2021-03-17 23:33:07 +00:00
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# Ethereum 2.0 Data Availability Sampling
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2021-01-04 21:22:17 +00:00
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**Notice**: This document is a work-in-progress for researchers and implementers.
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## Table of contents
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2021-03-17 23:07:15 +00:00
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<!-- TOC -->
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2021-01-04 21:22:17 +00:00
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
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<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
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2021-01-04 21:24:34 +00:00
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- [Data Availability Sampling](#data-availability-sampling)
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- [GossipSub](#gossipsub)
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- [Horizontal subnets](#horizontal-subnets)
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- [Vertical subnets](#vertical-subnets)
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- [Slow rotation: Backbone](#slow-rotation-backbone)
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- [Quick rotation: Sampling](#quick-rotation-sampling)
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- [DAS during network instability](#das-during-network-instability)
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- [Stage 0: Waiting on missing samples](#stage-0-waiting-on-missing-samples)
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- [Stage 1: Pulling missing samples from known peers](#stage-1-pulling-missing-samples-from-known-peers)
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- [Stage 2: Pulling missing data from validators with custody.](#stage-2-pulling-missing-data-from-validators-with-custody)
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2021-01-04 21:22:17 +00:00
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<!-- END doctoc generated TOC please keep comment here to allow auto update -->
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2021-03-17 23:07:15 +00:00
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<!-- /TOC -->
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2021-01-04 21:22:17 +00:00
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## Data Availability Sampling
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TODO: Summary of Data Availability problem
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TODO: Summary of solution, why 2x extension, and randomized samples
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## GossipSub
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### Horizontal subnets
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TODO
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### Vertical subnets
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#### Slow rotation: Backbone
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TODO
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#### Quick rotation: Sampling
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TODO
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### DAS during network instability
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The GossipSub based retrieval of samples may not always work.
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In such event, a node can move through below stages until it recovers data availability.
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#### Stage 0: Waiting on missing samples
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Wait for the sample to re-broadcast. Someone may be slow with publishing, or someone else is able to do the work.
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Any node can do the following work to keep the network healthy:
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- Common: Listen on a horizontal subnet, chunkify the block data in samples, and propagate the samples to vertical subnets.
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- Extreme: Listen on enough vertical subnets, reconstruct the missing samples by recovery, and propagate the recovered samples.
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This is not a requirement, but should improve the network stability with little resources, and without any central party.
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#### Stage 1: Pulling missing samples from known peers
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The more realistic option, to execute when a sample is missing, is to query any node that is known to hold it.
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Since *consensus identity is disconnected from network identity*, there is no direct way to contact custody holders
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without explicitly asking for the data.
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However, *network identities* are still used to build a backbone for each vertical subnet.
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These nodes should have received the samples, and can serve a buffer of them on demand.
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Although serving these is not directly incentivised, it is little work:
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1. Buffer any message you see on the backbone vertical subnets, for a buffer of up to two weeks.
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2. Serve the samples on request. An individual sample is just expected to be `~ 0.5 KB`, and does not require any pre-processing to serve.
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A validator SHOULD make a `DASQuery` request to random peers, until failing more than the configured failure-rate.
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TODO: detailed failure-mode spec. Stop after trying e.g. 3 peers for any sample in a configured time window (after the gossip period).
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#### Stage 2: Pulling missing data from validators with custody.
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Pulling samples directly from nodes with validators that have a custody responsibility,
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without revealing their identity to the network, is an open problem.
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