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

[Sebastian Moeller <[e-mail]>]

Hi Ruediger,


> On Sep 14, 2023, at 09:25, <[e-mail]> <[e-mail]> wrote:
> 
> Sebastian, Ingemar,
> 
> the draft says "limited domain". The method suggested resembles IPPM's IOAM (but the packet size remains fixed, certainly an advantage). Information like a port ID only makes sense, if the evaluation unit is aware of the domain-topology.

	[SM] Not sure I understand this, if on an internet-wide path all nodes would simply include the minimum of their egress bitrate (that is only include this if it is lower than the existing value), then an end-2end protocol like TCP could even as part of the initial 3 way handshake figure out a hard limit for maximum rasinsable cwin (by simply calculating the BDP). SUre that would not stop slow-start fuully from overshooting, but it should limit the maximum overshoot considerably....


> That will be restricted to a single domain, I think.

	[SM] As long as the forward and collection path requires L2 elements, sure this will not work over the internet.


> I'm also not sure, what is to be optimized on an operational carrier backbone.

	[SM] I trust you on this, as that is out of my league.


> Under expectable conditions, these are operated without congestion. Bottlenecks are likely at the access and, in a rather limited number of cases, at peering. There you'd need to know the RTT so that the available bandwidth calculation makes sense.

	[SM] But the protocol that is supposed to consume the congestion information will know the RTT, so can take this into account, the network really only needs to give information about its current state. That said, I can see network nodes that might want to know a flow's RTT, but I do not think there is a way of supplying that this is not both easy and efficient and not easily gamed. Passing on congestion informatin however is far less abusable (after all the node could just as well have dropped the packet instead of bothering to fudge the congestion info).


> Or you need to calculate several values for several RTTs and provide this information (RTTs and corresponding available bandwidths). Requires a fair amount of bytes, so then trust may be an issue (I'm not sure whether a carrier would be willing to add an extended Ethernet header to every encrypted IP packet indicating interest in getting CSIG information on bottleneck links).

	[SM] Yes encryption/privacy are reasons to stick to L2/L3. I would guess in a limited domain deploying CSIG as a pseudo VLAN header seems like a nice work-around, but this information really should be inside IP header to go end to end. IMHO it also should not be in an optional header, but should have been part of the mandatory header, but that ship has sailed long ago... It turns out that the mechanism intended here IPv6 extension headers was mis-deployed... (It should have been initiated with a really desirable use-case so it would actrually see usage immediately, protocol design is a bit like evolution, in both cases "use it or loose it" applies to some degree.)

Regards
	Sebastian


> 
> Regards, Ruediger
> 
> -----Ursprüngliche Nachricht-----
> Von: Sebastian Moeller <[e-mail]> 
> Gesendet: Donnerstag, 14. September 2023 08:49
> An: Ingemar Johansson S <[e-mail]>
> Cc: Geib, Rüdiger <[e-mail]>; [e-mail]; [e-mail]; [e-mail]; [e-mail]
> Betreff: Re: [tsvwg] [iccrg] New Internet Draft: Congestion Signaling (CSIG)
> 
> Hi Ingemar
> 
>> On Sep 14, 2023, at 08:42, Ingemar Johansson S <[e-mail]> wrote:
>> 
>> Hi Ruediger
>> 
>> Even though CSIG is on ethernet, it appears to be e2e as the feedback is on L4. So I guess somehow the CSIG info needs to jump from domain to domain somewhow, and that sounds to me like L3, albeit perhaps brief jumps ? 
> 
> 	[SM] rfc3168 and L4S ECN signaling faces the same hurdle, forward path uses a different layer than feed-back path. And I do not think that it is conceptually all that cleaner to have fewer layers between forward and reverse signaling... "clean" would be to put both forward and reverse information into the same layer, but that is not on the table as far as I can see.
> 	The other difference however, richness of signal, is a clear difference between 1 bit ECN and multibit CSIG (and alternatives). (Side-note, sure the ECN bitfield is two bits but it only carries 2 congestion states when ECN is in use, which makes it IMHO a 1-bit information channel, if we had followed the SCE proposal with its 3 congestion states we would be at 1.5 bits ;))
> 
> Regards
> 	Sebastian
> 
> 
> 
>> 
>> /Ingemar
>> 
>>> -----Original Message-----
>>> From: [e-mail] <[e-mail]>
>>> Sent: Wednesday, 13 September 2023 14:20
>>> To: [e-mail]; Ingemar Johansson S 
>>> <[e-mail]>
>>> Cc: [e-mail]; [e-mail]; [e-mail]; 
>>> [e-mail]; [e-mail]; [e-mail]; 
>>> [e-mail]; [e-mail]; [e-mail]; 
>>> [e-mail]; [e-mail]; [e-mail]
>>> Subject: AW: [tsvwg] [iccrg] New Internet Draft: Congestion Signaling
>>> (CSIG)
>>> 
>>> Hi Ingemar, hi Sebastian,
>>> 
>>> Skimming over the draft only, isn't CSIG about Ethernet-domains, 
>>> while L4S is E2E?
>>> 
>>> Regards,
>>> 
>>> Ruediger
>>> 
>>> -----Ursprüngliche Nachricht-----
>>> Von: tsvwg <[e-mail]> Im Auftrag von Sebastian Moeller
>>> Gesendet: Mittwoch, 13. September 2023 13:42
>>> An: Ingemar Johansson S
>>> <ingemar.s.johansson=[e-mail]>
>>> Cc: Vidhi Goel <vidhi_goel=[e-mail]>; 
>>> Shihang(Vincent) <shihang9=[e-mail]>; tsvwg 
>>> <[e-mail]>; Jai Kumar <[e-mail]>; IETF IPPM WG 
>>> <[e-mail]>; Tom Herbert <tom=[e-mail]>; 
>>> [e-mail]; Abhiram Ravi <abhiramr=[e-mail]>; 
>>> Nandita Dukkipati <[e-mail]>; [e-mail]; Huangyihong 
>>> (Rachel) <rachel.huang=[e-mail]>; Naoshad Mehta 
>>> <[e-mail]>
>>> Betreff: Re: [tsvwg] [iccrg] New Internet Draft: Congestion Signaling
>>> (CSIG)
>>> 
>>> 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
>> 
> 
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