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95 lines
4.4 KiB
Markdown
95 lines
4.4 KiB
Markdown
## Block Chain
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The following subsections briefly document core block details.
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### Block Contents
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{% autocrossref %}
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This section describes [version 2 blocks][v2 block]{:#term-v2-block}{:.term}, which are any blocks with a
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block height greater than 227,835. (Version 1 and version 2 blocks were
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intermingled for some time before that point.) Future block versions may
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break compatibility with the information in this section. You can determine
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the version of any block by checking its `version` field using
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bitcoind RPC calls.
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As of version 2 blocks, each block consists of four root elements:
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1. A [magic number][block header magic]{:#term-block-header-magic}{:.term} (0xd9b4bef9).
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2. A 4-byte unsigned integer indicating how many bytes follow until the
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end of the block. Although this field would suggest maximum block
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sizes of 4 GiB, max block size is currently capped at 1 MB and the
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default max block size (used by most miners) is 750 KB (although
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this will likely increase over time).
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3. An 80-byte block header described in the section below.
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4. One or more transactions.
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The first transaction in a block must be a [coinbase transaction][]{:#term-coinbase-tx}{:.term} which should collect and
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spend any transaction fees paid by transactions included in this block.
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All blocks with a block height less than 6,930,000 are entitled to
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receive a [block reward][]{:#term-block-reward}{:.term} of newly created bitcoin value, which also
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should be spent in the coinbase transaction. (The block reward started
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at 50 bitcoins and is being halved every 210,000 blocks---approximately once every four years. As of
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June 2014, it's 25 bitcoins.) A coinbase transaction is invalid if it
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tries to spend more value than is available from the transaction
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fees and block reward.
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The coinbase transaction has the same basic format as any other
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transaction, but it references a single non-existent UTXO and a special
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[coinbase field][]{:#term-coinbase-field}{:.term} replaces the field that would normally hold a signature script and
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secp256k1 signature. In version 2 blocks, the coinbase parameter must begin with
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the current block's block height and may contain additional arbitrary
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data or a script up to a maximum total of 100 bytes.
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{% endautocrossref %}
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### Block Header
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{% autocrossref %}
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The 80-byte block header contains the following six fields:
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| Field | Bytes | Format |
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|-------------------|--------|--------------------------------|
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| 1. Version | 4 | Unsigned Int |
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| 2. hashPrevBlock | 32 | Unsigned Int (SHA256 Hash) |
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| 3. hashMerkleRoot | 32 | Unsigned Int (SHA256 Hash) |
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| 4. Time | 4 | Unsigned Int (Epoch Time) |
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| 5. Bits | 4 | Internal Bitcoin Target Format |
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| 6. Nonce | 4 | (Arbitrary Data) |
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1. The *[block version][]{:#term-block-version}{:.term}* number indicates which set of block validation rules
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to follow so Bitcoin Core developers can add features or
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fix bugs. As of block height 227,836, all blocks use version number
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2.
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2. The *hash of the previous block header* puts this block on the
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block chain and ensures no previous block can be changed without also
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changing this block's header.
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3. The *merkle root* is a hash derived from hashes of all the
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transactions included in this block. It ensures no transactions can
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be modified in this block without changing the block header hash.
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4. The *[block time][]{:#term-block-time}{:.term}* is the approximate time when this block was created in
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Unix Epoch time format (number of seconds elapsed since
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1970-01-01T00:00 UTC). The time value must be greater than the
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median time of the previous 11 blocks. No peer will accept a block with a
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time currently more than two hours in the future according to the
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peer's clock.
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5. *Bits* translates into the target threshold value---the maximum allowed
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value for this block's hash. The bits value must match the network
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difficulty at the time the block was mined.
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6. The *[header nonce][]{:#term-header-nonce}{:.term}* is an arbitrary input that miners can change to test different
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hash values for the header until they find a hash value less than or
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equal to the target threshold. If all values within the nonce's four
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bytes are tested, the time can be updated or the
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coinbase transaction can be changed and the merkle
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root updated.
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{% endautocrossref %}
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