Broadcasting Transactions to the Network

9 min readarticleIncludes quiz · 4 questions

After creating and signing a transaction, you need to broadcast it to the Bitcoin network. Your transaction gets sent to nodes, which validate it and relay it to other nodes, eventually reaching miners who include it in a block. This peer-to-peer propagation is what makes Bitcoin decentralized.

What is Broadcasting?

  • •Definition: Sending your signed transaction to Bitcoin nodes on the network.
  • •Peer-to-peer: No central server—transactions spread node-to-node.
  • •Validation: Each node checks the transaction is valid before relaying.
  • •Speed: Propagates to most of the network within seconds.
  • •Irreversible: Once broadcast, you can't un-send (but can try to replace with RBF).
P2P Network
P2P Network

Key Definitions:

  • •Broadcasting: Sending a transaction to the network.
  • •Peer-to-peer (P2P): Network structure where nodes connect directly without central server.
  • •Node: A computer running Bitcoin software that validates and relays transactions.
  • •Relay: Passing a transaction to connected peers.
  • •Propagation: The process of a transaction spreading across the network.
  • •Mempool: Memory pool where unconfirmed transactions wait for confirmation.

How Broadcasting Works:

1. Create transaction: Your wallet constructs the transaction with inputs, outputs, signatures. 2. Connect to node: Your wallet connects to one or more Bitcoin nodes. 3. Send transaction: Your wallet sends the raw transaction data to connected nodes. 4. Node validates: Each node checks signatures, inputs, outputs, fees. 5. Node relays: If valid, node forwards the transaction to its peers (other nodes). 6. Propagation: Transaction spreads exponentially across the network. 7. Reaches miners: Mining nodes add the transaction to their mempool. 8. Included in block: A miner includes your transaction in the next block they mine.

Transaction Propagation:

Timeline:

  • •0 seconds: You broadcast to 1-8 connected nodes
  • •1-2 seconds: Those nodes relay to their peers (~hundreds of nodes)
  • •5-10 seconds: Reaches most of the network (~10,000+ nodes)
  • •Within 30 seconds: Nearly universal propagation

Exponential spread:

  • •Round 1: 8 nodes
  • •Round 2: 64 nodes
  • •Round 3: 512 nodes
  • •Round 4: 4,096 nodes
  • •Round 5: 32,768 nodes
Transaction Propagation
Transaction Propagation

Ways to Broadcast:

1. Via Your Wallet

  • •Most common method
  • •Wallet connects to nodes and broadcasts automatically
  • •Examples: Electrum connects to Electrum servers, Bitcoin Core to full node peers

2. Via Your Own Node

  • •Run Bitcoin Core or similar
  • •Connect wallet to your node
  • •More private (don't leak addresses to third parties)
  • •More reliable (you control the connection)

3. Via Public APIs/Block Explorers

  • •Websites like blockchain.com, mempool.space offer broadcasting
  • •Copy raw transaction hex, paste into website
  • •Less private (service knows your IP and transaction)
  • •Useful as backup if wallet connection fails

4. Via Tor

  • •Route through Tor network for IP privacy
  • •Hides your location from nodes
  • •Wasabi, Samourai wallets support this
  • •Slower but more private

Validation Before Relay:

Nodes check transactions before relaying:

Basic checks:

  • •Valid format and syntax
  • •Signatures are correct
  • •Inputs reference unspent outputs
  • •Inputs ≥ outputs (no negative fees)
  • •Output values ≥ dust limit
  • •Transaction size within limits

If invalid:

  • •Node rejects and doesn't relay
  • •Transaction dies (doesn't propagate)
  • •Wallet might show error or just fail silently

If valid:

  • •Node adds to its mempool
  • •Relays to peers
  • •Transaction propagates

Privacy Considerations:

What nodes learn:

  • •Your transaction details (inputs, outputs, amounts)
  • •Your IP address (if not using Tor/VPN)
  • •Timing of broadcast

How to improve privacy:

  • •Use your own node (don't trust third parties)
  • •Route through Tor or VPN
  • •Use CoinJoin before sending
  • •Don't link addresses (avoid input linking)
  • •Batch transactions to hide recipient count

What Can Go Wrong:

Transaction doesn't propagate:

  • •Invalid signature or format
  • •Spending already-spent output
  • •Fee too low (some nodes have minimum fee filters)
  • •Transaction too large (>100 KB rejected by default)
  • •Nonstandard transaction (unusual scripts)

Solution:

  • •Check wallet error messages
  • •Verify transaction is valid
  • •Try broadcasting via different node/service
  • •Increase fee if too low
  • •Wait and retry if temporary network issue

Test Your Knowledge

This lesson includes a 4-question quiz (passing score: 75%).

Quiz functionality available in the mobile app.