Internet DRAFT - draft-vilajosana-6tisch-globaltime
draft-vilajosana-6tisch-globaltime
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
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Drafts is at https://datatracker.ietf.org/drafts/current/.
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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
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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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