[CRIU] criu - Restoring TCP connections and timestamps

[Dilip Daya]

Hi Pavel,

On Tue, 2012-11-20 at 20:55 +0400, Pavel Emelyanov wrote:
> On 11/20/2012 08:21 PM, Dilip Daya wrote:
> > 
> > Re: tcp_time_stamp when restoring TCP connections
> > 
> > tcp_time_stamp:
> > This is the low-order 32 bits of the jiffies counter and is used to
> > generate the timestamp in the TCP timestamp option.  The timestamp
> > option is included in almost every outgoing TCP packet and is used for
> > two purposes.
> > 
> > For PAWS, it acts something like a high-order extension of the packet
> > sequence number.  If the receiver sees a timestamp value less than what
> > it previously got on the connection (modulo 2^31), it assumes the
> > sequence number is from a previous wrap and discards the packet.  The
> > timestamp generator must therefore never decrease.
> > 
> > The jiffies counters on separate systems can be any value when restore
> > occurs, so the restore system has a good chance of generating timestamps
> > less than that of the original system and the this will start dropping
> > packets.  This problem was discussed in the paper "TCP Connection
> > Passing" by Werner Almesberger, which is the basis of previous
> > connection migration code for Linux.  The solution given in that paper
> > is to save an offset for each connection that is applied to
> > tcp_time_stamp when generating the TCP timestamp option.  The offset is
> > calculated to ensure the timestamps never decrease.
> > 
> > Connection Repair does not appear to do anything special for timestamp
> > generation and I'm wondering how it gets away with that.
> > 
> > => So looking at Linux kernel code snippets, does the following take
> >    care of the this issue?
> 
> You're right, the tcp repair feature is not complete yet. Currently we're
> concentrated on making it work when doing c/r without changing the kernel.
> If you can participate in this things will get fixed faster :)

Sure I'm ready to participate, but will tread softly with the help of
your guidance.

Let me very briefly explain the motivation that stemmed from your great
work on c/r TCP connections: I'm investigating in a kernel-level
framework for live TCP connection failover at sockets-layer and hope to
make minimal kernel changes that are acceptable to c/r project as well
as include any changes to crtool.

Thanks.
-DilipD.

> 
> I think I should have a page on wiki that lists all the issues we haven't
> yet fixed with tcp repair.

That will be a great!

Thanks.
-DilipD.

> 
> Thanks,
> Pavel