Re: [mpls] entropy label indicator label indraft-kompella-mpls-entropy-label

[Shane Amante <[e-mail]>]

Lucy,

On Jul 27, 2010, at 17:06 GMT+02:00, Yong Lucy wrote:
> Hi John,
> I don’t get it. If egress LSR asks ingress LSR to place ELI on the stack, the ingress can insert a unique ELI label on the stack followed by entropy label. When packet arrives to egress LSR, the LSR will find ELI on the stack. Why is it important for egress LSP to allocate a value for ELI instead of using standardized value?

I think John already answered this question, but I'll give it a try, as well.  It's really a design choice on the part of the co-authors, in an attempt to avoid asking the MPLS WG to allocate a reserved label from the very scarce reserved label space.  I would note that there are only 8 reserved MPLS label values that are currently unassigned:
http://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml

So, if we believe it's "sound engineering" to use an ELI instead of a reserved label -- certainly a judgement call -- then, I believe, we should NOT request a reserved label in order that future applications that have a legitimate use (or, said differently, may not be able to function properly without a reserved label value) have an ability to acquire one.

-shane



> Regards,
> Lucy
>  
> From: John E Drake [mailto:[e-mail]] 
> Sent: Tuesday, July 27, 2010 8:31 AM
> To: Yong Lucy; Kireeti Kompella; 'Shane Amante'; [e-mail]
> Subject: RE: [mpls] entropy label indicator label indraft-kompella-mpls-entropy-label
>  
> Lucy,
>  
> Yes it does.  That is the whole point.  That is how the egress knows whether the ingress has placed an entropy label in the stack.  (That is why we termed it the ‘Entropy Label Indicator’.)
>  
> Thanks,
>  
> John
>  
> From: Yong Lucy [mailto:[e-mail]] 
> Sent: Tuesday, July 27, 2010 6:25 AM
> To: John E Drake; Kireeti Kompella; 'Shane Amante'; [e-mail]
> Subject: RE: [mpls] entropy label indicator label indraft-kompella-mpls-entropy-label
>  
> What is the benefit for such control? Egress LSR does not use this value.
> Lucy
>  
> From: John E Drake [mailto:[e-mail]] 
> Sent: Tuesday, July 27, 2010 8:10 AM
> To: Yong Lucy; Kireeti Kompella; 'Shane Amante'; [e-mail]
> Subject: RE: [mpls] entropy label indicator label indraft-kompella-mpls-entropy-label
>  
> Lucy, we discussed this and decided it was better to have the egress control the values of the ELI it advertises.  Plus, getting a reserved value would be difficult
>  
> From: [e-mail] [mailto:[e-mail]] On Behalf Of Yong Lucy
> Sent: Tuesday, July 27, 2010 5:31 AM
> To: Kireeti Kompella; 'Shane Amante'; [e-mail]
> Subject: [mpls] entropy label indicator label in draft-kompella-mpls-entropy-label
>  
> Hi Kireeti and Shane,
>  
> Could we consider use one of reserved label for ELI purpose? This will make implementation much easy, i.e. not need to keep the state for each ELI label. The approach can be used in general for RSVP-TE and BGP as well.
>  
> Regards,
> Lucy
>  
>  
>