Subject: [Megaco] Base Root Package Interpretation

From: "Dave Stephens" <[email-masked]>
Date: Fri, 8 Dec 2006 15:00:43 -0800

We are currently working through implementing the Base Root Package and
would appreciate hearing your interpretation of how
these elements should be applied to the transaction resend algorithm. Our
interest is only in the MG side although this would 
apply equally to the MGC.  We are using UDP/ALF.
 
We reviewed the previous posting below by Jerker Zetterlund and the follow
up discussion:
http://www1.ietf.org/mail-archive/web/megaco/current/msg06418.html
 
In particular, we are interested in this portion of the posting:
>> When MGC is responder:
>>   - MGC sends pending at the latest at expiry of MGC timer
"MGCNormalExecutionTime". The "MGCNormalExecutionTime" is started when the
transaction request is received in MGC.

>> - MG resends transaction request if it did not receive a pending or reply
at expiry of MG timer "MGCProvisioalResponseTimer". The
"MGCProvisionalResponseTimer" is started >> when transaction request is sent
from MG and when pending is recevied from MGC.
 
Subsequent discussion indicated there might be some follow up in SG16.
However, we have not seen any subsequent clarification of this issue.
 
Our interpretation is that in the absence of the Base Root Package, or the
setting of elements in the package, the retry algorithm as defined in
H.248.1 Appendix D .1.3  is used . In particular, "Implementations SHALL
ensure that the algorithm used to calculate retransmission timing performs
an exponentially increasing backoff of the retransmission timeout for each
retransmission or repetition after the first one."
 
The Base Root package defines the MGCProvisionalResponseTimer element. This
value is calculated by the MGC and represents the time the MGC would like
the MG to wait before retransmitting requests that have received neither a
provisional response  nor a reply to the actual outstanding transaction.
The value is constant and replaces the rate of retransmission as calculated
by the back-off algorithm.  It is in effect as soon as it is set.  One of
the ambiguities here is when does this value take effect.  Does it take
effect as soon as it is set or does it take effect only after the receipt of
a provisional response?  
Our assumption is that the behaviour associated with the
MGCProvisionalResponseTimer is:
1. A timer is started for the MGCProvisionalResponseTimer period after the
initial transaction request has been sent.
2. If it expires without receipt of a provisional response or transaction
reply then the transaction request is resent and the timer restarted for the
same period
3. If a provisional response is received the timer is stopped and restarted
for the same period.
4. If the transaction reply is received it is stopped and not restarted. 
 
Is anybody aware of any subsequent clarification of this issue?  
 
Thanks,
 
Dave
-------------------------------------------------------------------
Dave Stephens
Team Leader VXML Team
RadiSys Canada Inc.
Ph:  604 918 6381
Fax: 604 918 6400
 
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