feat: early muxer negotiation

content/concepts/multiplex/early-negotiation.md

@@ -0,0 +1,69 @@
+---
+title: "Early Multiplexer Negotiation"
+description: "Early stream multiplexer negotiation is an optimization in libp2p where peers can negotiate which multiplexer to use during the security protocol handshake, saving one round trip."
+weight: 162
+---
+
+## Vanilla stream multiplexer selection process
+
+One of libp2p's main guarantees is that no data sent over the wire is unencrypted.
+This means that transport protocols, like TCP or WebSocket, that don't support encryption
+by default must complete a cryptographic handshake. This process of adding the secure channel
+on top of the raw transport is called upgrading the connections and happens via the
+[multistream-select protocol](https://github.com/multiformats/multistream-select).
+
+In the unoptimized libp2p connection upgrade process, the security or encryption is negotiated
+first. After that is agreed upon, the stream multiplexer is negotiated. Again this only happens
+for transports that don't have native stream multiplexing.
+
+A standard connection upgrade process that negotiates the secure channel first and the multiplexer
+second
+[is shown in a diagram here](https://github.com/libp2p/specs/tree/master/connections#upgrading-connections).
+
+First, the security protocol is negotiated, then this protocol is used to perform a cryptographic
+handshake. libp2p currently supports [Noise](../secure-comm/noise) and [TLS 1.3](../secure-comm/tls).
+Once the cryptographic handshake completes, multistream-select runs again on top of
+the secured connection to negotiate a steam multiplexer, like [yamux](yamux) or [mplex](mplex).
+
+<!-- ADD DIAGRAM -->
+
+## Early muxer negotiation
+
+The libp2p project eliminates an unnecessary round trip in the standard negotiation protocol
+for selecting a stream multiplexer. This is achieved by combining the steps of agreeing on a
+secure channel and multiplexer. This is called "early" or "inlined" muxer negotiation.
+
+Early muxer negotiation is a feature in libp2p that allows for the simultaneous selection of a
+stream multiplexer during the cryptographic handshake of the TLS and Noise security protocols.
+This is achieved by sharing a list of supported muxer protocols as a part of the handshake payload.
+
+For example, if a libp2p node supports mplex and yamux, it will advertise both in the list.
+This **eliminates an extra round trip**, improving TTFB (time to first byte) in the libp2p handshake.
+Currently, this feature is only supported in go-libp2p and for TCP and WebSocket transports that do
+not have native encryption or multiplexing.
+
+<!-- ADD DIAGRAM -->
+
+### ALPN extension in TLS
+
+The [Application-Layer Protocol Negotiation (ALPN) extension](https://datatracker.ietf.org/doc/html/rfc7301)
+is a feature of TLS that allows for the negotiation of application-layer protocols during the TLS handshake.
+This allows the client and server to agree on the application-layer protocol for the rest of the TLS session.
+ALPN is typically used to negotiate the application-layer protocol for applications that use TLS, such as HTTP/2
+or QUIC. libp2p uses ALPN to negotiate the stream muxer and saves a roundtrip when upgrading a raw connection.
+
+### Extension registry in Noise
+
+Since there's no commonly used extension mechanism in Noise, libp2p defines an extension registry.
+We then defined an extension to negotiate the stream multiplexer, that is conceptually the equivalent
+of the ALPN extension in TLS.
+
+> The extension registry is modeled after
+> [RFC 6066](https://www.rfc-editor.org/rfc/rfc6066) (for TLS) and
+> [RFC 9000](https://datatracker.ietf.org/doc/html/rfc9000#section-19.21)
+> (for QUIC).
+
+More information is available in the
+[Noise specification](https://github.com/libp2p/specs/blob/master/noise/README.md#libp2p-data-in-handshake-messages).
+
+{{< alert icon="💡" context="note" text="See the inclined muxer negotiation <a class=\"text-muted\" href=\"https://github.com/libp2p/specs/blob/master/connections/inlined-muxer-negotiation.md\">specification</a> for more details." />}}