Subject: [Capwap] question on the discovery phase

From: "Yong Zhang" <[email-masked]>
Date: Thu, 3 Jul 2008 11:30:56 -0700

I have some question on the role of discovery in the capwap. Is it
used to find the AC, or used to find the best AC?

If static config AC or DHCP mode are used to get the AC list. Does the
WTP need to go into the Discovery phase?

Discovery response will provide some live load status of the AC such
as current connected WTPs, and STAs.

>From the following:

   Discovery to Discovery (#):  In the Discovery state, the WTP
      determines which AC to connect to.

      WTP:  This transition occurs when the DiscoveryInterval timer
         expires.  If the WTP is configured with a list of ACs, it
         transmits a Discovery Request message to every AC from which it
         has not received a Discovery Response message.  For every
         transition to this event, the WTP increments the DiscoveryCount
         counter.  See Section 5.1 for more information on how the WTP
         knows the ACs to which it should transmit the Discovery Request
         messages.  The WTP restarts the DiscoveryInterval timer
         whenever it transmits Discovery Request messages.

It looks like the WTP will send Disocovery REQ to the AC's one by one?

In the case of more than 1 AC, how to deal with DiscoveryCount (its
definition in 4.8.2 is for a single AC). MaxDiscoveries (4.8.6) is the for the
sum of all AC's, or apply it for each AC?

Thanks!

 Yong
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From: "Pat Calhoun (pacalhou)" <[email-masked]>
Date: Thu, 3 Jul 2008 13:02:03 -0700

Hi Yong,

The Discovery process is optional. If a WTP has a pre-configured list of
ACs it needs to communicate with, it has no need to perform a discovery.
However, there are possible issues with not doing a Discovery, since an
AC could be at maximum capacity, and may refuse the Join, which would
cause the WTP to try another AC. The Discovery process allows the WTP to
get more information about the ACs before it attempts to setup the DTLS
session. Section 3.3 of the spec says:

   The AC discovery phase allows the WTP to determine which ACs are
   available, and chose the best AC with which to establish a CAPWAP
   session.  The discovery phase occurs when the WTP enters the optional
   Discovery state.  A WTP does not need to complete the AC Discovery
   phase if it uses a pre-configured AC.  This section details the
   mechanism used by a WTP to dynamically discover candidate ACs.

> It looks like the WTP will send Disocovery REQ to the AC's one by one?
The WTP needs to send a Discovery to each of the ACs that it knows about
- and how it knows about it can be through various techniques, from
static configuration to DHCP/DNS to having the AC provide a list.
Section 3.3 of the spec goes into much more detail on the actual
discovery process. The state transition included below also shows that
when the Discovery phase is (re-)entered, a Discovery Request is sent to
all ACs - not just one.

> In the case of more than 1 AC, how to deal with DiscoveryCount (its
definition in 4.8.2 is for a single AC). 
> MaxDiscoveries (4.8.6) is the for the sum of all AC's, or apply it for
each AC?
The spec is pretty clear, I think, about this:

         counter.  

Basically, everytime the Discovery state is entered, the WTP sends a
Discovery Request to the ACs it doesn't have a response from, and
increments the counter.

PatC

-----Original Message-----
From: Yong Zhang [mailto:[email-masked]] 
Sent: Thursday, July 03, 2008 11:31 AM
To: [email-masked]
Subject: [Capwap] question on the discovery phase

I have some question on the role of discovery in the capwap. Is it used
to find the AC, or used to find the best AC?

Discovery response will provide some live load status of the AC such as
current connected WTPs, and STAs.

In the case of more than 1 AC, how to deal with DiscoveryCount (its
definition in 4.8.2 is for a single AC). MaxDiscoveries (4.8.6) is the
for the sum of all AC's, or apply it for each AC?

Thanks!

 Yong

---

From: "Yong Zhang" <[email-masked]>
Date: Thu, 3 Jul 2008 13:56:50 -0700

I see. Thanks for the explanation.

Yong