Subject: A Directory Agent question

From: Subbu Subramaniam <[email-masked]>
Date: Fri, 05 Feb 1999 12:08:54 -0800

Hi,

I have a question on SLP.

How does a User Agent discover that the Directory Agent information it
had is not valid any more
,esp. if the Dir Agent goes down unexpectedly, withouth being able to
send out the DAAdvert message (with time=0)?

It appears that the UA will send out Service Requests to the DA, and
will not be able to tell the difference between a DA being down or
the fact that there is no service in the network with the speicific
attributes that it requested for.

One way to do this can be to include an error code of some sort that
says "No such entries" in the Service Reply message.

Alternatively, there was a suggestion in the group a couple of days ago
about using a unicast Service Request to see if a DA is alive.

Thanks,

-Subbu

-- 
Subbu Subramaniam
Hewlett-Packard
Email:	[email-masked]

---

From: Erik Guttman <[email-masked]>
Date: Fri, 5 Feb 1999 22:16:14 +0100 (MET)

> attributes that it requested for.

If the DA is down, the SrvRqsts will not receive any SrvRply messages
from the DA.  The UA will give up on the DA and attempt to use a 
different DA in the same scope.  Only if there are no DAs in the scope
of the request will the UA multicast the service requests to SAs.   And
only in the case that it gets no reply will the UA assume that there are
no services in the network.

Perhaps you are asking a more subtle question:  What if a DA goes down
and then comes back up and the UA doesn't know that this occurred?  Are
you worried that the UA would then get a SrvRply from the DA which did
not include the services which the DA should have?

This can happen, but only transiently, if:  (i). The DA is stateless (ie.
it comes up every time forgetting about prior registrations.  (ii). The
registrations made by SAs with the DA has not been 'refreshed' yet.  
(iii).  Multicast routing is not active in the network so SAs did not
see the DA's initial unsolicited announcement upon coming up.  If they
had seen this announcement, they would have registered their services 
with the DA.

As you can see, providing either statefulness to DAs, lower refresh
intervals or multicast routing will solve the problem.  Even if there
is a problem, it will be of short duration (the maximum time that a
service can go without being reregistered with a DA is 18 hours and
the suggested length of time is 3 hours.)

> says "No such entries" in the Service Reply message.

There is a 0 results possibility in the reply if the DA genuinely doesn't
have information conforming to the service request.  This is not an error
condition, however.

> about using a unicast Service Request to see if a DA is alive.

You can do this, of course.

> -Subbu

Erik