1. Configuration Guide
  2. Session Plug-In Language
  3. Enterprise SPLs
  4. Suppression of Extraneous 18x Messages

Suppression of Extraneous 18x Messages

You can configure the ESBC to suppress some provisional 18x, specifically 180 or 183, messages from a UAS within call setup transactions to reduce excess signaling traffic. The system suppresses all of these 18x messages until it receives a 200 OK from the UAC. You configure this feature using an SPL option within the SuppressAdditionalProvisional SPL.

After the ESBC receives an initial 18x message, subsequent 18x messages may only increase the amount of traffic on the UAC network without furthering a call setup. When configured, however, the ESBC is capable of forwarding the first 18x to the client and responding to subsequent 18x, specifically 180 or 183, messages locally, thereby offloading the network from that traffic.

Examples of environments that benefit from this configuration includes MS Teams Direct Routing and AVAYA IP-PBX environments, which typically issue multiple 18x responses towards the caller while waiting for subsequent call setup signaling. Without this feature, the ESBC forwards all of these 18x responses.

You enable this feature by configuring the SPL option Drop-Additional-Provisional within a session-agent, realm-config, sip-interface, and/or the system elements. If the ESBC encounters any enabled Drop-Additional-Provisional SPL option, it triggers the feature for that agent, realm or interface. If you enable the SPL option at the system level, you enable the feature for all traffic.

With respect to provisional responses, the ESBC normally monitors this traffic for 180 responses that include the 100rel header. When you enable this feature, it performs a similar function, forwarding the first 180 response (with 100rel) to the other side, then suppressing the subsequent 180 responses (with 100rel). In addition, the ESBC sends a PRACK towards the UAS side using its +100rel interworking feature.

Additional operational behavior includes:

  • The ESBC suppresses all subsequent 18x responses with or without 100rel, sends a PRACK to the UAS, and accepts the subsequent 200OK for 100rel 180s.
  • If the ESBC receives a 18x message that it needs to suppress includes a 100rel tag in a Required header, the ESBC sends a PRACK for that message with the 100rel Required tag, assuming proper configuration.
  • Deployments using this feature can accept a 200 OK with a different ‘To’ tag than the one sent in the first 180 Ringing. For example, consider the scenario wherein the ESBC sends a 18x to a UAC with ‘to’ tag = t1, followed by subsequent 18x messages with the 'To' tags – ‘t2’, ‘t3’, ‘t4’. This feature suppresses these subsequent 18x messages. If a 200 OK then arrives at the ESBC with the 't4' ‘To’ tag = ‘t4’, both the ESBC and the UAC accept this 200 OK.

Required Conditions

This feature imposes the following requirements for proper operation:

  • The incoming INVITE must include SDP.
  • The egress side of the ESBC must send SDP in the 200 OK, even if SDP was included in the 18x.
  • Early media is not supported within the context of this feature operation because applicable 18x messages may include SDP.
  • This feature does not work within forked call flows.

Feature Configuration

When you enable the Drop-Additional-Provisional SPL option, the system automatically loads the SuppressAddionalProvisional.spl. 

ORACLE(session-agent)#spl-options +Drop-Additional-Provisional

Use the syntax below to disable this feature.

ORACLE(session-agent)#spl-options -Drop-Additional-Provisional

Related Configuration

The following additional features may be required for this feature:

  • If you need to support 18x suppression for scenarios that include the 100rel tag in a Require header, you must enable the 100rel-interworking option on the applicable sip-interface.
  • If you are using the SuppressAdditionalProvisional SPL loaded on an ESBC version that does not support this feature, and are upgrading to a version that does, remove this suppression SPL manually and reboot your system before you perform this upgrade.