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Versions: 00 01

6TiSCH                                                X. Vilajosana, Ed.
Internet-Draft                                                  P. Tuset
Intended status: Standards Track                             B. Martinez
Expires: December 21, 2018               Universitat Oberta de Catalunya
                                                                J. Munoz
                                                                   Inria
                                                           June 19, 2018


              Global Time Distribution in 6TiSCH Networks
                 draft-vilajosana-6tisch-globaltime-01

Abstract

   This specification defines an optional extension to the Constrained
   Join Protocol (CoJP) defined by the Minimal Security Framework for
   6TiSCH.  The extension aims at providing global time distribution
   support so nodes in the 6TiSCH network can exploit global time
   information instead of relying only in relative network time based on
   the Absolute Sequence Number (ASN).  The specification also defines a
   mechanism for resynchronization, to handle leap seconds or to enable
   periodic global time updates relying on a CoAP service.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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 https://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 21, 2018.





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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Global Time Source  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Global Time Extension to the Join Response  . . . . . . . . .   4
   4.  Resynchronization . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Leap Second handling  . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Time Synchronized Channel Hopping (TSCH) exploits node
   synchronization to build up deterministic access networks through
   scheduling [RFC7554].  6TiSCH defines a control plane architecture to
   enable IEEE802.15.4 TSCH networks to securely bootstrap and in a
   distributed manner self-organize in order to meet application traffic
   needs [I-D.ietf-6tisch-architecture].  The synchronization accuracy
   between the nodes' clocks in a 6TiSCH network is dependent on the
   network maintenance traffic (Keep Alives), application traffic, and
   MAC layer guard time duration.  It is well-known that, for a given
   traffic, the smaller the guard time, the smaller the tolerated drift
   between two nodes, and hence, the more precise their synchronization.

   The concept of network synchronization is achieved through a virtual
   counter referred as Absolute Sequence Number (ASN).  In a 6TiSCH
   network, each node updates its ASN at every slot, giving the nodes
   the same notion of time (with timeslot granularity).  This time is
   relative to the moment the network started or reset and hence cannot
   be used to compare tagged events from different networks.



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   This document defines a data structure to map ASN and absolute time.
   The document then defines the procedure to transport the structure to
   the 6TiSCH nodes:

   o  As an optional extension to the Constrained Join Protocol (CoJP)
      Join Response procedure within the Minimal Security Framework for
      6TiSCH [I-D.ietf-6tisch-minimal-security].
   o  As a CoAP Response [RFC7252] to a global time service exposed by
      the Join Registrar/Cordinator (JRC).

2.  Global Time Source

   In order to distribute global time information in a 6TiSCH network at
   least one component must be acting as a global time source and
   enabling nodes in the network to obtain the absolute time reference
   from it.  The way global time is obtained and maintained in this
   network component is out of scope of this specification.  As an
   example, this component can account for the global time in the
   network internally, can use an external source to obtain global time
   (e.g.  GPS, NTP [RFC5905]) or, can be synchronized through a
   precision time protocol (PTP) such the IEEE-1588 [IEEE1588] to
   another network.

   We use the example network in Figure 1 throughout this specification
   for illustration.  The Join Registrar/Coordinator (JRC) acts as the
   global time information source for a node when it joins.  How the JRC
   obtains such global time information is out of scope of this
   specification.  The JRC can be a component outside of the 6TiSCH
   network.  However it is required to be synchronize to it through the
   ASN.  How the JRC obtains such synchronization to the 6TiSCH network
   is out of the scope of this specification.  This specification
   defines how the JRC formats and distributes the absolute time
   reference to the 6TiSCH nodes in the network.


















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               ---+-------- ............
                  |      External Network
                  | NTP/GPS/PTP
               +-----+
               |     | LLN Border
               |     | router/JRC/global
               +-----+ time source
             o    o   o
         o     o   o     o    o
     JP o   o   6TiSCH   o    o
        |  o   o   o       o
        x         o  o
      Pledge

                       Figure 1: An example network

   A Pledge node obtains its global time reference during the Secure
   Join Process with the JRC [I-D.ietf-6tisch-minimal-security].  This
   specification extends the Join Response message with an optional data
   structure which includes the global time reference and optionally the
   period for absolute time updates.

   The global time reference is a mapping between the ASN of the network
   and the global time at the moment of processing the Join Response.
   After having obtained the global time reference, a 6TiSCH node
   maintains internally its timing until it updates it, resets or is
   disconnected from the network.  Optionally, periodic refresh messages
   can be issued by the 6TiSCH node to the JRC using the JRC URI
   exposing the time service.  These optional refresh messages MAY be
   used to cope with the clock drift caused by any possible imprecision
   between the configured timeslot length and the actual timeslot
   length.  As an example, a timeslot can be configured to be 10ms long
   but because of the nature of the crystal in the device the timeslot
   becomes 10.1ms.

3.  Global Time Extension to the Join Response

   This document extends the Join Response message within the
   Constrained Join Protocol (CoJP) defined in
   [I-D.ietf-6tisch-minimal-security] with:

   o  A byte string containing the ASN at which the CoAP Response (e.g,
      Join Response) is processed at the JRC.  The 5-byte ASN is carried
      in network byte order.
   o  An 8-bit unsigned integer containing an era counter.  The era
      counter is used to account for wraps of the seconds counter.  It
      starts at 0 and increments approximately every 136 years as per




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      definition of era in NTP [RFC5905].  Era 0 starts at 0h UTC 1st of
      January 1900.
   o  A 32-bit unsigned integer containing a timestamp in seconds,
      captured at the beginning of the timeslot at which the CoAP
      Response (e.g.  Join Response) is processed.  Carried in network
      byte order.  The seconds and fraction fields are based on the
      specification described in the NTP standard [RFC5905] for the
      Timestamp format.  The seconds field accounts for seconds elapsed
      since the 0h on the 1 January 1900 UTC, as described by the NTP
      standard [RFC5905].
   o  A 32-bit unsigned integer containing the number of picoseconds
      elapsed after the last entire second at the beginning of the
      timeslot at which the CoAP Response (e.g.  Join Response) is
      processed.  Carried in network byte order.  Its granularity is
      described in the NTP standard [RFC5905].
   o  Optionally, a byte string encoding a global time service URI in
      core-link format.  The default value is 'gt'.
   o  Optionally, an unsigned word lease value indicating the number of
      minutes of freshness of the assigned global time information.  If
      this value is not provided the lease time is considered to be
      infinite.  If this value is 0, a node does not refresh the global
      time information.

   global_time_option = {
            0 : bstr,            ; ASN
            1 : uint8,           ; era counter
            2 : uint32           ; seconds counter
            3 : uint32           ; fraction
          ? 4 : bstr             ; gt_service
          ? 5 : uint16           ; gt_lease
   }




















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      +------------+-------+----------+------------+
      |   Name     | Label |   CBOR   | Reference  |
      |            |       |   type   |            |
      +------------+-------+----------+------------+
      |  ASN       | 0     |     byte | [[this     |
      |            |       |   string | document]] |
      |            |       |          |            |
      |  era       | 1     | unsigned | [[this     |
      |  counter   |       |  integer | document]] |
      |            |       |  (uint8) |            |
      |            |       |          |            |
      |  seconds   | 2     | unsigned | [[this     |
      |  counter   |       |  integer | document]] |
      |            |       | (uint32) |            |
      |            |       |          |            |
      | fraction   | 3     | unsigned | [[this     |
      |            |       |  integer | document]] |
      |            |       | (uint32) |            |
      |            |       |          |            |
      | gt_service | 4     |     byte | [[this     |
      | (optional) |       |   string | document]] |
      |            |       |          |            |
      |            |       |          |            |
      | gt_lease   | 5     | unsigned | [[this     |
      | (optional) |       |  integer | document]] |
      |            |       | (uint16) |            |
      +------------+-------+----------+------------+

   To take into account possible leap seconds.  An optional
   leap_second_option is defined by:

   o  An 8-bit unsigned integer containing the action to be performed
      when the next leap second day is reached.
   o  A 16-bit unsigned integer containing an offset in days to the
      beginning of the day (0 h UTC) when the next leap second must be
      applied.  Carried in network byte order.

   leap_second_option = {
       0 : uint8,            ; leap_indicator
       1 : uint16,           ; leap_offset
   }










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      +----------------+-------+----------+------------+
      |   Name         | Label |   CBOR   | Reference  |
      |                |       |   type   |            |
      +----------------+-------+----------+------------+
      | leap_indicator | 0     | unsigned | [[this     |
      |                |       |  integer | document]] |
      |                |       |  (uint8) |            |
      |                |       |          |            |
      | leap_offset    | 1     | unsigned | [[this     |
      |                |       |  integer | document]] |
      |                |       | (uint16) |            |
      +----------------+-------+----------+------------+

   The optional leap_second_option defines a leap second indicator,
   which identifies the type of correction that needs to be applied once
   the next leap second day is reached.  The types are described in
   Figure 9 of the RFC5905 [RFC5905].

   A leap_offset contains the offset in days to when the next leap
   second needs to be applied, following the action described in the
   leap second indicator.

   The global_time_option and the leap_second_option, if present, SHOULD
   be appended following the Join Response Payload and MUST be encoded
   as a CBOR map objects [RFC7049].

4.  Resynchronization

   When a pledge receives the Join Response containing the
   global_time_option, it updates its internal absolute time clock/
   counter.  If present, it also stores the gt_service link-format URI
   and the lease time.

   After correcting a leap second or when the lease period is reached, a
   node MAY want to update the global time information values to keep
   track of the next leap second correction event or to renew its global
   time synchronization lease.  This resynchronization is conducted
   through a CoAP GET Request to the JRC address and gt_service URI.

   o  The request method is GET.
   o  The type is Non-confirmable (NON).
   o  The Proxy-Scheme option is set to "coap".
   o  The Uri-Host option is defined by the JRC URI.
   o  The Uri-Path option is set to gt_service.
   o  The payload is empty.

   The response is a CoAP Response Message with Response Code 2.05
   (Content) containing the global_time_option as payload.  The response



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   MAY contain a leap_second_option in case a leap second update is
   needed.  Both options if present are encoded as CBOR dictionaries.

5.  Leap Second handling

   When a 6TiSCH node receives a global time synchronization message and
   this response contains a leap_second_option, the node MUST store the
   values until the leap second offset is reached.

   When a leap second offset is reached, the leap second is corrected
   adding or substracting a second to the last minute of the day as
   indicated by the leap_indicator field.

6.  Security Considerations

   The global time synchronization option is transported as part of the
   payload of the 6P Join Response message and secured by OSCORE (Object
   Security for Constrained RESTful Environments) as per
   [I-D.ietf-6tisch-minimal-security].  Since the JRC acts as the global
   time synchronization server, comprimising the JRC will result in a
   compromise of the transported information.

   A malicious 6N attacker may use a short lease time to generate high
   volumes of traffic in the network or even to try to denial the
   service to the JRC.  Is up to the JRC to implement temporal
   blacklisting policies to avoid any DoS attack.

7.  References

7.1.  Normative References

   [I-D.ietf-6tisch-minimal-security]
              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Minimal Security Framework for 6TiSCH", draft-ietf-
              6tisch-minimal-security-06 (work in progress), May 2018.

   [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>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.






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   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [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>.

7.2.  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-14 (work
              in progress), April 2018.

   [IEEE1588]
              IEEE standard for Information Technology, "IEEE Standard
              for a Precision Clock Synchronization Protocol for
              Networked Measurement and Control Systems," in IEEE Std
              1588-2008 (Revision of IEEE Std 1588-2002) , vol., no.,
              pp.1-300", July 2008.

Authors' Addresses

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

   Email: xvilajosana@uoc.edu









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   Pere Tuset
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   Barcelona, Catalonia  08018
   Spain

   Email: peretuset@uoc.edu


   Borja Martinez
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   Barcelona, Catalonia  08018
   Spain

   Email: bmartinezh@uoc.edu


   Jonathan Munoz
   Inria
   2 rue Simone Iff
   Paris  75012
   France

   Email: jonathan.munoz@inria.fr


























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