Mobile User Plane Evolution
draft-zzhang-dmm-mup-evolution-04

dmm                                                             Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Informational                                  K. Patel
Expires: 14 September 2023                                        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
                                                           13 March 2023

                      Mobile User Plane Evolution
                   draft-zzhang-dmm-mup-evolution-04

Abstract

   [I-D.zzhang-dmm-5g-distributed-upf] 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.
   Building on top of that, this document further discusses potentially
   integrating UPF and Access Node (AN) in a future generation (xG) of
   mobile network.

   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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Table of Contents

   1.  MUP Evolution . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  UPF Distribution and RAN Decomposition  . . . . . . . . .   3
     1.2.  Integrated AN/UP Function in xG . . . . . . . . . . . . .   3
   2.  Some considerations with integrated ANUP  . . . . . . . . . .   6
     2.1.  Separate AN/UP Functions  . . . . . . . . . . . . . . . .   6
     2.2.  Simplified/reduced Signaling and optimized data plane . .   6
     2.3.  Mobility Handover . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Paging  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Microservice architecture . . . . . . . . . . . . . . . .   8
     2.6.  Increased burden on previously simple AN  . . . . . . . .   9
     2.7.  Use of ULCL I-UPF for MEC Purpose . . . . . . . . . . . .   9
     2.8.  VPN PE Function in AN/ANUP  . . . . . . . . . . . . . . .  10
     2.9.  QoS Handling  . . . . . . . . . . . . . . . . . . . . . .  11
     2.10. NAT . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  MUP Evolution

   [I-D.zzhang-dmm-5g-distributed-upf] describes evolution of mobile
   user plane in 5G [_3GPP-23.501], including distributed UPFs and
   alternative user plane implementations that some vendors/operators
   are pushing without changing 3GPP architecture/signaling.

   This document discusses potential MUP evolution in a future
   generation (referred to as xG) of mobile networks.  It does involve
   changes in 3GPP architecture and signaling, so the purpose of this
   section 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.

1.1.  UPF Distribution and RAN Decomposition

   In 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.

   This is also specified in [ORAN-Arch], with corresponding O-RU, O-DU
   and O-CU terms.

   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.

1.2.  Integrated AN/UP Function in xG

   While the AN (CU) and UPF can be co-located, 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.

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   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.

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   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 2.2
      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 2.8).

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2.  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.

2.1.  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

2.2.  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)

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   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.

2.3.  Mobility Handover

   Notice that ANUP is for the scenario of distributed UPFs (that are
   co-located with ANs) and the handover procedures for distributed UPFs
   (that are not integrated with ANs) applies to ANUP transparently as
   well.  UEs may have persistent IP addresses even when they re-anchor
   from one ANUP to another, as described in Section 2 of
   [I-D.zzhang-dmm-5g-distributed-upf], or they can just get a new
   address when they re-anchor to a different ANUP, in which case host
   routes are not needed.

2.4.  Paging

   In a mobile system like 5GS the UE may be in power-saving state when
   the mobile system receives a downlink packet targeted to the UE.  In
   5GS the UPF is responsible to buffer the packet and notify the SMF
   and AMF that a downlink data is pending.  AMF then instructs the RAN
   to page the UE, i.e. broadcast a signal to the UE to wake-up from the
   power-saving state (RRC-Idle or RRC-Inactive state).  After receiving
   the paging the UE reconnects to the gNB and N3 tunnel can be
   established between the UPF and gNB to deliver the buffered data to
   the UE.  The UE may also move under a new gNB while in a power-saving
   state; in this case the UE does not connect to a new gNB until
   receiving the paging message.

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   With integrated ANUP, the UP in ANUP would receive such downlink data
   packet while the UE is in power-saving state.  If the UE has moved
   out from this ANUP while in power-saving state, and is camping in
   another (target) ANUP when the source ANUP receives the downlink data
   packet, upon paging it reconnects to to the target ANUP and may
   preserve the IP address as described in section Section 2.3.  The
   source ANUP learns the new route for the UE and forwards the buffered
   data to the target ANUP.

   Another option is to re-use the RAN-based Notification Area as
   specified in 5GS.  In this case the ANUP that buffers the data is
   responsible to page the UE across all ANUPs within the RAN-based
   Notification Area, using the XnAP protocol over the Xn-C interface
   between the ANUPs.  If the UE wakes-up in a new target ANUP the UE
   could re-anchor to the target ANUP as described above.

   Again, notice that because ANUP is just the integration of previously
   separate but co-located AN and UPF functions, the above paging
   procedures are not different from when AN and UPF are separate.

2.5.  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.

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2.6.  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.

2.7.  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.

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   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.

2.8.  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.

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   *  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.

2.9.  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.

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   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.

2.10.  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.

3.  Security Considerations

   To be provided.

4.  Acknowledgements

   The authors thank Arda Akamn, Constantine Polychronopoulos, Sandeep
   Patel and Shraman Adhikary for their review, comments and suggestions
   to make this document and solution more complete.

5.  Informative References

   [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>.

   [ORAN-Arch]
              "O-RAN Architecture Description, V06.00", 2022.

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   [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-23.501]
              "System architecture for the 5G System (5GS), V17.3.0",
              December 2021.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net

   Keyur Patel
   Arrcus
   Email: keyur@arrcus.com

   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

Zhang, et al.           Expires 14 September 2023              [Page 13]
Internet-Draft                MUP Evolution                   March 2023

   Email: ORIP@xl.co.id

   Shay Zadok
   Broadcom
   Email: shay.zadok@broadcom.com

Zhang, et al.           Expires 14 September 2023              [Page 14]