I'm struggling to grasp how QUIC implements handshakes in contrast with 3 handshakes in TCP. I came across with this picture on Wikipedia. As QUIC uses UDP, I don't understand how the protocol ensures that a packet was delivered and a client doesn't need to resend it.
How does QUIC provide delivery guarantee
protocol-theorytransport-protocol
Related Solutions
I would give an answer of "no, but it is remarkably similar."
Here's some history and a largely complete explanation.
Circuits 101
Information networks can route traffic, basically, in terms of circuit switching or in terms of packet switching. Circuit switching offers many more guarantees than packet switching, but this comes at a cost, and so circuit switched networks can't degrade gracefully. The classic circuit-switched network is the PSTN, and a virtual circuit would be something like a DS0 on the PSTN.
A DS0 basically works as part of a bundle of connections, usually in a DS1. In a DS1, you will have a bundle of DS0's which are transmitted together, frame-by frame in time-division manner, so each DS0 is guaranteed a specific bandwidth, timeliness, etc. by the underlying network transport.
Another way to look at this is that a physical circuit would be something like a cat6 cable running between two terminals. You can send data back and forth over the wires at guaranteed speeds, and no other communications are going to interfere with that. Indeed the early telephone networks worked by connecting physical circuits (that is copper wires) using manual or electromechanical switches. As this was computerized, the circuits were virtualized and digital (as opposed to analog) information was sent down wires on a time division basis again with a circuit reserving a slot in the time division schedule.
What this means is that circuit switching is more about bandwidth reservation than it is about routing. The former leads to the latter. I.e. a circuit reserves bandwidth for the entire connection.
Why TCP Connections are not Virtual Circuits
TCP/IP is fully packet-switched. It makes no provisions for virtual circuits. This is why things like QoS are often necessary when trunking VOIP (a virtual circuit has built-in QoS guarantees). You have no guarantee that all packets will be routed alike. They may not come through in the same order. They may not come through in a timely manner (from a connection-oriented perspective). So you can't really build virtual circuits per se on top of a packet switched protocol like IP.
TCP comes somewhat close and in fact can work as a somewhat imperfect substitute. It offers as many of the guarantees as it can. This is why, when implemented on TCP/IP, H.323 uses TCP connections instead of the virtual circuits the protocol prefers.
But TCP connections still aren't circuits, because they don't reserve bandwidth during connection on every switch between the two nodes.
Of course TCP connections are more than just datagrams. They include routing information (as does UDP) but they also include the accounting information necessary to reconstruct the stream on the other side in order.
The Answer
Both TCP and UDP are datagram protocols. They send a packet of data with routing information to routers with none of the guarantees of that a circuit offers. TCP offers a subset of guarantees on the end points of what a circuit would offer by adding accounting information to allow the end points to handle errors and a series of data in order, but it is only a subset. Of datagram protocols, TCP is the closest thing one will find to a virtual circuit but it is still conceptually and operationally very different.
Assuming you mean to protect confidentiality of the communication at IP layer with IPsec:
How would the underlying network be able to differentiate between UDP and TCP since they're at the transport layer.
The next header field of the ESP header tells you the type of payload.
If you use tunnel mode (which is custom for VPNs), then without the necessary keys you cannot decide what's at transport layer because the next header field will tell you just that there's a whole IP packet encapsulated.
If you use transport mode, then the next header field tells you the type of payload at transport layer.
Will we still have TCP and UDP when we move the IPv6(Although I see that IPsec has been made optional for IPv6)?
TCP and UDP are agnostic to the layer-3 protocol. In fact, TCP and UDP (and SCTP and DCCP) exist also for IPv6.
What seems to puzzle you is that in IPsec tunnel (VPN) mode there is no way to inspect the content. This is supposed to happen at the tunnel end-points. An organization that is worried by this loss of control should not allow IPsec that is not under it's own control.
Further reading: An Illustrated Guide to IPsec
Best Answer
QUIC just uses unreliable UDP as underlying transport vehicle, in the same way that TCP uses unreliable IP transport. Using a new IP protocol number would have caused massive problems due to the ubiquity of NA(P)T, so UDP is used as a workaround.
Roughly, QUIC tracks data using sequence numbers within each connection ID in a very similar way to TCP: sent data is resent when it is not acknowledged in the expected time window. There are many differences in the details though, see https://datatracker.ietf.org/doc/html/rfc9000#section-13.