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0 pointspierrec1y ago0 comments

Interesting, how does that add a round trip? For the record here's what I believe to be the common definition of an additional "round trip", in a web development context:

  - client requests X
  - client gets X, which contains a reference to Y
  - therefore client requests Y

So you're starting a new request that depends on the client having received the first one. (although upon closer inspection I think the technique described in the blog post manages to fit everything into the first response, so I'm not sure how relevant this is)

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4 comments · 2 top-level

crote1y ago· 2 in thread

In reality it's more like:

  - client requests X
  - server sends bytes 0-2k of X
  - client acknowledges bytes 0-2k of X
  - server sends bytes 2k-6k of X
  - client acknowledges bytes 2k-6k of X
  - server sends bytes 6k-14k of X
  - client acknowledges bytes 6k-14k of X
  - server sends bytes 14k-30k of X
  - client acknowledges bytes 14k-30k of X
  - server sends bytes 30k-62k of X
  - client acknowledges bytes 30k-62k of X
  - server sends bytes 62k-83k of X
  - client acknowledges bytes 62k-83k of X
  - client has received X, which contains a reference to Y
  - therefore client requests Y

It's all about TCP congestion control here. There are dozens of algorithms used to handle it, but in pretty much all cases you want to have some kind of slow buildup in order to avoid completely swamping a slower connection and having all but the first few of your packets getting dropped.

notpushkin1y ago

> client acknowledges bytes 0-2k of X

Doesn’t client see reference to Y at this point? Modern browsers start parsing HTML even before they receive the whole document.

Timwi1y ago

Not just modern. This was even more significant on slow connections, so they've kind of always done that. One could even argue that HTML, HTTP (specifically, chunked encoding) and gzip are all intentionally designed to enable this.

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jsnell1y ago

Unless a resource is very small, it won't be transmitted in a single atomic unit. The sender will only send a part of it, wait the client to acknowledge having received them, and only then send more. That requires a network roundtrip. The larger the resource, the more network roundtrips will be required.

If you want to learn more, pretty much any resource on TCP should explain this stuff. Here's something I wrote years ago, the background section should be pretty applicable: https://www.snellman.net/blog/archive/2017-08-19-slow-ps4-do...

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