[Working Draft] Fetch protocol spec

fetch/README.md

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+# Fetch v0.0.1
+
+> The Fetch protocol is used for performing a direct peer-to-peer key-value lookup 
+
+| Lifecycle Stage | Maturity       | Status | Latest Revision |
+|-----------------|----------------|--------|-----------------|
+| 1A              | Working Draft  | Active | r1, 2019-08-13  |
+
+Authors: [@aschmahmann]
+
+Interest Group: [Insert Your Name Here]
+
+[@aschmahmann]: https://github.com/aschmahmann
+
+See the [lifecycle document][lifecycle-spec] for context about maturity level
+and spec status.
+
+[lifecycle-spec]: https://github.com/libp2p/specs/blob/master/00-framework-01-spec-lifecycle.md
+
+## Implementations
+
+- [go-libp2p-pubsub-router](https://github.com/libp2p/go-libp2p-pubsub-router/pull/33)
+
+## Table of Contents
+
+- [Overview](#overview)
+- [Spec Proposal Info](#spec-proposal-info)
+- [Wire protocol](#wire-protocol)
+
+## Overview
+
+The fetch protocol is a means for a libp2p node to get data from another based on a given key. It generally fulfills
+the contract:
+
+`Fetch(key) (value, statusCode)`
+
+## Spec Proposal Info
+### Context
+
+Currently there is no standard protocol for doing simple data retrieval. Sometimes a small data retrieval protocol is all
+that's needed to get the job done. This is especially true when it comes to augmenting or layering protocols together.
+
+### Motivation
+
+This specification is helpful because the protocol is already part of a proposal to add a persistence layer on top of
+[PubSub](../pubsub).
+
+There have been numerous discussion revolving around how to allow PubSub to be persistent, some of which go back multiple
+years (more background information is available [here](https://github.com/libp2p/go-libp2p/issues/698)).
+Initial suggestions included modifying or leveraging the existing PubSub spec and implementations.
+However, it was decided that PubSub should remain agnostic to any sort of persistence,
+and that persistence should be layered on top of PubSub as a separate protocol.
+
+There is currently an [IPNS-over-PubSub spec PR](https://github.com/ipfs/specs/pull/218) that describes how the Fetch
+protocol can be used to create a persistence layer on top of PubSub and therefore greatly improve IPNS performance.
+
+### Scope
+
+This is one small protocol that as of right now is only being used to improve the
+[go-libp2p-pubsub-router](https://github.com/libp2p/go-libp2p-pubsub-router). This protocol should not require any one
+who doesn't want to use it to change anything about their current use.
+
+### Potential Effects
+
+1. A persistent pubsub protocol easily built on top of Fetch
+2. A reusable fetch protocol that can be used to augment various other protocols
+(e.g. add the ability to directly ask peers for data to a DHT)
+3. Might be abused/overused given the simple nature of the protocol
+
+### Expected Feature Set and Tentative Technical Directions
+
+Should support: `Fetch(key) (value, statusCode)`
+
+However, the level of specificity in the types of the above variables has wiggle room if people are interested.
+The `go-libp2p-pubsub-router` implementation requires:
+
+ `key`: At least as generic as a UTF-8 string
+
+ `value`: At least as generic as a byte array
+
+ `statusCode`: Supports at least `OK`, `NOT_FOUND`, and `ERROR`
+
+## Wire protocol
+
+The libp2p protocol ID for this protocol is `/libp2p/fetch/0.0.1`
+
+### Message Format
+The messages in the Fetch protocol use on of the following protobufs (proto3 syntax):
+
+```
+message FetchRequest {
+	string identifier = 1;
+}
+
+message FetchResponse {
+	StatusCode status = 1;
+	enum StatusCode {
+		OK = 0;
+		NOT_FOUND = 1;
+		ERROR = 2;
+	}
+	bytes data = 2;
+}
+```
+
+### Protocol
+
+**Setup:**
+- Peers involved, `A`, `B`
+- `A` wants to fetch data from `B` corresponding to the key `k`
+
+**Assumptions:**
+- `A` has connection to `B`
+
+**Events:**
+- `A` sends a `RequestLatest{Identifier : k}` message to `B`
+- `B` does some internal lookup and responds with a `RespondLatest` message
+  - If `B` finds (and elects to send) some data `v` to `A` it sends `RespondLatest{status: SUCCESS, data: v}`
+  - If `B` has no data to send it responds with `RespondLatest{status: NOT_FOUND, data: null}`
+    - `A` ignores any information in the `data` field if the status is `NOT_FOUND` 
+
+Note: If at any time `A` or `B` break from the protocol in some way, either by disconnecting/closing streams or by sending
+invalid data there is no guarantee on the behavior from the other party.
+
+## Future work
+
+Would love this protocol to work over a packet instead of a stream oriented transport since it's very short lived and will
+have cases where the payloads are tiny.
+
+It could also be useful to have an aggregate version of this protocol where instead of fetching key-value pairs we fetch
+sets of key-value pairs. This will be more efficient, much more when using a stream oriented transport, than running fetch
+multiple times.