Internet DRAFT - draft-zuniga-lpwan-schc-over-sigfox
draft-zuniga-lpwan-schc-over-sigfox
lpwan Working Group JC. Zuniga
Internet-Draft SIGFOX
Intended status: Informational C. Gomez
Expires: September 12, 2019 Universitat Politecnica de Catalunya
L. Toutain
IMT-Atlantique
March 11, 2019
SCHC over Sigfox LPWAN
draft-zuniga-lpwan-schc-over-sigfox-06
Abstract
The Static Context Header Compression (SCHC) specification describes
a header compression scheme and a 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.
Status of This Memo
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This Internet-Draft will expire on September 12, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Static Context Header Compression . . . . . . . . . . . . . . 3
4. SCHC over Sigfox . . . . . . . . . . . . . . . . . . . . . . 4
4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Packet processing . . . . . . . . . . . . . . . . . . . . 4
5. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Fragmentation headers . . . . . . . . . . . . . . . . . . 5
5.2. Uplink fragment transmissions . . . . . . . . . . . . . . 5
5.2.1. Uplink No-ACK mode . . . . . . . . . . . . . . . . . 5
5.2.2. Uplink ACK-Always mode . . . . . . . . . . . . . . . 6
5.2.3. Uplink ACK-on-Error mode . . . . . . . . . . . . . . 6
5.3. Downlink fragment transmissions . . . . . . . . . . . . . 6
6. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Static Context Header Compression (SCHC) specification
[I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
scheme and a fragmentation functionality. Both can be used on top of
all the LWPAN systems defined in [RFC8376] . 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 recommended parameters and modes of
operation to be used when SCHC is implemented over a Sigfox LPWAN.
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2. Terminology
It is assumed that the reader is familiar with the terms and
mechanisms defined in [RFC8376] and in
[I-D.ietf-lpwan-ipv6-static-context-hc].
3. Static Context Header Compression
The Static Context Header Compression (SCHC) described in
[I-D.ietf-lpwan-ipv6-static-context-hc] takes advantage of the
predictability of data flows existing in LPWAN networks to avoid
context synchronization. Nonetheless, these 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 (for instance 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 and F/R| | |
| | | |
+--------+-------+ +-------+------+
$ +--+ +----+ +-----------+ .
+~~ |RG| === |NGW | === | SCHC |... Internet ..
+--+ +----+ |F/R and C/D|
+-----------+
Figure 1: Architecture
Figure 1 represents the architecture for compression/decompression
and fragmentation/reassembly, which is based on [RFC8376]
terminology, where the Radio Gateway is a Sigfox Base Station and the
Network Gateway is the Sigfox Cloud.
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).
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4. SCHC over Sigfox
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.
Uplink Sigfox transmissions occur in repetitions over different times
and frequencies. Besides these time and frequency diversities, the
Sigfox network also 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 same Core network (NGW), multiple input copies can be combined
at the NGW and hence provide for extra reliability based on the
triple diversity (i.e. time, space and frequency). A detailed
description of the Sigfox Radio Protocol can be found in
[sigfox-spec].
The NGW communicates with the Network SCHC C/D for compression/
decompression and/or for fragmentation/reassembly. The Network SCHC
C/D shares the same set of rules as the Dev SCHC C/D. The Network
SCHC C/D can be collocated with the NGW or it could be in another
place, as long 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.
4.1. 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. For
this reason, it is recommended to keep the number of rules that are
defined for a specific device to the minimum possible.
4.2. Packet processing
TBD
5. Fragmentation
The SCHC specification [I-D.ietf-lpwan-ipv6-static-context-hc]
defines a generic fragmentation functionality that allows sending
data packets larger than the maximum size of a Sigfox data frame.
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The functionality also defines a mechanism to send reliably multiple
frames, by allowing to resend selectively any lost frames.
The SCHC fragmentation supports several modes of operation. These
modes have different advantages and disadvantages depending on the
specifics of the underlying LPWAN technology and Use Case. This
section describes how the SCHC fragmentation functionality should
optimally be implemented when used over a Sigfox LPWAN for the most
typical use case applications.
5.1. Fragmentation headers
A list of fragmentation header fields, their sizes as well as
suggested modes for SCHC fragmentation over Sigfox are provided in
this section.
5.2. 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.
Since uplink transmissions occur asynchronously, an SCHC fragment can
be transmitted at any given time by the Dev.
5.2.1. Uplink No-ACK mode
No-ACK is RECOMMENDED to be used for transmitting short, non-critical
packets that require fragmentation.
The recommended Fragmentation Header size is 8 bits, and it is
composed as follows:
The recommended Rule ID size is: 2 bits
The recommended DTag size (T) is: 2 bits
Fragment Compressed Number (FCN) size (N): 4 bits
As per [I-D.ietf-lpwan-ipv6-static-context-hc], in the No-ACK mode
the W (window) field is not present.
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When fragmentation is used to transport IP frames, the Message
Integrity Check (MIC) size, M: TBD bits
The algorithm for computing the MIC field MUST be TBD.
5.2.2. Uplink ACK-Always mode
TBD
5.2.3. Uplink ACK-on-Error mode
ACK-on-Error is RECOMMENDED for larger packets that need to be sent
reliably, since it leads to a reduced number of ACKs in the lower
capacity downlink channel.
In the most generic case, the Fragmentation Header size is 8 bits and
it is composed as follows:
The recommended Rule ID size is: 2 bits.
The recommended DTag size (T) is: 1 bit.
The recommended Window (W) size is: 2 bits.
Fragment Compressed Number (FCN) size (N): 3 bits.
For the ACK-on-Error fragmentation mode(s), a single window size is
RECOMMENDED.
The value of MAX_ACK_REQUESTS SHOULD be 2, and the value of
MAX_WIND_FCN SHOULD be 6 (or 0b110, which allows a maximum window
size of 7 fragments).
When fragmentation is used to transport IP frames, the Message
Integrity Check (MIC) size, M: TBD bits
The algorithm for computing the MIC field MUST be TBD.
5.3. 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-enabled devices, which
can only listen to downlink communications after performing an uplink
transmission and requesting a downlink.
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When there are 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 downlink
fragment that is not the last one. Such uplink transmission MAY be
triggered by sending a SCHC message, such as a SCHC ACK. However,
other data messages can equally be used to trigger DL communications.
For reliable downlink fragment transmission, the ACK-Always mode is
RECOMMENDED.
The recommended Fragmentation Header size is: 8 bits
The recommended Rule ID size is: 2 bits.
The recommended DTag size (T) is: 2 bits.
Fragment Compressed Number (FCN) size (N): 3 bits.
As per [I-D.ietf-lpwan-ipv6-static-context-hc], in the ACK-Always
mode a Window (W) 1-bit field must be present.
For the ACK-Always fragmentation mode(s), a single window size is
RECOMMENDED.
The value of MAX_ACK_REQUESTS SHOULD be 2, and the value of
MAX_WIND_FCN SHOULD be 6 (or 0b110, which allows a maximum window
size of 7 fragments).
When fragmentation is used to transport IP frames, the Message
Integrity Check (MIC) size, M: TBD bits
The algorithm for computing the MIC field MUST be TBD.
Sigfox downlink frames have a fixed length of 8 bytes, which means
that default SCHC algorithm for padding cannot be used. Therefore,
the 3 last bits of the fragmentation header are used to indicate in
bytes the size of the padding. A size of 000 means that the full
ramaining frame is used to carry payload, a value of 001 indicates
that the last byte contains padding, and so on.
6. Padding
The Sigfox payload fields have different characteristics in uplink
and downlink.
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Uplink frames can contain a payload size from 0 to 96 bits, that is 0
to 12 bytes. The radio protocol allows sending zero bits or one
single bit of information for binary applications (e.g. status), or
an integer number of bytes. Therefore, 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 (i.e. 8 bytes). Hence, if less information
bits are to be transmitted, padding would be necessary and it should
be performed as described in the previous section.
7. 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. This flexibility
allows providing 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.
The radio protocol has protections against reply attacks, and the
cloud-based core network provides firewalling protection against
undesired incoming communications.
8. Acknowledgements
Carles Gomez has been funded in part by the ERDF and the Spanish
Government through project TEC2016-79988-P.
9. Informative References
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "LPWAN Static Context Header Compression (SCHC)
and fragmentation for IPv6 and UDP", draft-ietf-lpwan-
ipv6-static-context-hc-17 (work in progress), October
2018.
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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>.
[sigfox-spec]
Sigfox, "Sigfox Radio Specifications",
<https://build.sigfox.com/
sigfox-device-radio-specifications>.
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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