Add content for QUIC

content/concepts/transport.md

@@ -115,3 +115,109 @@ 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 between Google 
+services and Chrome in 2014, and was later standardized by the IETF in 
+[RFC 9000](https://datatracker.ietf.org/doc/html/rfc9000), 
+[RFC 9001](https://datatracker.ietf.org/doc/html/rfc9001), and
+[RFC 9002](https://datatracker.ietf.org/doc/html/rfc9002).
+
+### 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 head-of-line (HoL) blocking.
+
+   {{% notice "info" %}}
+   In TCP, head-of-line blocking occurs when a single packet is lost, and packets delivered 
+   after that need to wait in the kernel buffer until a retransmission for the lost packet 
+   is received.
+   {{% /notice %}}
+
+2. Ossification: Because the header of TCP packet is not encrypted, middleboxes can 
+   inspect and modify TCP header fields and may break unexpectedly when they encounter 
+   anything they don’t understand. This makes it practically impossible to deploy any 
+   changes to the TCP protocol that change the wire format.
+
+3. Handshake inefficiency: TCP spends one network round-trip (RTT) on verifying the 
+   client's address. Only after this can TLS start the cryptographic handshake, consuming 
+   another RTT. Setting up an encrypted connection therefore always takes 2 RTTs.
+
+QUIC was designed with the following goals in mind:
+
+- Making the transport layer aware of streams, so that packet loss doesn't cause HoL blocking 
+  between streams.
+- Reducing the latency of connection establishment to a single RTT for new connections, and to 
+  allow sending of 0 RTT application data for resumed connections.
+- Encrypting as much as possible. This eliminates the ossification risk, as middleboxes aren't 
+  able to read any encrypted fields. This allows future evolution of the protocol.
+
+### Comparing HTTP/2 and HTTP/3
+
+In addition to defining the QUIC transport, the IETF also standardized a new version of HTTP that runs on top of QUIC: HTTP/3 (
+[RFC 9114](https://datatracker.ietf.org/doc/html/rfc9114)). HTTP/3 combines the advantages 
+of the existing transfer protocols HTTP/2 and HTTP over QUIC in one standard for faster and 
+more stable data transmission.
+
+The following diagram illustrates the OSI model for HTTP/2 and HTTP/3 [1]:
+
+![HTTP/2 & HTTP/3 OSI model](https://cloudspoint.xyz/wp-content/uploads/2022/03/http3.png)
+
+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. Security layer: A TLS handshake runs on top of TCP to
+   establish 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.
+
+In contrast, HTTP/3 runs over [QUIC](##what-is-quic), where QUIC is similar to 
+TCP+TLS+HTTP/2 and runs over UDP. Building on UDP allows HTTP/3 to bypass the challenges 
+found in TCP and use all the advantages of HTTP/2 and HTTP over QUIC.
+
+### What is QUIC?
+
+QUIC combines the functionality of these layers. Instead of TCP, it builds on UDP. 
+When a UDP datagram is lost, it is not automatically retransmitted by the kernel. 
+QUIC therefore takes responsibility for loss detection and repair itself. By using 
+encryption, QUIC avoids ossified middleboxes. The TLS 1.3 handshake is performed in 
+the first flight, removing the cost of verifying the client’s address and saving an 
+RTT. QUIC also exposes multiple streams (and not just a single byte stream), so 
+no stream multiplexer is needed at the application layer. Part of the application 
+layer is also built directly into QUIC.
+
+In addition, a client can make use of QUIC's 0 RTT feature for subsequent connections 
+when it has already communicated with a certain server. The client can then send 
+(encrypted) application data even before the QUIC handshake has finished.
+
+### QUIC in libp2p
+
+libp2p only supports bidirectional streams and uses TLS 1.3 by default. 
+Since QUIC already provides an encrypted, stream-multiplexed connection, 
+libp2p directly uses QUIC streams, without any additional framing.
+
+To authenticate each others' peer IDs, peers encode their peer ID into a self-signed 
+certificate, which they sign using their host's private key. This is the same way peer 
+IDs are authenticated in the 
+[libp2p TLS handshake](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. This also means that establishing a libp2p connection between
+two nodes using QUIC only takes a single RTT.
+
+{{% /notice %}}
+
+Following the multiaddress format described earlier, a standard QUIC connection will
+look like: `/ip4/127.0.0.1/udp/65432/quic/`.
+
+## References
+
+[1] [What is HTTP/3 by Cloudspoint](https://cloudspoint.xyz/what-is-http3/)