Learning Outcomes

• Describe the structure of a block and the fields inside a block header.
• Understand how nodes maintain, propagate, and validate data.
• Explain mempool dynamics and how transactions flow through the network.
• Identify different node types: mining nodes, validator nodes, full nodes, light nodes.
• Analyse how peer-to-peer topology supports decentralisation and fault tolerance.

1. Block Structure

A blockchain block is a container for transactions and metadata. While specific block formats differ across protocols (Bitcoin, Ethereum, etc.), they share common elements:

• Block Header
• Block Body (Transactions)
• Metadata and Cryptographic Commitments

1.1 The Block Header

The block header contains the cryptographic commitments that define the block:

• Parent Hash: hash of the previous block
• Merkle Root: summarises all transactions
• Timestamp: when the block was mined/validated
• Nonce: for Proof-of-Work chains
• Difficulty Target: required to validate PoW
• State Root (Ethereum): summarises world state

1.2 Block Body

Contains the ordered list of valid transactions, along with optional metadata (receipts, logs).

2. Node Types and Roles

The blockchain network consists of heterogeneous nodes, each with distinct capabilities and responsibilities.

3. Mempool and Transaction Propagation

Before a transaction is included in a block, it resides in the mempool — a node’s local cache of unconfirmed transactions. Transaction flow follows these steps:

1. User signs and broadcasts a transaction.
2. Nodes validate the transaction (signature, nonce, balance).
3. The transaction enters the mempool.
4. Validators/miners select transactions from the mempool.
5. The transaction is included in a block.
6. The block propagates across the network, confirming the transaction.

4. Peer-to-Peer Network Topology

Blockchain networks are built on peer-to-peer (P2P) architectures where nodes connect directly to each other without central servers. Key aspects include:

• Gossip Protocols: messages propagate by nodes forwarding data to peers.
• Redundancy: multiple propagation paths reduce failure risk.
• Latency Variation: not all nodes receive data simultaneously.
• Topology Randomisation: prevents targeted attacks.

In a decentralised environment, P2P topology ensures robustness, censorship resistance, and persistence even under partial network partitioning.

5. State, Storage, and Pruning

Over time, blockchain data grows significantly. Different strategies address storage:

• Full History: nodes store every block and transaction.
• Pruned Nodes: keep recent blocks, discard old data safely.
• Archival Nodes: store entire state transitions (Ethereum).
• Light Clients: download only block headers.

6. Synthesis

Blockchain is not a single structure but a layered architecture involving:

1. Data Structures (blocks, Merkle trees, headers)
2. Nodes (full, light, archival, validator)
3. Transaction Flow (mempools, propagation)
4. Network Topology (peer-to-peer, gossip protocols)

Together, these components enable decentralised agreement, data integrity, and resilient operation at global scale. This concludes Course 1, establishing the cryptographic and structural foundations for studying blockchain mechanics and smart-contract engineering in the next courses.

7. Key Terms

Block Header

Metadata containing cryptographic commitments.

Mempool

Cache of unconfirmed transactions at each node.

Full Node

Node that stores full chain and validates everything.

P2P Network

Network where nodes connect directly without central servers.

Pruned Node

Node that stores only recent blocks to save space.

8. Self-Check Quiz

1. What is the purpose of the block header?
2. How do full nodes differ from light nodes?
3. Why are mempools not globally synchronised?
4. Explain why gossip protocols support decentralisation.
5. What is the advantage of using Merkle proofs for light clients?