151 lines
6.8 KiB
Markdown
151 lines
6.8 KiB
Markdown
---
|
|
title: 2/MVDS
|
|
name: Minimum Viable Data Synchronization
|
|
status: stable
|
|
editor: Sanaz Taheri \<sanaz@status.im\>
|
|
contributors:
|
|
- Dean Eigenmann \<dean@status.im\>
|
|
- Oskar Thorén \<oskarth@titanproxy.com\>
|
|
sidebar_position: 2
|
|
---
|
|
|
|
In this specification, we describe a minimum viable protocol for data synchronization inspired by the Bramble Synchronization Protocol[^1]. This protocol is designed to ensure reliable messaging between peers across an unreliable peer-to-peer (P2P) network where they may be unreachable or unresponsive.
|
|
|
|
We present a reference implementation[^2] including a simulation to demonstrate its performance.
|
|
|
|
## Definitions
|
|
|
|
| Term | Description |
|
|
|------------|-------------------------------------------------------------------------------------|
|
|
| **Peer** | The other nodes that a node is connected to. |
|
|
| **Record** | Defines a payload element of either the type `OFFER`, `REQUEST`, `MESSAGE` or `ACK` |
|
|
| **Node** | Some process that is able to store data, do processing and communicate for MVDS. |
|
|
|
|
## Wire Protocol
|
|
|
|
### Secure Transport
|
|
|
|
This specification does not define anything related to the transport of packets. It is assumed that this is abstracted in such a way that any secure transport protocol could be easily implemented. Likewise, properties such as confidentiality, integrity, authenticity and forward secrecy are assumed to be provided by a layer below.
|
|
|
|
### Payloads
|
|
|
|
Payloads are implemented using [protocol buffers v3](https://developers.google.com/protocol-buffers/).
|
|
|
|
```protobuf
|
|
syntax = "proto3";
|
|
|
|
package vac.mvds;
|
|
|
|
message Payload {
|
|
repeated bytes acks = 5001;
|
|
repeated bytes offers = 5002;
|
|
repeated bytes requests = 5003;
|
|
repeated Message messages = 5004;
|
|
}
|
|
|
|
message Message {
|
|
bytes group_id = 6001;
|
|
int64 timestamp = 6002;
|
|
bytes body = 6003;
|
|
}
|
|
```
|
|
|
|
*The payload field numbers are kept more "unique" to ensure no overlap with other protocol buffers.*
|
|
|
|
Each payload contains the following fields:
|
|
|
|
- **Acks:** This field contains a list (can be empty) of `message identifiers` informing the recipient that sender holds a specific message.
|
|
- **Offers:** This field contains a list (can be empty) of `message identifiers` that the sender would like to give to the recipient.
|
|
- **Requests:** This field contains a list (can be empty) of `message identifiers` that the sender would like to receive from the recipient.
|
|
- **Messages:** This field contains a list of messages (can be empty).
|
|
|
|
**Message Identifiers:** Each `message` has a message identifier calculated by hashing the `group_id`, `timestamp` and `body` fields as follows:
|
|
|
|
```
|
|
HASH("MESSAGE_ID", group_id, timestamp, body);
|
|
```
|
|
|
|
**Group Identifiers:** Each `message` is assigned into a **group** using the `group_id` field, groups are independent synchronization contexts between peers.
|
|
|
|
The current `HASH` function used is `sha256`.
|
|
|
|
## Synchronization
|
|
|
|
### State
|
|
|
|
We refer to `state` as set of records for the types `OFFER`, `REQUEST` and `MESSAGE` that every node SHOULD store per peer. `state` MUST NOT contain `ACK` records as we do not retransmit those periodically. The following information is stored for records:
|
|
|
|
- **Type** - Either `OFFER`, `REQUEST` or `MESSAGE`
|
|
- **Send Count** - The amount of times a record has been sent to a peer.
|
|
- **Send Epoch** - The next epoch at which a record can be sent to a peer.
|
|
|
|
### Flow
|
|
|
|
A maximum of one payload SHOULD be sent to peers per epoch, this payload contains all `ACK`, `OFFER`, `REQUEST` and `MESSAGE` records for the specific peer. Payloads are created every epoch, containing reactions to previously received records by peers or new records being sent out by nodes.
|
|
|
|
Nodes MAY have two modes with which they can send records: `BATCH` and `INTERACTIVE` mode. The following rules dictate how nodes construct payloads every epoch for any given peer for both modes.
|
|
|
|
\> ***NOTE:** A node may send messages both in interactive and in batch mode.*
|
|
|
|
#### Interactive Mode
|
|
|
|
- A node initially offers a `MESSAGE` when attempting to send it to a peer. This means an `OFFER` is added to the next payload and state for the given peer.
|
|
- When a node receives an `OFFER`, a `REQUEST` is added to the next payload and state for the given peer.
|
|
- When a node receives a `REQUEST` for a previously sent `OFFER`, the `OFFER` is removed from the state and the corresponding `MESSAGE` is added to the next payload and state for the given peer.
|
|
- When a node receives a `MESSAGE`, the `REQUEST` is removed from the state and an `ACK` is added to the next payload for the given peer.
|
|
- When a node receives an `ACK`, the `MESSAGE` is removed from the state for the given peer.
|
|
- All records that require retransmission are added to the payload, given `Send Epoch` has been reached.
|
|
|
|
\<p align="center"\>
|
|
\<img src="../assets/mvds/interactive.png" /\>
|
|
\<br /\>
|
|
Figure 1: Delivery without retransmissions in interactive mode.
|
|
\</p\>
|
|
|
|
#### Batch Mode
|
|
|
|
1. When a node sends a `MESSAGE`, it is added to the next payload and the state for the given peer.
|
|
2. When a node receives a `MESSAGE`, an `ACK` is added to the next payload for the corresponding peer.
|
|
3. When a node receives an `ACK`, the `MESSAGE` is removed from the state for the given peer.
|
|
4. All records that require retransmission are added to the payload, given `Send Epoch` has been reached.
|
|
|
|
\<!-- diagram --\>
|
|
|
|
\<p align="center"\>
|
|
\<img src="../assets/mvds/batch.png" /\>
|
|
\<br /\>
|
|
Figure 2: Delivery without retransmissions in batch mode.
|
|
\</p\>
|
|
|
|
|
|
\> ***NOTE:** Batch mode is higher bandwidth whereas interactive mode is higher latency.*
|
|
|
|
\<!-- Interactions with state, flow chart with retransmissions? --\>
|
|
|
|
### Retransmission
|
|
|
|
The record of the type `Type` SHOULD be retransmitted every time `Send Epoch` is smaller than or equal to the current epoch.
|
|
|
|
`Send Epoch` and `Send Count` MUST be increased every time a record is retransmitted. Although no function is defined on how to increase `Send Epoch`, it SHOULD be exponentially increased until reaching an upper bound where it then goes back to a lower epoch in order to prevent a record's `Send Epoch`'s from becoming too large.
|
|
|
|
\> ***NOTE:** We do not retransmission `ACK`s as we do not know when they have arrived, therefore we simply resend them every time we receive a `MESSAGE`.*
|
|
|
|
## Formal Specification
|
|
|
|
MVDS has been formally specified using TLA+: \<https://github.com/vacp2p/formalities/tree/master/MVDS\>.
|
|
|
|
## Acknowledgments
|
|
- Preston van Loon
|
|
- Greg Markou
|
|
- Rene Nayman
|
|
- Jacek Sieka
|
|
|
|
## Copyright
|
|
|
|
Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
|
|
|
|
## Footnotes
|
|
|
|
[^1]: akwizgran et al. [BSP](https://code.briarproject.org/briar/briar-spec/blob/master/protocols/BSP.md). Briar.
|
|
[^2]: \<https://github.com/vacp2p/mvds\>
|