use of network resources. DNS information is used in almost every node-to-node session initiation. Its use is so pervasive that Figure 4-3 on page 40 would be made very difficult to understand if lines were drawn to the DNS from every node that starts sessions. Those lines have, therefore, been omitted. 4.2.2 Use cases In this section, we discuss some grid access use cases that relate to access aspects of the solutions described in Chapter 8, "Use cases" on page 133. Security Single sign-on: Users need to sign on to the grid portal. The grid portal authenticates the user ID and password by querying the security server's user directory. At the same time, it retrieves the user authorization information. Extended enterprise single sign-on: If the user is from another security domain within the enterprise, there are two main possibilities. If the user already logged in to the single sign-on facility in the other domain, the portal validates the user's credentials by interacting with the security integration server of the other domain through the security server of this domain - all without another user ID and password challenge. If the user was not signed on, the portal presents a user id and password challenge, which the security server in this domain will validate