Technical Specs

The engineering behind Blip's BLE mesh network. Built for festival-scale crowds with end-to-end encryption, adaptive gossip routing, and Ed25519 packet authentication.

Mesh Network

TransportBluetooth Low Energy 5.0+

TopologyDual-role (central + peripheral simultaneously)

Max connections6 per device (configurable for bridge nodes)

Service UUIDFC000001-0000-1000-8000-00805F9B34FB

MTU512 bytes (517 requested, 512 effective)

Hop limit (TTL)0–7 hops per packet

Range per hop~40m (BLE 5.0, line-of-sight up to 100m)

Effective range~200–300m through crowd (7 hops)

Gossip Routing

AlgorithmProbabilistic gossip with adaptive relay

Deduplication3-tier Bloom filter (60s / 10min / 2hr windows)

Relay probability100% (<10 peers) → 20% (60+ peers)

SOS relayAlways 100%, TTL preserved for first 3 hops

Jitter8–25ms random per relay (prevents synchronized flooding)

Congestion backoffQueue fill >80% reduces relay to 20%

DM routing (at scale)DirectedRouter with 5-min route expiry

Store-and-forward10MB LRU cache, DMs cached 2hrs, SOS until resolved

Encryption

Key agreementNoise XX handshake (Curve25519 + ChaChaPoly + SHA256)

Packet signingEd25519 via libsodium (64-byte detached signatures)

Signable dataFull packet excluding TTL byte and signature (spec §7.3)

Relay compatibilityTTL excluded from signed data — relay nodes decrement without breaking sig

Key distributionNoise + signing public keys exchanged in announce payload

Session managementNoise session per peer pair, rekeying on reconnect

Packet Format

Header16 bytes fixed (version, type, TTL, timestamp, flags, length)

Sender ID8 bytes (SHA256 of noise public key, truncated)

Recipient ID8 bytes (optional, for addressed DMs)

Signature64 bytes (optional, Ed25519)

Max payload (broadcast signed)424 bytes

Max payload (addressed signed)416 bytes

Fragmentation threshold416 bytes

Byte orderBig-endian (network byte order)

Scale Design

Target100,000+ concurrent users

Crowd modesGather (<500) / Festival (500–5K) / Mega (5K–25K) / Massive (25K+)

RSSI pollingEvery 10 seconds for connected peers

Peer evaluation30-second cycle for connection quality scoring

Reconnect backoff30 seconds (reduced from 120s for density)

Stale peer cleanupDisconnected peers pruned after 5 minutes

Platform

iOS17.0+ (SwiftUI, SwiftData, MVVM)

PackagesBlipProtocol, BlipMesh, BlipCrypto (Swift Package Manager)

DependenciesCryptoKit (Apple), swift-sodium, swift-opus

BackendCloudflare Workers (auth, relay) + Neon Postgres

BuildXcodeGen + xcodebuild

Open development

Blip is built in the open. Check out the source, report issues, or contribute.

View on GitHub