SCHC over NBIoT
draft-ietf-lpwan-schc-over-nbiot-10
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9391.
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|
|---|---|---|---|
| Authors | Edgar Ramos , Ana Minaburo | ||
| Last updated | 2022-08-29 (Latest revision 2022-07-11) | ||
| Replaces | draft-minaburo-lpwan-nbiot-hc | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Stream | WG state | Submitted to IESG for Publication | |
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| Shepherd write-up | Show Last changed 2022-07-12 | ||
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draft-ietf-lpwan-schc-over-nbiot-10
lpwan Working Group E. Ramos
Internet-Draft Ericsson
Intended status: Standards Track A. Minaburo
Expires: 10 January 2023 Acklio
9 July 2022
SCHC over NBIoT
draft-ietf-lpwan-schc-over-nbiot-10
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 recommendations for the use cases and 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 10 January 2023.
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
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and restrictions with respect to this document. Code Components
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. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Data Transmission in the 3GPP Architecture . . . . . . . . . 5
5.1. Use of SCHC over the Radio link . . . . . . . . . . . . . 6
5.1.1. SCHC Entities Placing over the Radio Link . . . . . . 6
5.2. Use of SCHC over the No-Access Stratum (NAS) . . . . . . 7
5.2.1. SCHC Entities Placing over DoNAS . . . . . . . . . . 8
5.3. Parameters for Static Context Header Compression and
Fragmentation (SCHC) for the Radio and DONAS use-cases. . 9
5.4. End-to-End Compression . . . . . . . . . . . . . . . . . 11
5.4.1. SCHC Entities Placing over End-to-End Compression . . 11
5.4.2. Parameters for Static Context Header Compression and
Fragmentation (SCHC) . . . . . . . . . . . . . . . . 11
6. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 14
8. Security considerations . . . . . . . . . . . . . . . . . . . 14
9. Appendix NB-IoT User Plane protocol architecture . . . . . . 14
9.1. Packet Data Convergence Protocol (PDCP) . . . . . . . . . 14
9.2. Radio Link Protocol (RLC) . . . . . . . . . . . . . . . . 15
9.3. Medium Access Control (MAC) . . . . . . . . . . . . . . . 16
10. Appendix NB-IoT Data over NAS (DoNAS) . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. Normative References . . . . . . . . . . . . . . . . . . 20
11.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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 that 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. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
This document will follow the terms defined in [RFC8724], in
[RFC8376], and the [TR23720].
* CIoT. Cellular IoT.
* Dev-UE. Device - User Equipment.
* EPC. Evolved Packet Connectivity. Core network of 3GPP LTE
systems.
* EUTRAN. Evolved Universal Terrestrial Radio Access Network.
Radio access network of LTE-based systems.
* HSS. Home Subscriber Server. It is a database that performs
mobility management.
* 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.
* LCID. Logical Channel ID. Is the logical channel instance of the
corresponding MAC SDU.
* NGW-CSGN. Network Gateway - CIoT Serving Gateway Node.
* NGW-CSGW. Network Gateway - Serving Gateway. It routes and
forwards the user data packets through the access network.
* 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-MME. Network Gateway - Mobility Management Entity. An entity
in charge of handling mobility of the Dev-UE.
* NGW-PGW. Network Gateway - Packet Data Node Gateway. An
interface between the internal with the external network.
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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.
* PLMN. Public Land Mobile Network. Combination of wireless
communication services offered by a specific operator.
* PDU. Protocol Data Unit. A data packet including headers that
are transmitted between entities through a protocol.
* RGW-eNB. Radio Gateway - Node B. Base Station that controls the
UE.
* SDU. Service Data Unit. A data packet (PDU) from higher layer
protocols used by lower layer protocols as a payload of their own
PDUs.
4. Architecture
The Narrowband Internet of Things (NB-IoT) architecture has a complex
structure. It relies on different NGWs from different providers. It
can send data via different paths, each 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 a 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 [TS23222] and OneM2M [TR-0024]
define the northbound APIS [TR33203]. In this case, the path is
small for data transmission. The main functions of the NGW-SCEF are
Connectivity path and Device Monitoring.
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+---+ +------+
|Dev| \ +-----+ ----| HSS |
|-UE| \ | NGW | +------+
+---+ | |-MME |\__
\ / +-----+ \
+---+ \+-----+ / | +------+
|Dev| ----| RGW |- | | NGW- |
|-UE| |-eNB | | | SCEF |---------+
+---+ /+-----+ \ | +------+ |
/ \ +------+ |
/ \| NGW- | +-----+ +-----------+
+---+ / | CSGW |--| NGW-|---|Application|
|Dev| | | | PGW | | Server |
|-UE| +------+ +-----+ +-----------+
+---+
Figure 1: 3GPP network architecture
5. Data Transmission in the 3GPP Architecture
NB-IoT networks deal with end-to-end user data and in-band signaling
between the nodes and functions to configure, control, and monitor
the system functions and behaviors. The signaling 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 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 the standard specifying SCHC
support would not be backward compatible. A terminal or a network
supporting a version of the standard without SCHC or 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. Over the radio
transmission, see section Section 5.1. The Dev-UE and the RGW-eNB
may use the SCHC functionalities. Alternatively, the packets
transmitted over the path can use SCHC. When the transmissions go
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over the NGW-MME or NGW-SCEF, the NGW-CSGN uses the SCHC entity. See
sections Section 5.2 and Section 5.4. 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 tunneling 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.
5.1. Use of SCHC over the Radio link
Deploying SCHC over the radio link only 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
selected Modulation and Coding Schemes (MCS). Transport Block (TB)
transmissions 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 [TR36321] defines the Transport Blocks'
characteristics. The Radio link stack shown in Figure 2 comprises
the Packet Data Convergence Protocol (PDCP) [TS36323], Radio Link
Protocol (RLC) [TS36322], Medium Access Control protocol (MAC)
[TR36321], and the Physical Layer [TS36201]. The Section 9 gives
more details about these protocols.
5.1.1. SCHC Entities Placing over the Radio Link
The current architecture supports header compression in the PDCP
layer using [RFC5795]. Therefore the architecture can deploy SCHC
header compression entities similarly without the need for
significant changes in the 3GPP specifications.
The RLC layer has three modes of operation. 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
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additional protocol overhead. While sending IP packets, the Radio
link does not commonly use the RLC Transparent Mode. However, if
other protocol overhead optimizations are targeted for NB-IoT
traffic, SCHC fragmentation may be used in the future for this
transmission mode.
+---------+ +---------+ |
|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
5.2. Use of SCHC over the No-Access Stratum (NAS)
The NGW-MME conveys mainly signaling between the Dev-UE and the
cellular network [TR24301]. 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 Section 10 gives
additional details of DoNAS.
5.2.1. SCHC Entities Placing over DoNAS
SCHC resides in this scenario's Non-Access Stratum (NAS) protocol
layer. The same principles as for the section Section 5.1 apply here
as well. Because the NAS protocol already uses [RFC5795], it can
also adapt SCHC for header compression. The main difference compared
to the radio link, section Section 5.1, 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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5.3. Parameters for Static Context Header Compression and Fragmentation
(SCHC) for the Radio and DONAS use-cases.
These scenarios MUST use SCHC header compression capability to
optimize the data transmission.
* SCHC Context initialization.
The 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 [RFC5795]
operation [TS36323].
* SCHC Rules.
The network operator in these scenarios defines the number of rules
in a context. The network operator MUST be aware of the IP traffic
the device will carry. Implying that the network operator MIGHT use
provision sets of rules compatible with the device's use case. For
devices acting as gateways to 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 rules and
parameters. Additionally, deploying 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 to optimize the transmission. The packets
handled by 3GPP networks are byte-aligned, and therefore the smallest
payload possible (including padding) is 8 bits. Thus 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 capillary
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
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with the content distribution. The RuleID field size may range from
2 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.
An application may use a larger RuleID, 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 5.1 and Section 5.2 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 block 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 to 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 protocol 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 5.4.2.
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5.4. 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.
5.4.1. SCHC Entities Placing over End-to-End Compression
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
5.4.2. Parameters for Static Context Header Compression and
Fragmentation (SCHC)
These scenarios may use SCHC header compression capability to improve
the transmission of IPv6 packets.
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* 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 5.1 and Section 5.2
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 5.1 and Section 5.2, these scenarios
can dynamically set the RuleID size before the context delivery. For
example, negotiate 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 5.1 and Section 5.2 must be followed when
choosing a size value for the RuleID field.
* SCHC MAX_PACKET_SIZE.
In these scenarios, the maximum RECOMMENDED MTU size 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 arriving in
the 3GPP radio network transmission, for example, in between the
links of a capillary 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
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packets transmitted with a small trade-off on further transmissions
to signal the end-to-end arrival of the packets if no transport
protocol takes care of retransmission.
Also, the ACK-on-Error mode could be 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 5.3 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 2 bits. The transfer block size has a wide
range of values. Two configurations are RECOMMENDED for the
fragmentation parameters.
* For Transfer Blocks smaller or equal to 304bits using an 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 304 bits using a 16 bits-
Header_size configuration, with the size of the header fields as
follows:
- RulesID from 8 - 10 bits,
- DTag 1 or 2 bits,
- FCN 3 bits,
- W 2 or 3 bits.
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- W 2 or 3 bits.
* WINDOW_SIZE of 2^N-1 is RECOMMENDED.
* RCS will follow the default size defined in section 8.2.3 of the
[RFC8724], with a length equal to the L2 Word.
* MAX_ACK_REQ is RECOMMENDED to be 2, but applications may change
this value based on transmission conditions.
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 [TS24008] 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. The Inactivity Timer and the
Retransmission Timer be set based on these limits.
6. 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.
7. IANA considerations
This document has no IANA actions.
8. Security considerations
This document does not add any security considerations and follows
the [RFC8724] and the 3GPP access security document specified in
[TR33203].
9. Appendix NB-IoT User Plane protocol architecture
9.1. 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 [RFC5795]
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* 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
9.2. 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
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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.
9.3. 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.
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<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
(1) Segment One
(2) Segment Two
Figure 5: Example of User Plane packet encapsulation for two
transport blocks
10. Appendix 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)
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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
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[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>.
[TS36323] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Packet Data Convergence Protocol (PDCP)
specification", 2016, <https://www.3gpp.org/ftp/Specs/
archive/36_series/36.323/36323-d20.zip>.
11.2. Informative 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>.
[TR-0024] OneM2M, "3GPP_Interworking", 2020,
<https://ftp.onem2m.org/work%20programme/WI-0037/TR-0024-
3GPP_Interworking-V4_3_0.DOCX>.
[TR23720] 3GPP, "Study on architecture enhancements for Cellular
Internet of Things", 2015,
<https://www.3gpp.org/ftp/Specs/
archive/23_series/23.720/23720-d00.zip>.
[TR24301] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Medium Access Control (MAC) protocol
specification", 2016, <https://www.3gpp.org/ftp/Specs/
archive/36_series/36.321/36321-d20.zip>.
[TR33203] 3GPP, "3G security; Access security for IP-based
services", 2015, <https://www.3gpp.org/ftp/Specs/
archive/33_series/33.203/33203-d10.zip>.
[TR36321] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Medium Access Control (MAC) protocol
specification", 2016, <https://www.3gpp.org/ftp/Specs/
archive/36_series/36.321/36321-d20.zip>.
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[TS23222] OMA, "Common definitions for RESTful Network APIs", 2018,
<https://www.openmobilealliance.org/release/
REST_NetAPI_Common/V1_0-20180116-A/OMA-TS-
REST_NetAPI_Common-V1_0-20180116-A.pdf>.
[TS24008] 3GPP, "Mobile radio interface layer 3 specification.",
2018, <https://www.3gpp.org/ftp//Specs/
archive/24_series/24.008/24008-f50.zip>.
[TS36201] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); LTE physical layer; General description", 2018,
<https://www.3gpp.org/ftp/Specs/
archive/36_series/36.201/36201-f10.zip>.
[TS36322] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Radio Link Control (RLC) protocol
specification", 2018, <https://www.3gpp.org/ftp/Specs/
archive/36_series/36.322/36322-f01.zip>.
[_3GPPR15] 3GPP, "The Mobile Broadband Standard", 2019,
<https://www.3gpp.org/release-15>.
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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