What Is L4S?

The article doesn't actually explain anything about L4S other than "make latency go lower". Here's the actual RFC: https://www.rfc-editor.org/rfc/rfc9330.html

This is the first time I've heard of it, but it looks pretty cool. From what I can tell skimming through the RFC, it's a new congestion control standard that aims to minimise queuing latency. It co-exists with existing AQM technology, and is different in that it actively signals to other nodes about the state of its own queues.

> Below, we outline the three main components to the L4S architecture: 1) the Scalable congestion control on the sending host; 2) the AQM at the network bottleneck; and 3) the protocol between them.

> But first, the main point to grasp is that low latency is not provided by the network; low latency results from the careful behaviour of the Scalable congestion controllers used by L4S senders. The network does have a role, primarily to isolate the low latency of the carefully behaving L4S traffic from the higher queuing delay needed by traffic with preexisting Classic behaviour. The network also alters the way it signals queue growth to the transport. It uses the Explicit Congestion Notification (ECN) protocol, but it signals the very start of queue growth immediately, without the smoothing delay typical of Classic AQMs. Because ECN support is essential for L4S, senders use the ECN field as the protocol that allows the network to identify which packets are L4S and which are Classic.

> What L4S Adds to Existing Approaches: ... Diffserv ... State-of-the-art AQMs ... Per-flow queuing or marking ... Alternative Back-off ECN (ABE) ... Bottleneck Bandwidth and Round-trip propagation time (BBR)

My question, from a background with almost no networking, is this: there is a fundamental tradeoff between latency and throughput; what exactly is the tradeoff they're trying to make here? I understand the desire to move into lower latency, but how does this effect their utilisation?

L4S is an enhanced version of ECN (Explicit Congestion Notification). It spans layer 3 and 4 in the OSI model. ECN works by having a field in the IP header part of the packet that says if the packet experienced congestion on its way to the destination. A ECN-enabled router along the way changes the packet in transit, marking it as congested instead of dropping it. L4S is an advancement upon that, using a bigger field and special handling in routers, which allows for finer grained control over the congestion notification and more advanced algorithms to improve latency.

>What is L4S?

The answer is buried over halfway down the fucking, droning malaise of an article:

>L4S stands for Low Latency, Low Loss, Scalable Throughput, and its goal is to make sure your packets spend as little time needlessly waiting in line as possible by reducing the need for queuing.

As such, here's an archive version of this crap because it does not deserve user traffic: https://archive.is/XWzbL

I hope it's gonna go better than the rollout of IPV6 :-)

So L4S has to do with your device only?

I'm missing something because the ACK of a packet may be enough to indicate congestion.