Re: [Detnet] Magnus Westerlund's Discuss on draft-ietf-detnet-mpls-11: (with DISCUSS)

[Jeff Tantsura <[e-mail]>]

MPLS data plane formally operates on a FEC not a flow (while a flow can be a FEC).

Regards,
Jeff

> On Sep 9, 2020, at 17:21, Toerless Eckert <[e-mail]> wrote:
> 
> On Wed, Sep 09, 2020 at 01:50:34PM +0000, Bal?zs Varga A wrote:
>> Hi Toerless,
>> 
>> Many thanks for the comments. One remark:
>> - I disagree with your statement "DetNet like any other IP/MPLS network with per-flow forwarding provides"
>> Just as an example, PREOF functions are not available in current MPLS networks. 
> 
> PREOF is not subject of the sentence part in question. My concern is only about:
> 
> ... DetNet provides zero congestion loss and bounded latency and jitter
> 
> Of course, now you mention it: The MPLS forwarding plane of this spec does
> support PEROF, but the sentence only talks about "DetNet", for which at
> large in my assesment this is not true (no current PREOF for IPv4/IPv6 AFAIK).
> 
> Aka: also for the part of PREOF its better to re-scope the sentence to talk only
> the MPLS forwarding plane of this document instead of (unnecessarily?) make
> claims about DetNet at large.
> 
> Cheers
>    Toerless
> 
>> Thanks
>> Bala'zs
>> 
>> -----Original Message-----
>> From: Toerless Eckert <[e-mail]> 
>> Sent: Tuesday, September 8, 2020 9:13 PM
>> To: Magnus Westerlund <[e-mail]>
>> Cc: The IESG <[e-mail]>; [e-mail]; [e-mail]; [e-mail]; [e-mail]
>> Subject: Re: [Detnet] Magnus Westerlund's Discuss on draft-ietf-detnet-mpls-11: (with DISCUSS)
>> 
>> Thanks Magnus, *:
>> 
>> Related to your comments, i would like to raise a concern about the initial sentence in the spec:
>> 
>> ...DetNet provides zero congestion loss and bounded latency and jitter.
>> 
>> To me, this is overselling what DetNet actually "provides" or that uniquely distinguishes DetNet from other solutions. It sounds as if DetNet provides a novel solution whereas in reality it just allows to adopt existing or new solutions.
>> 
>> With the definitions DetNet has done today, any IP or MPLS network where end-to-end flows can be identified as e.g.: an IP 5-tuple or an LSP identifier and that manages to figure out how to implement or operationalize one of the solutions for bounded latency such as a PHB in support of rfc2212.
>> 
>> Aka: one could equally write:
>> 
>> ...DetNet like any other IP/MPLS network with per-flow forwarding provides zero congestion loss and bounded latency and jitter.
>> 
>> Which would be equally true and equally misleading.
>> 
>> So, here is proposed IMHO more technically correct text to replace the IMHO misleading "marketing" sentence segment:
>> 
>> ...DetNet MPLS sets up point-to-point LSPs end-to-end across DetNet domains.
>> 
>> Because of this, DetNet MPLS can integrate with pre-existing and/or future Per-Hop-Behavior
>> (PHB) (such one derived from RFC2212) that can provide per-flow (e.g.: LSP) bounded latency, bounded jitter and no congestion loss, as long as such a PHB does not require additional network packet header information beside the flow/LSP identification.
>> 
>> Cheers
>>    Toerless
>> 
>>> On Tue, Sep 08, 2020 at 08:09:21AM -0700, Magnus Westerlund via Datatracker wrote:
>>> Magnus Westerlund has entered the following ballot position for
>>> draft-ietf-detnet-mpls-11: Discuss
>>> 
>>> When responding, please keep the subject line intact and reply to all 
>>> email addresses included in the To and CC lines. (Feel free to cut 
>>> this introductory paragraph, however.)
>>> 
>>> 
>>> Please refer to 
>>> https://www.ietf.org/iesg/statement/discuss-criteria.html
>>> for more information about IESG DISCUSS and COMMENT positions.
>>> 
>>> 
>>> The document, along with other ballot positions, can be found here:
>>> https://datatracker.ietf.org/doc/draft-ietf-detnet-mpls/
>>> 
>>> 
>>> 
>>> ----------------------------------------------------------------------
>>> DISCUSS:
>>> ----------------------------------------------------------------------
>>> 
>>> I like to thank the TSV-ART reviewer for helping me consider one 
>>> aspect of the issue I see needing some discussion for this document.
>>> 
>>> This relates to Section 4.2.2.2. and 4.2.2.3.
>>> 
>>> So both of these section discuss the use of the sequence number for
>>> removing packet duplicates and handling reorder. As the text discusses 
>>> there can be a configured limit for how deep the buffer and state are 
>>> for performing these operations. We all know that the implementation 
>>> of this will have a practical limit in both buffer space for 
>>> reordering as well as state for tracking which sequence numbers that 
>>> have been forwarded. I think that should be more clearly expressed in
>>> the document that these practical limits exists. Thus, the 
>>> implementations will have tracking and determination of what are new packets (increasing sequence number within a window higher than previous largest seen.
>>> And consider sequence number form currently highest seen and a bit 
>>> backwards as older packets. Thus how this is implemented will impact 
>>> how this acts in cases of disruptions of the packet flow. Thus, I 
>>> wonder if there is actually need to be  a bit more specific in how 
>>> classification should be done. Especially if the wrap-around of the
>>> sequence number space approaches a small multiple of round trip times for the path which is likely for the 16-bit space.
>>> 
>>> Then  sections fails to discuss how the duplication removal, the 
>>> reordering buffering and bound latency interacts and affet each other.  
>>> So if the latency is bounded then the reordering has an hard time 
>>> limit for the maximum delay. If there is a boundary for reordering 
>>> then there are no point in de-duplicating packets that will not be 
>>> forwarded due to the reordering. And even if there are no bounded 
>>> latency the reordering buffer size will still impact the depth of 
>>> de-duplication. These practical limits will also be limitations on the guarantees that can be provided.
>>> 
>>> Thus, from my perspective there is need for more text on the 
>>> requirements of the implementation of these functions and their 
>>> interactions of creating limitations.
>>> 
>>> Another point on 4.2.2.2:
>>> 
>>> When configured, the
>>>   implementation MUST track the sequence number contained in received
>>>   d-CWs and MUST ensure that duplicate (replicated) instances of a
>>>   particular sequence number are discarded.
>>> 
>>> That second MUST I think is possible to meet given that one discard 
>>> all packets outside of the current window where one have information 
>>> if a packet sequence number have been forwarded or not. Given that a 
>>> very late packet beyond the amount of state for the flow likely anyway 
>>> have little utility that is likely the right choice. However, I think 
>>> it needs to be made explicit that this is okay.
>>> 
>>> In Section 4.2.2.3:
>>> 
>>> When configured, the
>>>   implementation MUST track the sequence number contained in received
>>>   d-CWs and MUST ensure that packets are processed in the order
>>>   indicated in the received d-CW sequence number field, which may not
>>>   be in the order the packets are received.
>>> 
>>> I think this part needs to be explicit that packets that are to fare 
>>> out of order for the implementation to handle will/shall be dropped.
>>> 
>>>   Note that an implementation MAY wish to constrain the maximum number
>>>   of out of order packets that can be processed, on platform-wide or
>>>   per flow basis.  Some implementations MAY support the provisioning of
>>>   this number on either a platform-wide or per flow basis.  The number
>>>   of out of order packets that can be processed also impacts the
>>>   latency of a flow.
>>> 
>>> If there exists a latency requirement then that will interact with 
>>> this when it comes to reordering. In fact a significant issue here is 
>>> that if the packet flow is not periodic at a steady pace the maximum 
>>> latency in the reordering buffers based on packet sequence numbers can 
>>> not be ensured. Instead some form of time limit needs to exist also. 
>>> If that time limit is only local then there exists a risk that over 
>>> multiple reordering buffers if multiple independent service labels are 
>>> used the jitter and latency becomes cumulative. If the goal is to 
>>> avoid this then the individual packets would need to carry a time 
>>> stamp to ensure that from ingress of the service label path until the egress a maximum latency is added.
>>> 
>>> 
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> detnet mailing list
>>> [e-mail]
>>> https://www.ietf.org/mailman/listinfo/detnet
>> 
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