Consensus Protocol
Decentralize the Infrastructure
Last updated
Decentralize the Infrastructure
Last updated
The consensus protocol in a decentralized system ensures the integrity of the network, allowing permissionless participation without the possibility of fraudulent activities. The consensus protocol is the most important component in any decentralized system, since it is where "decentralization" comes from.
The hardest part about the consensus protocol design is that Everyone Could Be Malicious. If a leader is selected, the leader could be malicious. If validators are chosen, the validators could be malicious. The goal of every participant is the same: maximizing the income while at the same time reducing the cost as much as possible. If vulnerability exists, even a minor one, it will be exploited, resulting in the losses for the honest participants. This situation can compel these participants to exit the network, leading to a network downfall eventually.
For example, consider a scenario in Crynux Network, where a malicious node submits a random image to the network without actually performing any computation. If we rely on the user to detect this fraud, allowing them to withhold payment until they have verified the result, it opens a loophole. A dishonest user could exploit this by denying all payments, effectively using the network services without paying.
The consensus protocol in the Crynux Network aims to verify the correctness of a task's output based on its input arguments. Additionally, it ensures that the node submitting the correct result gets the payment.
The consensus protocol must be enforced by the blockchain, which eliminates the need for a centralized authority. This decentralized approach safeguards against potential abuse of power by removing the temptation for any single party to cheat, given their control.
When the application sends a task to the blockchain, the blockchain will decide whether to validate the task based on a pre-defined probability (e.g., 10%). If chosen for validation, the task is sent to 3 nodes for independent execution. The computation results from all 3 nodes will be cross-validated on-chain to prevent cheating. If a node submits a fake result, it will be punished by slashing its staked tokens on the blockchain.
The random sampling result should be kept secret from nodes until they submit their computation results. If a node knows in advance whether a task will be validated, it could cheat by submitting fake results for tasks that won't be validated.
Hiding the random sampling process from the public while keeping it verifiable on-chain is a challenging task, given that all data on the blockchain is public and transparent. Crynux achieved this using a combination of VRF (Verifiable Random Function) and ZKP (Zero-Knowledge Proofs).
Comparing to validating all the tasks on chain, the secret task sampling significantly enhances network efficiency, rivaling centralized platforms while remaining decentralized and permissionless by effectively preventing fraudulent activities. Please find the details of the sampling algorithm in the following document:
For 3-task cross validation to function correctly, the execution of AI tasks needs to be deterministic. This means that, regardless of the GPU types, hardware, or operating systems used across different nodes, identical task parameters should consistently yield the same results.
The non-deterministic behaviors observed in current AI computations stem from two main sources:
For different types of GPUs, the non-determinism observed in AI computations can be pinpointed to specific nuances like floating-point precision disparities, execution strategies, and the tailored optimizations within math libraries and drivers.
The architectural distinctions across different GPUs can introduce slight precision variations, particularly noticeable when leveraging reduced precision formats (e.g., FP16 or BF16) to enhance computational speed. This approach, while efficient, may result in minor discrepancies after numerous calculations, a common scenario in deep learning tasks.
Moreover, GPUs exhibit unique processing strategies, where the scheduling and load management of parallel computations can differ, affecting the determinism due to the non-associative and non-distributive nature of floating-point arithmetic under rounding errors.
Additionally, Nvidia's continuous refinement of its CUDA toolkit, including specialized libraries like cuDNN for deep learning, introduces optimization-driven differences. These libraries are engineered to maximize efficiency and performance on hardware through sophisticated algorithmic choices and task partitioning strategies, which, while largely beneficial, can subtly influence the consistency of computational results.
The frameworks commonly used in AI computation, such as PyTorch, introduce non-deterministic behaviors through their handling of random number generation and the use of inherently non-deterministic algorithms. This randomness is pivotal in various stages, from initializing neural network weights to shuffling data for training.
Moreover, certain PyTorch operations and layers, especially those executed on GPUs, are designed with non-deterministic algorithms for efficiency, such as specific convolution implementations and atomic operations in parallel reductions. Although these features enrich PyTorch's flexibility and performance, they also sow the seeds of variability in outcomes, making exact reproducibility a challenge despite the ability to set global random seeds. This nuanced dance between enhancing performance and managing unpredictability underscores the complexity of achieving deterministic results in AI models developed with PyTorch.
More details about the non-deterministic behavior of PyTorch can be found in its docs and discussions.
Despite the aforementioned challenges, Crynux succeeded in achieving deterministic execution for specific AI tasks on identical GPU models. This was accomplished by thoroughly dissecting the frameworks to capture and control the random numbers, alongside substituting the non-deterministic algorithms with their deterministic counterparts.
Restricting the execution of validation tasks to the same GPU models curtails network performance by narrowing the pool of eligible candidates for a task, and it compromises network security by diminishing the number of honest nodes, thereby making it easier for attackers to launch Sybil attacks with fewer counterfeit nodes.
By tolerating slight discrepancies in computation results and employing specific similarity comparison methods, it becomes feasible to permit the execution of certain tasks across all GPU models, thereby optimizing both performance and security while still facilitating cross-validation of tasks.
Image generation tasks, including text-to-image and image-to-image, can be executed across a variety of GPU models. However, text generation tasks utilizing Large Language Models (LLMs) are restricted to identical model types. Further information is provided in the document below:
The Stable Diffusion fine-tuning tasks can be executed across a variety of GPU models. Read more in the document below:
Generating random numbers on the blockchain is then a critical step to the security of the whole network. Ethereum 2.0 has prevrando, which can be used as the source of the random number. On the other blockchains, the block hash of the last confirmed block is usually used. More advanced (and complex) methods exist such as the Verifiable Random Functions. Strictly speaking, however, none of these methods are safe enough in our scenario.
The attack one could perform, given that the result validation is effective, is for an attacker to host more nodes by himself, and try to have two or more of his own nodes selected for a single task. In which case the attacker could submit two identical fake results to cheat the blockchain.
If an attacker is hosting the blockchain node (and producing the blocks) himself, the last block hash, or prevrando, or the selection of the VRF, is known to him before the CreateTask transaction has been confirmed by the next block. This leaves a chance for the attacker to find out if his nodes are selected for a task ahead of time.
The attacker could then reject the CreateTask transactions in which it can not cheat, i.e. not having two or more of his own nodes selected in the task.
By carefully constructing and organizing more adjacent blocks, the attacker could even control who will be selected in the next task. Note that this does not apply to the VRF method, where the source of the randomness is not from the blockchain. Which is immune to this kind of attack, but introduces other risks which we will not cover in this article.
Considering that to make this attack practical, the attacker must control a significant large number of nodes in the whole network by himself. The Crynux Network chooses to ignore this problem and uses the prevrando on the supported blockchains, and uses the last block hash on other blockchains.
Nodes are required to stake a certain amount of tokens on the blockchain before joining the network. If a node exhibits malicious behavior, its tokens will be slashed.
Given the VSS task validation scheme above, it is then a calculation of the required number of tokens to stake to prevent attacking attempts. If the staked tokens are not enough, the attacker can still make profit even if some tokens will be slashed.
The attacker will start as many malicious nodes as he could. All the malicious nodes will do one thing: submitting the identical fake result for every task they received.
If the task is not selected for validation, the attacker gets the reward for free.
If the task is selected for validation:
If 2 or 3 nodes from the same attacker are selected for the task, the attacker gets the rewards for free.
If there is only 1 node from the attacker is selected, the attacker loses staked tokens.
The probability of an attacker getting more than 2 nodes of himself selected in a task could be calculated as:
And the expectation of the rewards from sybil attack is given by:
An attacker could identify the validation task group by decrypting and comparing the task parameters received by all the malicious nodes. If parameters are identical for two adjacent tasks from the same application, they likely belong to the same validation group. The attacker might then return identical fake results to gain rewards without effort.
However, identifying task groups doesn't provide the attacker with additional advantages in a Sybil attack. The attacker already receives rewards by submitting two identical fake results for all tasks, without needing to identify the validation groups.
Another attack method involves submitting fake results only when the validation group is detected, while behaving normally otherwise. The network cannot identify this behavior.
For this attack to be effective, all malicious nodes must be equipped with GPUs, significantly increasing the cost compared to the Sybil attack mentioned earlier.
Given that only a small portion of the network's tasks will be validated (targeted by this attack), and the chance of an attacker discovering the identification groups is even smaller, the attacker would need to control a significant portion of the nodes, making the attack impractical with low potential income. This scenario is therefore excluded in the consensus protocol.
Additionally, although the task parameters may be identical, the attacker cannot be certain that the tasks are part of the same validation group. There's still a possibility that they are independent tasks. If the attacker submits two fake results, they will be penalized.
Given that the network is a loosely coupled P2P system composed of home computers and laptops, we cannot assume the nodes are reliable. A node may lose contact with the network at any moment, even if it is still marked as available or executing a task on the blockchain.
The applications are also unreliable. Tasks submitted might be entirely inexecutable, such as combining the SD1.5 base model with an SDXL LoRA model.
When an exception occurred during the task execution on the node, if the exception is not recoverable, the node will report the error to the blockchain.
The error reporting will also be cross validated in a validation task group to prevent malicious behaviors from the nodes. If one of nodes reports error while the other two send the normal computation results, it will be penalized.
Crynux Network allows model downloads through an external link. However, network issues may occur during the download. It's challenging to determine if these issues affect all three nodes or if they are temporary.
To prevent mistakenly slashing honest nodes, reporting errors should only be used when the node is certain it's an issue with the task arguments, not a network problem. All other cases should be handled by the timeout mechanism below.
If errors are reported by the nodes, the task will be aborted. And the task fee will be refunded. The small cost of the transaction fee will prevent the applications from sending the invalid task parameters intentionally.
The consensus protocol requires the submission of the commitments of all the 3 nodes. If a selected node goes offline before submitting the commitment to the blockchain, the other 2 nodes will have to wait for an unlimited time, which is not tolerable for both the nodes and the applications.
The timeout mechanism is introduced to solve this problem. After a pre-defined period, all the 3 nodes, and the application, are allowed to submit the request to cancel the task on the blockchain. Once submitted, the blockchain will abort the task immediately.
The timeout mechanism introduces a new vulnerability to the network. An attacker could exploit this by returning fake results only when task validation groups are found. In other scenarios, rather than executing the tasks, the node could simply wait for the timeout to avoid penalties. And similar to a Sybil attack, the attacker can execute this attack without needing GPUs.
Similar to the discussion earlier, the risk of this attack is low and therefore it is excluded from the consensus protocol.
Where is the number of the honest nodes, and is the number of the dishonest nodes the attacker starts.
Where is the sampling rate given in VSS, is the price of the task, and is the number of the staked tokens for a node.
By increasing the number of the staked tokens , we could decrease the expectation down to zero or even below. If is below zero, there is no benefit to attack the system by starting more malicious nodes. The attacking will highly likely cause the attacker to lose money rather than earn.
The safety of the network now depends on the calculated value of the amount of the staked tokens . Given a network size (the number of the total nodes in the network), and a target ratio of the malicious nodes (under which the network is safe), the probability of a successful attack is then fixed. Setting to zero, the amount of the staked tokens required for a single node is determined by: