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\chapter {The QUIPU Nameservice}
\label {ns_design}

\section {Requirements}

Many applications make a relatively simple use of the directory, with the
following type of requirements:

\begin{itemize}
\item  Reading a single (or small number of) attribute(s).

\item  Public data only (i.e., no need for access control)

\item  Most data accessed is available locally
\end{itemize}

A typical usage is to look up and Application Entity Title to determine a
presentation address.
The DAP is a rather heavyweight procedure to achieve this.  
This chapter describes a lightweight mechanism to access a 
{\em local} DSA.   The protocol was designed by Marshall Rose.

\section {Design}

The QUIPU Name Server uses a lightweight interface to the QUIPU Directory
Service.

This gives an application the
ability to supply a distinguished name, and the Attribute Type required, in
IA5String format (because the encoding is more efficient).  The name server
will then use the lightweight protocol to pass a ``read'' operation onto the
directory.  The Directory will then perform a lookup of this data (giving
the operation a high priority status), and return the value.


The nameservice protocol is straight-forward.  The initiator formats a
query message for the nameservice.  This message is sent via a connectionless
protocol ``n'' times.  Each time after sending the message, the initiator
waits up to ``s'' seconds for a response.  If the response arrives, the
initiator processes it; otherwise, the initiator either sends another
message or gives up.  A definition of the PDUs for a ``query'' and the
``response'' to that query are shown in Figure~\ref{ns-h}.  

\tagrind {ns}{Name Service Protocol}{ns-h}

The request-id identifies this particular Query; the same value is used
in the message, regardless of how many times the message is sent.

The name identifies the DN being looked-up.  It is a string from the IA5
repertoire which expresses the DN in the QUIPU format.

The attribute identifies the desired attribute of the DN.  It is a
string from the IA5 repertoire which expresses the attribute as known in
it's user-friendly form.

The cache-ok tells the nameservice if it is permitted to use cached
information when responding to the query.

When the nameservice receives a Query, it processes it as follows:

\begin{enumerate}

\item It constructs an environment to process the Query with access rights
for publically-readable information.

\item It prepares a Response using the request-id from the Query.

\item It reads the indicated DN and attribute from the DIT.  If the DN
exists with the given attribute, it returns the value of that attribute.
In the event that multiple values are present, the nameservice returns
any one of those values.  The ASN.1 encoded distinguished name is also
returned, typically for use as the Application Entity Title.

\item It then sends the response to the initiator.
\end{enumerate}

A message called the ``Response'' is returned by the name server (see
Figure~\ref{ns-h}).

request-id corresponds to the Query which invoked the nameservice.

value is the answer to the Query.  It is present only if no errors
occurred during processing.

The transaction protocol used by the nameservice is very
simple: Response messages are not cached and there are no Keep-alive nor
In-Progress messages.

\section {Support}

This protocol can optionally be supported by a QUIPU DSA.

A simple procedural interface (DUA) s supplied which is used by
ISODE applications such as FTAM and IMISC to
look up Presentation addresses of remote Application Processes.
The interface is designed so that use of the lightweight protocol or DAP can
be chosen at compile time (the saving in code size is significant, and so
this is not a runtime choice).   This is described in the QUIPU Manual.