IMHO, you likely want the server side to be in the kernel, so you can get to performance similar to in-kernel TCP, and ossification is less of a big deal, because it's "easy" to modify the kernel on the server side.
OTOH, you want to be in user land on the client, because modifying the kernel on clients is hard. If you were Google, maybe you could work towards a model where Android clients could get their in-kernel protocol handling to be something that could be updated regularly, but that doesn't seem to be something Google is willing or able to do; Apple and Microsoft can get priority kernel updates out to most of their users quickly; Apple also can influence networks to support things they want their clients to use (IPv6, MP-TCP). </rant>
If you were happy with congestion control on both sides of TCP, and were willing to open multiple TCP connections like http/1, instead of multiplexing requests on a single connection like http/2, (and maybe transfer a non-pessimistic bandwidth estimate between TCP connections to the same peer), QUIC still gives you control over retransmission that TCP doesn't, but I don't think that would be compelling enough by itself.
Yes, there's still ossification in middle boxes doing TCP optimization. My information may be old, but I was under the impression that nobody does that in IPv6, so the push for v6 is both a way to avoid NAT and especially CGNAT, but also a way to avoid optimizer boxes as a benefit for both network providers (less expense) and services (less frustration).
One thing is that congestion control choice is sort of cursed in that it assumes your box/side is being switched but the majority of the rest of the internet continues with legacy limitations (aside from DCTCP, which is designed for intra-datacenter usage), which is an essential part of the question given that resultant/emergent network behavior changes drastically depending on whether or not all sides are using the same algorithm. (Cubic is technically another sort-of-exception, at least since it became the default Linux CC algorithm, but even then you’re still dealing with all sorts of middleware with legacy and/or pathological stateful behavior you can’t control.)
I mean, if you're trying to be a good netizen, you try to tune your congestion control so it's fair enough in at least a few scenarios. You want it to be fair relative to status quo streams when status quo is dominant or when your new system is dominant, and also fair relative to new streams in the same conditions. This is a challenge of course, and if something in the middle is doing its own congestion control, that's indeed its own layer of fun and pathology.
This is a perspective, but just one of many. The overwhelming majority of IP flows are within data centers, not over planet-scale networks between unrelated parties.
I've never been convinced by an explanation of how QUIC applies for flows in the data center.
Ossification doesn't apply (or it shouldn't, IMHO, the point of Open Source software is that you can change it to fit your needs... if you don't like what upstream is doing, you should be running a local fork that does what you want... yeah, it's nicer if it's upstreamed, but try running a local fork of Windows or MacOS); you can make congestion control work for you when you control both sides; enterprise switches and routers aren't messing with tcp flows. If you're pushing enough traffic that this is an issue, the cost of QUIC seems way too high to justify, even if it helps with some issues.
I don't see why this exception to the end-to-end principle should exist. At the scale of single hosts today, with hundreds of CPUs and hundreds of tenants in a single system sharing a kernel, the kernel itself becomes an unwanted middlebox.
Unless you're using QUIC as some kind of datacenter-to-datacenter protocol (basically as SCTP on steroids with TLS), I don't think QUIC in the datacenter makes much sense at all.
As very few server administrators bother turning on features like MPTCP, QUIC has an advantage on mobile phones with moderate to bad reception. That's not a huge issue for me most of the time, but billions of people are using mobile phones as their only access to the internet, especially in developing countries that are practically skipping widespread copper and fiber infrastructure and moving directly to 5G instead. Any service those people are using should probably consider implementing QUIC, and if they use it, they'd benefit from an in-kernel server.
All the data center operators can stick to (MP)TCP, the telco people can stick to SCTP, but the consumer facing side of the internet would do well to keep QUIC as an option.
> That's not a huge issue for me most of the time, but billions of people are using mobile phones as their only access to the internet, especially in developing countries that are practically skipping widespread copper and fiber infrastructure and moving directly to 5G instead.
For what it's worth: Romania, one of the piss poorest countries of Europe, has a perfectly fine mobile phone network, and even outback small villages have XGPON fiber rollouts everywhere. Germany? As soon as you cross into the country from Austria, your phone signal instantly drops, barely any decent coverage outside of the cities. And forget about PON, much less GPON or even XGPON.
Germany should be considered a developing country when it comes to expectations around telecommunication.