SIP Retry-After Header - (Transmit)  


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

Related Topics:

SIP Retry-After Header - Receive

SIP Signaling


SIP Retry-After Header Diagram:



Call Flow Diagram:





  1. 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.

  2. 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:


Additional Information:



Configuration of SIP Retry-After feature:

  1. Create a Logical IMG Group. See Logical IMG Group

  2. Create a Physical IMG Group. See Physical IMG Group

  3. Create a Signaling object under the Physical IMG Group. See Signaling

  4. Create a SIP signaling object by right clicking on the Signaling Object and select 'New SIP'. The following Pane will appear.


  5. 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)