Subject: [Capwap] AC Name with Index Confusion

From: "Pat Calhoun (pacalhou)" <[email-masked]>
Date: Mon, 8 Sep 2008 11:09:33 -0700

All,

I received this out of band message from Glenn, and looking into the
issue he has raised, he points out a couple of small issues that do need
to be resolved. Issue 178 has been created.

First, the index field in the AC Name with Index message element is not
intended to be restricted to 1 and 2. It was intended to have any value,
and the primary, secondary was only reference as an example. Further,
the WTP needs to be able to send its configuration to the AC, which is
clearly stated in the Configuration Status Request. The only issue there
is that the message element is a MUST for the Configuration Status
Request, while it needs to be a MAY (since the WTP may not have anything
configured). So I have made the following small changes:

<modified text>
4.6.5.  AC Name with Index

   The AC Name with Index message element is sent by the AC to the WTP
   to configure preferred ACs.  The number of instances of this message
   element is equal to the number of ACs configured on the WTP.  The WTP
   also uses this message element to send its configuration to the AC.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Priority  |   AC Name...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:   5 for AC Name with Index

   Length:   >= 2

   Priority:   A value between 1 and 255 specifying the priority order
      of the preferred AC.  For instance, the value of one (1) is used
      to set the primary AC, the value of two (2) is used to set the
      secondary, etc.

8.2.  Configuration Status Request
[...]
   The following message elements MAY be included in the Configuration
   Status Request message.

   o  AC Name with Index, see Section 4.6.5
</modified text>

PatC
------

Hi Pat,

While I believe I am clear regarding the intent of the AC Name with
Index message element, I am confused about the actual intended
implementation.  In the description provided in section 3.3 regarding AC
discovery, it states the following:

"The AC Name with Index message element (see Section 4.6.5), is used to
communicate a list of preferred ACs to the WTP."

Then in section 4.6.5, the description states:

 "The AC Name with Index message element is sent by the AC to the WTP to
configure preferred ACs.  The number of instances of this message
element is equal to the number of ACs configured on the WTP."

It appears that this message element is only used in the CONFIGURATION
STATUS and CONFIGURATION UPDATE messages.  The confusing part is that
the CONFIGURATION STATUS message is issued first by the WTP, so it isn't
immediately clear to me where the content of the message element (in
other words, the information regarding the primary and secondary ACs) is
supposed to enter into the WTP in the first place.  Further adding to my
confusion is the fact that there is supposed to be one such message per
configured AC.  I am not sure what is meant by "configured AC".

Finally, I find description of the index field in the message element to
be a bit confusing.  For clarity, the text reads:

"   Index:   The index of the preferred server (1=primary,
2=secondary)." 

I believe that the index is supposed to take only one of two values: 1 =
primary, 2=secondary.  Meaning the associated name will be the name of
either the primary or secondary AC.  However, index sort of implies an
index into an array, for example.  I think this is just a terminology
issue and I just want to make sure I am reading the spec correctly.
Also, the usage of the term "server" here is a bit confusing.  I assume
I can safely replace "server" with "AC".

Sorry if these are dumb questions, but I wanted to see if I could get
some clarity and highlight my difficulty in understanding this area in
case others will have similar difficulties.

Best,

Glenn

Glenn Chagnot
Chief Architect
VeriWave, Inc.
817-898-0369
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