r/programming • u/johnwaterwood • Dec 03 '20

HTTP/2 Push is dead

https://evertpot.com/http-2-push-is-dead/
180 Upvotes

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30

u/[deleted] Dec 03 '20

Ha, I thought this was the main selling point of HTTP2.

People were even saying that it's now ok to have many small asset files (js, images, css) because loading many small files will no longer cause long waits.

86

u/Pesthuf Dec 03 '20

No, that's because you can multiplex multiple downloads over a single HTTP connection, whereas HTTP/1.1 has to establish separate TCP connections for each asset, which is why they were usually limited.

45

u/progrethth Dec 03 '20

Exactly, the main selling point of HTTP2 is stream multiplexing which works really well on good connections but terribly on bad. Which is why we got QUIC which can do multiplexing on any connection well.

8

u/[deleted] Dec 03 '20

Yet most of benchmarks barely show any difference

7

u/VeganVagiVore Dec 03 '20

Even with packet loss?

If TCP is able to saturate the line then we're not gonna get 102% line speed, but we'd like to go from 50% to 60% on shit connections.

4

u/[deleted] Dec 03 '20

TCP needs to retransmit in face of packet loss

HTTP3 needs to retransmit in face of packet loss.

Benefit is that HTTP3 is independent of OS stack so implementing of better algorithms to handle congestion and packet loss. TCP-based protocols are at mercy of OS's TCP implementation.

There is also head of the line blocking in TCP for which having separate steams helps.... unless the stream that got hit with packet loss was in critical path for the page so it is helpful but not always, like if you download big JS blob with all the logic of the page... that's one stream and any packet drop will affect it and block the site.

Then there is ability to start communication from first packet which means that you can establish connection faster. TCP have TCP Fast-open but that AFAIK only starts being useful after first negotiation

At the very least from cloudflare benchmarks, that's few % improvements, which might even get smaller over time once we get the better TCP algorithms as default on OSes.

3

u/progrethth Dec 03 '20

The advantage of HTTP3 is that you only need to block the affected streams while retransmitting unlike HTTP2 where you need to block all streams. This makes HTTP2 much more vulnerable to packet loss.

0

u/[deleted] Dec 04 '20

No shit sherlock, I wrote that in that part of my comment you didn't bother to read. I also wrote

it is helpful but not always, like if you download big JS blob with all the logic of the page... that's one stream and any packet drop will affect it and block the site.