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

[Elwyn Davies via Datatracker <[e-mail]>]

Reviewer: Elwyn Davies
Review result: Not Ready

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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.  
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.  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.

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.

Firstly, I assume that this section covers the 'additional techniques'
mentioned 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.

Secondly, AFAICS these techniques are basically about measuring and
communicating  delay jitter in various ways.  It might be helpful to
link what is being offered here with RFC 5481 and the discussion of
delay variation measurement in RFC 6374.  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

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?  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.

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?

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.

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

s1, bullet 4:  Would it be helpful to refer to RFC  7190 with respect to
aggregation?

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

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!]

s3, last sentence: s/packet/packets/

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

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

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

s5, para 2: s/are relatively independent/are made relatively independent/

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

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

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

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

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

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.

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

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?

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

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

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.

s9: Expand GAL on first use.

s9.1: Expand FEC on first use.

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

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.

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'.)

s13: I am not sure I can identify the relevant issue in s5.

s14.2: s/request/requested/

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