Re: [DMM] Architecture Discussion on SRv6 Mobile User plane

Hi Miya,

As Joel pointed out, it is 3GPP not IETF who may adopt SRv6 as a user plane. Before then, we have to take GTP-U as granted. Of course, if IETF can reach consensus on the merit, we could recommend to 3GPP and they can decide whether to take it or not.

The draft talks about various advantages in various use cases, but I don't see why 3GPP needs to move away from GTP-U. If I understand it correctly, the draft mainly talks about two reasons:

1. 5G NF nodes (as GTP-U tunnel endpoints) are better off not being CEs off PEs
2. SRv6's TE and program capability solve lots of problems

However, it does not explain why it would not work if an NF node continues to use GTP-U but put it on top of SRv6 (w/o PE/CE separation). The way I (and perhaps some 3GPP folks) see it, a 5G NF may be better off not being concerned with how a GTP-U packet is steered across the network (e.g. figuring out and encoding the SRH) but leaving it to the network layer.

Note that this does not mean the NF has to be a host/CE separate from a PE. It could be that the 5G NF is the application layer (using GTP-U) on top of the network layer that uses SRv6.

In fact, the last paragraph of this document says "it is totally fine to keep ovelray underlay-agnostic":

   Note that the interaction with underlay infrastructure is not a
   mandatory in the data plane commonality.  It just gives a design
   option to interact with the underlay and optimize it, and it is
   totally fine to keep ovelray underlay-agnostic.

Additionally, for the drop-in mode described in section 5.4 of draft-ietf-dmm-srv6-mobile-uplane, the two SRGWs can be implemented either as standalone entities or as part of the network stack on the 5G NFs themselves. This achieves the same result as if 3GPP replaced GTP-U with SRv6 w/o any impact to existing 3GPP specifications or implementations.

So, what really matters is why the GTP-U encapsulation should be integrated/dissolved into SRv6 header itself, and make sure that the 3GPP (not IETF) folks are convinced of that.

Related to convincing 3GPP folks of the above, one question is - is MPLS dead already? Are there operators not using SRv6 transport?

As long as there are still operators not using SRv6 for transportation, why would 3GPP want to have two ways, when the existing GTP-U works for both?

Thanks.
Jeffrey

-----Original Message-----
From: dmm <dmm-bounces@ietf.org> On Behalf Of Joel M. Halpern
Sent: Friday, May 7, 2021 10:41 AM
To: Miya Kohno <miya.kohno@gmail.com>; dmm <dmm@ietf.org>
Subject: Re: [DMM] Architecture Discussion on SRv6 Mobile User plane

[External Email. Be cautious of content]

Without getting into the content, when it comes to whether GTP-U is the
mechanism for carrying cellular mobile user data, that is a 3GPP
decision, not an IETF decision.

Yours,
Joel

On 5/7/2021 10:35 AM, Miya Kohno wrote:
> Dear DMM WG,
>
> Following up the discussion at the IETF110
> (https://urldefense.com/v3/__https://codimd.ietf.org/notes-ietf-110-dmm__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBr19WLV0$
> <https://urldefense.com/v3/__https://codimd.ietf.org/notes-ietf-110-dmm__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBr19WLV0$ >), I would like to have your
> review on the draft -
> https://urldefense.com/v3/__https://tools.ietf.org/html/draft-kohno-dmm-srv6mob-arch-04__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBnDKFo0K$
> <https://urldefense.com/v3/__https://tools.ietf.org/html/draft-kohno-dmm-srv6mob-arch-04__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBnDKFo0K$ >.
>
> The purpose of this draft is to support the value of the SRv6 mobile
> user plane
> (https://urldefense.com/v3/__https://tools.ietf.org/html/draft-ietf-dmm-srv6-mobile-uplane-12__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBsfnapQb$
> <https://urldefense.com/v3/__https://tools.ietf.org/html/draft-ietf-dmm-srv6-mobile-uplane-12__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBsfnapQb$ >),
> and to be a trigger to revisit the current situation where GTP-U is
> taken for granted as a mobile user plane.
>
>
> Thanks,
> Miya - on behalf of the authors
>
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