lpwan Working Group JC. Zuniga
Internet-Draft SIGFOX
Intended status: Informational C. Gomez
Expires: September 20, 2018 Universitat Politecnica de Catalunya
L. Toutain
IMT-Atlantique
March 19, 2018
SCHC over Sigfox LPWAN
draft-zuniga-lpwan-schc-over-sigfox-02
Abstract
The Static Context Header Compression (SCHC) specification describes
a header compression scheme and fragmentation functionality for Low
Power Wide Area Network (LPWAN) technologies. SCHC offers a great
level of flexibility that can be tailored for different LPWAN
technologies.
The present document provides the optimal parameters and modes of
operation when SCHC is implemented over a Sigfox LPWAN.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Static Context Header Compression . . . . . . . . . . . . . . 3
3.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Packet processing . . . . . . . . . . . . . . . . . . . . 4
4. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Fragmentation headers . . . . . . . . . . . . . . . . . . 4
4.2. Uplink fragment transmissions . . . . . . . . . . . . . . 4
4.3. Downlink fragment transmissions . . . . . . . . . . . . . 5
5. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security considerations . . . . . . . . . . . . . . . . . . . 6
7. Informative References . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Static Context Header Compression (SCHC) specification
[I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
scheme and fragmentation functionality that can be used on top of all
the LWPAN systems defined in [I-D.ietf-lpwan-overview]. These LPWAN
systems have similar characteristics such as star-oriented
topologies, network architecture, connected devices with built-in
applications, etc.
SCHC offers a great level of flexibility to accommodate all these
LPWAN systems. Even though there are a great number of similarities
between LPWAN technologies, some differences exist with respect to
the transmission characteristics, payload sizes, etc. Hence, there
are optimal parameters and modes of operation that can be used when
SCHC is used on top of a specific LPWAN.
This document describes the optimal parameters and modes of operation
when SCHC is implemented over a Sigfox LPWAN.
2. Terminology
The reader is assumed to be familiar with the terms and mechanisms
defined in [I-D.ietf-lpwan-overview] and in
[I-D.ietf-lpwan-ipv6-static-context-hc].
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3. Static Context Header Compression
Static Context Header Compression (SCHC) avoids context
synchronization because data flows are highly predictable in LPWAN
networks. Contexts must be stored and configured on both ends. This
can be done either by using a provisioning protocol, by out of band
means, or by pre-provisioning them e.g. at manufacturing time. The
way the contexts are configured and stored on both ends is out of the
scope of this document.
Dev App
+--------------+ +--------------+
|APP1 APP2 APP3| |APP1 APP2 APP3|
| | | |
| UDP | | UDP |
| IPv6 | | IPv6 |
| | | |
| SCHC C/D | | |
| (context) | | |
+-------+------+ +-------+------+
| +--+ +----+ +---------+ .
+~~ |RG| === |NGW | === |SCHC C/D |... Internet ..
+--+ +----+ |(context)|
+---------+
Figure 1: Architecture
Figure 1 represents the architecture for compression/decompression
and fragmentation, which is based on [I-D.ietf-lpwan-overview]
terminology.
The Device is sending applications flows that are compressed (and/or
fragmented) by a Static Context Header Compression Compressor/
Decompressor (SCHC C/D) to reduce headers size and/or fragment the
packet. The resulting information is sent over a layer two (L2)
frame to a LPWAN Radio Gateway (RG) which forwards the frame to a
Network Gateway (NGW).
In the case of the global Sigfox network, RGs (or base stations) are
distributed over the multiple countries where the Sigfox LPWAN
service is provided. On the other hand, the NGW (or Cloud-based Core
network) is a single entity that connects to all Sigfox base stations
in the world.
The NGW communicates with the Network SCHC C/D for compression/
decompression (and/or fragmentation/reassembly). The Network SCHC C/
D shares the same set of rules with the Dev SCHC C/D. The Network
SCHC C/D can be collocated with the NGW or in another place, as long
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as a tunnel is established between the NGW and the SCHC C/D. After
decompression (and/or reassembly), the packet can be forwarded over
the Internet to one (or several) LPWAN Application Server(s) (App).
The SCHC C/D process is bidirectional, so the same principles can be
applied on both uplink and downlink.
3.1. SCHC Rules
The RuleID MUST be sent at the beginning of the SCHC header. The
total number of rules to be used affects directly the Rule ID field
size, and therefore the total size of the fragmentation header (R).
For this reason, it is recommended to keep the number of rules that
are defined for a specific device to the minimum possible.
3.2. Packet processing
TBD
4. Fragmentation
The SCHC specification [I-D.ietf-lpwan-ipv6-static-context-hc]
supports several modes of operation to fragment packets. These modes
have different advantages and disadvantages depending on the
specifics of the underlying LPWAN technology. This section describes
how the SCHC fragmentation functionality SHOULD optimally be used
over a Sigfox LPWAN.
4.1. Fragmentation headers
A list of fragmentation header fields, their sizes as well as
recommended modes for SCHC fragmentation over Sigfox are provided in
this section.
4.2. Uplink fragment transmissions
Uplink transmissions are completely asynchronous and can take place
in any random frequency of the allowed uplink bandwidth allocation.
Hence, devices can go to deep sleep mode, and then wake up and
transmit whenever there is a need to send any information to the
network. In that way, there is no need to perform any network
attachment, synchronization, or other procedure before transmitting a
data packet. All data packets are self contained with all the
required information for the network to process them accordingly.
Uplink transmissions occur in repetitions over different times and
frequencies (e.g. three times over three different frequencies).
Besides these time and frequency diversities, the Sigfox network also
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provides space diversity, as potentially an uplink message will be
received by several base stations. Since all messages are self-
contained and base stations forward them all back to the Core network
(NGW), multiple input copies can be combined at the NGW and hence
provide for extra reliability based on the triple diversity.
Since uplink transmissions occur asynchronously, an SCHC fragment can
be transmitted at any given time by the Dev.
For uplink fragment transmissions, the following mode(s) MUST be
supported: TBD.
For the TBD fragmentation mode(s), the default values for
MAX_ACK_REQUESTS, MAX_WIND_FCN, Retransmission Timer and Inactivity
Timer are TBD.
For the TBD fragmentation mode(s), the number of window sizes
supported is TBD.
The recommended Rule ID size is: TBD bits
The recommended Dtag size (T) is: TBD bits
Fragment Compressed Number (FCN) size, N: TBD bits
Message Integrity Check (MIC) size, M: TBD bits
The algorithm for computing the MIC field MUST be TBD.
4.3. Downlink fragment transmissions
In some LPWAN technologies, as part of energy-saving techniques,
downlink transmission is only possible immediately after an uplink
transmission. This allows the device to go in a very deep sleep mode
and preserve battery, without the need to listen to any information
from the network. This is the case for Sigfox devices, which can
only listen to downlink communications after performing an uplink
transmission.
When there are multiple fragments to be transmitted in the downlink,
an uplink message is required to trigger the downlink communication.
In order to avoid potentially high delay for fragmented datagram
transmission in the downlink, the fragment receiver MAY perform an
uplink transmission as soon as possible after reception of a fragment
that is not the last one. Such uplink transmission MAY be triggered
by sending a SCHC message, such as an ACK.
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For downlink fragment transmission, the following mode(s) MUST be
supported: TBD.
For the TBD fragmentation mode, the default value for
MAX_ACK_REQUESTS is TBD.
For the TBD fragmentation mode(s), the default values for
MAX_ACK_REQUESTS, MAX_WIND_FCN, Retransmission Timer and Inactivity
Timer are TBD.
For the TBD fragmentation mode(s), the number of window sizes
supported is TBD.
5. Padding
The Sigfox payload fields have different characteristics in uplink
and downlink.
Uplink frames can contain a payload from 0 to 96 bits (i.e. 12
bytes). The radio protocol allows sending zero bits or one single
bit of information for binary applications (e.g. status). However,
for 2 or more bits of payload it is required to add padding to the
next integer number of bytes. The reason for this flexibility is to
optimize transmission time and hence save battery consumption at the
device.
Downlink frames on the other hand have a fixed length. The payload
length must be 64 bits. Hence, if less information bits are to be
transmitted padding would be necessary.
The padding procedure is TBD.
6. Security considerations
The radio protocol authenticates and ensures the integrity of each
message. This is achieved by using a unique device ID and an AES-128
based message authentication code, ensuring that the message has been
generated and sent by the device with the ID claimed in the message.
Application data can be encrypted at the application level or not,
depending on the criticality of the use case, to provide a balance
between cost and effort vs. risk. AES-128 in counter mode is used
for encryption. Cryptographic keys are independent for each device.
These keys are associated with the device ID and separate integrity
and confidentiality keys are pre-provisioned. A confidentiality key
is only provisioned if confidentiality is to be used.
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The radio protocol has protections against reply attacks, and the
cloud-based core network provides firewalling protection against
undesired incoming communications.
7. Informative References
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., and C. Gomez, "LPWAN Static
Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP", draft-ietf-lpwan-ipv6-static-context-hc-07
(work in progress), October 2017.
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-07 (work in progress), October 2017.
Authors' Addresses
Juan Carlos Zuniga
SIGFOX
425 rue Jean Rostand
Labege 31670
France
Email: JuanCarlos.Zuniga@sigfox.com
URI: http://www.sigfox.com/
Carles Gomez
Universitat Politecnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carlesgo@entel.upc.edu
Laurent Toutain
IMT-Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
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