Skype supernodes sap bandwidth
- 24 October, 2005 21:00
Corporate network managers beware — Skype might be the next big thing but for now your users could very well be sapping your network's strength.
A recent column in Computerworldby Kevin Tolly on the Skype peer-to-peer internet voice service drew a large number of reader responses.
In his article, Tolly says that a VoIP call over Skype uses up between 24 and 128kbit/s service. This is the same as what Skype itself specifies. However, Tolly found that when a Skype "station" was used as a relay, the bandwidth usage doubled. If Skype is installed by a large number of users on a corporate network, the amount of bandwidth consumed could therefore be considerable — and it would be used by Skype users outside the corporate network.
Reader Julian Bond wrote in to say that the relay traffic in question only occurs if the Skype PC becomes a "supernode". This is only possible the PC is connected directly to the internet without an intervening firewall. Even then, Bond says that Skype only relays signalling and not voice traffic, to a maximum of 40kbit/s in both directions.
Skype's official End-User Licence Agreement (EULA) no longer mentions supernodes, but now says merely:
"4.1 Permission to utilise your computer. In order to receive the benefits provided by the Skype Software, you hereby grant permission for the Skype Software to utilise the processor and bandwidth of your computer for the limited purpose of facilitating the communication between Skype Software users. "
However, earlier descriptions of the supernodes say their main purpose is to help Skype traverse Network Address Translation (NAT) and firewalls. Supernodes are computers running Skype and have IP addresses routable on the internet. These then act as "go-betweens" for communications between Skype users behind NAT and firewalls, who would normally not be able to talk to each other. The deliberate "firewall busting" means Skype violates some organisations' security policies.
Although he says he doesn't mind his computer being used as a Skype supernode from time to time, Bond says he has noted a serious problem when this happens. According to Bond, when his computer goes into supernode mode, Skype opens up a great number of network connections over TCP (Transmission Control Protocol, part of the Internet Protocol suite). In a posting on the Skype user forum bulletin board, Bond describes how his Linksys router was overwhelmed by over 1,500 TCP connections being opened in supernode mode.
As there is no way presently to limit the number of connections Skype opens up, Bond had to turn off Skype and has since firewalled incoming connections to it. Others users say that by unticking the "use port 80 and 443" options in the Skype Tools menu, the computers will not become supernodes.
A Skype representative posting on the forum confirmed that some routers have problems with a large number of TCP connections being opened simultaneously.
Testing by Computerworld showed that Skype starts up a large number of TCP and UDP connections when it starts up, but the supernode issue didn't occur. However, reports elsewhere warn against using Skype on connections with high bandwidth and IP addresses routeable on the internet. In supernode mode, Skype is reputedly able to saturate 100Mbit/s connections. Universities with high-speed connections often ban Skype usage because of this.
Skype was contacted for comment on the supernode issue, but didn't respond before going to press.
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