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Skype Network Administrator’s Guide Skype 3.0 Beta 9

2006-10-31 Document version 2.0 Beta

Only a relatively small percentage of Skype nodes ever transform into supernodes, even

though supernode capabilities are built into every Skype client.

Where do the supernode’s additional capabilities come from? When the Skype client is

installed, only part of the application is visible to the end user. In addition to the portion of

the Skype client which the end user can see, the Skype software installation includes

functionality that evaluates the capabilities of the computer on which Skype is installed, as

well as its network connection to identify how capable the new resource might be to the

Skype network.

Under normal circumstances, a Skype client functions as an ordinary node in the Skype

network. However, a Skype client that is running on a high-performance computer with a

healthy connection directly to the Internet may, under certain conditions, “wake up” to

support the global index by acting as a supernode or relay host thus giving the Skype

network minor but essential additional capabilities above and beyond the simple

capabilities that end users are aware they actually need.

Skype Supernodes and Relay Hosts

When a Skype peer node comes alive as a supernode, it dynamically creates a local cluster

in a star-like pattern of up to several hundred peer nodes to leverage all the available

resources on the Skype P2P network.

Each supernodes holds the directory entries of up to several hundred Skype users. And

although supernodes need to accept a relatively small number of directory queries, they do

not actually carry content: voice, text, video, or file-transfer data. Supernodes are restricted

from using more than 5 kilobits per second of bandwidth.

Type of Traffic Bandwidth Limit Per Session

Signaling Info 5 Kbyte per second (strict)

Table 1. Supernode Bandwidth Limits Per Session

A relay host is similar to a supernode but is has a different role and purpose in the Skype

network. While each supernode functions as temporary directory index server for nearby

peer nodes in the cluster, a relay hosts acts as data-transfer relay station by connecting

Skype clients that are unable to connect directly with one another.

Relay hosts do carry Skype network traffic but they have strict limits in terms of how much

bandwidth they can consume on a per-session basis. Note also, that while in theory a relay

host can carry more than one relayed session, in practice this is uncommon and shouldn’t

have an impact on capacity planning.

Type of Traffic Bandwidth Limits Per Session

File Transfer 3 Kbyte per second (strict)

Voice call 4 kByte per second

Video call 10 kByte per second (strict)

Table 2. Relay Host Per-Session Bandwidth Limits

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 36 37

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Skype - 3.0 beta Guide de l'utilisateur Page 9