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

Lizhong Jin<> Tue, 19 June 2012 13:49 UTC

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Hi Elwyn,
Thank you for the review. It seems you are reviewing v-03, not v-04, but 
some comments are for v-04. I am confused. See my reply inline below.


Elwyn Davies <> wrote 2012/06/19 20:12:17:

> I am the assigned Gen-ART reviewer for this draft. For background on 
> Gen-ART, please see the FAQ at 
> <>. 
> Please resolve these comments along with any other Last Call comments 
> you may receive. 
> 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
> rather than just clumsy.
[Lizhong] We have two editors, and both Raymond and I are editing this 
document. Hope the below reply will make it clear.
> Apologies for the late submission of the review.
> 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.

> 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.

> 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.

> 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. SHOULD means highly recommended. We do 
not meet any problem when implementing RFC6073. Do you suggest to change 
this to MUST? 

> Nits/editorial comments: 
> General: In RFC 4447, the label message names use title case (e.g.,
> Label Mapping).  For consistency this should be followed throughout this
> document.
[Lizhong] yes, and thank you and Adrian.
> Abstract:  Should not contain references.  Best to give full title of
> RFC and number in round brackets.
[Lizhong] I removed brackets and do not intend to contain citations here. 
It seems the citations are generated automatically by IETF. Could the 
round brackets solve this? Or are you referring v-03?

> Abstract: s/problem of control/ the problem with control/ (if there is
> just one)
[Lizhong] are you reviewing v-03 of the draft? In v-04, the abstract has 
been revised in v-04.

> Abstract: The second two sentences say the same thing in different ways 
>    Based on the problem analysis, a
>    message exchanging mechanism is introduced to solve this control word
>    negotiation issue.  This document is to update [RFC4447] control word
>    negotiation mechanism.
> Based on the problem analysis, this document introduces a modified 
> message exchange 
> sequence updating the control word negotiation mechanism in RFC 4447.
> This issue also applies to s1.
[Lizhong] are you reviewing v-03 of the draft? In v-04, the abstract has 
been revised in v-04.

> s1: Subtle distinction - this is a practical problem with the 
> mechanism defined in RFC 4447 rather 
> than the abstract problem of how to do control word negotiation:
>  s/the problem of control word negotiation/problems identified in 
> the control word negotiation/
[Lizhong] has been revised in v-04.
> s1: Expand acronym PW.
[Lizhong] has been revised in v-04.
> s2: Expand acronym PE.
[Lizhong] accept

> s2, 2nd sentence: s/configurable/configured/
[Lizhong] configurable is right.

> 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.

> s2, bullet #2: s/PE1 send label mapping with C bit=0 finally./ultimately
> PE1 sends a Label Mapping message with the C bit set to 0./
[Lizhong] accept
> s2, bullet #4: s/send label withdraw/send a Label Withdraw/
[Lizhong] accept
> s2, bullet #5: s/the received/the previously received/, 
> s/indicates C bit=0/carried the C bit set to 0/, 
> s/send label mapping with C bit=0/send a label mapping message with the
> C bit set to 0/
[Lizhong] accept
> s3: It would be much clearer if s3 was divided into 3 sub-sections
> (possibly reordered):
> - 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.

> s3, Various acronyms need expanding: FEC, T-PE, S-PE
[Lizhong] accept
> s3, para 1: s/adding label request/adding a new label request/
[Lizhong] much difference here?
> s3 bullets '-i' to '-iii': s/Local PE/The local PE/, s/remote PR/the
> remote peer PE/.  Also a more usual labeling of the bullets would be
> appropriate (e.g., just i, ii and iii).
[Lizhong] accept, and I do not see "remote PR".
> s3, para 2: s/When Local/When the local/,
> s/the remote label mapping message with C bit=0, additional procedure
> will be added as follow:/a label mapping message from the PW peer with
> the C bits set to 0, the following additional procedure will be acrried
> out:/
[Lizhong] accept
> s3, bullet '-i': s/wait until receiving a label release/wait until it
> has received a Label Release message/ (but general clarification and
> rewording needed - see issues section above).
[Lizhong] do not see much difference here. Of couse the "label release" 
should be capitalized.
> s3, bullet '-ii':
> OLD:
>       Local PE MUST send a label request message to remote PE, and
>       wait until receiving a label mapping message containing the remote
>       PE locally configured preference for use of control word.
> NEW:
>       The local PE MUST send a Label Mapping message to the peer PE with 
>       C bit set to 1, and then wait until a Label Mapping message 
>       containing the peer's current configured preference for usage of 
>       control word.
[Lizhong] no, the procedure here is not right. The local PE MUST send a 
label request message, not label mapping.
> s3, para sfter item '-iii': s/successfully/has successfully/
[Lizhong] accept

> The following implies that there is memory of previous action although
> the label binding has been destroyed. Better would be:
> OLD:
> >    ...and removed the remote label binding, it MUST reset
> >    its use of control word with the locally configured preference, and
> >    send label mapping as a response of label request with locally
> >    configured preference for use of control word.
> NEW:
> ...and removed the remote label binding, subsequent Label Requests will
> naturally be treated as new requests and processed as described in
> Section 6 of RFC 4447.
[Lizhong] we got similar comments from Adrian, and will update this.

> s3, discussion of multi-segment PWs: s/case for T-PE is same./case for a
> T-PE operates similarly./
[Lizhong] accept
> 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.

> s3.1: The wording in this section is generally  rather 'loose'.  A more
> formal style would be helpful.
[Lizhong] this is a usecase to give an example, so we make it "loose".