DMM Working Group                                               S. Homma
Internet-Draft                                                       NTT
Intended status: Informational                               T. Miyasaka
Expires: May 6, 2021                                       KDDI Research
                                                           S. Matsushima
                                                                D. Voyer
                                                             Bell Canada
                                                        November 2, 2020

    User Plane Protocol and Architectural Analysis on 3GPP 5G System


   This document analyzes the mobile user plane protocol and the
   architecture specified in 3GPP 5G documents.  The analysis work is to
   clarify those specifications, extract protocol and architectural
   requirements and derive evaluation aspects for user plane protocols
   on IETF side.  This work is corresponding to the User Plane Protocol
   Study work on 3GPP side.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 6, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Current Status of Mobile User Plane for 5G  . . . . . . .   3
     1.2.  Our Way of Analysis Work  . . . . . . . . . . . . . . . .   4
   2.  Terms and Abbreviations . . . . . . . . . . . . . . . . . . .   4
   3.  GTP-U Specification and Observation . . . . . . . . . . . . .   6
     3.1.  GTP-U Tunnel  . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  GTP-U Header Format . . . . . . . . . . . . . . . . . . .   9
     3.3.  Control Plane Protocol for GTP-U  . . . . . . . . . . . .  12
     3.4.  GTP-U message . . . . . . . . . . . . . . . . . . . . . .  13
     3.5.  Packet Format . . . . . . . . . . . . . . . . . . . . . .  14
     3.6.  Observations Summary  . . . . . . . . . . . . . . . . . .  16
   4.  5GS Architectural Requirements for User Plane Protocols . . .  16
     4.1.  Overview of 5G System Architecture  . . . . . . . . . . .  16
       4.1.1.  UPF Functionalities . . . . . . . . . . . . . . . . .  18
       4.1.2.  UP Traffic Detection  . . . . . . . . . . . . . . . .  19
       4.1.3.  User Plane Configuration  . . . . . . . . . . . . . .  21
     4.2.  Architectural Requirements for User Plane     Protocols .  22
       4.2.1.  Fundamental Functionalities . . . . . . . . . . . . .  23
       4.2.2.  Supporting 5G Services  . . . . . . . . . . . . . . .  26
   5.  Evaluation Aspects  . . . . . . . . . . . . . . . . . . . . .  32
     5.1.  Supporting PDU Session Type Variations  . . . . . . . . .  32
     5.2.  Nature of Data Path . . . . . . . . . . . . . . . . . . .  33
     5.3.  Supporting Transport Variations . . . . . . . . . . . . .  33
     5.4.  Data Path Management  . . . . . . . . . . . . . . . . . .  34
     5.5.  QoS Control . . . . . . . . . . . . . . . . . . . . . . .  35
     5.6.  Traffic Detection and Flow Handling . . . . . . . . . . .  35
     5.7.  Supporting Network Slicing Diversity  . . . . . . . . . .  35
     5.8.  Reliable Communication support  . . . . . . . . . . . . .  36
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  37
   7.  Security Consideration  . . . . . . . . . . . . . . . . . . .  37
   8.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  37
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   This document analyzes the mobile user plane protocol and the
   architecture specified by 3GPP 5G documents.  The background of the
   work is that 3GPP requests through a liaison statement that the IETF

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   to provide any information for the User Plane Protocol Study work in
   3GPP [CP-180116-3GPP].  Justification and the objectives of the study
   can be found from [CP-173160-3GPP].

   We understand that the current user plane protocol, GTP-U
   [TS.29.281-3GPP], has been well developed in 3GPP, and deployed very
   widely as the successor of legacy network technologies, such as TDM
   circuit, or ATM virtual circuit.  That GTP-U success seems based on
   IP overlay technique that is dramatically scaled compare to the
   previous ones because it successfully isolates mobile session states
   from the user plane transport network.

   Even after that big success, it is definitely worth that 3GPP has
   decided to revisit user plane which seems to response to IPv6
   deployment growth and [IAB-Statement] that encourages the industry to
   develop strategies for IPv6-only operation.  It can be seen from the
   justification section in [CP-173160-3GPP].

   The study description mentions that the study would be based on
   Release 16 requirement while only Release 15 specifications has been
   available now.  However we believe that to provide adequate
   information for 3GPP, we need to clearly understand what the current
   user plane protocol is in Release 15, and architectural requirements
   for the user plane.

   As the liaison statement indicates 3GPP specifications related to
   user plane, those documents should be a good start point to clarify
   their specifications and to extract protocol and architectural
   requirements from them.

1.1.  Current Status of Mobile User Plane for 5G

   3GPP RAN and CT4 decided to use GTP-U as the 5G user plane
   encapsulation protocol over N3 and N9 that respectively described
   in[TS.38.300-3GPP] and [TR.29.891-3GPP].  N3 is an interface between
   RAN and UPF and N9 is an interface between different UPFs

   In [TR.29.891-3GPP], it captured user plane requirements and
   concluded that GTP-U is adopted for the user plane protocol.  It
   seems that GTP-U was only option to be chose and it focused on how to
   carry 5G specific QoS information between UPF and access networks.
   That is described in section 5.2 and 11.2 of [TR.29.891-3GPP].
   Another aspects of user plane requirements couldn't be found.

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1.2.  Our Way of Analysis Work

   First, we analyze [TS.29.281-3GPP] for clarifying it as the current
   user plane protocol in the 5G system.  [TR.29.891-3GPP] describes how
   GTP-U is selected as the user plane protocol for 5G in 3GPP.
   Clarified characteristics of the protocol are described in Section 3.

   Then, to clarify what are required to the user plane protocol in
   architecture level, we analyze [TS.23.501-3GPP] as the 5G system
   architecture specification.  [TS.23.502-3GPP] is the specification of
   system procedures that helps us to understand how the system works in
   the architecture.  [TS.23.503-3GPP] is also helpful to find the role
   of user plane in the architecture that influences user plane
   protocol.  Extracted architectural requirements are described in
   Section 4.

   Based on the results of above, we identify some aspects where there
   might be gap between the current user plane protocol and the
   architectural requirements on which [TR.29.891-3GPP] does not
   discuss.  That aspects are discussed Section 5.  That's what we
   intend to be as a part of the reply to 3GPP.  CT4 WG in 3GPP can
   utilize it as an input to evaluate the candidate protocols for user
   plane to the 5G system including the current protocol.

2.  Terms and Abbreviations

   This section describes terms of functions and interfaces relevant to
   user plane protocol which we extract from the 3GPP specifications
   since this document focuses on user plane.

   In those specifications, there are so many unique terms and
   abbreviations in the 3GPP context which IETF community seems not
   familiar with.  We will try to bring those terms with brief
   explanations to make sure common understanding for them.

   GTP:  GPRS Tunneling Protocol

   GTP-U:  User Plane part of GTP

      Noted that GTP version 1 (GTPv1-U) is the user-plane protocol
      specification which is defined in [TS.29.281-3GPP].  Unless there
      is no specific annotation, we refer GTP-U to GTPv1-U in this

   PDU:  Protocol Data Unit of end-to-end user protocol packet.

      Noted that the PDU in 3GPP includes IP header in case that PDU
      session type is IPv4 or IPv6.  In contrast, in IETF it is supposed

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      that PDU is the payload of IP packet so that it doesn't include
      IP/TCP/UDP header in end-to-end.

   T-PDU:  Transport PDU.

   G-PDU:  GTP encapsulated user Plane Data Unit.

      GTP-U has above two notions on PDU.  T-PDU is a PDU that GTP-U
      header encapsulates.  G-PDU is a PDU that includes GTP-U header.
      A G-PDU may include a T-PDU.  G-PDU can be sent without T-PDU, but
      just with extension headers or TLV elements.  It can be used for
      OAM related operations.

   PDU session:  Association between the UE and a Data Network that
      provides a PDU connectivity service.

   Data Network (DN):  The network of operator services, Internet access
      or 3rd party services.

   User Plane (UP):  Encapsulating user end-to-end PDU.

      In fact, we can't find exact text that defines UP in the
      architecture specification.  However when we see the figure
      8.3.1-1 in [TS.23.501-3GPP], we specify UP as the layer right
      under PDU that directly encapsulates PDU.  Underneath layers of UP
      are UP transport, such as IP/UDP, L2 and L1.

      However 3GPP is consistent to use the term user plane when they
      indicate that layer.  In IETF, we can see the terms data plane, or
      forwarding plane as variations which often makes us tend to be
      confused in terminology.

   QFI:  QoS Flow Identifier

   UPF:  User Plane Function

   SMF:  Session Management Function

      SMF is a control plane function which provides session management
      service that handling PDU sessions in the control plane.  SMF
      allocates tunnels corresponding to the PDU sessions and configure
      the tunnel to the UPF.

   PFCP:  Packet Forwarding Control Protocol

      PFCP is used on N4 interface between SMF and UPF to configure the
      rules of packet detection, forwarding action, QoS enforcement,
      usage report and buffering for each PDU session.

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   PDR:  Packet Detection Rule

   FAR:  Fowarding Action Rule

   RAN:  Radio Access Network

      Noted that UP protocol provides a RAN to connect UPF.  But the UP
      protocol is not appeared on the air in the RAN.

3.  GTP-U Specification and Observation

   In this section we analyze the GTP-U specification and summarize
   clarified characteristic of GTP-U to see if GTP-U meets the
   requirements of 5G architecture for user plane in later section.

3.1.  GTP-U Tunnel

   GTP-U is a tunneling protocol between given a pair of GTP-U tunnel
   endpoint nodes and encapsulates T-PDU from/to UE on top of IP/UDP.  A
   Tunnel Endpoint Identifier (TEID) value allocated on each end point
   indicates which tunnel a particular T-PDU belongs to.

   The receiving endpoint individually allocate a TEID and the sender
   tunnel endpoint node encapsulates the IP packet from/to UE with the
   TEID which is present in GTP-U header on top of IPv4 or IPv6, and
   UDP.  That is described in section 4.2.1 of [TS.29.281-3GPP].

   [GTP-U-1]:  GTP-U is an unidirectional Point-to-Point tunneling

   Figure 1 shows an example of GTP-U protocol stack for uplink (UL) and
   downlink (DL) traffic flow.  Two GTP-U tunnels are required to form
   one bi-directional tunnel.

   UL:  From RAN to UPF1 (TEID=1), and from UPF1 to UPF2 (TEID=2)

   DL:  From UPF2 to UPF1 (TEID=3), and from UPF1 to RAN (TEID=4)

   In 5GS, GTP-U tunnel is established at following interfaces to
   provide PDU Session between UE and 5GC.

   N3:  Between RAN and UPF

   N9:  Between different UPFs

   GTP-U allows one tunnel endpoint node to send out a G-PDU to be
   received by multiple tunnel endpoints by utilizing IP multicast
   capability of underlay IP networks.  That is described in section

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   4.2.6 of [TS.29.281-3GPP].  It looks GTP-U has Point-to-Multipoint
   (P2MP) tunneling capability.  The P2MP tunneling is used for MBMS
   (Multimedia Broadcast Multicast Service) through GTP-U tunnel.

   [GTP-U-2]:  GTP-U supports Point-to-Multipoint tunneling.

   UDP is utilized for GTP-U encapsulation and UDP destination port is
   2152 which is assigned by IANA.  Allocation of UDP source port
   depends on sender tunnel endpoint node and GTP-U supports dynamic
   allocation of UDP source port for load balancing objective.  The
   specification of this dynamic allocation is described in section of [TS.29.281-3GPP], however specific procedure, e.g.,
   5-tuple hashing, is not described in the document and depends on the
   implementation of GTP-U tunnel endpoint node.

   [GTP-U-3]:  GTP-U supports load balancing by using dynamic UDP source
               port allocation.

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          .-     .-+--------+ - - - - +--------+ - - - - +--------+
          |      | |Payload |         |Payload |         |Payload |
          | PDU <  +--------+         +--------+         +--------+
          |(3GPP)| |Inner IP|         |Inner IP|         |Inner IP|
          |      '-+--------+ - - - - +--------+ - - - - +--------+
          |        | GTP-U  |         | GTP-U  |         |   L2   |
          |        |(TEID=1)|         |(TEID=2)|         +--------+
      GTP<         +--------+         +--------+         |   L1   |
      Pkt |        |  UDP   |         |   UDP  |         +--------+
          |        +--------+         +--------+
          |        |Outer IP|         |Outer IP|
          |        +--------+         +--------+
          |        |   L2   |         |   L2   |
          |        +--------+         +--------+
          |        |   L1   |         |   L1   |
          '-       +--------+ - - - - +--------+

      ================   Traffic Direction  =======================>

          .-     .-+--------+ - - - - +--------+ - - - - +--------+
          |      | |Payload |         |Payload |         |Payload |
          | PDU <  +--------+         +--------+         +--------+
          |(3GPP)| |Inner IP|         |Inner IP|         |Inner IP|
          |      '-+--------+ - - - - +--------+ - - - - +--------+
          |        | GTP-U  |         | GTP-U  |         |   L2   |
          |        |(TEID=4)|         |(TEID=3)|         +--------+
      GTP<         +--------+         +--------+         |   L1   |
      Pkt |        |  UDP   |         |   UDP  |         +--------+
          |        +--------+         +--------+
          |        |Outer IP|         |Outer IP|
          |        +--------+         +--------+
          |        |   L2   |         |   L2   |
          |        +--------+         +--------+
          |        |   L1   |         |   L1   |
          '-       +--------+ - - - - +--------+

      <===============   Traffic Direction  ========================

            +-----+     N3    +------+     N9     +------+    N6
       -----+ RAN +-----------+ UPF1 +------------+ UPF2 +----------
            +-----+           +------+            +------+

    Figure 1: Protocol Stack by GTPv1-U for Uplink and Downlink Traffic

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   IPv6 flow label [RFC6437] is also candidate method for load balancing
   especially for IP-in-IPv6 tunnel [RFC6438] like GTP-U.  GTP-U also
   supports dynamic allocation of IPv6 flow label for load balancing
   objective.  The specification of this dynamic allocation is described
   in section of [TS.29.281-3GPP], however specific procedure,
   e.g., 5-tuple hashing, is not described in the document and depends
   on the implementation of GTP-U tunnel endpoint node.

   [GTP-U-4]:  GTP-U supports load balancing by using dynamic IPv6 flow
               label allocation.

   GTP-U supports both IPv4 and IPv6 as the underlying network layer
   protocol.  From Release 16, GTP-U updates their reference to IPv6
   specification from [RFC2460] to [RFC8200] which allows UDP zero
   checksum for the protocols that use UDP as a tunnel encapsulation,
   such as GTP-U.  As a result of the update, GTP-U over IPv6 also
   supports the UDP zero checksum if the sender and receiver tunnel
   endpoint node support the UDP zero checksum, which is described in
   section of [TS.29.281-3GPP].

   [GTP-U-5]:  GTP-U supports UDP zero checksum.

   "Unnecessary fragmentation should be avoided" is recommended and to
   avoid the fragmentation operator should configure MTU size at UE
   [TS.29.281-3GPP].  However, there's no reference and specification of
   Path MTU Discovery for IPv6 transport.  If encapsulated IPv6 packet
   is too big on a network link between tunnel endpoint nodes, UE may
   not receive ICMPv6 Packet Too Big message and causes Path MTU
   Discovery black hole.

   [GTP-U-6]:  GTP-U does not support to response ICMP PTB for Path MTU

   Section 9.3 of [TS.23.060-3GPP] specifies advertisement of inner IPv6
   link MTU size for UE by IPv6 RA message [RFC4861].  However, this
   document doesn't specify a procedure to measure MTU size in mobile
   network system and mobile network operator need to calculate MTU size
   for UE like Annex C of [TS.23.060-3GPP].  If link MTU of a router in
   a transport network is accidentally modified, UE cannot detect the
   event and send packet with initial MTU size, which may cause service
   disruption due to MTU exceed in the router link.

3.2.  GTP-U Header Format

   Figure 2 shows general and mandatory GTP-U header and Figure 3 shows
   extension GTP-U header.

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   [GTP-U-7]:  GTP-U supports sequence number option in the header, but
               it is not recommended to be used by almost GTP-U

   GTP-U header has Sequence Number field to reorder incoming packets
   based on the sequence number.  If Sequence Number Flag is set to '1'
   it indicates that Sequence Number Filed exists in GTP-U header and
   examined at receiving tunnel endpoint node to reorder incoming
   packets.  However, the sequence number flag is set to '1' only for
   RAT HO procedure and sequence number flag should be set to '0' in
   normal case.  Therefore, in normal case receiver tunnel endpoint node
   doesn't examine sequence number and can't reorder GTP-U packets based
   on the sequence number.  This specification is described in section
   5.1 of [TS.29.281-3GPP].  In 3GPP, sequential delivery is required
   only during handover procedure and is used by only RAN entities.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     | Ver |P|R|E|S|N| Message Type  |           Length              |
     |                Tunnel Endpoint Identifier                     |
     |      Sequence Number          | N-PDU Number  |  Next-Ext-Hdr |

                          Figure 2: GTP-U Header

   o  Ver: Version field (Set to '1')

   o  P: Protocol Type (Set to '1')

   o  R: Reserved bit (Set to '0')

   o  E: Extension Header Flag (Set to '1' if extension header exists)

   o  S: Sequence Number Flag (Set to '1' if sequence number exists)

   o  N: N-PDU Number Flag (Set to '1' if N-PDU number exists)

   o  Message Type: Indicates the type of GTP-U message

   o  Length: Indicates the length in octets of the payload

   o  Tunnel Endpoint Identifier (TEID)

   o  Sequence Number: Indicates increasing sequence number for T-PDUs
      is transmitted via GTP-U tunnels

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   o  N-PDU Number: It is used only for inter SGSN, 2G-3G handover case,

   o  Next-Ext-Hdr: Indicates following extension header type

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     | Ext-Hdr Length|                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                  Extension Header Content                     |
     .                                                               .
     .                                               +-+-+-+-+-+-+-+-+
     |                                               |  Next-Ext-Hdr |

                     Figure 3: Extension GTP-U Header

   o  Ext-Hdr Length: Represents the length of the Extension header in
      units of 4 octets

   o  Extension Header Content: Contains 3GPP related information

   o  Next-Ext-Hdr: Indicates following extension header type

   The extension GTP-U header is a variable-length and extendable header
   and contains 3GPP specific information.  Following list summarizes
   every extension header which is used for user plane protocol.  These
   extension headers are defined in [TS.29.281-3GPP].  In this list
   Next-Ext-Hdr is represented in binary.

   o  No more extension headers (Next-Ext-Hdr = 00000000)

   o  Service Class Indicator (Next-Ext-Hdr = 00100000)

   o  UDP Port (Next-Ext-Hdr = 01000000)

   o  RAN Container (Next-Ext-Hdr = 10000001)

   o  Long PDCP PDU Number (Next-Ext-Hdr = 10000010)

   o  Xw RAN Container (Next-Ext-Hdr = 10000011)

   o  NR RAN Container (Next-Ext-Hdr = 10000100)

   o  PDU Session Container (Next-Ext-Hdr = 10000101)

   o  PDCP PDU Number (Next-Ext-Hdr = 11000000)

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   [GTP-U-8]:  GTP-U supports carrying QoS Identifiers transparently for
               Access Networks in an extension header.

   GTP-U is designed to carry 3GPP specific information with extension
   headers. 3GPP creates PDU Session Container extension header for
   NGRAN of 5G to carry QFI.  It is described in section of

   [GTP-U-9]:  GTP-U supports DSCP marking based on the QFI.

   DSCP marking on outer IPv4 or IPv6 shall be set by sender tunnel
   endpoint node based on the QFI.  This specification is described in
   section 4.4.1 of [TS.29.281-3GPP].

   [GTP-U-10]: GTP-U does not specify extension header order.

   In general, multiple GTP-U extension headers are able to contained in
   one GTP-U packet and the order of those extension headers is not
   specified by [TS.29.281-3GPP].  Thereby the receiving endpoint can't
   predict exact position where the target extension headers are.  This
   could impact on header lookup performance on the node.

   As for PDU Session Container extension header, there is a note in
   [TS.29.281-3GPP] as "For a G-PDU with several Extension Headers, the
   PDU Session Container should be the first Extension Header".  This
   note was added at the version 15.3.0 of [TS.29.281-3GPP] which is
   published on June 2018 in order to accelerate the processing of GTP-U
   packet at UPF and RAN.  It is only one rule regarding the extension
   header order.

   [GTP-U-11]: GTP-U does not support to indicate next protocol type.

   When Next-Ext-Hdr is set to 0x00 it indicates that no more extension
   headers follow.  As GTP is designed to indicate protocol types for
   T-PDU by control-plane signaling, GTP-U doesn't have Next-Protocol-
   Header field to indicate the T-PDU type in the header.

3.3.  Control Plane Protocol for GTP-U

   Control plane protocol for GTP-U signals TEID between tunnel endpoint
   nodes.  GTPv2-C [TS.29.274-3GPP] is the original control plane
   protocol tied with GTP-U in previous generation architectures before
   CUPS (Control and User Plane Separation).

   3GPP decided to use extended PFCP (Packet Forwarding Control
   Protocol) [TS.29.244-3GPP] for N4 interface [TR.29.891-3GPP] to
   signal tunnel states from SMF to UPF.

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3.4.  GTP-U message

   GTP-U supports in-band messaging to signal OAM.  Currently GTP-U
   supports following messages [TS.29.281-3GPP].

   o  Echo Request

   o  Echo Response

   o  Supported Extension Headers Notification

   o  Error Indication

   o  End Marker

   [GTP-U-12]: GTP-U supports active OAM as a path management message
               "Echo Request/Response".

   A GTP-U tunnel endpoint node sends a Echo Request message to another
   nodes for keep-alive and received node sends a Echo Response message
   to sender node as acknowledgment.  Echo Request message and Echo
   Response message are described in section 7.2.1 and section 7.2.2 of
   [TS.29.281-3GPP] respectively.  [TR.29.891-3GPP] recommends not to
   send Echo Request message more often than 60s on each path.

   Supported Extension Headers Notification message indicates a list of
   supported that tunnel endpoint node can support.  This message is
   sent only in case a tunnel endpoint node receives GTP-U packet with
   unsupported extension header.

   [GTP-U-13]: GTP-U supports tunnel management messages "Error

   GTP-U has Error Indication message to notify that the receiving
   endpoint discard packets of which no session exist to the sending
   endpoint.  Error Indication message is described in section 7.3.1 of

   [GTP-U-14]: GTP-U supports tunnel management messages "End Marker".

   GTP-U has End Marker message to indicate the end of the payload
   stream that needs to be sent on a GTP-U tunnel.  End Marker message
   is described in section 7.3.2 of [TS.29.281-3GPP].

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3.5.  Packet Format

   Figure 4 shows a packet format example of GTP-U over IPv6 that
   carries an extension header for QFI and an IPv6 PDU.  All values in
   the example are illustration purpose only.  The encoding of PDU
   Session Container for QFI refers to [TS.38.415-3GPP].

   Outer IPv6 Header's DSCP value(EF) in Figure 4 is marked at sender
   tunnel endpoint node based on QFI value which is contained in GTP-U
   Extension Header (PDU Session Container).

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     Outer IPv6 Header
     |Version|     DSCP=EF   |               Flow Label              |
     |          Payload Length       | NxtHdr=17(UDP)|    Hop Limit  |
     |                                                               |
     +                                                               +
     |                                                               |
     +                   Source IPv6 Address                         +
     |                        2001:db8:1:1::1                        |
     +                                                               +
     |                                                               |
     |                                                               |
     +                                                               +
     |                                                               |
     +              Destination IPv6 Address                         +
     |                        2001:db8:1:2::1                        |
     +                                                               +
     |                                                               |

     Outer UDP Header
     |     Source Port = xxxx        |         Dest Port = 2152      |
     |         UDP Length            |    UDP Checksum (Non-zero)    |

     GTP-U header
     | 0x1 |1|0|1|0|0|     0xff      |           Length              |
     |                           TEID = 1654                         |

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     |    Sequence Number = 0        |N-PDU Number=0 |NextExtHdr=0x85|

     GTP-U Extension Header (PDU Session Container)
     |  ExtHdrLen=2  |Type=0 |0|0|   |0|0|   QFI     | PPI |  Spare  |
     |                    Padding                    |NextExtHdr=0x0 |

     Inner IPv6 Header
     |Version|     DSCP=0    |               Flow Label              |
     |          Payload Length       |    NexttHdr   |    Hop Limit  |
     |                                                               |
     +                                                               +
     |                                                               |
     +                   Source IPv6 Address                         +
     |                        2001:db8:2:1::1                        |
     +                                                               +
     |                                                               |
     |                                                               |
     +                                                               +
     |                                                               |
     +              Destination IPv6 Address                         +
     |                        2001:db8:3:1::1                        |
     +                                                               +
     |                                                               |

     |                                                               |
     |                                                               |
     .                        TCP/UDP/etc., Data                     .
     .                                                               .
     |                                                               |
     |                                                               |

                  Figure 4: GTP-U Protocol Stack Example

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3.6.  Observations Summary

   [GTP-U-1]:  An unidirectional Point-to-Point tunneling protocol.

   [GTP-U-2]:  Supports Point-to-Multipoint tunneling.

   [GTP-U-3]:  Supports load balancing by using dynamic UDP port

   [GTP-U-4]:  Does not support IPv6 flow label for load balancing in
               case of IPv6 transport.

   [GTP-U-5]:  UDP zero checksum is not available in case of IPv6

   [GTP-U-6]:  Does not support to response ICMP PTB for Path MTU

   [GTP-U-7]:  Supports sequence number option and sequence number flag
               in the header, but it is not recommended to be used by
               almost GTP-U entities.

   [GTP-U-8]:  Supports carrying QoS Identifiers transparently for
               Access Networks in extension headers.

   [GTP-U-9]:  Supports DSCP marking based on the QFI.

   [GTP-U-10]: Does not specify the rule for the extension header order.

   [GTP-U-11]: Does not support an indication of next-header type.

   [GTP-U-12]: Supports active OAM as a path management message "Echo

   [GTP-U-13]: Supports tunnel management messages "Error Indication".

   [GTP-U-14]: Supports tunnel management messages "End Marker".

4.  5GS Architectural Requirements for User Plane Protocols

4.1.  Overview of 5G System Architecture

   The 5G system is designed for applying to diverse devices and
   services due to factors such as the diffusion of IoT devices, and the
   UP protocol is required to have capabilities for satisfying their

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   As a principle of the 5G system, User Plane (UP) functions are
   separated from the Control Plane (CP) functions for allowing
   independent scalability, evolution and flexible deployments.

   Network slicing is also one of the fundamental concepts of the 5G
   system, and it provides logical network separation.  In terms of user
   plane, multiple network slices can be comprised of UPFs on top of
   same physical network resources.  Allocated resources and structures
   may be differentiated among the slices by which the required features
   or capabilities.

   The 3GPP 5G architecture [TS.23.501-3GPP] defines slice types which
   are eMBB, URLLC and MIoT from Rel-15.  In addition to that, V2X slice
   type is defined from Rel-16.

   The architecture overview is shown in Figure 5.  The details of
   functions are described in [TS.23.501-3GPP].  A UPF handles UP paths
   on N3, N9 and N6 interface, and the setup is controlled by SMF via N4
   interface.  A UP path will be manipulated based on application
   requirements for the PDU session corresponding to the path.  An SMF
   is also capable to receive information regarding routing path with
   API from AF via NEF, PCF, and SMF.

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      +-----+  +-----+  +-----+     +-----+  +-----+  +-----+
      |NSSF |  | NEF |  | NRF |     | PCF |  | UDM |  | AF  |
      +--+--+  +--+--+  +--+--+     +--+--+  +--+--+  +--+--+
    Nnssf|    Nnef|    Nnrf|       Npcf|    Nudm|        |Naf
                Nausf|    Namf|       Nsmf|
                  +--+--+  +--+--+     +--+-------+
                  |AUSR |  | AMF |     |    SMF   |
                  +-----+  ++-+--+     +--+-----+-+
                           /  |           |      \
   Control Plane        N1/   |N2         |N4     \N4
   User Plane           /     |           |         \
                    +--++  +--+--+ N3  +--+--+ N9  +-+---+ N6 +-----+
                    |UE +--+(R)AN+-----+ UPF +-----+ UPF +----+ DN  |
                    +---+  +-----+     +--+--+     +-----+    +-----+
                                       | DN  |

          Figure 5: 5GS Architecture and Service-based Interfaces

   This document mainly focuses on requirements for N9 interface as
   relevant to UP protocol of 5G system.

4.1.1.  UPF Functionalities

   UPF has a role to handle UP traffic, and provides functionalities to
   look up user data traffic and enforce the appropriate policies to it.

   The followings are defined as UPF functionalities defined in the
   section 6.2.3 of [TS.23.501-3GPP]

   o  Anchor point for Intra-/Inter-RAT mobility (when applicable).

   o  External PDU Session point of interconnect to Data Network.

   o  Packet routing and forwarding (e.g. support of Uplink classifier
      to route traffic flows to an instance of a data network, support
      of Branching point to support multi-homed PDU Session).

   o  Packet inspection (e.g.  Application detection based on service
      data flow template and the optional PFDs received from the SMF in

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   o  User Plane part of policy rule enforcement, e.g.  Gating,
      Redirection, Traffic steering).

   o  Lawful intercept (UP collection).

   o  Traffic usage reporting.

   o  QoS handling for user plane, e.g.  UL/DL rate enforcement,
      Reflective QoS marking in DL.

   o  Uplink Traffic verification (SDF to QoS Flow mapping).

   o  Transport level packet marking in the uplink and downlink.

   o  Downlink packet buffering and downlink data notification

   o  Sending and forwarding of one or more "end marker" to the source
      NG-RAN node.

   o  ARP proxying and / or IPv6 Neighbour Solicitation Proxying for the
      Ethernet PDUs.

   o  Packet duplication in downlink direction and elimination in uplink
      direction in UP protocol layer.

   o  TSN Translator functionality to hold and forward user plane
      packets for de-jittering when 5G System is integrated as a bridge
      with the TSN network.

4.1.2.  UP Traffic Detection

   The traffic detection is described in the section of
   [TS.23.501-3GPP].  In 3GPP UP packet forwarding model, UPF detects UP
   traffic flow which belong to a N4 session configured by SMF.

   The protocol of N4 interface, PFCP, brings a set of traffic detection
   information from SMF to UPF as Packet Detection Information (PDI) in
   a PDR to establish/modify the N4 PFCP session.  It is defined in
   section of [TS.29.244-3GPP].

   Combination of the following information is used for the traffic

   o  For IPv4 or IPv6 PDU Session type

      *  CN tunnel info (Tunnel ID and the endpoint IP address of 5G

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      *  Network instance

      *  QFI

      *  IP Packet Filter Set

      *  Application Identifier: The Application ID is an index to a set
         of application detection rules configured in UPF

   o  For Ethernet PDU Session type

      *  CN tunnel info(Tunnel ID and the endpoint IP address of 5G

      *  Network instance

      *  QFI

      *  Ethernet Packet Filter Set

   It is noted that Network Instance is encoded as Octet String in PFCP,
   and is NOT appeared in UP packet over the wire.  It is expected like
   an attribute of the receiving IP interface of the UPF.  It supports
   UPF to be able to connect to different IP domains of N3, N9 or N6,
   which run each independent policy in routing and addressing.  The UPF
   detects traffic flow with Network Instance which the receiving
   interface attributed to.

   The IP Packet Filter Set and Ethernet Packet Filter Set defined in
   clause 5.7.6 of [TS.23.501-3GPP] are following:

   o  IP Packet Filter Set:

      *  Source/destination IP address or IPv6 prefix

      *  Source/destination port number

      *  Protocol ID of the protocol above IP/Next header type

      *  Type of Service (TOS) (IPv4) / Traffic class (IPv6) and Mask.

      *  Flow Label (IPv6)

      *  Security parameter index

      *  Packet filter direction

   o  Ethernet Packet Filter Set:

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      *  Source/destination MAC address

      *  Ethertype as defined in IEEE 802.3

      *  Customer-VLAN tag(C-TAG) and/or Service-VLAN tag(S-TAG) VID
         fields as defined in IEEE 802.1Q

      *  Customer-VLAN tag(C-TAG) and/or Service-VLAN tag(S-TAG) PCP/DEI
         fields as defined in IEEE 802.1Q

      *  IP Packet Filter Set, in case Ethertype indicates IPv4/IPv6

      *  Packet filter direction

4.1.3.  User Plane Configuration

   User Plane configuration on a UPF is managed by an SMF through PFCP
   [TS.29.244-3GPP].  The SMF establishes PFCP sessions on the UPF per
   PDU session basis.  The UPF maintains each configured PFCP session
   states during the sessions exist.

   A PFCP session consists of the rules of packet detection, forwarding
   action, QoS enforcement, usage reporting and buffering action.
   Figure 6 depicts overview of the PFCP session state structure.

   The listed information in Section 4.1.2 indicates packet detection
   information of packet detection rule for that the rest of related
   rules within the PFCP session to be derived.  All rules are per
   session unique and no rules are shared with other sessions.

   PFCP-Session* [F-SEID]
     +- F-SEID(Full Qualified Session Endpoint ID)    uint64
     +- PDU-Session-Type               [IPv4|IPv6|IPv4v6|Ether|Unstrct]
     +- DNN(Data Network Name)
     +- PDR(Packet Detection Rule)* [PDR-ID]
     |  +- PDR-ID    uint16
     |  +- PDI (Packet Detection Information)
     |  |  +- Traffic-Endpoint-ID?   -> Traffic-Endpoint-ID reference
     |  |  +- ....
     |  +- FAR/URR/QER-ID            -> FAR/URR/QER-ID references
     +- FAR(Forwarding Action Rule)* [FAR-ID]
     |  +- FAR-ID                    uint32
     |  +- Forwarding-Parameters
     |  |  +- Network-Instance?        Octet String
     |  |  +- Outer-Header-Creation
     |  |  |  +- Outer-Hdr-Creation-Desc   [GTPoUDP/IPv4|IPv6, etc.,]

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     |  |  |  +- TEID, outer IP-Address for N3/N9
     |  |  |  +- C/S-TAG, UDP Port-number for N6
     |  |  +- Forwarding-Policy-ID?    Octet String
     |  |  +- ....
     |  +- Duplicating-Parameters
     |  |  +- ....
     |  +- BAR-ID?                   -> BAR-ID reference
     +- QER(QoS Enforcement Rule)* [QER-ID]
     |  +- QER-ID                    uint32
     |  +- MBR(Maximum Bit Rate)
     |  |  +- UL/DL-MBR?   bitrate_in_kbps (0..10000000)
     |  +- GBR(Guaranteed Bit Rate)
     |  |  +- UL/DL-GBR?   bitrate_in_kbps (0..10000000)
     |  +- QoS-flow-identifier?       QFI value(6-bits)
     |  +- Reflective-QoS?            boolean
     |  +- Paging-Policy-Indicator?   PPI value(3-bits)
     |  +- ....
     +- URR(Usage Reporting Rule)* [URR-ID]
     |  +- URR-ID                    uint32
     |  +- Measurement-Method, Period, Reporting-Triggers?
     |  +- Volume/Event/Time Threshold, Quota?
     |  +- Quota-Holding-Time?
     |  +- FAR-ID for Quota action?        -> FAR-ID reference
     |  +- ....
     +- BAR(Buffering Action Rule)* [BAR-ID]
     |  +- BAR-ID                    uint8
     |  +- Suggested-Buffering-Packets-Count
     +- Traffic-Endpoint* [Traffic-Endpoint-ID]
        +- Traffic-Endpoint-ID                   uint8
        +- TEID, Tunnle IP Address, UE Address...?

                 Figure 6: User Plane Configuration Model

4.2.  Architectural Requirements for User Plane Protocols

   This section lists the requirements for the UP protocol on the 5G
   system.  The requirements are picked up from [TS.23.501-3GPP].  In
   addition, some of service requirements described in [TS.22.261-3GPP]
   are referred to clarify the originations of architectural

   According to [TS.23.501-3GPP], the specifications potentially have
   assumptions that the UP protocol is a tunnel representing a single
   TEID between a pair of UPFs and it is corresponding to a single PDU
   session.  In short, the UP protocol is a tunnel and it is assumed to
   be managed under per PDU session handling.  Also, it should be a
   stateful tunnel in the UPFs along with the PDU session.

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4.2.1.  Fundamental Functionalities

   The fundamental requirements for UP protocols are described below:

   ARCH-Req-1:  Supporting IPv4, IPv6, Ethernet and Unstructured PDU

   The 5G system defines four types of PDU session as IPv4, IPv6,
   Ethernet, and Unstructured.  Therefore, UP protocol must support to
   convey all of these PDU session types.  This is described in

   Note: In TS 23.501 v15.2.0, IPv4v6 is added as a PDU session type.

   ARCH-Req-2:  Supporting IP connectivity for N3, N6, and N9 interfaces

   The 5G system requires IP connectivity for N3, N6, and N9 interfaces.
   The IP connectivity is assumed that it comprises of IP routing and
   L1/L2 transport networks which are outside of 3GPP specifications.

   It is desirable that the IP connectivity built on IPv6 networks when
   it comes to address space for end-to-end user plane coverage.  But it
   is expected to take certain time.  During the IPv6 networks are not
   deployed for all the coverage, UP protocol should support RANs and
   DNs running on IPv4 transport connect to UPF running on IPv6

   Furthermore, on N6 interface, point-to-point tunneling based on UDP/
   IPv6 may be used to deliver unstructured PDU type data.  Then, the
   content information of the PDU may be mapped into UDP port number,
   and the UDP port numbers is pre-configured in the UPF and DN.  This
   is described in the section 9.2 of [TS.29.561-3GPP].

   ARCH-Req-3:  Supporting deployment of multiple UPFs as anchors for a
                single PDU session

   The 5G system allows to deploy multiple UPFs as anchors for a single
   PDU session, and supports multihoming of a single PDU session for
   such anchor UPFs.

   Multihoming is provided with Branching Point (BP).  BP provides
   forwarding of UL traffic towards the different PDU Session Anchors
   based on the source IPv6 prefixes and merge of DL traffic to the UE.
   IPv6 multihoming only means multiple source IPv6 prefixes are used
   for a PDU session.  It is identical to one classified as scenario 1
   in [RFC7157].

   Up link classifier (UL CL) is to forward uplink packets to multiple
   anchor UPFs based on the destination IP of the T-PDU regardless of

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   the source IP address.  Noted that single source IP address/prefix
   PDU session is not defined as multihoming PDU session in 5GCS even
   though a PDU session has multiple anchor UPFs.

   On UL side, P2P tunnels are established per destination anchor UPFs
   basis from one UL CL UPF to the anchor UPFs for the PDU session.

   On DL side, one single multipoint-to-point (MP2P) tunnel exists from
   the source anchor UPFs to the destination BP UPF for the PDU session.
   It means that the paths from the anchor UPFs are merged into just one
   tunnel state at the destination BP UPF.

   Multiple P2P paths on DL could also be used for multihoming.  However
   it should be the multiple PDU sessions multihoming case where the
   destination gNB or UPF needs to maintain multiple tunnel states under
   the one PDU session to one UP tunnel architectural principle.  It
   causes increase of load on tunnel states management in UPF due to
   increment of the anchor UPF for the PDU session.

   However, P2P tunneling could increase explosively the number of
   states in UPF as the anchor UPF/DN incremented to the PDU session.
   Thereby single PDU session multihoming with MP2P path should be a
   better option for multihoming in terms of reducing total number of
   tunnel states.

   SSC mode 3 for session continuity in hand-over case uses a single PDU
   multihoming with BP to make sure make-before-break.  It is described
   in the section 5.6.4 and 5.6.9 of [TS.23.501-3GPP].

   Multihoming is also assumed to be used for edge computing scenario.
   Edge computing enables some services to be hosted close to the UE's
   access point of attachment, and achieves an efficient service
   delivery through the reduced end-to-end latency and load on the
   transport network.  In edge computing, local user's traffic is routed
   or steered to application in the local DN by UPF.  This refers the
   section 5.13 of [TS.23.501-3GPP].

   ARCH-Req-4:  Supporting flexible UPF selection for PDU

   The appropriate UPFs are selected for a PDU session based on
   parameters and information such as UPF's dynamic load or UE location
   information.  Examples of parameters and information are described in
   the section 6.3.3 of [TS.23.501-3GPP].

   This means that it is possible to make routing on user plane more
   efficient in the 5GS.  For example, in case that UPFs are distributed
   geographically, decision of the destination UPF based on locations of
   end hosts (e.g., UE or NF in DN) enables to forward PDUs with a route

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   connecting between UPFs nearby the hosts directly.  This would be
   useful UE-to-UE or UE-to-local_DN communication, and such usage is
   described in the section 6.5 of [TS.22.261-3GPP].

   The 5GS allows operators to select parameters used for UPF selection.
   (In other words, any specific schemes on UPF selection are not
   defined in the current 3GPP documents.)

   ARCH-Req-5:  No limitation for number of UPFs in a data path

   The number of UPF in the data path is not constrained by 3GPP
   specifications.  This specification is described in the section 8.3.1
   of [TS.23.501-3GPP].

   Putting multiple UPFs, which provides specific function, in a data
   path enables flexible function deployment to make sure load
   distribution optimizations, etc.

   Meanwhile, each UPF in a data path shall be controlled by an SMF via
   N4 interface.  Thus putting an excess of UPF for data paths might
   cause increase of load of an SMF.  Pragmatically, the number of UPF
   put in a data path is one or two (e.g., for MEC or roaming cases),
   and, at most, it would be three (e.g., for case where UE moves during
   a session).

   It is expected that multiple UPFs with per session tunnel handling
   for a PDU session becomes complicated task more and more for a SMF by
   increasing number of UPFs.

   ARCH-Req-6:  Supporting aggregation of multiple QoS Flow indicated
                with QFI into a PDU Session

   Against to the previous generation, 5G enables UPF to multiplex QoS
   Flows, equivalent with IP-CAN bearers in the previous generation,
   into one single PDU session.  That means that a single tunnel
   includes multiple QFIs contrast to just one QoS Flow (a bearer) to
   one tunnel before 5G.

   In even the 5GS, each flow is forwarded based on the appropriate QoS
   rules.  QoS rules are configured by SMF as QoS profiles to UP
   components and these components perform QoS controls to PDUs based on
   rules.  In downlink, a UPF pushes QFI into an extension header, and
   transmits the PDU to RAN or another UPF.  Then, such UPF may perform
   transport level QoS packet marking (e.g., DSCP marking in the outer
   header).  In uplink, each UE obtains the QoS rule from SMF, and
   transmit PDUs with QFI containing the QoS rules to the RAN.  The
   following RAN and UPFs perform enforcement of QoS control and
   charging based on the QFI.

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   This specification is described in 5.7.1 of [TS.23.501-3GPP].

   ARCH-Req-7:  Supporting network slicing

   The 5GS fundamentally supports network slicing for provision the
   appropriate end-to-end communication to various services.  In the
   relevant documents (e.g., [TS.23.501-3GPP], [TS.28.530-3GPP]), a
   network slice is defined as virtual network and it is structured with
   5GS NF instances, such as SMF, UPF including IP transport
   connectivity between RANs and DNS.  Each network slice is independent
   and its user plane (including network functions and links) should be
   noninteractive against the others.

   The 5G architecture specification has been updated with that Network
   Instance is defined as the glue of network slice between 5G slice and
   corresponding IP transport slice in addition to the original role of
   separating IP domains, which is described in Section 4.1.2.

   It has been appeared from version 15.2.0 of [TS.23.501-3GPP] in
   section 5.6.12.

   UP underlay transport networks and UPFs may be shared by 5G slices,
   as described in section 4 of [TS.28.530-3GPP].  The data model
   defined in [TS.29.510-3GPP] allows that a Network Instance, a UPF and
   its interfaces can belong to multiple slices as same as other type of
   NFs.  UP endpoint IP prefix/address of an interface can also be
   shared with multiple interfaces on the UPF as the model doesn't make
   them slice unique.

   The slice lifecycle managements is described in the relevant
   documents: [TS.28.531-3GPP], [TS.28.532-3GPP], and [TS.28.533-3GPP].

   ARCH-Req-8:  End Marker support

   The construction of End Marker packets specified in [TS.23.501-3GPP]
   may either be done in the CP/UP functions for indicating the end of
   the payload stream on a given UP tunnel.  PDU packets arrive after an
   End Marker message on the tunnel may be silently discarded.  For
   example, End Maker is used for handover procedures, and it can
   prevent reordering of arriving packets due to switch of anchor UPFs.

4.2.2.  Supporting 5G Services

   In the release 16 [TS.23.501-3GPP], some specifications have been
   added to support 5G specific services and communications.  This
   section describes overviews of the specifications relevant to use
   plane functionalities.

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   ARCHI-Req-9:  URLLC Support

   The 5GS supports Ultra-Reliable Low Latency Communication (URLLC) for
   mission critical applications.  The User Plane features are described

   o  Redundant UP transmission for URLLC

   The 5G is expected to support services which are latency sensitive
   and require high reliability.  Communication to realize such services
   is called Ultra-Reliable and Low-Latency Communication or URLLC.  In
   URLLC, redundancy of QoS flows is required for providing highly
   reliable communication.  For instance, a set of UP NFs (e.g., UPF or
   gNB) and interfaces between UE and DN are redundant, and packets are
   replicated and forwarded via each route.  UEs and DN support dual
   connectivity and drop duplicated received packets.  The scheme of
   packet dropping at UE is out of responsibility of 3GPP.  The overview
   is shown in Figure 7.

                           Namf|       Nsmf|      Nsmf|
                            +--+--+     +--+--+    +--+--+
                            | AMF |     | SMF1|    | SMF3|
                            +-+---+     +--+--+    +-+---+
                             /            /         /
                          N2/          N4/       N4/
                           /            /         /
                     +----++   N3   +--+---+     / N6     +----+
                     |M-RAN+--------+ UPF1 +--------------+    |
                     ++-+--+        +-+--+-+   /          |    |
                     /  |             |  |    /           |    |
             +----+ /   |             +--+   /            |    |
             | UE +<    |Xn            N9   /             | DN |
             +----+ \   |                  /              |    |
                     \  |                 /               |    |
                     ++-+--+   N3   +----+-+       N6     |    |
                     |S-RAN+--------+ UPF2 +--------------+    |
                     +-----+        +-+--+-+              +----+
                                      |  |
                M-RAN: Master RAN
                S-RAN: Secondary RAN

           Figure 7: Redundant UP paths using dual connectivity

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   Otherwise, in case that RAN nodes and UPFs have enough reliability
   and they are not redundant by dual devices, reliable connectivity of
   QoS flows is provided by dual N3 tunnels between RAN and UPFs.  Such
   tunnels are treated as individual ones, but they have the same
   sequence number.  UP NFs identifies the duplication of PDU packets
   based on sequence number content in the UP tunnel headers.  For
   uplink packets, a RAN node replicates each packet from a UE.  An
   anchor UPF receives the duplicated packets, and drops ones which
   reach later in each duplicated packet pare.  On the other hand, for
   downlink packets, a UPF replicates packets received from DN, and a
   RAN node drops the duplicated packets as well.  The overviews of the
   ways are shown in Figure 8.

                             Namf|       Nsmf|       Npcf|
                              +--+--+     +--+--+     +--+--+
                              | AMF |     | SMF |     | PCF |
                              +-+---+     +--+--+     +-----+
                               /             |
                            N2/            N4|
                             /  N3 Tunnel1   |
               +----+    +--+--+__________+--+--+  N6 +----+
               | UE +----+ RAN |__________| UPF |-----+ DN |
               +----+    +-----+          +-----+     +----+
                                N3 Tunnel2

          Figure 8: Redundant UP transmission with two N3 tunnels

   In addition, there is a case that two intermediate UPFs (I-UPFs)
   between anchor UPF and RAN are used to support the redundant
   transmission based on two N3 and N9 tunnels between single anchor UPF
   and RAN node.  The RAN node and anchor UPF support the packet
   replication and dropping of duplicated packets as described above.
   As described above, anchor UPF and RAN node detect packet duplication
   with sequence number of UP tunnels, and thus I-UPFs would forward the
   packets with the same sequence number on N3 and N9 tunnels.  The
   overview is shown in Figure 9.

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                   Namf|         Nsmf|          Npcf|
                    +--+--+      +---+---+       +--+--+
                    | AMF |      |  SMF  +       | PCF |
                    +--+--+      +-+-+-+-+       +-----+
                      /            | | |
                  N2 /           N4| | +----------------+
                    /              | |                  |
                   /  N3 Tunnel1 +-+-+---+  N9 Tunnel1  |
                  /        +-----+I-UPF1 +----+         |
     +----+   +--+--+______|     +-------+    |______+--+--+ N6  +----+
     | UE +---+ RAN |______          |         ______| UPF +-----+ DN |
     +----+   +-----+  N3  |     +---+---+    |  N9  +-----+     +----+
                           +-----+I-UPF2 +----+
                      N3 Tunnel2 +-------+  N9 Tunnel2

   Figure 9: Redundant UP transmission with two I-UPF and N3/N9 tunnels

   o  Supporting QoS Monitoring for URLLC

   QoS monitoring is also required for URLLC.  It means that the user
   plane should be able to measure packet delay between anchor UPF and
   UE.  The measurement would be in various granularities, in the basis
   of per QoS Flow per UE, or per UP path for example.

   To help the measurement at anchor UPF and RAN, UP protocol requires
   to have capability to convey necessary information to do that; such
   as time information at sending or reception of a measurement packet.
   That information should exist in per F-TEID and QFI basis which
   indicates QoS Flow of the packet.  UP protocol should also be able to
   indicate which packets include the corresponding information for each

   The QoS monitoring requirement has been appeared in section 5.33.3 of
   [TS.23.501-3GPP] from Rel-16, version 16.2.0.

   ARCHI-Req-10: Time Sensitive Communication Support

   The 5GS supports Time Sensitive Communications (TSC) for realtime
   applications, and it can be integrated transparently as a bridge in
   an IEEE 802.1 TSN network.  For TSN time synchronization, the E2E 5GS
   can be considered as a "time-aware system (ref [IEEE-Std-802.1AS])".
   The TSN Translators (TTs) at the edges of the 5GS need to support the
   [IEEE-Std-802.1AS] operations.  For instance, UE, gNB, NW-TT
   (Network-side TSN Translator) and DS-TTs (Device-side TSN
   Translators) are synchronized with the Grandmaster (GM) located in
   the 5GS.  In addition, the TTs fulfill some functions related to
   [IEEE-Std-802.1AS] (e.g., gPTP support, timestamping, rateRatio,

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   etc.).  An overview of the 5G and TSN GM clock distribution model via
   the 5GS is shown in Figure 10.

<-TSN-D->    <----          5G Time Domain              ---->  <-TSN-D->

                           +-----+                              +------+
                           |5G GM|                              |TSN GM|
                           +-----+                              +------+
                              |M                                    |M
                              |                   +---+-----+       VS
 +----+                       VS    ,--------.    |   |NW-TT|    ,-----.
 |End |   +-----+  +--+  Uu +---+  / PTP      \   |   +-----+   / TSN   \
 |Sta.|<==|DS-TT|<-|UE|<----|gNB|--|Compatible|-->|UPF      |<==|Working|
 +----+S M+-----+  +--+S   M+---+M \ 5G TN    /  S+---------+S M\ Domain/
                                    `--------'                   `-----'

  TSN-D   : Non-3GPP TSN Domain
  TN      : Transport Network
  End Sta.: End Station
  <--     : 5GS timing direction
  <==     : TSN timing direction
  M       : Master
  S       : Slave

   Figure 10: An overview of the 5G and TSN GM clock distribution model

   In this model, two independent synchronizations are processing, and
   gNB only needs to be synchronized to the 5G GM clock.  To enable TSN
   domain synchronization, the 5GS calculates and adds the measured
   residence time between the DS-TT and NW-TT into the Correction Field
   (CF) of the synchronization packet of the TSN working domain.  The
   details are described in section 5.27 in [TS.23.501-3GPP].

   From this feature, UP functions and protocol are needed to support
   TSN specified in [IEEE-Std-802.1AS] .

   ARCHI-Req-11: Cellular IoT Support

   For supporting Cellular IoT (CIoT) (ref.  [TS.22.261-3GPP]),
   optimizations of functionalities of the 5GS is needed.  CIoT is in
   earlier 3GPP release also referred to as Machine Type Communication
   (MTC).  Some of CIoT functionalities relevant to user plane are
   described in this section.  The details of CIoT support is described
   in section 5.31 in [TS.23.501-3GPP].

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   o  Non-IP Data Delivery (NIDD)

   The 5GS may support Non-IP Data Delivery (NIDD) to handle Mobile
   Originated (MO) and Mobile Terminated (MT) communication for
   unstructured data.  Thus, User Plane Protocol should be conveyable
   such unstructured data units.

   o  Reliable Data Service (RDS)

   Reliable Data Service (RDS) may be used for a PDU session of
   unstructured type.  The service provides a mechanism for the NEF or
   UPF to determine if the data was successfully delivered to the UE and
   for the UE to determine if the data was successfully delivered to the
   NEF or UPF.

   When the service is enabled, a protocol that uses a packet header to
   identify the requested acknowledgement from peered end-point may be
   used between end-points of the PDU session.  In addition, port
   numbers in the header are used to identify the applications on the
   originator and receiver.  The UE, NEF and the UPF may support
   reservation of the source and destination port numbers for their use
   and subsequent release of the reserved port numbers.

   Therefore, UP protocol is required to have fields for containing
   information to determine normality of unstructured PDU sessions and
   used applications.

   o  High Latency Communication

   Functions for High Latency Communication may be used to handle mobile
   terminated (MT) communication with UEs being unreachable while using
   power saving functions.  "High latency" refers to the initial
   response time before normal exchange of packets is established.  High
   latency communication is supported by extended buffering of downlink
   data in the UPF, SMF or NEF when a UE is using power saving functions
   in CM-IDL state and the UE is not reachable.

   o  Small Data Rate Control

   The SMF may apply Small Data Rate Control for PDU sessions based on,
   for example, operator policy, DNN, S-NSSAI, RAT type etc.  The rate
   control may indicate following parameters in each of uplink and

   - an integer number of packets per time unit

   - an integer number of additional allowed exception report packets
   per time unit once the rate control limit has been reached

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   The UE shall comply with this uplink rate control instruction.  If
   the UE exceeds the uplink number of packet per time, the UE may still
   send uplink exception report if allowed and the number exception
   reports per time unit has not been exceeded.

   For the UPF and NEF, Small Data Rate Control is based on a maximum
   allowed rate per direction.  The UPF or NEF may enforce the uplink
   rate by discarding or delaying packets that exceed the maximum
   allowed rate.  The UPF or NEF shall enforce the downlink rate by
   discarding or delaying packets that exceed the downlink part of the
   maximum allowed rate.

   o  User Plane CIoT 5GS Optimisation

   User Plane CIoT 5GS Optimization enables transfer of user plane data
   from CM-IDLE without the need for using the Service Request procedure
   by negotiation between UE and AMF in advance.  In case that there are
   many devices being CM-IDLE state for long time, it would be better
   that User Plane Protocol i s session less.

5.  Evaluation Aspects

   This section provides UP protocol evaluation aspects that are mainly
   we derived from the architectural requirements described in
   Section 4.  Those aspects are not prioritized by the order here.
   Expected deployment scenarios explain the evaluations purpose in the
   corresponding aspects.

   As we were noticed that the gaps between GTP-U specifications and 5G
   architectural requirements through the analysis, those each gap are
   briefly described in the evaluation aspect associated to it.

   Since it is obvious that 5G system should be able to interwork with
   existing previous generation based systems, any aspects from
   coexisting and interworking point of view are not particularly
   articulated here.  It may be described in a next version.

5.1.  Supporting PDU Session Type Variations

   Given that UP protocol is required to support all PDU session types:
   IPv4, IPv6, Ethernet, and Unstructured.  However, it is expected that
   some deployment cases allow candidate protocol to adopt only one or
   few PDU session type(s) for simplicity of operations.  As we can
   expect that IPv4 connectivity services will be available through
   IPv6-only PDU session that enabled by bunch of IPv6 transition
   solutions already available in the field.

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   For this, the expected evaluation points from this aspect should be
   whether there is substitutional means to cover other PDU session
   types.  And how much it makes simple the system than deploying
   original PDU session types.

5.2.  Nature of Data Path

   As it is described in Section 4.2, the single PDU session multi-
   homing case requires multipoint-to-point (MP2P) data path.  It should
   be much scalable than multi-homing with multiple PDU sessions because
   number of required path states in the UPFs are reduced as closed to
   egress endpoint.  Against that point-to-point (P2P) protocol requires
   same number of states in each UPF throughout the path, and it could
   increase explosively the load on management of tunnel states.

   From this point of view, the expected evaluation points from this
   aspect is whether the nature of candidate UP protocols are to utilize
   MP2P data path.  Supporting MP2P data path by GTP-U could be a gap
   since GTP-U is a point-to-point tunneling protocol as it is described
   in Section 3.

   Noted that 3GPP CT WG4 pointed out GTP-U was already required to
   allow one single tunnel endpoint to receive packets from multiple
   source endpoints ([C4-185491-3GPP]).  It was an architectural
   requirement of 3GPP system from a previous generation.  It means that
   MP2P data path requirement for UP protocol has been existed before
   the 5G system.

5.3.  Supporting Transport Variations

   The 5G system will be expected that the new radio spectrums in high
   frequency bands require operators to deploy their base stations much
   dense for much wider areas compare to previous generation footprints.
   To make sure that density and coverage, all available types of
   transport in the field must be employed between RAN to UPF, or UPF to

   It is also expected that MTU size of each transport could be varied.
   Because one could be own fiber which the operator configure the MTU
   size as they like while others are third-party provided L2/L3 VPN
   lines which MTU size can't be controlled by the operators.

   The MTU between RAN and UPF can be discovered by offline means and
   the operator takes into account the MTU that is transferable on the
   radio interface and based on this the operator configures the right
   MTU to be used.  That is then signaled to the UE either via PCO (for
   IPv4 case) or the IPv6 RA message (for IPv6 case).

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   In addition, for cases that third-parties provide VPN lines, it would
   be recommended MTU size discovery for each data path and dynamic MTU
   size adjustment mechanisms, while GTP-U does not support those

   As the study item in 3GPP mentioned, IPv6 is preferable address
   family not only for UEs, but also for the UP transport, in terms of
   size of available address space to support dense and wide footprint
   of base stations.  However it increases header size from 20bytes to
   40bytes compare to IPv4.  It could be a problem if the MTU size is
   uncontrollable, or only limited MTU size available to carry committed
   PDU size on the user plane.

   The expected evaluation points from this aspect should be that the
   candidate protocols are able to dynamically adjust path MTU size with
   appropriate MTU size discovery mechanism.  It also should be that how
   the candidate protocols leverage IPv6 to deal with header size

5.4.  Data Path Management

   As Section 4.2 described, the 5G systems allows user plane that
   flexible UPF selection, multiple anchor UPFs, and no limit on how
   many UPFs chained for the data path of the PDU session.  UPF
   deployments in the field will thereby be distributed to be able to
   optimize the data path based on various logics and service scenarios.

   That powerful user plane capability could make data path management
   through the control plane, or operation support systems (OSS) be
   complicated and difficult.  Perhaps it could be the case where the UP
   protocol nature is P2P and it only supports per session base data
   path handling.  Therefore it would be better that UP protocol could
   support to aggregate several PDU sessions into a tunnel or shall be a
   session-less tunnel.

   Because it increases data path states by number of sessions, and
   number of endpoints of UPFs that makes data path handling much hectic
   and the control plane tend to be overloaded by not only usual
   attach/detach/hand-over operations, but also existing session
   manipulation triggered by UPF and transport nodes/paths restoration,

   The expected evaluation points from this aspect should be that how
   much the candidate protocols can reduce data path management loads
   both on the control plane NFs and UPFs compare to the per session
   based handling for P2P paths.  It could possibly include N3 and N6 in
   addition to N9 while it supports flexible user plane data path
   optimizations for some example scenarios.

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5.5.  QoS Control

   The QoS model is based on QoS flows to which QFI indicates in the 5G
   system that allows multiple QoS flows are aggregated into a single
   PDU session.  So that it is given that the UP protocol should convey
   QFIs for a PDU session and the UPF needs to lookup them.  It makes
   sure that reflects QoS policy in the 5G system to corresponding
   forwarding policy in the user plane IP transports.

   The expected evaluation points from this aspect should be whether the
   candidate protocols can provide stable ID space for QFI which
   shouldn't be a big deal since QFI just requires 6-bits space.

   As we pointed out in Section 3.2, the lookup process could impact UPF
   performance if the QFI container position in the header is
   unpredictable.  It could happen many times along the path because the
   each UPFs should do it again and again in case that various different
   QoS policies are deployed in the networks under the UP as we
   discussed in Section 5.3.

   As [TS.29.281-3GPP] updated in version 15.3.0, it is recommended that
   the first extension header is the PDU session container in which QFI
   is present.

5.6.  Traffic Detection and Flow Handling

   As described in Section 4.1.1, UPF need to detect traffic flow
   specified by SMF within a PDU session, and enforce some processes to
   the PDU based on the pre-configured policy rule.

   As similar with QoS flow lookup described in Section 5.5, UPFs along
   the path are repeatedly detecting an specified traffic flow in inner
   PDU.  It could increase redundant flow detection load on every UPFs
   that could be avoided if the upstream UPF put some identifier which
   abstracts the detected flow into the packets.  It enables following
   UPFs just find the ID to detect the indicated flow from the packet.

   The expected evaluation points from this aspect should be whether the
   candidate protocols can provide means to reduce that redundant flow
   detection that could be enough bits space on stable ID space to put
   abstracted detected flow identifier.

5.7.  Supporting Network Slicing Diversity

   Network Instance has been defined as the glue of network slice
   between 5G and IP transport in addition to IP domain separation, as
   described in Section 4.1.2.  It is expected that SMF is able to
   configure UPF to send UP packet to corresponding transport slice by

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   indicating Network Instance in FAR so that UPF can determine outgoing
   interface for the UP packet.

   It is assumed that IP transport networks are Network Instance
   agnostic, i.e., transport slices are independently instantiated and
   not bound to specific IP address space in the 5GC, for preventing
   increase of routing table size.

   As a transport slice may be shared with multiple IP domains, Network
   Instance could be instantiated for all combination of IP domains and
   transport slice.  To indicate those combination in UP packet over the
   wire, the 5G architecture expects VPN solutions as described in
   section 5.6.12 of [TS.23.501-3GPP].

   Binding Network Instance with corresponding VPN would be varied per
   VPN solutions and FAR is not able to do.  Hence it is out of scope of
   3GPP and it may be covered by IETF, or other SDOs.

   Apart from binding, if it is the case where MPLS based VPNs, such as
   [RFC4364] and [RFC4664] are expected as the existing VPN solution
   which bound to Network Instance, there are some avaiable deployment
   options, such as 1).  PE router integrates UPF, 2).  CE router
   integrates UPF, 3).  UPF connects to the VPN behind the CE router.

   Option 1 could work since all legacy MPLS or Segment Routing
   [RFC8402] based solution are available for both VPN and transport
   slicing at the UPF integrated PE router.  However it is hard to
   expect it in multi-vendor deployment case, where the PE routers
   providing vendor is different from the vendor who provides UPFs, for

   Option 2 and 3 are expected as IP domain separation, but it is hard
   to see that it is able to indicate transport slice in addition to the
   IP domain.  Other L2 and tunneling solutions should be same with
   those options.

   The expected evaluation points from this aspect should be whether the
   candidate protocols can contain forwarding information associated to
   the assigned IP domain and transport slice for all possible
   deployment cases.

5.8.  Reliable Communication support

   As Section 4.2 described, more than two UP paths are required for a
   QoS flow of a PDU session between the anchor UPF and gNB.  Those UP
   paths are to convey redundant duplicated packets.

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   To support reliable communication with above requirements, UPF and
   gNB must replicate the sending UP packets and eliminate the received
   duplicated UP packets.  Not to mention that UP protocol should be
   able to make sure that the paths are not in fate sharing, the UP
   packet must have sequence number to indicate duplicate packets per
   QoS flow basis.

   The expected evaluation points from this aspect should be whether the
   candidate protocols can indicate packet sequence and diversed paths
   in the context of QoS flow, not in UP tunnel context.  The packet
   sequence information should be transparent through I-UPF(s) exist in
   the middle of the path even in case that the UP tunnels are
   terminated at the I-UPF(s).

6.  Conclusion

   We analyzed the 3GPP specifications of the 5G architecture in terms
   of user plane and the current protocol adopted to the user plane.
   After the analysis work, we believe that the results described in
   this document shows that we reach at certain level of understanding
   on the 5G systems and ready to provide our inputs to 3GPP.

   We clarified GTP-U through the analysis and listed observed
   characteristics in Section 3.6.  We also clarified the architectural
   requirements for UP protocol described in Section 4.2.

   Our conclusion here is that it is hopefull if the evaluation aspects
   described in Section 5 help for the study progress.  It is worth to
   study possible candidate UP protocols for the 5G system including
   current one based from the aspects.

7.  Security Consideration


8.  Acknowledgement

   The authors would like to thank Tom Herbert, Takashi Ito, John Leddy,
   Pablo Camarillo, Daisuke Yokota, Satoshi Watanabe, Koji Tsubouchi and
   Miya Kohno for their detailed reviews, comments, and contributions.

   A special thank you goes to Arashmid Akhavain for his technical
   review and feedback.

   Lastly, the authors would like to thank 3GPP CT WG4 folks for their
   review and feedback.

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9.  Informative References

              3rd Generation Partnership Project (3GPP), "LS OUT on User
              Plane Analysis", July 2018,

              3rd Generation Partnership Project (3GPP), "New Study Item
              on User Plane Protocol in 5GC", December 2017,

              3rd Generation Partnership Project (3GPP), "LS on user
              plane protocol study", March 2018,

              Internet Architecture Board (IAB), "IAB Statement on
              IPv6", November 2016,

              Institute of Electrical and Electronics Engineers (IEEE),
              "Timing and Synchronization for Time-Sensitive
              Applications in Bridged Local Area Networks", March 2011,

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <>.

   [RFC4664]  Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
              2 Virtual Private Networks (L2VPNs)", RFC 4664,
              DOI 10.17487/RFC4664, September 2006,

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

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   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,

   [RFC7157]  Troan, O., Ed., Miles, D., Matsushima, S., Okimoto, T.,
              and D. Wing, "IPv6 Multihoming without Network Address
              Translation", RFC 7157, DOI 10.17487/RFC7157, March 2014,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <>.

              3rd Generation Partnership Project (3GPP), "3GPP TR 29.891
              (V15.0.0): 5G System Phase 1, CT WG4 Aspects", December
              2017, <

              3rd Generation Partnership Project (3GPP), "3GPP TS 22.261
              (V15.7.0): Service requirements for 5G system stage 1",
              December 2018, <

              3rd Generation Partnership Project (3GPP), "3GPP TS 23.060
              (V15.3.0): General Packet Radio Service (GPRS); Service
              description; Stage 2", June 2018,

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              3rd Generation Partnership Project (3GPP), "3GPP TS 23.501
              (V16.2.0): System Architecture for 5G System; Stage 2",
              September 2019, <

              3rd Generation Partnership Project (3GPP), "3GPP TS 23.502
              (V15.4.0): Procedures for 5G System; Stage 2", December
              2018, <

              3rd Generation Partnership Project (3GPP), "3GPP TS 23.503
              (V15.4.0): Policy and Charging Control System for 5G
              Framework; Stage 2", December 2018,

              3rd Generation Partnership Project (3GPP), "3GPP TS 28.530
              (V15.1.0): Management and orchestration of networks and
              network slicing; Concepts, use cases and requirements
              (work in progress)", December 2018,

              3rd Generation Partnership Project (3GPP), "3GPP TS 28.531
              (V15.1.0): Management and orchestration of networks and
              network slicing; Provisioning; Stage 1 (Release 15)",
              December 2018, <

              3rd Generation Partnership Project (3GPP), "3GPP TS 28.532
              (V15.1.0): Management and orchestration of networks and
              network slicing; Provisioning; Stage 2 and stage 3
              (Release 15)", Decempber 2018,

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              3rd Generation Partnership Project (3GPP), "3GPP TS 28.533
              (V15.1.0): Management and orchestration of networks and
              network slicing; Management and orchestration architecture
              (Release 15)", December 2018,

              3rd Generation Partnership Project (3GPP), "3GPP TS 29.244
              (V15.1.0): Interface between the Control Plane and the
              User Plane Nodes; Stage 3", December 2018,

              3rd Generation Partnership Project (3GPP), "3GPP TS 29.274
              (V15.4.0): 3GPP Evolved Packet System (EPS); Evolved
              General Packet Radio Service (GPRS) Tunneling Protocol for
              Control plane (GTPv2-C); Stage 3", June 2018,

              3rd Generation Partnership Project (3GPP), "3GPP TS 29.281
              (V16.1.0): GPRS Tunneling Protocol User Plane (GTPv1-U)",
              September 2020, <

              3rd Generation Partnership Project (3GPP), "3GPP TS 29.510
              (V15.2.0): 5G System; Network Function Repository
              Services; Stage 3", December 2018,

              3rd Generation Partnership Project (3GPP), "3GPP TS 29.561
              (V15.1.0): 5G System; Interworking between 5G Network and
              external Data Networks; Stage 3", September 2018,

              3rd Generation Partnership Project (3GPP), "3GPP TS 38.300
              (v15.4.0): NR and NG-RAN Overall Description; Stage 2",
              December 2018, <

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              3rd Generation Partnership Project (3GPP), "3GPP TS 38.401
              (v15.4.0): NG-RAN; Architecture Description", December
              2018, <

              3rd Generation Partnership Project (3GPP), "3GPP TS 38.415
              (v16.2.0): NG-RAN; PDU Session User Plane protocol",
              October 2020, <

Authors' Addresses

   Shunsuke Homma


   Takuya Miyasaka
   KDDI Research


   Satoru Matsushima


   Daniel Voyer
   Bell Canada


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