Subject: Re: [PWE3] VCCV usage

From: Lizhong Jin <[email-masked]>
Date: Mon, 15 Nov 2010 00:03:00 +0800

Hi Sasha,
If CW + TTL expiry is used for segmented PW OAM, I wonder why we also call
the TTL expiry here as VCCV type 3 (although I know this is described in
RFC5085). This is the TTL expiry function which should be supported by all
standard MPLS forwarding box. And when the node receives packets with TTL =
1 and CW first nibble = 0, the packets should be discarded, and receives
packets with TTL = 1 and CW first nibble = 1, the packets should be sent to
control plane. This kind of TTL expiry function should be a default function
of S-PE for all kinds of labels.
Then if CW is mandatory, VCCV type3 is not necessary to be required. It
should be consider it as a default function in MS-PW case.

BR
Lizhong

 ------------------------------

Yaakov, Sri and all,
I am not sure why going to a different MPLS network in Yaakov's
example would result in replacing a single-segment PW with a
multi-segment one. IMO protection would probably happen at the tunnel
level, not at the PW level.
And in any case, the ingress PE would be aware of the change because
the PW label it transmits would change (it has been allocated by the
remote T-PE before the switch, but after the switch it is allocated by
the nearest S-PE).

I would further notice that as long as we are dealing with SS-PWs
(which indeed represent most of the current PW deployments), VCCV Type
3 using TTL = 1 in the PW label entry looks safe enough (I do not
think we should worry about non-standard equipment that does not check
TTL in the label stack entries). It is only MS-PWs that are
potentially problematic when it comes to leaking VCCV packets to ACs.
Hence I would like us to be internally consistent among themselves:
either we do not care about MS-PWs - and hence VCCV Type 3 is
sufficient, or we want to cover MS-PWs - and then segment VCCV is
important. One use case that comes to mind is propagation of PW status
across a MS-PW comprised of both static and dynamically signaled
segments.

As for original Yaakov's question - whether the ingress PE "knows"
with which case it deals, and in the case of a MS-PW, how many
segments there are: IMO this is a very important question indeed. One
way to answer it would be IMO by defining MS-PW extensions to the PW
MIB. According to the WG Charter this was supposed to be ready in July
2009, but I have not found any signs of related activity.
Another answer (actually suggested by Yaakov) would be to use trace
route for MS-PWs as described in the Segmented PWs draft.

Regards,
     Sasha

-----Original Message-----
From: pwe3-bounces at ietf.org [mailto:pwe3-bounces <pwe3-bounces> at
ietf.org] On Behalf Of Yaakov Stein
Sent: Sunday, November 14, 2010 10:18 AM
To: Sriganesh Kini
Cc: pwe3 at ietf.org
Subject: Re: [PWE3] VCCV usage

Sri

I hear that you are saying that it should not change.

I already agreed that this is the typical case,
but as I said I am not sure that it is the only case.

Let me explain in more detail what I mentioned in my previous email.
Consider a SP that has several MPLS networks, each of which is
separate at the MPLS layer,
but across any combination of which you can set up PWs.
Now take, for example, a PW that is initially set up as a SS one.
Due to a failure the PW can only be maintained by going outside the
original MPLS network
to a transit network, and then back in.
So from a single segment PW it becomes a 3-segment PW.  PW-label TTL changes.

Furthermore, as I asked Sasha, the ingress PE does not know if it is SS,
or if MS how many segments there are.

Y(J)S

-----Original Message-----
From: sriganeshkini at gmail.com [mailto:sriganeshkini <sriganeshkini>
at gmail.com] On Behalf Of Sriganesh Kini
Sent: Sunday, November 14, 2010 10:08
To: Yaakov Stein
Cc: Sriganesh Kini; pwe3 at ietf.org
Subject: Re: [PWE3] VCCV usage

Hi Yaakov,

Each of the SS-PW and the (backup) MS-PW should have their own TTL and
VCCV. The number of PW hops should not change for either of the PWs.

On Sunday, November 14, 2010, Yaakov Stein <yaakov_s at rad.com> wrote:
> Sri,
>
> Almost.
>
> In theory a failure may cause a simple PW to be backed up by a MS one,
> thus changing the number of PW hops.
>
>
> Y(J)S
>
> -----Original Message-----
> From: sriganeshkini at gmail.com [mailto:sriganeshkini <sriganeshkini> at gmail.com] On Behalf Of Sriganesh Kini
> Sent: Friday, November 12, 2010 23:27
> To: Yaakov Stein
> Cc: Alexander Vainshtein; pwe3 at ietf.org
> Subject: Re: [PWE3] VCCV usage
>
> Yaakov,
>
> The tunnel setup protocol should not matter since the TTL expiry VCCV
> depends on the PW label TTL. When the PW is placed in a tunnel setup
> by LDP and the number of hops of the LDP tunnel change, the same PW
> label TTL must be used for VCCV.
>
>
> - Sri
>
> _______________________________________________
> pwe3 mailing list
> pwe3 at ietf.org
> https://www.ietf.org/mailman/listinfo/pwe3
>
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