II. modeling of BIND (cont.)

how does a client select a root server? (courtesy vix)
        1) rtt sorting
try every NS+A until you find best one
        2) aged rtt sorting  
gradually depref the "best NS+A" to force rescans
        3) priming
only use "root.cache" to do an initial ". NS" query sweep
        4) static
use servers in order on EVERY query, stop when answer heard
5) random selection among known authorative (including roots)
        6) round robin  
rotate the NS and/or A list every time one is used.

BIND4/BIND8, and modern BIND9 do (2) and (3)
early BIND9 did (1) and (3)
djbdns does (5)
win2k does (5) [although it may tend to prefer A if it responds quickly]
win2003 "(2)-like"  [per usoft: tries to balance across NSes per node]
nobody does (6)