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Versions: (draft-petrov-lpwan-ipv6-schc-over-lorawan) 00 01 02

lpwan Working Group                                      O. Gimenez, Ed.
Internet-Draft                                                   Semtech
Intended status: Informational                            I. Petrov, Ed.
Expires: January 9, 2020                                          Acklio
                                                           July 08, 2019


         Static Context Header Compression (SCHC) over LoRaWAN
                 draft-ietf-lpwan-schc-over-lorawan-02

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, 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
   (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
   5.  SCHC-over-LoRaWAN . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  LoRaWAN FPort . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Rule ID management  . . . . . . . . . . . . . . . . . . .   9
     5.3.  IID computation . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .   9
     5.5.  DTag  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
       5.5.1.  Uplink fragmentation: From device to SCHC gateway . .  10
       5.5.2.  Downlink fragmentation: From SCHC gateway to 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
   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.



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   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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   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 a end-device
   statically or dynamically after a Join Procedure (depending on the
   activation mode)

   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 contexts must be stored in both
   ends, and it can either be learned by a provisioning protocol or by
   out-of-band means or it can be pre-provisioned, etc.  The way the
   context is learned on both sides is out of the scope of this
   document.










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          Dev                                               App
  +----------------+                                +----+ +----+ +----+
  | App1 App2 App3 |                                |App1| |App2| |App3|
  |                |                                |    | |    | |    |
  |       UDP      |                                |UDP | |UDP | |UDP |
  |      IPv6      |                                |IPv6| |IPv6| |IPv6|
  |                |                                |    | |    | |    |
  |SCHC C/D and F/R|                                |    | |    | |    |
  +--------+-------+                                +----+ +----+ +----+
           |  +--+     +----+    +----+    +----+     .      .      .
           +~ |RG| === |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 fragemented (SCHC F/R), and compressed by an Static Context
   Header Compression Compressor/Decompressor (SCHC C/D) to reduce
   headers size.  Resulting information is sent on a layer two (L2)
   frame to a LPWAN Radio Network (RG) 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 map 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 and F/R|                                          |    | |    | |    |
+--------+-------+                                          +----+ +----+ +----+
         |  +-------+     +-------+    +----------------+     .      .      .
         +~ |Gateway| === |Network| == |Application     |...... Internet ....
            +-------+     |server |    |server F/R - C/D|
                          +-------+    +----------------+

                          Figure 2: Architecture

4.  LoRaWAN Architecture

   An overview of LoRaWAN [lora-alliance-spec] protocol and architecture
   is described in [RFC8376].  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 end point 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 classA, some end-devices
   implement classA+B or class A+C.  ClassB and classC are mutually
   exclusive.

   o  *ClassA*: The classA 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 classA is the lowest power end-device class.

   o  *ClassB*: classB end-devices implement all the functionalities of
      classA end-devices, but also schedule periodic listen windows.
      Therefore, as opposed the classA end-devices, classB end-devices
      can receive downlink that are initiated by the network and not
      following an uplink.  There is a trade-off between the periodicity
      of those scheduled classB listen windows and the power consumption
      of the end-device.  The lower the downlink latency, the higher the
      power consumption.

   o  *ClassC*: classC end-devices implement all the functionalities of
      classA end-devices, but keep their receiver open whenever they are



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      not transmitting.  ClassC end-devices can receive downlinks at any
      time at the expense of a higher power consumption.  Battery
      powered end-devices can only operate in classC for a limited
      amount of time (for example for a firmware upgrade over-the-air).
      Most of the classC end-devices are main powered (for example Smart
      Plugs).

4.2.  End-Device addressing

   LoRaWAN end-devices use a 32 bits network address (devAddr) to
   communicate with the network over-the-air.  However, that address
   might be reused several time on the same network at the same time for
   different end-devices.  End-devices using the same devAddr are
   distinguish 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
   64bits device ID 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.  ex: 24bits OUI is
   concatenated with a 40 bits 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 a 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 a 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 layers 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 network and application to identify data.

   A fragmentation session with application payload transferred from
   device to server, is called uplink fragmentation session.  It uses
   FPortUpShort or FPortUpDefault for data uplink and its associated
   SCHC control downlinks.  The other way, a fragmentation session with
   application payload transferred from server to device, is called
   downlink fragmentation session.  It uses FPortDown for data downlink
   and its associated SCHC control uplinks.

   FPorts can use arbitrary values inside the allowed FPort range and
   must be shared by the end-device, the Network Server and SCHC
   gateway.  The uplink and downlink SCHC ports must be different.  In
   order to improve interoperability, it is recommended to use:

   o  FPortUpShort = 20

   o  FPortUpDefault = 21

   o  FPortDown = 22

   Those are recommended values and are application defined.  Also
   application can have multiple fragmentation session between a device
   and one or several SCHC gateways.  A set of three FPort values is
   required for each gateway instance the device is required to
   communicate with.





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   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
   FPortUpShort or FPortUpDefault ports are the fragmentation SCHC
   control messages of an uplink fragmentation session.

5.2.  Rule ID management

   SCHC-over-LoRaWAN SHOULD support encoding RuleID on 6 bits (64
   possible rules).

   The RuleID 0 is reserved for fragmentation.  The RuleID 63 is used to
   tag packets 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 different from
   0 means that the fragmentation is not used, thus the packet should be
   send to C/D layer.

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-bits rule.  TODO:
   Derive IID from DevEUI with privacy constraints ? Ask working group ?

5.4.  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.

5.5.  DTag

   A LoRaWAN device cannot interleave several fragmented SCHC datagrams.
   This one bit field is used to distinguish two consecutive
   fragmentation sessions.





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   _Note_: While it is used to recover faster from transmission errors,
   it SHALL not be considered as the only way to distinguish two
   fragmentation sessions.

5.5.1.  Uplink fragmentation: From device to SCHC gateway

   In that case the device is the fragmentation transmitter, and the
   SCHC gateway the fragmentation receiver.  Two fragmentation rules are
   defined regarding the *FPort*:

   o  *FPortUpShort*: SCHC header is only one byte.  Used when
      fragmentation is required and payload size is less than 381 bytes.

   o  *FPortUpDefault*: SCHC header is two bytes.  Used for all other
      cases: no fragmentation required or payload size is between 382
      and 1524 byte.

   *Both rules share common parameters:*

   o  *SCHC fragmentation reliability mode*: "ACK-on-Error"

   o  *DTag*: size is 1 bit.

   o  *FCN*: The FCN field is encoded on N = 7 bits, so WINDOW_SIZE =
      127 tiles are allowed in a window (FCN=All-1 is reserved for
      SCHC).

   o  *MIC 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 3 bytes (24 bits)

   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 the
      end-device 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




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      mainly driven by application requirements and may be changed by
      the application.

   *The following fields are different:*

   o  RuleID size

   o  Window index size W

5.5.1.1.  FPortUpShort - 1 byte header

   In that case RuleID size is 0, the rule is the FPort=FPortUpShort and
   only fragmented payload can be transported.

   o  *RuleID*: size is 0 bit in SCHC header, not used.

   o  *Window index*: encoded on W = 0 bit, not used

   With this set of parameters, the SCHC fragment header overhead is 1
   byte (8 bits).  MTU is: _127 tiles * 3 bytes per tile = 381 bytes_

   *Regular fragments*

   | DTag  | FCN    | Payload |
   + ----- + ------ + ------- +
   | 1 bit | 7 bits |         |


   Figure 5: All fragment except the last one.  Header size is 8 bits (1
                                  byte).

   *SCHC ACK*

   | DTag  | C     | Encoded bitmap (if C = 0) | Padding (0s) |
   + ----- + ----- + ------------------------- + ------------ +
   | 1 bit | 1 bit | 0 to 127 bits             | 7 or 0 bits  |


               Figure 6: SCHC ACK format, failed mic check.

5.5.1.2.  FPortUpDefault - 2 bytes header

   o  *RuleID*: size is 6 bits (64 possible rules, 62 available for
      compression)

   o  *Window index*: encoded on W = 2 bits.  So 4 windows are
      available.




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   With this set of parameters, the SCHC fragment header overhead is 2
   bytes (16 bits).  MTU is: _4 windows * 127 tiles * 3 bytes per tile =
   1524 bytes_

   _Note_: Even if it is less efficient, this rule can also be used for
   fragmented payload size less than 382 bytes.

   *Regular fragments*

   | RuleID | DTag  | W      | FCN    | Payload |
   + ------ + ----- + ------ + ------ + ------- +
   | 6 bits | 1 bit | 2 bits | 7 bits |         |


    Figure 7: All fragment except the last one.  Header size is 16 bits
                                (2 bytes).

   *Last fragment (All-1)*

   | RuleID | DTag  | W      | FCN=All-1 | MIC     | Payload           |
   + ------ + ----- + ------ + --------- + ------- + ----------------- +
   | 6 bits | 1 bit | 2 bits | 7 bits    | 32 bits | Last tile, if any |


      Figure 8: All-1 fragment detailed format for the last fragment.

   *SCHC ACK*

   | RuleID | DTag  | W     | C     | Encoded bitmap (if C = 0) |
   + ------ + ----- + ----- + ----- + ------------------------- +
   | 6 bits | 1 bit | 2 bit | 1 bit | 0 to 127 bits             |


                 Figure 9: SCHC formats, failed MIC check.

   *Receiver-Abort*

   | RuleID | DTag  | W = b'11 | C = 1 | b'111111 | 0xFF (all 1's) |
   + ------ + ----- + -------- + ------+--------- + ---------------+
   | 6 bits | 1 bit | 2 bits   | 1 bit | 6 bits   | 8 bits         |


                     Figure 10: Receiver-Abort format.

   *SCHC acknowledge request*






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   | RuleID | DTag  | W      | FCN = b'0000000 |
   + ------ + ----- + ------ + --------------- +
   | 6 bits | 1 bit | 2 bits | 7 bits          |



                      Figure 11: SCHC ACK REQ format.

5.5.2.  Downlink fragmentation: From SCHC gateway to 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.

   o  *SCHC fragmentation reliability mode*: ACK-Always.

   o  *RuleID*: size is 6 bits (64 possible rules, 62 for compression).

   o  *Window index*: encoded on W=1 bit, as per
      [I-D.ietf-lpwan-ipv6-static-context-hc].

   o  *DTag*: Not used, so its size is 0 bit.

   o  *FCN*: The FCN field is encoded on N=1 bits, so WINDOW_SIZE = 1
      tile (FCN=All-1 is reserved for SCHC).

   o  *MIC 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 minus header (1 byte)

   _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 in but not SCHC needs.

   *Regular fragments*










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


     Figure 12: All fragments but the last one.  Header size 1 byte (8
                                  bits).

   *Last fragment (All-1)*

   | RuleID | W     | FCN = b'1 | MIC     | Payload           |
   + ------ + ----- + --------- + ------- + ----------------- +
   | 6 bits | 1 bit | 1 bit     | 32 bits | Last tile, if any |


     Figure 13: All-1 SCHC ACK detailed format for the last fragment.

   *SCHC acknowledge*

   | RuleID | W     | C = b'1 |
   + ------ + ----- + ------- +
   | 6 bits | 1 bit | 1 bit   |


                Figure 14: SCHC ACK format, MIC is correct.

   *Receiver-Abort*

   | RuleID | W    | C = b'0 | b'11111111 |
   + ------ + ----- + ------- + ---------- +
   | 6 bits | 1 bit | 1 bits  | 8 bits     |


     Figure 15: Receiver-Abort packet (following an all-1 packet with
                              incorrect MIC).

   Class A and classB&C end-devices do not manage retransmissions and
   timers in the same way.

5.5.2.1.  ClassA 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 classA 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 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 a FCN=All-1 fragment (the last fragment of
   a datagram) and if the MIC is correct, the device shall transmit the
   ACK with the "MIC 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 MIC 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 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.

5.5.2.2.  Class B or C end-devices

   Class B&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 a
   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



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   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 a FCN=All-1 fragment (the last fragment of
   a datagram) and if the MIC is correct, the device shall transmit the
   ACK with the "MIC 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.

   The device SHALL keep the SCHC ACK message in memory until it
   receives a downlink from the SCHC gateway different from the last
   (FCN>0 and different DTag) fragment indicating 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 if the MIC 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 classB end-devices, this timer
   trigger is a function of the periodicity of the classB ping slots.
   The recommended value is equal to 3 times the classB ping slot
   periodicity.  For classC 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 classB&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 parameters
   does not contribute to any new security issues in addition of those
   identified in [I-D.ietf-lpwan-ipv6-static-context-hc].







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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: J.  Catalano name: Julien Catalano org: Kerlink street: 1 rue
      Jacqueline Auriol city: 35235 Thorigne-Fouillard country: France
      email: j.catalano@kerlink.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>.

   [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-19 (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 16 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: 21 bits residue + 38 bytes
payload.


| RuleID | Compression residue |  Payload  | Padding=0b000 |
+ ------ + ------------------- + --------- + ------------- +
|   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 FPortUpDefault


| LoRaWAN Header | RuleID | Compression residue |  Payload  | Padding=b'000 |
+ -------------- + ------ + ------------------- + --------- + ------------- +
|       XXXX     |   1    |       21 bits       |  38 bytes |    3 bits     |


       Figure 16: Uplink example: compression without fragmentation

A.2.  Uplink - Compression and fragmentation example

   Figure 17 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 440 bytes: 21 bits residue + 138 bytes
payload.


| RuleID | Compression residue |  Payload  |
+ ------ + ------------------- + --------- +
|   1    |       21 bits       | 138 bytes |


Given the size of the payload, FPortUpDefault will be used.
The current LoRaWAN MTU is 11 bytes, although 2 bytes FOpts are used by
LoRaWAN protocol: 9 bytes are available for SCHC payload.
SCHC header is 2 bytes so 2 tiles are send in first fragment.

| LoRaWAN Header |  FOpts  | RuleID | DTag  |   W    |  FCN   | 2 tiles |
+ -------------- + ------- + ------ + ----- + ------ + ------ + ------- +
|       XXXX     | 2 bytes |   0    |   0   |   0    |  126   | 6 bytes |

Content of the two tiles is:
| RuleID | Compression residue |  Payload           |



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+ ------ + ------------------- + ------------------ +
|   1    |       21 bits       |  2 bytes + 5 bits  |


Next transmission MTU is 242 bytes, no FOpts. 80 tiles are transmitted:

| LoRaWAN Header | RuleID | DTag  |   W    |  FCN   | 80 tiles  |
+ -------------- + ------ + ----- + ------ + ------ + --------- +
|       XXXX     |   0    |   0   |   0    |  124   | 240 bytes |


Next transmission MTU is 242 bytes, no FOpts. All 65 remaining tiles are
transmitted, last tile is only 2 bytes. Padding is added for the remaining 6 bits.

| LoRaWAN Header | RuleID | DTag  |   W    |  FCN   |  MIC  | 65 tiles  | Padding=b'000 |
+ -------------- + ------ + ----- + ------ + ------ + ----- + --------- + ---------------- +
|       XXXX     |   0    |   0   |   0    |  127   | CRC32 | 194 bytes |     3 bits       |


All packets have been received by the SCHC gateway, computed MIC is correct so
the following ACK is send to the device:

| LoRaWAN Header | RuleID | DTag  |   W    |  C  |
+ -------------- + ------ + ----- + ------ + --- +
|       XXXX     |   0    |   0   |   0    |  1  |


         Figure 17: Uplink example: compression and fragmentation

A.3.  Downlink

An applicative payload of 43 bytes is passed to SCHC compression layer using
rule 1, allowing to compress it to 24 bytes and 5 bits: 21 bits residue + 22 bytes
payload.


| RuleID | Compression residue |  Payload  |
+ ------ + ------------------- + --------- +
|   1    |       21 bits       |  18 bytes |


The current LoRaWAN MTU is 11 bytes, although 2 bytes FOpts are used by
LoRaWAN protocol: 9 bytes are available for SCHC payload => it has to be fragmented.


| LoRaWAN Header |  FOpts  | RuleID |   W    |  FCN   | 1 tile  |
+ -------------- + ------- + ------ + ------ + ------ + ------- +
|       XXXX     | 2 bytes |   0    |   0    |   0    | 8 bytes |



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Content of the two tiles is:
| RuleID | Compression residue |        Payload     |
+ ------ + ------------------- + ------------------ +
|   1    |       21 bits       |  2 bytes + 5 bits  |


The receiver answers with an SCHC ACK


| RuleID | W = 0 | C = b'1 |
+ ------ + ----- + ------- +
| 6 bits | 1 bit | 1 bit   |


The second downlink is send, no FOpts:


| LoRaWAN Header | RuleID |   W    |  FCN   |  1 tile  |
+ -------------- + ------ + ------ + ------ + -------- +
|       XXXX     |   0    |   1    |   0    | 10 bytes |


The receiver answers with an SCHC ACK


| RuleID | W = 1 | C = b'1 |
+ ------ + ----- + ------- +
| 6 bits | 1 bit | 1 bit   |


The third downlink is send, no FOpts:


| LoRaWAN Header | RuleID |   W    |  FCN   |  1 tile  |
+ -------------- + ------ + ------ + ------ + -------- +
|       XXXX     |   0    |   0    |   0    | 10 bytes |


The receiver answers with an SCHC ACK


| RuleID | W = 0 |  C = 1  |
+ ------ + ----- + ------- +
| 6 bits | 1 bit | 1 bit   |


The last downlink is send, no FOpts:




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| LoRaWAN Header | RuleID |   W    |  FCN   |  1 tile  |
+ -------------- + ------ + ------ + ------ + -------- +
|       XXXX     |   0    |   1    |   1    | 2 bytes  |


The receiver answers with an SCHC ACK


| RuleID | W = 1 |  C = 1  |
+ ------ + ----- + ------- +
| 6 bits | 1 bit | 1 bit   |


        Figure 18: 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
   2bis rue de la Chataigneraie
   35510 Cesson-Sevigne Cedex
   France

   Email: ivaylo@ackl.io
















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