Re: [PWE3] WG Poll on draft-jin-pwe3-cbit-negotiation-03.txt ( comment )

[Luca Martini <[e-mail]>]

Sorry for the late response , comments below:
Luca


On 03/03/11 08:41, [e-mail] wrote:
>
> Luca,
> After discussing with the authors, we decide to add the wait for label
> release before sending label request, this wouldn't harm and bring
> some secure.
>
> Then are you OK with the following text?
>
> When Local PE changes its control word from NOT PREFERRED to PREFERRED
> and only if it already received the remote label mapping message with
> C-bit=0, additional procedure will be added as follow:
> -i Local PE MUST send a label withdraw message to remote PE if it has
> previously sent a label mapping, and wait until receiving a label
> release from the remote PE.
> -ii Local PE MUST send a label request message to remote PE, and wait
> until receiving a label mapping message containing the remote PE
> configured control word setting.
> -iii After receiving remote PE label mapping with control word
> setting, Local PE MUST follow procedures defined in [RFC4447] section
> 6 when sending it's label mapping message.
>
Ok the above text is fine.
However you need to address the case when the Local PE changes its
control word from  PREFERRED to NOT PREFERRED as well.


> Correspondingly, the modified procedure of PE in figure 1 will be as
> follows:
>
> 1. PE2 changes locally configured control word to PREFERRED.
> 2. PE2 will then send label withdraw message to PE1.
> 3. PE1 MUST send label release in reply to label withdraw message from
> PE2.
> 4. Upon receipt of Label release message from PE1, PE2 MUST send label
> request messages to PE1 although it already received the label mapping
> with C-bit=0.
> 5. PE1 MUST send label mapping message with C-bit=1 again to PE2
> (Note: PE1 MUST send label mapping with locally configured CW parameter).
> 6. PE2 receives the label mapping from PE1 and updates the remote
> label binding information. PE2 MUST wait for PE1 label binding before
> sending its label binding with C-bit set, only if it previously had a
> label binding with C-bit = 0 from PE1.
> 7. PE2 will send label mapping to PE1 with C-bit=1.
>
No, the above procedure will result in a LDP error and session shutdown.
In LDP you  cannot "update" the FEC.  In step 5 PE1 can only send the
label mapping message it has previously sent. So PE1 cannot change the
C-Bit without first issuing a label withdraw.

> It is to be noted that the above assume that PE1 is configured to
> support CW, however in step 5 if PE1 doesn't support CW, PE1 would
> send the label mapping message with C-bit =0, this would result in PE2
> in step 7 sending a label mapping with C-bit=0 as per [RFC4447] CW
> negotiation procedure.
>
What is needed is some way to send a message to the remote PE that the
C-bit negotiation procedure needs to be re-started.

Solution 1

one way to get this procedure working is to have PE2 release the label
mapping previously sent by PE1.
If you do this today, most implementations will simply go to a dead
state requiring operator intervention.
What should happen is that upon receiving the label release , PE1 should
withdraw it's label mapping , and reset the c-bit negotiation state
machine , and re-advertise the new label mapping restarting the
procedure described in rfc4447 from the beginning.

Solution 2

Whenever a PE withdraws a FEC mapping , the remote PE resets the C-bit
negotiation procedure. If this results in a different C-bit local value
then previously advertised , the local PE MUST go though a label
withdraw/label release/label advertisement cycle.


Both solutions above should work within the  LDP rules and are backward
compatible. I like solution 2 a bit better.

Luca

> Thanks
> Lizhong
>
>
> ------------send by Luca
> Martini-------------------------------------------
> Matthew & Andy,
>
>
> Although I support clarifying this procedure , I believe this document
> needs some work.
> In section 3 for example :
>
>    When Local PE changes its control word from NOT PREFERRED to
>    PREFERRED and only if it already received the remote label mapping
>    message with C-bit=0, additional procedure will be added as follow:
>
>          -i. Local PE sends label withdraw message to the remote if it
>              already sends label mapping message, for it has changed its
>              control word parameter.
>
>         -ii. Local PE MUST send a label request messages to peer PE to
>              get peer's configured control word parameter before sending
>              new label mapping message to peer PE.
>
>        -iii. After receiving the new label mapping message from peer PE
>              and updating the remote label binding information, the
>              Local PE should send label mapping to peer PE according to
>              procedures defined in [RFC4447].
>
> What does the item i) mean ? the English is not clear.
> Why does the PE have to do ii) ?
>
> All we really need is a statement on how the c-bit negotiation state
> machine can be reset if one end has changed "his mind" .
>
> Can we fix this document before  issuing the -00 WG draft version ?
>
> Thanks.
> Luca
>  
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