diff --git a/specs/altair/p2p-interface.md b/specs/altair/p2p-interface.md
index d6f8a0dfe..49082452f 100644
--- a/specs/altair/p2p-interface.md
+++ b/specs/altair/p2p-interface.md
@@ -74,6 +74,15 @@ New topics are added in Altair to support the sync committees and the beacon blo
 
 The specification around the creation, validation, and dissemination of messages has not changed from the Phase 0 document.
 
+The derivation of the `message-id` has changed starting with Altair to incorporate the message topic along with the message data.
+The `message-id` MUST be the following 20 byte value computed from the message:
+* 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 and the topic name,
+  i.e. `SHA256(MESSAGE_DOMAIN_VALID_SNAPPY + snappy_decompress(message.data) + message.topic)[: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 and the topic name,
+  i.e. `SHA256(MESSAGE_DOMAIN_INVALID_SNAPPY + message.data + message.topic)[:20]`.
+
 The new topics along with the type of the `data` field of a gossipsub message are given in this table:
 
 | Name | Message Type |