One document matched: draft-thubert-6tisch-architecture-00.xml
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<rfc category="std" docName="draft-thubert-6tisch-architecture-00" ipr="trust200902">
<front>
<title abbrev="6TiSCH-architecture">An Architecture for IPv6 over the TSCH mode of IEEE IEEE802.15.4e</title>
<author fullname="Pascal Thubert" initials="P" role="editor" surname="Thubert">
<organization abbrev="cisco">Cisco Systems, Inc</organization>
<address>
<postal>
<street>Building D</street>
<street>45 Allee des Ormes - BP1200 </street>
<city>MOUGINS - Sophia Antipolis</city>
<code>06254</code>
<country>FRANCE</country>
</postal>
<phone>+33 497 23 26 34</phone>
<email>pthubert@cisco.com</email>
</address>
</author>
<author fullname="Robert Assimiti" initials="RA" surname="Assimiti">
<organization abbrev="Centero">Centero</organization>
<address>
<postal>
<street>961 Indian Hills Parkway</street>
<city>Marietta</city>
<region>GA</region>
<code>30068</code>
<country>USA</country>
</postal>
<phone>+1 404 461 9614</phone>
<email>robert.assimiti@centerotech.com</email>
</address>
</author>
<author initials="T" surname="Watteyne" fullname="Thomas Watteyne">
<organization>Linear Technology / Dust Networks</organization>
<address>
<postal>
<street>30695 Huntwood Avenue</street>
<city>Hayward</city>
<region>CA</region>
<code>94544</code>
<country>USA</country>
</postal>
<phone>+1 (510) 400-2978</phone>
<email>twatteyne@linear.com</email>
</address>
</author>
<date/>
<area>Internet Area</area>
<workgroup>6TiSCH</workgroup>
<keyword>Draft</keyword>
<abstract>
<t>
This document presents an architecture for an IPv6 multilink 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, in a distributed fashion using the
Routing Protocol for Low Power and Lossy Networks, or in a mixed mode.
The Backbone Routers perform proxy Neighbor discovery operations over the
backbone on behalf of the wireless device, so they can share a same subnet
and appear to be connected to the same backbone as classical devices.
</t>
</abstract>
<note title="Requirements Language">
<t>
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 <xref target="RFC2119">RFC 2119</xref>.
</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>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 could be less
than practical, for instance rotating devices.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>Though at a different time scale, both standards provide
Deterministic capabilities to the point that a packet that pertains
to a certain flow will cross 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 a unicast or
a 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).
</t>
<t>
This document presents an architecture for an IPv6 multilink 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
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.
</t>
</section>
<section anchor="Terminology" title="Terminology">
<t>
The draft uses terminology defined in
<xref target="I-D.palattella-6TiSCH-terminology"/>,
<xref target="I-D.chakrabarti-nordmark-6man-efficient-nd"/>,
<xref target="RFC5191"/>
and <xref target="RFC4080"/>.
</t>
<t>
It conforms to the terms and models described for IPv6 in
<xref target="RFC5889"/> and uses the vocabulary and the concepts
defined in <xref target="RFC4291"/> for IPv6.
</t>
</section>
<section anchor="Goals" title="Applications and Goals">
<t>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 and the centralized path computation element.
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.
</t>
<t>Additional industrial use cases are addressed with the addition of a more
autonomic and distributed routing based on RPL. These use cases include
plant setup and decommissioning, as well as monitoring of lots of lesser
importance measurements such as corrosion and events.
RPL also enables mobile use cases such as mobile workers and cranes.
</t>
<t>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.
</t>
<t>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.).
</t>
</section>
<section anchor="Scope" title="Overview and Scope">
<t>
The scope of the present work is a subnet that, in its basic
configuration, is made of a
<xref target="I-D.watteyne-6TiSCH-tsch-lln-context">
IEEE802.15.4e Timeslotted Channel Hopping (TSCH)</xref>
MAC Route-Over Low Power Lossy Network (LLN).
<figure anchor="fig1" title="Basic Configuration">
<artwork><![CDATA[
---+-------- ............ ------------
| External Network |
| +-----+
+-----+ | NME |
| | LLN Border | |
| | router +-----+
+-----+
o o o
o o o o
o o LLN o o o
o o o o
o
]]></artwork>
</figure>
</t>
<t>
The LLN devices communicate over IPv6 <xref target="RFC2460"/>
using the <xref target="RFC6282">6LoWPAN Header Compression (6LoWPAN HC)</xref>.
From the Layer 3 perspective, a single LLN interface
(typically an IEEE802.15.4 radio) may be seen as a collection of Links with
different capabilities for unicast or multicast services. An IPv6 subnet will
span over multiple links, effectively forming a multilink subnet. Within that
subnet, Neighbor Devices are discovered with <xref target="RFC6775"> 6LoWPAN
Neighbor Discovery (6LoWPAN ND)</xref>. <xref target="RFC6550">
The Routing Protocol for Low Power and Lossy Networks (RPL) </xref>
enables routing within the LLN, typically within the multilink 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 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
<xref target="RFC6550"/> sections "3.1.2. RPL Identifiers"
and "3.1.3. Instances, DODAGs, and DODAG Versions".
</t>
<t>
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
<xref target="RFC4861"/> and <xref target="RFC4862"/>.
The backbone can also support
<xref target="I-D.chakrabarti-nordmark-6man-efficient-nd">
Efficiency aware IPv6 Neighbor Discovery Optimizations </xref>
in mixed mode as described in
<xref target="I-D.thubert-6lowpan-backbone-router"/>.
</t>
<t>
Security is often handled at layer 2 and Layer 4. Authentication
during the join process is handled with the <xref target="RFC5191">
Protocol for Carrying Authentication for Network access (PANA)</xref>.
</t>
<t>
The LLN devices are time-synchronized at 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.
</t>
<t>
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
<xref target="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.
</t>
<t>
A Network Management Entity may participate to tre
<figure anchor="fig2" title="Extended Configuration">
<artwork><![CDATA[
---+-------- ............ ------------
| 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
]]></artwork>
</figure>
</t>
<t>
The main architectural blocks are arranged as follows:
<figure anchor="fig4" title="6TiSCH stack">
<artwork><![CDATA[
+-----+-----+-----+-----+-------+-----+
|PCEP | CoAP |PANA |6LoWPAN| RPL |
| PCE |DTLS | | | ND | |
+-----+-----+-----+-----+-------+-----+-----+
| TCP | UDP | ICMP |RSVP |
+-----+-----+-----+-----+-------+-----+-----+
| IPv6 |
+-------------------------------------------+
| 6LoWPAN HC |
+-------------------------------------------+
| 6top |
+-------------------------------------------+
| IEEE802.15.4e TSCH |
+-------------------------------------------+
]]></artwork>
</figure>
</t>
<t>RPL is the routing protocol of choice for LLNs. (TBD RPL) whether there
is a need to define a 6TiSCH OF.</t>
<t>(tbd NME) COMAN is working on network Management for LLN.
They are considering the Open Mobile Alliance (OMA) Lightweight M2M (LWM2M) Objet system.
This standard includes DTLS, CoAP (core plus the Block and Observe patterns),
SenML and CoAP Resource Directory.</t>
<t>(tbd PCE) need to work with PCE WG to define flows to PCE, and define how to
accomodate PCE routes and reservation. Will probably look a lot like GMPLS</t>
<t>(tbd Backbone Router) need to work with 6MAN to define ND proxy.
Also need BBR sync sync between deterministic ethernet and 6TiSCH LLNs.</t>
<t>IEEE802.1TSN: external, maintain consistency.</t>
<t>IEEE802.15.4: external, (tbd need updates?).</t>
<t>ISA100.20 Common Network Management: external, maintain consistency.</t>
<t>IoT6 European Project: external, maintain consistency.</t>
</section>
<section anchor="PCEvsRPL" title="Centralized vs. Distributed Routing">
<t>6TiSCH supports a mix model of centralized routes and distributed routes.
Centralized routes are typically computed by a entity such as the PCE.
Distributed routes are computed by the RPL routing protocol.
</t>
<t>Both RPL and the PCE 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 <xref target="RFC6550">RPL</xref>. RPL and PCE rely on shared sources to define
Global and Local RPLInstanceIDs.
</t>
<t>
It is possible for RPL and PCE to share a same topology, in which case the PCE routes have
precedence over RPL routes in case of a conflict.
</t> <t>
Inside the 6TiSCH domain, the flow label is used to indicate the topology that must be used for
routing and the associated Routing Tables as discussed in <xref target="I-D.thubert-roll-flow-label"/>.
</t>
</section>
<section anchor="FwdMdl" title="Forwarding Models">
<t>
6TiSCH supports three different forwarding model, G-MPLS Track Forwarding (TF),
6LoWPAN Fragment Forwarding (FF) and IPv6 Forwarding (6F).
</t>
<section anchor="Trkfwd" title="Track Forwarding">
<t>
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 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.
</t>
<t>
A Track is defined end-to-end as a succession of timeslots and 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.
</t> <t>
A frame that is forwarded along a Track has a destination MAC address set to broadcast
or a multicast address depending on the MAC support.
This way, the MAC layer in the intermediate nodes will accept the incoming frame and
6top will switch 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.
</t> <t>
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 just ignored at the MAC layer and thus is not received
at the 6top sublayer.
</t> <t>
There are 2 modes for a Track, transport mode and tunnel mode.
</t>
<section anchor="Tranmode" title="Transport Mode">
<t>
In transport mode, the PDU is associated with 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, the identification of that flow information is transported in the
Flow Label in 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.
<figure anchor="fig5" title="Track Forwarding, Transport Mode">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | |
+--------------+ ingress egress
| 6top | sets +----+ +----+ restores
+--------------+ dmac to | | | | dmac to
| TSCH MAC | brdcst | | | | self
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
</t>
</section>
<section anchor="Trktun" title="Tunnel Mode">
<t>
In tunnel mode, the frames originate from an arbitrary protocol over a compatible MAC
that may or may not be perfectly 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.
</t> <t>
In that mode, the PCE may coordinate with a WirelessHART Network Manager or an ISA100.11a
System Manager in order to specify the flows that are to be transported transparently
over the Track.
<figure anchor="fig6" title="Track Forwarding, Tunnel Mode">
<artwork><![CDATA[
+--------------+
| IPv6 |
+--------------+
| 6LoWPAN HC |
+--------------+ set restore
| 6top | +dmac+ +dmac+
+--------------+ | | | |
| TSCH MAC | | | | |
+--------------+ | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+ | ingress egress |
| |
+--------------+ | |
| LLN PHY | | |
+--------------+ | |
| TSCH MAC | | |
+--------------+ | |
|ISA100/WiHART | | v
+--------------+
]]></artwork>
</figure>
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.
</t>
<t>
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.
</t>
</section>
<section anchor="Tundata" title="Tunnel Metadata">
<t>
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.
</t> <t>
If the tunnel egress point does not have a MAC address that matches the configuration,
the Track installation fails.
</t> <t>
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 that operation to the MAC address obtained
from the tunnel metadata.
</t>
</section>
</section>
<section anchor="Frgfwd" title="Fragment Forwarding">
<t>Considering that 6LoWPAN packets can be as large as 1280 bytes, which is 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.
</t> <t>
This level of fragmentation is much higher than that traditionally experienced over the Internet
with IPv4 fragments, where fragmentation is already known as harmful.
</t> <t>
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.
</t> <t>
<xref target="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.
In that model, the first fragment is routed based on the IPv6 header that is present in that fragment.
<figure anchor="fig7" title="Forwarding First Fragment">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | +----+ +----+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | learn learn |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
</t> <t>
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.
<figure anchor="fig8" title="Forwarding Next Fragment">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | |
+--------------+ | |
| 6LoWPAN HC | | replay replay |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
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.
</t>
</section>
<section title="IPv6 Forwarding"> <t>
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 <xref target="I-D.svshah-tsvwg-lln-diffserv-recommendations"/>.
<figure anchor="fig9" title="IP Forwarding">
<artwork><![CDATA[
| ^
+--------------+ | |
| IPv6 | | +-QoS+ +-QoS+ |
+--------------+ | | | | | |
| 6LoWPAN HC | | | | | | |
+--------------+ | | | | | |
| 6top | | | | | | |
+--------------+ | | | | | |
| TSCH MAC | | | | | | |
+--------------+ | | | | | |
| LLN PHY | +-------+ +--...-----+ +-------+
+--------------+
]]></artwork>
</figure>
</t>
</section>
</section>
<section anchor="Flows" title="Functional Flows">
<t>
<list hangIndent="6" style="hanging">
<t hangText="Join:"></t>
<t hangText="Time Synchronization:"></t>
<t hangText="Setup for routing:"></t>
<t hangText="PCE reservation:"></t>
<t hangText="Distributed reservation:"></t>
<t hangText="Dynamic slot (de)allocation:"></t>
<t hangText="DSCP mapping:"></t>
</list>
</t>
</section>
<section anchor="Sync" title="Network Synchronization">
<t>
Nodes in a TSCH are time synchronized. A node keeps synchronized to its time source neighbor(s)
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 synch 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 the Device Management Entity.
</t>
<!--t>
Time synchronization in TSCH is based on three mechanisms:
<list>
<t>Enhanced Beacons</t>
<t>Enhanced ACKs </t>
<t>Frame based synchronization </t>
</list>
If a node communicates intermittently (sleepy, battery operated) it
can also proactively ping its time source and receive time stamps.
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 synch packets (especially with enhanced
beacons). This could be a function of the 6top sublayer or it could
be deferred to the device management entity. Any suggestions,
ideas on this topic?
</t-->
<t>Time distribution requires a loopless structure. Nodes taken in a 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 mode that are detailed in
<xref target="RFC6550">RPL</xref> 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.
</t>
<t>
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
<xref target="RFC6550"/> section "3.5.1. Rank Comparison".
</t>
<t>
A root is configured or obtains by some external mean 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 concertation with IEEE authors, it was asserted that 6TiSCH can make
a full use of the octet to carry an integer value up to 0xFF.
</t>
<t>
A node that reads a Join Priority of less than 0xFF should join the neighbor with
the lesser Join Priority and use is 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.
</t>
</section>
<section anchor="SixTOP" title="TSCH and 6top">
<section title="6top">
<t>
6top is a sublayer which is the next higher layer to TSCH and which offers a set of commands
defining data and management interfaces. 6top is defined in <xref target="I-D.draft-wang-6TiSCH-6top"/>
</t>
<t>
The management interface of 6top enables an upper layer to schedule cells and Slotframes in
the TSCH schedule.
</t>
<t>
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 typically used by a central PCE.
</t>
<t>
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 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.
</t>
</section>
<section anchor="Slotframes" title="Slotframes and Priorities">
<t>
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.
</t>
</section>
<section anchor="PCEFlow" title="Centralized Flow Reservation">
<t> In a centralized setting, an entity (typically a PCE) is responsible for computing
the TSCH schedule, and communicates with the different nodes in the network to configure
their TSCH schedule. Since it has full knowledge of the network's topology, the PCE can
compute a collision-free schedule, which results in a high degree of communication determinism.
</t>
<t>
The protocol for the PCE to communicate with the motes is not yet defined. This protocol typically reserves hard cells on the transmitter side of a dedicated cell, and the negotiation protocol of 6top takes care of reserving the same cell on the receiver node.
</t>
</section>
<section anchor="SettingUpAFlow" title="Distributed Flow Reservation">
<t>
In a distributed setting, no central PCE is present in the network. Nodes use 6top to reserve soft cells with their neighbors. Since no node has full knowledge of the network's topology and the traffic requirements, scheduling collisions are possible, for example because of a hidden terminal problem.
</t>
<t>
A schedule collision can be detected if two motes have multiple dedicated cells schedule to one another. The monitoring process of 6top can be configured to continuously compute the packet delivery ratio of those cells, and it can declare a soft cell to perform bad when the statistics for that cell are significantly worse than for the other cells to the same neighbor.
</t>
<t>
When this happens, the monitoring process of 6top moves the cell to another location in the 6TiSCH schedule, through a re-negotiation procedure with the neighbor.
</t>
<t>
The entity that builds and maintains the schedule in a distributed fashion is not yet defined.
</t>
</section>
<section anchor="Packet" title="Packet Marking and Handling">
<t>
</t>
<t>
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)
</t>
</section>
</section>
<section anchor="MGT" title="Monitoring and Management">
<t>
For the purpose of operations and management, a given LLN node
interacts through the Backbone Router with an NME and optionally
a PCE if centralized routing operations are enabled. Both a PCE and an
NME may require information about the LLN node and its link operations, and may
control that operation for Instance by assigning new bundles or new tracks.
In order to avoid duplication, and simplify the interaction with the node,
the Backbone Router may perform some proxy, publish/subscribe and/or translational
operations on behalf of the LLN node.
<figure anchor="fig10" title="Management">
<artwork><![CDATA[
---+-------- ............ ------------
| 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
]]></artwork>
</figure>
</t>
<t>
The architecture supports variations on the deployment model, and focusses
on the flows rather than the whether there is a proxy or a translational
operation on the way.
<list hangIndent="6" style="hanging">
<t hangText="Discovery:">
PCE discovery; SCADA/DCS discovery; actuator discovery
</t>
<t hangText="Request">
To ask the PCE to change schedule, typically directed to a PCE, from
a LLN node or an NME.
</t>
<t hangText="Action">
For the PCE to change the schedule of an LLN node, typically directed
from the PCE, to a LLN node.
</t>
<t hangText="Report">
For an LLN node to report periodic information or stats, eventually based
on a profile, typically directed to a PCE or an NME from an LLN node.
</t>
<t hangText="Event">
For an LLN node to report an exception, eventually based on a profile,
typically directed to a PCE or an NME from an LLN node.
</t>
<t hangText="Query">
For the PCE or an NME to ask for schedule information from an LLN node,
typically directed from the PCE, to a LLN node.
</t>
</list>
</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>
This specification does not require IANA action.
</t>
</section>
<section anchor="Sec" title="Security Considerations">
<t>
This specification is not found to introduce new security threat.
</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>
</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.2460"?>
<?rfc include="reference.RFC.4080"?>
<?rfc include="reference.RFC.4291"?>
<?rfc include="reference.RFC.4861"?>
<?rfc include="reference.RFC.4862"?>
<?rfc include="reference.RFC.5191"?>
<?rfc include="reference.RFC.5889"?>
<?rfc include="reference.RFC.5974"?>
<?rfc include="reference.RFC.6282"?>
<?rfc include="reference.RFC.6550"?>
<?rfc include="reference.RFC.6775"?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.watteyne-6TSCH-tsch-lln-context'?>
<?rfc include='reference.I-D.palattella-6TSCH-terminology'?>
<reference anchor="I-D.draft-wang-6TiSCH-6top">
<front>
<title>
6TiSCH Operation Sublayer (6top). draft-wang-6TiSCH-6top-00 (work in progress)
</title>
<author initials="Q" surname="Wang" fullname="Qin Wang" role="editor"/>
<author initials="X" surname="Vilajosana" fullname="Xavier Vilajosana"/>
<author initials="T" surname="Watteyne" fullname="Thomas Watteyne"/>
<date month="July" year="2013"/>
</front>
</reference>
<?rfc include='reference.I-D.vilajosana-6TSCH-basic'?>
<?rfc include='reference.I-D.ohba-6TSCH-security'?>
<?rfc include='reference.I-D.svshah-tsvwg-lln-diffserv-recommendations'?>
<?rfc include='reference.I-D.chakrabarti-nordmark-6man-efficient-nd.xml'?>
<?rfc include='reference.I-D.thubert-6lowpan-backbone-router.xml'?>
<?rfc include='reference.I-D.thubert-roll-forwarding-frags.xml'?>
<?rfc include='reference.I-D.svshah-tsvwg-lln-diffserv-recommendations.xml'?>
<?rfc include='reference.I-D.thubert-roll-flow-label.xml'?>
</references>
<references title="External Informative References">
<reference anchor="IEEE802.1TSNTG" target="http://www.ieee802.org/1/pages/avbridges.html">
<front>
<title>IEEE 802.1 Time-Sensitive Networks Task Group</title>
<author>
<organization>IEEE Standards Association</organization>
</author>
<date day="08" month="March" year="2013" />
</front>
</reference>
<reference anchor="HART">
<front>
<title>Highway Addressable Remote Transducer, a group of specifications for industrial process and control devices administered by the HART Foundation</title>
<author>
<organization>www.hartcomm.org</organization>
</author>
<date></date>
</front>
</reference>
<reference anchor="ISA100.11a" target="http://www.isa.org/Community/SP100WirelessSystemsforAutomation">
<front>
<title>ISA100, Wireless Systems for Automation</title>
<author>
<organization>ISA</organization>
</author>
<date day="05" month="May" year="2008" />
</front>
</reference>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-22 03:35:43 |