3.1 Peer-to-Peer Networks and Distributed Systems

P2P networks are like a group of friends sharing resources without a leader. Each person can give and receive, creating a system where everyone's equal. This setup allows for direct sharing and decision-making among all participants.

Distributed systems are like a team working on a project together. They're designed to handle big tasks, keep working even if someone drops out, and share the workload. They use special ways to talk and make decisions as a group.

Peer-to-Peer (P2P) Networks and Decentralization

P2P Network Architecture

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• P2P networks consist of interconnected nodes that share resources and perform tasks without relying on a central server
• Nodes in a P2P network act as both clients and servers, allowing them to consume and provide services simultaneously
• P2P networks enable direct communication and resource sharing between nodes, eliminating the need for intermediaries (BitTorrent)
• Decentralization is a key characteristic of P2P networks, where control and decision-making are distributed among participating nodes

Network Topology and Node Roles

• Network topology refers to the arrangement and interconnection of nodes in a P2P network
  • Common P2P network topologies include unstructured (Gnutella) and structured (Chord) networks
  • Unstructured networks have a random or ad-hoc organization, while structured networks follow a specific overlay structure
• Nodes in a P2P network can assume different roles based on their capabilities and responsibilities
  • Full nodes maintain a complete copy of the network's data and participate in validating and propagating transactions (Bitcoin full nodes)
  • Light nodes, also known as thin clients, rely on full nodes for network interactions and only store relevant data (SPV clients in Bitcoin)

Distributed Systems Fundamentals

Characteristics and Benefits

• Distributed systems consist of multiple interconnected computers that work together to achieve a common goal
• Key characteristics of distributed systems include scalability, fault tolerance, and resource sharing
• Scalability enables distributed systems to handle increasing workloads by adding more nodes to the network (horizontal scaling)
• Fault tolerance ensures that the system continues to operate correctly even in the presence of node failures or network disruptions

Communication and Coordination

• Network protocols define the rules and formats for communication between nodes in a distributed system
  • Examples of network protocols include TCP/IP for reliable data transfer and HTTP for web-based interactions
• Nodes in a distributed system coordinate their actions and share information to maintain consistency and achieve desired outcomes
  • Coordination mechanisms, such as consensus algorithms (Paxos, Raft), enable nodes to agree on a shared state or make collective decisions
• Message passing is a fundamental communication paradigm in distributed systems, where nodes exchange messages to share data and trigger actions

Advanced P2P Concepts

Distributed Hash Tables (DHTs)

• DHTs are a class of decentralized systems that provide efficient lookup and storage of key-value pairs across a P2P network
• DHTs enable scalable and fault-tolerant storage and retrieval of data without relying on a central authority
• Key features of DHTs include distributed indexing, self-organization, and load balancing among participating nodes
• Examples of DHT implementations include Kademlia, which is used in BitTorrent for peer discovery, and Chord, which provides a ring-based topology for efficient lookups

Overlay Networks and Routing

• Overlay networks are logical networks built on top of existing physical networks to provide additional functionality and abstractions
  • P2P networks often utilize overlay networks to enable efficient routing and resource discovery
  • Examples of overlay networks include Freenet for anonymous and censorship-resistant communication
• Routing in P2P networks involves determining the path for messages to travel from source to destination nodes
  • Structured P2P networks employ deterministic routing algorithms based on the overlay topology (Chord's finger tables)
  • Unstructured P2P networks rely on flooding or random walk techniques to propagate queries and locate resources (Gnutella's query flooding)