The point of the article is that calling a protocol reliable doesn't make sense. You can (and should) use UDP if you don't care about (or you have a special way of handling) packet loss, and instead need to receive the latest packet as soon as possible. In the context of multiplayer FPS games and remote controlling drones -- it is actually more reliable.

The protocol is reliable in the sense that the protocol provides guarantees where either party can know that the other party received something. UDP provides no such guarantees. I'm a fan of using UDP where applicable, I loathe TCP-in-TCP, not at all a fan of head of line blocking, but I don't need to call UDP reliable just because it is a great protocol with it's own usecases.

TCP does not guarantee delivery, and there are cases where getting confirmation of receipt actually makes the /channel/ less reliable.

Again, the point is not to call UDP reliable, it's that it doesn't make sense for the reasons I stated above. If you choose the wrong tool for a usecase then the communication channel will be unreliable in either case. There's a lot of confusion about this -- it is valid to call a communication channel unreliable for the system to respond accordingly, not the protocol itself.

TCP does not guarantee delivery, but it guarantees that if you receive an ACK, then the acknowledged bytes were received. UDP provides no such guarantee. Yes, it is OK to call it unreliable, because you can't rely on it to reliably deliver packets - the sender will never know if the packets it sent were ever delivered without any external validation. Game servers and clients work around this by being fault-tolerant. QUIC runs on UDP but the way it achieves reliability is by implementing reliability of TCP in the application layer. You cannot expect to receive all UDP packets you send to yourself on localhost, I think that it is reasonable to call such a network protocol unreliable, in the sense that one cannot reliably know if sent packets have ever been delivered within the confines of the protocol itself.

Note that TCP doesn't actually provide that guarantee to the application layer. There's no way to know if some bytes you sent on a TCP socket ever arrived. So, if you want a reliable packet delivery, you have to build that at the application layer yourself anyway. That's why HTTP requires the 204 No Content response code, for example - instead of relying on a TCP ACK to achieve the same.

The only real guarantee that TCP provides is in-order delivery. That is, it really guarantees that if you sent two requests A and B to a server, the server will never see request B unless it saw request A first. Again looking at HTTP, this is why request pipelining theoretically works without including any request ID in the response: the client knows that if it sent reqA first and reqB second, the first response it gets from the server will be respA, and the second will be respB (if either of them ever arrives, of course).

Note that the fact ACK is not shown to the application layer is not an implementation limitation, it is a fundamental part of the design of TCP. The ACK is generated by the TCP stack once the packet was received. This doesn't guarantee in any way that the application will ever see this packet: the application could crash, for example, and never receive (and thus act on) this message. The in-order guarantee works instead because the transport layer can simply avoid ever showing reqB to the application layer if it knows that there was a reqA before it that it didn't receive. So the transport can in fact ensure that it's impossible for the application to see out-of-order data, but it can't ensure that the application has actually received any data.

(I don't know if you've seen my edit. I re-read your response to answer more relevantly to your point about ACK. But to be honest I still don't think it's correct.)

I see, and I still believe that as far as network protocols go, UDP allows for unreliable communication and TCP allows for reliable communication, at least at a naive first glance. I don't see anything wrong with UDP being labeled as unreliable. Is there a particular reason you believe it is reliable? Say if UDP was a reliable network protocol, what would be an unreliable network protocol?

> Is there a particular reason you believe it is reliable?

I don't, nor do I think it is unreliable. Protocols exist in an abstract space, but they are instantiated in a system that works within physical /constraints/ (noise, faults, oxidation, etc.). These constraints vary because different systems work in different environments.

The systems we make also have to provide utility for the /purposes/ that it's made. Reliability is necessary to trust the utility of a system.

If I'm playing a multiplayer FPS game and due to backoff I see players snapping back to the position of a lost packet once it is received, I will not think that the game is reliable (and I will be especially sad since this is known issue with known improvements).

If I don't care about old packets and only care about the latest packet, I don't work with old information -- but the information is incomplete. Latency will still be noticeable on a noisy connection, but my brain can at least anticipate -- guess where to look next -- once the last packet is received.

That incomplete information can be used to extrapolate movements. So the final step is to use a model to predict movements in (short) moments when a packet is not received. You can't do this when the information isn't relevant anymore. When that's done, I -- the player -- will feel like that game is more reliable than the first TCP case.

(Games, and real-time interfaces in general, have some interesting aspects that I think is well covered in Game Feel by Steve Swink).

I wouldn’t say either is absolute. I’d call TCP more reliable than UDP, but not necessarily reliable exactly. I’d also call UDP unreliable compared to TCP. However, that doesn’t make it broken. Reliable ordering and delivery was not part of the design or spec. It’s fit for purpose sometimes better than TCP - when you don’t need the extra reliability or have built your own around it.

For the case examples being discussed, like drones, TCP is unreliable in the sense that it may be death penalty for your app. Isn't death penalty a severe kind of unreliable? In that case UDP is much more reliable in not murdering your app.

TCP will be reliably too slow. There are situations in which it’s better to drop a packet than wait for a reordering buffer. That doesn’t mean UDP is reliable. It just means sometimes it’s the right solution anyway.

Too slow underplays how bad it is. It'll be unbounded backup. That is death penalty. There's no point in talking about reliability at that point.

The article willfully misinterprets the way "reliable" is used to talk about the protocol, as if people use the word to insult UDP.

Either that or it's slop that somehow made it to the front page.

We use UDP a lot. I mean, a whole lot. Sometimes it’s bare UDP within a site. Some of that is unicast and some is multicast. Sometimes it’s UDP with a bunch of custom protocol logic around it between sites, handling buffering, reordering, retransmission, and more.

I never said UDP wasn’t sometimes fit for purpose. I said it’s unreliable compared to TCP. Those are not the same statement. Sometimes reliability isn’t part of the spec, so you do without that or build your own reliability around it if the less reliable option is still a better fit.

This is common in the industry, actually. The ‘I’ in RAID stands for inexpensive, because it’s a redundant array. We scale horizontally these days where we can because more cheaper servers can sometimes scale further and more reliably than fewer more premium servers. Heck, a paper cup is unreliable compared to ceramic or stainless steel, but Starbucks and Tim Horton’s move a lot of product in them.

> A colleague is working on a kernel patch to bypass part of the UDP stack to restore proper ordering, which will probably need to be maintained in-house or at least hidden behind a kernel config knob upstream since this reordering is considered acceptable under the guarantees for UDP.

My understanding is that in Linux, TCP on localhost (including packets sent to a public address on a local interface) bypasses the stack; I don't see why it would be a problem for UDP to do the same.

This is contrast with FreeBSD, where TCP to localhost is actually packetized and queued on the loopback interface, and you can experience congestion collapse on loopback if you do things right(wrong).

TCP on localhost does bypass a certain part of the stack. That’s the same part he’s looking to bypass.

Some of these things turn out to be death penalty depending on the kind of app you're doing, and it can be quite surprising when it happens too. For example, redelivery with ordering with bad internet can lead to unbounded backup.