Table of Contents

The Retry-After header field was added to reduce SIP congestion. The SIP layer receives an INVITE and checks its congestion level to determine whether SIP is congested. If the SIP layer shows no congestion then the call will go through. If it is determined that there is congestion on the SIP layer then IMG 2020 will determine whether it is minor, major, or critical. Once this is determined a 503 (Service Unavailable) response is sent. The response will contain the Retry-After header which includes a time to wait before sending the SIP INVITE back to the IMG 2020. The time is configurable from 1 - 65535 seconds (1 second - 18.2 Hours). The Retry Header field will also send a comment as to whether the congestion is minor, major, or critical. Refer to SIP Retry-After Header - Receive for information on the receive side.

SIP Retry-After Header Diagram

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Call Flow Diagram

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• A remote gateway sends a SIP INVITE message to the IMG 2020. The IMG 2020 will determine whether SIP is congested. If SIP Layer 3 is not congested the call will proceed as normal.

• If IMG 2020 determines that SIP is congested, a 503 - Service Unavailable response will be returned. Within this response will be the Retry-After Header specifying the number of seconds to wait before sending the SIP INVITE to the IMG 2020 again. The Retry-After Header will specify whether the congestion is one of the following:

  • Minor SIP Msg Overload

  • Major SIP Msg Overload

  • Critical SIP Msg Overload

Configuration of SIP Retry-After feature

Initial Configuration

The procedure below explains how to configure the IMG 2020 to transmit a SIP Retry-After Header back to a remote gateway if SIP congestion is determined. Before configuring the outbound SIP Retry-After feature, the IMG 2020 must have an initial configuration created on it. Follow the Basic Configurations procedure before proceeding.

Create SIP Profile (SGP)

• Right click on the Profiles object and select New SIP Profiles. The SIP Profiles object is a parent or container object. No configuration is needed here. Refer to the SIP Profiles topic for more information on this object.

• Right click on the SIP Profiles object and select New SIP Profile. The first profile that gets created is a Default SIP Profile and cannot be modified. Disregard this profile.

• Right click on the SIP Profiles object and select New SIP Profile again. In the SIP Profile object that appears, either enter a name that identifies this SIP Profile or accept the default name already entered. In this example the SIP Profile was labeled Retry_After_Profile.

• Right click on the SIP Profile object just created and select New SIP Advanced Settings. Verify the Retry-After Support field is defaulted to Enabled.

Create SIP Signaling

The Retry-After functionality is configured in the SIP Signaling object as described below. When creating the SIP signaling object,

• Right click on the IMG 2020 physical node and select New Signaling.  The Signaling object is a parent or container object and no configuration is needed here. Refer to the topic for more information on this object.

• Right click on the Signaling object created and select New SIP. In the IP Operation Mode field, select whether the SIP stack will be configured for Single IP or Multiple IP Mode.

• In the Retry-After field, enter the number of seconds (Range = 1 to 65535 seconds). This value will be transmitted back to the remote inbound gateway via the Retry-After header that is part of the 503 Service Unavailable response. Below in the Additional Information section there are two examples of how the Retry-After headers are formatted.

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• Right click on the SIP Signaling object and select New SIP IP Address. Refer to the SIP Signaling - IP Address topic for more information on configuring the fields in this object.

Configure SIP Signaling (Cont)

The configuration above displays how to configure the SIP Profile and SIP Signaling stack so the SIP Retry-After Header feature is configured. The next step is to configure the SIP Channel Group, SIP remote gateway, and any other remaining SIP objects not displayed in the configuration above. Refer to the Configure SIP (Single SIP IP) topic for a procedure that describes how to configure the remaining SIP functionality. This procedure can be used to finish the SIP configuration for this feature. 

Additional Information

• This feature is enabled by default and cannot be disabled.

• The default number of retry after timer is 5 seconds.

• A retry-after value in the form of a SIP date as shown below is supported. 

• The duration parameter used to indicate how long the IMG 2020 will be available after the specified retry-after time is sent is NOT supported. See example below.

• The retry-after comment will always be either: Minor SIP Msg Overload, Major SIP Msg Overload, or Critical SIP Msg Overload.