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Overview

In cloud environments, SBC instances are added and removed based on call traffic volume. There are situations when the call volume is low and instances are removed for optimum utilization of resources. SBC instances with minimal impact on the existing calls and registrations are removed by allocating time for those sessions to terminate on their own. The following object and parameters are added under Global  object  object to support this feature:

• System
  • Mode - This parameter specifies the desired new global system state and acts according to the settings of the adjoining parameters. The following options are available:
    
    • In Service - When this option is selected, the SBC functions normally. The other two parameters (Action and Dry Up Timeout) are ignored. The SBC accepts new calls and refreshes the existing calls per existing SBC functionality.
    • Out of Service - When this option is selected, the SBC rejects any new calls, registrations, or subscriptions. Existing calls are dealt with in accordance with the remaining parameters.
  • Action - This parameter specifies the SBC actions to handle the existing calls when the Mode is set to Out of Service. The following options are available:
    
    • Dry up - When this option is selected, the SBC enters an Out Of Service Pending state and rejects all new calls. For existing calls, a timer is started, at the end of which, any remaining calls are force terminated. The duration of that timer is determined by the Dry Up Timeout parameter, below.  During the dry up period, messages involved in the maintenance of existing call legs such as reINVITEs or UPDATEs are allowed. Out-of-session messages are rejected. Specific scenarios are covered below. Once all the existing calls/registrations/subscriptions are terminated/cleared or the Dry Up Timeout value expires (whichever happens first), the SBC goes to Out of Service state.
    • Force - When this option is selected, the SBC immediately goes to Out of Service state. The existing calls are terminated. The Dry Up Timeout parameter is bypassed.
  • Dry Up Timeout - This option specifies the time (in minutes) until the SBC goes to Out Of Service state by setting the duration of the dry up timer. Once set, you cannot change the value of the running timer unless you revert the system back to In Service mode. Changing the value of this parameter after starting the dry up  action has no effect on the system. The dry up timeout value may range from 15 minutes to 1440 minutes (1 day).

If the mode is changed from Out of Service to In Service when the dry up times is running, The Dry Up Timeout gets cancelled and the system is reverted to In Service mode.

If the mode is changed from Out of Service to In Service and If there is a dry up timer running and the sessions are not removed completely, all the unterminated calls, registrations and subscriptions are recovered. The SBC does not recover any calls, subscriptions, and registrations for which a tear-down message is initiated.

If the system mode is changed from Out of Service > Dry up to Out of Service > Force before the Dry up Timeout expires, the Dry up Timeout timer is cancelled. The system behaves as though the Dry up Timeout has expired and takes all the actions for removing the calls.

If the mode is Out of Service Pending state and the system switches over, the Dry-up timer continues on and is not reset.

REGISTER Message in Out of Service Mode

In Out of Service mode, all new REGISTER messages are rejected with a 503 response code. The 503 response does not contain a retry-after header. Any DEREGISTER messages received are passed through to the registrar.

Refresh REGISTER requests are also rejected, with the 503 response that does not contain a retry-after header, based on the following conditions:

• The refresh register may be rejected if the register occurs within the Dry up Timeout.
  The probability of rejection is based on an internal statistical method.

• The refresh register will be rejected if it occurs after the Dry up Timeout value.

An RCB destruction timer is started once the 503 response is sent for refresh REGISTER. The RCB is not destroyed immediately since the contents are required for routing calls to that endpoint through a different SBC instance. The timeout value is chosen in such a manner that the endpoint has completed registration through an alternate SBC instance before this RCB is destroyed. Once the RCB destruction timer expires, the RCB is destroyed immediately. The SBC does not send any message, especially the unREGISTER, to the inside registrar.

At the end of the Dry Up Timer, any remaining connected registrations are destroyed with no further messages sent or received.

SUBSCRIBE Message in Out of Service Mode

In Out of Service mode, all new SUBSCRIBE messages are rejected with a 503 response code. The 503 response does not contain a retry-after header. If there is a matching SCB, then a randomized SCB destruction timer between 10 seconds to 5 minutes is started upon sending the 503 response. This allows the SBC to relay NOTIFIES messages until the endpoint re-subscribes through an alternate SBC instance. Once the SCB destruction timer expires, the SBC sends an unSUBSCRIBE to Egress peer and destroys the SCB. Any response to the unSUBSCRIBE, including a 2xx, 401, or 403 response, is ignored. As stated earlier, a NOTIFY message matching an existing subscription is forwarded.

A refresh SUBSCRIBE which does not match an existing subscription is rejected with a 503 response. The 503 response does not include a retry-after header.

NOTIFY Message in Out of Service Mode

In  Out of Service mode, a NOTIFY messages matching an existing subscription are forwarded. The NOTIFY message is associated with a SUBSCRIBE and until the endpoint has learned about out-of-service and re-subscribed through an alternate SBC instance, the NOTIFY is forwarded.

A refresh NOTIFY which does not match an existing subscription is rejected with a 503 response. The 503 response does not include a retry-after header.

When existing calls are terminated a result of the Dry-up timer expiring or the use of the Force option, the calls are terminated with a BYE message.

OPTIONS Message in Out of Service Mode

In Out of Service mode, all new OPTION messages are rejected with a 503 response which does not contain a retry-after header.

R-Factor Computation

In the SBC, the R-Factor computation is decoupled from QoS Routing. As a result, the SBC computes a call's R-Factor and adds it to the Call Data Records (CDRs) even if the QoS routing is not enabled. The parameter rFactorComputation is  added to the Global system level configuration CLI. If enabled, the SBC computes a call's R-Factor and adds it to the STOP CDR.

The R-factor is populated in the STOP CDR in the following fields:

• Ingress Inbound R-Factor
• Ingress Outbound R-Factor
• Egress Inbound R-Factor
• Egress Outbound R-Factor

Command Syntax

% set global system
    mode <inService	action <dryup | outOfService>force>
    actionanonymizeDtmfLogging <dryup<disabled | force>
    enabled>
	dryupTimeout <15-1440 mins>
	mode <inService | outOfService>	
	rFactorComputation <disabled | enabled>

Command Parameters

The following table represents Set Global System

command parameters:

mode

Specifies the service state of the SBC.

action

• dryUp (default
• force

Command Example

In this example, the system is pushed to out of service mode with dry up action (Dry Up Timeout = 180 mins). The system immediately goes to out of service standby mode and stops accepting new calls and registrations. Once all the existing calls/registrations/subscriptions are terminated/cleared or the Dry Up Timeout value expires (whichever happens first), the SBC goes to Out of Service mode. The R-Factor computation is enabled to compute the R-Factor and add it to the STOP CDR even if the QoS routing is not enabled.

% set global system mode outOfService action dryUp dryupTimeout 180

 rFactorComputation enabled

set global system anonymizeDtmfLogging enabled
commit