lpwan Working Group N. Sornin, Ed.
Internet-Draft M. Coracin
Intended status: Informational Semtech
Expires: January 3, 2019 I. Petrov
Acklio
A. Yegin
Actility
J. Catalano
Kerlink
V. Audebert
EDF R&D
July 02, 2018
Static Context Header Compression (SCHC) over LoRaWAN
draft-petrov-lpwan-ipv6-schc-over-lorawan-02
Abstract
The Static Context Header Compression (SCHC) specification describes
generic header compression and fragmentation techniques for LPWAN
(Low Power Wide Area Networks) technologies. SCHC is a generic
mechanism designed for great flexibility, so that it can be adapted
for any of the LPWAN technologies.
This document provides the adaptation of SCHC for use in LoRaWAN
networks, and provides elements such as efficient parameterization
and modes of operation.
Status of This Memo
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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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Drafts is at https://datatracker.ietf.org/drafts/current/.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 3, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Static Context Header Compression Overview . . . . . . . . . 3
4. LoRaWAN Architecture . . . . . . . . . . . . . . . . . . . . 4
4.1. Device classes (A, B, C) and interactions . . . . . . . . 5
4.2. Device addressing . . . . . . . . . . . . . . . . . . . . 6
4.3. General Message Types . . . . . . . . . . . . . . . . . . 6
4.4. LoRaWAN MAC Frames . . . . . . . . . . . . . . . . . . . 6
5. SCHC over LoRaWAN . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Rule ID management . . . . . . . . . . . . . . . . . . . 6
5.2. IID computation . . . . . . . . . . . . . . . . . . . . . 6
5.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 6
5.3.1. Reliability options . . . . . . . . . . . . . . . . . 6
5.3.2. Supporting multiple window sizes . . . . . . . . . . 11
5.3.3. Downlink fragment transmission . . . . . . . . . . . 11
5.3.4. SCHC behavior for devices in class A, B and C . . . . 11
6. Security considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 12
Appendix B. Note . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Static Context Header Compression (SCHC) specification
[I-D.ietf-lpwan-ipv6-static-context-hc] describes generic header
compression and fragmentation techniques that can be used on all
LPWAN (Low Power Wide Area Networks) technologies defined in
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[I-D.ietf-lpwan-overview]. Even though those technologies share a
great number of common features like start-oriented topologies,
network architecture, devices with mostly quite predictable
communications, etc; they do have some slight differences in respect
of payload sizes, reactiveness, etc.
SCHC gives a generic framework that enables those devices to
communicate with other Internet networks. However, for efficient
performance, some parameters and modes of operation need to be set
appropriately for each of the LPWAN technologies.
This document describes the efficient parameters and modes of
operation when SCHC is used over LoRaWAN networks.
2. Terminology
This section defines the terminology and acronyms used in this
document. For all other definitions, please look up the SCHC
specification [I-D.ietf-lpwan-ipv6-static-context-hc].
o DevEUI: an IEEE EUI-64 identifier used to identify the device
during the procedure while joining the network (Join Procedure)
o DevAddr: a 32-bit non-unique identifier assigned to a device
statically or dynamically after a Join Procedure (depending on the
activation mode)
o TBD: all significant LoRaWAN-related terms.
3. Static Context Header Compression Overview
This section contains a short overview of Static Context Header
Compression (SCHC). For a detailed description, refer to the full
specification [I-D.ietf-lpwan-ipv6-static-context-hc].
Static Context Header Compression (SCHC) avoids context
synchronization, which is the most bandwidth-consuming operation in
other header compression mechanisms such as RoHC [RFC5795]. Based on
the fact that the nature of data flows is highly predictable in LPWAN
networks, some static contexts may be stored on the Device (Dev).
The contexts must be stored in both ends, and it can either be
learned by a provisioning protocol or by out of band means or it can
be pre-provisioned, etc. The way the context is learned on both
sides is out of the scope of this document.
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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,
it is based on [I-D.ietf-lpwan-overview] terminology. The Device is
sending applications flows using IPv6 or IPv6/UDP protocols. These
flows are compressed by an Static Context Header Compression
Compressor/Decompressor (SCHC C/D) to reduce headers size. Resulting
information is sent on a layer two (L2) frame to a LPWAN Radio
Network (RG) which forwards the frame to a Network Gateway (NGW).
The NGW sends the data to a SCHC C/D for decompression which shares
the same rules with the Dev. The SCHC C/D can be located on the
Network Gateway (NGW) or in another place as long as a tunnel is
established between the NGW and the SCHC C/D. The SCHC C/D in both
sides must share the same set of Rules. After decompression, the
packet can be sent on the Internet to one or several LPWAN
Application Servers (App).
The SCHC C/D process is bidirectional, so the same principles can be
applied in the other direction.
In a LoRaWAN network, the RG is called a Gateway, the NGW is Network
Server, and the SCHC C/D can be embedded in different places, for
example in the Network Server and/or the Application Server.
Next steps for this section: detailed overview of the LoRaWAN
architecture and its mapping to the SCHC architecture.
4. LoRaWAN Architecture
An overview of LoRaWAN [lora-alliance-spec] protocol and architecture
is described in [I-D.ietf-lpwan-overview]. Mapping between the LPWAN
architecture entities as described in
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[I-D.ietf-lpwan-ipv6-static-context-hc] and the ones in
[lora-alliance-spec] is as follows:
o Devices (Dev) are the end-devices or hosts (e.g. sensors,
actuators, etc.). There can be a very high density of devices per
radio gateway. This entity maps to the LoRaWAN End-device.
o The Radio Gateway (RGW), which is the end point of the constrained
link. This entity maps to the LoRaWAN Gateway.
o The Network Gateway (NGW) is the interconnection node between the
Radio Gateway and the Internet. This entity maps to the LoRaWAN
Network Server.
o LPWAN-AAA Server, which controls the user authentication and the
applications. This entity maps to the LoRaWAN Join Server.
o Application Server (App). The same terminology is used in LoRaWAN.
() () () | +------+
() () () () / \ +---------+ | Join |
() () () () () / \======| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |Application|
() () () () / \==========| v |=============| Server |
() () () / \ +---------+ +-----------+
End-Devices Gateways Network Server
Figure 1: LPWAN/LoRaWAN Architecture
SCHC C/D (Compressor/Decompressor) and SCHC Fragmentation are
performed on the LoRaWAN End-device and the Application Server.
While the point-to-point link between the End-device and the
Application Server constitutes single IP hop, the ultimate end-point
of the IP communication may be an Internet node beyond the
Application Server. In other words, the LoRaWAN Application Server
acts as the first hop IP router for the End-device. Note that the
Application Server and Network Server may be co-located, which
effectively turns the Network/Application Server into the first hop
IP router.
4.1. Device classes (A, B, C) and interactions
TBD
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4.2. Device addressing
TBD
4.3. General Message Types
TBD
4.4. LoRaWAN MAC Frames
TBD
5. SCHC over LoRaWAN
5.1. Rule ID management
Rule ID can be stored and transported in the FPort field of the
LoRaWAN MAC frame or as the first bytes of the payload.
TBD
5.2. IID computation
TBD
5.3. Fragmentation
TBD
5.3.1. Reliability options
5.3.1.1. Uplinks: From device to gateway
In that case the device is the fragmentation transmitter, and the
SCHC gateway the fragmentation receiver.
o SCHC fragmentation reliability mode : "ACK_ALWAYS"
o Window size: 8, the FCN field is encoded on 3 bits
o DTag : 1bit. this field is used to clearly separate two
consecutive fragmentation sessions. A LoRaWAN device cannot
interleave several fragmented SCHC datagrams.
o MIC calculation algorithm: CRC32 using 0xEDB88320 (i.e. the
reverse representation of the polynomial used e.g. in the Ethernet
standard [RFC3385])
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o Retransmission Timer and inactivity Timer: LoRaWAN devices do not
implement a "retransmission timer". At the end of a window the
ACK corresponding to this window is transmitted by the network
gateway in the RX1 or RX2 receive slot of the device. If this ACK
is not received the device sends an all-0 (or an all-1) fragment
with no payload to request an ACK retransmission. The periodicity
between retransmission of the all-0/all-1 fragments is device/
application specific and may be different for each device (not
specified). The gateway implements an "inactivity timer". The
default recommended duration of this timer is 12h. This value is
mainly driven by application requirements and may be changed.
| RuleID | DTag | W | FCN | Payload |
+ ------ + ----- + ----- | ------ + ------- +
| 3 bits | 1 bit | 1 bit | 3 bits | |
Figure 2: All fragment except the last one. Header size is 8 bits.
| RuleID | DTag | W | FCN | MIC | Payload |
+ ------ + ----- + ----- | ------ + ------- + ------- +
| 3 bits | 1 bit | 1 bit | 3 bits | 32 bits | |
Figure 3: All-1 fragment detailed format for the last fragment.
Header size is 8 bits.
The format of an all-0 or all-1 acknowledge is:
| RuleID | DTag | W | Encoded bitmap | Padding (0s) |
+ ------ + ----- + ----- | -------------- + ------------ +
| 3 bits | 1 bit | 1 bit | up to 8 bits | 0 to 3 bits |
Figure 4: ACK format for All-0 windows. Header size is 1 or 2 bytes.
| RuleID | DTag | W | C | Encoded bitmap (if C = 0) | Padding (0s) |
+ ------ + ----- + ----- + ----- + ------------------------- + ------------ +
| 3 bits | 1 bit | 1 bit | 1 bit | up to 8 bits | 0 to 2 bits |
Figure 5: ACK format for All-1 windows. Header size is 1 or 2 bytes.
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5.3.1.2. Downlinks: From gateway to device
In that case the device is the fragmentation receiver, and the SCHC
gateway the fragmentation transmitter. The following fields are
common to all devices.
o SCHC fragmentation reliability mode : ACK_ALWAYS
o Window size : 1 , The FCN field is encoded on 1 bits
o DTag : 1bit. This field is used to clearly separate two
consecutive fragmentation sessions. A LoRaWAN device cannot
interleave several fragmented SCHC datagrams.
o MIC calculation algorithm: CRC32 using 0xEDB88320 (i.e. the
reverse representation of the polynomial used e.g. in the Ethernet
standard [RFC3385])
o MAX_ACK_REQUESTS : 8
| RuleID | DTag | W | FCN | Payload | Padding |
+ ------ + ----- + ----- | ------ + ------- + ------- +
| 3 bits | 1 bit | 1 bit | 1 bits | X bytes | 2 bits |
Figure 6: All fragments but the last one. Header size is 6 bits.
| RuleID | DTag | W | FCN | MIC | Payload | Padding |
+ ------ + ----- + ----- | ------ + ------- + ------- + ------- +
| 3 bits | 1 bit | 1 bit | 1 bits | 32 bits | X bytes | 2 bits |
Figure 7: All-1 Fragment Detailed Format for the Last Fragment.
Header size is 6 bits.
The format of an all-0 or all-1 acknowledge is:
| RuleID | DTag | W | Encoded bitmap | Padding (0s) |
+ ------ + ----- + ----- | -------------- + ------------ +
| 3 bits | 1 bit | 1 bit | 1 bit | 2 bits |
Figure 8: ACK format for All-0 windows. Header size is 8 bits.
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| RuleID | DTag | W | C = 1 | Padding (0s) |
+ ------ + ----- + ----- + ----- + ------------ +
| 3 bits | 1 bit | 1 bit | 1 bit | 2 bits |
Figure 9: ACK format for All-1 windows, MIC is correct. Header size
is 8 bits.
| RuleID | DTag | W | b'111 | 0xFF (all 1's) |
+ ------ + ----- + ----- + ------ + -------------- +
| 3 bits | 1 bit | 1 bit | 3 bits | 8 bits |
Figure 10: Receiver ABORT packet (following an all-1 packet with
incorrect MIC). Header size is 16 bits.
Class A and classB&C device do not manage retransmissions and timers
in the same way.
5.3.1.2.1. Class A devices
Class A devices can only receive in an RX slot following the
transmission of an uplink. Therefore there cannot be a concept of
"retransmission timer" for a gateway talking to classA devices for
downlink fragmentation.
The device replies with an ACK fragment to every single fragment
received from the gateway (because the window size is 1). Following
the reception of a FCN=0 fragment (fragment that is not the last
fragment of the packet or ACK-request), the device MUST transmit the
ACK fragment until it receives the fragment of the next window. The
device shall transmit up to MAX_ACK_REQUESTS ACK fragments before
aborting. The device should transmit those ACK as soon as possible
while taking into consideration eventual local radio regulation on
duty-cycle, to progress the fragmentation session as quickly as
possible. The ACK bitmap is 1 bit long and is always 1.
Following the reception of a FCN=1 fragment (the last fragment of a
datagram) and if the MIC is correct, the device shall transmit the
ACK with the "MIC is correct" indicator bit set. This message might
be lost therefore the gateway may request a retransmission of this
ACK in the next downlink. The device SHALL keep this ACK message in
memory until it receives a downlink from the gateway different from
an ACK-request indicating that the gateway has received the ACK
message.
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Following the reception of a FCN=1 fragment (the last fragment of a
datagram) and if the MIC is NOT correct, the device shall transmit a
receiver-ABORT fragment. The device SHALL keep this ABORT message in
memory until it receives a downlink from the gateway different from
an ACK-request indicating that the gateway has received the ABORT
message. The fragmentation receiver (device) does not implement
retransmission timer and inactivity timer.
The fragmentation sender (the gateway) implements an inactivity timer
with default duration 12 hours. Once a fragmentation session is
started, if the gateway has not received any ACK or receiver-ABORT
message 12 hours fater the last message from the device was received,
the gateway may flush the fragmentation context. For devices with
very low transmission rates (example 1 packet a day in normal
operation) , that duration may be extended, but this is application
specific.
5.3.1.3. Class B or C devices
Class B&C devices can receive in scheduled RX slots or in RX slots
following the transmission of an uplink. The device replies with an
ACK fragment to every single fragment received from the gateway
(because the window size is 1). Following the reception of a FCN=0
fragment (fragment that is not the last fragment of the packet or
ACK-request), the device MUST always transmit the corresponding ACK
fragment even if that fragment has already been received. The ACK
bitmap is 1 bit long and is always 1. If the gateway receives this
ACK, it proceeds to send the next window fragment If the
retransmission timer elapses and the gateway has not received the ACK
of the current window it retransmits the last fragment. The gateway
tries retransmitting up to MAX_ACK_REQUESTS times before aborting.
Following the reception of a FCN=1 fragment (the last fragment of a
datagram) and if the MIC is correct, the device shall transmit the
ACK with the "MIC is correct" indicator bit set. If the gateway
receives this ACK, the current fragmentation session has succeeded
and its context can be cleared.
If the retransmission timer elapses and the gateway has not received
the all-1 ACK it retransmits the last fragment with the payload (not
an ACK-request without payload). The gateway tries retransmitting up
to MAX_ACK_REQUESTS times before aborting.
The device SHALL keep the all-1 ACK message in memory until it
receives a downlink from the gateway different from the last (FCN=1)
fragment indicating that the gateway has received the ACK message.
Following the reception of a FCN=1 fragment (the last fragment of a
datagram) and if the MIC is NOT correct, the device shall transmit a
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receiver-ABORT fragment. The retransmission timer is used by the
gateway (the sender), the optimal value is very much application
specific but here are some recommended default values. For classB
devices, this timer trigger is a function of the periodicity of the
classB ping slots. The recommended value is equal to 3 times the
classB ping slot periodicity. (modify 128sec) For classC devices
which are nearly constantly receiving, the recommended value is 30
seconds. This means that the device shall try to transmit the ACK
within 30 seconds of the reception of each fragment. The inactivity
timer is implemented by the device to flush the context in-case it
receives nothing from the gateway over an extended period of time.
The recommended value is 12 hours for both classB&C devices.
5.3.2. Supporting multiple window sizes
TBD
5.3.3. Downlink fragment transmission
TBD
5.3.4. SCHC behavior for devices in class A, B and C
TBD
6. Security considerations
TBD
7. Acknowledgements
TBD
8. References
8.1. Normative References
[RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
"Internet Protocol Small Computer System Interface (iSCSI)
Cyclic Redundancy Check (CRC)/Checksum Considerations",
RFC 3385, DOI 10.17487/RFC3385, September 2002,
<https://www.rfc-editor.org/info/rfc3385>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
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[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>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>.
8.2. Informative References
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., and D. Barthel,
"LPWAN Static Context Header Compression (SCHC) and
fragmentation for IPv6 and UDP", draft-ietf-lpwan-ipv6-
static-context-hc-16 (work in progress), June 2018.
[I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-10 (work in progress), February 2018.
[lora-alliance-spec]
Alliance, L., "LoRaWAN Specification Version V1.0.2",
<http://portal.lora-
alliance.org/DesktopModules/Inventures_Document/
FileDownload.aspx?ContentID=1398>.
Appendix A. Examples
Appendix B. Note
Authors' Addresses
Nicolas Sornin (editor)
Semtech
14 Chemin des Clos
Meylan
France
Email: nsornin@semtech.com
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Michael Coracin
Semtech
14 Chemin des Clos
Meylan
France
Email: mcoracin@semtech.com
Ivaylo Petrov
Acklio
2bis rue de la Chataigneraie
35510 Cesson-Sevigne Cedex
France
Email: ivaylo@ackl.io
Alper Yegin
Actility
.
Paris, Paris
France
Email: alper.yegin@actility.com
Julien Catalano
Kerlink
1 rue Jacqueline Auriol
35235 Thorigne-Fouillard
France
Email: j.catalano@kerlink.fr
Vincent AUDEBERT
EDF R&D
7 bd Gaspard Monge
91120 PALAISEAU
FRANCE
Email: vincent.audebert@edf.fr
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