Re: [Detnet] TSN scheduling/queuing algorithms and DetNet

[Florian Kauer <[e-mail]>]

Thank you a lot David and Toerless for your explanations!

On 05.10.23 18:39, Toerless Eckert wrote:
> As i tried to explain in the last email reply to Florian:
> 
> I think for large-scale/high-speed detnets, that the TSN algorithms will not work well,
> but with proposal like we have on the tavble, i think we know how to adopt what we learned from TSN.
> (aka: TCQF/CSQF vs. CQF/ECQF).

Yes, as you have explained detailed in your first mail to me, just TSN is certainly not sufficient for
the large-scale/high-speed cases and I think we all agree on that and it is the primary motivation
for the "open working meetings".

> Even if something like timed gates on ingres is something we could reuse directly from TSN, i still
> prefer we have IETF docs explaining how to adopt to IETF networks.> For low-speed/limited-bandwdith DetNet, such as in manufacturing floors, i wonder if we are going to see
> any insance where we need to adopt TSN mechanisms directly to L3:

I think the discussion is "just" about terminology. So what it really means in the end to
"adopt TSN mechanisms directly to L3".

> I do not think we are going to see any such limited-scale networks without L2 switches involved. I for
> once am not aware of any campus/industrial environments where sender/receivers directly attach to
> a router. IETF has just done a (sh#$y - censored) job to make deployment of L3 easy. You immediately
> need an IP/IPv6 addrssing expert, manage addressing space, 

Following your argument, you need that addressing expert anyway either for configuring the senders/receivers
or for configuring the bridges, so what is the difference?
Why does that imply that the senders/receiver will not directly attach to a router?
Do you refer to the complexity to implement such functionality on the end stations?

> and if you want to have device mobility you
> need complex solutions that most likely will break any services guarantees (like TSN or DetNet).

I think device mobility for time-sensitive/deterministic networks is a completely different topic
where IPv6 addressing might be even the smallest problem you need to solve...


> Aka: between any sender/receiver and any router will be L2 switches whcih would then have to be TSN
> anyhow. And we have the RFCs explaining how to run DetNet "over" such L2 TSN subnets.
> 
> How about a larger industrial network/campus: 
> 
>     Src --- L2sw --- Rtr1 ---- backbone --- Rtr2 --- L2sw -- Rcvr
> 
> Would we not we need "DetNet only" between Rtr1 and Rtr2  - no TSN involved ?
> 
> No. Because any Rtr equipment that vendors would sell into such environment will already have an
> L2 switch integrated inyo any of these routers. So in reality the actual forwarding would be:
> 
>     Src --- L2sw --- [L2sw-Rtr1-L2sw] ---- backbone --- [L2sw-Rtr2-L2sw] --- L2sw -- Rcvr

What exactly do you mean by "DetNet only"? Even in the "large-scale/high-speed case"
there will always be SOME L2 layer (and L1), otherwise you won't be able to transmit
anything on the wire (or wireless).

Of course, that does not necessarily need to be TSN according to IEEE 802.*, but at least
SOME L2 layer that provides basic RT guarantees (such as not delaying or dropping packets
arbitrarily).


> In effect we do have 3 TSN domains where Rtr1 / Rtr2 really just need to do non-queuing
> forwarding, make sure that queuing happens on the outgoing TSN L2sw side on the same box,
> and the only DetNet job on the sending side is to aply the right L2 tagging to ensure the
> packet is put into th right TSN service instance.

I don't know if I understand you correctly, but JUST connecting two completely uncoordinated
TSNs together with a L3 forwarding function, will IMHO not provide you deterministic latency.
So if, just for the sake of the example, both networks use 1 second cycle time, but the
gate for a certain traffic class only opens for 1 ms in every cycle, it depends on the relative
time position of the gate opening if the latency over the hop is ~1 ms or ~1 s.

So in this case you need both time synchronization between the networks as well as some
overarching coordination that is aware of the fact that there are two networks that shall be
connected. Nevertheless, there is nothing fancy that this "coordination" needs to consider
that is DetNet-specific or uses other algorithms that are different from those
applied in a "pure" TSN.


> Only when the "backbone" links become fast / lage enough that the L2sw approach on the box
> won't work would we need "direct L3" interface queuing. 

Probably the same question as the "DetNet only" one above, but what exactly do you mean "direct L3" here?

In principle, I can just implement a single silicon and put it in a black box
that looks from outside just the same as two separate L2 bridges and a separate "L3 forwarding device"
in a box, but does that very efficiently and fast internally.
How would that be different than a "direct L3" in silicon given that also this one needs
to implement some L2 + L1 functionality to output any meaningful electrical signals?

Thanks,
Florian

> 
> Cheers
>     toerless
> 
> 
> On Thu, Oct 05, 2023 at 01:46:44PM +0000, Black, David wrote:
>> Hi Florian,
>>
>> Still posting as an individual - the two of us are on the same page ...
>>
>>> Since I was not able to attend the meeting and I can not find any minutes or recording, I would love to get more background about this question, especially why you explicitly marked **algorithms** as bold.
>>
>> I was drawing a distinction between the entire scope of TSN technology, which is a layer-2 subnet when fully deployed, versus just the TSN scheduling/queuing algorithms (e.g., Qbv) which are readily deployable at layer 3.
>>
>>> And for those scenarios my current working assumption is actually that all DetNet nodes that route on L3 between L2 TSN
>>> networks would be able to directly apply all TSN techniques (like Qbv and CQF) without any special consideration for them on L3.
>>
>> +1 on "directly apply all TSN techniques (like Qbv and CQF) without any special consideration for them on L3" - and in particular, I believe that this direct application is possible now (i.e., no need to wait for IETF to publish a detnet RFC that says it's ok to do this obvious thing ...).
>>
>>> The only exception is that the L3 processing MIGHT take more time than L2 bridging and this might have to be taken into account e.g. when calculating a Qbv schedule.
>>> Do you agree or is there something I overlook?
>>
>> I don't think you've missed anything major ...
>>
>> Thanks, --David
>>
>> -----Original Message-----
>> From: Florian Kauer <[e-mail]> 
>> Sent: Thursday, October 5, 2023 3:22 AM
>> To: Black, David; [e-mail]
>> Cc: Black, David
>> Subject: Re: [Detnet] TSN scheduling/queuing algorithms and DetNet
>>
>>
>> [EXTERNAL EMAIL] 
>>
>> Hi David,
>> thanks for bringing up this point!
>> Since I was not able to attend the meeting and I can not find any minutes or recording, I would love to get more background about this question, especially why you explicitly marked **algorithms** as bold.
>>
>> For the use cases I am currently considering for DetNet (industrial control/vPLC and pro audio) the scaling requirements (time asynchrony, propagation latency...) are IMHO of lower importance at the moment. And for those scenarios my current working assumption is actually that all DetNet nodes that route on L3 between L2 TSN networks would be able to directly apply all TSN techniques (like Qbv and CQF) without any special consideration for them on L3. The only exception is that the L3 processing MIGHT take more time than L2 bridging and this might have to be taken into account e.g. when calculating a Qbv schedule. Do you agree or is there something I overlook?
>>
>> Thanks,
>> Florian
>>
>> On 04.10.23 23:30, Black, David wrote:
>>> For clarity, this is posted as an individual contributor …
>>>
>>>  
>>>
>>> In the most recent open working meeting, Toerless and I had a discussion about whether DetNet nodes could use the TSN scheduling/queuing algorithms.
>>>
>>>  
>>>
>>> Checking the DetNet RFCs, what I believe I see is:
>>>
>>>   * Toerless’s assertion that all DetNet RFC references to TSN refer to layer-2 TSN subnets appears to be correct.
>>>   * OTOH, I did not see anything that prohibits implementation and use of the TSN scheduling/queuing **algorithms** at layer 3 in DetNet nodes.
>>>
>>>  
>>>
>>> I would hope that it’s not necessary to publish an RFC to bless the use of the TSN scheduling/queuing algorithms at layer 3 in DetNet Nodes …
>>>
>>>  
>>>
>>> Thanks, --David
>>>
>>> * *
>>>
>>> *David L. Black, *Sr. Distinguished Engineer, Technology & Standards
>>>
>>> Infrastructure Solutions Group,*Dell Technologies*
>>>
>>> mobile [phone] [e-mail] <mailto:[e-mail]>
>>>
>>>  
>>>
>>>
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>