Subject: [Ieprep] Megaco

From: "King, Kimberly S." <[email-masked]>
Date: Thu, 16 Jan 2003 11:17:28 -0500

Hi!

Anybody want to investigate how Megaco can be used to support 
preferential treatment for certain calls (e.g., emergency 
services not limited to 911)?

Part of what would be nice to understand is does a priority 
indicator need to be the Megaco protocol or could 
preferential treatment be done based upon something 
the MGC understands?  

Here is a specific example.  With the GETS service in the 
circuit-switched world, a PSTN switch will allow a GETS call 
a greater chance of success by giving it a larger timeout 
window to obtain a trunk. Now suppose we are in some kind of 
IP Bridging scenario and a MGC receive a SS7 message for 
setting up a GETS call.  (The part of SS7 that GETS uses for 
conveying priority is a codepoint in the CPC of the IAM.)  
Can a MGC do a similar thing of allowing some small interval 
for a gateway port to become available before returning a 
fast busy or something like that? 

If so, this doesn't seem to require any 
Megaco protocol changes and 
I believe MGC's could have access to multiple 
gateways and hence even poll other gateways 
(in addition to trying the same gateway again later) 
based upon policy.  This would provide both 
queuing for resources and alternate routing.  
Plus, it isn't dependent upon the gateways which 
I think are fairly diverse in the number of 
vendors/gateways/etc. With this solution, the 
gateways themselves would have to understand 
no priority indicator or do anything about it.

Alternatively, is this kind of timeout 
(if performed at all) done in the gateway 
whereby perhaps Megaco would need to pass 
some priority indicator? 

This solution might involve a Megaco protocol 
change (see below), would potentially be more 
difficult to implement because MG and MGC 
would have to understand the indicator and 
act upon it. Etc.  

Related questions:

1. Can a MG have certain assets (e.g., ports) reserved 
exclusively for emergency services where the 
indicator triggers the MG's use of this asset? 

2. Is there anything to forbid an MGC from tracking 
priority of on-going calls (e.g., MLPP scenario) 
and pre-empting calls using gateway resources?

3. Is the Boolean emergency indication reserved for 911?  

Could a MG be setup to place certain policy_data 
elements within an associated RSVP request for 
transport of IP media when it receives either a 
certain priority or emergency indicator? 

Background: 
 
The framework http://www.ietf.org/rfc/rfc2805.txt states " 
  k.   Allow the MGC to specify that a given connection has higher 
      priority than other connections." 
 
6.1.1 Context attributes and descriptors of 
 http://www.ietf.org/internet-drafts/draft-ietf-megaco-h248v2-01.txt 
 "ò  The priority is used for a Context in order to provide 
 the MG with 
information about a certain precedence handling for a Context. The 
MGC can also use the priority to control autonomously the traffic 
precedence in the MG in a smooth way in certain situations (e.g. 
restart), when a lot of Contexts must be handled simultaneously. 
Priority 0 is the lowest priority and a priority of 15 is 
the highest priority. 
 
ò  An indicator for an emergency call is also provided to allow a 
preference handling in the MG." 

Ideas?

Kimberly
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