Static Context Header Compression and Fragmentation (SCHC) over LoRaWANSemtech14 Chemin des ClosMeylanFranceogimenez@semtech.comAcklio1137A Avenue des Champs BlancsCesson-Sévigné Cedex35510Franceivaylo@ackl.iolpwan Working Groupheader compressioncompressionfragmentationstatic contextrule-basedLPWANLPWANslow powerlow-powerLoRaLoRaWANIoTInternet of Thingsadaptation layerUDPIPv6sensor networkwireless sensor network802.15.4constrained networkconstrained nodeconstrained-node networkSCHCThe Static Context Header Compression and fragmentation (SCHC) specification (RFC 8724) describes
generic header compression and fragmentation techniques for Low-Power
Wide Area Network (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 defines a profile of SCHC (RFC 8724) for use in
LoRaWAN networks and provides elements such as efficient
parameterization and modes of operation.IntroductionThe SCHC specification describes
generic header compression and fragmentation techniques that can be used on all
Low-Power Wide Area Network (LPWAN) technologies defined in
. Even though those technologies share a great
number of common features like star-oriented topologies, network architecture,
devices with communications that are mostly quite predictable, 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. The LoRaWAN protocol is specified by
the LoRa Alliance in .TerminologyThe 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 when, and
only when, they appear in all capitals, as shown here.This section defines the terminology and abbreviations used in this document. For
all other definitions, please look up the SCHC specification
.
AppKey:
Application Key. An AES-128 root key specific to each device.
AppSKey:
Application Session Key. An AES-128 key derived from the AppKey for
each new session. It is used to encrypt the payload field of a LoRaWAN
applicative frame.
DevAddr:
A 32-bit non-unique identifier assigned to a device either:
Statically:
by the device manufacturer in "Activation-by-Personalization"
mode, or
Dynamically:
after a LoRaWAN "Join Procedure" by the Network Gateway in "Over-the-Air-Activation" mode.
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.
Downlink:
A LoRaWAN term for a frame transmitted by the network and received by the device.
EUI:
Extended Unique Identifier
FRMPayload:
Application data in a LoRaWAN frame
IID:
Interface Identifier
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: .
MSB:
Most Significant Byte
NGW:
Network Gateway
OUI:
Organizationally Unique Identifier. IEEE-assigned prefix for EUI.
RCS:
Reassembly Check Sequence. Used to verify the integrity of the fragmentation-reassembly process.
RGW:
Radio Gateway
RX:
A device's reception window.
RX1/RX2:
LoRaWAN class A devices open two RX windows following an uplink, called "RX1" and "RX2".
SCHC C/D:
SCHC Compression/Decompression
SCHC F/R:
SCHC Fragmentation/Reassembly
SCHC gateway:
The LoRaWAN Application Server that manages translation between an IPv6
network and the Network Gateway (LoRaWAN Network Server).
Tile:
A piece of a fragmented packet as described in .
Uplink:
LoRaWAN term for a frame transmitted by the device and received by the network.
SCHC OverviewThis section contains a short overview of SCHC. For a detailed
description, refer to the full specification .It defines:
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 the "SCHC Compression/Decompression" (SCHC C/D).
Fragmentation mechanisms to allow SCHC Packet transportation on a
small, and potentially variable, MTU. This component is called the "SCHC
Fragmentation/Reassembly" (SCHC F/R).
Context exchange or pre-provisioning is out of scope of this document. represents the
architecture for compression/decompression; it is based on the terminology from . The device is sending
application flows using IPv6 or IPv6/UDP protocols. These flows might
be compressed by a SCHC C/D to reduce header size, and fragmented
by the SCHC F/R. The resulting
information is sent on a Layer 2 (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 a SCHC C/D for
decompression. The SCHC C/D shares the same rules with the device. The
SCHC C/D and SCHC F/R can be located on the 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 SCHC C/D. The SCHC C/D and SCHC 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 SCHC 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 a
"Network Server", and the SCHC C/D and SCHC F/R are one or more
"Application Servers".
Application servers can be provided by the NGW or any third-party
software. can be mapped in
LoRaWAN terminology to:LoRaWAN ArchitectureAn overview of the LoRaWAN protocol and architecture is described in . The mapping between the LPWAN
architecture entities as described in and the ones in is as follows:
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.
The RGW is the endpoint of the constrained
link. This entity maps to the LoRaWAN Gateway.
The NGW is the interconnection node between the Radio
Gateway and the SCHC gateway (LoRaWAN Application Server). This entity maps to
the LoRaWAN Network Server.
The SCHC C/D and SCHC F/R are handled by the LoRaWAN Application Server.
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.
The SCHC C/D and SCHC F/R are performed on the LoRaWAN end
device and the Application Server (called the SCHC gateway). While the
point-to-point link between the device and the Application Server
constitutes a single IP hop, the ultimate endpoint 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.Device Classes (A, B, C) and InteractionsThe LoRaWAN Medium Access Control (MAC) layer supports three
classes of devices named A, B, and C. All devices implement Class
A, and some devices may implement Class B or Class C. Class B and Class C
are mutually exclusive.
Class A:
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 two 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. Class A is the lowest power consumption class.
Class B:
Class B devices implement all the functionalities of Class A devices but
also schedule periodic listen windows. Therefore, as opposed to 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:
High periodicity:
Downlinks from the NGW will be sent faster but the device wakes up more
often and power consumption is increased.
Low periodicity:
Downlinks from the NGW will have higher latency but lower power consumption.
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).
Device AddressingLoRaWAN 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. For example, a 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.General Frame TypesLoRaWAN implements the possibility to send confirmed or unconfirmed frames:
Confirmed frame:
The sender asks the receiver to acknowledge the frame.
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 (FType = 0b100 as per
).LoRaWAN MAC FramesIn addition to regular data frames, LoRaWAN implements JoinRequest and JoinAccept
frame types, which are used by a device to join a network:
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.
JoinAccept:
To onboard 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 (among other fields) the network's major
settings and a random nonce used to derive the session keys.
Data:
This refers to MAC and application data. Application data is protected with AES-128
encryption. MAC-related data is AES-128 encrypted with another key.
LoRaWAN FPortThe 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.LoRaWAN Empty FrameA LoRaWAN empty frame is a LoRaWAN frame without FPort (cf. ) and FRMPayload.Unicast and Multicast TechnologyLoRaWAN technology supports unicast downlinks but also multicast; a
multicast packet sent over a LoRaWAN radio link can be received by several
devices. It is useful to address many devices with the same content:
either a large binary file (firmware upgrade) or the same command (e.g.,
lighting control). As IPv6 is also a multicast technology, this
feature can be used to address a group of devices.SCHC over LoRaWANLoRaWAN FPort and RuleIDThe FPort field is part of the SCHC Message, as shown in
. The SCHC C/D and the SCHC F/R SHALL concatenate
the FPort field with the LoRaWAN payload to recompose the SCHC Message.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 a "downlink-fragmented datagram". It uses another
FPort for data downlink and its associated SCHC control uplinks, named "FPortDown"
in this document.All RuleIDs can use arbitrary values inside the FPort range allowed
by the 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.RuleID ManagementThe RuleID MUST be 8 bits and encoded in the LoRaWAN
FPort as described in .
LoRaWAN supports up to 223 application FPorts in
the range [1..223] as defined in Section 4.3.2 of ; it implies that the 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.In order to improve interoperability, RECOMMENDED fragmentation RuleID values are:
RuleID = 20 (8-bit) for uplink fragmentation, named FPortUp.
RuleID = 21 (8-bit) for downlink fragmentation, named FPortDown.
RuleID = 22 (8-bit) for which SCHC compression was not possible (i.e., no matching
compression Rule was found), as described in .
The FPortUp value MUST be different from the FPortDown value. 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.Interface IDentifier (IID) ComputationIn order to mitigate the risks described in and ,
implementations MUST implement the following algorithm and SHOULD use it.
key = LoRaWAN AppSKey
cmac = aes128_cmac(key, DevEUI)
IID = cmac[0..7]
The aes128_cmac algorithm is described in . It has been chosen as it is already used by
devices for the LoRaWAN protocol.As AppSKey is renewed each time a device joins or rejoins a LoRaWAN
network, the IID will change over time; this
mitigates privacy concerns, for example, location tracking or correlation
over time. Join periodicity is defined at the application level.Address-scan risk is mitigated thanks to the entropy added to
the IID by the inclusion of AppSKey.Using this algorithm will also ensure that there is no correlation
between the hardware identifier (DevEUI) and the IID, so an
attacker cannot use the manufacturer OUI to target devices.Example with:
DevEUI: 0x1122334455667788
AppSKey: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
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 the L2 level (i.e., triggering a LoRaWAN join). The way the device is
rekeyed is out of scope of this document and left to the
implementation.PaddingAll padding bits MUST be 0.DecompressionThe SCHC C/D MUST concatenate FPort and LoRaWAN payload
to retrieve the SCHC Packet as per .RuleIDs matching FPortUp and FPortDown are reserved for SCHC fragmentation.FragmentationThe 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.DTag describes the possibility to interleave several
fragmented SCHC datagrams for the same RuleID. This is not used in
the 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.Uplink Fragmentation: From Device to SCHC GatewayIn this case, the device is the fragment transmitter and the SCHC
gateway is the fragment receiver. A single fragmentation rule is
defined. The SCHC F/R MUST concatenate FPort and LoRaWAN
payload to retrieve the SCHC Packet, as per .
SCHC fragmentation reliability mode:
ACK-on-Error.
SCHC header size:
2 bytes (the FPort byte + 1 additional byte).
RuleID:
8 bits stored in the LoRaWAN FPort (cf. ).
DTag:
Size T = 0 bits, not used (cf. ).
Window index:
4 windows are used, encoded on M = 2 bits.
FCN:
The FCN field is encoded on N = 6 bits, so WINDOW_SIZE = 63 tiles
are allowed in a window.
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. Implementations must ensure that:
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 an All-1 SCHC Message, the current L2 MTU
MUST be big enough to fit the All-1 header and the last
tile.
Penultimate tile:
MUST be equal to the regular size.
RCS:
Use the recommended calculation algorithm in , Integrity Checking.
Tile:
Size is 10 bytes.
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.
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.
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 ,
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:
The SCHC receiver SHOULD send a SCHC ACK after every window even if there is no
missing tile.
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.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.Regular Fragments
is an example of a regular fragment for all fragments except
the last one. SCHC Header Size is 16 Bits, including the
LoRaWAN FPort.
Last Fragment (All-1)Following figures are examples of All-1 messages.
is without the last tile,
is with the last tile.
SCHC ACKReceiver-AbortSCHC Acknowledge RequestDownlink Fragmentation: From SCHC Gateway to DeviceIn this case, the device is the fragmentation receiver and the
SCHC gateway is the fragmentation transmitter. The following fields are
common to all devices. The SCHC F/R MUST concatenate
FPort and LoRaWAN payload to retrieve the SCHC Packet as described
in .
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 for the SCHC gateway and REQUIRED for the device.
RuleID:
8 bits stored in the LoRaWAN FPort (cf. ).
DTag:
Size T = 0 bit, not used (cf. ).
FCN:
The FCN field is encoded on N = 1 bit, so WINDOW_SIZE = 1 tile.
RCS:
Use the recommended calculation algorithm in , Integrity Checking.
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:
Retransmission timer:
See .
MAX_ACK_REQUESTS:
8.
Window index (unicast only):
encoded on M = 1 bit, as per .
As only one 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 in . As this uplink is sent only to open an RX window, any
LoRaWAN uplink frame from the device MAY reset this
counter.Regular Fragments
is an example of a regular fragment for all fragments except
the last one. SCHC Header Size is 10 Bits, including the
LoRaWAN FPort.
Last Fragment (All-1)SCHC ACKReceiver-Abort
is an example of a Receiver-Abort packet, following an All-1
SCHC Fragment with incorrect RCS.
Downlink Retransmission TimerClass A, Class B, and Class C devices do not manage
retransmissions and timers the same way.Class A DevicesClass 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 (for 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 the 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 messages if no SCHC downlink has been
received during the RX1 and RX2 windows. The LoRaWAN layer will respect
the applicable local spectrum regulation.SCHC All-1 (FCN = 1)SCHC All-1 is the last fragment of a datagram, and 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 interval of 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 ), 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 the opening of a
downlink opportunity to any downlink with higher priority than
the SCHC ACK REQ message.Class B or Class C DevicesClass 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 values are:
Class B:
3 times the ping slot periodicity.
Class C:
30 seconds.
The RECOMMENDED inactivity timer value is 12
hours for both Class B and Class C devices.SCHC Fragment FormatAll-0 SCHC FragmentUplink Fragmentation (Ack-on-Error):All-0 is distinguishable from a SCHC ACK REQ, as states "This condition is also
met if the SCHC Fragment Header is a multiple of L2 Words", the
following condition being met: SCHC header is 2 bytes.Downlink fragmentation (ACK-Always):As per , SCHC All-1
MUST contain the last tile, and implementations
MUST ensure that SCHC All-0 message Payload will be
at least the size of an L2 Word.All-1 SCHC FragmentAll-1 is distinguishable from a SCHC Sender-Abort, as states "This condition is met
if the RCS is present and is at least the size of an L2 Word",
the following condition being met: RCS is 4 bytes.Delay after Each LoRaWAN Frame to Respect Local RegulationThis profile does not define a delay to be added after each
LoRaWAN frame; local regulation compliance is expected to be
enforced by the LoRaWAN stack.Security ConsiderationsThis document is only providing parameters that are expected to be best
suited for LoRaWAN networks for . IID
security is discussed in . As such, this document does not contribute to
any new security issues beyond those already identified in
.
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 (i.e., each join or rejoin to the LoRaWAN network).IANA ConsiderationsThis document has no IANA actions.ReferencesNormative ReferencesLoRaWAN 1.0.4 Specification PackageLoRa AllianceInformative ReferencesLoRaWAN Remote Multicast Setup Specification v1.0.0LoRa AllianceExamplesIn the following examples, "applicative data" refers to the IPv6 payload
sent by the application to the SCHC layer.Uplink - Compression Example - No FragmentationThis example represents an applicative data going through SCHC over
LoRaWAN; no fragmentation required.An applicative data of 78 bytes is passed to the SCHC compression
layer. Rule 1 is used by the SCHC C/D layer, allowing to compress it
to 40 bytes and 5 bits: 1 byte RuleID, 21 bits residue + 37 bytes
payload.The current LoRaWAN MTU is 51 bytes, although 2-byte FOpts are
used by the LoRaWAN protocol: 49 bytes are available for SCHC payload; no
need for fragmentation. The payload will be transmitted through FPort
= 1.Uplink - Compression and Fragmentation ExampleThis example represents an applicative data going through SCHC, with
fragmentation.An applicative data of 300 bytes is passed to the SCHC compression layer. Rule 1
is used by the SCHC C/D layer, allowing to compress it to 282 bytes and 5 bits: 1 byte
RuleID, 21 bits residue + 279 bytes payload.The current LoRaWAN MTU is 11 bytes; 0-byte FOpts are used by
the LoRaWAN protocol: 11 bytes are available for SCHC payload + 1 byte
FPort field. The SCHC header is 2 bytes (including FPort), so 1 tile is
sent in the first fragment.The tile content is described in Next transmission MTU is 11 bytes, although 2-byte FOpts are used
by the LoRaWAN protocol: 9 bytes are available for SCHC payload + 1
byte FPort field, a tile does not fit inside so the LoRaWAN stack will
send only FOpts.Next transmission MTU is 242 bytes, 4-byte FOpts. 23 tiles are transmitted: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.Then All-1 message can be transmitted: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:DownlinkAn applicative data of 155 bytes is passed to the SCHC compression layer. Rule 1
is used by the SCHC C/D layer, allowing to compress it to 130 bytes and 5 bits: 1 byte
RuleID, 21 bits residue + 127 bytes payload.The current LoRaWAN MTU is 51 bytes; no FOpts are used by the
LoRaWAN protocol: 51 bytes are available for SCHC payload + FPort
field; the applicative data has to be fragmented.The tile content is described in The receiver answers with a SCHC ACK:The second downlink is sent, two FOpts:The receiver answers with a SCHC ACK:The last downlink is sent, no FOpts:The receiver answers to the sender with a SCHC ACK:AcknowledgementsThanks to all those listed in the Contributors Section for the
excellent text, insightful discussions, reviews, and suggestions, and
also to (in alphabetical order) ,
, , , , and for
useful design considerations, reviews, and comments.LoRaWAN is a registered trademark of the LoRa Alliance.ContributorsContributors ordered by family name.EDF R&Dvincent.audebert@edf.frKerlinkj.catalano@kerlink.frSemtechmcoracin@semtech.comSagemcommarc.legourrierec@sagemcom.comChirp Foundationnicolas.sornin@chirpfoundation.orgActilityalper.yegin@actility.com