# Explaining core system Ethereum Virtual Machine #### What is EVM (Ethereum Virtual Machine)? EVM is a runtime environment for Ethereum blockchain. It allows smart contract code to run by compiling to EVM bytecode. EVM plays a core role in Ethereum blockchain to ensure a trustless mechanism without having any central administrator. EVM keeps each node systemically isolated from others in order to avoid security risks. Even if one node is compromised, it does not influence any other nodes and blockchain network. EVM ensures Ethereum blockchain to be secured. As called world computer, Ethereum can solve more complex computational logics differently from Bitcoin blockchain(it only can simply transfer money). EVM reads the code, calculates Gas, executes, and mines to record to Block. Again, it plays a core and essential role in Ethereum. #### Why do we need EVM? EVM enables smart contract(solidity) to be executed on any computer(OS agnostic). EVM is installed on the computer(operating system) and works as a middle layer between a smart contract and operating system. Once Solidity code is compiled to bytecode, EVM can read to execute. If you are a developer, JVM(Java Virtual Machine) might let you understand more easily. ![](https://i.imgur.com/8o8dKbn.jpg) If we do not have EVM, we need to develop respective compilers for each operating system. EVM makes Ethereum ecosystem compatible and efficient. #### EVM bytecode Here, I’ll clearly break down two terms: “bytecode” and “EVM bytecode”. In computer science, “bytecode” is a computer language which is compiled from source code and run on Virtual Machine. Bytecode is not human-readable but computer-readable. ``` [Flow] Source Code -> Bytecode -> Machine Code Source Code: File written in programming language such as Java, Solidity. Bytecode: Compiled from source code and run on Virtual Machine such as JVM, EVM Machine Code: Code that only operating system can read. Bytecode is converted to Machine Code and finally executed. ``` Once, you understand what bytecode is, it is easy to understand what “EVM bytecode” is. EVM byte code is compiled from Solidity and executed on EVM. #### What is Opcodes? In computer science, Opcode(operation code) is a set of instruction code for computers. We can tell hardware directly what it should do. It is a portion of machine language instruction. Ethereum has original opcode system, called [Ethereum Virtual Machine Opcodes](https://ethervm.io/). Internally, EVM converts “EVM bytecode” to “Ethereum Virtual Machine Opcodes” to execute specific tasks. A developer also can directly write opcode with [Solidity Assembly](https://docs.soliditylang.org/en/develop/assembly.html). But it might be a lot harder to understand than solidity. #### Contract ABI ABI(Application Binary Interface) is an interface between two program modules; often, between an operating system and user programs. In Ethereum, Contract ABI is a standard(or scheme), which is defined as data structure and functions with JSON array format, to interact with a smart contract from external application. It enables application-to-contract and contract-to-contract interaction. When developers make their wallet or DApp that interact with the smart contract, the application would call Contract ABI. #### Solidity Solidity is a high-level programming language on which executes EVM. It needs to be compiled to bytecode to be executed on EVM with compiler such as solc. It is designed for Smart Contract and the syntax is based on ECMAScript so that similar to JavaScript. But it is not only influenced by JavaScript but also C++, Python, and PowerShell. Solidity creates “Contract” as a base frame like “Class” in object-oriented programming. All programs start from “Contract” as object-oriented programming does from “Class”. As of today, Solidity is the primary language on Ethereum as well as other blockchain platforms such as Monax and Hyperledger Burrow. #### Gas and EVM [Gas in Ethereum](https://medium.com/@eiki1212/what-is-ethereum-gas-simple-explanation-2f0ae62ed69c) is the fuel that is consumed for computation. Whenever the users execute a smart contract on Ethereum, they need to pay Gas as a fee. Each and every line of code in Solidity requires a certain amount of gas to be executed. Amount of Gas in each instruction is clearly defined in [Ethereum Yellow Paper](http://paper.gavwood.com/) like below. ![](https://i.imgur.com/HZSYDu0.png) Ethereum Gas system plays a very important role to incentivize miners. Ethereum blockchain is run by someone who runs EVM by using their own computer resources. Therefore, the user needs to pay for their computation effort. The reason why they are not paid with ETH is to hedge from market volatility. If you want to know more details, please refer here. #### What is Client Ethereum client is a software that implements EVM. It has a variety of implementations such as Geth(Go-Ethereum) and Parity(Rust implementation). It provides not only EVM but also other functions such as a command-line interface to interact Ethereum blockchain. Here, I’ll explain a bit more deeply what Ethereum Client is by starting a definition of “Client”. In computer science, a term “client” means software/hardware which connects to the server. The client uses the resources and services that the server provides. In a blockchain, a client is a software/hardware that connects to other clients in a peer-to-peer way. The client is also called “node” in a similar way. However, “client” refers to more like software installed on hardware and “Node” to whole physical computer including hardware and software(“client”). The client has two types: full node client and lightweight client. Full node client downloads all blockchain data on disk and receives blocks and transaction to validate. On the other hand, the lightweight client downloads only blockchain header in order to save disk space. Below the functions of each node are listed. ##### Full Node Client * Stores the full blockchain data available on disk and can serve the network with any data on request. * Receives new transactions and blocks while participating in block validation. * Verifies all blocks and states. * Stores recent state only for more efficient initial sync. * All state can be derived from a full node. * Once fully synced, stores all state moving forward similar to archive nodes (more below). ##### Lightweight Client * Stores the header chain and requests everything else on demand. * Can verify the validity of the data against the state roots in the block headers References [What Is An Ethereum (ETH) Virtual Machine (EVM)?](https://xbt.net/blog/ethereum-blog/what-is-an-ethereum-eth-virtual-machine-evm/) [What is Ethereum Gas? [The Most Comprehensive Step-By-Step Guide Ever!]](https://blockgeeks.com/guides/ethereum-gas/) [Running an Ethereum Node](https://blockgeeks.com/guides/ethereum-gas/)
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Nhlanhla Hasane
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Update 24 September 2021 - 30 September 2021

This week I am focused on learning about LES(Light Ethereum subprotocol), from what i have gathered so far with limited resources I have come across internet,is that they only download block headers as they appear and fetch other parts of the blockchain on-demand. They provide full functionality in terms of safely accessing the blockchain, but do not mine and therefore do not take part in the consensus process. Full and archive nodes can also support the 'les' protocol besides 'eth' in order to be able to serve light nodes. LES use Cononial Hash trie structure (specifically Patricia verkle tree) for quick initial syncing and secure on-demand retrieval of the Cononial Hash mapping, block headers and total difficulty values. The BloomBits data structure optimizes log searching by doing a bitwise transformation that makes it cheaper to retrieve bloom filter data relevant to a specific filter. When searching in a long section of the block history, we are checking three specific bits of each bloom filter per queried address/topic. In order to do that, LES must retrieve a ~550 byte block header per filtered block. BloomBits data is stored in compressed form. The compression algorithm is optimized for sparse input data which contains a lot of zero bytes. Decompression requires knowledge of the decompressed data length. Any node which takes on a server role in the the LES protocol needs to be able to somehow limit the amount of work it does for each client peer during a given time period. They can always just serve requests slowly if they are overloaded, but it is beneficial to give some sort of flow control feedback to the clients. This way, clients could (and would have incentive to) behave nicely and not send requests too quickly in the first place (and then possibly timeout and resend while the server is still working on them). They could also distribute requests better between multiple servers they are connected to. And if clients can do this, servers can expect them to do this and throttle or drop them if they break the flow control rules.

9/30/2021
Light Ethereum Subprotocol (LES)

The Light Ethereum Subprotocol (LES) is the protocol used by "light" clients, which only download block headers as they appear and fetch other parts of the blockchain on-demand. They provide full functionality in terms of safely accessing the blockchain, but do not mine and therefore do not take part in the consensus process. Full and archive nodes can also support the 'les' protocol besides 'eth' in order to be able to serve light nodes. The current protocol version is les/4. See end of document for a list of changes in past protocol versions. Some of the les protocol messages are similar to of the Ethereum Wire Protocol, with the addition of a few new fields.

9/27/2021
Ethereum Wire Protocol (ETH)

'eth' is a protocol on the RLPx transport that facilitates exchange of Ethereum blockchain information between peers. The current protocol version is eth/66. See end of document for a list of changes in past protocol versions. Basic Operation Once a connection is established, a Status message must be sent. Following the reception of the peer's Status message, the Ethereum session is active and any other message may be sent. Within a session, three high-level tasks can be performed: chain synchronization, block propagation and transaction exchange. These tasks use disjoint sets of protocol messages

9/20/2021
The RLPx Transport Protocol

This specification defines the RLPx transport protocol, a TCP-based transport protocol used for communication among Ethereum nodes. The protocol carries encrypted messages belonging to one or more 'capabilities' which are negotiated during connection establishment. RLPx is named after the RLP serialization format. The name is not an acronym and has no particular meaning. The current protocol version is 5. You can find a list of changes in past versions at the end of this document. Notation

9/13/2021
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