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This page describes how we handle established TCP connections

This page describes how we handle established TCP connections.

== TCP repair mode in kernel ==

== TCP repair mode in kernel ==

The sockoption called TCP_REPAIR was recently added to the kernel and helps with doing C/R for TCP sockets.

The <code>TCP_REPAIR</code> socket option was added to the kernel 3.5 to help with C/R for TCP sockets.

When set this option turn the socket into a special state in which any action performed on it doesn't

When this option is used, a socket is switched into a special mode, in which any action performed on it

result in any defined by protocol actions, but instead directly puts the socket into a state, which

does not result in anything defined by an appropriate protocol actions, but rather directly puts the socket

should be at the end of the successfully finished operation.

into a state, in which the socket is expected to be at the end of the successfully finished operation.

E.g. calling connect() on a repaired socket just switches one to the ESTABLISHED state with the peer set as requested.

For example, calling <code>connect()</code> on a repaired socket just changes its state to <code>ESTABLISHED</code>,

The bind() call forcibly binds the socket to a given address (ignoring any potential conflicts). Close()-ing the

with the peer address set as requested.

socket under repair happens without any transient FIN_WAIT/TIME_WAIT/etc states. Socket is silently killed.

The <code>bind()</code> call forcibly binds the socket to a given address (ignoring any potential conflicts).

The <code>close()</code> call closes the socket without any transient <code>FIN_WAIT</code>/<code>TIME_WAIT</code>/etc states,

socket is silently killed.

=== Sequences ===

=== Sequences ===

In order to restore the connection properly only binding and connecting it is not enough. One also needs to restore the

To restore the connection properly, bind() and connect() is not enough. One also needs to restore the

TCP sequence numbers. To do so the TCP_REPAIR_QUEUE and TCP_QUEUE_SEQ options were introduced.

TCP sequence numbers. To do so, the <code>TCP_REPAIR_QUEUE</code> and <code>TCP_QUEUE_SEQ</code> options were introduced.

The former one selects which queue (input or output) will be repaired and the latter gets/sets the sequence. Note, that

The former one selects which queue (input or output) will be repaired and the latter gets/sets the sequence. Note

setting the sequence is only possible on CLOSE-d socket.

setting the sequence is only possible on CLOSE-d socket.