Re: [DMM] [Apn] Regarding APN Usecase in Mobile Core

Many thanks to Sridhar and John 's insightful description of GTP-u tunnel for both 4G and 5G.
IMHO, APN should focus on the 5G case, i.e. potentially hundreds of thousands (or even millions) of GTP-u tunnels between a pair of one gNB and one UPF.
I think two scenarios worth exploring further separately:

  1.  5G core can exchange signal with the IP network (the N6 interface), more likely the 5G Core and N6 network are administrated by the same network operator.
     *   5G core can tag the packets before handing off to the N6 interface (i.e. the IP network), such as inserting a metadata to the data packets
     *   inform the IP network of the meaning of the inserted Metadata.
     *   The IP network needs to remove the metadata before handing the packets to their destinations

  1.  There is no communication between the 5G core and the IP network, then the IP network can only depend on the priority bits added by the UE, which can be very unpredictable.

The Section 4 (Mobility Packet Transition in the Data Network) and the Section 5 (Transport Network characteristics Mapping to SR-TE Paths) have good description on how to extend 5G core's TN aware mobility information for various TE paths within IP networks. https://datatracker.ietf.org/doc/draft-mcd-rtgwg-extension-tn-aware-mobility/

Linda Dunbar
From: Apn <apn-bounces@ietf.org> On Behalf Of Sridhar Bhaskaran
Sent: Thursday, January 21, 2021 10:39 PM
To: Kaippallimalil John <john.kaippallimalil@futurewei.com>
Cc: apn@ietf.org; Gengxuesong (Geng Xuesong) <gengxuesong@huawei.com>; Uma Chunduri <umac.ietf@gmail.com>; Lizhenbin <lizhenbin@huawei.com>; dmm <dmm@ietf.org>
Subject: Re: [Apn] [DMM] Regarding APN Usecase in Mobile Core

Hi Xuesong,

In addition to what John said, in 3GPP networks there is one GTP-u tunnel per bearer (in case of 4G) and one GTP-u tunnel per PDU session (in case of 5G).

One UE (user equipment - i.e mobile device) may have multiple PDU sessions and hence in the network there may be more than one tunnel for a UE. The scope of GTP-u tunnel is from UPF to gNB only. GTP-u does not go all the way upto UE. The GTP-u header has a field called "TEID" (tunnel endpoint identifier). The TEID in the header identifies a context in UPF and gNB. The context gets established through signalling plane. The context provides information on the QoS to be provided for the bearer,  PDCP ciphering keys applicable for the bearer context etc.,. If there are a million UE that are getting connected to a UPF, there could be few million GTP-u tunnels (TEID).

In summary:
1. The 5QI / QFI marking in the GTP-u extension header provides a lookup for the general QoS characteristic applicable for that 5QI
2. TEID in the GTP-u header provides a lookup for UE and bearer specific contextual information for any differentiated treatment.

Regards
Sridhar

On Fri, Jan 22, 2021 at 2:55 AM Kaippallimalil John <john.kaippallimalil@futurewei.com<mailto:john.kaippallimalil@futurewei.com>> wrote:
Hi Xuesong,

Traffic policy for subscribers is managed per PDU session at the UPF (and gNB).
GTP-u does provide encapsulation between the end points, but its control fields are meant for conveying control semantics between the GTP endpoints: they were not intended for IP transport/ traffic underlays. 5QI/QCI etc are in the GTP extension header which may not be ideal to lookup to classify each packet in the transport network.

The entity that classifies data packets (upstream at gNB and downstream at UPF-PSA) also inserts the DSCP for that GTP packet. The classification is based on subscriber aspects but may also on be based on its content (e.g., using DPI).

Best Regards,
John

From: dmm <dmm-bounces@ietf.org<mailto:dmm-bounces@ietf.org>> On Behalf Of Gengxuesong (Geng Xuesong)
Sent: Wednesday, January 20, 2021 8:23 PM
To: Uma Chunduri <umac.ietf@gmail.com<mailto:umac.ietf@gmail.com>>; Lizhenbin <lizhenbin@huawei.com<mailto:lizhenbin@huawei.com>>
Cc: apn@ietf.org<mailto:apn@ietf.org>; dmm <dmm@ietf.org<mailto:dmm@ietf.org>>
Subject: Re: [DMM] [Apn] Regarding APN Usecase in Mobile Core

Hi Uma and all,

I have read the document and got a few questions:
In my understanding, in the UPF where traffic policy is enforced, the fine-granularity services are provided. Then what fields in the GTP-u encapsulation indicates the traffic's service requirements? When a GTP-u tunnel goes into a SRv6 policy, according to which fields in the GTP-u encapsulation the DSCP is generated? We know that there are parameters such as 5QI/QCI and QFI, whether they are associated with a GTP-u tunnel?

Best
Xuesong
From: Apn [mailto:apn-bounces@ietf.org] On Behalf Of Uma Chunduri
Sent: Tuesday, January 19, 2021 3:17 AM
To: Lizhenbin <lizhenbin@huawei.com<mailto:lizhenbin@huawei.com>>
Cc: apn@ietf.org<mailto:apn@ietf.org>; dmm <dmm@ietf.org<mailto:dmm@ietf.org>>
Subject: Re: [Apn] [DMM] Regarding APN Usecase in Mobile Core

Hi Robin,

In-line..

Cheers!
--
Uma C.

On Mon, Jan 18, 2021 at 5:25 AM Lizhenbin <lizhenbin@huawei.com<mailto:lizhenbin@huawei.com>> wrote:
Hi APNers and DMMers,
I remember that in the mobile core scenarios the GTP-u tunnel can be set up according to the user and application requirements, but I do not understand the details.

[Uma]: Obviously, the best reference for GTP-U is TS 29.281. However, uou should look into https://datatracker.ietf.org/doc/draft-ietf-dmm-5g-uplane-analysis/<https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fdraft-ietf-dmm-5g-uplane-analysis%2F&data=04%7C01%7Clinda.dunbar%40futurewei.com%7C5985b13c46f440f9c3af08d8be8fc90e%7C0fee8ff2a3b240189c753a1d5591fedc%7C1%7C0%7C637468872069926165%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=x4dOKXMZutPMV2%2BXZltRgnXKhVzJOktjqWAt%2FuDswHs%3D&reserved=0> where lot more details and other references related this topic was analyzed (primarily started after/during REL-15, when  any other use plane other than GTP-U is worthwhile is debated for 5G N9 interface).

I think when the packet tunneled by GTP-u traverses the APN-based transport network, it may be mapped to the corresponding tunnel according to the user and application requirements to implement the uniform service. If you are familiar with the principle of GTP-u in the mobile core, please help provide some details.

Best Regards,
Zhenbin (Robin)

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