Low Latency DOCSIS

fmd

Comcast has now begun the mass rollout of LLD.

With a little hope, Jason Livingood will join us here to keep us updated. The thread is ready... :-)

hitherelolz

fmd Just wondering, has Comcast been open about this widespread deployment? I can’t find any recent information on LLD.

fmd

To get some interest going. How L4S can help...

To the left is your regular network queue when you load it with data. The exact nature of it depends on how it is managed, but the current AQMs will let the queue grow to a certain size (larger than when using L4S), and they drop packets to control the sender's rate.

When LLD is enabled, you get an additional queue, where any L4S traffic goes through. L4S sees the bottleneck much sooner and controls the sender's rate more gradually. It only drops packets as a last resort. The endpoints must also support L4S.

There is a L4S discussion club here: https://mailarchive.ietf.org/arch/browse/l4s-discuss/ Livingood also comes by now and then.

cypherstream

Are you fm from DSLR?

fmd

cypherstream Yes. The forum wouldn't let me use a two letter username.

fmd

hitherelolz In the other thread, Livingood talked about the ramp up before the holidays and how it would continue in the new year.

Upstream LLD requires firmwaresupport by the modem, but Livingood confirmed that they will enable it downstream for modems that haven't got the firmware (yet).

adlr

For anyone who wants to run tracebox on Linux to see if the low latency marks are preserved, this was the command I got working:

sudo podman run -it --rm --cap-add=NET_RAW --network=host -v "${PWD}:${PWD}" -w "${PWD}" matttbe/tracebox:latest -v -p "ip{ecn=1}/udp{dst=33451}" apple.com

fmd

adlr To add: If using DSCP, the value is 45.

DSCP-40, 46, and 56 may also be recognized. Comcast recommends those to be supported initially.

videomatic3

may as well start this again. my modem has a new software file again. last rebooted 3 or so days ago

im also curious what docsis 1 downstream frequencies they are using for docsis 4.0, they moved my downstream channels starting from 378 through 576 mhz. when i look at this and see its 372mhz for their uhs-300 example, and knowing im in a n+0 area, im almost positive my area is fdx ready and ill get symmetrical once they start releasing xb10s or whatever they are waiting for.

fmd

videomatic3

Comcast has presented these two configs:

Edit:
In legacy plants, the OFDM channel above 1 GHz will be hit by roll-off (1 GHz taps, MoCA filter). 3.1 modems with FDX-L firmware can use the FDX OFDM channel instead. At least Comcast's own modems will get that firmware.

videomatic3

videomatic3
another new image.

jlivingood

I expect another update on our low latency deployment later this week. As we are scaling up, we are able to run a lot more measurements - so some interesting stats:

upstream latency under load is cut by ₇₆%
downstream latency under load is cut by ₇₈%

Not bad! ;-) But that data is from active measurements. Better IMO is actual app-layer QoE stats from within a user session for something like a game session or video call (since it is inside an encrypted session, only the app provide has that data). I expect those stats may improve over time as app developers iterate their server config and client code.

videomatic3

jlivingood
is this just for the no delay queue or all traffic?

jlivingood

videomatic3

When LLD deploys on a vCMTS, all devices get DS AQM. So that is a 50% cut in working latency right off the bat - obviously benefiting delay-sensitive ("classic") traffic. Layer on LLD and it goes down half again (to roughly 75% down vs no AQM/no LLD). I am pleased to be able to get DS AQM enabled as part of this, since it does not require any app developer work to implement and it clearly helps (just like US AQM did during COVID in 2020).

ka2107

@jlivingood

Modem: Hitron CODA56
Router: OpenWrt
Plan: 150 Mbps download, 100 Mbps Upload (mid-split enabled)

Does LLD work in this setup?

Also is there any change in OpenWrt CAKE SQM and/or qosify config that needs to be done in order to make it work in sync with LLD?

Thanks in advance.

jlivingood

ka2107

CODA56 won't work out of the gate, but its a next-gen modem and is high on the list to get tested and deployed as our deployment progresses.

Man-of-Leisure

jlivingood

Are any retail 4.0 modems expected?

The recent LightReading article said:

A number of retail DOCSIS gateway suppliers are in the lab now iterating their code to work with FDX, he said.

But I'm not aware of any announced.

ka2107

@jlivingood Does this conversation have any relevance to Low Latency DOCSIS?

https://lists.bufferbloat.net/pipermail/cake/2025-January/006201.html

fmd

ka2107 Does this conversation have any relevance to Low Latency DOCSIS?

https://lists.bufferbloat.net/pipermail/cake/2025-January/006201.html

Dave Taht is talking about how Cake groups traffic into different queues based on the DSCP value. LLD does that too, although it only has two queues, whereas Cake has several.

Edit: He refers to them by name. To compare them to the DSCP values I mentioned earlier:
DSCP value = name
40 = CS5
45 = NQB
46 = EF
56 = CS7

46(EF) was used by DOCSIS initially.

cypherstream

I’m curious if it’s even necessary to turn on or mess with any traffic limiters / QoS bufferbloat mitigation techniques in the router once LLD is fully enabled.

videomatic3

cypherstream
it might to some if you have stuff thats using a ton of bandwidth, but as we get symmetrical speeds it wont matter anymore. at 200mbit upload i still felt the need to use qos, to prioritize some computers over others such as plex, at 300mbit upload a lot less. cant wait for symmetrical.

cypherstream

fmd Yes DSCP46 (Expedited Forwarding) is what I use on our voice vlans in our network at work. We have plenty of bandwidth, so this is just being kinda overcautious.

jlivingood

FYI - fresh off the press!
https://corporate.comcast.com/press/releases/comcast-introduces-nations-first-ultra-low-lag-xfinity-internet-experience-with-meta-nvidia-and-valve

telcodad

jlivingood

Jason - You forgot to also mention this item of yours: 🙂

What is Latency?
....We spoke with Jason Livingood, VP, Technology Policy, Product and Standards, who has spearheaded Comcast’s industry-leading work on the technology responsible for delivering this ultra-low-lag experience about what it means for customers and the future of their connected experiences....

https://corporate.comcast.com/stories/latency-speed-explainer