SIP Retry-After Header - Receive

The 2020 IMG has the ability to react when receiving the Retry-After Header in any one of the following responses after an INVITE message has been sent. Refer to SIP Retry-After Header - Transmit for information on the transmit side.

Response Value



Not Found (User not found)


Request Entity Too Large


Temporarily Unavailable


Busy Here


Server Internal Error


Service Unavailable


Busy Everywhere




When routed to a single gateway, the 2020 IMG will not re-send an INVITE message back to this particular gateway for the time specified in the Retry-After header field received in one of the above responses. The maximum amount of time after receiving the Retry-After Header 2020 IMG will wait is 65535 seconds. This feature is configured in the SIP SGP Profile pane. See below for more information.

Note: (When Route List is Configured): If a route list is configured, the call will be rerouted to the next gateway entry in the Route List. The 2020 IMG will not route subsequent calls to this initial gateway until the timer value indicated in the Retry-After header has elapsed. After timer has elapsed, the gateway will be available again and calls can be routed to it.

SIP Retry-After Header Diagram --> (Receive Retry-After)


Call Flow Diagram


  1. The 2020 IMG sends an invite message to a remote gateway.
  2. The remote gateway cannot process the INVITE and sends a 4xx, 5xx, or 6xx response value as shown in Call Flow above. Within the response message is a SIP Retry-After Header with a wait time. See message below

    Retry-After: 17

  3. When the retry header is validated the call is either rerouted or released depending on the configuration. Any subsequent calls will not be routed to this gateway until after the Timer expires. At that time normal routing and call setup will resume (This example would be 17 seconds).

Configuration of SIP Retry-After feature: (Receive)

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

  2. Right-click External Network Elements and select New External Gateways. Refer to External Gateways for more information.

  3. Right-click 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