Re: [ippm] [tsvwg] [iccrg] New Internet Draft: Congestion Signaling (CSIG)

[Sebastian Moeller <[e-mail]>]

Hi Ingemar,


> On Sep 13, 2023, at 12:30, Ingemar Johansson S <ingemar.s.johansson=[e-mail]> wrote:
> 
> Hi
>  
> I agree with Vihdi
>  
> L4S is recently standardised

	[SM] In experimental track, the goal is currently to test whether it can/should be deployed at scale....

> and it is definitely gaining traction also in 3GPP. We have an echo system that is looking forward to having L4S widely deployed.
> Still, the congestion control aspects are not fully explored yet. One interesting topic is if L4S allows to more safely deviate from additive increase to make congestion control algorithms more quickly converge to higher link capacity. There are a number of study topics around L4S congestion control that are listed in e.g the TCP Prague draft.
>  
> I cannot dictate what others should do with their time and money but personally I'd prefer that the IETF explores L4S and its possibilities and downsides before jumping on the next idea.  

	[SM] L4S can be described as taking the ideas behind DCTCP and making them fit for use over the internet*. Yet the signaling discussed here is to be used in e.g. data center contexts where DCTCP is already used and found lacking compared to newer methods operating on richer congestion information (HPCC, Swift, Poseidon, ...).
	Given that L4S essentially uses a multi-packet signal(**) to report the "queue filling state" that is then stochastically distributed over all concurrent flows, it seems obvious to me that reconstructing a reliable estimate of on-path queueing will take some time and averaging for each individual flow, I would guess that in some environments this delay simply is too costly.
	So L4S and CSIG seem complementary and in no way mutually exclusive.


Regards
	Sebastian



*) I will not further discuss whether that is achieved or not as it seems irrelevant here.
**) In essence transmission of congestion state via a 1-bit serial channel, clocked at the (variable***) packet rate at the bottleneck.
***) as packets are not of uniform size

>  
> CSIG sounds to me like something that belongs more in ICCRG or ?
>  
> /Ingemar
>  
> From: tsvwg <[e-mail]> On Behalf Of Vidhi Goel
> Sent: Wednesday, 13 September 2023 00:59
> To: Shihang(Vincent) <shihang9=[e-mail]>
> Cc: Huangyihong (Rachel) <rachel.huang=[e-mail]>; Tom Herbert <tom=[e-mail]>; Abhiram Ravi <abhiramr=[e-mail]>; IETF IPPM WG <[e-mail]>; tsvwg <[e-mail]>; [e-mail]; [e-mail]; Nandita Dukkipati <[e-mail]>; Naoshad Mehta <[e-mail]>; Jai Kumar <[e-mail]>
> Subject: Re: [tsvwg] [iccrg] New Internet Draft: Congestion Signaling (CSIG)
>  
> Not sure why we are coming up with so many new techniques when ECN just works fine. 
> ECN is a 2 bit field (not 1 bit) and seems to be sufficient to indicate extent of congestion by marking it per packet. Adding more complexity to any layer whether it is L2 or L3 doesn’t work well in deployments. Our goal should be to simplify things and only add new headers if absolutely necessary.
>  
> Vidhi
> 
> 
> On Sep 12, 2023, at 3:12 AM, Shihang(Vincent) <shihang9=[e-mail]> wrote:
>  
> Hi,
> I agree L2 may not be the best choice to carry the congestion signaling end-to-end and more bits are needed. We have submitted a draft to carry the multi-bits congestion signaling in L3. We call it Advanced ECN. See https://datatracker.ietf.org/doc/draft-shi-ccwg-advanced-ecn/. 
>  
> Thanks,
> Hang
>  
> From: CCWG <[e-mail]> On Behalf Of Huangyihong (Rachel)
> Sent: Tuesday, September 12, 2023 5:41 PM
> To: Tom Herbert <tom=[e-mail]>; Abhiram Ravi <abhiramr=[e-mail]>
> Cc: IETF IPPM WG <[e-mail]>; tsvwg <[e-mail]>; [e-mail]; [e-mail]; Nandita Dukkipati <[e-mail]>; Naoshad Mehta <[e-mail]>; Jai Kumar <[e-mail]>
> Subject: Re: [CCWG] [iccrg] [tsvwg] New Internet Draft: Congestion Signaling (CSIG)
>  
> Hi,
>  
> I also have the same feeling. Implementing in L2 may be difficult to be used in e2e transport. Of course it can work well in limited domain, like DC or HPC clusters. However, I also look for some solutions that could be able to go through internet. We have submitted a draft to describe the transport challenges. See https://datatracker.ietf.org/doc/html/draft-huang-tsvwg-transport-challenges.
>  
> I share the same opinion that the congestion signal is useful and current 1-bit ECN solution is not fully sufficient. But I also feel like the more straight way is to extend L3, or l4, like update IOAM, to carry the information. For L2 solution, it should be developed together with IEEE 802.1.
>  
> BR，
> Rachel
>  
> 发件人: iccrg <[e-mail]> 代表 Tom Herbert
> 发送时间: 2023年9月10日 0:10
> 收件人: Abhiram Ravi <abhiramr=[e-mail]>
> 抄送: IETF IPPM WG <[e-mail]>; tsvwg <[e-mail]>; [e-mail]; [e-mail]; Nandita Dukkipati <[e-mail]>; Naoshad Mehta <[e-mail]>; Jai Kumar <[e-mail]>
> 主题: Re: [iccrg] [tsvwg] New Internet Draft: Congestion Signaling (CSIG)
>  
> Hi, thanks for draft!
>  
> The first thing that stands out to me is the carrier of the new packet headers. In the forward path it would be in L2 and in reflection it would be L4. As the draft describes, this would entail having to support the protocol in multiple L2 and multiple L4 protocols-- that's going to be a pretty big lift! Also, L2 is not really an end-to-end protocol (would legacy switches in the path also forward the header)l?).
>  
> The signaling being described in the draft is network layer information, and hence IMO should be conveyed in network layer headers. That's is L3 which conveniently is the average of L2+L4 :-)
>  
> IMO, the proper carrier of the signal data is Hop-by-Hop Options. This is end-to-end and allows modification of data in-flight. The typical concern with Hop-by-Hop Options is high drop rates on the Internet, however in this case the protocol is explicitly confined to a limited domain so I don't see that as a blocking issue for this use case.
>  
> The information being carried seems very similar to that of IOAM (IOAM uses Hop-by-Hop Options and supports reflection). I suppose the differences are that this protocol is meant to be consumed by the transport Layer and the data is a condensed summary of path characteristics. IOAM seems pretty extensible, so maybe it could be adapted to carry the signals of this draft?
>  
> A related proposal might be FAST draft-herbert-fast. Where the CSIG is network to host signaling, FAST is host to network signaling for the purposes of requesting network services. These might be complementary and options for both may be in the same packet. FAST also uses reflection, so we might be able to leverage some common implementation at a destination.
>  
> Tom
>  
> On Fri, Sep 8, 2023, 7:43 PM Abhiram Ravi <abhiramr=[e-mail]> wrote:
> Hi IPPM folks,
>  
> I am pleased to announce the publication of a new internet draft, Congestion Signaling (CSIG): https://datatracker.ietf.org/doc/draft-ravi-ippm-csig/ 
>  
> CSIG is a new end-to-end packet header mechanism for in-band signaling that is simple, efficient, deployable, and grounded in concrete use cases of congestion control, traffic management, and network debuggability. We believe that CSIG is an important new protocol that builds on top of existing in-band network telemetry protocols.
>  
> We encourage you to read the CSIG draft and provide your feedback and comments. We have also cc'd the TSVWG, CCWG, and ICCRG mailing lists, as we believe that this work may be of interest to their members as well.
>  
> Thank you for your time and consideration.
>  
> Sincerely,
> Abhiram Ravi
> On behalf of the CSIG authors