Re: [Gen-art] [PWE3] Gen-art last call review of draft-ietf-pwe3-cbit-negotiation-04

[Alexander Vainshtein <[e-mail]>]

Elwyn, Stewart and all,

I do not see any technical issues with the proposed solution for PREFERRED --> NOT PREFERRED configuration change in draft-ietf-pwe3-cbit-negotiation.
To the best of my understanding, it does not really require any additions to the procedures defined in RFC 4447 and would work like following:

1. I assume that originally the two PEs have agreed to use the CW. (If they didn't because the remote PE did not want to support it, there is nothing to re-negotiate).
2. Local PE (where the configuration of the PW endpoint has changed):
	- Sends a Label Withdraw message for its original PW label mapping (where C-bit has been set)
	- Waits for the Label Release acknowledging this Label Withdraw message as mandated by 5036
	- Sends a Label Mapping message with the C-bit cleared.
3. Remote PE, upon receiving the new Label Mapping with the C-bit cleared does what RFC 4447 says, namely:
	- Withdraws its original Label Mapping with C-bit 
	- Waits for Label Release acknowledging this withdrawal
	- Sends a new Label Mapping message with C-bit cleared to the Local PW.
4. At this moment re-negotiation is complete.

This said, adding an explicit description to the text could be useful. But IMHO this is an editorial issue, not a technical one.

My 2c,
     Sasha


> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]] On Behalf
> Of Stewart Bryant
> Sent: Thursday, June 21, 2012 4:56 PM
> To: Elwyn Davies
> Cc: [e-mail]; General Area Review
> Team; Lizhong Jin; pwe3; [e-mail]
> Subject: Re: [PWE3] Gen-art last call review of draft-ietf-pwe3-cbit-
> negotiation-04
> 
> Bringing this to the attention of the PWE3 WG
> since that need to know that this discussion is happening
> and will have a view as to what is already well
> known in terms of RFC4447 behavior and what
> needs clarification via this draft.
> 
> Stewart
> 
> On 21/06/2012 13:38, Elwyn Davies wrote:
> > Hi, Lizhong.
> >
> > I have removed the bits that we are agreed on to shorten the mail a bit.
> >
> > I don't think we have converged just yet.
> >
> > Regards,
> > Elwyn
> >
> > On Wed, 2012-06-20 at 01:17 +0800, Lizhong Jin wrote:
> >> Hi Elwyn,
> >> Thank you for the prompt reply. See inline below.
> >>
> >> Lizhong
> >>
> >>
> >> Elwyn Davies <[e-mail]> wrote 2012/06/19 23:16:08:
> >>>>> Document: draft-ietf-pwe3-cbit-negotiation-04.txt
> >>>>> Reviewer: Elwyn Davies
> >>>>> Review Date: 19 June 2012
> >>>>> IETF LC End Date: 15 June 2012
> >>>>> IESG Telechat date: 21 June 2012
> >>>>>
> >>>>> Summary:
> >>>>> Not ready. The proposal for the NOT PREFERRED to PREFERRED
> >>>> transition
> >>>>> case does not appear to be compatible with the existing RFC 4447
> >>>>> standard in the way stated and there are a number of other minor
> >>>> issues.
> >>>>> The draft is also in need of an editing pass by an author whose
> >>>> mother
> >>>>> tongue is English as there are parts where the syntax is misleading
> >>>>> Major issues:
> >>>>> s3: The discussion of the NOT PREFERRED to PREFERRED transition
> case
> >>>>> (buried at the end of s3 - causing me to ask 'what about this case?'
> >>>>> during reading of s2 and most of s3)  implies that some existing RFC
> >>>>> 4447 procedure applies. Clearly, if the PW is not using the control
> >>>> word
> >>>>> then there is nothing to do.  On the other hand, inspection of s6.2
> >>>> of
> >>>>> RFC4447 indicates that once the two PEs have agreed on c = 1, 'setup
> >>>> is
> >>>>> complete' and Label Mapping messages would therefore be
> >>>>> 'unexpected' (see item '-i' in second set of bullets in s6.2 of RFC
> >>>>> 4447). So, what procedure is to be used? And what implications does
> >>>> this
> >>>>> have for backwards compatibility?  Wouldn't it be generally simpler
> >>>> to
> >>>>> apply the PREFERRED to NOT PREFERRED mechanism to all case?
> >>>> [Lizhong] this draft is to solve the case to change a PW from c=0 to
> >>>> c=1, that means one PE should change its use of control word from
> NOT
> >>>> PREFERRED to PREFERRED. RFC4447 is already there and deployed, and
> the
> >>>> PE will not always send its locally configured preferrence according
> >>>> RFC4447, see PE1 behavior in section 2 step 1&2. That's why we could
> >>>> not simply apply the mechanism at the end of section 3. Hope it is
> >>>> clear.
> >>> There are two issues here:
> >>> - Clarity: RFC 4447 does not have any discussion of what might happen if
> >>> the configuration value changes.  The draft focusses on on transition
> >>> direction but does not mention the other except for one paragaph right
> >>> at the end of s3.  It is therefore reasonable that somebody looking at
> >>> this draft would wonder 'what about the other transition direction?'.
> >>> Whether or not anything needs to be done it would save people
> wondering
> >>> if you put in a sentence in the introduction to explain that the other
> >>> direction is not (or is) a problem.
> >> [Lizhong] accept. Add following:
> >> When PE changes the preference for the use of control word from
> >> PREFERRED to NOT PREFERRED, it should follow [RFC4447], and there is
> >> no problem.
> > See below...
> >>> - Technical: The paragraph in s3 implies that the PREFERRED to NOT
> >>> PREFERRED direction will result in some part of the RFC 4447 protocol
> >>> being (re-)invoked.  What part is not made very clear.  As far as I can
> >>> see sending more messages other than Label Release or Label Withdraw
> for
> >>> this PW is not part of the RFC 4447 protocol and hence will cause an
> >>> error.  Alternatively if it isn't reinvoked, the PW will continue to use
> >>> control words.  Please explain what is going on here.  The fact that RFC
> >>> 4447 has been deployed doesn't explain what is going on.
> >> [Lizhong] now I understand you concern. The last sentance of s3 is not
> >> clear. How about the following:
> >> In that case, local PE will always send Label Withdraw message if
> >> already sending Label Mapping message, and then send new Label
> Mapping
> >> message with C-bit value following the procedures defined in
> >> [RFC4447].
> > I think we have reached the heart of the issue here:  RFC 4447 does not
> > mention changing the control word configuration.  The combination of the
> > existing text in the draft and what you have just weittrn above
> > indicates that *in both transition directions* the PE changing
> > configuration notifies its peer by sending a Label Withdraw. It is not
> > immediately obvious to me that you could deduce this from RFC 4447 - if
> > it is then there needs to be a pointer to the relevant section in RFC
> > 4447 to enlighten the ignorant like me.  Otherwise the PREFERRED to NOT
> > PREFERRED transition is *not* just 'follow the RFC 4447 procedures'.
> >
> > If I now understand correctly, the situation is as follows:
> > - in both cases when there is a control word preference transition, send
> > a Label Withdraw.
> > - in the NOT PREFERRED to PREFERRED case send Label Release also (order
> > not important), wait for a responses from peer, then send Label Request
> > and negotiate use of control word using Label Mapping C bit as for a new
> > PW (might result in either using or not using control word depending on
> > state in peer).
> > - in the PREFERRED to NOT PREFERRED case, just negotiate C bit to 0
> > using Label Mapping - label will be maintained. [Need to check that RFC
> > 4447 is expecting/will allow this sequence.]
> >
> > If I have this straight, I think that the PREFERRED to NOT PREFERRED
> > case needs some more words in s3 and an introduction in s2. (And it
> > would be easier to understand with this case in a separate sub-section -
> > sorry to harp on about this).
> >
> >>>>> Minor issues:
> >>>>> s3: Has there been any discussion on possible race conditions?
> >>>>   Changing
> >>>>> the configuration value during the message exchange strikes me as
> >>>>> dangerous - it is probably sufficient to note that changes should be
> >>>>> suppressed during the Label Mapping message exchange but I am not
> >>>>> totally sure about this.
> >>>> [Lizhong] The message would be processed in sequence in TCP-based
> LDP
> >>>> session. I do not see a problem here. But we do have a note in section
> >>>> 3 for multi-segment PW for sequence processing.
> >>> There are several network round trips in the message exchanges.  The
> >>> protocol and the configuration mechanism in a PE are potentially
> >>> separate threads.  There is scope, depending on the implementation, for
> >>> the user at the 'remote' PE to change the configuration during the
> >>> message exchange making for a potential race condition.
> >> [Lizhong] we discussed the race condition on the PWE3 maillist from
> >> Spike, and for multi-segment PW, we add a note to ensure the sequence
> >> processing for implementation.
> > I look forward to the revised text.
> >>
> >>
> >>>>> s3, bullet '-i':  I completely misparsed this section on first
> >>>> reading
> >>>>> and I am still not absolutely sure what message sequence is being
> >>>>> specified.  Working back from later sections I *think* that the
> >>>>> intention is:
> >>>>> IF Mapping sent THEN { send Withdraw; send Release;}
> >>>>> Wait to receive Release
> >>>>> The implication at present is that a Mapping might not have been
> >>>> sent
> >>>>> and then only a Release is needed: is this a possibility? Please
> >>>>> clarify.
> >>>>> The picture in Appendix A suffers from the same problem.
> >>>> [Lizhong] Adrian raise the same comment, and I change it as below:
> >>>>         -i.  PE MUST send a Label Release message to remote PE. If a
> >>>> PE
> >>>>              has previously sent a Label Mapping message to a remote
> >>>> PE,
> >>>>              it MUST also send a Label Withdraw message to the remote
> >>>> PE,
> >>>>              and wait until it receives a Label Release message from
> >>>> the
> >>>>             remote PE.
> >>> What does it send first if both must be sent?  The text in the rest of
> >>> the document implies withdraw then release.  This new text sort of
> >>> implies release then withdraw but isn't really clear.
> >> [Lizhong] both should be sent, and does not require the sending
> >> sequence. The two messages does not have dependence.
> > In that case, it is important to say so.
> >>>>> s3, discussion of multi-segment PWs: The statement that S-PE's
> >>>> SHOULD
> >>>>> assume an initial passive role seems to have several problems:
> >>>>> - Does this mean that changing the configuration of an S-PE would
> >>>> not
> >>>>> provoke the new mechanisms?
> >>>>> - The passive role situation is only specified for some sorts of
> >>>> linked
> >>>>> FECs in RFC 6073 - what about other cases?
> >>>>> - What are the consequences for ignoring the SHOULD in this case? (I
> >>>>> have to say I am unsure that RFC 6073 deals with this problem
> >>>> either.)
> >>>> [Lizhong] we follow RFC6073, and passive role is only applied for the
> >>>> PW FEC, other cases are out of scope.
> >>> Why? This needs an explanation. Also this doesn't cover the point about
> >>> whether reconfig of an S-PE is allowed and how this squares with the
> >>> passive role.
> >>>> SHOULD means highly recommended. We do not meet any problem
> when
> >>>> implementing RFC6073. Do you suggest to change this to MUST?
> >>> Whenever a specification includes a 'SHOULD' the question arises of
> what
> >>> alternatives there might be and what is the reason for preferring the
> >>> suggested solution.  In the original setup, it is fairly clear that life
> >>> is easier letting setup progress from  one end to the other (although
> >>> this is not spelt out - I would have argued for this had  I reviewed the
> >>> doc).  For the configuration change this is less obvious.  Regarding the
> >>> point about reconfig of the  S-PE, it is going to make life more
> >>> complicated if the S-PE has to tell the T-PE to be active so it can be
> >>> passive.  I would therefore argue that probably the notion of passivity
> >>> is irrelevant to the transition use case.  Whether it should be SHOULD
> >>> or MUST is therefore moot.
> >> [Lizhong] how about the following? Because we just refer to RFC6073,
> and does not add anything new.
> >> An initial passive role is defined in [RFC6073] for S-PE.
> > I can't see that this really helps.
> > There are still two points at issue here:
> > - Why are the cases other than the one in RFC 4447 that refers to a
> > passive S-PE out of scope?  There aren't any statements to this effect
> > in the document.
> > - It is difficult for a S-PE that has its control word configuration
> > change to act passively.  If it is really passive than nothing will
> > happen until the label is withdrawn.  Otherwise it can't be descibed as
> > passive for this action.  The text in the current draft actually
> > describes what is specified in RFC 6073 when talking 'passive' rather
> > than explaining what happens when the control word configuration
> > changes.
> >>
> >>>> [Lizhong] has been revised in v-04.
> >>> Not as far as I can see.
> >> [Lizhong] write "pseudowire" directly in the text, not PW.
> > There are lots of instances of PW still left in v04. Expand acronym =>
> > define what it means on first usage i.e pseudowire (PW).  Same for the
> > various other acronyms that aren't defined in the text.
> >>>>> s2: Expand acronym PE.
> >>>> [Lizhong] accept
> >>>>
> >>>>> s2, 2nd sentence: s/configurable/configured/
> >>>> [Lizhong] configurable is right.
> >>> Leave that one to the RFc Editor.
> >>>>> s2, 3rd and 4th sentences: I *think* this text is trying to say:
> >>>>>    The intention of the control word negotiation is that the control
> >>>> word
> >>>>> will be used when both endpoints are configured with control word
> >>>> usage
> >>>>> PREFERRED.  However if one endpoint is initially configured with
> >>>> control
> >>>>> word usage NOT PREFERRED but later changes to PREFERRED, a PW
> >>>> between
> >>>>> the endpoints will not transition to usage of the control word as
> >>>>> explained below.
> >>>> [Lizhong] no, the case is that operator deploys PW with control word
> >>>> used in the first phase. In the second phase, they want to upgrade
> >>>> their PW service to use control word. Two different deployment
> >>>> timeframes.
> >>> That is what my sentence says.
> >> [Lizhong] ok.
> >>
> >>>>> s3: It would be much clearer if s3 was divided into 3 sub-sections
> >>>>> (possibly reordered):
> >>>>> - PREFERRED to NOT PREFERRED transition
> >>>>> - NOT PREFERRED to PREFERRED transition
> >>>>> - Multi-segment case (which should refer to both previous cases)
> >>>>> The pointer to the diagram in Appendix A could usefully occur in the
> >>>>> introduction to s3.  If this was adopted s3.1 could either be a
> >>>> fourth
> >>>>> sub-section or a sub-sub-section of the PREFERRED to NOT
> PREFERRED
> >>>>> section.
> >>>> [Lizhong] PREFERRED to NOT PREFERRED does not introduce any new.
> >>>> Multi-segment case is fully inherited from single-segment case. It
> >>>> would be redundancy to have sub-sections here.
> >>> I disagree strongly.  The multi-segment case affects the other RFC and
> >>> needs to be made to stand out so that implementors can see where the
> >>> changes occur.  The PREFERRED to NOT PREFERRED case also ought to
> be
> >>> separated whether or not I am right about this case.
> >> [Lizhong] how about only divide multi-segment? The case of PREFERRED
> to
> >> NOT PREFERRED case is only for reference, and the text is not many.
> > That is so, but it is a change of subject and the fact that it will then
> > be in the table of contents makes it easier for implementors to find the
> > relevant text.  I wouldn't call it 'redundant'.
> >
> >>>>> s3, last para/Appendix A:  The diagram doesn't cover the PREFERRED
> >>>> to
> >>>>> NOT PREFERRED transition.
> >>>> [Lizhong] no, there is a "no" branch under "NOT Preferred to
> >>>> Preferred" to cover "PREFERRED to NOT PREFERRED". Anyway, the
> diagram
> >>>> discription should be "NOT Preferred to Preferred" which is wrong in
> >>>> v-04.
> > Need to check it reflects the Label Withdraw message needed in the
> > PREFERRED to NOT PREFERRED case.
> >
> >
> >
> 
> 
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