[Bloat] Re: Vol 185, Issue 5

[bob.mcmahon@umbernetworks.com]

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
>>>>> _______________________________________________
>>>>> Bloat mailing list -- bloat@lists.bufferbloat.net
>>>>> To unsubscribe send an email to bloat-leave@lists.bufferbloat.net
>>>>> 
>>>> _______________________________________________
>>>> Bloat mailing list -- bloat@lists.bufferbloat.net
>>>> To unsubscribe send an email to bloat-leave@lists.bufferbloat.net
>>> 
>> 
> _______________________________________________
> Bloat mailing list -- bloat@lists.bufferbloat.net
> To unsubscribe send an email to bloat-leave@lists.bufferbloat.net