IMG 1010 - SIP Retry-After Header - Transmit

Overview:

The Retry-After header field was added to the IMG to reduce SIP congestion. The IMG SIP layer receives an INVITE. The IMG would then check its congestion level and determine whether SIP is congested or not. If the SIP layer shows no congestion then the call will go through. If the IMG determines that there is congestion on the SIP layer then it will determine whether it is minor, major, or critical. Once this is determined the IMG will send a 503 (Service Unavailable) response. Within this response the IMG will include the Retry-After header which includes a time to wait before sending the SIP INVITE back to the IMG. 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. See the links in the Related Topics section below for more information

SIP Retry-After Header Diagram:

Call Flow Diagram:

Procedure:

A remote gateway sends a SIP INVITE message to the IMG. The IMG will determine whether SIP is congested. If SIP Layer 3 is not congested the call will proceed as normal.
If IMG determines that SIP is congested, the IMG will return a 503 - Service Unavailable response. Within this response will be the Retry-After Header specifying the amount of seconds to wait before sending the SIP INVITE to the IMG again. Also 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

Additional Information:

This feature is enabled by default and cannot be disabled.
The default number of retry after timer is 5 seconds.
The IMG will NOT support a retry-after value in the form of a SIP date as shown below.
- Retry-After: Mon, 2 Mar 2008 13:30:00 GMT
The IMG will NOT support a duration parameter used to indicate how long the IMG will be available for after the specified retry-after time. See example below.
- Retry-After: 5; duration=3600
The retry-after comment is not configurable. It will always be either: Minor SIP Msg Overload, Major SIP Msg Overload, or Critical SIP Msg Overload.

Configuration of SIP Retry-After feature:

Create a Logical IMG Group. See IMG 1010 - Logical IMG
Create a Physical IMG Group. See IMG 1010 - Physical IMG
Create a Signaling object under the Physical IMG Group. See IMG 1010 - Signaling Object
Create a SIP signaling object by right clicking on the Signaling Object and select New SIP. The following Pane will appear.
Select from drop-down menu the amount of seconds that will be transmitted back to the remote inbound gateway via the Retry-After header in the 503 Service Unavailable response. See examples of Retry-After headers below.
- Retry-After: 5 (Minor SIP Msg Overload)
- Retry-After: 120 (Major SIP Msg Overload)