How Does TCP Keep-Alive Work and How Long Do Socket Connections Remain Open?

Does a TCP Socket Connection Have a "Keep Alive"?

Unlike HTTP keep-alives, TCP socket connections do not remain open indefinitely. Instead, they are subject to a timeout limit enforced by TCP Keep-Alive.

Introduction

TCP connections involve two sockets, each residing on one end of the connection. Normally, these sockets remain open until one end terminates the connection, indicated by an FIN packet. However, if unintended socket termination occurs, TCP Keep-Alive exists to detect and close stale connections.

Keep-Alive Process

On Linux systems, TCP Keep-Alive operates with the following default settings:

• tcp_keepalive_time: 7200 seconds (2 hours)
• tcp_keepalive_probes: 9
• tcp_keepalive_intvl: 75 seconds

If a connection remains idle for over tcp_keepalive_time, an ACK packet is sent to the other end. If there is no response after a specified number of probes, the connection is terminated via RST.

Gotchas

2 Hour Default: By default, the Keep-Alive process only initiates after 2 hours of inactivity, which may allow stale connections to persist.

Keep-Alive is Optional: Responding to Keep-Alive packets is optional, and some implementations may drop them.

Changing TCP Timeouts

To modify TCP timeouts, refer to the following OS-specific instructions:

Linux:

• Per-socket: Currently not supported in Java
• System-wide: Use echo to adjust /proc/sys/net/ipv4/[tcp_keepalive_time|tcp_keepalive_probes|tcp_keepalive_intvl]

Mac OS X:

• System-wide: Use sysctl -w to set net.inet.tcp.[keepidle|keepintvl|keepcnt]
• Persistent: Add settings to /etc/sysctl.conf

Windows:

• System-wide: Edit registry values in HKEY_LOCAL_MACHINESystemCurrentControlSetServicesTCPIPParameters

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