IPv4 and IPv6 traffic throttling by the SBC Core towards a SIP peer is initiated on receipt of a 503 response from the SIP peer. The 503 message indicates that the peer is overloaded. Once no 503 responses are received during the configured sampling period, the traffic rate is gradually increased to normal levels.

 Traffic throttling is implemented through a configurable parameter, Peer-Overload Throttling, associated with the SIP trunk group for egress signaling. The throttling behavior is controlled with the following parameters:

  • Sampling period—Period in which the received 503 responses are counted.
  • Discard period—The period during which the received 503 responses are not counted for traffic adjustment calculations. This period is used to absorb 503 responses sent by the SIP peer before rate reduction but received by the SBC after rate reduction.
  • Step-down gain—Controls rate of traffic decline when reject threshold is exceeded.
  • Reject threshold—Indicates the number of 503 responses required in a sampling period to further reduce the allowed traffic rate to a SIP peer.

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