diff --git a/src/transport/SecureSession.cpp b/src/transport/SecureSession.cpp
index 0a878244bc5df6..e350029b7c7b1a 100644
--- a/src/transport/SecureSession.cpp
+++ b/src/transport/SecureSession.cpp
@@ -46,6 +46,7 @@ void SecureSession::Activate(const ScopedNodeId & localNode, const ScopedNodeId
     mPeerSessionId   = peerSessionId;
     mRemoteMRPConfig = config;
     SetFabricIndex(peerNode.GetFabricIndex());
+    MarkActiveRx(); // Initialize SessionTimestamp and ActiveTimestamp per spec.
 
     Retain(); // This ref is released inside MarkForEviction
     MoveToState(State::kActive);
diff --git a/src/transport/SecureSession.h b/src/transport/SecureSession.h
index ed8eae3a400454..4428ee98bf41d4 100644
--- a/src/transport/SecureSession.h
+++ b/src/transport/SecureSession.h
@@ -304,8 +304,13 @@ class SecureSession : public Session, public ReferenceCounted<SecureSession, Sec
     uint16_t mPeerSessionId = 0;
 
     PeerAddress mPeerAddress;
-    System::Clock::Timestamp mLastActivityTime     = System::SystemClock().GetMonotonicTimestamp(); ///< Timestamp of last tx or rx
-    System::Clock::Timestamp mLastPeerActivityTime = System::SystemClock().GetMonotonicTimestamp(); ///< Timestamp of last rx
+
+    /// Timestamp of last tx or rx. @see SessionTimestamp in the spec
+    System::Clock::Timestamp mLastActivityTime = System::SystemClock().GetMonotonicTimestamp();
+
+    /// Timestamp of last rx. @see ActiveTimestamp in the spec
+    System::Clock::Timestamp mLastPeerActivityTime = System::SystemClock().GetMonotonicTimestamp();
+
     ReliableMessageProtocolConfig mRemoteMRPConfig = GetDefaultMRPConfig();
     CryptoContext mCryptoContext;
     SessionMessageCounter mSessionMessageCounter;