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Versions: (draft-vilajosana-6tisch-minimal) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21

6TiSCH                                                X. Vilajosana, Ed.
Internet-Draft                           Universitat Oberta de Catalunya
Intended status: Best Current Practice                         K. Pister
Expires: August 24, 2017               University of California Berkeley
                                                             T. Watteyne
                                                       Linear Technology
                                                       February 20, 2017


                      Minimal 6TiSCH Configuration
                      draft-ietf-6tisch-minimal-21

Abstract

   This document describes a minimal mode of operation for an IPv6 over
   the TSCH mode of IEEE 802.15.4e (6TiSCH) Network.  This minimal mode
   of operation specifies the baseline set of protocols that need to be
   supported, recommended configurations and modes of operation
   sufficient to enable a 6TiSCH functional network.  6TiSCH provides
   IPv6 connectivity over a Time Synchronized Channel Hopping (TSCH)
   mesh composed of IEEE Std 802.15.4 TSCH links.  This minimal mode
   uses a collection of protocols with the respective configurations,
   including the 6LoWPAN framework, enabling interoperable IPv6
   connectivity over IEEE Std 802.15.4 TSCH.  This minimal configuration
   provides the necessary bandwidth for network and security bootstrap,
   and defines the proper link between the IETF protocols that interface
   to IEEE Std 802.15.4 TSCH.  This minimal mode of operation should be
   implemented by all 6TiSCH compliant devices.

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 August 24, 2017.






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Copyright Notice

   Copyright (c) 2017 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
   (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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IEEE Std 802.15.4 Settings  . . . . . . . . . . . . . . . . .   4
     4.1.  TSCH Schedule . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Cell Options  . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Retransmissions . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Timeslot Timing . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Frame Contents  . . . . . . . . . . . . . . . . . . . . .   7
       4.5.1.  IEEE Std 802.15.4 Header  . . . . . . . . . . . . . .   8
       4.5.2.  Enhanced Beacon Frame . . . . . . . . . . . . . . . .   8
       4.5.3.  Acknowledgment Frame  . . . . . . . . . . . . . . . .   9
     4.6.  Link-Layer Security . . . . . . . . . . . . . . . . . . .   9
   5.  RPL Settings  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Objective Function  . . . . . . . . . . . . . . . . . . .  10
       5.1.1.  Rank Computation  . . . . . . . . . . . . . . . . . .  11
       5.1.2.  Rank Computation Example  . . . . . . . . . . . . . .  12
     5.2.  Mode of Operation . . . . . . . . . . . . . . . . . . . .  13
     5.3.  Trickle Timer . . . . . . . . . . . . . . . . . . . . . .  13
     5.4.  Packet Contents . . . . . . . . . . . . . . . . . . . . .  13
   6.  Network Formation and Lifetime  . . . . . . . . . . . . . . .  13
     6.1.  Value of the Join Metric Field  . . . . . . . . . . . . .  13
     6.2.  Time Source Neighbor Selection  . . . . . . . . . . . . .  14
     6.3.  When to Start Sending EBs . . . . . . . . . . . . . . . .  14
     6.4.  Hysteresis  . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Implementation Recommendations  . . . . . . . . . . . . . . .  15
     7.1.  Neighbor Table  . . . . . . . . . . . . . . . . . . . . .  15
     7.2.  Queues and Priorities . . . . . . . . . . . . . . . . . .  15
     7.3.  Recommended Settings  . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18



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   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
     11.3.  External Informative References  . . . . . . . . . . . .  21
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  21
     A.1.  Example: EB with Default Timeslot Template  . . . . . . .  21
     A.2.  Example: EB with Custom  Timeslot Template  . . . . . . .  23
     A.3.  Example: Link-layer Acknowledgment  . . . . . . . . . . .  25
     A.4.  Example: Auxiliary Security Header  . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   A 6TiSCH network provides IPv6 connectivity [RFC2460] over a Time
   Synchronized Channel Hopping (TSCH) mesh [RFC7554] composed of IEEE
   Std 802.15.4 TSCH links [IEEE802154-2015].  IPv6 connectivity is
   obtained by the use of the 6LoWPAN framework ([RFC4944], [RFC6282],
   [RFC8025],[I-D.ietf-roll-routing-dispatch] and [RFC6775]), RPL
   [RFC6550], and its Objective Function 0 (OF0) [RFC6552].

   This specification defines operational parameters and procedures for
   a minimal mode of operation to build a 6TiSCH Network.  Any 6TiSCH
   compliant device should implement this mode of operation.  This
   operational parameter configuration provides the necessary bandwidth
   for nodes to bootstrap the network.  The bootstrap process includes
   initial network configuration and security bootstrap.  In this
   specification, the 802.15.4 TSCH mode, the 6LoWPAN framework, RPL
   [RFC6550], and its Objective Function 0 (OF0) [RFC6552] are used
   unmodified.  Parameters and particular operations of TSCH are
   specified to guarantee interoperability between nodes in a 6TiSCH
   Network.

   In a 6TiSCH network, nodes follow a communication schedule as per
   802.15.4 TSCH.  In it, nodes learn the schedule of the network when
   joining.  When following this specification, the learned schedule is
   the same for all nodes and does not change over time.  Future
   specifications may define mechanisms for dynamically managing the
   communication schedule.  Dynamic scheduling solutions are out of
   scope of this document.

   IPv6 addressing and compression are achieved by the 6LoWPAN
   framework.  The framework includes [RFC4944], [RFC6282], [RFC8025],
   the 6LoWPAN Routing Header dispatch [I-D.ietf-roll-routing-dispatch]
   for addressing and header compression, and [RFC6775] for duplicate
   address detection (DAD) and address resolution.





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   More advanced work is expected in the future to complement the
   Minimal Configuration with dynamic operations that can adapt the
   schedule to the needs of the traffic at run time.

2.  Requirements Language

   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 RFC 2119 [RFC2119].

3.  Terminology

   This document uses terminology from [I-D.ietf-6tisch-terminology].
   The following concepts are used in this document:

   802.15.4:  We use "802.15.4" as a short version of "IEEE Std
      802.15.4" in this document.

   SFD:  Start of Frame Delimiter.

   RX:  Reception.

   TX:  Transmission.

   IE:  Information Element.

   EB:  Enhanced Beacon.

   ASN:  Absolute Slot Number.

   Join Metric:  Field in the TSCH Synchronization IE representing the
      topological distance between the node sending the EB and the PAN
      coordinator.

4.  IEEE Std 802.15.4 Settings

   An implementation compliant to this specification MUST implement IEEE
   Std 802.15.4 [IEEE802154-2015] in "timeslotted channel hopping"
   (TSCH) mode.

   The remainder of this section details the RECOMMENDED TSCH settings,
   which are summarized in Figure 1.  Any of the properties marked in
   the EB column are announced in the Enhanced Beacons (EB) the nodes
   send [IEEE802154-2015] and learned by those joining the network.
   Changing their value hence means changing the contents of the EB.






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   In case of discrepancy between the values in this specification and
   IEEE Std 802.15.4 [IEEE802154-2015], the IEEE standard has
   precedence.

   +--------------------------------+------------------------------+---+
   |           Property             |     Recommended Setting      |EB*|
   +--------------------------------+------------------------------+---+
   | Slotframe Size                 | Tunable. Trades-off          | X |
   |                                | bandwidth against energy.    |   |
   +--------------------------------+------------------------------+---+
   | Number of scheduled cells**    | 1                            | X |
   | (active)                       | Timeslot        0x0000       |   |
   |                                | Channel Offset  0x0000       |   |
   |                                | Link Options = (TX Link = 1, |   |
   |                                | RX Link = 1, Shared Link = 1,|   |
   |                                | Timekeeping = 1)             |   |
   +--------------------------------+------------------------------+---+
   | Number of unscheduled cells    | All remaining cells in the   | X |
   | (off)                          | slotframe                    |   |
   +--------------------------------+------------------------------+---+
   | Max Number MAC retransmissions | 3 (4 transmission attempts)  |   |
   +--------------------------------+------------------------------+---+
   | Timeslot template              | IEEE Std 802.15.4 default    | X |
   |                                | (macTimeslotTemplateId=0)    |   |
   +--------------------------------+------------------------------+---+
   | Enhanced Beacon Period         | Tunable. Trades-off join     |   |
   | (EB_PERIOD)                    | time against energy.         |   |
   +--------------------------------+------------------------------+---+
   | Number used frequencies        | IEEE Std 802.15.4 default    | X |
   | (2.4 GHz O-QPSK PHY)           | (16)                         |   |
   +--------------------------------+------------------------------+---+
   | Channel Hopping sequence       | IEEE Std 802.15.4 default    | X |
   | (2.4 GHz O-QPSK PHY)           | (macHoppingSequenceID = 0)   |   |
   +--------------------------------+------------------------------+---+
     * an "X" in this column means this property's value is announced
       in the EB; a new node hence learns it when joining.
    ** This cell LinkType is set to ADVERTISING.

          Figure 1: Recommended IEEE Std 802.15.4 TSCH Settings.

4.1.  TSCH Schedule

   This minimal mode of operation uses a single slotframe.  The TSCH
   slotframe is composed of a tunable number of timeslots.  The
   slotframe size (i.e. the number of timeslots it contains) trades off
   bandwidth for energy consumption.  The slotframe size needs to be
   tuned; the way of tuning it is out of scope of this specification.
   The slotframe size is announced in the EB.  The RECOMMENDED value for



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   the slotframe handle (macSlotframeHandle) is 0x00.  An implementation
   MAY choose to use a different slotframe handle, for example to add
   other slotframes with higher priority.  The use of other slotframes
   is out of the scope of this document.

   There is only a single scheduled cell in the slotframe.  This cell
   MAY be scheduled at any slotOffset/channelOffset within the
   slotframe.  The location of that cell in the schedule is announced in
   the EB.  The LinkType of the scheduled cell is ADVERTISING to allow
   EBs to be sent on it.

   Figure 2 shows an example of a slotframe of length 101 timeslots,
   resulting in a radio duty cycle below 0.99%.

      Chan.  +----------+----------+          +----------+
      Off.0  | TxRxS/EB |   OFF    |          |   OFF    |
      Chan.  +----------+----------+          +----------+
      Off.1  |   OFF    |   OFF    |   ...    |   OFF    |
             +----------+----------+          +----------+
                 .
                 .
                 .
      Chan.  +----------+----------+          +----------+
      Off.15 |   OFF    |   OFF    |          |   OFF    |
             +----------+----------+          +----------+

   slotOffset     0          1                    100

   EB:  Enhanced Beacon
   Tx:  Transmit
   Rx:  Receive
   S:   Shared
   OFF: Unscheduled by this specification

           Figure 2: Example slotframe of length 101 timeslots.

   A node MAY use the scheduled cell to transmit/receive all types of
   link-layer frames.  EBs are sent to the link-layer broadcast address
   and not acknowledged.  Data frames are sent unicast, and acknowledged
   by the receiving neighbor.

   All remaining cells in the slotframe are unscheduled.  Dynamic
   scheduling solutions may be defined in the future which schedule
   those cells.  One example is the 6top Protocol (6P)
   [I-D.ietf-6tisch-6top-protocol].  Dynamic scheduling solutions are
   out of scope of this document.





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   The default values of the TSCH Timeslot template (defined in
   [IEEE802154-2015] Section 8.4.2.2.3) and Channel Hopping sequence
   (defined in [IEEE802154-2015] Section 6.2.10) SHOULD be used.  A node
   MAY use different values by properly announcing them in its Enhanced
   Beacon.

4.2.  Cell Options

   In the scheduled cell, a node transmits if there is a packet to
   transmit, listens otherwise (both "TX" and "RX" bits are set).  When
   a node transmits, requesting a link-layer acknowledgment per
   [IEEE802154-2015], and does not receive it, it uses a back-off
   mechanism to resolve possible collisions ("Shared" bit is set).  A
   node joining the network maintains time synchronization to its
   initial time source neighbor using that cell ("Timekeeping" bit is
   set).

   This translates into a Link Option for this cell:

      b0 = TX Link = 1 (set)
      b1 = RX Link = 1 (set)
      b2 = Shared Link = 1 (set)
      b3 = Timekeeping = 1 (set)
      b4 = Priority = 0 (clear)
      b5-b7 = Reserved = 0 (clear)

4.3.  Retransmissions

   Per Figure 1, the RECOMMENDED maximum number of link-layer
   retransmissions is 3.  This means that, for packets requiring an
   acknowledgment, if none are received after a total of 4 attempts, the
   transmission is considered failed and the link layer MUST notify the
   upper layer.  Packets not requiring an acknowledgment (including EBs)
   are not retransmitted.

4.4.  Timeslot Timing

   Per Figure 1, the RECOMMENDED timeslot template is the default one
   (macTimeslotTemplateId=0) defined in [IEEE802154-2015].

4.5.  Frame Contents

   [IEEE802154-2015] defines the format of frames.  Through a set of
   flags, [IEEE802154-2015] allows for several fields to be present or
   not, to have different lengths, and to have different values.  This
   specification details the RECOMMENDED contents of 802.15.4 frames,
   while strictly complying to [IEEE802154-2015].




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4.5.1.  IEEE Std 802.15.4 Header

   The Frame Version field MUST be set to 0b10 (Frame Version 2).  The
   Sequence Number field MAY be elided.

   EB Destination Address field MUST be set to 0xFFFF (short broadcast
   address).  The EB Source Address field SHOULD be set as the node's
   short address if this is supported.  Otherwise the long address MUST
   be used.

   The PAN ID Compression bit SHOULD indicate that the Source PAN ID is
   "Not Present" and the Destination PAN ID is "Present".  The value of
   the PAN ID Compression bit is specified in Table 7-2 of the IEEE Std
   802.15.4-2015 specification, and depends on the type of the
   destination and source link-layer addresses (short, extended, not
   present).

   Nodes follow the reception and rejection rules as per Section 6.7.2
   of [IEEE802154-2015].

   The Nonce is formatted according to [IEEE802154-2015].  In the IEEE
   Std 802.15.4 specification [IEEE802154-2015], nonce generation is
   described in Section 9.3.2.2, and byte ordering in Section 9.3.1,
   Annex B.2 and Annex B.2.2.

4.5.2.  Enhanced Beacon Frame

   After booting, a TSCH node starts in an unsynchronized, unjoined
   state.  Initial synchronization is achieved by listening for EBs.
   EBs from multiple networks may be heard.  Many mechanisms exist for
   discrimination between networks, the details of which are out of
   scope.

   The IEEE Std 802.15.4 specification does not define how often EBs are
   sent, nor their contents [IEEE802154-2015].  In a minimal TSCH
   configuration, a node SHOULD send an EB every EB_PERIOD.  Tuning
   EB_PERIOD allows a trade-off between joining time and energy
   consumption.

   EBs should be used to obtain information about local networks, and to
   synchronize ASN and time offset of the specific network that the node
   decides to join.  Once joined to a particular network, a node MAY
   choose to continue to listen for EBs, to gather more information
   about other networks, for example.  During the joining process,
   before secure connections to time parents have been created, a node
   MAY maintain synchronization using EBs.  [RFC7554] discusses
   different time synchronization approaches.




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   The IEEE Std 802.15.4 specification requires EBs to be send in order
   to enable nodes to join the network.  The EBs SHOULD carry the
   Information Elements (IEs) listed below [IEEE802154-2015].

   TSCH Synchronization IE:  Contains synchronization information such
      as ASN and Join Metric.  The value of the Join Metric field is
      discussed in Section 6.1.

   TSCH Timeslot IE:  Contains the timeslot template identifier.  This
      template is used to specify the internal timing of the timeslot.
      This specification RECOMMENDS the default timeslot template.

   Channel Hopping IE:  Contains the channel hopping sequence
      identifier.  This specification RECOMMENDS the default channel
      hopping sequence.

   TSCH Slotframe and Link IE:  Enables joining nodes to learn the
      initial schedule to be used as they join the network.  This
      document RECOMMENDS the use of a single cell.

   If a node strictly follows the recommended setting from Figure 1, the
   EB it sends has the exact same contents as an EB it has received when
   joining, except for the Join Metric field in the TSCH Synchronization
   IE.

   When a node has already joined a network, i.e. it has received an EB,
   synchronized to the EB sender and configured its schedule following
   this specification, the node SHOULD ignore subsequent EBs which try
   to change the configured parameters.  This does not preclude
   listening EBs from other networks.

4.5.3.  Acknowledgment Frame

   Per [IEEE802154-2015], each acknowledgment contain an ACK/NACK Time
   Correction IE.

4.6.  Link-Layer Security

   When securing link-layer frames, link-layer frames MUST be secured by
   the link-layer security mechanisms defined in IEEE Std 802.15.4
   [IEEE802154-2015].  Link-layer authentication MUST be applied to the
   entire frame, including the 802.15.4 header.  Link-layer encryption
   MAY be applied to 802.15.4 payload IEs and the 802.15.4 payload.

   This specification assumes the existence of two cryptographic keys:






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      Key K1 is used to authenticate EBs.  EBs MUST be authenticated
      only (no encryption), and their contents is defined in
      Section 4.5.2.

      Key K2 is used to authenticate and encrypt DATA and ACKNOWLEDGMENT
      frames.

   These keys can be pre-configured, or learned during a key
   distribution phase.  Key distribution mechanisms are defined for
   example in [I-D.ietf-6tisch-minimal-security] and
   [I-D.ietf-6tisch-dtsecurity-secure-join].  Key distribution is out of
   scope of this document.

   The behavior of a Joining Node (JN) is different depending on which
   key(s) are pre-configured:

      If both keys K1 and K2 are pre-configured, the JN does not rely on
      a key distribution phase to learn K1 or K2.

      If key K1 is pre-configured but not key K2, the JN authenticates
      EBs using K1, and relies on the key distribution phase to learn
      K2.

      If neither key K1 nor key K2 is pre-configured, the JN accepts EBs
      as defined in Section 6.3.1.2 of IEEE Std 802.15.4
      [IEEE802154-2015], i.e., they are passed forward even "if the
      status of the unsecuring process indicated an error".  The JN then
      runs key distribution phase to learn K1 and K2.  During that
      process, the node JN is talking to uses the secExempt mechanism
      (IEEE Std 802.15.4, Section 9.2.4) to process frames from JN.
      Once the key distribution phase is done, the node which has
      installed secExempts for the JN MUST clear the installed exception
      rules.

   In the event of a network reset, the new network MUST either use new
   cryptographic keys, or ensure that the ASN remains monotonically
   increasing.

5.  RPL Settings

   In a multi-hop topology, the RPL routing protocol [RFC6550] MAY be
   used.

5.1.  Objective Function

   If RPL is used, nodes MUST implement the RPL Objective Function Zero
   (OF0) [RFC6552].




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5.1.1.  Rank Computation

   The Rank computation is described at [RFC6552], Section 4.1.  A
   node's Rank (see Figure 4 for an example) is computed by the
   following equations:

      R(N) = R(P) + rank_increment

      rank_increment = (Rf*Sp + Sr) * MinHopRankIncrease

   Figure 3 lists the OF0 parameter values that MUST be used if RPL is
   used.

       +----------------------+-------------------------------------+
       |    OF0 Parameters    |              Value                  |
       +----------------------+-------------------------------------+
       | Rf                   |                                   1 |
       +----------------------+-------------------------------------+
       | Sp                   |                           (3*ETX)-2 |
       +----------------------+-------------------------------------+
       | Sr                   |                                   0 |
       +----------------------+-------------------------------------+
       | MinHopRankIncrease   | DEFAULT_MIN_HOP_RANK_INCREASE (256) |
       +----------------------+-------------------------------------+
       | MINIMUM_STEP_OF_RANK |                                   1 |
       +----------------------+-------------------------------------+
       | MAXIMUM_STEP_OF_RANK |                                   9 |
       +----------------------+-------------------------------------+
       | ETX limit to select  |                                   3 |
       | a parent             |                                     |
       +----------------------+-------------------------------------+

                         Figure 3: OF0 parameters.

   The step_of_rank (Sp) uses Expected Transmission Count (ETX)
   [RFC6551].

   An implementation MUST follow OF0's normalization guidance as
   discussed in Section 1 and Section 4.1 of [RFC6552].  Sp SHOULD be
   calculated as (3*ETX)-2.  The minimum value of Sp
   (MINIMUM_STEP_OF_RANK) indicates a good quality link.  The maximum
   value of Sp (MAXIMUM_STEP_OF_RANK) indicates a poor quality link.
   The default value of Sp (DEFAULT_STEP_OF_RANK) indicates an average
   quality link.  Candidate parents with ETX greater than 3 SHOULD NOT
   be selected.  This avoids having ETX values on used links which are
   larger that the maximum allowed transmission attempts.





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5.1.2.  Rank Computation Example

   This section illustrates the use of the Objective Function Zero (see
   Figure 4).  We have:

      rank_increment = ((3*numTx/numTxAck)-2)*minHopRankIncrease = 512

       +-------+
       |   0   | R(minHopRankIncrease) = 256
       |       | DAGRank(R(0)) = 1
       +-------+
           |
           |
       +-------+
       |   1   | R(1)=R(0) + 512 = 768
       |       | DAGRank(R(1)) = 3
       +-------+
           |
           |
       +-------+
       |   2   | R(2)=R(1) + 512 = 1280
       |       | DAGRank(R(2)) = 5
       +-------+
           |
           |
       +-------+
       |   3   | R(3)=R(2) + 512 = 1792
       |       | DAGRank(R(3)) = 7
       +-------+
           |
           |
       +-------+
       |   4   | R(4)=R(3) + 512 = 2304
       |       | DAGRank(R(4)) = 9
       +-------+
           |
           |
       +-------+
       |   5   | R(5)=R(4) + 512 = 2816
       |       | DAGRank(R(5)) = 11
       +-------+

   Figure 4: Rank computation example for 5-hop network where numTx=100
                      and numTxAck=75 for all links.







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5.2.  Mode of Operation

   When RPL is used, nodes MUST implement the non-storing ([RFC6550]
   Section 9.7) mode of operation.  The storing ([RFC6550] Section 9.8)
   mode of operation SHOULD be implemented by nodes with enough
   capabilities.  Nodes not implementing RPL MUST join as leaf nodes.

5.3.  Trickle Timer

   RPL signaling messages such as DIOs are sent using the Trickle
   Algorithm [RFC6550] (Section 8.3.1) and [RFC6206] (Section 4.2).  For
   this specification, the Trickle Timer MUST be used with the RPL
   defined default values [RFC6550] (Section 8.3.1).

5.4.  Packet Contents

   RPL information and hop-by-hop extension headers MUST follow
   [RFC6553] and [RFC6554].  For cases in which the packets formed at
   the LLN need to cross through intermediate routers, these MUST follow
   the IP-in-IP encapsulation requirement specified by [RFC6282] and
   [RFC2460].  Routing extension headers such as RPI [RFC6550] and SRH
   [RFC6554], and outer IP headers in case of encapsulation MUST be
   compressed according to [I-D.ietf-roll-routing-dispatch] and
   [RFC8025].

6.  Network Formation and Lifetime

6.1.  Value of the Join Metric Field

   The Join Metric of the TSCH Synchronization IE in the EB MUST be
   calculated based on the routing metric of the node, normalized to a
   value between 0 and 255.  A lower value of the Join Metric indicates
   the node sending the EB is topologically "closer" to the root of the
   network.  A lower value of the Join Metric hence indicates higher
   preference for a joining node to synchronize to that neighbor.

   In case the network uses RPL, the Join Metric of any node (including
   the DAG root) MUST be set to DAGRank(rank)-1.  According to
   Section 5.1.1, DAGRank(rank(0)) = 1.  DAGRank(rank(0))-1 = 0 is
   compliant with 802.15.4's requirement of having the root use Join
   Metric = 0.

   In case the network does not use RPL, the Join Metric value MUST
   follow the rules specified by [IEEE802154-2015].







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6.2.  Time Source Neighbor Selection

   When a node joins a network, it may hear EBs sent by different nodes
   already in the network.  The decision of which neighbor to
   synchronize to (e.g. which neighbor becomes the node's initial time
   source neighbor) is implementation-specific.  For example, after
   having received the first EB, a node MAY listen for at most
   MAX_EB_DELAY seconds until it has received EBs from
   NUM_NEIGHBOURS_TO_WAIT distinct neighbors.  Recommended values for
   MAX_EB_DELAY and NUM_NEIGHBOURS_TO_WAIT are defined in Figure 5.
   When receiving EBs from distinct neighbors, the node MAY use the Join
   Metric field in each EB to select the initial time source neighbor,
   as described in IEEE Std 802.15.4 [IEEE802154-2015], Section 6.3.6.

   At any time, a node MUST maintain synchronization to at least one
   time source neighbor.  A node's time source neighbor MUST be chosen
   among the neighbors in its RPL routing parent set when RPL is used.
   In the case a node cannot maintain connectivity to at least one time
   source neighbor, the node looses synchronization and needs to join
   the network again.

6.3.  When to Start Sending EBs

   When a RPL node joins the network, it MUST NOT send EBs before having
   acquired a RPL Rank to avoid inconsistencies in the time
   synchronization structure.  This applies to other routing protocols
   with their corresponding routing metrics.  As soon as a node acquires
   routing information (e.g. a RPL Rank, see Section 5.1.1), it SHOULD
   start sending Enhanced Beacons.

6.4.  Hysteresis

   Per [RFC6552] and [RFC6719], the specification RECOMMENDS the use of
   a boundary value (PARENT_SWITCH_THRESHOLD) to avoid constant changes
   of the parent when ranks are compared.  When evaluating a parent that
   belongs to a smaller path cost than the current minimum path, the
   candidate node is selected as new parent only if the difference
   between the new path and the current path is greater than the defined
   PARENT_SWITCH_THRESHOLD.  Otherwise, the node MAY continue to use the
   current preferred parent.  Per [RFC6719], the PARENT_SWITCH_THRESHOLD
   SHOULD be set to 192 when ETX metric is used (in the form 128*ETX),
   the recommendation for this document is to use
   PARENT_SWITCH_THRESHOLD equal to 640 if the metric being used is
   ((3*ETX)-2)*minHopRankIncrease, or a proportional value.  This deals
   with hysteresis both for routing parent and time source neighbor
   selection.





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7.  Implementation Recommendations

7.1.  Neighbor Table

   The exact format of the neighbor table is implementation-specific.
   The RECOMMENDED per-neighbor information is (taken from the [openwsn]
   implementation):

   identifier: Identifier(s) of the neighbor (e.g.  EUI-64).

   numTx:      Number of link-layer transmission attempts to that
               neighbor.

   numTxAck:   Number of transmitted link-layer frames that have been
               link-layer acknowledged by that neighbor.

   numRx:      Number of link-layer frames received from that neighbor.

   timestamp:  When the last frame was received from that neighbor.
               This can be based on the ASN counter or any other time
               base.  It can be used to trigger a keep-alive message.

   routing metric:  Such as the RPL Rank of that neighbor.

   time source neighbor:  A flag indicating whether this neighbor is a
               time source neighbor.

7.2.  Queues and Priorities

   The IEEE Std 802.15.4 specification [IEEE802154-2015] does not define
   the use of queues to handle upper-layer data (either application or
   control data from upper layers).  The following rules are
   RECOMMENDED:

      A node is configured to keep in the queues a configurable number
      of upper-layer packets per link (default NUM_UPPERLAYER_PACKETS)
      for a configurable time that should cover the join process
      (default MAX_JOIN_TIME).

      Frames generated by the 802.15.4 layer (including EBs) are queued
      with a priority higher than frames coming from higher-layers.

      Frame type BEACON is queued with higher priority than frame types
      DATA.







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7.3.  Recommended Settings

   Figure 5 lists RECOMMENDED values for the settings discussed in this
   specification.

           +-------------------------+-------------------+
           | Parameter               | RECOMMENDED Value |
           +-------------------------+-------------------+
           | MAX_EB_DELAY            |               180 |
           +-------------------------+-------------------+
           | NUM_NEIGHBOURS_TO_WAIT  |                 2 |
           +-------------------------+-------------------+
           | PARENT_SWITCH_THRESHOLD |               640 |
           +-------------------------+-------------------+
           | NUM_UPPERLAYER_PACKETS  |                 1 |
           +-------------------------+-------------------+
           | MAX_JOIN_TIME           |               300 |
           +-------------------------+-------------------+

                      Figure 5: Recommended Settings.

8.  Security Considerations

   This document is concerned only with link-layer security.

   By their nature, many IoT networks have nodes in physically
   vulnerable locations.  We should assume that nodes will be physically
   compromised, their memories examined, and their keys extracted.
   Fixed secrets will not remain secret.  This impacts the node joining
   process.  Provisioning a network with a fixed link key K2 is not
   secure.  For most applications, this implies that there will be a
   joining phase during which some level of authorization will be
   allowed for nodes which have not been authenticated.  Details are out
   of scope, but the link layer must provide some flexibility here.

   If an attacker has obtained K1 it can generate fake EBs to attack
   whole network by sending authenticated EBs.  The attacker can cause
   the joining node to initiate the joining process to the attacker.  In
   the case that the joining process includes authentication and
   distribution of a K2, then the joining process will fail and the JN
   will notice the attack.  If K2 is also compromised the JN will not
   notice the attack and the network will be compromised.

   Even if an attacker does not know the value of K1 and K2
   (Section 4.6), it can still generate fake EB frames, authenticated
   with an arbitrary key.  We here discuss the impact these fake EBs can
   have, depending on what key(s) are pre-provisioned.




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      If both K1 and K2 are pre-provisioned, a joining node can
      distinguish legitimate from fake EBs, and join the legitimate
      network.  The fake EBs have no impact.

      The same holds if K1 is pre-provisioned but not K2.

      If neither K1 nor K2 is pre-provisioned, a joining node may
      mistake a fake EB for a legitimate one and initiate a joining
      process to the attacker.  That joining process will fail, as the
      joining node will not be able to authenticate the attacker during
      the security handshake.  This will force the joining node to start
      over listening for an EB.  So while the joining node never joins
      the attacker, this costs the joining node time and energy, and is
      a vector of attack.

   Choosing what key(s) to pre-provision need to balance the different
   discussions above.

   Once the joining process is over, the node that has joined can
   authenticate EBs (it knows K1).  This means it can process their
   contents and use EBs for synchronization.

   ASN provides a nonce for security operations in a slot.  Any re-use
   of ASN with a given key exposes information about encrypted packet
   contents, and risks replay attacks.  Replay attacks are prevented
   because, when the network resets, either the new network uses new
   cryptographic key(s), or ensures that the ASN increases monotonically
   (Section 4.6).

   Maintaining accurate time synchronization is critical for network
   operation.  Accepting timing information from unsecured sources MUST
   be avoided during normal network operation, as described in
   Section 4.5.2.  During joining, a node may be susceptible to timing
   attacks before key K1 and K2 are learned.  During network operation,
   a node MAY maintain statistics on time updates from neighbors and
   monitor for anomalies.

   Denial of Service (DoS) attacks at the MAC layer in an LLN are easy
   to achieve simply by RF jamming.  This is the base case against which
   more sophisticated DoS attacks should be judged.  For example,
   sending fake EBs announcing a very low Join Metric may cause a node
   to waste time and energy trying to join a fake network even when
   legitimate EBs are being heard.  Proper join security will prevent
   the node from joining the false flag, but by then the time and energy
   will have been wasted.  However, the energy cost to the attacker
   would be lower and the energy cost to the joining node higher if the
   attacker simply sent loud short packets in the middle of any valid EB
   that it hears.



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   ACK reception probability is less than 100%, due to changing channel
   conditions and unintentional or intentional jamming.  This will cause
   the sending node to retransmit the same packet until it is
   acknowledged or a retransmission limit is reached.  Upper layer
   protocols should take this into account, possibly using a sequence
   number to match retransmissions.

   The 6TiSCH layer SHOULD keep track of anomalous events and report
   them to a higher authority.  For example, EBs reporting low Join
   Metrics for networks which cannot be joined, as described above, may
   be a sign of attack.  Additionally, in normal network operation,
   message integrity check failures on packets with valid CRC will occur
   at a rate on the order of once per million packets.  Any significant
   deviation from this rate may be a sign of network attack.  Along the
   same lines, time updates in ACKs or EBs that are inconsistent with
   the MAC-layer's sense of time and its own plausible time error drift
   rate may also be a result of network attack.

9.  IANA Considerations

   This document requests no immediate action by IANA.

10.  Acknowledgments

   The authors acknowledge the guidance and input from Rene Struik, Pat
   Kinney, Michael Richardson, Tero Kivinen, Nicola Accettura, Malisa
   Vucinic and Jonathan Simon.  Thanks to Charles Perkins, Brian E.
   Carpenter, Ralph Droms, Warren Kumari, Mirja Kuehlewind, Ben
   Campbell, Benoit Claise and Suresh Krishnan for the exhaustive and
   detailed reviews.  Thanks to Simon Duquennoy, Guillaume Gaillard,
   Tengfei Chang and Jonathan Munoz for the detailed review of the
   examples section.  Thanks to 6TiSCH co-chair Pascal Thubert for his
   guidance and advice.

11.  References

11.1.  Normative References

   [I-D.ietf-roll-routing-dispatch]
              Thubert, P., Bormann, C., Toutain, L., and R. Cragie,
              "6LoWPAN Routing Header", draft-ietf-roll-routing-
              dispatch-05 (work in progress), October 2016.

   [IEEE802154-2015]
              IEEE standard for Information Technology, "IEEE Std
              802.15.4-2015 Standard for Low-Rate Wireless Personal Area
              Networks (WPANs)", December 2015.




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   [RFC8025]  Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
              RFC 8025, DOI 10.17487/RFC8025, November 2016,
              <http://www.rfc-editor.org/info/rfc8025>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <http://www.rfc-editor.org/info/rfc6775>.

   [RFC6719]  Gnawali, O. and P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 6719,
              DOI 10.17487/RFC6719, September 2012,
              <http://www.rfc-editor.org/info/rfc6719>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <http://www.rfc-editor.org/info/rfc6554>.

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,
              <http://www.rfc-editor.org/info/rfc6553>.

   [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,
              <http://www.rfc-editor.org/info/rfc6552>.

   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,
              <http://www.rfc-editor.org/info/rfc6551>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <http://www.rfc-editor.org/info/rfc6550>.






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   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <http://www.rfc-editor.org/info/rfc6282>.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
              March 2011, <http://www.rfc-editor.org/info/rfc6206>.

   [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,
              <http://www.rfc-editor.org/info/rfc4944>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

11.2.  Informative References

   [I-D.ietf-6tisch-6top-protocol]
              Wang, Q. and X. Vilajosana, "6top Protocol (6P)", draft-
              ietf-6tisch-6top-protocol-03 (work in progress), October
              2016.

   [I-D.ietf-6tisch-terminology]
              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-08 (work in
              progress), December 2016.

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., and K. Pister, "Minimal Security
              Framework for 6TiSCH", draft-ietf-6tisch-minimal-
              security-01 (work in progress), February 2017.

   [I-D.ietf-6tisch-dtsecurity-secure-join]
              Richardson, M., "6tisch Secure Join protocol", draft-ietf-
              6tisch-dtsecurity-secure-join-00 (work in progress),
              December 2016.






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   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,
              <http://www.rfc-editor.org/info/rfc7554>.

11.3.  External Informative References

   [openwsn]  Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F.,
              Weekly, K., Wang, Q., Glaser, S., and K. Pister, "OpenWSN:
              a Standards-Based Low-Power Wireless Development
              Environment", Transactions on Emerging Telecommunications
              Technologies , August 2012.

Appendix A.  Examples

   This section contains several example packets.  Each example contains
   (1) a schematic header diagram, (2) the corresponding bytestream, (3)
   a description of each of the IEs that form the packet.  Packet
   formats are specific for the [IEEE802154-2015] revision and may vary
   in future releases of the IEEE standard.  In case of differences
   between the packet content presented in this section and
   [IEEE802154-2015], the latter has precedence.

   The MAC header fields are described in a specific order.  All field
   formats in this examples are depicted in the order in which they are
   transmitted, from left to right, where the leftmost bit is
   transmitted first.  Bits within each field are numbered from 0
   (leftmost and least significant) to k - 1 (rightmost and most
   significant), where the length of the field is k bits.  Fields that
   are longer than a single octet are sent to the PHY in the order from
   the octet containing the lowest numbered bits to the octet containing
   the highest numbered bits (little endian).

A.1.  Example: EB with Default Timeslot Template

                       1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Len1 =   0  |Element ID=0x7e|0|    Len2 = 26        |GrpId=1|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Len3 =   6    |Sub ID = 0x1a|0|           ASN
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                ASN                                | Join Metric   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Len4 = 0x01  |Sub ID = 0x1c|0| TT ID = 0x00  |   Len5 = 0x01
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |ID=0x9 |1| CH ID = 0x00  | Len6 = 0x0A   |Sub ID = 0x1b|0|



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   #SF = 0x01  | SF ID = 0x00  |   SF LEN = 0x65 (101 slots)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | #Links = 0x01 |      SLOT OFFSET = 0x0000     |    CHANNEL
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    OFF  = 0x0000  |Link OPT = 0x0F|         NO MAC PAYLOAD
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Bytestream:

       00 3F 1A 88 06 1A ASN#0 ASN#1 ASN#2 ASN#3 ASN#4 JP 01 1C 00
       01 C8 00 0A 1B 01 00 65 00 01 00 00 00 00 0F

   Description of the IEs:

       #Header IE Header
           Len1 = Header IE Length (0)
           Element ID = 0x7e - termination IE indicating Payload IE
               coming next
           Type 0

       #Payload IE Header (MLME)
           Len2 = Payload IE Len (26 Bytes)
           Group ID = 1 MLME (Nested)
           Type = 1

       #MLME-SubIE TSCH Synchronization
           Len3 = Length in bytes of the sub-IE payload (6 Bytes)
           Sub-ID = 0x1a (MLME-SubIE TSCH Synchronization)
           Type = Short (0)
           ASN  = Absolute Sequence Number (5 Bytes)
           Join Metric = 1 Byte

       #MLME-SubIE TSCH Timeslot
           Len4 = Length in bytes of the sub-IE payload (1 Byte)
           Sub-ID = 0x1c (MLME-SubIE Timeslot)
           Type = Short (0)
           Timeslot template ID = 0x00 (default)

       #MLME-SubIE Channel Hopping
           Len5 = Length in bytes of the sub-IE payload (1 Byte)
           Sub-ID = 0x09 (MLME-SubIE Channel Hopping)
           Type = Long (1)
           Hopping Sequence ID = 0x00 (default)

       #MLME-SubIE TSCH Slotframe and Link
           Len6 = Length in bytes of the sub-IE payload (10 Bytes)
           Sub-ID = 0x1b (MLME-SubIE TSCH Slotframe and Link)



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           Type = Short (0)
           Number of slotframes = 0x01
           Slotframe handle = 0x00
           Slotframe size = 101 slots (0x65)
           Number of Links (Cells) = 0x01
           Timeslot = 0x0000 (2B)
           Channel Offset = 0x0000 (2B)
           Link Options = 0x0F
           (TX Link = 1, RX Link = 1, Shared Link = 1,
            Timekeeping = 1 )

A.2.  Example: EB with Custom Timeslot Template

   Using a custom timeslot template in EBs: setting timeslot length to
   15ms.

                     1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Len1 =   0  |Element ID=0x7e|0|    Len2 = 53        |GrpId=1|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Len3 =   6    |Sub ID = 0x1a|0|           ASN
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                ASN                                | Join Metric   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Len4 = 25    |Sub ID = 0x1c|0| TT ID = 0x01  | macTsCCAOffset
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     = 2700        |  macTsCCA = 128               | macTsTxOffset
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     = 3180        |  macTsRxOffset = 1680         | macTsRxAckDelay
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     = 1200        |  macTsTxAckDelay = 1500       | macTsRxWait
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     = 3300        |  macTsAckWait = 600           | macTsRxTx
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     = 192         |  macTsMaxAck  = 2400          | macTsMaxTx
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     = 4256        | macTsTimeslotLength = 15000   | Len5 = 0x01
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |ID=0x9 |1| CH ID = 0x00  | Len6 = 0x0A   | ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Bytestream:

   00 3F 1A 88 06 1A ASN#0 ASN#1 ASN#2 ASN#3 ASN#4 JP 19 1C 01 8C 0A 80
   00 6C 0C 90 06 B0 04 DC 05 E4 0C 58 02 C0 00 60 09 A0 10 98 3A 01 C8
   00 0A ...




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   Description of the IEs:

       #Header IE Header
           Len1 = Header IE Length (none)
           Element ID = 0x7e - termination IE indicating Payload IE
               coming next
           Type 0

       #Payload IE Header (MLME)
           Len2 = Payload IE Len (53 Bytes)
           Group ID = 1 MLME (Nested)
           Type = 1

       #MLME-SubIE TSCH Synchronization
           Len3 = Length in bytes of the sub-IE payload (6 Bytes)
           Sub-ID = 0x1a (MLME-SubIE TSCH Synchronization)
           Type = Short (0)
           ASN  = Absolute Sequence Number (5 Bytes)
           Join Metric = 1 Byte

       #MLME-SubIE TSCH Timeslot
           Len4 = Length in bytes of the sub-IE payload (25 Bytes)
           Sub-ID = 0x1c (MLME-SubIE Timeslot)
           Type = Short (0)
           Timeslot template ID = 0x01 (non-default)

           The 15ms timeslot announced:
           +--------------------------------+------------+
           | IEEE 802.15.4 TSCH parameter   | Value (us) |
           +--------------------------------+------------+
           | macTsCCAOffset                 |       2700 |
           +--------------------------------+------------+
           | macTsCCA                       |        128 |
           +--------------------------------+------------+
           | macTsTxOffset                  |       3180 |
           +--------------------------------+------------+
           | macTsRxOffset                  |       1680 |
           +--------------------------------+------------+
           | macTsRxAckDelay                |       1200 |
           +--------------------------------+------------+
           | macTsTxAckDelay                |       1500 |
           +--------------------------------+------------+
           | macTsRxWait                    |       3300 |
           +--------------------------------+------------+
           | macTsAckWait                   |        600 |
           +--------------------------------+------------+
           | macTsRxTx                      |        192 |
           +--------------------------------+------------+



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           | macTsMaxAck                    |       2400 |
           +--------------------------------+------------+
           | macTsMaxTx                     |       4256 |
           +--------------------------------+------------+
           | macTsTimeslotLength            |      15000 |
           +--------------------------------+------------+

       #MLME-SubIE Channel Hopping
           Len5 = Length in bytes of the sub-IE payload. (1 Byte)
           Sub-ID = 0x09 (MLME-SubIE Channel Hopping)
           Type = Long (1)
           Hopping Sequence ID = 0x00 (default)

A.3.  Example: Link-layer Acknowledgment

   Enhanced Acknowledgment packets carry the Time Correction IE (Header
   IE).

                       1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Len1 =   2  |Element ID=0x1e|0|        Time Sync Info         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Bytestream:

       02 0F TS#0 TS#1

   Description of the IEs:

       #Header IE Header
           Len1 = Header IE Length (2 Bytes)
           Element ID = 0x1e - ACK/NACK Time Correction IE
           Type 0

A.4.  Example: Auxiliary Security Header

   802.15.4 Auxiliary Security Header with security Level set to ENC-
   MIC-32.












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                       1
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |L = 5|M=1|1|1|0|Key Index = IDX|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Bytestream:

       6D IDX#0

   Security Auxiliary Header fields in the example:

       #Security Control (1 byte)
           L = Security Level ENC-MIC-32 (5)
           M = Key Identifier Mode (0x01)
           Frame Counter Suppression = 1 (omitting Frame Counter field)
           ASN in Nonce = 1 (construct Nonce from 5 byte ASN)
           Reserved = 0

       #Key Identifier (1 byte)
           Key Index = IDX (deployment-specific KeyIndex parameter that
                      identifies the cryptographic key)

Authors' Addresses

   Xavier Vilajosana (editor)
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   Barcelona, Catalonia  08018
   Spain

   Email: xvilajosana@uoc.edu


   Kris Pister
   University of California Berkeley
   512 Cory Hall
   Berkeley, California  94720
   USA

   Email: pister@eecs.berkeley.edu










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   Thomas Watteyne
   Linear Technology
   32990 Alvarado-Niles Road, Suite 910
   Union City, CA  94587
   USA

   Email: twatteyne@linear.com












































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