Static Context Header Compression over Narrowband Internet of Things
draft-ietf-lpwan-schc-over-nbiot-15
| Document | Type | Active Internet-Draft (lpwan WG) | |
|---|---|---|---|
| Authors | Edgar Ramos , Ana Minaburo | ||
| Last updated | 2022-12-22 (Latest revision 2022-12-15) | ||
| Replaces | draft-minaburo-lpwan-nbiot-hc | ||
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draft-ietf-lpwan-schc-over-nbiot-15
lpwan Working Group E. Ramos
Internet-Draft Ericsson
Intended status: Standards Track A. Minaburo
Expires: 18 June 2023 Acklio
15 December 2022
Static Context Header Compression over Narrowband Internet of Things
draft-ietf-lpwan-schc-over-nbiot-15
Abstract
This document describes Static Context Header Compression and
Fragmentation (SCHC) specifications, RFC 8724 and RFC 8824,
combination with the 3rd Generation Partnership Project (3GPP) and
the Narrowband Internet of Things (NB-IoT).
This document has two parts. One normative to specify the use of
SCHC over NB-IoT. And one informational, which recommends some
values if 3GPP wanted to use SCHC inside their architectures.
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
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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 18 June 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
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. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. NB-IoT Architecture . . . . . . . . . . . . . . . . . . . . . 5
5. Data Transmission in the 3GPP Architecture . . . . . . . . . 6
5.1. Normative Part. . . . . . . . . . . . . . . . . . . . . . 7
5.1.1. SCHC over Non-IP Data Delivery (NIDD) . . . . . . . . 7
5.2. Informational Part. . . . . . . . . . . . . . . . . . . . 10
5.2.1. Use of SCHC over the Radio link . . . . . . . . . . . 11
5.2.2. Use of SCHC over the Non-Access Stratum (NAS) . . . . 12
5.2.3. Parameters for Static Context Header Compression and
Fragmentation (SCHC) for the Radio link and DONAS
use-cases. . . . . . . . . . . . . . . . . . . . . . 13
6. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Security considerations . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. NB-IoT User Plane protocol architecture . . . . . . 17
A.1. Packet Data Convergence Protocol (PDCP) TS36323 . . . . . 18
A.2. Radio Link Protocol (RLC) TS36322 . . . . . . . . . . . . 18
A.3. Medium Access Control (MAC) TR36321 . . . . . . . . . . . 19
Appendix B. NB-IoT Data over NAS (DoNAS) . . . . . . . . . . . . 20
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
This document defines the scenarios where the Static Context Header
Compression and fragmentation (SCHC) [RFC8724] and [RFC8824] are
suitable for 3rd Generation Partnership Project (3GPP) and Narrowband
Internet of Things (NB-IoT) protocol stacks.
In the 3GPP and the NB-IoT networks, header compression efficiently
brings Internet connectivity to the Device-User Equipment (Dev-UE),
the radio (RGW-eNB) and network (NGW-MME) gateways, and the
Application Server. This document describes the SCHC parameters
supporting static context header compression and fragmentation over
the NB-IoT architecture.
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This document assumes functionality for NB-IoT of 3GPP release 15
[_3GPPR15]. Otherwise, the text explicitly mentions other versions'
functionality.
This document has two parts, a standard end-to-end scenario
describing how any application must use SCHC over the 3GPP public
service. And informational scenarios about how 3GPP could use SCHC
in their protocol stack network.
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].
* Capillary Gateway. A capillary gateway facilitates seamless
integration because it has wide area connectivity through cellular
and provides wide area access as a proxy to other devices using
LAN technologies (BT, Wi-Fi, Zigbee, or others.)
* CIoT EPS. Cellular IoT Evolved Packet System. It is a
functionality to improve the support of small data transfers.
* Dev-UE. Device - User Equipment.
* DoNAS. Data over Non-Access Stratum.
* EPC. Evolved Packet Connectivity. Core network of 3GPP LTE
systems.
* EUTRAN. Evolved Universal Terrestrial Radio Access Network.
Radio access network of LTE-based systems.
* HARQ. Hybrid Automatic Repeat Request.
* HSS. Home Subscriber Server. It is a database that contains
users' subscription data, including data needed for mobility
management.
* IP address. IPv6 or IPv4 address used.
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* 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.
* L2. Layer-2 in the 3GPP architectures it includes MAC, RLC and
PDCP layers see Appendix A.
* LCID. Logical Channel ID. Is the logical channel instance of the
corresponding MAC SDU.
* MAC. Medium Access Control protocol, part of L2.
* NAS. Non-Access Stratum.
* 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-CSGN. Network Gateway - CIoT Serving Gateway Node.
* NGW-CSGW. Network Gateway - Cellular Serving Gateway. It routes
and forwards the user data packets through the access network.
* NGW-MME. Network Gateway - Mobility Management Entity. An entity
in charge of handling mobility of the Dev-UE.
* NGW-PGW. Network Gateway - Packet Data Network Gateway. An
interface between the internal with the external network.
* NGW-SCEF. Network Gateway - Service Capability Exposure Function.
EPC node for exposure of 3GPP network service capabilities to 3rd
party applications.
* NIDD. Non-IP Data Delivery.
* PDCP. Packet Data Convergence Protocol part of L2.
* 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.
* RLC. Radio Link Protocol part of L2.
* RGW-eNB. Radio Gateway - evolved Node B. Base Station that
controls the UE.
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* 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. NB-IoT 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 bandwidth, acknowledgments, and layer-2
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 Network
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. The Open Mobile Alliance (OMA)
[OMA0116] and the One Machine to Machine (OneM2M) [TR-0024] define
the northbound APIs. [TS23222] defines architecture for the common
API framework for 3GPP northbound APIs and [TS33122] defines security
aspects for common API framework for 3GPP northbound APIs. 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| | +------+| +-----+ +-----------+
+---+ | |
|NGW-CSGN |
+---------+
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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, see Section 5.2.3. However, the
recommended 3GPP 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.
This document identifies the use cases of SCHC over the NB-IoT
architecture.
First, the radio transmission where, see Section 5.2.1, the Dev-UE
and the RGW-eNB can use the SCHC functionalities.
Second, the packets transmitted over the control path can also use
SCHC when the transmission goes over the NGW-MME or NGW-SCEF. See
Section 5.2.2.
These two use cases are also valid for any 3GPP architecture and not
only for NB-IoT. And as the 3GPP internal network is involved, they
have been put in the informational part of this section.
And third, over the SCHC over Non-IP Data Delivery (NIDD) connection
or at least up to the operator network edge, see Section 5.1.1. In
this case, 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. NGW-PGW or NGW-SCEF transmit
the packets which are non-IP traffic, using IP tunneling or API
calls. It is also possible to benefit legacy devices with SCHC by
using the non-IP transmission features of the operator network.
A non-IP transmission refers to other layer-2 transport different
from NB-IoT.
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5.1. Normative Part.
This scenarios does not modify the 3GPP architecture or any of its
components, it only use it as a layer-2 transmission.
5.1.1. SCHC over Non-IP Data Delivery (NIDD)
This section specifies the use of SCHC over Non-IP Data Delivery
(NIDD) services of 3GPP. The NIDD services of 3GPP enable the
transmission of SCHC packets compressed by the application layer.
The packets can be delivered between the NGW-PGW and the Application
Server or between the NGW-SCEF and the Application Server, using IP-
tunnels or 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 over the layer-2.
5.1.1.1. SCHC Entities Placing over NIDD
In the two scenarios using NIDD compression, SCHC entities are
located almost on top of the stack. The NB-IoT connectivity services
implement SCHC in the Dev-UE, 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 |
+---------+ +--------+
Dev-UE Application
Server
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Figure 2: End-to End Compression. SCHC entities placed when
using Non-IP Delivery (NIDD) 3GPP Services
5.1.1.2. Parameters for Static Context Header Compression and
Fragmentation (SCHC)
These scenarios MAY use SCHC header compression capability to improve
the transmission of IPv6 packets.
* 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.
For devices acting as a capillary gateway, several rules match the
diversity of devices and protocols used by the devices associated
with the gateway. Meanwhile, simpler devices may have predetermined
protocols and fixed parameters.
* Rule ID.
This scenario 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. 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. Thus, to use the smallest TB, the maximum SCHC header
size is 12 bits. 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. The configuration may be part of the agreed operation
profile and 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 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.
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* 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 e.g. 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
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 activated for all packets transmitted by the applications.
SCHC ACK-on-Error fragmentation MAY be activated 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 5.2.3 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.
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* For Transfer Blocks smaller or equal to 304 bits using an 8-bit
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-bit
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.
* 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 use the
Power Save Mode and the Idle Mode DRX, which govern how often the
device wakes up, stays up, and is reachable. The use of the
different modes allows the battery to last ten years.
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.
5.2. Informational Part.
These scenarios shows how 3GPP could use SCHC for their
transmissions.
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5.2.1. 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 3 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 Appendix A gives
more details about these protocols.
+---------+ +---------+ |
|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 3: SCHC over the Radio link
5.2.1.1. SCHC Entities Placing over the Radio Link
The 3GPP architecture supports Robust Header Compression (ROHC)
[RFC5795] in the PDCP layer. Therefore, the architecture can deploy
SCHC header compression entities similarly without the need for
significant changes in the 3GPP specifications.
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The RLC layer has three functional modes Transparent Mode (TM),
Unacknowledged Mode (UM), and Acknowledged Mode (AM). The mode of
operation controls the functionalities of the RLC layer. TM only
applies to signaling packets, while AM or UM carry signaling and data
packets.
The RLC layer takes care of fragmentation unless for the Transparent
Mode. In AM or UM modes, the SCHC fragmentation is unnecessary and
SHOULD NOT be used. 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 for TM transmission mode in the future.
5.2.2. Use of SCHC over the Non-Access Stratum (NAS)
This section consists of IETF suggestions to the 3GPP. 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 Non-Access Stratum (DoNAS) or
Control Plane Cellular Internet of Things (CIoT) evolved packet
system (EPS) optimizations. 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.
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 B gives
additional details of DoNAS.
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5.2.2.1. 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 5.2.1 apply
here as well. Because the NAS protocol already uses ROHC [RFC5795],
it can also adapt SCHC for header compression. The main difference
compared to the radio link, section Section 5.2.1, 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 SGi
Dev-UE RGW-eNB NGW-MME NGW-PGW
*PDCP is bypassed until AS security is activated TGPP36300.
Figure 4: SCHC entities placement in the 3GPP CIOT radio protocol
architecture for DoNAS transmissions
5.2.3. Parameters for Static Context Header Compression and
Fragmentation (SCHC) for the Radio link and DONAS use-cases.
If 3GPP incorporates SCHC, it is recommended that these scenarios use
SCHC header compression [RFC8724] capability to optimize the data
transmission.
* SCHC Context initialization.
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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 ROHC [RFC5795]
operation [TS36323].
* SCHC Rules.
The network operator in these scenarios defines the number of rules.
For this, the network operator must know the IP traffic the device
will carry. The operator might supply rules compatible with the
device's use case. For devices acting as a capillary gateway,
several rules match the diversity of devices and protocols used by
the devices associated with the gateway. Meanwhile, simpler devices
may have predetermined protocols and fixed parameters. The use of
IPv6 and IPv4 may force to get more 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. As for Section 5.1.1.2, these scenarios
must optimize the physical layer where the smallest TB 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 flexibility on the number and
type of rules independently supported by each device; consequently,
these scenarios require a configurable value. The configuration may
be part of the agreed operation profile 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 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.
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* Fragmentation.
For the Radio link Section 5.2.1 and DoNAS' Section 5.2.2 scenarios,
the SCHC fragmentation functions are disabled. The RLC layer of NB-
IoT can segment packets into 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 intrinsically.
* Fragmentation in RLC Transparent Mode.
The RLC Transparent Mode mostly applies to control signaling
transmissions. When RLC operates in Transparent Mode, the MAC layer
mechanisms ensure reliability and generate overhead. This additional
reliability implies sending repetitions or automatic retransmissions.
The ACK-Always fragmentation mode of SCHC may reduce this overhead in
future operations when data transmissions may use this mode. ACK-
Always mode may 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 5.1.1.2.
6. Padding
NB-IoT and 3GPP wireless access, in general, assumes byte-aligned
payload. Therefore, the layer 2 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
[TS33122].
9. References
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9.1. 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>.
[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>.
[RFC8824] Minaburo, A., Toutain, L., and R. Andreasen, "Static
Context Header Compression (SCHC) for the Constrained
Application Protocol (CoAP)", RFC 8824,
DOI 10.17487/RFC8824, June 2021,
<https://www.rfc-editor.org/info/rfc8824>.
9.2. Informative References
[OMA0116] 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>.
[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>.
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[TR24301] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Medium Access Control (MAC) protocol
specification", 2019, <https://www.3gpp.org/ftp//Specs/
archive/24_series/24.301/24301-f80.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>.
[TS23222] 3GPP, "Common API Framework for 3GPP Northbound APIs",
2022, <https://www.3gpp.org/ftp/Specs/
archive/23_series/23.222/23222-f60.zip>.
[TS24008] 3GPP, "Mobile radio interface layer 3 specification.",
2018, <https://www.3gpp.org/ftp//Specs/
archive/24_series/24.008/24008-f50.zip>.
[TS33122] 3GPP, "Security aspects of Common API Framework (CAPIF)
for 3GPP northbound APIs", 2018,
<https://www.3gpp.org/ftp//Specs/
archive/33_series/33.122/33122-f30.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>.
[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>.
[TS36331] 3GPP, "Evolved Universal Terrestrial Radio Access
(E-UTRA); Radio Resource Control (RRC); Protocol
specification", 2018, <https://www.3gpp.org/ftp//Specs/
archive/36_series/36.331/36331-f51.zip>.
[_3GPPR15] 3GPP, "The Mobile Broadband Standard", 2019,
<https://www.3gpp.org/release-15>.
Appendix A. NB-IoT User Plane protocol architecture
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A.1. Packet Data Convergence Protocol (PDCP) [TS36323]
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 [RFC5795]
* 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
A.2. Radio Link Protocol (RLC) [TS36322]
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
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opportunity by the lower layer (MAC) and is octet-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.
A.3. Medium Access Control (MAC) [TR36321]
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 Hybrid
Automatic Repeat reQuest (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
Appendix B. NB-IoT Data over NAS (DoNAS)
The Access Stratum (AS) protocol stack used by DoNAS is specific
because the radio network still needs to establish the security
associations and reduce the protocol overhead, so the 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 HARQ is available. In this case, the
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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 1600 bytes. NAS packets are encapsulated within an RRC (Radio
Resource Control) [TS36331] 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
Appendix C. Acknowledgements
The authors would like to thank (in alphabetic order): Carles Gomez,
Antti Ratilainen, Tuomas Tirronen, Pascal Thubert, Eric Vyncke.
Authors' Addresses
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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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