Internet Engineering Task Force (IETF) O. Gimenez, Ed.

Internet Engineering Task Force (IETF) O. Gimenez, Ed.
Request for Comments: 9011 Semtech
Category: Standards Track I. Petrov, Ed.
ISSN: 2070-1721 Acklio
April 2021

Static Context Header Compression and Fragmentation (SCHC) over LoRaWAN

Abstract

The 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.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9011.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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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. Terminology
3. SCHC Overview
4. LoRaWAN Architecture
4.1. Device Classes (A, B, C) and Interactions
4.2. Device Addressing
4.3. General Frame Types
4.4. LoRaWAN MAC Frames
4.5. LoRaWAN FPort
4.6. LoRaWAN Empty Frame
4.7. Unicast and Multicast Technology
5. SCHC over LoRaWAN
5.1. LoRaWAN FPort and RuleID
5.2. RuleID Management
5.3. Interface IDentifier (IID) Computation
5.4. Padding
5.5. Decompression
5.6. Fragmentation
5.6.1. DTag
5.6.2. Uplink Fragmentation: From Device to SCHC Gateway
5.6.3. Downlink Fragmentation: From SCHC Gateway to Device
5.7. SCHC Fragment Format
5.7.1. All-0 SCHC Fragment
5.7.2. All-1 SCHC Fragment
5.7.3. Delay after Each LoRaWAN Frame to Respect Local
Regulation
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Examples
A.1. Uplink - Compression Example - No Fragmentation
A.2. Uplink - Compression and Fragmentation Example
A.3. Downlink
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

The SCHC specification [RFC8724] describes generic header compression
and fragmentation techniques that can be used on all Low-Power Wide
Area Network (LPWAN) technologies defined in [RFC8376]. 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 [LORAWAN-SPEC].

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

This section defines the terminology and abbreviations used in this
document. For all other definitions, please look up the SCHC
specification [RFC8724].

| Note: The SCHC acronym is pronounced like "sheek" in English
| (or "chic" in French). Therefore, this document writes "a SCHC
| Packet" instead of "an SCHC Packet".

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: <https://www.lora-
alliance.org>.

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 Section 8.2.2.1
of [RFC8724].

Uplink: LoRaWAN term for a frame transmitted by the device and
received by the network.

3. SCHC Overview

This section contains a short overview of SCHC. For a detailed
description, refer to the full specification [RFC8724].

It defines:

1. Compression mechanisms to avoid transporting information known by
both sender and receiver over the air. Known information is part
of the "context". This component is called the "SCHC
Compression/Decompression" (SCHC C/D).

2. 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.

Device App
+----------------+ +----+ +----+ +----+
| App1 App2 App3 | |App1| |App2| |App3|
| | | | | | | |
| UDP | |UDP | |UDP | |UDP |
| IPv6 | |IPv6| |IPv6| |IPv6|
| | | | | | | |
|SCHC C/D and F/R| | | | | | |
+--------+-------+ +----+ +----+ +----+
| +---+ +----+ +----+ +----+ . . .
+~ |RGW| === |NGW | == |SCHC| == |SCHC|...... Internet ....
+---+ +----+ |F/R | |C/D |
+----+ +----+
|<- - - - LoRaWAN - - ->|

Figure 1: Architecture

Figure 1 represents the architecture for compression/decompression;
it is based on the terminology from [RFC8376]. 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. Figure 1 can be mapped in LoRaWAN
terminology to:

End Device App
+--------------+ +----+ +----+ +----+
|App1 App2 App3| |App1| |App2| |App3|
| | | | | | | |
| UDP | |UDP | |UDP | |UDP |
| IPv6 | |IPv6| |IPv6| |IPv6|
| | | | | | | |
|SCHC C/D & F/R| | | | | | |
+-------+------+ +----+ +----+ +----+
| +-------+ +-------+ +-----------+ . . .
+~ |Gateway| == |Network| == |Application|..... Internet ....
+-------+ |server | |server |
+-------+ | F/R - C/D |
+-----------+
|<- - - - - LoRaWAN - - - ->|

Figure 2: SCHC Architecture Mapped to LoRaWAN

4. LoRaWAN Architecture

An overview of the LoRaWAN protocol and architecture [LORAWAN-SPEC]
is described in [RFC8376]. The mapping between the LPWAN
architecture entities as described in [RFC8724] and the ones in
[LORAWAN-SPEC] 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.

(LPWAN-AAA Server)
() () () | +------+
() () () () / \ +---------+ | Join |
() () () () () / \======| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |Application|
() () () () / \==========| v |=============| Server |
() () () / \ +---------+ +-----------+
End devices Gateways Network Server (SCHC C/D and F/R)
(devices) (RGW) (NGW)

Figure 3: LPWAN Architecture

| Note: Figure 3 terms are from LoRaWAN, with [RFC8376]
| terminology in brackets.

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.

4.1. Device Classes (A, B, C) and Interactions

The 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).

4.2. Device Addressing

LoRaWAN end devices use a 32-bit network address (DevAddr) to
communicate with the Network Gateway over the air; this address might
not be unique in a LoRaWAN network. Devices using the same DevAddr
are distinguished by the Network Gateway based on the cryptographic
signature appended to every LoRaWAN frame.

To communicate with the SCHC gateway, the Network Gateway MUST
identify the devices by a unique 64-bit device identifier called the
"DevEUI".

The DevEUI is assigned to the device during the manufacturing process
by the device's manufacturer. It is built like an Ethernet MAC
address by concatenating the manufacturer's IEEE OUI field with a
vendor unique number. 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.

+--------+ +---------+ +---------+ +----------+
| Device | <=====> | Network | <====> | SCHC | <======> | Internet |
| | DevAddr | Gateway | DevEUI | Gateway | IPv6/UDP | |
+--------+ +---------+ +---------+ +----------+

Figure 4: LoRaWAN Addresses

4.3. General Frame Types

LoRaWAN implements the possibility to send confirmed or unconfirmed
frames:

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-SPEC]).

4.4. LoRaWAN MAC Frames

In addition to regular data frames, LoRaWAN implements JoinRequest
and JoinAccept frame types, which are used by a device to join a
network:

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.

4.5. LoRaWAN FPort

The LoRaWAN MAC layer features a frame port field in all frames.
This field (FPort) is 8 bits long and the values from 1 to 223 can be
used. It allows LoRaWAN networks and applications to identify data.

4.6. LoRaWAN Empty Frame

A LoRaWAN empty frame is a LoRaWAN frame without FPort (cf.
Section 5.1) and FRMPayload.

4.7. Unicast and Multicast Technology

LoRaWAN technology supports unicast downlinks but also multicast; a
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.

| Note 1: IPv6 multicast addresses must be defined as per
| [RFC4291]. The LoRaWAN multicast group definition in a Network
| Gateway and the relation between those groups and IPv6 groupID
| are out of scope of this document.

| Note 2: The LoRa Alliance defined
| [LORAWAN-REMOTE-MULTICAST-SET] as the RECOMMENDED way to set up
| multicast groups on devices and create a synchronized reception
| window.

5. SCHC over LoRaWAN

5.1. LoRaWAN FPort and RuleID

The FPort field is part of the SCHC Message, as shown in Figure 5.
The SCHC C/D and the SCHC F/R SHALL concatenate the FPort field with
the LoRaWAN payload to recompose the SCHC Message.

| FPort | LoRaWAN payload |
+ ------------------------ +
| SCHC Message |

Figure 5: SCHC Message in LoRaWAN

| Note: The SCHC Message is any datagram sent by the SCHC C/D or
| F/R layers.

A fragmented datagram with application payload transferred from
device to Network Gateway is called an "uplink-fragmented datagram".
It uses an FPort for data uplink and its associated SCHC control
downlinks, named "FPortUp" in this document. The other way, a
fragmented datagram with application payload transferred from Network
Gateway to device is called 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 [LORAWAN-SPEC] and MUST be shared by the
device and SCHC gateway prior to the communication with the selected
rule. The uplink and downlink fragmentation FPorts MUST be
different.

5.2. RuleID Management

The RuleID MUST be 8 bits and 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
[LORAWAN-SPEC]; 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
Section 6 of [RFC8724].

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.

5.3. Interface IDentifier (IID) Computation

In order to mitigate the risks described in [RFC8064] and [RFC8065],
implementations MUST implement the following algorithm and SHOULD use
it.

1. key = LoRaWAN AppSKey

2. cmac = aes128_cmac(key, DevEUI)

3. IID = cmac[0..7]

The aes128_cmac algorithm is described in [RFC4493]. 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

1. key: 0x00AABBCCDDEEFF00AABBCCDDEEFFAABB
2. cmac: 0x4E822D9775B2649928F82066AF804FEC
3. IID: 0x4E822D9775B26499

Figure 6: Example of IID Computation

There is a small probability of IID collision in a LoRaWAN network.
If this occurs, the IID can be changed by rekeying the device at 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.

| Note: Implementations also using another IID source MUST ensure
| that the same IID is shared between the device and the SCHC
| gateway in the compression and decompression of the IPv6
| address of the device.

5.4. Padding

All padding bits MUST be 0.

5.5. Decompression

The 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 device cannot support simultaneous interleaved fragmented
datagrams in the same direction (uplink or downlink).

The fragmentation parameters are different for uplink- and downlink-
fragmented datagrams and are successively described in the next
sections.

5.6.1. DTag

Section 8.2.4 of [RFC8724] 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.

| Note: The device can still have several parallel fragmented
| datagrams with more than one SCHC gateway thanks to distinct
| sets of FPorts, cf. Section 5.2.

5.6.2. Uplink Fragmentation: From Device to SCHC Gateway

In this case, the device is the fragment transmitter and the SCHC
gateway 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 Section 5.1.

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. Section 5.2).

DTag: Size T = 0 bits, not used (cf. Section 5.6.1).

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 Section 8.2.3 of
[RFC8724], 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
[RFC8724], for uplink rules, an additional context parameter is
added: whether or not to ack after each window. For battery powered
devices, it is RECOMMENDED to use the ACK mechanism at the end of
each window instead of waiting until the end of all windows:

* 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.

5.6.2.1. Regular Fragments

Figure 7 is an example of a regular fragment for all fragments except
the last one. SCHC Header Size is 16 Bits, including the LoRaWAN
FPort.

| FPort | LoRaWAN payload |
+ ------ + ------------------------- +
| RuleID | W | FCN | Payload |
+ ------ + ------ + ------ + ------- +
| 8 bits | 2 bits | 6 bits | |

Figure 7: All Fragments Except the Last One.

5.6.2.2. Last Fragment (All-1)

Following figures are examples of All-1 messages. Figure 8 is
without the last tile, Figure 9 is with the last tile.

| FPort | LoRaWAN payload |
+ ------ + ---------------------------- +
| RuleID | W | FCN=All-1 | RCS |
+ ------ + ------ + --------- + ------- +
| 8 bits | 2 bits | 6 bits | 32 bits |

Figure 8: All-1 SCHC Message without Last Tile

| FPort | LoRaWAN payload |
+ ------ + ---------------------------------------------------------- +
| RuleID | W | FCN=All-1 | RCS | Last tile | Opt. padding |
+ ------ + ------ + --------- + ------- + ------------ + ------------ +
| 8 bits | 2 bits | 6 bits | 32 bits | 1 to 80 bits | 0 to 7 bits |

Figure 9: All-1 SCHC Message with Last Tile

5.6.2.3. SCHC ACK

| FPort | LoRaWAN payload |
+ ------ + --------------------------+
| RuleID | W | C = 1 | padding |
| | | | (b'00000) |
+ ------ + ----- + ----- + --------- +
| 8 bits | 2 bit | 1 bit | 5 bits |

Figure 10: SCHC ACK Format - Correct RCS Check

| FPort | LoRaWAN payload |
+ ------ + --------------------------------- + ---------------- +
| RuleID | W | C = 0 | Compressed bitmap | Optional padding |
| | | | (C = 0) | (b'0...0) |
+ ------ + ----- + ----- + ----------------- + ---------------- +
| 8 bits | 2 bit | 1 bit | 5 to 63 bits | 0, 6, or 7 bits |

Figure 11: SCHC ACK Format - Incorrect RCS Check

| Note: Because of the bitmap compression mechanism and L2 byte
| alignment, only the following discrete values are possible for
| the compressed bitmap size: 5, 13, 21, 29, 37, 45, 53, 61, 62,
| and 63. Bitmaps of 63 bits will require 6 bits of padding.

5.6.2.4. Receiver-Abort

| FPort | LoRaWAN payload |
+ ------ + -------------------------------------------- +
| RuleID | W = b'11 | C = 1 | b'11111 | 0xFF (all 1's) |
+ ------ + -------- + ------+-------- + ----------------+
| 8 bits | 2 bits | 1 bit | 5 bits | 8 bits |
next L2 Word boundary ->| <-- L2 Word --> |

Figure 12: Receiver-Abort Format

5.6.2.5. SCHC Acknowledge Request

| FPort | LoRaWAN payload |
+------- +------------------------- +
| RuleID | W | FCN = b'000000 |
+ ------ + ------ + --------------- +
| 8 bits | 2 bits | 6 bits |

Figure 13: SCHC ACK REQ Format

5.6.3. Downlink Fragmentation: From SCHC Gateway to Device

In 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
Section 5.1.

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. Section 5.2).

DTag: Size T = 0 bit, not used (cf. Section 5.6.1).

FCN: The FCN field is encoded on N = 1 bit, so WINDOW_SIZE = 1 tile.

RCS: Use the recommended calculation algorithm in Section 8.2.3 of
[RFC8724], 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 Section 5.6.3.5.

MAX_ACK_REQUESTS: 8.

Window index (unicast only): encoded on M = 1 bit, as per [RFC8724].

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 Section 4.6. As this
uplink is sent only to open an RX window, any LoRaWAN uplink frame
from the device MAY reset this counter.

| Note: The FPending bit included in the LoRaWAN protocol SHOULD
| NOT be used for the SCHC-over-LoRaWAN protocol. It might be
| set by the Network Gateway for other purposes but not SCHC
| needs.

5.6.3.1. Regular Fragments

Figure 14 is an example of a regular fragment for all fragments
except the last one. SCHC Header Size is 10 Bits, including the
LoRaWAN FPort.

| FPort | LoRaWAN payload |
+ ------ + ------------------------------------ +
| RuleID | W | FCN = b'0 | Payload |
+ ------ + ----- + --------- + ---------------- +
| 8 bits | 1 bit | 1 bit | X bytes + 6 bits |

Figure 14: All Fragments but the Last One.

5.6.3.2. Last Fragment (All-1)

| FPort | LoRaWAN payload |
+ ------ + --------------------------- + ------------------------- +
| RuleID | W | FCN = b'1 | RCS | Payload | Opt padding |
+ ------ + ----- + --------- + ------- + ----------- + ----------- +
| 8 bits | 1 bit | 1 bit | 32 bits | 6 to X bits | 0 to 7 bits |

Figure 15: All-1 SCHC Message: The Last Fragment

5.6.3.3. SCHC ACK

| FPort | LoRaWAN payload |
+ ------ + ---------------------------------- +
| RuleID | W | C = b'1 | Padding b'000000 |
+ ------ + ----- + ------- + ---------------- +
| 8 bits | 1 bit | 1 bit | 6 bits |

Figure 16: SCHC ACK Format - Correct RCS Check

| FPort | LoRaWAN payload |
+ ------ + ------------------------------------------------- +
| RuleID | W | C = b'0 | Bitmap = b'1 | Padding b'000000 |
+ ------ + ----- + ------- + ------------ + ---------------- +
| 8 bits | 1 bit | 1 bit | 1 bit | 5 bits |

Figure 17: SCHC ACK Format - Incorrect RCS Check

5.6.3.4. Receiver-Abort

Figure 18 is an example of a Receiver-Abort packet, following an
All-1 SCHC Fragment with incorrect RCS.

| FPort | LoRaWAN payload |
+ ------ + ---------------------------------------------- +
| RuleID | W = b'1 | C = b'1 | b'111111 | 0xFF (all 1's) |
+ ------ + ------- + ------- + -------- + --------------- +
| 8 bits | 1 bit | 1 bits | 6 bits | 8 bits |
next L2 Word boundary ->| <-- L2 Word --> |

Figure 18: Receiver-Abort Packet

5.6.3.5. Downlink Retransmission Timer

Class A, Class B, and Class C devices do not manage retransmissions
and timers the same way.

5.6.3.5.1. Class A Devices

Class A devices can only receive in an RX slot following the
transmission of an uplink.

The SCHC gateway implements an inactivity timer with a RECOMMENDED
duration of 36 hours. For devices with very low transmission rates
(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.

| Note: The ACK bitmap is 1 bit long and is always 1.

*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 Section 4.6), but it is application specific and
will be used by the NGW to transmit a potential SCHC ACK REQ.
SCHC_ACK_REQ_DN_OPPORTUNITY is application specific and its
recommended value is 2. It MUST be greater than 1. This allows the
opening of a downlink opportunity to any downlink with higher
priority than the SCHC ACK REQ message.

| Note: The device MUST keep this SCHC ACK message in memory
| until it receives a downlink SCHC Fragmentation Message (with
| FPort == FPortDown) that is not a SCHC ACK REQ; this indicates
| that the SCHC gateway has received the SCHC ACK message.

5.6.3.6. Class B or Class C Devices

Class B devices can receive in scheduled RX slots or in RX slots
following the transmission of an uplink. Class C devices are almost
in constant reception.

RECOMMENDED retransmission timer 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.

5.7. SCHC Fragment Format

5.7.1. All-0 SCHC Fragment

*Uplink Fragmentation (Ack-on-Error)*:

All-0 is distinguishable from a SCHC ACK REQ, as [RFC8724] states
"This condition is also met if the SCHC Fragment Header is a multiple
of L2 Words", the following condition being met: SCHC header is 2
bytes.

*Downlink fragmentation (ACK-Always)*:

As per [RFC8724], 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.

5.7.2. All-1 SCHC Fragment

All-1 is distinguishable from a SCHC Sender-Abort, as [RFC8724]
states "This condition is met if the RCS is present and is at least
the size of an L2 Word", the following condition being met: RCS is 4
bytes.

5.7.3. Delay after Each LoRaWAN Frame to Respect Local Regulation

This profile does not define a delay to be added after each LoRaWAN
frame; local regulation compliance is expected to be enforced by the
LoRaWAN stack.

6. Security Considerations

This document is only providing parameters that are expected to be
best suited for LoRaWAN networks for [RFC8724]. IID security is
discussed in Section 5.3. As such, this document does not contribute
to any new security issues beyond those already identified in
[RFC8724]. Moreover, SCHC data (LoRaWAN payload) are protected at
the LoRaWAN level by an AES-128 encryption with a session key shared
by the device and the SCHC gateway. These session keys are renewed
at each LoRaWAN session (i.e., each join or rejoin to the LoRaWAN
network).

7. IANA Considerations

This document has no IANA actions.

8. References

8.1. Normative References

[LORAWAN-SPEC]
LoRa Alliance, "LoRaWAN 1.0.4 Specification Package",
<https://lora-alliance.org/resource_hub/lorawan-104-
specification-package/>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.

[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, <https://www.rfc-editor.org/info/rfc4493>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zúñiga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.

8.2. Informative References

[LORAWAN-REMOTE-MULTICAST-SET]
LoRa Alliance, "LoRaWAN Remote Multicast Setup
Specification v1.0.0", <https://lora-
alliance.org/resource_hub/lorawan-remote-multicast-setup-
specification-v1-0-0/>.

[RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu,
"Recommendation on Stable IPv6 Interface Identifiers",
RFC 8064, DOI 10.17487/RFC8064, February 2017,
<https://www.rfc-editor.org/info/rfc8064>.

[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <https://www.rfc-editor.org/info/rfc8065>.

[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.

Appendix A. Examples

In the following examples, "applicative data" refers to the IPv6
payload sent by the application to the SCHC layer.

A.1. Uplink - Compression Example - No Fragmentation

This example represents an applicative data going through SCHC over
LoRaWAN; no fragmentation required.

An applicative data of 78 bytes is passed to 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.

| RuleID | Compression residue | Payload | Padding=b'000 |
+ ------ + ------------------- + --------- + ------------- +
| 1 | 21 bits | 37 bytes | 3 bits |

Figure 19: Uplink Example: SCHC Message

The current LoRaWAN MTU is 51 bytes, although 2-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.

| LoRaWAN Header | LoRaWAN payload (40 bytes) |
+ ------------------------- + --------------------------------------- +
| | FOpts | RuleID=1 | Compression | Payload | Padding=b'000 |
| | | | residue | | |
+ ---- + ------- + -------- + ----------- + --------- + ------------- +
| XXXX | 2 bytes | 1 byte | 21 bits | 37 bytes | 3 bits |

Figure 20: Uplink Example: LoRaWAN Packet

A.2. Uplink - Compression and Fragmentation Example

This example represents an applicative data going through SCHC, with
fragmentation.

An applicative data of 300 bytes is passed to 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.

| RuleID | Compression residue | Payload |
+ ------ + ------------------- + --------- +
| 1 | 21 bits | 279 bytes |

Figure 21: Uplink Example: SCHC Message

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.

| LoRaWAN Header | LoRaWAN payload (11 bytes) |
+ -------------------------- + -------------------------- +
| | RuleID=20 | W | FCN | 1 tile |
+ -------------- + --------- + ----- + ------ + --------- +
| XXXX | 1 byte | 0 0 | 62 | 10 bytes |

Figure 22: Uplink Example: LoRaWAN Packet 1

The tile content is described in Figure 23

Content of the tile is:
| RuleID | Compression residue | Payload |
+ ------ + ------------------- + ----------------- +
| 1 | 21 bits | 6 bytes + 3 bits |

Figure 23: Uplink Example: First Tile Content

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:

| LoRaWAN Header | LoRaWAN payload (231 bytes) |
+ --------------------------------------+ --------------------------- +
| | FOpts | RuleID=20 | W | FCN | 23 tiles |
+ -------------- + ------- + ---------- + ----- + ----- + ----------- +
| XXXX | 4 bytes | 1 byte | 0 0 | 61 | 230 bytes |

Figure 24: Uplink Example: LoRaWAN Packet 2

Next transmission MTU is 242 bytes, no FOpts. All 5 remaining tiles
are transmitted, the last tile is only 2 bytes + 5 bits. Padding is
added for the remaining 3 bits.

| LoRaWAN Header | LoRaWAN payload (44 bytes) |
+ ---- + ---------- + ----------------------------------------------- +
| | RuleID=20 | W | FCN | 5 tiles | Padding=b'000 |
+ ---- + ---------- + ----- + ----- + --------------- + ------------- +
| XXXX | 1 byte | 0 0 | 38 | 42 bytes+5 bits | 3 bits |

Figure 25: Uplink Example: LoRaWAN Packet 3

Then All-1 message can be transmitted:

| LoRaWAN Header | LoRaWAN payload (44 bytes) |
+ ---- + -----------+ -------------------------- +
| | RuleID=20 | W | FCN | RCS |
+ ---- + ---------- + ----- + ----- + ---------- +
| XXXX | 1 byte | 0 0 | 63 | 4 bytes |

Figure 26: Uplink Example: LoRaWAN Packet 4 - All-1 SCHC Message

All packets have been received by the SCHC gateway, computed RCS is
correct so the following ACK is sent to the device by the SCHC
receiver:

| LoRaWAN Header | LoRaWAN payload |
+ -------------- + --------- + ------------------- +
| | RuleID=20 | W | C | Padding |
+ -------------- + --------- + ----- + - + ------- +
| XXXX | 1 byte | 0 0 | 1 | 5 bits |

Figure 27: Uplink Example: LoRaWAN Packet 5 - SCHC ACK

A.3. Downlink

An applicative data of 155 bytes is passed to 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.

| RuleID | Compression residue | Payload |
+ ------ + ------------------- + --------- +
| 1 | 21 bits | 127 bytes |

Figure 28: Downlink Example: SCHC Message

The current LoRaWAN MTU is 51 bytes; no FOpts are used by the LoRaWAN
protocol: 51 bytes are available for SCHC payload + FPort field; the
applicative data has to be fragmented.

| LoRaWAN Header | LoRaWAN payload (51 bytes) |
+ ---- + ---------- + -------------------------------------- +
| | RuleID=21 | W = 0 | FCN = 0 | 1 tile |
+ ---- + ---------- + ------ + ------- + ------------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 50 bytes and 6 bits |

Figure 29: Downlink Example: LoRaWAN Packet 1 - SCHC Fragment 1

The tile content is described in Figure 30

| RuleID | Compression residue | Payload |
+ ------ + ------------------- + ------------------ +
| 1 | 21 bits | 48 bytes and 1 bit |

Figure 30: Downlink Example: First 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 31: Downlink Example: LoRaWAN Packet 2 - SCHC ACK

The second downlink is sent, two FOpts:

| LoRaWAN Header | LoRaWAN payload (49 bytes) |
+ --------------------------- + ------------------------------------- +
| | FOpts | RuleID=21 | W = 1 | FCN = 0 | 1 tile |
+ ---- + ------- + ---------- + ----- + ------- + ------------------- +
| XXXX | 2 bytes | 1 byte | 1 bit | 1 bit | 48 bytes and 6 bits |

Figure 32: Downlink Example: LoRaWAN Packet 3 - SCHC Fragment 2

The receiver answers with a SCHC ACK:

| LoRaWAN Header | LoRaWAN payload |
+ ---- + --------- + -------------------------------- +
| | RuleID=21 | W = 1 | C = 1 | Padding=b'000000 |
+ ---- + --------- + ----- + ----- + ---------------- +
| XXXX | 1 byte | 1 bit | 1 bit | 6 bits |

Figure 33: Downlink Example: LoRaWAN Packet 4 - SCHC ACK

The last downlink is sent, no FOpts:

| LoRaWAN Header | LoRaWAN payload (37 bytes) |
+ ---- + ------- + -------------------------------------------------- +
| | RuleID | W | FCN | RCS | 1 tile | Padding |
| | 21 | 0 | 1 | | | b'00000 |
+ ---- + ------- + ----- + ----- + ------- + -------------- + ------- +
| XXXX | 1 byte | 1 bit | 1 bit | 4 bytes | 31 bytes+1 bit | 5 bits |

Figure 34: Downlink Example: LoRaWAN Packet 5 - All-1 SCHC Message

The receiver answers to the sender 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 35: Downlink Example: LoRaWAN Packet 6 - SCHC ACK

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, and
Laurent Toutain for useful design considerations, reviews, and
comments.

LoRaWAN is a registered trademark of the LoRa Alliance.

Contributors

Contributors ordered by family name.

Vincent Audebert
EDF R&D

Email: [email protected]

Julien Catalano
Kerlink

Email: [email protected]

Michael Coracin
Semtech

Email: [email protected]

Marc Le Gourrierec
Sagemcom

Email: [email protected]

Nicolas Sornin
Chirp Foundation

Email: [email protected]

Alper Yegin
Actility

Email: [email protected]

Authors' Addresses

Olivier Gimenez (editor)
Semtech
14 Chemin des Clos
Meylan
France

Email: [email protected]

Ivaylo Petrov (editor)
Acklio
1137A Avenue des Champs Blancs
35510 Cesson-Sévigné Cedex
France

Email: [email protected]