book: update Event to have layer field

doc/src/misc/event_graph/network_protocol.md

@@ -2,26 +2,30 @@
 
 ## Common Structures
 
+### Event ID
+
+Is [blake3::Hash](https://docs.rs/blake3/latest/blake3/struct.Hash.html) 
+of the event, we use those IDs to request and reply 
+events and tips (tips being childless events in the graph).
+
 ### Event
 
 Representation of an event in the Event Graph.
 This is either sent when a new event is created, or in response to `EventReq`.
 
-| Description   | Data Type      	   | Comments           		    |
-|-------------- | -------------------- | ------------------------------ |
-| timestamp	  	| `u64`                | Timestamp of the event    	    |
-| content	  	| `Vec<u8>`            | Content of the event    	    |
-| parents	  	| `u64`                | Parent nodes in the event DAG  |
+| Description   | Data Type      	                | Comments           		     |
+|-------------- | --------------------------------- | ------------------------------ |
+| timestamp	  	| `u64`                             | Timestamp of the event    	 |
+| content	  	| `Vec<u8>`                         | Content of the event    	     |
+| parents	  	| `[blake3::Hash; N_EVENT_PARENTS]` | Parent nodes in the event DAG  |
+| layer	  	    | `u64`                             | DAG layer index of the event   |
+
+Events could have multiple parents, `N_EVENT_PARENTS` is the maximum 
+number of parents an event could have.
 
 Receiving an event with missing parents, the node will issue `EventReq`
 requesting the missing parent from a peer.
 
-### Event ID
-
-Is [blake3::Hash](https://docs.rs/blake3/latest/blake3/struct.Hash.html) 
-of the event, we use those IDs to request and reply 
-events and tips (tips being childless events in the graph).
-
 
 ## P2P Messages