Privacy Relay Protocols

Overview

Bitcoin Commons implements multiple privacy-preserving and performance-optimized transaction relay protocols: Dandelion++, Fibre, and Package Relay. These protocols improve privacy, reduce bandwidth, and enable efficient transaction propagation.

Dandelion++

Overview

Dandelion++ provides privacy-preserving transaction relay with formal anonymity guarantees against transaction origin analysis. It operates in two phases: stem phase (obscures origin) and fluff phase (standard diffusion).

Code: dandelion.rs

Architecture

Dandelion++ operates in two phases:

1. Stem Phase: Transaction relayed along a random path (obscures origin)
2. Fluff Phase: Transaction broadcast to all peers (standard diffusion)

Stem Path Management

Each peer maintains a stem path to a randomly selected peer:

#![allow(unused)]
fn main() {
pub struct StemPath {
    pub next_peer: String,
    pub expiry: Instant,
    pub hop_count: u8,
}
}

Code: dandelion.rs

Stem Phase Behavior

• Transactions relayed to next peer in stem path
• Random path selection obscures transaction origin
• Stem timeout: 10 seconds (default)
• Fluff probability: 10% per hop (default)
• Maximum stem hops: 2 (default)

Code: dandelion.rs

Fluff Phase Behavior

• Transaction broadcast to all peers
• Standard Bitcoin transaction diffusion
• Triggered by:
  • Random probability at each hop
  • Stem timeout expiration
  • Maximum hop count reached

Code: dandelion.rs

Configuration

[network.dandelion]
enabled = true
stem_timeout_secs = 10
fluff_probability = 0.1  # 10%
max_stem_hops = 2

Code: dandelion.rs

Benefits

1. Privacy: Obscures transaction origin
2. Formal Guarantees: Anonymity guarantees against origin analysis
3. Backward Compatible: Falls back to standard relay if disabled
4. Configurable: Adjustable timeouts and probabilities

Fibre

Overview

Fibre (Fast Internet Bitcoin Relay Engine) provides high-performance block relay using UDP transport with Forward Error Correction (FEC) encoding for packet loss tolerance.

Code: fibre.rs

Architecture

Fibre uses:

• UDP Transport: Low-latency UDP for block relay
• FEC Encoding: Reed-Solomon erasure coding for packet loss tolerance
• Chunk-based Transmission: Blocks split into chunks with parity shards
• Automatic Recovery: Missing chunks recovered via FEC

FEC Encoding

Blocks are encoded using Reed-Solomon erasure coding:

• Data Shards: Original block data split into shards
• Parity Shards: Redundant shards for error recovery
• Shard Size: Configurable (default: 1024 bytes)
• Parity Ratio: Configurable (default: 0.2 = 20% parity)

Code: fibre.rs

Block Encoding Process

1. Serialize block to bytes
2. Split into data shards
3. Generate parity shards via FEC
4. Create FEC chunks for transmission
5. Send chunks via UDP

Code: fibre.rs

Block Assembly Process

1. Receive FEC chunks via UDP
2. Track received chunks per block
3. When enough chunks received (data shards), reconstruct block
4. Verify block hash matches

Code: fibre.rs

UDP Transport

Fibre uses UDP for low-latency transmission:

• Connection Tracking: Per-peer connection state
• Retry Logic: Automatic retry for lost chunks
• Sequence Numbers: Duplicate detection
• Timeout Handling: Connection timeout management

Code: fibre.rs

Configuration

[network.fibre]
enabled = true
bind_addr = "0.0.0.0:8334"
chunk_timeout_secs = 5
max_retries = 3
fec_parity_ratio = 0.2  # 20% parity
max_assemblies = 100

Code: fibre.rs

Statistics

Fibre tracks comprehensive statistics:

• Blocks sent/received
• Chunks sent/received
• FEC recoveries
• UDP errors
• Average latency
• Success rate

Code: fibre.rs

Benefits

1. Low Latency: UDP transport reduces latency
2. Packet Loss Tolerance: FEC recovers from lost chunks
3. High Throughput: Efficient chunk-based transmission
4. Automatic Recovery: No manual retry needed

Package Relay (BIP331)

Overview

Package Relay (BIP331) allows nodes to relay and validate groups of transactions together, enabling efficient fee-bumping (RBF) and CPFP (Child Pays For Parent) scenarios.

Code: package_relay.rs

Package Structure

A transaction package contains:

• Transactions: Ordered list (parents before children)
• Package ID: Combined hash of all transactions
• Combined Fee: Sum of all transaction fees
• Combined Weight: Total weight for fee rate calculation

Code: package_relay.rs

Package Validation

Packages are validated for:

• Size Limits: Maximum 25 transactions (BIP331)
• Weight Limits: Maximum 404,000 WU (BIP331)
• Fee Rate: Minimum fee rate requirement
• Ordering: Parents must precede children
• No Duplicates: No duplicate transactions

Code: package_relay.rs

Use Cases

1. Fee-Bumping (RBF): Parent + child transaction for fee increase
2. CPFP: Child transaction pays for parent’s fees
3. Atomic Sets: Multiple transactions that must be accepted together

Code: package_relay.rs

Package ID Calculation

Package ID is calculated as double SHA256 of all transaction IDs:

#![allow(unused)]
fn main() {
pub fn from_transactions(transactions: &[Transaction]) -> PackageId {
    // Hash all txids, then double hash
}
}

Code: package_relay.rs

Configuration

[network.package_relay]
enabled = true
max_package_size = 25
max_package_weight = 404000  # 404k WU
min_fee_rate = 1000  # 1 sat/vB

Code: package_relay.rs

Benefits

1. Efficient Fee-Bumping: Better fee rate calculation for packages
2. Reduced Orphans: Reduces orphan transactions in mempool
3. Atomic Validation: Package validated as unit
4. DoS Resistance: Size and weight limits prevent abuse

Integration

Relay Manager

The RelayManager coordinates all relay protocols:

• Standard block/transaction relay
• Dandelion++ integration (optional)
• Fibre integration (optional)
• Package relay support

Code: relay.rs

Protocol Selection

Relay protocols are selected based on:

• Feature flags (dandelion, fibre)
• Peer capabilities
• Configuration settings
• Runtime preferences

Code: relay.rs

Components

The privacy relay system includes:

• Dandelion++ stem/fluff phase management
• Fibre UDP transport with FEC encoding
• Package Relay (BIP331) validation
• Relay manager coordination
• Statistics tracking

Location: blvm-node/src/network/dandelion.rs, blvm-node/src/network/fibre.rs, blvm-node/src/network/package_relay.rs, blvm-node/src/network/relay.rs