Q. How does InterMapper computer errors for a link?

A. The Percent Err values are computed as follows:

The one-way percent errors under the Receive section are computed by totalling { ifInUcastPkts, ifInNUcastPkts, ifInErrors, ifInDiscards } as follows:

  • PERCENT ERROR = totalErrors / totalPkts;


    totalErrors = dErrs + dDis and
    totalPkts = dUcast + dNUcast + totalErrors


    dUcast = ifCurrStats.inUcastPkts - ifPrevStats.inUcastPkts
    dNUcast = ifCurrStats.inNUcastPkts - ifPrevStats.inNUcastPkts
    dErrs = ifCurrStats.inErrors - ifPrevStats.inErrors
    dDis = ifCurrStats.inDiscards - ifPrevStats.inDiscards

Note: Either of 'dErrs' or 'dDis' may be forced to zero if you have "IgnoreInterface Errors" or "Ignore Interface Discards" checked.

The one-way percent errors for outgoing traffic are similarly computed from the { ifOutUcastPkts, ifOutNUcastPkts, ifOutErrors, ifOutDiscards } statistics.

The two-way Percent error number (just below Utilization on the Interface information pop-up) is the probability given both one-way error percentages that a packet will be lost making the round-trip across the link and back. If the probability of successful transmission is T and the probability of successful receipt is R (and assuming the act of transmission and receive are relatively independent), then the probability of a successful round-trip is T * R. The probability of error is (1 - T*R).

T and R are computed from the complement of the one-way percent errors above.