HTTP3 Explained
(http3-explained.haxx.se)87 points by weinzierl 5 hours ago
87 points by weinzierl 5 hours ago
Worth nothing, that react application (using React Server Components?)! If you have javascript enabled, it renders as a single page app, fetching each additional page via an API. If you disable JS, it renders it all on the server.
The introduction has a "help out" section which links to the github repo: https://github.com/bagder/http3-explained
After a quick google: https://github.com/bagder/http3-explained
(using a search engine is faster than asking for a link on HN)
> As the packet loss rate increases, HTTP/2 performs less and less well. At 2% packet loss (which is a terrible network quality, mind you), tests have proven that HTTP/1 users are usually better off - because they typically have up to six TCP connections to distribute lost packets over. This means for every lost packet the other connections can still continue.
Why doesn't HTTP/2 use more than one socket?
Here’s a conceptual background about how and why HTTP/3 came to be (recollected from memory):
HTTP/1.0 was built primarily as a textual request-response protocol over the very suitable TCP protocol which guaranteed reliable byte stream semantics. The usual pattern was to use a TCP connection to exchange a request and response pair.
As websites grew more complex, a web page was no longer just one document but a collection of resources stitched together into a main document. Many of these resources came from the same source, so HTTP/1.1 came along with one main optimisation — the ability to reuse a connection for multiple resources using Keep Alive semantics.
This was important because TCP connections and TLS (nee SSL) took many round-trips to get established and transmitting at optimal speed. Latency is one thing that cannot be optimised by adding more hardware because it’s a function of physical distance and network topology.
HTTP/2 came along as a way to improve performance for dynamic applications that were relying more and more on continuous bi-directional data exchange and not just one-and-done resource downloads. Two of its biggest advancements were faster (fewer round-trips) TLS negotiation and the concept of multiple streams over the same TCP connection.
HTTP/2 fixed pretty much everything that could be fixed with HTTP performance and semantics for contemporary connected applications but it was still a protocol that worked over TCP. TCP is really good when you have a generally stable physical network (think wired connections) but it performs really badly with frequent interruptions (think Wi-Fi with handoffs and mobile networks).
Besides the issues with connection reestablishment, there was also the challenge of “head of the line blocking” — since TCP has no awareness of multiplexed HTTP/2 streams, it blocks everything if a packet is dropped, instead of blocking only the stream to which the packet belonged. This renders HTTP/2 multiplexing a lot less effective.
In parallel with HTTP/2, work was also being done to optimise the network connection experience for devices on mobile and wireless networks. The outcome was QUIC — another L4 protocol over UDP (which itself is barebones enough to be nicknamed “the null protocol”). Unlike TCP, UDP just tosses data packets between endpoints without much consideration of their fate or the connection state.
QUIC’s main innovation is to integrate encryption into the transport layer and elevate connection semantics to the application space, and allow for the connection state to live at the endpoints rather than in the transport components. This allows retaining context as devices migrate between access points and cellular towers.
So HTTP/3? Well, one way to think about it is that it is HTTP/2 semantics over QUIC transport. So you get excellent latency characteristics over frequently interrupted networks and you get true stream multiplexing semantics because QUIC doesn’t try to enforce delivery order or any such thing.
Is HTTP/3 the default option going forward? Maybe not until we get the level of support that TCP enjoys at the hardware level. Currently, managing connection state in application software means that over controlled environments (like E-W communications within a data centre), HTTP/3 may not have as good a throughput as HTTP/2.
Thank you for a great overview! I wish HTTP3/QUIC was the "default option" and had much wider adoption.
Unfortunately, software implementations of QUIC suffer from dealing with UDP directly. Every UDP packet involves one syscall, which is relatively expensive in modern times. And accounting for MTU further makes the situation ~64 times worse.
In-kernel implementations and/or io-uring may improve this unfortunate situation, but today in practice it's hard to achieve the same throughput as with plain TCP. I also vaguely remember that QUIC makes load-balancing more challenging for ISPs, since they can not distinguish individual streams as with TCP.
Finally, QUIC arrived a bit too late and it gets blocked in some jurisdictions (e.g. Russia) and corporate environments similarly to ESNI.
Thanks for taking the time to make this, that was helpful!
Glad you found it helpful! Most of it is distilled from High Performance Browser Networking (https://hpbn.co/). It’s a very well organised, easy to follow book. Highly recommended!
Unfortunately, it’s not updated to include QUIC and HTTP/3 so I had to piece together the info from various sources.
It's hidden in the "Copy" drop down at the top right.
Tracking sessions across different physical connections has some non-trivial privacy implications:
https://http3-explained.haxx.se/en/quic/quic-connections#con...
Yes, obviously, but we already know how that is used. This is a more complex protocol that might enable attack vectors that were not possible before and we do not think about when accessing websites:
But see the notes taken from the HTTP/3 RFC itself, written by the authors:
10.11. Privacy Considerations
Several characteristics of HTTP/3 provide an observer an opportunity
to correlate actions of a single client or server over time. These
include the value of settings, the timing of reactions to stimulus,
and the handling of any features that are controlled by settings.
As far as these create observable differences in behavior, they could
be used as a basis for fingerprinting a specific client.
HTTP/3's preference for using a single QUIC connection allows
correlation of a user's activity on a site. Reusing connections for
different origins allows for correlation of activity across those
origins.
Several features of QUIC solicit immediate responses and can be used
by an endpoint to measure latency to their peer; this might have
privacy implications in certain scenarios.Sounds overly complicated, I doubt this will have a widespread adoption
About 30% percent of traffic to Cloudflare uses HTTP/3 [0], so it seems pretty popular already. For comparison, this is 3× as much traffic as HTTP/1.1.
[0]: https://radar.cloudflare.com/adoption-and-usage#http1x-vs-ht...
"As of September 2024, HTTP/3 is supported by more than 95% of major web browsers in use and 34% of the top 10 million websites."
Yes and, at the same time practical support within programming language standard libraries & common tooling lags way behind: https://httptoolkit.com/blog/http3-quic-open-source-support-...
A lot of servers still don't support that.
Go http webserver doesn't support http 3 without external libraries. Nginx doesn't support http 3. Apache doesn't support http 3. node.js doesn't support http 3. Kubernetes ingress doesn't support http 3.
should I go on?
edit: even curl itself - which created the original document linked above - has http 3 just in an experimental build.
Well this statement have to be precised.
caddyserver v2 supports HTTP/3 and it's an webserver written in go https://caddyserver.com/features
FYI: There is also an rust webserver which supports HTTP/3. https://v2.ferronweb.org/
Damn it's nice reading a simple static site like this. Links open instantly to the next fully laid out page of content. If only the rest of the web could be like this..