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Versions: (draft-minaburo-lpwan-nbiot-hc)  00 01         Standards Track
          02 03 04 05 06 07 08 09                                       
lpwan Working Group                                             E. Ramos
Internet-Draft                                                  Ericsson
Intended status: Standards Track                             A. Minaburo
Expires: 20 November 2022                                         Acklio
                                                             19 May 2022


                            SCHC over NBIoT
                  draft-ietf-lpwan-schc-over-nbiot-08

Abstract

   The Static Context Header Compression and Fragmentation (SCHC)
   specification describes header compression and fragmentation
   functionalities for LPWAN (Low Power Wide Area Networks)
   technologies.  The Narrowband Internet of Things (NB-IoT)
   architecture may adapt SCHC to improve its capacities.

   This document describes the use of SCHC over the NB-IoT wireless
   access and provides use-cases for efficient parameterization.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 20, 2022.

Copyright Notice

   Copyright (c) 2022 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components



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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Data Transmission in the 3GPP Architecture  . . . . . . . . .   5
     4.1.  Use of SCHC over the Radio link . . . . . . . . . . . . .   6
       4.1.1.  SCHC Entities Placing . . . . . . . . . . . . . . . .   6
     4.2.  Use of SCHC over the No-Access Stratum (NAS)  . . . . . .   7
       4.2.1.  SCHC Entities Placing . . . . . . . . . . . . . . . .   8
       4.2.2.  Parameters for Static Context Header Compression and
               Fragmentation (SCHC) for the Section 4.1 and
               Section 4.2.  . . . . . . . . . . . . . . . . . . . .   9
     4.3.  End-to-End Compression  . . . . . . . . . . . . . . . . .  11
       4.3.1.  SCHC Entities Placing . . . . . . . . . . . . . . . .  11
       4.3.2.  Parameters for Static Context Header Compression and
               Fragmentation (SCHC)  . . . . . . . . . . . . . . . .  12
   5.  Padding . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  14
   7.  Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  NB-IoT User Plane protocol architecture . . . . . . . . .  14
       7.1.1.  Packet Data Convergence Protocol (PDCP) . . . . . . .  14
       7.1.2.  Radio Link Protocol (RLC) . . . . . . . . . . . . . .  15
       7.1.3.  Medium Access Control (MAC) . . . . . . . . . . . . .  16
     7.2.  NB-IoT Data over NAS (DoNAS)  . . . . . . . . . . . . . .  17
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The Static Context Header Compression (SCHC) [RFC8724] defines a
   header compression scheme, and fragmentation functionality, suitable
   for the Low Power Wide Area Networks (LPWAN) networks described in
   [RFC8376].

   In a Narrowband Internet of Things (NB-IoT) network, header
   compression efficiently brings Internet connectivity to the Device -
   User Equipment (Dev-UE).  This document describes the SCHC parameters
   used to support the static context header compression and
   fragmentation over the NB-IoT wireless access.  This document assumes
   functionality for NB-IoT of 3GPP release 15 (3GPPR15).  Otherwise,
   the text explicitly mentions other versions' functionality.





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2.  Terminology

   This document will follow the terms defined in [RFC8724], in
   [RFC8376], and the [TGPP23720].

   *  CIoT.  Cellular IoT.

   *  NGW-C-SGN.  Network Gateway - CIoT Serving Gateway Node.

   *  Dev-UE.  Device - User Equipment.

   *  RGW-eNB.  Radio Gateway - Node B.  Base Station that controls the
      UE.

   *  EPC.  Evolved Packet Connectivity.  Core network of 3GPP LTE
      systems.

   *  EUTRAN.  Evolved Universal Terrestrial Radio Access Network.
      Radio access network of LTE-based systems.

   *  NGW-MME.  Network Gateway - Mobility Management Entity.  An entity
      in charge of handling mobility of the Dev-UE.

   *  NB-IoT.  Narrowband IoT.  A 3GPP LPWAN technology based on the LTE
      architecture but with additional optimization for IoT and using a
      Narrowband spectrum frequency.

   *  NGW-SGW.  Network Gateway - Serving Gateway.  It routes and
      forwards the user data packets through the access network.

   *  HSS.  Home Subscriber Server.  It is a database that performs
      mobility management.

   *  NGW-PGW.  Network Gateway - Packet Data Node Gateway.  An
      interface between the internal with the external network.

   *  PDU.  Protocol Data Unit.  A data packet including headers that
      are transmitted between entities through a protocol.

   *  SDU.  Service Data Unit.  A data packet (PDU) from higher layer
      protocols used by lower layer protocols as a payload of their own
      PDUs.

   *  IWK-SCEF.  InterWorking Service Capabilities Exposure Function.
      It is used in roaming scenarios, it is located in the Visited PLMN
      and serves for interconnection with the SCEF of the Home PLMN.





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   *  NGW-SCEF.  Network Gateway - Service Capability Exposure Function.
      EPC node for exposure of 3GPP network service capabilities to 3rd
      party applications.

3.  Architecture

   The Narrowband Internet of Things (NB-IoT) architecture has a complex
   structure.  It relies on different NGWs from different providers and
   can send data via different paths, each path with different
   characteristics in terms of bandwidths, acknowledgments, and layer
   two reliability and segmentation.

   Figure 1 shows this architecture, where the Network Gateway Cellular
   Internet of Things Serving Gateway Node (NGW-CSGN) optimizes co-
   locating entities in different paths.  For example, a Dev-UE using
   the path formed by the Network Gateway Mobility Management Entity
   (NGW-MME), the NGW-CSGW, and Network Gateway Packet Data Node Gateway
   (NGW-PGW) may get a limited bandwidth transmission from few bytes/s
   to one thousand bytes/s only.

   Another node introduced in the NB-IoT architecture is the Network
   Gateway Service Capability Exposure Function (NGW-SCEF), which
   securely exposes service and network capabilities to entities
   external to the network operator.  OMA and OneM2M define the
   northbound APIS [TGPP33203].  In this case, the path is small for
   data transmission.  The main functions of the NGW-SCEF are:
   Connectivity path and Device Monitoring.

     +---+                            +------+
     |Dev| \              +-----+ ----| HSS  |
     |-UE|  \             | NGW |     +------+
     +---+  |             |-MME |\__
             \          / +-----+   \
     +---+    \+-----+ /    |       +------+
     |Dev| ----| RGW |-     |       | NGW- |
     |-UE|     |-eNB |      |       | SCEF |---------+
     +---+    /+-----+ \    |       +------+         |
             /          \ +------+                   |
            /            \| NGW- |  +-----+   +-----------+
     +---+ /              | CSGW |--| NGW-|---|Application|
     |Dev|                |      |  | PGW |   |   Server  |
     |-UE|                +------+  +-----+   +-----------+
     +---+

                    Figure 1: 3GPP network architecture






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4.  Data Transmission in the 3GPP Architecture

   NB-IoT networks deal with end-to-end user data and in-band signalling
   between the nodes and functions to configure, control, and monitor
   the system functions and behaviors.  The signalling data uses a
   different path with specific protocols, handling processes, and
   entities but can transport end-to-end user data for IoT services.  In
   contrast, the end-to-end application only transports end-to-end data.

   The maximum recommended 3GPP MTU size is 1358 Bytes.  The radio
   network protocols limit the packet sizes over the air, including
   radio protocol overhead, to 1600 Bytes.  However, the recommended MTU
   is smaller to avoid fragmentation in the network backbone due to the
   payload encryption size (multiple of 16) and the additional core
   transport overhead handling.

   3GPP standardizes NB-IoT and, in general, the cellular technologies
   interfaces and functions.  Therefore the introduction of SCHC
   entities to Dev-UE, RGW-eNB, and NGW-CSGN needs to be specified in
   the NB-IoT standard, which implies that standard specifying SCHC
   support would not be backward compatible.  A terminal or a network
   supporting a version of the standard without SCHC or without an
   implementation capability (in case of not being standardized as
   mandatory capability) cannot utilize SCHC with this approach.

   SCHC could be deployed differently depending on where the header
   compression and the fragmentation are applied.  The SCHC
   functionalities can be used over the radio transmission only, between
   the Dev-UE and the RGW-eNB.  Alternatively, the packets transmitted
   over the path can use SCHC.  Else, when the transmissions go over the
   NGW-MME or NGW-SCEF, the NGW-CSGN uses SCHC entity.  For these two
   cases, the functions need to be standardized by 3GPP.

   Another possibility is to apply SCHC functionalities to the end-to-
   end connection or at least up to the operator network edge.  SCHC
   functionalities are available in the application layer of the Dev-UE
   and the Application Servers or a broker function at the edge of the
   operator network.  The radio network transmits the packets as non-IP
   traffic using IP tunnelling or SCEF services.  Since this option does
   not necessarily require 3GPP standardization, it is possible to also
   benefit legacy devices with SCHC by using the non-IP transmission
   features of the operator network.









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4.1.  Use of SCHC over the Radio link

   Deploying SCHC only over the radio link would require placing it as
   part of the protocol stack for data transfer between the Dev-UE and
   the RGW-eNB.  This stack is the functional layer responsible for
   transporting data over the wireless connection and managing radio
   resources.  There is support for features such as reliability,
   segmentation, and concatenation.  The transmissions use link
   adaptation, meaning that the system will optimize the transport
   format used according to the radio conditions, the number of bits to
   transmit, and the power and interference constraints.  That means
   that the number of bits transmitted over the air depends on the
   Modulation and Coding Schemes (MCS) selected.  The transmissions of
   Transport Block (TB) happen in the physical layer at network
   synchronized intervals called Transmission Time Interval (TTI).  Each
   Transport Block has a different MCS and number of bits available to
   transmit.  The MAC layer [TGPP36321] defines the Transport Blocks
   characteristics.  The Radio link stack shown in Figure 2 comprises
   the Packet Data Convergence Protocol (PDCP) [TGPP36323], Radio Link
   Protocol (RLC) [TGPP36322], Medium Access Control protocol (MAC)
   [TGPP36321], and the Physical Layer [TGPP36201].  The
   Appendix gives more details of these protocols.

4.1.1.  SCHC Entities Placing

   The current architecture provides support for header compression in
   the PDCP layer using RoHC [RFC5795].  Therefore SCHC header
   compression entities can be deployed similarly without the need for
   significant changes in the 3GPP specifications.

   In this scenario, the RLC layer takes care of fragmentation unless
   for the Transparent Mode.  When packets exceed the Transport Block
   size at transmission, SCHC fragmentation is unnecessary and should
   not be used to avoid the additional protocol overhead.  The RLC
   Transparent Mode is not commonly used while sending IP packets in the
   Radio link.  However, given the case in the future, SCHC
   fragmentation may be used.  In that case, a SCHC tile would match the
   minimum transport block size minus the PDCP and MAC headers.













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     +---------+                              +---------+  |
     |IP/non-IP+------------------------------+IP/non-IP+->+
     +---------+   |   +---------------+   |  +---------+  |
     | PDCP    +-------+ PDCP  | GTP|U +------+ GTP-U   |->+
     | (SCHC)  +       + (SCHC)|       +      +         |  |
     +---------+   |   +---------------+   |  +---------+  |
     | RLC     +-------+ RLC   |UDP/IP +------+ UDP/IP  +->+
     +---------+   |   +---------------+   |  +---------+  |
     | MAC     +-------+ MAC   | L2    +------+ L2      +->+
     +---------+   |   +---------------+   |  +---------+  |
     | PHY     +-------+ PHY   | PHY   +------+ PHY     +->+
     +---------+       +---------------+      +---------+  |
                C-Uu/                    S1-U             SGi
       Dev-UE               RGW-eNB             NGW-CSGN
               Radio Link

                     Figure 2: SCHC over the Radio link

4.2.  Use of SCHC over the No-Access Stratum (NAS)

   The NGW-MME conveys mainly control signaling between the Dev-UE and
   the cellular network [TGPP24301].  The network transports this
   traffic on top of the radio link.

   This kind of flow supports data transmissions to reduce the overhead
   when transmitting infrequent small quantities of data.  This
   transmission is known as Data over No-Access Stratum (DoNAS) or
   Control Plane CIoT EPS optimization.  In DoNAS, the Dev-UE uses the
   pre-established security and can piggyback small uplink data into the
   initial uplink message and uses an additional message to receive a
   downlink small data response.

   The NGW-MME performs the data encryption from the network side in a
   DoNAS PDU.  Depending on the data type signaled indication (IP or
   non-IP data), the network allocates an IP address or establishes a
   direct forwarding path.  DoNAS is regulated under rate control upon
   previous agreement, meaning that a maximum number of bits per unit of
   time is agreed upon per device subscription beforehand and configured
   in the device.












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   The system will use DoNAS when a terminal in a power-saving state
   requires a short transmission and receives an acknowledgment or short
   feedback from the network.  Depending on the size of buffered data to
   transmit, the Dev-UE might deploy the connected mode transmissions
   instead, limiting and controlling the DoNAS transmissions to
   predefined thresholds and a good resource optimization balance for
   the terminal and the network.  The support for mobility of DoNAS is
   present but produces additional overhead.  The Appendix gives
   additional details of DoNAS.

4.2.1.  SCHC Entities Placing

   In this scenario, SCHC may reside in the Non-Access Stratum (NAS)
   protocol layer.  The same principles as for Radio link transmissions
   apply here as well.  Because the NAS protocol already uses RoHC it
   can adapt SCHC for header compression too.  The main difference
   compared to the radio link is the physical placing of the SCHC
   entities.  On the network side, the NGW-MME resides in the core
   network and is the terminating node for NAS instead of the RGW-eNB.

   +--------+                       +--------+--------+  +  +--------+
   | IP/    +--+-----------------+--+  IP/   |   IP/  +-----+   IP/  |
   | Non-IP |  |                 |  | Non-IP | Non-IP |  |  | Non-IP |
   +--------+  |                 |  +-----------------+  |  +--------+
   | NAS    +-----------------------+   NAS  |GTP-C/U +-----+GTP-C/U |
   |(SCHC)  |  |                 |  | (SCHC) |        |  |  |        |
   +--------+  |  +-----------+  |  +-----------------+  |  +--------+
   | RRC    +-----+RRC  |S1|AP+-----+ S1|AP  |        |  |  |        |
   +--------+  |  +-----------+  |  +--------+  UDP   +-----+  UDP   |
   | PDCP*  +-----+PDCP*|SCTP +-----+ SCTP   |        |  |  |        |
   +--------+  |  +-----------+  |  +-----------------+  |  +--------+
   | RLC    +-----+ RLC | IP  +-----+ IP     | IP     +-----+ IP     |
   +--------+  |  +-----------+  |  +-----------------+  |  +--------+
   | MAC    +-----+ MAC | L2  +-----+ L2     | L2     +-----+ L2     |
   +--------+  |  +-----------+  |  +-----------------+  |  +--------+
   | PHY    +--+--+ PHY | PHY +--+--+ PHY    | PHY    +-----+ PHY    |
   +--------+     +-----+-----+     +--------+--------+  |  +--------+
              C-Uu/           S1-lite                   SGi
    Dev-UE           RGW-eNB               NGW-MME             NGW-PGW



       *PDCP is bypassed until AS security is activated TGPP36300.

     Figure 3: SCHC entities placement in the 3GPP CIOT radio protocol
                    architecture for DoNAS transmissions





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4.2.2.  Parameters for Static Context Header Compression and
        Fragmentation (SCHC) for the Section 4.1 and Section 4.2.

   These scenarios MUST use SCHC header compresion capability to improve
   the transmission of IPv6 packets.  The 3GPP Architecture currently
   provides Header Compression using the [RFC5795] but the use of SCHC
   for IoT application MUST be considered to improve the devices
   connectivity.

   *  SCHC Context initialization RRC (Radio Resource Control) protocol
      is the main tool used to configure the parameters of the Radio
      link.  It will configure SCHC and the static context distribution
      as it has made for RoHC operation [TGPP36323].

   *  SCHC Rules The network operator in these scenarios defines the
      number of rules in a context.  The operator must be aware of the
      type of IP traffic that the device will carry out.  Implying that
      the operator might use provision sets of rules compatible with the
      use case of the device.  For devices acting as gateways of other
      devices, several rules may match the diversity of devices and
      protocols used by the devices associated with the gateway.
      Meanwhile, simpler devices (for example, an electricity meter) may
      have a predetermined set of fixed protocols and parameters.
      Additionally, the deployment of IPv6 addresses may force different
      rules to deal with each case.

   *  RuleID There is a reasonable assumption of 9 bytes of radio
      protocol overhead for these transmission scenarios in NB-IoT,
      where PDCP uses 5 bytes due to header and integrity protection,
      and RLC and MAC use 4 bytes.  The minimum physical Transport
      Blocks (TB) that can withhold this overhead value according to
      3GPP Release 15 specifications are 88, 104, 120, and 144 bits.  A
      transmission optimization may require only one physical layer
      transmission.  SCHC overhead should not exceed the available
      number of effective bits of the smallest physical TB available.
      The packets handled by 3GPP networks are byte-aligned, and
      therefore the minimum payload possible (including padding) is 8
      bits.  Therefore in order to use the smallest TB, the maximum SCHC
      header size is 12 bits.  These 12 bits must include the
      Compression Residue in addition to the RuleID.  On the other hand,
      more complex NB-IoT devices (such as a capillarity gateway) might
      require additional bits to handle the variety and multiple
      parameters of higher-layer protocols deployed.  In that sense, the
      operator may want to have flexibility on the number and type of
      rules supported by each device independently, and consequently,
      these scenarios require a configurable value.  The configuration
      may be part of the operation profile agreed together with the
      content distribution.  The RuleID field size may range from 2



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      bits, resulting in 4 rules to an 8-bit value that would yield up
      to 256 rules that can be used together with the operators and
      seems quite a reasonable maximum limit even for a device acting as
      a NAT.  More bits could be configured, but it should consider the
      byte-alignment of the expected Compression Residue.  In the
      minimum TB size case, 2 bits of RuleID leave only 6 bits available
      for Compression Residue.

   *  SCHC MAX_PACKET_SIZE The Radio Link can handle the fragmentation
      of SCHC packets if needed, including reliability.  Hence the
      packet size is limited by the MTU handled by the radio protocols
      which corresponds to 1600 bytes for 3GPP Release 15.

   *  Fragmentation For the Section 4.1 and Section 4.2 scenarios, the
      SCHC fragmentation functions are disabled.  The RLC layer of NB-
      IoT can segment packets in suitable units that fit the selected
      transport blocks for transmissions of the physical layer.  The
      blocks selection is made according to the link adaptation input
      function in the MAC layer and the quantity of data in the buffer.
      The link adaptation layer may produce different results at each
      Time Transmission Interval (TTI), resulting in varying physical
      transport blocks that depend on the network load, interference,
      number of bits transmitted, and QoS.  Even if setting a value that
      allows the construction of data units following the SCHC tiles
      principle, the protocol overhead may be greater or equal than
      allowing the Radio link protocols to take care of the
      fragmentation natively.

   *  Fragmentation in RLC Transparent Mode If RLC operates in
      Transparent Mode, there could be a case to activate a
      fragmentation function together with a light reliability function
      such as the ACK-Always mode.  In practice, it is uncommon to
      transmit radio link data using this configuration.  It mainly
      targets signaling transmissions.  In those cases, the MAC layer
      mechanisms ensure reliability, such as repetitions or automatic
      retransmissions, and additional reliability might only generate
      protocol overhead.

   SCHC may reduce radio network protocols overhead in future
   operations, support reliable transmissions, and transmit compressed
   data with fewer possible transmissions by using fixed or limited
   transport blocks compatible with the tiling SCHC fragmentation
   handling.  For SCHC fragmentation parameters see section
   Section 4.3.2.







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4.3.  End-to-End Compression

   The Non-IP Data Delivery (NIDD) services of 3GPP enable the
   transmission of SCHC packets compressed by the application layer.
   The packets can be delivered using IP-tunnels to the 3GPP network or
   NGW-SCEF functions (i.e., API calls).  In both cases, as compression
   occurs before transmission, the network will not understand the
   packet, and the network does not have context information of this
   compression.  Therefore the network will treat the packet as Non-IP
   traffic and deliver it to the other side without any other protocol
   stack element, directly under the L2.

4.3.1.  SCHC Entities Placing

   In the two scenarios using End-to-End compression, SCHC entities are
   located almost on top of the stack.  The NB-IoT connectivity services
   implement SCHC in the Dev, an in the Application Server.  The IP
   tunneling scenario requires that the Application Server send the
   compressed packet over an IP connection terminated by the 3GPP core
   network.  If the transmission uses the NGW-SCEF services, it is
   possible to utilize an API call to transfer the SCHC packets between
   the core network and the Application Server.  Also, an IP tunnel
   could be established by the Application Server if negotiated with the
   NGW-SCEF.

   +---------+       XXXXXXXXXXXXXXXXXXXXXXXX             +--------+
   | SCHC    |      XXX                    XXX            | SCHC   |
   |(Non-IP) +-----XX........................XX....+--*---+(Non-IP)|
   +---------+    XX                  +----+  XX   |  |   +--------+
   |         |    XX                  |SCEF+-------+  |   |        |
   |         |   XXX     3GPP RAN &   +----+  XXX     +---+  UDP   |
   |         |   XXX    CORE NETWORK          XXX     |   |        |
   |  L2     +---+XX                  +------------+  |   +--------+
   |         |     XX                 |IP TUNNELING+--+   |        |
   |         |      XXX               +------------+  +---+  IP    |
   +---------+       XXXX                 XXXX        |   +--------+
   | PHY     +------+ XXXXXXXXXXXXXXXXXXXXXXX         +---+  PHY   |
   +---------+                                            +--------+
      UE                                                       AS

      Figure 4: SCHC entities placed when using Non-IP Delivery (NIDD)
                                3GPP Sevices









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4.3.2.  Parameters for Static Context Header Compression and
        Fragmentation (SCHC)

   These scenarios may use SCHC header compresion capability to improve
   the transmission of IPv6 packets.  The use of SCHC for IoT
   application MUST be considered to improve the devices connectivity.

   *  SCHC Context initialization.  The application layer handles the
      static context; consequently, the context distribution must be
      according to the application's capabilities, perhaps utilizing IP
      data transmissions up to context initialization.  Also, the static
      contexts delivery may use the same IP tunneling or NGW-SCEF
      services used later for the SCHC packets transport.

   *  SCHC Rules.  Even when the transmission content is not visible for
      the 3GPP network, the same limitations as for Section 4.1 and
      Section 4.2 transmissions apply in these scenarios in terms of
      aiming to use the minimum number of transmissions and minimize the
      protocol overhead.

   *  Rule ID Similar to the case of Section 4.1 and Section 4.2, the
      RuleID size can be dynamically set before the context delivery.
      For example, negotiated between the applications when choosing a
      profile according to the type of traffic and application deployed.
      The same considerations related to the transport block size and
      performance mentioned for the Section 4.1 and Section 4.2 must be
      followed when choosing a size value for the RuleID field.

   *  SCHC MAX_PACKET_SIZE In these scenarios, the maximum recommended
      MTU size that applies is 1358 Bytes since the SCHC packets (and
      fragments) are traversing the whole 3GPP network infrastructure
      (core and radio), not only the radio as the IP transmissions case.

   *  Fragmentation Packets larger than 1358 bytes need the SCHC
      fragmentation function.  Since the 3GPP uses reliability
      functions, the No-ACK fragmentation mode may be enough in point-
      to-point connections.  Nevertheless, additional considerations are
      described below for more complex cases.

   *  Fragmentation modes A global service assigns a QoS to the packets
      depending on the billing.  Packets with very low QoS may get lost
      before they arrive in the 3GPP radio network transmission, for
      example, in between the links of a capillarity gateway or due to
      buffer overflow handling in a backhaul connection.  The use of
      SCHC fragmentation with the ACK-on-Error mode is recommended to
      secure additional reliability on the packets transmitted with a
      small trade-off on additional transmissions to signal the end-to-
      end arrival of the packets if no transport protocol takes care of



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      retransmission.
      Also, the ACK-on-Error mode is even desirable to keep track of all
      the SCHC packets delivered.  In that case, the fragmentation
      function could be active for all packets transmitted by the
      applications.  SCHC ACK-on-Error fragmentation may be active in
      transmitting non-IP packets on the NGW-MME.  A non-IP packet will
      use SCHC reserved RuleID for non-compressing packets as [RFC8724]
      allows it.

   *  Fragmentation Parameters SCHC profile will have specific Rules for
      the fragmentation modes.  The rule will identify, which
      fragmentation mode is in use, and section Section 4.2.2 defines
      the RuleID size.

   SCHC parametrization considers that NBIoT aligns the bit and uses
   padding and the size of the Transfer Block.  SCHC will try to reduce
   padding to optimize the compression of the information.  The Header
   size needs to be multiple of 4, and the Tiles may keep a fixed value
   of 4 or 8 bits to avoid padding except for transfer block equals 16
   bits where Tiles may be of 2 bits.  The transfer block size has a
   wide range of values.  Two configurations may be used for the
   fragmentation parameters.

   *  For Transfer Blocks smaller or equal to 300bits using a 8 bits-
      Header_size configuration, with the size of the header fields as
      follows:

      -  RuleID from 1 - 3 bits,

      -  DTag 1 bit,

      -  FCN 3 bits,

      -  W 1 bits.

   *  For Transfer Blocks bigger than 300 bits using a 16 bits-
      Header_size configuration, with the size of the header fields as
      follows:

      -  RulesID from 1 to 8 or 10 bits,

      -  DTag 1 or 2 bits,

      -  FCN 3 bits,

      -  W 2 or 3 bits.





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   The IoT devices communicate with small data transfer and have a
   battery life of 10 years.  These devices use the Power Save Mode and
   the Idle Mode DRX, which govern how often the device wakes up, stays
   up, and is reachable.  Table 10.5.163a in {3GPP-TS_24.088} specifies
   a range for the radio timers as N to 3N in increments of one where
   the units of N can be 1 hour or 10 hours.  To adapt SCHC to the NB-
   IoT activities, the Inactivity Timer and the Retransmission Timer be
   set based on these limits.

5.  Padding

   NB-IoT and 3GPP wireless access, in general, assumes byte-aligned
   payload.  Therefore the L2 word for NB-IoT MUST be considered 8 bits,
   and the padding treatment should use this value accordingly.

6.  Security considerations

   This document does not add any security considerations and follows
   the [RFC8724] and the 3GPP access security document specified in
   [TGPP33203].

7.  Appendix

7.1.  NB-IoT User Plane protocol architecture

7.1.1.  Packet Data Convergence Protocol (PDCP)

   Each of the Radio Bearers (RB) is associated with one PDCP entity.
   Moreover, a PDCP entity is associated with one or two RLC entities
   depending on the unidirectional or bi-directional characteristics of
   the RB and RLC mode used.  A PDCP entity is associated with either a
   control plane or a user plane with independent configuration and
   functions.  The maximum supported size for NB-IoT of a PDCP SDU is
   1600 octets.  The primary services and functions of the PDCP sublayer
   for NB-IoT for the user plane include:

   *  Header compression and decompression using ROHC (Robust Header
      Compression)

   *  Transfer of user and control data to higher and lower layers

   *  Duplicate detection of lower layer SDUs when re-establishing
      connection (when RLC with Acknowledge Mode in use for User Plane
      only)

   *  Ciphering and deciphering

   *  Timer-based SDU discard in uplink



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7.1.2.  Radio Link Protocol (RLC)

   RLC is a layer-2 protocol that operates between the UE and the base
   station (eNB).  It supports the packet delivery from higher layers to
   MAC, creating packets transmitted over the air, optimizing the
   Transport Block utilization.  RLC flow of data packets is
   unidirectional, and it is composed of a transmitter located in the
   transmission device and a receiver located in the destination device.
   Therefore to configure bi-directional flows, two sets of entities,
   one in each direction (downlink and uplink), must be configured and
   effectively peered to each other.  The peering allows the
   transmission of control packets (ex., status reports) between
   entities.  RLC can be configured for data transfer in one of the
   following modes:

   *  Transparent Mode (TM).  RLC does not segment or concatenate SDUs
      from higher layers in this mode and does not include any header to
      the payload.  RLC receives SDUs from upper layers when acting as a
      transmitter and transmits directly to its flow RLC receiver via
      lower layers.  Similarly, a TM RLC receiver would only deliver
      without processing the packets to higher layers upon reception.

   *  Unacknowledged Mode (UM).  This mode provides support for
      segmentation and concatenation of payload.  The RLC packet's size
      depends on the indication given at a particular transmission
      opportunity by the lower layer (MAC) and is octets aligned.  The
      packet delivery to the receiver does not include reliability
      support, and the loss of a segment from a packet means a complete
      packet loss.  Also, in the case of lower layer retransmissions,
      there is no support for re-segmentation in case of change of the
      radio conditions triggering the selection of a smaller transport
      block.  Additionally, it provides PDU duplication detection and
      discards, reordering of out-of-sequence, and loss detection.

   *  Acknowledged Mode (AM).  In addition to the same functions
      supported by UM, this mode also adds a moving windows-based
      reliability service on top of the lower layer services.  It also
      supports re-segmentation, and it requires bidirectional
      communication to exchange acknowledgment reports called RLC Status
      Report and trigger retransmissions.  This model also supports
      protocol error detection.  The mode used depends on the operator
      configuration for the type of data to be transmitted.  For
      example, data transmissions supporting mobility or requiring high
      reliability would be most likely configured using AM.  Meanwhile,
      streaming and real-time data would be mapped to a UM
      configuration.





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7.1.3.  Medium Access Control (MAC)

   MAC provides a mapping between the higher layers abstraction called
   Logical Channels comprised by the previously described protocols to
   the Physical layer channels (transport channels).  Additionally, MAC
   may multiplex packets from different Logical Channels and prioritize
   what to fit into one Transport Block if there is data and space
   available to maximize data transmission efficiency.  MAC also
   provides error correction and reliability support through HARQ,
   transport format selection, and scheduling information reporting from
   the terminal to the network.  MAC also adds the necessary padding and
   piggyback control elements when possible and the higher layers data.

                                               <Max. 1600 bytes>
                       +---+         +---+           +------+
   Application         |AP1|         |AP1|           |  AP2 |
   (IP/non-IP)         |PDU|         |PDU|           |  PDU |
                       +---+         +---+           +------+
                       |   |         |   |           |      |
   PDCP           +--------+    +--------+      +-----------+
                  |PDCP|AP1|    |PDCP|AP1|      |PDCP|  AP2 |
                  |Head|PDU|    |Head|PDU|      |Head|  PDU |
                  +--------+    +--------+      +--------+--\
                  |    |   |    |     |  |      |    |   |\  `----\
            +---------------------------+      |    |(1)| `-----\(2)'-\
   RLC      |RLC |PDCP|AP1|RLC |PDCP|AP1| +-------------+    +----|---+
            |Head|Head|PDU|Head|Head|PDU| |RLC |PDCP|AP2|    |RLC |AP2|
            +-------------|-------------+ |Head|Head|PDU|    |Head|PDU|
            |         |   |         |   | +---------|---+    +--------+
            |         |   | LCID1   |   | /         /   /   /         /
           /         /   /        _/  _//        _/  _/    / LCID2   /
           |        |   |        |   | /       _/  _/     /      ___/
           |        |   |        |   ||       |   |      /      /
       +------------------------------------------+ +-----------+---+
   MAC |MAC|RLC|PDCP|AP1|RLC|PDCP|AP1|RLC|PDCP|AP2| |MAC|RLC|AP2|Pad|
       |Hea|Hea|Hea |PDU|Hea|Hea |PDU|Hea|Hea |PDU| |Hea|Hea|PDU|din|
       |der|der|der |   |der|der |   |der|der |   | |der|der|   |g  |
       +------------------------------------------+ +-----------+---+
                         TB1                               TB2

        Figure 5: Example of User Plane packet encapsulation for two
                              transport blocks









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7.2.  NB-IoT Data over NAS (DoNAS)

   The Access Stratum (AS) protocol stack used by DoNAS is somehow
   particular.  Since the security associations are not established yet
   in the radio network, to reduce the protocol overhead, PDCP (Packet
   Data Convergence Protocol) is bypassed until AS security is
   activated.  RLC (Radio Link Control protocol) uses by default the AM
   mode, but depending on the network's features and the terminal, it
   may change to other modes by the network operator.  For example, the
   transparent mode does not add any header or process the payload to
   reduce the overhead, but the MTU would be limited by the transport
   block used to transmit the data, which is a couple of thousand bits
   maximum.  If UM (only Release 15 compatible terminals) is used, the
   RLC mechanisms of reliability are disabled, and only the reliability
   provided by the MAC layer by Hybrid Automatic Repeat reQuest (HARQ)
   is available.  In this case, the protocol overhead might be smaller
   than the AM case because of the lack of status reporting but with the
   same support for segmentation up to 16000 Bytes.  NAS packets are
   encapsulated within an RRC (Radio Resource Control) TGPP36331
   message.

   Depending on the data type indication signaled (IP or non-IP data),
   the network allocates an IP address or establishes a direct
   forwarding path.  DoNAS is regulated under rate control upon previous
   agreement, meaning that a maximum number of bits per unit of time is
   agreed upon per device subscription beforehand and configured in the
   device.  The use of DoNAS is typically expected when a terminal in a
   power-saving state requires a short transmission and receiving an
   acknowledgment or short feedback from the network.  Depending on the
   size of buffered data to transmit, the UE might be instructed to
   deploy the connected mode transmissions instead, limiting and
   controlling the DoNAS transmissions to predefined thresholds and a
   good resource optimization balance for the terminal the network.  The
   support for mobility of DoNAS is present but produces additional
   overhead.
















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       +--------+   +--------+   +--------+
       |        |   |        |   |        |       +-----------------+
       |   UE   |   |  C-BS  |   |  C-SGN |       |Roaming Scenarios|
       +----|---+   +--------+   +--------+       |  +--------+     |
            |            |            |           |  |        |     |
        +----------------|------------|+          |  |  P-GW  |     |
        |        Attach                |          |  +--------+     |
        +------------------------------+          |       |         |
            |            |            |           |       |         |
     +------|------------|--------+   |           |       |         |
     |RRC Connection Establishment|   |           |       |         |
     |with NAS PDU transmission   |   |           |       |         |
     |& Ack Rsp                   |   |           |       |         |
     +----------------------------+   |           |       |         |
            |            |            |           |       |         |
            |            |Initial UE  |           |       |         |
            |            |message     |           |       |         |
            |            |----------->|           |       |         |
            |            |            |           |       |         |
            |            | +---------------------+|       |         |
            |            | |Checks Integrity     ||       |         |
            |            | |protection, decrypts ||       |         |
            |            | |data                 ||       |         |
            |            | +---------------------+|       |         |
            |            |            |       Small data packet     |
            |            |            |------------------------------->
            |            |            |       Small data packet     |
            |            |            |<-------------------------------
            |            | +----------|---------+ |       |         |
            |            | Integrity protection,| |       |         |
            |            | encrypts data        | |       |         |
            |            | +--------------------+ |       |         |
            |            |            |           |       |         |
            |            |Downlink NAS|           |       |         |
            |            |message     |           |       |         |
            |            |<-----------|           |       |         |
    +-----------------------+         |           |       |         |
    |Small Data Delivery,   |         |           |       |         |
    |RRC connection release |         |           |       |         |
    +-----------------------+         |           |       |         |
                                                  |                 |
                                                  |                 |
                                                  +-----------------+

   Figure 6: DoNAS transmission sequence from an Uplink initiated access






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                      +---+ +---+ +---+                  +----+
    Application       |AP1| |AP1| |AP2|                  |AP2 |
   (IP/non-IP)        |PDU| |PDU| |PDU|  ............... |PDU |
                      +---+ +---+ +---+                  +----+
                      |   |/   /  |    \                 |    |
   NAS /RRC      +--------+---|---+----+            +---------+
                 |NAS/|AP1|AP1|AP2|NAS/|            |NAS/|AP2 |
                 |RRC |PDU|PDU|PDU|RRC |            |RRC |PDU |
                 +--------+-|-+---+----+            +---------|
                 |          |\         |            |         |
                 |<--Max. 1600 bytes-->|__          |_        |
                 |          |  \__        \___        \_       \_
                 |          |     \           \         \__      \_
            +---------------|+-----|----------+             \      \
   RLC      |RLC | NAS/RRC  ||RLC  | NAS/RRC  |       +----|-------+
            |Head|  PDU(1/2)||Head | PDU (2/2)|       |RLC |NAS/RRC|
            +---------------++----------------+       |Head|PDU    |
            |    |          | \               |       +------------+
            |    |    LCID1 |  \              |       |           /
            |    |          |   \              \      |           |
            |    |          |    \              \     |           |
            |    |          |     \              \     \          |
       +----+----+----------++-----|----+---------++----+---------|---+
   MAC |MAC |RLC |    RLC   ||MAC  |RLC |  RLC    ||MAC |  RLC    |Pad|
       |Head|Head|  PAYLOAD ||Head |Head| PAYLOAD ||Head|  PDU    |   |
       +----+----+----------++-----+----+---------++----+---------+---+
                TB1                   TB2                     TB3

       Figure 7: Example of User Plane packet encapsulation for Data
                                  over NAS

8.  Normative References

   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,
              <https://www.rfc-editor.org/info/rfc5795>.

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
              <https://www.rfc-editor.org/info/rfc8376>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/info/rfc8724>.




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   [TGPP23720] Meredith, J., "TR 23.720 v13.0.0 - Study on architecture
               enhancements for Cellular Internet of Things", TR 23.720
               v13.0.0, 2015,
               <https://www.3gpp.org/ftp/Specs/archive/23_series/23.720
               /23720-d00.zip>.

   [TGPP33203] Soveri, M., "TR 33.203 v13.0.1 : 3G security; Access
               security fro IP-based services", TR 33.203 v13.0.1, 2020
               <https://www.3gpp.org/ftp/Specs/archive/33_series/33.203
               /33203-d10.zip>.

   [TGPP36321] Chung, Y., "TR 36.321 v13.2.0 : Evolved Universal
               Terrestrial Radio Access (E-UTRA); Medium Access Control
               (MAC) protocol specification", TR 36.321 V13.2.0, 2016,
               <https://www.3gpp.org/ftp/Specs/archive/36_series/36.321
               /36321-d20.zip>.

   [TGPP36323] Chung, Y., "TS 36.323 v13.2.0 : Evolved Universal
               Terrestrial Radio Access (E-UTRA); Packet Data Convergence
               Protocol (PDCP) specification", TS 36.323 v13.2.0, 2016,
               <https://www.3gpp.org/ftp/Specs/archive/36_series/36.323
               /36323-d20.zip>.

   [TGPP24301] Firmin. F., "TS 24.301 v13.2.0 : Non-Access-Stratum (NAS)
               protocol for Evolved Packet System (EPS); Stage 3, TS
               24.301 v13.2.0, 2015.
               <https://www.3gpp.org/ftp/Specs/archive/24_series/24.301
               /24301-d20.zip


Authors' Addresses

   Edgar Ramos
   Ericsson
   Hirsalantie 11
   FI- 02420 Jorvas, Kirkkonummi
   Finland
   Email: edgar.ramos@ericsson.com


   Ana Minaburo
   Acklio
   1137A Avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex
   France
   Email: ana@ackl.io



































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