Frequency of traffic encryption key change, based upon number
of seconds elapsed since last key change; in operation, this
is effectively rounded up to the next full packet. Note that
the key change rate is also influenced by number of bytes
encrypted. The so-called 'best' value will be dependent upon
the algorithms that use the TEK; also, for a given algorithm
the 'best' value will change over time due to the progresses
made in cryptanalysis. The strongest value of this or the
corresponding line setting will be used. Units are seconds.