Add README for PF-1.2 Policy-based traffic GRE Encapsulation to IPv4 …

…GRE tunnel (#3002)

* Add README for PF-1.2 Policy-based traffic GRE Encapsulation to IPv4 GRE tunnel

feature/policy_forwarding/encapsulation/otg_tests/encap_gre_ipv4/README.md

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+# PF-1.2: Policy-based traffic GRE Encapsulation to IPv4 GRE tunnel
+
+## Summary
+
+The test verifies policy forwarding(PF) encapsulation action to IPv4 GRE tunnel when matching on source/destination.
+
+## Testbed type
+
+*  [`featureprofiles/topologies/atedut_4.testbed`](https://github.com/openconfig/featureprofiles/blob/main/topologies/atedut_4.testbed)
+
+## Procedure
+
+### Test environment setup
+
+*   DUT has an ingress port and 2 egress ports.
+
+    ```
+                             |         | --eBGP-- | ATE Port 2 |
+        [ ATE Port 1 ] ----  |   DUT   |          |            |
+                             |         | --eBGP-- | ATE Port 3 |
+    ```
+
+*   Traffic is generated from ATE Port 1.
+*   ATE Port 2 is used as the destination port for encapsulated 
+    traffic.
+*   ATE Port 3 is used as the fallback destination for
+    pass-through traffic.
+
+#### Configuration
+
+1.  All DUT Ports are configured as a singleton IP interfaces. Configure MTU of 9216 (L2) on ATE Port1, MTU 2000 on ATE Port 2, 3
+
+2.  IPv4 and IPv6 static routes to test destination networks IPV4-DST/IPV6-DST are configured on DUT towards ATE Port 3.
+
+3.  Another set of IPv4 static routes to 32x IPv4 GRE encap destinations towards ATE Port 2.
+
+4.  2 IPv4 and 2 IPv6 source prefixes will be used to generate tests traffic 
+(SRC1-SRC2). Apply policy-forwarding with 4 rules to DUT Port 1:
+    - Match IPV4-SRC1 and accept/foward.
+    - Match IPV6-SRC1 and accept/foward.
+    - Match IPV4-SRC2 and encapsulate to 32 IPv4 GRE destinations.
+    - Match IPV6-SRC2 and encapsulate to 32 IPv4 GRE destinations.
+
+5.   Set GRE encap source to device's loopback interface.
+6.   Either `identifying-prefix` or `targets/target/config/destination` can be used to configure GRE destinations based on vendor implementation.
+7.   Configure QoS classifier for incoming traffic on ATE Port1 for IPv4 and IPv6 traffic. 
+     QoS classifier should remark egress packet to the matching ingress DSCP value (eg. match DSCP 32, set egress DSCP 32).
+     Match and remark all values for 3 leftmost DSCP bits [0, 8, 16, 24, 32, 40, 48, 56].
+
+
+### PF-1.1.1: Verify PF GRE encapsulate action for IPv4 traffic
+Generate traffic on ATE Port 1 from IPV4-SRC2 from a random combination of 1000 source addresses to IPV4-DST at linerate.
+Use 512 bytes frame size.
+
+Verify:
+
+*  All traffic received on ATE Port 2 GRE-encapsulated.
+*  No packet loss when forwarding.
+*  Traffic equally load-balanced across 32 GRE destinations.
+*  Verify PF packet counters matching traffic generated.
+
+### PF-1.1.2: Verify PF GRE encapsulate action for IPv6 traffic
+Generate traffic on ATE Port 1 from IPV6-SRC2 from a random combination of 1000 source addresses to IPV6-DST.
+Use 512 bytes frame size.
+
+Verify:
+
+*  All traffic received on ATE Port 2 GRE-encapsulated.
+*  No packet loss when forwarding.
+*  Traffic equally load-balanced across 32 GRE destinations.
+*  Verify PF packet counters matching traffic generated.
+
+### PF-1.1.3: Verify PF IPV4 forward action
+Generate traffic on ATE Port 1 from sources IPV4-SRC1 to IPV4-DST.
+
+Verify:
+
+*  All traffic received on ATE Port 3.
+*  No packet loss when forwarding.
+
+### PF-1.1.4: Verify PF IPV6 forward action
+Generate traffic on ATE Port 1 from sources IPV6-SRC1 to IPV6-DST.
+
+Verify:
+
+*  All traffic received on ATE Port 3.
+*  No packet loss when forwarding.
+
+### PF-1.1.5: Verify PF GRE DSCP copy to outer header for IPv4 traffic
+Generate traffic on ATE Port 1 from IPV4-SRC1 source for every DSCP value in [0, 8, 16, 24, 32, 40, 48, 56]
+
+Verify:
+
+*  All traffic received on ATE Port 2 GRE-encapsulated.
+*  Outer GRE IPv4 header has same marking as ingress non-encapsulated IPv4 packet.
+
+### PF-1.1.6: Verify PF GRE DSCP copy to outer header for IPv6 traffic
+Generate traffic on ATE Port 1 from IPV6-SRC1 for every IPv6 TC 8-bit value [0, 32, 64, 96, 128, 160, 192, 224]
+
+Verify:
+
+*  All traffic received on ATE Port 2 GRE-encapsulated.
+*  Outer GRE IPv4 header has DSCP match to ingress IPv6 TC packet.
+
+### PF-1.1.7: Verify MTU handling during GRE encap
+* Generate traffic on ATE Port 1 from IPV4-SRC1 with frame size of 4000 with DF-bit set.
+* Generate traffic on ATE Port 1 from IPV6-SRC1 with frame size of 4000 with DF-bit set.
+
+Verify:
+
+*  DUT generates "Fragmentation Needed" message back to ATE source.
+
+## OpenConfig Path and RPC Coverage
+
+```yaml
+paths:
+    # match condition
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/ipv4/config/source-address:
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/ipv6/config/source-address:
+    # encap action
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/action/encapsulate-gre/targets/target/config/id:
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/action/encapsulate-gre/targets/target/config/source:
+    # either destination or identifying-prefix can be specified based on specific vendor implementation.
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/action/encapsulate-gre/targets/target/config/destination:
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/action/encapsulate-gre/config/identifying-prefix:
+    # application to the interface
+    /network-instances/network-instance/policy-forwarding/interfaces/interface/config/apply-forwarding-policy:
+
+    # telemetry
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/state/matched-pkts:
+    /network-instances/network-instance/policy-forwarding/policies/policy/rules/rule/state/matched-octets:
+
+rpcs:
+  gnmi:
+    gNMI.Set:
+      union_replace: true
+      replace: true
+    gNMI.Subscribe:
+      on_change: true
+```
+
+## Required DUT platform
+
+* MFF
+* FFF