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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 RFC 8180

6TiSCH                                                X. Vilajosana, Ed.
Internet-Draft                           Universitat Oberta de Catalunya
Intended status: Standards Track                               K. Pister
Expires: December 17, 2015             University of California Berkeley
                                                           June 15, 2015


                      Minimal 6TiSCH Configuration
                      draft-ietf-6tisch-minimal-09

Abstract

   This document describes the minimal set of rules to operate an IEEE
   802.15.4 Timeslotted Channel Hopping (TSCH) network.  This minimal
   mode of operation can be used during network bootstrap, as a fall-
   back mode of operation when no dynamic scheduling solution is
   available or functioning, or during early interoperability testing
   and development.

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 December 17, 2015.

Copyright Notice

   Copyright (c) 2015 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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Minimal Schedule Configuration  . . . . . . . . . . . . . . .   3
     3.1.  Slotframe . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Cell Options  . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Retransmissions . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Time Slot timing  . . . . . . . . . . . . . . . . . . . .   6
   4.  Enhanced Beacons Configuration and Content  . . . . . . . . .   7
   5.  Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Neighbor information  . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Neighbor Table  . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Time Source Neighbor Selection  . . . . . . . . . . . . .   9
   7.  Queues and Priorities . . . . . . . . . . . . . . . . . . . .  10
   8.  Security  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  RPL on TSCH . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  RPL Objective Function Zero . . . . . . . . . . . . . . .  11
       9.1.1.  Rank computation  . . . . . . . . . . . . . . . . . .  11
       9.1.2.  Rank computation Example  . . . . . . . . . . . . . .  13
     9.2.  RPL Configuration . . . . . . . . . . . . . . . . . . . .  14
       9.2.1.  Mode of Operation . . . . . . . . . . . . . . . . . .  15
       9.2.2.  Trickle Timer . . . . . . . . . . . . . . . . . . . .  15
       9.2.3.  Hysteresis  . . . . . . . . . . . . . . . . . . . . .  15
       9.2.4.  Variable Values . . . . . . . . . . . . . . . . . . .  15
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Example 1. Information Elements in EBs . . . . . . . . .  16
     10.2.  Example 2. Information Elements in EBs not using default
            timeslot template  . . . . . . . . . . . . . . . . . . .  18
     10.3.  Example 3. Information Elements in ACKs  . . . . . . . .  20
     10.4.  Example 4. Auxiliary Security Header . . . . . . . . . .  21
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     13.2.  Informative References . . . . . . . . . . . . . . . . .  24
     13.3.  External Informative References  . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   The nodes in a IEEE 802.15.4 TSCH network follow a communication
   schedule.  The entity (centralized or decentralized) responsible for
   building and maintaining that schedule has precise control over the
   trade-off between the network's latency, bandwidth, reliability and



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   power consumption.During early interoperability testing and
   development, however, simplicity is more important than efficiency.
   One goal of this document is to define the simplest set of rules for
   building a TSCH-compliant network, at the necessary price of lesser
   efficiency.  Yet, this minimal mode of operation MAY also be used
   during network bootstrap before any schedule is installed into the
   network so nodes can self-organize and the management and
   configuration information be distributed.  In addition, the minimal
   configuration MAY be used as a fall-back mode of operation, ensuring
   connectivity of nodes in case that dynamic scheduling mechanisms fail
   or are not available.  [IEEE802154] provides a mechanism whereby the
   details of slotframe length, timeslot timing, and channel hopping
   pattern are communicated when a node synchronizes to the network.
   This document describes specific settings for these parameters.
   Nodes MUST broadcast properly-formed Enhanced Beacons to announce
   these values.

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.  Minimal Schedule Configuration

   In order to form a network, a minimum schedule configuration is
   required so nodes can advertise the presence of the network, and
   allow other nodes to join.

3.1.  Slotframe

   The slotframe, as defined in [I-D.ietf-6tisch-terminology], is an
   abstraction of the link layer that defines a collection of time slots
   of equal length, and which repeats over time.  In order to set up a
   minimal TSCH network, nodes need to be synchronized with the same
   slotframe configuration so they can communicate.  This document
   recommends the following slotframe configuration.














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   Minimal configuration

   +------------------------------------+----------------------+
   |           Property                 |       Value          |
   +------------------------------------+----------------------+
   | Number of time slots per Slotframe | Variable             |
   +------------------------------------+----------------------+
   | Number of available frequencies    | 16                   |
   +------------------------------------+----------------------+
   | Number of scheduled cells          | 1 (slotOffset 0)     |
   |                                    | (macLinkType NORMAL) |
   +------------------------------------+----------------------+
   | Number of unscheduled cells        | The remainder of the |
   |                                    | slotframe            |
   +------------------------------------+----------------------+
   | Number of MAC retransmissions (max)| 3 (4 transmission    |
   |                                    |    attempts)         |
   +------------------------------------+----------------------+

   The slotframe is composed of a configurable number of time slots.
   Choosing the number of time slots per slotframe needs to take into
   account network requirements such as density, bandwidth per node,
   etc.  In the minimal configuration, there is only a single active
   slot in slotframe, used to transmit/receive both EBs and data link-
   layer frames.  The trade-off between bandwidth, latency and energy
   consumption can be controlled by choosing a different slotframe
   length.  The active slot MAY be scheduled at the slotOffset 0x00 and
   channelOffset 0x00 and MUST be announced in the EBs.  EBs are sent
   using this active slot to the link-layer broadcast address (and are
   therefore not acknowledged).  Data packets, as described in
   Section 3.2, use the same active slot.  Per [IEEE802154], data
   packets sent unicast on this cell are acknowledged by the receiver.
   The remaining cells in the slotframe are unscheduled, and MAY be used
   by dynamic scheduling solutions.  Details about such dynamic
   scheduling solution are out of scope of this document.

   The slotframe length (expressed in number of time slots) is
   configurable.  The length used determines the duty cycle of the
   network.  For example, a network with a 0.99% duty cycle is composed
   of a slotframe of 101 slots, which includes 1 active slot.  The
   present document RECOMMENDS the use of a default slot duration set to
   10ms and its corresponding default timeslot timings defined by the
   [IEEE802154] macTimeslotTemplate.  The use of the default
   macTimeslotTemplate MUST be announced in the EB by using the Timeslot
   IE containing only the default macTimeslotTemplateId.  Other time
   slot durations MAY be supported and MUST be announced in the EBs.  If
   one uses a timeslot duration different than 10ms, EBs MUST contain
   the complete TimeSlot IE as described in Section 3.4.  This document



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   also recommends to clearly indicate nodes not supporting the default
   timeslot value.

   Example schedule with 0.99% duty cycle

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

   slotOffset     0          1                    100

   EB:  Enhanced Beacon
   Tx:  Transmit
   Rx:  Receive
   S:   Shared
   OFF: Unscheduled (MAY be used by a dynamic scheduling mechanism)

3.2.  Cell Options

   Per [IEEE802154] TSCH, each scheduled cell has an associated bitmap
   of cell options, called LinkOptions.  The scheduled cell in the
   minimal schedule is configured as a Hard cell
   [RFC7554][I-D.ietf-6tisch-6top-interface].  Additional available
   cells MAY be scheduled by a dynamic scheduling solution.  The dynamic
   scheduling solution is out of scope, and this specification does not
   make any restriction on the LinkOption associated with those
   dynamically scheduled cells (i.e. they can be hard cells or soft
   cells).

   The active cell is assigned the bitmap of cell options below.
   Because both the "Transmit" and "Receive" bits are set, a node
   transmits if there is a packet in its queue, listens otherwise.
   Because the "shared" bit is set, the back-off mechanism defined in
   [IEEE802154] is used to resolve contention when transmitting.  This
   results in "Slotted Aloha" behavior.  The "Timekeeping" flag is never
   set, since the time source neighbor is selected using the DODAG
   structure of the network (detailed below).

      b0 = Transmit = 1 (set)




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      b1 = Receive = 1 (set)

      b2 = Shared = 1 (set)

      b3 = Timekeeping = 0 (clear)

      b4-b7 = Reserved (clear)

   All remaining cells are unscheduled.  In unscheduled cells, the nodes
   SHOULD keep their radio off.  In a memory-efficient implementation,
   scheduled cells can be represented by a circular linked list.
   Unscheduled cells SHOULD NOT occupy any memory.

3.3.  Retransmissions

   The maximum number of link layer retransmissions is set to 3.  For
   packets which require an acknowledgment, if none is received after a
   total of 4 attempts, the transmission is considered failed and the
   link layer MUST notify the upper layer.  Packets sent to the
   broadcast MAC address (including EBs) are not acknowledged and
   therefore not retransmitted.

3.4.  Time Slot timing

   The figure below shows an active timeslot in which a packet is sent
   from the transmitter node (TX) to the receiver node (RX).  A link-
   layer acknowledgment is sent by the RX node to the TX node when the
   packet is to be acknowledged.  The TsTxOffset duration defines the
   instant in the timeslot when the first bit after the Start of Frame
   Delimiter (SFD) of the transmitted packet leaves the radio of the TX
   node.  The radio of the RX node is turned on tsRxWait/2 before that
   instant, and listens for at least tsRxWait.  This allows for a de-
   synchronization between the two nodes of at most tsRxWait/2 in either
   direction (early or late).  The RX node needs to send the first bit
   after the SFD of the MAC acknowledgment exactly TsTxAckDelay after
   the end of the last byte of the received packet.  TX's radio has to
   be turned on tsAckWait/2 before that time, and keep listening for at
   least tsAckWait.  The TX node can perform a Clear Channel Assessment
   (CCA) if required, this does not interfere with the scope of this
   draft.  As for a minimal configuration, CCA is OPTIONAL.











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   Time slot internal timing diagram

      /---------------------- Time Slot Duration ----------------------/
      |                                                  / (5) /       |
      |                   |              / tsRxAckDelay /|  |  |       |
      |-------------------+--------------+------------------+------+---|
   TX |/(1)/  (2)  / (3) /|   TX frame   |                  |RX ACK|   |
      |----+-------+------+--------------+------------------+------+---|
      |/    tsTxOffset   /|              |                  |      |   |
      |                   |              |                  |      |   |
      |-------------------+--------------+------------------+------+---|
   RX |                |  |  | RX frame  |                  |TX ACK|   |
      |----------------+--+--+-----------+------------------+------+---|
      |                |  |  |           |                  |      |   |
      |                / (4) /           /   tsTxAckDelay   /      |   |
      Start                                                          End
      of                                                              of
      Slot                                                          Slot
   /(1)/ tsCCAOffset
   /(2)/ tsCCA
   /(3)/ tsRxTx
   /(4)/ tsRxWait
   /(5)/ tsAckWait

   The timing parameters for the default macTimeslotTemplate
   (macTimeslotTemplateId = 0) MUST be used when utilizing the default
   time slot duration.  In this case, the Timeslot IE only transports
   the macTimeslotTemplateId with value 0x00.  If a timeslot template
   other than the default is used, the EB MUST contain a complete
   TimeSlot IE indicating the timeslot duration and the corresponding
   timeslot timings.  Note however that in case of discrepancy between
   the values in this document and [IEEE802154], the IEEE standard has
   precedence.

4.  Enhanced Beacons Configuration and Content

   [IEEE802154] does not define how often EBs are sent, nor their
   contents.  EBs MUST NOT in general be used for synchronization.
   Synchronization is achieved via acknowledgements to normal packet
   traffic, and keepalives.  For a minimal TSCH configuration, a mote
   SHOULD send an EB every EB_PERIOD.  For additional reference see
   [RFC7554] where different synchronization approaches are summarized.
   EBs are only authenticated and neither Payload IEs nor the frame
   payload are encrypted.  Refer to the 6TiSCH architecture document
   [I-D.ietf-6tisch-architecture] for further details on security
   aspects.





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   EBs MUST be sent as per [IEEE802154]  and MUST carry the Information
   Elements (IEs) listed below.  Refer to Section 10.1 for an example of
   the Information Elements Header Content.

      Synchronization IE: Contains synchronization information such as
      ASN and Join Priority.  The value of Join Priority is discussed in
      Section 6.2.

      TSCH Timeslot IE: Contains the timeslot template identifier.  This
      specification uses the default timeslot template as defined in
      [IEEE802154].  In the case that a non-default timeslot template is
      used, the IE Content MUST follow the specification as defined in
      [IEEE802154] . Refer to Section 10.1 for an illustrative example
      of non default timeslot template.

      Channel Hopping IE: Contains the channel hopping template
      identifier.  This specification uses the default channel hopping
      template, as defined in [IEEE802154].The default sequence for the
      2.4GHz OQPSK PHY is [5, 6, 12, 7, 15, 4, 14, 11, 8, 0, 1, 2, 13,
      3, 9, 10]  [IEEE802154].  Note however that in case of discrepancy
      between the values in this document and [IEEE802154], the IEEE
      standard specification has preference.

      Frame and Link IE: Each node MUST indicate the schedule in each EB
      through a Frame and Link IE.  This enables nodes which implement
      [IEEE802154] to learn the schedule used in the network as they
      join it.  This draft defines the use of a single cell set at
      channel offset 0x00, slot offset 0x00 and with linkOption 0xE0
      (TX, RX, SHARED bits set).

5.  Acknowledgment

   Link-layer acknowledgment frames are built according to [IEEE802154].
   Unicast frames sent to a unicast MAC destination address request an
   acknowledgment.  The sender node MUST set the ACK requested bit in
   the MAC header.  The acknowledgment frame is of type ACK (version
   0x10).  Each acknowledgment contains the following IE:

      ACK/NACK Time Correction IE: The ACK/NACK time correction IE
      carries the measured de-synchronization between the sender and the
      receiver.  Refer to Section 10.3 for an example of the Header IE
      content.  The possible values for the Time Synchronization
      Information and ACK status are described in [IEEE802154].








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6.  Neighbor information

   [IEEE802154] does not define how and when each node in the network
   keeps information about its neighbors.  Keeping the following
   information in the neighbor table is RECOMMENDED:

6.1.  Neighbor Table

   The exact format of the neighbor table is implementation-specific,
   but it SHOULD contain the following information for each neighbor:

      Neighbor statistics:

         numTx: number of transmitted packets to that neighbor

         numTxAck: number of transmitted packets that have been
         acknowledged by that neighbor

         numRx: number of received packets from that neighbor

      The EUI64 of the neighbor.

      Timestamp of the last frame 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.

      RPL rank of that neighbor.

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

      Connectivity statistics (e.g., RSSI), which can be used to
      determine the quality of the link.

   In addition to that information, each node has to be able to compute
   some RPL Objective Function (OF), taking into account the neighbor
   and connectivity statistics.  An example RPL objective function is
   the OF Zero as described in [RFC6552] and Section 9.1.1.

6.2.  Time Source Neighbor Selection

   Each node MUST select at least one Time Source Neighbor among the
   nodes in its RPL routing parent set.  When a node joins a network, it
   has no routing information.  To select its time source neighbor, it
   uses the Join Priority field in the EB, as described in [IEEE802154].
   The Sync IE contains the ASN and 1 Byte field named Join Priority.
   The Join Priority of any node MUST be equivalent to the result of the
   function DAGRank(rank) as defined by [RFC6550] and Section 9.1.1.
   The Join Priority of the DAG root is zero, i.e., EBs sent from the



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   DAG root are sent with Join Priority equal to 0.  A lower value of
   the Join Priority indicates higher preference to connect to that
   device.  When a node joins the network, it MUST NOT send EBs before
   having acquired a RPL rank.  This avoids routing loops and matches
   RPL topology with underlying mesh topology.  As soon as a node
   acquires a RPL rank (see [RFC6550] and Section 9.1.1), it SHOULD send
   Enhanced Beacons including a Sync IE with Join Priority field set to
   DAGRank(rank), where rank is the node's rank.  If a node receives EBs
   from different nodes with equal Join Priority, the time source
   neighbor selection SHOULD be assessed by other metrics that can help
   determine the better connectivity link.  Time source neighbor
   hysteresis SHOULD be used, according to the rules defined in
   Section 9.2.3.  If connectivity to the time source neighbor is lost,
   a new time source neighbor MUST be chosen among the neighbors in the
   RPL routing parent set.

   The decision for a node to select one Time Source Neighbor when
   multiple EBs are received is implementation-specific.

   For example, a node MAY wait until one EB from NUM_NEIGHBOURS_TO_WAIT
   neighbors have been received to select the best Time Source Neighbor.
   This condition MAY apply unless a second EB is not received after
   MAX_EB_DELAY seconds.  This avoids initial hysteresis when selecting
   a first Time Source Neighbor.

   Optionally, some form of hysteresis SHOULD be implemented to avoid
   frequent changes in time source neighbors.

7.  Queues and Priorities

   [IEEE802154] does not define the use of queues to handle upper layer
   data (either application or control data from upper layers).  The use
   of a single queue with the following rules is RECOMMENDED:

      When the node is not synchronized to the network, higher layers
      are not able to insert packets into the queue.

      Frames generated by the MAC layer (e.g., EBs and ACK) have a
      higher queuing priority than packets received from a higher layer.

      Frame types Beacon and Command have a higher queuing priority than
      frame types Data and ACK.

      One entry in the queue is reserved at all times for frames of
      types Beacon or Command frames.






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8.  Security

   As this document refers to the interaction between Layer 3 and Layer
   2 protocols, this interaction MUST be secured by L2 security
   mechanisms as defined by [IEEE802154].  Two security mechanisms are
   considered, authentication and encryption, authentication applies to
   all packet content while encryption applies to header IEs and MAC
   payload.  Key distribution is out of scope of this document, but
   examples include pre-configured keys at the nodes, shared keys among
   peers or well-known keys.  Refer to the 6TiSCH architecture document
   [I-D.ietf-6tisch-architecture] for further details on key
   distribution and advanced security aspects.

   The present document assumes the existence of two cryptographic keys,
   which can be pre-configured.  One of the keys (K1) is used to
   authenticate EBs.  As defined in Section 4, EBs MUST be
   authenticated, with no payload encryption.  This facilitates logical
   segregation of distinct networks.  A second key (K2) is used to
   authenticate DATA, ACKNOWLEDGEMENT, MAC COMMAND frame types and
   respective header IEs, with payload encryption.  Depending on
   security policy, these keys could be the same (i.e., K1=K2).

   For early interoperability, K1 MAY be set to 36 54 69 53 43 48 20 6D
   69 6E 69 6D 61 6C 31 35 ("6TiSCH minimal15").

9.  RPL on TSCH

   Nodes in the network MUST use the RPL routing protocol [RFC6550].

9.1.  RPL Objective Function Zero

   Nodes in the network MUST use the RPL routing protocol [RFC6550] and
   implement the RPL Objective Function Zero [RFC6552].

9.1.1.  Rank computation

   The rank computation is described at [RFC6552], Section 4.1.  A node
   rank is computed by the following equation:

   R(N) = R(P) + rank_increment

   rank_increment = (Rf*Sp + Sr) * MinHopRankIncrease

   Where:

      R(N): Rank of the node.

      R(P): Rank of the parent obtained as part of the DIO information.



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      rank_increment: The result of a function that determines the rank
      increment.

      Rf (rank_factor): A configurable factor that is used to multiply
      the effect of the link properties in the rank_increment
      computation.  If none is configured, rank_factor of 1 is used.  In
      this specification, a rank_factor of 1 MUST be used.

      Sp (step_of_rank): (strictly positive integer) - an intermediate
      computation based on the link properties with a certain neighbor,
      i.e., the Expected Transmission Count (ETX) which provides an
      average of number of packet transmissions between two nodes.  ETX
      is defined in detail by [decouto03high] and [RFC6551].  The ETX is
      computed as the inverse of the Packet Delivery Ratio (PDR), this
      is the number of transmitted packets, divided by the number of
      acknowledged packets to a certain node (e.g., ETX = numTX/
      numTXAck).  According to this specification, Sp MUST be set to
      2*ETX, for the reason of preferring shorter routes.

      Sr (stretch_of_rank): (unsigned integer) - the maximum increment
      to the step_of_rank of a preferred parent, to allow the selection
      of an additional feasible successor.  If none is configured to the
      device, then the step_of_rank is not stretched.  In this
      specification, stretch_of_rank MUST be set to 0.

      MinHopRankIncrease: the MinHopRankIncrease is set to the fixed
      constant DEFAULT_MIN_HOP_rank_increment [RFC6550].
      DEFAULT_MIN_HOP_rank_increment has a value of 256.

      DAGRank(rank): Equivalent to the floor of (Rf*Sp + Sr) as defined
      by [RFC6550].  Specifically, when an Objective Function computes
      Rank, this is defined as an unsigned integer (i.e., a 16-bit
      value) Rank quantity.  When the Rank is compared, e.g. to
      determine parent relationships or loop detection, the integer
      portion of the Rank is used.  The integer portion of the Rank is
      computed by the DAGRank() macro as floor(x) where floor(x) is the
      function that evaluates to the greatest integer less than or equal
      to x.  DAGRank(rank) = floor(rank/MinHopRankIncrease)













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   Rank computation scenario

       +-------+
       |   P   | R(P)
       |       |
       +-------+
           |
           |
       +-------+
       |   N   | R(N)=R(P) + rank_increment
       |       | rank_increment = (Rf*Sp + Sr) * MinHopRankIncrease
       +-------+ Sp=2*ETX

9.1.2.  Rank computation Example

   This section illustrates with an example the use of the Objective
   Function Zero.  Assume the following parameters:

      Rf = 1

      Sp = 2* ETX

      Sr = 0

      minHopRankIncrease = 256 (default in RPL)

      ETX=(numTX/numTXAck)

      r(n) = r(p) + rank_increment

      rank_increment = (Rf*Sp + Sr) * minHopRankIncrease

      rank_increment = 512*numTx/numTxAck


















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   Rank computation example for 5 hop network where numTx=100 and
   numTxAck=75 for all nodes

       +-------+
       |   0   | R(minHopRankIncrease) = 256
       |       | DAGRank(R(0)) = 1
       +-------+
           |
           |
       +-------+
       |   1   | R(1)=R(0) + 683 = 939
       |       | DAGRank(R(1)) = 3
       +-------+
           |
           |
       +-------+
       |   2   | R(2)=R(1) + 683 = 1622
       |       | DAGRank(R(2)) = 6
       +-------+
           |
           |
       +-------+
       |   3   | R(3)=R(2)+683=2305
       |       | DAGRank(R(3)) = 9
       +-------+
           |
           |
       +-------+
       |   4   | R(4)=R(3)+683=2988
       |       | DAGRank(R(4)) = 11
       +-------+
           |
           |
       +-------+
       |   5   | R(5)=R(4)+683=3671
       |       | DAGRank(R(5)) = 14
       +-------+

9.2.  RPL Configuration

   In addition to the Objective Function (OF), a minimal configuration
   for RPL SHOULD indicate the preferred mode of operation (either
   Storing Mode or Non-Storing Mode) so different RPL implementations
   can inter-operate.  RPL information and hop-by-hop extension headers
   MUST follow [RFC6553] and [RFC6554] specification.  In the case that
   the packets formed at the LLN need to cross through intermediate
   routers, these MUST obey to the IP in IP encapsulation requirement
   specified by the [RFC6282] and [RFC2460].  RPI and RH3 extension



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   headers and inner IP headers MUST be compressed according to
   [RFC6282].

9.2.1.  Mode of Operation

   For downstream route maintenance, in a minimal configuration, RPL
   SHOULD be set to operate in the Non-Storing mode as described by
   [RFC6550] Section 9.7.  Storing mode ([RFC6550] Section 9.8) MAY be
   supported in less constrained devices.

9.2.2.  Trickle Timer

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

9.2.3.  Hysteresis

   According to [RFC6552], [RFC6719] recommends the use of a boundary
   value (PARENT_SWITCH_THRESHOLD) to avoid constant changes of parent
   when ranks are compared.  When evaluating a parent that belongs to a
   smaller path cost than 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.  As for [RFC6719]  the recommended value
   for PARENT_SWITCH_THRESHOLD is 192 when ETX metric is used (in the
   form 128*ETX), the recommendation for this document is to use
   PARENT_SWITCH_THRESHOLD equal to 768 as the metric being used is
   2*ETX*minHopRankIncrease.  This is mechanism is suited to deal with
   parent hysteresis in both cases routing parent and time source
   neighbor selection.

9.2.4.  Variable Values

   The following table presents the RECOMMENDED values for the RPL-
   related variables defined in the previous section.












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   Recommended variable values

   +-------------------------+-------+
   | Variable                | Value |
   +-------------------------+-------+
   | EB_PERIOD               |   10s |
   +-------------------------+-------+
   | MAX_EB_DELAY            |   180 |
   +-------------------------+-------+
   | NUM_NEIGHBOURS_TO_WAIT  |     2 |
   +-------------------------+-------+
   | PARENT_SWITCH_THRESHOLD |   768 |
   +-------------------------+-------+

10.  Examples

   Several examples are provided to illustrate the content of the
   packets used by the minimal configuration as proposed by this
   document.  Each example follows the same structure presenting first a
   schematic header diagram, then the LSB stream of bytes that conform
   the header and finally a description of each of the IEs the form the
   packet.  The packet formats are specific for the [IEEE802154-2012]
   revision and MAY vary in future releases of the IEEE standard.  In
   case of differences between the packet content presented in this
   section and the [IEEE802154-2012], the later has presedence.

   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 by the PHY, from left to right, where the leftmost bit is
   transmitted first in time.  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, hence LSB format.

10.1.  Example 1.  Information Elements in EBs

   Mandatory content for the EB as proposed by this draft.  The example
   uses a slotframe of 101 slots.  The following figure represents
   schematically the Header IE and Payload IE content of an EB.


    Schematic representation of the IE header in an EB:

                         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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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    | Len1 =   0  |Element ID=0x7e|0|    Len2 = 26        |GrpId=1|1|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Len3 =   6    |Sub ID = 0x1a|0|           ASN
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  ASN                               | Join Priority |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Len4 = 1     |Sub ID = 0x1c|0| TT ID = 0x00  |   Len5 = 0x01
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |ID=0x9 |1| CH ID = 0x00  | Len6 = 0x0A   |Sub ID = 0x1b|0|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   #SF = 0x01  | SF ID = 0x01  |   SF LEN = 0x65 (101 slots)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | #Links = 0x01 |      SLOT OFFSET = 0x0000     |    CHANNEL
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      OFF  = 0x0000 |Link OPT = 0x07|    Len7 = 0x00      |ID=0xf |1|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         REST OF MAC PAYLOAD ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Stream of bytes (in LSB format) that derive from the previous
    schematic header:

    00 FC 1A 88 06 34 ASN#0 ASN#1 ASN#2 ASN#3 ASN#4 JP 01 38 00
    00 33 00 0A 36 01 01 65 00 01 00 00 00 00 07 00 1F ...

    Description of the IE fields in the example:

    #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)
    GroupID = 1 MLME (Nested)
    Type = 1

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

    #MLME-SubIE TSCH TimeSlot
    Len4 = Length in bytes of the sub-IE payload (1 Byte)
    SubID = 0x1c (MLME-SubIE Timeslot)
    Type = Short (0)



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    TimeSlot template ID = 0x00 (default)

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

    #MLME-SubIE TSCH Slotframe and Link
    Len6 = Length in bytes of the sub-IE payload (10 Bytes)
    SubID = 0x1b (MLME-SubIE TSCH Slotframe and Link)
    Type = Short (0)
    Number of slotframes = 0x01
    SlotFrame Handle = 0x00
    SlotFrame Size = 101 slots (0x65)
    Number of Links = 0x01
    Timeslot = 0x0000 (2B)
    Channel Offset = 0x0000 (2B)
    Link Option = 0x07 (tx,rx,shared)

    #Payload IE Header (Termination IE) (MAY be ommitted)
    Len7 = Payload IE Len (0)
    GroupID = 0xf Termination
    Type = 0


10.2.  Example 2.  Information Elements in EBs not using default
       timeslot template

   Using a non-default timeslot template in EBs.  Timeslot length set to
   15ms instead of the 10ms default.

    Schematic representation of the IE header in an EB:

                         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 Priority |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Len4 = 25    |Sub ID = 0x1c|0| TT ID = 0x01  | macTsCCAOffset
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       = 2700       |  macTsCCA = 128               | macTsTxOffset
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       = 3180       |  macTsRxOffset = 1680         | macTsRxAckDelay



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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       = 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   | ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Stream of bytes (in LSB format) that derive from the previous
    schematic header:

    00 FC 1A 88 06 34 ASN#0 ASN#1 ASN#2 ASN#3 ASN#4 JP 19 38 00 33 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 ...

    Description of the IE fields in the example:

    #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)
    GroupID = 1 MLME (Nested)
    Type = 1

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

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

    Example timeslot timming using 15ms timeslot.
    +--------------------------------+------------+
    | IEEE802.15.4 TSCH parameter    | Value (us) |
    +--------------------------------+------------+



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    | tsCCAOffset                    |    2700    |
    +--------------------------------+------------+
    | tsCCA                          |     128    |
    +--------------------------------+------------+
    | tsTxOffset                     |    3180    |
    +--------------------------------+------------+
    | tsRxOffset                     |    1680    |
    +--------------------------------+------------+
    | tsRxAckDelay                   |    1200    |
    +--------------------------------+------------+
    | tsTxAckDelay                   |    1500    |
    +--------------------------------+------------+
    | tsRxWait                       |    3300    |
    +--------------------------------+------------+
    | tsAckWait                      |     600    |
    +--------------------------------+------------+
    | tsRxTx                         |     192    |
    +--------------------------------+------------+
    | tsMaxAck                       |    2400    |
    +--------------------------------+------------+
    | tsMaxTx                        |    4256    |
    +--------------------------------+------------+
    | Time Slot duration             |   15000    |
    +--------------------------------+------------+

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


10.3.  Example 3.  Information Elements in ACKs

   Acknowledgement packets carry the ACK/NACK Time Correction IE (Header
   IE).  The following example illustrates the IE format as specified in
   [IEEE802154-2012].














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                           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         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Stream of bytes (in LSB format) that derive from the previous
      schematic header:

      40 F0 TS#0 TS#1

      Description of the IE fields in the example:

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


10.4.  Example 4.  Auxiliary Security Header

   The example illustrates content of the Auxiliary Security Header as
   mandated by this document, if security is enabled.  Security Level in
   the example is set to ENC-MIC-32 (5).



























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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|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Stream of bytes (in LSB format) that derive from the previous
     schematic header:

     6D IDX#0

     Description of the 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)
     Frame Counter Size = 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)


11.  IANA Considerations

   This document requests no immediate action by IANA.

12.  Acknowledgments

   The authors would like to acknowledge the guidance and input provided
   by Rene Struik, Pat Kinney, Michael Richardson, Tero Kivinen, Nicola
   Accettura, Malisa Vucinic and the 6TiSCH Chairs Pascal Thubert and
   Thomas Watteyne.

13.  References

13.1.  Normative References

   [RFC6719]  Gnawali, O. and P. Levis, "The Minimum Rank with
              Hysteresis Objective Function", RFC 6719, September 2012.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.





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   [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, March
              2012.

   [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, March
              2012.

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

   [RFC6551]  Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
              Barthel, "Routing Metrics Used for Path Calculation in
              Low-Power and Lossy Networks", RFC 6551, March 2012.

   [RFC6550]  Winter, T., Thubert, P., 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, March 2012.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [IEEE802154-2012]
              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.
              802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", April
              2012.

   [IEEE802154]
              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".



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   [decouto03high]
              De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
              High-Throughput Path Metric for Multi-Hop Wireless
              Routing", ACM International Conference on Mobile Computing
              and Networking (MobiCom) , June 2003.

13.2.  Informative References

   [RFC7554]  Watteyne, T., 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,
              May 2015.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, January 2014.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, September 2003.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-08 (work
              in progress), May 2015.

   [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-04 (work in
              progress), March 2015.

   [I-D.ietf-6tisch-6top-interface]
              Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
              Operation Sublayer (6top) Interface", draft-ietf-6tisch-
              6top-interface-03 (work in progress), March 2015.

13.3.  External Informative References

   [CCM]      National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation: The
              CCM Mode for Authentication and Confidentiality. SP
              800-38C", May 2004.

   [CCM-Star]
              Struik, R., "Formal Specification of the CCM* Mode of
              Operation, IEEE P802.15 Working Group for Wireless
              Personal Area Networks (WPANs).", September 2005.





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   [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.

Authors' Addresses

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

   Phone: +34 (646) 633 681
   Email: xvilajosana@uoc.edu


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

   Email: pister@eecs.berkeley.edu


























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