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lpwan                                                         S. Farrell
Internet-Draft                                    Trinity College Dublin
Intended status: Informational                                  A. Yegin
Expires: May 1, 2017                                            Actility
                                                        October 28, 2016


                            LoRaWAN Overview
                  draft-farrell-lpwan-lora-overview-01

Abstract

   Low Power Wide Area Networks (LPWAN) are wireless technologies
   covering different Internet of Things (IoT) applications.  The common
   characteristics for LPWANs are large coverage, low bandwidth, small
   packet and application layer data sizes and long battery life
   operation.  One of these technologies is LoRaWAN developed by the
   LoRa Alliance.  LoRaWAN targets key requirements of the Internet of
   things such as secure bi-directional communication, mobility and
   localization services.  This memo is an informational overview of
   LoRaWAN and gives the principal characteristics of this technology in
   order to help with the IETF work for providing IPv6 connectivity over
   LoRaWAN along with other LPWANs.

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-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 May 1, 2017.

Copyright Notice

   Copyright (c) 2016 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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   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.  Radio Spectrum  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  MAC Layer . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Names and Addressing  . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     6.1.  Payload Encryption and Data Integrity . . . . . . . . . .  10
     6.2.  Key Derivation  . . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   LoRaWAN is a wireless technology for long-range low-power low-data-
   rate applications developed by the LoRa Alliance, a membership
   consortium.  <https://www.lora-alliance.org/> LoRaWAN networks are
   typically organized in a star-of-stars topology in which gateways
   relay messages between end-devices and a central "network server" in
   the backend.  Gateways are connected to the network server via IP
   links while end-devices use single-hop LoRaWAN communication that can
   be received at one or more gateways.  All communication is generally
   bi-directional, although uplink communication from end-devices to the
   network server are favoured in terms of overall bandwidth
   availability.

   In LoRaWAN networks, end-device transmissions may be received at
   multiple gateways, so during nominal operation a network server may
   see multiple instances of the same uplink message from an end-device.

   To maximize both battery life of end-devices and overall network
   capacity, the LoRaWAN network infrastructure manages the data rate
   and RF output power for each end-device individually by means of an
   adaptive data rate (ADR) scheme.  End-devices may transmit on any
   channel allowed by local regulation at any time, using any of the
   currently available data rates.



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   This memo provides an overview of the LoRaWAN technology for the
   Internet community, but the definitive specification [LoRaSpec] is
   that produced by the LoRa Alliance.  This draft is based on version
   1.0.2 of the LoRa specification.  (Note that version 1.0.2 is
   expected to be published in a few weeks.  We will update this draft
   when that has happened.  For now, version 1.0 is available at
   [LoRaSpec1.0])

2.  Terminology

   This section introduces some LoRaWAN terms.  Figure 1 shows the
   entities involved in a LoRaWAN network.

   +----------+
   |End-device| * * *
   +----------+       *   +---------+
                        * | Gateway +---+
   +----------+       *   +---------+   |   +---------+
   |End-device| * * *                   +---+ Network +--- Application
   +----------+       *                 |   | Server  |
                        * +---------+   |   +---------+
   +----------+       *   | Gateway +---+
   |End-device| * * *   * +---------+
   +----------+
       Key: *      LoRaWAN radio
            +---+  IP connectivity

                      Figure 1: LoRaWAN architecture

   o  End-device: a LoRa client device, sometimes called a mote.
      Communicates with gateways.

   o  Gateway: a radio on the infrastructure-side, sometimes called a
      concentrator or base-station.  Communicates with end-devices and,
      via IP, with a network server.

   o  Network Server: The Network Server (NS) terminates the LoRaWAN MAC
      layer for the end-devices connected to the network.  It is the
      center of the star topology.

   o  Uplink message: refers to communications from end-device to
      network server or appliction via one or more gateways.

   o  Downlink message: refers to communications from network server or
      application via one gateway to a single end-device or a group of
      end-devices (considering multicasting).





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   o  Application: refers to application layer code both on the end-
      device and running "behind" the network server.  For LoRaWAN,
      there will generally only be one application running on most end-
      devices.  Interfaces between the network server and application
      are not further described here.

   o  Classes A, B and C define different device capabilities and modes
      of operation for end-devices.  End-devices can transmit uplink
      messages at any time in any mode of operation (so long as e.g.,
      ISM band restrictions are honoured).  An end-device in Class A can
      only receive downlink messages at predetermined timeslots after
      each uplink message transmission.  Class B allows the end-device
      to receive downlink messages at periodically scheduled timeslots.
      Class C allows receipt of downlink messages at anytime.  Class
      selection is based on the end-devices' application use case and
      its power supply.  (While Classes B and C are not further
      described here, readers may have seen those terms elsewhere so we
      include them for clarity.)

3.  Radio Spectrum

   LoRaWAN radios make use of ISM bands, for example, 433MHz and 868MHz
   within the European Union and 915MHz in the Americas.

   The end-device changes channel in a pseudo-random fashion for every
   transmission to help make the system more robust to interference and/
   or to conform to local regulations.

   As with other LPWAN radio technologies, LoRaWAN end-devices respect
   the frequency, power and maximum transmit duty cycle requirements for
   the sub-band imposed by local regulators.  In most cases, this means
   an end-device is only transmitting for 1% of the time, as specified
   by ISM band regulations.  And in some cases the LoRaWAN specification
   calls for end-devices to transmit less often than is called for by
   the ISM band regulations in order to avoid congestion.

   Figure 2 below shows that after a transmission slot a Class A device
   turns on its receiver for two short receive windows that are offset
   from the end of the transmission window.  The frequencies and data
   rate chosen for the first of these receive windows match those used
   for the transmit window.  The frequency and data-rate for the second
   receive window are configurable.  If a downlink message preamble is
   detected during a receive window, then the end-device keeps the radio
   on in order to receive the frame.

   End-devices can only transmit a subsequent uplink frame after the end
   of the associated receive windows.  When a device joins a LoRaWAN




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   network (see Section 4 for details), there are similar timeouts on
   parts of that process.

   |----------------------------|         |--------|     |--------|
   |             Tx             |         |   Rx   |     |   Rx   |
   |----------------------------|         |--------|     |--------|
                                |---------|
                                 Rx delay 1
                                |------------------------|
                                 Rx delay 2

        Figure 2: LoRaWAN Class A transmission and reception window

   Given the different regional requirements the detailed specification
   for the LoRaWAN physical layer (taking up more than 30 pages of the
   specification) is not reproduced here.  Instead and mainly to
   illustrate the kinds of issue encountered, in Table 1 we present some
   of the default settings for one ISM band (without fully explaining
   those here) and in Table 2 we describe maxima and minima for some
   parameters of interest to those defining ways to use IETF protocols
   over the LoRaWAN MAC layer.

   +------------------------+------------------------------------------+
   |       Parameters       |              Default Value               |
   +------------------------+------------------------------------------+
   |       Rx delay 1       |                   1 s                    |
   |                        |                                          |
   |       Rx delay 2       |    2 s (must be RECEIVE_DELAY1 + 1s)     |
   |                        |                                          |
   |      join delay 1      |                   5 s                    |
   |                        |                                          |
   |      join delay 2      |                   6 s                    |
   |                        |                                          |
   |     868MHz Default     | 3 (868.1,868.2,868.3), date rate: 0.3-5  |
   |        channels        |                   kbps                   |
   +------------------------+------------------------------------------+

                Table 1: Default settings for EU868MHz band













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   +-----------------------------------------------+--------+----------+
   | Parameter/Notes                               |  Min   |   Max    |
   +-----------------------------------------------+--------+----------+
   | Duty Cycle: some but not all ISM bands impose |   1%   | no-limit |
   | a limit in terms of how often an end-device   |        |          |
   | can transmit. In some cases LoRaWAN is more   |        |          |
   | stringent in an attempt to avoid congestion.  |        |          |
   |                                               |        |          |
   | EU 868MHz band data rate/frame-size           |  250   |  50000   |
   |                                               | bits/s | bits/s : |
   |                                               |  : 59  |   250    |
   |                                               | octets |  octets  |
   |                                               |        |          |
   | US 915MHz band data rate/frame-size           |  980   |  21900   |
   |                                               | bits/s | bits/s : |
   |                                               |  : 19  |   250    |
   |                                               | octets |  octets  |
   +-----------------------------------------------+--------+----------+

         Table 2: Minima and Maxima for various LoRaWAN Parameters

   Note that in the case of the smallest frame size (19 octets), 8
   octets are required for LoRa MAC layer headers leaving only 11 octets
   for payload (including MAC layer options).  However, those settings
   do not apply for the join procedure - end-devices are required to use
   a channel that can send the 23 byte Join-request message for the join
   procedure.

4.  MAC Layer

   Uplink and downlink higher layer data is carried in a MACPayload.
   There is a concept of "ports" (an optional 8 bit value) to handle
   different applications on an end-device.  Port zero is reserved for
   LoRaWAN specific messaging, such as the join procedure.

   The header also distinguishes the uplink/downlink directions and
   whether or not an acknowledgement ("confirmation") is required from
   the peer.

   All payloads are encrypted and ciphertexts are protected with a
   cryptographic Message Integrity Check (MIC) - see Section 6 for
   details.

   In addition to carrying higher layer PDUs there are Join-Request and
   Join-Response (aka Join-Accept) messages for handling network access.
   And so-called "MAC commands" (see below) up to 15 bytes long can be
   piggybacked in an options field ("FOpts").




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   LoRaWAN end-devices can choose various different data rates from a
   menu of available rates (dependent on the frequencies in use).  It is
   however, recommended that end-devices set the Adaptive Data Rate
   ("ADR") bit in the MAC layer which is a signal that the network
   should control the data rate (via MAC commands to the end-device).
   The network can also assert the ADR bit and control data rates at
   it's discretion.  The goal is to ensure minimal on-time for radios
   whilst increasing throughput and reliability when possible.  Other
   things being equal, the effect should be that end-devices closer to a
   gateway can successfully use higher data rates, whereas end-devices
   further from all gateways still receive connectivity though at a
   lower data rate.

   Data rate changes can be validated via a scheme of acks from the
   network with a fall-back to lower rates in the event that downlink
   acks go missing.

   There are 16 (or 32) bit frame counters maintained in each direction
   that are incremented on each transmission (but not re-transmissions)
   that are not re-used for a given key.  When the device supports a 32
   bit counter, then only the least significant 16 bits are sent in the
   MAC header, but all 32 bits are used in cryptographic operations.
   (If an end-device only supports a 16 bit counter internally, then the
   topmost 16 bits are set to zero.)

   There are a number of MAC commands for: Link and device status
   checking, ADR and duty-cycle negotiation, managing the RX windows and
   radio channel settings.  For example, the link check response message
   allows the network server (in response to a request from an end-
   device) to inform an end-device about the signal attenuation seen
   most recently at a gateway, and to also tell the end-device how many
   gateways received the corresponding link request MAC command.

   Some MAC commands are initiated by the network server.  For example,
   one command allows the network server to ask an end-device to reduce
   it's duty-cycle to only use a proportion of the maximum allowed in a
   region.  Another allows the network server to query the end-device's
   power status with the response from the end-device specifying whether
   it has an external power source or is battery powered (in which case
   a relative battery level is also sent to the network server).

   The network server can also inform an end-device about channel
   assignments (mid-point frequencies and data rates).  Of course, these
   must also remain within the bands assigned by local regulation.







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5.  Names and Addressing

   A LoRaWAN network has a short network identifier ("NwkID") which is a
   seven bit value.  A private network (common for LoRaWAN) can use the
   value zero.  If a network wishes to support "foreign" end-devices
   then the NwkID needs to be registered with the LoRA Alliance, in
   which case the NwkID is the seven least significant bits of a
   registered 24-bit NetID.  (Note however, that the methods for
   "roaming" are currently being enhanced within the LoRA Alliance, so
   the situation here is somewhat fluid.)

   In order to operate nominally on a LoRaWAN network, a device needs a
   32-bit device address, which is the catentation of the NwkID and a
   25-bit device-specific network address that is assigned when the
   device "joins" the network (see below for the join procedure) or that
   is pre-provisioned into the device.

   End-devices are assumed to work with one or a quite limited number of
   applications, which matches most LoRaWAN use-cases.  The applications
   are identified by a 64-bit AppEUI, which is assumed to be a
   registered IEEE EUI64 value.

   In addition, a device needs to have two symmetric session keys, one
   for protecting network artefacts (port=0), the NwkSKey, and another
   for protecting appliction layer traffic, the AppSKey.  Both keys are
   used for 128 bit AES cryptpgraphic operations.  (See Section 6 for
   details.)

   So, one option is for an end-device to have all of the above, plus
   channel information, somehow (pre-)provisioned, in which case the
   end-device can simply start transmitting.  This is achievable in many
   cases via out-of-band means given the nature of LoRaWAN networks.
   Table 3 summarises these values.

   +---------+---------------------------------------------------------+
   | Value   | Description                                             |
   +---------+---------------------------------------------------------+
   | DevAddr | DevAddr (32-bits) =  NwkId (7-bits) + device-specific   |
   |         | network address (25 bits)                               |
   |         |                                                         |
   | AppEUI  | IEEE EUI64 naming the application                       |
   |         |                                                         |
   | NwkSKey | 128 bit network session key for use with AES            |
   |         |                                                         |
   | AppSKey | 128 bit application session key for use with AES        |
   +---------+---------------------------------------------------------+

              Table 3: Values required for nominal operation



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   As an alternative, end-devices can use the LoRaWAN join procedure in
   order to setup some of these values and dynamically gain access to
   the network.

   To use the join procedure, an end-device must still know the AppEUI.
   In addition to the AppEUI, end-devices using the join procedure need
   to also know a different (long-term) symmetric key that is bound to
   the AppEUI - this is the application key (AppKey), and is distinct
   from the application session key (AppSKey).  The AppKey is required
   to be specific to the device, that is, each end-device should have a
   different AppKey value.  And finally the end-device also needs a
   long-term identifier for itself, syntactically also an EUI-64, and
   known as the device EUI or DevEUI.  Table 4 summarises these values.

     +---------+----------------------------------------------------+
     | Value   | Description                                        |
     +---------+----------------------------------------------------+
     | DevEUI  | IEEE EUI64 naming the device                       |
     |         |                                                    |
     | AppEUI  | IEEE EUI64 naming the application                  |
     |         |                                                    |
     | AppKey  | 128 bit long term application key for use with AES |
     +---------+----------------------------------------------------+

                Table 4: Values required for join procedure

   The join procedure involves a special exchange where the end-device
   asserts the AppEUI and DevEUI (integrity protected with the long-term
   AppKey, but not encrypted) in a Join-request uplink message.  This is
   then routed to the network server which interacts with an entity that
   knows that AppKey to verify the Join-request.  All going well, a
   Join-accept downlink message is returned from the network server to
   the end-device that specifies the 24-bit NetID, 32-bit DevAddr and
   channel information and from which the AppSKey and NwkSKey can be
   derived based on knowledge of the AppKey.  This provides the end-
   device with all the values listed in Table 3.

   There is some special handling related to which channels to use and
   for multiple transmissions for the join-request which is intended to
   ensure a successful join in as many cases as possible.  Join-request
   and Join-accept messages also include some random values (nonces) to
   both provide some replay protection and to help ensure the session
   keys are unique per run of the join procedure.  If a Join-request
   fails validation, then no Join-accept message (indeed no message at
   all) is returned to the end-device.  For example, if an end-device is
   factory-reset then it should end up in a state in which it can re-do
   the join procedure.




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6.  Security Considerations

   In this section we describe the use of cryptography in LoRaWAN.  This
   section is not intended as a full specification but to be sufficient
   so that future IETF specifications can encompass the required
   security considerations.  The emphasis is on describing the
   externally visible characteristics of LoRaWAN.

6.1.  Payload Encryption and Data Integrity

   All payloads are encrypted and have data integrity.  Frame options
   (used for MAC commands) when sent as a payload (port zero) are
   therefore protected.  MAC commands piggy-backed as frame options
   ("FOpts") are however sent in clear.  Since MAC commands may be sent
   as options and not only as payload, any values sent in that manner
   are visible to a passive attacker but are not malleable for an active
   attacker due to the use of the MIC.

   For LoRaWAN version 1.0.x, the NWkSkey session key is used to provide
   data integrity between the end-device and the network server.  The
   AppSKey is used to provide data confidentiality between the end-
   device and network server, or to the application "behind" the network
   server, depending on the implementation of the network.

   All MAC layer messages have an outer 32-bit Message Integrity Code
   (MIC) calculated using AES-CMAC calculated over the ciphertext
   payload and other headers and using the NwkSkey.

   Payloads are encrypted using AES-128, with a counter-mode derived
   from IEEE 802.15.4 using the AppSKey.

   Gateways are not expected to be provided with the AppSKey or NwkSKey,
   all of the infrastructure-side cryptography happens in (or "behind")
   the network server.

6.2.  Key Derivation

   When session keys are derived from the AppKey as a result of the join
   procedure the Join-accept message payload is specially handled.

   The long-term AppKey is directly used to protect the Join-accept
   message content, but the function used is not an aes-encrypt
   operation, but rather an aes-decrypt operation.  The justification is
   that this means that the end-device only needs to implement the aes-
   encrypt operation.  (The counter mode variant used for payload
   decryption means the end-device doesn't need an aes-decrypt
   primitive.)




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   The Join-accept plaintext is always less than 16 bytes long, so
   electronic code book (ECB) mode is used for protecting Join-accept
   messages.

   The Join-accept contains an AppNonce (a 24 bit value) that is
   recovered on the end-device along with the other Join-accept content
   (e.g.  DevAddr) using the aes-encrypt operation.

   Once the Join-accept payload is available to the end-device the
   session keys are derived from the AppKey, AppNonce and other values,
   again using an ECB mode aes-encrypt operation, with the plaintext
   input being a maximum of 16 octets.

7.  IANA Considerations

   There are no IANA considerations related to this memo.

8.  Acknowledgements

   The authors re-used some text from [I-D.vilajosana-lpwan-lora-hc]

   Stephen Farrell's work on this memo was supported by the Science
   Foundation Ireleand funded CONNECT centre
   <https://connectcentre.ie/>.

9.  Contributors

   The following members of the LoRa Alliance reviewed this draft and
   contributed (much more than SF) to the definition of LoRaWAN.

      Name, Affiliation, email (optional)

      Chun-Yeow Yeoh, VADS LYFE SDN BHD, yeow@tmrnd.com.my

      Olivier Hersent, Actility, olivier.hersent@actility.com

      Dave Kjendal, Senet Inc, dkjendal@senetco.com

      Paul Duffy, Cisco, paduffy@cisco.com

      Joachim Ernst, Swisscom Broadcast Ltd, joachim.ernst@swisscom.com

      Nicolas Sornin, Semtech, nsornin@semtech.com

      Phillippe Christin, Orange, philippe.christin@orange.com






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10.  Informative References

   [I-D.vilajosana-lpwan-lora-hc]
              Vilajosana, X., Dohler, M., and A. Yegin, "Transmission of
              IPv6 Packets over LoRaWAN", draft-vilajosana-lpwan-lora-
              hc-00 (work in progress), July 2016.

   [LoRaSpec]
              LoRa Alliance, "LoRaWAN Specification Version V1.0.2", Nov
              2016, <URL TBD>.

   [LoRaSpec1.0]
              LoRa Alliance, "LoRaWAN Specification Version V1.0", Jan
              2015, <https://www.lora-alliance.org/portals/0/specs/
              LoRaWAN%20Specification%201R0.pdf>.

Authors' Addresses

   Stephen Farrell
   Trinity College Dublin
   Dublin  2
   Ireland

   Phone: +353-1-896-2354
   Email: stephen.farrell@cs.tcd.ie


   Alper Yegin
   Actility
   Paris, Paris
   FR

   Email: alper.yegin@actility.com


















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