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Internet Engineering Task Force                              A. Karaagac
Internet-Draft                                                J. Hoebeke
Intended status: Standards Track                 Ghent University - imec
Expires: January 14, 2021                                  July 13, 2020


             In-band Network Telemetry for 6TiSCH Networks
                      draft-karaagac-6tisch-int-01

Abstract

   This document describes In-band Network Telemetry for 6TiSCH
   Networks, offering a flexible monitoring solution with minimal
   resource consumption and communication overhead while supporting a
   wide range of monitoring operations and strategies for dealing with
   various network scenarios and use cases.  It enables 6TiSCH networks
   to collect per-packet and per-hop monitoring information by
   piggybacking telemetry information onto the data packets by
   exploiting the remaining space in the IEEE 802.15.4e frames, thus not
   impacting network behavior and performance.  This document also
   discusses the data fields and associated data types for 6TiSCH INT
   mechanism.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 14, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  In-band Network Telemetry for 6TiSCH  . . . . . . . . . . . .   4
     2.1.  Capacity-Neutral Network Monitoring . . . . . . . . . . .   5
     2.2.  INT Data Model, Format and Encoding . . . . . . . . . . .   6
       2.2.1.  INT Sub-IE Format . . . . . . . . . . . . . . . . . .   8
       2.2.2.  Telemetry Data Model  . . . . . . . . . . . . . . . .  10
     2.3.  INT Strategies  . . . . . . . . . . . . . . . . . . . . .  12
       2.3.1.  Opportunistic Logic . . . . . . . . . . . . . . . . .  12
       2.3.2.  Probabilistic Logic . . . . . . . . . . . . . . . . .  12
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  IETF IE Subtype INT . . . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  14
     6.3.  External Informative References . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   For continuous, persistent and problem-free operation of "IPv6 over
   the TSCH mode of IEEE 802.15.4e" (6TiSCH) Networks
   [I-D.ietf-6tisch-architecture], it is critical to have visibility and
   awareness into what is happening on the network at any one time.  For
   centrally managed 6TiSCH networks, it is required to collect and
   analyze network performance data, often as close to real time as
   possible.  For TiSCH networks with distributed management solutions,
   it is still vital to monitor network nodes continuously or
   periodically to ensure their functioning, detect relevant problems,
   perform traffic engineering and network optimization.

   Nevertheless, efficient monitoring and management mechanisms for
   these networks have not been addressed adequately.  First,
   traditional active network and health monitoring systems (i.e.
   statistical polling, active probing) are of limited applicability in
   these constrained and dynamic networks due to their static and
   inefficient design.  Especially, considering the constrained nature



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   of sensor networks, the introduced control traffic can occupy
   extensive network resources, impact network behavior and/or interfere
   with the scheduled application traffic flow.  Secondly, the passive
   health monitoring and tomography methods can only offer limited
   capabilities for collecting in-network state information and
   telemetry data, thus are not sufficient for advanced network
   monitoring and fine-grained management operations.  In addition, the
   6TiSCH WG is defining a management interface, based on CoAP
   Management Interface (CoMI) [I-D.ietf-core-comi], which can be used
   to monitor network performance and perform network configurations
   [I-D.ietf-6tisch-coap].  However, performing telemetry via CoMI
   interfaces will result in a polling-based monitoring scheme which may
   cause a large amount of control traffic.

   This document specifies an In-Band Network Telemetry (INT) mechanism
   adapted to 6TiSCH Networks.  It provides the definition of telemetry
   semantics and data models for 6TiSCH Networks and their efficient
   encoding in the IEEE 802.15.4e [IEEE802154e] frames.  Additionally,
   it defines a set of novel telemetry operations and strategies for
   dealing with various network scenarios and system interactions.

   The proposed INT-based network monitoring solution creates an
   efficient, adaptive and flexible design which offers several novel
   monitoring functionalities and telemetry operations for 6TiSCH
   Networks.

   o  Opportunistic piggybacking mechanism that eliminates the need for
      artificial probing packets and resource reservation for monitoring
      data in 6TiSCH Networks.

   o  Real-time monitoring capabilities where the collected telemetry
      data reflects the momentary network performance and the exact
      treatment that an application packet encounters.

   o  The combination of real-time edge-to-edge packet-level network
      information (e.g. reliability, latency) and hop-by-hop telemetry
      data (e.g. per-hop latencies, queue states and link qualities).

   o  Flexibility in terms of telemetry initiation and addition
      approaches: continuous, periodic, event-driven or query-driven.

   o  Flexibility for forwarding nodes to initiate an INT operation on a
      packet with another source.

   o  Flexibility for source and forwarding nodes to decide what to add:
      even a subset of INT entries if not all of them fit in the frame.





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1.1.  Terminology

   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 BCP 14 [RFC2119]
   [RFC8174] when, and only when, they appear in all capitals, as shown
   here.

   Readers are expected to be familiar with terms and concepts defined
   in [IEEE802154e] and [I-D.ietf-6tisch-architecture].

   "RPL", "RPL Dag Rank", MaxRankIncrease, MinRankIncrease and RootRank
   are defined in the "RPL: IPv6 Routing Protocol for Low-Power and
   Lossy Networks" [RFC6550] specification.

   This document refers also to the following terminology.

   INT : In-band Network Telemetry

   E2E : End-to-End

   HBH : Hop-by-Hop

2.  In-band Network Telemetry for 6TiSCH

   INT, or also referred to as In-situ Operations, Administration, and
   Maintenance (iOAM) [I-D.ietf-ippm-ioam-data], is created to
   complement current out-of-band monitoring mechanisms and allows for
   telemetry metadata to be collected as packets traverse a network.
   The term "in-band" refers to the fact that telemetry data is carried
   within data packets rather than being sent within specifically
   dedicated packets.  Therefore, it does not require artificial probing
   packets or dedicated middle-boxes, and the network state is obtained
   at the exact point in time the real user traffic passes through.
   Also, the insertion of in-band information does not change the
   forwarding behavior of the packet.  However, it might impact the
   packet delivery ratios (PDR) due to the increase in the length of the
   transmitted frames.

   The 6TiSCH INT mechanism collects the telemetry data while a packet
   is traversing towards the Backbone Router.  This measurement data can
   typically be node or network state information such as health/failure
   reports, link/neighbor statistics, network topology and node/link
   occupancy.  When the packets reach the edge (backbone router) of the
   network, the telemetry metadata is removed and telemetry reports are
   generated to be used by the Network Management Entity (NME) for
   further visualization, analysis and management.




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2.1.  Capacity-Neutral Network Monitoring

   In a Timeslotted Channel Hopping (TSCH) [RFC7554] network, time is
   globally synchronized and is sliced up into time slots.  The time
   synchronization in the network means that all nodes share a timeslot
   counter, named Absolute Slot Number (ASN), indicating the total
   number of slots which have passed since the network has started
   [IEEE802154e].  The overall communication is orchestrated by a
   schedule which instructs each node what to do (transmit, receive,
   sleep) in each timeslot [IEEE802154e].  In this TSCH schedule, a
   single element, named cell, is identified by a pair of slotOffset and
   channelOffset, which is used to define the communication time and
   frequency.

   The duration of a time slot is not defined by the standard, but it is
   defined to be long enough to send a data frame, handle the radio
   turnaround and receive an ACK, typically being 10ms.  With radios
   that are compliant with IEEE 802.15.4 operating in the 2.4 GHz
   frequency band, a maximum-length frame of 127 bytes is considered,
   which takes around 4 ms to transmit [RFC7554].  Whatever size that a
   node is sending, the resources are reserved for that node so that it
   can transmit a data frame of 127 bytes.  If the node has a shorter
   frame to send, there will be remaining time for that node to sleep or
   stay idle.  That means the reserved time/bandwidth resources are
   wasted, instead of being used for other good reasons.  Therefore,
   this paper proposes a mechanism that collects the monitoring
   information for each node by piggybacking telemetry information on
   the data packets in order to leverage these remaining resources, as
   presented in Figure 1.  If there is no or insufficient remaining
   space in the transmitted frame, the node cannot add any telemetry
   information.




















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           :            COMMUNICATION SLOT OPERATION           :
           :                                                   :
           :  +-------+---------------------+---+     +-----+  :
           :  |  MAC  |        Frame        |FCS|     | ACK |  :
           :  | Header|       Payload       |   |     |     |  :
     ______:__|_______|_____________________|___|_____|_____|__:______
           :                                                   :
           :  +-------+---+---+---+---------+---+     +-----+  :
           :  |  MAC  |INT|INT|INT|  Frame  |FCS|     | ACK |  :
           :  | Header|   |   |   | Payload |   |     |     |  :
     ______:__|_______|___|___|___|_________|___|_____|_____|__:______
           :                                                   :
           :  +-------+---+-----------------+---+     +-----+  :
           :  |  MAC  |INT|      Frame      |FCS|     | ACK |  :
           :  | Header|   |     Payload     |   |     |     |  :
     ______:__|_______|___|_________________|___|_____|_____|__:______
           :                                                   :

              Figure 1: Capacity-Neutral Network Monitoring.

   Regarding the cost of the INT operation, there will only be a limited
   amount of extra energy consumption for the transmitting and receiving
   nodes in order to transmit/receive extra bytes in the frame.
   However, it will not use any resource (i.e. slot, bandwidth) reserved
   for other application or control traffic and it will not have any
   effect on the network capacity, network behavior and traffic flows.

2.2.  INT Data Model, Format and Encoding

   For the insertion of telemetry data in IEEE 802.15.4 MAC frames, the
   Information Elements (IEs) are used, which are positioned between the
   end of the MAC Header and the Frame Payload.  The IEs are intended to
   extend 802.15.4 in an interoperable manner and they can be exchanged
   between one-hop neighbors or forwarded for communication towards
   further away devices, thus allowing several optimizations
   [IEEE802154].  The IEs are structured containers as Type, Length,
   Value fields (TLV) and they have two types, named Header IEs and
   Payload IEs [IEEE802154].  Header IEs are part of the MAC header and
   most of their processing is done by the MAC, so IETF protocols should
   not have any direct effect on that processing.  Contrary, Payload IEs
   are part of the MAC payload and they may be encrypted and
   authenticated.  According to the standard, each frame can include one
   or more Header or Payload IEs that contain information.

   IETF has formulated a request towards the IEEE 802.15 Assigned
   Numbers Authority (ANA) to allocate a registry number and described
   how IETF IEs should be formatted with their subtypes [RFC8137].
   Also, 6TiSCH WG has expressed the need for IEs and a temporary



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   assignment is already provided [RFC8137].  For the design of IEs for
   INT data, an IETF INT sub-IE type is created by following the IETF IE
   subtype format.

   For inserting an INT sub-IE in a MAC frame, the node first must set
   the "Information Elements Present" field in the 802.15.4 header.
   Next, Header IEs must be added which will be terminated by a Header
   Termination 1 IE (2 Bytes).  If there is no Header IE, the Header
   Termination 1 IE must still be added in order to indicate the start
   of Payload IEs [IEEE802154].  After that, the IETF IE descriptor (2
   Bytes: type, id, length) must be added, where the IETF IE Group ID is
   assigned as 0x5 in IEEE 802.15 ANA [ana2019].  Then, the INT sub-IEs
   must be added including the INT sub-IE descriptors (1 Byte: sub-IE
   ID) and the relevant INT data.  At the end of the payload IEs, a
   Payload Termination IE (2 Byte) must be added.  Considering all these
   necessary IEs, 7 Bytes of overhead will be added to the frame in
   order to insert any size of INT data.  The resulting frame format
   after the INT sub-IE insertion is provided in Figure 2.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Frame Control        |    Seq. No    |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
     |                                                               |
     ~   Addressing Fields  &  Aux. Security Header  &  Header IEs   ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Header Termination 1 IE    |    IETF IE descriptor (0x5)   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | INT sub-IE ID |                                               |
     +-+-+-+-+-+-+-+-+                                               +
     ~                      INT sub-IE Content                       ~
     |                                                               |
     |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               |     Payload Termination IE    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      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
                          1                   2                   3

             Figure 2: The frame format with inserted INT IE.

   The following subsections describe the approach and format for
   embedding telemetry information in the body of an active data packet
   via IETF INT sub-IEs.








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2.2.1.  INT Sub-IE Format

   The INT-extended packets in transit must contain telemetry
   instructions, so the network nodes can process and insert relevant
   telemetry data according to these instructions when processing the
   packets.  In this regard, based on the requirements and targeted
   telemetry functionalities for 6TiSCH networks, the INT sub-IE format
   is designed with its headers and content, as shown in Figure 3.  In
   this format, the Subtype Id represents the IETF IEs subtype
   identifier as defined in [RFC8137]: IANA_IETF_IE_INT.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Subtype ID   |  INT Control  | Seq. No (8b)  | Bitmap (8b)*  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         INT content                           ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      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
                          1                   2                   3

             Figure 3: The format of the IETF INT IE Subtype.

   The INT Header consists of three parts; INT Control header, Sequence
   Number and Bitmap.  The INT Control header will be used to instruct
   the other nodes about the telemetry modes and functions considered in
   the particular packet.  The detailed format of this field is provided
   in Figure 4.  The sequence number is an 8-bit counter for the INT
   source, in order to differentiate between different INT data entries
   from the same node and to detect the end-to-end delivery ratio for
   data packets with INT entries.  Finally, the Bitmap is the optional
   INT request vector where each bit represents another type of INT
   data.  It is used to inform middle nodes about the relevant telemetry
   data to add or determine the content of the INT metadata during the
   decoding.  The details of the INT control header is provided in the
   remainder of this subsection.

    +-------+-------+-------+--------+--------+-------+-------+-------+
    |Bits:0 |   1   |   2   |   3    |   4    |   5   |   6   |   7   |
    +-------+-------+-------+--------+--------+-------+-------+-------+
    |  INT  |      HBH      |Encoding| Bitmap | Over  | Loop  | Query |
    |  Mode |      Mode     |  Mode  |  Mode  | flow  | back  |       |
    +-------+---------------+--------+--------+-------+-------+-------+

              Figure 4: The format of the INT Control Field.

   INT Mode (1b):  defines the mode of telemetry operation: End-to-End
       (E2E) or Hop-by-Hop (HBH).  In E2E mode, the middle nodes may



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       only forward the INT data without any processing or addition.
       This mode may be used to monitor end-to-end network performance
       or notify a central entity about local performance issues.  On
       the other hand, HBH mode may be used to perform per-hop telemetry
       operation which allows all or a subset of the traversing nodes to
       add telemetry data if any space is left in the current frame.

   HBH Mode (2b):  defines the behavior of middle nodes in Hop-by-Hop
       telemetry operations.  It must be 0 if End-to-End INT Mode is
       selected.  If Mode 1 (Opportunistic) is selected, then all the
       nodes will try to add telemetry data in a opportunistic manner.
       Mode 2 (Probabilistic) will trigger the middle nodes to follow a
       probabilistic approach for telemetry addition.  So the nodes may
       add telemetry data with a certain probability which can
       dynamically change based on the last time it added a telemetry,
       the available space in the forwarded frame and the remaining
       number of hops.  This approach can be beneficial when attempts to
       add INT data frequently lead to frame size overflows and can
       enable collecting data from a more diverse set of nodes in the
       network.  Finally, Mode 3 enables middle nodes to decide to add
       or skip telemetry data in distributed manner.  In this mode, the
       nodes which detect performance drops/issues may add telemetry
       data to packets as a middle node.  This will also avoid the usage
       of resources for already known/not important data.

   Encoding Mode (1b):  determines the encoding mode that will be used
       in the INT content.  The first option is using the Bitmap mode
       (Content or Node) which must be followed by telemetry data as
       byte array.  The type of each data will determine the length of
       that field which will be used to process/decode the data.  The
       second option is using a TLV encoding, where each entry must be
       encoded with its type, length and value.  This will bring
       flexibility to insert data with variable length and enable nodes
       to decide on the INT content to insert.  In order to reveal the
       owner of each INT entry, each node must add a Node Id entry
       before the other telemetry data.  In addition, the whole INT
       content should be processed to understand what kind of telemetry
       data is added by each node.

   Bitmap Mode (1b):  defines what kind of bitmap will be used: Content
       Bitmap vs Node Bitmap.  If it is Content Bitmap, then that bitmap
       will apply for each node that adds INT data.  Each node must
       follow the given bitmap and concatenate the relevant entries to
       the end of the current INT content.  The content must include all
       fields mentioned in the bitmap with correct sizes.  So, the
       bitmap can be used to detect the length of each field during
       decoding.  Alternatively, the Node Bitmap option enables each
       node to add its own bitmap along with the INT data which will



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       bring independence to nodes for adding different kinds of INT
       data.  During decoding, each node bitmap can be used to detect
       the length of each field.

   Overflow (1b):  states if any INT entry overflow has happened until
       that particular hop.  If it is set, all of the following hops
       will know that they won't be able to add any INT entry, and so
       they can avoid any kind of INT processing.

   Loopback (1b):  may be used by the central entity to achieve downlink
       INT operation towards an end node.  The central entity may insert
       an INT sub-IE entry with enabled loopback and then middle nodes
       may add INT data until it arrives at the destination node.  After
       that, that node must forward the collected INT data to the
       central management entity in any of the following uplink data
       messages as INT entry.  This downlink INT operation will still
       happen fully in-band.

   Query (1b):  may be used by the central unit to trigger an uplink INT
       operation with given configuration.  When a node receives a
       packet with attached INT sub-IE including Query bit set, then it
       should create an INT operation using the received bitmap.  This
       can be used to create a polling-based INT operation triggered by
       central entity.  For instance, there can be the case that the
       central management entity detects a problem in the network, but
       there is not sufficient data to troubleshoot or isolate it.  Then
       it can send a query to certain nodes to collect more insight
       about the problem.

2.2.2.  Telemetry Data Model

   Based on a number of monitoring and management scenarios for 6TiSCH
   Networks, a number of Telemetry Data types are defined.  The proposed
   telemetry data model with limited scope is provided in Table 1 with
   details about their bitmap id, name, size and description.  One can
   extend the INT Metadata by defining any relevant telemetry data types
   in order to collect other network status information; such as link
   quality, number of neighbors, number of incoming/outgoing cells,
   number of re-transmissions.  As it is shown in Table 1, four of the
   bitmap ids are reserved for any further type definition.











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   +--------+------------------+------+--------------------------------+
   | Bitmap | Name             | Size | Description                    |
   |   ID   |                  |      |                                |
   +--------+------------------+------+--------------------------------+
   |   0    | Node ID          |  2B  | Device identifier (e.g.        |
   |        |                  |      | 802.15.4 16bit short address)  |
   |   1    | Receive Channel  |  2B  | Channel (4b) & Reception or    |
   |        | & Timestamp      |      | Generation time (12b)          |
   |   2    | Utilization      |  1B  | Transit Delay (4b), Queue      |
   |        | indicator        |      | Depth (4b)                     |
   |   3    | RSSI             |  1B  | Received Signal Strength       |
   |        |                  |      | (-127...0...127)               |
   |  4-7   | Reserved         |  -   | Reserved for other telemetry   |
   |        |                  |      | data                           |
   +--------+------------------+------+--------------------------------+

                       Table 1: Telemetry Data Model

   Node Id is one of the fundamental telemetry information types and
   represents the unique identifier of the node that inserts the
   telemetry data.  In the scope of 6TiSCH networks, IEEE 802.15.4
   16-bit short addresses can be used.

   Receive Channel and Timestamp constitute a combined telemetry entry.
   The first 4 bits of this field represent the channel (0...15) the
   packet is received on, i.e. one of the available 16 IEEE 802.15.4
   channels.  The Timestamp represents the 12 least significant bits of
   the time (expressed in ASN which is 5 bytes) at which a packet that
   needs to be forwarded is received.  For the source node, this time
   represents the time the packet is generated.  Since all of the
   network nodes share the same ASN, the timestamps on each node are
   inherently synchronized.  Assuming a 10ms slot length, 12 bits are
   enough to represent 40.96 seconds which is sufficient to detect all
   of the timestamps based on the reception ASN at the border router.
   The packet generation time can also allow us to understand the age of
   telemetry data and evaluate its validity.

   Utilization indicator illustrates the node occupation when the packet
   traverses that node.  The first 4 bits of this field represent the
   transit delay which is the delay (in slots) between the reception of
   a frame and its entry to the outgoing queue to be transmitted to the
   next hop.  For the source node, this field will be 0.  The remaining
   4 bits constitute the Queue Depth value which is the number of
   packets in the outgoing queue at the time.

   RSSI represents the received signal strength for that frame measured
   at the particular hop.  It must take values between -127 dBm and 127




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   dBm.  This value is 0 for the source node and will be ignored during
   INT processing.

2.3.  INT Strategies

   During INT entry initiation and addition process, the nodes can
   follow various INT strategies via making use of several locally
   calculated indicators.  For instance, the nodes may avoid adding
   repetitive INT entries by checking the last time a similar INT
   operation is performed.  Additionally, they may continuously process
   all locally collected telemetry data, detect events/misbehavior and
   assign an importance/relevance metric to each of them, then trigger
   an INT operation respectively.

2.3.1.  Opportunistic Logic

   In this strategy, each node tries to exploit immediate telemetry
   insertion opportunities, regardless of any planning or principle, in
   a greedy manner.  So, the nodes will take every chance to insert
   telemetry in any suitable outgoing packet towards the border router.

   Although this approach will maximize the total amount of collected
   telemetry, the source node and the nodes which are closer to the
   source will have a higher chance to insert telemetry data and
   subsequent nodes may not even get any chance to add any telemetry.
   This results in an unfair telemetry distribution and different INT
   inter-arrival times for different nodes.  Therefore, for certain
   network scenarios, especially for large networks with limited
   telemetry opportunities, this approach may result in an inadequate
   network view due to the telemetry information that comes from only a
   limited part of the network.

2.3.2.  Probabilistic Logic

   In this strategy, the nodes are following a probabilistic approach
   where each node may insert or skip INT entries with certain
   probabilities which must be dynamically calculated in a distributed
   manner.  This probability can be calculated based on the current
   frame size (including headers, payload, current INT), the size of a
   newly to be added INT entry based on Bitmap, and the remaining hop
   count that can be calculated based on the RPL Dag Rank and RPL link
   parameters (i.e.  MaxRankIncrease, MinRankIncrease, RootRank)
   [RFC6550].

   This approach assures each node with equal opportunity to insert
   telemetry data, despite their different distances.  Although this
   approach may result in a lower amount of telemetry data, it will
   result in a better distribution of the telemetry data across nodes



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   and thus a more diverse set of telemetries and a more clear/wider
   network image.

3.  Acknowledgements

   TBD!

4.  IANA Considerations

4.1.  IETF IE Subtype INT

   This document requires a number assignment in the "IEEE Std 802.15.4
   IETF IE Subtype IDs" registry for IANA_IETF_IE_INT.

5.  Security Considerations

   Regarding the security of the INT entries, the INT protocol does not
   define its own security mechanisms.  However, since INT fields are
   carried as Payload IEs, they can be encrypted and authenticated
   through link-layer security through CCM* with the same level of
   security as any other Payload IE.

   INT mechanism makes use of Payload IEs in order to transfer/collect
   the telemetry information from network nodes.  However, a malicious
   agent can exploit the contents of the INT Sub-IEs in order to
   implement a Covert Channel attack and transfer information for other
   purposes.  Based on the INT Sub-IE Control Fields and INT request
   vector (Bitmap), a validation process can be applied at border router
   to detect and prevent possible covert/hidden channels.

   Since the content of the INT sub-IE is modified at each hop, INT
   mechanism does not guarantee the preservation of the original
   telemetry information, thus creates an opportunity for a modification
   attack.

6.  References

6.1.  Normative References

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.




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6.2.  Informative References

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-28 (work
              in progress), October 2019.

   [I-D.ietf-6tisch-coap]
              Sudhaakar, R. and P. Zand, "6TiSCH Resource Management and
              Interaction using CoAP", draft-ietf-6tisch-coap-03 (work
              in progress), March 2015.

   [I-D.ietf-core-comi]
              Veillette, M., Stok, P., Pelov, A., Bierman, A., and I.
              Petrov, "CoAP Management Interface", draft-ietf-core-
              comi-08 (work in progress), September 2019.

   [I-D.ietf-ippm-ioam-data]
              Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
              Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
              P., remy@barefootnetworks.com, r., daniel.bernier@bell.ca,
              d., and J. Lemon, "Data Fields for In-situ OAM", draft-
              ietf-ippm-ioam-data-08 (work in progress), October 2019.

   [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,
              <https://www.rfc-editor.org/info/rfc6550>.

   [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,
              <https://www.rfc-editor.org/info/rfc7554>.

   [RFC8137]  Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
              Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May
              2017, <https://www.rfc-editor.org/info/rfc8137>.

6.3.  External Informative References

   [ana2019]  Alfvin, R., "802.15.4 ANA database", January 2019.







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   [IEEE802154]
              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", October 2019.

   [IEEE802154e]
              IEEE standard for Information Technology, "IEEE std.
              802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
              Networks (LR-WPANs) Amendment 1: MAC sublayer", April
              2012.

Authors' Addresses

   Abdulkadir Karaagac
   Ghent University - imec
   iGent Tower
   Technologiepark-Zwijnaarde 126
   Gent  B-9052
   Belgium

   Email: abdulkadir.karaagac@gmail.com


   Jeroen Hoebeke
   Ghent University - imec
   iGent Tower
   Technologiepark-Zwijnaarde 126
   Gent  B-9052
   Belgium

   Email: jeroen.hoebeke@ugent.be



















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