Added the 'ttl_mode' attribute for VxLANs. (#24657)

Why I did it
To allow specifying the decap TTL mode used for each VxLAN tunnel.

Work item tracking
Microsoft ADO (number only): 35743756
How I did it
Added the new attribtue ttl_mode for each VxLAN, which can be set to uniform or pipe.
sonic-swss PR: sonic-net/sonic-swss#3963

How to verify it
Run the tests modified in this PR, or build an image with these changes and see if this attribute can be set without any errors.

Author: mramezani95

src/sonic-yang-models/doc/Configuration.md

@@ -2837,7 +2837,8 @@ Single tunnel example:
     "VXLAN_TUNNEL": {
         "vtep1": {
             "src_ip": "10.10.10.10",
-            "dst_ip": "12.12.12.12"
+            "dst_ip": "12.12.12.12",
+            "ttl_mode": "pipe"
         }
     },
     "VXLAN_TUNNEL_MAP": {
@@ -2864,7 +2865,8 @@ Dual tunnel example:
     "VXLAN_TUNNEL": {
         "vtep1": {
             "src_ip": "10.10.10.10",
-            "dst_ip": "12.12.12.12"
+            "dst_ip": "12.12.12.12",
+            "ttl_mode": "uniform"
         },
         "vtep2": {
             "src_ip": "10.20.10.10",

src/sonic-yang-models/tests/yang_model_tests/tests/vxlan.json

@@ -41,5 +41,12 @@
     },
     "VXLAN_TUNNEL_NAME_STRING_VALID_TEST": {
         "desc": "Tunnel name as arbitrary string is valid"
+    },
+    "VXLAN_TUNNEL_VALID_TTL_MODE_TEST": {
+        "desc": "Valid config with 'pipe' and 'uniform' TTL modes"
+    },
+    "VXLAN_TUNNEL_INVALID_TTL_MODE_TEST": {
+        "desc": "Invalid config with an unsupported TTL mode",
+        "eStrKey": "Pattern"
     }
 }

src/sonic-yang-models/tests/yang_model_tests/tests_config/vxlan.json

@@ -351,5 +351,36 @@
                 ]
             }
         }
+    },
+    "VXLAN_TUNNEL_VALID_TTL_MODE_TEST": {
+        "sonic-vxlan:sonic-vxlan": {
+            "sonic-vxlan:VXLAN_TUNNEL": {
+                "VXLAN_TUNNEL_LIST": [
+                    {
+                        "name": "tunnel_v4-1",
+                        "src_ip": "5.6.7.8",
+                        "ttl_mode": "pipe"
+                    },
+                    {
+                        "name": "tunnel_v4-2",
+                        "src_ip": "1.2.3.4",
+                        "ttl_mode": "uniform"
+                    }
+                ]
+            }
+        }
+    },
+    "VXLAN_TUNNEL_INVALID_TTL_MODE_TEST": {
+        "sonic-vxlan:sonic-vxlan": {
+            "sonic-vxlan:VXLAN_TUNNEL": {
+                "VXLAN_TUNNEL_LIST": [
+                    {
+                        "name": "invalid_tunnel",
+                        "src_ip": "1.2.3.4",
+                        "ttl_mode": "invalid"
+                    }
+                ]
+            }
+        }
     }
-}
+}

src/sonic-yang-models/yang-models/sonic-vxlan.yang

@@ -59,6 +59,13 @@ module sonic-vxlan {
                 leaf dst_ip {
                     type inet:ip-address;
                 }
+
+                leaf ttl_mode {
+                    description "Decap TTL mode";
+                    type string {
+                        pattern "uniform|pipe";
+                    }
+                }
             }
         }