Mobile User Plane Evolution

2.1. Distributed UPFs

With MEC, ULCL UPFs are deployed closer to gNBs, while centralized PSA UPFs are still used to provide persistent IP addresses to UEs.¶

In fact, even PSA UPFs could be distributed closer to gNBs and then the N3 interface becomes very simple – over a direct or short transport connection between gNB and UPF (or even an internal connection if the gNB and UPF are hosted on the same server). On the other hand, since the UPF to DN connection is direct, the DN becomes a VPN (e.g., IP VPN in case of IP PDU sessions or EVPN in case of Ethernet PDU sessions) over a transport infrastructure, most likely the same transport infrastructure for the VPN supporting the N3/N9 tunneling in centralized PSA UPF case, as shown in the following picture:¶

                          N3             N6
    UE1         AN1/gNB1   |  PSA UPF1    |
+---------+                |              |
|App Layer|                |    routing   |
+---------+                |+--/---+---\-+|
|PDU Layer|      relay     || PDU  |     ||      PE1
+---------+ +---/--+--\---+|+------+IP+L2||    +----+--+
|         | |      |GTP-U |||GTP-U |     ||----+VRF1|  |
| 5G-AN   | |5G-AN +------+||------+  or ||    +----+  |
|         | |      |UDP+IP|||UDP+IP|     ||    |VRFn|  |
| Proto   | |Proto +------+||------+Ether||    +----+--+
|         | |      |  L2  |||  L2  |     ||   (         )
| Layers  | |Layers+------+||------+-----+|  (           )
|         | |      |  L1  |||  L1  |  L1 ||  ( Transport  )
+---------+ +------+------+|+------+-----+|  (            )
                           |              |  ( Network    )  PE3
                           |              |  (            +--+----+
    UE2         AN2/gNB2   |  PSA UPF2    |  (            |  |VRF1|
+---------+                |              |  (            |  |----+
|App Layer|                |    routing   |  (            |  |VRFn|
+---------+                |+--/---+---\-+|  (            +--+----+
|PDU Layer|      relay     || PDU  |     ||  (            )
+---------+ +---/--+--\---+|+------+IP+L2||  (           )
|         | |      |GTP-U |||GTP-U |     ||   (         )
| 5G-AN   | |5G-AN +------+||------+  or ||    +----+--+
|         | |      |UDP+IP|||UDP+IP|     ||----+VRF1|  |
| Proto   | |Proto +------+||------+Ether||    +----+  |
|         | |      |  L2  |||  L2  |     ||    |VRFn|  |
| Layers  | |Layers+------+||------+-----+|    +----+--+
|         | |      |  L1  |||  L1  |  L1 ||      PE2
+---------+ +------+------+|+------+-----+|
                           |              |

The central PSA UPF is no longer needed in this case. Distributed UPF1/UPF2 connect to VRF1 on PE1/PE2 and VRF1 is for the VPN of the DN that UE1/UE2 access. There is also a PE3 for other sites of the VPN, which could be wireline sites including sites providing Internet access.¶

UEs may keep their persistent IP addresses even when they re-anchor from one PSA UPF to another. In that case, for downlink traffic to be sent to the right UPF, when a UE anchors at a UPF the UPF advertises a host route for the UE and when a UE de-achors from a UPF the UPF withdraws the host route.¶

While this relies on host routes to direct to-UE traffic to the right UPF, it does not introduce additional scaling burden compared to centralized PSA UPF model, as the centralized UPFs need to maintain per-UE forwarding state (in the form of PDRs/FARs) and the number is the same as the number of host routes that a hub DN router (e.g. vrf1 on PE3 for internet access) need to maintain in the distributed PSA UPFs model. Since the host routes may be lighter-weighted than the PDRs/FARs, the total amount of state may be actually smaller in the distributed model.¶

For UE-UE traffic, the distributed PSA UPFs may maintain host routes that they learn from each other. With that the UE-UE traffic may take direct UPF-UPF path instead of going through a hub router in the DN (equivalent of central UPF). That is important in LAN-type services that require low delay. Alternatively, the distributed UPFs may maintain only a default route pointing to the hub router like PE3 (besides the host routes for locally anchored UEs). That way, they don't need to maintain many host routes though UPF-UPF traffic has to go through the hub router (and that is similar to all traffic going through a central PSA UPF).¶

Optionally, even the host routes for locally anchored UEs can be omitted in the FIB of local UPF. Traffic among local UEs can be simply routed to the hub router following the default route, who will then send back to local UPF using VPN tunnels (MPLS or SRv6) that are stitched to GTP tunnels for destination UEs.¶

2.2. Alternative Transport Options for 5G

3.1. UPF Distribution and RAN Decomposition

As described earlier, with 5G, in the opposite direction of UPF distribution, some RAN components are becoming centralized as a result of the disaggregation and decomposition of baseband processing functions. The AN functionality is now divided into the Radio Unit (RU, comprising the antenna and radiating elements), the Distributed Unit (DU, comprising the functions for the real time processing of the signal), and the Centralized Unit (CU, comprising the remaining signal processing functions). CU is the AN function that handles N3 GTP-U encapsulation for UpLink (UL) traffic and decapsulation for DownLink (DL) traffic.¶

The placement of the decomposed CU component can converge with the distribution process of the UPF to some optimal and convenient location in the network - they become co-located in an edge or far edge data center (DC) either with direct/short local connections in between or both running as virtual functions on the same compute server.¶

3.2. Integrated AN/UP Function (ANUP)

While the AN (CU) and UPF can be co-located, in 5G they are still separate functions connected by N3 tunneling over a short/internal transport connection. Routing happens on the UPF between the DN and UEs over the N3 tunnels, and relay happens on the AN between the N3 tunnels and AN protocol stack.¶

With AN and UPF functions more and more disaggregated and virtualized even in 5G, it is becoming more and more feasible and attractive to integrate the AN and UPF functions, eliminating the N3 tunneling and the relay on AN entirely. The combined function is referred to as ANUP in this document, which does routing between DN and UEs over the AN protocol stack directly:¶

                         N6
    UE1          ANUP     |
+---------+               |
|App Layer|     routing   |
+---------+ +--/---+---\-+|
|PDU Layer| | PDU  |     ||      PE1
+---------+ +------+IP+L2||    +----+--+
|         | |      |     ||----+VRF1|  |
| xG-AN   | |xG-AN +  or ||    +----+  |
|         | |      |     ||    |VRFn|  |
| Proto   | |Proto +Ether||    +----+--+
|         | |      |     ||   (         )
| Layers  | |Layers+-----+|  (           )
|         | |      |  L1 ||  ( Transport  )
+---------+ +------+-----+|  (            )
                          |  ( Network    )  PE3
                          |  (            +--+----+
    UE2          ANUP     |  (            |  |VRF1|
+---------+               |  (            |  |----+
|App Layer|     routing   |  (            |  |VRFn|
+---------+ +--/---+---\-+|  (            +--+----+
|PDU Layer| | PDU  |     ||  (            )
+---------+ +------+IP+L2||  (           )
|         | |      |     ||   (         )
| xG-AN   | |xG-AN +  or ||    +----+--+
|         | |      |     ||----+VRF1|  |
| Proto   | |Proto +Ether||    +----+  |
|         | |      |     ||    |VRFn|  |
| Layers  | |Layers+-----+|    +----+--+
|         | |      |  L1 ||      PE2
+---------+ +------+-----+|
                          |

With this architecture, 3GPP and IETF technologies are applied where they are best applicable: 3GPP technologies responsible for radio access and IETF technologies for the rest. As IETF technologies continue to evolve, they can be automatically applied in mobile networks without any changes in 3GPP architecture/specification.¶

One way to view this is that the ANUP is a router/switch with wireless and wired interfaces and it routes/switches traffic among those interfaces. The wireless interface is established by 3GPP technologies (just like an Ethernet interface is established by IEEE technologies) and the routing/switching function follows IETF/IEEE standards.¶

Some advantages of this new architecture include:¶

• 5G-LAN and MEC become transparent applications that wireline networks have been supporting (PDU sessions terminate into the closest ANUP and routed/switched to various DNs).¶

• MBS becomes very simple – the ANUP gets the multicast traffic in the DN and then use either shared radio bearer or individual bearers to send to interested UEs.¶

• Simplified signaling - instead of seven-steps of separate N2/N4 signaling from separate AMF/SMF to separate AN/UPF and N11 signaling between AMF and SMF to set up the N3 tunneling for a PDU session, a two-step signaling between a new single control plane entity to the single integrated ANUP is enough - see Section 4.2 for details.¶

• Simplified/Optimized data plane - AN-UPF connection and GTP-U encapsulation/decapsulation are not needed anymore. This can significantly improve throughput, especially when compared to AN/UPF functions running on servers.¶

• Natural local break-out in traffic forwarding, by allowing the more efficient routing/switching of traffic according to its destination.¶

• Any kind of tunnels can be used for the DN VPN, whether it is MPLS or SRv6, w/o the overhead of UDP/GTP encapsulation compared to GTP tunneling. Network slicing and QoS functions are still supported (even with current GTP tunneling the transport network need to instantiate slices and implement QoS for N3/N9 tunnels as well).¶

Because the ANUP already implement the routing/switching functions, even the PE functions (for the DN VPN) could be optionally integrated into it, further streamlining end-to-end communication by reducing NFs and connections between them. While integrating PE function is optional, it is desired and today's AN can be already considered as a PE (Section 4.6).¶

3.3. ANUP Potential Use-case: 5G-A Satellite Services

The 3GPP SA2 working group has several projects to study & standardize the 5G advanced services whose wireless connectivities are provided via satellite networks. These projects cover various aspects of satellite services, e.g., one focusing on the support of wireless access considering the satellite-based discontinuous coverage, while the 2nd-one studying the service requirements via satellite backhaul taking into account 5G new capabilities.¶

Still, there is a 3rd project exploring the scenario that a gNB will be on board satellite while the corresponding anchor UPF may (i.e., on-board a satellite) or may not (i.e., on the ground). Evidently, this is a very challenging case that requires the seamless integration among AN (i.e., gNB), UPF & 5GS.¶

An on-board UPF might not share the same underlaying satellite as the matching gNB. For this case, thanks to the everlasting movement of (LEO-based) satellites, the highly mobile satellite constellation network will significantly impact the signaling performance between the gNB and the UPF. Therefore, some measures must be adopted to reduce the signalling impact to the AN/RAT, to the UP (UPF) and to the CP (5GS).¶

Further, a latest 5G-service, the satellite-based store & foward (S&F) feature for (on-ground) UEs via intermittent (satellite) service-link and/or feed-link connectivities [_3GPP-23.700-29], has embraced quite a few proposals in which the AN (i.e. gNB), the CP (i.e., 5GS/EPC) and the UP (i.e., UPF/S-GW,P-GW) could be either deployed together (being less challenging) or distributed (being much more complicated). In some proposal(s), even an individual CP and/or UP NF (network function) might be decomposed into multiple (sub)-instances to accomodate the complexity of distributedness. However, if we plug into the above S&F service requirements into the integrated ANUP architecture, there is no more implication of the distribution of gNB and UPF. The complexity of both the CP signaling exchanges and the UP data transport will be greatly relieved.¶

Given the ubiquitous discussion of the satellite communication for 5G, beyond-5G and imminent 6G, we do believe our proposal ANUP will benefit materially both the IETF and the 3GPP communities.¶

3.4. ANUP-like Feature in 4G: Local IP Access (LIPA)

While Section 3.2 proposed the integrated AN and UPF, or ANUP, for the evolution of 6G MUP, the 3GPP specification 23.401 [_3GPP-23.401] has already standardized an ANUP-like function, i.e., the Local IP Access or LIPA, that fundamentally integrates together the 4G RAN entity 'HeNB or Home eNodeB' and the traffic switching gateway 'L-GW or Local Gateway'.¶

     LIPA @ DN            DN: Data Network
    ^     |               UP: User Plane
    |     |SGi
    |  +--+---+    S5
    |  | L-GW |-----------\
    |  +------+   S1-U     \+-----+  S5  +------+ SGi  /----\
    |  | HeNB +-------------+ SGW +------+ P-GW +-----<  DN  >
    |  +--+---+\            +-----+      +------+      \----/
  UP|     |     \S1-MME    /S11
    |     |Uu    \        /
    |  +-----+    +------+
    |  |     |    |  MME |
    +--+ UE  |    +------+
       +-----+

The above figure shows the LIPA architecture. It enables a UE (on the bottom-left) that can connect via a HeNB to access the DN without the user plane traversing the mobile operator's network (e.g., SGW->P-GW). The LIPA feature is achieved using a L-GW (on the top-left) that is collocated with the HeNB. The functionalities of HeNB and L-GW are integrated together to provide the direct User-Plane (UP) path between the HeNB and the L-GW. There is NO reference interface between HeNB and L-GW. That is, they are truly an integrated entity.¶

As of now, while the LIPA feature has not yet been deployed extensively by MNO's, it does give somewhat promising indicator that the ANUP-like integration solution has been studied before by 3GPP and it is worthy of the continuous exploration.¶

4.1. Separate AN/UP Functions

There are still cases where separate AN/UP functions are desired/required:¶

• An MNO may want to deploy one UPF for a cluster of ANs in proximity in some scenarios/locations¶

• An MNO may support MVNOs who have their own UP functions but make use of the hosting MNO's ANs¶

• Home Routed roaming requires separate HPLMN UPs and VPLMN ANs¶

Therefore, the integration does not have to be always used. Rather, it is "integration when desired and feasible, separation when necessary".¶

Note that, the same ANUP can handle both situations - some PDU sessions may be tunneled to a separate UPF while other sessions are terminated and then traffic is routed/switched to either local DN or remote/central DN.¶

This is also the basis of interworking between 5G and xG:¶

• A 5G AN can have N3 tunneling to an xG UPF¶

• An xG ANUP can have N3 tunneling to a 5G/xG UPF¶

4.2. Simplified/reduced Signaling and optimized data plane

One may ask why bother with integration when it is still needed to support separate AN and UPF anyway.¶

When AN and UPF are separate, to set up the N3 tunnel the following seven steps are needed, involving four NFs and three Nx interfaces:¶

1. SMF sends request to UPF (N4)¶

2. UPF responds with UPF-TEID (N4)¶

3. SMF passes <UPF, UPF-TEID> to AMF (N11)¶

4. AMF sends request to gNB, passing <UPF, UPF-TEID> (N2)¶

5. gNB responds with AN-TEID (N2)¶

6. AMF passes <AN, AN-TEID> to SMF (N11)¶

7. SMF sends <AN, AN-TEID> to UPF (N4)¶

With integrated ANUP, there is no need for N3 tunnel anymore. A new control plane NF only needs to tell the ANUP which DN that PDU session belongs to.¶

Additionally, the N3 tunnel is maintained by periodical signaling refreshes - otherwise timeout will happen. This causes significant control plane load on the NFs and interfaces, which no longer exists with ANUP since N3 tunneling is eliminated.¶

As mentioned before, with ANUP the AN-UPF connection and GTP-U encapsulation/decapsulation are not needed anymore. This can significantly improve performance/throughput, especially when compared to AN/UPF functions running on servers.¶

4.3. Microservice architecture

One may argue that the integration of AN and UP functions are against the microservice trend.¶

The following is a verbatim quote from https://microservices.io/:¶

  Microservices - also known as the microservice architecture -
  is an architectural style that structures an application as a
  collection of services that are:

  - Highly maintainable and testable
  - Loosely coupled
  - Independently deployable
  - Organized around business capabilities
  - Owned by a small team
  - The microservice architecture enables the rapid, frequent
    and reliable delivery of large, complex applications.
    It also enables an organization to evolve its technology stack.

The counter argument is that microservice is about decomposing complex "applications". ANUP is about integrating co-located and mature data plane entities to streamline and optimize forwarding. It has real and significant benefits of simplified signaling and optimized data plane - it does not make sense to force microservice here for data plane. Note that microservices can still be utilized in the control plane for ANUP.¶

4.4. Increased burden on previously simple AN

One may think that the AN only needed to do simple traffic stitching functions while now the ANUP has added UPF burden. However, the main use case of ANUP is where the AN and UPF are co-located even if they are separate functions. Therefore, the ANUP only absorbs the whatever functionalities that the separate UPF at the same site need to do anyway, with reduced signaling and data plane handling - the overall processing at the site actually decreases. While a particular ANUP now has more processing to do, it can offload some sessions to additional ANUPs that are now made possible because of removal of separate UPFs at the same site.¶

This may also make it easier to allocate resources at the edge DC. Previously, an operator needs to consider how much resources to allocate for the separate UPFs and assign which sessions to which UPFs. Now it simply is to decide which sessions are assigned to which ANUP (just like to decide which sessions are assigned to which AN).¶

In addition, there are some similar or even overlapping functionalities in the current UPF and AN in 5GS; in integrated ANUP these functions can be re-designed. For example for a rate control enforcement, UPF supports the enforcement of the aggregated MBR for the session (Session-AMBR) in UL/DL directions, while AN can enforce the aggregated MBR for the UE (UE-AMBR) in UL/DL directions. Both UPF and AN support the enforcement of the QoS Flow MBR (MFBR) and GBR (GFBR) in both UL/DL directions (for the GBR flows), while AN can in additon to ensure the UE-Slice-MBR is not exceeded in UL/DL directions. With ANUP, these previously separate functions may be optimized now that they are in the same entity.¶

4.5. Use of ULCL I-UPF for MEC Purpose

Notice that the ANUP is to integrate AN and distributed UPF that are co-located in edge DCs, and one use case of distributed UPF (in those edge DCs) is MEC. UpLink CLassifier Intermediate UPF (ULCL I-UPF) is an existing way to achieve local breakout routing for MEC purpose, but it is not an optimized/elegant solution compared to ANUP.¶

The ULCL I-UPF is placed between an AN and a central UPF as a filtering device. While called an UPF it is different from a typical UPF - It inspects all GTP-U UL traffic, and based on N4 signaling from SMF certain traffic is intercepted and forwarded to local DN. This places additional control plane burden on SMF in addition to the need of the special traffic-filtering UPF. For example, the SMF will need to know which traffic (to some particular destination address) is to be intercepted.¶

For comparison, with ANUP there is no need for the additional special UPF and corresponding N4 signaling at all. Everything is standard routing/filtering w/o relying on SMF to determine which traffic is delivered locally:¶

• For some PDU sessions, all traffic may be tunneled to a separate UPF.¶

• For a particular PDU session, some traffic may be delivered locally while some other delivered to the central/remote DN all based on routing/filtering in the DN.¶

4.6. VPN PE Function in AN/ANUP

As previously mentioned, the ANUP can optionally have the VPN PE function integrated, instead of being a standalone CE device for the VPN for the DN.¶

While optional, it is a desired optimization. Moreover, even the separate AN itself can be considered as a spoke PE for a hub-and-spoke VPN [RFC7024] for the DN.¶

Consider a hub-and-spoke VPN outside the mobile network context:¶

• A spoke PE only imports a default route from a hub PE and therefore sends all traffic from its CEs to the hub PE¶

• A hub PE imports routes from all PEs and sends traffic to appropriate PEs or its CEs, whether the traffic is from a local CE or another PE¶

Additionally, consider that a spoke PE advertise different per-prefix (vs. per VRF) VPN labels. When it receives traffic with a per-prefix label, it can send traffic to a local CE purely based on the label without having to do a route lookup in the VRF.¶

Now consider the AN and the central UPF in a mobile network. Effectively the AN is a spoke PE and the central UPF is a hub PE for the DN:¶

• The GTP-U tunnel corresponds to the MPLS label stack.¶

• For UL traffic, there is no need for route lookup on the AN because all is to be tunneled to the UPF. The UPF TEID is used by the UPF to determine which DN the traffic belongs to, just like how a VPN label is used to determine VPN the traffic belongs to.¶

• For DL traffic, the UPF does a lookup based on the destination address (e.g., that of a UE) and a corresponding GTP-U tunnel is used to send traffic to an AN. When traffic arrives on the AN, the per-UE TEID allows traffic to be relayed to the UE without a route lookup.¶

In other words, the separate ANs and UPF form a hub-and-spoke VPN for the DN with per-prefix "labels", though no VRF is present on the ANs because there is no need for route lookup at all.¶

For ANUP with VPN PE function integrated, the only difference is the addition of VRF in the AN. That's so that some sessions will be locally terminated and traffic is locally routed. For DL traffic, the ANUP can either advertise per-VRF label (or SID in case of SR) and do a lookup for DL traffic, or advertises per-prefix/UE label (or SID in case of SR) - just like per-UE TEID - so that it does not to do a lookup before sending traffic to a UE.¶

4.7. QoS Handling

With separate AN and UPF, the QoS handling happens in the following segments:¶

• Between UE and AN over the air interface¶

• Between AN and UPF over the N3 tunnel, which can be:¶

  • through a transport network, or¶

  • through a local/internal link in co-location case¶

The QoS over the air interface is the same for both AN and ANUP cases.¶

For the trivial QoS previously over N3 tunnel through a local/internal link in co-location case, it is now completely eliminated with ANUP.¶

The QoS over N3 tunnel through a transport network is realized through QoS mechanisms in the transport network. With ANUP, it's likely that similar QoS is needed between the ANUP and a hub router in the DN, which is a VPN over the same transport network. Therefore, it is similar to the QoS over N3 tunnel - only that now it is QoS over VPN tunnel and realized through QoS mechanisms in the transport network.¶

A central UPF may have rate limiting for N3 tunnels so that each PDU session's DL traffic is limited and the AN won't be overwhelmed by DL traffic. With distributed UPF (whether integrated into AN or not), the routes advertised to the hub DN router may carry QoS information like rate limiting parameters, so that the hub DN router can do rate limiting.¶

4.8. NAT

Addresses assigned to UEs may be from a private address space and NAT is needed between the private space and public space. In case of central UPFs, the NAT can be done on a central UPF (though NAT is still a logically separate function) or by a separate NAT Gateway (GW) connected to the central UPF.¶

With distributed UPFs (whether it is a separate UPF or an integrated ANUP), NAT can be done by a central NAT GW connected to the hub router, just like a NAT GW on or next to the previously central UPF.¶

A large operator may have multiple central UPFs for different regions, and the regions may have overlapping private address spaces. Each UPF will have its own NAT GW, and UE to UE traffic across regions will go throw two NAT GWs. With distributed UPFs, each region will have its own hub router with its own NAT GW, and UE to UE traffic across regions will go through two NAT GWs and two hub routers.¶

5.1. User-plane/UP vs. Control-plane/CP

Stepping from the IETF perspective, this draft centers around the ANUP innovations along with its implications to 3GPP SDO. Because IETF focuses more on the connectivity of transport network (TN), this draft addresses mainly the mobile user plane or UP, e.g., re-design of the hub-and-spoke VPN settings different from those over the current separate AN & UPF architecture, alternative UP protocol(s) to GTP-U tunnel between AN and UPF (in the TN domain), etc. However, while this draft does not limit the discussions only to UP, but given the complexities of the 5G CP and the on-going discussions of the evolution of the 6G system architecture, the draft does not dive into the CP of the mobile wireless domain. All those mobility related CP details, e.g., RM, MM, SM, paging, handover, QoS settings, etc., are left to the 3GPP's further exploration & refinement. Certainly, the results from the UP investigation would benefit the CP design in 6G evolution.¶

5.2. Impacts & Intentions to 5G/6G CP

As set forth at the beginning, this draft does not intend to do the 3GPP 5G/6G work in IETF. In comparison, it actually acknowledges the principle that the complete studies should be done in the 3GPP SDO. The I.D. has argued that the innovative ANUP architecture does have certain advantageous impacts to the current 5GS CP (and likely to the future 6G evolution). But, given the complexity of 5GS, any ANUP related achievement in the IETF domain shall only serve as a reference to the 3GPP, possibly via the liaison exchange between the two SDO's.¶