Re: [PWE3] [mpls] ELI as a reserved label

Curtis,

On Jul 30, 2010, at 11:06 GMT+02:00, Curtis Villamizar wrote:
> The Entropy Label Indicator (ELI) is called for in
> draft-kompella-mpls-entropy-label-01.txt
> 
> After brief discussion with Stewart and with Kireeti and Lucy Yong,
> there were three uses of ELI that would benefit from making the ELI a
> reserved label.  It may be premature to make any such request and the
> remaining reserved label number space is sparse, so this will be
> floated as a separate draft, and put on hold awaiting advancement of
> the drafts that depend on it.
> 
> The three benefits of making ELI a reserved label are in:
> 
>  1.  Extending any use of entropy-label to P2MP (where there are
>      multiple egress and the possibility of each egress requiring a
>      separate entropy label value is problematic for the ingress).
> 
>  2.  Extending any use of entropy-label to any arbitrary position in
>      the label stack such that in an aggregation LSP with aggregates
>      both MPLS-TP and MPLS LSP, load split on the MPLS LSPs is
>      simplified and not impacted by limiting label stack depth for
>      the MPLS-TP LSPs.  This is when label stack hash is limited to
>      support MPLS-TP as proposed in
>      draft-villamizar-mpls-tp-multipath-00.txt .

I agree there are benefits to a reserved label for the above two applications.  Most importantly, a receiving node would have pretty clearly semantics that what [immediately] follows a reserved ELI is the entropy-label, regardless of where the ELI is in the MPLS label stack and without needing to be in the signaling path to know what is the ELI value.

>  3.  Providing a means to carry indication of large flow as described
>      in draft-yong-pwe3-enhance-ecmp-lfat-01.txt and
>      draft-yong-pwe3-lfc-fat-pw-01.txt without changing the use of TC
>      in a forwarding LSP label.

I'm unclear how a single, reserved label would be a means of signaling a large vs. small flow.  How can a single label represent two states (large vs. small flow)?

Thanks,

-shane

> This would reuse one reserved label for three purposes.
> 
> This email is to gauge the reaction to this proposal (preferably among
> those familiar with these drafts) before taking the time to write a
> new draft proposing that a reserved label be allocated.
> 
> Curtis
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