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Versions: 00 01 02

lpwan Working Group                                       N. Sornin, Ed.
Internet-Draft                                                M. Coracin
Intended status: Informational                                   Semtech
Expires: January 3, 2019                                       I. Petrov
                                                                  Acklio
                                                                A. Yegin
                                                                Actility
                                                             J. Catalano
                                                                 Kerlink
                                                             V. Audebert
                                                                 EDF R&D
                                                           July 02, 2018


         Static Context Header Compression (SCHC) over LoRaWAN
              draft-petrov-lpwan-ipv6-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.

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 https://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 January 3, 2019.






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Copyright Notice

   Copyright (c) 2018 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
   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  . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Device classes (A, B, C) and interactions . . . . . . . .   5
     4.2.  Device addressing . . . . . . . . . . . . . . . . . . . .   6
     4.3.  General Message Types . . . . . . . . . . . . . . . . . .   6
     4.4.  LoRaWAN MAC Frames  . . . . . . . . . . . . . . . . . . .   6
   5.  SCHC over LoRaWAN . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Rule ID management  . . . . . . . . . . . . . . . . . . .   6
     5.2.  IID computation . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Fragmentation . . . . . . . . . . . . . . . . . . . . . .   6
       5.3.1.  Reliability options . . . . . . . . . . . . . . . . .   6
       5.3.2.  Supporting multiple window sizes  . . . . . . . . . .  11
       5.3.3.  Downlink fragment transmission  . . . . . . . . . . .  11
       5.3.4.  SCHC behavior for devices in class A, B and C . . . .  11
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Note . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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



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   [I-D.ietf-lpwan-overview].  Even though those technologies share a
   great number of common features like start-oriented topologies,
   network architecture, devices with mostly quite predictable
   communications, etc; they do have some slight differences in respect
   of payload sizes, reactiveness, etc.

   SCHC gives a generic framework that enables those devices to
   communicate with other Internet networks.  However, for efficient
   performance, some parameters and modes of operation need to be set
   appropriately for each of the LPWAN technologies.

   This document describes the efficient parameters and modes of
   operation when SCHC is used over LoRaWAN networks.

2.  Terminology

   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 device
   during the procedure while joining the network (Join Procedure)

   o DevAddr: a 32-bit non-unique identifier assigned to a 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, which is the most bandwidth-consuming operation in
   other header compression mechanisms such as RoHC [RFC5795].  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      |
   |     IPv6     |                                  |    IPv6      |
   |              |                                  |              |
   |   SCHC C/D   |                                  |              |
   |   (context)  |                                  |              |
   +-------+------+                                  +-------+------+
            |   +--+     +----+     +---------+              .
            +~~ |RG| === |NGW | === |SCHC C/D |... Internet ..
                +--+     +----+     |(context)|
                                    +---------+

                          Figure 1: Architecture

   Figure 1 represents the architecture for compression/decompression,
   it is based on [I-D.ietf-lpwan-overview] terminology.  The Device is
   sending applications flows using IPv6 or IPv6/UDP protocols.  These
   flows are 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 C/D for decompression which shares
   the same rules with the Dev. The SCHC 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 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 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 can be embedded in different places, for
   example in the Network Server and/or the Application Server.

   Next steps for this section: detailed overview of the LoRaWAN
   architecture and its mapping to the SCHC architecture.

4.  LoRaWAN Architecture

   An overview of LoRaWAN [lora-alliance-spec] protocol and architecture
   is described in [I-D.ietf-lpwan-overview].  Mapping between the LPWAN
   architecture entities as described in




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   [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.  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.

   ()   ()   ()       |                      +------+
    ()  () () ()     / \       +---------+   | Join |
   () () () () ()   /   \======|    ^    |===|Server|  +-----------+
    () ()  ()      |           | <--|--> |   +------+  |Application|
   () ()  ()  ()  / \==========|    v    |=============|  Server   |
    ()  ()  ()   /   \         +---------+             +-----------+
   End-Devices  Gateways     Network Server


                      Figure 1: LPWAN/LoRaWAN Architecture

   SCHC C/D (Compressor/Decompressor) and SCHC Fragmentation are
   performed on the LoRaWAN End-device and the Application Server.
   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
   acts as the first hop IP router for the End-device.  Note that 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

   TBD







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4.2.  Device addressing

   TBD

4.3.  General Message Types

   TBD

4.4.  LoRaWAN MAC Frames

   TBD

5.  SCHC over LoRaWAN

5.1.  Rule ID management

   Rule ID can be stored and transported in the FPort field of the
   LoRaWAN MAC frame or as the first bytes of the payload.

   TBD

5.2.  IID computation

   TBD

5.3.  Fragmentation

   TBD

5.3.1.  Reliability options

5.3.1.1.  Uplinks: From device to gateway

   In that case the device is the fragmentation transmitter, and the
   SCHC gateway the fragmentation receiver.

   o  SCHC fragmentation reliability mode : "ACK_ALWAYS"

   o  Window size: 8, the FCN field is encoded on 3 bits

   o  DTag : 1bit. this field is used to clearly separate two
      consecutive fragmentation sessions.  A LoRaWAN device cannot
      interleave several fragmented SCHC datagrams.

   o  MIC calculation algorithm: CRC32 using 0xEDB88320 (i.e. the
      reverse representation of the polynomial used e.g. in the Ethernet
      standard [RFC3385])




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   o  Retransmission Timer and inactivity Timer: LoRaWAN devices do not
      implement a "retransmission timer".  At the end of a window the
      ACK corresponding to this window is transmitted by the network
      gateway in the RX1 or RX2 receive slot of the device.  If this ACK
      is not received the device sends an all-0 (or an all-1) fragment
      with no payload to request an 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 gateway implements an "inactivity timer".  The
      default recommended duration of this timer is 12h.  This value is
      mainly driven by application requirements and may be changed.

   | RuleID | DTag  | W     | FCN    | Payload |
   + ------ + ----- + ----- | ------ + ------- +
   | 3 bits | 1 bit | 1 bit | 3 bits |         |



    Figure 2: All fragment except the last one.  Header size is 8 bits.

   | RuleID | DTag  | W     | FCN    | MIC     | Payload |
   + ------ + ----- + ----- | ------ + ------- + ------- +
   | 3 bits | 1 bit | 1 bit | 3 bits | 32 bits |         |



      Figure 3: All-1 fragment detailed format for the last fragment.
                          Header size is 8 bits.

   The format of an all-0 or all-1 acknowledge is:

   | RuleID | DTag  | W     | Encoded bitmap | Padding (0s) |
   + ------ + ----- + ----- | -------------- + ------------ +
   | 3 bits | 1 bit | 1 bit | up to 8 bits   | 0 to 3 bits  |



   Figure 4: ACK format for All-0 windows.  Header size is 1 or 2 bytes.

| RuleID | DTag  | W     | C     | Encoded bitmap (if C = 0) | Padding (0s) |
+ ------ + ----- + ----- + ----- + ------------------------- + ------------ +
| 3 bits | 1 bit | 1 bit | 1 bit | up to 8 bits              | 0 to 2 bits  |



   Figure 5: ACK format for All-1 windows.  Header size is 1 or 2 bytes.





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5.3.1.2.  Downlinks: From 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  Window size : 1 , The FCN field is encoded on 1 bits

   o  DTag : 1bit.  This field is used to clearly separate two
      consecutive fragmentation sessions.  A LoRaWAN device cannot
      interleave several fragmented SCHC datagrams.

   o  MIC calculation algorithm: CRC32 using 0xEDB88320 (i.e. the
      reverse representation of the polynomial used e.g. in the Ethernet
      standard [RFC3385])

   o  MAX_ACK_REQUESTS : 8

   | RuleID | DTag  | W     | FCN    | Payload | Padding |
   + ------ + ----- + ----- | ------ + ------- + ------- +
   | 3 bits | 1 bit | 1 bit | 1 bits | X bytes | 2 bits  |



     Figure 6: All fragments but the last one.  Header size is 6 bits.

   | RuleID | DTag  | W     | FCN    | MIC     | Payload | Padding |
   + ------ + ----- + ----- | ------ + ------- + ------- + ------- +
   | 3 bits | 1 bit | 1 bit | 1 bits | 32 bits | X bytes | 2 bits  |



      Figure 7: All-1 Fragment Detailed Format for the Last Fragment.
                          Header size is 6 bits.

   The format of an all-0 or all-1 acknowledge is:

   | RuleID | DTag  | W     | Encoded bitmap | Padding (0s) |
   + ------ + ----- + ----- | -------------- + ------------ +
   | 3 bits | 1 bit | 1 bit | 1 bit          | 2 bits       |



      Figure 8: ACK format for All-0 windows.  Header size is 8 bits.





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   | RuleID | DTag  | W     | C = 1 | Padding (0s) |
   + ------ + ----- + ----- + ----- + ------------ +
   | 3 bits | 1 bit | 1 bit | 1 bit | 2 bits       |



   Figure 9: ACK format for All-1 windows, MIC is correct.  Header size
                                is 8 bits.

   | RuleID | DTag  | W     | b'111  | 0xFF (all 1's) |
   + ------ + ----- + ----- + ------ + -------------- +
   | 3 bits | 1 bit | 1 bit | 3 bits | 8 bits         |



     Figure 10: Receiver ABORT packet (following an all-1 packet with
                 incorrect MIC).  Header size is 16 bits.

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

5.3.1.2.1.  Class A devices

   Class A devices can only receive in an RX slot following the
   transmission of an uplink.  Therefore there cannot be a concept of
   "retransmission timer" for a gateway talking to classA devices for
   downlink fragmentation.

   The device replies with an ACK fragment to every single fragment
   received from the 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 transmit the
   ACK fragment until it receives the fragment of the next window.  The
   device shall transmit up to MAX_ACK_REQUESTS ACK fragments before
   aborting.  The device should transmit those ACK as soon as possible
   while taking into consideration eventual 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=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.  This message might
   be lost therefore the 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 from the gateway different from
   an ACK-request indicating that the gateway has received the ACK
   message.




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   Following the reception of a FCN=1 fragment (the last fragment of a
   datagram) and if 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 from the gateway different from
   an ACK-request indicating that the gateway has received the ABORT
   message.  The fragmentation receiver (device) does not implement
   retransmission timer and inactivity timer.

   The fragmentation sender (the gateway) implements an inactivity timer
   with default duration 12 hours.  Once a fragmentation session is
   started, if the gateway has not received any ACK or receiver-ABORT
   message 12 hours fater the last message from the device was received,
   the 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.

5.3.1.3.  Class B or C devices

   Class B&C devices can receive in scheduled RX slots or in RX slots
   following the transmission of an uplink.  The device replies with an
   ACK fragment to every single fragment received from the 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 ACK
   fragment even if that fragment has already been received.  The ACK
   bitmap is 1 bit long and is always 1.  If the gateway receives this
   ACK, it proceeds to send the next window fragment If the
   retransmission timer elapses and the gateway has not received the ACK
   of the current window it retransmits the last fragment.  The gateway
   tries retransmitting up to MAX_ACK_REQUESTS times before aborting.

   Following the reception of a FCN=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 gateway
   receives this ACK, the current fragmentation session has succeeded
   and its context can be cleared.

   If the retransmission timer elapses and the gateway has not received
   the all-1 ACK it retransmits the last fragment with the payload (not
   an ACK-request without payload).  The gateway tries retransmitting up
   to MAX_ACK_REQUESTS times before aborting.

   The device SHALL keep the all-1 ACK message in memory until it
   receives a downlink from the gateway different from the last (FCN=1)
   fragment indicating that the gateway has received the ACK message.
   Following the reception of a FCN=1 fragment (the last fragment of a
   datagram) and if the MIC is NOT correct, the device shall transmit a



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   receiver-ABORT fragment.  The retransmission timer is used by the
   gateway (the sender), the optimal value is very much application
   specific but here are some recommended default values.  For classB
   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. (modify 128sec) For classC devices
   which are nearly constantly receiving, the recommended value is 30
   seconds.  This means that the device shall try to transmit the ACK
   within 30 seconds of the reception of each fragment.  The inactivity
   timer is implemented by the device to flush the context in-case it
   receives nothing from the gateway over an extended period of time.
   The recommended value is 12 hours for both classB&C devices.

5.3.2.  Supporting multiple window sizes

   TBD

5.3.3.  Downlink fragment transmission

   TBD

5.3.4.  SCHC behavior for devices in class A, B and C

   TBD

6.  Security considerations

   TBD

7.  Acknowledgements

   TBD

8.  References

8.1.  Normative References

   [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>.

   [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>.




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   [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>.

8.2.  Informative References

   [I-D.ietf-lpwan-ipv6-static-context-hc]
              Minaburo, A., Toutain, L., Gomez, C., and D. Barthel,
              "LPWAN Static Context Header Compression (SCHC) and
              fragmentation for IPv6 and UDP", draft-ietf-lpwan-ipv6-
              static-context-hc-16 (work in progress), June 2018.

   [I-D.ietf-lpwan-overview]
              Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
              overview-10 (work in progress), February 2018.

   [lora-alliance-spec]
              Alliance, L., "LoRaWAN Specification Version V1.0.2",
              <http://portal.lora-
              alliance.org/DesktopModules/Inventures_Document/
              FileDownload.aspx?ContentID=1398>.

Appendix A.  Examples

Appendix B.  Note

Authors' Addresses

   Nicolas Sornin (editor)
   Semtech
   14 Chemin des Clos
   Meylan
   France

   Email: nsornin@semtech.com











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   Michael Coracin
   Semtech
   14 Chemin des Clos
   Meylan
   France

   Email: mcoracin@semtech.com


   Ivaylo Petrov
   Acklio
   2bis rue de la Chataigneraie
   35510 Cesson-Sevigne Cedex
   France

   Email: ivaylo@ackl.io


   Alper Yegin
   Actility
   .
   Paris, Paris
   France

   Email: alper.yegin@actility.com


   Julien Catalano
   Kerlink
   1 rue Jacqueline Auriol
   35235 Thorigne-Fouillard
   France

   Email: j.catalano@kerlink.fr


   Vincent AUDEBERT
   EDF R&D
   7 bd Gaspard Monge
   91120 PALAISEAU
   FRANCE

   Email: vincent.audebert@edf.fr








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