Global Time Distribution in 6TiSCH Networks
draft-vilajosana-6tisch-globaltime-00

6TiSCH                                                X. Vilajosana, Ed.
Internet-Draft                                                  P. Tuset
Intended status: Standards Track                             B. Martinez
Expires: September 2, 2018               Universitat Oberta de Catalunya
                                                                J. Munoz
                                                                   Inria
                                                           March 1, 2018

              Global Time Distribution in 6TiSCH Networks
                 draft-vilajosana-6tisch-globaltime-00

Abstract

   This specification defines an optional extension to the Join Response
   message 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
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   This Internet-Draft will expire on September 2, 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
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   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 . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Leap Second handling  . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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 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 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 a
      node in the network and announced through the Join Response.

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 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.  This specification defines how the JRC formats and
   distributes the absolute time reference to the 6TiSCH nodes in the
   network.

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

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   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 an optional
   CoAP link-format and IPv6 address pointing to the network global time
   service to support periodic absolute time updates.

   The global time reference is a 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 resets or is
   disconnected from the network.  Optionally, periodic refresh messages
   can be issued by the 6TiSCH node to the node that acts as global time
   source, through a provided CoAP URI exposing the time service.  The
   JRC or any other node in the network can expose that service.

3.  Global Time Extension to the Join Response

   This document extended the Join Response 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.
   o  A 32-bit unsigned integer of 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.
   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.
   o  Optionally, a byte string encoding the IPv6 address of the global
      time source.
   o  Optionally, a byte string encoding a global time service URI in
      core-link format.
   o  Optionally, an unsigned word lease value indicating the number of
      days of freshness of the assigned global time information.

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   global_time_option = [
       asn: bstr,
       era: uint8,
       seconds: uint32,
       fraction: uint32,
       ? gt_address : bstr,
       ? gt_service: link-format (bstr),
       ? gt_lease: uint16,
   ]

   To take into account possible leap seconds.  A 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 = [
       leap_indicator: uint8,
       leap_offset: uint16,
   ]

   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
   definition of era in [RFC5905].  Era 0 starts at 0h UTC 1st of
   January 1900.  The seconds and fraction fields are based on the
   specification described in the [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 [RFC5905].  The fraction field
   provides synchronization precisions in the order of hundreds of
   picoseconds.  Its granularity is described in [RFC5905].  If the
   global time service is co-located at the JRC, the gt_address field
   can be omitted as the address is known through the Join Response.  An
   optional byte string, indicating the global time synchronization
   service URI MAY be present.  The URI is defined using the CORE link-
   format as per [RFC6690].  An optional gt_lease value in days
   indicating the mandatory refresh period MAY be present.  If this
   value is 0, a node does not refresh the global time information.

   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 [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.

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   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 array object [RFC7049].  A CBOR array contains as a first
   element the number of items contained by the array.  This element
   then enables to determine if the optional elements are present in the
   option.

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_address, the 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 gt_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 gt_address.
   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
   MAY contain a leap_second_option in case a leap second update is
   needed.  Both options if present are encoded as CBOR arrays.

5.  Leap Second handling

   When a 6TiSCH node receives a global time synchronization message,
   either being a Join Response or a CoAP Response message for a time
   synchronization Request, 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.

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6.  References

6.1.  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-04 (work in progress), October
              2017.

   [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>.

   [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>.

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-13 (work
              in progress), November 2017.

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   [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

   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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