and used them to increase the efficiency of its desktop client. These changes have been integrated into the latest versions of the client, but will be invisible to end users, Rosenberg said.
Skype made the deal with Facebook, despite having to pay for most of the IT overhead of the video-calling tool, because it hopes users will convert from Facebook into Skype proper.
"What this partnership brings to us now is a chance to use the Facebook user base and ecosystem as a [method of] distribution for Skype products," Rosenberg said.
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