[Bloat] Re: Vol 185, Issue 5

[bob.mcmahon@umbernetworks.com]

David,

These are the right implementation questions.

We hope to demo this at IETF 127 using the UAX-8 eval system. The 2027 
plan is up to 96 radio heads over fiber using a C-RAN-style intra-MAC 
split. The fronthaul is PCIe from concentrator to radio head, with 
shared memory and DMA under DPDK, and no Ethernet switch in the path or 
in the concentrator. APs are deprecated in this architecture.

The 802.11 resource being scheduled is TXOPs. The orthogonal dimensions 
are time, space, including spatial eigenmodes, and frequency. More 
carriers are one way to create more simultaneous TXOP opportunities. 
Spatial reuse creates another. CASP can be used to coordinate 
eligibility across all three.

https://www.umbernetworks.com/

Bob


> while something like that could run on APs that are connected with fast 
> enough wires, the latency in this would be a problem (not just wired 
> network latency but processing latency, you are after all talking about 
> scheduling many different stations for every transmit window)
> 
> but you do not have control of when the mobile devices transmit (unless 
> you do some sort of token passing protocol, which would need more 
> transmit slots to tell the mobile devices when to transmit, compounding 
> your airtime problems)
> 
> It's not as simple as "only one station can transmit on a given channel 
> at a time", if you have a good network layout over a large area, that 
> channel is getting re-used many times where multiple stations can 
> transmit on it at once without interference.
> 
> you are taking a hard real-time task (if your are delayed, the airtime 
> is wasted and you clobber someone else who wasn't delayed). wired 
> ethernet networks and general purpose operating systems are not good at 
> that sort of timing. This is why high speed network cards (wired and 
> wireless) have their own internal queues so that the OS can pass the 
> data to them as it's avaialble and the cards do the real-time tasks of 
> scheduling the transmission.
> 
> pushing that scheduling not just back to the main CPU/OS, but across 
> the network to a central CPU/OS to make the decision for lots of places 
> does not sound scalable[1]
> 
> 
> In practice, small cells and lots of different channels work very well.
> 
> The new wifi standards push for a small number of very wideband 
> channels that give a single device a huge amount of available bandwidth 
> (and is fantastic if you live out in the country)
> 
> but when you have congestion, borrow the idea from the cell companies, 
> implement micro-cells, lots of low-power, low-footprint stations 
> allowing you to re-use channels more overall, and have each station use 
> narrow channels instead of wide ones. This is optimizing for overall 
> throughput rather than individual throughput and works pretty darn well 
> today.
> 
> the current problem is that some mobile devices really don't like to 
> change stations, so they will transmit louder to try to maintain 
> connectivity to their current station rather than switching stations 
> (Intel and Apple come to mind)
> 
> this doesn't require expensive equipment, it just needs all the APs to 
> be on the same SSID, DHCP managed centrally so that the device doesn't 
> need to change IPs when it moves from one AP to another, and some 
> smarts in allocating the channels to minimize contention between APs.
> 
> I'm not saying that it can't be further improved (I am looking at DAWN 
> and usteer to try and push devices to better APs faster)
> 
> David Lang
> 
> [1] as always, "the person saying something cannot be done should not 
> interrupt the person doing it". I'd be happy to be proven wrong, but 
> there do seem to be some fundamental limits you will run up against 
> here.
> 
> On Sun, 20 Sep 2026, bob.mcmahon@umbernetworks.com wrote:
> 
>> Date: Sun, 20 Sep 2026 16:17:22 -0700
>> From: bob.mcmahon@umbernetworks.com
>> To: David Lang <david@lang.hm>
>> Cc: D.Goncz-FAX-571-395-3575@replikon.net, bloat@lists.bufferbloat.net
>> Subject: Re: [Bloat] Re: Vol 185, Issue 5
>> 
>> David's deployment experience is exactly the kind of knowledge the 
>> industry should be engineering into better products. Cell size, 
>> transmit power, hidden stations, the number of active stations, and 
>> the resulting contention all determine how well the shared medium 
>> behaves under load.
>> 
>> I've been working on an interface proposal aimed at that problem. It 
>> has the working name Contended Access Scheduling Protocol (CASP), and 
>> I'm starting the discussion around it here:
>> 
>> https://www.umbernetworks.com/ietf-fiwi-problem-statement.html
>> 
>> The introduction:
>> 
>> Every 802.11 station contends for the medium independently, following 
>> carrier sense multiple access with collision avoidance (CSMA/CA) ahead 
>> of transmitting. It applies listen before talk (LBT), and after a 
>> transmission, or on finding the medium busy, it must back off 
>> regardless of whether anyone is on the air by then. Three properties 
>> define the algorithm. Distributed: each station runs it alone on what 
>> it can hear, answering to no arbiter anywhere in the system. 
>> Stochastic: each draws its own random delay, so chance settles the 
>> order. Exponential: each failure doubles the previous range, so 
>> consecutive failures compound rather than add. As more stations hold 
>> traffic, the odds that several draw the same slot climb steeply, and 
>> collisions, retries and widening latency tails follow. The CSMA/CA 
>> algorithm runs whenever the driver has packets queued for the 802.11 
>> MAC, and it goes quiet only while an upper layer withholds them.
>> 
>> This note proposes an interface that lets a scheduler control when 
>> traffic becomes eligible to enter that contention process. It asks two 
>> things. The scheduler sends grants, and the endpoint follows them, 
>> holding traffic back and releasing it when its grant opens. Grant 
>> times are stated in the TSF domain that 802.11 already synchronizes. 
>> The protocol has the working name CASP, and it defines neither the 
>> scheduler nor any part of the 802.11 MAC. Section 6 sets out the asks.
>> 
>> Bob
>> 
>> On 2026-09-20 15:49, David Lang wrote:
>>> lower latency lets stations transmit sooner, which could increase the 
>>> airtime usage, but it's more pulling it forward in time than 
>>> amplification.
>>> 
>>> the thing with a guitar amp is that the speaker signal gets back into 
>>> the input and the higher the output from the amp, the more gets into 
>>> the input. that doesn't happen with packets, more acks do not result 
>>> in more signal the same way
>>> 
>>> David Lang
>>> 
>>> On Sun, 20 Sep 2026, David Lang wrote:
>>> 
>>>> Date: Sun, 20 Sep 2026 15:46:23 -0700 (MST)
>>>> From: David Lang <david@lang.hm>
>>>> To: bob.mcmahon@umbernetworks.com
>>>> Cc: David Lang <david@lang.hm>, 
>>>> D.Goncz-FAX-571-395-3575@replikon.net,
>>>>     bloat@lists.bufferbloat.net
>>>> Subject: Re: [Bloat] Re: Vol 185, Issue 5
>>>> 
>>>> wifi is a very different thing than general network latency (which 
>>>> is what we were talking about before)
>>>> 
>>>> A large part of it is that wifi is designed or weak signals and a 
>>>> low number of users, a very different environment than most people 
>>>> see today.
>>>> 
>>>> It interprets loss as "this is a weak signal, let me do things that 
>>>> will help in the weak signal case", slowing the transmission speed, 
>>>> which helps in the face of general noise, but when the problem is 
>>>> interference from other wifi transmitters, it instead expands the 
>>>> window where the interference will clobber the retransmission.
>>>> 
>>>> This is why wifi doesn't degrade gracefully, it goes from "works 
>>>> well" to "doesn't work at all" in a very short time and it doesn't 
>>>> recover until enough people give up (and retries time out).
>>>> 
>>>> AFAIK newer protocols don't fix this, they just make it possible to 
>>>> cram more data into a given transmission
>>>> 
>>>> This is not a latency issue, just an airtime utilization problem.
>>>> Stations do listen and do not transmit if they hear another station 
>>>> talking, but it's very easy to be in a situation where stations A 
>>>> and B cannot hear each other, but station C can hear both (the 
>>>> better the antennas on the AP, the more likely you are to run into 
>>>> this problem, especially when combined with higher transmit levels 
>>>> on the AP than on the devices it's talking to)
>>>> 
>>>> This is why at the Scale conference, we use a LOT of APs, all turned 
>>>> down to very low power and with default or worse antennas on them to 
>>>> shrink the transmission range of the APs.
>>>> 
>>>> David Lang
>>>> 
>>>> On Sun, 20 Sep 2026, bob.mcmahon@umbernetworks.com wrote:
>>>> 
>>>>> Date: Sun, 20 Sep 2026 12:30:40 -0700
>>>>> From: bob.mcmahon@umbernetworks.com
>>>>> To: David Lang <david@lang.hm>
>>>>> Cc: D.Goncz-FAX-571-395-3575@replikon.net, 
>>>>> bloat@lists.bufferbloat.net
>>>>> Subject: Re: [Bloat] Re: Vol 185, Issue 5
>>>>> 
>>>>> David is right that packets don't superpose. The useful math here 
>>>>> is feedback control.
>>>>> 
>>>>> The guitar amp is a useful analogy for feedback instability. A 
>>>>> squeal occurs when the loop gain and phase satisfy the conditions 
>>>>> for sustained oscillation. The amplifier keeps supplying energy to 
>>>>> the loop, and the oscillation grows until nonlinearities and 
>>>>> saturation bound it.
>>>>> 
>>>>> Networks have feedback loops too. The delay through that loop is 
>>>>> the RTT. Tail-drop synchronization, congestion-window sawtooths, 
>>>>> and other oscillatory behaviors arise when multiple controllers 
>>>>> react to delayed information about a shared bottleneck. That is why 
>>>>> control theory is the useful toolbox here.
>>>>> 
>>>>> The key state is at the bottleneck queue. Endpoints need a timely 
>>>>> congestion signal derived from that state. Loss communicates 
>>>>> congestion after the queue has filled far enough to drop a packet. 
>>>>> ECN lets the bottleneck communicate that information earlier and 
>>>>> more frequently, giving the senders time to adjust their rates 
>>>>> while keeping the queue shallow.
>>>>> 
>>>>> The difficult segment is the wireless last hop. AQM at an AP 
>>>>> manages its queue, while contention arbitrates access to the air. 
>>>>> Managing the air itself requires scheduling: grants issued from a 
>>>>> point with enough state to make the decision, including wireless 
>>>>> state across the relevant radios and bottleneck queue state.
>>>>> 
>>>>> An 802.11ax AP has detailed state about its own BSS and can use 
>>>>> trigger frames to schedule uplink transmissions within that BSS. 
>>>>> Building-wide scheduling requires a broader view: the air state 
>>>>> across multiple radios together with the forwarding and bottleneck 
>>>>> queue state. The scheduler therefore belongs at the point where 
>>>>> those two kinds of state come together.
>>>>> 
>>>>> Bob
>>>>> 
>>>>> 
>>>>> On 2026-09-19 02:16, David Lang wrote:
>>>>>> Explain how you think this resonance would happen? unlike sound, 
>>>>>> packets don't 'echo', they deliver messages, which can (and 
>>>>>> frequently do) trigger reply messages but unlike audio resonance, 
>>>>>> the incoming packets don't add to the outgoing packets to make a 
>>>>>> larger signal than the outgoing packets would if the timing was a 
>>>>>> little difference.
>>>>>> 
>>>>>> David Lang
>>>>>> 
>>>>>> 
>>>>>> On Sat, 19 Sep 2026, Replikon Research, D-U-N-S/FCN 77-387-4974 
>>>>>> wrote:
>>>>>> 
>>>>>>> There are two realms of analysis where things can blow up
>>>>>>> 
>>>>>>> One is synergy and synergetics meaning the interactions between 
>>>>>>> presumably
>>>>>>> interchangeable parts relevant to the interactions between 
>>>>>>> presumably
>>>>>>> interchangeable routers and node points
>>>>>>> 
>>>>>>> The other is combinatorics and that's my thing I love 
>>>>>>> combinatorics!!!! The
>>>>>>> only thing that matters is what you choose to count and how much 
>>>>>>> each part
>>>>>>> of what you count matters to you to put it concisely.
>>>>>>> 
>>>>>>> So while I have very little to contribute I am encouraged to hear 
>>>>>>> that the
>>>>>>> bottleneck is in the latency but the causes of the latency are
>>>>>>> multi-dimensional there are at least we know where the problem is 
>>>>>>> not where
>>>>>>> that would mean we knew the causes no that's not right let me 
>>>>>>> know is what
>>>>>>> the problem is yes the problem is latency
>>>>>>> 
>>>>>>> What comes to mind is it for certain latencies there may be an 
>>>>>>> echo like
>>>>>>> the feedback of a guitar when the amplifiers turned up too high
>>>>>>> 
>>>>>>> Does that have any intuitional relationship to these very high 
>>>>>>> level
>>>>>>> Network analyzes that you all do everyday
>>>>>>> 
>>>>>>> This question of residence no not residents got darn it in the 
>>>>>>> microphone
>>>>>>> resonance like my voice yes
>>>>>>> 
>>>>>>> I think resonance and spectral radius are two characteristics of 
>>>>>>> behind
>>>>>>> level network analysis which apply here to this question of 
>>>>>>> latency because
>>>>>>> when spectral radius is one resonance is nearly inevitable
>>>>>>> 
>>>>>>> Is the internet we have built evolving to an internet in which 
>>>>>>> some
>>>>>>> parameterizable characteristic has a special radius of 1?
>>>>>>> 
>>>>>>> I sure as hell hope not
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