October 25, 2005

An Idle Back of the Envelop Computation

We all know that Skype uses P2P technology and heard the claim that this has allowed them to scale very inexpensively. The crux of the idea is to conscript a few, select users to act as “supernodes” and “relay nodes” that assist other users during establishment of calls and for traversing NATs. People have theorized and have collected empirical evidence that Skype has seeded the network with a few of their own supernodes. I have a suspicion that eBay may conclude, for corporate and commercial reasons, it will play safe and deploy all the needed supernodes and host them in their data centers. So I wanted to find out how expensive would that be.

Previously Zennstrom had stated that a conscripted supernode will manage about 100 clients. Tithing suggests that such a supernode will use one tenth of the computer’s resource. The modern religion, also called secular government, takes away about one fifth of its citizens’ income. So we can assume that a computer that is dedicated only to be a supernode can support 500 to 1000 users. Let us assume that the cost of a computer is $1000. Currently Skype claims it has 60 million registered users and at any time about 4 million users are logged onto Skype. If we use this ratio, then it costs around 6 to 13 cents to add a user.

We are also told that a supernode consumes 5 Kbps. I think there is a typo and that the bandwidth consumption is 5 KB/s. This is for 100 users. Hence a dedicated computer needs a net connection of 25 to 50 KB/s and will consume about 4 MB per month. I contend that even the operational cost is marginal.

In short, it is neither capital intensive nor cost much operationally to deploy and operate supernodes for Skype. More importantly, this is not unique to Skype. A SIP based system can also be deployed using a similar architecture.

Posted by aswath at October 25, 2005 11:40 PM
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I once worked for a P2P company, and we employed our own "supernodes" in the field too. They played a variety of roles, amnog them, act as a distribution point for latest upgrades to the desktop P2P software. So, yes, this is not unique to Skype.

Posted by: Manoj Sati at October 26, 2005 11:56 AM


There is a company called "damaka" (http://www.damaka.com) that have figured out a better way to crack the P2P puzzle. They have completely eliminated the concept of supernodes and thus removed any dependency/cost involved in maintaining such relays.

Using supernodes (at eBay or anywhere else) to relay data is potentially dangerous and can be considered unethical by some. This explains why Skype was not able to penetrate the enterprise market.

According the Podcast by damaka's senior advisor Alastair explains how they are different from (better than) Skype. damaka uses the concept of "direct peering". (Check ou tthe podcast at http://www.damaka.com/news.htm)

I have used damaka with my friends in the US and in UK, Canada and Australia. Its audio quality is much better than Skype. And they claim to be completely standards-based. Give it a shot!

...may the force be with you...

Posted by: sippedoutyoda at October 31, 2005 12:32 PM


I have known damaka, but haven't quite figured out how they are doing what they claim they are doing. For example, in the podcast, we are told that initially the ysers register with a central server and at that time the client is informed of the buddies' status. So I presume that subsequently the clients query each other. But then, when the host commented that you offer presence service, the guest agreed. So I am not sure.

They say and you seem to suggest that there is no need for relay nodes ever. The details are hidden due to the pending patent. Unfortunately, the application is not yet available at the patent office for public viewing. But without the details, it is not clear how one can assure "ALL" NAT/FW can be traversed. Remember, Skype maintained this as well; but later on others narrated the restrictions. I firmly beleive that either you need relay nodes (at least temporarily) or you have to do something special somewhere. So I remain skeptical.

Posted by: Aswath at October 31, 2005 03:13 PM

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