Internet DRAFT - draft-ietf-lpwan-schc-over-lorawan
draft-ietf-lpwan-schc-over-lorawan
lpwan Working Group O. Gimenez, Ed.
Internet-Draft Semtech
Intended status: Standards Track I. Petrov, Ed.
Expires: July 29, 2021 Acklio
January 25, 2021
Static Context Header Compression (SCHC) over LoRaWAN
draft-ietf-lpwan-schc-over-lorawan-14
Abstract
The Static Context Header Compression (SCHC) specification describes
generic header compression and fragmentation techniques for Low Power
Wide Area Networks (LPWAN) technologies. SCHC is a generic mechanism
designed for great flexibility so that it can be adapted for any of
the LPWAN technologies.
This document specifies a profile of RFC8724 to use SCHC in
LoRaWAN(R) 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.
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Copyright Notice
Copyright (c) 2021 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
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Static Context Header Compression Overview . . . . . . . . . 4
4. LoRaWAN Architecture . . . . . . . . . . . . . . . . . . . . 6
4.1. Device classes (A, B, C) and interactions . . . . . . . . 7
4.2. Device addressing . . . . . . . . . . . . . . . . . . . . 8
4.3. General Frame Types . . . . . . . . . . . . . . . . . . . 8
4.4. LoRaWAN MAC Frames . . . . . . . . . . . . . . . . . . . 9
4.5. LoRaWAN FPort . . . . . . . . . . . . . . . . . . . . . . 9
4.6. LoRaWAN empty frame . . . . . . . . . . . . . . . . . . . 9
4.7. Unicast and multicast technology . . . . . . . . . . . . 9
5. SCHC-over-LoRaWAN . . . . . . . . . . . . . . . . . . . . . . 10
5.1. LoRaWAN FPort and RuleID . . . . . . . . . . . . . . . . 10
5.2. Rule ID management . . . . . . . . . . . . . . . . . . . 10
5.3. Interface IDentifier (IID) computation . . . . . . . . . 11
5.4. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.5. Decompression . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 13
5.6.1. DTag . . . . . . . . . . . . . . . . . . . . . . . . 13
5.6.2. Uplink fragmentation: From device to SCHC gateway . . 13
5.6.3. Downlink fragmentation: From SCHC gateway to device . 17
5.7. SCHC Fragment Format . . . . . . . . . . . . . . . . . . 20
5.7.1. All-0 SCHC fragment . . . . . . . . . . . . . . . . . 20
5.7.2. All-1 SCHC fragment . . . . . . . . . . . . . . . . . 21
5.7.3. Delay after each LoRaWAN frame to respect local
regulation . . . . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . 23
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 23
A.1. Uplink - Compression example - No fragmentation . . . . . 23
A.2. Uplink - Compression and fragmentation example . . . . . 24
A.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
SCHC specification [RFC8724] describes generic header compression and
fragmentation techniques that can be used on all Low Power Wide Area
Networks (LPWAN) technologies defined in [RFC8376]. Even though
those technologies share a great number of common features like star-
oriented topologies, network architecture, devices with mostly quite
predictable communications, etc; they do have some slight differences
with respect to payload sizes, reactiveness, etc.
SCHC provides a generic framework that enables those devices to
communicate on IP 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 parameters and modes of operation when
SCHC is used over LoRaWAN networks. LoRaWAN protocol is specified by
the LoRa Alliance(R) in [lora-alliance-spec]
2. Terminology
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.
This section defines the terminology and acronyms used in this
document. For all other definitions, please look up the SCHC
specification [RFC8724].
o DevEUI: Device Extended Unique Identifier, an IEEE EUI-64
identifier used to identify the device during the procedure while
joining the network (Join Procedure). It is assigned by the
manufacturer or the device owner and provisioned on the Network
Gateway.
o DevAddr: a 32-bit non-unique identifier assigned to a device
either:
* Statically: by the device manufacturer in _Activation by
Personalization_ mode.
* Dynamically: after a Join Procedure by the Network Gateway in
_Over The Air Activation_ mode.
o Downlink: LoRaWAN term for a frame transmitted by the network and
received by the device.
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o EUI: Extended Unique Identifier
o LoRaWAN: LoRaWAN is a wireless technology based on Industrial,
Scientific, and Medical (ISM) radio bands that is used for long-
range, low-power, low-data-rate applications developed by the LoRa
Alliance, a membership consortium: https://www.lora-alliance.org
[1].
o FRMPayload: Application data in a LoRaWAN frame.
o MSB: Most Significant Byte
o OUI: Organisation Unique Identifier. IEEE assigned prefix for
EUI.
o RCS: Reassembly Check Sequence. Used to verify the integrity of
the fragmentation-reassembly process.
o RX: Device's reception window.
o RX1/RX2: LoRaWAN class A devices open two RX windows following an
uplink, called RX1 and RX2.
o SCHC gateway: The LoRaWAN Application Server that manages
translation between IPv6 network and the Network Gateway (LoRaWAN
Network Server).
o Tile: Piece of a fragmented packet as described in [RFC8724]
section 8.2.2.1
o Uplink: LoRaWAN term for a frame transmitted by the device and
received by the network.
3. Static Context Header Compression Overview
This section contains a short overview of SCHC. For a detailed
description, refer to the full specification [RFC8724].
It defines:
1. Compression mechanisms to avoid transporting information known by
both sender and receiver over the air. Known information is part
of the "context". This component is called SCHC Compressor/
Decompressor (SCHC C/D).
2. Fragmentation mechanisms to allow SCHC Packet transportation on
small, and potentially variable, MTU. This component is called
SCHC Fragmentation/Reassembly (SCHC F/R).
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Context exchange or pre-provisioning is out of scope of this
document.
Device App
+----------------+ +----+ +----+ +----+
| App1 App2 App3 | |App1| |App2| |App3|
| | | | | | | |
| UDP | |UDP | |UDP | |UDP |
| IPv6 | |IPv6| |IPv6| |IPv6|
| | | | | | | |
|SCHC C/D and F/R| | | | | | |
+--------+-------+ +----+ +----+ +----+
| +---+ +----+ +----+ +----+ . . .
+~ |RGW| === |NGW | == |SCHC| == |SCHC|...... Internet ....
+---+ +----+ |F/R | |C/D |
+----+ +----+
|<- - - - LoRaWAN - - ->|
Figure 1: Architecture
Figure 1 represents the architecture for compression/decompression,
it is based on [RFC8376] terminology. The device is sending
applications flows using IPv6 or IPv6/UDP protocols. These flows
might be compressed by a Static Context Header Compression
Compressor/Decompressor (SCHC C/D) to reduce headers size and
fragmented by the SCHC Fragmentation/Reassembly (SCHC F/R). The
resulting information is sent on a layer two (L2) frame to an LPWAN
Radio Gateway (RGW) that forwards the frame to a Network Gateway
(NGW). The NGW sends the data to a SCHC F/R for reassembly, if
required, then to SCHC C/D for decompression. The SCHC C/D shares
the same rules with the device. The SCHC C/D and F/R can be located
on the Network Gateway (NGW) or in another place as long as a
communication is established between the NGW and the SCHC F/R, then
SCHC F/R and C/D. The SCHC C/D and F/R in the device and the SCHC
gateway 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 and F/R process is bidirectional, so the same principles
can be applied to the other direction.
In a LoRaWAN network, the RGW is called a Gateway, the NGW is Network
Server, and the SCHC C/D and F/R are an Application Server. It can
be provided by the Network Gateway or any third party software.
Figure 1 can be mapped in LoRaWAN terminology to:
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End Device App
+--------------+ +----+ +----+ +----+
|App1 App2 App3| |App1| |App2| |App3|
| | | | | | | |
| UDP | |UDP | |UDP | |UDP |
| IPv6 | |IPv6| |IPv6| |IPv6|
| | | | | | | |
|SCHC C/D & F/R| | | | | | |
+-------+------+ +----+ +----+ +----+
| +-------+ +-------+ +-----------+ . . .
+~ |Gateway| === |Network| == |Application|..... Internet ....
+-------+ |server | |server |
+-------+ | F/R - C/D |
+-----------+
|<- - - - - LoRaWAN - - - ->|
Figure 2: SCHC Architecture mapped to LoRaWAN
4. LoRaWAN Architecture
An overview of LoRaWAN [lora-alliance-spec] protocol and architecture
is described in [RFC8376]. The mapping between the LPWAN
architecture entities as described in [RFC8724] and the ones in
[lora-alliance-spec] is as follows:
o Devices are LoRaWAN End Devices (e.g. sensors, actuators, etc.).
There can be a very high density of devices per radio gateway
(LoRaWAN gateway). This entity maps to the LoRaWAN end-device.
o The Radio Gateway (RGW), which is the endpoint 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 SCHC gateway (LoRaWAN Application server).
This entity maps to the LoRaWAN Network Server.
o SCHC C/D and F/R are handled by LoRaWAN Application Server; ie the
LoRaWAN application server will do the SCHC C/D and F/R.
o The LPWAN-AAA Server is the LoRaWAN Join Server. Its role is to
manage and deliver security keys in a secure way, so that the devices
root key is never exposed.
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(LPWAN-AAA Server)
() () () | +------+
() () () () / \ +---------+ | Join |
() () () () () / \======| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |Application|
() () () () / \==========| v |=============| Server |
() () () / \ +---------+ +-----------+
End-devices Gateways Network Server (SCHC C/D and F/R)
(devices) (RGW) (NGW)
Figure 3: LPWAN Architecture
_Note_: Figure 3 terms are from LoRaWAN, with [RFC8376] terminology
in brackets.
SCHC Compressor/Decompressor (SCHC C/D) and SCHC Fragmentation/
Reassembly (SCHC F/R) are performed on the LoRaWAN end-device and the
Application Server (called SCHC gateway). While the point-to-point
link between the device and the Application Server constitutes a
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 (SCHC gateway) acts as the first hop IP
router for the device. 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
The LoRaWAN MAC layer supports 3 classes of devices named A, B and C.
All devices implement the Class A, some devices may implement Class B
or Class C. Class B and Class C are mutually exclusive.
o Class A: The Class A is the simplest class of devices. The device
is allowed to transmit at any time, randomly selecting a
communication channel. The Network Gateway may reply with a
downlink in one of the 2 receive windows immediately following the
uplinks. Therefore, the Network Gateway cannot initiate a
downlink, it has to wait for the next uplink from the device to
get a downlink opportunity. The Class A is the lowest power
consumption class.
o Class B: Class B devices implement all the functionalities of
Class A devices, but also schedule periodic listen windows.
Therefore, opposed to the Class A devices, Class B devices can
receive downlinks that are initiated by the Network Gateway and
not following an uplink. There is a trade-off between the
periodicity of those scheduled Class B listen windows and the
power consumption of the device: if the periodicity is high
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downlinks from the NGW will be sent faster, but the device wakes
up more often: it will have higher power consumption.
o Class C: Class C devices implement all the functionalities of
Class A devices, but keep their receiver open whenever they are
not transmitting. Class C devices can receive downlinks at any
time at the expense of a higher power consumption. Battery-
powered devices can only operate in Class C for a limited amount
of time (for example for a firmware upgrade over-the-air). Most
of the Class C devices are grid powered (for example Smart Plugs).
4.2. Device addressing
LoRaWAN end-devices use a 32-bit network address (devAddr) to
communicate with the Network Gateway over-the-air, this address might
not be unique in a LoRaWAN network. Devices using the same devAddr
are distinguished by the Network Gateway based on the cryptographic
signature appended to every LoRaWAN frame.
To communicate with the SCHC gateway, the Network Gateway MUST
identify the devices by a unique 64-bit device identifier called the
DevEUI.
The DevEUI is assigned to the device during the manufacturing process
by the device's manufacturer. It is built like an Ethernet MAC
address by concatenating the manufacturer's IEEE OUI field with a
vendor unique number. e.g.: 24-bit OUI is concatenated with a 40-bit
serial number. The Network Gateway translates the devAddr into a
DevEUI in the uplink direction and reciprocally on the downlink
direction.
+--------+ +---------+ +---------+ +----------+
| Device | <=====> | Network | <====> | SCHC | <======> | Internet |
| | devAddr | Gateway | DevEUI | Gateway | IPv6/UDP | |
+--------+ +---------+ +---------+ +----------+
Figure 4: LoRaWAN addresses
4.3. General Frame Types
LoRaWAN implements the possibility to send confirmed or unconfirmed
frames:
o Confirmed frame: The sender asks the receiver to acknowledge the
frame.
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o Unconfirmed frame: The sender does not ask the receiver to
acknowledge the frame.
As SCHC defines its own acknowledgment mechanisms, SCHC does not
require the use of LoRaWAN Confirmed frames (MType=0b100 as per
[lora-alliance-spec])
4.4. LoRaWAN MAC Frames
In addition to regular data frames, LoRaWAN implements JoinRequest
and JoinAccept frame types, which are used by a device to join a
network:
o JoinRequest: This frame is used by a device to join a network. It
contains the device's unique identifier DevEUI and a random nonce
that will be used for session key derivation.
o JoinAccept: To on-board a device, the Network Gateway responds to
the JoinRequest issued by a device with a JoinAccept frame. That
frame is encrypted with the device's AppKey and contains (amongst
other fields) the network's major settings and a random nonce used
to derive the session keys.
o Data: MAC and application data. Application data are protected
with AES-128 encryption. MAC related data are AES-128 encrypted
with another key.
4.5. LoRaWAN FPort
The LoRaWAN MAC layer features a frame port field in all frames.
This field (FPort) is 8 bits long and the values from 1 to 223 can be
used. It allows LoRaWAN networks and applications to identify data.
4.6. LoRaWAN empty frame
A LoRaWAN empty frame is a LoRaWAN frame without FPort (cf
Section 5.1) and FRMPayload.
4.7. Unicast and multicast technology
LoRaWAN technology supports unicast downlinks, but also multicast: a
packet sent over LoRaWAN radio link can be received by several
devices. It is useful to address many devices with same content,
either a large binary file (firmware upgrade), or same command (e.g:
lighting control). As IPv6 is also a multicast technology this
feature can be used to address a group of devices.
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_Note 1_: IPv6 multicast addresses must be defined as per [RFC4291].
LoRaWAN multicast group definition in a Network Gateway and the
relation between those groups and IPv6 groupID are out of scope of
this document.
_Note 2_: LoRa Alliance defined [lora-alliance-remote-multicast-set]
as the RECOMMENDED way to setup multicast groups on devices and
create a synchronized reception window.
5. SCHC-over-LoRaWAN
5.1. LoRaWAN FPort and RuleID
The FPort field is part of the SCHC Message, as shown in Figure 5.
The SCHC C/D and the SCHC F/R SHALL concatenate the FPort field with
the LoRaWAN payload to recompose the SCHC Message.
| FPort | LoRaWAN payload |
+ ------------------------ +
| SCHC Message |
Figure 5: SCHC Message in LoRaWAN
Note: SCHC Message is any datagram sent by SCHC C/D or F/R layers.
A fragmented datagram with application payload transferred from
device to Network Gateway, is called an uplink fragmented datagram.
It uses an FPort for data uplink and its associated SCHC control
downlinks, named FPortUp in this document. The other way, a
fragmented datagram with application payload transferred from Network
Gateway to device, is called downlink fragmented datagram. It uses
another FPort for data downlink and its associated SCHC control
uplinks, named FPortDown in this document.
All RuleID can use arbitrary values inside the FPort range allowed by
LoRaWAN specification and MUST be shared by the device and SCHC
gateway prior to the communication with the selected rule. The
uplink and downlink fragmentation FPorts MUST be different.
5.2. Rule ID management
RuleID MUST be 8 bits, encoded in the LoRaWAN FPort as described in
Section 5.1. LoRaWAN supports up to 223 application FPorts in the
range [1;223] as defined in section 4.3.2 of [lora-alliance-spec], it
implies that RuleID MSB SHOULD be inside this range. An application
can send non SCHC traffic by using FPort values different from the
ones used for SCHC.
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In order to improve interoperability, RECOMMENDED fragmentation
RuleID values are:
o RuleID = 20 (8-bit) for uplink fragmentation, named FPortUp.
o RuleID = 21 (8-bit) for downlink fragmentation, named FPortDown.
o RuleID = 22 (8-bit) for which SCHC compression was not possible
(i.e., no matching compression Rule was found), as described in
[RFC8724] section 6.
FPortUp value MUST be different from FPortDown. The remaining
RuleIDs are available for compression. RuleIDs are shared between
uplink and downlink sessions. A RuleID not in the set(s) of FPortUp
or FPortDown means that the fragmentation is not used, thus, on
reception, the SCHC Message MUST be sent to the SCHC C/D layer.
The only uplink frames using the FPortDown port are the fragmentation
SCHC control messages of a downlink fragmented datagram (for example,
SCHC ACKs). Similarly, the only downlink frames using the FPortUp
port are the fragmentation SCHC control messages of an uplink
fragmented datagram.
An application can have multiple fragmented datagrams between a
device and one or several SCHC gateways. A set of FPort values is
REQUIRED for each SCHC gateway instance the device is required to
communicate with. The application can use additional uplinks or
downlink fragmented parameters but SHALL implement at least the
parameters defined in this document.
The mechanism for context distribution across devices and gateways is
outside the scope of this document.
5.3. Interface IDentifier (IID) computation
In order to mitigate the risks described in [RFC8064] and [RFC8065],
implementation MUST implement the following algorithm and SHOULD use
it.
1. key = LoRaWAN AppSKey
2. cmac = aes128_cmac(key, DevEUI)
3. IID = cmac[0..7]
aes128_cmac algorithm is described in [RFC4493]. It has been chosen
as it is already used by devices for LoRaWAN protocol.
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As AppSKey is renewed each time a device joins or rejoins a LoRaWAN
network, the IID will change over time; this mitigates privacy,
location tracking and correlation over time risks. Join periodicity
is defined at the application level.
Address scan risk is mitigated thanks to AES-128, which provides
enough entropy bits of the IID.
Using this algorithm will also ensure that there is no correlation
between the hardware identifier (IEEE-64 DevEUI) and the IID, so an
attacker cannot use manufacturer OUI to target devices.
Example with:
o DevEUI: 0x1122334455667788
o appSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
2. cmac: 0xBA59F4B196C6C3432D9383C145AD412A
3. IID: 0xBA59F4B196C6C343
Figure 6: Example of IID computation.
There is a small probability of IID collision in a LoRaWAN network.
If this occurs, the IID can be changed by rekeying the device at L2
level (ie: trigger a LoRaWAN join). The way the device is rekeyed is
out of scope of this document and left to the implementation.
Note: Implementation also using another IID source MUST ensure that
the same IID is shared between the device and the SCHC gateway in the
compression and decompression of the IPv6 address of the device.
5.4. Padding
All padding bits MUST be 0.
5.5. Decompression
SCHC C/D MUST concatenate FPort and LoRaWAN payload to retrieve the
SCHC Packet as per Section 5.1.
RuleIDs matching FPortUp and FPortDown are reserved for SCHC
Fragmentation.
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5.6. Fragmentation
The L2 Word Size used by LoRaWAN is 1 byte (8 bits). The SCHC
fragmentation over LoRaWAN uses the ACK-on-Error mode for uplink
fragmentation and Ack-Always mode for downlink fragmentation. A
LoRaWAN device cannot support simultaneous interleaved fragmented
datagrams in the same direction (uplink or downlink).
The fragmentation parameters are different for uplink and downlink
fragmented datagrams and are successively described in the next
sections.
5.6.1. DTag
[RFC8724] section 8.2.4 describes the possibility to interleave
several fragmented SCHC datagrams for the same RuleID. This is not
used in SCHC over LoRaWAN profile. A device cannot interleave
several fragmented SCHC datagrams on the same FPort. This field is
not used and its size is 0.
Note: The device can still have several parallel fragmented datagrams
with more than one SCHC gateway thanks to distinct sets of FPorts, cf
Section 5.2.
5.6.2. Uplink fragmentation: From device to SCHC gateway
In this case, the device is the fragment transmitter, and the SCHC
gateway the fragment receiver. A single fragmentation rule is
defined. SCHC F/R MUST concatenate FPort and LoRaWAN payload to
retrieve the SCHC Packet, as per Section 5.1.
o SCHC fragmentation reliability mode: "ACK-on-Error".
o SCHC header size is two bytes (the FPort byte + 1 additional
byte).
o RuleID: 8 bits stored in LoRaWAN FPort. cf Section 5.2
o DTag: Size T=0 bit, not used. cf Section 5.6.1
o Window index: 4 windows are used, encoded on M = 2 bits
o FCN: The FCN field is encoded on N = 6 bits, so WINDOW_SIZE = 63
tiles are allowed in a window.
o Last tile: it can be carried in a Regular SCHC Fragment, alone in
an All-1 SCHC Fragment or with any of these two methods.
Implementation must ensure that:
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* The sender MUST ascertain that the receiver will not receive
the last tile through both a Regular SCHC Fragment and an All-1
SCHC Fragment during the same session.
* If the last tile is in All-1 SCHC message: current L2 MTU MUST
be big enough to fit the All-1 header and the last tile.
o Penultimate tile MUST be equal to the regular size.
o RCS: Use recommended calculation algorithm in [RFC8724] (S.8.2.3.
Integrity Checking).
o Tile: size is 10 bytes.
o Retransmission timer: Set by the implementation depending on the
application requirements. The default RECOMMENDED duration of
this timer is 12 hours; this value is mainly driven by application
requirements and MAY be changed by the application.
o Inactivity timer: The SCHC gateway implements an "inactivity
timer". The default RECOMMENDED duration of this timer is 12
hours; this value is mainly driven by application requirements and
MAY be changed by the application.
o MAX_ACK_REQUESTS: 8. With this set of parameters, the SCHC
fragment header is 16 bits, including FPort; payload overhead will
be 8 bits as FPort is already a part of LoRaWAN payload. MTU is:
_4 windows * 63 tiles * 10 bytes per tile = 2520 bytes_
In addition to the per-rule context parameters specified in
[RFC8724], for uplink rules, an additional context parameter is
added: whether or not to ack after each window.
For battery powered devices, it is RECOMMENDED to use the ACK
mechanism at the end of each window instead of waiting until the end
of all windows:
o The SCHC receiver SHOULD send a SCHC ACK after every window even
if there is no missing tile.
o The SCHC sender SHOULD wait for the SCHC ACK from the SCHC
receiver before sending tiles from the next window. If the SCHC
ACK is not received, it SHOULD send a SCHC ACK REQ up to
MAX_ACK_REQUESTS times, as described previously.
This will avoid useless uplinks if the device has lost network
coverage.
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For non-battery powered devices, the SCHC receiver MAY also choose to
send a SCHC ACK only at the end of all windows. This will reduce
downlink load on the LoRaWAN network, by reducing the number of
downlinks.
SCHC implementations MUST be compatible with both behaviors, and this
selection is part of the rule context.
5.6.2.1. Regular fragments
| FPort | LoRaWAN payload |
+ ------ + ------------------------- +
| RuleID | W | FCN | Payload |
+ ------ + ------ + ------ + ------- +
| 8 bits | 2 bits | 6 bits | |
Figure 7: All fragments except the last one. SCHC header size is 16
bits, including LoRaWAN FPort.
5.6.2.2. Last fragment (All-1)
| FPort | LoRaWAN payload |
+ ------ + ---------------------------- +
| RuleID | W | FCN=All-1 | RCS |
+ ------ + ------ + --------- + ------- +
| 8 bits | 2 bits | 6 bits | 32 bits |
Figure 8: All-1 SCHC Message: the last fragment without last tile.
| FPort | LoRaWAN payload |
+ ------ + ---------------------------------------------------------- +
| RuleID | W | FCN=All-1 | RCS | Last tile | Opt. padding |
+ ------ + ------ + --------- + ------- + ------------ + ------------ +
| 8 bits | 2 bits | 6 bits | 32 bits | 1 to 80 bits | 0 to 7 bits |
Figure 9: All-1 SCHC Message: the last fragment with last tile.
5.6.2.3. SCHC ACK
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| FPort | LoRaWAN payload |
+ ------ + --------------------------+
| RuleID | W | C = 1 | padding |
| | | | (b'00000) |
+ ------ + ----- + ----- + --------- +
| 8 bits | 2 bit | 1 bit | 5 bits |
Figure 10: SCHC ACK format, correct RCS check.
| FPort | LoRaWAN payload |
+ ------ + --------------------------------- + ---------------- +
| RuleID | W | C = 0 | Compressed bitmap | Optional padding |
| | | | (C = 0) | (b'0...0) |
+ ------ + ----- + ----- + ----------------- + ---------------- +
| 8 bits | 2 bit | 1 bit | 5 to 63 bits | 0, 6 or 7 bits |
Figure 11: SCHC ACK format, failed RCS check.
Note: Because of the bitmap compression mechanism and L2 byte
alignment, only the following discrete values are possible for the
compressed bitmap size: 5, 13, 21, 29, 37, 45, 53, 61, 62 and 63.
Bitmaps of 63 bits will require 6 bits of padding.
5.6.2.4. Receiver-Abort
| FPort | LoRaWAN payload |
+ ------ + -------------------------------------------- +
| RuleID | W = b'11 | C = 1 | b'11111 | 0xFF (all 1's) |
+ ------ + -------- + ------+-------- + ----------------+
| 8 bits | 2 bits | 1 bit | 5 bits | 8 bits |
next L2 Word boundary ->| <-- L2 Word --> |
Figure 12: Receiver-Abort format.
5.6.2.5. SCHC acknowledge request
| FPort | LoRaWAN payload |
+------- +------------------------- +
| RuleID | W | FCN = b'000000 |
+ ------ + ------ + --------------- +
| 8 bits | 2 bits | 6 bits |
Figure 13: SCHC ACK REQ format.
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5.6.3. Downlink fragmentation: From SCHC gateway to device
In this case, the device is the fragmentation receiver, and the SCHC
gateway the fragmentation transmitter. The following fields are
common to all devices. SCHC F/R MUST concatenate FPort and LoRaWAN
payload to retrieve the SCHC Packet as described in Section 5.1.
o SCHC fragmentation reliability mode:
* Unicast downlinks: ACK-Always.
* Multicast downlinks: No-ACK, reliability has to be ensured by
the upper layer. This feature is OPTIONAL and may not be
implemented by SCHC gateway.
o RuleID: 8 bits stored in LoRaWAN FPort. cf Section 5.2
o DTag: Size T=0 bit, not used. cf Section 5.6.1
o FCN: The FCN field is encoded on N=1 bit, so WINDOW_SIZE = 1 tile.
o RCS: Use recommended calculation algorithm in [RFC8724] (S.8.2.3.
Integrity Checking).
o Inactivity timer: The default RECOMMENDED duration of this timer
is 12 hours; this value is mainly driven by application
requirements and MAY be changed by the application.
The following parameters apply to ACK-Always (Unicast) only:
o Retransmission timer: See Section 5.6.3.5.
o MAX_ACK_REQUESTS: 8.
o Window index (unicast only): encoded on M=1 bit, as per [RFC8724].
As only 1 tile is used, its size can change for each downlink, and
will be the currently available MTU.
Class A devices can only receive during an RX slot, following the
transmission of an uplink. Therefore the SCHC gateway cannot
initiate communication (e.g., start a new SCHC session). In order to
create a downlink opportunity it is RECOMMENDED for Class A devices
to send an uplink every 24 hours when no SCHC session is started,
this is application specific and can be disabled. The RECOMMENDED
uplink is a LoRaWAN empty frame as defined Section 4.6. As this
uplink is to open an RX window, any LoRaWAN uplink frame from the
device MAY reset this counter.
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_Note_: The Fpending bit included in LoRaWAN protocol SHOULD NOT be
used for SCHC-over-LoRaWAN protocol. It might be set by the Network
Gateway for other purposes but not SCHC needs.
5.6.3.1. Regular fragments
| FPort | LoRaWAN payload |
+ ------ + ------------------------------------ +
| RuleID | W | FCN = b'0 | Payload |
+ ------ + ----- + --------- + ---------------- +
| 8 bits | 1 bit | 1 bit | X bytes + 6 bits |
Figure 14: All fragments but the last one. Header size 10 bits,
including LoRaWAN FPort.
5.6.3.2. Last fragment (All-1)
| FPort | LoRaWAN payload |
+ ------ + --------------------------- + ------------------------- +
| RuleID | W | FCN = b'1 | RCS | Payload | Opt padding |
+ ------ + ----- + --------- + ------- + ----------- + ----------- +
| 8 bits | 1 bit | 1 bit | 32 bits | 6 to X bits | 0 to 7 bits |
Figure 15: All-1 SCHC Message: the last fragment.
5.6.3.3. SCHC ACK
| FPort | LoRaWAN payload |
+ ------ + ---------------------------------- +
| RuleID | W | C = b'1 | Padding b'000000 |
+ ------ + ----- + ------- + ---------------- +
| 8 bits | 1 bit | 1 bit | 6 bits |
Figure 16: SCHC ACK format, RCS is correct.
| FPort | LoRaWAN payload |
+ ------ + ------------------------------------------------- +
| RuleID | W | C = b'0 | Bitmap = b'1 | Padding b'000000 |
+ ------ + ----- + ------- + ------------ + ---------------- +
| 8 bits | 1 bit | 1 bit | 1 bit | 5 bits |
Figure 17: SCHC ACK format, RCS is incorrect.
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5.6.3.4. Receiver-Abort
| FPort | LoRaWAN payload |
+ ------ + ---------------------------------------------- +
| RuleID | W = b'1 | C = b'1 | b'111111 | 0xFF (all 1's) |
+ ------ + ------- + ------- + -------- + --------------- +
| 8 bits | 1 bit | 1 bits | 6 bits | 8 bits |
next L2 Word boundary ->| <-- L2 Word --> |
Figure 18: Receiver-Abort packet (following an All-1 SCHC Fragment
with incorrect RCS).
5.6.3.5. Downlink retransmission timer
Class A and Class B or Class C devices do not manage retransmissions
and timers the same way.
5.6.3.5.1. Class A devices
Class A devices can only receive in an RX slot following the
transmission of an uplink.
The SCHC gateway implements an inactivity timer with a RECOMMENDED
duration of 36 hours. For devices with very low transmission rates
(example 1 packet a day in normal operation), that duration may be
extended: it is application specific.
RETRANSMISSION_TIMER is application specific and its RECOMMENDED
value is INACTIVITY_TIMER/(MAX_ACK_REQUESTS + 1).
*SCHC All-0 (FCN=0)*
All fragments but the last have an FCN=0 (because window size is 1).
Following an All-0 SCHC Fragment, the device MUST transmit the SCHC
ACK message. It MUST transmit up to MAX_ACK_REQUESTS SCHC ACK
messages before aborting. In order to progress the fragmented
datagram, the SCHC layer should immediately queue for transmission
those SCHC ACK if no SCHC downlink have been received during RX1 and
RX2 window. LoRaWAN layer will respect the applicable local spectrum
regulation.
_Note_: The ACK bitmap is 1 bit long and is always 1.
*SCHC All-1 (FCN=1)*
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SCHC All-1 is the last fragment of a datagram, the corresponding SCHC
ACK message might be lost; therefore the SCHC gateway MUST request a
retransmission of this ACK when the retransmission timer expires. To
open a downlink opportunity the device MUST transmit an uplink every
RETRANSMISSION_TIMER/(MAX_ACK_REQUESTS *
SCHC_ACK_REQ_DN_OPPORTUNITY). The format of this uplink is
application specific. It is RECOMMENDED for a device to send an
empty frame (see Section 4.6) but it is application specific and will
be used by the NGW to transmit a potential SCHC ACK REQ.
SCHC_ACK_REQ_DN_OPPORTUNITY is application specific and its
recommended value is 2. It MUST be greater than 1. This allows to
open a downlink opportunity to any downlink with higher priority than
the SCHC ACK REQ message.
_Note_: The device MUST keep this SCHC ACK message in memory until it
receives a downlink SCHC Fragmentation Message (with FPort ==
FPortDown) that is not a SCHC ACK REQ: it indicates that the SCHC
gateway has received the SCHC ACK message.
5.6.3.6. Class B or Class C devices
Class B devices can receive in scheduled RX slots or in RX slots
following the transmission of an uplink. Class C devices are almost
in constant reception.
RECOMMENDED retransmission timer value:
o Class B: 3 times the ping slot periodicity.
o Class C: 30 seconds.
The RECOMMENDED inactivity timer value is 12 hours for both Class B
and Class C devices.
5.7. SCHC Fragment Format
5.7.1. All-0 SCHC fragment
*Uplink fragmentation (Ack-On-Error)*:
All-0 is distinguishable from a SCHC ACK REQ as [RFC8724] states
_This condition is also met if the SCHC Fragment Header is a multiple
of L2 Words_; this condition met: SCHC header is 2 bytes.
*Downlink fragmentation (Ack-always)*:
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As per [RFC8724] the SCHC All-1 MUST contain the last tile,
implementation must ensure that SCHC All-0 message Payload will be at
least the size of an L2 Word.
5.7.2. All-1 SCHC fragment
All-1 is distinguishable from a SCHC Sender-Abort as [RFC8724] states
_This condition is met if the RCS is present and is at least the size
of an L2 Word_; this condition met: RCS is 4 bytes.
5.7.3. Delay after each LoRaWAN frame to respect local regulation
This profile does not define a delay to be added after each LoRaWAN
frame, local regulation compliance is expected to be enforced by
LoRaWAN stack.
6. Security Considerations
This document is only providing parameters that are expected to be
best suited for LoRaWAN networks for [RFC8724]. IID security is
discussed in Section 5.3. As such, this document does not contribute
to any new security issues beyond those already identified in
[RFC8724]. Moreover, SCHC data (LoRaWAN payload) are protected at
the LoRaWAN level by an AES-128 encryption with a session key shared
by the device and the SCHC gateway. These session keys are renewed
at each LoRaWAN session (ie: each join or rejoin to the LoRaWAN
network)
7. IANA Considerations
This document has no IANA actions.
Acknowledgements
Thanks to all those listed in the Contributors section for the
excellent text, insightful discussions, reviews and suggestions, and
also to (in alphabetical order) Dominique Barthel, Arunprabhu
Kandasamy, Rodrigo Munoz, Alexander Pelov, Pascal Thubert, Laurent
Toutain for useful design considerations, reviews and comments.
Contributors
Contributors ordered by family name.
Vincent Audebert
EDF R&D
Email: vincent.audebert@edf.fr
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Julien Catalano
Kerlink
Email: j.catalano@kerlink.fr
Michael Coracin
Semtech
Email: mcoracin@semtech.com
Marc Le Gourrierec
Sagemcom
Email: marc.legourrierec@sagemcom.com
Nicolas Sornin
Semtech
Email: nsornin@semtech.com
Alper Yegin
Actility
Email: alper.yegin@actility.com
10. References
10.1. Normative References
[lora-alliance-spec]
Alliance, L., "LoRaWAN Specification Version V1.0.4",
<https://lora-alliance.org/resource_hub/lorawan-104-
specification-package/>.
[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>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, <https://www.rfc-editor.org/info/rfc4493>.
[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>.
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[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>.
10.2. Informative References
[lora-alliance-remote-multicast-set]
Alliance, L., "LoRaWAN Remote Multicast Setup
Specification Version 1.0.0", <https://lora-
alliance.org/sites/default/files/2018-09/
remote_multicast_setup_v1.0.0.pdf>.
[RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu,
"Recommendation on Stable IPv6 Interface Identifiers",
RFC 8064, DOI 10.17487/RFC8064, February 2017,
<https://www.rfc-editor.org/info/rfc8064>.
[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <https://www.rfc-editor.org/info/rfc8065>.
[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>.
10.3. URIs
[1] https://www.lora-alliance.org
Appendix A. Examples
In following examples "applicative data" refers to the IPv6 payload
sent by the application to the SCHC layer.
A.1. Uplink - Compression example - No fragmentation
This example represents an applicative data going through SCHC over
LoRaWAN, no fragmentation required
An applicative data of 78 bytes is passed to SCHC compression layer.
Rule 1 is used by SCHC C/D layer, allowing to compress it to 40 bytes
and 5 bits: 1 byte RuleID, 21 bits residue + 37 bytes payload.
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| RuleID | Compression residue | Payload | Padding=b'000 |
+ ------ + ------------------- + --------- + ------------- +
| 1 | 21 bits | 37 bytes | 3 bits |
Figure 19: Uplink example: SCHC Message
The current LoRaWAN MTU is 51 bytes, although 2 bytes FOpts are used
by LoRaWAN protocol: 49 bytes are available for SCHC payload; no need
for fragmentation. The payload will be transmitted through FPort =
1.
| LoRaWAN Header | LoRaWAN payload (40 bytes) |
+ ------------------------- + --------------------------------------- +
| | FOpts | RuleID=1 | Compression | Payload | Padding=b'000 |
| | | | residue | | |
+ ---- + ------- + -------- + ----------- + --------- + ------------- +
| XXXX | 2 bytes | 1 byte | 21 bits | 37 bytes | 3 bits |
Figure 20: Uplink example: LoRaWAN packet
A.2. Uplink - Compression and fragmentation example
This example represents an applicative data going through SCHC, with
fragmentation.
An applicative data of 300 bytes is passed to SCHC compression layer.
Rule 1 is used by SCHC C/D layer, allowing to compress it to 282
bytes and 5 bits: 1 byte RuleID, 21 bits residue + 279 bytes payload.
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + --------- +
| 1 | 21 bits | 279 bytes |
Figure 21: Uplink example: SCHC Message
The current LoRaWAN MTU is 11 bytes, 0 bytes FOpts are used by
LoRaWAN protocol: 11 bytes are available for SCHC payload + 1 byte
FPort field. SCHC header is 2 bytes (including FPort) so 1 tile is
sent in first fragment.
| LoRaWAN Header | LoRaWAN payload (11 bytes) |
+ -------------------------- + -------------------------- +
| | RuleID=20 | W | FCN | 1 tile |
+ -------------- + --------- + ----- + ------ + --------- +
| XXXX | 1 byte | 0 0 | 62 | 10 bytes |
Figure 22: Uplink example: LoRaWAN packet 1
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Content of the tile is:
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + ----------------- +
| 1 | 21 bits | 6 bytes + 3 bits |
Figure 23: Uplink example: LoRaWAN packet 1 - Tile content
Next transmission MTU is 11 bytes, although 2 bytes FOpts are used by
LoRaWAN protocol: 9 bytes are available for SCHC payload + 1 byte
FPort field, a tile does not fit inside so LoRaWAN stack will send
only FOpts.
Next transmission MTU is 242 bytes, 4 bytes FOpts. 23 tiles are
transmitted:
| LoRaWAN Header | LoRaWAN payload (231 bytes) |
+ --------------------------------------+ --------------------------- +
| | FOpts | RuleID=20 | W | FCN | 23 tiles |
+ -------------- + ------- + ---------- + ----- + ----- + ----------- +
| XXXX | 4 bytes | 1 byte | 0 0 | 61 | 230 bytes |
Figure 24: Uplink example: LoRaWAN packet 2
Next transmission MTU is 242 bytes, no FOpts. All 5 remaining tiles
are transmitted, the last tile is only 2 bytes + 5 bits. Padding is
added for the remaining 3 bits.
| LoRaWAN Header | LoRaWAN payload (44 bytes) |
+ ---- + ---------- + ----------------------------------------------- +
| | RuleID=20 | W | FCN | 5 tiles | Padding=b'000 |
+ ---- + ---------- + ----- + ----- + --------------- + ------------- +
| XXXX | 1 byte | 0 0 | 38 | 42 bytes+5 bits | 3 bits |
Figure 25: Uplink example: LoRaWAN packet 3
Then All-1 message can be transmitted:
| LoRaWAN Header | LoRaWAN payload (44 bytes) |
+ ---- + -----------+ -------------------------- +
| | RuleID=20 | W | FCN | RCS |
+ ---- + ---------- + ----- + ----- + ---------- +
| XXXX | 1 byte | 0 0 | 63 | 4 bytes |
Figure 26: Uplink example: LoRaWAN packet 4 - All-1 SCHC message
All packets have been received by the SCHC gateway, computed RCS is
correct so the following ACK is sent to the device by the SCHC
receiver:
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| LoRaWAN Header | LoRaWAN payload |
+ -------------- + --------- + ------------------- +
| | RuleID=20 | W | C | Padding |
+ -------------- + --------- + ----- + - + ------- +
| XXXX | 1 byte | 0 0 | 1 | 5 bits |
Figure 27: Uplink example: LoRaWAN packet 5 - SCHC ACK
A.3. Downlink
An applicative data of 155 bytes is passed to SCHC compression layer.
Rule 1 is used by SCHC C/D layer, allowing to compress it to 130
bytes and 5 bits: 1 byte RuleID, 21 bits residue + 127 bytes payload.
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + --------- +
| 1 | 21 bits | 127 bytes |
Figure 28: Downlink example: SCHC Message
The current LoRaWAN MTU is 51 bytes, no FOpts are used by LoRaWAN
protocol: 51 bytes are available for SCHC payload + FPort field => it
has to be fragmented.
| LoRaWAN Header | LoRaWAN payload (51 bytes) |
+ ---- + ---------- + -------------------------------------- +
| | RuleID=21 | W = 0 | FCN = 0 | 1 tile |
+ ---- + ---------- + ------ + ------- + ------------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 50 bytes and 6 bits |
Figure 29: Downlink example: LoRaWAN packet 1 - SCHC Fragment 1
Content of the tile is:
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + ------------------ +
| 1 | 21 bits | 48 bytes and 1 bit |
Figure 30: Downlink example: LoRaWAN packet 1: Tile content
The receiver answers with a SCHC ACK:
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| LoRaWAN Header | LoRaWAN payload |
+ ---- + --------- + -------------------------------- +
| | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 6 bits |
Figure 31: Downlink example: LoRaWAN packet 2 - SCHC ACK
The second downlink is sent, two FOpts:
| LoRaWAN Header | LoRaWAN payload (49 bytes) |
+ --------------------------- + ------------------------------------- +
| | FOpts | RuleID=21 | W = 1 | FCN = 0 | 1 tile |
+ ---- + ------- + ---------- + ----- + ------- + ------------------- +
| XXXX | 2 bytes | 1 byte | 1 bit | 1 bit | 48 bytes and 6 bits |
Figure 32: Downlink example: LoRaWAN packet 3 - SCHC Fragment 2
The receiver answers with an SCHC ACK:
| LoRaWAN Header | LoRaWAN payload |
+ ---- + --------- + -------------------------------- +
| | RuleID=21 | W = 1 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 6 bits |
Figure 33: Downlink example: LoRaWAN packet 4 - SCHC ACK
The last downlink is sent, no FOpts:
| LoRaWAN Header | LoRaWAN payload (37 bytes) |
+ ---- + ------- + --------------------------------------------------- +
| | RuleID | W | FCN | RCS | 1 tile | Padding |
| | 21 | 0 | 1 | | | b'00000 |
+ ---- + ------- + ----- + ----- + ------- + --------------- + ------- +
| XXXX | 1 byte | 1 bit | 1 bit | 4 bytes | 31 bytes+1 bits | 5 bits |
Figure 34: Downlink example: LoRaWAN packet 5 - All-1 SCHC message
The receiver answers to the sender with an SCHC ACK:
| LoRaWAN Header | LoRaWAN payload |
+ ---- + --------- + -------------------------------- +
| | RuleID=21 | W = 0 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 6 bits |
Figure 35: Downlink example: LoRaWAN packet 6 - SCHC ACK
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Authors' Addresses
Olivier Gimenez (editor)
Semtech
14 Chemin des Clos
Meylan
France
Email: ogimenez@semtech.com
Ivaylo Petrov (editor)
Acklio
1137A Avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ivaylo@ackl.io
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