Re: [Detnet] Extended WG Last Call: draft-ietf-detnet-architecture-06

["Pascal Thubert (pthubert)" <[e-mail]>]

Hello Deborah 

The architecture as published does not have a CPE. 

This was introduced in this thread and never needs to see the light. 

The original text had PCE because conceptually this is the IETF base for that entity, even if we’ll need to evolve it, and that there are other entities we'll need for management and automation.

I’m still unclear When CPE came up ?
I saw it first in Lou’s proposal. Was that voluntary or a typo ?


Regards,

Pascal

> Le 13 juil. 2018 à 13:44, Pascal Thubert (pthubert) <[e-mail]> a écrit :
> 
> Works for me, Lou,
> 
> I think that the best would be a terminology that would define this operational plane, leaving it open to network control and feedback, and then change in situ the text as you proposed.
> 
> 
> Regards,
> 
> Pascal
> 
>> Le 13 juil. 2018 à 06:29, Lou Berger <[e-mail]> a écrit :
>> 
>> Pascal,
>> 
>> Keeping in mind that LMI is generally considered a form of OAM, it sounds like changing it to "Operational Plane (e.g., OAM)" would sufficiently cover your intent and be a fairly trivial change to the document.  Would this change work for you?
>> 
>> Lou
>> 
>>> On 7/13/2018 12:19 AM, Pascal Thubert (pthubert) wrote:
>>> Hello Lou and Norm:
>>> 
>>> I meant control protocols over the LMI as well as measurement (OAM) and automation such as (reflex) reactions that do no pass via a controller.
>>> 
>>> The LMI provides information on the status of a DetNet path which can act as go/nogo for data and trigger fallback. It may in the future enable flow setup if one day we go for a more distributed design. It may provide time though for DetNet it is not in scope. It may provide rate control as well which is the object of a draft I have to split.
>>> 
>>> So operation there was meant as a generic term for train data traffic overhead inside the network as opposed to in relation with a controller.
>>> 
>>> Should we expand to clarify?
>>> 
>>> Regards,
>>> 
>>> Pascal
>>> 
>>>> Le 13 juil. 2018 à 00:04, Norman Finn <[e-mail]> a écrit :
>>>> 
>>>> Pascal wrote that chunk.  I always assumed that "Operational Plane (control plane)" was some sort of IETF phrase I just didn't understand.  If it's OAM, I don't see what the "(control plane)" is for.
>>>> 
>>>> Pascal?
>>>> 
>>>> -- Norm
>>>> ________________________________________
>>>> From: detnet [[e-mail]] on behalf of Lou Berger [[e-mail]]
>>>> Sent: Thursday, July 12, 2018 9:47 AM
>>>> To: [e-mail]
>>>> Cc: DetNet WG
>>>> Subject: Re: [Detnet] Extended WG Last Call: draft-ietf-detnet-architecture-06
>>>> 
>>>> Hi,
>>>> 
>>>> I have the following comments/questions:
>>>> 
>>>> - WRT 4.4.2
>>>> 
>>>> I think CPE and PCE are a bit conflated.  To clarify, hiw about:
>>>> 
>>>> OLD
>>>>   to any device operating in that plane, whether is it a Path
>>>>   Computation entity, or a Network Management entity (NME)), or a
>>>>   distributed control plane.  The Path Computation Element (PCE)
>>>>   [RFC4655] is a core element of a controller, in charge of computing
>>>>   Deterministic paths to be applied in the Network Plane.
>>>> 
>>>> NEW
>>>>   to any device operating in that plane, whether is it a Path
>>>>   Computation Element [RFC4655] or entity, or a Network Management
>>>> entity (NME)), or a
>>>>   distributed control plane.  The CPE
>>>>    is a core element of a controller, in charge of computing
>>>>   Deterministic paths to be applied in the Network Plane.
>>>> 
>>>> and s/PCE/CPE in the next paragraph, specifically:
>>>> 
>>>> OLD
>>>>   One or more PCE(s) collaborate to implement the requests from the FME
>>>>   as Per-Flow Per-Hop Behaviors installed in the intermediate nodes for
>>>>   each individual flow.  The PCEs place each flow along a deterministic
>>>> NEW
>>>>   One or more CPE(s) collaborate to implement the requests from the FME
>>>>   as Per-Flow Per-Hop Behaviors installed in the intermediate nodes for
>>>>   each individual flow.  The CPEs place each flow along a deterministic
>>>> 
>>>> - WRT Section 4.4.3
>>>> I'm unclear as to what "Operational Plane (control plane)" means in the
>>>> first paragraph.  Should it perhaps read "Operational Plane (OAM)"? If
>>>> not, what is the intent of "control plane" in this paragraph (and section)?
>>>> 
>>>> Thank you,
>>>> Lou
>>>> 
>>>>> On 6/28/2018 10:35 PM, Lou Berger wrote:
>>>>> Authors,
>>>>> 
>>>>>     Thank you for the update!
>>>>> 
>>>>> WG,
>>>>> 
>>>>>     This document has changed to a sufficient degree that I think a 2nd
>>>>> last call is warranted.  Typically I would start a 1 week LC in these
>>>>> circumstances - but given the proximity to the meeting I'd like to start
>>>>> a 3 week LC right ending with the IETF meeting -- that is on July 20th.
>>>>> This should allow for both adequate review of the changes and discussion
>>>>> of the changes in our WG session (Monday July 16.)
>>>>> 
>>>>> As always, please send LC comment to the list and positive comments,
>>>>> e.g., "I've reviewed this document
>>>>> and believe it is ready for publication", are welcome!
>>>>> 
>>>>> Thank you,
>>>>> 
>>>>> Lou (as Shepherd)
>>>>> 
>>>>>> On 6/28/2018 9:08 PM, János Farkas wrote:
>>>>>> Dear all,
>>>>>> 
>>>>>> Off-line discussions among Lou, Stewart, and authors followed the
>>>>>> discussions to properly address the WGLC comments, including the
>>>>>> detailed comments.
>>>>>> 
>>>>>> A new revision of the draft has been uploaded:
>>>>>> draft-ietf-detnet-architecture-06.
>>>>>> 
>>>>>> In addition to the changes already described in this thread, the
>>>>>> following bigger changes have been made to the draft:
>>>>>> 
>>>>>> 
>>>>>> *Section 2.1 Terms used in this document*
>>>>>> 
>>>>>> Some definitions refined as suggested by the detailed comments
>>>>>> 
>>>>>> New definitions have been added:
>>>>>> 
>>>>>>   "allocation
>>>>>>           Resources are dedicated to support a DetNet flow. Depending
>>>>>>           on an implementation, the resource may be reused by non-
>>>>>>           DetNet flows when it is not used by the DetNet flow.
>>>>>> 
>>>>>> 
>>>>>>   PEF     A Packet Elimination Function (PEF) eliminates duplicate
>>>>>>           copies of packets to prevent excess packets flooding the
>>>>>>           network or duplicate packets being sent out of the DetNet
>>>>>>           domain.  PEF can be implemented by an edge node, a relay
>>>>>>           node, or an end system.
>>>>>> 
>>>>>>  PRF     A Packet Replication Function (PRF) replicates DetNet flow
>>>>>>           packets and forwards them to one or more next hops in the
>>>>>>           DetNet domain.  The number of packet copies sent to each next
>>>>>>           hop is a DetNet flow specific parameter at the node doing the
>>>>>>           replication.  PRF can be implemented by an edge node, a relay
>>>>>>           node, or an end system.
>>>>>> 
>>>>>>   PREOF   Collective name for Packet Replication, Elimination, and
>>>>>>           Ordering Functions.
>>>>>> 
>>>>>>   POF     A Packet Ordering Function (POF) re-orders packets within a
>>>>>>           DetNet flow that are received out of order.  This function
>>>>>>           can be implemented by an edge node, a relay node, or an end
>>>>>>           system.
>>>>>> 
>>>>>>  DetNet service proxy
>>>>>>           Maps between App-flows and DetNet flows.
>>>>>> 
>>>>>>   bridged path
>>>>>>           A VLAN bridge uses the VLAN ID and the destination MAC
>>>>>>           address to select the outbound port hence the path for a
>>>>>>           frame."
>>>>>> 
>>>>>> 
>>>>>> *Section 3.1 Primary goals defining the DetNet QoS*
>>>>>> 
>>>>>> A new QoS aspect has been added:
>>>>>>   "o  An upper bound on out-of-order packet delivery.  It is worth
>>>>>>      noting that some DetNet applications are unable to tolerate any
>>>>>>      out-of-order delivery."
>>>>>> 
>>>>>> 
>>>>>> The 3rd paragraph on loss on page 8 after the bullet list has been
>>>>>> extended to:
>>>>>>   "After congestion, the most important contributions to packet loss are
>>>>>>   typically from random media errors and equipment failures. Service
>>>>>>   protection is the name for the mechanisms used by DetNet to address
>>>>>>   these losses.  The mechanisms employed are constrained by the
>>>>>>   requirement to meet the users' latency requirements.  Packet
>>>>>>   replication and elimination (Section 3.2.2) and packet encoding
>>>>>>   (Section 3.2.2.3) are described in this document to provide service
>>>>>>   protection; others may be found.  For instance, packet encoding can
>>>>>>   be used to provide service protection against random media errors,
>>>>>>   packet replication and elimination can be used to provide service
>>>>>>   protection against equipment failures.  This mechanism distributes
>>>>>>   the contents of DetNet flows over multiple paths in time and/or
>>>>>>   space, so that the loss of some of the paths does need not cause the
>>>>>>   loss of any packets."
>>>>>> 
>>>>>> 
>>>>>> *3.2.2.  Service Protection*
>>>>>> 
>>>>>> Service protection is used as a more generic term. Introductory text
>>>>>> added:
>>>>>>   "Service protection aims to mitigate or eliminate packet loss due to
>>>>>>   equipment failures, random media and/or memory faults.  These types
>>>>>>   of packet loss can be greatly reduced by spreading the data over
>>>>>>   multiple disjoint forwarding paths.  Various service protection
>>>>>>   methods are described in [RFC6372], e.g., 1+1 linear protection.
>>>>>>   This section describes the functional details of an additional method
>>>>>>   in Section 3.2.2.2, which can be implemented as described in
>>>>>>   Section 3.2.2.3 or as specified in [I-D.ietf-detnet-dp-sol-mpls] in
>>>>>>   order to provide 1+n hitless protection.  The appropriate service
>>>>>>   protection mechanism depends on the scenario and the requirements."
>>>>>> 
>>>>>> 
>>>>>> New sub-section added:
>>>>>> 
>>>>>> "3.2.2.1.  In-Order Delivery
>>>>>> 
>>>>>>   Out-of-order packet delivery can be a side effect of service
>>>>>>   protection.  Packets delivered out-of-order impact the amount of
>>>>>>   buffering needed at the destination to properly process the received
>>>>>>   data.  Such packets also influence the jitter of a flow.  The DetNet
>>>>>>   service includes maximum allowed misordering as a constraint. Zero
>>>>>>   misordering would be a valid service constraint to reflect that the
>>>>>>   end system(s) of the flow cannot tolerate any out-of-order delivery.
>>>>>>   Service protection may provide a mechanism to support in-order
>>>>>>   delivery."
>>>>>> 
>>>>>> 
>>>>>> *3.2.2.2. Packet Replication and Elimination*
>>>>>> 
>>>>>> New bullet added as the last one:
>>>>>>   "o  The Packet Ordering Function (POF) uses the sequencing information
>>>>>>      to re-order a DetNet flow's packets that are received out of
>>>>>>      order."
>>>>>> 
>>>>>> New sentence added after the bullet list:
>>>>>> "The order in which a node applies the PEF and the PRF to a DetNet
>>>>>> flow is implementation specific."
>>>>>> 
>>>>>> 2nd paragraph after the bullet list has been updated to:
>>>>>> "Some service protection mechanisms rely on switching from one flow to
>>>>>>   another when a failure of a flow is detected.  Contrarily, packet
>>>>>>   replication and elimination combines the DetNet member flows sent
>>>>>>   along multiple different paths, and performs a packet-by-packet
>>>>>>   selection of which to discard, e.g., based on sequencing information."
>>>>>> 
>>>>>> 
>>>>>> *3.2.3.  Explicit routes*
>>>>>> 
>>>>>> Out-of-order aspect added to the first paragraph, which is about
>>>>>> distributed routing:
>>>>>> "Furthermore, out-of-order
>>>>>> packet delivery can be a side effect of route changes."
>>>>>> 
>>>>>> New sentence added to the 3rd paragraph:
>>>>>> "Explicit routes can be established various
>>>>>> ways, e.g., with RSVP-TE [RFC3209], with Segment Routing (SR)
>>>>>> [I-D.ietf-spring-segment-routing], via a Software Defined Networking
>>>>>> approach [RFC7426], with IS-IS [RFC7813], etc."
>>>>>> 
>>>>>> New paragraph added:
>>>>>>   "Out-of-order packet delivery can be a side effect of distributing a
>>>>>>   single flow over multiple paths especially when there is a change
>>>>>>   from one path to another when combining the flow.  This is
>>>>>>   irrespective of the distribution method used, also applies to service
>>>>>>   protection over explicit routes.  As described in Section 3.2.2.1,
>>>>>>   out-of-order packets influence the jitter of a flow and impact the
>>>>>>   amount of buffering needed to process the data; therefore, DetNet
>>>>>>   service includes maximum allowed misordering as a constraint. The
>>>>>>   use of explicit routes helps to provide in-order delivery because
>>>>>>   there is no immediate route change with the network topology, but the
>>>>>>   changes are plannable as they are between the different explicit
>>>>>>   routes."
>>>>>> 
>>>>>> *
>>>>>> **4.1.1. Representative Protocol Stack Model*
>>>>>> 
>>>>>> "Explicit routes" have been added to Figure 2 with the corresponding
>>>>>> explanation:
>>>>>> "Explicit routes
>>>>>>           The DetNet transport layer provides mechanisms to ensure that
>>>>>>           fixed paths are provided for DetNet flows.  These explicit
>>>>>>           paths avoid the impact of network convergence."
>>>>>> 
>>>>>> 
>>>>>> Section 4.11 Connected islands vs. networks of v05 has been deleted
>>>>>> because it was just a leftover from early drafts on what DetNet WG
>>>>>> should do; which are covered by the charter anyways.
>>>>>> 
>>>>>> 
>>>>>> References have been cleaned up and brought up-to-date.
>>>>>> 
>>>>>> 
>>>>>> Refinements have been implemented in the draft according to Lou's
>>>>>> detailed comments. They are not listed here because they are minor
>>>>>> changes.
>>>>>> 
>>>>>> 
>>>>>> Best regards,
>>>>>> Janos
>>>>>> 
>>>>>> 
>>>>>>> On 6/12/2018 2:48 PM, Lou Berger wrote:
>>>>>>> Balázs,
>>>>>>> 
>>>>>>> Thanks for the response -- please see below.
>>>>>>> 
>>>>>>> ----------
>>>>>>> On June 12, 2018 4:07:35 AM Balázs Varga A
>>>>>>> <[e-mail]> wrote:
>>>>>>> 
>>>>>>>> Hi Lou, Thanks for the comments. See reactions inline. Document
>>>>>>>> update in progress. Cheers Bala'zs
>>>>>>>> 
>>>>>>>> -----Original Message-----
>>>>>>>> From: detnet <[e-mail]> On Behalf Of Lou Berger
>>>>>>>> Sent: 2018. június 1. 22:42
>>>>>>>> To: DetNet WG <[e-mail]>;
>>>>>>>> [e-mail]
>>>>>>>> Subject: [Detnet] Promised comments on draft-ietf-detnet-architecture
>>>>>>>> 
>>>>>>>> Hi,
>>>>>>>> 
>>>>>>>>     I have a number of high level comments on the document that I'd
>>>>>>>> like to  raise below.  I also have a number of more
>>>>>>>> editorial/specific comments that  I'd like to review directly with
>>>>>>>> the authors and then have them report back  on changes -- if any
>>>>>>>> turn out to be more substantive discussions from the  author's
>>>>>>>> perspective, I'll raise these on the list separately.
>>>>>>>> 
>>>>>>> ...
>>>>>>>> - WRT Section 4.4.3, I think the inclusion of a distributed control
>>>>>>>> plane in the "Network Plane" is inconsistent with other functional
>>>>>>>> definitions and conflates where a function resides from the actual
>>>>>>>> function and that whether control is implemented in a fully
>>>>>>>> centralized, fully distributed or some hybrid form doesn't
>>>>>>>> fundamentally change  the control function's role -- therefore I
>>>>>>>> think the sections 4.4.2 and .3 should be revised accordingly
>>>>>>>> [Balázs Varga A] Agree in principal. Let's discuss the details.
>>>>>>>> 
>>>>>>> Okay - I'll work with you off line and we can report back the
>>>>>>> results/proposed changes.
>>>>>>> 
>>>>>>>> - In several places it's not clear that DetNet service is always a
>>>>>>>> L3 service which is controlled using L3 identifiers, i.e., IP
>>>>>>>> addresses -- this is true even in the MPLS service case and the TSN
>>>>>>>> over MPLS case. I think this is an important point to be clear on in
>>>>>>>> the document.
>>>>>>>> [Balázs Varga A] I am not sure. DetNet service is always provided
>>>>>>>> over a L3 network (IP or MPLS), that is fine. However the service
>>>>>>>> itself can be L2 or L3. In case of TSN Ethernet frames are
>>>>>>>> transported, so it is a L2 service. In case of IP end systems IP
>>>>>>>> packets are transported so it is a L3 service.
>>>>>>>> 
>>>>>>> Humm - While I agree that DetNet is providing an (enhanced) L2VPN
>>>>>>> service, it is not itself providing control or service of L2 devices
>>>>>>> -- this is TSN's job.  The fact that DetNet is all about behavior of
>>>>>>> L3 nodes (i.e., IP and PW/MPLS) and not L2 nodes (i.e., TSN bridges)
>>>>>>> is something the architecture should make unambiguous.
>>>>>>> 
>>>>>>> Thanks,
>>>>>>> Lou
>>>>>>> 
>>>>>>>> Please let me know what you think.
>>>>>>>> 
>>>>>>>> Cheers,
>>>>>>>> 
>>>>>>>> Lou
>>>>>>>> 
>>>>>>>> 
>>>>>>>> _______________________________________________
>>>>>>>> detnet mailing list
>>>>>>>> [e-mail]
>>>>>>>> https://www.ietf.org/mailman/listinfo/detnet
>>>>>>>> _______________________________________________
>>>>>>>> detnet mailing list
>>>>>>>> [e-mail]
>>>>>>>> https://www.ietf.org/mailman/listinfo/detnet
>>>>>> 
>>>>>> 
>>>>>>> On 6/12/2018 6:27 PM, Stewart Bryant wrote:
>>>>>>> Dear Bala'zs
>>>>>>> 
>>>>>>> Thank you your for your consideration of these points. I will just
>>>>>>> pick up a few that need some further thought:
>>>>>>> 
>>>>>>> 
>>>>>>>>    DetNet transit node
>>>>>>>> 
>>>>>>>>           A node operating at the DetNet transport layer, that utilizes
>>>>>>>> 
>>>>>>>>           link layer and/or network layer switching across multiple
>>>>>>>> 
>>>>>>>>           links and/or sub-networks to provide paths for DetNet service
>>>>>>>> 
>>>>>>>>           layer functions. Optionally provides congestion protection
>>>>>>>> 
>>>>>>>>           over those paths.  An MPLS LSR is an example of a DetNet
>>>>>>>> 
>>>>>>>>           transit node.
>>>>>>>> 
>>>>>>>> SB> In that example it would have to be a DetNet enable/aware LSR. An
>>>>>>>> 
>>>>>>>> SB> ordinary LSR would not know anything about DetNet.
>>>>>>>> 
>>>>>>>> [Balázs Varga A] No, A DetNet aware LSR would be a relay node (S-PE).
>>>>>>>> 
>>>>>>> I think the confusion is what "DetNet Transport Layer" means. This
>>>>>>> technology touches on Transport Layer in  the L4 sense, and the
>>>>>>> Transport Network Layer as in the packet network that carries
>>>>>>> L3 packets.
>>>>>>> 
>>>>>>> So I think that we need to clarify whether a DetNet transit node
>>>>>>> is an S-PE (i.e. a a transit node in the DetNet layer), or a P node
>>>>>>> (i.e. a transit node that is carrying DetNet packets but could be
>>>>>>> carrying any sort of MPLS packet)
>>>>>>> 
>>>>>>>> ============
>>>>>>>> 
>>>>>>>>   These three techniques can be applied independently, giving eight
>>>>>>>> 
>>>>>>>>   possible combinations, including none (no DetNet), although some
>>>>>>>> 
>>>>>>>>   combinations are of wider utility than others.  This separation keeps
>>>>>>>> 
>>>>>>>>   the protocol stack coherent and maximizes interoperability with
>>>>>>>> 
>>>>>>>>   existing and developing standards in this (IETF) and other Standards
>>>>>>>> 
>>>>>>>>   Development Organizations.  Some examples of typical expected
>>>>>>>> 
>>>>>>>>   combinations:
>>>>>>>> 
>>>>>>>>   o  Explicit routes plus service protection are exactly the techniques
>>>>>>>> 
>>>>>>>>      employed by [HSR-PRP]. Explicit routes are achieved by limiting
>>>>>>>> 
>>>>>>>>      the physical topology of the network, and the sequentialization,
>>>>>>>> 
>>>>>>>>      replication, and duplicate elimination are facilitated by packet
>>>>>>>> 
>>>>>>>>      tags added at the front or the end of Ethernet frames.
>>>>>>>> 
>>>>>>>> SB> ER can be done virtually as well as physically. RSVP is a type of
>>>>>>>> 
>>>>>>>> SB> ER, as is Adj-SID based SR, and we can design other types.
>>>>>>>> 
>>>>>>>> [Balázs Varga A] Agree, but these are examples. Statement is for
>>>>>>>> HSR-PRP.
>>>>>>>> 
>>>>>>> So the question is whether we should expand the set of examples,
>>>>>>> particularly
>>>>>>> to more accessible ones.
>>>>>>> 
>>>>>>> ============
>>>>>>>>                    Packet replication and elimination
>>>>>>>> 
>>>>>>>>>>>>>>>>> relay > > > > > > > >
>>>>>>>>> /------------+ R node E +------------\ >
>>>>>>>>> /                  v + ^               \ >
>>>>>>>>      end    R +                   v | ^                + E end
>>>>>>>> 
>>>>>>>>      system   +                   v | ^                +   system
>>>>>>>> 
>>>>>>>>> \                  v + ^               / >
>>>>>>>>> \------------+ R relay E +-----------/ >
>>>>>>>>>>>>>>>>>  node > > > > > > > >
>>>>>>>> Figure 1
>>>>>>>> 
>>>>>>>>   Packet replication and elimination does not react to and correct
>>>>>>>> 
>>>>>>>>   failures; it is entirely passive.  Thus, intermittent failures,
>>>>>>>> 
>>>>>>>> SB> I think it copes with intermittent failures OK.
>>>>>>>> 
>>>>>>>> [Balázs Varga A] Yes, PRF and PEF can eliminate the effect of such
>>>>>>>> failures. But text
>>>>>>>> 
>>>>>>>> intends to say that it is passive. It is not reacting to such
>>>>>>>> failures. No change in text.
>>>>>>>> 
>>>>>>> You say that PREF does not correct failures. I would contend that is
>>>>>>> exactly
>>>>>>> what it does. Sure it does not react but it does correct, and it does
>>>>>>> address intermittent failures.
>>>>>>>> ===========
>>>>>>>> 
>>>>>>>>   transported between the peer end systems.  Therefore, the following
>>>>>>>> 
>>>>>>>>   possible types / formats of a DetNet flow are distinguished in this
>>>>>>>> 
>>>>>>>>   document:
>>>>>>>> 
>>>>>>>>   o  App-flow: native format of a DetNet flow.  It does not contain any
>>>>>>>> 
>>>>>>>>      DetNet related attributes.
>>>>>>>> 
>>>>>>>>   o  DetNet-t-flow: specific format of a DetNet flow.  Only requires
>>>>>>>> 
>>>>>>>>      the congestion / latency features provided by the Detnet transport
>>>>>>>> 
>>>>>>>>      layer.
>>>>>>>> 
>>>>>>>>   o  DetNet-s-flow: specific format of a DetNet flow.  Only requires
>>>>>>>> 
>>>>>>>>      the replication/elimination feature ensured by the DetNet service
>>>>>>>> 
>>>>>>>>      layer.
>>>>>>>> 
>>>>>>>>   o  DetNet-st-flow: specific format of a DetNet flow.  It requires
>>>>>>>> 
>>>>>>>>      both DetNet service layer and DetNet transport layer functions
>>>>>>>> 
>>>>>>>>      during forwarding.
>>>>>>>> 
>>>>>>>> SB> I find the relisting of these types confusing. Wheren't they
>>>>>>>> defined
>>>>>>>> 
>>>>>>>> SB> in the section above?
>>>>>>>> 
>>>>>>>> [Balázs Varga A] This is inline with the previous section " Grouping
>>>>>>>> of end systems ".
>>>>>>>> 
>>>>>>> Surely if we have defined them we never need to do anything but name
>>>>>>> them in
>>>>>>> later sections. Redefinition is never a good idea because it often
>>>>>>> leads to
>>>>>>> conflicting definitions.
>>>>>>> 
>>>>>>>> ============
>>>>>>>> 
>>>>>>>>   [HSR-PRP]  IEC, "High availability seamless redundancy (HSR) is a
>>>>>>>> 
>>>>>>>>              further development of the PRP approach, although HSR
>>>>>>>> 
>>>>>>>>              functions primarily as a protocol for creating media
>>>>>>>> 
>>>>>>>>              redundancy while PRP, as described in the previous
>>>>>>>> 
>>>>>>>>              section, creates network redundancy.  PRP and HSR are both
>>>>>>>> 
>>>>>>>>              described in the IEC 62439 3 standard.",
>>>>>>>> 
>>>>>>>>              <http://webstore.iec.ch/webstore/webstore.nsf/
>>>>>>>> 
>>>>>>>> artnum/046615!opendocument>.
>>>>>>>> 
>>>>>>>> SB> Not available at the time of this review, but my recollection is
>>>>>>>> 
>>>>>>>> SB> that this is not readily available without paying a large fee.
>>>>>>>> 
>>>>>>> If we decide that this is essential as a key reference, there needs to be
>>>>>>> some suitable text, as this will get raised a number of times between
>>>>>>> here an publication as first the directorates and then the ADs run
>>>>>>> into this.
>>>>>>> 
>>>>>>>> ===========
>>>>>>>> 
>>>>>>>>   [ISA95]    ANSI/ISA, "Enterprise-Control System Integration Part 1:
>>>>>>>> 
>>>>>>>>              Models and Terminology", 2000,
>>>>>>>> 
>>>>>>>>              <https://www.isa.org/isa95/>.
>>>>>>>> 
>>>>>>>> SB> Should that be 2000, or 2010.
>>>>>>>> 
>>>>>>>> SB> Note that this seems to be a very expensive document to access.
>>>>>>>> 
>>>>>>> You did not comment on the correctness of the reference.
>>>>>>> Best Regards
>>>>>>> 
>>>>>>> Stewart
>>>>> _______________________________________________
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