Why does it take so long to detect that the peer died?
From Andrew Gierth (email@example.com):
Because by default, no packets are sent on the TCP connection unless
there is data to send or acknowledge.
So, if you are simply waiting for data from the peer, there is no way
to tell if the peer has silently gone away, or just isn't ready to
send any more data yet. This can be a problem (especially if the peer
is a PC, and the user just hits the Big Switch...).
One solution is to use the SO_KEEPALIVE option. This option enables
periodic probing of the connection to ensure that the peer is still
present. BE WARNED: the default timeout for this option is AT LEAST 2
HOURS. This timeout can often be altered (in a system-dependent
fashion) but not normally on a per-connection basis (AFAIK).
RFC1122 specifies that this timeout (if it exists) must be
configurable. On the majority of Unix variants, this configuration
may only be done globally, affecting all TCP connections which have
keepalive enabled. The method of changing the value, moreover, is
often difficult and/or poorly documented, and in any case is different
for just about every version in existence.
If you must change the value, look for something resembling
tcp_keepidle in your kernel configuration or network options
If you're sending to the peer, though, you have some better
guarantees; since sending data implies receiving ACKs from the peer,
then you will know after the retransmit timeout whether the peer is
still alive. But the retransmit timeout is designed to allow for
various contingencies, with the intention that TCP connections are not
dropped simply as a result of minor network upsets. So you should
still expect a delay of several minutes before getting notification of
The approach taken by most application protocols currently in use on
the Internet (e.g. FTP, SMTP etc.) is to implement read timeouts on
the server end; the server simply gives up on the client if no
requests are received in a given time period (often of the order of 15
minutes). Protocols where the connection is maintained even if idle
for long periods have two choices:
1. use SO_KEEPALIVE
2. use a higher-level keepalive mechanism (such as sending a null
request to the server every so often).