ブロックヘッダーの取得とヘッダーレスポンス

どのノードも初めてネットワークに接続する際、取得して検証すべき最も重要なデータはブロックヘッダーである。
ブロックヘッダーをダウンロード数ルト、複数のノードに完全なブロック非同期に要求してブロックのダウンロードを並列化できる
ブロックヘッダーを取得するコマンドはgetheadersと呼ばれる

7f110100: Protocol version, 4bytes, little-endian, 70015
01: Number of hashes, varint
a35b…00: Starting block, little-endian
0000…00: Ending block, little-endian

class GetHeaderMessage:
    command = b'getheaders'

    def __init__(self, version=70015, num_hashes=1,
        start_block=None, end_block=None):
        self.version = version
        self.num_hashes = num_hashes
        if start_block is None:
            raise RuntimeError('a start block is required')
        self.start_block = start_block
        if end_block is None:
            self.end_block = b'\x00' * 32
        else:
            self.end_block = end_block

    def serialize(self):
        result = int_to_little_endian(self.version, 4)
        result += encode_varint(self.num_hashes)
        result += self.start_block[::-1]
        result += self.end_block[::-1]
        return result

### ヘッダーレスポンス
ノードを作成し、ハンドシェイクした後、ヘッダーを要求できる

from io import BytesIO
from block import Block, GENESIS_BLOCK
from network import SimpleNode, GetHeadersMessage
node = SimpleNone('', testnet=False)
node.handshake()
genesis = Block.parse(BytesIO(GENESIS_BLOCK))
getheaders = GetHeadersMessage(start_block=genesis.hash())
node.send(getheaders)

## headrsメッセージ
02: Number of block headers
00…67: Block header
00: Number of transactions (always 0)

headersメッセージはvarintのヘッダー数(1~2000)
各ブロックヘッダーは80bytes

class HeadersMessage:
    command = b'headers'

    def __init__(self, blocks):
        self.blocks = blocks

    @classmethod
    def parse(cls, stream):
        num_headers = read_varint(stream)
        blocks = []
        for _ in range(num_headers):
            blocks.append(Block.parse(stream))
            num_txs = read_varint(stream)
            if num_txs != 0:
                raise RuntimeError('number of txs not 0')
        return cls(blocks)

from io import BytesIO
from network import SimpleNode, GetHeaderMessage, HeadersMessage
from block import Block, GENESIS_BLOCK, LOWEST_BITS
from helper import calculate_new_bits
previous = Block.parse(BytesIO(GENESIS_BLOCK))
first_epoch_timestamp = previous.timestamp
expected_bits = LOWEST_BITS
count = 1
node = SimpleNode('', testnet=False)
node.handshake()
for _ in range(19):
    getheaders = GetHeadersMessage(start_block=previous.hash())
    node.send(getheaders)
    headers = node.wait_for(HeadersMessage)
    for header in headers.blocks:
        if not header.check_pow():
            raise RuntimeError('bad PoW at block {}'.format(count))
        if header.prev_block != previous.hash():
            raise RuntimeError('discontinuous block at {}'.format(count))
        if count % 2016 == 0:
            time_diff = previous.timestamp - first_epoch_timestamp
            expected_bits = calculate_new_bits(previous.bits, time_diff)
            print(expected_bits.hex())
            first_epoch_timestamp = header.timestamp
        if header.bits != expected_bits:
            raise RuntimeError('bad bits at block {}'.format(count))
        previous = header
        count += 1

VerAckMessageは最小のネットワークメッセージ。AckはAcknoldgeの略称で確認の意味
VerAckMessage :The verack message acknowledges a previously-received version message, informing the connecting node that it can begin to send other messages.
※https://bitcoin-s.org/api/org/bitcoins/core/p2p/index.html