How broadcasting a transaction works today

This is Bob.

Bob wants to send a bitcoin transaction, the reason why isn’t relevant for this explainer, but you can make an educated guess.

He obviously wants his transaction to be included in the Blockchain as soon as possible. To achieve this, it helps if many of the participants in the Bitcoin network know that Bob’s wallet has broadcast his transaction. This will increase the chance of his transaction being included in the next block of transactions that is created, which happens every 10 minutes on average.

As Bob is more interested in doing other things with his money, he does not have his own node in the Bitcoin network. Instead, he trusts Alice’s node to broadcast the transaction it receives from his wallet.

Bob knows there is no way for Alice to steal his money when using her node.

Alice’s node first checks Bob’s transaction, to see if his wallet signed it.

It also checks whether Bob has spent this bitcoin before, by looking through its records of currently unspent transactions, known as “UTXOs”.

The “What are UTXOs” explainer