Gdp packetout (openconfig#3111)

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feature/experimental/p4rt/otg_tests/google_discovery_protocol_packetout_lag_test/README.md

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+# P4RT-3.21: Google Discovery Protocol: PacketOut with LAG
+
+## Summary
+
+Verify that GDP packets can be sent by the controller.
+
+## Testbed Type
+
+* https://github.com/openconfig/featureprofiles/blob/main/topologies/atedut_2.testbed
+
+## Procedure
+
+* Configure two lag bundles between ATE and DUT with one member port in each of the LAG.
+  A[ATE:LAG1] <---> B[LAG1:DUT];
+  C[ATE:LAG2] <---> D[LAG2:DUT];
+*	Enable the P4RT server on the device.
+*	Connect two P4RT clients in a master/secondary configuration.
+*	Configure the forwarding pipeline and install the P4RT table entry required for GDP.
+*	Send a GDP packet from the master with egress_singleton_port set to one of the connected interfaces.
+*	Verify that the GDP packet is received on the ATE port connected to the indicated interface.
+*	Repeat sending the packet in the same way but from the secondary connection.
+*	Verify that the packet is not received on the ATE.
+
+## OpenConfig Path and RPC Coverage
+
+The below yaml defines the OC paths intended to be covered by this test.
+
+```yaml
+paths:
+    ## Config parameter coverage
+    /interfaces/interface/ethernet/config/port-speed:
+    /interfaces/interface/ethernet/config/duplex-mode:
+    /interfaces/interface/ethernet/config/aggregate-id:
+    /interfaces/interface/aggregation/config/lag-type:
+    /network-instances/network-instance/vlans/vlan/state/vlan-id:
+
+    ## Telemetry parameter coverage
+    /lacp/interfaces/interface/members/member/state/counters/lacp-in-pkts:
+    /lacp/interfaces/interface/members/member/state/counters/lacp-out-pkts:
+    /lacp/interfaces/interface/members/member/state/counters/lacp-rx-errors:
+    /lacp/interfaces/interface/name:
+    /lacp/interfaces/interface/state/name:
+    /lacp/interfaces/interface/members/member/interface:
+    /lacp/interfaces/interface/members/member/state/interface:
+
+
+    ## Protocol/RPC Parameter Coverage
+rpcs:
+  gnmi:
+    gNMI.Subscribe:
+    gNMI.Set:
+    gNMI.Get:
+```
+
+## Required DUT platform
+
+* FFF