Compact Headers & Compact Blocks - BIP 152 Implementation

[blondfrogs]

Compact Headers & Compact Blocks - BIP 152 Implementation

Implements BIP 152 style bandwidth optimization for both header sync and block relay.

Features

1. Compact Headers (cmpheaders):

• New message for efficient header sync of checkpointed blocks
• Omits Equihash nSolution (~1344 bytes) from POW block headers
• ~80% bandwidth reduction (344KB vs 1.7MB per 2000 headers)
• PON blocks send full data (already compact)

2. Compact Blocks (cmpctblock - BIP 152):

• Send blocks with short transaction IDs instead of full transactions
• Reconstruct blocks from mempool + short IDs
• Request missing transactions via getblocktxn/blocktxn messages
• High-bandwidth mode for low-latency block relay
• Ring buffer of 100 recent transactions for better reconstruction

3. Headers-First Sync:

• Download all headers before requesting blocks
• Multi-peer parallel block downloads after header sync
• CanDirectFetch() gates block downloads until synced
• Periodic disk flush (every 250k headers) prevents data loss

Network Protocol

• Protocol version 170021 (SENDCMPCT_VERSION/CMPHEADERS_VERSION)
• Messages: cmpheaders, sendcmpct, cmpctblock, getblocktxn, blocktxn
• Checkpoint validation for security (compact headers only below checkpoint)
• Initialize pindexBestKnownBlock from peer nStartingHeight for multi-peer sync

Database

• Stores actual block hash for compact headers (can't compute without nSolution)
• Backwards compatible serialization
• Old nodes ↔ new nodes: fully compatible

Implementation

New files:

• blockencodings.cpp/h - BIP 152 compact block structures
• crypto/siphash.cpp/h - Short transaction ID generation

Modified:

• main.cpp - Protocol handlers, sync logic
• chain.h - CDiskBlockIndex with hashBlock storage
• version.h, main.h, net.cpp/h, and 6 others

Testing

• ✅ Synced to 2M+ blocks at ~45 blocks/sec
• ✅ Backwards compatible with protocol < 170021
• ✅ Database compatibility verified