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<title>OpenSLP Programmers Guide - Divergence from RFC 2614</title>
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<h2></h2>
<h2>Divergence from RFC 2614<br />
<span id="breadcrumbs"><a href="index.html">OpenSLP Programmer's Guide</a> » Misc. Information » <a href="Divergence.html">Divergence from RFC 2614</a></span></h2>
<h3>SLP Service URLs are required</h3>
<p>SLP Service URL syntax is required for all functions that
accept URL strings. The following is a list of affected
functions:</p>
<ul>
<li>
<tt>SLPReg</tt></li>
<li>
<tt>SLPDeReg</tt></li>
<li>
<tt>SLPDelAttrs</tt></li>
<li>
<tt>SLPFindAttrs</tt></li>
<li>
<tt>SLPParseSrvURL</tt></li>
<li>
<tt>SLPSrvURLCallback</tt></li>
</ul>
<p>The decision to require SLP Service URL syntax was made
based on in part the following scenario:</p>
<blockquote>Suppose that two calls were made to SLPReg with <tt>srvurl</tt>
and <tt>srvtype</tt> of "192.168.100.2","http"
and also "192.168.100.2","ftp".
Now the developer wants to deregister one service with out deregistering the
other. How can it be done? The SLPDeReg call does not have a service type
parameter, so it would be impossible for the underlying implementation to
distinguish between the two registrations. In an attempt to standardize,
OpenSLP expects valid Service URLS.</blockquote>
<p>OpenSLP requires strict Service URL syntax because a Service
URL can be treated as a unique database key that identifies
a registered service. Not requiring Service URL syntax
allows for several ambiguities like the one mentioned above.</p>
<h3>Scopelist may be the empty string ("")</h3>
<p>In the calls where a scope list is accepted as a parameter,
RFC 2614 says that this parameter may not be the empty
string or a NULL pointer value. OpenSLP allows scope list to
be NULL or the empty string. If the empty string is passed
in as a scopelist, then OpenSLP will use the scopelist the
system administrator has configured for the machine. This
saves 99% of all developers the time of calling
SLPFindScopes and parsing the result.</p>
<p>Scoping is almost entirely an administrative task that is only required for
scalebility of the SLP wire protocol. Having to deal with and understand scopes
will be a burden to the large majority of programmers. Unless they are writing
some sort of SLP browser, they will be very content to use the scope that the
machine is configured to use.</p>
<p>This extension by no means mandates the use of NULL or empty scope strings.
Programmers who want complete implementations of SLP applications may indeed use
the API to acquire knowledge about existing scopes on the network, and then
present a scoping choice (if more than one is available) to the user.</p>
<h3>The SLPSetProperty is ignored</h3>
<p>The SLPSetProperty and SLPGetProperty calls are impossible
to implement in a way that would be both scalable and thread
safe. The SLPGetProperty call could never be made thread
safe unless return value was a pointer to a buffer
dynamically allocated by the library and freed by the
caller. The SLPSetProperty call would still access the data
store in such a way that mutexes would be required to ensure
that SLPSetProperty and SLPGetProperty were never using the
same buffers at the same time. Even if a thread-safe data
store were devised, the SLPGetProperty call would be used so
frequently during internal operations of the library that
performance might be adversely affected due to mutex
bottlenecking or the amount of processing required to
resolve the attribute name to a value.</p>
<h3>NULL and empty string are acceptable parameters</h3>
<p>According to RFC 2614, NULL is not exceptable value for any
parameter. Instead programmers are instructed to passed the
empty string (""). OpenSLP allows programmers to use either
NULL or the empty string. It is very easy to deal with both
NULL and empty string in the implementation, and allows
developers to write more familiar and slightly more
efficient code. There should not be any reason why the
compiler should be required to pass a pointer to a static
constant empty string when NULL would do just as well. This
is why the vast majority of C APIs use NULL, and not the
empty string, to indicate an ignored parameter or default
value.</p>
<h3>Incremental registrations and de-registrations</h3>
<p>The only reason I can think of ever wanting to expose the
functionality of incremental registration and deregistration
is to represent dynamic data via SLP attributes. I can think
of a long list of reasons why this is a very bad idea.
Without a doubt, it is best to instruct SLP developers to
minimize whenever possible, the number of calls that
ultimately generate SrvReg and SrvDereg messages. If dynamic
data is to be represented, it is best to do it via a
specialized protocol optimized for the given service.
OpenSLP does not support incremental registrations and
de-registrations via SLPReg and SLPDelAttrs because we have
found that when developers really learn what happens "under
the SLP covers" they are very careful *not* to call them
very often.</p>
<p>In addition to poor usage of network resources, incremental registrations and
de-registrations require additional code that decreases the efficiency of, and
increases the size and complexity of the OpenSLP API and agent implementations.</p>
<p>The work around for this behavior involves the following:</p>
<ul>
<li>
Design application usage of SLP such that SLP is not used to store great
quantities of data</li>
<li>
Design application usage of SLP such that SLP is not used to store dynamic
data</li>
<li>
If the need does arise to add or remove an attribute from an existing
registration simply re-register the service with new attributes as a "fresh"
registration.</li>
</ul>
<h3>Addition of a very simple attribute parsing function</h3>
<p>The following function is included, without explicit
documentation, with the OpenSLP library, and has proven to
be very useful:</p>
<pre>
<>
/*=========================================================================*/
SLPEXP SLPError SLPAPI SLPParseAttrs(
const char * pcAttrList,
const char * pcAttrId,
char ** ppcAttrVal );
/* Used to get individual attribute values from an attribute string that */
/* is passed to the SLPAttrCallback */
/* */
/* pcAttrList (IN) A character buffer containing a comma separated, null */
/* terminated list of attribute id/value assignments, in */
/* SLP wire format; i.e. "(attr-id=attr-value-list)" */
/* */
/* pcAttrId (IN) The string indicating which attribute value to return. */
/* MUST not be null. MUST not be the empty string (""). */
/* */
/* ppcAttrVal (OUT) A pointer to a pointer to the buffer to receive */
/* attribute value. The memory should be freed by a call */
/* to SLPFree() when no longer needed. */
/* */
/* Returns: Returns SLP_PARSE_ERROR if an attribute of the specified id */
/* was not found otherwise SLP_OK */
/*=========================================================================*/
</>
</pre>
<h3>Some .conf properties are ignored</h3>
<p>See the <a href="../UsersGuide/SlpConf.html">OpenSLP Users Guide</a>
for more details.</p>
<p id="breadcrumbs0">Prepared by: <a href="http://www.calderasystems.com">Caldera Systems Inc</a><br />
Maintained by: <a href="http://www.openslp.org/">openslp.org</a></p>
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