Re: Static VPLS MAC withdrawal draft

[Sami Boutros <[e-mail]>]

Hi Sasha,

>> 
>> Thanks for your comments, Please see responses inline..
>> 
>>> 
>>> 
>>>> -----Original Message-----
>>>> From: Alexander Vainshtein
>>>> Sent: Thursday, May 10, 2012 5:08 PM
>>>> To: '[e-mail]:'; Sami Boutros ([e-mail]);
>>>> '[e-mail]'
>>>> Cc: [e-mail]; 'Giles Heron'; Rotem Cohen; Andrew Sergeev;
>> Mishael
>>>> Wexler; Gideon Agmon
>>>> Subject: RE: Static VPLS MAC withdrawal draft
>>>> 
>>>> Hi all,
>>>> I've read the draft in question and I would like to clarify several issues
>> that
>>>> look either undefined or ambiguous to me.
>>>> 
>>>> 1. To the best of my understanding, the draft requires each PE
>> participating
>>>> in a given VPLS instance to set up two counters for each static PW
>>>> connecting it to a peer PE device in this VPLS instance: one for static MAC
>>>> withdrawal messages it is going to send and another - for static MAC
>>>> withdrawal messages it receives. Is this understanding correct?
>>>> 
>> 
>> Sami: This is correct.
>> 
>>>> 2. Assuming a positive answer to the previous questions, how are these
>>>> counters initialized? The draft seems to be moot on this point.
>> 
>> Sami: This is what the draft has for setting up the sequence # in section 3.
>>   Only half of the sequence number space is used. Modular arithmetic
>>   is used to detect wrapping of sequence number. When sequence number
>>   wraps (i.e., when it becomes 0), all MAC addresses are flushed and
>>   the sequence number is reset.
>> Sami: Can you please explain what's vague in the above?
> [[[Sasha]]]  This does not say too much about initialization (at least, not to me).
> And it would be nice if the draft was more specific here.

Sami: Agreed, we need to add the initialization value to the text.
 
>> 
>>>> The more
>>>> interesting question is, of course, about initialization of the counter of
>>>> received static MAC withdrawal messages, since, with static PWs,
>> generally
>>>> speaking, there is no synchronization between setting up one of its end
>>>> points and setting up another one.
>> 
>> 
>> Sami: Initially the rx sequence # can be assumed to be 0.
> [[[Sasha]]] But you've said that Rx number 0 results in flushing all MAC addresses?

Sami: Good point, but this is why the receiving PE should always flush on receiving 0.
Sami: The Transmitter PE would then increment the sequence # and subsequent 
Sami: Transmission will be higher than 0. 

>> 
>>>> 
>>>> 3. The draft specifies that:
>>>> 	- The transmitter sends the MAC withdrawal message with the same
>>>> sequence number until it receives an ACK for it (Section 4.1.1)
>>>> 	- The receiver, after having acknowledged a MAC withdrawal
>>>> message, ignores refresh messages with the same sequence number
>>>> (Section 4.1.2)
>>>> What is supposed to happen in the following scenario:
>>>> 	- Transmitter finds out that some MACs have to be withdrawn (e.g.,
>>>> because the AC from which they have been learned fails)
>>>> 	   and sends a MAC withdrawal message to its peers with sequence
>>>> number N
>>>> 	- Immediately after that (i.e., before it receives an ACK for this
>>>> message) it finds out that yet another set of MAC addresses has to be
>>>> withdrawn
>>>> 	- Retransmission timer expires without ACK received.
>>>> 
>> 
>> Sami: A new sequence # will be transmitted that will include both sets of
>> MAC addresses
>> Sami: that need to be flushed, and the transmitter will wait for an ack for
>> this new sequence #.
>> Sami: We can add this to the draft.
> [[[Sasha]]] First of all, I think something like that MUST be added to the draft.
> And I am not sure what you say is really OK: consider the case when ACK for the first withdrawal message is received after the 2nd message has been sent.
> If one of the ACKed (and withdrawn) MAC addresses has been then re-learned they will be flushed again…

Sami: Yes, this will lead to deleting again, and then the macs will be learnt again, keep in mind that we are talking about using a mac list (not an empty mac list), and deleting 2 sets of macs and we didn't Sami: get the ack for the 1st set in time before we ask to delete the 2nd set. I would think this should be ok, however am open to better schemes.


>> 
>>>> 4. Suppose that only one end point of a static PW between a given pair of
>>>> peers has been set up. In this case transmitting the static MAC
>> withdrawal
>>>> messages via such a PW is possible, but ACKs will never be received.
>> Should
>>>> retransmission of these messages go on "ad infinitum"?
>>>> 
>> 
>> Sami: Yes, this will be the case, the mechanism will be no different then the
>> static PW status one for this case.
> [[[Sasha]]] But ACK is not mandatory in the PW status message 9indeed, it is not clear if it is ever needed).
> Here you may end with multiple retransmitting messages that are sent to a black hole.

Sami: Wouldn't that be TRUE too for PW status messages? however is there a scheme you have in mind to solve this?


Thanks,

Sami
>> 
>>>> 5. The draft does not specify any default value for the retransmit time.
>>>> 
>> 
>> Sami: Sure will add.
>> 
>>>> 6. A nit:  Heading of Section 4 is followed by headings for Sections 4.1.1
>> and
>>>> 4.1.2 without any heading for 4.1.
>>>> 
>> 
>> Sami: Sure will fix.
>> 
>>>> Hopefully these questions will be useful.
>>>> 
>> 
>> Thanks,
>> 
>> Sami
>> 
>> 
>>>> Regards,
>>>>    Sasha
>>>> 
>>>>> -----Original Message-----
>>>>> From: [e-mail] [mailto:[e-mail]] On
>> Behalf
>>>>> Of Giles Heron
>>>>> Sent: Tuesday, May 08, 2012 10:06 PM
>>>>> To: [e-mail]
>>>>> Subject: Static VPLS MAC withdrawal draft
>>>>> 
>>>>> Hi everyone,
>>>>> 
>>>>> The following draft was presented at IETF in Paris:
>>>>> 
>>>>> http://tools.ietf.org/html/draft-boutros-l2vpn-mpls-tp-mac-wd-00
>>>>> 
>>>>> The authors have asked for it to be adopted as a WG draft, but at this
>> stage
>>>>> the chairs' view is that it probably hasn't had enough sets of eyes on it.
>>>>> 
>>>>> So this email is to request that those on the list take a read of the draft
>>>>> and send comments back to the list.  Depending on the response we get
>>>> we
>>>>> may
>>>>> then ask the WG if they feel happy adopting this as a WG draft.
>>>>> 
>>>>> Nabil and Giles
>>>>> 
>>> 
>>> 
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> 
> 
> This e-mail message is intended for the recipient only and contains information which is CONFIDENTIAL and which may be proprietary to ECI Telecom. If you have received this transmission in error, please inform us by e-mail, phone or fax, and then delete the original and all copies thereof.
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