when RTO retransmissions remain unacknowledged…

$cat /proc/sys/net/ipv4/tcp_orphan_retries

tcp_orphan_retries - INTEGER
This value influences the timeout of a locally closed TCP connection,
        when RTO retransmissions remain unacknowledged. See
 tcp_retries2 for more details.

The default value is 7

If your machine is a loaded WEB server, you should think about
 lowering this value, such sockets may consume significant resources.
 Cf. tcp_max_orphans.

source :
Linux Kernel Documentation . 2.6.32
net/ipv4/tcp_timer.c - 39 identical
    99: static int tcp_orphan_retries(struct sock *sk, int alive)
   100: {
   101:         int retries = sysctl_tcp_orphan_retries; /* May be zero. */
   157:                         retry_until = tcp_orphan_retries(sk, alive);
android.git.kernel.org/kernel/msm.git - GPL - C - More from msm.git »

   176:   echo "Set number of orphant retries to 5"
   177:   echo 5 > /proc/sys/net/ipv4/tcp_orphan_retries
www.chronox.de/tc+filter/shaper-0.2.tar.bz2 - Unknown - Shell - More from shaper-0.2.tar.bz2 »

   282: .TP
   283: .B tcp_orphan_retries
   284: The maximum number of attempts made to probe the other
www2.cddc.vt.edu/linux/distributions/7linux/7v6/7base/7v6a11.tar.bz2 - Unknown - Troff -

3.3.15. tcp_orphan_retries
The tcp_orphan_retries variable tells the TCP/IP stack how many times
 to retry to kill connections on the other side before killing it on our
 own side. If your machine runs as a highly loaded http server it may
 be worth thinking about lowering this value. http sockets will consume
 large amounts of resources if not checked.

This variable takes an integer value. The default value for this variable
 is 7, which would approximately correspond to 50 seconds through 16
 minutes depending on the Retransmission Timeout (RTO). For a
 complete explanation of the RTO, read the "3.7. Data Communication"
 section in RFC 793 - Transmission Control Protocol.

source :
Ipsysctl tutorial 1.0.4
Oskar Andreasson
Copyright © 2002 by Oskar Andreasson