73/WAKU2-INCENTIVIZATION: add Waku incentivization RFC (#642)

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+---
+slug: "73"
+title: 73/WAKU2-INCENTIVIZATION
+name: Incentivization for Waku Light Protocols
+status: raw
+category: Standards Track
+tags:
+  - incentivization
+editor: Sergei Tikhomirov <sergei@status.im>
+contributors:
+---
+# Abstract
+
+This document describes an approach to incentivization of Waku request-response protocols.
+Incentivization is necessary for economically sustainable growth of Waku.
+In an incentivized request-response protocol, only eligible (e.g., paying) clients receive the service.
+Clients include eligibility proofs in their requests.
+
+Eligibility proofs are designed to be used in multiple Waku protocols, such as Store, Lightpush, and Filter.
+Store is planned to become the first Waku protocol to support incentivization.
+We discuss the proof-of-concept implementation of incentivization for Store in a later section.
+
+# Background / Rationale / Motivation
+
+Decentralized protocols require incentivization to be economically sustainable.
+While some aspects of a P2P network can successfully operate in a tit-for-tat model,
+we believe that nodes that run the protocol in good faith need to be tangibly rewarded.
+Motivating servers to expand resources on handling clients' requests allows us to scale the network beyond its initial altruism-based phase.
+
+Incentivization is not necessarily limited to monetary rewards.
+Reputation may also play a role.
+For Waku request-response (i.e., client-server) protocols, we envision a combination of monetary and reputation-based incentivization.
+See a [write-up on incentivization](https://github.com/waku-org/research/blob/1e3ed6a5cc47e6d1e7cb99271ddef9bf38429518/docs/incentivization.md) for our high-level reasoning on the topic.
+
+# Theory / Semantics
+
+The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”,  “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt).
+
+Consider a request-response protocol with two roles: a client and a server.
+A server MAY indicate to a client that it expects certain eligibility criteria to be met.
+In that case, a client MUST provide a valid eligibility proof as part of its request.
+
+Forms of eligibility proofs include:
+- Proof of payment: for paid non-authenticated requests. A proof of payment, in turn, may also take different forms, such as a transaction hash or a ZK-proof. In order to interpret a proof of payment, the server needs information about its type.
+- Proof of membership: for services for a predefined group of users. An example use case: an application developer pays in bulk for their users' requests. A client then prove that they belong to the user set of that application. A similar concept (RLN) is used in Waku Relay for spam prevention.
+- Service credential: a proof of membership in a set of clients who have prepaid for the service (which may be considered a special case of proof of membership).
+
+Upon a receiving a request:
+- the server SHOULD check if the eligibility proof is included and valid;
+- if that proof is absent or invalid, the server SHOULD send back response with a corresponding error code and error description;
+- if the proof is valid, the server SHOULD send back the response that the client has requested.
+
+Note that the protocol does not ensure atomicity.
+It is technically possible for a server to fail to respond to an eligible request (in violation of the protocol).
+Addressing this issue is left for future work.
+
+# Wire Format Specification / Syntax
+
+A client includes an `EligibilityProof` in its request.
+A server includes an `EligibilityStatus` in its response.
+
+```protobuf
+syntax = "proto3";
+
+message EligibilityProof {
+  optional bytes proof_of_payment = 1;  // e.g., a txid
+  // may be extended with other eligibility proof types, such as:
+  //optional bytes proof_of_membership = 2;  // e.g., an RLN proof
+}
+
+message EligibilityStatus {
+  optional uint32 status_code = 1;
+  optional string status_desc = 2;
+}
+```
+
+We include the `other_eligibility_proof` field in `EligibilityProof` to reflect other types of eligibility proofs that could be added to the protocol later.
+
+# Implementation in Store (PoC version)
+
+This Section describes a proof-of-concept (PoC) implementation of incentivization in the Store protocol.
+Note: this section may later be moved to Store RFC.
+
+Store is one of Waku's request-response protocols.
+A Store client queries the server for historic messages.
+A Store server responds with a list of messages that pass the user's filter.
+See [13/WAKU2-STORE](/spec/13) for the definitions of `HistoryQuery` and `HistoryResponse`.
+
+The PoC Store incentivization makes the following simplifying assumptions:
+- the client knows the server's on-chain address `A`;
+- the client and the server have agreed on a constant price `r` per hour of message history.
+
+To query messages from a period of length `t`, the client:
+1. calculates the total price `p` as `p = r * t`;
+2. pays `p` to the server's address `A` with an on-chain transaction;
+3. waits until the transaction is confirmed with identifier `txid`;
+4. includes `txid` in the request as a proof of payment.
+
+It is the server's responsibility to keep track of the `txid`s from prior requests and to make sure they are not reused.
+
+Note that `txid` may not always be practical as proof of payment due to on-chain confirmation latency.
+To address this issue, future versions of the protocol may involve:
+- paying for multiple requests in one transaction;
+- using faster (likely L2-based) payment mechanisms.
+
+## Wire Format Specifications for Store PoC incentivization
+
+### Request
+
+We extend `HistoryQuery` to include an eligibility proof:
+```protobuf
+message HistoryQuery {
+  // the first field is reserved for future use
+  string pubsubtopic = 2;
+  repeated ContentFilter contentFilters = 3;
+  PagingInfo pagingInfo = 4;
+  // numbering gap left for non-eligibility-related protocol extensions
+  + optional bytes eligibility_proof = 10;
+}
+```
+
+An example of usage with `txid` as a proof of payment:
+```protobuf
+HistoryQuery history_query {
+  // the first field is reserved for future use
+  pubsubtopic: "example_pubsub_topic"
+  contentFilters: []
+  pagingInfo: {
+    // provide values for PagingInfo fields
+  }
+  eligibility_proof: {
+	  proof_of_payment: 0xabc123  // txid for the client's payment
+	  // eligibility proofs of other types are not included
+  };
+}
+```
+
+### Response
+
+We extend the `HistoryResponse` to indicate the eligibility status:
+```protobuf
+message HistoryResponse {
+  // the first field is reserved for future use
+  repeated WakuMessage messages = 2;
+  PagingInfo pagingInfo = 3;
+  enum Error {
+    NONE = 0;
+    INVALID_CURSOR = 1;
+    + NON_ELIGIBLE = 2;
+  }
+  Error error = 4;
+  + EligibilityStatus eligibility_status = 5;
+}
+```
+
+Example of a response if the client is eligible:
+```protobuf
+HistoryResponse response_example {
+  messages: [message_1, message_2]
+  pagingInfo: paging_info
+  error: NONE
+  eligibility_status: {
+	  status_code: 200
+	  status_desc: "OK"
+  }
+}
+```
+
+Example of a response if the client is not eligible:
+```protobuf
+HistoryResponse response_example {
+  messages: [] // no messages sent to non-eligible clients
+  pagingInfo: paging_info
+  error: NON_ELIGIBLE
+  eligibility_status: {
+    status_code: 402
+    status_desc: "PAYMENT_REQUIRED"
+  }
+}
+```
+
+# Security/Privacy Considerations
+
+Eligibility proofs may reveal private information about the client.
+In particular, a transaction identifier used as a proof of payment links the client's query to their on-chain activity.
+Potential countermeasures may include using one-time addresses or ZK-based privacy-preserving protocols.
+
+# Limitations and Future Work
+
+This document is intentionally simplified in its initial version.
+It assumes a shared understanding of prices and the blockchain addresses of servers.
+Additionally, the feasibility of paying for each query is hindered by on-chain fees and confirmation delays.
+
+We will address these challenges as the specification evolves alongside the corresponding PoC implementation.
+The following ideas will be explored:
+- Batch Payment: instead of paying for an individual query, the client would make a consolidated payment for multiple messages.
+- Price Negotiation: rather than receiving prices off-band, the client would engage in negotiation with the server to determine costs.
+- Dynamic Pricing: the price per message would be variable, based on the total size (in bytes) of all received messages.
+- Subscriptions: the client would pay for a defined time period during which they can query any number of messages, subject to DoS protection.
+
+# Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
+
+# References
+
+## normative
+- A high-level [incentivization outline](https://github.com/waku-org/research/blob/master/incentivization.md)
+- [13/WAKU2-STORE](/spec/13) (for Store-specific sections)
+
+## informative
+
+RFCs of request-response protocols:
+- [12/WAKU2-FILTER](/spec/12)
+- [13/WAKU2-STORE](/spec/13)
+- [19/WAKU2-LIGHTPUSH](/spec/19)
+
+RFCs of Relay and RLN-Relay:
+- [11/WAKU2-RELAY](/spec/11)
+- [17/WAKU2-RLN-RELAY](/spec/17)