tcp_keepalive_probes the number of unacknowledged probes to send before considering the connection dead and notifying the application layer source : www.tldp.org
$cat /proc/sys/net/ipv4/tcp_keepalive_probes 9 $
trunk/os/ip/tcp.c - 7 identical 2436: /* Compute usable segment based on offered window and limit 2437: * window probes to one 2438: */ 3064: return tcphangup(c); 30100: if(n >= 1 && strcmp(f[0], "keepalive") == 0) 3066: return tcpstartka(c, f, n); inferno-os.googlecode.com/svn - MIT - C
Taken from Research Paper. TCP Keep-alives The TCP speci?cation does not include a mecha- nism for probing idle connections. In theory, if a host crashes after establishing a connection to another host, the second machine will continue to hold the idle con- nection forever. Some TCP implementations include a mechanism that tests an idle connection and releases it if the remote host has crashed. Called TCP keep-alive, the mechanism periodically sends a probe segment to elicit response from the peer. If the peer responds to the probe by sending an ACK, the connection is alive. If the peer TCP fails to respond to probe segments for longer than a ?xed threshold,the connection is declared down and the connection is closed. Probing TCP Implementations