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

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Elwyn, thanks for your review. Stewart, thanks for your response. I entered a No Objection ballot as it seems the major issues have been corrected or clarified. However, Elwyn, it would be good if you can reply to Stewart with any remaining comments.

Thanks,
Alissa


On Feb 10, 2021, at 11:26 AM, Stewart Bryant <stewart.bryant@xxxxxxxxx> wrote:



On 2 Feb 2021, at 15:27, Elwyn Davies via Datatracker <noreply@xxxxxxxx> 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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