Internet-Draft | 5G dUPFs and 5MBS | March 2023 |
Jiang & Han | Expires 10 September 2023 | [Page] |
The drafts [I-D.zzhang-dmm-5g-distributed-upf] and [I-D.zzhang-dmm-mup-evolution] have described the 5G mobile user plane (MUP) via the refinement of distributed UPFs and a more radical proposal by integrating gNB & UPF as a single network function (NF). Some user plane implementation requirements that vendors and operators are exploring are not introducing changes to 3GPP architecture & signaling, if possible. 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, specifically if coupled with the increasingly popular satellite-related requirements, these would certainly 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 DNN.¶
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 user plane from the MB-UPF to gNB directly (i.e., the lower-path in the above figure for the shared delivery) and the user plane from the MB-UPF to UPFs then to gNB (i.e., the upper path in the figure for individual delivery) may use IP multicast transport via a common GTP-U tunnel per MBS session, or use unicast transport via separate GTP-U tunnels at gNB or at UPF per MBS session. When using the IP multicast transport, GTP-U Multicast Tunnels shall be used for unidirectional transfer of the encapsulated T-PDUs from one GTP-U Tunnel Endpoint (i.e., acting as the sender) to multiple GTP-U Tunnel Endpoints (i.e., acting as receivers). The Common Tunnel Endpoint ID (C-TEID) which is present in the GTP header shall indicate which tunnel a particular T-PDU belongs to. The C-TEID value to be used in the TEID field shall be allocated at the source Tunnel Endpoint (e.g., the sender) and signaled to the destination Tunnel Endpoints (e.g., receivers) using a control plane protocol, e.g., GTPv1-C & GTPv2-C. One C-TEID shall be allocated per MBMS bearer service or per MBS session [_3GPP-23.247][_3GPP-29.281]. As we have explained in the draft [I-D.zzhang-dmm-mup-evolution], the signaling overhead to establish a N3 GTP unicast tunnel has reached seven steps, let alone the case of the more complicated MBS tunnel creation.¶
The 5G service via the satellite constellation has become a popular topic in 3GPP. There are currently three major satellite-related projects in SA workgroups, i.e., the satellite access (SAT_Ph2) [_3GPP-23.700-28] and the satellite backhaul (SATB) [_3GPP-23.700-27] in SA2 as well as the Phase-3 enhancement via the satellite-based store-and-forward technology (SAT_Ph3) in SA1 WG [_3GPP-22.865]. These projects study various 5GS requirements when either a gNB or a UPF or both are on-board satellites. Evidently, the continuously-moving satellite constellations introduce another dimension of challenges to UE registration, session management and traffic routing. The GTP-U tunnel end points have to be changed frequently when the satellite providing the on-board service for a UPF rotates away from the corresponding gNB of the same GTP-U tunnel. For the SAT_access case, the ground station (GS) has to find a new gNB on-board another satellite every couple of minutes (e.g., being around 7-8 minutes for the LEO category) to hand over UEs. There are significantly large amount of singalling messages involved even for unicast case via satellite constellation, let alone if we extend the similar scenarios to 5G MBS communication.¶
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 distant from customer sites. If the deployed site happens to be on-board satellites, the additional complexities and moving dynamics will certainly worsen the operations. In these scenarios, the efficiency of multicast transmission will be compromised. On the other aspect, a 5G dUPF deployed closer to gNB, or even more radically applying 'ANUP' via the possible integration of gNB & UPF [I-D.zzhang-dmm-mup-evolution], might lead to more efficient implementation:¶
When we take into consideration the above plausible arguments and accordingly apply them to different 3GPP satellite-related projects, e.g., SATB (backhaul), SAT_Ph2 & SAT_Ph3 (access), we can certainly draw the conclusion that the extra burden of signalling messages, the complexity of control plane as well as the excessive encapsulations of user plane, as introduced by 5MBS, can be relieved dramatically.¶
Both drafts [I-D.zzhang-dmm-5g-distributed-upf] [I-D.zzhang-dmm-mup-evolution] discussed and compared briefly different tunneling mechanisms to implement the 3GPP GTP-U UP, i.e., SRv6, MPLS as the underlay, or in [I-D.mhkk-dmm-srv6mup-architecture] specifying a new SRv6 based MUP architecture to replace the GTP-U. While these proposals may experience different issues upon 5MBS transport implementation, the application of distributed or 'integrated' UPF might make it more feasible.¶
This document requests no IANA actions.¶