6TiSCH P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track T. Watteyne
Expires: August 16, 2014 Linear Technology
RA. Assimiti
Centero
February 14, 2014
An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4e
draft-ietf-6tisch-architecture-01
Abstract
This document presents an architecture for an IPv6 Multi-Link subnet
that is composed of a high speed powered backbone and a number of
IEEE802.15.4e TSCH wireless networks attached and synchronized by
Backbone Routers. The TSCH schedule can be static or dynamic.
6TiSCH defines mechanisms to establish and maintain the routing and
scheduling operations in a centralized, distributed, or mixed
fashion. Backbone Routers perform proxy Neighbor Discovery
operations over the backbone on behalf of the wireless devices, so
they can share a same subnet and appear to be connected to the same
backbone as classical devices
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 RFC
2119 [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 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 August 16, 2014.
Copyright Notice
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Copyright (c) 2014 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
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applications and Goals . . . . . . . . . . . . . . . . . . . . 4
4. Overview and Scope . . . . . . . . . . . . . . . . . . . . . . 5
5. Communication Paradigms and Interaction Models . . . . . . . . 8
6. Forwarding Models . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Track Forwarding . . . . . . . . . . . . . . . . . . . . . 9
6.1.1. Transport Mode . . . . . . . . . . . . . . . . . . . . 9
6.1.2. Tunnel Mode . . . . . . . . . . . . . . . . . . . . . 10
6.1.3. Tunnel Metadata . . . . . . . . . . . . . . . . . . . 11
6.2. Fragment Forwarding . . . . . . . . . . . . . . . . . . . 12
6.3. IPv6 Forwarding . . . . . . . . . . . . . . . . . . . . . 13
7. TSCH and 6top . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. 6top . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2. 6top and RPL Objective Function operations . . . . . . . . 14
7.3. Network Synchronization . . . . . . . . . . . . . . . . . 15
7.4. Slotframes and Priorities . . . . . . . . . . . . . . . . 16
7.5. Packet Marking and Handling . . . . . . . . . . . . . . . 16
7.6. Distributing the reservation of timeslots . . . . . . . . 17
8. Schedule Management Mechanisms . . . . . . . . . . . . . . . . 18
8.1. Minimal Static Scheduling . . . . . . . . . . . . . . . . 18
8.2. Neighbor-to-neighbor Scheduling . . . . . . . . . . . . . 19
8.3. Remote Monitoring and Schedule Management . . . . . . . . 19
8.4. Hop-by-hop Scheduling . . . . . . . . . . . . . . . . . . 20
9. Centralized vs. Distributed Routing . . . . . . . . . . . . . 20
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. Security Considerations . . . . . . . . . . . . . . . . . . . 21
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 22
13.3. External Informative References . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
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The emergence of radio technology enabled a large variety of new
types of devices to be interconnected, at a very low marginal cost
compared to wire, at any range from Near Field to interplanetary
distances, and in circumstances where wiring would be less than
practical, for instance rotating devices.
At the same time, a new breed of Time Sensitive Networks is being
developed to enable traffic that is highly sensitive to jitter and
quite sensitive to latency. Such traffic is not limited to voice and
video, but also includes command and control operations such as found
in industrial automation or in-vehicle sensors and actuators.
At IEEE802.1, the "Audio/Video Task Group", was renamed TSN for Time
Sensitive Networking to address Deterministic Ethernet. The
IEEE802.15.4 Medium access Control (MAC) has evolved with
IEEE802.15.4e that provides in particular the Timeslotted Channel
Hopping (TSCH) mode for industrial-type applications.
Though at a different time scale, both standards provide
Deterministic capabilities to the point that a packet that pertains
to a certain flow crosses the network from node to node following a
very precise schedule, as a train that leaves intermediate stations
at precise times along its path. With TSCH, time is formatted into
timeslots, and an individual timeslot is allocated to unicast or
broadcast communication at the MAC level. The time slotted operation
reduces collisions, saves energy, and enables to more closely
engineer the network for deterministic properties. The channel
hopping aspect is a simple and efficient technique to combat
multipath fading and external interference (for example by WiFi
emitters).
This document presents an architecture for an IPv6 Multi-Link subnet
that is composed of a high speed powered backbone and a number of
IEEE802.15.4e TSCH wireless networks attached and synchronized by
backbone routers. Route Computation may be achieved in a centralized
fashion by a Path Computation Element (PCE), in a distributed fashion
using the Routing Protocol for Low Power and Lossy Networks (RPL), or
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in a mixed mode. The Backbone Routers perform proxy IPv6 neighbor
Discovery (ND) operations over the backbone on behalf of the wireless
devices, so they can share a same IPv6 subnet and appear to be
connected to the same backbone as classical devices. Timeslots and
other device resources are managed by an abstract Network Management
Entity (NME) that may cooperate with the PCE in order to minimize the
interaction with and the load on the constrained device.
2. Terminology
Readers are expected to be familiar with all the terms and concepts
that are discussed in "neighbor Discovery for IP version 6"
[RFC4861], "IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs): Overview, Assumptions, Problem Statement, and Goals"
[RFC4919], neighbor Discovery Optimization for Low-power and Lossy
Networks [RFC6775] and "Multi-link Subnet Support in IPv6" [I-D.ietf-
ipv6-multilink-subnets].
Readers may benefit from reading the "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks" [RFC6550] specification; "Multi-Link
Subnet Issues" [RFC4903]; "Mobility Support in IPv6" [RFC6275];
"neighbor Discovery Proxies (ND Proxy)" [RFC4389]; "IPv6 Stateless
Address Autoconfiguration" [RFC4862]; "FCFS SAVI: First-Come, First-
Served Source Address Validation Improvement for Locally Assigned
IPv6 Addresses" [RFC6620]; and "Optimistic Duplicate Address
Detection" [RFC4429] prior to this specification for a clear
understanding of the art in ND-proxying and binding.
The draft uses terminology defined or referenced in [I-D.ietf-6tisch-
terminology], [I-D.chakrabarti-nordmark-6man-efficient-nd], [I-D
.roll-rpl-industrial-applicability], [RFC5191] and [RFC4080].
The draft also conforms to the terms and models described in
[RFC3444] and [RFC5889] and uses the vocabulary and the concepts
defined in [RFC4291] for the IPv6 Architecture.
3. Applications and Goals
The architecture derives from existing industrial standards for
Process Control by its focus on Deterministic Networking, in
particular with the use of the IEEE802.15.4e TSCH MAC [IEEE802154e]
and the centralized PCE. This approach leverages the TSCH MAC
benefits for high reliability against interference, low-power
consumption on deterministic traffic, and its Traffic Engineering
capabilities. Deterministic Networking applies in particular to open
and closed control loops, as well as supervisory control flows and
management.
An incremental set of industrial requirements are addressed with the
addition of an autonomic and distributed routing operation based on
RPL. These use cases include plant setup and decommissioning, as well
as monitoring of lots of lesser importance measurements such as
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corrosion and events. RPL also enables mobile use cases such as
mobile workers and cranes.
A Backbone Router is included in order to scale the factory plant
subnet to address large deployments, with proxy ND and time
synchronization over a high speed backbone.
The architecture also applies to building automation that leverage
RPL's storing mode to address multipath over a large number of hops,
in-vehicle command and control that can be as demanding as industrial
applications, commercial automation and asset Tracking with mobile
scenarios, home automation and domotics which become more reliable
and thus provide a better user experience, and resource management
(energy, water, etc.).
4. Overview and Scope
The scope of the present work is a subnet that, in its basic
configuration, is made of a IEEE802.15.4e Timeslotted Channel Hopping
(TSCH) [I-D.ietf-6tisch-tsch] MAC Low Power Lossy Network (LLN).
---+-------- ............ ------------
| External Network |
| +-----+
+-----+ | NME |
| | LLN Border | |
| | router +-----+
+-----+
o o o
o o o o
o o LLN o o o
o o o o
o
The LLN devices communicate over IPv6 [RFC2460] using the 6LoWPAN
Header Compression (6LoWPAN HC) [RFC6282]. From the perspective of
Layer 3, a single LLN interface (typically an IEEE802.15.4-compliant
radio) may be seen as a collection of Links with different
capabilities for unicast or multicast services. An IPv6 subnet spans
over multiple links, effectively forming a Multi-Link subnet. Within
that subnet, neighbor Devices are discovered with 6LoWPAN neighbor
Discovery (6LoWPAN ND) [RFC6775]. RPL [RFC6550] enables routing
within the LLN, typically within the Multi-Link subnet in the so
called Route Over fashion. RPL forms Destination Oriented Directed
Acyclic Graphs (DODAGs) within Instances of the protocol, each
Instance being associated with an Objective Function (OF) to form a
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routing topology. A particular LLN device, the LLN Border Router
(LBR), acts as RPL root, 6LoWPAN HC terminator, and LLN Border Router
(LBR) to the outside. The LBR is usually powered. More on RPL
Instances can be found in RPL [RFC6550], sections "3.1.2. RPL
Identifiers" and "3.1.3. Instances, DODAGs, and DODAG Versions".
An extended configuration of the subnet comprises multiple LLNs. The
LLNs are interconnected and synchronized over a backbone, that can be
wired or wireless. The backbone can be a classical IPv6 network,
with neighbor Discovery operating as defined in [RFC4861] and
[RFC4862]. The backbone can also support Efficiency-aware IPv6
neighbor Discovery Optimizations [I-D.chakrabarti-nordmark-6man-
efficient-nd] in mixed mode as described in [I-D.thubert-6lowpan-
backbone-router].
Security is often handled at layer 2 and Layer 4. Authentication
during the join process can be handled by the Protocol for Carrying
Authentication for Network access (PANA) [RFC5191].
The LLN devices are time-synchronized at the MAC level. The LBR that
serves as time source is a RPL parent in a particular RPL instance
that serves for time synchronization; this way, the time
synchronization starts at the RPL root and follows the RPL DODAGs
with no timing loop.
In the extended configuration, the functionality of the LBR is
enhanced to that of Backbone Router (BBR). A BBR is an LBR, but also
an Energy Aware Default Router (NEAR) as defined in [I-D.chakrabarti-
nordmark-6man-efficient-nd]. The BBR performs ND proxy operations
between the registered devices and the classical ND devices that are
located over the backbone. 6TiSCH BBRs synchronize with one another
over the backbone, so as to ensure that the multiple LLNs that form
the IPv6 subnet stay tightly synchronized. If the Backbone is
Deterministic (such as defined by the Time Sensitive Networking WG at
IEEE), then the Backbone Router ensures that the end-to-end
deterministic behavior is maintained between the LLN and the
backbone.
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---+-------- ............ ------------
| External Network |
| +-----+
| +-----+ | NME |
+-----+ | +-----+ | |
| | Router | | PCE | +-----+
| | +--| |
+-----+ +-----+
| |
| Subnet Backbone |
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | Backbone | | Backbone | | Backbone
o | | router | | router | | router
+-----+ +-----+ +-----+
o o o o o
o o o o o o o o o o o
o o o LLN o o o o
o o o o o o o o o o o o
The main architectural blocks are arranged as follows:
+-----+-----+-----+-----+-------+-----+
|PCEP | CoAP |PANA |6LoWPAN| RPL |
| PCE |DTLS | | | ND | |
+-----+-----+-----+-----+-------+-----+-----+
| TCP | UDP | ICMP |RSVP |
+-----+-----+-----+-----+-------+-----+-----+
| IPv6 |
+-------------------------------------------+
| 6LoWPAN HC |
+-------------------------------------------+
| 6top |
+-------------------------------------------+
| IEEE802.15.4e TSCH |
+-------------------------------------------+
RPL is the routing protocol of choice for LLNs. (TBD RPL) whether
there is a need to define a 6TiSCH OF.
(tbd NME) COMAN is working on network Management for LLN. They are
considering the Open Mobile Alliance (OMA) Lightweight M2M (LWM2M)
Object system. This standard includes DTLS, CoAP (core plus Block
and Observe patterns), SenML and CoAP Resource Directory.
(tbd PCE) need to work with PCE WG to define flows to PCE, and define
how to accommodate PCE routes and reservation. Will probably look a
lot like GMPLS.
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(tbd PANA) There is a debate whether PANA (layer 3) or IEEE802.1x
(layer 2) should be used in the join process. There is also a debate
whether the node should be able to send any unprotected packet on the
medium. Regardless, the security model must ensure that, prior to a
join process, packets from a untrusted device must be controlled in
volume and in reachability.
(tbd Backbone Router) need to work with 6MAN to define ND proxy.
Also need BBR sync sync between deterministic Ethernet and 6TiSCH
LLNs.
IEEE802.1TSN: external, maintain consistency. See also AVnu.
IEEE802.15.4: external, (tbd need updates?).
ISA100.20 Common Network Management: external, maintain consistency.
The 6TiSCH Operation sublayer (6top) [I-D.wang-6tisch-6top-sublayer]
is an Logical Link Control (LLC) or a portion thereof that provides
the abstraction of an IP link over a TSCH MAC.
5. Communication Paradigms and Interaction Models
[I-D.ietf-6tisch-terminology] defines the terms of Communication
Paradigms and Interaction Models, which can be placed in parallel to
the Information Models and Data Models that are defined in
[RFC3444].
A Communication Paradigms would be an abstract view of a protocol
exchange, and would come with an Information Model for the
information that is being exchanged. In contrast, an Interaction
Models would be more refined and could point on standard operation
such as a Representational state transfer (REST) "GET" operation and
would match a Data Model for the data that is provided over the
protocol exchange.
[I-D.roll-rpl-industrial-applicability] section 2.1.3. and next
discusses appplication-layer paradigms, such as Source-sink (SS) that
is a Multipeer to Multipeer (MP2MP) model that is primarily used for
alarms and alerts, Publish-subscribe (PS, or pub/sub) that is
typically used for sensor data, as well as Peer-to-peer (P2P) and
Peer-to-multipeer (P2MP) communications. Additional considerations
on Duocast and its N-cast generalization are also provided. Those
paradigms are frequently used in industrial automation, which is a
major use case for IEEE802.15.4e TSCH wireless networks with
[ISA100.11a] and [HART].
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This specification focuses on Communication Paradigms and Interaction
Models for packet forwarding and TSCH resources (cells) management.
L ink-layer and Network-layer Packet forwarding interactions are
discussed in Section 6, whereas Link-layer (one-hop), Network-layer
(multithop along a track), and Application-layer (remote control)
management mechanisms for the TSCH schedule are discussed in Section
8.
6. Forwarding Models
6TiSCH supports three different forwarding model, G-MPLS Track
Forwarding (TF), 6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding
(6F).
6.1. Track Forwarding
Track Forwarding is the simplest and fastest. A set of input cells
are uniquely bound to a set of output cells, representing a
forwarding state that can be used regardless of the upper layer
protocol. This model can effectively be seen as a G-MPLS operation
in that the information used to switch is not an explicit label, but
rather related to other properties of the way the packet was
received, a particular cell in the case of 6TiSCH. As a result, as
long as the TSCH MAC (and Layer 2 security) accepts a frame, that
frame can be switched regardless of the protocol, whether this is an
IPv6 packet, a 6LoWPAN fragment, or a frame from an alternate
protocol such as WirelessHART of ISA100.11a.
A Track is defined end-to-end as a succession of timeslots. A
timeslot belongs to at most one Track. For a given iteration of a
slotframe, the timeslot is associated uniquely with a cell, which
indicates the channel at which the timeslot operates for that
iteration.
A data frame that is forwarded along a Track has a destination MAC
address set to broadcast or a multicast address depending on MAC
support. This way, the MAC layer in the intermediate nodes accepts
the incoming frame and 6top switches it without incurring a change in
the MAC header. In the case of IEEE802.15.4e, this means effectively
broadcast, so that along the Track the short address for the
destination is set to 0xFFFF.
Conversely, a frame that is received along a Track with a destination
MAC address set to this node is extracted from the Track stream and
delivered to the upper layer. A frame with an unrecognized MAC
address is ignored at the MAC layer and thus is not received at the
6top sublayer.
There are 2 modes for a Track, transport mode and tunnel mode.
6.1.1. Transport Mode
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In transport mode, the PDU is associated flow information that refers
uniquely to the Track, so the 6top sublayer can place the frame in
the appropriate timeslot without ambiguity. In the case of IPv6
traffic, flow identification is transported in the Flow Label of the
IPv6 header. Associated with the source IPv6 address, the flow label
forms a globally unique identifier for that particular Track that is
validated at egress before restoring the destination MAC address
(dmac) and punting to the upper layer.
| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | |
+--------------+ ingress egress
| 6top | sets +----+ +----+ restores
+--------------+ dmac to | | | | dmac to
| TSCH MAC | brdcst | | | | self
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
6.1.2. Tunnel Mode
In tunnel mode, the frames originate from an arbitrary protocol over
a compatible MAC that may or may not be synchronized with the 6TiSCH
network. An example of this would be a router with a dual radio that
is capable of receiving and sending WirelessHART or ISA100.11a frames
with the second radio, by presenting itself as an access Point or a
Backbone Router, respectively.
In that mode, some entity (e.g. PCE) can coordinate with a
WirelessHART Network Manager or an ISA100.11a System Manager to
specify the flows that are to be transported transparently over the
Track.
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+--------------+
| IPv6 |
+--------------+
| 6LoWPAN HC |
+--------------+ set restore
| 6top | +dmac+ +dmac+
+--------------+ | | | |
| TSCH MAC | | | | |
+--------------+ | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+ | ingress egress |
| |
+--------------+ | |
| LLN PHY | | |
+--------------+ | |
| TSCH MAC | | |
+--------------+ | |
|ISA100/WiHART | | v
+--------------+
In that case, the flow information that identifies the Track is
uniquely derived from the information at the receiving end, for
instance the incoming timeslots, or an ISA100.11a ContractId. At the
ingress 6TiSCH router, the packet destination is recognized as self
but the flow information indicates that the frame must be tunnelled
over a particular 6top Track so the packet is not punted to upper
layer. Instead, it is passed to the 6top sublayer for switching.
The 6top sublayer in the ingress router overrides the destination MAC
to broadcast and forwards.
At the egress 6top router, the reverse operation occurs. Based on
metadata associated to the Track, the frame is passed to the
appropriate link layer with the destination MAC restored.
6.1.3. Tunnel Metadata
Metadata coming with the Track configuration is expected to provide
the destination MAC address of the egress endpoint as well as the
tunnel mode and specific data depending on the mode, for instance a
service access point for frame delivery at egress. If the tunnel
egress point does not have a MAC address that matches the
configuration, the Track installation fails.
In transport mode, if the final layer 3 destination is the tunnel
termination, then it is possible that the IPv6 address of the
destination is compressed at the 6LoWPAN sublayer based on the MAC
address. It is thus mandatory at the ingress point to validate that
the MAC address that was used at the 6LoWPAN sublayer for compression
matches that of the tunnel egress point. For that reason, the node
that injects a packet on a Track checks that the destination is
effectively that of the tunnel egress point before it overwrites it
to broadcast. The 6top sublayer at the tunnel egress point reverts
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that operation to the MAC address obtained from the tunnel metadata.
6.2. Fragment Forwarding
Considering that 6LoWPAN packets can be as large as 1280 bytes (the
IPv6 MTU), and that the non-storing mode of RPL implies Source
Routing that requires space for routing headers, and that a
IEEE802.15.4 frame with security may carry in the order of 80 bytes
of effective payload, an IPv6 packet might be fragmented into more
than 16 fragments at the 6LoWPAN sublayer.
This level of fragmentation is much higher than that traditionally
experienced over the Internet with IPv4 fragments, where
fragmentation is already known as harmful.
In the case to a multihop route within a 6TiSCH network, Hop-by-Hop
recomposition occurs at each hop in order to reform the packet and
route it. This creates additional latency and forces intermediate
nodes to store a portion of a packet for an undetermined time, thus
impacting critical resources such as memory and battery.
[I-D.thubert-roll-forwarding-frags] describes a mechanism whereby the
datagram tag in the 6LoWPAN Fragment is used as a label for switching
at the 6LoWPAN sublayer. The draft allows for a degree of flow
control base on an Explicit Congestion Notification, as well as end-
to-end individual fragment recovery.
| ^
+--------------+ | |
| IPv6 | | +----+ +----+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | learn learn |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
In that model, the first fragment is routed based on the IPv6 header
that is present in that fragment. The 6LoWPAN sublayer learns the
next hop selection, generates a new datagram tag for transmission to
the next hop, and stores that information indexed by the incoming MAC
address and datagram tag. The next fragments are then switched based
on that stored state.
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| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | replay replay |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
A bitmap and an ECN echo in the end-to-end acknowledgement enable the
source to resend the missing fragments selectively. The first
fragment may be resent to carve a new path in case of a path failure.
The ECN echo set indicates that the number of outstanding fragments
should be reduced.
6.3. IPv6 Forwarding
As the packets are routed at layer 3, traditional QoS and RED
operations are expected to prioritize flows with differentiated
services. A new class of service for Deterministic Forwarding is
being defined to that effect in [I-D.svshah-tsvwg-lln-diffserv-
recommendations].
| ^
+--------------+ | |
| IPv6 | | +-QoS+ +-QoS+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | | | | | |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
7. TSCH and 6top
7.1. 6top
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6top is a logical link control sitting between the IP layer and the
TSCH MAC layer, which provides the link abstraction that is required
for IP operations. The 6top operations are specified in [I-D.wang-
6tisch-6top-sublayer]. In particular, 6top provides a management
interface that enables an external management entity to schedule
cells and Slotframes, and allows the addition of complementary
functionality, for instance to support a dynamic schedule management
based on observed resource usage as discussed in section Section 8.2.
The 6top data model and management interfaces are further discussed
in Section 8.3.
If the scheduling entity explicitly specifies the slotOffset/
channelOffset of the cells to be added/deleted, those cells are
marked as "hard". 6top cannot move hard cells in the TSCH schedule.
Hard cells are for example used by a central PCE.
6top contains a monitoring process which monitors the performance of
cells, and can move a cell in the TSCH schedule when it performs bad.
This is only applicable to cells which are marked as "soft". To
reserve a soft cell, the higher layer does not indicate the exact
slotOffset/channelOffset of the cell to add, but rather the resulting
bandwidth and QoS requirements. When the monitoring process triggers
a cell reallocation, the two neighbor motes communicating over this
cell negotiate its new position in the TSCH schedule.
7.2. 6top and RPL Objective Function operations
An implementation of a RPL [RFC 6550] Objective Function (OF), such
as the RPL Objective Function Zero (OF0) [RFC 6552] that is used in
the Minimal 6TiSCH Configuration [I-D.ietf-6tisch-minimal] to
support RPL over a static schedule, may leverage, for its internal
computation, the information maintained by 6top.
In particular, 6top creates and maintains an abstract neighbor table.
A neighbor table entry contains a set of statistics with respect to
that specific neighbor including the ASN when the last packet has
been received from that neighbor, a set of cell quality metrics
(RSSI, LQI), the number of packets sent to the neighbor or the number
of packets received from it. This information can be obtained
through 6top management APIs as detailed in the 6top sublayer
specification [I-D.wang-6tisch-6top-sublayer] and used to compute a
Rank Increment that will determine the selection of the preferred
parent.
6top provides statistics about the underlying layer so the OF can be
tuned to the nature of the TSCH MAC layer. 6top also enables the RPL
OF to influence the MAC behaviour, for instance by configuring the
periodicity of EBs. By augmenting the EB periodicity, it is possible
to change the network dynamics so as to improve the support of mobile
devices.
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Some RPL control messages, such as the DODAG Information Object (DIO)
are broadcast to all neighbor nodes. The broadcast channel
requirement is addressed by 6top by configuring TSCH to provide such
a channel, as opposed to, for instance, carrying DIO messages in
Enhance Beacons.
In the TSCH schedule, each cell has the LinkType attribute. Setting
the LinkType to ADVERTISING indicates that the cell MAY be used to
send an Enhanced Beacon. When a node forms its Enhanced Beacon, the
cell, with LinkType=ADVERTISING, SHOULD be included in the
FrameAndLinkIE, and its LinkOption field SHOULD be set to the
combination of "Receive" and "Timekeeping". The receiver of the
Enhanced Beacon MAY be listening at the cell to get the Enhanced
Beacon ([IEEE802154e]). 6top takes this way to establish broadcast
channel, which not only allows TSCH to broadcast Enhanced Beacons,
but also allows an upper layer like RPL.
To support DIO and DAO broadcasts, 6top uses the payload of a Data
Packet to carry the DIO or DAO. The message is inserted into the
queue associated with the cells which LinkType is set to ADVERTISING.
Then, taking advantage of the broadcast cell feature established with
FrameAndLinkIE (as described above), the data packet with DIO or DAO
in the payload can be received by neighbors, which enforces the
maintenance of DODAG.
A LinkOption combining "Receive" and "Timekeeping" bits indicates to
the receivers of the Enhanced Beacon that the cell MUST be used as a
broadcast cell. The frequency of sending Enhanced Beacons or other
broadcast messages by the upper layer is determined by the timers
associated with the messages. For example, the transmission of
Enhance Beacons is triggered by a timer in 6top; transmission of a
DIO message is triggered by the trickle timer of RPL.
7.3. Network Synchronization
Nodes in a TSCH network must be time synchronized. A node keeps
synchronized to its time source neighbor through a combination of
frame-based and acknowledgement-based synchronization. In order to
maximize battery life and network throughput, it is advisable that
RPL ICMP discovery and maintenance traffic (governed by the trickle
timer) be somehow coordinated with the transmission of time
synchronization packets (especially with enhanced beacons). This
could be achieved through an interaction of the 6top sublayer and the
RPL objective Function, or could be controlled by a management
entity.
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Time distribution requires a loop-less structure. Nodes taken in a
synchronization loop will rapidly desynchronize from the network and
become isolated. It is expected that a RPL DAG with a dedicated
global Instance is deployed for the purpose of time synchronization.
That Instance is referred to as the Time Synchronization Global
Instance (TSGI). The TSGI can be operated in either of the 3 modes
that are detailed in RPL [RFC6550] section "3.1.3. Instances,
DODAGs, and DODAG Versions". Multiple uncoordinated DODAGs with
independent roots may be used if all the roots share a common time
source such as the Global Positioning System (GPS). In the absence of
a common time source, the TSGI should form a single DODAG with a
virtual root. A backbone network is then used to synchronize and
coordinate RPL operations between the backbone routers that act as
sinks for the LLN.
A node that has not joined the TSGI advertises a MAC level Join
Priority of 0xFF to notify its neighbors that is is not capable of
serving as time parent. A node that has joined the TSGI advertises a
MAC level Join Priority set to its DAGRank() in that Instance, where
DAGRank() is the operation specified in [RFC6550], section "3.5.1.
Rank Comparison".
A root is configured or obtains by some external means the knowledge
of the RPLInstanceID for the TSGI. The root advertises its DagRank in
the TSGI, that MUST be less than 0xFF, as its Join Priority (JP) in
its IEEE802.15.4e Extended Beacons (EB). We'll note that the JP is
now specified between 0 and 0x3F leaving 2 bit sin the octet unused
in the IEEE802.15.4e specification. After consultation with IEEE
authors, it was asserted that 6TiSCH can make a full use of the octet
to carry an integer value up to 0xFF.
A node that reads a Join Priority of less than 0xFF should join the
neighbor with the lesser Join Priority and use it as time parent. If
the node is configured to serve as time parent, then the node should
join the TSGI, obtain a Rank in that Instance and start advertising
its own DagRank in the TSGI as its Join Priority in its EBs.
7.4. Slotframes and Priorities
6top uses priority queues to manage concurrent data flows of
different priorities. When a packet is received from an higher layer
for transmission, the I-MUX module of 6top inserts that packet in the
outgoing queue which matches the packet best (DSCP can therefore be
used). At each scheduled transmit slot, the MUX module looks for the
frame in all the outgoing queues that best matches the cells. If a
frame is found, it is given to TSCH for transmission.
7.5. Packet Marking and Handling
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reservation Deterministic flow allocation (hard reservation of
timeslots) eg centralized RSVP? metrics? Hop-by-hop interaction with
6top. Lazy reservation (use shared slots to transport extra burst
and then dynamically (de)allocate) Classical QoS (dynamic based on
observation)
7.6. Distributing the reservation of timeslots
6TiSCH expects a high degree of scalability together with a
distributed routing functionality based on the RPL routing protocol.
To achieve this goal, the spectrum must be allocated in a way that
allows for spatial reuse between zones that will not interfere with
one another. In a large and spatially distributed network, a 6TiSCH
node is often in a good position to determine usage of spectrum in
its vicinity.
Use cases for distributed routing are often associated with a
statistical distribution of best-effort traffic with variable needs
for bandwidth on each individual link. With 6TiSCH, the link
abstraction is implemented as a bundle of cells; the size of a bundle
is optimal when both the energy wasted idle listening and the packet
drops due to congestion loss are minimized. This can be maintained
if the number of cells in a bundle is adapted dynamically, and with
enough reactivity, to match the variations of best-effort traffic.
In turn, the agility to fulfill the needs for additional cells
improves when the number of interactions with other devices and the
protocol latencies are minimized.
6TiSCH limits that interaction to RPL parents that will only
negotiate with other RPL parents, and performs that negotiation by
groups of cells as opposed to individual cells. The 6TiSCH
architecture allows RPL parents to adjust dynamically, and
independently from the PCE, the amount of bandwidth that is used to
communicate between themselves and their children, in both
directions; to that effect, an allocation mechanism enables a RPL
parent to obtain the exclusive use of a portion of an abstract
channel usage/distribution (CUD) matrix of timeslots within its
interference domain.
The 6TiSCH architecture introduces the concept of chunks [I-D.ietf-
6tisch-terminology]) to operate such spectrum distribution for a
whole group of cells at a time. The CUD matrix is formatted into a
set of chunks, each of them identified uniquely by a chunk-ID. The
knowledge of this formatting is shared between all the nodes in a
6TiSCH network. 6TiSCH also defines the process of chunk ownership
appropriation whereby a RPL parent discovers a chunk that is not used
in its interference domain (e.g lack of energy detected in reference
cells in that chunk); then claims the chunk, and then defends it in
case another RPL parent would attempt to appropriate it while it is
in use. The chunks is the basic unit of ownership that is used in
that process.
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+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 0 |chnkA|chnkP|chnk7|chnkO|chnk2|chnkK|chnk1| ... |chnkZ|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 1 |chnkB|chnkQ|chnkA|chnkP|chnk3|chnkL|chnk2| ... |chnk1|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
...
+-----+-----+-----+-----+-----+-----+-----+ +-----+
chan.Off. 15 |chnkO|chnk6|chnkN|chnk1|chnkJ|chnkZ|chnkI| ... |chnkG|
+-----+-----+-----+-----+-----+-----+-----+ +-----+
0 1 2 3 4 5 6 M
As a result of the process of chunk ownership appropriation, the RPL
parent has exclusive authority to decide which cell in the
appropriated chunk can be used by which node in its interference
domain. In other words, it is implicitly delegated the right to
manage the portion of the slotframe that is represented by the chunk.
The RPL parent may thus orchestrate which transmissions occur in any
of the cells in the chunk, by allocating cells from the chunk to any
form of communication (unicast, multicast) in any direction between
itself and its children. Initially, those cells are added to the
heap of free cells, then dynamically placed into existing bundles, in
new bundles, or allocated opportunistically for one transmission.
The appropriation of a chunk can also be requested explicitly by the
PCE to any node. In that case, the node still may need to perform
the appropriation process to validate that no other node has claimed
that chunk already. After a successful appropriation, the PCE owns
the cells in that chunk, and may use them as hard cells to set up
tracks.
8. Schedule Management Mechanisms
6TiSCH uses 4 paradigms to manage the TSCH schedule of the LLN nodes:
Static Scheduling, neighbor-to-neighbor Scheduling, remote monitoring
and scheduling management, and Hop-by-hop scheduling. Multiple
mechanisms are defined that implement the associated Interaction
Models, and can be combined and used in the same LLN. Which
mechanism(s) to use depends on application requirements.
8.1. Minimal Static Scheduling
In the simplest instantiation of a 6TiSCH network, a common fixed
schedule may be shared by all nodes in the network. Cells are
shared, and nodes contend for slot access in a slotted aloha manner.
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A static TSCH schedule can be used to bootstrap a network, as an
initial phase during implementation, or as a fall-back mechanism in
case of network malfunction. This scheduled can be preconfigured or
learnt by a node when joining the network. Regardless, the schedule
remains unchanged after the node has joined a network. The Routing
Protocol for LLNs (RPL) is used on the resulting network. This
"minimal" scheduling mechanism that implements this paradigm is
detailed in [I-D.ietf-6tisch-minimal].
8.2. Neighbor-to-neighbor Scheduling
In the simplest instantiation of a 6TiSCH network described in
Section 8.1, nodes may expect a packet at any cell in the schedule
and will waste energy idle listening. In a more complex
instantiation of a 6TiSCH network, a matching portion of the schedule
is established between peers to reflect the observed amount of
transmissions between those nodes. The aggregation of the cells
between a node and a peer forms a bundle that the 6top layer uses to
implement the abstraction of a link for IP. The bandwidth on that
link is proportional to the number of cells in the bundle.
If the size of a bundle is configured to fit an average amount of
bandwidth, peak emissions will be destroyed. If the size is
configured to allow for peak emissions, energy is be wasted idle
listening.
In the most efficient instantiation of a 6TiSCH network, the size of
the bundles that implement the links may be changed dynamically in
order to adapt to the need of end-to-end flows routed by RPL. An
optional On-The-Fly (OTF) component may be used to monitor bandwidth
usage and perform requests for dynamic allocation by the 6top
sublayer. The OTF component is not part of the 6top sublayer. It
may be collocated on the same device or may be partially or fully
offloaded to an external system.
The 6top sublayer [I-D.wang-6tisch-6top-sublayer] defines a protocol
for neighbor nodes to reserve soft cells to one another. Because
this reservation is done without global knowledge of the schedule of
nodes in the LLN, scheduling collisions are possible. 6top defines a
monitoring process which continuously tracks the packet delivery
ratio of soft cells. It uses these statistics to trigger the
relocation of a soft cell in the schedule, using a negotiation
protocol between the neighbors nodes communicating over that cell.
Monitoring and relocation is done in the 6top layer. For the upper
layer, the connection between two neighbor nodes appears as an number
of cells. Depending on traffic requirements, the upper layer can
request 6top to add or delete a number of cells scheduled to a
particular neighbor, without being responsible for choosing the exact
slotOffset/channelOffset of those cells.
8.3. Remote Monitoring and Schedule Management
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The 6top interface document [I-D.wang-6tisch-6top-interface]
specifies the generic data model that can be used to monitor and
manage resources at the 6top sublayer. Abstract methods are
suggested for use by a management entity in the device. The data
model also enables remote control operations on the 6top sublayer.
Being able to interact with the 6top sublayer of a node multiple hops
away can be used for monitoring, scheduling, or a combination of
both. The architecture supports variations on the deployment model,
and focuses on the flows rather than whether there is a proxy or a
translational operation on the way.
[I-D.sudhaakar-6tisch-coap] defines an mapping of 6top's set of
commands described in [I-D.wang-6tisch-6top-interface] to CoAP
resources. This allows an entity to interact with the 6top layer of
a node that is multiple hops away in a RESTful fashion.
[I-D.sudhaakar-6tisch-coap] defines a basic set CoAP resources and
associated RESTful access methods (GET/PUT/POST/DELETE). The payload
(body) of the CoAP messages is encoded using the CBOR format. The
draft also defines the concept of "profiles" to allow for future or
specific extensions, as well as a mechanism for a CoAP client to
discover the profiles installed on a node.
The entity issuing the CoAP requests can be a central scheduling
entity (e.g. a PCE), a node multiple hops away with the authority to
modify the TSCH schedule (e.g. the head of a local cluster), or a
external device monitoring the overall state of the network (e.g.
NME). The architecture allows for different types of interactions
between this CoAP client and a node in the network:
8.4. Hop-by-hop Scheduling
A node can reserve a track to a destination node multiple hops away
by installing soft cells at each intermediate node. This forms a
track of soft cells. It is the responsibility of the 6top sublayer
of each node on the track to monitor these soft cells and trigger
relocation when needed.
This hop-by-hop reservation mechanism is similar to [RFC2119] and
[RFC5974]. The protocol for a node to trigger hop-by-hop scheduling
is not yet defined.
9. Centralized vs. Distributed Routing
6TiSCH supports a mixed model of centralized routes and distributed
routes. Centralized routes can for example computed by a entity such
as a PCE. Distributed routes are computed by the RPL routing
protocol.
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Both may inject routes in the Routing Tables of the 6TiSCH routers.
In either case, each route is associated with a topology that is
indexed by an RPLInstanceID, as defined in RPL [RFC6550]. RPL and
PCE rely on shared sources to define Global and Local RPLInstanceIDs.
It is possible for centralized and distributed routing to share a
same topology. In this case, centralized routes have precedence over
distributed routes in case of conflict.
Inside the 6TiSCH domain, the flow label is used to indicate the
topology that must be used for routing. The associated Routing
Tables are discussed in [I-D.thubert-roll-flow-label].
10. IANA Considerations
This specification does not require IANA action.
11. Security Considerations
This specification is not found to introduce new security threat.
12. Acknowledgements
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S.E. and R.M. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC3444] Pras, A. and J. Schoenwaelder, "On the Difference between
Information Models and Data Models", RFC 3444, January
2003.
[RFC4080] Hancock, R., Karagiannis, G., Loughney, J. and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC
4080, June 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W. and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
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[RFC5191] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H. and A.
Yegin, "Protocol for Carrying Authentication for Network
Access (PANA)", RFC 5191, May 2008.
[RFC5889] Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
Hoc Networks", RFC 5889, September 2010.
[RFC5974] Manner, J., Karagiannis, G. and A. McDonald, "NSIS
Signaling Layer Protocol (NSLP) for Quality-of-Service
Signaling", RFC 5974, October 2010.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[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.
[RFC6552] Thubert, P., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)", RFC
6552, March 2012.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E. and C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
November 2012.
13.2. Informative References
[I-D.chakrabarti-nordmark-6man-efficient-nd]
Chakrabarti, S., Nordmark, E., Thubert, P. and M.
Wasserman, "Wired and Wireless IPv6 Neighbor Discovery
Optimizations", Internet-Draft draft-chakrabarti-nordmark-
6man-efficient-nd-04, October 2013.
[I-D.ietf-6tisch-minimal]
Vilajosana, X. and K. Pister, "Minimal 6TiSCH
Configuration", Internet-Draft draft-ietf-6tisch-
minimal-00, November 2013.
[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", Internet-Draft draft-ietf-6tisch-
terminology-00, November 2013.
[I-D.ietf-6tisch-tsch]
Watteyne, T., Palattella, M. and L. Grieco, "Using
IEEE802.15.4e TSCH in an LLN context: Overview, Problem
Statement and Goals", Internet-Draft draft-ietf-6tisch-
tsch-00, November 2013.
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[I-D.ietf-roll-rpl-industrial-applicability]
Phinney, T., Thubert, P. and R. Assimiti, "RPL
applicability in industrial networks", Internet-Draft
draft-ietf-roll-rpl-industrial-applicability-02, October
2013.
[I-D.ohba-6tisch-security]
Chasko, S., Das, S., Lopez, R., Ohba, Y., Thubert, P. and
A. Yegin, "Security Framework and Key Management Protocol
Requirements for 6TiSCH", Internet-Draft draft-ohba-
6tisch-security-00, October 2013.
[I-D.sudhaakar-6tisch-coap]
Sudhaakar, R. and P. Zand, "6TiSCH Data Model for CoAP",
Internet-Draft draft-sudhaakar-6tisch-coap-00, October
2013.
[I-D.svshah-tsvwg-lln-diffserv-recommendations]
Shah, S. and P. Thubert, "Differentiated Service Class
Recommendations for LLN Traffic", Internet-Draft draft-
svshah-tsvwg-lln-diffserv-recommendations-01, August 2013.
[I-D.thubert-6lowpan-backbone-router]
Thubert, P., "6LoWPAN Backbone Router", Internet-Draft
draft-thubert-6lowpan-backbone-router-03, February 2013.
[I-D.thubert-roll-flow-label]
Thubert, P., "Use of the IPv6 Flow Label within an LLN",
Internet-Draft draft-thubert-roll-flow-label-02, November
2012.
[I-D.thubert-roll-forwarding-frags]
Thubert, P. and J. Hui, "LLN Fragment Forwarding and
Recovery", Internet-Draft draft-thubert-roll-forwarding-
frags-02, September 2013.
[I-D.wang-6tisch-6top-interface]
Wang, Q., Vilajosana, X. and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Interface", Internet-Draft
draft-wang-6tisch-6top-interface-01, February 2014.
[I-D.wang-6tisch-6top-sublayer]
Wang, Q., Vilajosana, X. and T. Watteyne, "6TiSCH
Operation Sublayer (6top)", Internet-Draft draft-wang-
6tisch-6top-00, October 2013.
13.3. External Informative References
[HART] www.hartcomm.org, "Highway Addressable Remote Transducer,
a group of specifications for industrial process and
control devices administered by the HART Foundation", .
[IEEE802.1TSNTG]
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IEEE Standards Association, "IEEE 802.1 Time-Sensitive
Networks Task Group", March 2013, <http://www.ieee802.org/
1/pages/avbridges.html>.
[IEEE802154e]
IEEE standard for Information Technology, "IEEE std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendament 1: MAC sublayer", April
2012.
[ISA100.11a]
ISA, "ISA100, Wireless Systems for Automation", May 2008,
<http://www.isa.org/Community/
SP100WirelessSystemsforAutomation>.
Authors' Addresses
Pascal Thubert, editor
Cisco Systems, Inc
Building D
45 Allee des Ormes - BP1200
MOUGINS - Sophia Antipolis, 06254
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Thomas Watteyne
Linear Technology, Dust Networks Product Group
30695 Huntwood Avenue
Hayward, CA 94544
USA
Phone: +1 (510) 400-2978
Email: twatteyne@linear.com
Robert Assimiti
Centero
961 Indian Hills Parkway
Marietta, GA 30068
USA
Phone: +1 404 461 9614
Email: robert.assimiti@centerotech.com
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