SIP Retry-After Header - Transmit

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 2020 IMG 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 2020 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. Refer to SIP Retry-After Header - Receive for information on the receive side.

SIP Retry-After Header Diagram


Call Flow Diagram


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

  2. If 2020 IMG 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 2020 IMG again. The Retry-After Header will specify whether the congestion is one of the following:

Configuration of SIP Retry-After feature

Initial Configuration

The procedure below explains how to configure the 2020 IMG 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 2020 IMG must have an initial configuration created on it. Follow the Basic Configurations procedure before proceeding.

Create SIP Profile (SGP)

  1. Create a SIP Profile. Right Click on 2020 IMG EMS and select New Profiles.

  2. Right Click on the Profiles object just created and select SIP SGP. Change the SIP Profile ID to something other than 0. This will allow other fields in this object to be modified. Refer to SIP Signaling Gateway Profile (SGP) for more information on configuring this object.

  3. The Retry-After Support field is defaulted to enable. Verify enable is the selection displayed.

  4. Enter a name in the SIP Profile Name field.

Create SIP Channel Groups

  1. Right Click on 2020 IMG EMS and select New Routing Configuration. See Routing Configuration for more information.

  2. Right Click on Routing Configuration and select New Channel Groups. This is a container object and no configuration is needed. Refer to Channel Groups topic for more information.

  3. Right Click on Channel Groups object and select New Channel Group. Enter a name to identify this channel group and change the Signaling Type to SIP using the drop down menu. Refer to SIP Channel Group for more information on configuring a SIP channel group.

Create SIP Signaling

  1. Right Click on 2020 IMG Name: and select New Signaling.  Refer to Signaling topic for more information on this object.

  2. Right Click on the Signaling object created and select New SIP.

  3. Select the SIP SGP Profile created above in the Remote 2020 IMG SIP Profile field.

  4. Select from drop-down menu the number 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)See screen capture below.

Refer to SIP Signaling topic for more information.



Create Remote Gateway

Create an external network element. To associate the SIP profile created with a remote SIP gateway, an external network element must be created. Follow procedure below to communicate with a remote gateway.

  1. Right Click on 2020 IMG EMS and select New External Network Elements. Refer to External Network Elements topic for more information.

  2. Right Click on External Network Elements and select New External Gateways. Refer to External Gateways for more information.

  3. Right Click on External Gateways and select New External Gateway. Select SIP as the Gateway Signaling Protocol.

  4. Enter in the Gateway IP Address field an IP address of the gateway that the 2020 IMG will be communicating with.

  5. Select the SIP SGP created above from the drop down menu in the SIP Profile field. Refer to External Gateway topic for more information.

  6. Once the External Network Elements are created, they can be associated with Channel Groups through the IP Network Elements object. See below

Insert SIP External Network Element into SIP Channel Group:

  1. Right Click on the SIP Channel group created and select New IP Network Element. Within the IP Network Element field, select from drop down menu, the gateway created. See screen capture below.



Additional Information