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

[Lou Berger <[e-mail]>]

The  doc currently reads (asterisks indicate the sentence under discussion):

    1.  Introduction

    Deterministic Networking (DetNet) is a service that can be offered by
    a network to DetNet flows.  *DetNet provides these flows with
    extremely low packet loss rates and assured maximum end-to-end
    delivery latency.*  General background and concepts of DetNet can be
    found in [RFC8655].

The sentence in question was copied from the draft version of RFC8655, 
which now reads slightly differently:

    ... which provides a capability for the delivery of
    data flows with extremely low packet loss rates and bounded end-to-
    end delivery latency.

I suggest either (a) updating the draft to match the RFC text or (b) 
dropping it altogether and let the reference to RFC8655 stand alone.

Lou

On 9/9/2020 11:20 AM, Toerless Eckert 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.
>>>
>>>
>>>
>>>
>>>
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>>> https://www.ietf.org/mailman/listinfo/detnet
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