Subject: Re: [PWE3] Missing operation in draft-ietf-pwe3-static-pw-status-00

From: Luca Martini <[email-masked]>
Date: Tue, 08 Jun 2010 12:55:35 -0600

[email-masked] wrote:
>
> Hi Luca,
> For the static PW that using G-ACH to signal PW status, when the PW is 
> locally deleted by operator, it is necessary for the PE to send a PW 
> status TLV with the PW status code "0x00000001 - Pseudowire Not 
> Forwarding" to the adjacent PW on the static PW segment. This 
> operation is missing in the draft, and should be explicitly described.
What do you mean by "Deleted" ?
If the PW is un-provisioned, then you should probably send a label 
withdraw, not a status message.
If the PW is just taken out of service , then the above status code is 
correct.

>
> What's more, as I suggested at IETF 77 Anaheim, when LDP session is in 
> graceful restart state and can not be used for PW status signaling, it 
> is also valuable to for this PW to signal PW status through G-ACH. To 
> be clear, PW status signaling through G-ACH will only be enabled when 
> LDP session is in graceful restart state.
>
Currently Graceful restart for LDP has not been designed to support PWs. 
That does not mean it cannot be designed, to do so , but it might be 
quite detrimental to use graceful restart. For example , let's suppose 
that a PE reboots unexpectedly, in the PW case , the remote PE can 
switch immediately to a backup PW , and provide minimal service 
disruption. If we have graceful restart , the PE would have to wait for 
the expiration time to pass , which would cause a significant service 
interruption.

We cannot use the GACH just because the LDP session is in GR state. IT 
ould make sense that the GACH communication channel would not be 
operational as well.

What I think we need to do is write a separate document on how Graceful 
restart of the LDP or more in general other control protocols is to be 
handled by the PW.

Let me start by pointing out that LDP is not a controlling protocol in 
the case of a simple IP FEC. Hence the procedures for graceful restart 
in rfc3478 cannot be used as stated for PW FECs.
There was a draft (draft-jiang-pwe3-ldp-gr-00 
<https://datatracker.ietf.org/doc/draft-jiang-pwe3-ldp-gr/>) last year 
that attempted to approach this topic, but failed.

I believe that we need to answer several questions , for example:

Is Graceful restart even useful for PWs ?
How do we avoid extended traffic interruptions if we want to support 
Graceful restart for the PW FECs ?
Maybe we can just support a planned restart instead ?
Since LDP is the control protocol for PWs, but does not normally control 
the destination of IP packets, are the procedures in rfc3478 even going 
to work properly ?
Graceful restart is supposed to allow for faster convergence in an IP 
network, however do we even care about fast switch back of PWs ? ( in 
most cases local policy does nto even allow PWs to switch back to the 
main PW after a service interruption. )

Luca

> Best Regards
> Lizhong Jin
> --------------------------------------------------------
> ZTE Information Security Notice: The information contained in this mail is solely property of the sender's organization. This mail communication is confidential. Recipients named above are obligated to maintain secrecy and are not permitted to disclose the contents of this communication to others.
> This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the originator of the message. Any views expressed in this message are those of the individual sender.
> This message has been scanned for viruses and Spam by ZTE Anti-Spam system.
>

---

From: Greg Mirsky <[email-masked]>
Date: Tue, 8 Jun 2010 14:45:32 -0700

Dear Luca,
I agree, that GR for PW LDP as well as utilization of G-ACH to send PW
Status wouldn't address scenarios when a PE failed. But it might and, I
think, will improve stability of the network and services if only Control
plane of a PE failed and NSF, separation of CP and DP are in place. To
address your concern regarding extended service un-availability when LDP GR
is in effect I think that CV/CC would provide verification of the Data Plane
for particular PW and should trigger PW switchover if the failure is in the
Data plane.
And I'd note that the goal of GR is not faster convergence but avoidance of
convergence when that is possible, preserving stability of the Data plane.

Regards,
Greg

On Tue, Jun 8, 2010 at 11:55 AM, Luca Martini <[email-masked]> wrote:

>> locally deleted by operator, it is necessary for the PE to send a PW status
>> TLV with the PW status code "0x00000001 - Pseudowire Not Forwarding" to the
>> adjacent PW on the static PW segment. This operation is missing in the
>> draft, and should be explicitly described.
>>
>> graceful restart state and can not be used for PW status signaling, it is
>> also valuable to for this PW to signal PW status through G-ACH. To be clear,
>> PW status signaling through G-ACH will only be enabled when LDP session is
>> in graceful restart state.
>>
> That does not mean it cannot be designed, to do so , but it might be quite
> detrimental to use graceful restart. For example , let's suppose that a PE
> reboots unexpectedly, in the PW case , the remote PE can switch immediately
> to a backup PW , and provide minimal service disruption. If we have graceful
> restart , the PE would have to wait for the expiration time to pass , which
> would cause a significant service interruption.
>
> We cannot use the GACH just because the LDP session is in GR state. IT ould
> make sense that the GACH communication channel would not be operational as
> well.
>
> Let me start by pointing out that LDP is not a controlling protocol in the
> case of a simple IP FEC. Hence the procedures for graceful restart in
> rfc3478 cannot be used as stated for PW FECs.
> There was a draft (draft-jiang-pwe3-ldp-gr-00 <
> https://datatracker.ietf.org/doc/draft-jiang-pwe3-ldp-gr/>) last year that
> attempted to approach this topic, but failed.
>
> the destination of IP packets, are the procedures in rfc3478 even going to
> work properly ?
> network, however do we even care about fast switch back of PWs ? ( in most
> cases local policy does nto even allow PWs to switch back to the main PW
> after a service interruption. )
>
> Luca
>
>
>
>  Best Regards
>> Lizhong Jin
>> ZTE Information Security Notice: The information contained in this mail is
>> solely property of the sender's organization. This mail communication is
>> confidential. Recipients named above are obligated to maintain secrecy and
>> are not permitted to disclose the contents of this communication to others.
>> This email and any files transmitted with it are confidential and intended
>> solely for the use of the individual or entity to whom they are addressed.
>> If you have received this email in error please notify the originator of the
>> message. Any views expressed in this message are those of the individual
>> sender.
> _______________________________________________
> pwe3 mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/pwe3
>