Ethereum - Metropolis & the Road to Proof of Stake

This is just a short little thread outlining some key things in Metropolis that we know of so far.

What is Metropolis?

Metropolis is the next Hard Fork planned for Ethereum, due sometime in September or October, 2017 (anywhere around block number 4.3 million - 4.4 million). A Hard Fork is a change to the protocol that makes previously valid blocks/transactions invalid (or vice-versa), and as such requires all nodes or users to upgrade to the latest version of the protocol software.

Metropolis will be rolled out via two forks, Byzantium and Constantinople.

Byzantium

Byzantium Hard Fork will include:

Code:
EIP 100 (Change difficulty adjustment to target mean block time including uncles)
EIP 140 (REVERT instruction in the Ethereum Virtual Machine)
EIP 196 (Precompiled contracts for addition and scalar multiplication on the elliptic curve alt_bn128)
EIP 197 (Precompiled contracts for optimal ate pairing check on the elliptic curve alt_bn128)
EIP 198 (Precompiled contract for bigint modular exponentiation)
EIP 211 (New opcodes: RETURNDATASIZE and RETURNDATACOPY)
EIP 214 (New opcode STATICCALL)
EIP 649 (Difficulty Bomb Delay and Issuance Reduction)
EIP 658 (Embedding transaction return data in receipts)

The Difficulty Bomb

Also known as the Ice Age. Some of you may have noticed that block times have slightly increased. This is a planned event and has probable cause for to prevent a miner-driven fork as Ethereum leaves Proof of Work and approaches a Proof of Stake protocol. EIP 649 proposes to delay the difficulty bomb for 42 million seconds (or 1.4 years) and to reduce the block rewards with the Byzantium fork from 5 ETH to 3 ETH. The block times will steadily increase as follows:

Code:
23 seconds now (block #4200000)
30 seconds on Sept. 22nd (block #4300000)
39 seconds on Oct. 27th (block #4400000)

You can see the decline in rewards here: https://etherscan.io/chart/ethersupply

Don't worry, near the end of 2018 the block times will be back around 30 seconds.

Constantinople

The main parts of the Byzantium Hard Fork pretty much include the Ice Age/Difficulty Bomb. Some things we can hopefuly expect to see in the Constantinople Hard Fork is

Masking, for security enthusiasts, will allow users to determine the address for which they have a private key.

zk-SNARKs (Zero-knowledge proofs). Users will be able to perform anonymous transactions at higher levels than in the past.

People receiving Ether will be able to pay gas fees instead of just the sender. (I cannot find where I read this in the GitHub right now, but will update when I do find it).

And more as ideas/plans are published.

Source: https://github.com/ethereum/EIPs/pull/669