Directory System Agents (DSA)

The following is a description of the Directory system at the second level of abstraction . It involves the interaction of the DSA's to provide the service requested by the user , and to maintain the the Directory Database . Again the interaction between these components is standarised and is called the DSA Abstract Service . Combining this with ACSE and ROSE , a protocol is formed called the Directory System Protocol (DSP) . The DSP consists of the operations , result and errors defined for the DAP , so that the users requsted operation , and the appropriate result can be passed between the user and DSA .

Extra information is added to the users request by the DSA . This information is state related and is used to notify each of the DSA's how much of the operation has been processed so far . The reason for this is so that duplication does not result . This state information is added to the first DSA and gradually modified by other DSA's and finally removed from the result obtained by the first DSA before returning to the user . Each DSA passes on the request and waits for the result to be returned before passing it to the user . The process that has been described is known as chaining . (see fig. 2.0)

But how does the current DSA know where to chain next ?
Another method of implementing chaining is that of referral . This interaction mode for DSA's implies that distribution and location transparency are no longer possible in relation to DSA communication . How this type of interaction works is that a DSA returns to it's requestor , either a DSA or DUA , the names and addresses of the DSA's that would have been chained if the chaining method had been used . Or said another way , it is where the DSA addressed returns to the DUA together with a reference to the alternative DSA to be addressed . This name and address of a DSA is called it's Access Point . Because of this , the requestor can hence recreate the chained interaction and the process is resumed as normal ( as if no referral had taken place) . A DSA in this mode does not actually have to respond to a referral and can simply return the request to the requestor .
When the request is returned to the DUA , it asks the user for further information for the next stages of the process . To get a better understanding of this idea , consider the following diagram . Referral has some advantages over chaining in that the DUA directly contacts the DSA where the data is stored rather than always going via it's home DSA . This results in more efficiency and also is more secure because of the direct binding to the required DSA .