Re: [Detnet] WG Last Call: draft-ietf-detnet-architecture-05

["Black, David" <[e-mail]>]

Hi Janos,

That text works nicely.  In briefly talking to Lou, a related suggestion surfaced: “congestion protection” -> “congestion loss protection” where possible (may need to be abbreviated in the Figure) .

This change would make it clearer that the primary focus is protection from losses caused by congestion as opposed to protection from all congestion.  DetNet should be able to tolerate some modest congestion (e.g., in nodes that don’t have reserved DetNet resources) as long as delivery is reliable and timely.

Thanks, --David

From: Janos Farkas [mailto:[e-mail]]
Sent: Wednesday, July 18, 2018 11:45 AM
To: Black, David
Cc: DetNet WG
Subject: RE: [Detnet] WG Last Call: draft-ietf-detnet-architecture-05

Hi David,

This implies updating the definition of the DetNet service layer, e.g., to:
DetNet service layer
The layer at which A DetNet Service, e.g., service protection is provided.

The definition of the DetNet transport layer remains the same:
DetNet transport layer
The layer that optionally provides congestion protection for
DetNet flows over paths provided by the underlying network.

Regards,
Janos


From: Black, David <[e-mail]<mailto:[e-mail]>>
Sent: Monday, July 16, 2018 3:21 PM
To: Janos Farkas <[e-mail]<mailto:[e-mail]>>
Cc: DetNet WG <[e-mail]<mailto:[e-mail]>>
Subject: RE: [Detnet] WG Last Call: draft-ietf-detnet-architecture-05

Don’t use “congestion protection” in the definitions for both levels.

Thanks, --David

From: János Farkas [mailto:[e-mail]]
Sent: Sunday, July 15, 2018 10:43 PM
To: Black, David
Cc: DetNet WG
Subject: Re: [Detnet] WG Last Call: draft-ietf-detnet-architecture-05

Thank you David!

I think we are set. :-)

You got a point, we should update
"DetNet service layer is the layer at which A DetNet Service, such as congestion or service protection is provided."
to
"DetNet service layer is the layer at which A DetNet Service, such as congestion protection and/or service protection is provided."
in the definitions as well to make it clear.

Thanks,
Janos
On 7/16/2018 3:03 AM, Black, David wrote:
Moving the only two open items to the top …

-- Section 4

> We could add some introductory text to 4.1 or 4.1.1, for instance:
> The functionality needed for DetNet (Section 3) are implemented in the DetNet layer which comprises the DetNet service layer and the DetNet transport layer.
> The DetNet service layer functions rely on flow identification and sequencing. The DetNet transport layer function may rely on flow identification.
>
> What do you think?

I prefer functional (what it does) characterization to mechanistic (how it works) characterization in an overview, e.g., excerpting from what I originally wrote:

·       The DetNet Service layer provides timely reliable in-order DetNet flow delivery to applications that use DetNet.

·       The DetNet Transport layer insulates DetNet flows from interference by network mechanisms, primarily queuing and routing
I understand the principle.

What about just copying the two definitions here:
DetNet service layer is the layer at which A DetNet Service, such as congestion or service protection is provided.
DetNet transport layer optionally provides congestion protection for DetNet flows over paths provided by the underlying network.

David> Yes, that works, although I don’t think “congestion” was meant to be included in the first definition …


   Many real-time networks rely on physical rings or chains of two-port

   devices, with a relatively simple ring control protocol.

How does a “ring control protocol” control “chains”?  Suggest: “ring control protocol” -> “control protocol”
Also, are paired chains between common endpoints intended, or a more general notion of chains?
This text refers to typical factory deployment, which is chain or ring.
It seems better to keep “ring control protocol” because they are often very different from generic control protocols applicable to mesh networks.

Would flipping chain and ring resolve your concern?

   Many real-time networks rely on physical chains or rings of two-port

   devices, with a relatively simple ring control protocol.

[David>] It feels like “ring control protocol” is an important phrase to retain.   In the original text, would it be reasonable to add “(applies to both rings and chains)” to the end of the sentence?
Well, I guess the point is that there are many specific ring protocols, but we are looking for a generic solution.
What about deleting "chains" from the sentence?

David> Yes, if the sentence is all about rings, it’s clear.

Thanks, --David

From: János Farkas [mailto:[e-mail]]
Sent: Sunday, July 15, 2018 7:25 PM
To: Black, David
Cc: DetNet WG
Subject: Re: [Detnet] WG Last Call: draft-ietf-detnet-architecture-05

Hi David,

Please see in-line.

Thank you!
Janos
On 7/6/2018 11:50 PM, Black, David wrote:
-- Bridged path definition

>  Or should we do something else?
> There is only one occurrence of "bridged path" in the text in brackets.

That sounds like a better suggestion – change that occurrence and remove the definition of “bridged path” from the draft.  That occurrence is in Section 4.1.1:

   Congestion protection
           The DetNet transport layer provides congestion protection.
           See Section 4.5.  The actual queuing and shaping mechanisms
           are typically provided by underlying subnet layers, these can
           be closely associated with the means of providing paths for
          DetNet flows (e.g., MPLS LSPs or bridged paths), the path and
           the congestion protection are conflated in this figure.

Perhaps: (e.g., MPLS LSPs or bridged paths) -> (MPLS LSPs or other network paths)
or just delete the entire parenthetical example.
OK. Let's just delete the entire parenthetical example.


-- Section 4

> We could add some introductory text to 4.1 or 4.1.1, for instance:
> The functionality needed for DetNet (Section 3) are implemented in the DetNet layer which comprises the DetNet service layer and the DetNet transport layer.
> The DetNet service layer functions rely on flow identification and sequencing. The DetNet transport layer function may rely on flow identification.
>
> What do you think?

I prefer functional (what it does) characterization to mechanistic (how it works) characterization in an overview, e.g., excerpting from what I originally wrote:

  *   The DetNet Service layer provides timely reliable in-order DetNet flow delivery to applications that use DetNet.
  *   The DetNet Transport layer insulates DetNet flows from interference by network mechanisms, primarily queuing and routing
I understand the principle.

What about just copying the two definitions here:
DetNet service layer is the layer at which A DetNet Service, such as congestion or service protection is provided.
DetNet transport layer optionally provides congestion protection for DetNet flows over paths provided by the underlying network.



> Should we also rearrange, e.g., indent the text after Figure 2 such that
> Packet sequencing, Duplicate elimination, Flow replication, Flow merging, Packet encoding, and Packet decoding get under Service layer
> and
> Congestion protection and Explicit routes get under Transport layer?

No, I don’t think that’s necessary – Figure 2 provides sufficient structure.
OK.


Plus a few more minor comments inline …

Thanks, --David

From: János Farkas [mailto:[e-mail]]
Sent: Wednesday, July 4, 2018 2:15 PM
To: Black, David
Cc: DetNet WG
Subject: Re: [Detnet] WG Last Call: draft-ietf-detnet-architecture-05

Hi David,

Thank you very much for your review and comments!

Please see in-line.
On 6/29/2018 4:10 PM, Black, David wrote:
The -06 version of this draft is much improved over the -05 version – lots of diligent work by the authors is evident and appreciated.

I have two minor content concerns and some smaller editorial items.

** Content

-- Section 2.2

The definition of “bridged path” looks like a definition of “bridged forwarding” – the definition needs to encompass the notion of a path that involves more than one bridge.


   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.

You are right, it looks like the emphasis is on bridge forwarding rather than path.
However, the superposition of the outbound ports provides a path, which was tried to be captured by "hence the path".
The idea was to keep it simple not dive into the details, which are not necessary in this document.

Would it be good to update to:

   bridged path

           A VLAN bridge uses the VLAN ID and the destination MAC

           address to select the outbound port. The superposition

           of the outbound ports is the path for a frame.
?

Or should we do something else?
There is only one occurrence of "bridged path" in the text in brackets.


-- Section 4

Figure 2 in Section 4.1.1 introduces the Detnet service and transport layers.   The associated text quickly dives into explaining the functionality in those layers, but it would be helpful to start with an initial high level description of each of those layers that helps motivate their separation.  It appears that:
- The DetNet Service layer provides timely reliable in-order DetNet flow delivery to applications that use DetNet.   A DetNet flow at the top of the DetNet Service layer may or may not be a compound flow at the bottom of that layer.
- The DetNet Transport layer insulates DetNet flows (at the bottom of the DetNet Service layer) from interference by network mechanisms, primarily queuing and routing, where congestion is an indication of queueing interference.  A DetNet flow at the top of the DetNet Transport layer may or may not be a member flow that is part of a compound flow, depending on what is done in the DetNet Service layer.

You are right, section 4 jumps into the details without much of an introduction. I guess we had a kind of chicken and egg problem here. Figure 4 provides a higher level view than Figure 2. However, Figure 4 captures different aspects, dives into the details of data plane options. So, it seems better to keep the order.

We could add some introductory text to 4.1 or 4.1.1, for instance:
The functionality needed for DetNet (Section 3) are implemented in the DetNet layer which comprises the DetNet service layer and the DetNet transport layer. The DetNet service layer functions rely on flow identification and sequencing. The DetNet transport layer function may rely on flow identification.

What do you think?

Should we also rearrange, e.g., indent the text after Figure 2 such that
Packet sequencing, Duplicate elimination, Flow replication, Flow merging, Packet encoding, and Packet decoding get under Service layer
and
Congestion protection and Explicit routes get under Transport layer?




There’s a related paragraph of text between Figures 3 and 4 in Section 4.1.2, but it doesn’t provide useful intuition about which functionality belongs in which layer.

** Editorial


-- Section 2.1

“expected” seems off in the definition of reservation:

   reservation
           The set of resources allocated between a source and one or
           more destinations through transit nodes and subnets
           associated with a DetNet flow, to provide the expected DetNet
           Service.

Suggest either “to provide the provisioned DetNet Service to that flow”
I like this one, will update to this one.


or “to provide the specified DetNet Service to that flow.”

-- Section 3.1


   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.

Is that a sequence bound or a time bound or both?   I suspect that a sequence bound (maximum degree of reordering in a flow, independent of flow rate) was intended.  An analogous clarification would be helpful in section 3.2.2.1 .
It is intentionally kept high level in the architecture draft. Details are to be provided by the flow information model draft. Please see my previous mail: https://mailarchive.ietf.org/arch/msg/detnet/AWG2eemg_AWsvF2FwtREyd_xtC8.

[David>] OK.

-- Section 3.2.3

I don’t understand “defined” in:


   Even the use of

   redundant paths through a network defined, e.g., by [RFC6372] do not

   eliminate the chances of packet loss.

Was “e.g., as defined by [RFC6372],” intended?
Yes, this was the intention, will update to this one.


   Many real-time networks rely on physical rings or chains of two-port

   devices, with a relatively simple ring control protocol.

How does a “ring control protocol” control “chains”?  Suggest: “ring control protocol” -> “control protocol”
Also, are paired chains between common endpoints intended, or a more general notion of chains?
This text refers to typical factory deployment, which is chain or ring.
It seems better to keep “ring control protocol” because they are often very different from generic control protocols applicable to mesh networks.

Would flipping chain and ring resolve your concern?

   Many real-time networks rely on physical chains or rings of two-port

   devices, with a relatively simple ring control protocol.

[David>] It feels like “ring control protocol” is an important phrase to retain.   In the original text, would it be reasonable to add “(applies to both rings and chains)” to the end of the sentence?
Well, I guess the point is that there are many specific ring protocols, but we are looking for a generic solution.
What about deleting "chains" from the sentence?



   Explicit routes can be established various ways,

established various -> established in various
OK.




   This is

   irrespective of the distribution method used, also applies to service

   protection over explicit routes.

used, also -> used, and also
OK.



-- Figure 5 (Section 4.1.3)

The UNI acronym is introduced without expansion or definition – please correct that, e.g., by defining UNI in Section 2.1.
DetNet-UNI is there among the definitions. It seems enough to me.

[David>] That’s fine – I overlooked that definition, sorry.

-- Figure 9 (Section 4.7.2)

I think I understand what’s going on, but the use of “add/remove” in the figure should be explained in associated text.
What about updating the sentence above Figure 9 to:
A packet with corresponding Flow-IDs is illustrated in Figure 9, which also indicates where Flow-ID can be added or removed.

[David>] Sure, with one minor edit: “where Flow-ID” -> “where each Flow-ID”
OK.


Thanks, --David

Thank you!
Janos




From: detnet [mailto:[e-mail]] On Behalf Of János Farkas
Sent: Thursday, June 28, 2018 9:09 PM
To: Lou Berger; Stewart Bryant; [e-mail]<mailto:[e-mail]> >> DetNet WG
Subject: Re: [Detnet] WG Last Call: draft-ietf-detnet-architecture-05

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]><mailto:[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]><mailto:[e-mail]> On Behalf Of Lou Berger
Sent: 2018. június 1. 22:42
To: DetNet WG <[e-mail]><mailto:[e-mail]>; [e-mail]<mailto:[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


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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