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

[Sriganesh Kini <[e-mail]>]

Hi Carlos,

The applicability of draft-kini-pwe3-inband-cc-offset is when CW is
not present, but the intermediate LSR looks beyond the PW label to do
multipath decisions. The draft is independent of any (even proprietary
logic) for multipath decision taken by intermediate nodes looking
beyond PW label and also independent of any heuristic the intermediate
routers may use to guess the packet type (beyond label stack) being
carried. The most common heuristic is of course looking at the first
nibble beyond label stack to determine that it is an IP packet. So
this would also apply for an IP PW, IPLS, etc in addition to
draft-kini-pwe3-pkt-encap-efficient-ip-mpls. LSP-Ping uses the 127
address and it is not easy to co-relate this to a problem reported for
real (non 127) addresses.

Thanks


On Thu, Apr 21, 2011 at 9:48 PM, Carlos Pignataro <[e-mail]> wrote:
> Hi Sriganesh,
>
> Do you see the applicability of draft-kini-pwe3-inband-cc-offset
> strictly limited to the encapsulation proposed in
> draft-kini-pwe3-pkt-encap-efficient-ip-mpls?
>
> Specifically, for a PW that carries say Ethernet without CW (or any of
> the other encap modes with non-mandatory CW), there is no "flow" that is
> being ECMP-ed in the core that you want to mimic/imitate in a
> load-balancing pseudo-header. Conversely, for "IP PWs", LSP-Ping can
> take different Equal-cost paths (all of them, one by one) based on
> choosing the destination IP address. And there is no need for this when
> there is a CW.
>
> I am trying to understand how common is the use case for what this
> optimizes for.
>
> Thanks,
>
> -- Carlos.
>
> On 3/31/2011 4:08 AM, Sriganesh Kini wrote:
>> 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
>>> _______________________________________________
>>> pwe3 mailing list
>>> [e-mail]
>>> https://www.ietf.org/mailman/listinfo/pwe3
>>>
>>
>>
>>
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-- 
- Sri