Re: [spring] TSV-ART review of draft-ietf-spring-segment-routing-msdc-08

[Martin Stiemerling <[e-mail]>]

Hi Gaurav,

This got lost on my end, sorry for this. The filter just moved these 
messages out of my sight... :-/

Am 15.02.18 um 05:47 schrieb Gaurav Dawra:
> Hey Martin,
> 
> Sorry for late reply. Please see some comments inline[Gaurav]
> 
>> On Jan 9, 2018, at 2:25 PM, Martin Stiemerling <[e-mail] 
>> <mailto:[e-mail]>> wrote:
>>
>> Hi all,
>>
>> I've reviewed this document as part of the transport area review 
>> team's ongoing effort to review key IETF documents. These comments 
>> were written primarily for the transport area directors, but are 
>> copied to the document's authors for their information and to allow 
>> them to address any issues raised. When done at the time of IETF Last 
>> Call, the authors should consider this review together with any other 
>> last-call comments they receive. Please always CC tsv-art@… if you 
>> reply to or forward this review.
>>
>> Summary:
>> This draft has serious issues in Section 7.1, 7.2 and in one part of 
>> Section3, described in the review, and needs to be rethought. The 
>> other sections are good AFAIK.
>>
>>
>> Technicals:
>> The overall draft looks ok, but the three points below look strange 
>> and need a fix before publication IMHO:
>>
>> Both Sections, 7.1. and 7.2., are describing ideas, but not well 
>> proven funcationality and not even safe to use functionality. Both are 
>> some sort discussing that different paths in the network could be used 
>> by the end host traffic. This sounds pretty much like the Path Aware 
>> Networking Proposed Research Group (https://irtf.org/panrg) and hints 
>> to the fact that there is no commonly understand and accepted 
>> engineering solution in this space.
>>
>> Section 7.1:
>> [KANDULA04] is a really old reference that hasn't been followed up in 
>> recent times and even worse there is no evidence that this is going to 
>> work good enough or stable enough under real Internet traffic. 
>> Additionally, it is more than unclear how any modern TCP 
>> implementation will react to this
> [Gaurav] Will get back on this.

I will reply to the other email dicussing this.

>>
>> Section 7.2:
>> This section describes an idea without detailing too much about any 
>> further aspects. Further it changes the commonly accepted notion of 
>> what an end host can do with the network. At best this would require a 
>> good definition of what an end host in your setting is, e.g., a highly 
>> modified piece of (at least) software that usually not found in OS 
>> availble on the market (yet?)
>> Further communicating instantaneous path characteristics to a central 
>> point is potentially a bad idea, as the data is already outdated when 
>> reported by any node.
> [Gaurav] I believe Authors are trying to highlight that Host which is 
> defined in 
> (https://tools.ietf.org/html/draft-ietf-spring-segment-routing-15) can 
> influence the traffic based on the Calculations locally or jointly with 
> the controller. Implementations can decide how much Centralized -vs- 
> localized control is allowed at Host based on performance data collection.

Performance data collection (monitoring?) isn't crucial when it comes to 
timely (actually real-time) reaction. However, this section isn't just 
about performance data collection as it is about "Performance-aware 
routing" this seems to try to interact in real-time with the network 
behavior of TCP. This isn't even close to acceptable.

I would be ok to say that it is useful to collect performance data for 
offline analysis and improvement of the data network. However, this is 
at completely different magnitues of time scales.

I would recommend to remove the TCP part from this section entirely.


>>
>> Section 3, 3rd bullet point:
>> It is the foundation of TCP that the network is regarded as a black 
>> box and that you infer from the transmission of packets what the 
>> current state of the network path is. Inferring network path metrics 
>> (you mention SRTT, MSS, CWND ) is a bad idea, as this would required 
>> that all paths exhibit this and if not what is going to happen?
>> It could be an interesting research field to change many points in 
>> TCP's behavior, but this once again points to the fact that this not 
>> the IETF works but IRTF or elsewhere.
> [Gaurav] Martin, Authors are trying to suggest that TCP is rightly 
> treating Network as Black Box. Authors are implying per path performance 
> metrics as not cached. Is there some change in text you are suggesting?


I would recommend to remove the 3rd bullet point completey. TCP isn't 
the place to remember "good" or "bad" paths. This is something the 
network could decide, e.g., rerouting TCP flows within the network or 
changing the forwarding path in the network for particular flows (if it 
is not routed).

Kind regards,

   Martin