Re: [PWE3] GAL above and below the PW label

[Sriganesh Kini <[e-mail]>]

All,

Just an FYI that a proposal for fate-sharing PW OAM and data given in
draft-kini-pwe3-inband-cc-offset

Just to summarize the proposal.
1. It does not require GAL in PW. TTL expiry is used to alert the S/T-PE.
2. It is mainly applicable when CW is not used in the PW.
3. It uses a fixed offset (negotiated between PW endpoints) after the
label stack before starting the OAM msg.
4. The bytes between the label-stack and the fixed offset is referred
to as a pseudoflow header and is filled with byte-values (by the PE)
that represent the flow for which OAM is desired. This helps PW OAM
and data to fate-share even when the intermediate node looks beyond
label stack to do multipath forwarding decisions.


On Thu, Mar 31, 2011 at 12:03 AM, Curtis Villamizar <[e-mail]> wrote:
>
> Loa, Dave, Sasha,
>
> I've snipped from various posts on the thread that Sasha started.  See
> inline.
>
> In message <[e-mail]>
> Loa Andersson writes:
>>
>> All,
>>
>> missed a nuance in Sasha subject line.
>>
>> We have two issues
>>
>> - where the GAL is placed relative to the PW label, I  believe it is
>>    necessary to have the GAL below the PW label.
>>
>> - whether the the GAL label needs to be bottom of stack or not, it
>>    figure that this is really a discussion if it is possible to have
>>    a FAT label below the GAL or not. I'm not sure  about my preferences
>>    but I think it is possible.
>>
>> /Loa
>
> I agree with Loa's summary.  Not putting GAL at the bottom may confuse
> some LSR, but putting it above the PW label is likely to be even more
> problematic.
>
>
> In message <[e-mail]>
> David Allan I writes:
>>
>> It does become a new behavior, GAL with S=3D0. However the combination of G=
>> AL being top label at the S-PE and TTL being encoded in the PW label means =
>> that fate sharing is broken at every S-PE. Life only gets a little more str=
>> ange if there is a FAT label as well...
>>
>> That being said, GAL as bottom label is broken in any ECMP environment, whi=
>> ch is why GAL is a TP construct.
>>
>> my 2 cents
>> D
>
> Dave,
>
> If GAL is broken for ECMP, which it is, then all TP OAM is broken.
> If all it takes to fix it is simple then lets just fix it.
>
> This is what we'd have to do.
>
>  1) Relax the requirement that GAL be at the bottom
>
>  2) Have the ingress insert GAL in the stack immediately below the
>     label for which the measurement is made, keeping the rest of the
>     label stack in place.
>
> The only thing the midpoing LSR at a multipath (LAG, link bundle for
> MPLS) has to do is skip over the GAL when hashing, as if the GAL
> wasn't there.  That will yield the same hash value and it will
> preserve fate-sharing across multipath.
>
> I prefer that we fix things rather than complain that they are broken.
>
> Curtis
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-- 
- Sri