The Nagle algorithm was created back in the day of multi-point networking. Multiple hosts were all tied to the same communications (Ethernet) channel, so they would use CSMA (https://en.wikipedia.org/wiki/Carrier-sense_multiple_access_...) to avoid collisions. CSMA is no longer necessary on Ethernet today because all modern connections are point-to-point with only two "hosts" per channel. (Each host can have any number of "users.") In fact, most modern (copper) (Gigabit+) Ethernet connections have both ends both transmitting and receiving AT THE SAME TIME ON THE SAME WIRES. A hybrid is used on the PHY at each end to subtract what is being transmitted from what is being received. Older (10/100 Base-T) can do the same thing because each end has dedicated TX/RX pairs. Fiber optic Ethernet can use either the same fiber with different wavelengths, or separate TX/RX fibers. I haven't seen a 10Base-2 Ethernet/DECnet interface for more than 25 years. If any are still operating somewhere, they are still using CSMA. CSMA is also still used for digital radio systems (WiFi and others). CSMA includes a "random exponential backoff timer" which does the (poor) job of managing congestion. (More modern congestion control methods exist today.) Back in the day, disabling the random backoff timer was somewhat equivalent to setting TCP_NODELAY.

Dumping the Nagle algorithm (by setting TCP_NODELAY) almost always makes sense and should be enabled by default.

I found this article while debugging some networking delays for a game that I'm working on.

It turns out that in my case it wasn't TCP_NODELAY - my backend is written in go, and go sets TCP_NODELAY by default!

But I still found the article - and in particular Nagle's acknowledgement of the issues! - to be interesting.

There's a discussion from two years ago here: https://news.ycombinator.com/item?id=40310896 - but I figured it'd been long enough that others might be interested in giving this a read too.

The problem is actually that nobody uses the generic solution to these classes of problems and then everybody complains that the special-case for one set of parameters works poorly for a different set of parameters.

Nagle’s algorithm is just a special case solution of the generic problem of choosing when and how long to batch. We want to batch because batching usually allows for more efficient batched algorithms, locality, less overhead etc. You do not want to batch because that increases latency, both when collecting enough data to batch and because you need to process the whole batch.

One class of solution is “Work or Time”. You batch up to a certain amount of work or up to a certain amount of time, whichever comes first. You choose your amount of time as your desired worst case latency. You choose your amount of work as your efficient batch size (it should be less than max throughput * latency, otherwise you will always hit your timer first).

Nagle’s algorithm is “Work” being one packet (~1.5 KB) with “Time” being the time until all data gets a ack (you might already see how this degree of dynamism in your timeout might pose a problem already) which results in the fallback timer of 500 ms when delayed ack is on. It should be obvious that is a terrible set of parameters for modern connections. The problem is that Nagle’s algorithm only deals with the “Work” component, but punts on the “Time” component allowing for nonsense like delayed ack helpfully “configuring” your effective “Time” component to a eternity resulting in “stuck” buffers which is what the timeout is supposed to avoid. I will decline to discuss the other aspect which is choosing when to buffer and how much of which Nagle’s algorithm is again a special case.

Delayed ack is, funnily enough, basically the exact same problem but done on the receive side. So both sides set timeouts based on the other side going first which is obviously a recipe for disaster. They both set fixed “Work”, but no fixed “Time” resulting in the situation where both drivers are too polite to go first.

What should be done is use the generic solutions that are parameterized by your system and channel properties which holistically solve these problems which would take too long to describe in depth here.

Wildly, the Polish word "nagle" (pronounced differently) means "suddenly" or "all at once", which is just astonishingly apropos for what I'm almost certain is pure coincidence.

(2024) - previously discussed at https://news.ycombinator.com/item?id=40310896

I've always thought a problem with Nagel's algorithm is, that the socket API does not (really) have a function to flush the buffers and send everything out instantly, so you can use that after messages that require a timely answer.

For stuff where no answer is required, Nagel's algorithm works very well for me, but many TCP channels are mixed use these days. They send messages that expect a fast answer and other that are more asynchronous (from a users point of view, not a programmers).

Wouldn't it be nice if all operating systems, (home-)routers, firewalls and programming languages would have high quality implementations of something like SCTP...

> The bigger problem is that TCP_QUICKACK doesn’t fix the fundamental problem of the kernel hanging on to data longer than my program wants it to.

Well, of course not; it tries to reduce the problem of your kernel hanging on to an ack (or genearting an ack) longer than you would like. That pertains to received data. If the remote end is sending you data, and is paused due to filling its buffers due to not getting an ack from you, it behooves you to send an ack ASAP.

The original Berkeley Unix implementation of TCP/IP, I seem to recall, had a single global 500 ms timer for sending out acks. So when your TCP connection received new data eligible for acking, it could be as long as 500 ms before the ack was sent. If we reframe that in modern realities, we can imagine every other delay is negligible, and data is coming at the line rate of a multi gigabit connection, 500 ms represents a lot of unacknowledged bits.

Delayed acks are similar to Nagle in spirit in that they promote coalescing at the possible cost of performance. Under the assumption that the TCP connection is bidirectional and "chatty" (so that even when the bulk of the data transfer is happening in one direction, there are application-level messages in the other direction) the delayed ack creates opportunities for the TCP ACK to be piggy backed on a data transfer. A TCP segment carrying no data, only an ACK, is prevented.

As far as portability of TCP_QUICKACK goes, in C code it is as simple as #ifdef TCP_QUICKACK. If the constant exists, use it. Otherwise out of luck. If you're in another language, you have to to through some hoops depending on whether the network-related run time exposes nonportable options in a way you can test, or whether you are on your own.

Ha ha, well that's a relief. I thought the article was going to say that enabling TCP_NODELAY is causing problems in distributed systems. I am one of those people who just turn on TCP_NODELAY and never look back because it solves problems instantly and the downsides seem minimal. Fortunately, the article is on my side. Just enable TCP_NODELAY if you think it's a good idea. It apparently doesn't break anything in general.

I first ran into this years ago after working on a database client library as an intern. Having not heard of this option beforehand, I didn't think to enable it in the connections the library opened, and in practice that often led to messages in the wire protocol being entirely ready for sending without actually getting sent immediately. I only found out about it later when someone using it investigated why the latency was much higher than they expected, and I guess either they had run into this before or were able to figure out that it might be the culprit, and it turned out that pretty much all of the existing clients in other languages set NODELAY unconditionally.

I've always thought that Nagle's algorithm is putting policy in the kernel where it doesn't really belong.

If userspace applications want to make latency/throughput tradeoffs they can already do that with full awareness and control using their own buffers, which will also often mean fewer syscalls too.

Why doesn’t linux just add a kconfig that enables TCP_NODELAY system wide? It could be enabled by default on modern distros.

https://oxide-and-friends.transistor.fm/episodes/mr-nagles-w...

oxide and friends episode on it! It's quite good

Wouldn't distributed systems benefit from using UDP instead of TCP?

I fondly remember a simple simulation project we had to do with a group of 5 students in a second year class which had a simulation and some kind of scheduler which communicated via TCP. I was appalled at the perfomance we were getting. Even on the same machine it was way too slow for what it was doing. After hours of debugging in turned out it was indeed Nagle's algorithm causing the slowness, which I never heard about at the time. Fixed instantly with TCP_NODELAY. It was one of the first times it was made abundantly clear to me the teachers at that institution didn't know what they were teaching. Apparently we were the only group that had noticed the slow performance, and the teachers had never even heard of TCP_NODELAY.

<waits for animats to show up>