Add content for QUIC

content/concepts/transport.md

@@ -115,3 +115,98 @@ and some places in the codebase still use the "swarm" terminology.
 {{% /notice %}}
 
 [definition_switch]: /reference/glossary/#switch
+
+## QUIC
+
+QUIC is a new transport protocol that provides an always-encrypted, stream-multiplexed 
+connection built on top of UDP. It started as an experiment by Google on Google Chrome 
+in 2014, and was later standardized by the IETF in 
+[RFC 9000](https://datatracker.ietf.org/doc/html/rfc9000).
+
+### Key challenges with TCP
+
+1. Head-of-line blocking (HoL blocking): TCP is a single byte stream exposed by the kernel, 
+   so streams layered on top of TCP experience HoL blocking.
+
+   {{% notice "info" %}}
+   HoL blocking occurs when a the head packet holds up the other packets in the transmission 
+   queue. 
+   {{% /notice %}}
+
+2. Ossification: Because TCP is unencrypted, middleboxes can inspect and modify
+   TCP header fields and may break unexpectedly when they encounter anything they don’t like.
+
+   {{% notice "info" %}}
+   Middleboxes are intermediary networking devices that can perform special functions to 
+   inspect, transform, and even manipulate network traffic. Examples of middleboxes are 
+   firewalls, NATs, proxies, and load balancers.
+   {{% /notice %}}
+
+3. Handshake inefficiency: the 3-way handshake is inefficient, as it spends 1-RTT on verifying 
+   the client’s address.
+
+   {{% notice "info" %}}
+   TCP requires a 3-way handshake to establish a reliable, bidirectional connection as it intends to.
+   {{% /notice %}}
+
+We need a transport protocol that:
+
+- Understands streams 
+- Overcomes HOL blocking (Head-of-line blocking)
+- Overcomes the latency of connection setup
+- Overcomes the ossification risks of TCP
+
+and, ideally, keeps the guarantees of TCP.
+
+A web browser connection typically entails the following (TCP+TLS+HTTP/2):
+
+1. Transport layer: TCP runs on top of the IP layer to provide a reliable 
+   byte stream.
+   - TCP provides a reliable, bidirectional connection between two end systems.
+2. Secure communication layer: A TLS handshake runs on top of TCP to,
+   establishing an encrypted and authenticated connection.
+   - Standard TLS over TCP requires 3-RTT. A typical TLS 1.3 handshake takes 1-RTT.
+3. Application layer: HTTP runs on a secure transport connection to transfer 
+   information and applies a stream muxer to serve multiple requests.
+   - Application data starts to flow.
+
+<!-- to add diagram -->
+
+### What is QUIC?
+
+QUIC combines the functionality of these layers: it sends UDP packets. Therefore, 
+it is responsible for loss detection and repair itself. By using encryption, 
+QUIC avoid middleboxes. The TLS 1.3 handshake is performed in the first flight, 
+removing the 1-RTT cost of verifying the client’s address. QUIC also exposes multiple 
+streams, so no stream multiplexer is needed at the application layer. Part of the application 
+layer is also built directly into QUIC; when you run HTTP on top of QUIC; only a small shim 
+layer exists that maps 
+[HTTP semantics](https://httpwg.org/http-core/draft-ietf-httpbis-semantics-latest.html) 
+onto QUIC streams.
+
+QUIC supports the resumption of connections (0-RTT connections), allowing a 
+client to send application data right away, even before the QUIC handshake has finished.
+
+<!-- to add diagram -->
+
+### QUIC in libp2p
+
+libp2p only supports bidirectional streams and uses TLS 1.3 by default. 
+The streams in libp2p map cleanly to QUIC streams.
+
+When a connection starts, peers will take their host key and create a self-signed CA 
+certificate. They then sign an intermediate chain using their self-signed CA and put it 
+as a certificate chain in the TLS handshake. View the full TLS specification
+[here](https://github.com/libp2p/specs/blob/master/tls/tls.md).
+
+{{% notice "note" %}}
+
+To be clear, there is no additional security handshake and stream muxer needed as QUIC 
+provides all of this by default.
+
+{{% /notice %}}
+
+Following the multiaddress format described earlier, a standard QUIC connection will
+look like: `/ip4/127.0.0.1/udp/65432/quic/`.
+
+In this section, we offered an overview of QUIC and how QUIC works in libp2p.