[Nick-Craver]

Well, yes and no - it depends on the length. Let's take 3 common examples. Here's GitHub's relevant headers (that we don't have):

Content-Security-Policy:default-src 'none'; base-uri 'self'; block-all-mixed-content; child-src render.githubusercontent.com; connect-src 'self' uploads.github.com status.github.com collector.githubapp.com api.github.com www.google-analytics.com github-cloud.s3.amazonaws.com github-production-repository-file-5c1aeb.s3.amazonaws.com github-production-user-asset-79cafe.s3.amazonaws.com wss://live.github.com; font-src assets-cdn.github.com; form-action 'self' github.com gist.github.com; frame-ancestors 'none'; img-src 'self' data: assets-cdn.github.com identicons.github.com collector.githubapp.com github-cloud.s3.amazonaws.com *.githubusercontent.com; media-src 'none'; script-src assets-cdn.github.com; style-src 'unsafe-inline' assets-cdn.github.com

Public-Key-Pins:max-age=5184000; pin-sha256="WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18="; pin-sha256="RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho="; pin-sha256="k2v657xBsOVe1PQRwOsHsw3bsGT2VzIqz5K+59sNQws="; pin-sha256="K87oWBWM9UZfyddvDfoxL+8lpNyoUB2ptGtn0fv6G2Q="; pin-sha256="IQBnNBEiFuhj+8x6X8XLgh01V9Ic5/V3IRQLNFFc7v4="; pin-sha256="iie1VXtL7HzAMF+/PVPR9xzT80kQxdZeJ+zduCB3uj0="; pin-sha256="LvRiGEjRqfzurezaWuj8Wie2gyHMrW5Q06LspMnox7A="; includeSubDomains

Those are 1220 bytes. I'm not sure what they'll compress down to, but it's still non-trivial and not near 0 (anyone want to run the numbers?).

The same pair of headers are 969 bytes for facebook.com and 2,772 for gmail.com.

I don't know what ours would be - since we're open-ended on the image domain side it's a bit apples-to-oranges compared to the big players.

When you take into account that you can only send 10 packets down the first response (in almost all cases today) due to TCP congestion window specifications (google: CWND), they get more expensive as a percentage of what you can send. It may be that you can't send enough of the page to render, or the browser isn't getting to a critical stylesheet link until the second wave of packets after the ACK. This can greatly affect load times.

Does HPACK affect this? Yeah absolutely, but I disagree on "negligible". It depends, and if something critical gets pushed to that 11th packet as a result, you can drastically increase actual page render time for users.

If it helps, I did a blog post with some details about this a while back: https://nickcraver.com/blog/2015/03/24/optimization-consider...

[hdhzy]

Oh I wasn't clear - I meant that for the same connection headers are not sent for every page but just references for previous values (see [0]). The initial page load is a different matter but that's part of the cost/risk analysis if you need CSP or HPKP (I agree it's not necessary and very easy to mess up).

[0]: https://http2.github.io/http2-spec/compression.html#indexed....

> When you take into account that you can only send 10 packets down the first response (in almost all cases today) due to TCP congestion window specifications (google: CWND), they get more expensive as a percentage of what you can send. It may be that you can't send enough of the page to render, or the browser isn't getting to a critical stylesheet link until the second wave of packets after the ACK. This can greatly affect load times.

I wonder how much of the page can be rendered in 10 packets...

Do you send Link preload headers?

[DamonHD]

I explicitly try to ensure that for my sites the first 10kB sent (so less than 10 packets typically) is enough to render all the information above the fold. Anything essential should make it out in the first 2 packets for old TCP slow-start rules. (Lipstick and ads can arrive later, once the user is happy reading or whatever, IMHO.) Has been my policy since about the mid '90s!