[mpls] mpls open dt & the first nibble discussion

"Fomin, Sergey (Nokia - US/Mountain View)" <sergey.fomin@nokia.com> Thu, 15 April 2021 19:04 UTC

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From: "Fomin, Sergey (Nokia - US/Mountain View)" <sergey.fomin@nokia.com>
To: "mpls@ietf.org" <mpls@ietf.org>
Thread-Topic: mpls open dt & the first nibble discussion
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Date: Thu, 15 Apr 2021 19:04:27 +0000
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Subject: [mpls] mpls open dt & the first nibble discussion
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Hi,
this "first nibble" discussion took up a lot of time in today's meeting, but I'm not quite sold on the idea.
Perhaps I'm missing some context, but what exactly are we trying to achieve with it?

Let me share a few thoughts, starting with a relevant paragraph from Stewart’s draft-bryant-mpls-dev-primer/meeting agenda
>    Note that when we define new first nibbles we are technically taking
   IP versions away from the IETF Internet Area.  When PWE3 first
   proposed this we agreed with the IETF of the day that we would only
   take 0b0000 and 0b0001.  I am looking to see if this agreement was
   documented.
I don't think this is the case.

0x4 and 0x6 have meaning in the context of IP header.
After an MPLS header with BoS=1 we may encounter any type of payload (with currently dominant IP & Ethernet). And we all know that this the reason how we got into the whole ECMP/CW mess in the first place. And that the first nibble was never intended to be used as a protocol identifier from MPLS point of view, it’s just a byproduct of load-balancing needs.
Which leads me to a few points:

  1.  We are not 'taking' anything from IP folks as long as we are not claiming that an IP header follows (VTN folks do not make such claim, instead using their own VTN header; same as BIER-MPLS and bier header).
  2.  We already put all sorts of things into that first nibble (e.g. when encapsulating ethernet frames without CW).
  3.  We can't directly/solely use "first nibble" after as an analogue to ethertype or next protocol/next header/etc. Its meaning by itself is not defined; and I don’t think we should (or can, for that matter) change that logic.

--
Sergey