diff --git a/specs/phase0/p2p-interface.md b/specs/phase0/p2p-interface.md
index 8bde2b80a..b214c458d 100644
--- a/specs/phase0/p2p-interface.md
+++ b/specs/phase0/p2p-interface.md
@@ -547,7 +547,8 @@ Requests count beacon blocks from the peer starting from `start_slot`, leading u
 The request MUST be encoded as an SSZ-container.
 
 The response MUST consist of zero or more `response_chunk`. Each _successful_ `response_chunk` MUST contain a single `SignedBeaconBlock` payload.
-Clients MUST support requesting blocks since the start of the weak subjectivity period and up to the given `head_block_root`.
+
+Clients MUST keep a record of signed blocks seen since the since the start of the weak subjectivity period and MUST support requesting blocks up to the given `head_block_root`.
 
 Clients MUST respond with at least one block, if they have it and it exists in the range. Clients MAY limit the number of blocks in the response.