Re: [Qemu-devel] PATCH: enabling TCP keepalives - v3

[David Ahern <dsahern@gmail.com>]

Richard W.M. Jones wrote:
> On Thu, Apr 30, 2009 at 01:40:42PM -0600, David Ahern wrote:
>> Did not see a response to the last version.
>>
>> This patch enables TCP keepalives on VNC connections and TCP-based char
>> devices.
>>
>> Default parameters have keep alive probes sent after 60-seconds of idle
>> time. Probes are sent every 12 seconds with the connection resetting
>> after 5 failed probes (ie., connection is closed if no response received
>> in 60-seconds).
> 
> IMHO this should be optional, and firmly default to _OFF_.  Brief
> network outages shouldn't result in connections failing all over the
> place.  In addition, does this negatively impact migration?

It's not a matter of connections failing; it's a matter of cleaning them
up for a variety of reasons. Besides the VPN example which motivated
this patch (i.e, VPN connection drops and when re-established you get a
differnt IP), there are a lot of networks with very aggressive firewalls
(e.g., 60-minute timers). Without some sort of keepalive mechanisms
those firewalls will close the holes and the connections will hang.

I'll take a look at adding yet another command line option to enable
this. sshd for example, does not specify individual timer and count
values, only on/off. So for char devices, how about something like:

-serial tcp:<ip>:<port>[,server][,nowait][,tcpkeep]

-vnc display[,tcpkeep]

If timer and counters are to be configurable, I could do something like
tcpkeep=i,j,k, where i is the idle time, j is the interval for sending
probes and k is the count of missed probes.

I have not run, and not setup to run, migration tests. Will migrations
work as expected if the network were to stall for 2 minutes? The current
patch would only drop the connection after 2 minutes of no response.

david

> 
> Rich.
>