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Skype Network Administrator’s Guide Skype 3.0 Beta 14

2006-10-31 Document version 2.0 Beta

When a Skype user logs in using a Skype name and password, the user’s Skype client

attempts to connect to a centralized resource; that is, the Skype authentication server. If

and when the authentication server validates the connection, it gives the user’s Skype client

a signed digital credential—signed using a private key which is maintained by Skype

Technologies S.A.

The public key required to verify another Skype user’s digital credential is maintained in

each Skype client. Signed digital credentials are valid for only a limited period of time. In

addition, Skype Technologies S.A., periodically renews them to further enhance security.

At the point when a Skype client gets a signed digital credential and can validate its

authenticity, the Skype client may (on behalf of a Skype user) present it to other Skype

clients. When the authentication process is complete, there is no reason for the recipient to

re-verify the authenticity of the caller’s credential by checking in with the authentication

server or any other piece of centralized infrastructure.

Note: To the Skype network, a Skype user is simply represented by a Skype name that has

been authenticated properly to the network. Although a Skype user typically runs only one

Skype client instance, an individual person can have multiple Skype accounts with unique

Skype names, passwords, and profiles.

How are Skype Sessions Established?

When a Skype user wants to communicate with another Skype user, each connection and

session are established uniquely.

While a Skype user is online, the user’s Skype client maintains a persistent connection to a

supernode. This enables presence on the Skype network; put simply, this is how one Skype

client is able to constantly inform other Skype clients of the user’s availability and on-line

status.

Moreover, when a Skype user attempts to communicate with others, the caller’s Skype

client checks with the global index—the distributed database of users that is maintained in

the hierarchy of supernodes—to see whether the intended recipient is, in fact, online,

regardless of how the recipient has set his or her on-line status.

Note: For brevity, the following sequence will not distinguish between voice / video calls,

instant messages (IMs), and file transfers. It will simply refer to a caller and recipient to

keep things simple.

If the intended recipient is online, the caller’s Skype client collects both the recipient’s

Skype client’s network address and the network address for the recipient’s Skype client’s

supernode from the global index.

Next, the caller’s Skype client attempts to make a direct connection to the recipient’s

Skype client. Assuming the Skype clients can connect directly to one another,

communication begins.

It is not always possible to establish a direct connection right away. This can happen, for

example, when a recipient’s Skype client is behind a firewall or NAT device. So, by

design, if the first attempt at a direction connection fails, the caller’s Skype client transmits

a message to the intended recipient’s client by way of supernodes.

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Skype - 3.0 beta Guide de l'utilisateur Page 14

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