Support PV63 fast sync mode

[bobsummerwill]

From @chriseth on February 8, 2016 23:48

This is implemented at the responding side but not at the asking side.
Explanation (implementation in go-ethereum): ethereum/go-ethereum#1889

Description of older issue:

PV63 fastsync mode, launched if PV63 peers available && (start || a new block is noted with height > N + currentHeight)

1. Get last N block headers by hashes in ascending tree order. Pick block X = Latest - N
2. In parallel:
   • Download state tree nodes for block X from PV63 peers
   • Download transaction receipts for blocks 1 to X from PV63 peers
   • Download 1..X block bodies and headers from PV62 and PV63 peers
3. Once done
   • Insert blocks
   • Insert state

PV62 sync mode, launched if no PV62 peers available && (start || a new block is noted with height > N + currentHeight)
2. Split chain into chunks of size M
Download chunks in parallel. For each chunk:

• Download block headers in ascending tree order until a known block hash is met
• Download block bodies
  1. Reorder blocks and insert them into block queue

Keep up mode, launched when a new block is noted with height < N + currentHeight

• Download headers from a single peer in ascending tree order until a known block hash is met
• Download bodies

General:

• Validate all attack vectors, review code audit report.

Copied from original issue: ethereum/webthree-umbrella#177

[axic]

@gumb0 I think this must be implemented otherwise it wouldn't be able to sync with latest clients

[gumb0]

@axic No, this is fast sync description, still not implemented, we sync only through full sync (supported by all full clients)

But I haven't seen this issue, thanks.