Re: [ovs-dev] [PATCH net-next] openvswitch: add TTL decrement action

[Matteo Croce]

On Mon, Nov 18, 2019 at 5:20 PM Ben Pfaff <blp@xxxxxxx> wrote:
>
> On Tue, Nov 12, 2019 at 04:46:12PM +0100, Matteo Croce wrote:
> > On Tue, Nov 12, 2019 at 4:00 PM Simon Horman <simon.horman@xxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Nov 12, 2019 at 11:25:18AM +0100, Matteo Croce wrote:
> > > > New action to decrement TTL instead of setting it to a fixed value.
> > > > This action will decrement the TTL and, in case of expired TTL, send the
> > > > packet to userspace via output_userspace() to take care of it.
> > > >
> > > > Supports both IPv4 and IPv6 via the ttl and hop_limit fields, respectively.
> > > >
> > >
> > > Usually OVS achieves this behaviour by matching on the TTL and
> > > setting it to the desired value, pre-calculated as TTL -1.
> > > With that in mind could you explain the motivation for this
> > > change?
> > >
> >
> > Hi,
> >
> > the problem is that OVS creates a flow for each ttl it see. I can let
> > vswitchd create 255 flows with like this:
> >
> > $ for i in {2..255}; do ping 192.168.0.2 -t $i -c1 -w1 &>/dev/null & done
> > $ ovs-dpctl dump-flows |fgrep -c 'set(ipv4(ttl'
> > 255
>
> Sure, you can easily invent a situation.  In real traffic there's not
> usually such a variety of TTLs for a flow that matches on the number of
> fields that OVS usually needs to match.  Do you see a real problem given
> actual traffic in practice?
>

Hi Ben,

yes, my situation was a bit artificious, but you can get a similar
situation in practice.

Imagine a router with some subnetworks behind it on N levels, with
some nodes hosting virtual machines.
Windows and Linux have different default TTL values, 128 and 64
respectively, so you could see N*2 different TTL values.

Bye,



--
Matteo Croce
per aspera ad upstream