Description of problem:

OVS currently supports an OpenFlow extension (OFPRAW_NXT_CT_FLUSH_ZONE) to flush all conntrack entries in a given zone:

https://github.com/openvswitch/ovs/blob/ff55e8f3858ae153ef1996df3d52938f3df6d950/include/openvswitch/ofp-msgs.h#L515

/* NXT_CT_FLUSH_ZONE.
 *
 * Flushes the connection tracking table. */
struct nx_zone_id {
    uint8_t zero[6];            /* Must be zero. */
    ovs_be16 zone_id;           /* Connection tracking zone. */
}

It would be desirable to be able to flush a subset of the conntrack entries in a zone.  The subset could be defined by a (potentially partial) tuple of the form: IP, L4 proto, L4 port.

For example, OVN would use this feature to clear conntrack entries that correspond to a load balancer backend ("IP:PROTO:PORT") that was recently removed (bug 1839103).  This means that all conntrack records that use IP, PROTO and PORT either in the original or in the reply direction need to be flushed.

Note: OVS must be able to indicate to the controller whether this extension is supported or not, for example, via the vswitchd.Datapath.capabilities column in the local OVSDB.