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

["Jeffrey (Zhaohui) Zhang" <[e-mail]>]

Hi Miya,

Please see zzh> below.

From: Miya Kohno <[e-mail]>
Sent: Friday, May 14, 2021 11:04 PM
To: Jeffrey (Zhaohui) Zhang <[e-mail]>
Cc: Joel M. Halpern <[e-mail]>; [e-mail]
Subject: Re: [DMM] Architecture Discussion on SRv6 Mobile User plane

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Hi Jeffrey,

Thank you very much for your review and comments.

The two points you picked up are somewhat important, but what's more important is that if we are tied to the GTP-U, we cannot break-through the current tunnel-session convention, where:
 - tunnel-session gateways become a scaling bottleneck.
 - it is not optimal for distributed data and applications.

Zzh> Not exactly clear what “gateways” you’re referring to, but I have the following thoughts on the tunnel-session convention.
Zzh> Consider the typical wireline/IETF VPN scenario:

CE1001 ---- PE1 ------- P -------- PE2 ---- CE2000
…         /
CE1999 –-/
Zzh> PE1 has 1000 CEs connected in a VRF, say via VLANs. IP adjacencies from the CEs terminate on PEs, therefore PE1 only need to advertise one label/SID for the VRF. Traffic from CE2000 to any of the CEs off PE1 will have an IP lookup in PE2’S VRF first, and a tunnel with that single label/SID is used to reach PE1, who will do an IP lookup in the VRF to determine where to send the traffic.
Zzh> Now consider 5G:

UE1001 ---- gNB1 –----- P -------- UPF1 ---- DN
…         /
UE1999 –-/
Zzh> gNBs are not IP terminating points (wrt PDU sessions from UEs). UPFs terminate the PDU sessions and they need tunnels to the UEs for those PDU sessions. As long as UPFs need to distinguish which UE the traffic is to/from, it does not matter whether the PDU sessions are instantiated via GTP-U or SRv6 (and if some UEs do not need to be distinguished for uplink traffic, then the same TEID can be used with GTP-U as well, similar to wireline/IETF VPN case or as mentioned in draft-ietf-dmm-srv6-mobile-uplane section 5.2.3). In other words, 3GPP would need to decide if the tunnel-session convention will continue and before that changes, the two points you mention won’t be changed by SRv6.
Zzh> UPFs can be distributed, e.g. to gNB sites. Even with that, the tunnels still exist unless 3GPP changes the tunnel-session convention (and before then SRv6 won’t help), just that they do not go across a large transport network. However, the distributed UPFs do optimize for distributed data and applications.

We will improve the section 2 "problem statement" to be clearer.

Zzh> Will see how it turn out. I do not agree with current statements in the section – at least GTP-U transport over SRv6 should work well for non-MPLS operators.

I never think MPLS is dead. But I don't think that's a reason to discourage new options.

As access technologies become more diverse and computing is more distributed, the importance of FMC (Fixed Mobile Convergence) increases more than ever.
Currently, FMC is discussed exclusively in 3GPP/BBF, but I hope that the IETF community, knowing the strength of IP as a stateless common data plane, will influence the industry a bit more.

Zzh> My point is that, if you are a 3GPP person, would you specify two ways for PDU sessions? The SRv6 way won’t work for MPLS operators, while the GTP-U way should work well for any operators.
Zzh> Clean layering has its architecture advantages. Even if an operator does not like many layers of IPv6/UDP/GTP headers, the drop-in mode in draft-ietf-dmm-srv6-mobile-uplane can address that transparently.
Zzh> Again, the arguments need to be made in 3GPP, not in IETF.
Zzh> Thanks.
Zzh> Jeffrey

Thanks,
Miya

On Tue, May 11, 2021 at 3:57 AM Jeffrey (Zhaohui) Zhang <[e-mail]<mailto:[e-mail]>> wrote:
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 <[e-mail]<mailto:[e-mail]>> On Behalf Of Joel M. Halpern
Sent: Friday, May 7, 2021 10:41 AM
To: Miya Kohno <[e-mail]<mailto:[e-mail]>>; dmm <[e-mail]<mailto:[e-mail]>>
Subject: Re: [DMM] Architecture Discussion on SRv6 Mobile User plane

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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$<https://urldefense.com/v3/__https:/codimd.ietf.org/notes-ietf-110-dmm__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBr19WLV0$>
> <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$<https://urldefense.com/v3/__https:/tools.ietf.org/html/draft-kohno-dmm-srv6mob-arch-04__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBnDKFo0K$>
> <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$<https://urldefense.com/v3/__https:/tools.ietf.org/html/draft-ietf-dmm-srv6-mobile-uplane-12__;!!NEt6yMaO-gk!SzV0kRt5R1BtZ6iXWrwQL2PSnxSFw0e-sTZ2WKE6-yG-eF_Ugx6Nj5tSBsfnapQb$>
> <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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