Re: [PWE3] VCCV usage

[Sriganesh Kini <[e-mail]>]

Hi Carlos, pls see inline

On Mon, Nov 29, 2010 at 2:35 PM, Carlos Pignataro <[e-mail]> wrote:
> Hi Sri,
>
> Thanks for the summary. Please see inline.
>
> On 11/29/2010 4:10 PM, Sriganesh Kini wrote:
>> Carlos, that is useful clarification. If I were to summarize,
>>
>> 1. When a PW has a CW, CC Type 1 is obvious/implicit choice.
>
> Or put a different way, CC Type 1 can only be used when the PW has a CW.
> There's no implicit choice standardized though.
>
>> However,
>> PW label TTL expiry should trigger examining the ACH for an VCCV
>> packet (both S-PE and T-PE).
>
> A PW label TTL expiry should trigger the exception mechanism to dealing
> with a TTL expiration; this may include examining the PW-ACH for a VCCV
> packet (or not), and may include detecting the VCCV packet but not
> processing it for security if not advertised.
>
>> 2. When PW does not have CW, CC Type 3 should be used.
>
> According to RFC 5085, that should be Type 2 as preferred over Type 3
> (see S5.1.2. and S7)

Correct. I was referring to it more in the context of this thread
where Type-2 deprecation has been proposed.

>
>> When PWs were
>> single segment, the 'PW demultiplexer' TTL was defined as
>> application-specific (RFC 3985). With MS-PW it is important for it to
>> have consistent TTL decrement operations and treat TTL expiry (i.e.
>> TTL=1) packets as VCCV packets (both at S-PE and T-PE). This requires
>> the number of PW hops to be known to execute OAM targeted to a
>> particular S/T-PE and that is straightforward.
>
> I agree.
>
>>
>> Note that TTL expiry VCCV is not inband when CW is not used since
>> intermediate nodes can look beyond the label stack and have different
>> ECMP behavior.
>
> Exactly. Which prompts the question:
>
> If the VCCV packet takes a different path than the PW data, can this
> different path have a different number of hops and have the VCCV expire
> at a PE different than the target PE?

There can be different paths in the underlying MPLS PSN. However that
should not affect the TTL expiry of VCCV since the number of PW hops
must remain the same.

>
>> A draft that addresses this condition by extending TTL
>> expiry VCCV was submitted at the last IETF
>> (draft-kini-pwe3-inband-cc-offset).
>
> Scanning now through this I-D, I could not find why a 'flow/entropy
> label' [fat-pw] would not work.

Even with FAT-PW, when a CW is not present, an intermediate hop in the
MPLS PSN can look beyond the label stack and the data packet can take
a different path from the VCCV packet.

>
> Thanks,
>
> -- Carlos.
>
>>
>> Thanks
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-- 
- Sri