lpwan Working Group O. Gimenez, Ed.
Internet-Draft Semtech
Intended status: Informational I. Petrov, Ed.
Expires: April 11, 2020 Acklio
J. Catalano
Kerlink
October 09, 2019
Static Context Header Compression (SCHC) over LoRaWAN
draft-ietf-lpwan-schc-over-lorawan-03
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. This is called a profile.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 11, 2020.
Copyright Notice
Copyright (c) 2019 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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Static Context Header Compression Overview . . . . . . . . . 3
4. LoRaWAN Architecture . . . . . . . . . . . . . . . . . . . . 5
4.1. End-Device classes (A, B, C) and interactions . . . . . . 6
4.2. End-Device addressing . . . . . . . . . . . . . . . . . . 7
4.3. General Message Types . . . . . . . . . . . . . . . . . . 7
4.4. LoRaWAN MAC Frames . . . . . . . . . . . . . . . . . . . 8
5. SCHC-over-LoRaWAN . . . . . . . . . . . . . . . . . . . . . . 8
5.1. LoRaWAN FPort . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Rule ID management . . . . . . . . . . . . . . . . . . . 9
5.3. IID computation . . . . . . . . . . . . . . . . . . . . . 9
5.4. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5. Compression . . . . . . . . . . . . . . . . . . . . . . . 10
5.6. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 10
5.6.1. DTag . . . . . . . . . . . . . . . . . . . . . . . . 10
5.6.2. Uplink fragmentation: From device to SCHC gateway . . 10
5.6.3. Downlink fragmentation: From SCHC gateway to a device 13
6. Security considerations . . . . . . . . . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 18
A.1. Uplink - Compression example - No fragmentation . . . . . 18
A.2. Uplink - Compression and fragmentation example . . . . . 19
A.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix B. Note . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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
[RFC8376]. Even though those technologies share a great number of
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common features like star-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
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 [I-D.ietf-lpwan-ipv6-static-context-hc].
o DevEUI: an IEEE EUI-64 identifier used to identify the end-device
during the procedure while joining the network (Join Procedure)
o DevAddr: a 32-bit non-unique identifier assigned to an end-device
statically or dynamically after a Join Procedure (depending on the
activation mode)
o RCS: Reassembly Check Sequence. Used to verify the integrity of
the fragmentation-reassembly process
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, 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 context MUST be stored in both ends,
and it can either be learned by a provisioning protocol or by out-of-
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band means or it can be pre-provisioned, etc. The way the context is
learned on both sides is outside the scope of this document.
Dev 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 |
+----+ +----+
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 flow
might be compressed by an Static Context Header Compression
Compressor/Decompressor (SCHC C/D) to reduce headers size and
fragmented (SCHC F/R). The resulting information is sent on a layer
two (L2) frame to an LPWAN Radio Gateway (RGW) which forwards the
frame to a Network Gateway (NGW). The NGW sends the data to a SCHC
F/R for defragmentation, if required, then C/D for decompression
which shares the same rules with the device. The SCHC F/R and 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 F/R,
then SCHC F/R and 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 F/R and 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 is an Application Server. It can be
provided by the Network Server or any third party software. Figure 1
can be mapped in LoRaWAN terminology to:
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Dev 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 |
+-----------+
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
[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 (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 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.
In that case, the application server will be the SCHC gateway, doing
C/D and F/R.
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() () () | +------+
() () () () / \ +---------+ | Join |
() () () () () / \======| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |Application|
() () () () / \==========| v |=============| Server |
() () () / \ +---------+ +-----------+
End-Devices Gateways Network Server
Figure 3: LPWAN Architecture
SCHC C/D (Compressor/Decompressor) and SCHC F/R (Fragmentation/
Reassembly) are performed on the LoRaWAN End-Device and the
Application Server (called SCHC gateway). 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 (SCHC gateway) acts as the first hop IP
router for the End-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. End-Device classes (A, B, C) and interactions
The LoRaWAN MAC layer supports 3 classes of end-devices named A, B
and C. All end-devices implement the Class A, some end-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 end-devices. The
end-device is allowed to transmit at any time, randomly selecting
a communication channel. The network may reply with a downlink in
one of the 2 receive windows immediately following the uplinks.
Therefore, the network cannot initiate a downlink, it has to wait
for the next uplink from the end-device to get a downlink
opportunity. The Class A is the lowest power end-device class.
o Class B: Class B end-devices implement all the functionalities of
Class A end-devices, but also schedule periodic listen windows.
Therefore, opposed to the Class A end-devices, Class B end-devices
can receive downlinks that are initiated by the network 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 end-device. The lower the downlink latency,
the higher the power consumption.
o Class C: Class C end-devices implement all the functionalities of
Class A end-devices, but keep their receiver open whenever they
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are not transmitting. Class C end-devices can receive downlinks
at any time at the expense of a higher power consumption.
Battery-powered end-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 end-devices are grid powered (for
example Smart Plugs).
4.2. End-Device addressing
LoRaWAN end-devices use a 32-bit network address (devAddr) to
communicate with the network over-the-air. However, that address
might be reused several times on the same network at the same time
for different end-devices. End-devices using the same devAddr are
distinguished by the Network Server based on the cryptographic
signature appended to every single LoRaWAN MAC frame, as all end-
devices use different security keys. To communicate with the SCHC
gateway the Network Server MUST identify the end-devices by a unique
64-bit device identifier called the devEUI. Unlike devAddr, devEUI
is guaranteed to be unique for every single end-device across all
networks. The devEUI is assigned to the end-device during the
manufacturing process by the end-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 Server
translates the devAddr into a devEUI in the uplink direction and
reciprocally on the downlink direction.
+--------+ +----------+ +---------+ +----------+
| End- | <=====> | Network | <====> | SCHC | <========> | Internet |
| Device | devAddr | Server | devEUI | Gateway | IPv6/UDP | |
+--------+ +----------+ +---------+ +----------+
Figure 4: LoRaWAN addresses
4.3. General Message Types
o Confirmed messages: The sender asks the receiver to acknowledge
the message.
o Unconfirmed messages: The sender does not ask the receiver to
acknowledge the message.
As SCHC defines its own acknowledgment mechanisms, SCHC does not
require to use confirmed messages.
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4.4. LoRaWAN MAC Frames
o JoinRequest: This message is used by an end-device to join a
network. It contains the end-device's unique identifier devEUI
and a random nonce that will be used for session key derivation.
o JoinAccept: To on-board an end-device, the Network Server responds
to the JoinRequest end-device's message with a JoinAccept message.
That message is encrypted with the end-device's AppKey and
contains (amongst other fields) the major network's settings and a
network random nonce used to derive the session keys.
o Data
5. SCHC-over-LoRaWAN
5.1. LoRaWAN FPort
The LoRaWAN MAC layer features a frame port field in all frames.
This field (FPort) is 8-bit long and the values from 1 to 223 can be
used. It allows LoRaWAN networks and applications to identify data.
The FPort field is part of the SCHC Packet or the SCHC Fragment, as
shown in Figure 5. The SCHC C/D and the SCHC F/R SHALL concatenate
the FPort field with the LoRaWAN payload to retrieve their payload as
it is used as a part of the ruleId field.
| FPort | LoRaWAN payload |
+ ------------------------ +
| SCHC payload |
Figure 5: SCHC payload in LoRaWAN
A fragmentation session with application payload transferred from
device to server, is called uplink fragmentation session. It uses an
FPort for data uplink and its associated SCHC control downlinks,
named FPortUp in this document. The other way, a fragmentation
session with application payload transferred from server to device,
is called downlink fragmentation session. It uses another FPort for
data downlink and its associated SCHC control uplinks, named
FPortDown in this document.
FPorts can use arbitrary values inside the allowed FPort range and
MUST be shared by the end-device, the Network Server and SCHC gateway
prior to the communication. The uplink and downlink fragmentation
FPorts MUST be different.
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5.2. Rule ID management
RuleID minimum length MUST be 8 bits, and RECOMMENDED length is 8
bits. RuleID MSB is 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
MAY reserve some FPort values for other needs as long as they don't
conflict with FPorts used for SCHC C/D and SCHC F/R.
A RuleID SHOULD be reserved to tag packets for which SCHC compression
was not possible (no matching Rule was found). RuleIDs FPortUp and
FPortDown are reserved for fragmentation, in order to improve
interoperability RECOMMENDED 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
The remaining RuleIDs are available for compression. RuleIDs are
shared between uplink and downlink sessions. A RuleID different from
FPortUp or FPortDown means that the fragmentation is not used, thus
the packet SHOULD be sent to C/D layer.
The only uplink messages using the FPortDown port are the
fragmentation SCHC control messages of a downlink fragmentation
session (ex ACKs). Similarly, the only downlink messages using the
FPortUp port are the fragmentation SCHC control messages of an uplink
fragmentation session.
An application can have multiple fragmentation sessions 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 mechanism for sharing those RuleID values is outside the scope of
this document.
5.3. IID computation
As LoRaWAN network uses unique EUI-64 per end-device, the Interface
IDentifier is the LoRaWAN DevEUI. It is compliant with [RFC4291] and
IID starting with binary 000 must enforce the 64-bit rule.
TODO: Derive IID from DevEUI with privacy constraints ? Ask working
group ?
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5.4. Padding
All padding bits MUST be 0.
5.5. Compression
SCHC C/D MUST concatenate FPort and LoRaWAN payload to retrieve the
SCHC packet as per Section 5.1.
SCHC C/D RuleID size SHOULD be 8 bits to fit the LoRaWAN FPort field.
RuleIDs matching FPortUp and FPortDown are reserved for SCHC
Fragmentation.
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 for uplink
fragmentation and Ack-Always for downlink fragmentation. A LoRaWAN
end-device cannot support simultaneous interleaved fragmentation
sessions in the same direction (uplink or downlink). This means that
only a single fragmented IPv6 datagram may be transmitted and/or
received by the end-device at a given moment.
The fragmentation parameters are different for uplink and downlink
fragmentation sessions and are successively described in the next
sections.
5.6.1. DTag
A LoRaWAN 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 fragmentation
sessions with one or more SCHC gateway(s) thanks to distinct sets of
FPorts, cf Section 5.2
5.6.2. Uplink fragmentation: From device to SCHC gateway
In that case the device is the fragmentation transmitter, and the
SCHC gateway the fragmentation receiver. A single fragmentation rule
is defined. SCHC F/R MUST concatenate FPort and LoRaWAN payload to
retrieve the SCHC fragment as per Section 5.1.
o Minimum SCHC header is two bytes (the FPort byte + 1 additional
byte) and the RECOMMENDED header size is two bytes.
o RuleID: Recommended size is 8 bits in SCHC header.
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o SCHC fragmentation reliability mode: "ACK-on-Error"
o DTag: Size is 0 bit, not used
o FCN: The FCN field is encoded on N = 6 bits, so WINDOW_SIZE = 64
tiles are allowed in a window
o Window index: encoded on W = 2 bits. So 4 windows are available.
o RCS calculation algorithm: CRC32 using 0xEDB88320 (i.e. the
reverse representation of the polynomial used e.g. in the Ethernet
standard [RFC3385]) as suggested in
[I-D.ietf-lpwan-ipv6-static-context-hc].
o MAX_ACK_REQUESTS: 8
o Tile: size is 5 bytes
o Retransmission and inactivity timers: LoRaWAN end-devices do not
implement a "retransmission timer". At the end of a window or a
fragmentation session, corresponding ACK(s) is (are) transmitted
by the network gateway (LoRaWAN application server) in the RX1 or
RX2 receive slot of end-device. If this ACK is not received by
the end-device at the end of its RX windows, it sends an all-0 (or
an all-1) fragment with no payload to request an SCHC 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 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 Last tile: The last tile can be carried in the All-1 fragment.
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 * 64 tiles * 5 bytes
per tile = 1280 bytes_
5.6.2.1. Regular fragments
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| FPort | LoRaWAN payload |
+ ------ + ------------------------- +
| RuleID | W | FCN | Payload |
+ ------ + ------ + ------ + ------- +
| 8 bits | 2 bits | 6 bits | |
Figure 6: 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 | Payload |
+ ------ + ------ + --------- + ------- + ----------------- +
| 8 bits | 2 bits | 6 bits | 32 bits | Last tile, if any |
Figure 7: All-1 fragment detailed format for the last fragment.
5.6.2.3. SCHC ACK
| FPort | LoRaWAN payload |
+ ------ + ----------------------------------------- +
| RuleID | W | C | Encoded bitmap (if C = 0) |
+ ------ + ----- + ----- + ------------------------- +
| 8 bits | 2 bit | 1 bit | 0 to 127 bits |
Figure 8: SCHC formats, failed RCS check.
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 9: Receiver-Abort format.
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5.6.2.5. SCHC acknowledge request
| FPort | LoRaWAN payload |
+------- +------------------------- +
| RuleID | W | FCN = b'000000 |
+ ------ + ------ + --------------- +
| 8 bits | 2 bits | 6 bits |
Figure 10: SCHC ACK REQ format.
5.6.3. Downlink fragmentation: From SCHC gateway to a 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. SCHC F/R MUST concatenate FPort and LoRaWAN
payload to retrieve the SCHC fragment as described in Section 5.1.
o SCHC fragmentation reliability mode: ACK-Always.
o RuleID: Recommended size is 8 bits in SCHC header.
o Window index: encoded on W=1 bit, as per
[I-D.ietf-lpwan-ipv6-static-context-hc].
o DTag: Size is 0 bit, not used
o FCN: The FCN field is encoded on N=1 bit, so WINDOW_SIZE = 1 tile
(FCN=All-1 is reserved for SCHC).
o RCS calculation algorithm: CRC32 using 0xEDB88320 (i.e. the
reverse representation of the polynomial used e.g. in the Ethernet
standard [RFC3385]), as per
[I-D.ietf-lpwan-ipv6-static-context-hc].
o MAX_ACK_REQUESTS: 8
As only 1 tile is used, its size can change for each downlink, and
will be maximum available MTU.
_Note_: The Fpending bit included in LoRaWAN protocol SHOULD NOT be
used for SCHC-over-LoRaWAN protocol. It might be set by the Network
Server for other purposes but not SCHC needs.
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5.6.3.1. Regular fragments
| FPort | LoRaWAN payload |
+ ------ + ----------------------------------- +
| RuleID | W | FCN = b'0 | Payload |
+ ------ + ----- + --------- + --------------- +
| 8 bits | 1 bit | 1 bit | X bytes |
Figure 11: 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 |
+ ------ + ----- + --------- + ------- + ----------------- +
| 8 bits | 1 bit | 1 bit | 32 bits | Last tile, if any |
Figure 12: All-1 SCHC ACK detailed format for the last fragment.
5.6.3.3. SCHC acknowledge
| FPort | LoRaWAN payload |
+ ------ + ---------------------------------- +
| RuleID | W | C = b'1 | Padding b'000000 |
+ ------ + ----- + ------- + ---------------- +
| 8 bits | 1 bit | 1 bit | 6 bits |
Figure 13: SCHC ACK format, RCS is correct.
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 14: Receiver-Abort packet (following an all-1 packet with
incorrect RCS).
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Class A and Class B or Class C end-devices do not manage
retransmissions and timers in the same way.
5.6.3.5. Class A end-devices
Class A end-devices can only receive in an RX slot following the
transmission of an uplink. Therefore there cannot be a concept of
"retransmission timer" for an SCHC gateway. The SCHC gateway cannot
initiate communication to a Class A end-device.
The device replies with an ACK message to every single fragment
received from the SCHC 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, but the end of a window),
the device MUST transmit the SCHC ACK fragment until it receives the
fragment of the next window. The device SHALL transmit up to
MAX_ACK_REQUESTS ACK messages before aborting. The device should
transmit those ACK as soon as possible while taking into
consideration potential 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 an FCN=All-1 fragment (the last fragment
of a datagram) and if the RCS is correct, the device SHALL transmit
the ACK with the "RCS is correct" indicator bit set (C=1). This
message might be lost therefore the SCHC 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, on SCHC
FPortDown from the SCHC gateway different from an ACK-request: it
indicates that the SCHC gateway has received the ACK message.
Following the reception of a FCN=All-1 fragment (the last fragment of
a datagram), if all fragments have been received and the RCS 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, on SCHC FPortDown, from the SCHC gateway different from an
ACK-request indicating that the SCHC gateway has received the Abort
message. The fragmentation receiver (device) does not implement
retransmission timer and inactivity timer.
The fragmentation sender (the SCHC gateway) implements an inactivity
timer with a default duration of 12 hours. Once a fragmentation
session is started, if the SCHC gateway has not received any ACK or
Receiver-Abort message 12 hours after the last message from the
device was received, the SCHC 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.
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5.6.3.6. Class B or Class C end-devices
Class B and Class C end-devices can receive in scheduled RX slots or
in RX slots following the transmission of an uplink. The device
replies with an ACK message to every single fragment received from
the SCHC gateway (because the window size is 1). Following the
reception of an FCN=0 fragment (fragment that is not the last
fragment of the packet or ACK-request), the device MUST always
transmit the corresponding SCHC ACK message even if that fragment has
already been received. The ACK bitmap is 1 bit long and is always 1.
If the SCHC gateway receives this ACK, it proceeds to send the next
window fragment. If the retransmission timer elapses and the SCHC
gateway has not received the ACK of the current window it retransmits
the last fragment. The SCHC gateway tries retransmitting up to
MAX_ACK_REQUESTS times before aborting.
Following the reception of an FCN=All-1 fragment (the last fragment
of a datagram) and if the RCS is correct, the device SHALL transmit
the ACK with the "RCS is correct" indicator bit set. If the SCHC
gateway receives this ACK, the current fragmentation session has
succeeded and its context can be cleared.
If the retransmission timer elapses and the SCHC gateway has not
received the SCHC ACK it retransmits the last fragment with the
payload (not an ACK-request without payload). The SCHC gateway tries
retransmitting up to MAX_ACK_REQUESTS times before aborting.
Following the reception of an FCN=All-1 fragment (the last fragment
of a datagram), if all fragments have been received and if the RCS is
NOT correct, the device SHALL transmit a Receiver-Abort fragment.
The retransmission timer is used by the SCHC gateway (the sender),
the optimal value is very much application specific but here are some
recommended default values. For Class B end-devices, this timer
trigger is a function of the periodicity of the Class B ping slots.
The RECOMMENDED value is equal to 3 times the Class B ping slot
periodicity. For Class C end-devices which are nearly constantly
receiving, the RECOMMENDED value is 30 seconds. This means that the
end-device shall try to transmit the ACK within 30 seconds of the
reception of each fragment. The inactivity timer is implemented by
the end-device to flush the context in-case it receives nothing from
the SCHC gateway over an extended period of time. The RECOMMENDED
value is 12 hours for both Class B and Class C end-devices.
6. Security considerations
This document is only providing parameters that are expected to be
better suited for LoRaWAN networks for
[I-D.ietf-lpwan-ipv6-static-context-hc]. As such, this document does
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not contribute to any new security issues in addition to those
identified in [I-D.ietf-lpwan-ipv6-static-context-hc].
Acknowledgements
Thanks to all those listed in the Contributors section for the
excellent text, insightful discussions, reviews and suggestions.
Contributors
Contributors ordered by family name.
o ins: V. Audebert name: Vincent AUDEBERT org: EDF R&D street: 7 bd
Gaspard Monge city: 91120 PALAISEAU country: FRANCE email:
vincent.audebert@edf.fr
o ins: M. Coracin name: Michael Coracin org: Semtech street: 14
Chemin des Clos city: Meylan country: France email:
mcoracin@semtech.com
o ins: M. Le Gourrierec name: Marc Le Gourrierec org: SagemCom
street: 250 Route de l'Empereur city: 92500 Rueil Malmaison
country: FRANCE email: marc.legourrierec@sagemcom.com
o ins: N. Sornin name: Nicolas Sornin org: Semtech street: 14
Chemin des Clos city: Meylan country: France email:
nsornin@semtech.com
o ins: A. Yegin name: Alper Yegin org: Actility street: . city:
Paris, Paris country: France email: alper.yegin@actility.com
9. References
9.1. Normative References
[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[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>.
9.2. Informative References
[I-D.ietf-lpwan-ipv6-static-context-hc]
Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
Zuniga, "Static Context Header Compression (SCHC) and
fragmentation for LPWAN, application to UDP/IPv6", draft-
ietf-lpwan-ipv6-static-context-hc-21 (work in progress),
July 2019.
[lora-alliance-spec]
Alliance, L., "LoRaWAN Specification Version V1.0.3",
<https://lora-alliance.org/sites/default/files/2018-07/
lorawan1.0.3.pdf>.
Appendix A. Examples
A.1. Uplink - Compression example - No fragmentation
Figure 15 is representing an applicative payload going through SCHC,
no fragmentation required
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An applicative payload of 78 bytes is passed to SCHC compression layer
using rule 1, allowing to compress it to 40 bytes and 5 bits: 1 byte
ruleID, 21 bits residue + 37 bytes payload.
| RuleID | Compression residue | Payload | Padding=b'000 |
+ ------ + ------------------- + --------- + ------------- +
| 1 | 21 bits | 38 bytes | 3 bits |
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 15: Uplink example: compression without fragmentation
A.2. Uplink - Compression and fragmentation example
Figure 16 is representing an applicative payload going through SCHC,
with fragmentation.
An applicative payload of 478 bytes is passed to SCHC compression layer
using rule 1, 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 |
The current LoRaWAN MTU is 11 bytes, although 2 bytes FOpts are used by
LoRaWAN protocol: 9 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 (6 bytes) |
+ ------------------------------------- + ------------------------- +
| | FOpts | RuleID=20 | W | FCN | 1 tile |
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+ -------------- + ------- + ---------- + ----- + ------ + -------- +
| XXXX | 2 bytes | 1 byte | 0 0 | 62 | 5 bytes |
Content of the tile is:
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + ----------------- +
| 1 | 21 bits | 1 byte + 3 bits |
Next transmission MTU is 242 bytes, no FOpts. 48 tiles are transmitted:
| LoRaWAN Header | LoRaWAN payload (241 bytes) |
+ -------------- + -----------+ --------------------------- +
| | RuleID=20 | W | FCN | 48 tiles |
+ -------------- + ---------- + ----- + ------ + ---------- +
| XXXX | 1 byte | 0 0 | 61 | 240 bytes |
Next transmission MTU is 242 bytes, no FOpts. All 8 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 (39 bytes) |
+ ---- + -----------+ ----------------------------------------------- +
| | RuleID=20 | W | FCN | 8 tiles | Padding=b'000 |
+ ---- + ---------- + -- + ------ + ----------------- + ------------- +
| XXXX | 1 byte | 00 | 13 | 37 bytes + 5 bits | 3 bits |
All packets have been received by the SCHC gateway, computed RCS is
correct so the following ACK is sent to the device:
| LoRaWAN Header | LoRaWAN payload |
+ -------------- + --------- + ------------------- +
| | RuleID=20 | W | C | Padding |
+ -------------- + --------- + ----- + - + ------- +
| XXXX | 1 byte | 0 0 | 1 | 5 bits |
Figure 16: Uplink example: compression and fragmentation
A.3. Downlink
An applicative payload of 443 bytes is passed to SCHC compression layer
using rule 1, allowing to compress it to 130 bytes and 5 bits: 1 byte
ruleId, 21 bits residue + 127 bytes payload.
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| RuleID | Compression residue | Payload |
+ ------ + ------------------- + --------- +
| 1 | 21 bits | 127 bytes |
The current LoRaWAN MTU is 51 bytes, no FOpts are used by LoRaWAN
protocol: 48 bytes are available for SCHC payload + FPort field => it
has to be fragmented.
| LoRaWAN Header | LoRaWAN payload (51 bytes) |
+ ---- + ---------- + --------------------------------------------- +
| | RuleID=21 | W | FCN | 1 tile | Padding=b'000000 |
+ ---- + ---------- + --- + --- + -------------- + ---------------- +
| XXXX | 1 byte | 0 | 0 | 50 bytes | 6 bits |
Content of the tile is:
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + ------------------ +
| 1 | 21 bits | 46 bytes + 3 bits |
The receiver answers with an SCHC ACK
| FPortDown | LoRaWAN payload |
+ --------- + ---------------------------------- +
| RuleID | W = 0 | C = b'1 | Padding=b'000000 |
+ --------- + ----- + ------- + ---------------- +
| 1 byte | 1 bit | 1 bit | 6 bits |
The second downlink is sent, two FOpts:
| LoRaWAN Header | LoRaWAN payload (49 bytes) |
+ --------------------------- + ------------------ + ---------------- +
| | FOpts | RuleID=21 | W | FCN | 1 tile | Padding=b'000000 |
+ ---- + ------- + ---------- + - + --- + -------- + ---------------- +
| XXXX | 2 bytes | 1 byte | 1 | 0 | 48 bytes | 6 bits |
The receiver answers with an SCHC ACK
| FPortDown | LoRaWAN payload |
+ --------- + ---------------------------------- +
| RuleID | W = 1 | C = b'1 | Padding=b'000000 |
+ --------- + ----- + ------- + ---------------- +
| 1 byte | 1 bit | 1 bit | 6 bits |
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The last downlink is sent, no FOpts:
| LoRaWAN Header | LoRaWAN payload (33 bytes) |
+ ---- + ---------- + ----------------------------------------------- +
| | RuleID=21 | W | FCN | 1 tile | Padding=b'0 |
+ ---- + ---------- + --- + --- + --------------------- + ----------- +
| XXXX | 1 byte | 0 | 1 | 32 bytes + 5 bits | 1 bit |
The receiver answers with an SCHC ACK
| FPortDown | LoRaWAN payload |
+ --------- + ---------------------------------- +
| RuleID | W = 0 | C = b'1 | Padding=b'000000 |
+ --------- + ----- + ------- + ---------------- +
| 1 byte | 1 bit | 1 bit | 6 bits |
Figure 17: Downlink example: compression and fragmentation
Appendix B. Note
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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Julien Catalano
Kerlink
1 rue Jacqueline Auriol
35235 Thorigne-Fouillard
France
Email: j.catalano@kerlink.fr
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