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

[Toerless Eckert <[e-mail]>]

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
> 
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> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
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> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-detnet-mpls/
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> 
> 
> ----------------------------------------------------------------------
> 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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