SIP Overload Control Overview

IMG 2020

Feature F-6296 Overload Control support uses the SIP OPTIONS message to control SIP signaling traffic between two SIP networks or entities. To accomplish this, the SIP method OPTIONS is utilized. Feature F-6296 uses this method to perform the following functions:

1. Transmit or Inform the entity in a network that the IMG 2020 has reset or failed and has come back into service. At this point, calls can now resume.

2. Block or Unblock calls coming from a network.

3. Monitor resource usage and inform entities in a network and restrict calls when a maximum number is reached.

4. Monitor and inform entities in a network and resume calls when the resources available go below a set threshold after reaching a set maximum.

Call Flows

Transmission of Node Reset

In the first call flow below, the IMG 2020 transmits the SIP OPTIONS (node reset) message

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• When the functionality is first configured, the IMG 2020 transmits the SIP OPTIONS (node reset) message to inform the entity on the incoming side that the SIP OPTIONS (node reset) functionality is enabled.

• The IMG 2020 resets and then comes back into service. Once back in service, the IMG 2020 will transmit the SIP OPTIONS (node reset) message to inform the entity on the incoming side that the node is back in service and can now accept calls.

• As part of the protocol exchange, Network A transmits an SIP OPTIONS message to the IMG 2020 informing the IMG 2020 that it can start initiating calls. The IMG 2020 then acks the SIP OPTIONS message. Since the IMG 2020 doesn't initiate calls, the message is rather meaningless. The IMG 2020 simply acks the SIP OPTIONS message as it would for any SIP OPTIONS message received. 

Reception of Node Reset

In this call flow the IMG 2020 receives a SIP OPTIONS message informing it that an entity in Network A has reset.

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• In the call flow above, the IMG 2020 receives the SIP OPTIONS (node reset) message from an external entity within Network A. The IMG 2020 releases any resources/calls between Network A and the IMG 2020.

• The IMG 2020 then sends the BYE message for any calls connected between the IMG 2020 and entity in Network B. At this point all calls are dropped between these two entities.

• Once all calls are released, the IMG 2020 transmits the SIP OPTIONS (call accept) message indicating it is ready to accept calls.

Manual Block/Unblock of Incoming calls

Through use of the SIP OPTIONS message, the IMG 2020 can be configured to block calls coming from an external gateway.

• Through the Web UI, when the user configures the IMG 2020 to Block Incoming Callson the Network A leg. The IMG 2020 transmits the SIP OPTIONS message to the incoming network informing it that a call restriction ofBlockis configured on the IMG 2020.

• If an INVITE messages received from Network A, it will respond with a 503 Service Unavailable message.

• When the user removes the block, the IMG 2020 will transmit another SIP OPTIONS message to the incoming network that the call restriction of Block has been removed. Any INVITE messages received after the Block has been removed will be accepted.

Node Congestion Monitoring

In the call flow below the IMG 2020 monitors the CPU usage, SIP message handles usage, and IP Channels usage. If any one of these meet a specific level (see table below) the IMG 2020 will send the SIP OPTIONS message with Overload-Control equal to 100. Refer to Call Flow diagram below.

• The IMG 2020 monitors CPU usage, SIP message handles usage, and IP Channels usage. If any of these go above a specific level, a SIP OPTIONS message will be transmitted to inform the incoming network. This message will have an Overload value of 100 and a call restriction will be put into place.

• The IMG 2020 will respond with a 503 Service Unavailable message if any calls are received while the IMG 2020 is in the Overload state.

• If the function (CPU usage, SIP message Handles, or IP Channels usage) go below a lower limit threshold, the IMG 2020 will transmit a second SIP OPTIONS message with an Overload-Control value of 0 and calls will resume.

• It only takes one usage level to go above its threshold to trigger the SIP OPTIONS message with Overload-Control:100 to be transmitted.

• If more than one usage goes above the threshold, both usage levels must come below their respective lower thresholds limit for the SIP OPTIONS message (Overload-Control:0) to be transmitted.

• The SIP message handles usage information can be viewed and modified through the IMG 2020 CLI (Command Line Interface). To view or modify any of this usage parameter, contact Sangoma Support Personnel and they can assist you.

Configuration

The configuration is applied to each individual SIP Channel group. The configuration is applied to each individual Packet Facility configured within a SIP Channel Group. Refer to the procedure below to configure.

Before proceeding, it is assumed that the Basic Configurations topic has been applied as well as the SIP Signaling functionality.

Configure External Gateway

The external SIP gateway that the SIP OPTIONS messaging will be sent to needs to be configured. Follow the steps below to configure an external gateway.

1. Right click on the Dialogic object and select New External Network Elements. The External Network Elements object is a container object and no configuration is needed here. For more information on this object refer to the External Network Elements topic.

2. Right click on External Network Elements and select New External Gateways. The External Gateways object is a container object and no configuration is needed here. For more information on this object refer to the External Gateways topic.

3. Right click on External Gateways object and select New External Gateway. Enter a name that identifies this gateway.

4. Select SIP from drop down menu in the Protocol field.

5. Select either Gateway IP Address or Gateway Host name from the Address Type field. In this procedure the IP Address will be how the IMG 2020 will communicate with external gateway.

6. Enter the IP address of the external gateway in the IP Address field.

7. Select the SIP Profile that will be utilized from the drop down menu of the Profile field . For more information on configuring the remaining fields in the External Gateway object, refer to the External Gateway topic.

Configure SIP Channel Group and

1. Right click on the Dialogic object and select New Routing Configuration. The Routing Configuration object is a container or parent object and no configuration is required here. Refer to the Routing Configuration topic for more information on this object.

2. Right click on the Routing Configuration object and select New Channel Groups. The Channel Groups object is a container object for multiple channel groups. No configuration is needed here. For more information refer to the Channel Groups topic.

3. Right click on the Channel Groups object and select New Channel Group. In the Name field, either except the default name or enter a name that identifies the Channel Group being configured. In this procedure, the channel group was labeled ChGrp1_SIP.

4. Select SIP from the drop down menu in the Signaling Type field.

5. Select the Incoming and Outgoing IP Profiles from drop down menus. The IP Profile was initially created in the Basic Configurations topic. For more information on configuring the remaining fields in the channel group object, refer to the Channel Group topic.

6. Right click on the SIP Channel Group and select New IP Network Element. Select from drop down menu the EXTERNAL gateway configured in the Configure External Gateway section above. This will be the external gateway that the channel group being configured will communicate with. Refer to the IP Network Element  topic for more information on this object.

7. Right Click on the IP Network Element and select New Node Association.

8. In the Node field, select from drop down menu the IMG 2020 node that the OPTIONS feature is getting configured on.

9. The Service IP Address field will display all the SIP IP addresses configured above in its drop down menu. Select the IP address that will be utilized for SIP Signaling.

10. The Media Packet Facility field will display all the packet facilities configured on this IMG 2020 node. Select the Packet Facility that will be utilizes to communicate with the External Gateway configured above. At this point, the SIP Signaling IP address selected in step 9 will control the messaging for the Packet Facility selected. Refer to the SIP Signaling - Node Association object for more information on configuring the Node Association object.

Configure Overload Control

Configure Overload Control to configure the Node Reset Notification, Block Incoming Calls, and/or the Congestion Monitoring functionality.

1. Right click on the Node Association object and select New Overload Control. Select the functionality required from the drop down menus of each field. Refer to the information above as well as the SIP Signaling - Overload Control topic for information of configuring each of the individual fields.

2. The Channel Groups object will now have an yellow exclamation point displayed in the configuration tree icon. The exclamation point indicates that the configuration has been created but not yet sent to the node. Click on the Channel Groups object and then click on the Download Resource Tables button in the Channel Groups object pane. The configuration will now get sent to the IMG 2020 node and the yellow exclamation point will change to a green check.
   The Overload functionality is now configured within a SIP channel group and this channel group is configured to communicate with an external gateway. Overload Control will now function between the SIP channel group and the external gateway configured. At this point routing and translations can be configured. Below is screen capture displaying the channel group section of the configuration tree after the feature has been configured.