[ovs-discuss] Query on DEC_ttl action implementation in datapath

bindiya Kurle bindiyakurle at gmail.com
Wed Aug 28 06:25:19 UTC 2019


Hi Justin,

Thanks for the clarification. I agree to your point ,but consider a
use-case if I have routing decision only based on destination ip then as
per current implementation, ovs will add 2 flows (in data path)for packets
coming from different source and if the TTL happens to be different for
them .This will reduce number of flows that can be supported with ovs. If
decrement TTL  was done in kernel ,it would have ended in adding one flow
only.

Regards,
Bindiya

Regards,
Bindiya

On Tue, Aug 27, 2019 at 9:48 PM Justin Pettit <jpettit at ovn.org> wrote:

> I think it was considered cleaner from an ABI perspective, since it
> doesn't require another action, since "set" was already supported.  In
> practice, I don't think it's a problem, since usually a TTL decrement is
> associated with a routing decision, and TTLs tend to be fairly static
> between two hosts.
>
> --Justin
>
>
> > On Aug 27, 2019, at 1:11 AM, bindiya Kurle <bindiyakurle at gmail.com>
> wrote:
> >
> > hi ,
> > I have a question related to dec_ttl action implemented in datapath.
> > when  dec_ttl action is configured in OVS following action get added in
> datapath.
> >
> > recirc_id(0),in_port(2),eth(),eth_type(0x0800),ipv4(ttl=64,frag=no),
> packets:3, bytes:294, used:0.068s, actions:set(ipv4(ttl=63)),3,
> >
> > if packet comes with different TTL on same port then one more action get
> added in datapath.
> > for ex:
> > recirc_id(0),in_port(2),eth(),eth_type(0x0800),ipv4(ttl=9,frag=no),
> packets:3, bytes:294, used:0.068s, actions:set(ipv4(ttl=8)),3,
> >
> > Could someone please explain why dec_ttl is implemeted as a set action
> rather than dec_ttl action.
> >
> >
> > I mean , why for different ttl one more rule get added rather than  just
> adding it as  following as done in userspace
> >
> > recirc_id(0),in_port(3),eth(),eth_type(0x0800),ipv4(frag=no), packets:3,
> bytes:294, used:0.737s, actions:dec_ttl,2
> >
> > Regards,
> > Bindiya
> > _______________________________________________
> > discuss mailing list
> > discuss at openvswitch.org
> > https://mail.openvswitch.org/mailman/listinfo/ovs-discuss
>
>
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