[ovs-discuss] Is there an option to disable metadata propagation on patch ports?

[Ben Pfaff]

What metadata is propagating?

On Thu, Apr 05, 2018 at 08:14:51PM +0200, Alan Kayahan wrote:
> I introduced a new 32bit field in flow.h to match on. A push_header action
> appends a header, which contains TLVs, to the packet and sets this field
> for the first time. An increment action sets this field to a value that
> resides in the first TLV, if increment action is called again, the field is
> set to the value in the second TLV and so on.
> 
> I have a chain of br0-br1-br2 connected via patch ports. In br0 flow rule
> calls push_header and out to br1. In br1, the flow rule matches on the port
> connected to br0, plus match on the 32bit field.The action for this rule is
> increment, then out to br2. Just like br1, br2 matches on the port from br1
> plus the match on the 32bit field for the value in the TLV.
> 
> Everything works well until br2. But the match on the field for the value
> in the TLV doesn't work. If I remove the match on the field at br2 and just
> use in_port and redirect the traffic to packet hex printer, I do see that
> the value in the TLV has been set correctly at the offset of this 32bit
> field.
> 
> The increment action depends on the iteration of TLVs in the header at each
> packet, therefore is at datapath and can not update the flow context. So if
> the flow metadata is propagating (which seems to be the only explanation),
> the new field is set to the old value, which is the culprit. Perhaps a
> better approach is to create an own context just like nat, iterate the TLVs
> and populate the context with all available values when push_header is
> first called, then redesign the increment action so that it just pops the
> next value from the context and emits the data-plane action. Eliminates
> TLVs iteration per packet at the dataplane.
> 
> But just to save the day, do you have a trick to stop this propagation? Or
> do you think the problem might be something else?
> 
> Thanks!
> 
> 2018-04-05 19:41 GMT+02:00 Ben Pfaff <blp at ovn.org>:
> 
> > On Thu, Apr 05, 2018 at 07:31:35PM +0200, Alan Kayahan wrote:
> > > OVS patch ports allow the propagation of the metadata (e.g. flow context)
> > > across the connected switches.
> > >
> > > Is there an option to disable the metadata propagation feature? I need
> > this
> > > for my research to benchmark certain behavior. Or patch ports become
> > > nothing but veth pairs when this feature is disabled?
> >
> > Patch ports zero out most metadata.  What additional metadata do you
> > want them to clear?
> >