dmm Z. Zhang
Internet-Draft Juniper Networks
Intended status: Informational K. Patel
Expires: 26 August 2024 Arrcus
L. Contreras
Telefonica
K. Islam
Redhat
J. Mutikainen
NTT Docomo
T. Jiang
China Mobile
L. Jalil
Verizon
O. Sejati
XL Axiata
S. Zadok
Broadcom
23 February 2024
Mobile User Plane Evolution
draft-zzhang-dmm-mup-evolution-08
Abstract
This document describes evolution of mobile user plane in 5G,
including distributed User Plane Functions (UPFs) and alternative
user plane implementations that some vendors/operators are promoting
without changing 3GPP architecture/signaling, and further discusses
potentially integrating UPF and Access Node (AN) in 6G mobile
networks.
This document is not an attempt to do 3GPP work in IETF. Rather, it
discusses potential integration of IETF/wireline and 3GPP/wireless
technologies - first among parties who are familiar with both areas
and friendly with IETF/wireline technologies. If the ideas in this
document are deemed reasonable, feasible and desired among these
parties, they can then be brought to 3GPP for further discussions.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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document authors. All rights reserved.
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Table of Contents
1. Current User Plane in 5G . . . . . . . . . . . . . . . . . . 3
2. MUP Evolution in 5G: Distributed UPFs . . . . . . . . . . . . 4
2.1. Advantages of Distributed PSA UPFs . . . . . . . . . . . 5
2.2. Enablers of Distributed PSA UPFs . . . . . . . . . . . . 6
3. MUP Evolution in 5G: Alternative Transport Options . . . . . 7
3.1. GTP vs. SRv6 vs. MPLS tunneling . . . . . . . . . . . . . 7
3.2. Routing Based UPF-Lite . . . . . . . . . . . . . . . . . 8
4. MUP Evolution for 6G . . . . . . . . . . . . . . . . . . . . 9
4.1. UPF Distribution and RAN Decomposition . . . . . . . . . 9
4.2. Integrated AN/UP Function (ANUP) . . . . . . . . . . . . 9
4.3. ANUP Potential Use-case: 5G-A Satellite Services . . . . 11
4.4. An Existing ANUP-like Feature in 4G: Local IP Access
(LIPA) . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Some considerations with integrated ANUP . . . . . . . . . . 13
5.1. Separate AN/UP Functions . . . . . . . . . . . . . . . . 13
5.2. Simplified/reduced Signaling and optimized data plane . . 14
5.3. Microservice architecture . . . . . . . . . . . . . . . . 15
5.4. Increased burden on previously simple AN . . . . . . . . 15
5.5. Use of ULCL I-UPF for MEC Purpose . . . . . . . . . . . . 16
5.6. VPN PE Function in AN/ANUP . . . . . . . . . . . . . . . 17
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5.7. QoS Handling . . . . . . . . . . . . . . . . . . . . . . 18
5.8. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
8. Informative References . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Current User Plane in 5G
Mobile User Plane (MUP) in 5G [_3GPP-23.501] has two distinct parts:
the Access Network part between UE and AN/gNB, and the Core Network
part between AN/gNB and UPF.
N3 N9 N6
UE AN(gNB) | I-UPF | PSA UPF |
+---------+ | | |
|App Layer| | | routing | __
+---------+ | |+--/---+---\-+| ( )
|PDU Layer| relay | relay || PDU | || ( )
+---------+ +---/--+--\---+|+---/--+--\---+|+------+IP+L2|| ( )
| | | |GTP-U |||GTP-U |GTP-U |||GTP-U | || ( DN )
| 5G-AN | |5G-AN +------+||------+------+||------+ or || ( )
| | | |UDP+IP|||UDP+IP|UDP+IP|||UDP+IP| || ( )
| Proto | |Proto +------+||------+------+||------+Ether|| ( )
| | | | L2 ||| L2 | L2 ||| L2 | || --
| Layers | |Layers+------+||------+------+||------+-----+|
| | | | L1 ||| L1 | L1 ||| L1 | L1 ||
+---------+ +------+------+|+------+------+|+------+-----+|
| | |
For the core network (CN) part, N3 interface extends the PDU layer
from AN/gNB towards the PSA UPF, optionally through I-UPFs and in
that case N9 interface is used between I-UPF and PSA UPF.
Traditionally, UPFs are deployed at central locations and the N3/N9
tunnels extend the PDU layer to them. The N3/N9 interface uses GTP-U
tunnels that are typically over a VPN over a transport
infrastructure. While N6 is a 3GPP defined interface, it is for
reference only and there is no tunneling or specification involved.
It is simply a direct IP (in case of IP PDU session) or Ethernet (in
case of Ethernet PDU session) connection to the DN.
At the AN/gNB, relay is done between the radio layer and the GTP-U
layer. At the PSA UPF, routing/switching is done for IP/Ethernet
before GTP-U encapsulation (for downlink traffic) or after GTP-U
decapsulation (for uplink traffic).
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2. MUP Evolution in 5G: 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
+---------+ +------+------+|+------+-----+|
| |
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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.
2.1. Advantages of Distributed PSA UPFs
Distributed PSA UPFs have the following advantages:
* MEC becomes much simpler - no need for centralized PSA UPF plus
ULCL UPFs, and no need for special procedures for location based
edge server discovery.
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* For LAN-type services, UE-UE traffic can be optimized (no need to
go through centralized PSA UPFs) when UPFs maintain host routes.
It also allows seamless integration of services across wireline/
wireless-connected customer sites.
* N3/N9 tunneling is simplified
In particular, there is now only short/simple N3 tunneling between
AN/gNB and local UPFs in proximity. Among the distributed UPFs and
other DN sites, versatile IETF/wireline VPN technologies are used
instead. For example:
* Any tunneling technology - MPLS, SR-MPLS or SRV6 - with any
traffic engineering/differentiation capabilities can be used.
Removal of the GTP/UDP header (and IPv4/IPv6 header in case of
MPLS data plane) brings additional bandwidth savings in the
transport infrastructure.
* Any control plane model for VPN can be used - traditional
distributed or newer controller based route advertisement.
In short, the distributed PSA UPFs model achieves "N3/N9/N6 shortcut
and central UPF bypass", which is desired by many operators.
Notice that, since UPF has routing functions, depending on the
capability of a UPF device, it may even be possible for a UPF device
to act as a VPN PE. That can be done in one of the two models:
* The UPF function and VPN PE function are separate but co-hosted on
the same device with a logical/internal N6 connection between
them.
* The UPF and VPN PE function are integrated and the PDU sessions
become VPN PE-CE links.
The second model is especially useful when a UE is multi-homed to
different EVPN PEs in case of Ethernet PDU sessions - EVPN's all-
active multihoming procedures can be utilized.
2.2. Enablers of Distributed PSA UPFs
To distribute PSA UPFs, if persistent addresses must be used for UEs,
the SMF must be able to allocate persistent IP addresses from a
central pool even when a UE re-anchors at different PSA UPFs (e.g.
due to mobility). If DHCPv4 is used, either the SMF acts as a
central DHCP server or it relays DCHP requests to a central DHCP
server on the DN.
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The distributed PSA UPFs must be able to advertise host routes in the
DN. This should not be a problem since a UPF is essentially a router
in that it routes traffic between DN and UEs (that are connected via
PDU sessions).
Notice that, advertising host routes for persistent IP addresses is
no different from advertising MAC addresses in case of Ethernet PDU
sessions.
3. MUP Evolution in 5G: Alternative Transport Options
3.1. GTP vs. SRv6 vs. MPLS tunneling
3GPP specifies that all tunneling (e.g. N3/N9) use GTP, whose
encapsulation includes IP header, UDP header and GTP header. The
tunnel is between 3GPP NFs (e.g. gNBs and UPFs) over an IP transport,
and the IP transport may be a VPN over the multi-service transport
infrastructure of an operator.
There have been proposals to replace GTP with SRv6 tunnels for the
following benefits:
* Traffic Engineering (TE) and Service Function Chaining (SFC)
capability provided by SRv6
* Bandwidth savings because UDP and GTP headers are no longer needed
While 3GPP has not adopted the proposal, and GTP can be transported
over SRv6 (as overlay, instead of SRv6 replacing GTP), some operators
still prefer to replace GTP with SRv6 "under the hood". That is,
while RAN/UPF still use N2/N4 signaling, the actual tunnel are no
longer GTP but SRv6 based on GTP parameters signaled by N2/N4. The
SRv6 tunnel could be between two NFs, or a GW could be attached to an
NF that still use traditional GTP and the GW will convert GTP to/from
SRv6. This is specified in [I-D.ietf-dmm-srv6-mobile-uplane].
Similarly, if an operator prefers to use MPLS, a GTP tunnel can also
be replaced with an MPLS PW instead of an SRv6 tunnel. Compared with
SRv6, it is even more bandwidth efficient (no need for a minimum
40-byte IPv6 header) and SR-MPLS can also provide TE/SFC
capabilities. This is specified in
[I-D.zzhang-pals-pw-for-ip-udp-payload].
Note that, While only IPv6 can scale to the 5G requirements for the
transport infrastructure, it does not mean MPLS can not be used as
data plane in the IPv6 network.
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3.2. Routing Based UPF-Lite
Traditionally, a UPF is implemented to follow 3GPP specifications.
Specifically, N4 signaling is used for SMF to instruct a UPF to set
up its session state in terms of PDRs/FARs. On N6 side, a UPF
receives downlink traffic with destination addresses that are covered
by the UPF's address range for its anchored UEs. The packet is
matched against the installed PDRs and forwarded according to the
associated FARs. On N3 side, a UPF decapsulates GTP+UDP+IP header of
uplink traffic and uses the TEID to identify the DN where inner IP
routing or Ethernet switching is done.
[I-D.mhkk-dmm-srv6mup-architecture] specifies a new SRv6 based MUP
architecture. When it is applied to a 3GPP based mobile
architecture:
* BGP signaling from a MUP Controller replaces N4 signaling from
SMF. N4 signaling is still used between the MUP Controller and
SMF - from SMF's point of view it is just interacting with a
traditional UPF as usual.
* A MUP GW becomes a distributed UPF for uplink traffic.
* A MUP PE, which is different from a usually central PSA UPF,
becomes a UPF for downlink traffic, in that traffic to each UE is
placed into a different tunnel that is stitched to a GTP tunnel
for that UE by a MUP GW (no route lookup is needed on the MUP GW
for the downlink traffic).
In this approach UE to UE traffic may still optionally go through the
central PSA UPF. This is similar to that a hub router may be used in
Section 2.
This approach can be viewed as a specific way of implementing/
deploying a subset of functionalities of distributed UPFs discussed
in Section 2, specifically the routing/switching functionalities,
hence often referred to as UPF-Lite. It does have the advantage that
from SMF's point of view, nothing is different from before - both
from N4 signaling and deployment model point of view.
While the above is specific to SRv6, a similar MPLS based approach
will be specified separately for operators who prefer MPLS data
plane, and it can even be SR-agnostic.
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4. MUP Evolution for 6G
This section discusses potential MUP evolution in 6G mobile networks.
It does involve changes in 3GPP architecture and signaling, so the
purpose is to share the ideas in IETF/wireline community first. If
it gains consensus within IETF/wireline community especially among
mobile operators, then the proposal may be brought to 3GPP community
for further discussions.
4.1. 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.
4.2. 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:
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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:
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* 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 5.2
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 5.6).
4.3. 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.
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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.
4.4. An Existing ANUP-like Feature in 4G: Local IP Access (LIPA)
While Section 4.2 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'.
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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.
5. Some considerations with integrated ANUP
Various considerations/concerns were brought up during the
discussions of the ANUP proposal. They are documented in the
following sections.
5.1. 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
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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
5.2. 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.
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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.
5.3. 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.
5.4. 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.
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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.
5.5. 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.
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5.6. 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,
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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.
5.7. 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.
5.8. 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.
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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.
6. Security Considerations
To be provided.
7. Acknowledgements
The authors thank Arda Akman, Constantine Polychronopoulos, Sandeep
Patel and Shraman Adhikary for their review, comments and suggestions
to make this document and solution more complete.
8. Informative References
[I-D.ietf-dmm-srv6-mobile-uplane]
Matsushima, S., Filsfils, C., Kohno, M., Camarillo, P.,
and D. Voyer, "Segment Routing IPv6 for Mobile User
Plane", Work in Progress, Internet-Draft, draft-ietf-dmm-
srv6-mobile-uplane-24, 17 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dmm-
srv6-mobile-uplane-24>.
[I-D.mhkk-dmm-srv6mup-architecture]
Matsushima, S., Horiba, K., Khan, A., Kawakami, Y.,
Murakami, T., Patel, K., Kohno, M., Kamata, T., Camarillo,
P., Horn, J., Voyer, D., Zadok, S., Meilik, I., Agrawal,
A., and K. Perumal, "Mobile User Plane Architecture using
Segment Routing for Distributed Mobility Management", Work
in Progress, Internet-Draft, draft-mhkk-dmm-srv6mup-
architecture-06, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-mhkk-dmm-
srv6mup-architecture-06>.
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[I-D.zzhang-dmm-5g-distributed-upf]
Zhang, Z. J., Patel, K., Jiang, T., and L. M. Contreras,
"5G Distributed UPFs", Work in Progress, Internet-Draft,
draft-zzhang-dmm-5g-distributed-upf-01, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-zzhang-dmm-
5g-distributed-upf-01>.
[I-D.zzhang-pals-pw-for-ip-udp-payload]
Zhang, Z. J. and K. Patel, "PW for IP/UDP Payload without
IP/UDP Headers", Work in Progress, Internet-Draft, draft-
zzhang-pals-pw-for-ip-udp-payload-01, 4 March 2022,
<https://datatracker.ietf.org/doc/html/draft-zzhang-pals-
pw-for-ip-udp-payload-01>.
[ORAN-Arch]
"O-RAN Architecture Description, V06.00", 2022.
[RFC7024] Jeng, H., Uttaro, J., Jalil, L., Decraene, B., Rekhter,
Y., and R. Aggarwal, "Virtual Hub-and-Spoke in BGP/MPLS
VPNs", RFC 7024, DOI 10.17487/RFC7024, October 2013,
<https://www.rfc-editor.org/info/rfc7024>.
[_3GPP-23.401]
"General Packet Radio Service (GPRS) enhancements for
Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access, V18.2.0", June 2023.
[_3GPP-23.501]
"System architecture for the 5G System (5GS), V18.4.0",
December 2023.
[_3GPP-23.700-29]
"Study on integration of satellite components in the 5G
architecture; Phase 3, V19.2.0", February 2024.
Authors' Addresses
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
Keyur Patel
Arrcus
Email: keyur@arrcus.com
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Luis M. Contreras
Telefonica
Email: luismiguel.contrerasmurillo@telefonica.com
Kashif Islam
Redhat
Email: kislam@redhat.com
Jari Mutikainen
NTT Docomo
Email: mutikainen@docomolab-euro.com
Tianji Jiang
China Mobile
Email: tianjijiang@chinamobile.com
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
Ori Prio Sejati
XL Axiata
Email: ORIP@xl.co.id
Shay Zadok
Broadcom
Email: shay.zadok@broadcom.com
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