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lpwan Working Group                                           JC. Zuniga
Internet-Draft                                                    SIGFOX
Intended status: Informational                                  C. Gomez
Expires: January 3, 2019            Universitat Politecnica de Catalunya
                                                              L. Toutain
                                                          IMT-Atlantique
                                                           July 02, 2018


                         SCHC over Sigfox LPWAN
                 draft-zuniga-lpwan-schc-over-sigfox-03

Abstract

   The Static Context Header Compression (SCHC) specification describes
   a header compression scheme and fragmentation functionality for Low
   Power Wide Area Network (LPWAN) technologies.  SCHC offers a great
   level of flexibility that can be tailored for different LPWAN
   technologies.

   The present document provides the optimal parameters and modes of
   operation when SCHC is implemented over a Sigfox LPWAN.

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
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   This Internet-Draft will expire on January 3, 2019.

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



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   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 . . . . . . . . . . . . . .   3
   4.  SCHC over Sigfox  . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  SCHC Rules  . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Packet processing . . . . . . . . . . . . . . . . . . . .   4
   5.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Fragmentation headers . . . . . . . . . . . . . . . . . .   5
     5.2.  Uplink fragment transmissions . . . . . . . . . . . . . .   5
       5.2.1.  Uplink No-ACK mode  . . . . . . . . . . . . . . . . .   5
       5.2.2.  Uplink ACK-Always mode  . . . . . . . . . . . . . . .   6
       5.2.3.  Uplink ACK-on-Error mode  . . . . . . . . . . . . . .   6
     5.3.  Downlink fragment transmissions . . . . . . . . . . . . .   6
   6.  Padding . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Static Context Header Compression (SCHC) specification
   [I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
   scheme and fragmentation functionality that can be used on top of all
   the LWPAN systems defined in [I-D.ietf-lpwan-overview].  These LPWAN
   systems have similar characteristics such as star-oriented
   topologies, network architecture, connected devices with built-in
   applications, etc.

   SCHC offers a great level of flexibility to accommodate all these
   LPWAN systems.  Even though there are a great number of similarities
   between LPWAN technologies, some differences exist with respect to
   the transmission characteristics, payload sizes, etc.  Hence, there
   are optimal parameters and modes of operation that can be used when
   SCHC is used on top of a specific LPWAN.

   This document describes the optimal parameters and modes of operation
   when SCHC is implemented over a Sigfox LPWAN.




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2.  Terminology

   The reader is assumed to be familiar with the terms and mechanisms
   defined in [I-D.ietf-lpwan-overview] and in
   [I-D.ietf-lpwan-ipv6-static-context-hc].

3.  Static Context Header Compression

   Static Context Header Compression (SCHC) avoids context
   synchronization because data flows are highly predictable in LPWAN
   networks.  Contexts must be stored and configured on both ends.  This
   can be done either by using a provisioning protocol, by out of band
   means, or by pre-provisioning them e.g. at manufacturing time.  The
   way the contexts are configured and stored on both ends is out of the
   scope of this document.

        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
   and fragmentation, which is based on [I-D.ietf-lpwan-overview]
   terminology.

   The Device is sending applications flows that are compressed (and/or
   fragmented) by a Static Context Header Compression Compressor/
   Decompressor (SCHC C/D) to reduce headers size and/or fragment the
   packet.  The resulting information is sent over a layer two (L2)
   frame to a LPWAN Radio Gateway (RG) which forwards the frame to a
   Network Gateway (NGW).








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4.  SCHC over Sigfox

   In the case of the global Sigfox network, RGs (or base stations) are
   distributed over the multiple countries where the Sigfox LPWAN
   service is provided.  On the other hand, the NGW (or Cloud-based Core
   network) is a single entity that connects to all Sigfox base stations
   in the world.

   Uplink transmissions occur in repetitions over different times and
   frequencies.  Besides these time and frequency diversities, the
   Sigfox network also provides space diversity, as potentially an
   uplink message will be received by several base stations.  Since all
   messages are self-contained and base stations forward them all back
   to the same Core network (NGW), multiple input copies can be combined
   at the NGW and hence provide for extra reliability based on the
   triple diversity.

   The NGW communicates with the Network SCHC C/D for compression/
   decompression (and/or fragmentation/reassembly).  The Network SCHC C/
   D shares the same set of rules as the Dev SCHC C/D.  The Network SCHC
   C/D can be collocated with the NGW or in another place, as long as a
   tunnel is established between the NGW and the SCHC C/D.  After
   decompression (and/or reassembly), the packet can be forwarded over
   the Internet to one (or several) LPWAN Application Server(s) (App).

   The SCHC C/D process is bidirectional, so the same principles can be
   applied on both uplink and downlink.

4.1.  SCHC Rules

   The RuleID MUST be sent at the beginning of the SCHC header.  The
   total number of rules to be used affects directly the Rule ID field
   size, and therefore the total size of the fragmentation header.  For
   this reason, it is recommended to keep the number of rules that are
   defined for a specific device to the minimum possible.

4.2.  Packet processing

   TBD

5.  Fragmentation

   The SCHC specification [I-D.ietf-lpwan-ipv6-static-context-hc]
   defines a generic fragmentation functionality that allows sending
   data packets larger than the maximum size of a Sigfox data frame.
   The functionality also defines a mechanism to send reliably multiple
   frames, by allowing to resend selectively any lost frames.




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   The SCHC fragmentation supports several modes of operation.  These
   modes have different advantages and disadvantages depending on the
   specifics of the underlying LPWAN technology and Use Case.  This
   section describes how the SCHC fragmentation functionality should
   optimally be implemented when used over a Sigfox LPWAN for the most
   typical use case applications.

5.1.  Fragmentation headers

   A list of fragmentation header fields, their sizes as well as
   recommended modes for SCHC fragmentation over Sigfox are provided in
   this section.

5.2.  Uplink fragment transmissions

   Uplink transmissions are completely asynchronous and can take place
   in any random frequency of the allowed uplink bandwidth allocation.
   Hence, devices can go to deep sleep mode, and then wake up and
   transmit whenever there is a need to send any information to the
   network.  In that way, there is no need to perform any network
   attachment, synchronization, or other procedure before transmitting a
   data packet.  All data packets are self contained with all the
   required information for the network to process them accordingly.

   Since uplink transmissions occur asynchronously, an SCHC fragment can
   be transmitted at any given time by the Dev.

5.2.1.  Uplink No-ACK mode

   No-ACK is RECOMMENDED to be used for transmitting short, non-critical
   packets that require fragmentation.

   Fragmentation Header size: 8 bits

   The recommended Rule ID size is: 3 bits

   The recommended DTag size (T) is: 0 bits, as the number of available
   Rule IDs are sufficient to interleave fragmented packets.

   Fragment Compressed Number (FCN) size (N): 4 bits

   As per [REF SCHC], in the No-ACK mode the W (window) 1-bit field is
   not present.

   When fragmentation is used to transport IP frames, the Message
   Integrity Check (MIC) size, M: TBD bits

   The algorithm for computing the MIC field MUST be TBD.



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5.2.2.  Uplink ACK-Always mode

   TBD

5.2.3.  Uplink ACK-on-Error mode

   ACK-on-Error is RECOMMENDED for larger packets, since it leads to a
   reduced number of ACKs to be sent in the lower capacity downlink
   channel.

   The recommended Fragmentation Header size is: 8 bits

   The recommended Rule ID size is: 3 bits.

   The recommended DTag size (T) is: 0 bits, as the number of available
   Rule IDs are sufficient to interleave fragmented packets.

   Fragment Compressed Number (FCN) size (N): 4 bits.

   As per [REF SCHC], in the ACK-on-Error mode the Window (W) 1-bit
   field must be present.

   For the ACK-on-Error fragmentation mode(s), a single window size is
   RECOMMENDED.

   The value of MAX_ACK_REQUESTS SHOULD be 2, and the value of
   MAX_WIND_FCN SHOULD be 14 (which allows a maximum window size with 15
   fragments).

   When fragmentation is used to transport IP frames, the Message
   Integrity Check (MIC) size, M: TBD bits

   The algorithm for computing the MIC field MUST be TBD.

5.3.  Downlink fragment transmissions

   In some LPWAN technologies, as part of energy-saving techniques,
   downlink transmission is only possible immediately after an uplink
   transmission.  This allows the device to go in a very deep sleep mode
   and preserve battery, without the need to listen to any information
   from the network.  This is the case for Sigfox-enabled devices, which
   can only listen to downlink communications after performing an uplink
   transmission.

   When there are multiple fragments to be transmitted in the downlink,
   an uplink message is required to trigger the downlink communication.
   In order to avoid potentially high delay for fragmented datagram
   transmission in the downlink, the fragment receiver MAY perform an



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   uplink transmission as soon as possible after reception of a fragment
   that is not the last one.  Such uplink transmission MAY be triggered
   by sending a SCHC message, such as an ACK.  In this sense, ACK-Always
   is the preferred fragmentation mode for downlink communications.

   For downlink fragment transmission, the ACK-Always mode MUST be
   supported.

   The recommended Fragmentation Header size is: 8 bits

   The recommended Rule ID size is: 3 bits.

   The recommended DTag size (T) is: 0 bits, as the number of available
   Rule IDs are sufficient to interleave fragmented packets.

   Fragment Compressed Number (FCN) size (N): 4 bits.

   As per [REF SCHC], in the ACK-on-Error mode the Window (W) 1-bit
   field must be present.

   For the ACK-Always fragmentation mode(s), a single window size is
   RECOMMENDED.

   The value of MAX_ACK_REQUESTS SHOULD be 2, and the value of
   MAX_WIND_FCN SHOULD be 14 (which allows a maximum window size with 15
   fragments).

   When fragmentation is used to transport IP frames, the Message
   Integrity Check (MIC) size, M: TBD bits

   The algorithm for computing the MIC field MUST be TBD.

   Sigfox downlink frames have a fixed length of 8 bytes, which means
   that default SCHC algorithm for padding cannot be used.  Therefore,
   the 3 last bits of the fragmentation header are used to indicate in
   bytes the size of the padding.  A size of 000 means that the full
   ramaining frame is used to carry payload, a value of 001 indicates
   that the last byte contains padding, and so on.

6.  Padding

   The Sigfox payload fields have different characteristics in uplink
   and downlink.

   Uplink frames can contain a payload from 0 to 96 bits (i.e. 12
   bytes).  The radio protocol allows sending zero bits or one single
   bit of information for binary applications (e.g. status).  However,
   for 2 or more bits of payload it is required to add padding to the



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   next integer number of bytes.  The reason for this flexibility is to
   optimize transmission time and hence save battery consumption at the
   device.

   Downlink frames on the other hand have a fixed length.  The payload
   length must be 64 bits (i.e. 8 bytes).  Hence, if less information
   bits are to be transmitted padding would be necessary and it should
   be performed as described in the previous section.

7.  Security considerations

   The radio protocol authenticates and ensures the integrity of each
   message.  This is achieved by using a unique device ID and an AES-128
   based message authentication code, ensuring that the message has been
   generated and sent by the device with the ID claimed in the message.

   Application data can be encrypted at the application level or not,
   depending on the criticality of the use case, to provide a balance
   between cost and effort vs. risk.  AES-128 in counter mode is used
   for encryption.  Cryptographic keys are independent for each device.
   These keys are associated with the device ID and separate integrity
   and confidentiality keys are pre-provisioned.  A confidentiality key
   is only provisioned if confidentiality is to be used.

   The radio protocol has protections against reply attacks, and the
   cloud-based core network provides firewalling protection against
   undesired incoming communications.

8.  Acknowledgements

   Carles Gomez has been funded in part by the ERDF and the Spanish
   Government through project TEC2016-79988-P.

9.  Informative References

   [I-D.ietf-lpwan-ipv6-static-context-hc]
              Minaburo, A., Toutain, L., and C. Gomez, "LPWAN Static
              Context Header Compression (SCHC) and fragmentation for
              IPv6 and UDP", draft-ietf-lpwan-ipv6-static-context-hc-07
              (work in progress), October 2017.

   [I-D.ietf-lpwan-overview]
              Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
              overview-07 (work in progress), October 2017.







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Authors' Addresses

   Juan Carlos Zuniga
   SIGFOX
   425 rue Jean Rostand
   Labege  31670
   France

   Email: JuanCarlos.Zuniga@sigfox.com
   URI:   http://www.sigfox.com/


   Carles Gomez
   Universitat Politecnica de Catalunya
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain

   Email: carlesgo@entel.upc.edu


   Laurent Toutain
   IMT-Atlantique
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   France

   Email: Laurent.Toutain@imt-atlantique.fr






















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