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Grammar fixes
The <code>TCP_REPAIR</code> socket option was added to the kernel 3.5 to help with 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 this option is used, a socket is switched into a special mode, in which any action performed on it
When this option is used, the socket is switched into a special mode, in which any action performed on it
does not result in anything defined by an appropriate protocol actions, but rather directly puts the socket
does not result in anything defined by an appropriate protocol actions, but rather directly puts the socket
into a state, in which the socket is expected to be at the end of the successfully finished operation.
into the state that the socket is expected to be in at the end of a successfully finished operation.
For example, calling <code>connect()</code> on a repaired socket just changes its state to <code>ESTABLISHED</code>,
For example, calling <code>connect()</code> on a repaired socket just changes its state to <code>ESTABLISHED</code>,
All four can be read with <code>getsockopt()</code> calls to a socket and in order to restore them the <code>TCP_REPAIR_OPTIONS</code> sockoption is introduced.
All four can be read with <code>getsockopt()</code> calls to a socket and in order to restore them the <code>TCP_REPAIR_OPTIONS</code> sockoption is introduced.
== Timestamp ==
"The sender's timestamp clock is used as a source of monotonic non-decreasing values to stamp the segments"(rfc7323). The Linux kernel uses the jiffies counter as the tcp timestamp.
<code>#define tcp_time_stamp ((__u32)(jiffies))</code>
We add the <code>TCP_TIMESTAMP</code> options to be able to compensate a difference between jiffies counters, when a connection is migrated on another host. When a connection is dumped, criu calls <code>getsockopt(TCP_TIMESTAMP)</code> to get a current timestamp, then on restore it calls <code>setsockopt(TCP_TIMESTAMP)</code> to set this timestamp as a starting point.
== Checkpoint and restore TCP connection ==
== Checkpoint and restore TCP connection ==
could isolate the whole netns from network. Typically this is done by downing the respective veth pair end.
could isolate the whole netns from network. Typically this is done by downing the respective veth pair end.
== More info ==
== States ==
=== TCP_SYN_SENT ===
There is only one difference with TCP_ESTABLISHED, we have to restore a socket and disable the repair mode before calling <code>connect()</code>. The kernel will send a one syn-sent packet with the same initial sequence number and sets the TCP_SYN_SENT state for the socket.
=== Half-closed sockets ===
A socket is half-closed when it sent or received a fin packet. These sockets are in one for these states: TCP_FIN_WAIT1, TCP_FIN_WAIT2, TCP_CLOSING, TCP_LAST_ACL, TCP_CLOSE_WAIT. To restore these states, we restore a socket into the TCP_ESTABLISHED state and then we call shutfown(SHUT_WR), if a socket has sent a fin packet and we send a fake fin packet, if a socket has received it before. For example, if we want to restore the TCP_FIN_WAIT1 state, we have to call shutfown(SHUT_WR) and we can send a fake ack to the fin packet to restore the TCP_FIN_WAIT2 state.
== See also ==
* [[Simple TCP pair]]
* [[TCP repair TODO]]
* [[CLI/opt/--tcp-close|Dropping the connection]]
== External links ==
* http://lwn.net/Articles/495304/
* http://lwn.net/Articles/495304/
[[Category:Under the hood]]
[[Category:Under the hood]]
[[Category:Network]]
[[Category:Sockets]]
[[Category: Editor help needed]]