TCP/IP Tutorial and Technical Overview - IBM Redbooks

In order to find a resource in another cell, a communication path needs to exist
between the two cells. This communication path can currently be one of two
types:
► CCITT X.500
► Internet Domain Name Services (DNS)
In order for intercell communications to be accomplished, another component,
the Global Directory Agent, is required. The Global Directory Agent (GDA) is the
intermediary between the local cell and the Global Directory Service. In
Figure 13-8, if the CDS does not know the location of a resource, it tells the client
to ask the GDA for assistance. The GDA knows to which global namespace it is
connected and queries the GDS (either DNS or X.500) for the name of the
foreign cell directory server with which to communicate. When in direct
communication with the foreign cell directory server, the network name of the
resource requested can be found. The Global Directory Agent is the component
that provides communications support for either DNS or X.500 environments.
CDS
Cell A
GDA
Client
Figure 13-8 DCE: Global Directory Agent
DCE security service
Security is always a concern in a networked environment. In a large, distributed
environment, it is even more crucial to ensure that all participants are valid users
who access only the data with which they are permitted to work. The two primary
concerns are authentication and authorization. Authentication is the process of
proving or confirming the identity of a user or service. Authorization is the
process of checking a user's level of authority when an access attempt is made.
For example, if a user tries to make a change when read-only access has been
granted, the update attempt will fail.
500 TCP/IP Tutorial and Technical Overview
DNS
or X.500
GDA
Cell B
CDS