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Parallel-Block-Downloads.md

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Parallel Block Downloads

New network protocol & strategy.

Goals

  • Achieve parallel downloads of chain
  • Introduce framework that can form basis of Swarm
  • Minimise unnecessary block transfers

Basic Overview

  • Two peers connect & say Hello. Hello includes the TD & hash of their best block.
  • The client with the worst TD asks peer for full chain of just block hashes.
  • Chain of hashes is stored in space shared by all peer connections, and used as a "work pool".
  • While there are hashes in the chain of hashes that we don't have in our chain:
    • Ask for N blocks from our peer using the hashes. Mark them as on their way so we don't get them from another peer.

Required Changes

Network protocol: 33

Additional parameters to Hello:

  • TD: Total Difficulty of the best chain. Integer.
  • BestHash: The hash of the best (i.e. highest TD) known block.
  • GenesisHash: The hash of the Genesis block.

Additional Message types:

  • 0x17: GetBlockHashes [ hash : B_32, maxBlocks: P ]: Requests a BlockHashes message of at most maxBlocks entries, of block hashes from the blockchain, starting at the parent of block hash. Does not require the peer to give maxBlocks hashes - they could give somewhat fewer.
  • 0x18:BlockHashes [ hash_0: B_32, hash_1: B_32, .... ]: Gives a series of hashes of blocks (each the child of the next). This implies that the blocks are ordered from youngest to oldest.
  • 0x19:GetBlocks [ hash_0: B_32, hash_1: B_32, .... ]: Requests a Blocks message detailing a number of blocks to be sent, each referred to by a hash. Note: Don't expect that the peer necessarily give you all these blocks in a single message - you might have to re-request them.

Remove Message types:

  • GetChain
  • NotInChain