Static Context Header Compression (SCHC) over LoRaWAN
draft-ietf-lpwan-schc-over-lorawan-06
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| Authors | Olivier Gimenez , Ivaylo Petrov | ||
| Last updated | 2020-03-30 | ||
| Replaces | draft-petrov-lpwan-ipv6-schc-over-lorawan | ||
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draft-ietf-lpwan-schc-over-lorawan-06
lpwan Working Group O. Gimenez, Ed.
Internet-Draft Semtech
Intended status: Informational I. Petrov, Ed.
Expires: October 2, 2020 Acklio
March 31, 2020
Static Context Header Compression (SCHC) over LoRaWAN
draft-ietf-lpwan-schc-over-lorawan-06
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 2, 2020.
Copyright Notice
Copyright (c) 2020 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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
4.5. Unicast and multicast technology . . . . . . . . . . . . 8
5. SCHC-over-LoRaWAN . . . . . . . . . . . . . . . . . . . . . . 8
5.1. LoRaWAN FPort . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Rule ID management . . . . . . . . . . . . . . . . . . . 9
5.3. IID computation . . . . . . . . . . . . . . . . . . . . . 10
5.4. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.5. Decompression . . . . . . . . . . . . . . . . . . . . . . 11
5.6. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 11
5.6.1. DTag . . . . . . . . . . . . . . . . . . . . . . . . 11
5.6.2. Uplink fragmentation: From device to SCHC gateway . . 12
5.6.3. Downlink fragmentation: From SCHC gateway to a device 15
6. Security considerations . . . . . . . . . . . . . . . . . . . 18
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 18
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 21
A.1. Uplink - Compression example - No fragmentation . . . . . 21
A.2. Uplink - Compression and fragmentation example . . . . . 22
A.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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
common features like star-oriented topologies, network architecture,
devices with mostly quite predictable communications, etc; they do
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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.
o OUI: Organisation Unique Identifier. IEEE assigned prefix for EUI
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].
It defines:
1. Compression mechanisms to avoid transport of known data by both
sender and receiver over the air. Known data are part of the
"context"
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2. Fragmentation mechanisms to allow SCHC Packet transportation on
small, and potentially variable, MTU
Context exchange or pre-provisioning is out of 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 flows
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) that 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 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, this address might not be
unique in a LoRaWAN network; end-devices using the same devAddr are
distinguished by the Network Server based on the cryptographic
signature appended to every LoRaWAN frame.
To communicate with the SCHC gateway the Network Server MUST identify
the end-devices by a unique 64-bit device identifier called the
devEUI.
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 issued by 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: MAC and application data. Application data are protected
with AES-128 encryption, MAC related data are AES-128 encrypted
with another key.
4.5. Unicast and multicast technology
LoRaWAN technology supports unicast downlinks, but also multicast: a
packet send over LoRaWAN radio link can be received by several
devices. It is useful to address many end-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.
_Note 1_: IPv6 multicast addresses must be defined as per [RFC4291].
LoRaWAN multicast group definition in a network server 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 RECOMMENDED way to setup multicast groups on devices and create a
synchronized reception window.
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 bits 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 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 retrieve their payload as it is used as a part
of the RuleID field.
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| FPort | LoRaWAN payload |
+ ------------------------ +
| SCHC payload |
Figure 5: SCHC Message 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.
All ruleId can use arbitrary values inside the FPort range allowed by
LoRaWAN specification and MUST be shared by the end-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.
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
(no matching rule was found)
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 C/D
layer.
The only uplink messages using the FPortDown port are the
fragmentation SCHC control messages of a downlink fragmentation
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session (for example, SCHC 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
In order to mitigate risks described in [RFC8064] and [RFC8065] IID
MUST be created regarding the following algorithm:
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 procotol.
As AppSKey is renewed each time a device joins or rejoins a 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
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1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
2. cmac: 0x4E822D9775B2649928F82066AF804FEC
3. IID: 0x28F82066AF804FEC
Figure 6: Example of IID computation.
There is a small probability of IID collision in a network, if such
event occurs the IID can be changed by rekeying the device on 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.
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.
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 end-device cannot support simultaneous interleaved
fragmentation sessions in the same direction (uplink or downlink).
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
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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 Packet, as per Section 5.1.
o SCHC header size is two bytes (the FPort byte + 1 additional
byte).
o RuleID: 8 bits stored in LoRaWAN FPort.
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 = 63
tiles are allowed in a window
o Window index: encoded on W = 2 bits. So 4 windows are available.
o RCS: Use recommended calculation algorithm in
[I-D.ietf-lpwan-ipv6-static-context-hc].
o MAX_ACK_REQUESTS: 8
o Tile: size is 10 bytes
o Retransmission timer: LoRaWAN end-devices MUST NOT implement a
"retransmission timer", this changes the specification of
[I-D.ietf-lpwan-ipv6-static-context-hc], see Section 5.6.3.5. It
must transmit MAX_ACK_REQUESTS time the SCHC ACK REQ at it own
timing; ie the periodicity between retransmission of SCHC ACK REQs
is device specific and can vary depending on other application
uplinks and regulations.
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 Penultimate tile MUST be equal to the regular size.
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.
* If last tile is in All-1 message: current L2 MTU MUST be big
enough to fit the All-1 and the last tile.
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_
_Note_: As LoRaWAN is a radio communication, it is RECOMMENDED for an
implementation to use ACK mechanism at the end of each window:
o SCHC receiver sends an SCHC ACK after every window even if there
is no missing tiles.
o SCHC sender waits for the SCHC ACK from the SCHC receiver before
sending tiles from next window. If the SCHC ACK is not received,
it should send an SCHC ACK REQ up to MAX_ACK_REQUESTS time as
described previously.
This OPTIONAL feature allows the implementation to select between: *
SCHC ACK after every window: Save battery life by preventing a device
to transmit full payload if the network cannot be reached *
Otherwise: Reduce downlink load on the network by reducing the number
of downlinks
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)
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| FPort | LoRaWAN payload |
+ ------ + ---------------------------- +
| RuleID | W | FCN=All-1 | RCS |
+ ------ + ------ + --------- + ------- +
| 8 bits | 2 bits | 6 bits | 32 bits |
Figure 8: 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 63 bits |
Figure 9: SCHC ACK format, 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 10: 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 11: SCHC ACK REQ format.
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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 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.
o Window index (unicast only): 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: Use recommended calculation algorithm in
[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.
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 12: All fragments but the last one. Header size 10 bits,
including LoraWAN FPort.
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5.6.3.2. Last fragment (All-1)
| FPort | LoRaWAN payload |
+ ------ + --------------------------- +
| RuleID | W | FCN = b'1 | RCS |
+ ------ + ----- + --------- + ------- +
| 8 bits | 1 bit | 1 bit | 32 bits |
Figure 13: All-1 SCHC Message: 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 14: 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 15: Receiver-Abort packet (following an All-1 SCHC Fragment
with incorrect RCS).
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).
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Following the reception of a FCN=0 fragment (fragment that is not the
last fragment of the packet or SCHC ACK REQ, 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 SCHC ACK REQ: it
indicates that the SCHC gateway has received the ACK message.
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.
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 SCHC ACK REQ), 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
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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 SCHC ACK REQ 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]. IID security is discussed
in Section 5.3. As such, this document does not contribute to any
new security issues in addition to those identified in
[I-D.ietf-lpwan-ipv6-static-context-hc]. Moreover SCHC data (LoRaWAN
payload) are protected on LoRaWAN level by an AES-128 encryption with
key shared by device and SCHC gateway. Those keys are renew each
LoRaWAN session (ie: each join or rejoin to the network)
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.
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Contributors
Contributors ordered by family name.
Vincent Audebert
EDF R&D
Email: vincent.audebert@edf.fr
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
9. References
9.1. Normative 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-24 (work in progress),
December 2019.
[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>.
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[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>.
[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>.
[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>.
[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>.
[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>.
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9.2. 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>.
[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
This example represents an applicative payload going through SCHC
over LoRaWAN, no fragmentation required
An applicative payload of 78 bytes is passed to SCHC compression
layer. Rule 1 is used by C/D layer, 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 | 37 bytes | 3 bits |
Figure 16: 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 17: Uplink example: LoRaWAN packet
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A.2. Uplink - Compression and fragmentation example
This example represents an applicative payload going through SCHC,
with fragmentation.
An applicative payload of 478 bytes is passed to SCHC compression
layer. Rule 1 is used by 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 18: 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 19: Uplink example: LoRaWAN packet 1
Content of the tile is:
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + ----------------- +
| 1 | 21 bits | 6 byte + 3 bits |
Figure 20: 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:
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| LoRaWAN Header | LoRaWAN payload (231 bytes) |
+ --------------------------------------+ --------------------------- +
| | FOpts | RuleID=20 | W | FCN | 23 tiles |
+ -------------- + ------- + ---------- + ----- + ----- + ----------- +
| XXXX | 4 bytes | 1 byte | 0 0 | 61 | 230 bytes |
Figure 21: 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 22: 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 23: Uplink example: LoRaWAN packet 4 - All-1 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:
| LoRaWAN Header | LoRaWAN payload |
+ -------------- + --------- + ------------------- +
| | RuleID=20 | W | C | Padding |
+ -------------- + --------- + ----- + - + ------- +
| XXXX | 1 byte | 0 0 | 1 | 5 bits |
Figure 24: Uplink example: LoRaWAN packet 5 - SCHC ACK
A.3. Downlink
An applicative payload of 443 bytes is passed to SCHC compression
layer. Rule 1 is used by C/D layer, 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 |
Figure 25: 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 26: 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 27: Downlink example: LoRaWAN packet 1: Tile content
The receiver answers with a 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 28: 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 29: Downlink example: LoRaWAN packet 3 - SCHC Fragment 2
The receiver answers with an SCHC ACK:
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| LoRaWAN Header | LoRaWAN payload |
+ ---- + --------- + -------------------------------- +
| | RuleID=21 | W = 1 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 6 bits |
Figure 30: Downlink example: LoRaWAN packet 4 - SCHC ACK
The last downlink is sent, no FOpts:
| LoRaWAN Header | LoRaWAN payload (37 bytes) |
+ ---- + --------- + ----------------------------------------------------------------- +
| | RuleID=21 | W = 0 | FCN = 1 | RCS | 1 tile | Padding=b'00000 |
+ ---- + --------- + ------- + ------- + ------- + ----------------- + --------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 4 bytes | 31 bytes + 1 bits | 5 bits |
Figure 31: Uplink example: LoRaWAN packet 5 - All-1 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 32: Uplink example: LoRaWAN packet 6 - SCHC ACK
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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