The protocols being considered for DUA - DSA and DSA - DSA
interactions are based on Remote Operations Service (ROS).
They are known as the Directory access Protocol (DAP) and the
Directory Service Protocol (DSP).
The data held in the Directory is in the form of Attribute/Value
pairs, and thus can be arbitrarily complex. The set of operations
allowed on the directory include searching on keys as well as the
usual resolver type interaction.
Recursion versus Iterative, ;SPM_quot;Chaining and Referral;SPM_quot;
Information in the Directory is arranged to form a tree, as are the
directory servers themselves. Each server holds some part of the
directory tree, most of which is information relevant to the local
operations of the system the directory server resides in.
It is possible for servers to hold information on behalf of other
parts of the Directory Information Tree, and is reasonable in some
circumstances (e.g. a small site without the capacity to run a
directory service - i.e. cost reasons, or to provide mutual backup
between two sites - i.e. reliability).
When a Directory User accesses a server and makes a query, it is
possible that the query is for information held in other directories.
There are two different approaches for how to proceed:
There are two important engineering design decisions here. One is the choice
of cache timeouts. The other is when to chain and when to refer. A
Model implementation [ref QUIPU] relies on the slow changing nature of
Directory Information and uses large cache timeouts. QUIPU uses
chaining for the first interaction (e.g. DUA to DSA) and thereafter
uses referrals. This is justified on the grounds that the DUA is
simplified and that only the ;SPM_quot;first;SPM_quot; DSA need ;SPM_quot;switch;SPM_quot; connections -
i.e. hold complex state about complex operations non-local parts of
the tree.
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Table:
Figure:
These two mechanism permit location transparency, although the second
requires more intelligence in the Directory User Agent.
In both cases, the results may be cached by the user or indeed all
along a chain. This achieves some performance transparency as well as
location transparency. The mechanism by which the directory servers choose
other servers to chain or refer too is self consistent. The mapping of the
structure of the directory information tree onto the tree of directory
servers must be held in the Directory itself.
Figure:
Figure: