Networks - XWiki

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= Peer-for-Peer Networks =

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P4P, short for Peer-4-Peer (which in turn is short for Peer-for-Peer) are a family of networks which build on principles of local-first, peer-2-peer, open-source, routing agnostic (offline-first) and mutual-aid principles. The above is a lot of terms which in and of themselves carry a lot of meaning, yet when combined they enable censorship-resistant, resilient and adaptive, sustainable and energy-efficient communication infrastructures.

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== Building Blocks of P4P Networks ==

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To fully assemble a P4P network one needs a few different building blocks, below is an overview of 15 of those building blocks. Lost in translation? Take a look at the [[terminology>>doc:P4P.Definitions.WebHome]].

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==== **1. Data Synchronization** ====

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> Synchronization answers **how updates flow between peers** and how they determine what data to exchange. This layer is about **diffing, reconciliation, order, causality tracking, and efficient exchange**, not persistence or user-facing collaboration semantics.

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* //How do peers detect differences and synchronize state?//

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* Examples: Range-Based Set Reconciliation, RIBLT, Gossip-based sync, State-based vs op-based sync, Lamport/Vector/HLC clocks, Braid Protocol

==== **2. Collaborative Data Structures & Conflict Resolution** ====

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> This layer defines **how shared data evolves** when multiple peers edit concurrently. It focuses on **conflict-free merging, causality, and consistency of meaning**, not transport or storage. CRDTs ensure deterministic convergence, while event-sourced or stream-driven models maintain a history of all changes and derive consistent state from it.

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* //How do peers collaboratively change shared data and merge conflicts?//

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* Examples: CRDTs (Yjs, Automerge), OT, Event Sourcing, Stream Processing, Version Vectors, Peritext

==== **3. Data Storage & Replication** ====

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> This layer focuses on **durability, consistency, and redundancy**. It handles write-paths, crash-resilience, and replication semantics across nodes. It is the “database/storage engine” layer where **data lives and survives over time**, independent of sync or merging logic.

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* //How is data persisted locally and replicated between peers?//

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* Examples: SQLite, IndexedDB, LMDB, Hypercore (append-only logs), WALs, Merkle-DAGs (IPFS/IPLD), Blob/media storage

==== **4. Peer & Content Discovery** ====

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> Discovery occurs in two phases:

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> 1. **Peer Discovery** → finding _any_ nodes

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> 2. **Topic Discovery** → finding _relevant_ nodes or resources

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> These mechanisms enable decentralized bootstrapping and interest-based overlays.

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* //How do peers find each other, and how do they discover content in the network?//

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* Examples: DHTs (Kademlia, Pastry), mDNS, DNS-SD, Bluetooth scanning, QR bootstrapping, static peer lists, Interest-based routing, PubSub discovery (libp2p), Rendezvous protocols

==== **5. Identity & Trust** ====

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> Identity systems ensure reliable mapping between peers and cryptographic keys. They underpin authorization, federated trust, and secure overlays.

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* //How peers identify themselves, authenticate, and establish trustworthy relationships?//

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* Examples: PKI, Distributed Identities (DIDs), Web-of-Trust, TOFU (SSH-style), Verifiable Credentials (VCs), Peer key fingerprints (libp2p PeerIDs), Key transparency logs

==== **6. Transport Layer** ====

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> This layer provides logical connections and flow control. QUIC and WebRTC bring modern congestion control and encryption defaults; Interpeer explores transport beyond IP assumptions.

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* //How do peers establish end-to-end byte streams and reliable delivery?//

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* Examples: TCP, UDP, QUIC, SCTP, WebRTC DataChannels, Interpeer transport stack

==== **7. Underlying Transport (Physical/Link Layer)** ====

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> Highly relevant for **offline-first / edge networks**, device-to-device communication, and mesh networks and relates to the hardware which facilitates connections.

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* //How does data move across the medium?//

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* Examples: Ethernet, Wi-Fi Direct / Wi-Fi Aware (post-AWDL), Bluetooth Mesh, LoRa, NFC, Cellular, CSMA/CA, TDMA, FHSS

==== **8. Session & Connection Management** ====

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> Manages **connection lifecycle**, including authentication handshakes, reconnection after drops, and session continuation—especially important in lossy or mobile networks.

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* //How are connections initiated, authenticated, resumed, and kept alive?//

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* Examples: TLS handshake semantics, Noise IK/XX patterns, session tokens, keep-alive heartbeats, reconnection strategies, session resumption tickets

==== **9. Content Addressing** ====

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> Content addressing ensures **immutability, verifiability, and deduplication**. Identity of data = cryptographic hash, enabling offline-first and tamper-evident systems.

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* //How is data addressed and verified by content, not location?//

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* Examples: IPFS CIDs, BitTorrent infohashes, Git hashes, SHA-256 addressing, Named Data Networking (NDN)

==== **10. P2P Connectivity** ====

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> Connectivity ensures peers bypass NATs/firewalls to reach each other.

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* //How can two peers connect directly across networks, firewalls, and NATs?//

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* Examples: IPv6 direct, NAT Traversal, STUN, TURN, ICE (used in WebRTC), UDP hole punching, UPnP

==== **11. Session & Connection Management** ====

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> Manages **connection lifecycle**, including authentication handshakes, reconnection after drops, and session continuation.

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* //How are connections initiated, authenticated, resumed, and kept alive?//

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* Examples: TLS handshake semantics, Noise IK/XX patterns, session tokens, keep-alive heartbeats, reconnection strategies, session resumption tickets

==== **12. Message Format & Serialization** ====

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> Serialization ensures **portable data representation**, forward-compatible schemas, and efficient messaging. IPLD provides content-addressed structuring for P2P graph data.

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* //How is data encoded, structured, and made interoperable between peers?//

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* Examples: CBOR, Protocol Buffers, Cap’n Proto, JSON, ASN.1, IPLD schemas, Flatbuffers

==== **13. File / Blob Synchronization** ====

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> Bulk data syncing has **different trade-offs** than small collaborative state (chunking, deduplication, partial transfer, resume logic). Critical for media and archival P2P use-cases.

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//How are large objects transferred and deduplicated efficiently across peers?//

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Examples: BitTorrent chunking, IPFS block-store, NDN segments, rsync-style delta sync, ZFS send-receive, streaming blob transfers

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==== **14. Local Storage & Processing Primitives** ====

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> Provides durable on-device state and local computation (event sourcing, materialization, compaction). Enables offline-first writes and deterministic replay.

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* //How do nodes persist, index, and process data locally—without external servers?//

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* Examples: RocksDB, LevelDB, SQLite, LMDB, local WALs/append-only logs, embedded stream processors (NATS Core JetStream mode, Actyx-like edge runtimes), Kafka-like libraries

==== **15. Crash Resilience & Abortability** ====

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> Ensures P2P apps don’t corrupt state on crashes. Tied to **local storage & stream-processing**, and critical in offline-first and distributed update pipelines. Abortability is the updated term for Atomicity as part of the ACID abbreviation.

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* //How do nodes recover and maintain correctness under failure?//

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* Examples: WALs, idempotent ops, partial log replay, transactional journaling, write fences

== Distributed Network Types ==

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[[Flowchart depicting distributed network variants, under development. Building on work from Z. Elfen, 2024: ~[~[https:~~~~/~~~~/doi.org/10.17613/naj7d-6g984~>~>https://doi.org/10.17613/naj7d-6g984~]~]>>image:P4P_Typology.png||alt="Flowchart depicting typologies of distributed networks, such as Friend-2-Friend, Grassroots Networks, Federated Networks, Local-First, P2P and P4P Networks" data-xwiki-image-style-alignment="center" height="649" width="639"]]

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