Congestion Control

MTP2 monitors the level of messages queued in buffers (both output and retransmission) and alerts SNM in case of congestion.

Onset of congestion messages are sent to SNM when the threshold value for the buffers is exceeded. The SNM process considers all destinations across the link to be congested.

Now consider congestion from the signaling endpoint and STP perspective:

• Signaling endpoints (SSP, SCP) receive congestion information from MTP2 onset of congestion indications. Excessive higher-layer messages can cause congestion over signal endpoint (SSP and SCP) links. In this case, SNM sends status messages to applications indicating which DPCs are affected. The application should reduce outgoing messages for a period of time. SNM continues to send the congestion status message until MTP2 receives the end of congestion indication. At this point, SNM stops sending the status messages, and after the timeout period, user applications resume normal activity.

• If the STP SNM process receives an onset of congestion alert concerning a particular link, it considers that the route to its adjacent node is congested. When messages are received for the affected node, the STP SNM process sends a Transfer Controlled (TFC) message to the SNM of the transmitting endpoint. The STP indicates the affected node in the TFC message. This enables the signaling endpoint to choose an alternate route to the affected node. When the SNM process receives the end of congestion indication, it stops sending the status indications to the transmitting endpoint.

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