diff --git a/specs/phase0/p2p-interface.md b/specs/phase0/p2p-interface.md
index cb84bef6c..7f548a6f7 100644
--- a/specs/phase0/p2p-interface.md
+++ b/specs/phase0/p2p-interface.md
@@ -176,6 +176,9 @@ This section outlines constants that are used in this spec.
 | `RESP_TIMEOUT` | `10s` | The maximum time for complete response transfer. |
 | `ATTESTATION_PROPAGATION_SLOT_RANGE` | `32` | The maximum number of slots during which an attestation can be propagated. |
 | `MAXIMUM_GOSSIP_CLOCK_DISPARITY` | `500ms` | The maximum milliseconds of clock disparity assumed between honest nodes. |
+| `MESSAGE_DOMAIN_INVALID_SNAPPY` | `0x00000000` | 4-byte domain for gossip message-id isolation of *invalid* snappy messages |
+| `MESSAGE_DOMAIN_VALID_SNAPPY`  | `0x01000000` | 4-byte domain for gossip message-id isolation of *valid* snappy messages |
+
 
 ## MetaData
 
@@ -241,11 +244,17 @@ Each gossipsub [message](https://github.com/libp2p/go-libp2p-pubsub/blob/master/
 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 the first 8 bytes of the SHA-256 hash of the message data, i.e.:
+The `message-id` of a gossipsub message MUST be the following 20 byte value computed from the message data:
+* If `message.data` has a valid snappy decompression, set `message-id` to the first 20 bytes of the `SHA256` hash of
+  the concatenation of `MESSAGE_DOMAIN_VALID_SNAPPY` with the snappy decompressed message data,
+  i.e. `SHA256(MESSAGE_DOMAIN_VALID_SNAPPY + snappy_decompress(message.data))[:20]`.
+* Otherwise, set `message-id` to the first 20 bytes of the `SHA256` hash of
+  the concatenation of `MESSAGE_DOMAIN_INVALID_SNAPPY` with the raw message data,
+  i.e. `SHA256(MESSAGE_DOMAIN_INVALID_SNAPPY + message.data)[:20]`.
 
-```python
-   message-id: SHA256(message.data)[0:8]
-```
+*Note*: The above logic handles two exceptional cases:
+(1) multiple snappy `data` can decompress to the same value,
+and (2) some message `data` can fail to snappy decompress altogether.
 
 The payload is carried in the `data` field of a gossipsub message, and varies depending on the topic: