fixed section heading

ch06.asciidoc

@@ -54,7 +54,7 @@ These conditions can be seen in detail in the functions AcceptToMemoryPool, Chec
 By independently verifying each transaction as it is received and before propagating it, every node builds a pool of valid new transactions (the transaction pool), roughly in the same order. 
 
 [[transaction_pools]]
-== Transaction Pools
+=== Transaction Pools
 
 Almost every node on the bitcoin network maintains a temporary list of unconfirmed transactions called the memory pool or transaction pool. Once a transaction is verified using the detailed checklist introduced in the section above, it is added to the transaction pool. Nodes use this pool to keep track of transactions that are known to the network but are not yet included in the blockchain. For example, a node that holds a users wallet will use the transaction pool to track incoming payments to the users wallet that have been received on the network but are not yet confirmed. Every node also maintains a separate pool of orphaned transactions as detailed in <<orphan_transactions>>. If a transactions inputs refer to a transaction that is not yet known, a missing parent, then it will be stored temporarily in the orphan pool until the parent transaction arrives. Both the transaction pool and orphan pool are stored in local memory and are not saved on persistent storage, rather they are dynamically populated from incoming network messages. When a node starts, both pools are empty and are gradually populated with new transactions received on the network.