docs: add WebSockets spec

Specifies the WebSockets transport with notes on certificates,
multiplexing, etc.

Fixes #594

websockets/README.md

@@ -0,0 +1,69 @@
+# libp2p WebSockets
+
+| Lifecycle Stage | Maturity                 | Status | Latest Revision |
+|-----------------|--------------------------|--------|-----------------|
+| 2A              | Candidate Recommendation | Active | r0, 2022-10-12  |
+
+Authors: [@achingbrain]
+
+Interest Group: [@MarcoPolo]
+
+[@achingbrain]: https://github.com/achingbrain
+[@MarcoPolo]: https://github.com/MarcoPolo
+
+See the [lifecycle document](../00-framework-01-spec-lifecycle.md) for context about maturity level
+and spec status.
+
+## Introduction
+
+[WebSockets](https://websockets.spec.whatwg.org/) are a way for web applications to maintain bidirectional communications with server-side processes.
+
+All major browsers have shipped WebSocket support and the implementations are both robust and well understood.
+
+A WebSocket request starts as a regular HTTP request, which is renegotiated as a WebSocket connection using the [HTTP protocol upgrade mechanism](https://developer.mozilla.org/en-US/docs/Web/HTTP/Protocol_upgrade_mechanism).
+
+## Drawbacks
+
+WebSockets suffer from [head of line blocking](https://en.wikipedia.org/wiki/Head-of-line_blocking) and provide no mechanism for stream multiplexing, encryption or authentication so additional features must be added by the developer or by libp2p.
+
+In practice they only run over TCP so are less effective with [DCuTR Holepunching](../relay/DCUtR.md).
+
+## Certificates
+
+With [some exceptions](https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts#when_is_a_context_considered_secure) browsers will prevent making connections to unencrypted WebSockets when the request is made from a [Secure Context](https://www.w3.org/TR/secure-contexts/).
+
+Given that libp2p makes extensive use of the [SubtleCrypto API](https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto), and that API is only available in Secure Contexts, it's safe to assume that any incoming libp2p connections initiated over WebSockets originate from a Secure Context.
+
+Consequently server-side processes listening for incoming libp2p connections via WebSockets must use TLS certificates that can be verified by the connecting user agent.
+
+These must be obtained externally and configured in the same way as you would for an HTTP server.
+
+The only exception to this is if both server and client are operating exclusively on loopback or localhost addresses such as in a testing or offline environment. Such addresses should not be shared outside of these environments.
+
+## Stream Multiplexing
+
+WebSockets have no built in stream multiplexing. Server-side processes listening for incoming libp2p connections via WebSockets should support [multi-stream select](https://github.com/multiformats/multistream-select) and negotiate an appropriate stream multiplexer such as [yamux](../yamux/README.md).
+
+## Authentication
+
+WebSockets have no built in authentication mechanism. Server-side processes listening for incoming libp2p connections via WebSockets should support [multi-stream select](https://github.com/multiformats/multistream-select) and negotiate an appropriate authentication mechanism such as [noise](../noise/README.md).
+
+## Encryption
+
+At the time of writing, the negotiated authentication mechanism should also be used to encrypt all traffic sent over the WebSocket even if TLS certificates are also used at the transport layer.
+
+A mechanism to avoid this but also maintain backwards compatibility with existing server-side processes will be specified in a future revision to this spec.
+
+## Addressing
+
+A WebSocket address contains `/ws`, `/tls/ws` or `/wss` and runs over TCP. If a TCP port is omitted, a secure WebSocket (e.g. `/tls/ws` or `/wss` is assumed to run on TCP port 443), an insecure WebSocket is assumed to run on TCP port 80 similar to HTTP addresses.
+
+Examples:
+
+* `/ip4/192.0.2.0/tcp/1234/ws` (an insecure address with a TCP port)
+* `/ip4/192.0.2.0/tcp/1234/tls/ws` (a secure address with a TCP port)
+* `/ip4/192.0.2.0/ws` (an insecure address that defaults to TCP port 80)
+* `/ip4/192.0.2.0/tls/ws` (a secure address that defaults to TCP port 443)
+* `/ip4/192.0.2.0/wss` (`/tls` may be omitted when using `/wss`)
+* `/dns/example.com/wss` (a DNS address)
+* `/dns/example.com/wss/http-path/path%2Fto%2Fendpoint` (an address with a path)