代写代考 FIT5214: Blockchain

FIT5214: Blockchain
Lecture 11: Blockchain Network Lecturer:

https://dowsley.net

Unit Structure
• Lecture 1: Introduction to Blockchain
• Lecture 2: Bitcoin
• Lecture 3: Ethereum and Smart Contracts
• Lecture 4: Proof-of-Work (PoW)
• Lecture 5: Attacks on Blockchains
• Lecture 6: Class Test/Alternatives to PoW
• Lecture 7: Proof-of-Stake (PoS)
• Lecture 8: Privacy
• Lecture 9: Byzantine Agreement
• Lecture 10: Algorand
• Lecture 11: Blockchain Network
• Lecture 12: Payment Channels

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Blockchain Network
1. Centralisation v.s. Decentralisation. (Why P2P network?)
2. Blockchain Network Overview. (What is a blockchain network?) 3. Network Protocol. (How does it work?)

1. Centralisation vs Decentralisation. (Why P2P network?)

Centralised System
1. Trusted third party helps users to verify transactions.
2. Trusted third party protects users’ privacy.
3. Trusted third party guarantees honest users’
safety and security to avoid the attacks of malicious nodes.

Centralised System
1. The centralised authority could reject any transaction without reason.
2. The centralised authority could modify any user’s account.
3. The system will be collapsed if the centralised authority fails (single point of failure).

Decentralised/Distributed System
Decentralised System
1. No single point of failure.
2. System correctness is guaranteed
by multiple nodes.

Decentralised/Distributed System
Distributed System
1. No single point of failure.
2. System correctness is guaranteed
by the majority of nodes.

Decentralised/Distributed System
Distributed Network
1. No single point of failure.
2. System correctness is guaranteed
by the majority of nodes.
A fully-connected distributed network is robust against failures, however, what if there are more than 10M nodes? Not every node can maintain more than 10M connections.

Peer to Peer (P2P) Network
Source: http://www.processmodelcanvas.com/network/
1. Each node maintains a number of connections depending on its connectivity.
2. Each node follows a membership discovery protocol to refresh its connections.
3. Each node randomly selects a known peer as its new neighbour.
Blockchain peer to peer network is a dynamic distributed/decentralised network, that can disseminate messages in order to tolerant faults.

Peer to Peer (P2P) Network
Source: http://www.processmodelcanvas.com/network/
Bitcoin is a peer-to-peer digital cash system by design, and the network architecture is both a reflection and a foundation of that core characteristic.

2. Blockchain Network Overview

Blockchain network consists of different nodes to generate, validate, and disseminate messages.

Participants
Users can exchange coins with each other.

Participants
Users then broadcast the new transaction to their neighbours.

Participants
Miners select different transactions to do a computing competition. The transactions will be recorded in blocks. Only the winner can add his block into the chain of blocks.

Participants
As long as a miner discovers new block (n+1), it immediately “broadcasts” this block to all its neighbours, which could be normal users or other miners.

Participants
• Blockchain network consists of normal users and miners.
• Mining pool owns multiple front-end nodes to connect with users or other miners/pools.
• Each participant is allowed to have different connections.

• Transactions are generated from users, and validated by miners.
• Blocks are generated from miners/mining pools, and validated by the majority of participants.

Each participant owns its unique membership message. Participants can find new peers by exchanging membership messages.

Network Infrastructure

Network Infrastructure
User B is the neighbour of user A in this P2P network. The messages from user A to user B have to go through several hops in Internet.

Network Infrastructure
Bitcoin and other blockchain platforms are structured as a peer-to-peer network architecture on top of the Internet (overlay network).

Network Infrastructure
As long as a block is found by a miner, it will immediately be sent to the neighbours in P2P overlay network, and then relay to neighbours’ neighbours.
The block messages have to go through several hops in Internet.

3. Peer to Peer Protocol

Connection Initialisation
If the node is new, then it initialises the connections through:
• Hardcoded seed nodes. • Hardcoded DNS nodes.
Where are other peers?
Source: github/bitcoin
maintained by the core developers

Connection Initialisation If the node joined the network before, then
it initialises the connections through: • Its peer list.
Where are other peers?
If there are no active nodes in its peer list, then through:
• Hardcoded seed nodes. • Hardcoded DNS nodes.
Source: github/bitcoin

Membership Discovery
• Each node learns other members’ information from its neighbours.
• Each node maintains a database to manage members’ information.
• Each node disseminates its members’ information to its neighbours.
If a connection is dropped, the node can select a member from the database, and establish a new connection.

Membership Discovery
For each participant:
• The memberships are dynamic.
• Database is limited.
• There is a timing-based eviction mechanism to keep the freshness.
• Only its neighbours can receive its membership messages.

Communication Protocol
• Stratum protocol: for supporting pooled mining.

Communication Protocol
Connected (handshake used)
Connectionless
Error Free
Error Packets Discarded
Ordered Data Delivery
No Sequence Guarantee

Communication Protocol • TCP
Most blockchain networks are using TCP.
Membership, blocks and transactions messages can be sent using TCP.

Communication Protocol • UDP
Ethereum defines 4 types UDP-based messages: Ping, Pong, FindNode, and Neighbors for membership discovery.
Source: http://block.bendakhlia.org/main-blockchains/blockchains/ethereum/ethereum-discovery

• Bitcoin uses a multi-hop broadcast to propagate transactions and blocks through the network (i.e., each node propagates the information to its neighbours, and so on).
• At each hop in the broadcast the message incurs a propagation delay (sum of transmission time plus verification time)

• After receiving a valid block or transaction in the Bitcoin network, a node sends an “inv” message to its neighbours to check if they already have the transaction or block.
• If the neighbour doesn’t have, it sends a “getdata” response back to request the details of the transaction or block.
• “getblocks” message allows a peer which has been disconnected or started for the first time to request the blocks it hasn’t seen (at most 500 blocks are sent in the answer to “getblocks”, possibly multiple “getblocks” messages are needed).

Additional Reading
• https://developer.bitcoin.org/reference/ p2p_networking.html
• Information Propagation in the Bitcoin network https://ieeexplore.ieee.org/document/6688704

Next Week • Payment Channels

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