[Teas] Re: Late WGLC review of draft-ietf-teas-5g-ns-ip-mpls

[Adrian Farrel <[e-mail]>]

Hi Med,

 

Sorry for the delay. Thanks for posting the updates.

 

I looked at the diff between -05 (which I reviewed) and -07 (that you just posted).

 

A lot of really good changes. Many thanks. 

 

I hope the colour coding works. Please shout if it doesn’t and I’ll do something else.

 

Cheers,

Adrian

 

[Deborah] I agree with Adrian – it is not helpful for good SDO relationships to include in an IETF document (even as an appendix) a description of another SDO’s technology without requesting a review. While it may appear to be “process”, in the past, when another, unnamed, SDO made assumptions on an IETF technology, IETF was not happy. It became a very contentious technical argument. Recommend, either send a liaison (can be done in parallel with IESG review/request for publication) or remove (can enhance the current figures/terms if needed). While some have commented they found this Appendix helpful, I find it too detailed. With all the on-going architectural discussion in ORAN and 3GPP on these interfaces and components (and resulting presumptions on implementation), if want to keep, it should be scoped only to what is relevant to IETF.

 

I don’t know how we’re going to resolve this. Obviously, Deborah and I are unconvinced about including the Appendix, certainly in its current form. The chairs have called “consensus” to include the appendix. I’m a little disappointed with that call as I didn’t see the arguments in favour except “We find it helpful.”

 

[Deborah] Looking at the updated document on Adrian’s comments, I also find what Adrian commented as still not being clear in 3.3.3. To say in this document, that another TEAS working group document has shortcomings, is not a fair statement.

[Med] That’s not the intent of the text. We only explain why we don’t mention PBR or relying on source port numbers for slice identification purposes. There is nothing against the app I-D. 

[Deborah] If don’t want to include the proposals of the other document, suggest simply delete this paragraph. The paragraph above already says this document lists a few (lists few/s/lists a few).

[Med] Let’s try that and avoid spending more cycles on this.

 

I’m happy with that solution.

 

The document could really benefit from the addition of a section

called "Scalability Considerations."

 

draft-ietf-teas-nrp-scalability says...

 

[snip]

[Med] I hear the comment even if the NRP advice does not directly apply here. We added a new section about scalability implications and added new text to remind that we inherit scalability properties of current technologies. We added pointers for readers interested in such scalability assessment.

[AF] Even your choice to have just one NRP is still an NRP, and thinking about scalability is important especially as the chosen approach does have some scaling limits. So thanks for the section.

[Med] ACK.

 

Your new section is nice. Thanks.

 

3.1

 

  The term "Transport Network" is used for disambiguation with

  5G network (e.g., IP, packet-based forwarding vs RAN and CN).

  Consequently, the disambiguation applies to Transport Network

  Slicing vs. 5G End-to-End Network Slicing (Section 3.2) as well the

  management domains: RAN, CN, and TN domains.

 

I thought I understood what was meant by TN in this document

until I reached this paragraph. The previous text in 3.1 (and in

the references) seems clear as to what a TN is. This text,

however, confuses me and I can't extract anything useful from it.

After all, haven't you just explained that:

 

  Appendix B provides an overview of 5G network building blocks:

  the Radio Access Network (RAN), Core Network (CN), and

  Transport Network (TN).  The Transport Network is defined by

 the 3GPP as the "part supporting connectivity within and between

 CN and RAN parts" (Section 1 of [TS-28.530]).

 

[Med] This is still under discussion among authors.

[Med] With the updated Intro, we do think that this text is not needed anymore. So, deleted it.

 

Yup. OK.

 

Figure 5 finally makes it clear that you are trying to

distinguish a "network slice" from a "TN slice".

 

[Med] Bingo, but it is unfortunate to see that readers may find that mention too late. Updated the intro to call that out early in the doc.

[AF] Excellent, but still dangling is…

 

In practice, I think you are trying to say that the slices of the different

domains may be combined to form an end-to-end slice in the

IP/MPLS technology. This is certainly supported by 3.4.2 and is

consistent with draft-li-teas-composite- network-slices, but you

need to work out which way you are slicing (sic)

this:

 

[snip]

 

[Med] I hope this is now better articulated with the changes.

 

Yes, the new mini-paragraph just before Figure 6 is good.

 

In 3.4.2 and with reference to Figure 5, it appears that your

realisation is based on RFC 9543 Figure 1 Type 3. That's great,

could you say so somewhere early in the document? It would help.

[Med] Added a statement that the realization is based on types 3/4. 

 

Good.

 

By the time we get to Figure 6, you are talking about "slice

segments" and that is really helping because now we can

consider stitching those segments together.

 

[Med] Moved that figure to the introduction.

 

Yeah, that is a good call. Makes the reader pay attention to the architecture.

 

3.4.2

 

  In other words, the main

  focus for the enforcement of end-to-end SLOs is managed at the

  Network Slice between PE interfaces connected to the AC.

 

Would that be more clearly stated with reference to the SDP?

[Med] I think this is covered by the note about types 3/4.

 

OK

 

3.5

 

There seems to be a difference between the title of the

section...

     Mapping Schemes Between 5G Network Slices and Transport

    Network Slices

...and the first line of text

  There are multiple options for mapping 5G Network Slices to TN

  slices:

That is, the text talks about a unidirectional mapping (5G to TN)

while the title says "between".

[Med] Updated to “Mapping 5G Network Slices to Transport Network Slices” for consistency.

 

So far, so good…

 

But I think I object to the word "mapping".

While, in one

direction, the word is fine and clearly describes how one type of

slice is projected onto another type of slice, the problem is

more complicated because in the other direction (at the receiving

end of the data flow) we need to "un-map".

[Med] Why should we be concerned with that? Isn’t that part of the non-TN job?

 

This depends on whether you are simply tunnelling (“map” means which 5G slices will be carried by which TN slice) or if you are aggregating (“map” means that a set of 5G slices “become” a TN slice). As you exit the TN slice, you need to go back to processing the individual 5G slices, and that is easy in the tunnelling case. But it is more complicated to demux when the mapping does not preserve the identity of the 5G slice.

 

[snip]

 

Section 4 is pretty clear and helpful. Thanks. I think it is

where the real work of the draft begins (23 pages in). I wonder

whether we can do something to get here more quickly.

[AF] Seems like you’re not rising to this :-)

I wonder whether the introduction can steal a few lines from this section to set the document up a bit better.

[Med] Good suggestion. Moved some text around.

 

Thanks. Good.

 

In Section 6, have you invented the Filter Topology when you use

the term "transport plane"? I think you have, and it would be

helpful

either:

- to say "when we say transport plane, this is equivalent to the

   term Filter Topology defined in RFC 9542"

- to replace all mentions of "transport plane"

 

I prefer the second of these.

[Med] I'm not sure filtered topology is exactly identical to. I heard other comments that this is similar to NRP. We prefer to use a term that is close to what is currently used in deployments. For example, this is consistent with RFC9182 and several RFCs out there which include the following:

 

  'underlay-transport':  Describes the preference for the transport

     technology to carry the traffic of the VPN service.  This

     preference is especially useful in networks with multiple domains

     and Network-to-Network Interface (NNI) types.  The underlay

     transport can be expressed as an abstract transport instance

     (e.g., an identifier of a VPN+ instance, a virtual network

     identifier, or a network slice name) or as an ordered list of the

     actual protocols to be enabled in the network.

 

     A rich set of protocol identifiers that can be used to refer to an

     underlay transport are defined in [RFC9181].

[AF] Two points here:

1.       If this sounds like NRPs, then you are acknowledging multiple NRPs, which is OK but is counter to your assertion that there is a single NRP in all aspects of this document.

[Med] I wasn’t saying that I agree with that NRP comment. 

2.       The quoted text from 9182 sounds exactly like filtered topology to me

[Med] Still this can be done using the same topology. We updated the text with a new text to explain the notion of “transport plane”:

 

NEW:

A transport plane refers to a specific forwarding behavior between PEs in order to provide packet delivery that is consistent with the corresponding SLOs.

 

Well, I don’t think we are converging :-(

This is a document about IETF network slices, so it should link back to the terminology in RFC 9543. It doesn’t have to use that terminology, but it should link to it.

So, keep all your text about “transport plane” if you like (noting that ITU-T people may find this a little confusing), but let’s still try to understand where this fits in the architecture and in this document.

You have: “A network operator can define multiple transport planes.”

So, does a transport plane map to:

*	A TN slice
*	An NRP
*	A filtered topology

Re-reading, I see that the transport plane could be a collection of tunnels. That certainly sounds like an NRP. It is partitioning the links that might be selected by a filtered topology, so it isn’t a filtered topology. But it is providing connectivity mechanisms that could be used by multiple TN slices, so it isn’t a TN slice. Hence, NRP.

But the document is pretty adamant that “The realization model described in this document uses a single Network Resource Partition (NRP) (Section 7.1 of [RFC9543]).  The applicability to multiple NRPs is out of scope.” So why talk about multiple transport planes?

 

[snip]