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.

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gerdesj 2 hours ago | parent | next [-]

I think you are confusing network layers and their functionality.

"CSMA is no longer necessary on Ethernet today because all modern connections are point-to-point with only two "hosts" per channel."

Ethernet really isn't ptp. You will have a switch at home (perhaps in your router) with more than two ports on it. At layer 1 or 2 how do you mediate your traffic, without CSMA? Take a single switch with n ports on it, where n>2. How do you mediate ethernet traffic without CSMA - its how the actual electrical signals are mediated?

"Ethernet connections have both ends both transmitting and receiving AT THE SAME TIME ON THE SAME WIRES."

That's full duplex as opposed to half duplex.

Nagle's algo has nothing to do with all that messy layer 1/2 stuff but is at the TCP layer and is an attempt to batch small packets into fewer larger ones for a small gain in efficiency. It is one of many optimisations at the TCP layer, such as Jumbo Frames and mini Jumbo Frames and much more.

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anonymousiam 2 hours ago | parent | next [-]

It's P2P as far as the physical layer (L1) is concerned.

Usually, full duplex requires two separate channels. The introduction of a hybrid on each end allows the use of the same channel at the same time.

Some progress has been made in doing the same thing with radio links, but it's harder.

Nagle's algorithm is somewhat intertwined with the backoff timer in the sense that it prevents transmitting a packet until some condition is met. IIRC, setting the TCP_NODELAY flag will also disable the backoff timer, at least this is true in the case of TCP/IP over AX25.

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gerdesj 2 hours ago | parent [-]

Sorry?

Ethernet has had the concept of full duplex for several decades and I have no idea what you mean by: "hybrid on each end allows the use of the same channel at the same time."

The physical electrical connections between a series of ethernet network ports (switch or end point - it doesn't matter) are mediated by CSMA.

No idea why you are mentioning radios. That's another medium.

	▲	switchbak an hour ago \| parent \| next [-]
		My understanding is that no one used hubs anymore, so your collision domain goes from a number of machines on a hub to a dedicated channel between the switch and the machine. There obviously won’t be collisions if you’re the only one talking and you’re able to do full duplex communications without issue.
	▲	Dylan16807 2 hours ago \| parent \| prev [-]
		> Ethernet has had the concept of full duplex for several decades and I have no idea what you mean by: "hybrid on each end allows the use of the same channel at the same time." Gigabit (and faster) is able to do full duplex without needing separate wires in each direction. That's the distinction they're making. > The physical electrical connections between a series of ethernet network ports (switch or end point - it doesn't matter) are mediated by CSMA. Not in a modern network, where there's no such thing as a wired collision. > Take a single switch with n ports on it, where n>2. How do you mediate ethernet traffic without CSMA - its how the actual electrical signals are mediated? Switches are not hubs. Switches have a separate receiver for each port, and each receiver is attached to one sender.

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saltcured 40 minutes ago | parent | prev [-]

In modern ethernet, there is also flow-control via the PAUSE frame. This is not for collisions at the media level, but you might think of it as preventing collisions at the buffer level. It allows the receiver to inform the sender to slow down, rather than just dropping frames when its buffers are full.

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immibis 3 hours ago | parent | prev | next [-]

Nagle is quite sensible when your application isn't taking any care to create sensibly-sized packets, and isn't so sensitive to latency. It avoids creating stupidly small packets unless your network is fast enough to handle them.

	▲	silisili 2 hours ago \| parent \| next [-]
		At this point, this is an application level problem and not something the kernel should be silently doing for you IMO. An option for legacy systems or known problematic hosts fine, but off by default and probably not a per SOCKOPT. Every modern language has buffers in their stdlib. Anyone writing character at a time to the wire lazily or unintentionally should fix their application.
	▲	CamperBob2 23 minutes ago \| parent \| prev [-]
		If by "latency" you mean a hundred milliseconds or so, that's one thing, but I've seen Nagle delay packets by several seconds. Which is just goofy, and should never have been enabled by default, given the lack of an explicit flush function. A smarter implementation would have been to call it TCP_MAX_DELAY_MS, and have it take an integer value with a well-documented (and reasonably low) default.

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Hikikomori 3 hours ago | parent | prev [-]

Just to add, ethernet uses csma/cd, WiFi uses csma/ca.

Upgraded our DC switches to new ones around 2014 and needed to keep a few old ones because the new ones didn't support 10Mbit half duplex.

	▲	anonymousiam 3 hours ago \| parent [-]
		Thanks for the clarification. They're so close to being the same thing that I always call it CSMA/CD. Avoiding a collision is far more preferable than just detecting one. Yeah, many enterprise switches don't even support 100Base-T or 10Base-T anymore. I've had to daisy chain an old switch that supports 100Base-T onto a modern one a few times myself. If you drop 10/100 support, you can also drop HD (simplex) support. In my junk drawer, I still have a few old 10/100 hubs (not switches), which are by definition always HD.