Re: [PWE3] VCCV usage

[Carlos Pignataro <[e-mail]>]

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)

> 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?

> 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.

Thanks,

-- Carlos.

> 
> Thanks