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+---
+title: WAKU-SYNC
+name: Waku Sync
+editor: Simon-Pierre Vivier <simvivier@status.im>
+contributors:
+ - Prem Chaitanya Prathi <prem@status.im>
+ - Hanno Cornelius <hanno@status.im>
+---
+
+## Abstract
+This specification explains the `WAKU-SYNC` protocol
+which enables the reconciliation of two sets of message hashes
+in the context of keeping multiple Store nodes synchronized.
+Waku Sync is a wrapper around
+[Negentropy](https://github.com/hoytech/negentropy) a [range-based set reconciliation protocol](https://logperiodic.com/rbsr.html).
+
+## Specification
+
+**Protocol identifier**: `/vac/waku/sync/1.0.0`
+
+### Terminology
+The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, 
+“RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119](https://www.ietf.org/rfc/rfc2119.txt).
+
+The term Negentropy refers to the protocol of the same name.
+Negentropy payload refers to
+the messages created by the Negentropy protocol.
+Client always refers to the initiator
+and the server the receiver of the first payload.
+
+### Design Requirements
+Nodes enabling Waku Sync SHOULD
+manage and keep message hashes in a local cache
+for the range of time
+during which synchronization is required.
+Nodes SHOULD use the same time range,
+for Waku we chose one hour as the global default.
+Waku Relay or Light Push protocol MAY be enabled
+and used in conjunction with Sync
+as a source of new message hashes
+for the cache.
+
+Nodes MAY use the Store protocol
+to request missing messages once reconciliation is complete
+or to provide messages to requesting clients.
+
+### Payload
+
+```protobuf
+syntax = "proto3";
+
+package waku.sync.v1;
+
+message SyncPayload {
+  optional bytes negentropy = 1;
+
+  repeated bytes hashes = 20;
+}
+```
+
+### Session Flow
+A client initiates a session with a server
+by sending a `SyncPayload` with
+only the `negentropy` field set.
+This field MUST contain
+the first negentropy payload
+created by the client
+for this session.
+
+The server receives a `SyncPayload`.
+A new negentropy payload is computed from the received one.
+The server sends back a `SyncPayload` to the client.
+
+The client receives a `SyncPayload`.
+A new negentropy payload OR an empty one is computed.
+If a new payload is computed then
+the exchanges between client and server continues until
+the client computes an empty payload.
+This client computation also outputs any hash differences found,
+those MUST be stored.
+In the case of an empty payload,
+the reconciliation is done,
+the client MUST send back a `SyncPayload`
+with all the missing server hashes in the `hashes` field and
+an empty `nengentropy` field.
+
+## Attack Vectors
+Nodes using `WAKU-SYNC` are fully trusted.
+Message hashes are assumed to be of valid messages received via Waku Relay or Light push.
+
+Further refinements to the protocol are planned
+to reduce the trust level required to operate.
+Notably by verifying messages RLN proof at reception.
+
+## Implementation
+The following is not part of the specifications but good to know implementation details.
+
+### Peer Choice
+Peering strategies can lead to inadvertently segregating peers and reduce sampling diversity.
+We randomly select peers to sync with for simplicity and robustness.
+
+A good strategy can be devised but we chose not to.
+
+### Interval
+Ad-hoc syncing can be useful in some cases but continuous periodic sync
+minimize the differences in messages stored across the network.
+Syncing early and often is the best strategy.
+The default used in nwaku is 5 minutes interval between sync with a range of 1 hour.
+
+### Range
+We also offset the sync range by 20 seconds in the past.
+The actual start of the sync range is T-01:00:20 and the end T-00:00:20
+This is to handle the inherent jitters of GossipSub.
+In other words, it is the amount of time needed to confirm if a message is missing or not.
+
+### Storage
+The storage implementation should reflect the Waku context.
+Most messages that will be added will be recent and
+all removed messages will be older ones.
+When differences are found some messages will have to be inserted randomly.
+It is expected to be a less likely case than time based insertion and removal.
+Last but not least it must be optimized for sequential read
+as it is the most often used operation.
+
+## Copyright
+
+Copyright and related rights waived via
+[CC0](https://creativecommons.org/publicdomain/zero/1.0/).
+
+## References
+ - https://logperiodic.com/rbsr.html
+ - https://github.com/hoytech/negentropy
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