| Internet-Draft | 5G dUPFs and 5MBS | March 2022 |
| Jiang | Expires 8 September 2022 | [Page] |
The companion draft [I-D.zzhang-dmm-5g-distributed-upf] has described the 5G mobile user plane (MUP) via the refinement of distributed UPFs, along with various user plane implementations that some vendors and operators are exploring, with the requirement of not introducing changes to 3GPP architecture & signaling. The document 3GPP TS 23.247 [_3GPP-23.247] for 5G multicast and broadcast services, or 5MBS, specifies the 5GS architecture to support MBS communication. Thanks to the addition of new 5GS network functions (NFs) and MB-interfaces on 5G CP & UP, this might post additional provisioning & implementation challenges to the underlay transport infrastructure.¶
This document is not an attempt to do 3GPP SDO work in IETF. Instead, it discusses how to potentially integrate distributed UPFs with the delivery of 5MBS communication, as well as the benefits of using distributed UPFs to handle 5MBS traffic delivery.¶
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Mobile User Plane (MUP) in 5G has two distinct parts: the Access Network part between UE and gNB, and the Core Network part between gNB and UPF. UPFs are traditionally deployed at central locations, with UEs' PDU sessions encapsulated and extended thru GTP-U tunnels via the N3 (and potentially N9) interfaces in 5GS. The interface N6 supports fundamentally a direct IP or Ethernet connection to the data network or DN.¶
Actually, UPFs could be distributed & deployed closer to gNBs.
The draft [I-D.zzhang-dmm-5g-distributed-upf] has described
the 5G mobile user plane (MUP) via the refinement of distributed UPFs or dUPFs.
The following picture shows the dUPF architecture:¶
N3 N6
UE1 gNB1 | dUPF1 |
+---------+ |+------+-----+|
| PDU | || PDU | || PE1
+---------+ +------+------+|+------+ IP/ || +-----+--+
| | | |GTP-U |||GTP-U | ||----+ IP/ | |
| 5G-AN | |5G-AN +------+|+------+Ether|| |Ether| |
| xHaul | |xHaul |L3/2/1|||L3/2/1| || +-----+--+
+---------+ +------|------+|+------------+| ( )
| | ( Transport ) PE3
| | ( Network +--+-----+
UE2 gNB2 | dUPF2 | ( | | IP/ |
+---------+ |+------+-----+| ( (DN) | |Ether|
| PDU | || PDU | || ( +--+-----+
+---------+ +------+------+|+------+ IP/ || +-----+--+
| | | |GTP-U |||GTP-U | || | IP/ | |
| 5G-AN | |5G-AN +------+|+------+Ether|| |Ether| |
| xHaul | |xHaul |L3/2/1|||L3/2/1| || +-----+--+
+---------+ +-------------+|+------------+| PE2
¶
In distributed UPF architecture, the central (PSA) UPF is no longer needed. dUPF1 and UPF2 connect via PE1 and PE2, respectively, to the DN VPN (or network instance/NI) that UE1 and UE2 intend to access. There could exist other PEs, like PE3 in the picture, for other sites of the same network domain(VPN or NI) or for global Internet access.¶
There are some benefits of distributed UPFs:¶
In short, the distributed UPFs model achieves "N3/N9/N6 shortcut and central UPF bypass", which is desired by many operators.¶
The 3GPP document TS 23.247 [_3GPP-23.247] for 5G multicast and broadcast services, or 5MBS, specifies the 5GS architecture to support MBS communication. The following picture shows the brief system architecture of 5MBS:¶
----+----------(SBA for 5GC) ---------+-----
| | |
+--+--+ +---+---+ +---+----+
| AMF | | SMF | | MB-SMF |
+--+--+ +-+-+-+-+ +---+----+
/ | |
N2 / N4 | N4mb|
/ | |
/ N3 +-+-+---+ N19mb +---+----+ N6mb +----+
+-----+---------+ UPF +--------------| MB-UPF |------| DN |
+----+ | | +-------+ (Individual) +---+----+ +----+
| UE +---+ gNB | |
+----+ +-----+ |
|_________N3mb (shared delivery)_____|
¶
TS 23.247 [_3GPP-23.247] adds new 5GS network functions (NFs) on both 5G control-plane (CP) and user-plane (UP). For example, the CP NF MB-SMF is, in collaboration with the regular SMF, to provision and signal to the UP NF MB-UPF (via the interface N4mb) for setting up MBS delivery path.¶
5MBS has specified two data delivery modes, individual delivery vs. shared delivery:¶
The 5MBS architecture in TS 23.247 [_3GPP-23.247] introduces some network challenges:¶
The REQ8 of [RFC7333] talks about the multicast efficiency between non-optimal and optimal routes, where it states that, in term of multicast considerations, DMM SHOULD enable multicast solutions to be developed to avoid network inefficiency in multicast traffic delivery.¶
The current 5MBS architecture requires all DL multicast traffic go through the (centralized) MB-UPF, regardless of using the individual or shared delivery. In many operators' networks, 5GS might be deployed in a location that is relatively distant from customer (edge) sites. In this scenario, the efficiency of multicast transmission will be compromised. On the other aspect, 5G dUPF, deployed closer to gNB, will make the implementation more efficient:¶
The draft [I-D.zzhang-dmm-5g-distributed-upf] discussed and compared briefly different tunneling mechanisms to implement 3GPP GTP, i.e., SRv6, MPLS as the underlay, or in [I-D.mhkk-dmm-srv6mup-architecture] specifying a new SRv6 based MUP architecture to replace GTP. While these proposals may experience different issues upon 5MBS transport implementation, dUPF will make it more feasible.¶
This document requests no IANA actions.¶