TCP keepalive only applies to idle connections. Its purpose primarily
for cases where the local process waits for data from the peer
indefinitely. If the peer disappears or reboots, the connection can tie
up resources forever, so you need a keepalive. However, if the local
TCP sends data, the connection will be terminated after a certain number
of timeouts (controlled by tcp_retries2), usually much shorter than the
keepalive timeout.
If you really want to shorten the time a connection dies during a
network disconnection, you can pull down tcp_retries2, but do so at your
own risk...
-John
Jeff Weber wrote:
If I:
a)
enable TCP keepalive on a server,
lower the default keepalive parameters,
establish a connection from a client,
physically disconnect the server network,
monitor client socket read status with select()
the select() indeed notices the client socket is dead in an interval
consistent with the keepalive parameters.
However, if I:
b)
enable TCP keepalive on a server,
lower the default keepalive parameters,
establish a connection from a client,
physically disconnect the server network,
periodically send data to the disconnected client socket <-- NEW
monitor client socket read status with select()
the select() is not able to see the client socket is dead. That is, it
appears that writing to a socket associated with a physically disconnected
network interferes with the keepalive algorithm. I would expect that the
inactivity timer associated with the socket keepalive algorithm is reset upon
actually receiving traffic from the remote socket.
Is this the intended behavior of the TCP keepalive algorithm, or a bug?
[kernel = 2.4.27 on i686]
TIA,
Jeff
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