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UDP vs TCP

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Re: UDP vs TCP

#21

Post by vvanders »

wtallis wrote: post #15
exDM69 wrote: post #8

Earlier quoted context omitted.

Another article on this site addresses TCP plus UDP mix. The tl;dr is that they should not be used at the same time because TCP retransmission will interfere with UDP (and routers give TCP priority). As long as TCP happens outside of the "hot" loop, it's fine.

> (and routers give TCP priority). What routers? Is this something that stupid routers did in the '90s and people are still worried about, or are modern routers actually behaving this way? (And if so, why?) TCP retransmission interfering with UDP sounds like something that shouldn't be a problem on any sane network, especially if the application authors are smart and use something like LEDBAT for their TCP traffic wh…

You'd be shocked at the things home routers do. There's a reason most people don't try to implement NAT traversal and use common middleware.

Since UDP isn't as common of a protocol I can certainly see some routers treating it a second class traffic when it comes to packet shaping.

Re: UDP vs TCP

#22

Post by blackflame7000 »

mikepurvis wrote: post #7

The recommendation to avoid TCP altogether is surprising to me. Having encountered a number of video-conferencing systems which are in a similar space, it seems pretty standard to have separate real-time and control sockets on UDP and TCP respectively. I skimmed the linked paper and didn't find it conclusive; can someone summarize how it is that having a TCP socket can affect UDP traffic on the same interface? All th…

I too was wondering how TCP and UDP can affect one another on the same interface and the explanation I read is that at the lowest level both UDP and TCP segment data into packets and place them onto the same network adapter's queue which can affect they way they are transmitted under congestion depending on the device/drivers.

Re: UDP vs TCP

#23

Post by lobster_johnson »

mjevans wrote: post #6

The main difference between TCP and UDP, as this programmer discovered, relates to quality of realtime service. Times to use UDP over TCP * When you need the lowest latency * When LATE data is worse than GAPS (loss of) in data. * When you want to implement your own form of error correction to handle late/missing/mangled data. TCP is best when * You need all of the data to arrive, period. * You want to automatically m…

I always wondered why nobody used UDP for bulk data transfer: Ship all the chunks over with a sequential number identifying each, then when you've reached the end, have the client request any lost packets, then repeat the cycle until all are transferred. After all, the client doesn't need everything in sequential order if it knows the size, and it can deal with holes as long as they're eventually repaired.

This gives you (in theory, I've never tried it) better throughout, and you get the equivalent of TCP's retransmission without the window semantics, since you move all the retransmissions to the end. On the other hand, it's probably bad for everyone else, since there's no congestion control.

Re: UDP vs TCP

#24

Post by dabockster »

mjevans wrote: post #6

The main difference between TCP and UDP, as this programmer discovered, relates to quality of realtime service. Times to use UDP over TCP * When you need the lowest latency * When LATE data is worse than GAPS (loss of) in data. * When you want to implement your own form of error correction to handle late/missing/mangled data. TCP is best when * You need all of the data to arrive, period. * You want to automatically m…

Isn't UDP more secure than TCP since it does not implement a sequence number (among other things)?

Re: UDP vs TCP

#25

Post by vvanders »

lobster_johnson wrote: post #23
mjevans wrote: post #6

The main difference between TCP and UDP, as this programmer discovered, relates to quality of realtime service. Times to use UDP over TCP * When you need the lowest latency * When LATE data is worse than GAPS (loss of) in data. * When you want to implement your own form of error correction to handle late/missing/mangled data. TCP is best when * You need all of the data to arrive, period. * You want to automatically m…

I always wondered why nobody used UDP for bulk data transfer: Ship all the chunks over with a sequential number identifying each, then when you've reached the end, have the client request any lost packets, then repeat the cycle until all are transferred. After all, the client doesn't need everything in sequential order if it knows the size, and it can deal with holes as long as they're eventually repaired. This gives…

You've kinda answered your own question there, congestion control becomes a big issue(also MTU discovery to a lesser degree).

Games sit in this happy space where the data rate is ~10-20kb at most so they don't need to worry about congestion control as much.

Re: UDP vs TCP

#26

Post by problems »

mjevans wrote: post #6

The main difference between TCP and UDP, as this programmer discovered, relates to quality of realtime service. Times to use UDP over TCP * When you need the lowest latency * When LATE data is worse than GAPS (loss of) in data. * When you want to implement your own form of error correction to handle late/missing/mangled data. TCP is best when * You need all of the data to arrive, period. * You want to automatically m…

You can also use something like enet which allows for both reliable and non reliable channels.

Re: UDP vs TCP

#27

Post by sytelus »

milansuk wrote: post #12

> they may arrive out of order, be duplicated, or not arrive at all! This is first time what I read that datagram can be duplicated. Is it true? It's duplicated by network or does it mean that peer send it again?

A classic example is FPV video from drone. There may be reflections in surroundings and you may get packets multiple times!

Re: UDP vs TCP

#28

Post by wtallis »

vvanders wrote: post #21
wtallis wrote: post #15

Earlier quoted context omitted.

> (and routers give TCP priority). What routers? Is this something that stupid routers did in the '90s and people are still worried about, or are modern routers actually behaving this way? (And if so, why?) TCP retransmission interfering with UDP sounds like something that shouldn't be a problem on any sane network, especially if the application authors are smart and use something like LEDBAT for their TCP traffic wh…

You'd be shocked at the things home routers do. There's a reason most people don't try to implement NAT traversal and use common middleware. Since UDP isn't as common of a protocol I can certainly see some routers treating it a second class traffic when it comes to packet shaping.

I'm well aware of how dumb home routers can be, but a general de-prioritizing of the transport protocol that DNS uses is both really dumb and pretty easy to detect. Are you aware of any router vendors that have actually shipped such a configuration? (Or for that matter, any consumer router that has shipped with any prioritization rules enabled out of the box?)

Re: UDP vs TCP

#29

Post by wtallis »

blackflame7000 wrote: post #22
mikepurvis wrote: post #7

The recommendation to avoid TCP altogether is surprising to me. Having encountered a number of video-conferencing systems which are in a similar space, it seems pretty standard to have separate real-time and control sockets on UDP and TCP respectively. I skimmed the linked paper and didn't find it conclusive; can someone summarize how it is that having a TCP socket can affect UDP traffic on the same interface? All th…

I too was wondering how TCP and UDP can affect one another on the same interface and the explanation I read is that at the lowest level both UDP and TCP segment data into packets and place them onto the same network adapter's queue which can affect they way they are transmitted under congestion depending on the device/drivers.

That's not an explanation. That's just pointing in the general direction of where to look for an explanation.

Re: UDP vs TCP

#30

Post by frutiger »

dabockster wrote: post #24
mjevans wrote: post #6

The main difference between TCP and UDP, as this programmer discovered, relates to quality of realtime service. Times to use UDP over TCP * When you need the lowest latency * When LATE data is worse than GAPS (loss of) in data. * When you want to implement your own form of error correction to handle late/missing/mangled data. TCP is best when * You need all of the data to arrive, period. * You want to automatically m…

Isn't UDP more secure than TCP since it does not implement a sequence number (among other things)?

Questions like "more secure" and "less secure" only make sense in some specific context with a well-defined threat model: what it is that you want to keep safe from which classes of threats.

For many (if not most) threat models, the lack of a sequence number does not provide adequate security.

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