Re: [Gen-art] [mpls] Genart last call review of draft-ietf-mpls-rfc6374-sfl-08

> On 2 Feb 2021, at 15:27, Elwyn Davies via Datatracker <noreply@ietf.org> wrote:
> 
> Reviewer: Elwyn Davies
> Review result: Not Ready
> 
> I am the assigned Gen-ART reviewer for this draft. The General Area
> Review Team (Gen-ART) reviews all IETF documents being processed
> by the IESG for the IETF Chair.  Please treat these comments just
> like any other last call comments.
> 
> For more information, please see the FAQ at
> 
> <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
> 
> Document: draft-ietf-mpls-rfc6374-sfl-??
> Reviewer: Elwyn Davies
> Review Date: 2021-02-02
> IETF LC End Date: 2021-01-26
> IESG Telechat date: Not scheduled for a telechat
> 
> Summary:  Not Ready.  Apologies that this review is rather late, but I found this 
> document extremely hard to work with.  There appear to be a number of areas where 
> the work is rather too much in progress rather than ready for publication as an RFC.  

That was some old text from an early version that was missed.

> I also found it very difficult, not just as someone who is not at all familiar with this
> area of work, but at a basic technical level to work out what the protocol was going 
> to be able to achieve and whether a LSR would garner the information it appeared 
> to need to deliver what was clamed.  

This is a document where you need to understand MPLS, coloured marking and  packet delay characteristics, but I think anyone seeking to deploy this would already be familiar with that.

> Part of this appeared to be due to multiple 
> names being used for the same thing and being used with other than their natural 
> meaning particulaly in sections 7.1 and 7,2. 
> 
> Major Issues:
> 
> s7, What is being standardized?:
> 
>>    A number of methods are described.  The expectation is that the MPLS
>>    WG possibly with the assistance of the IPPM WG will select one or
>>    maybe more than one of these methods for standardization.
>> 
> I find this statement very confusing.  This document is intended for
> standards track, so if it goes ahead as is, the three methods are
> standardised and implementors would be expected to provide support for
> all of them unless there are to be words to indicate that not all need
> to be supported.   Is this the intention? Or is it that this document
> should only support the methods chosen by the MPLS working group?  In
> the latter case, this document is definitely not ready for
> standardization; I assume the unused method(s) would be removed in this
> case.  Otherwise the second sentence is speculative and should be removed.

I have changed this text in response to other comments received:

It now says 

A number of methods are described. Each of these methods has different
characteristics and different processing demands on the packet forwader.
The choice of method will depend on the type of diagnostic that the operator seeks. 

> 
> s7, Title, purpose and general method:
> 
> Note that I have very limited knowledge of this area of performance
> measurement so there may be misunderstandings here. However, given that
> caveat, I did not find the document very helpful in enlightening me and
> a considerable amount of background reading was needed to try and
> determine what was going on.

It is always difficult to get the balance right between a concise document for subject matter experts and a detailed description.

> 
> Firstly, I assume that this section covers the 'additional techniques'
> mentioned in the Abstract

That term does not seem to be in the abstract.

>  and described as 'more sophisticated
> measurements' in s1. [Perhaps common phraseology would be desirable
> between the two cases.]  I suggest a sentence to make this clear would
> be desirable.

I am afraid I cannot see the conflict that you are concerned about.

> 
> Secondly, AFAICS these techniques are basically about measuring and
> communicating  delay jitter in various ways.  

SB> No, Method 1 is measuring jitter. Method 2 is measuring delay as is Method 3

> It might be helpful to
> link what is being offered here with RFC 5481 and the discussion of
> delay variation measurement in RFC 6374.  

SB> I think we need to assume that the reader is familiar with RFC6374

> Section 7.1 is, as I
> understand it, covering IPDV measurement using (in general) normal
> service packets rather than just specialised RFC 6374 packets and
> working primarily on one LSR.  I assume that the technique in s7.2 is
> primarily a means for reporting measurements derived from s7.1 and/or
> s7.4, but given that actual delays are mentioned rather than
> inter-packet gaps, the

SB> All measurements take place on user service data using SFL to 
SB> indicate different groups of packets. We are using RFC6374 to
SB> trigger measurements and collective results.

> 
> s7.1: After the first sentence, the first paragraph talks about delay. 
> Since the receiving LSR has no knowledge of the transmission time of
> each individual packet, it is not possible for the LSR to calculate
> actual delays without additional information - I take it that the
> packets are not intended to be RFC 6374 Delay Measurement Packets as
> these would require corresponding responses which would contravene the
> query interval setting  and there does not appear to be a way for the
> LSR doing the measurements to be told the inter-packet transmission
> interval.  Should this be written in terms of inter-packet gaps rather
> than delays throughout?  

SB> 7.1 is measuring the inter packet gaps so as you say is measuring
The variation in the delay rather than the absolute delay. However this
Is made clear in the text.

> Further, The first paragraph describes two
> methods of operation without saying which one should be standardised or
> AFAICS providing a selection flag to allow either to be used.

SB> We could do that but there is a need for the operator to configure 
SB> other characteristics of the measurement, for example the size of the 
SB> time increments that the buckets represent, so this would just be another
SB> such characteristic. The math in the analytics engine to convert one
SB> method into the other is trivial (the difference in the techniques is
SB> about collection hardware optimisation) so I don’t think we need to
SB> pick one,

> 
> It seems to me that an outline of how this facility might be used is
> pretty much essential.  Would I be right in thinking that to measure the
> delay jitter between a source LSR (S) and destination LSR (D), the
> operator decides to send a set of packets at equally spaced intervals
> from S to D and decides on the interval and the number of packets.  S
> then issues a Query setting the query interval to a time greater than
> that needed to send the  set of packets and using the Bucket Jitter
> Measurement Message to set the bucket delay intervals around the
> sending  interval according to the Operator's expectations of the
> network.  D then sets up to measure the inter-packet delays up until the
> next Bucket Jitter Measurement message arrives after the elapse of the
> query interval when D returns  the profile of inter-packet delays.
> 
> Does the arrival of this second Bucket Jitter Measurement Message
> trigger a further set of measurements?  And if so, how is the sequence
> ended?

SB> No, you send packets in one color then you change color and then
SB> send an Query message and the response refers to the set of packets
SB> before the colour changed.

SB> The hardware continuously makes the measurement and the 
SB> measurement system collects the results when it wants a test result.

> 
> s9.1: This section is headed by an Editor's Note saying that the section
> needs review which may alter the format of the TLV.  It is thus
> impossible to say if this section is ready.

SB> That is a note I had forgotten to remove

> 
> Minor Issues:
> 
> s7.2: As with s7.1, there seems to be some confusion bettween delay and
> inter-packet gap.
> 
> Nits/editorial comments:
> 
> Abstract:  The primary purpose of this document, as set out in s1, is to
> extend RFC 6374 to cover general MPLS networks rather than primarily
> MPLS-TP networks and in particular to add support for
> multi-point-to-point LSPs.  I think that it would be helpful for the
> casual reader to make this somewhat clearer in the abstract.  I suggest:
> 
> OLD:
> 
>   This document describes a method of making RFC6374 performance
>   measurements on flows carried over an MPLS Label Switched path.  This
>   allows loss and delay measurements to be made on multi-point to point
>   LSPs and allows the measurement of flows within an MPLS construct
>   using RFC6374.
> NEW:
> 
>   RFC 6374 describes methods of making loss and delay measurements on
>   Label Switched Paths (LSPs) primarily as used in MPLS TransportProfile (MPLS-TP)
>   networks.  This document describes a method of making RFC6374 performance
>   measurements on flows carried over general  MPLS LSPs.  In particular, it extends
>   the technique to  allow loss and delay measurements to be made on multi-point to point
>   LSPs and introduces some additional techniques to allow more sophisticated
>   measurements to be made in both MPLS-TP and general MPLS networks.
> 
> ENDS

SB> Thank you that is a good proposal
> 
> s1, bullet 4:  Would it be helpful to refer to RFC  7190 with respect to
> aggregation?

SB> Yes, I will add the ref.

> 
> s1, bullet 5: s/counter again/counter, again/

SB> Fixed

> 
> s3, last sentence: s/co-responding/corresponding/ [co-responding means
> responding together rather than matching.  Look up co-respondent in
> cases of adultery in the divorce courts!]
> 
SB> Thanks. Co-responding seems like a good term to get into a protocol description.

> s3, last sentence: s/packet/packets/
> 
SB> Fixed

> s4, para 1: Expand TC: s/TC/Trafic Class (TC)/
> 
SB> Fixed

> s5, para 1: s/proxy data service packets Section 4./proxy data service
> packets (see Section 4)./
> 
SB> fixed

> s5, para 2: s/This it is/Thus it is/
> 

SB> Fixed

> s5, para 2: s/are relatively independent/are made relatively independent/
> 
SB> Text fixed

> s5, para 3: s/arises for the potential/arises from the potential/
> 
SB> Fixed

> s5, para after Figure 1: s/were/where/

SB> Fixed

> s5, next to last para: s/which ever/whichever/

SB> Fixed 

> 
> s6, para 1: s/measurement type/measurement types/;
> s/combination/combinations/
> 

SB> Fixed

> s7: I assume these are the additional facilities mentioned in the
> Introduction.  It would be helpful to make this clear.

SB> Text added

> 
> s7.1, para after Figure 2:  The acrronyms QTF, RTF, RPTF and DS should
> be expanded.  There is no section 3.7 in RFC 6374.  These items are
> defined in Section 3.2.

SB> Sorry Typo - thanks - fixed.

> 
> s7.1: The formats of the various numerical fields are not specified.  I
> assume they are unsigned integers.

SB> Yes, note added

> 
> s7.1, Number of Buckets:  I assume that an LSR is likely to have a limit
> for this value.  If the query requests an unsupported amount should
> there be a specific error code?
> 

         0x1A: Error - Resource Unavailable.  Indicates that the
         operation failed because node resources were not available.

SB> Would be the normal error message

> s7.3: s/In other that exception/In other than exceptional/

SB> Fixed

> 
> s7.4: The formats for the time fields in the and the Sum of Timestamps
> field are not specified.

SB> The subject of the various timestamp formats is discussed in RFC6374.

> 
> s8, first sentence: I am unable to parse 'a delay measurement interval
> defined by an SL of constant colour' before being introduced to RFC
> 8321.   Even then I don't know what SL stands for - it is not used in
> RFC 8321 or RFC 6374.

SB> That should be SFL

> 
> s9: Expand GAL on first use.

SB> Done

> 
> s9.1: Expand FEC on first use.

SB> Done
> 
> s9.1, para 2: Where is the concept of well-defined array of SFLs defined?
> 

SB> I have added the following text:

              Multiple SFLs can be assigned to a FEC each
	      with different actions. This index is an optional
	      convenience for use in mapping between the TLV
	      and the associated data structures in the LSRs.

> s9.1, Specification of FEC field: 'This is encoded as per Section 3.4.1
> of TBD'...  Er, there doesn't seem to be a reference for TBD.
> 

SB> Fixed

> s10: 'A future version of the *this document*...'  Is this a sign of
> unfinishedness or an indication that further documents will address this
> issue? (apart from the 'the this'.)

SB> Text removed - it was old text

> 
> s13: I am not sure I can identify the relevant issue in s5.
> 
SB> It should have pointed to the privacy section - fixed.

> s14.2: s/request/requested/
SB> Done

> 
> s14.2, RFC Editor note:  I presume the RFC Editor should be asked to
> delete two lines - the ones before and after the request.

SB> I have changed it to para. It is a markdown device to include something referenced in a figure. 
> 
> 
> 
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