diff --git a/specs/networking/p2p-interface.md b/specs/networking/p2p-interface.md
index 38e9b366b..2220740d9 100644
--- a/specs/networking/p2p-interface.md
+++ b/specs/networking/p2p-interface.md
@@ -212,6 +212,13 @@ Topics are plain UTF-8 strings and are encoded on the wire as determined by prot
 
 Each gossipsub [message](https://github.com/libp2p/go-libp2p-pubsub/blob/master/pb/rpc.proto#L17-L24) has a maximum size of `GOSSIP_MAX_SIZE`. Clients MUST reject (fail validation) messages that are over this size limit. Likewise, clients MUST NOT emit or propagate messages larger than this limit.
 
+The message-id of a gossipsub message MUST be:
+
+```python
+   message-id: base64(SHA256(message.data))
+```
+where `base64` is the [URL-safe base64 alphabet](https://tools.ietf.org/html/rfc4648#section-3.2) with padding characters omitted.
+
 The payload is carried in the `data` field of a gossipsub message, and varies depending on the topic:
 
 | Topic                                  | Message Type      |