Re: [mpls] Comments on draft-andersson-mpls-mna-fwk-01

"Dongjie (Jimmy)" <> Mon, 30 May 2022 04:18 UTC

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From: "Dongjie (Jimmy)" <>
To: Tony Li <>
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Thread-Topic: Comments on draft-andersson-mpls-mna-fwk-01
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Date: Mon, 30 May 2022 04:18:37 +0000
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Subject: Re: [mpls] Comments on draft-andersson-mpls-mna-fwk-01
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Hi Tony,

Please see inline with [Jie]:

From: Tony Li [] On Behalf Of Tony Li
Sent: Sunday, May 29, 2022 12:51 AM
To: Dongjie (Jimmy) <>
Subject: Re: Comments on draft-andersson-mpls-mna-fwk-01

Hi Jimmy,

1.      Concepts and terminologies

-        Network Action vs forwarding action. Since this document introduces the concept “network actions”, and it says network action is more general than forwarding action, I’m not sure the concept “forwarding action” needs to be defined here, actually it is only mentioned in the introduction. I’d suggest to just introduce one new concept (network action) instead of two.

Simply repeating yourself is not constructive.

[Jie] I was highlighting the major remaining comments by categorizing them into a few classes in email. Hope this is more efficient to WG participants who didn’t have time to check the doc file.

-        Ancillary data. The major question is: are the network actions without further parameters considered as ancillary data? This was recorded as one of the comments to the requirement document, and this document shows that inconsistency understanding of this basic concept could result in different interpretation of other terminologies. Thus I’d suggest the DT and the WG to have further review and discussion about the definition and the scope of ancillary data.

A network action is an abstract concept, so it is never considered data.  This is why we have Network Action Indicators.  NAI are not ancillary data.

[Jie] There are network actions which don’t have any associated data and only need an indicator in the packet. According to the ancillary data definition, such indicator also belong to ancillary data (data relating to the MPLS packet that may be used to affect the forwarding or other processing of that packet). Or do you want to modify the definition of ancillary data to exclude the network action indicators?

-        Network Action Sub-stack Indicator (NSI) and MNA label. These terms are used to refer to the indicator of the network action sub-stack. However, there is no clear text about whether they can indicate the existence of PSD or not. In the framework a general term for the indicator of the ancillary data (either ISD or PSD) is needed.

That’s because they’re orthogonal to PSD.  The indication of PSD is an NAI for a network action that requires PSD.  Similarly, the indication of ISD is an NAI for a network action that requires ISD.

[Jie] What you said is just one solution to indicate the existence of PSD. The indication of PSD may not rely on the NAI for specific network actions. It is suggested the framework document be generalized to allow different solutions of indicating the PSD.

2.      Considerations about ISD and PSD in the framework

According to the recent DT and mail list discussion, it seems the consensus is that the framework and solution need to include the mechanism for carrying PSD. Whether ISD is needed in the framework and the specific solutions is still under discussion. Thus it is suggested the framework document align with the DT’s discussion on this point: have some text to indicate that PSD is the necessary component of the framework. For the text about ISD, it is suggested to indicate ISD is an optional component, and for some solutions ISD may not be used.

PSD is not required in every packet.  A solution that may chooses to support network actions that require PSD.  However, we also have solutions (e.g., Bruno’s) that choose NOT to support PSD.

[Jie] Neither ISD nor PSD is required in every MPLS packet. From the miad use cases we collected so far, it is clear that some types of network actions need to be carried using PSD. Thus PSD is required in the mna framework to meet the requirement of all the use cases. A solution document may choose to only support a subset of the use cases.

The framework currently requires that NAI appear in stack.  I have sent a request to the mailing list (below) to see if the group would be willing to generalize this, but to date, only John Drake has responded.  You have responded in this email, but you have not given a clear answer.

[Jie] My personal opinion is that the framework can be generalized on the positioning of the NAI.

3.      Changes to MPLS forwarding/processing

The potential changes introduced by MNA to MPLS architecture is not only in the data plane encoding, but also in the forwarding and processing behaviors. This is especially the case for the processing of the ISD data. IMO this is one of the most important things the framework should cover. There is a placeholder section on the development of MPLS forwarding model, the processing of the indicator, the ISD and/or PSD data based on MPLS forwarding model needs to be specified.

Again, repetition does not improve understanding.

The section on the MPLS forwarding model is currently part of the editorial attic.  IMHO, it adds no value and will be discarded.

We can expect a solution to describe how its particular encoding is to be processed. This is not something that can be dealt with by the framework.

As of right now, there are no architectural bounds on a network action. If someone wants to define a network action to compute the Nth digit of pi, that’s 100% acceptable.

[Jie] The changes to MPLS forwarding model is architectural and not just related to a specific solution or network action. For example, the introduction of NAS to MPLS label stack requires additional processing at the ingress node, and the forwarding (in terms of label stack manipulation) at some transit nodes could be different from traditional label swap or pop.  Such changes to MPLS forwarding needs to be described in the document where NAS is introduced to MPLS.

4.      Incorporation of related existing work

One of the comments I made is that draft-song-mpls-eh-indicator describes the alternatives of the indicator of the extension header. Please note that most of that text is about general analysis and comparison, and is not specific to any solution (i.e. not PSD only). Thus incorporation of such text would be helpful to this framework document.

Again, the framework is not an analysis or survey document.

[Jie] Maybe I was not accurate enough with the above statement, my reading of the content in draft-song-mpls-eh-indicator is mainly the description and summary of the alternatives of the indicator, thus some of which could fit into section 3.1 of the mna framework.

Best regards,


Another existing work which may be incorporated is draft-andersson-mpls-eh-architecture, which describes the architecture of MPLS extension header. Some of the text in that document may be reused for the description of the PSD part, and some text may even be generic for both ISD and PSD.

Hope this helps.

Best regards,

From: Tony Li [] On Behalf Of Tony Li
Sent: Friday, May 27, 2022 1:50 AM
To: Dongjie (Jimmy) <<>>
Subject: Re: Comments on draft-andersson-mpls-mna-fwk-01

Hi Jimmy,

Thank you for your comments.  I’ve replied to each and every one of them.  As you commented inside of a Word document, I’ve replied in kind.  Please see the attached document.

I have made a number of the changes that you’ve suggested.  I will send a separate post to the list with the updated document and a diff.

Many of the changes that you suggested hinge on a single question which you did not raise directly.  That question should have the input of the broader group, so I’ll raise it explicitly now:

        Currently the framework document implicitly precludes some of the mechanisms found in draft-song-mpls-eh-indicator.
        Should the framework draft be broadened to encompass this draft?

Speaking personally (co-author hat off), I don’t see an architectural reason to disagree. Please don’t take this as support of the draft.  I still believe that this is a sub-optimal approach, but could it work architecturally?  I have to say that yes, it could, and thus the framework draft could and should be broadened to encompass this.

If the group agrees with this, the changes to implement this are minimal and straightforward.

Could I please get input from the group?


> On May 25, 2022, at 7:34 AM, Dongjie (Jimmy) <<>> wrote:
> Dear authors,
> Thanks for the effort in organizing and updating this document. I've reviewed the current version and have a number of comments and suggestions.
> In my view, the most important thing for this framework document is to describe the possible changes introduced to the MPLS architecture, the MPLS data plane encoding, and the processing behaviors of the LSRs.
> Please find the attached file for the details of my review notes.
> Hope it helps and looking forward to your feedback.
> Best regards,
> Jie
> <draft-andersson-mpls-mna-fwk-01_Jie Dong_0525.docx>