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

[Loa Andersson <[e-mail]>]

All,

While I won't cry if this is dropped, I'm a little bit more than a
bit concerned that every time we try to state what we can expect of
a DetNet service we seem to stumble on the finish line.

If it works for MPLS and is updated to match RFC 8655, I'd say we leave
it in.

/Loa

On 09/09/2020 23:59, Stewart Bryant wrote:
> I think it has to be dropped, because as the work stands I cannot see that we have a way of bounding the latency.
> 
> I know that we are talking about MPLS, but of course we need to look at all of the data plane drafts in this respect.
> 
> - Stewart
> 
> 
> 
>> On 9 Sep 2020, at 16:39, Lou Berger <[e-mail]> wrote:
>>
>> 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.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> detnet mailing list
>>>>> [e-mail]
>>>>> https://www.ietf.org/mailman/listinfo/detnet
>>>> --
>>>> ---
>>>> [e-mail]
> 
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-- 

Loa Andersson                        email: [e-mail]
Senior MPLS Expert                          [e-mail]
Bronze Dragon Consulting             phone: [phone]