網頁大小 14KB 與 15KB 的速度差異

Hacker News 上看到「Why your website should be under 14kB in size」這篇,對應的討論在「A 14kb page can load much faster than a 15kb page (endtimes.dev)」,在講網頁大小 14KB/15KB 的速度差異比 15KB/16KB 大很多的問題:

What is surprising is that a 14kB page can load much faster than a 15kB page — maybe 612ms faster — while the difference between a 15kB and a 16kB page is trivial.

原因是 TCP slow start 造成的:

This is because of the TCP slow start algorithm.

而網頁這邊 TCP slow start 目前大多數的實做都是 10 packets 後發動:

Most web servers TCP slow start algorithm starts by sending 10 TCP packets.

然後再組合 1500 bytes/packet 以及 overhead,就差不多是 14KB 了:

The maximum size of a TCP packet is 1500 bytes.

This this maximum is not set by the TCP specification, it comes from the ethernet standard

Each TCP packet uses 40 bytes in its header — 16 bytes for IP and an additional 24 bytes for TCP

That leaves 1460 bytes per TCP packet. 10 x 1460 = 14600 bytes or roughly 14kB!

然後 HTTP/3 也可以看到類似的設計 (出自「QUIC Loss Detection and Congestion Control」:

Sending multiple packets into the network without any delay between them creates a packet burst that might cause short-term congestion and losses. Implementations MUST either use pacing or limit such bursts to the initial congestion window, which is recommended to be the minimum of 10 * max_datagram_size and max(2* max_datagram_size, 14720)), where max_datagram_size is the current maximum size of a datagram for the connection, not including UDP or IP overhead.

算是一個小知識... 但對於現在肥滋滋的網頁效果來說就沒辦法了,而且考慮到大一點的網站會在一個 TCP 連線裡面可能會傳很多 request,其實早就超過 TCP slow start 的門檻了。

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