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6TiSCH                                                     J. Munoz, Ed.
Internet-Draft                                                     Inria
Intended status: Informational                             X. Vilajosana
Expires: January 3, 2019                 Universitat Oberta de Catalunya
                                                                T. Chang
                                                            July 2, 2018

       Problem Statement for Generalizing 6TiSCH to Multiple PHYs


   The present document describes the needs that arise when considering
   to use more than one PHY in a IPv6 over the TSCH mode of
   IEEE802.15.4e (6TiSCH) network.  These considerations are present in:
   the choice of the PHY, the MAC layer -TSCH- configuration, the 6top
   protocol, 6LoWPAN and RPL.

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
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   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.  Neighbor Considerations . . . . . . . . . . . . . . . . . . .   3
   3.  MAC Sub-Layer Considerations  . . . . . . . . . . . . . . . .   4
     3.1.  Network Formation . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Discovering Node PHY Capabilities . . . . . . . . . . . .   4
     3.3.  TSCH Configuration  . . . . . . . . . . . . . . . . . . .   5
       3.3.1.  Timeslot Duration . . . . . . . . . . . . . . . . . .   5
       3.3.2.  Channel Hopping Sequence  . . . . . . . . . . . . . .   5
   4.  6top Sub-Layer Considerations . . . . . . . . . . . . . . . .   6
     4.1.  Resource Allocation . . . . . . . . . . . . . . . . . . .   6
     4.2.  Duty Cycle Regulations  . . . . . . . . . . . . . . . . .   6
   5.  6LoWPAN Considerations  . . . . . . . . . . . . . . . . . . .   6
   6.  RPL Considerations  . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
     9.3.  Other Informative References  . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The protocol stack in a IPv6 over the TSCH mode of IEEE802.15.4e
   (6TiSCH) network is defined by multiple protocols covering multiple
   layers starting from the link layer, up to the application layer
   [I-D.ietf-6tisch-architecture].  This protocol stack sits on top of
   the IEEE802.15.4 O-QPSK PHY, at 2.4 GHz, that allows the exchange of
   frames of 127 B over 16 frequencies at 250 kbps.  Since 2012, more
   PHYs are available within the IEEE802.15.4 specification, e.g. the
   IEEE802.15.4g amendment [IEEE802154g] of the IEEE802.15.4 standard,
   designed for Smart Utility Networks (SUN) application, introducing
   the SUN-OFDM, SUN-FSK and SUN-O-QPSK PHYs.  The main differences with
   the previous IEEE802.15.4 O-QPSK PHY is support of link-layer frames
   up to 2047 B long, the possibility of being used either at the same
   2.4 GHz band or in sub-GHz, regionally defined bands, and variable
   data rate that goes from 6.25 kbps up to 800 kbps.

   Radio chips supporting all these new PHY configurations are now
   available, giving the opportunity to implementers to exploit the
   benefits of this diversity in terms of throughput, range and
   reliability that each PHY brings with it.

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   However, the adoption of a PHY different from IEEE802.15.4 O-QPSK
   poses new design considerations across the 6TiSCH protocol stack.
   Even though layer separation exists between protocols, from the link
   layer upwards, 6TiSCH protocols have been designed considering one
   PHY only.  This approach of having links over multiple PHYs in the
   same network is new, and poses up-to-now unknown considerations for
   network designers.

   This document describes how the behavior of each item of the 6TiSCH
   protocol stack may be impacted by the inclusion of multiple PHYs with
   such different properties.

   This document makes the assumption that the reader is familiar with
   the [I-D.ietf-6tisch-terminology] and [I-D.ietf-6tisch-architecture],
   as well as the protocols mentioned there.

   Solutions for the considerations here exposed are out of the scope of
   this document.  This document is to be considered only for
   informative purposes.

2.  Neighbor Considerations

   In a low-power wireless network with a single PHY, a neighbor node to
   a particular node A is any node within its interference domain.  If
   nodes are able to use multiple PHYs, a pair of nodes using a specific
   PHY may be within the same interference domain and when using another
   PHY, they may not.  In addition, nodes also can communicate over
   different frequency bands.  So now the definition of a neighbor node
   changes to any device within the same interference domain for a given
   PHY configuration and frequency band.  This modifies how nodes can
   manage their neighbors' information.

   Neighbor information is accessed by both the MAC and routing layers.
   Letting which layer to handle the multiple PHY information changes
   the network protocol stack significantly.  In case of handling by the
   MAC layer, an entity between MAC and Routing to choose which PHY
   layer to use is required.  This entity could be part of Scheduling
   Function (Section 4.3).  In case of handling by Routing layer, each
   PHY layer could be considered as a neighbor.  For RPL, if only one
   DODAG exist through the network, a dedicated Objective Function for
   multiPHY features is required.  If each PHY layer has a DODAG
   corresponding with, the OF for 6TiSCH could be used with little
   modification.  However, this increases the complexity of the network.

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3.  MAC Sub-Layer Considerations

   The considerations that arise according to the used PHY include
   network formation, node discovery, and TSCH configuration.

3.1.  Network Formation

   Getting nodes to join the network as fast as possible is a major
   interest to minimize energy consumption.  Radio activity is the most
   power consuming task for nodes, therefore the more time nodes spend
   listening to get an Enhanced Beacon (EB) the more energy they
   consume, reducing their lifespan.  Considering a current 6TiSCH
   network, with just one PHY and one frequency band (16 frequencies),
   nodes have to tune their radios in one frequency wait for an EB.
   Nodes which are already part of the network transmit EBs in a round-
   robin fashion on these 16 frequencies.  If the node did not receive
   any EB after some time, it may tune its radio on a different
   frequency and listen again for an EB.  This means a node listen for a
   long time before hearing an EB.

   In the case of multiple PHYs, nodes attempting to join the network
   need to over even more PHYs, until hearing an EB.  A mechanism might
   be needed to reduce join time, for example use a particular PHY for

3.2.  Discovering Node PHY Capabilities

   For 6TiSCH networks using one PHY configuration, discovering the PHY
   neighbor node's capabilities is not necessary.  But in a new multiPHY
   network context, knowing the capabilities of neighbor nodes is
   important.  Once a node is part of the network, it may have not have
   joined using the most convenient PHY configuration for this pair of
   nodes.  Any of these nodes might then (a) unicast a request its
   neighbors to get the information about their PHY capabilities, or (b)
   discover the PHY capabilities of the neighbors by listening for EBs
   at specific times over other PHYs.

   If using (a), further choices need to be taken to decide whether
   nodes would use shared slots or negotiate dedicated timeslots to test
   the connectivity over other PHYs.  Agreeing on which PHY to test and
   when has to be done under the already tested PHY configuration, and
   the energy consumption footprint of this process may be too heavy.
   If using (b), it may take long time until the most efficient PHY
   configuration is discovered between two nodes.

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3.3.  TSCH Configuration

   A multi-PHY approach has an impact on timeslot duration and channel
   hopping sequence.

3.3.1.  Timeslot Duration

   The diversity of data rates of the PHYs in the IEEE802.15.4-2015
   standard makes it challenging to find a timeslot duration that is
   both efficient and fits all PHYs options.  In current 6TiSCH
   networks, a common practice is to have a timeslot of 10 ms.  This is
   time enough for transmitting a 127 B frame using IEEE802.15.4 O-QPSK,
   taking roughly 4 ms, to wait for the acknowledgment, leaving a
   handful of milliseconds for data processing with proper guard times.

   But for multiple PHYs with data rates going from 6.25 kbps up to 800
   kbps and with maximum frame size of 2047 B, the time of transmission
   for a full size packet varies from 0.020 s (800 kbps) to 2.62 s (6.25
   kbps).  With such disparity, considering a timeslot long enough (>
   2.62 s) to allow the transmission (and its acknowledge frame) of the
   maximum frame size with the slowest data rate results in a waste of
   time (network resources) if faster PHY can be used, by leaving the
   most part of the timeslot unused.  Such a long timeslot would cause
   slotframes to have a duration in the order of minutes (considering
   for example a slotframe of 101 timeslots), and as tight
   synchronization is mandatory, multiple KA frames would have to be
   sent within the same slotframe, considerably reducing the network
   resources and efficiency.

   On the other hand, choosing a shorter timeslot poses a rigid
   limitation in the size of the frames when slow data rates PHYs are
   used.  By having timeslots in the order of 10's of ms, the frame size
   for slow data rate is heavily reduced: with a 100 ms timeslot, only
   78 B can be transmitted using 6.25 kbps, without considering time for
   acknowledgment and guard times.

   Multi-PHY designs should therefore tune these parameters to find the
   right trade-off between shorter or longer timeslots (limiting sizes
   of frames with some PHYs), as well as the size of the slotframe.

3.3.2.  Channel Hopping Sequence

   Current 6TiSCH implementations use the 2.4 GHz band, with 16
   frequencies separated by 5 MHz and 2 MHz wide.  Channel hopping
   sequences use only the frequency number identification.  By
   introducing multiple PHYs, these do not have the same characteristics
   of channel spacing, bandwidth nor channel numbering.  Moreover,
   channels from different PHYs may overlap.

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   As a result, by only referring to a channel by some index doesn't
   carry over to multiple PHYs.  Multi-PHY designs need to solve how to
   identify channels.

4.  6top Sub-Layer Considerations

   The 6top sub-layer [I-D.ietf-6tisch-6top-protocol] is responsible for
   allocating cells between pairs of neighbor nodes.  In a multi-PHY
   environment, cells have different capabilities depending on the PHY
   used.  Moreover, in some frequency bands, duty cycle regulation must
   be met.

4.1.  Resource Allocation

   Current 6TiSCH networks account the network resources allocation in
   the amount of cells per slotframe a pair of nodes needs.  In a multi-
   PHY design, allocating cells does not provide enough information,
   since depending on the PHY used, more or less data can fit in a
   timeslot.  Multi-PHY designs have to define how to network resource
   needs are measured.

4.2.  Duty Cycle Regulations

   Duty cycle regulations apply to most frequency bands.  These
   regulations vary from country to country, so multi-PHY designs need
   to comply with local regulations.

5.  6LoWPAN Considerations

   6LoWPAN has been initially designed with IEEE802.15.4 O-QPSK in mind.
   Header compression, fragmentation and reassembly are the main tasks
   of this adaptive layer.  However, in this new context, other PHYs
   allow to send more than 127 bytes in one frame.  6LoWPAN
   functionalities should be adapted to efficiently fit in the layer
   below.  This includes the sizes of the fragments, that should be
   calculated depending on the PHY to be used and the maximum amount of
   data that can transport, given the length of the timeslot.

6.  RPL Considerations

   In multi-PHY design, RPL is impacted in several ways: Objective
   Functions must now consider more than one PHY, and each node's rank
   must be calculated accordingly.

   A multi-PHY design may consider new Objective Functions that take
   into account the difference in throughput, resource occupancy and
   energy consumption of each PHY.  For example, in OF0 [RFC6552] , the

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   'rank_factor' can have a different value for each PHY, depending on
   its characteristics.

7.  Security Considerations

   This document discusses a number of elements to consider when
   designing a multi-PHY solution based on 6TiSCH.  It does not define a
   new protocol.

8.  IANA Considerations

   This document does not require any IANA actions.

9.  References

9.1.  Normative References

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-14 (work
              in progress), April 2018.

              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terms Used in IPv6 over the TSCH mode of IEEE 802.15.4e",
              draft-ietf-6tisch-terminology-10 (work in progress), March

   [RFC6552]  Thubert, P., Ed., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, DOI 10.17487/RFC6552, March 2012,

9.2.  Informative References

              Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer Protocol (6P)", draft-ietf-6tisch-6top-
              protocol-12 (work in progress), June 2018.

9.3.  Other Informative References

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              IEEE standard for Information Technology, "IEEE standard
              for Information Technology, IEEE Std. 802.15.4, Part.
              15.4: Wireless Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications for Low-Rate Wireless Personal
              Area Networks, June 2011 as amended by IEEE Std.
              802.15.4g, Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 3: Physical Layer (PHY)
              Specifications for Low Data-Rate, Wireless, Smart Metering
              Utility Networks", April 2012.

Authors' Addresses

   Jonathan Munoz (editor)
   2 rue Simone Iff
   Paris 12  75012

   Email: jonathan.munoz@inria.fr

   Xavier Vilajosana
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   Barcelona, Catalonia  08018

   Email: xvilajosana@uoc.edu

   Tengfei Chang
   2 rue Simone Iff
   Paris 12  75012

   Email: tengfei.chang@inria.fr

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