Chooses the mechanism, if any, SSG uses to detect that a logged-in subscriber's host has gone away, so it can automatically log the session off rather than leaving it open. ICMP mode pings the host and ARP mode watches ARP reachability, and picking the wrong one for the access technology (e.g. ARP on a link where ARP doesn't apply) means dead sessions never get reaped, silently consuming session slots and skewing cssgStatsActiveSessions upward. On an Ethernet-aggregated DSL network where subscriber CPEs answer ARP but sometimes block ICMP, an admin would set cssgCfgAutoLogOffMode to arp(3) so a powered-off modem gets logged off within cssgCfgAutoLogOffInterval instead of lingering as a phantom session.