dmm                                                             T. Jiang
Internet-Draft                                              China Mobile
Intended status: Informational                              7 March 2022
Expires: 8 September 2022


   5G Distributed UPFs for 5G Multicast and Broadcast Services (5MBS)
                    draft-tjiang-dmm-5g-dupf-5mbs-00

Abstract

   The companion draft [I-D.zzhang-dmm-5g-distributed-upf] has described
   the 5G mobile user plane (MUP) via the refinement of distributed
   UPFs, along with various user plane implementations that some vendors
   and operators are exploring, with the requirement of not introducing
   changes to 3GPP architecture & signaling.  The document 3GPP TS
   23.247 [_3GPP-23.247] for 5G multicast and broadcast services, or
   5MBS, specifies the 5GS architecture to support MBS communication.
   Thanks to the addition of new 5GS network functions (NFs) and MB-
   interfaces on 5G CP & UP, this might post additional provisioning &
   implementation challenges to the underlay transport infrastructure.

   This document is not an attempt to do 3GPP SDO work in IETF.
   Instead, it discusses how to potentially integrate distributed UPFs
   with the delivery of 5MBS communication, as well as the benefits of
   using distributed UPFs to handle 5MBS traffic delivery.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 8 September 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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Table of Contents

   1.  Distributed UPFs in 5G User Plane . . . . . . . . . . . . . .   2
   2.  5G Multicast and Broadcast Services (5MBS)  . . . . . . . . .   4
   3.  5G Distributed UPF for 5G MBS Communication . . . . . . . . .   5
     3.1.  5MBS Transport Challenges . . . . . . . . . . . . . . . .   5
     3.2.  5G Distributed UPF for 5MBS Implementation  . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Distributed UPFs in 5G User Plane

   Mobile User Plane (MUP) in 5G has two distinct parts: the Access
   Network part between UE and gNB, and the Core Network part between
   gNB and UPF.  UPFs are traditionally deployed at central locations,
   with UEs' PDU sessions encapsulated and extended thru GTP-U tunnels
   via the N3 (and potentially N9) interfaces in 5GS.  The interface N6
   supports fundamentally a direct IP or Ethernet connection to the data
   network or DN.

   Actually, UPFs could be distributed & deployed closer to gNBs.
   The draft [I-D.zzhang-dmm-5g-distributed-upf] has described the 5G
   mobile user plane (MUP) via the refinement of distributed UPFs or
   dUPFs.  The following picture shows the dUPF architecture:














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                             N3             N6
       UE1          gNB1      |     dUPF1    |
   +---------+                |+------+-----+|
   |   PDU   |                || PDU  |     ||      PE1
   +---------+ +------+------+|+------+ IP/ ||    +-----+--+
   |         | |      |GTP-U |||GTP-U |     ||----+ IP/ |  |
   | 5G-AN   | |5G-AN +------+|+------+Ether||    |Ether|  |
   | xHaul   | |xHaul |L3/2/1|||L3/2/1|     ||    +-----+--+
   +---------+ +------|------+|+------------+|   (          )
                              |              |  ( Transport  )  PE3
                              |              |  (  Network   +--+-----+
       UE2          gNB2      |     dUPF2    |  (            |  | IP/ |
   +---------+                |+------+-----+|  (   (DN)     |  |Ether|
   |   PDU   |                || PDU  |     ||   (           +--+-----+
   +---------+ +------+------+|+------+ IP/ ||    +-----+--+
   |         | |      |GTP-U |||GTP-U |     ||    | IP/ |  |
   | 5G-AN   | |5G-AN +------+|+------+Ether||    |Ether|  |
   | xHaul   | |xHaul |L3/2/1|||L3/2/1|     ||    +-----+--+
   +---------+ +-------------+|+------------+|      PE2

   In distributed UPF architecture, the central (PSA) UPF is no longer
   needed. dUPF1 and UPF2 connect via PE1 and PE2, respectively, to the
   DN VPN (or network instance/NI) that UE1 and UE2 intend to access.
   There could exist other PEs, like PE3 in the picture, for other sites
   of the same network domain(VPN or NI) or for global Internet access.

   There are some benefits of distributed UPFs:

   *  The N3 interface becomes very simple - over a direct or short
      transport connection between gNB and dUPF.

   *  The transport infrastructure off N3/N9 and N6 are straightforward,
      most likely over the same underlay VPN (MPLS, SR-MPLS or SRv6)
      supporting the traditional N3/N9 tunneling as in centralized PSA
      UPF case.

   *  MEC becomes much simpler since no need to deploy centralized PSA
      UPF plus ULCL UPFs; UE-UE traffic can be optimized for LAN-type
      services (via host-route).

   In short, the distributed UPFs model achieves "N3/N9/N6 shortcut and
   central UPF bypass", which is desired by many operators.









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2.  5G Multicast and Broadcast Services (5MBS)

   The 3GPP document TS 23.247 [_3GPP-23.247] for 5G multicast and
   broadcast services, or 5MBS, specifies the 5GS architecture to
   support MBS communication.  The following picture shows the brief
   system architecture of 5MBS:

                ----+----------(SBA for 5GC) ---------+-----
                    |          |                      |
                 +--+--+   +---+---+              +---+----+
                 | AMF |   |  SMF  |              | MB-SMF |
                 +--+--+   +-+-+-+-+              +---+----+
                   /           |                      |
               N2 /         N4 |                  N4mb|
                 /             |                      |
                /    N3    +-+-+---+     N19mb    +---+----+ N6mb +----+
           +-----+---------+  UPF  +--------------| MB-UPF |------| DN |
  +----+   |     |         +-------+ (Individual) +---+----+      +----+
  | UE +---+ gNB |                                    |
  +----+   +-----+                                    |
                 |_________N3mb (shared delivery)_____|

   TS 23.247 [_3GPP-23.247] adds new 5GS network functions (NFs) on both
   5G control-plane (CP) and user-plane (UP).  For example, the CP NF
   MB-SMF is, in collaboration with the regular SMF, to provision and
   signal to the UP NF MB-UPF (via the interface N4mb) for setting up
   MBS delivery path.

   5MBS has specified two data delivery modes, individual delivery vs.
   shared delivery:

   *  Individual delivery: When the (downlink or DL) MBS packets are
      received by the MB-UPF from the interface N6mb, MB-UPF replicates
      & forwards those packets towards (multiple) UPFs, via the
      interface N19mb, through either unicast (requiring multiple GTP
      tunnels if unicast underlay transport is applied) or multicast (if
      multicast underlay transport over N19mb is applied) transmission.

   *  Shared delivery: When the (DL) MBS packets are received by the MB-
      UPF from N6mb, MB-UPF replicates & forwards those packets towards
      (multiple) gNBs, via the interface N3mb (the lower-path in the
      picture), through either (multiple) separate GTP tunnels if
      unicast underlay transport over N3mb is applied, or a single GTP
      tunnel if multicast underlay over N3mb is supported.







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3.  5G Distributed UPF for 5G MBS Communication

3.1.  5MBS Transport Challenges

   The 5MBS architecture in TS 23.247 [_3GPP-23.247] introduces some
   network challenges:

   *  Because of the addition of new CP and UP NFs, this will post
      additional provisioning & implementation challenges to the
      underlay transport infrastructure.  For example, in the individual
      delivery mode, both SMF and MB-SMF have to synchronize with each
      other to help set up the relay/stitching path between UPF, MB-UPF
      and DN.

   *  The picture in previous section shows three new interface types
      corresponding to three different segments: N3mb, N6mb and N19mb.
      Based on the traffic delivery mode, once MB-UPF receives DL
      traffic from N6mb, it will have to do either individual or shared
      delivery.

   *  In accordance with TS 23.247 [_3GPP-23.247], the underlay
      transport infrastructure of all three segments can use either
      unicast or multicast transmission, based on the capabilities of
      underlay networks.  For example, for the DL _shared_ delivery from
      MB-UPF to gNB via the interface N3mb, 5G MBS packets can be
      transmitted to multiple gNBs via multicast transmission if the
      underlay network supports.  Otherwise, MB-UPF will have to use
      unicast to transmit separately to (multiple) gNBs.  Considering
      that this unicast/multicast flexibility is applicable to all the
      three above-mentioned segments, the implementation will have to
      face more challenges.

3.2.  5G Distributed UPF for 5MBS Implementation

   The REQ8 of [RFC7333] talks about the multicast efficiency between
   non-optimal and optimal routes, where it states that, in term of
   multicast considerations, DMM SHOULD enable multicast solutions to be
   developed to avoid network inefficiency in multicast traffic
   delivery.

   The current 5MBS architecture requires all DL multicast traffic go
   through the (centralized) MB-UPF, regardless of using the individual
   or shared delivery.  In many operators' networks, 5GS might be
   deployed in a location that is relatively distant from customer
   (edge) sites.  In this scenario, the efficiency of multicast
   transmission will be compromised.  On the other aspect, 5G dUPF,
   deployed closer to gNB, will make the implementation more efficient:




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   *  For shared delivery, the MB-UPF can be distributed closer to gNB.
      The N6mb is a normal IP interface which is connected to DN over
      underlay network.  This transport connection will most likely use
      the VPN infrastructure that has been provisioned by operators for
      5GS.  As a dUPF, the N3mb tunnel off MB-UPF could be made much
      simpler.  In some field edge sites, a dUPF could co-locate on-prem
      with gNB, which can even remove the usage of complex (inter-site)
      VPN to favor native IP transport.

   *  For individual delivery, it involves two UPFs, one regular UPF and
      one MB-UPF.  To follow the current 3GPP specification, we can
      distribute and deploy both UPFs closer to gNB.  While the DL
      traffic off the N6mb interface may achieve the same gain as in the
      shared-delivery mode, the transport for N19mb tunnel and (regular)
      N3 tunnel can be significantly simplified.  Remember we have
      mentioned that either unicast or multicast (underlay) transmission
      can be used for N19mb (and actually also for N6mb and N3mb).
      Therefore, applying dUPF will help simplify the N19mb VPN
      transmission.

   *  For individual delivery, if we expand the scope beyond the current
      3GPP spec, we could integrate the regular UPF and MB-UPF together
      as a distributed UPF, and then deploy the dUPF closer to gNB.  In
      this scenario, both the N19mb and N3 tunnels can be simplified
      significantly.  TS 23.247 [_3GPP-23.247] specifies the behaviors
      of MB-UPF, as a standalone NF.  Indeed, all the features and
      behaviors that would be implemented by a MB-UPF can be
      collaboratively integrated into a regular UPF.  This type of
      'merging' will lead to more network efficiency and better
      multicast traffic forwarding, conforming the [RFC7333] REQ8.

   The draft [I-D.zzhang-dmm-5g-distributed-upf] discussed and compared
   briefly different tunneling mechanisms to implement 3GPP GTP, i.e.,
   SRv6, MPLS as the underlay, or in [I-D.mhkk-dmm-srv6mup-architecture]
   specifying a new SRv6 based MUP architecture to replace GTP.  While
   these proposals may experience different issues upon 5MBS transport
   implementation, dUPF will make it more feasible.

4.  Security Considerations

   TBD.

5.  IANA Considerations

   This document requests no IANA actions.

6.  References




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6.1.  Normative References

   [RFC7333]  Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
              Korhonen, "Requirements for Distributed Mobility
              Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
              <https://www.rfc-editor.org/info/rfc7333>.

6.2.  Informative References

   [I-D.mhkk-dmm-srv6mup-architecture]
              Matsushima, S., Horiba, K., Khan, A., Kawakami, Y.,
              Murakami, T., Patel, K., Kohno, M., Kamata, T., Camarillo,
              P., Voyer, D., Zadok, S., Meilik, I., Agrawal, A., and K.
              Perumal, "Segment Routing IPv6 Mobile User Plane
              Architecture for Distributed Mobility Management", Work in
              Progress, Internet-Draft, draft-mhkk-dmm-srv6mup-
              architecture-01, 10 November 2021,
              <https://www.ietf.org/archive/id/draft-mhkk-dmm-srv6mup-
              architecture-01.txt>.

   [I-D.zzhang-dmm-5g-distributed-upf]
              Zhang, Z., Patel, K., and T. Jiang, "5G Distributed UPFs",
              Work in Progress, Internet-Draft, draft-zzhang-dmm-5g-
              distributed-upf-00, 6 March 2022,
              <https://www.ietf.org/archive/id/draft-zzhang-dmm-5g-
              distributed-upf-00.txt>.

   [_3GPP-23.247]
              "Architectural enhancements for 5G multicast-broadcast
              services; V17.1.0", December 2021.

Author's Address

   Tianji Jiang
   China Mobile
   Email: tianjijiang@chinamobile.com















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