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

Justin Pettit jpettit at ovn.org
Tue Aug 27 16:18:43 UTC 2019

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.


> 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
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