RFC-0001/Overview

Overview of Tari Network

Maintainer(s): Cayle Sharrock

Licence

The 3-Clause BSD Licence.

Copyright 2018 The Tari Development Community

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of this document must retain the above copyright notice, this list of conditions and the following disclaimer.
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Language

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY" and "OPTIONAL" in this document are to be interpreted as described in BCP 14 (covering RFC2119 and RFC8174) when, and only when, they appear in all capitals, as shown here.

Disclaimer

This document and its content are intended for information purposes only and may be subject to change or update without notice.

This document may include preliminary concepts that may or may not be in the process of being developed by the Tari community. The release of this document is intended solely for review and discussion by the community of the technological merits of the potential system outlined herein.

Goals

The aim of this proposal is to provide a very high-level perspective of the moving parts of the Tari protocol.

Related Requests for Comment

• RFC-0303: Digital asset network
• RFC-0131: Tari mining
• RFC-0201: TariScript
• RFC-0202: TariScript Opcodes

Description

Abstract

Tari's aims to be the most useful, decentralized platform that empowers anyone to create digitally scarce things people love.

This statement packages some important concepts.

To be useful, creators and users need to be able to interact with their digital assets in the way that they've been accustomed to in Web 2.0. This means that there is a safe, secure, responsive interface between human and the machines that manage their information. It means that the user experience is smooth and intuitive. It means that the network protocol itself is flexible and capable enough to provide every type of assets creators can imagine.

It also scales, so that the entire network does not become a victim of its own success and grind to a halt when more than a few hundred people actually try to use it.

Decentralisation is central to Tari's philosophy (pun intended). The tape is wearing thin on the newsreel that reports yet another centralised custodian, gatekeeper or "authority" becoming a single point of failure. This has led to thousands of people losing their crypto-assets, as well as their trust in the ideas of crypto-assets.

Tari has a strong focus on developer experience. The Tari community obsesses over building beautiful, clean, intuitive APIs and templates, which lead to beautiful developer tools. The idea is to widen the pool and allow anyone to create safe, secure, performant digital assets, rather than provide a gated community of rent-extracting "smart-contract developers".

In general, some of these goals are in direct opposition to each other. Speed, security and decentralisation typically form a trilemma that means that you need to settle for two out of the three.

Tari attempts to resolve the dilemma by splitting operations over two discrete layers. Underpinning everything is a base layer that focuses on security and manages global state, and a second, digital assets layer that focuses on rapid finalisation and scalability.

Multiple Layers

The distributed system trilemma tells us that these requirements are mutually exclusive.

We can't have fast, cheap digital assets and also highly secure and decentralized currency tokens on a single system.

Tari overcomes this constraint by building two layers:

1. A base layer that provides a public ledger of Tari coin transactions, secured by PoW to maximize security.
2. A DAN consisting of a highly scalable, efficient side-chain that each manages the state of all digital asset.

The DAN layer gives up some security guarantees in exchange for performance. However, in the case of a liveness failure (wherein parts of the DAN cannot make progress), the base layer intervenes to break the deadlock and allow the DAN to continue.

Base Layer

The Tari base layer has the following primary features:

• PoW-based blockchain using Nakamoto consensus
• Transactions and blocks based on the Mimblewimble protocol

Mimblewimble is a blockchain protocol that offers some key advantages over other UTXO-based cryptocurrencies such as Bitcoin:

• Transactions are private. This means that casual observers cannot ascertain the amounts being transferred or the identities of the parties involved.
• Mimblewimble has a different set of security guarantees to Bitcoin. The upshot of this is that you can throw away UTXOs once they are spent and still verify the integrity of the ledger.
• Multi-signature transactions can be easily aggregated, making such transactions very compact, and completely hiding the parties involved, or the fact that there were multiple parties involved at all.

"Mimblewimble is the most sound, scalable 'base layer' protocol we know" -- @fluffypony

In addition to this, Tari has made some novel additions to the basic Mimblewimble protocol. Primarily, these were invented to allow the DAN to be built on top of Tari, but have found some great applications generally:

• TariScript. Similar to Bitcoin script, TariScript (RFC-201, RFC-202) provides limited "smart contract" functionality on the base layer protocol.
• One-sided payments. Vanilla Mimblewimble requires both sender and receiver to participate in the creation of transactions. It is impossible in MimblewimbleCoin, for example, to post a "tip jar" address and let people unilaterally send you funds. However, this is possible in Tari, thanks to TariScript.
• Stealth addresses. Want to allow people to send you funds using one-sided payments without revealing your public key (and thus, who you are) to the world? Stealth addresses have you covered.
• Covenants. Covenants allow you to construct complex chains of transactions that follow predefined rules.
• Burn transactions. Tari offers unequivocal "burn" transactions that render the burnt outputs permanently unspendable. This is an important mechanism that underpins the DAN economy.

Proof of Work

Tari is mined using a hybrid approach. On average, 50% of block rewards come from Monero merge-mining, while 50% come from the Sha3x algorithm. Blocks are produced every 2 minutes, on average.

The role of the base layer

The Base Layer fulfils these, and only these, major roles:

1. It manages and enforces the accounting and consensus rules of the base Tari (XTR) token. This includes standard payments, and simple smart contracts such as one-sided payments and cross-chain atomic swaps.
2. It maintains the Validator node register.
3. Maintain a register of smart contract templates. This allows users to verify that digital assets are running the code that they expect and includes functionality like version tracking.
4. Provides a global reference clock for the digital assets layer to help it resolve certain operational failure modes.

Digital Assets Network

The DAN is focused on achieving high speed and scalability while maintaining a high degree of decentralisation.

The DAN itself is made up of two conceptual levels. On the more fundamental level, the consensus layer uses Cerberus and emergent HotStuff to reach consensus on state changes in the DAN in a highly scalable, decentralised way.

A big, and probably the biggest, advantage of this approach is that assets in disparate smart contracts can easily interact, without the need for slow, honey pot-shaped bridges.

Then there is the semantic layer, which is where the Tari contracts are compiled, run and verified inside sandboxed Tari virtual machines.

Together, these levels provide that smart contract enabled digital assets layer, that we've simply been calling the DAN.

Interplay of the layers

In general, the base layer knows nothing about the specifics of what is happening on the side-chain. It only cares that no Tari is created or destroyed, and that the flow of funds in and out of side chains are carried out by the appropriate authorised agents.

This is by design: the network cannot scale if details of digital asset contracts have to be tracked on the base layer. We envisage that there could be hundreds of thousands of contracts deployed on Tari. Some of those contracts may be enormous; imagine controlling every piece of inventory and their live statistics for a massively multiplayer online role-playing game (MMORPG). The base layer is also too slow. If any state relies on base layer transactions being confirmed, there is an immediate lag before that state change can be considered final, which kills the latency properties we seek for the DAN.

It is better to keep the two networks almost totally decoupled from the outset, and allow each network to play to its strength.

Change Log