TCP keepalive under linux

TCP keepalive During TCP connection idle
Main goal:

• Checking for dead peers
• Preventing disconnection due to network inactivity- TCP connections through NATs or proxy but they keep limited memory resources (mantain a few TCP connections, discard old connections)

How to use TCP keepalive under Linux

• tcp_keepalive_time - the interval between the last data packet sent (simple ACKs are not considered data) and the first keepalive probe; after the connection is marked to need keepalive
• tcp_keepalive_intvl - the interval between subsequential keepalive probes, regardless of what the connection has exchanged in the meantime
• tcp_keepalive_probes - the number of unacknowledged probes to send before considering the connection dead and notifying the application layer

procfs interface

echo 20 > /proc/sys/net/ipv4/tcp_keepalive_probes echo 600 > /proc/sys/net/ipv4/tcp_keepalive_time echo 60 > /proc/sys/net/ipv4/tcp_keepalive_intvl

sysctl interface

sysctl -w \ > net.ipv4.tcp_keepalive_time=600 \ > net.ipv4.tcp_keepalive_intvl=60 \ > net.ipv4.tcp_keepalive_probes=20

How to program network applications with TCP keepalive

• System call :setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, optlen)

• Other three socket options for the current socket overide system-wide variables as the follwoing

TCP_KEEPCNT: overrides tcp_keepalive_probes

TCP_KEEPIDLE: overrides tcp_keepalive_time

TCP_KEEPINTVL: overrides tcp_keepalive_intvl
reference from http://tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/#whatis