Internet DRAFT - draft-minaburo-lpwan-nbiot-hc
draft-minaburo-lpwan-nbiot-hc
lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Informational E. Ramos
Expires: September 8, 2019 Ericsson
S. Shanmugalingam
Acklio
March 07, 2019
LPWAN Static Context Header Compression (SCHC) over NB-IoT
draft-minaburo-lpwan-nbiot-hc-02
Abstract
The Static Context Header Compression (SCHC) specification describes
a header compression and fragmentation functionalities for LPWAN (Low
Power Wide Area Networks) technologies. SCHC was designed to be
adapted over any of the LPWAN technologies.
This document describes the use of SCHC over the NB-IoT wireless
access, and provides elements for an efficient parameterization.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Data Transmission . . . . . . . . . . . . . . . . . . . . . . 6
5. IP based Data Transmission . . . . . . . . . . . . . . . . . 7
5.1. SCHC over User Plane transmissions . . . . . . . . . . . 8
5.1.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 8
5.2. Data Over Control Plane . . . . . . . . . . . . . . . . . 9
5.2.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 10
5.3. Parameters for Static Context Header Compression (SCHC) . 10
5.3.1. SCHC Context initialization . . . . . . . . . . . . . 11
5.3.2. SCHC Rules . . . . . . . . . . . . . . . . . . . . . 11
5.3.3. Rule ID . . . . . . . . . . . . . . . . . . . . . . . 11
5.3.4. SCHC MAX_PACKET_SIZE . . . . . . . . . . . . . . . . 12
5.3.5. Fragmentation . . . . . . . . . . . . . . . . . . . . 12
6. Non-IP based Data Transmission . . . . . . . . . . . . . . . 13
6.1. SCHC Entities Placing . . . . . . . . . . . . . . . . . . 13
6.2. Parameters for Static Context Header Compression . . . . 14
6.2.1. SCHC Context initialization . . . . . . . . . . . . . 14
6.2.2. SCHC Rules . . . . . . . . . . . . . . . . . . . . . 14
6.2.3. Rule ID . . . . . . . . . . . . . . . . . . . . . . . 14
6.2.4. SCHC MAX_PACKET_SIZE . . . . . . . . . . . . . . . . 14
6.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 14
6.3.1. Fragmentation modes . . . . . . . . . . . . . . . . . 15
6.3.2. Fragmentation Parameters(TBD) . . . . . . . . . . . . 15
7. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Security considerations . . . . . . . . . . . . . . . . . . . 16
9. 3GPP References . . . . . . . . . . . . . . . . . . . . . . . 16
10. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. NB-IoT User Plane protocol architecture . . . . . . . . 16
10.1.1. Packet Data Convergence Protocol (PDCP) . . . . . . 16
10.1.2. Radio Link Protocol (RLC) . . . . . . . . . . . . . 17
10.1.3. Medium Access Control (MAC) . . . . . . . . . . . . 18
10.2. NB-IoT Data over NAS (DoNAS) . . . . . . . . . . . . . . 19
11. Informative References . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
The Static Context Header Compression (SCHC)
[I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
scheme and fragmentation functionality, both specially tailored for
Low Power Wide Area Networks (LPWAN) networks defined in [RFC8376].
Header compression is needed to efficiently bring Internet
connectivity to the node within an NB-IoT network. SCHC uses a
static context to performs header compression with specific
parameters that need to be adapted into the NB-IoT wireless access.
This document assumes functionality for NB-IoT of 3GPP release 15
otherwise other versions functionality is explicitly mentioned in the
text.
This document describes the use of SCHC and its parameterizing over
the NB-IoT wireless access.
2. Terminology
This document will follow the terms defined in
[I-D.ietf-lpwan-ipv6-static-context-hc], in [RFC8376], and the
[TGPP23720].
o CIoT. Cellular IoT
o C-SGN. CIoT Serving Gateway Node
o UE. User Equipment
o eNB. Node B. Base Station that controls the UE
o EPC. Evolved Packet Connectivity. Core network of 3GPP LTE
systems.
o EUTRAN. Evolved Universal Terrestrial Radio Access Network.
Radio network from LTE based systems.
o MME. Mobility Management Entity. Handle mobility of the UE
o NB-IoT. Narrow Band IoT. Referring to 3GPP LPWAN technology
based in LTE architecture but with additional optimization for IoT
and using a Narrow Band spectrum frequency.
o SGW. Serving Gateway. Routes and forwards the user data packets
through the access network
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o HSS. Home Subscriber Server. It is a database that performs
mobility management
o PGW. Packet Data Node Gateway. An interface between the internal
with the external network
o PDU. Protocol Data Unit. Data packets including headers that are
transmitted between entities through a protocol.
o SDU. Service Data Unit. Data packets (PDUs) from higher layers
protocols used by lower layer protocols as a payload of their own
PDUs that has not yet been encapsulated.
o IWK-SCEF. InterWorking Service Capabilities Exposure Function.
Used in roaming scenarios and serves for interconnection with the
SCEF of the Home PLMN and is located in the Visited PLMN
o SCEF. Service Capability Exposure Function. EPC node for
exposure of 3GPP network service capabilities to 3rd party
applications.
3. Architecture
+--+
|UE| \ +-----+ +------+
+--+ \ | MME |-----| HSS |
\ / +-----+ +------+
+--+ \+-----+ / |
|UE| ----| eNB |- |
+--+ /+-----+ \ |
/ \ +------+
/ \| | +------+ Service PDN
+--+ / | S-GW |--| P-GW |-- e.g. Internet
|UE| | | +------+
+--+ +------+
Figure 1: 3GPP network architecture
The architecture for 3GPP LTE network has been reused for NB-IoT with
some optimizations and simplifications known as Cellular IoT (CIoT).
Considering the typical use cases for CIoT devices here are described
some of the additions to the LTE architecture specific for CIoT.
C-SGN(CIoT Serving Gateway Node) is a deployment option co-locating
EPS entities in the control plane and user plane paths (for example,
MME + SGW + P-GW) and the external interfaces of the entities
supported. The C-SGN also supports at least some of the following
CIoT EPS Optimizations:
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o Control Plane CIoT EPS Optimization for small data transmission.
o User Plane CIoT EPS Optimization for small data transmission.
o Necessary security procedures for efficient small data
transmission.
o SMS without combined attach for NB-IoT only UEs.
o Paging optimizations for coverage enhancements.
o Support for non-IP data transmission via SGi tunneling and/or
SCEF.
o Support for Attach without PDN (Packet Data Network) connectivity.
Another node introduced in the CIOT architecture is the SCEF (Service
Capability Exposure Function) that provide means to securely expose
service and network capabilities to entities external to the network
operator. The northbound APIS are defined by OMA and OneM2M. The
main functions of a SCEF are:
o Non-IP Data Delivery (NIDD) established through the SCEF.
o Monitoring and exposure of event related to UE reachability, loss
of connectivity, location reporting, roaming status, communication
failure and change of IMEI-IMSI association.
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+-------+
| HSS |
+-+-----+
/
+---------+ __/S6a
+--------+ | +-----+ +_/
+----+ C-Uu | +---+-+ MME | | T6i+--------+ T7 +----+
|CIOT+--------+ eNB |S1 | | +-+----+IWK-SCEF+----+SCEF|
|UE | |(NB-IoT)| | +---+-+ | +--------+ +----+
+----+ +--------+ | | |
|C-SGN| |
| |S11|
+------+ | | |
+--------+LTE-Uu| | | +--+-+ |
|LTE eMTC|(eMTC)|eNB +---+--+SGW | | S8+---+ +-----------+
| UE +------+(eMTC)|S1 | | +-+---+PGW|SGi |Application|
+--------+ +------+ | +----+ | | +----+Server (AS)|
+---------+ +---+ +-----------+
Figure 2: 3GPP optimized CIOT network architecture
4. Data Transmission
3GPP networks deal not only with data transmitted end-to-end but also
with in-band signaling that is used between the nodes and functions
to configure, control and monitor the system functions and behaviors.
The control data is handled using a Control Plane which has a
specific set of protocols, handling processes and entities. In
contrast, the end-to-end or user data utilize a User Plane with
characteristics of its own separated from the Control Plane. The
handling and setup of the Control Plane and User Plane spans over the
whole 3GPP network and it has particular implications in the radio
network (i.e., EUTRAN) and in the packet core (ex., EPC).
For the CIOT cases, additionally to transmissions of data over User
Plane, 3GPP has specified optimizations for small data transmissions,
allowing to transport user data (IP, Non-IP) within signaling on the
access network (Data transmission over Control Plane or Data Over
NAS).
The maximum recommended MTU size is 1358 Bytes. The radio network
protocols limit the packet sizes to be transmitted over the air
including radio protocol overhead to 1600 Octets. But the value is
reduced further to avoid fragmentation in the backbone of the network
due to the payload encryption size (multiple of 16) and handling of
the additional core transport overhead.
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NB-IoT and in general the cellular technologies interfaces and
functions are standardized by 3GPP. Therefore the introduction of
SCHC entities to UE, eNB and C-SGN does need to be specified in the
NB-IoT standard. This implies that standard specified SCHC support
would not be backwards compatible. A terminal or a network
supporting a version of the standard without support of SCHC or
without capability implementation (in case of not being standardized
as mandatory capability) is not able to utilize the compression
services with this approach.
SCHC could be deployed differently depending on where the header
compression and the fragmentation are applied. The SCHC
functionalities could be applied to the packets about to be
transmitted over the air, or to the whole end-to-end link. To
accomplish the first, it is required to place SCHC compression and
decompression entities in the eNB and in the UE for transmissions
over the User Plane. Additionally, to handle the case of the
transmissions over Control Plane or Data Over NAS, the network SCHC
entity has to be placed in the C-SGN as well. For these two cases,
the functions are 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. In that
case, the SCHC entities would be placed in the application layer of
the terminal in one end, and either in the application servers or in
a broker function in the edge of the operator network in the other
end. For the radio network, the packets are transmitted as non-IP
traffic, which can be currently served utilizing 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 utilizing the non-IP transmission features of the operator
network.
Accordingly, there are four different scenarios where SCHC can be
used in the NB-IoT architecture. IP header compression on the data
transmission over User Plane, IP header compression on the optimized
transmissions over Control Plane (i.e.,DoNAS), non-IP transmissions
of SCHC packets by IP tunneling, and non-IP transmissions of SCHC
packets by SCEF forwarding. The following sections describe each of
them in more detail. The first two scenarios refer to transmissions
using the 3GPP IP transmission capabilities and the last two refers
to transmission using the Non-IP capabilities.
5. IP based Data Transmission
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5.1. SCHC over User Plane transmissions
Deploying SCHC only over the radio link would require to place it as
part of the User Plane data transmission. The User Plane utilizes
the protocol stack of the Access Stratum (AS) for data transfer. AS
(Access Stratum) is the functional layer responsible for transporting
data over wireless connection and managing radio resources. The user
plane AS has support for features such as reliability, segmentation
and concatenation. The transmissions of the AS make use of link
adaptation, meaning that the transport format utilized for the
transmissions are optimized according to the radio conditions, the
number of bits to transmit and the power and interference constrains.
That means that the number of bits transmitted over the air depends
of the Modulation and Coding Schemes (MCS) selected. The
transmissions in the physical layer happens at network synchronized
intervals of times called TTI (Transmission Time Interval). The
transmission of a Transport Block (TB) is completed during, at least,
one TTI. Each Transport Block has a different MCS and number of bits
available to transmit. The Transport Blocks characteristics are
defined by the MAC technical specification [TGPP36321]. The Access
Stratum for User Plane is comprised by Packet Data Convergence
Protocol (PDCP) [TGPP36323], Radio Link Protocol (RLC)[TGPP36322],
Medium Access Control protocol (MAC)[TGPP36321] and the Physical
Layer [TGPP36201]. More details of this protocols are given in the
Appendix.
5.1.1. SCHC Entities Placing
The current architecture provides support for header compression in
PDCP utilizing RoHC [RFC5795]. Therefore SCHC entities can be
deployed in similar fashion without need for major changes in the
3GPP specifications.
In this scenario, RLC takes care of the handling of fragmentation (if
transparent mode is not configured) when packets exceeds the
transport block size at the time of transmission. Therefore SCHC
fragmentation is not needed and should not be used to avoid
additional protocol overhead. It is not common to configure RLC in
Transparent Mode for IP based user plane data. But 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
CIOT/ LTE+Uu C-BS/eNB C-SGN
LTE eMTC
UE
Figure 3: SCHC entities placement in the 3GPP CIOT radio protocol
architecture for data over user plane
5.2. Data Over Control Plane
The Non-Access Stratum (NAS), conveys mainly control signaling
between the UE and the cellular network [TGPP24301]. NAS is
transported on top of the Access Stratum (AS) already mentioned in
the previous section.
NAS has been adapted to provide support for user plane data
transmissions to reduce the overhead when transmitting infrequent
small quantities of data. This is known as Data over NAS (DoNAS) or
Control Plane CIoT EPS optimization. In DoNAS the UE makes use of
the pre-established NAS security and piggyback uplink small data into
the initial NAS uplink message, and uses an additional NAS message to
receive downlink small data response.
The data encryption from the network side is performed by the C-SGN
in a NAS PDU. Depending on the data type signaled indication (IP or
non-IP data), the network allocates an IP address or just establish a
direct forwarding path. DoNAS (Data over NAS) is regulated under
rate control upon previous agreement, meaning that a maximum number
of bits per unit of time is agreed 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 to do a short transmission and receive an
acknowledgment or short feedback from the network. Depending on the
size of buffered data to transmit, the UE might be instructed to
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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. Additional details of DoNAS are given in the Appendix.
5.2.1. SCHC Entities Placing
In this scenario SCHC can be applied in the NAS protocol layer
instead of PDCP. The same principles than for user plane
transmissions applies here as well. The main difference is the
physical placing of the SCHC entities in the network side as the
C-SGN (placed in the core network) is the terminating node for NAS
instead of the 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
CIOT/ LTE-Uu C-BS/eNB C-SGN PGW
LTE eMTC
UE
*PDCP is bypassed until AS security is activated [TGPP36300].
Figure 4
5.3. Parameters for Static Context Header Compression (SCHC)
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5.3.1. SCHC Context initialization
RRC (Radio Resource Control) protocol is the main tool used to
configure the operation parameters of the AS transmissions for 3GPP
technologies. RoHC operation is configured with this protocol and it
is to expect that SCHC will be configured and the static context
distributed in similar fashion for these scenarios.
5.3.2. SCHC Rules
The number of rules in a context are defined by the network operator
in these scenarios. For this, the operator must be aware of the type
of IP traffic that the device will carry out. This means that the
operator might provision sets of rules compatible with the use case
of the device. For devices acting as gateways of other devices
several rules that match the diversity of devices and protocols used
by the devices associated to the gateway. Meanwhile than simpler
devices (for example an electricity meter) may have a predetermined
set of protocols and parameters fixed. Additionally, the deployment
of IPV4 addresses in addition to IPV6 may force to provision separate
rules to deal with each of the cases.
5.3.3. Rule ID
For these transmission scenarios in NB-IoT, a reasonable assumption
of 9 bytes of radio protocol overhead can be expected. PDCP 5 bytes
due to header and integrity protection, and 4 bytes of RLC and MAC.
The minimum physical Transport Block (TB) that can withhold this
overhead value according to 3GPP Release 15 specifications are: 88,
104, 120 and 144 bits. If it is wished to optimize the number of
transmissions of a very small application packet so that in some
cases can be transmitted using only one physical layer transmission,
then the SCHC overhead should not exceed the available number of bits
of the smallest utile 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
utilize the smallest TB the maximum SCHC is 8 bits. This must
include the Compression Residue in addition to the Rule ID. In the
other hand, it is possible that more complex NB-IoT devices (such as
a capillarity gateway) might require additional bits to handle the
variety and multiple parameters the 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 a configurable value is preferred for
these scenarios. The configuration may be set as part of the
operation profile agreed together with the context distribution. The
Rule Id field size may range for example from 2 bits resulting in 4
rules to a 8 bits value that would yield up to 256 rules which can be
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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 take in account the byte-alignment of the
expected Compression Residue too. In the minimum TB size case, 2
bits size of Rule Id leave only 6 bits available for Compression
Residue.
5.3.4. SCHC MAX_PACKET_SIZE
The Access Stratum can handle the fragmentation of SCHC packets if
needed including reliability. Hence the packet size is limited by
the MTU possible to be handled by the AS radio protocols that
corresponds to 1600 bytes for 3GPP Release 15.
5.3.5. Fragmentation
For these scenarios the SCHC fragmentation functions are recommend to
be 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 selection of the blocks is done according to the
input of the link adaptation 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 depends of the
network load, interference and number of bits to be transmitted and
QoS. Even if setting a value that allows the construction of data
units following SCHC tiles principle, the protocol overhead may be
greater or equal than allowing the AS radio protocols to take care of
the fragmentation natively.
5.3.5.1. Fragmentation in Transparent Mode
If RLC is configured to operate 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 very rare to transmit user plane data using this configuration and
it is mainly targeting control plane transmissions. In those cases
the reliability is normally ensured by MAC based mechanisms, such as
repetitions or automatic retransmissions, and additional reliability
might only generate protocol overhead.
In future operations, it could be devised the utilization of SCHC to
reduce radio network protocols overhead and support the reliability
of the transmissions, and targeting small data with the fewer
possible transmissions. This could be realized by using fixed or
limited set of transport blocks compatible with the tiling SCHC
fragmentation handling.
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6. Non-IP based Data Transmission
The Non-IP Data Delivery (NIDD) services of 3GPP enable the
possibility of transmitting SCHC packets compressed by the
application layer. The packets can be delivered by means of IP-
tunnels to the 3GPP network or using SCEF functions (i.e., API
calls). In both cases the packet IP is not understood by the 3GPP
network since it is already compressed and the network does not has
information of the context used for compression. Therefore the
network will treat the packet as a Non-IP traffic and deliver it to
the UE without any other stack element, directly under the L2.
6.1. SCHC Entities Placing
In the two scenarios using NIDD, SCHC entities are located almost in
top of the stack. In the terminal, it may be implemented by a
application utilizing the NB-IoT connectivity services. In the
network side, the SCHC entities are located in the Application Server
(AS). The IP tunneling scenario requires that the Application Server
sends the compressed packet over an IP connection that is terminated
by the 3GPP core network. If instead the SCEF services are used,
then it is possible to utilize a API call to transfer the SCHC
packets between the core network and the AS, also an IP tunnel could
be established by the AS, if negotiated with the 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 5: SCHC entities placed when using Non-IP Delivery (NIDD) 3GPP
Sevices
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6.2. Parameters for Static Context Header Compression
6.2.1. SCHC Context initialization
The static context is handled in the application layer level,
consequently the contexts are required to be distributed according to
the applications own capabilities, perhaps utilizing IP data
transmissions up to context initialization. Also the same IP
tunneling or SCEF services used later for the SCHC packets transport
may be used by the applications in both ends to deliver the static
contexts to be used.
6.2.2. SCHC Rules
Even when the transmissions content are not visible for the 3GPP
network, the same limitations than for IP based data transmissions
applies in these scenarios in terms of aiming to use the minimum
number of transmission and minimize the protocol overhead.
6.2.3. Rule ID
Similarly to the case of IP transmissions, the Rule ID size can be
dynamically set prior the context delivery. For example negotiated
between the applications when choosing a profile according to the
type of traffic and type of application deployed. Same
considerations related to the transport block size and performance
mentioned for the IP type of traffic has to be follow when choosing a
size value for the Rule ID field.
6.2.4. 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), and not only the
radio as the IP transmissions case.
6.3. Fragmentation
In principle the fragmentation function should be activated for
packets greater than 1358 Bytes. Since the 3GPP reliability
functions take great deal care of it, for simple point to point
connections may be enough a NO-ACK mode. Nevertheless additional
considerations for more complex cases are mentioned in the next
subsection to be taken in account.
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6.3.1. Fragmentation modes
Depending of the QoS that has been assigned to the packets, it is
possible that packets are lost before they arrive to 3GPP radio
network transmission, for example in between the links of a
capillarity gateway, or due to buffer overflow handling in a backhaul
connection. In consequence, it is possible to secure additional
reliability on the packets transmitted with a small trade-off on
additional transmissions to signal the packets arrival indication
end-to-end if no transport protocol takes care of retransmission. To
achieve this, the packets fragmentation is activated with the ACK-on-
Error mode enabled. In some cases, it 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 MAX_PACKET_SIZE == 1 Byte) and the ACK-on-
Error mode.
6.3.2. Fragmentation Parameters(TBD)
o Rule ID
o DTag
o FCN
o W (number of bits)
o WINDOW_SIZE
o Retransmission Timer
o Inactivity Timer
o MAX_ACK_Retries
o MAX_ATTEMPS
o MIC (size and algorithm)
7. 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 treatment of padding should use this value accordingly.
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8. Security considerations
3GPP access security is specified in (TGPP33203).
9. 3GPP References
o [TGPP23720] 3GPP, "TR 23.720 v13.0.0 - Study on architecture
enhancements for Cellular Internet of Things", 2016.
o [TGPP33203] 3GPP, "TS 33.203 v13.1.0 - 3G security; Access
security for IP-based services", 2016.
o [TGPP36321] 3GPP, "TS 36.321 v13.2.0 - Evolved Universal
Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC)
protocol specification", 2016
o [TGPP36323] 3GPP, "TS 36.323 v13.2.0 - Evolved Universal
Terrestrial Radio Access (E-UTRA); Packet Data Convergence
Protocol (PDCP) specification", 2016.
o [TGPP36331] 3GPP, "TS 36.331 v13.2.0 - Evolved Universal
Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC);
Protocol specification", 2016.
o [TGPP36300] 3GPP, "TS 36.300 v15.1.0 - Evolved Universal
Terrestrial Radio Access (E-UTRA) and Evolved Universal
Terrestrial Radio Access Network (E-UTRAN); Overall description;
Stage 2", 2018
o [TGPP24301] 3GPP "TS 24.301 v15.2.0 - Non-Access-Stratum (NAS)
protocol for Evolved Packet System (EPS); Stage 3", 2018
10. Appendix
10.1. NB-IoT User Plane protocol architecture
10.1.1. Packet Data Convergence Protocol (PDCP)
Each of the Radio Bearers (RB) are associated with one PDCP entity.
And a PDCP entity is associated with one or two RLC entities
depending of the unidirectional or bi-directional characteristics of
the RB and RLC mode used. A PDCP entity is associated either control
plane or user plane which independent configuration and functions.
The maximum supported size for NB-IoT of a PDCP SDU is 1600 octets.
The main services and functions of the PDCP sublayer for NB-IoT for
the user plane include:
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o Header compression and decompression by means of ROHC (Robust
Header Compression)
o Transfer of user and control data to higher and lower layers
o Duplicate detection of lower layer SDUs when re-establishing
connection (when RLC with Acknowledge Mode in use for User Plane
only)
o Ciphering and deciphering
o Timer-based SDU discard in uplink
10.1.2. 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 that are 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 set of entities, one
in each direction (downlink and uplink) must be configured and they
are 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:
o Transparent Mode (TM). In this mode RLC do not segment or
concatenate SDUs from higher layers and do not include any header
to the payload. When acting as a transmitter, RLC receives SDUs
from upper layers and transmit directly to its flow RLC receiver
via lower layers. Similarly, an TM RLC receiver would only
deliver without additional processing the packets to higher layers
upon reception.
o Unacknowledged Mode (UM). This mode provides support for
segmentation and concatenation of payload. The size of the RLC
packet depends of the indication given at a particular
transmission opportunity by the lower layer (MAC) and are octets
aligned. The packet delivery to the receiver do not include
support for reliability and the lost of a segment from a packet
means a whole packet loss. Also in 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 discard, reordering of out of sequence and loss
detection.
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o Acknowledged Mode (AM). Additional to the same functions
supported from UM, this mode also adds a moving windows based
reliability service on top of the lower layer services. It also
provides support for re-segmentation and it requires bidirectional
communication to exchange acknowledgment reports called RLC Status
Report and trigger retransmissions is needed. Protocol error
detection is also supported by this mode. The mode uses depends
of 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 map to a
UM configuration.
10.1.3. 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 the efficiency of data transmission. MAC also
provides error correction and reliability support by means of 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 additional to 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
Figure 6: Example of User Plane packet encapsulation for two
transport blocks
10.2. NB-IoT Data over NAS (DoNAS)
The AS protocol stack used by DoNAS is somehow special. 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) is configured by default in AM mode, but
depending of the features supported by the network and the terminal
it may be configured in other modes by the network operator. For
example, the transparent mode does not add any header or does not
process the payload in any way reducing the overhead, but the MTU
would be limited by the transport block used to transmit the data
which is couple of thousand of bits maximum. If UM (only Release 15
compatible terminals) is used, the RLC mechanisms of reliability is
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 for the AM case because the
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lack of status reporting but with the same support for segmentation
up to 16000 Bytes. NAS packet are encapsulated within a RRC (Radio
Resource Control)[TGPP36331] message.
Depending of the data type indication signaled (IP or non-IP data),
the network allocates an IP address or just establish 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 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 to do a short transmission and receive an
acknowledgment or short feedback from the network. Depending of 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 and 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 7: 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 8: Example of User Plane packet encapsulation for Data over
NAS
11. Informative References
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
Zuniga, "LPWAN Static Context Header Compression (SCHC)
and fragmentation for IPv6 and UDP", draft-ietf-lpwan-
ipv6-static-context-hc-18 (work in progress), December
2018.
[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>.
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[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>.
Authors' Addresses
Ana Minaburo
Acklio
2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Edgar Ramos
Ericsson
Hirsalantie 11
02420 Jorvas, Kirkkonummi
Finland
Email: edgar.ramos@ericsson.com
Sivasothy Shanmugalingam
Acklio
2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex
France
Email: sothy@ackl.io
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