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

==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==

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

==Specification==

# Treat transactions with a version greater than 1 as non-standard (official Satoshi client will not mine or relay them).
# 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).
# 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)
# 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==

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==

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