feat(gateway): add specification for running the gateway over libp2p

src/http-gateways/libp2p-gateway.md

@@ -0,0 +1,82 @@
+---
+title: libp2p+HTTP Transport Gateway Specification
+description: >
+  The libp2p Transport Gateway specification describes how HTTP Gateway semantics can be used over libp2p transports.
+date: 2023-10-04
+maturity: draft
+editors:
+  - name: Adin Schmahmann
+    github: aschmahmann
+    affiliation:
+        name: Protocol Labs
+        url: https://protocol.ai/
+  - name: Marcin Rataj
+    github: lidel
+    url: https://lidel.org/
+    affiliation:
+        name: Protocol Labs
+        url: https://protocol.ai/
+xref:
+  - path-gateway
+  - trustless-gateway
+tags: ['httpGateways', 'lowLevelHttpGateways', 'exchange', 'transport']
+order: 3
+---
+
+## Introduction
+
+This specification describes how HTTP Gateway semantics
+and APIs can be used over [libp2p](https://github.com/libp2p/specs) transports.
+
+## Specification
+
+The [libp2p+HTTP specification](https://github.com/libp2p/specs/pull/508)
+describes how to use HTTP semantics over stream transports, as well as how
+to do discovery of what protocols are available (and where they are mounted).
+
+### `.well-known/libp2p`
+
+libp2p application sub-protocols exposed behind `/http/1.1` protocol can be
+discovered by the well-known resource (:cite[rfc8615]) at `.well-known/libp2p`.
+
+#### Protocol identifier
+
+In order for a given HTTP Gateway protocol like the :cite[trustless-gateway] to
+work in this environment it requires a protocol identifier to act as a key in
+the `.well-known/libp2p` mapping file.
+
+The `/http/1.1` sub-protocol identifier for the IPFS Gateway when used over libp2p is:
+
+```
+/ipfs/gateway
+```
+
+#### Protocol mounting
+
+A reference `.well-known/libp2p` JSON body with mapping that assumes the gateway to be mounted at `/`:
+
+```js
+{
+  "protocols": {
+    "/ipfs/gateway": {"path": "/"},
+  }
+}
+```
+
+## Gateway type detection
+
+The protocol identifier is shared among Gateway specifications.
+
+HTTP server mounted behind the `/ipfs/gateway` identifier MUST expose
+:cite[trustless-gateway], but is free to also support other gateway types and
+features.
+
+:::note
+
+Signaling Features on HTTP Gateways is wip in [IPIP-425](https://github.com/ipfs/specs/pull/425).
+
+Until the IPIP is finalized, client implementations SHOULD perform feature
+detection on their own, or assume only the most basic [block (application/vnd.ipld.raw)](https://specs.ipfs.tech/http-gateways/trustless-gateway/#block-responses-application-vnd-ipld-raw)
+response type from :cite[trustless-gateway] is available.
+
+:::