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Comment by CJefferson | Hacker News Reader

Out of interest, what kind of applications are you thinking of? What systems are common written using TCP, that could switch to UDP?

Clearly websites, audio and video generally don't work with out-of-order frames -- most people don't want dropped audio and video.

Some video games are happy to ignore missed packets, but when they can they are already written in UDP.

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Any data block that must be fully received before processing comes to mind (e.g. websites’ html), just request retransmission of parts that didn’t make it in first pass. Funnily enough it’s (realtime) video and audio that already uses UDP due to preferring timely data to complete data.

On the contrary, for me it’s hard to imagine video game with missed packets as the state can get out of sync too easily, you’ll need eventual consistency via retransmission or some clever data structures (I know least about this domain though)

I'm not expert in this area, but my understanding is that for latency sensitive video games, on the client side, state is predicted between updates, e.g. the state may be updated via the network, at say, roughly 10Hz (probably faster, but probably less than 60Hz), but updated locally at 60Hz via interpolation, dead reckoning and other predictive/smoothing heuristics, a few missed updates just means a bit more prediction is used. "State" is not usually transmitted as one lump "frame" at a time, but rather per-game unit (per spaceship, asteroid, robot, or whatever is in the game) or per some-small-group of units. When some updates are delayed too long, you might get some visible artifacts, "rubber-banding", "teleportation" or "warping", etc. often lumped together by players under the umbrella term "lag". For out-of-order packets, older packets might be dropped as newer packets may already have been applied to the state of some game units (typically there's some timestamps or monotonically increasing counter associated with state updates for each unit used by the prediction/interpolation/smoothing heuristics) and the state is usually represented in absolute terms rather than relative terms (e.g. (x,y) = (100,100), rather than x+=10, y+=10, so that any update may be applied in isolation.)

Many video games just transmit the whole state the player should be able to see every tick. While some games send a diff of some kind, sometimes that turns out to not be worth it. This is particularly true in the games which care most about lag (think mario kart, or street fighter and friends).

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