What is Peer-to-Peer (P2P)?

Peer-to-peer (P2P) is a network architecture where nodes or participants share their resources directly with each other, rather than relying on a centralized server or authority. In a P2P network, each node, often referred to as a peer, acts both as a client and a server. This means that each peer in the network is capable of sharing resources, such as bandwidth, storage space, and processing power, as well as consuming resources provided by other peers.

Dissecting Peer-to-Peer (P2P)

In the early days of computing, in the 1960s and 1970s, systems were primarily centralized, with mainframes serving terminals. However, as personal computers gained popularity in the 1980s, the concept of connecting them directly to each other without the need for central servers started to gain traction. One of the earliest examples of P2P technology was the USENET system, created in 1979, which allowed users to exchange files and messages.

The P2P concept as we know it today really took off in the late 1990s with the advent of file-sharing networks. Napster, created by Shawn Fanning in 1999, is widely recognized as one of the first modern P2P file-sharing networks. Napster allowed users to share music files with each other directly. While Napster did rely on centralized servers for indexing, the actual file transfers were P2P.

After the legal troubles and eventual shutdown of Napster, other P2P networks like Gnutella, Kazaa, and eDonkey emerged. These were more decentralized compared to Napster and did not rely on a central server for indexing.

How Peer-to-Peer (P2P) Works

P2P networks work by allowing peers to connect directly with each other and share resources without the need for a central server. They can be used for a variety of applications and are particularly useful in scenarios where decentralization, scalability, and fault tolerance are important.

1. Establishing Connections: When a participant (peer) joins a P2P network, it typically connects to one or several other peers in the network. Some P2P networks use a central server for initial peer discovery, while others use distributed methods like Distributed Hash Tables (DHT) or network gossip to discover other peers.
2. Resource Sharing and Discovery: Each peer in the P2P network shares some of its resources, which can include files, bandwidth, or computing power. To find a specific resource in the network, a peer may broadcast a request to the peers it is connected to, who can then either respond with the resource if they have it or forward the request to peers they are connected to. This process can continue until the resource is found or a certain number of hops or time limit is reached. In more structured P2P networks, resources may be indexed using DHTs or similar systems, allowing for more efficient searches without broadcasting the request to the entire network.
3. Data Transfer: Once the resource is located, the peer that requested it establishes a direct connection with the peer that has the resource. Data is then transferred directly between the two peers. In the case of file sharing, the file may be divided into smaller pieces, and these pieces may be transferred in a non-sequential order and reassembled once the transfer is complete.
4. Handling Peer Churn: Peer churn refers to the phenomenon where peers frequently join and leave the network. P2P networks need to handle this to maintain network stability and resource availability. This is usually managed through redundancy, where resources are replicated across multiple peers, and through algorithms that ensure the network can reorganize itself as peers join and leave.
5. Security and Anonymity: Security is a concern in P2P networks since the open nature of the network can make it vulnerable to various attacks. Some P2P networks use encryption to secure data transfers. For anonymity, some networks employ techniques to obfuscate the source and destination of data transfers. This is common in privacy-focused P2P networks like Tor.

Peer-to-Peer (P2P) Application and Protocols

Peer-to-peer (P2P) networks have been adopted in various applications and domains. Some notable applications and the protocols they use:

File Sharing

• BitTorrent: It's one of the most popular P2P protocols for file-sharing. Users create a small torrent descriptor file which other users can download and use to connect to each other. BitTorrent efficiently manages bandwidth by breaking files into pieces, allowing peers to upload and download pieces from multiple sources concurrently.
• eDonkey2000 (ed2k): This is an older P2P file sharing network that also uses a hash-based approach similar to BitTorrent. Its client eMule became more popular than the original eDonkey2000 client.
• Gnutella: This protocol was used by early P2P file-sharing applications like LimeWire and BearShare. It is a decentralized protocol that doesn't rely on a central server for indexing, but it is less efficient than BitTorrent for sharing large files.

Media Streaming

• Ace Stream: This is a P2P media streaming application protocol that is based on BitTorrent. It allows users to both download and upload parts of files simultaneously, which is useful for live streaming and large media files.
• PPStream and SopCast: These are P2P streaming protocols that allow users to broadcast and watch streaming media content. They are popular for streaming live events, particularly sports.

Cryptocurrencies and Blockchains

• Bitcoin Protocol: The Bitcoin network operates as a P2P network where transactions and blocks are propagated between nodes. It utilizes a cryptographic protocol to ensure the integrity and security of transactions.
• Ethereum: Similar to Bitcoin, Ethereum operates on a P2P network but with added functionality of smart contracts, allowing more complex transactions and applications to run on its blockchain.

Communication

• WebRTC: Stands for Web Real-Time Communication. It is an open-source project that provides P2P communication capabilities to web browsers and mobile applications via simple APIs. It is widely used for video conferencing and real-time communication applications.
• Tox: It is a protocol used for secure P2P messaging and video and audio calls. It aims to provide secure communication channels that are free from central control.

Anonymous Networks

• Tor (The Onion Router): While Tor primarily operates over a network of relay nodes, it incorporates P2P characteristics. It allows users to browse the internet anonymously by routing traffic through a series of encrypted relays.
• I2P (Invisible Internet Project): It is a P2P network that provides an anonymous layer, allowing applications to send messages and establish connections anonymously across the network.

Distributed Computing

• Folding@home: This is a distributed computing project that uses the spare processing power of volunteers' computers to simulate protein folding. It is P2P in the sense that it distributes computing tasks to multiple nodes.

Content Delivery Networks (CDNs)

• Peer5: Peer5 operates as a hybrid CDN that combines traditional CDN with a P2P network. It is particularly useful for scaling video delivery without the need for additional infrastructure investments.