Merge pull request #550 from libp2p/marco/http-transport-component

Document `/http`

http/transport-component.md

@@ -0,0 +1,98 @@
+# HTTP Transport Component <!-- omit in toc -->
+
+| Lifecycle Stage | Maturity      | Status | Latest Revision |
+| --------------- | ------------- | ------ | --------------- |
+| 1A              | Working Draft | Active | r0, 2023-05-31  |
+
+Authors: [@marcopolo]
+
+Interest Group: [@marcopolo], [@mxinden], [@marten-seemann]
+
+[@marcopolo]: https://github.com/marcopolo
+[@mxinden]: https://github.com/mxinden
+[@marten-seemann]: https://github.com/marten-seemann
+
+## Table of Contents <!-- omit in toc -->
+- [Context](#context)
+- [What is an HTTP transport](#what-is-an-http-transport)
+- [Multiaddr representation](#multiaddr-representation)
+- [HTTP Paths (and other HTTP Semantics)](#http-paths-and-other-http-semantics)
+  - [Recommendation on including HTTP semantics in multiaddrs](#recommendation-on-including-http-semantics-in-multiaddrs)
+
+
+## Context
+
+This document is only about advertising support for an HTTP transport. It
+doesn't make any assertions about how libp2p should interact with that
+transport. That will be defined in a future document.
+
+This exists to clarify the role of the `/http` component in Multiaddrs early to
+avoid confusion and conflicting interpretations.
+
+## What is an HTTP transport
+
+An HTTP transport is simply a node that can speak some standardized version of
+HTTP. Intuitively if you can `curl` it with HTTP, then it speaks HTTP.
+
+Most environments will have a way to create an HTTP Client and Server, and the
+specific HTTP version used will be opaque. We use the `/http` component at the
+end of the multidadr to signal that this server supports an HTTP transport. The
+end user agent decides on HTTP version to use, based on the multiaddr prefix,
+application, server negotiation, and specific use case. This follows what
+existing `http://` URL implementations do.
+
+## Multiaddr representation
+
+The multiaddr of a node with an HTTP transport ends with `/http` and is prefixed
+by information that would let an HTTP client know how to reach the server
+(remember that multiaddrs are [interpreted right to
+left](https://github.com/multiformats/multiaddr#interpreting-multiaddrs)). 
+
+The following are examples of multiaddrs for HTTP transport capable nodes:
+
+* `/dns/example.com/tls/http`
+* `/ip4/1.2.3.4/tcp/443/tls/http`
+* `/ip6/2001:0db8:85a3:0000:0000:8a2e:0370:7334/tcp/443/tls/http`
+* `/ip4/1.2.3.4/udp/50781/quic-v1/http`
+
+Note: When we use `/quic-v1/http` or `/tcp/443/tls/http` (or any other
+transport) implementations MUST use the correct HTTP ALPN (e.g. `h3` or `h2`
+respectively) and not `libp2p` when using the HTTP transport.
+
+## HTTP Paths (and other HTTP Semantics)
+
+It may be tempting to add an HTTP path to end of the multiaddr to specify some
+information about a user protocol. However the `/http` component is not a user
+protocol, and it doesn't accept any parameters. It only signals that a node is
+capable of an HTTP transport.
+
+The HTTP Path exists in the semantics level. HTTP Semantics are
+transport-agnostic, and defined by [RFC
+9110](https://httpwg.org/specs/rfc9110.html). You can use these semantics on any
+transport including, but not limited to, the HTTP transports like
+[HTTP/1.1](https://www.rfc-editor.org/info/rfc7235),
+[HTTP/2](https://www.rfc-editor.org/info/rfc9113), or
+[HTTP/3](https://www.rfc-editor.org/info/rfc9114).
+
+### Recommendation on including HTTP semantics in multiaddrs
+
+In general, it's better to keep the multiaddrs as a way of addressing an
+endpoint and keep the semantics independent of any specific transport. This way
+you can use the same semantics among many specific transports.
+
+However, sometimes it's helpful to share a single multiaddr that contains some
+extra application-level data (as opposed to transport data). The recommendation
+is to use a new [multicodec in the private
+range](https://github.com/multiformats/multicodec#private-use-area) for your
+application. Then apply whatever application parameters to the right of your new
+multicodec and transport information to the left. E.g.
+`<transport>/myapp/<parameters>`
+or `/ip4/127.0.0.1/tcp/8080/http/myapp/custom-prefix/foo%2fbar`. Your
+application has the flexibility to handle the parameters in any way it wants
+(e.g. set HTTP headers, an HTTP path prefix, cookies, etc).
+
+This is a bit cumbersome when you are trying to use multiple transports since
+you may end up with many multiaddrs with different transports but the same
+suffix. A potential solution here is to keep them separate. A list of multiaddrs
+for the transports being used, and another multiaddr for the application-level
+data. This is one suggestion, and many other strategies would work as well.