6TiSCH X. Vilajosana, Ed.
Internet-Draft Universitat Oberta de Catalunya
Intended status: Informational K. Pister
Expires: September 9, 2015 University of California Berkeley
March 8, 2015
Minimal 6TiSCH Configuration
draft-ietf-6tisch-minimal-06
Abstract
This document describes the minimal set of rules to operate an IEEE
802.15.4e Timeslotted Channel Hopping (TSCH) network. This minimal
mode of operation can be used during network bootstrap, as a fall-
back mode of operation when no dynamic scheduling solution is
available or functioning, or during early interoperability testing
and development.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 9, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Minimal Schedule Configuration . . . . . . . . . . . . . . . 3
3.1. Slotframe . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Cell Options . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Retransmissions . . . . . . . . . . . . . . . . . . . . . 6
3.4. Time Slot timing . . . . . . . . . . . . . . . . . . . . 6
4. Enhanced Beacons Configuration and Content . . . . . . . . . 8
4.1. Sync IE . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 9
4.1.2. IE Content . . . . . . . . . . . . . . . . . . . . . 9
4.2. TSCH Timeslot IE . . . . . . . . . . . . . . . . . . . . 9
4.2.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 9
4.2.2. IE Content . . . . . . . . . . . . . . . . . . . . . 9
4.3. Channel Hopping IE . . . . . . . . . . . . . . . . . . . 10
4.3.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 10
4.3.2. IE Content . . . . . . . . . . . . . . . . . . . . . 10
4.4. Frame and Link IE . . . . . . . . . . . . . . . . . . . . 11
4.4.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 11
4.4.2. IE Content . . . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. ACK/NACK Time Correction IE . . . . . . . . . . . . . . . 12
5.1.1. IE Header . . . . . . . . . . . . . . . . . . . . . . 12
5.1.2. IE Content . . . . . . . . . . . . . . . . . . . . . 12
6. Neighbor information . . . . . . . . . . . . . . . . . . . . 12
6.1. Neighbor Table . . . . . . . . . . . . . . . . . . . . . 12
6.2. Time Source Neighbor Selection . . . . . . . . . . . . . 13
7. Queues and Priorities . . . . . . . . . . . . . . . . . . . . 14
8. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9. RPL on TSCH . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. RPL Objective Function Zero . . . . . . . . . . . . . . . 15
9.1.1. Rank computation . . . . . . . . . . . . . . . . . . 15
9.1.2. Rank computation Example . . . . . . . . . . . . . . 16
9.2. RPL Configuration . . . . . . . . . . . . . . . . . . . . 18
9.2.1. Mode of Operation . . . . . . . . . . . . . . . . . . 18
9.2.2. Trickle Timer . . . . . . . . . . . . . . . . . . . . 18
9.2.3. Hysteresis . . . . . . . . . . . . . . . . . . . . . 18
9.2.4. Variable Values . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . 20
11.3. External Informative References . . . . . . . . . . . . 21
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The nodes in a [IEEE802154e] TSCH network follow a communication
schedule. The entity (centralized or decentralized) responsible for
building and maintaining that schedule has precise control over the
trade-off between the network's latency, bandwidth, reliability and
power consumption.During early interoperability testing and
development, however, simplicity is more important than efficiency.
One goal of this document is to define the simplest set of rules for
building a [IEEE802154e] TSCH-compliant network, at the necessary
price of lesser efficiency. Yet, this minimal mode of operation MAY
also be used during network bootstrap before any schedule is
installed into the network so nodes can self-organize and the
management and configuration information be distributed. In
addition, the minimal configuration MAY be used as a fall-back mode
of operation, ensuring connectivity of nodes in case that dynamic
scheduling mechanisms fail or are not available. [IEEE802154e]
provides a mechanism whereby the details of slotframe length,
timeslot timing, and channel hopping pattern are communicated when a
node synchronizes to the network. This document describes specific
settings for these parameters. Nodes MUST broadcast properly-formed
Enhanced Beacons to announce these values.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Minimal Schedule Configuration
In order to form a network, a minimum schedule configuration is
required so nodes can advertise the presence of the network, and
allow other nodes to join.
3.1. Slotframe
The slotframe, as defined in [I-D.ietf-6tisch-terminology], is an
abstraction of the link layer that defines a collection of time slots
of equal length, and which repeats over time. In order to set up a
minimal TSCH network, nodes need to be synchronized with the same
slotframe configuration so they can communicate. This document
recommends the following slotframe configuration.
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Minimal configuration
+------------------------------------+----------------------+
| Property | Value |
+------------------------------------+----------------------+
| Number of time slots per Slotframe | Variable |
+------------------------------------+----------------------+
| Number of available frequencies | 16 |
+------------------------------------+----------------------+
| Number of scheduled cells | 1 (slotOffset 0) |
| | (macLinkType NORMAL) |
+------------------------------------+----------------------+
| Number of unscheduled cells | The remainder of the |
| | slotframe |
+------------------------------------+----------------------+
| Number of MAC retransmissions (max)| 3 (4 attempts to tx) |
+------------------------------------+----------------------+
The slotframe is composed of a configurable number of time slots.
Choosing the number of time slots per slotframe needs to take into
account network requirements such as density, bandwidth per node,
etc. In the minimal configuration, there is only a single active
slot in slotframe, used to transmit/receive both EBs and data link-
layer frames. The trade-off between bandwidth, latency and energy
consumption can be controlled by choosing a different slotframe
length. The active slot MAY be scheduled at the slotOffset 0x00 and
channelOffset 0x00 and MUST be announced in the EBs. EBs are sent
using this active slot to the link-layer broadcast address (and are
therefore not acknowledged). Data packets, as described in
Section 3.2, use the same active slot. Per [IEEE802154e], data
packets sent unicast on this cell are acknowledged by the receiver.
The remaining cells in the slotframe are unscheduled, and MAY be used
by dynamic scheduling solutions. Details about such dynamic
scheduling solution are out of scope of this document.
The slotframe length (expressed in number of time slots) is
configurable. The length used determines the duty cycle of the
network. For example, a network with a 0.99% duty cycle is composed
of a slotframe of 101 slots, which includes 1 active slot. The
present document RECOMMENDS the use of a default slot duration set to
10ms and its corresponding default timeslot timings defined by the
[IEEE802154e] macTimeslotTemplate. The use of the default
macTimeslotTemplate MUST be announced in the EB by using the Timeslot
IE containing only the default macTimeslotTemplateId. Other time
slot durations MAY be supported and MUST be announced in the EBs. If
one uses a timeslot duration different than 10ms, EBs MUST contain
the complete TimeSlot IE as described in Section 3.4. This document
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also recommends to clearly indicate nodes not supporting the default
timeslot value.
Example schedule with 0.99% duty cycle
Chan. +----------+----------+ +----------+
Off.0 | TxRxS/EB | OFF | | OFF |
Chan. +----------+----------+ +----------+
Off.1 | | | ... | |
+----------+----------+ +----------+
.
.
.
Chan. +----------+----------+ +----------+
Off.15 | | | | |
+----------+----------+ +----------+
slotOffset 0 1 100
EB: Enhanced Beacon
Tx: Transmit
Rx: Receive
S: Shared
OFF: Unscheduled (MAY be used by a dynamic scheduling mechanism)
3.2. Cell Options
Per [IEEE802154e] TSCH, each scheduled cell has an associated bitmap
of cell options, called LinkOptions. The scheduled cell in the
minimal schedule is configured as a Hard cell
[I-D.ietf-6tisch-tsch][I-D.ietf-6tisch-6top-interface]. Additional
available cells MAY be scheduled by a dynamic scheduling solution.
The dynamic scheduling solution is out of scope, and this
specification does not make any restriction on the LinkOption
associated with those dynamically scheduled cells (i.e. they can be
hard cells or soft cells).
The active cell is assigned the bitmap of cell options below.
Because both the "Transmit" and "Receive" bits are set, a node
transmits if there is a packet in its queue, listens otherwise.
Because the "shared" bit is set, the back-off mechanism defined in
[IEEE802154e] is used to resolve contention when transmitting. This
results in "Slotted Aloha" behavior. The "Timekeeping" flag is never
set, since the time source neighbor is selected using the DODAG
structure of the network (detailed below).
b0 = Transmit = 1 (set)
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b1 = Receive = 1 (set)
b2 = Shared = 1 (set)
b3 = Timekeeping = 0 (clear)
b4-b7 = Reserved (clear)
All remaining cells are unscheduled. In unscheduled cells, the nodes
SHOULD keep their radio off. In a memory-efficient implementation,
scheduled cells can be represented by a circular linked list.
Unscheduled cells SHOULD NOT occupy any memory.
3.3. Retransmissions
The maximum number of link layer retransmissions is set to 3. For
packets which require an acknowledgment, if none is received after a
total of 4 attempts, the transmissions is considered failed and the
link layer MUST notify the upper layer. Packets sent to the
broadcast MAC address (including EBs) are not acknowledged and
therefore not retransmitted.
3.4. Time Slot timing
The figure below shows an active timeslot in which a packet is sent
from the transmitter node (TX) to the receiver node (RX). A link-
layer acknowledgment is sent by the RX node to the TX node when the
packet is to be acknowledged. The TsTxOffset duration defines the
instant in the timeslot when the first bit after the Start of Frame
Delimiter (SFD) of the transmitted packet leaves the radio of the TX
node. The radio of the RX node is turned on tsRxWait/2 before that
instant, and listens for at least tsRxWait. This allows for a de-
synchronization between the two nodes of at most tsRxWait/2 in either
direction (early or late). The RX node needs to send the first bit
after the SFD of the MAC acknowledgment exactly TsTxAckDelay after
the end of the last byte of the received packet. TX's radio has to
be turned on tsAckWait/2 before that time, and keep listening for at
least tsAckWait. The TX node can perform a Clear Channel Assessment
(CCA) if required, this does not interfere with the scope of this
draft. As for a minimal configuration, CCA is OPTIONAL.
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Time slot internal timing diagram
/---------------------- Time Slot Duration ----------------------/
| / (5) / |
| | / tsRxAckDelay /| | | |
|-------------------+--------------+------------------+------+---|
TX |/(1)/ (2) / (3) /| TX frame | |RX ACK| |
|----+-------+------+--------------+------------------+------+---|
|/ tsTxOffset /| | | | |
| | | | | |
|-------------------+--------------+------------------+------+---|
RX | | | | RX frame | |TX ACK| |
|----------------+--+--+-----------+------------------+------+---|
| | | | | | | |
| / (4) / / tsTxAckDelay / | |
Start End
of of
Slot Slot
/(1)/ tsCCAOffset
/(2)/ tsCCA
/(3)/ tsRxTx
/(4)/ tsRxWait
/(5)/ tsAckWait
A 10ms time slot length is the default value defined by
[IEEE802154e]. Section 6.4.3.3.3 of [IEEE802154e] defines a default
macTimeslotTemplate, i.e. the different duration within the slot.
These values are summarized in the following table and MUST be used
when utilizing the default time slot duration. In this case, the
Timeslot IE only transports the macTimeslotTemplateId (0x00) as the
timing values are well known. If a timeslot template other than the
default is used, the EB MUST contain a complete TimeSlot IE
indicating the timeslot duration and the corresponding timeslot
timings, requiring 25 bytes. Note however that in case of
discrepancy between the values in this document and [IEEE802154e],
the IEEE standard specification has precedence.
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Default timeslot durations (per [IEEE802154e], Section 6.4.3.3.3)
+--------------------------------+------------+
| IEEE802.15.4e TSCH parameter | Value (us) |
+--------------------------------+------------+
| tsCCAOffset | 1800 |
+--------------------------------+------------+
| tsCCA | 128 |
+--------------------------------+------------+
| tsTxOffset | 2120 |
+--------------------------------+------------+
| tsRxOffset | 1120 |
+--------------------------------+------------+
| tsRxAckDelay | 800 |
+--------------------------------+------------+
| tsTxAckDelay | 1000 |
+--------------------------------+------------+
| tsRxWait | 2200 |
+--------------------------------+------------+
| tsAckWait | 400 |
+--------------------------------+------------+
| tsRxTx | 192 |
+--------------------------------+------------+
| tsMaxAck | 2400 |
+--------------------------------+------------+
| tsMaxTx | 4256 |
+--------------------------------+------------+
| Time Slot duration | 10000 |
+--------------------------------+------------+
4. Enhanced Beacons Configuration and Content
[IEEE802154e] does not define how often EBs are sent, nor their
contents. EBs should not in general be used for synchronization.
Synchronization is achieved via acknowledgements to normal packet
traffic, and keepalives. For a minimal TSCH configuration, a mote
SHOULD send an EB every EB_PERIOD. For additional reference see
[I-D.ietf-6tisch-tsch] where different synchronization approaches are
summarized. EBs are only authenticated and payload is not encrypted.
Refer to the 6TiSCH architecture document
[I-D.ietf-6tisch-architecture] for further details on security
aspects.
EBs MUST be sent with the Beacon IEEE802.15.4 frame type and this EB
MUST carry the Information Elements (IEs) listed below.
The content of the IEs is presented here for completeness, however
this information is redundant with [IEEE802154e].
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4.1. Sync IE
Contains synchronization information such as ASN and Join Priority.
The value of Join Priority is discussed in Section 6.2.
4.1.1. IE Header
Length (b0-b7) = 0x06
Sub-ID (b8-b14) = 0x1a
Type (b15) = 0x00 (short)
4.1.2. IE Content
ASN Byte 1 (b16-b23)
ASN Byte 2 (b24-b31)
ASN Byte 3 (b32-b39)
ASN Byte 4 (b40-b47)
ASN Byte 5 (b48-b55)
Join Priority (b56-b63)
4.2. TSCH Timeslot IE
Contains the timeslot template identifier. This specification uses
the default timeslot template as defined in [IEEE802154e],
Section 5.2.4.15.
4.2.1. IE Header
Length (b0-b7) = 0x01
Sub-ID (b8-b14) = 0x1c
Type (b15) = 0x00 (short)
4.2.2. IE Content
Timeslot Template ID (b0-b7) = 0x00
In the case that a different than the default timeslot template is
used, the IE Content MUST follow the following specification as
defined in [IEEE802154e], Section 5.2.4.15.
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Timeslot Template ID (b0-b7)
macTsCCAOffset (b8-b23)
macTsCCA (b24-b39)
macTsTxOffset (b40-b55)
macTsRxOffset (b56-b71)
macTsRxAckDelay (b72-b87)
macTsTxAckDelay (b88-b103)
macTsRxWait (b104-b119)
macTsAckWait (b120-b135)
macTsRxTx (b136-b151)
macTsMaxAck (b152-b167)
macTsMaxTx (b168-b183)
macTsTimeslotLength (b184-b199)
4.3. Channel Hopping IE
Contains the channel hopping template identifier. This specification
uses the default channel hopping template, as defined in
[IEEE802154e], Section 5.2.4.16.
4.3.1. IE Header
Length (b0-b7) = 0x01
Sub-ID (b8-b14) = 0x1d
Type (b15) = 0x00 (short)
4.3.2. IE Content
Channel Hopping Template ID (b0-b7) = 0x00
The default sequence for the 2.4GHz OQPSK PHY is [5, 6, 12, 7, 15, 4,
14, 11, 8, 0, 1, 2, 13, 3, 9, 10] per section 5.1.1a of
[IEEE802154e]. Note however that in case of discrepancy between the
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values in this document and [IEEE802154e], the IEEE standard
specification has preference.
4.4. Frame and Link IE
Each node MUST indicate the schedule in each EB through a Frame and
Link IE. This enables nodes which implement [IEEE802154e] to learn
the schedule used in the network as they join it.
4.4.1. IE Header
Length (b0-b7) = variable
Sub-ID (b8-b14) = 0x1b
Type (b15) = 0x00 (short)
4.4.2. IE Content
# Slotframes (b16-b23) = 0x01
Slotframe ID (b24-b31) = 0x01
Size Slotframe (b32-b47) = variable
# Links (b48-b55) = 0x01
For the active cell in the minimal schedule:
Channel Offset (2B) = 0x00
Slot Number (2B) = 0x00
LinkOption (1B) = as described in Section 3.2
5. Acknowledgment
Link-layer acknowledgment frames are built according to
[IEEE802154e]. Unicast frames sent to a unicast MAC destination
address request an acknowledgment. The sender node MUST set the ACK
requested bit in the IEEE802.15.4 header. The acknowledgment frame
is of type ACK (0x10). Each acknowledgment contains the following
IE:
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5.1. ACK/NACK Time Correction IE
The ACK/NACK time correction IE carries the measured de-
synchronization between the sender and the receiver.
5.1.1. IE Header
Length (b0-b7) = 0x02
Sub-ID (b8-b14) = 0x1e
Type (b15) = 0x00 (short)
5.1.2. IE Content
Time Synchronization Information and ACK status (b16-b31)
The possible values for the Time Synchronization Information and ACK
status are described in [IEEE802154e] and reproduced in the following
table:
ACK status and Time Synchronization Information.
+-----------------------------------+-----------------+
| ACK Status | Value |
+-----------------------------------+-----------------+
| ACK with positive time correction | 0x0000 - 0x07ff |
+-----------------------------------+-----------------+
| ACK with negative time correction | 0x0800 - 0x0fff |
+-----------------------------------+-----------------+
| NACK with positive time correction| 0x8000 - 0x87ff |
+-----------------------------------+-----------------+
| NACK with negative time correction| 0x8800 - 0x8fff |
+-----------------------------------+-----------------+
6. Neighbor information
[IEEE802154e] does not define how and when each node in the network
keeps information about its neighbors. Keeping the following
information in the neighbor table is RECOMMENDED:
6.1. Neighbor Table
The exact format of the neighbor table is implementation-specific,
but it SHOULD contain the following information for each neighbor:
Neighbor statistics:
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numTx: number of transmitted packets to that neighbor
numTxAck: number of transmitted packets that have been
acknowledged by that neighbor
numRx: number of received packets from that neighbor
The EUI64 of the neighbor.
Timestamp when that neighbor was heard for the last time. This
can be based on the ASN counter or any other time base. It can be
used to trigger a keep-alive message.
RPL rank of that neighbor.
A flag indicating whether this neighbor is a time source neighbor.
Connectivity statistics (e.g., RSSI), which can be used to
determine the quality of the link.
In addition to that information, each node has to be able to compute
some RPL Objective Function (OF), taking into account the neighbor
and connectivity statistics. An example RPL objective function is
the OF Zero as described in [RFC6552] and Section 9.1.1.
6.2. Time Source Neighbor Selection
Each node MUST select at least one Time Source Neighbor among the
nodes in its RPL routing parent set. When a node joins a network, it
has no routing information. To select its time source neighbor, it
uses the Join Priority field in the EB, as described in
Section 5.2.4.13 and Table 52b of [IEEE802154e]. The Sync IE
contains the ASN and 1 Byte field named Join Priority. The Join
Priority of any node MUST be equivalent to the result of the function
DAGRank(rank) as defined by [RFC6550] and Section 9.1.1. The Join
Priority of the DAG root is zero, i.e., EBs sent from the DAG root
are sent with Join Priority equal to 0. A lower value of the Join
Priority indicates higher preference to connect to that device. When
a node joins the network, it MUST NOT send EBs before having acquired
a RPL rank. This avoids routing loops and matches RPL topology with
underlying mesh topology. As soon as a node acquires a RPL rank (see
[RFC6550] and Section 9.1.1), it SHOULD send Enhanced Beacons
including a Sync IE with Join Priority field set to DAGRank(rank),
where rank is the node's rank. If a node receives EBs from different
nodes with equal Join Priority, the time source neighbor selection
SHOULD be assessed by other metrics that can help determine the
better connectivity link. Time source neighbor hysteresis SHOULD be
used, according to the rules defined in Section 9.2.3. If
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connectivity to the time source neighbor is lost, a new time source
neighbor MUST be chosen among the neighbors in the RPL routing parent
set.
The decision for a node to select one Time Source Neighbor when
multiple EBs are received is implementation-specific.
For example, a node MAY wait until one EB from NUM_NEIGHBOURS_TO_WAIT
neighbors have been received to select the best Time Source Neighbor.
This condition MAY apply unless a second EB is not received after
MAX_EB_DELAY seconds. This avoids initial hysteresis when selecting
a first Time Source Neighbor.
Optionally, some form of hysteresis SHOULD be implemented to avoid
frequent changes in time source neighbors.
7. Queues and Priorities
[IEEE802154e] does not define the use of queues to handle upper layer
data (either application or control data from upper layers). The use
of a single queue with the following rules is RECOMMENDED:
When the node is not synchronized to the network, higher layers
are not able to insert packets into the queue.
Frames generated by the MAC layer (e.g., EBs and ACK) have a
higher queuing priority than packets received from a higher layer.
IEEE802.15.4 frame types Beacon and Command have a higher queuing
priority than IEEE802.15.4 frame types Data and ACK.
One entry in the queue is reserved at all times for an
IEEE802.15.4 frames of types Beacon or Command frames.
8. Security
As this document refers to the interaction between Layer 3 and Layer
2 protocols, this interaction MUST be secured by L2 security
mechanisms as defined by [IEEE802154e]. Two security mechanisms are
considered, authentication and encryption, authentication applies to
the all packet content while encryption applies to header IEs and MAC
payload. Key distribution is out of scope of this document, but
examples include pre-configured keys at the nodes, shared keys among
peers or well-known keys. Refer to the 6TiSCH architecture document
[I-D.ietf-6tisch-architecture] for further details on key
distribution and advanced security aspects.
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The present document assumes the existence of two keys, which can be
well-known by the network devices and/or pre-configured. One of the
keys (K1) is used to authenticate EBs (all frame). As defined in
Section 4 EBs MUST be authenticated but payload not encrypted. This
prevents two independent networks to interfere or enable non-allowed
nodes to join a particular network. A second key (K2) is used to
authenticate and encrypt the payload of DATA, ACKNOWLEDGEMENT, MAC
COMMAND frame types and respective header IEs.
9. RPL on TSCH
Nodes in the network MUST use the RPL routing protocol [RFC6550].
9.1. RPL Objective Function Zero
Nodes in the network MUST use the RPL routing protocol [RFC6550] and
implement the RPL Objective Function Zero [RFC6552].
9.1.1. Rank computation
The rank computation is described at [RFC6552], Section 4.1. A node
rank is computed by the following equation:
R(N) = R(P) + rank_increment
rank_increment = (Rf*Sp + Sr) * MinHopRankIncrease
Where:
R(N): Rank of the node.
R(P): Rank of the parent obtained as part of the DIO information.
rank_increment: The result of a function that determines the rank
increment.
Rf (rank_factor): A configurable factor that is used to multiply
the effect of the link properties in the rank_increment
computation. If none is configured, rank_factor of 1 is used. In
this specification, a rank_factor of 1 MUST be used.
Sp (step_of_rank): (strictly positive integer) - an intermediate
computation based on the link properties with a certain neighbor.
In this specification, 2*ETX (Expected Transmissions) as defined
by [decouti03high] and [RFC6551] MUST be used. The ETX is
computed as the inverse of the Packet Delivery Ratio (PDR), and
MAY be computed as the number of acknowledged packets, divided by
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the number of transmitted packets to a certain node. E.g:
Sp=2*numTX/numTXAck
Sr (stretch_of_rank): (unsigned integer) - the maximum increment
to the step_of_rank of a preferred parent, to allow the selection
of an additional feasible successor. If none is configured to the
device, then the step_of_rank is not stretched. In this
specification, stretch_of_rank MUST be set to 0.
MinHopRankIncrease: the MinHopRankIncrease is set to the fixed
constant DEFAULT_MIN_HOP_rank_increment [RFC6550].
DEFAULT_MIN_HOP_rank_increment has a value of 256.
DAGRank(rank): Equivalent to the floor of (Rf*Sp + Sr) as defined
by [RFC6550]. Specifically, when an Objective Function computes
Rank, this is defined as an unsigned integer (i.e., a 16-bit
value) Rank quantity. When the Rank is compared, e.g. to
determine parent relationships or loop detection, the integer
portion of the Rank is used. The integer portion of the Rank is
computed by the DAGRank() macro as floor(x) where floor(x) is the
function that evaluates to the greatest integer less than or equal
to x. DAGRank(rank) = floor(rank/MinHopRankIncrease)
Rank computation scenario
+-------+
| P | R(P)
| |
+-------+
|
|
+-------+
| N | R(N)=R(P) + rank_increment
| | rank_increment = (Rf*Sp + Sr) * MinHopRankIncrease
+-------+ Sp=2*ETX
9.1.2. Rank computation Example
This section illustrates with an example the use of the Objective
Function Zero. Assume the following parameters:
Rf = 1
Sp = 2* ETX
Sr = 0
minHopRankIncrease = 256 (default in RPL)
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ETX=(numTX/numTXAck)
r(n) = r(p) + rank_increment
rank_increment = (Rf*Sp + Sr) * minHopRankIncrease
rank_increment = 512*numTx/numTxACK
Rank computation example for 5 hop network where numTx=100 and
numTxAck=75 for all nodes
+-------+
| 0 | R(0)=0
| | DAGRank(R(0)) = 0
+-------+
|
|
+-------+
| 1 | R(1)=R(0)+683=683
| | DAGRank(R(1)) = 2
+-------+
|
|
+-------+
| 2 | R(2)=R(1)+683=1366
| | DAGRank(R(2)) = 5
+-------+
|
|
+-------+
| 3 | R(3)=R(2)+683=2049
| | DAGRank(R(3)) = 8
+-------+
|
|
+-------+
| 4 | R(4)=R(3)+683=2732
| | DAGRank(R(4)) = 10
+-------+
|
|
+-------+
| 5 | R(5)=R(4)+683=3415
| | DAGRank(R(5)) = 13
+-------+
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9.2. RPL Configuration
In addition to the Objective Function (OF), a minimal configuration
for RPL SHOULD indicate the preferred mode of operation (either
Storing Mode or Non-Storing Mode) so different RPL implementations
can inter-operate. RPL information and hop-by-hop extension headers
MUST follow [RFC6553] and [RFC6554] specification. In the case that
the packets formed at the LLN need to cross through intermediate
routers, these MUST obey to the IP in IP encapsulation requirement
specified by the [RFC6282] and [RFC2460]. RPI and RH3 extension
headers and inner IP headers MUST be compressed according to
[RFC6282].
9.2.1. Mode of Operation
For downstream route maintenance, in a minimal configuration, RPL
SHOULD be set to operate in the Non-Storing mode as described by
[RFC6550] Section 9.7. Storing mode ([RFC6550] Section 9.8) MAY be
supported in less constrained devices.
9.2.2. Trickle Timer
RPL signaling messages such as DIOs are sent using the Trickle
Algorithm [RFC6550] (Section 8.3.1) and [RFC6206]. For this
specification, the Trickle Timer MUST be used with the RPL defined
default values [RFC6550] (Section 8.3.1). For a description of the
Trickle timer operation see Section 4.2 on [RFC6206].
9.2.3. Hysteresis
According to [RFC6552], [RFC6719] recommends the use of a boundary
value (PARENT_SWITCH_THRESHOLD) to avoid constant changes of parent
when ranks are compared. When evaluating a parent that belongs to a
smaller path cost than current minimum path, the candidate node is
selected as new parent only if the difference between the new path
and the current path is greater than the defined
PARENT_SWITCH_THRESHOLD. Otherwise the node MAY continue to use the
current preferred parent. As for [RFC6719] the recommended value for
PARENT_SWITCH_THRESHOLD is 192 when ETX metric is used, the
recommendation for this document is to use PARENT_SWITCH_THRESHOLD
equal to 394 as the metric being used is 2*ETX. This is mechanism is
suited to deal with parent hysteresis in both cases routing parent
and time source neighbor selection.
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9.2.4. Variable Values
The following table presents the RECOMMENDED values for the RPL-
related variables defined in the previous section.
Recommended variable values
+-------------------------+-------+
| Variable | Value |
+-------------------------+-------+
| EB_PERIOD | 10s |
+-------------------------+-------+
| MAX_EB_DELAY | 180 |
+-------------------------+-------+
| NUM_NEIGHBOURS_TO_WAIT | 2 |
+-------------------------+-------+
| PARENT_SWITCH_THRESHOLD | 394 |
+-------------------------+-------+
10. Acknowledgments
The authors would like to acknowledge the guidance and input provided
by the 6TiSCH Chairs Pascal Thubert and Thomas Watteyne.
11. References
11.1. Normative References
[RFC6719] Gnawali, O. and P. Levis, "The Minimum Rank with
Hysteresis Objective Function", RFC 6719, September 2012.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, March
2012.
[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
Power and Lossy Networks (RPL) Option for Carrying RPL
Information in Data-Plane Datagrams", RFC 6553, March
2012.
[RFC6552] Thubert, P., "Objective Function Zero for the Routing
Protocol for Low-Power and Lossy Networks (RPL)", RFC
6552, March 2012.
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[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
[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.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, March 2011.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[I-D.ietf-6tisch-tsch]
Watteyne, T., Palattella, M., and L. Grieco, "Using
IEEE802.15.4e TSCH in an IoT context: Overview, Problem
Statement and Goals", draft-ietf-6tisch-tsch-05 (work in
progress), January 2015.
[I-D.ietf-6tisch-architecture]
Thubert, P., Watteyne, T., Struik, R., and M. Richardson,
"An Architecture for IPv6 over the TSCH mode of IEEE
802.15.4e", draft-ietf-6tisch-architecture-05 (work in
progress), January 2015.
[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", draft-ietf-6tisch-terminology-03 (work in
progress), January 2015.
[I-D.ietf-6tisch-6top-interface]
Wang, Q., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Interface", draft-ietf-6tisch-
6top-interface-02 (work in progress), October 2014.
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[I-D.richardson-6tisch-security-architecture]
Richardson, M., "security architecture for 6top:
requirements and structure", draft-richardson-6tisch-
security-architecture-02 (work in progress), April 2014.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terms used in Routing for Low power And
Lossy Networks", draft-ietf-roll-terminology-13 (work in
progress), October 2013.
11.3. External Informative References
[IEEE802154e]
IEEE standard for Information Technology, "IEEE std.
802.15.4e, Part. 15.4: Low-Rate Wireless Personal Area
Networks (LR-WPANs) Amendment 1: MAC sublayer", April
2012.
[IEEE802154]
IEEE standard for Information Technology, "IEEE std.
802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks", June 2011.
[CCM] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation: The
CCM Mode for Authentication and Confidentiality. SP
800-38C", May 2004.
[CCM-Star]
Struik, R., "Formal Specification of the CCM* Mode of
Operation, IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs).", September 2005.
[decouti03high]
De Couto, D., Aguayo, D., Bicket, J., and R. Morris, "A
High-Throughput Path Metric for Multi-Hop Wireless
Routing", ACM International Conference on Mobile Computing
and Networking (MobiCom) , June 2003.
[OpenWSN] Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F.,
Weekly, K., Wang, Q., Glaser, S., and K. Pister, "OpenWSN:
a Standards-Based Low-Power Wireless Development
Environment", Transactions on Emerging Telecommunications
Technologies , August 2012.
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Authors' Addresses
Xavier Vilajosana (editor)
Universitat Oberta de Catalunya
156 Rambla Poblenou
Barcelona, Catalonia 08018
Spain
Phone: +34 (646) 633 681
Email: xvilajosana@uoc.edu
Kris Pister
University of California Berkeley
490 Cory Hall
Berkeley, California 94720
USA
Email: pister@eecs.berkeley.edu
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