Update P4Runtime-Spec.mdk with backup replicas being optional and pro…

…perties of replicas and backup replicas

Signed-off-by: Matthew Lam <[email protected]>

docs/v1/P4Runtime-Spec.mdk

@@ -4596,12 +4596,14 @@ semantics.
   `instance` ID is also a `uint32`, and its value may not exceed the maximum
   allowed by the target for the `EgressInstance_t` type (0 is allowed), or the
   server must return an `INVALID_ARGUMENT` error. The egress port (`port` field)
-  must be an SDN port and must refer to a singleton port. No two replicas may
-  have identical values of *both* `port` and `instance`, or the server must
-  return `INVALID_ARGUMENT`. The `metadata` field is an arbitrary `bytes` value
-  which is opaque to the target. There is no requirement of where this is
-  stored, but it must be returned by the server along with the rest of the entry
-  when the client performs a read on the entry.
+  must be an SDN port and must refer to a singleton port. A replica cannot have
+  the same port as any of its backup replicas, or the server must return
+  `INVALID_ARGUMENT`. All replicas and backup replicas in a particular
+  multicast group requires uniqueness among all (port, instance)-pairs, or the
+  server must return `INVALID_ARGUMENT`. The `metadata` field is an arbitrary
+  `bytes` value, which is opaque to the target. There is no requirement of
+  where this is stored, but it must be returned by the server along with the
+  rest of the entry when the client performs a read on the entry.
 * `MODIFY`: Modify the set of replicas and metadata for a given multicast group
   entry, indexed by the given `multicast_group_id`. Same restrictions as
   `INSERT` apply here.
@@ -4611,6 +4613,8 @@ semantics.
   metadata in the data plane set to the deleted `multicast_group_id` will be
   dropped.
 
+The support for back replicas is optional by the target and if the target does
+not support backup replicas, it must return an `UNIMPLEMENTED` error.
 When reading a multicast group, only `multicast_group_id` is considered. All
 other fields in `MulticastGroupEntry` are ignored. To perform a *wildcard*
 `Read` on all configured multicast group entries, the `multicast_group_id` field