Designing and Managing Behavior Models - Using Other Data Sources - NerveCenter's Built-In Triggers - Matching Errors with Pending SNMP and Ping Requests -
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Matching Errors with Pending SNMP and Ping Requests

Each poll packet that NerveCenter sends on a socket includes a unique identifier (the IP field Sequence Number). When a poll returns ICMP errors within its configured number of retries, NerveCenter collects the error messages that are returned. Each error message includes the sequence number as well as the destination address of the associated node. Certain fields in the ICMP error packet enable NerveCenter to attempt to match SNMP/ICMP error messages with a poll's pending SNMP/ping requests as follows:

• NerveCenter compares a reply on the SNMP socket to its list of pending SNMP requests and attempts to match the reply with the sequence number of an SNMP request. If a match cannot be found with a pending SNMP request, then NerveCenter discards the reply.
• NerveCenter compares a reply on the ICMP socket to its list of pending ICMP requests and attempts to match the reply with the sequence number of an ICMP request. Table 27 summarizes how NerveCenter attempts to match ICMP replies to ICMP pending requests:

  Matching ICMP Replies with ICMP Requests

  Sequence Number Match?	Destination Address In DB?	Action
  Yes	Yes	NerveCenter fires the appropriate built-in trigger for the poll.
  No	Yes	NerveCenter saves reply to attempt to match with a pending SNMP request.
  No	No	NerveCenter discards the reply.

  If NerveCenter cannot match the sequence number of an ICMP reply with any pending ICMP requests, but NerveCenter recognizes the destination address, the reply is saved because it might be an error response to an SNMP request for that node; therefore, at regular intervals, NerveCenter compares the destination address of saved ICMP error replies with pending SNMP requests. NerveCenter attempts to match each ICMP reply with the destination address of the oldest pending SNMP request. Only after attempting to match ICMP replies with both pending ICMP and SNMP requests does NerveCenter finally discard the reply when it finds no matches.