--------------------------------------------------
  BIP: 34
  Title: Block v2, Height in Coinbase
  Author: Gavin Andresen <gavinandresen@gmail.com>
  Status: Accepted
  Type: Standards Track
  Created: 2012-07-06
--------------------------------------------------

[[abstract]]
Abstract
~~~~~~~~

Bitcoin blocks and transactions are versioned binary structures. Both
currently use version 1. This BIP introduces an upgrade path for
versioned transactions and blocks. A unique value is added to newly
produced coinbase transactions, and blocks are updated to version 2.

[[motivation]]
Motivation
~~~~~~~~~~

1.  Clarify and exercise the mechanism whereby the bitcoin network
collectively consents to upgrade transaction or block binary structures,
rules and behaviors.
2.  Enforce block and transaction uniqueness, and assist unconnected
block validation.

[[specification]]
Specification
~~~~~~~~~~~~~

1.  Treat transactions with a version greater than 1 as non-standard
(official Satoshi client will not mine or relay them).
2.  Add height as the first item in the coinbase transaction's
scriptSig, and increase block version to 2. The format of the height is
"serialized CScript" -- first byte is number of bytes in the number
(will be 0x03 on main net for the next 300 or so years), following bytes
are little-endian representation of the number. Height is the height of
the mined block in the block chain, where the genesis block is height
zero (0).
3.  75% rule: If 750 of the last 1,000 blocks are version 2 or greater,
reject invalid version 2 blocks. (testnet3: 51 of last 100)
4.  95% rule ("Point of no return"): If 950 of the last 1,000 blocks are
version 2 or greater, reject all version 1 blocks. (testnet3: 75 of last
100)

[[backward-compatibility]]
Backward compatibility
~~~~~~~~~~~~~~~~~~~~~~

All older clients are compatible with this change. Users and merchants
should not be impacted. Miners are strongly recommended to upgrade to
version 2 blocks. Once 95% of the miners have upgraded to version 2, the
remainder will be orphaned if they fail to upgrade.

[[implementation]]
Implementation
~~~~~~~~~~~~~~

https://github.com/bitcoin/bitcoin/pull/1526
