Create README.md

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+---
+slug: 4
+title: 4/MVDS-META
+name: MVDS Metadata Field
+status: draft
+editor: Sanaz Taheri <sanaz@status.im>
+contributors:
+  - Dean Eigenmann <dean@status.im>
+  - Andrea Maria Piana <andreap@status.im>
+  - Oskar Thorén <oskarth@titanproxy.com>
+---
+
+In this specification, we describe a method to construct message history that will aid the consistency guarantees of [2/MVDS](/spec/2). Additionally, we explain how data sync can be used for more lightweight messages that do not require full synchronization.
+
+## Motivation
+
+In order for more efficient synchronization of conversational messages, information should be provided allowing a node to more effectively synchronize the dependencies for any given message.
+
+## Format
+
+We introduce the metadata message which is used to convey information about a message and how it SHOULD be handled.
+
+```protobuf
+package vac.mvds;
+
+message Metadata {
+  repeated bytes parents = 1;
+  bool ephemeral = 2;
+}
+```
+
+Nodes MAY transmit a `Metadata` message by extending the MVDS [message](/spec/2#payloads) with a `metadata` field.
+
+```diff
+message Message {
+  bytes group_id = 6001;
+  int64 timestamp = 6002;
+  bytes body = 6003;
+ Metadata metadata = 6004;
+}
+```
+
+### Fields
+
+| Name                   |   Description                                                                                                                    |
+| ---------------------- | -------------------------------------------------------------------------------------------------------------------------------- |
+| `parents`               |   list of parent [`message identifier`s](/spec/2#payloads) for the specific message. |            
+| `ephemeral`         |   indicates whether a message is ephemeral or not.                                                             |
+
+## Usage
+
+### `parents`
+
+This field contains a list of parent [`message identifier`s](/spec/2#payloads) for the specific message. It MUST NOT contain any messages as parent whose `ack` flag was set to `false`. This establishes a directed acyclic graph (DAG)[^1] of persistent messages.
+
+Nodes MAY buffer messages until dependencies are satisfied for causal consistency[^2], they MAY also pass the messages straight away for eventual consistency[^3].
+
+A parent is any message before a new message that a node is aware of that has no children.
+
+The number of parents for a given message is bound by [0, N], where N is the number of nodes participating in the conversation, therefore the space requirements for the `parents` field is O(N).
+
+If a message has no parents it is considered a root. There can be multiple roots, which might be disconnected, giving rise to multiple DAGs.
+
+### `ephemeral`
+
+When the `ephemeral` flag is set to `false`, a node MUST send an acknowledgment when they have received and processed a message. If it is set to `true`, it SHOULD NOT send any acknowledgment. The flag is `false` by default.
+
+Nodes MAY decide to not persist ephemeral messages, however they MUST NOT be shared as part of the message history.
+
+Nodes SHOULD send ephemeral messages in batch mode. As their delivery is not needed to be guaranteed.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
+
+## Footnotes
+
+[^1]: <https://en.wikipedia.org/wiki/Directed_acyclic_graph>
+[^2]: Jepsen. [Causal Consistency](https://jepsen.io/consistency/models/causal). Jepsen, LLC.
+[^3]: <https://en.wikipedia.org/wiki/Eventual_consistency>