Re: [mpls] Thoughts on Ancillary Data Indicators.

Kireeti Kompella <> Mon, 12 July 2021 20:43 UTC

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From: Kireeti Kompella <>
Date: Mon, 12 Jul 2021 13:42:49 -0700
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To: Loa Andersson <>
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Subject: Re: [mpls] Thoughts on Ancillary Data Indicators.
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Hi Loa,

See inline.

On Thu, Jul 8, 2021 at 2:11 AM Loa Andersson <> wrote:

> Design Team,
> We have discussed how to carry ancillary data in  in an MPLS packet and
> how to include indicators on the presences of such data in the label
> stack since the design team started. I think we have some convergence
> and that it is time to float an idea that could be the basis for a
> consensus. Please note that the actual consensus call will be made on
> the mpls wg mailing list, while this is for discussion preparing such a
> consensus call in the design team.
> The proposals we have for indicators often rely on assignment of a base
> special purpose label. The number of such labels are limited.
> I think we can agree on the following:
> - We want to limit the number of new assignments of base special purpose
>    labels as much as possible
> - We have a standardized associated channel, the GAL/GACH. We are not
>    going to change the associated channel. The associated will remain as
>    specified in RFC 5586. The ACH is found immediately after the label
>    carrying the Bottom of Stack (BoS) bit.
> - We therefore need a (base) special purpose label to serve as the
>    forwarding action indicator carrier.

The proposal in draft-kompella-mpls-mspl4fa does this, and at the same
time, collapses the requests of 5 or 6 SPLs to a single SPL.

- It is attractive to "re-purpose" the ELI to also fill the role as a
>    general (generic?) forwarding action indicator. This may be done by
>    using the TTL and TC fields of the ELI as flags indicating which
>    service is requested.

While I'd be happy if this was possible, backward compatibility MUST be
maintained without compromising functionality.

>    However, in order to do this it must be shown that there is no
>    potential interference between the normal service supplied by the
>    ELI/EL and the services indicated by the flags.
> - If we can't demonstrate the non-interference then we should assign
>    specific forwarding indicator action base special purpose label and
>    leave the ELI/EL unchanged.
> - The flag field of the re-purposed ELI of the the specially assigned
>    forwarding action indicator label must be made extendable. These
>    labels "only" have 11 bits (8 bit TTL and 3 bit TC fields) that can
>    be used as flags. Already in the very preliminary discussions we have
>    had so far are almost using all the 11 bits, making the it necessary
>    to make the indicator extendable.

Actually, the draft already had such an extension in place; the details
have been fleshed out a bit better in the new version (just posted).

The bit fields have changed as a consequence of my chat with Haoyu; some
have been removed, but more added, so all 11 bits have been used up. So the
extension header will be needed.

The -01 draft is clearer than the -00 draft on several fronts, and lists
issues needing resolution.  I'm sure more will come up after the MPLS WG


> /Loa
> --
> Loa Andersson                        email:
> Senior MPLS Expert                
> Bronze Dragon Consulting             phone: +46 739 81 21 64
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