💾 Archived View for gmi.noulin.net › rfc › rfc6550.gmi captured on 2022-01-08 at 21:30:38. Gemini links have been rewritten to link to archived content
-=-=-=-=-=-=-
Updated by:
Keywords: [--------], WSN for Wireless Sensor Network, L3 Mesh for Layer 3 Mesh Network, Routing Protocol, Subnet Routing, Distance Vector, Objective Function, DAG for Directed Acyclic Graph
Internet Engineering Task Force (IETF) T. Winter, Ed.
Request for Comments: 6550
Category: Standards Track P. Thubert, Ed.
ISSN: 2070-1721 Cisco Systems
A. Brandt
Sigma Designs
J. Hui
Arch Rock Corporation
R. Kelsey
Ember Corporation
P. Levis
Stanford University
K. Pister
Dust Networks
R. Struik
Struik Security Consultancy
JP. Vasseur
Cisco Systems
R. Alexander
Cooper Power Systems
March 2012
RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks
Abstract
Low-Power and Lossy Networks (LLNs) are a class of network in which
both the routers and their interconnect are constrained. LLN routers
typically operate with constraints on processing power, memory, and
energy (battery power). Their interconnects are characterized by
high loss rates, low data rates, and instability. LLNs are comprised
of anything from a few dozen to thousands of routers. Supported
traffic flows include point-to-point (between devices inside the
LLN), point-to-multipoint (from a central control point to a subset
of devices inside the LLN), and multipoint-to-point (from devices
inside the LLN towards a central control point). This document
specifies the IPv6 Routing Protocol for Low-Power and Lossy Networks
(RPL), which provides a mechanism whereby multipoint-to-point traffic
from devices inside the LLN towards a central control point as well
as point-to-multipoint traffic from the central control point to the
devices inside the LLN are supported. Support for point-to-point
traffic is also available.
Winter, et al. Standards Track [Page 1]
RFC 6550 RPL March 2012
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6550.
Copyright Notice
Copyright (c) 2012 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 as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Winter, et al. Standards Track [Page 2]
RFC 6550 RPL March 2012
Table of Contents
1. Introduction ....................................................8
1.1. Design Principles ..........................................8
1.2. Expectations of Link-Layer Type ...........................10
2. Terminology ....................................................10
3. Protocol Overview ..............................................13
3.1. Topologies ................................................13
3.1.1. Constructing Topologies ............................13
3.1.2. RPL Identifiers ....................................14
3.1.3. Instances, DODAGs, and DODAG Versions ..............14
3.2. Upward Routes and DODAG Construction ......................16
3.2.1. Objective Function (OF) ............................17
3.2.2. DODAG Repair .......................................17
3.2.3. Security ...........................................17
3.2.4. Grounded and Floating DODAGs .......................18
3.2.5. Local DODAGs .......................................18
3.2.6. Administrative Preference ..........................18
3.2.7. Data-Path Validation and Loop Detection ............18
3.2.8. Distributed Algorithm Operation ....................19
3.3. Downward Routes and Destination Advertisement .............19
3.4. Local DODAGs Route Discovery ..............................20
3.5. Rank Properties ...........................................20
3.5.1. Rank Comparison (DAGRank()) ........................21
3.5.2. Rank Relationships .................................22
3.6. Routing Metrics and Constraints Used by RPL ...............23
3.7. Loop Avoidance ............................................24
3.7.1. Greediness and Instability .........................24
3.7.2. DODAG Loops ........................................26
3.7.3. DAO Loops ..........................................27
4. Traffic Flows Supported by RPL .................................27
4.1. Multipoint-to-Point Traffic ...............................27
4.2. Point-to-Multipoint Traffic ...............................27
4.3. Point-to-Point Traffic ....................................27
5. RPL Instance ...................................................28
5.1. RPL Instance ID ...........................................29
6. ICMPv6 RPL Control Message .....................................30
6.1. RPL Security Fields .......................................32
6.2. DODAG Information Solicitation (DIS) ......................38
6.2.1. Format of the DIS Base Object ......................38
6.2.2. Secure DIS .........................................38
6.2.3. DIS Options ........................................38
6.3. DODAG Information Object (DIO) ............................38
6.3.1. Format of the DIO Base Object ......................39
6.3.2. Secure DIO .........................................41
6.3.3. DIO Options ........................................41
6.4. Destination Advertisement Object (DAO) ....................41
6.4.1. Format of the DAO Base Object ......................42
Winter, et al. Standards Track [Page 3]
RFC 6550 RPL March 2012
6.4.2. Secure DAO .........................................43
6.4.3. DAO Options ........................................43
6.5. Destination Advertisement Object Acknowledgement
(DAO-ACK) .................................................43
6.5.1. Format of the DAO-ACK Base Object ..................44
6.5.2. Secure DAO-ACK .....................................45
6.5.3. DAO-ACK Options ....................................45
6.6. Consistency Check (CC) ....................................45
6.6.1. Format of the CC Base Object .......................46
6.6.2. CC Options .........................................47
6.7. RPL Control Message Options ...............................47
6.7.1. RPL Control Message Option Generic Format ..........47
6.7.2. Pad1 ...............................................48
6.7.3. PadN ...............................................48
6.7.4. DAG Metric Container ...............................49
6.7.5. Route Information ..................................50
6.7.6. DODAG Configuration ................................52
6.7.7. RPL Target .........................................54
6.7.8. Transit Information ................................55
6.7.9. Solicited Information ..............................58
6.7.10. Prefix Information ................................59
6.7.11. RPL Target Descriptor .............................63
7. Sequence Counters ..............................................63
7.1. Sequence Counter Overview .................................63
7.2. Sequence Counter Operation ................................64
8. Upward Routes ..................................................66
8.1. DIO Base Rules ............................................67
8.2. Upward Route Discovery and Maintenance ....................67
8.2.1. Neighbors and Parents within a DODAG Version .......67
8.2.2. Neighbors and Parents across DODAG Versions ........68
8.2.3. DIO Message Communication ..........................73
8.3. DIO Transmission ..........................................74
8.3.1. Trickle Parameters .................................75
8.4. DODAG Selection ...........................................75
8.5. Operation as a Leaf Node ..................................75
8.6. Administrative Rank .......................................76
9. Downward Routes ................................................77
9.1. Destination Advertisement Parents .........................77
9.2. Downward Route Discovery and Maintenance ..................78
9.2.1. Maintenance of Path Sequence .......................79
9.2.2. Generation of DAO Messages .........................79
9.3. DAO Base Rules ............................................80
9.4. Structure of DAO Messages .................................80
9.5. DAO Transmission Scheduling ...............................83
9.6. Triggering DAO Messages ...................................83
9.7. Non-Storing Mode ..........................................84
9.8. Storing Mode ..............................................85
9.9. Path Control ..............................................86
Winter, et al. Standards Track [Page 4]
RFC 6550 RPL March 2012
9.9.1. Path Control Example ...............................88
9.10. Multicast Destination Advertisement Messages .............89
10. Security Mechanisms ...........................................90
10.1. Security Overview ........................................90
10.2. Joining a Secure Network .................................91
10.3. Installing Keys ..........................................92
10.4. Consistency Checks .......................................93
10.5. Counters .................................................93
10.6. Transmission of Outgoing Packets .........................94
10.7. Reception of Incoming Packets ............................95
10.7.1. Timestamp Key Checks ..............................97
10.8. Coverage of Integrity and Confidentiality ................97
10.9. Cryptographic Mode of Operation ..........................98
10.9.1. CCM Nonce .........................................98
10.9.2. Signatures ........................................99
11. Packet Forwarding and Loop Avoidance/Detection ................99
11.1. Suggestions for Packet Forwarding ........................99
11.2. Loop Avoidance and Detection ............................101
11.2.1. Source Node Operation ............................102
11.2.2. Router Operation .................................102
12. Multicast Operation ..........................................104
13. Maintenance of Routing Adjacency .............................105
14. Guidelines for Objective Functions ...........................106
14.1. Objective Function Behavior .............................106
15. Suggestions for Interoperation with Neighbor Discovery .......108
16. Summary of Requirements for Interoperable Implementations ....109
16.1. Common Requirements .....................................109
16.2. Operation as a RPL Leaf Node (Only) .....................110
16.3. Operation as a RPL Router ...............................110
16.3.1. Support for Upward Routes (Only) .................110
16.3.2. Support for Upward Routes and Downward
Routes in Non-Storing ............................110
16.3.3. Support for Upward Routes and Downward
Routes in Storing Mode ...........................111
16.4. Items for Future Specification ..........................111
17. RPL Constants and Variables ..................................112
18. Manageability Considerations .................................113
18.1. Introduction ............................................114
18.2. Configuration Management ................................115
18.2.1. Initialization Mode ..............................115
18.2.2. DIO and DAO Base Message and Options
Configuration ....................................115
18.2.3. Protocol Parameters to Be Configured on
Every Router in the LLN ..........................116
18.2.4. Protocol Parameters to Be Configured on
Every Non-DODAG-Root .............................117
18.2.5. Parameters to Be Configured on the DODAG Root ....117
Winter, et al. Standards Track [Page 5]
RFC 6550 RPL March 2012
18.2.6. Configuration of RPL Parameters Related
to DAO-Based Mechanisms ..........................118
18.2.7. Configuration of RPL Parameters Related
to Security Mechanisms ...........................119
18.2.8. Default Values ...................................119
18.3. Monitoring of RPL Operation .............................120
18.3.1. Monitoring a DODAG Parameters ....................120
18.3.2. Monitoring a DODAG Inconsistencies and
Loop Detection ...................................121
18.4. Monitoring of the RPL Data Structures ...................121
18.4.1. Candidate Neighbor Data Structure ................121
18.4.2. Destination-Oriented Directed Acyclic
Graph (DODAG) Table ..............................122
18.4.3. Routing Table and DAO Routing Entries ............122
18.5. Fault Management ........................................123
18.6. Policy ..................................................124
18.7. Fault Isolation .........................................125
18.8. Impact on Other Protocols ...............................125
18.9. Performance Management ..................................126
18.10. Diagnostics ............................................126
19. Security Considerations ......................................126
19.1. Overview ................................................126
20. IANA Considerations ..........................................128
20.1. RPL Control Message .....................................128
20.2. New Registry for RPL Control Codes ......................128
20.3. New Registry for the Mode of Operation (MOP) ............129
20.4. RPL Control Message Option ..............................130
20.5. Objective Code Point (OCP) Registry .....................131
20.6. New Registry for the Security Section Algorithm .........131
20.7. New Registry for the Security Section Flags .............132
20.8. New Registry for Per-KIM Security Levels ................132
20.9. New Registry for DODAG Informational
Solicitation (DIS) Flags ................................133
20.10. New Registry for the DODAG Information Object
(DIO) Flags ............................................134
20.11. New Registry for the Destination Advertisement
Object (DAO) Flags .....................................134
20.12. New Registry for the Destination Advertisement
Object (DAO) Flags .....................................135
20.13. New Registry for the Consistency Check (CC) Flags ......135
20.14. New Registry for the DODAG Configuration Option Flags ..136
20.15. New Registry for the RPL Target Option Flags ...........136
20.16. New Registry for the Transit Information Option Flags ..137
20.17. New Registry for the Solicited Information
Option Flags ...........................................137
20.18. ICMPv6: Error in Source Routing Header .................138
20.19. Link-Local Scope Multicast Address .....................138
21. Acknowledgements .............................................138
Winter, et al. Standards Track [Page 6]
RFC 6550 RPL March 2012
22. Contributors .................................................139
23. References ...................................................139
23.1. Normative References ....................................139
23.2. Informative References ..................................140
Appendix A. Example Operation ....................................143
A.1. Example Operation in Storing Mode with Node-Owned
Prefixes .................................................143
A.1.1. DIO Messages and PIO ..............................144
A.1.2. DAO Messages ......................................145
A.1.3. Routing Information Base ..........................145
A.2. Example Operation in Storing Mode with Subnet-Wide
Prefix ...................................................146
A.2.1. DIO Messages and PIO ..............................147
A.2.2. DAO Messages ......................................148
A.2.3. Routing Information Base ..........................148
A.3. Example Operation in Non-Storing Mode with Node-Owned
Prefixes .................................................149
A.3.1. DIO Messages and PIO ..............................150
A.3.2. DAO Messages ......................................150
A.3.3. Routing Information Base ..........................151
A.4. Example Operation in Non-Storing Mode with
Subnet-Wide Prefix .......................................151
A.4.1. DIO Messages and PIO ..............................152
A.4.2. DAO Messages ......................................153
A.4.3. Routing Information Base ..........................153
A.5. Example with External Prefixes ...........................154
Winter, et al. Standards Track [Page 7]
RFC 6550 RPL March 2012
1. Introduction
Low-power and Lossy Networks (LLNs) consist largely of constrained
nodes (with limited processing power, memory, and sometimes energy
when they are battery operated or energy scavenging). These routers
are interconnected by lossy links, typically supporting only low data
rates, that are usually unstable with relatively low packet delivery
rates. Another characteristic of such networks is that the traffic
patterns are not simply point-to-point, but in many cases point-to-
multipoint or multipoint-to-point. Furthermore, such networks may
potentially comprise up to thousands of nodes. These characteristics
offer unique challenges to a routing solution: the IETF ROLL working
group has defined application-specific routing requirements for a
Low-power and Lossy Network (LLN) routing protocol, specified in
[RFC5867], [RFC5826], [RFC5673], and [RFC5548].
This document specifies the IPv6 Routing Protocol for LLNs (RPL).
Note that although RPL was specified according to the requirements
set forth in the aforementioned requirement documents, its use is in
no way limited to these applications.
1.1. Design Principles
RPL was designed with the objective to meet the requirements spelled
out in [RFC5867], [RFC5826], [RFC5673], and [RFC5548].
A network may run multiple instances of RPL concurrently. Each such
instance may serve different and potentially antagonistic constraints
or performance criteria. This document defines how a single instance
operates.
In order to be useful in a wide range of LLN application domains, RPL
separates packet processing and forwarding from the routing
optimization objective. Examples of such objectives include
minimizing energy, minimizing latency, or satisfying constraints.
This document describes the mode of operation of RPL. Other
companion documents specify routing Objective Functions. A RPL
implementation, in support of a particular LLN application, will
include the necessary Objective Function(s) as required by the
application.
RPL operations require bidirectional links. In some LLN scenarios,
those links may exhibit asymmetric properties. It is required that
the reachability of a router be verified before the router can be
used as a parent. RPL expects an external mechanism to be triggered
during the parent selection phase in order to verify link properties
and neighbor reachability. Neighbor Unreachability Detection (NUD)
is such a mechanism, but alternates are possible, including
Winter, et al. Standards Track [Page 8]
RFC 6550 RPL March 2012
Bidirectional Forwarding Detection (BFD) [RFC5881] and hints from
lower layers via Layer 2 (L2) triggers like [RFC5184]. In a general
fashion, a detection mechanism that is reactive to traffic is favored
in order to minimize the cost of monitoring links that are not being
used.
RPL also expects an external mechanism to access and transport some
control information, referred to as the "RPL Packet Information", in
data packets. The RPL Packet Information is defined in Section 11.2
and enables the association of a data packet with a RPL Instance and
the validation of RPL routing states. The RPL option [RFC6553] is an
example of such mechanism. The mechanism is required for all packets
except when strict source routing is used (that is for packets going
Downward in Non-Storing mode as detailed further in Section 9), which
by nature prevents endless loops and alleviates the need for the RPL
Packet Information. Future companion specifications may propose
alternate ways to carry the RPL Packet Information in the IPv6
packets and may extend the RPL Packet Information to support
additional features.
RPL provides a mechanism to disseminate information over the
dynamically formed network topology. This dissemination enables
minimal configuration in the nodes, allowing nodes to operate mostly
autonomously. This mechanism uses Trickle [RFC6206] to optimize the
dissemination as described in Section 8.3.
In some applications, RPL assembles topologies of routers that own
independent prefixes. Those prefixes may or may not be aggregatable
depending on the origin of the routers. A prefix that is owned by a
router is advertised as on-link.
RPL also introduces the capability to bind a subnet together with a
common prefix and to route within that subnet. A source can inject
information about the subnet to be disseminated by RPL, and that
source is authoritative for that subnet. Because many LLN links have
non-transitive properties, a common prefix that RPL disseminates over
the subnet must not be advertised as on-link.
In particular, RPL may disseminate IPv6 Neighbor Discovery (ND)
information such as the [RFC4861] Prefix Information Option (PIO) and
the [RFC4191] Route Information Option (RIO). ND information that is
disseminated by RPL conserves all its original semantics for router
to host, with limited extensions for router to router, though it is
not to be confused with routing advertisements and it is never to be
directly redistributed in another routing protocol. A RPL node often
combines host and router behaviors. As a host, it will process the
options as specified in [RFC4191], [RFC4861], [RFC4862], and
[RFC6275]. As a router, the RPL node may advertise the information
Winter, et al. Standards Track [Page 9]
RFC 6550 RPL March 2012
from the options as required for the specific link, for instance, in
an ND Router Advertisement (RA) message, though the exact operation
is out of scope.
A set of companion documents to this specification will provide
further guidance in the form of applicability statements specifying a
set of operating points appropriate to the Building Automation, Home
Automation, Industrial, and Urban application scenarios.
1.2. Expectations of Link-Layer Type
In compliance with the layered architecture of IP, RPL does not rely
on any particular features of a specific link-layer technology. RPL
is designed to be able to operate over a variety of different link
layers, including ones that are constrained, potentially lossy, or
typically utilized in conjunction with highly constrained host or
router devices, such as but not limited to, low-power wireless or PLC
(Power Line Communication) technologies.
Implementers may find [RFC3819] a useful reference when designing a
link-layer interface between RPL and a particular link-layer
technology.
2. Terminology
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].
Additionally, this document uses terminology from [ROLL-TERMS], and
introduces the following terminology:
DAG: Directed Acyclic Graph. A directed graph having the property
that all edges are oriented in such a way that no cycles exist.
All edges are contained in paths oriented toward and
terminating at one or more root nodes.
DAG root: A DAG root is a node within the DAG that has no outgoing
edge. Because the graph is acyclic, by definition, all DAGs
must have at least one DAG root and all paths terminate at a
DAG root.
Destination-Oriented DAG (DODAG): A DAG rooted at a single
destination, i.e., at a single DAG root (the DODAG root) with
no outgoing edges.
Winter, et al. Standards Track [Page 10]
RFC 6550 RPL March 2012
DODAG root: A DODAG root is the DAG root of a DODAG. The DODAG root
may act as a border router for the DODAG; in particular, it may
aggregate routes in the DODAG and may redistribute DODAG routes
into other routing protocols.
Virtual DODAG root: A Virtual DODAG root is the result of two or more
RPL routers, for instance, 6LoWPAN Border Routers (6LBRs),
coordinating to synchronize DODAG state and act in concert as
if they are a single DODAG root (with multiple interfaces),
with respect to the LLN. The coordination most likely occurs
between powered devices over a reliable transit link, and the
details of that scheme are out of scope for this specification
(to be defined in future companion specifications).
Up: Up refers to the direction from leaf nodes towards DODAG roots,
following DODAG edges. This follows the common terminology
used in graphs and depth-first-search, where vertices further
from the root are "deeper" or "down" and vertices closer to the
root are "shallower" or "up".
Down: Down refers to the direction from DODAG roots towards leaf
nodes, in the reverse direction of DODAG edges. This follows
the common terminology used in graphs and depth-first-search,
where vertices further from the root are "deeper" or "down" and
vertices closer to the root are "shallower" or "up".
Rank: A node's Rank defines the node's individual position relative
to other nodes with respect to a DODAG root. Rank strictly
increases in the Down direction and strictly decreases in the
Up direction. The exact way Rank is computed depends on the
DAG's Objective Function (OF). The Rank may analogously track
a simple topological distance, may be calculated as a function
of link metrics, and may consider other properties such as
constraints.
Objective Function (OF): An OF defines how routing metrics,
optimization objectives, and related functions are used to
compute Rank. Furthermore, the OF dictates how parents in the
DODAG are selected and, thus, the DODAG formation.
Objective Code Point (OCP): An OCP is an identifier that indicates
which Objective Function the DODAG uses.
RPLInstanceID: A RPLInstanceID is a unique identifier within a
network. DODAGs with the same RPLInstanceID share the same
Objective Function.
Winter, et al. Standards Track [Page 11]
RFC 6550 RPL March 2012
RPL Instance: A RPL Instance is a set of one or more DODAGs that
share a RPLInstanceID. At most, a RPL node can belong to one
DODAG in a RPL Instance. Each RPL Instance operates
independently of other RPL Instances. This document describes
operation within a single RPL Instance.
DODAGID: A DODAGID is the identifier of a DODAG root. The DODAGID is
unique within the scope of a RPL Instance in the LLN. The
tuple (RPLInstanceID, DODAGID) uniquely identifies a DODAG.
DODAG Version: A DODAG Version is a specific iteration ("Version") of
a DODAG with a given DODAGID.
DODAGVersionNumber: A DODAGVersionNumber is a sequential counter that
is incremented by the root to form a new Version of a DODAG. A
DODAG Version is identified uniquely by the (RPLInstanceID,
DODAGID, DODAGVersionNumber) tuple.
Goal: The Goal is an application-specific goal that is defined
outside the scope of RPL. Any node that roots a DODAG will
need to know about this Goal to decide whether or not the Goal
can be satisfied. A typical Goal is to construct the DODAG
according to a specific Objective Function and to keep
connectivity to a set of hosts (e.g., to use an Objective
Function that minimizes a metric and is connected to a specific
database host to store the collected data).
Grounded: A DODAG is grounded when the DODAG root can satisfy the
Goal.
Floating: A DODAG is floating if it is not grounded. A floating
DODAG is not expected to have the properties required to
satisfy the goal. It may, however, provide connectivity to
other nodes within the DODAG.
DODAG parent: A parent of a node within a DODAG is one of the
immediate successors of the node on a path towards the DODAG
root. A DODAG parent's Rank is lower than the node's. (See
Section 3.5.1).
Sub-DODAG: The sub-DODAG of a node is the set of other nodes whose
paths to the DODAG root pass through that node. Nodes in the
sub-DODAG of a node have a greater Rank than that node. (See
Section 3.5.1).
Local DODAG: Local DODAGs contain one and only one root node, and
they allow that single root node to allocate and manage a RPL
Instance, identified by a local RPLInstanceID, without
Winter, et al. Standards Track [Page 12]
RFC 6550 RPL March 2012
coordination with other nodes. Typically, this is done in
order to optimize routes to a destination within the LLN. (See
Section 5).
Global DODAG: A Global DODAG uses a global RPLInstanceID that may be
coordinated among several other nodes. (See Section 5).
DIO: DODAG Information Object (see Section 6.3)
DAO: Destination Advertisement Object (see Section 6.4)
DIS: DODAG Information Solicitation (see Section 6.2)
CC: Consistency Check (see Section 6.6)
As they form networks, LLN devices often mix the roles of host and
router when compared to traditional IP networks. In this document,
"host" refers to an LLN device that can generate but does not forward
RPL traffic; "router" refers to an LLN device that can forward as
well as generate RPL traffic; and "node" refers to any RPL device,
either a host or a router.
3. Protocol Overview
The aim of this section is to describe RPL in the spirit of
[RFC4101]. Protocol details can be found in further sections.
3.1. Topologies
This section describes the basic RPL topologies that may be formed,
and the rules by which these are constructed, i.e., the rules
governing DODAG formation.
3.1.1. Constructing Topologies
LLNs, such as Radio Networks, do not typically have predefined
topologies, for example, those imposed by point-to-point wires, so
RPL has to discover links and then select peers sparingly.
In many cases, because Layer 2 ranges overlap only partially, RPL
forms non-transitive / Non-Broadcast Multi-Access (NBMA) network
topologies upon which it computes routes.
RPL routes are optimized for traffic to or from one or more roots
that act as sinks for the topology. As a result, RPL organizes a
topology as a Directed Acyclic Graph (DAG) that is partitioned into
Winter, et al. Standards Track [Page 13]
RFC 6550 RPL March 2012
one or more Destination Oriented DAGs (DODAGs), one DODAG per sink.
If the DAG has multiple roots, then it is expected that the roots are
federated by a common backbone, such as a transit link.
3.1.2. RPL Identifiers
RPL uses four values to identify and maintain a topology:
o The first is a RPLInstanceID. A RPLInstanceID identifies a set of
one or more Destination Oriented DAGs (DODAGs). A network may
have multiple RPLInstanceIDs, each of which defines an independent
set of DODAGs, which may be optimized for different Objective
Functions (OFs) and/or applications. The set of DODAGs identified
by a RPLInstanceID is called a RPL Instance. All DODAGs in the
same RPL Instance use the same OF.
o The second is a DODAGID. The scope of a DODAGID is a RPL
Instance. The combination of RPLInstanceID and DODAGID uniquely
identifies a single DODAG in the network. A RPL Instance may have
multiple DODAGs, each of which has an unique DODAGID.
o The third is a DODAGVersionNumber. The scope of a
DODAGVersionNumber is a DODAG. A DODAG is sometimes reconstructed
from the DODAG root, by incrementing the DODAGVersionNumber. The
combination of RPLInstanceID, DODAGID, and DODAGVersionNumber
uniquely identifies a DODAG Version.
o The fourth is Rank. The scope of Rank is a DODAG Version. Rank
establishes a partial order over a DODAG Version, defining
individual node positions with respect to the DODAG root.
3.1.3. Instances, DODAGs, and DODAG Versions
A RPL Instance contains one or more DODAG roots. A RPL Instance may
provide routes to certain destination prefixes, reachable via the
DODAG roots or alternate paths within the DODAG. These roots may
operate independently, or they may coordinate over a network that is
not necessarily as constrained as an LLN.
A RPL Instance may comprise:
o a single DODAG with a single root
* For example, a DODAG optimized to minimize latency rooted at a
single centralized lighting controller in a Home Automation
application.
Winter, et al. Standards Track [Page 14]
RFC 6550 RPL March 2012
o multiple uncoordinated DODAGs with independent roots (differing
DODAGIDs)
* For example, multiple data collection points in an urban data
collection application that do not have suitable connectivity
to coordinate with each other or that use the formation of
multiple DODAGs as a means to dynamically and autonomously
partition the network.
o a single DODAG with a virtual root that coordinates LLN sinks
(with the same DODAGID) over a backbone network.
* For example, multiple border routers operating with a reliable
transit link, e.g., in support of an IPv6 Low-Power Wireless
Personal Area Network (6LoWPAN) application, that are capable
of acting as logically equivalent interfaces to the sink of the
same DODAG.
o a combination of the above as suited to some application scenario.
Each RPL packet is associated with a particular RPLInstanceID (see
Section 11.2) and, therefore, RPL Instance (Section 5). The
provisioning or automated discovery of a mapping between a
RPLInstanceID and a type or service of application traffic is out of
scope for this specification (to be defined in future companion
specifications).
Figure 1 depicts an example of a RPL Instance comprising three DODAGs
with DODAG roots R1, R2, and R3. Each of these DODAG roots
advertises the same RPLInstanceID. The lines depict connectivity
between parents and children.
Figure 2 depicts how a DODAGVersionNumber increment leads to a new
DODAG Version. This depiction illustrates a DODAGVersionNumber
increment that results in a different DODAG topology. Note that a
new DODAG Version does not always imply a different DODAG topology.
To accommodate certain topology changes requires a new DODAG Version,
as described later in this specification.
In the following examples, please note that tree-like structures are
depicted for simplicity, although the DODAG structure allows for each
node to have multiple parents when the connectivity supports it.
Winter, et al. Standards Track [Page 15]
RFC 6550 RPL March 2012
+----------------------------------------------------------------+
| |
| +--------------+ |
| | | |
| | (R1) | (R2) (R3) |
| | / \ | /| \ / | \ |
| | / \ | / | \ / | \ |
| | (A) (B) | (C) | (D) ... (F) (G) (H) |
| | /|\ |\ | / | / |\ |\ | | |
| | : : : : : | : (E) : : : `: : |
| | | / \ |
| +--------------+ : : |
| DODAG |
| |
+----------------------------------------------------------------+
RPL Instance
Figure 1: RPL Instance
+----------------+ +----------------+
| | | |
| (R1) | | (R1) |
| / \ | | / |
| / \ | | / |
| (A) (B) | \ | (A) |
| /|\ / |\ | ------\ | /|\ |
| : : (C) : : | \ | : : (C) |
| | / | \ |
| | ------/ | \ |
| | / | (B) |
| | | |\ |
| | | : : |
| | | |
+----------------+ +----------------+
Version N Version N+1
Figure 2: DODAG Version
3.2. Upward Routes and DODAG Construction
RPL provisions routes Up towards DODAG roots, forming a DODAG
optimized according to an Objective Function (OF). RPL nodes
construct and maintain these DODAGs through DODAG Information Object
(DIO) messages.
Winter, et al. Standards Track [Page 16]
RFC 6550 RPL March 2012
3.2.1. Objective Function (OF)
The Objective Function (OF) defines how RPL nodes select and optimize
routes within a RPL Instance. The OF is identified by an Objective
Code Point (OCP) within the DIO Configuration option. An OF defines
how nodes translate one or more metrics and constraints, which are
themselves defined in [RFC6551], into a value called Rank, which
approximates the node's distance from a DODAG root. An OF also
defines how nodes select parents. Further details may be found in
Section 14, [RFC6551], [RFC6552], and related companion
specifications.
3.2.2. DODAG Repair
A DODAG root institutes a global repair operation by incrementing the
DODAGVersionNumber. This initiates a new DODAG Version. Nodes in
the new DODAG Version can choose a new position whose Rank is not
constrained by their Rank within the old DODAG Version.
RPL also supports mechanisms that may be used for local repair within
the DODAG Version. The DIO message specifies the necessary
parameters as configured from and controlled by policy at the DODAG
root.
3.2.3. Security
RPL supports message confidentiality and integrity. It is designed
such that link-layer mechanisms can be used when available and
appropriate; yet, in their absence, RPL can use its own mechanisms.
RPL has three basic security modes.
In the first, called "unsecured", RPL control messages are sent
without any additional security mechanisms. Unsecured mode does not
imply that the RPL network is unsecure: it could be using other
present security primitives (e.g., link-layer security) to meet
application security requirements.
In the second, called "preinstalled", nodes joining a RPL Instance
have preinstalled keys that enable them to process and generate
secured RPL messages.
The third mode is called "authenticated". In authenticated mode,
nodes have preinstalled keys as in preinstalled mode, but the
preinstalled key may only be used to join a RPL Instance as a leaf.
Joining an authenticated RPL Instance as a router requires obtaining
a key from an authentication authority. The process by which this
key is obtained is out of scope for this specification. Note that
this specification alone does not provide sufficient detail for a RPL
Winter, et al. Standards Track [Page 17]
RFC 6550 RPL March 2012
implementation to securely operate in authenticated mode. For a RPL
implementation to operate securely in authenticated mode, it is
necessary for a future companion specification to detail the
mechanisms by which a node obtains/requests the authentication
material (e.g., key, certificate) and to determine from where that
material should be obtained. See also Section 10.3.
3.2.4. Grounded and Floating DODAGs
DODAGs can be grounded or floating: the DODAG root advertises which
is the case. A grounded DODAG offers connectivity to hosts that are
required for satisfying the application-defined goal. A floating
DODAG is not expected to satisfy the goal; in most cases, it only
provides routes to nodes within the DODAG. Floating DODAGs may be
used, for example, to preserve interconnectivity during repair.
3.2.5. Local DODAGs
RPL nodes can optimize routes to a destination within an LLN by
forming a Local DODAG whose DODAG root is the desired destination.
Unlike global DAGs, which can consist of multiple DODAGs, local DAGs
have one and only one DODAG and therefore one DODAG root. Local
DODAGs can be constructed on demand.
3.2.6. Administrative Preference
An implementation/deployment may specify that some DODAG roots should
be used over others through an administrative preference.
Administrative preference offers a way to control traffic and
engineer DODAG formation in order to better support application
requirements or needs.
3.2.7. Data-Path Validation and Loop Detection
The low-power and lossy nature of LLNs motivates RPL's use of on-
demand loop detection using data packets. Because data traffic can
be infrequent, maintaining a routing topology that is constantly up
to date with the physical topology can waste energy. Typical LLNs
exhibit variations in physical connectivity that are transient and
innocuous to traffic, but that would be costly to track closely from
the control plane. Transient and infrequent changes in connectivity
need not be addressed by RPL until there is data to send. This
aspect of RPL's design draws from existing, highly used LLN protocols
as well as extensive experimental and deployment evidence on its
efficacy.
Winter, et al. Standards Track [Page 18]
RFC 6550 RPL March 2012
The RPL Packet Information that is transported with data packets
includes the Rank of the transmitter. An inconsistency between the
routing decision for a packet (Upward or Downward) and the Rank
relationship between the two nodes indicates a possible loop. On
receiving such a packet, a node institutes a local repair operation.
For example, if a node receives a packet flagged as moving in the
Upward direction, and if that packet records that the transmitter is
of a lower (lesser) Rank than the receiving node, then the receiving
node is able to conclude that the packet has not progressed in the
Upward direction and that the DODAG is inconsistent.
3.2.8. Distributed Algorithm Operation
A high-level overview of the distributed algorithm, which constructs
the DODAG, is as follows:
o Some nodes are configured to be DODAG roots, with associated DODAG
configurations.
o Nodes advertise their presence, affiliation with a DODAG, routing
cost, and related metrics by sending link-local multicast DIO
messages to all-RPL-nodes.
o Nodes listen for DIOs and use their information to join a new
DODAG (thus, selecting DODAG parents), or to maintain an existing
DODAG, according to the specified Objective Function and Rank of
their neighbors.
o Nodes provision routing table entries, for the destinations
specified by the DIO message, via their DODAG parents in the DODAG
Version. Nodes that decide to join a DODAG can provision one or
more DODAG parents as the next hop for the default route and a
number of other external routes for the associated instance.
3.3. Downward Routes and Destination Advertisement
RPL uses Destination Advertisement Object (DAO) messages to establish
Downward routes. DAO messages are an optional feature for
applications that require point-to-multipoint (P2MP) or point-to-
point (P2P) traffic. RPL supports two modes of Downward traffic:
Storing (fully stateful) or Non-Storing (fully source routed); see
Section 9. Any given RPL Instance is either storing or non-storing.
In both cases, P2P packets travel Up toward a DODAG root then Down to
the final destination (unless the destination is on the Upward
route). In the Non-Storing case, the packet will travel all the way
to a DODAG root before traveling Down. In the Storing case, the
Winter, et al. Standards Track [Page 19]
RFC 6550 RPL March 2012
packet may be directed Down towards the destination by a common
ancestor of the source and the destination prior to reaching a DODAG
root.
As of the writing of this specification, no implementation is
expected to support both Storing and Non-Storing modes of operation.
Most implementations are expected to support either no Downward
routes, Non-Storing mode only, or Storing mode only. Other modes of
operation, such as a hybrid mix of Storing and Non-Storing mode, are
out of scope for this specification and may be described in other
companion specifications.
This specification describes a basic mode of operation in support of
P2P traffic. Note that more optimized P2P solutions may be described
in companion specifications.
3.4. Local DODAGs Route Discovery
Optionally, a RPL network can support on-demand discovery of DODAGs
to specific destinations within an LLN. Such Local DODAGs behave
slightly differently than Global DODAGs: they are uniquely defined by
the combination of DODAGID and RPLInstanceID. The RPLInstanceID
denotes whether a DODAG is a Local DODAG.
3.5. Rank Properties
The Rank of a node is a scalar representation of the location of that
node within a DODAG Version. The Rank is used to avoid and detect
loops and, as such, must demonstrate certain properties. The exact
calculation of the Rank is left to the Objective Function. Even
though the specific computation of the Rank is left to the Objective
Function, the Rank must implement generic properties regardless of
the Objective Function.
In particular, the Rank of the nodes must monotonically decrease as
the DODAG Version is followed towards the DODAG destination. In that
regard, the Rank can be considered a scalar representation of the
location or radius of a node within a DODAG Version.
The details of how the Objective Function computes Rank are out of
scope for this specification, although that computation may depend,
for example, on parents, link metrics, node metrics, and the node
configuration and policies. See Section 14 for more information.
The Rank is not a path cost, although its value can be derived from
and influenced by path metrics. The Rank has properties of its own
that are not necessarily those of all metrics:
Winter, et al. Standards Track [Page 20]
RFC 6550 RPL March 2012
Type: The Rank is an abstract numeric value.
Function: The Rank is the expression of a relative position within a
DODAG Version with regard to neighbors, and it is not
necessarily a good indication or a proper expression of a
distance or a path cost to the root.
Stability: The stability of the Rank determines the stability of the
routing topology. Some dampening or filtering is RECOMMENDED
to keep the topology stable; thus, the Rank does not
necessarily change as fast as some link or node metrics would.
A new DODAG Version would be a good opportunity to reconcile
the discrepancies that might form over time between metrics and
Ranks within a DODAG Version.
Properties: The Rank is incremented in a strictly monotonic fashion,
and it can be used to validate a progression from or towards
the root. A metric, like bandwidth or jitter, does not
necessarily exhibit this property.
Abstract: The Rank does not have a physical unit, but rather a range
of increment per hop, where the assignment of each increment is
to be determined by the Objective Function.
The Rank value feeds into DODAG parent selection, according to the
RPL loop-avoidance strategy. Once a parent has been added, and a
Rank value for the node within the DODAG has been advertised, the
node's further options with regard to DODAG parent selection and
movement within the DODAG are restricted in favor of loop avoidance.
3.5.1. Rank Comparison (DAGRank())
Rank may be thought of as a fixed-point number, where the position of
the radix point between the integer part and the fractional part is
determined by MinHopRankIncrease. MinHopRankIncrease is the minimum
increase in Rank between a node and any of its DODAG parents. A
DODAG root provisions MinHopRankIncrease. MinHopRankIncrease creates
a trade-off between hop cost precision and the maximum number of hops
a network can support. A very large MinHopRankIncrease, for example,
allows precise characterization of a given hop's effect on Rank but
cannot support many hops.
When an Objective Function computes Rank, the Objective Function
operates on the entire (i.e., 16-bit) Rank quantity. When Rank is
compared, e.g., for determination of parent relationships or loop
detection, the integer portion of the Rank is to be used. The
Winter, et al. Standards Track [Page 21]
RFC 6550 RPL March 2012
integer portion of the Rank is computed by the DAGRank() macro as
follows, where floor(x) is the function that evaluates to the
greatest integer less than or equal to x:
DAGRank(rank) = floor(rank/MinHopRankIncrease)
For example, if a 16-bit Rank quantity is decimal 27, and the
MinHopRankIncrease is decimal 16, then DAGRank(27) = floor(1.6875) =
1. The integer part of the Rank is 1 and the fractional part is
11/16.
Following the conventions in this document, using the macro
DAGRank(node) may be interpreted as DAGRank(node.rank), where
node.rank is the Rank value as maintained by the node.
A Node A has a Rank less than the Rank of a Node B if DAGRank(A) is
less than DAGRank(B).
A Node A has a Rank equal to the Rank of a Node B if DAGRank(A) is
equal to DAGRank(B).
A Node A has a Rank greater than the Rank of a Node B if DAGRank(A)
is greater than DAGRank(B).
3.5.2. Rank Relationships
Rank computations maintain the following properties for any nodes M
and N that are neighbors in the LLN:
DAGRank(M) is less than DAGRank(N):
In this case, the position of M is closer to the DODAG root than
the position of N. Node M may safely be a DODAG parent for Node N
without risk of creating a loop. Further, for a Node N, all
parents in the DODAG parent set must be of a Rank less than
DAGRank(N). In other words, the Rank presented by a Node N MUST
be greater than that presented by any of its parents.
DAGRank(M) equals DAGRank(N):
In this case, the positions of M and N within the DODAG and with
respect to the DODAG root are similar or identical. Routing
through a node with equal Rank may cause a routing loop (i.e., if
that node chooses to route through a node with equal Rank as
well).
Winter, et al. Standards Track [Page 22]
RFC 6550 RPL March 2012
DAGRank(M) is greater than DAGRank(N):
In this case, the position of M is farther from the DODAG root
than the position of N. Further, Node M may in fact be in the
sub-DODAG of Node N. If Node N selects Node M as DODAG parent,
there is a risk of creating a loop.
As an example, the Rank could be computed in such a way so as to
closely track ETX (expected transmission count, a fairly common
routing metric used in LLN and defined in [RFC6551]) when the metric
that an Objective Function minimizes is ETX, or latency, or in a more
complicated way as appropriate to the Objective Function being used
within the DODAG.
3.6. Routing Metrics and Constraints Used by RPL
Routing metrics are used by routing protocols to compute shortest
paths. Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
and OSPF ([RFC4915]) use static link metrics. Such link metrics can
simply reflect the bandwidth or can also be computed according to a
polynomial function of several metrics defining different link
characteristics. Some routing protocols support more than one
metric: in the vast majority of the cases, one metric is used per
(sub-)topology. Less often, a second metric may be used as a
tiebreaker in the presence of Equal Cost Multiple Paths (ECMPs). The
optimization of multiple metrics is known as an NP-complete problem
and is sometimes supported by some centralized path computation
engine.
In contrast, LLNs do require the support of both static and dynamic
metrics. Furthermore, both link and node metrics are required. In
the case of RPL, it is virtually impossible to define one metric, or
even a composite metric, that will satisfy all use cases.
In addition, RPL supports constraint-based routing where constraints
may be applied to both link and nodes. If a link or a node does not
satisfy a required constraint, it is "pruned" from the candidate
neighbor set, thus leading to a constrained shortest path.
An Objective Function specifies the objectives used to compute the
(constrained) path. Furthermore, nodes are configured to support a
set of metrics and constraints and select their parents in the DODAG
according to the metrics and constraints advertised in the DIO
messages. Upstream and Downstream metrics may be merged or
advertised separately depending on the OF and the metrics. When they
are advertised separately, it may happen that the set of DIO parents
Winter, et al. Standards Track [Page 23]
RFC 6550 RPL March 2012
is different from the set of DAO parents (a DAO parent is a node to
which unicast DAO messages are sent). Yet, all are DODAG parents
with regard to the rules for Rank computation.
The Objective Function is decoupled from the routing metrics and
constraints used by RPL. Whereas the OF dictates rules such as DODAG
parent selection, load balancing, and so on, the set of metrics
and/or constraints used, and thus those that determine the preferred
path, are based on the information carried within the DAG container
option in DIO messages.
The set of supported link/node constraints and metrics is specified
in [RFC6551].
Example 1: Shortest path: path offering the shortest end-to-end
delay.
Example 2: Shortest Constrained path: the path that does not traverse
any battery-operated node and that optimizes the path
reliability.
3.7. Loop Avoidance
RPL tries to avoid creating loops when undergoing topology changes
and includes Rank-based data-path validation mechanisms for detecting
loops when they do occur (see Section 11 for more details). In
practice, this means that RPL guarantees neither loop-free path
selection nor tight delay convergence times, but it can detect and
repair a loop as soon as it is used. RPL uses this loop detection to
ensure that packets make forward progress within the DODAG Version
and trigger repairs when necessary.
3.7.1. Greediness and Instability
A node is greedy if it attempts to move deeper (increase Rank) in the
DODAG Version in order to increase the size of the parent set or
improve some other metric. Once a node has joined a DODAG Version,
RPL disallows certain behaviors, including greediness, in order to
prevent resulting instabilities in the DODAG Version.
Suppose a node is willing to receive and process a DIO message from a
node in its own sub-DODAG and, in general, a node deeper than itself.
In this case, a possibility exists that a feedback loop is created,
wherein two or more nodes continue to try and move in the DODAG
Version while attempting to optimize against each other. In some
cases, this will result in instability. It is for this reason that
RPL limits the cases where a node may process DIO messages from
deeper nodes to some form of local repair. This approach creates an
Winter, et al. Standards Track [Page 24]
RFC 6550 RPL March 2012
"event horizon", whereby a node cannot be influenced beyond some
limit into an instability by the action of nodes that may be in its
own sub-DODAG.
3.7.1.1. Example: Greedy Parent Selection and Instability
(A) (A) (A)
|\ |\ |\
| `-----. | `-----. | `-----.
| \ | \ | \
(B) (C) (B) \ | (C)
\ | | /
`-----. | | .-----'
\| |/
(C) (B)
-1- -2- -3-
Figure 3: Greedy DODAG Parent Selection
Figure 3 depicts a DODAG in three different configurations. A usable
link between (B) and (C) exists in all three configurations. In
Figure 3-1, Node (A) is a DODAG parent for Nodes (B) and (C). In
Figure 3-2, Node (A) is a DODAG parent for Nodes (B) and (C), and
Node (B) is also a DODAG parent for Node (C). In Figure 3-3, Node
(A) is a DODAG parent for Nodes (B) and (C), and Node (C) is also a
DODAG parent for Node (B).
If a RPL node is too greedy, in that it attempts to optimize for an
additional number of parents beyond its most preferred parents, then
an instability can result. Consider the DODAG illustrated in
Figure 3-1. In this example, Nodes (B) and (C) may most prefer Node
(A) as a DODAG parent, but we will consider the case when they are
operating under the greedy condition that will try to optimize for
two parents.
o Let Figure 3-1 be the initial condition.
o Suppose Node (C) first is able to leave the DODAG and rejoin at a
lower Rank, taking both Nodes (A) and (B) as DODAG parents as
depicted in Figure 3-2. Now Node (C) is deeper than both Nodes
(A) and (B), and Node (C) is satisfied to have two DODAG parents.
o Suppose Node (B), in its greediness, is willing to receive and
process a DIO message from Node (C) (against the rules of RPL),
and then Node (B) leaves the DODAG and rejoins at a lower Rank,
Winter, et al. Standards Track [Page 25]
RFC 6550 RPL March 2012
taking both Nodes (A) and (C) as DODAG parents. Now Node (B) is
deeper than both Nodes (A) and (C) and is satisfied with two DAG
parents.
o Then, Node (C), because it is also greedy, will leave and rejoin
deeper, to again get two parents and have a lower Rank then both
of them.
o Next, Node (B) will again leave and rejoin deeper, to again get
two parents.
o Again, Node (C) leaves and rejoins deeper.
o The process will repeat, and the DODAG will oscillate between
Figure 3-2 and Figure 3-3 until the nodes count to infinity and
restart the cycle again.
o This cycle can be averted through mechanisms in RPL:
* Nodes (B) and (C) stay at a Rank sufficient to attach to their
most preferred parent (A) and don't go for any deeper (worse)
alternate parents (Nodes are not greedy).
* Nodes (B) and (C) do not process DIO messages from nodes deeper
than themselves (because such nodes are possibly in their own
sub-DODAGs).
These mechanisms are further described in Section 8.2.2.4.
3.7.2. DODAG Loops
A DODAG loop may occur when a node detaches from the DODAG and
reattaches to a device in its prior sub-DODAG. In particular, this
may happen when DIO messages are missed. Strict use of the
DODAGVersionNumber can eliminate this type of loop, but this type of
loop may possibly be encountered when using some local repair
mechanisms.
For example, consider the local repair mechanism that allows a node
to detach from the DODAG, advertise a Rank of INFINITE_RANK (in order
to poison its routes / inform its sub-DODAG), and then reattach to
the DODAG. In some of these cases, the node may reattach to its own
prior-sub-DODAG, causing a DODAG loop, because the poisoning may fail
if the INFINITE_RANK advertisements are lost in the LLN environment.
(In this case, the Rank-based data-path validation mechanisms would
eventually detect and trigger correction of the loop).
Winter, et al. Standards Track [Page 26]
RFC 6550 RPL March 2012
3.7.3. DAO Loops
A DAO loop may occur when the parent has a route installed upon
receiving and processing a DAO message from a child, but the child
has subsequently cleaned up the related DAO state. This loop happens
when a No-Path (a DAO message that invalidates a previously announced
prefix, see Section 6.4.3) was missed and persists until all state
has been cleaned up. RPL includes an optional mechanism to
acknowledge DAO messages, which may mitigate the impact of a single
DAO message being missed. RPL includes loop detection mechanisms
that mitigate the impact of DAO loops and trigger their repair. (See
Section 11.2.2.3.)
4. Traffic Flows Supported by RPL
RPL supports three basic traffic flows: multipoint-to-point (MP2P),
point-to-multipoint (P2MP), and point-to-point (P2P).
4.1. Multipoint-to-Point Traffic
Multipoint-to-point (MP2P) is a dominant traffic flow in many LLN
applications ([RFC5867], [RFC5826], [RFC5673], and [RFC5548]). The
destinations of MP2P flows are designated nodes that have some
application significance, such as providing connectivity to the
larger Internet or core private IP network. RPL supports MP2P
traffic by allowing MP2P destinations to be reached via DODAG roots.
4.2. Point-to-Multipoint Traffic
Point-to-multipoint (P2MP) is a traffic pattern required by several
LLN applications ([RFC5867], [RFC5826], [RFC5673], and [RFC5548]).
RPL supports P2MP traffic by using a destination advertisement
mechanism that provisions Down routes toward destinations (prefixes,
addresses, or multicast groups), and away from roots. Destination
advertisements can update routing tables as the underlying DODAG
topology changes.
4.3. Point-to-Point Traffic
RPL DODAGs provide a basic structure for point-to-point (P2P)
traffic. For a RPL network to support P2P traffic, a root must be
able to route packets to a destination. Nodes within the network may
also have routing tables to destinations. A packet flows towards a
root until it reaches an ancestor that has a known route to the
destination. As pointed out later in this document, in the most
constrained case (when nodes cannot store routes), that common
ancestor may be the DODAG root. In other cases, it may be a node
closer to both the source and destination.
Winter, et al. Standards Track [Page 27]
RFC 6550 RPL March 2012
RPL also supports the case where a P2P destination is a 'one-hop'
neighbor.
RPL neither specifies nor precludes additional mechanisms for
computing and installing potentially more optimal routes to support
arbitrary P2P traffic.
5. RPL Instance
Within a given LLN, there may be multiple, logically independent RPL
Instances. A RPL node may belong to multiple RPL Instances, and it
may act as a router in some and as a leaf in others. This document
describes how a single instance behaves.
There are two types of RPL Instances: Local and Global. RPL divides
the RPLInstanceID space between Global and Local instances to allow
for both coordinated and unilateral allocation of RPLInstanceIDs.
Global RPL Instances are coordinated, have one or more DODAGs, and
are typically long-lived. Local RPL Instances are always a single
DODAG whose singular root owns the corresponding DODAGID and
allocates the local RPLInstanceID in a unilateral manner. Local RPL
Instances can be used, for example, for constructing DODAGs in
support of a future on-demand routing solution. The mode of
operation of Local RPL Instances is out of scope for this
specification and may be described in other companion specifications.
The definition and provisioning of RPL Instances are out of scope for
this specification. Guidelines may be application and implementation
specific, and they are expected to be elaborated in future companion
specifications. Those operations are expected to be such that data
packets coming from the outside of the RPL network can unambiguously
be associated to at least one RPL Instance and be safely routed over
any instance that would match the packet.
Control and data packets within RPL network are tagged to
unambiguously identify of which RPL Instance they are a part.
Every RPL control message has a RPLInstanceID field. Some RPL
control messages, when referring to a local RPLInstanceID as defined
below, may also include a DODAGID.
Data packets that flow within the RPL network expose the
RPLInstanceID as part of the RPL Packet Information that RPL
requires, as further described in Section 11.2. For data packets
coming from outside the RPL network, the ingress router determines
the RPLInstanceID and places it into the resulting packet that it
injects into the RPL network.
Winter, et al. Standards Track [Page 28]
RFC 6550 RPL March 2012
5.1. RPL Instance ID
A global RPLInstanceID MUST be unique to the whole LLN. Mechanisms
for allocating and provisioning global RPLInstanceID are out of scope
for this specification. There can be up to 128 Global instance in
the whole network. Local instances are always used in conjunction
with a DODAGID (which is either given explicitly or implicitly in
some cases), and up 64 Local instances per DODAGID can be supported.
Local instances are allocated and managed by the node that owns the
DODAGID, without any explicit coordination with other nodes, as
further detailed below.
A global RPLInstanceID is encoded in a RPLInstanceID field as
follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0| ID | Global RPLInstanceID in 0..127
+-+-+-+-+-+-+-+-+
Figure 4: RPLInstanceID Field Format for Global Instances
A local RPLInstanceID is autoconfigured by the node that owns the
DODAGID and it MUST be unique for that DODAGID. The DODAGID used to
configure the local RPLInstanceID MUST be a reachable IPv6 address of
the node, and it MUST be used as an endpoint of all communications
within that Local instance.
A local RPLInstanceID is encoded in a RPLInstanceID field as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1|D| ID | Local RPLInstanceID in 0..63
+-+-+-+-+-+-+-+-+
Figure 5: RPLInstanceID Field Format for Local Instances
The 'D' flag in a local RPLInstanceID is always set to 0 in RPL
control messages. It is used in data packets to indicate whether the
DODAGID is the source or the destination of the packet. If the 'D'
flag is set to 1, then the destination address of the IPv6 packet
MUST be the DODAGID. If the 'D' flag is cleared, then the source
address of the IPv6 packet MUST be the DODAGID.
For example, consider a Node A that is the DODAG root of a Local RPL
Instance, and has allocated a local RPLInstanceID. By definition,
all traffic traversing that Local RPL Instance will either originate
or terminate at Node A. In this case, the DODAGID will be the
Winter, et al. Standards Track [Page 29]
RFC 6550 RPL March 2012
reachable IPv6 address of Node A. All traffic will contain the
address of Node A, and thus the DODAGID, in either the source or
destination address. Thus, the local RPLInstanceID may indicate that
the DODAGID is equivalent to either the source address or the
destination address by setting the 'D' flag appropriately.
6. ICMPv6 RPL Control Message
This document defines the RPL control message, a new ICMPv6 [RFC4443]
message. A RPL control message is identified by a code and composed
of a base that depends on the code (and a series of options).
Most RPL control messages have the scope of a link. The only
exception is for the DAO / DAO-ACK messages in Non-Storing mode,
which are exchanged using a unicast address over multiple hops and
thus uses global or unique-local addresses for both the source and
destination addresses. For all other RPL control messages, the
source address is a link-local address, and the destination address
is either the all-RPL-nodes multicast address or a link-local unicast
address of the destination. The all-RPL-nodes multicast address is a
new address with a value of ff02::1a.
In accordance with [RFC4443], the RPL Control Message consists of an
ICMPv6 header followed by a message body. The message body is
comprised of a message base and possibly a number of options as
illustrated in Figure 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Base .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Option(s) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: RPL Control Message
The RPL control message is an ICMPv6 information message with a Type
of 155.
Winter, et al. Standards Track [Page 30]
RFC 6550 RPL March 2012
The Code field identifies the type of RPL control message. This
document defines codes for the following RPL control message types
(see Section 20.2)):
o 0x00: DODAG Information Solicitation (Section 6.2)
o 0x01: DODAG Information Object (Section 6.3)
o 0x02: Destination Advertisement Object (Section 6.4)
o 0x03: Destination Advertisement Object Acknowledgment
(Section 6.5)
o 0x80: Secure DODAG Information Solicitation (Section 6.2.2)
o 0x81: Secure DODAG Information Object (Section 6.3.2)
o 0x82: Secure Destination Advertisement Object (Section 6.4.2)
o 0x83: Secure Destination Advertisement Object Acknowledgment
(Section 6.5.2)
o 0x8A: Consistency Check (Section 6.6)
If a node receives a RPL control message with an unknown Code field,
the node MUST discard the message without any further processing, MAY
raise a management alert, and MUST NOT send any messages in response.
The checksum is computed as specified in [RFC4443]. It is set to
zero for the RPL security operations specified below and computed
once the rest of the content of the RPL message including the
security fields is all set.
The high order bit (0x80) of the code denotes whether the RPL message
has security enabled. Secure RPL messages have a format to support
confidentiality and integrity, illustrated in Figure 7.
Winter, et al. Standards Track [Page 31]
RFC 6550 RPL March 2012
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Security .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Base .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Option(s) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Secure RPL Control Message
The remainder of this section describes the currently defined RPL
control message Base formats followed by the currently defined RPL
Control Message options.
6.1. RPL Security Fields
Each RPL message has a secure variant. The secure variants provide
integrity and replay protection as well as optional confidentiality
and delay protection. Because security covers the base message as
well as options, in secured messages the security information lies
between the checksum and base, as shown in Figure 7.
The level of security and the algorithms in use are indicated in the
protocol messages as described below:
Winter, et al. Standards Track [Page 32]
RFC 6550 RPL March 2012
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T| Reserved | Algorithm |KIM|Resvd| LVL | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Key Identifier .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Security Section
Message Authentication Codes (MACs) and signatures provide
authentication over the entire unsecured ICMPv6 RPL control message,
including the Security section with all fields defined, but with the
ICMPv6 checksum temporarily set to zero. Encryption provides
confidentiality of the secured RPL ICMPv6 message starting at the
first byte after the Security section and continuing to the last byte
of the packet. The security transformation yields a secured ICMPv6
RPL message with the inclusion of the cryptographic fields (MAC,
signature, etc.). In other words, the security transformation itself
(e.g., the Signature and/or Algorithm in use) will detail how to
incorporate the cryptographic fields into the secured packet. The
Security section itself does not explicitly carry those cryptographic
fields. Use of the Security section is further detailed in Sections
19 and 10.
Counter is Time (T): If the counter's Time flag is set, then the
Counter field is a timestamp. If the flag is cleared, then the
counter is an incrementing counter. Section 10.5 describes the
details of the 'T' flag and Counter field.
Reserved: 7-bit unused field. The field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Security Algorithm (Algorithm): The Security Algorithm field
specifies the encryption, MAC, and signature scheme the network
uses. Supported values of this field are as follows:
Winter, et al. Standards Track [Page 33]
RFC 6550 RPL March 2012
+-----------+-------------------+------------------------+
| Algorithm | Encryption/MAC | Signature |
+-----------+-------------------+------------------------+
| 0 | CCM with AES-128 | RSA with SHA-256 |
| 1-255 | Unassigned | Unassigned |
+-----------+-------------------+------------------------+
Figure 9: Security Algorithm (Algorithm) Encoding
Section 10.9 describes the algorithms in greater detail.
Key Identifier Mode (KIM): The Key Identifier Mode is a 2-bit field
that indicates whether the key used for packet protection is
determined implicitly or explicitly and indicates the
particular representation of the Key Identifier field. The Key
Identifier Mode is set one of the values from the table below:
Winter, et al. Standards Track [Page 34]
RFC 6550 RPL March 2012
+------+-----+-----------------------------+------------+
| Mode | KIM | Meaning | Key |
| | | | Identifier |
| | | | Length |
| | | | (octets) |
+------+-----+-----------------------------+------------+
| 0 | 00 | Group key used. | 1 |
| | | Key determined by Key Index | |
| | | field. | |
| | | | |
| | | Key Source is not present. | |
| | | Key Index is present. | |
+------+-----+-----------------------------+------------+
| 1 | 01 | Per-pair key used. | 0 |
| | | Key determined by source | |
| | | and destination of packet. | |
| | | | |
| | | Key Source is not present. | |
| | | Key Index is not present. | |
+------+-----+-----------------------------+------------+
| 2 | 10 | Group key used. | 9 |
| | | Key determined by Key Index | |
| | | and Key Source Identifier. | |
| | | | |
| | | Key Source is present. | |
| | | Key Index is present. | |
+------+-----+-----------------------------+------------+
| 3 | 11 | Node's signature key used. | 0/9 |
| | | If packet is encrypted, |
| | | it uses a group key, Key | |
| | | Index and Key Source | |
| | | specify key. | |
| | | | |
| | | Key Source may be present. | |
| | | Key Index may be present. | |
+------+-----+-----------------------------+------------+
Figure 10: Key Identifier Mode (KIM) Encoding
In Mode 3 (KIM=11), the presence or absence of the Key Source and Key
Identifier depends on the Security Level (LVL) described below. If
the Security Level indicates there is encryption, then the fields are
present; if it indicates there is no encryption, then the fields are
not present.
Resvd: 3-bit unused field. The field MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Winter, et al. Standards Track [Page 35]
RFC 6550 RPL March 2012
Security Level (LVL): The Security Level is a 3-bit field that
indicates the provided packet protection. This value can be
adapted on a per-packet basis and allows for varying levels of
data authenticity and, optionally, for data confidentiality.
The KIM field indicates whether signatures are used and the
meaning of the Level field. Note that the assigned values of
Security Level are not necessarily ordered -- a higher value of
LVL does not necessarily equate to increased security. The
Security Level is set to one of the values in the tables below:
+---------------------------+
| KIM=0,1,2 |
+-------+--------------------+------+
| LVL | Attributes | MAC |
| | | Len |
+-------+--------------------+------+
| 0 | MAC-32 | 4 |
| 1 | ENC-MAC-32 | 4 |
| 2 | MAC-64 | 8 |
| 3 | ENC-MAC-64 | 8 |
| 4-7 | Unassigned | N/A |
+-------+--------------------+------+
+---------------------+
| KIM=3 |
+-------+---------------+-----+
| LVL | Attributes | Sig |
| | | Len |
+-------+---------------+-----+
| 0 | Sign-3072 | 384 |
| 1 | ENC-Sign-3072 | 384 |
| 2 | Sign-2048 | 256 |
| 3 | ENC-Sign-2048 | 256 |
| 4-7 | Unassigned | N/A |
+-------+---------------+-----+
Figure 11: Security Level (LVL) Encoding
The MAC attribute indicates that the message has a MAC of the
specified length. The ENC attribute indicates that the message is
encrypted. The Sign attribute indicates that the message has a
signature of the specified length.
Winter, et al. Standards Track [Page 36]
RFC 6550 RPL March 2012
Flags: 8-bit unused field reserved for flags. The field MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Counter: The Counter field indicates the non-repeating 4-octet value
used to construct the cryptographic mechanism that implements
packet protection and allows for the provision of semantic
security. See Section 10.9.1.
Key Identifier: The Key Identifier field indicates which key was used
to protect the packet. This field provides various levels of
granularity of packet protection, including peer-to-peer keys,
group keys, and signature keys. This field is represented as
indicated by the Key Identifier Mode field and is formatted as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Key Source .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Key Index .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Key Identifier
Key Source: The Key Source field, when present, indicates the logical
identifier of the originator of a group key. When present,
this field is 8 bytes in length.
Key Index: The Key Index field, when present, allows unique
identification of different keys with the same originator. It
is the responsibility of each key originator to make sure that
actively used keys that it issues have distinct key indices and
that all key indices have a value unequal to 0x00. Value 0x00
is reserved for a preinstalled, shared key. When present this
field is 1 byte in length.
Unassigned bits of the Security section are reserved. They MUST be
set to zero on transmission and MUST be ignored on reception.
Winter, et al. Standards Track [Page 37]
RFC 6550 RPL March 2012
6.2. DODAG Information Solicitation (DIS)
The DODAG Information Solicitation (DIS) message may be used to
solicit a DODAG Information Object from a RPL node. Its use is
analogous to that of a Router Solicitation as specified in IPv6
Neighbor Discovery; a node may use DIS to probe its neighborhood for
nearby DODAGs. Section 8.3 describes how nodes respond to a DIS.
6.2.1. Format of the DIS Base Object
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Reserved | Option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: The DIS Base Object
Flags: 8-bit unused field reserved for flags. The field MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Reserved: 8-bit unused field. The field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Unassigned bits of the DIS Base are reserved. They MUST be set to
zero on transmission and MUST be ignored on reception.
6.2.2. Secure DIS
A Secure DIS message follows the format in Figure 7, where the base
format is the DIS message shown in Figure 13.
6.2.3. DIS Options
The DIS message MAY carry valid options.
This specification allows for the DIS message to carry the following
options:
0x00 Pad1
0x01 PadN
0x07 Solicited Information
6.3. DODAG Information Object (DIO)
The DODAG Information Object carries information that allows a node
to discover a RPL Instance, learn its configuration parameters,
Winter, et al. Standards Track [Page 38]
RFC 6550 RPL March 2012
select a DODAG parent set, and maintain the DODAG.
6.3.1. Format of the DIO Base Object
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |Version Number | Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|0| MOP | Prf | DTSN | Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 14: The DIO Base Object
Grounded (G): The Grounded 'G' flag indicates whether the DODAG
advertised can satisfy the application-defined goal. If the
flag is set, the DODAG is grounded. If the flag is cleared,
the DODAG is floating.
Mode of Operation (MOP): The Mode of Operation (MOP) field identifies
the mode of operation of the RPL Instance as administratively
provisioned at and distributed by the DODAG root. All nodes
who join the DODAG must be able to honor the MOP in order to
fully participate as a router, or else they must only join as a
leaf. MOP is encoded as in the figure below:
Winter, et al. Standards Track [Page 39]
RFC 6550 RPL March 2012
+-----+-----------------------------------------------------+
| MOP | Description |
+-----+-----------------------------------------------------+
| 0 | No Downward routes maintained by RPL |
| 1 | Non-Storing Mode of Operation |
| 2 | Storing Mode of Operation with no multicast support |
| 3 | Storing Mode of Operation with multicast support |
| | |
| | All other values are unassigned |
+-----+-----------------------------------------------------+
A value of 0 indicates that destination advertisement messages are
disabled and the DODAG maintains only Upward routes.
Figure 15: Mode of Operation (MOP) Encoding
DODAGPreference (Prf): A 3-bit unsigned integer that defines how
preferable the root of this DODAG is compared to other DODAG
roots within the instance. DAGPreference ranges from 0x00
(least preferred) to 0x07 (most preferred). The default is 0
(least preferred). Section 8.2 describes how DAGPreference
affects DIO processing.
Version Number: 8-bit unsigned integer set by the DODAG root to the
DODAGVersionNumber. Section 8.2 describes the rules for
DODAGVersionNumbers and how they affect DIO processing.
Rank: 16-bit unsigned integer indicating the DODAG Rank of the node
sending the DIO message. Section 8.2 describes how Rank is set
and how it affects DIO processing.
RPLInstanceID: 8-bit field set by the DODAG root that indicates of
which RPL Instance the DODAG is a part.
Destination Advertisement Trigger Sequence Number (DTSN): 8-bit
unsigned integer set by the node issuing the DIO message. The
Destination Advertisement Trigger Sequence Number (DTSN) flag
is used as part of the procedure to maintain Downward routes.
The details of this process are described in Section 9.
Flags: 8-bit unused field reserved for flags. The field MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Reserved: 8-bit unused field. The field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Winter, et al. Standards Track [Page 40]
RFC 6550 RPL March 2012
DODAGID: 128-bit IPv6 address set by a DODAG root that uniquely
identifies a DODAG. The DODAGID MUST be a routable IPv6
address belonging to the DODAG root.
Unassigned bits of the DIO Base are reserved. They MUST be set to
zero on transmission and MUST be ignored on reception.
6.3.2. Secure DIO
A Secure DIO message follows the format in Figure 7, where the base
format is the DIO message shown in Figure 14.
6.3.3. DIO Options
The DIO message MAY carry valid options.
This specification allows for the DIO message to carry the following
options:
0x00 Pad1
0x01 PadN
0x02 DAG Metric Container
0x03 Routing Information
0x04 DODAG Configuration
0x08 Prefix Information
6.4. Destination Advertisement Object (DAO)
The Destination Advertisement Object (DAO) is used to propagate
destination information Upward along the DODAG. In Storing mode, the
DAO message is unicast by the child to the selected parent(s). In
Non-Storing mode, the DAO message is unicast to the DODAG root. The
DAO message may optionally, upon explicit request or error, be
acknowledged by its destination with a Destination Advertisement
Acknowledgement (DAO-ACK) message back to the sender of the DAO.
Winter, et al. Standards Track [Page 41]
RFC 6550 RPL March 2012
6.4.1. Format of the DAO Base Object
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |K|D| Flags | Reserved | DAOSequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID* +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
The '*' denotes that the DODAGID is not always present, as described
below.
Figure 16: The DAO Base Object
RPLInstanceID: 8-bit field indicating the topology instance
associated with the DODAG, as learned from the DIO.
K: The 'K' flag indicates that the recipient is expected to send a
DAO-ACK back. (See Section 9.3.)
D: The 'D' flag indicates that the DODAGID field is present. This
flag MUST be set when a local RPLInstanceID is used.
Flags: The 6 bits remaining unused in the Flags field are reserved
for flags. The field MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
Reserved: 8-bit unused field. The field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
DAOSequence: Incremented at each unique DAO message from a node and
echoed in the DAO-ACK message.
DODAGID (optional): 128-bit unsigned integer set by a DODAG root that
uniquely identifies a DODAG. This field is only present when
the 'D' flag is set. This field is typically only present when
a local RPLInstanceID is in use, in order to identify the
DODAGID that is associated with the RPLInstanceID. When a
global RPLInstanceID is in use, this field need not be present.
Winter, et al. Standards Track [Page 42]
RFC 6550 RPL March 2012
Unassigned bits of the DAO Base are reserved. They MUST be set to
zero on transmission and MUST be ignored on reception.
6.4.2. Secure DAO
A Secure DAO message follows the format in Figure 7, where the base
format is the DAO message shown in Figure 16.
6.4.3. DAO Options
The DAO message MAY carry valid options.
This specification allows for the DAO message to carry the following
options:
0x00 Pad1
0x01 PadN
0x05 RPL Target
0x06 Transit Information
0x09 RPL Target Descriptor
A special case of the DAO message, termed a No-Path, is used in
Storing mode to clear Downward routing state that has been
provisioned through DAO operation. The No-Path carries a Target
option and an associated Transit Information option with a lifetime
of 0x00000000 to indicate a loss of reachability to that Target.
6.5. Destination Advertisement Object Acknowledgement (DAO-ACK)
The DAO-ACK message is sent as a unicast packet by a DAO recipient (a
DAO parent or DODAG root) in response to a unicast DAO message.
Winter, et al. Standards Track [Page 43]
RFC 6550 RPL March 2012
6.5.1. Format of the DAO-ACK Base Object
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |D| Reserved | DAOSequence | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID* +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
The '*' denotes that the DODAGID is not always present, as described
below.
Figure 17: The DAO ACK Base Object
RPLInstanceID: 8-bit field indicating the topology instance
associated with the DODAG, as learned from the DIO.
D: The 'D' flag indicates that the DODAGID field is present. This
would typically only be set when a local RPLInstanceID is used.
Reserved: The 7-bit field, reserved for flags.
DAOSequence: Incremented at each DAO message from a node, and echoed
in the DAO-ACK by the recipient. The DAOSequence is used to
correlate a DAO message and a DAO ACK message and is not to be
confused with the Transit Information option Path Sequence that
is associated to a given Target Down the DODAG.
Status: Indicates the completion. Status 0 is defined as unqualified
acceptance in this specification. The remaining status values
are reserved as rejection codes. No rejection status codes are
defined in this specification, although status codes SHOULD be
allocated according to the following guidelines in future
specifications:
0: Unqualified acceptance (i.e., the node receiving the
DAO-ACK is not rejected).
Winter, et al. Standards Track [Page 44]
RFC 6550 RPL March 2012
1-127: Not an outright rejection; the node sending the DAO-ACK
is willing to act as a parent, but the receiving node is
suggested to find and use an alternate parent instead.
127-255: Rejection; the node sending the DAO-ACK is unwilling to
act as a parent.
DODAGID (optional): 128-bit unsigned integer set by a DODAG root that
uniquely identifies a DODAG. This field is only present
when the 'D' flag is set. Typically, this field is only
present when a local RPLInstanceID is in use in order to
identify the DODAGID that is associated with the
RPLInstanceID. When a global RPLInstanceID is in use,
this field need not be present.
Unassigned bits of the DAO-ACK Base are reserved. They MUST be set
to zero on transmission and MUST be ignored on reception.
6.5.2. Secure DAO-ACK
A Secure DAO-ACK message follows the format in Figure 7, where the
base format is the DAO-ACK message shown in Figure 17.
6.5.3. DAO-ACK Options
This specification does not define any options to be carried by the
DAO-ACK message.
6.6. Consistency Check (CC)
The CC message is used to check secure message counters and issue
challenge-responses. A CC message MUST be sent as a secured RPL
message.
Winter, et al. Standards Track [Page 45]
RFC 6550 RPL March 2012
6.6.1. Format of the CC Base Object
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |R| Flags | CC Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option(s)...
+-+-+-+-+-+-+-+-+
Figure 18: The CC Base Object
RPLInstanceID: 8-bit field indicating the topology instance
associated with the DODAG, as learned from the DIO.
R: The 'R' flag indicates whether the CC message is a response. A
message with the 'R' flag cleared is a request; a message with
the 'R' flag set is a response.
Flags: The 7 bits remaining unused in the Flags field are reserved
for flags. The field MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
CC Nonce: 16-bit unsigned integer set by a CC request. The
corresponding CC response includes the same CC nonce value as
the request.
DODAGID: 128-bit field, contains the identifier of the DODAG root.
Destination Counter: 32-bit unsigned integer value indicating the
sender's estimate of the destination's current security counter
value. If the sender does not have an estimate, it SHOULD set
the Destination Counter field to zero.
Unassigned bits of the CC Base are reserved. They MUST be set to
zero on transmission and MUST be ignored on reception.
Winter, et al. Standards Track [Page 46]
RFC 6550 RPL March 2012
The Destination Counter value allows new or recovered nodes to
resynchronize through CC message exchanges. This is important to
ensure that a Counter value is not repeated for a given security key
even in the event of devices recovering from a failure that created a
loss of Counter state. For example, where a CC request or other RPL
message is received with an initialized counter within the message
Security section, the provision of the Incoming Counter within the CC
response message allows the requesting node to reset its Outgoing
Counter to a value greater than the last value received by the
responding node; the Incoming Counter will also be updated from the
received CC response.
6.6.2. CC Options
This specification allows for the CC message to carry the following
options:
0x00 Pad1
0x01 PadN
6.7. RPL Control Message Options
6.7.1. RPL Control Message Option Generic Format
RPL Control Message options all follow this format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Option Type | Option Length | Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 19: RPL Option Generic Format
Option Type: 8-bit identifier of the type of option. The Option Type
values are assigned by IANA (see Section 20.4.)
Option Length: 8-bit unsigned integer, representing the length in
octets of the option, not including the Option Type and Length
fields.
Option Data: A variable length field that contains data specific to
the option.
Winter, et al. Standards Track [Page 47]
RFC 6550 RPL March 2012
When processing a RPL message containing an option for which the
Option Type value is not recognized by the receiver, the receiver
MUST silently ignore the unrecognized option and continue to process
the following option, correctly handling any remaining options in the
message.
RPL message options may have alignment requirements. Following the
convention in IPv6, options with alignment requirements are aligned
in a packet such that multi-octet values within the Option Data field
of each option fall on natural boundaries (i.e., fields of width n
octets are placed at an integer multiple of n octets from the start
of the header, for n = 1, 2, 4, or 8).
6.7.2. Pad1
The Pad1 option MAY be present in DIS, DIO, DAO, DAO-ACK, and CC
messages, and its format is as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0x00 |
+-+-+-+-+-+-+-+-+
Figure 20: Format of the Pad1 Option
The Pad1 option is used to insert a single octet of padding into the
message to enable options alignment. If more than one octet of
padding is required, the PadN option should be used rather than
multiple Pad1 options.
NOTE! The format of the Pad1 option is a special case -- it has
neither Option Length nor Option Data fields.
6.7.3. PadN
The PadN option MAY be present in DIS, DIO, DAO, DAO-ACK, and CC
messages, and its format is as follows:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 0x01 | Option Length | 0x00 Padding...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 21: Format of the Pad N Option
Winter, et al. Standards Track [Page 48]
RFC 6550 RPL March 2012
The PadN option is used to insert two or more octets of padding into
the message to enable options alignment. PadN option data MUST be
ignored by the receiver.
Option Type: 0x01
Option Length: For N octets of padding, where 2 <= N <= 7, the Option
Length field contains the value N-2. An Option Length of 0
indicates a total padding of 2 octets. An Option Length of 5
indicates a total padding of 7 octets, which is the maximum
padding size allowed with the PadN option.
Option Data: For N (N > 1) octets of padding, the Option Data
consists of N-2 zero-valued octets.
6.7.4. DAG Metric Container
The DAG Metric Container option MAY be present in DIO or DAO
messages, and its format is as follows:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 0x02 | Option Length | Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 22: Format of the DAG Metric Container Option
The DAG Metric Container is used to report metrics along the DODAG.
The DAG Metric Container may contain a number of discrete node, link,
and aggregate path metrics and constraints specified in [RFC6551] as
chosen by the implementer.
The DAG Metric Container MAY appear more than once in the same RPL
control message, for example, to accommodate a use case where the
Metric Data is longer than 256 bytes. More information is in
[RFC6551].
The processing and propagation of the DAG Metric Container is
governed by implementation specific policy functions.
Option Type: 0x02
Option Length: The Option Length field contains the length in octets
of the Metric Data.
Winter, et al. Standards Track [Page 49]
RFC 6550 RPL March 2012
Metric Data: The order, content, and coding of the DAG Metric
Container data is as specified in [RFC6551].
6.7.5. Route Information
The Route Information Option (RIO) MAY be present in DIO messages,
and it carries the same information as the IPv6 Neighbor Discovery
(ND) RIO as defined in [RFC4191]. The root of a DODAG is
authoritative for setting that information and the information is
unchanged as propagated down the DODAG. A RPL router may trivially
transform it back into an ND option to advertise in its own RAs so a
node attached to the RPL router will end up using the DODAG for which
the root has the best preference for the destination of a packet. In
addition to the existing ND semantics, it is possible for an
Objective Function to use this information to favor a DODAG whose
root is most preferred for a specific destination. The format of the
option is modified slightly (Type, Length, Prefix) in order to be
carried as a RPL option as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x03 | Option Length | Prefix Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Prefix (Variable Length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23: Format of the Route Information Option
The RIO is used to indicate that connectivity to the specified
destination prefix is available from the DODAG root.
In the event that a RPL control message may need to specify
connectivity to more than one destination, the RIO may be repeated.
[RFC4191] should be consulted as the authoritative reference with
respect to the RIO. The field descriptions are transcribed here for
convenience:
Option Type: 0x03
Winter, et al. Standards Track [Page 50]
RFC 6550 RPL March 2012
Option Length: Variable, length of the option in octets excluding the
Type and Length fields. Note that this length is expressed in
units of single octets, unlike in IPv6 ND.
Prefix Length: 8-bit unsigned integer. The number of leading bits in
the prefix that are valid. The value ranges from 0 to 128.
The Prefix field has the number of bytes inferred from the
Option Length field, that must be at least the Prefix Length.
Note that in RPL, this means that the Prefix field may have
lengths other than 0, 8, or 16.
Prf: 2-bit signed integer. The Route Preference indicates whether to
prefer the router associated with this prefix over others, when
multiple identical prefixes (for different routers) have been
received. If the Reserved (10) value is received, the RIO MUST
be ignored. Per [RFC4191], the Reserved (10) value MUST NOT be
sent. ([RFC4191] restricts the Preference to just three values
to reinforce that it is not a metric.)
Resvd: Two 3-bit unused fields. They MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Route Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all one
bits (0xFFFFFFFF) represents infinity.
Prefix: Variable-length field containing an IP address or a prefix of
an IPv6 address. The Prefix Length field contains the number
of valid leading bits in the prefix. The bits in the prefix
after the prefix length (if any) are reserved and MUST be
initialized to zero by the sender and ignored by the receiver.
Note that in RPL, this field may have lengths other than 0, 8,
or 16.
Unassigned bits of the RIO are reserved. They MUST be set to zero on
transmission and MUST be ignored on reception.
Winter, et al. Standards Track [Page 51]
RFC 6550 RPL March 2012
6.7.6. DODAG Configuration
The DODAG Configuration option MAY be present in DIO messages, and
its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x04 |Opt Length = 14| Flags |A| PCS | DIOIntDoubl. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntMin. | DIORedun. | MaxRankIncrease |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MinHopRankIncrease | OCP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Def. Lifetime | Lifetime Unit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24: Format of the DODAG Configuration Option
The DODAG Configuration option is used to distribute configuration
information for DODAG Operation through the DODAG.
The information communicated in this option is generally static and
unchanging within the DODAG, therefore it is not necessary to include
in every DIO. This information is configured at the DODAG root and
distributed throughout the DODAG with the DODAG Configuration option.
Nodes other than the DODAG root MUST NOT modify this information when
propagating the DODAG Configuration option. This option MAY be
included occasionally by the DODAG root (as determined by the DODAG
root), and MUST be included in response to a unicast request, e.g. a
unicast DODAG Information Solicitation (DIS) message.
Option Type: 0x04
Option Length: 14
Flags: The 4-bits remaining unused in the Flags field are reserved
for flags. The field MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
Authentication Enabled (A): 1-bit flag describing the security mode
of the network. The bit describes whether a node must
authenticate with a key authority before joining the network as
a router. If the DIO is not a secure DIO, the 'A' bit MUST be
zero.
Winter, et al. Standards Track [Page 52]
RFC 6550 RPL March 2012
Path Control Size (PCS): 3-bit unsigned integer used to configure the
number of bits that may be allocated to the Path Control field
(see Section 9.9). Note that when PCS is consulted to
determine the width of the Path Control field, a value of 1 is
added, i.e., a PCS value of 0 results in 1 active bit in the
Path Control field. The default value of PCS is
DEFAULT_PATH_CONTROL_SIZE.
DIOIntervalDoublings: 8-bit unsigned integer used to configure Imax
of the DIO Trickle timer (see Section 8.3.1). The default
value of DIOIntervalDoublings is
DEFAULT_DIO_INTERVAL_DOUBLINGS.
DIOIntervalMin: 8-bit unsigned integer used to configure Imin of the
DIO Trickle timer (see Section 8.3.1). The default value of
DIOIntervalMin is DEFAULT_DIO_INTERVAL_MIN.
DIORedundancyConstant: 8-bit unsigned integer used to configure k of
the DIO Trickle timer (see Section 8.3.1). The default value
of DIORedundancyConstant is DEFAULT_DIO_REDUNDANCY_CONSTANT.
MaxRankIncrease: 16-bit unsigned integer used to configure
DAGMaxRankIncrease, the allowable increase in Rank in support
of local repair. If DAGMaxRankIncrease is 0, then this
mechanism is disabled.
MinHopRankIncrease: 16-bit unsigned integer used to configure
MinHopRankIncrease as described in Section 3.5.1. The default
value of MinHopRankInc is DEFAULT_MIN_HOP_RANK_INCREASE.
Objective Code Point (OCP): 16-bit unsigned integer. The OCP field
identifies the OF and is managed by the IANA.
Reserved: 7-bit unused field. The field MUST be initialized to zero
by the sender and MUST be ignored by the receiver.
Default Lifetime: 8-bit unsigned integer. This is the lifetime that
is used as default for all RPL routes. It is expressed in
units of Lifetime Units, e.g., the default lifetime in seconds
is (Default Lifetime) * (Lifetime Unit).
Lifetime Unit: 16-bit unsigned integer. Provides the unit in seconds
that is used to express route lifetimes in RPL. For very
stable networks, it can be hours to days.
Winter, et al. Standards Track [Page 53]
RFC 6550 RPL March 2012
6.7.7. RPL Target
The RPL Target option MAY be present in DAO messages, and its format
is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x05 | Option Length | Flags | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Target Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25: Format of the RPL Target Option
The RPL Target option is used to indicate a Target IPv6 address,
prefix, or multicast group that is reachable or queried along the
DODAG. In a DAO, the RPL Target option indicates reachability.
A RPL Target option MAY optionally be paired with a RPL Target
Descriptor option (Figure 30) that qualifies the target.
A set of one or more Transit Information options (Section 6.7.8) MAY
directly follow a set of one or more Target options in a DAO message
(where each Target option MAY be paired with a RPL Target Descriptor
option as above). The structure of the DAO message, detailing how
Target options are used in conjunction with Transit Information
options is further described in Section 9.4.
The RPL Target option may be repeated as necessary to indicate
multiple targets.
Option Type: 0x05
Option Length: Variable, length of the option in octets excluding the
Type and Length fields.
Flags: 8-bit unused field reserved for flags. The field MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Prefix Length: 8-bit unsigned integer. Number of valid leading bits
in the IPv6 Prefix.
Winter, et al. Standards Track [Page 54]
RFC 6550 RPL March 2012
Target Prefix: Variable-length field identifying an IPv6 destination
address, prefix, or multicast group. The Prefix Length field
contains the number of valid leading bits in the prefix. The
bits in the prefix after the prefix length (if any) are
reserved and MUST be set to zero on transmission and MUST be
ignored on receipt.
6.7.8. Transit Information
The Transit Information option MAY be present in DAO messages, and
its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x06 | Option Length |E| Flags | Path Control |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path Sequence | Path Lifetime | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| |
+ Parent Address* +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The '*' denotes that the DODAG Parent Address subfield is not always
present, as described below.
Figure 26: Format of the Transit Information Option
The Transit Information option is used for a node to indicate
attributes for a path to one or more destinations. The destinations
are indicated by one or more Target options that immediately precede
the Transit Information option(s).
The Transit Information option can be used for a node to indicate its
DODAG parents to an ancestor that is collecting DODAG routing
information, typically, for the purpose of constructing source
routes. In the Non-Storing mode of operation, this ancestor will be
the DODAG root, and this option is carried by the DAO message. In
the Storing mode of operation, the DODAG Parent Address subfield is
not needed, since the DAO message is sent directly to the parent.
The option length is used to determine whether or not the DODAG
Parent Address subfield is present.
Winter, et al. Standards Track [Page 55]
RFC 6550 RPL March 2012
A non-storing node that has more than one DAO parent MAY include a
Transit Information option for each DAO parent as part of the non-
storing destination advertisement operation. The node may distribute
the bits in the Path Control field among different groups of DAO
parents in order to signal a preference among parents. That
preference may influence the decision of the DODAG root when
selecting among the alternate parents/paths for constructing Downward
routes.
One or more Transit Information options MUST be preceded by one or
more RPL Target options. In this manner, the RPL Target option
indicates the child node, and the Transit Information option(s)
enumerates the DODAG parents. The structure of the DAO message,
further detailing how Target options are used in conjunction with
Transit Information options, is further described in Section 9.4.
A typical non-storing node will use multiple Transit Information
options, and it will send the DAO message thus formed directly to the
root. A typical storing node will use one Transit Information option
with no parent field and will send the DAO message thus formed, with
additional adjustments, to Path Control as detailed later, to one or
multiple parents.
For example, in a Non-Storing mode of operation let Tgt(T) denote a
Target option for a Target T. Let Trnst(P) denote a Transit
Information option that contains a parent address P. Consider the
case of a non-storing Node N that advertises the self-owned targets
N1 and N2 and has parents P1, P2, and P3. In that case, the DAO
message would be expected to contain the sequence ((Tgt(N1),
Tgt(N2)), (Trnst(P1), Trnst(P2), Trnst(P3))), such that the group of
Target options {N1, N2} is described by the Transit Information
options as having the parents {P1, P2, P3}. The non-storing node
would then address that DAO message directly to the DODAG root and
forward that DAO message through one of the DODAG parents: P1, P2, or
P3.
Option Type: 0x06
Option Length: Variable, depending on whether or not the DODAG Parent
Address subfield is present.
External (E): 1-bit flag. The 'E' flag is set to indicate that the
parent router redistributes external targets into the RPL
network. An external Target is a Target that has been learned
through an alternate protocol. The external targets are listed
in the Target options that immediately precede the Transit
Information option. An external Target is not expected to
support RPL messages and options.
Winter, et al. Standards Track [Page 56]
RFC 6550 RPL March 2012
Flags: The 7 bits remaining unused in the Flags field are reserved
for flags. The field MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
Path Control: 8-bit bit field. The Path Control field limits the
number of DAO parents to which a DAO message advertising
connectivity to a specific destination may be sent, as well as
providing some indication of relative preference. The limit
provides some bound on overall DAO message fan-out in the LLN.
The assignment and ordering of the bits in the Path Control
also serves to communicate preference. Not all of these bits
may be enabled as according to the PCS in the DODAG
Configuration. The Path Control field is divided into four
subfields that contain two bits each: PC1, PC2, PC3, and PC4,
as illustrated in Figure 27. The subfields are ordered by
preference, with PC1 being the most preferred and PC4 being the
least preferred. Within a subfield, there is no order of
preference. By grouping the parents (as in ECMP) and ordering
them, the parents may be associated with specific bits in the
Path Control field in a way that communicates preference.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|PC1|PC2|PC3|PC4|
+-+-+-+-+-+-+-+-+
Figure 27: Path Control Preference Subfield Encoding
Path Sequence: 8-bit unsigned integer. When a RPL Target option is
issued by the node that owns the Target prefix (i.e., in a DAO
message), that node sets the Path Sequence and increments the
Path Sequence each time it issues a RPL Target option with
updated information.
Path Lifetime: 8-bit unsigned integer. The length of time in
Lifetime Units (obtained from the Configuration option) that
the prefix is valid for route determination. The period starts
when a new Path Sequence is seen. A value of all one bits
(0xFF) represents infinity. A value of all zero bits (0x00)
indicates a loss of reachability. A DAO message that contains
a Transit Information option with a Path Lifetime of 0x00 for a
Target is referred as a No-Path (for that Target) in this
document.
Winter, et al. Standards Track [Page 57]
RFC 6550 RPL March 2012
Parent Address (optional): IPv6 address of the DODAG parent of the
node originally issuing the Transit Information option. This
field may not be present, as according to the DODAG Mode of
Operation (Storing or Non-Storing) and indicated by the Transit
Information option length.
Unassigned bits of the Transit Information option are reserved. They
MUST be set to zero on transmission and MUST be ignored on reception.
6.7.9. Solicited Information
The Solicited Information option MAY be present in DIS messages, and
its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x07 |Opt Length = 19| RPLInstanceID |V|I|D| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ DODAGID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version Number |
+-+-+-+-+-+-+-+-+
Figure 28: Format of the Solicited Information Option
The Solicited Information option is used for a node to request DIO
messages from a subset of neighboring nodes. The Solicited
Information option may specify a number of predicate criteria to be
matched by a receiving node. This is used by the requester to limit
the number of replies from "non-interesting" nodes. These predicates
affect whether a node resets its DIO Trickle timer, as described in
Section 8.3.
The Solicited Information option contains flags that indicate which
predicates a node should check when deciding whether to reset its
Trickle timer. A node resets its Trickle timer when all predicates
are true. If a flag is set, then the RPL node MUST check the
associated predicate. If a flag is cleared, then the RPL node MUST
NOT check the associated predicate. (If a flag is cleared, the RPL
node assumes that the associated predicate is true.)
Winter, et al. Standards Track [Page 58]
RFC 6550 RPL March 2012
Option Type: 0x07
Option Length: 19
V: The 'V' flag is the Version predicate. The Version predicate is
true if the receiver's DODAGVersionNumber matches the requested
Version Number. If the 'V' flag is cleared, then the Version
field is not valid and the Version field MUST be set to zero on
transmission and ignored upon receipt.
I: The 'I' flag is the InstanceID predicate. The InstanceID
predicate is true when the RPL node's current RPLInstanceID
matches the requested RPLInstanceID. If the 'I' flag is
cleared, then the RPLInstanceID field is not valid and the
RPLInstanceID field MUST be set to zero on transmission and
ignored upon receipt.
D: The 'D' flag is the DODAGID predicate. The DODAGID predicate is
true if the RPL node's parent set has the same DODAGID as the
DODAGID field. If the 'D' flag is cleared, then the DODAGID
field is not valid and the DODAGID field MUST be set to zero on
transmission and ignored upon receipt.
Flags: The 5 bits remaining unused in the Flags field are reserved
for flags. The field MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
Version Number: 8-bit unsigned integer containing the value of
DODAGVersionNumber that is being solicited when valid.
RPLInstanceID: 8-bit unsigned integer containing the RPLInstanceID
that is being solicited when valid.
DODAGID: 128-bit unsigned integer containing the DODAGID that is
being solicited when valid.
Unassigned bits of the Solicited Information option are reserved.
They MUST be set to zero on transmission and MUST be ignored on
reception.
6.7.10. Prefix Information
The Prefix Information Option (PIO) MAY be present in DIO messages,
and carries the information that is specified for the IPv6 ND Prefix
Information option in [RFC4861], [RFC4862], and [RFC6275] for use by
RPL nodes and IPv6 hosts. In particular, a RPL node may use this
option for the purpose of Stateless Address Autoconfiguration (SLAAC)
from a prefix advertised by a parent as specified in [RFC4862], and
Winter, et al. Standards Track [Page 59]
RFC 6550 RPL March 2012
advertise its own address as specified in [RFC6275]. The root of a
DODAG is authoritative for setting that information. The information
is propagated down the DODAG unchanged, with the exception that a RPL
router may overwrite the Interface ID if the 'R' flag is set to
indicate its full address in the PIO. The format of the option is
modified (Type, Length, Prefix) in order to be carried as a RPL
option as follows:
If the only desired effect of a received PIO in a DIO is to provide
the global address of the parent node to the receiving node, then the
sender resets the 'A' and 'L' bits and sets the 'R' bit. Upon
receipt, the RPL will not autoconfigure an address or a connected
route from the prefix [RFC4862]. As in all cases, when the 'L' bit
is not set, the RPL node MAY include the prefix in PIOs it sends to
its children.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x08 |Opt Length = 30| Prefix Length |L|A|R|Reserved1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29: Format of the Prefix Information Option
The PIO may be used to distribute the prefix in use inside the DODAG,
e.g., for address autoconfiguration.
[RFC4861] and [RFC6275] should be consulted as the authoritative
reference with respect to the PIO. The field descriptions are
transcribed here for convenience:
Option Type: 0x08
Winter, et al. Standards Track [Page 60]
RFC 6550 RPL March 2012
Option Length: 30. Note that this length is expressed in units of
single octets, unlike in IPv6 ND.
Prefix Length: 8-bit unsigned integer. The number of leading bits in
the Prefix field that are valid. The value ranges from 0 to
128. The Prefix Length field provides necessary information
for on-link determination (when combined with the 'L' flag in
the PIO). It also assists with address autoconfiguration as
specified in [RFC4862], for which there may be more
restrictions on the prefix length.
L: 1-bit on-link flag. When set, it indicates that this prefix
can be used for on-link determination. When not set, the
advertisement makes no statement about on-link or off-link
properties of the prefix. In other words, if the 'L' flag is
not set, a RPL node MUST NOT conclude that an address derived
from the prefix is off-link. That is, it MUST NOT update a
previous indication that the address is on-link. A RPL node
acting as a router MUST NOT propagate a PIO with the 'L' flag
set. A RPL node acting as a router MAY propagate a PIO with
the 'L' flag not set.
A: 1-bit autonomous address-configuration flag. When set, it
indicates that this prefix can be used for stateless address
configuration as specified in [RFC4862]. When both protocols
(ND RAs and RPL DIOs) are used to carry PIOs on the same link,
it is possible to use either one for SLAAC by a RPL node. It
is also possible to make either protocol ineligible for SLAAC
operation by forcing the 'A' flag to 0 for PIOs carried in that
protocol.
R: 1-bit router address flag. When set, it indicates that the
Prefix field contains a complete IPv6 address assigned to the
sending router that can be used as parent in a target option.
The indicated prefix is the first prefix length bits of the
Prefix field. The router IPv6 address has the same scope and
conforms to the same lifetime values as the advertised prefix.
This use of the Prefix field is compatible with its use in
advertising the prefix itself, since Prefix Advertisement uses
only the leading bits. Interpretation of this flag bit is thus
independent of the processing required for the on-link (L) and
autonomous address-configuration (A) flag bits.
Reserved1: 5-bit unused field. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Winter, et al. Standards Track [Page 61]
RFC 6550 RPL March 2012
Valid Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that the
prefix is valid for the purpose of on-link determination. A
value of all one bits (0xFFFFFFFF) represents infinity. The
Valid Lifetime is also used by [RFC4862].
Preferred Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that
addresses generated from the prefix via stateless address
autoconfiguration remain preferred [RFC4862]. A value of all
one bits (0xFFFFFFFF) represents infinity. See [RFC4862].
Note that the value of this field MUST NOT exceed the Valid
Lifetime field to avoid preferring addresses that are no longer
valid.
Reserved2: This field is unused. It MUST be initialized to zero by
the sender and MUST be ignored by the receiver.
Prefix: An IPv6 address or a prefix of an IPv6 address. The Prefix
Length field contains the number of valid leading bits in the
prefix. The bits in the prefix after the prefix length are
reserved and MUST be initialized to zero by the sender and
ignored by the receiver. A router SHOULD NOT send a prefix
option for the link-local prefix, and a host SHOULD ignore such
a prefix option. A non-storing node SHOULD refrain from
advertising a prefix till it owns an address of that prefix,
and then it SHOULD advertise its full address in this field,
with the 'R' flag set. The children of a node that so
advertises a full address with the 'R' flag set may then use
that address to determine the content of the DODAG Parent
Address subfield of the Transit Information option.
Unassigned bits of the PIO are reserved. They MUST be set to zero on
transmission and MUST be ignored on reception.
Winter, et al. Standards Track [Page 62]
RFC 6550 RPL March 2012
6.7.11. RPL Target Descriptor
The RPL Target option MAY be immediately followed by one opaque
descriptor that qualifies that specific target.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x09 |Opt Length = 4 | Descriptor
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Descriptor (cont.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30: Format of the RPL Target Descriptor Option
The RPL Target Descriptor option is used to qualify a target,
something that is sometimes called "tagging".
At most, there can be one descriptor per target. The descriptor is
set by the node that injects the Target in the RPL network. It MUST
be copied but not modified by routers that propagate the Target Up
the DODAG in DAO messages.
Option Type: 0x09
Option Length: 4
Descriptor: 32-bit unsigned integer. Opaque.
7. Sequence Counters
This section describes the general scheme for bootstrap and operation
of sequence counters in RPL, such as the DODAGVersionNumber in the
DIO message, the DAOSequence in the DAO message, and the Path
Sequence in the Transit Information option.
7.1. Sequence Counter Overview
This specification utilizes three different sequence numbers to
validate the freshness and the synchronization of protocol
information:
DODAGVersionNumber: This sequence counter is present in the DIO Base
to indicate the Version of the DODAG being formed. The
DODAGVersionNumber is monotonically incremented by the root
each time the root decides to form a new Version of the DODAG
in order to revalidate the integrity and allow a global repair
to occur. The DODAGVersionNumber is propagated unchanged Down
Winter, et al. Standards Track [Page 63]
RFC 6550 RPL March 2012
the DODAG as routers join the new DODAG Version. The
DODAGVersionNumber is globally significant in a DODAG and
indicates the Version of the DODAG in which a router is
operating. An older (lesser) value indicates that the
originating router has not migrated to the new DODAG Version
and cannot be used as a parent once the receiving node has
migrated to the newer DODAG Version.
DAOSequence: This sequence counter is present in the DAO Base to
correlate a DAO message and a DAO ACK message. The DAOSequence
number is locally significant to the node that issues a DAO
message for its own consumption to detect the loss of a DAO
message and enable retries.
Path Sequence: This sequence counter is present in the Transit
Information option in a DAO message. The purpose of this
counter is to differentiate a movement where a newer route
supersedes a stale one from a route redundancy scenario where
multiple routes exist in parallel for the same target. The
Path Sequence is globally significant in a DODAG and indicates
the freshness of the route to the associated target. An older
(lesser) value received from an originating router indicates
that the originating router holds stale routing states and the
originating router should not be considered anymore as a
potential next hop for the target. The Path Sequence is
computed by the node that advertises the target, that is the
Target itself or a router that advertises a Target on behalf of
a host, and is unchanged as the DAO content is propagated
towards the root by parent routers. If a host does not pass a
counter to its router, then the router is in charge of
computing the Path Sequence on behalf of the host and the host
can only register to one router for that purpose. If a DAO
message containing the same Target is issued to multiple
parents at a given point in time for the purpose of route
redundancy, then the Path Sequence is the same in all the DAO
messages for that same target.
7.2. Sequence Counter Operation
RPL sequence counters are subdivided in a 'lollipop' fashion
[Perlman83], where the values from 128 and greater are used as a
linear sequence to indicate a restart and bootstrap the counter, and
the values less than or equal to 127 used as a circular sequence
number space of size 128 as in [RFC1982]. Consideration is given to
the mode of operation when transitioning from the linear region to
the circular region. Finally, when operating in the circular region,
if sequence numbers are detected to be too far apart, then they are
not comparable, as detailed below.
Winter, et al. Standards Track [Page 64]
RFC 6550 RPL March 2012
A window of comparison, SEQUENCE_WINDOW = 16, is configured based on
a value of 2^N, where N is defined to be 4 in this specification.
For a given sequence counter:
1. The sequence counter SHOULD be initialized to an implementation
defined value, which is 128 or greater prior to use. A
recommended value is 240 (256 - SEQUENCE_WINDOW).
2. When a sequence counter increment would cause the sequence
counter to increment beyond its maximum value, the sequence
counter MUST wrap back to zero. When incrementing a sequence
counter greater than or equal to 128, the maximum value is 255.
When incrementing a sequence counter less than 128, the maximum
value is 127.
3. When comparing two sequence counters, the following rules MUST be
applied:
1. When a first sequence counter A is in the interval [128..255]
and a second sequence counter B is in [0..127]:
1. If (256 + B - A) is less than or equal to
SEQUENCE_WINDOW, then B is greater than A, A is less than
B, and the two are not equal.
2. If (256 + B - A) is greater than SEQUENCE_WINDOW, then A
is greater than B, B is less than A, and the two are not
equal.
For example, if A is 240, and B is 5, then (256 + 5 - 240) is
21. 21 is greater than SEQUENCE_WINDOW (16); thus, 240 is
greater than 5. As another example, if A is 250 and B is 5,
then (256 + 5 - 250) is 11. 11 is less than SEQUENCE_WINDOW
(16); thus, 250 is less than 5.
2. In the case where both sequence counters to be compared are
less than or equal to 127, and in the case where both
sequence counters to be compared are greater than or equal to
128:
1. If the absolute magnitude of difference between the two
sequence counters is less than or equal to
SEQUENCE_WINDOW, then a comparison as described in
[RFC1982] is used to determine the relationships greater
than, less than, and equal.
Winter, et al. Standards Track [Page 65]
RFC 6550 RPL March 2012
2. If the absolute magnitude of difference of the two
sequence counters is greater than SEQUENCE_WINDOW, then a
desynchronization has occurred and the two sequence
numbers are not comparable.
4. If two sequence numbers are determined not to be comparable,
i.e., the results of the comparison are not defined, then a node
should consider the comparison as if it has evaluated in such a
way so as to give precedence to the sequence number that has most
recently been observed to increment. Failing this, the node
should consider the comparison as if it has evaluated in such a
way so as to minimize the resulting changes to its own state.
8. Upward Routes
This section describes how RPL discovers and maintains Upward routes.
It describes the use of DODAG Information Objects (DIOs), the
messages used to discover and maintain these routes. It specifies
how RPL generates and responds to DIOs. It also describes DODAG
Information Solicitation (DIS) messages, which are used to trigger
DIO transmissions.
As mentioned in Section 3.2.8, nodes that decide to join a DODAG MUST
provision at least one DODAG parent as a default route for the
associated instance. This default route enables a packet to be
forwarded Upward until it eventually hits a common ancestor from
which it will be routed Downward to the destination. If the
destination is not in the DODAG, then the DODAG root may be able to
forward the packet using connectivity to the outside of the DODAG; if
it cannot forward the packet outside, then the DODAG root has to drop
it.
A DIO message can also transport explicit routing information:
DODAGID: The DODAGID is a Global or Unique Local IPv6 address of the
root. A node that joins a DODAG SHOULD provision a host route
via a DODAG parent to the address used by the root as the
DODAGID.
RIO Prefix: The root MAY place one or more Route Information options
in a DIO message. The RIO is used to advertise an external
route that is reachable via the root, associated with a
preference, as presented in Section 6.7.5, which incorporates
the RIO from [RFC4191]. It is interpreted as a capability of
the root as opposed to a routing advertisement, and it MUST NOT
be redistributed in another routing protocol though it SHOULD
be used by an ingress RPL router to select a DODAG when a
packet is injected in a RPL domain from a node attached to that
Winter, et al. Standards Track [Page 66]
RFC 6550 RPL March 2012
RPL router. An Objective Function MAY use the routes
advertised in RIO or the preference for those routes in order
to favor a DODAG versus another one for the same instance.
8.1. DIO Base Rules
1. For the following DIO Base fields, a node that is not a DODAG
root MUST advertise the same values as its preferred DODAG parent
(defined in Section 8.2.1). In this way, these values will
propagate Down the DODAG unchanged and advertised by every node
that has a route to that DODAG root. These fields are as
follows:
1. Grounded (G)
2. Mode of Operation (MOP)
3. DAGPreference (Prf)
4. Version
5. RPLInstanceID
6. DODAGID
2. A node MAY update the following fields at each hop:
1. Rank
2. DTSN
3. The DODAGID field each root sets MUST be unique within the RPL
Instance and MUST be a routable IPv6 address belonging to the
root.
8.2. Upward Route Discovery and Maintenance
Upward route discovery allows a node to join a DODAG by discovering
neighbors that are members of the DODAG of interest and identifying a
set of parents. The exact policies for selecting neighbors and
parents is implementation dependent and driven by the OF. This
section specifies the set of rules those policies must follow for
interoperability.
8.2.1. Neighbors and Parents within a DODAG Version
RPL's Upward route discovery algorithms and processing are in terms
of three logical sets of link-local nodes. First, the candidate
neighbor set is a subset of the nodes that can be reached via link-
local multicast. The selection of this set is implementation and OF
dependent. Second, the parent set is a restricted subset of the
candidate neighbor set. Finally, the preferred parent is a member of
the parent set that is the preferred next hop in Upward routes.
Conceptually, the preferred parent is a single parent; although, it
may be a set of multiple parents if those parents are equally
preferred and have identical Rank.
Winter, et al. Standards Track [Page 67]
RFC 6550 RPL March 2012
More precisely:
1. The DODAG parent set MUST be a subset of the candidate neighbor
set.
2. A DODAG root MUST have a DODAG parent set of size zero.
3. A node that is not a DODAG root MAY maintain a DODAG parent set
of size greater than or equal to one.
4. A node's preferred DODAG parent MUST be a member of its DODAG
parent set.
5. A node's Rank MUST be greater than all elements of its DODAG
parent set.
6. When Neighbor Unreachability Detection (NUD) [RFC4861], or an
equivalent mechanism, determines that a neighbor is no longer
reachable, a RPL node MUST NOT consider this node in the
candidate neighbor set when calculating and advertising routes
until it determines that it is again reachable. Routes through
an unreachable neighbor MUST be removed from the routing table.
These rules ensure that there is a consistent partial order on nodes
within the DODAG. As long as node Ranks do not change, following the
above rules ensures that every node's route to a DODAG root is loop-
free, as Rank decreases on each hop to the root.
The OF can guide candidate neighbor set and parent set selection, as
discussed in [RFC6552].
8.2.2. Neighbors and Parents across DODAG Versions
The above rules govern a single DODAG Version. The rules in this
section define how RPL operates when there are multiple DODAG
Versions.
8.2.2.1. DODAG Version
1. The tuple (RPLInstanceID, DODAGID, DODAGVersionNumber) uniquely
defines a DODAG Version. Every element of a node's DODAG parent
set, as conveyed by the last heard DIO message from each DODAG
parent, MUST belong to the same DODAG Version. Elements of a
node's candidate neighbor set MAY belong to different DODAG
Versions.
Winter, et al. Standards Track [Page 68]
RFC 6550 RPL March 2012
2. A node is a member of a DODAG Version if every element of its
DODAG parent set belongs to that DODAG Version, or if that node
is the root of the corresponding DODAG.
3. A node MUST NOT send DIOs for DODAG Versions of which it is not a
member.
4. DODAG roots MAY increment the DODAGVersionNumber that they
advertise and thus move to a new DODAG Version. When a DODAG
root increments its DODAGVersionNumber, it MUST follow the
conventions of Serial Number Arithmetic as described in
Section 7. Events triggering the increment of the
DODAGVersionNumber are described later in this section and in
Section 18.
5. Within a given DODAG, a node that is a not a root MUST NOT
advertise a DODAGVersionNumber higher than the highest
DODAGVersionNumber it has heard. Higher is defined as the
greater-than operator in Section 7.
6. Once a node has advertised a DODAG Version by sending a DIO, it
MUST NOT be a member of a previous DODAG Version of the same
DODAG (i.e., with the same RPLInstanceID, the same DODAGID, and a
lower DODAGVersionNumber). Lower is defined as the less-than
operator in Section 7.
When the DODAG parent set becomes empty on a node that is not a root,
(i.e., the last parent has been removed, causing the node no longer
to be associated with that DODAG), then the DODAG information should
not be suppressed until after the expiration of an implementation-
specific local timer. During the interval prior to suppression of
the "old" DODAG state, the node will be able to observe if the
DODAGVersionNumber has been incremented should any new parents
appear. This will help protect against the possibility of loops that
may occur if that node were to inadvertently rejoin the old DODAG
Version in its own prior sub-DODAG.
As the DODAGVersionNumber is incremented, a new DODAG Version spreads
outward from the DODAG root. A parent that advertises the new
DODAGVersionNumber cannot belong to the sub-DODAG of a node
advertising an older DODAGVersionNumber. Therefore, a node can
safely add a parent of any Rank with a newer DODAGVersionNumber
without forming a loop.
For example, suppose that a node has left a DODAG with
DODAGVersionNumber N. Suppose that a node had a sub-DODAG and did
attempt to poison that sub-DODAG by advertising a Rank of
INFINITE_RANK, but those advertisements may have become lost in the
Winter, et al. Standards Track [Page 69]
RFC 6550 RPL March 2012
LLN. Then, if the node did observe a candidate neighbor advertising
a position in that original DODAG at DODAGVersionNumber N, that
candidate neighbor could possibly have been in the node's former sub-
DODAG, and there is a possible case where adding that candidate
neighbor as a parent could cause a loop. In this case, if that
candidate neighbor is observed to advertise a DODAGVersionNumber N+1,
then that candidate neighbor is certain to be safe, since it is
certain not to be in that original node's sub-DODAG, as it has been
able to increment the DODAGVersionNumber by hearing from the DODAG
root while that original node was detached. For this reason, it is
useful for the detached node to remember the original DODAG
information, including the DODAGVersionNumber N.
Exactly when a DODAG root increments the DODAGVersionNumber is
implementation dependent and out of scope for this specification.
Examples include incrementing the DODAGVersionNumber periodically,
upon administrative intervention, or on application-level detection
of lost connectivity or DODAG inefficiency.
After a node transitions to and advertises a new DODAG Version, the
rules above make it unable to advertise the previous DODAG Version
(prior DODAGVersionNumber) once it has committed to advertising the
new DODAG Version.
8.2.2.2. DODAG Roots
1. A DODAG root without possibility to satisfy the application-
defined goal MUST NOT set the Grounded bit.
2. A DODAG root MUST advertise a Rank of ROOT_RANK.
3. A node whose DODAG parent set is empty MAY become the DODAG root
of a floating DODAG. It MAY also set its DAGPreference such that
it is less preferred.
In a deployment that uses non-LLN links to federate a number of LLN
roots, it is possible to run RPL over those non-RPL links and use one
router as a "backbone root". The backbone root is the virtual root
of the DODAG and exposes a Rank of BASE_RANK over the backbone. All
the LLN roots that are parented to that backbone root, including the
backbone root if it also serves as the LLN root itself, expose a Rank
of ROOT_RANK to the LLN. These virtual roots are part of the same
DODAG and advertise the same DODAGID. They coordinate
DODAGVersionNumbers and other DODAG parameters with the virtual root
over the backbone. The method of coordination is out of scope for
this specification (to be defined in future companion
specifications).
Winter, et al. Standards Track [Page 70]
RFC 6550 RPL March 2012
8.2.2.3. DODAG Selection
The Objective Function and the set of advertised routing metrics and
constraints of a DAG determine how a node selects its neighbor set,
parent set, and preferred parents. This selection implicitly also
determines the DODAG within a DAG. Such selection can include
administrative preference (Prf) as well as metrics or other
considerations.
If a node has the option to join a more preferred DODAG while still
meeting other optimization objectives, then the node will generally
seek to join the more preferred DODAG as determined by the OF. All
else being equal, it is left to the implementation to determine which
DODAG is most preferred (since, as a reminder, a node must only join
one DODAG per RPL Instance).
8.2.2.4. Rank and Movement within a DODAG Version
1. A node MUST NOT advertise a Rank less than or equal to any member
of its parent set within the DODAG Version.
2. A node MAY advertise a Rank lower than its prior advertisement
within the DODAG Version.
3. Let L be the lowest Rank within a DODAG Version that a given node
has advertised. Within the same DODAG Version, that node MUST
NOT advertise an effective Rank higher than L +
DAGMaxRankIncrease. INFINITE_RANK is an exception to this rule:
a node MAY advertise an INFINITE_RANK within a DODAG Version
without restriction. If a node's Rank were to be higher than
allowed by L + DAGMaxRankIncrease, when it advertises Rank, it
MUST advertise its Rank as INFINITE_RANK.
4. A node MAY, at any time, choose to join a different DODAG within
a RPL Instance. Such a join has no Rank restrictions, unless
that different DODAG is a DODAG Version of which this node has
previously been a member; in which case, the rule of the previous
bullet (3) must be observed. Until a node transmits a DIO
indicating its new DODAG membership, it MUST forward packets
along the previous DODAG.
5. A node MAY, at any time after hearing the next DODAGVersionNumber
advertised from suitable DODAG parents, choose to migrate to the
next DODAG Version within the DODAG.
Winter, et al. Standards Track [Page 71]
RFC 6550 RPL March 2012
Conceptually, an implementation is maintaining a DODAG parent set
within the DODAG Version. Movement entails changes to the DODAG
parent set. Moving Up does not present the risk to create a loop but
moving Down might, so that operation is subject to additional
constraints.
When a node migrates to the next DODAG Version, the DODAG parent set
needs to be rebuilt for the new Version. An implementation could
defer to migrate for some reasonable amount of time, to see if some
other neighbors with potentially better metrics but higher Rank
announce themselves. Similarly, when a node jumps into a new DODAG,
it needs to construct a new DODAG parent set for this new DODAG.
If a node needs to move Down a DODAG that it is attached to,
increasing its Rank, then it MAY poison its routes and delay before
moving as described in Section 8.2.2.5.
A node is allowed to join any DODAG Version that it has never been a
prior member of without any restrictions, but if the node has been a
prior member of the DODAG Version, then it must continue to observe
the rule that it may not advertise a Rank higher than
L+DAGMaxRankIncrease at any point in the life of the DODAG Version.
This rule must be observed so as not to create a loophole that would
allow the node to effectively increment its Rank all the way to
INFINITE_RANK, which may have impact on other nodes and create a
resource-wasting count-to-infinity scenario.
8.2.2.5. Poisoning
1. A node poisons routes by advertising a Rank of INFINITE_RANK.
2. A node MUST NOT have any nodes with a Rank of INFINITE_RANK in
its parent set.
Although an implementation may advertise INFINITE_RANK for the
purposes of poisoning, doing so is not the same as setting Rank to
INFINITE_RANK. For example, a node may continue to send data packets
whose RPL Packet Information includes a Rank that is not
INFINITE_RANK, yet still advertise INFINITE_RANK in its DIOs.
When a (former) parent is observed to advertise a Rank of
INFINITE_RANK, that (former) parent has detached from the DODAG and
is no longer able to act as a parent, nor is there any way that
another node may be considered to have a Rank greater-than
INFINITE_RANK. Therefore, that (former) parent cannot act as a
parent any longer and is removed from the parent set.
Winter, et al. Standards Track [Page 72]
RFC 6550 RPL March 2012
8.2.2.6. Detaching
1. A node unable to stay connected to a DODAG within a given DODAG
Version, i.e., that cannot retain non-empty parent set without
violating the rules of this specification, MAY detach from this
DODAG Version. A node that detaches becomes the root of its own
floating DODAG and SHOULD immediately advertise this new
situation in a DIO as an alternate to poisoning.
8.2.2.7. Following a Parent
1. If a node receives a DIO from one of its DODAG parents,
indicating that the parent has left the DODAG, that node SHOULD
stay in its current DODAG through an alternative DODAG parent, if
possible. It MAY follow the leaving parent.
A DODAG parent may have moved, migrated to the next DODAG Version, or
jumped to a different DODAG. A node ought to give some preference to
remaining in the current DODAG, if possible via an alternate parent,
but ought to follow the parent if there are no other options.
8.2.3. DIO Message Communication
When a DIO message is received, the receiving node must first
determine whether or not the DIO message should be accepted for
further processing, and subsequently present the DIO message for
further processing if eligible.
1. If the DIO message is malformed, then the DIO message is not
eligible for further processing and a node MUST silently discard
it. (See Section 18 for error logging).
2. If the sender of the DIO message is a member of the candidate
neighbor set and the DIO message is not malformed, the node MUST
process the DIO.
8.2.3.1. DIO Message Processing
As DIO messages are received from candidate neighbors, the neighbors
may be promoted to DODAG parents by following the rules of DODAG
discovery as described in Section 8.2. When a node places a neighbor
into the DODAG parent set, the node becomes attached to the DODAG
through the new DODAG parent node.
Winter, et al. Standards Track [Page 73]
RFC 6550 RPL March 2012
The most preferred parent should be used to restrict which other
nodes may become DODAG parents. Some nodes in the DODAG parent set
may be of a Rank less than or equal to the most preferred DODAG
parent. (This case may occur, for example, if an energy-constrained
device is at a lesser Rank but should be avoided per an optimization
objective, resulting in a more preferred parent at a greater Rank.)
8.3. DIO Transmission
RPL nodes transmit DIOs using a Trickle timer [RFC6206]. A DIO from
a sender with a lesser DAGRank that causes no changes to the
recipient's parent set, preferred parent, or Rank SHOULD be
considered consistent with respect to the Trickle timer.
The following packets and events MUST be considered inconsistencies
with respect to the Trickle timer, and cause the Trickle timer to
reset:
o When a node detects an inconsistency when forwarding a packet, as
detailed in Section 11.2.
o When a node receives a multicast DIS message without a Solicited
Information option, unless a DIS flag restricts this behavior.
o When a node receives a multicast DIS with a Solicited Information
option and the node matches all of the predicates in the Solicited
Information option, unless a DIS flag restricts this behavior.
o When a node joins a new DODAG Version (e.g., by updating its
DODAGVersionNumber, joining a new RPL Instance, etc.).
Note that this list is not exhaustive, and an implementation MAY
consider other messages or events to be inconsistencies.
A node SHOULD NOT reset its DIO Trickle timer in response to unicast
DIS messages. When a node receives a unicast DIS without a Solicited
Information option, it MUST unicast a DIO to the sender in response.
This DIO MUST include a DODAG Configuration option. When a node
receives a unicast DIS message with a Solicited Information option
and matches the predicates of that Solicited Information option, it
MUST unicast a DIO to the sender in response. This unicast DIO MUST
include a DODAG Configuration option. Thus, a node MAY transmit a
unicast DIS message to a potential DODAG parent in order to probe for
DODAG Configuration and other parameters.
Winter, et al. Standards Track [Page 74]
RFC 6550 RPL March 2012
8.3.1. Trickle Parameters
The configuration parameters of the Trickle timer are specified as
follows:
Imin: learned from the DIO message as (2^DIOIntervalMin) ms. The
default value of DIOIntervalMin is DEFAULT_DIO_INTERVAL_MIN.
Imax: learned from the DIO message as DIOIntervalDoublings. The
default value of DIOIntervalDoublings is
DEFAULT_DIO_INTERVAL_DOUBLINGS.
k: learned from the DIO message as DIORedundancyConstant. The
default value of DIORedundancyConstant is
DEFAULT_DIO_REDUNDANCY_CONSTANT. In RPL, when k has the value
of 0x00, this is to be treated as a redundancy constant of
infinity in RPL, i.e., Trickle never suppresses messages.
8.4. DODAG Selection
The DODAG selection is implementation and OF dependent. In order to
limit erratic movements, and all metrics being equal, nodes SHOULD
keep their previous selection. Also, nodes SHOULD provide a means to
filter out a parent whose availability is detected as fluctuating, at
least when more stable choices are available.
When connection to a grounded DODAG is not possible or preferable for
security or other reasons, scattered DODAGs MAY aggregate as much as
possible into larger DODAGs in order to allow connectivity within the
LLN.
A node SHOULD verify that bidirectional connectivity and adequate
link quality is available with a candidate neighbor before it
considers that candidate as a DODAG parent.
8.5. Operation as a Leaf Node
In some cases, a RPL node may attach to a DODAG as a leaf node only.
One example of such a case is when a node does not understand or does
not support (policy) the RPL Instance's OF or advertised metric/
constraint. As specified in Section 18.6, related to policy
function, the node may either join the DODAG as a leaf node or may
not join the DODAG. As mentioned in Section 18.5, it is then
recommended to log a fault.
Winter, et al. Standards Track [Page 75]
RFC 6550 RPL March 2012
A leaf node does not extend DODAG connectivity; however, in some
cases, the leaf node may still need to transmit DIOs on occasion, in
particular, when the leaf node may not have always been acting as a
leaf node and an inconsistency is detected.
A node operating as a leaf node must obey the following rules:
1. It MUST NOT transmit DIOs containing the DAG Metric Container.
2. Its DIOs MUST advertise a DAGRank of INFINITE_RANK.
3. It MAY suppress DIO transmission, unless the DIO transmission has
been triggered due to detection of inconsistency when a packet is
being forwarded or in response to a unicast DIS message, in which
case the DIO transmission MUST NOT be suppressed.
4. It MAY transmit unicast DAOs as described in Section 9.2.
5. It MAY transmit multicast DAOs to the '1 hop' neighborhood as
described in Section 9.10.
A particular case that requires a leaf node to send a DIO is if that
leaf node was a prior member of another DODAG and another node
forwards a message assuming the old topology, triggering an
inconsistency. The leaf node needs to transmit a DIO in order to
repair the inconsistency. Note that due to the lossy nature of LLNs,
even though the leaf node may have optimistically poisoned its routes
by advertising a Rank of INFINITE_RANK in the old DODAG prior to
becoming a leaf node, that advertisement may have become lost and a
leaf node must be capable to send a DIO later in order to repair the
inconsistency.
In the general case, the leaf node MUST NOT advertise itself as a
router (i.e., send DIOs).
8.6. Administrative Rank
In some cases, it might be beneficial to adjust the Rank advertised
by a node beyond that computed by the OF based on some
implementation-specific policy and properties of the node. For
example, a node that has a limited battery should be a leaf unless
there is no other choice, and may then augment the Rank computation
specified by the OF in order to expose an exaggerated Rank.
Winter, et al. Standards Track [Page 76]
RFC 6550 RPL March 2012
9. Downward Routes
This section describes how RPL discovers and maintains Downward
routes. RPL constructs and maintains Downward routes with
Destination Advertisement Object (DAO) messages. Downward routes
support P2MP flows, from the DODAG roots toward the leaves. Downward
routes also support P2P flows: P2P messages can flow toward a DODAG
root (or a common ancestor) through an Upward route, then away from
the DODAG root to a destination through a Downward route.
This specification describes the two modes a RPL Instance may choose
from for maintaining Downward routes. In the first mode, called
"Storing", nodes store Downward routing tables for their sub-DODAG.
Each hop on a Downward route in a storing network examines its
routing table to decide on the next hop. In the second mode, called
"Non-Storing", nodes do not store Downward routing tables. Downward
packets are routed with source routes populated by a DODAG root
[RFC6554].
RPL allows a simple one-hop P2P optimization for both storing and
non-storing networks. A node may send a P2P packet destined to a
one-hop neighbor directly to that node.
9.1. Destination Advertisement Parents
To establish Downward routes, RPL nodes send DAO messages Upward.
The next-hop destinations of these DAO messages are called "DAO
parents". The collection of a node's DAO parents is called the "DAO
parent set".
1. A node MAY send DAO messages using the all-RPL-nodes multicast
address, which is an optimization to provision one-hop routing.
The 'K' bit MUST be cleared on transmission of the multicast DAO.
2. A node's DAO parent set MUST be a subset of its DODAG parent set.
3. In Storing mode operation, a node MUST NOT address unicast DAO
messages to nodes that are not DAO parents.
4. In Storing mode operation, the IPv6 source and destination
addresses of a DAO message MUST be link-local addresses.
5. In Non-Storing mode operation, a node MUST NOT address unicast
DAO messages to nodes that are not DODAG roots.
6. In Non-Storing mode operation, the IPv6 source and destination
addresses of a DAO message MUST be a unique-local or a global
address.
Winter, et al. Standards Track [Page 77]
RFC 6550 RPL March 2012
The selection of DAO parents is implementation and Objective Function
specific.
9.2. Downward Route Discovery and Maintenance
Destination Advertisement may be configured to be entirely disabled,
or operate in either a Storing or Non-Storing mode, as reported in
the MOP in the DIO message.
1. All nodes who join a DODAG MUST abide by the MOP setting from the
root. Nodes that do not have the capability to fully participate
as a router, e.g., that do not match the advertised MOP, MAY join
the DODAG as a leaf.
2. If the MOP is 0, indicating no Downward routing, nodes MUST NOT
transmit DAO messages and MAY ignore DAO messages.
3. In Non-Storing mode, the DODAG root SHOULD store source routing
table entries for destinations learned from DAOs. The DODAG root
MUST be able to generate source routes for those destinations
learned from DAOs that were stored.
4. In Storing mode, all non-root, non-leaf nodes MUST store routing
table entries for destinations learned from DAOs.
A DODAG can have one of several possible modes of operation, as
defined by the MOP field. Either it does not support Downward
routes, it supports Downward routes through source routing from DODAG
roots, or it supports Downward routes through in-network routing
tables.
When Downward routes are supported through source routing from DODAG
roots, it is generally expected that the DODAG root has stored the
source routing information learned from DAOs in order to construct
the source routes. If the DODAG root fails to store some
information, then some destinations may be unreachable.
When Downward routes are supported through in-network routing tables,
the multicast operation defined in this specification may or may not
be supported, also as indicated by the MOP field.
When Downward routes are supported through in-network routing tables,
as described in this specification, it is expected that nodes acting
as routers have been provisioned sufficiently to hold the required
routing table state. If a node acting as a router is unable to hold
the full routing table state then the routing state is not complete,
Winter, et al. Standards Track [Page 78]
RFC 6550 RPL March 2012
messages may be dropped as a consequence, and a fault may be logged
(Section 18.5). Future extensions to RPL may elaborate on refined
actions/behaviors to manage this case.
As of the writing of this specification, RPL does not support mixed-
mode operation, where some nodes source route and other store routing
tables: future extensions to RPL may support this mode of operation.
9.2.1. Maintenance of Path Sequence
For each Target that is associated with (owned by) a node, that node
is responsible to emit DAO messages in order to provision the
Downward routes. The Target+Transit information contained in those
DAO messages subsequently propagates Up the DODAG. The Path Sequence
counter in the Transit information option is used to indicate
freshness and update stale Downward routing information as described
in Section 7.
For a Target that is associated with (owned by) a node, that node
MUST increment the Path Sequence counter, and generate a new DAO
message, when:
1. the Path Lifetime is to be updated (e.g., a refresh or a no-
Path).
2. the DODAG Parent Address subfield list is to be changed.
For a Target that is associated with (owned by) a node, that node MAY
increment the Path Sequence counter, and generate a new DAO message,
on occasion in order to refresh the Downward routing information. In
Storing mode, the node generates such a DAO to each of its DAO
parents in order to enable multipath. All DAOs generated at the same
time for the same Target MUST be sent with the same Path Sequence in
the Transit Information.
9.2.2. Generation of DAO Messages
A node might send DAO messages when it receives DAO messages, as a
result of changes in its DAO parent set, or in response to another
event such as the expiry of a related prefix lifetime. In the case
of receiving DAOs, it matters whether the DAO message is "new" or
contains new information. In Non-Storing mode, every DAO message a
node receives is "new". In Storing mode, a DAO message is "new" if
it satisfies any of these criteria for a contained Target:
1. it has a newer Path Sequence number,
2. it has additional Path Control bits, or
Winter, et al. Standards Track [Page 79]
RFC 6550 RPL March 2012
3. it is a No-Path DAO message that removes the last Downward route
to a prefix.
A node that receives a DAO message from its sub-DODAG MAY suppress
scheduling a DAO message transmission if that DAO message is not new.
9.3. DAO Base Rules
1. If a node sends a DAO message with newer or different information
than the prior DAO message transmission, it MUST increment the
DAOSequence field by at least one. A DAO message transmission
that is identical to the prior DAO message transmission MAY
increment the DAOSequence field.
2. The RPLInstanceID and DODAGID fields of a DAO message MUST be the
same value as the members of the node's parent set and the DIOs
it transmits.
3. A node MAY set the 'K' flag in a unicast DAO message to solicit a
unicast DAO-ACK in response in order to confirm the attempt.
4. A node receiving a unicast DAO message with the 'K' flag set
SHOULD respond with a DAO-ACK. A node receiving a DAO message
without the 'K' flag set MAY respond with a DAO-ACK, especially
to report an error condition.
5. A node that sets the 'K' flag in a unicast DAO message but does
not receive a DAO-ACK in response MAY reschedule the DAO message
transmission for another attempt, up until an implementation-
specific number of retries.
6. Nodes SHOULD ignore DAOs without newer sequence numbers and MUST
NOT process them further.
Unlike the Version field of a DIO, which is incremented only by a
DODAG root and repeated unchanged by other nodes, DAOSequence values
are unique to each node. The sequence number space for unicast and
multicast DAO messages can be either the same or distinct. It is
RECOMMENDED to use the same sequence number space.
9.4. Structure of DAO Messages
DAOs follow a common structure in both storing and non-storing
networks. In the most general form, a DAO message may include
several groups of options, where each group consists of one or more
Target options followed by one or more Transit Information options.
Winter, et al. Standards Track [Page 80]
RFC 6550 RPL March 2012
The entire group of Transit Information options applies to the entire
group of Target options. Later sections describe further details for
each mode of operation.
1. RPL nodes MUST include one or more RPL Target options in each DAO
message they transmit. One RPL Target option MUST have a prefix
that includes the node's IPv6 address if that node needs the
DODAG to provision Downward routes to that node. The RPL Target
option MAY be immediately followed by an opaque RPL Target
Descriptor option that qualifies it.
2. When a node updates the information in a Transit Information
option for a Target option that covers one of its addresses, it
MUST increment the Path Sequence number in that Transit
Information option. The Path Sequence number MAY be incremented
occasionally to cause a refresh to the Downward routes.
3. One or more RPL Target options in a unicast DAO message MUST be
followed by one or more Transit Information options. All the
transit options apply to all the Target options that immediately
precede them.
4. Multicast DAOs MUST NOT include the DODAG Parent Address subfield
in Transit Information options.
5. A node that receives and processes a DAO message containing
information for a specific Target, and that has prior information
for that Target, MUST use the Path Sequence number in the Transit
Information option associated with that Target in order to
determine whether or not the DAO message contains updated
information per Section 7.
6. If a node receives a DAO message that does not follow the above
rules, it MUST discard the DAO message without further
processing.
In Non-Storing mode, the root builds a strict source routing header,
hop-by-hop, by recursively looking up one-hop information that ties a
Target (address or prefix) and a transit address together. In some
cases, when a child address is derived from a prefix that is owned
and advertised by a parent, that parent-child relationship may be
inferred by the root for the purpose of constructing the source
routing header. In all other cases, it is necessary to inform the
root of the transit-Target relationship from a reachable target, so
as to later enable the recursive construction of the routing header.
An address that is advertised as a Target in a DAO message MUST be
collocated in the same router, or reachable on-link by the router
Winter, et al. Standards Track [Page 81]
RFC 6550 RPL March 2012
that owns the address that is indicated in the associated Transit
Information. The following additional rules apply to ensure the
continuity of the end-to-end source route path:
1. The address of a parent used in the transit option MUST be taken
from a PIO from that parent with the 'R' flag set. The 'R' flag
in a PIO indicates that the prefix field actually contains the
full parent address but the child SHOULD NOT assume that the
parent address is on-link.
2. A PIO with an 'A' flag set indicates that the RPL child node may
use the prefix to autoconfigure an address. A parent that
advertises a prefix in a PIO with the 'A' flag set MUST ensure
that the address or the whole prefix in the PIO is reachable from
the root by advertising it as a DAO target. If the parent also
sets the 'L' flag indicating that the prefix is on-link, then it
MUST advertise the whole prefix as Target in a DAO message. If
the 'L' flag is cleared and the 'R' flag is set, indicating that
the parent provides its own address in the PIO, then the parent
MUST advertise that address as a DAO target.
3. An address that is advertised as Target in a DAO message MUST be
collocated in the same router or reachable on-link by the router
that owns the address that is indicated in the associated Transit
Information.
4. In order to enable an optimum compression of the routing header,
the parent SHOULD set the 'R' flag in all PIOs with the 'A' flag
set and the 'L' flag cleared, and the child SHOULD prefer to use
as transit the address of the parent that is found in the PIO
that is used to autoconfigure the address that is advertised as
Target in the DAO message.
5. A router might have targets that are not known to be on-link for
a parent, either because they are addresses located on an
alternate interface or because they belong to nodes that are
external to RPL, for instance connected hosts. In order to
inject such a Target in the RPL network, the router MUST
advertise itself as the DODAG Parent Address subfield in the
Transit Information option for that target, using an address that
is on-link for that nodes DAO parent. If the Target belongs to
an external node, then the router MUST set the External 'E' flag
in the Transit Information.
A child node that has autoconfigured an address from a parent PIO
with the 'L' flag set does not need to advertise that address as a
DAO Target since the parent ensures that the whole prefix is already
reachable from the root. However, if the 'L' flag is not set, then
Winter, et al. Standards Track [Page 82]
RFC 6550 RPL March 2012
it is necessary, in Non-Storing mode, for the child node to inform
the root of the parent-child relationship, using a reachable address
of the parent, so as to enable the recursive construction of the
routing header. This is done by associating an address of the parent
as transit with the address of the child as Target in a DAO message.
9.5. DAO Transmission Scheduling
Because DAOs flow Upward, receiving a unicast DAO can trigger sending
a unicast DAO to a DAO parent.
1. On receiving a unicast DAO message with updated information, such
as containing a Transit Information option with a new Path
Sequence, a node SHOULD send a DAO. It SHOULD NOT send this DAO
message immediately. It SHOULD delay sending the DAO message in
order to aggregate DAO information from other nodes for which it
is a DAO parent.
2. A node SHOULD delay sending a DAO message with a timer
(DelayDAO). Receiving a DAO message starts the DelayDAO timer.
DAO messages received while the DelayDAO timer is active do not
reset the timer. When the DelayDAO timer expires, the node sends
a DAO.
3. When a node adds a node to its DAO parent set, it SHOULD schedule
a DAO message transmission.
DelayDAO's value and calculation is implementation dependent. A
default value of DEFAULT_DAO_DELAY is defined in this specification.
9.6. Triggering DAO Messages
Nodes can trigger their sub-DODAG to send DAO messages. Each node
maintains a DAO Trigger Sequence Number (DTSN), which it communicates
through DIO messages.
1. If a node hears one of its DAO parents increment its DTSN, the
node MUST schedule a DAO message transmission using rules in
Sections 9.3 and 9.5.
2. In Non-Storing mode, if a node hears one of its DAO parents
increment its DTSN, the node MUST increment its own DTSN.
In a Storing mode of operation, as part of routine routing table
updates and maintenance, a storing node MAY increment DTSN in order
to reliably trigger a set of DAO updates from its immediate children.
Winter, et al. Standards Track [Page 83]
RFC 6550 RPL March 2012
In a Storing mode of operation, it is not necessary to trigger DAO
updates from the entire sub-DODAG, since that state information will
propagate hop-by-hop Up the DODAG.
In a Non-Storing mode of operation, a DTSN increment will also cause
the immediate children of a node to increment their DTSN in turn,
triggering a set of DAO updates from the entire sub-DODAG.
Typically, in a Non-Storing mode of operation, only the root would
independently increment the DTSN when a DAO refresh is needed but a
global repair (such as by incrementing DODAGVersionNumber) is not
desired. Typically, in a Non-Storing mode of operation, all non-root
nodes would increment their DTSN only when their parent(s) are
observed to do so.
In general, a node may trigger DAO updates according to
implementation-specific logic, such as based on the detection of a
Downward route inconsistency or occasionally based upon an internal
timer.
In a storing network, selecting a proper DelayDAO for triggered DAOs
can greatly reduce the number of DAOs transmitted. The trigger flows
Down the DODAG; in the best case, the DAOs flow Up the DODAG such
that leaves send DAOs first, with each node sending a DAO message
only once. Such a scheduling could be approximated by setting
DelayDAO inversely proportional to Rank. Note that this suggestion
is intended as an optimization to allow efficient aggregation (it is
not required for correct operation in the general case).
9.7. Non-Storing Mode
In Non-Storing mode, RPL routes messages Downward using IP source
routing. The following rule applies to nodes that are in Non-Storing
mode. Storing mode has a separate set of rules, described in
Section 9.8.
1. The DODAG Parent Address subfield of a Transit Information option
MUST contain one or more addresses. All of these addresses MUST
be addresses of DAO parents of the sender.
2. DAOs are sent directly to the root along a default route
installed as part of the parent selection.
3. When a node removes a node from its DAO parent set, it MAY
generate a new DAO message with an updated Transit Information
option.
Winter, et al. Standards Track [Page 84]
RFC 6550 RPL March 2012
In Non-Storing mode, a node uses DAOs to report its DAO parents to
the DODAG root. The DODAG root can piece together a Downward route
to a node by using DAO parent sets from each node in the route. The
Path Sequence information may be used to detect stale DAO
information. The purpose of this per-hop route calculation is to
minimize traffic when DAO parents change. If nodes reported complete
source routes, then on a DAO parent change, the entire sub-DODAG
would have to send new DAOs to the DODAG root. Therefore, in Non-
Storing mode, a node can send a single DAO, although it might choose
to send more than one DAO message to each of multiple DAO parents.
Nodes pack DAOs by sending a single DAO message with multiple RPL
Target options. Each RPL Target option has its own, immediately
following, Transit Information options.
9.8. Storing Mode
In Storing mode, RPL routes messages Downward by the IPv6 destination
address. The following rules apply to nodes that are in Storing
mode:
1. The DODAG Parent Address subfield of a Transmit Information
option MUST be empty.
2. On receiving a unicast DAO, a node MUST compute if the DAO would
change the set of prefixes that the node itself advertises. This
computation SHOULD include consultation of the Path Sequence
information in the Transit Information options associated with
the DAO, to determine if the DAO message contains newer
information that supersedes the information already stored at the
node. If so, the node MUST generate a new DAO message and
transmit it, following the rules in Section 9.5. Such a change
includes receiving a No-Path DAO.
3. When a node generates a new DAO, it SHOULD unicast it to each of
its DAO parents. It MUST NOT unicast the DAO message to nodes
that are not DAO parents.
4. When a node removes a node from its DAO parent set, it SHOULD
send a No-Path DAO message (Section 6.4.3) to that removed DAO
parent to invalidate the existing route.
5. If messages to an advertised Downward address suffer from a
forwarding error, Neighbor Unreachable Detection (NUD), or
similar failure, a node MAY mark the address as unreachable and
generate an appropriate No-Path DAO.
Winter, et al. Standards Track [Page 85]
RFC 6550 RPL March 2012
DAOs advertise to which destination addresses and prefixes a node has
routes. Unlike in Non-Storing mode, these DAOs do not communicate
information about the routes themselves: that information is stored
within the network and is implicit from the IPv6 source address.
When a storing node generates a DAO, it uses the stored state of DAOs
it has received to produce a set of RPL Target options and their
associated Transmit Information options.
Because this information is stored within each node's routing tables,
in Storing mode, DAOs are communicated directly to DAO parents, who
store this information.
9.9. Path Control
A DAO message from a node contains one or more Target options. Each
Target option specifies either a prefix advertised by the node, a
prefix of addresses reachable outside the LLN, the address of a
destination in the node's sub-DODAG, or a multicast group to which a
node in the sub-DODAG is listening. The Path Control field of the
Transit Information option allows nodes to request or allow for
multiple Downward routes. A node constructs the Path Control field
of a Transit Information option as follows:
1. The bit width of the Path Control field MUST be equal to the
value (PCS + 1), where PCS is specified in the control field of
the DODAG Configuration option. Bits greater than or equal to
the value (PCS + 1) MUST be cleared on transmission and MUST be
ignored on reception. Bits below that value are considered
"active" bits.
2. The node MUST logically construct groupings of its DAO parents
while populating the Path Control field, where each group
consists of DAO parents of equal preference. Those groups MUST
then be ordered according to preference, which allows for a
logical mapping of DAO parents onto Path Control subfields (see
Figure 27). Groups MAY be repeated in order to extend over the
entire bit width of the patch control field, but the order,
including repeated groups, MUST be retained so that preference is
properly communicated.
3. For a RPL Target option describing a node's own address or a
prefix outside the LLN, at least one active bit of the Path
Control field MUST be set. More active bits of the Path Control
field MAY be set.
Winter, et al. Standards Track [Page 86]
RFC 6550 RPL March 2012
4. If a node receives multiple DAOs with the same RPL Target option,
it MUST bitwise-OR the Path Control fields it receives. This
aggregated bitwise-OR represents the number of Downward routes
the prefix requests.
5. When a node sends a DAO message to one of its DAO parents, it
MUST select one or more of the bits that are set active in the
subfield that is mapped to the group containing that DAO parent
from the aggregated Path Control field. A given bit can only be
presented as active to one parent. The DAO message it transmits
to its parent MUST have these active bits set and all other
active bits cleared.
6. For the RPL Target option and DAOSequence number, the DAOs a node
sends to different DAO parents MUST have disjoint sets of active
Path Control bits. A node MUST NOT set the same active bit on
DAOs to two different DAO parents.
7. Path Control bits SHOULD be allocated according to the preference
mapping of DAO parents onto Path Control subfields, such that the
active Path Control bits, or groupings of bits, that belong to a
particular Path Control subfield are allocated to DAO parents
within the group that was mapped to that subfield.
8. In a Non-Storing mode of operation, a node MAY pass DAOs through
without performing any further processing on the Path Control
field.
9. A node MUST NOT unicast a DAO message that has no active bits in
the Path Control field set. It is possible that, for a given
Target option, a node does not have enough aggregate Path Control
bits to send a DAO message containing that Target to each of its
DAO parents, in which case those least preferred DAO Parents may
not get a DAO message for that Target.
The Path Control field allows a node to bound how many Downward
routes will be generated to it. It sets a number of bits in the Path
Control field equal to the maximum number of Downward routes it
prefers. At most, each bit is sent to one DAO parent; clusters of
bits can be sent to a single DAO parent for it to divide among its
own DAO parents.
A node that provisions a DAO route for a Target that has an
associated Path Control field SHOULD use the content of that Path
Control field in order to determine an order of preference among
multiple alternative DAO routes for that Target. The Path Control
field assignment is derived from preference (of the DAO parents), as
determined on the basis of this node's best knowledge of the "end-to-
Winter, et al. Standards Track [Page 87]
RFC 6550 RPL March 2012
end" aggregated metrics in the Downward direction as per the
Objective Function. In Non-Storing mode the root can determine the
Downward route by aggregating the information from each received DAO,
which includes the Path Control indications of preferred DAO parents.
9.9.1. Path Control Example
Suppose that there is an LLN operating in Storing mode that contains
a Node N with four parents, P1, P2, P3, and P4. Let N have three
children, C1, C2, and C3 in its sub-DODAG. Let PCS be 7, such that
there will be 8 active bits in the Path Control field: 11111111b.
Consider the following example:
The Path Control field is split into four subfields, PC1 (11000000b),
PC2 (00110000b), PC3 (00001100b), and PC4 (00000011b), such that
those four subfields represent four different levels of preference
per Figure 27. The implementation at Node N, in this example, groups
{P1, P2} to be of equal preference to each other and the most
preferred group overall. {P3} is less preferred to {P1, P2}, and more
preferred to {P4}. Let Node N then perform its Path Control mapping
such that:
{P1, P2} -> PC1 (11000000b) in the Path Control field
{P3} -> PC2 (00110000b) in the Path Control field
{P4} -> PC3 (00001100b) in the Path Control field
{P4} -> PC4 (00000011b) in the Path Control field
Note that the implementation repeated {P4} in order to get complete
coverage of the Path Control field.
1. Let C1 send a DAO containing a Target T with a Path Control
10000000b. Node N stores an entry associating 10000000b with
the Path Control field for C1 and Target T.
2. Let C2 send a DAO containing a Target T with a Path Control
00010000b. Node N stores an entry associating 00010000b with
the Path Control field for C1 and Target T.
3. Let C3 send a DAO containing a Target T with a Path Control
00001100b. Node N stores an entry associating 00001100b with
the Path Control field for C1 and Target T.
4. At some later time, Node N generates a DAO for Target T. Node N
will construct an aggregate Path Control field by ORing together
the contribution from each of its children that have given a DAO
for Target T. Thus, the aggregate Path Control field has the
active bits set as: 10011100b.
Winter, et al. Standards Track [Page 88]
RFC 6550 RPL March 2012
5. Node N then distributes the aggregate Path Control bits among
its parents P1, P2, P3, and P4 in order to prepare the DAO
messages.
6. P1 and P2 are eligible to receive active bits from the most
preferred subfield (11000000b). Those bits are 10000000b in the
aggregate Path Control field. Node N must set the bit to one of
the two parents only. In this case, Node P1 is allocated the
bit and gets the Path Control field 10000000b for its DAO.
There are no bits left to allocate to Node P2; thus, Node P2
would have a Path Control field of 00000000b and a DAO cannot be
generated to Node P2 since there are no active bits.
7. The second-most preferred subfield (00110000b) has the active
bits 00010000b. Node N has mapped P3 to this subfield. Node N
may allocates the active bit to P3, constructing a DAO for P3
containing Target T with a Path Control of 00010000b.
8. The third-most preferred subfield (00001100b) has the active
bits 00001100b. Node N has mapped P4 to this subfield. Node N
may allocate both bits to P4, constructing a DAO for P4
containing Target T with a Path Control of 00001100b.
9. The least preferred subfield (00000011b) has no active bits.
Had there been active bits, those bits would have been added to
the Path Control field of the DAO constructed for P4.
10. The process of populating the DAO messages destined for P1, P2,
P3, P4 with other targets (other than T) proceeds according to
the aggregate Path Control fields collected for those targets.
9.10. Multicast Destination Advertisement Messages
A special case of DAO operation, distinct from unicast DAO operation,
is multicast DAO operation that may be used to populate '1-hop'
routing table entries.
1. A node MAY multicast a DAO message to the link-local scope all-
RPL-nodes multicast address.
2. A multicast DAO message MUST be used only to advertise
information about the node itself, i.e., prefixes directly
connected to or owned by the node, such as a multicast group that
the node is subscribed to or a global address owned by the node.
3. A multicast DAO message MUST NOT be used to relay connectivity
information learned (e.g., through unicast DAO) from another
node.
Winter, et al. Standards Track [Page 89]
RFC 6550 RPL March 2012
4. A node MUST NOT perform any other DAO-related processing on a
received multicast DAO message; in particular, a node MUST NOT
perform the actions of a DAO parent upon receipt of a multicast
DAO.
o The multicast DAO may be used to enable direct P2P communication,
without needing the DODAG to relay the packets.
10. Security Mechanisms
This section describes the generation and processing of secure RPL
messages. The high-order bit of the RPL message code identifies
whether or not a RPL message is secure. In addition to secure
versions of basic control messages (DIS, DIO, DAO, DAO-ACK), RPL has
several messages that are relevant only in networks that are security
enabled.
Implementation complexity and size is a core concern for LLNs such
that it may be economically or physically impossible to include
sophisticated security provisions in a RPL implementation.
Furthermore, many deployments can utilize link-layer or other
security mechanisms to meet their security requirements without
requiring the use of security in RPL.
Therefore, the security features described in this document are
OPTIONAL to implement. A given implementation MAY support a subset
(including the empty set) of the described security features, for
example, it could support integrity and confidentiality, but not
signatures. An implementation SHOULD clearly specify which security
mechanisms are supported, and it is RECOMMENDED that implementers
carefully consider security requirements and the availability of
security mechanisms in their network.
10.1. Security Overview
RPL supports three security modes:
o Unsecured. In this security mode, RPL uses basic DIS, DIO, DAO,
and DAO-ACK messages, which do not have Security sections. As a
network could be using other security mechanisms, such as link-
layer security, unsecured mode does not imply all messages are
sent without any protection.
o Preinstalled. In this security mode, RPL uses secure messages.
To join a RPL Instance, a node must have a preinstalled key.
Nodes use this to provide message confidentiality, integrity, and
authenticity. A node may, using this preinstalled key, join the
RPL network as either a host or a router.
Winter, et al. Standards Track [Page 90]
RFC 6550 RPL March 2012
o Authenticated. In this security mode, RPL uses secure messages.
To join a RPL Instance, a node must have a preinstalled key.
Nodes use this key to provide message confidentiality, integrity,
and authenticity. Using this preinstalled key, a node may join
the network as a host only. To join the network as a router, a
node must obtain a second key from a key authority. This key
authority can authenticate that the requester is allowed to be a
router before providing it with the second key. Authenticated
mode cannot be supported by symmetric algorithms. As of the
writing of this specification, RPL supports only symmetric
algorithms: authenticated mode is included for the benefit of
potential future cryptographic primitives. See Section 10.3.
Whether or not the RPL Instance uses unsecured mode is signaled by
whether it uses secure RPL messages. Whether a secured network uses
the preinstalled or authenticated mode is signaled by the 'A' bit of
the DAG Configuration option.
This specification specifies CCM -- Counter with CBC-MAC (Cipher
Block Chaining - Message Authentication Code) -- as the cryptographic
basis for RPL security [RFC3610]. In this specification, CCM uses
AES-128 as its underlying cryptographic algorithm. There are bits
reserved in the Security section to specify other algorithms in the
future.
All secured RPL messages have either a MAC or a signature.
Optionally, secured RPL messages also have encryption protection for
confidentiality. Secured RPL message formats support both integrated
encryption/authentication schemes (e.g., CCM) as well as schemes that
separately encrypt and authenticate packets.
10.2. Joining a Secure Network
RPL security assumes that a node wishing to join a secured network
has been pre-configured with a shared key for communicating with
neighbors and the RPL root. To join a secure RPL network, a node
either listens for secure DIOs or triggers secure DIOs by sending a
secure DIS. In addition to the DIO/DIS rules in Section 8, secure
DIO and DIS messages have these rules:
1. If sent, this initial secure DIS MUST set the Key Identifier Mode
field to 0 (00) and MUST set the Security Level field to 1 (001).
The key used MUST be the pre-configured group key (Key Index
0x00).
2. When a node resets its Trickle timer in response to a secure DIS
(Section 8.3), the next DIO it transmits MUST be a secure DIO
with the same security configuration as the secure DIS. If a
Winter, et al. Standards Track [Page 91]
RFC 6550 RPL March 2012
node receives multiple secure DIS messages before it transmits a
DIO, the secure DIO MUST have the same security configuration as
the last DIS to which it is responding.
3. When a node sends a DIO in response to a unicast secure DIS
(Section 8.3), the DIO MUST be a secure DIO.
The above rules allow a node to join a secured RPL Instance using the
pre-configured shared key. Once a node has joined the DODAG using
the pre-configured shared key, the 'A' bit of the Configuration
option determines its capabilities. If the 'A' bit of the
Configuration option is cleared, then nodes can use this
preinstalled, shared key to exchange messages normally: it can issue
DIOs, DAOs, etc.
If the 'A' bit of the Configuration option is set and the RPL
Instance is operating in authenticated mode:
1. A node MUST NOT advertise a Rank besides INFINITE_RANK in secure
DIOs secured with Key Index 0x00. When processing DIO messages
secured with Key Index 0x00, a processing node MUST consider the
advertised Rank to be INFINITE_RANK. Any other value results in
the message being discarded.
2. Secure DAOs using a Key Index 0x00 MUST NOT have a RPL Target
option with a prefix besides the node's address. If a node
receives a secured DAO message using the preinstalled, shared key
where the RPL Target option does not match the IPv6 source
address, it MUST discard the secured DAO message without further
processing.
The above rules mean that in RPL Instances where the 'A' bit is set,
using Key Index 0x00, a node can join the RPL Instance as a host but
not a router. A node must communicate with a key authority to obtain
a key that will enable it to act as a router.
10.3. Installing Keys
Authenticated mode requires a would-be router to dynamically install
new keys once they have joined a network as a host. Having joined as
a host, the node uses standard IP messaging to communicate with an
authorization server, which can provide new keys.
The protocol to obtain such keys is out of scope for this
specification and to be elaborated in future specifications. That
elaboration is required for RPL to securely operate in authenticated
mode.
Winter, et al. Standards Track [Page 92]
RFC 6550 RPL March 2012
10.4. Consistency Checks
RPL nodes send Consistency Check (CC) messages to protect against
replay attacks and synchronize counters.
1. If a node receives a unicast CC message with the 'R' bit cleared,
and it is a member of or is in the process of joining the
associated DODAG, it SHOULD respond with a unicast CC message to
the sender. This response MUST have the 'R' bit set, and it MUST
have the same CC nonce, RPLInstanceID, and DODAGID fields as the
message it received.
2. If a node receives a multicast CC message, it MUST discard the
message with no further processing.
Consistency Check messages allow nodes to issue a challenge-response
to validate a node's current counter value. Because the CC nonce is
generated by the challenger, an adversary replaying messages is
unlikely to be able to generate a correct response. The counter in
the Consistency Check response allows the challenger to validate the
counter values it hears.
10.5. Counters
In the simplest case, the counter value is an unsigned integer that a
node increments by one or more on each secured RPL transmission. The
counter MAY represent a timestamp that has the following properties:
1. The timestamp MUST be at least six octets long.
2. The timestamp MUST be in 1024 Hz (binary millisecond)
granularity.
3. The timestamp start time MUST be January 1, 1970, 12:00:00AM UTC.
4. If the counter represents a timestamp, the counter value MUST be
a value computed as follows. Let T be the timestamp, S be the
start time of the key in use, and E be the end time of the key in
use. Both S and E are represented using the same three rules as
the timestamp described above. If E > T < S, then the counter is
invalid and a node MUST NOT generate a packet. Otherwise, the
counter value is equal to T-S.
5. If the counter represents such a timestamp, a node MAY set the
'T' flag of the Security section of secured RPL packets.
6. If the Counter field does not present such a timestamp, then a
node MUST NOT set the 'T' flag.
Winter, et al. Standards Track [Page 93]
RFC 6550 RPL March 2012
7. If a node does not have a local timestamp that satisfies the
above requirements, it MUST ignore the 'T' flag.
If a node supports such timestamps and it receives a message with the
'T' flag set, it MAY apply the temporal check on the received message
described in Section 10.7.1. If a node receives a message without
the 'T' flag set, it MUST NOT apply this temporal check. A node's
security policy MAY, for application reasons, include rejecting all
messages without the 'T' flag set.
The 'T' flag is present because many LLNs today already maintain
global time synchronization at sub-millisecond granularity for
security, application, and other reasons. Allowing RPL to leverage
this existing functionality when present greatly simplifies solutions
to some security problems, such as delay protection.
10.6. Transmission of Outgoing Packets
Given an outgoing RPL control packet and the required security
protection, this section describes how RPL generates the secured
packet to transmit. It also describes the order of cryptographic
operations to provide the required protection.
The requirement for security protection and the level of security to
be applied to an outgoing RPL packet shall be determined by the
node's security policy database. The configuration of this security
policy database for outgoing packet processing is implementation
specific.
Where secured RPL messages are to be transmitted, a RPL node MUST set
the Security section (T, Sec, KIM, and LVL) in the outgoing RPL
packet to describe the protection level and security settings that
are applied (see Section 6.1). The Security subfield bit of the RPL
Message Code field MUST be set to indicate the secure RPL message.
The counter value used in constructing the AES-128 CCM nonce
(Figure 31) to secure the outgoing packet MUST be an increment of the
last counter transmitted to the particular destination address.
Where security policy specifies the application of delay protection,
the Timestamp counter used in constructing the CCM nonce to secure
the outgoing packet MUST be incremented according to the rules in
Section 10.5. Where a Timestamp counter is applied (indicated with
the 'T' flag set), the locally maintained Timestamp counter MUST be
included as part of the transmitted secured RPL message.
Winter, et al. Standards Track [Page 94]
RFC 6550 RPL March 2012
The cryptographic algorithm used in securing the outgoing packet
shall be specified by the node's security policy database and MUST be
indicated in the value of the Sec field set within the outgoing
message.
The security policy for the outgoing packet shall determine the
applicable KIM and Key Identifier specifying the security key to be
used for the cryptographic packet processing, including the optional
use of signature keys (see Section 6.1). The security policy will
also specify the algorithm (Algorithm) and level of protection
(Level) in the form of authentication or authentication and
encryption, and potential use of signatures that shall apply to the
outgoing packet.
Where encryption is applied, a node MUST replace the original packet
payload with that payload encrypted using the security protection,
key, and CCM nonce specified in the Security section of the packet.
All secured RPL messages include integrity protection. In
conjunction with the security algorithm processing, a node derives
either a MAC or signature that MUST be included as part of the
outgoing secured RPL packet.
10.7. Reception of Incoming Packets
This section describes the reception and processing of a secured RPL
packet. Given an incoming secured RPL packet, where the Security
subfield bit of the RPL Message Code field is set, this section
describes how RPL generates an unencrypted variant of the packet and
validates its integrity.
The receiver uses the RPL security control fields to determine the
necessary packet security processing. If the described level of
security for the message type and originator is unknown or does not
meet locally maintained security policies, a node MUST discard the
packet without further processing, MAY raise a management alert, and
MUST NOT send any messages in response. These policies can include
security levels, keys used, source identifiers, or the lack of
timestamp-based counters (as indicated by the 'T' flag). The
configuration of the security policy database for incoming packet
processing is out of scope for this specification (it may, for
example, be defined through DIO Configuration or through out-of-band
administrative router configuration).
Where the message Security Level (LVL) indicates an encrypted RPL
message, the node uses the key information identified through the KIM
field as well as the CCM nonce as input to the message payload
decryption processing. The CCM nonce shall be derived from the
Winter, et al. Standards Track [Page 95]
RFC 6550 RPL March 2012
message Counter field and other received and locally maintained
information (see Section 10.9.1). The plaintext message contents
shall be obtained by invoking the inverse cryptographic mode of
operation specified by the Sec field of the received packet.
The receiver shall use the CCM nonce and identified key information
to check the integrity of the incoming packet. If the integrity
check fails against the received MAC, a node MUST discard the packet.
If the received message has an initialized (zero value) counter value
and the receiver has an incoming counter currently maintained for the
originator of the message, the receiver MUST initiate a counter
resynchronization by sending a Consistency Check response message
(see Section 6.6) to the message source. The Consistency Check
response message shall be protected with the current full outgoing
counter maintained for the particular node address. That outgoing
counter will be included within the security section of the message
while the incoming counter will be included within the Consistency
Check message payload.
Based on the specified security policy, a node MAY apply replay
protection for a received RPL message. The replay check SHOULD be
performed before the authentication of the received packet. The
counter, as obtained from the incoming packet, shall be compared
against the watermark of the incoming counter maintained for the
given origination node address. If the received message counter
value is non-zero and less than the maintained incoming counter
watermark, a potential packet replay is indicated and the node MUST
discard the incoming packet.
If delay protection is specified as part of the incoming packet
security policy checks, the Timestamp counter is used to validate the
timeliness of the received RPL message. If the incoming message
Timestamp counter value indicates a message transmission time prior
to the locally maintained transmission time counter for the
originator address, a replay violation is indicated and the node MUST
discard the incoming packet. If the received Timestamp counter value
indicates a message transmission time that is earlier than the
Current time less the acceptable packet delay, a delay violation is
indicated and the node MUST discard the incoming packet.
Once a message has been decrypted, where applicable, and has
successfully passed its integrity check, replay check, and optionally
delay-protection checks, the node can update its local security
information, such as the source's expected counter value for replay
comparison.
Winter, et al. Standards Track [Page 96]
RFC 6550 RPL March 2012
A node MUST NOT update its security information on receipt of a
message that fails security policy checks or other applied integrity,
replay, or delay checks.
10.7.1. Timestamp Key Checks
If the 'T' flag of a message is set and a node has a local timestamp
that follows the requirements in Section 10.5, then a node MAY check
the temporal consistency of the message. The node computes the
transmit time of the message by adding the counter value to the start
time of the associated key. If this transmit time is past the end
time of the key, the node MAY discard the message without further
processing. If the transmit time is too far in the past or future
compared to the local time on the receiver, it MAY discard the
message without further processing.
10.8. Coverage of Integrity and Confidentiality
For a RPL ICMPv6 message, the entire packet is within the scope of
RPL security.
MACs and signatures are calculated over the entire unsecured IPv6
packet. When computing MACs and signatures, mutable IPv6 fields are
considered to be filled with zeroes, following the rules in Section
3.3.3.1 of [RFC4302] (IPsec Authenticated Header). MAC and signature
calculations are performed before any compression that lower layers
may apply.
When a RPL ICMPv6 message is encrypted, encryption starts at the
first byte after the Security section and continues to the last byte
of the packet. The IPv6 header, ICMPv6 header, and RPL message up to
the end of the Security section are not encrypted, as they are needed
to correctly decrypt the packet.
For example, a node sending a message with LVL=1, KIM=0, and
Algorithm=0 uses the CCM algorithm [RFC3610] to create a packet with
attributes ENC-MAC-32: it encrypts the packet and appends a 32-bit
MAC. The block cipher key is determined by the Key Index. The CCM
nonce is computed as described in Section 10.9.1; the message to
authenticate and encrypt is the RPL message starting at the first
byte after the Security section and ends with the last byte of the
packet. The additional authentication data starts with the beginning
of the IPv6 header and ends with the last byte of the RPL Security
section.
Winter, et al. Standards Track [Page 97]
RFC 6550 RPL March 2012
10.9. Cryptographic Mode of Operation
The cryptographic mode of operation described in this specification
(Algorithm = 0) is based on CCM and the block-cipher AES-128
[RFC3610]. This mode of operation is widely supported by existing
implementations. CCM mode requires a nonce (CCM nonce).
10.9.1. CCM Nonce
A RPL node constructs a CCM nonce as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Source Identifier +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|KIM|Resvd| LVL |
+-+-+-+-+-+-+-+-+
Figure 31: CCM Nonce
Source Identifier: 8 bytes. Source Identifier is set to the logical
identifier of the originator of the protected packet.
Counter: 4 bytes. Counter is set to the (uncompressed) value of the
corresponding field in the Security option of the RPL control
message.
Key Identifier Mode (KIM): 2 bits. KIM is set to the value of the
corresponding field in the Security option of the RPL control
message.
Security Level (LVL): 3 bits. Security Level is set to the value of
the corresponding field in the Security option of the RPL
control message.
Unassigned bits of the CCM nonce are reserved. They MUST be set to
zero when constructing the CCM nonce.
All fields of the CCM nonce are represented in most significant octet
and most significant bit first order.
Winter, et al. Standards Track [Page 98]
RFC 6550 RPL March 2012
10.9.2. Signatures
If the KIM indicates the use of signatures (a value of 3), then a
node appends a signature to the data payload of the packet. The
Security Level (LVL) field describes the length of this signature.
The signature scheme in RPL for Security Mode 3 is an instantiation
of the RSA algorithm (RSASSA-PSS) as defined in Section 8.1 of
[RFC3447]. As public key, it uses the pair (n,e), where n is a
2048-bit or 3072-bit RSA modulus and where e=2^{16}+1. It uses CCM
mode [RFC3610] as the encryption scheme with M=0 (as a stream-
cipher). Note that although [RFC3610] disallows the CCM mode with
M=0, RPL explicitly allows the CCM mode with M=0 when used in
conjunction with a signature, because the signature provides
sufficient data authentication. Here, the CCM mode with M=0 is
specified as in [RFC3610], but where the M' field in Section 2.2 MUST
be set to 0. It uses the SHA-256 hash function specified in Section
6.2 of [FIPS180]. It uses the message encoding rules of Section 8.1
of [RFC3447].
Let 'a' be a concatenation of a 6-byte representation of counter and
the message header. The packet payload is the right-concatenation of
packet data 'm' and the signature 's'. This signature scheme is
invoked with the right-concatenation of the message parts a and m,
whereas the signature verification is invoked with the right-
concatenation of the message parts a and m and with signature s.
RSA signatures of this form provide sufficient protection for RPL
networks. If needed, alternative signature schemes that produce more
concise signatures is out of scope for this specification and may be
the subject of a future specification.
An implementation that supports RSA signing with either 2048-bit or
3072-bit signatures SHOULD support verification of both 2048-bit and
3072-bit RSA signatures. This is in consideration of providing an
upgrade path for a RPL deployment.
11. Packet Forwarding and Loop Avoidance/Detection
11.1. Suggestions for Packet Forwarding
This document specifies a routing protocol. These non-normative
suggestions are provided to aid in the design of a forwarding
implementation by illustrating how such an implementation could work
with RPL.
When forwarding a packet to a destination, precedence is given to
selection of a next-hop successor as follows:
Winter, et al. Standards Track [Page 99]
RFC 6550 RPL March 2012
1. This specification only covers how a successor is selected from
the DODAG Version that matches the RPLInstanceID marked in the
IPv6 header of the packet being forwarded. Routing outside the
instance can be done as long as additional rules are put in place
such as strict ordering of instances and routing protocols to
protect against loops. Such rules may be defined in a separate
document.
2. If a local administrative preference favors a route that has been
learned from a different routing protocol than RPL, then use that
successor.
3. If the packet header specifies a source route by including an RH4
header as specified in [RFC6554], then use that route. If the
node fails to forward the packet with that specified source
route, then that packet should be dropped. The node MAY log an
error. The node may send an ICMPv6 error in Source Routing
Header message to the source of the packet (see Section 20.18).
4. If there is an entry in the routing table matching the
destination that has been learned from a multicast destination
advertisement (e.g., the destination is a one-hop neighbor), then
use that successor.
5. If there is an entry in the routing table matching the
destination that has been learned from a unicast destination
advertisement (e.g., the destination is located Down the sub-
DODAG), then use that successor. If there are DAO Path Control
bits associated with multiple successors, then consult the Path
Control bits to order the successors by preference when choosing.
If, for a given DAO Path Control bit, multiple successors are
recorded as having asserted that bit, precedence should be given
to the successor who most recently asserted that bit.
6. If there is a DODAG Version offering a route to a prefix matching
the destination, then select one of those DODAG parents as a
successor according to the OF and routing metrics.
7. Any other as-yet-unattempted DODAG parent may be chosen for the
next attempt to forward a unicast packet when no better match
exists.
8. Finally, the packet is dropped. ICMP Destination Unreachable MAY
be invoked (an inconsistency is detected).
Hop Limit MUST be decremented when forwarding per [RFC2460].
Winter, et al. Standards Track [Page 100]
RFC 6550 RPL March 2012
Note that the chosen successor MUST NOT be the neighbor that was the
predecessor of the packet (split horizon), except in the case where
it is intended for the packet to change from an Upward to a Downward
direction, as determined by the routing table of the node making the
change, such as switching from DIO routes to DAO routes as the
destination is neared in order to continue traveling toward the
destination.
11.2. Loop Avoidance and Detection
RPL loop avoidance mechanisms are kept simple and designed to
minimize churn and states. Loops may form for a number of reasons,
e.g., control packet loss. RPL includes a reactive loop detection
technique that protects from meltdown and triggers repair of broken
paths.
RPL loop detection uses RPL Packet Information that is transported
within the data packets, relying on an external mechanism such as
[RFC6553] that places in the RPL Packet Information in an IPv6 Hop-
by-Hop option header.
The content of RPL Packet Information is defined as follows:
Down 'O': 1-bit flag indicating whether the packet is expected to
progress Up or Down. A router sets the 'O' flag when the
packet is expected to progress Down (using DAO routes), and
clears it when forwarding toward the DODAG root (to a node with
a lower Rank). A host or RPL leaf node MUST set the 'O' flag
to 0.
Rank-Error 'R': 1-bit flag indicating whether a Rank error was
detected. A Rank error is detected when there is a mismatch in
the relative Ranks and the direction as indicated in the 'O'
bit. A host or RPL leaf node MUST set the 'R' bit to 0.
Forwarding-Error 'F': 1-bit flag indicating that this node cannot
forward the packet further towards the destination. The 'F'
bit might be set by a child node that does not have a route to
destination for a packet with the Down 'O' bit set. A host or
RPL leaf node MUST set the 'F' bit to 0.
RPLInstanceID: 8-bit field indicating the DODAG instance along which
the packet is sent.
SenderRank: 16-bit field set to zero by the source and to
DAGRank(rank) by a router that forwards inside the RPL network.
Winter, et al. Standards Track [Page 101]
RFC 6550 RPL March 2012
11.2.1. Source Node Operation
If the source is aware of the RPLInstanceID that is preferred for the
packet, then it MUST set the RPLInstanceID field associated with the
packet accordingly; otherwise, it MUST set it to the
RPL_DEFAULT_INSTANCE.
11.2.2. Router Operation
11.2.2.1. Instance Forwarding
The RPLInstanceID is associated by the source with the packet. This
RPLInstanceID MUST match the RPL Instance onto which the packet is
placed by any node, be it a host or router. The RPLInstanceID is
part of the RPL Packet Information.
A RPL router that forwards a packet in the RPL network MUST check if
the packet includes the RPL Packet Information. If not, then the RPL
router MUST insert the RPL Packet Information. If the router is an
ingress router that injects the packet into the RPL network, the
router MUST set the RPLInstanceID field in the RPL Packet
Information. The details of how that router determines the mapping
to a RPLInstanceID are out of scope for this specification and left
to future specification.
A router that forwards a packet outside the RPL network MUST remove
the RPL Packet Information.
When a router receives a packet that specifies a given RPLInstanceID
and the node can forward the packet along the DODAG associated to
that instance, then the router MUST do so and leave the RPLInstanceID
value unchanged.
If any node cannot forward a packet along the DODAG associated with
the RPLInstanceID, then the node SHOULD discard the packet and send
an ICMP error message.
11.2.2.2. DAG Inconsistency Loop Detection
The DODAG is inconsistent if the direction of a packet does not match
the Rank relationship. A receiver detects an inconsistency if it
receives a packet with either:
the 'O' bit set (to Down) from a node of a higher Rank.
the 'O' bit cleared (for Up) from a node of a lower Rank.
Winter, et al. Standards Track [Page 102]
RFC 6550 RPL March 2012
When the DODAG root increments the DODAGVersionNumber, a temporary
Rank discontinuity may form between the next DODAG Version and the
prior DODAG Version, in particular, if nodes are adjusting their Rank
in the next DODAG Version and deferring their migration into the next
DODAG Version. A router that is still a member of the prior DODAG
Version may choose to forward a packet to a (future) parent that is
in the next DODAG Version. In some cases, this could cause the
parent to detect an inconsistency because the Rank-ordering in the
prior DODAG Version is not necessarily the same as in the next DODAG
Version, and the packet may be judged not to be making forward
progress. If the sending router is aware that the chosen successor
has already joined the next DODAG Version, then the sending router
MUST update the SenderRank to INFINITE_RANK as it forwards the
packets across the discontinuity into the next DODAG Version in order
to avoid a false detection of Rank inconsistency.
One inconsistency along the path is not considered a critical error
and the packet may continue. However, a second detection along the
path of the same packet should not occur and the packet MUST be
dropped.
This process is controlled by the Rank-Error bit associated with the
packet. When an inconsistency is detected on a packet, if the Rank-
Error bit was not set, then the Rank-Error bit is set. If it was set
the packet MUST be discarded and the Trickle timer MUST be reset.
11.2.2.3. DAO Inconsistency Detection and Recovery
DAO inconsistency loop recovery is a mechanism that applies to
Storing mode of operation only.
In Non-Storing mode, the packets are source routed to the
destination, and DAO inconsistencies are not corrected locally.
Instead, an ICMP error with a new code "Error in Source Routing
Header" is sent back to the root. The "Error in Source Routing
Header" message has the same format as the "Destination Unreachable
Message", as specified in [RFC4443]. The portion of the invoking
packet that is sent back in the ICMP message should record at least
up to the routing header, and the routing header should be consumed
by this node so that the destination in the IPv6 header is the next
hop that this node could not reach.
A DAO inconsistency happens when a router has a Downward route that
was previously learned from a DAO message via a child, but that
Downward route is not longer valid in the child, e.g., because that
related state in the child has been cleaned up. With DAO
inconsistency loop recovery, a packet can be used to recursively
explore and clean up the obsolete DAO states along a sub-DODAG.
Winter, et al. Standards Track [Page 103]
RFC 6550 RPL March 2012
In a general manner, a packet that goes Down should never go Up
again. If DAO inconsistency loop recovery is applied, then the
router SHOULD send the packet back to the parent that passed it with
the Forwarding-Error 'F' bit set and the 'O' bit left untouched.
Otherwise, the router MUST silently discard the packet.
Upon receiving a packet with a Forwarding-Error bit set, the node
MUST remove the routing states that caused forwarding to that
neighbor, clear the Forwarding-Error bit, and attempt to send the
packet again. The packet may be sent to an alternate neighbor, after
the expiration of a user-configurable implementation-specific timer.
If that alternate neighbor still has an inconsistent DAO state via
this node, the process will recurse, this node will set the
Forwarding-Error 'F' bit, and the routing state in the alternate
neighbor will be cleaned up as well.
12. Multicast Operation
This section describes a multicast routing operation over an IPv6 RPL
network and, specifically, how unicast DAOs can be used to relay
group registrations. The same DODAG construct can be used to forward
unicast and multicast traffic. This section is limited to a
description of how group registrations may be exchanged and how the
forwarding infrastructure operates. It does not provide a full
description of multicast within an LLN and, in particular, does not
describe the generation of DODAGs specifically targeted at multicast
or the details of operating RPL for multicast -- that will be the
subject of further specifications.
The multicast group registration uses DAO messages that are identical
to unicast except for the type of address that is transported. The
main difference is that the multicast traffic going down is copied to
all the children that have registered with the multicast group,
whereas unicast traffic is passed to one child only.
Nodes that support the RPL Storing mode of operation SHOULD also
support multicast DAO operations as described below. Nodes that only
support the Non-Storing mode of operation are not expected to support
this section.
The multicast operation is controlled by the MOP field in the DIO.
o If the MOP field requires multicast support, then a node that
joins the RPL network as a router must operate as described in
this section for multicast signaling and forwarding within the RPL
network. A node that does not support the multicast operation
required by the MOP field can only join as a leaf.
Winter, et al. Standards Track [Page 104]
RFC 6550 RPL March 2012
o If the MOP field does not require multicast support, then
multicast is handled by some other way that is out of scope for
this specification. (Examples may include a series of unicast
copies or limited-scope flooding).
A router might select to pass a listener registration DAO message to
its preferred parent only; in which case, multicast packets coming
back might be lost for all of its sub-DODAGs if the transmission
fails over that link. Alternatively, the router might select copying
additional parents as it would do for DAO messages advertising
unicast destinations; in which case, there might be duplicates that
the router will need to prune.
As a result, multicast routing states are installed in each router on
the way from the listeners to the DODAG root, enabling the root to
copy a multicast packet to all its children routers that had issued a
DAO message including a Target option for that multicast group.
For a multicast packet sourced from inside the DODAG, the packet is
passed to the preferred parents, and if that fails, then to the
alternates in the DODAG. The packet is also copied to all the
registered children, except for the one that passed the packet.
Finally, if there is a listener in the external infrastructure, then
the DODAG root has to further propagate the packet into the external
infrastructure.
As a result, the DODAG root acts as an automatic proxy Rendezvous
Point for the RPL network and as source towards the non-RPL domain
for all multicast flows started in the RPL domain. So, regardless of
whether the root is actually attached to a non-RPL domain, and
regardless of whether the DODAG is grounded or floating, the root can
serve inner multicast streams at all times.
13. Maintenance of Routing Adjacency
The selection of successors, along the default paths Up along the
DODAG, or along the paths learned from destination advertisements
Down along the DODAG, leads to the formation of routing adjacencies
that require maintenance.
In IGPs, such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance
of a routing adjacency involves the use of keepalive mechanisms
(Hellos) or other protocols such as the Bidirectional Forwarding
Detection (BFD) [RFC5881] and the MANET Neighborhood Discovery
Protocol (NHDP) [RFC6130]. Unfortunately, such a proactive approach
is often not desirable in constrained environments where it would
lead to excessive control traffic in light of the data traffic with a
negative impact on both link loads and nodes resources.
Winter, et al. Standards Track [Page 105]
RFC 6550 RPL March 2012
By contrast with those routing protocols, RPL does not define any
keepalive mechanisms to detect routing adjacency failures: this is
because in many cases, such a mechanism would be too expensive in
terms of bandwidth and, even more importantly, energy (a battery-
operated device could not afford to send periodic keepalives). Still
RPL requires an external mechanisms to detect that a neighbor is no
longer reachable. Such a mechanism should preferably be reactive to
traffic in order to minimize the overhead to maintain the routing
adjacency and focus on links that are actually being used.
Example reactive mechanisms that can be used include:
The Neighbor Unreachability Detection [RFC4861] mechanism.
Layer 2 triggers [RFC5184] derived from events such as association
states and L2 acknowledgements.
14. Guidelines for Objective Functions
An Objective Function (OF), in conjunction with routing metrics and
constraints, allows for the selection of a DODAG to join, and a
number of peers in that DODAG as parents. The OF is used to compute
an ordered list of parents. The OF is also responsible to compute
the Rank of the device within the DODAG Version.
The Objective Function is indicated in the DIO message using an
Objective Code Point (OCP), and it indicates the method that must be
used to construct the DODAG. The Objective Code Points are specified
in [RFC6552] and related companion specifications.
14.1. Objective Function Behavior
Most Objective Functions are expected to follow the same abstract
behavior at a node:
o The parent selection is triggered each time an event indicates
that a potential next-hop information is updated. This might
happen upon the reception of a DIO message, a timer elapse, all
DODAG parents are unavailable, or a trigger indicating that the
state of a candidate neighbor has changed.
o An OF scans all the interfaces on the node. Although, there may
typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer
dependent and are out of scope for this specification. Finally,
Winter, et al. Standards Track [Page 106]
RFC 6550 RPL March 2012
an interface might or might not match a required criterion for an
Objective Function, for instance, a degree of security. As a
result, some interfaces might be completely excluded from the
computation, for example, if those interfaces cannot satisfy some
advertised constraints, while others might be more or less
preferred.
o An OF scans all the candidate neighbors on the possible interfaces
to check whether they can act as a router for a DODAG. There
might be many of them and a candidate neighbor might need to pass
some validation tests before it can be used. In particular, some
link layers require experience on the activity with a router to
enable the router as a next hop.
o An OF computes Rank of a node for comparison by adding to the Rank
of the candidate a value representing the relative locations of
the node and the candidate in the DODAG Version.
* The increase in Rank must be at least MinHopRankIncrease.
* To keep loop avoidance and metric optimization in alignment,
the increase in Rank should reflect any increase in the metric
value. For example, with a purely additive metric, such as
ETX, the increase in Rank can be made proportional to the
increase in the metric.
* Candidate neighbors that would cause the Rank of the node to
increase are not considered for parent selection.
o Candidate neighbors that advertise an OF incompatible with the set
of OFs specified by the policy functions are ignored.
o As it scans all the candidate neighbors, the OF keeps the current
best parent and compares its capabilities with the current
candidate neighbor. The OF defines a number of tests that are
critical to reach the objective. A test between the routers
determines an order relation.
* If the routers are equal for that relation, then the next test
is attempted between the routers,
* Else the best of the two routers becomes the current best
parent, and the scan continues with the next candidate
neighbor.
* Some OFs may include a test to compare the Ranks that would
result if the node joined either router.
Winter, et al. Standards Track [Page 107]
RFC 6550 RPL March 2012
o When the scan is complete, the preferred parent is elected and the
node's Rank is computed as the preferred parent Rank plus the step
in Rank with that parent.
o Other rounds of scans might be necessary to elect alternate
parents. In the next rounds:
* Candidate neighbors that are not in the same DODAG are ignored.
* Candidate neighbors that are of greater Rank than the node are
ignored.
* Candidate neighbors of an equal Rank to the node are ignored
for parent selection.
* Candidate neighbors of a lesser Rank than the node are
preferred.
15. Suggestions for Interoperation with Neighbor Discovery
This specification directly borrows the Prefix Information Option
(PIO) and the Route Information Option (RIO) from IPv6 ND. It is
envisioned that, as future specifications build on this base, there
may be additional cause to leverage parts of IPv6 ND. This section
provides some suggestions for future specifications.
First and foremost, RPL is a routing protocol. One should take great
care to preserve architecture when mapping functionalities between
RPL and ND. RPL is for routing only. That said, there may be
persuading technical reasons to allow for sharing options between RPL
and IPv6 ND in a particular implementation/deployment.
In general, the following guidelines apply:
o RPL Type codes must be allocated from the RPL Control Message
Options registry.
o RPL Length fields must be expressed in units of single octets, as
opposed to ND Length fields, which are expressed in units of 8
octets.
o RPL options are generally not required to be aligned to 8-octet
boundaries.
o When mapping/transposing an IPv6 ND option for redistribution as a
RPL option, any padding octets should be removed when possible.
For example, the Prefix Length field in the PIO is sufficient to
describe the length of the Prefix field. When mapping/transposing
Winter, et al. Standards Track [Page 108]
RFC 6550 RPL March 2012
a RPL option for redistribution as an IPv6 ND option, any such
padding octets should be restored. This procedure must be
unambiguous.
16. Summary of Requirements for Interoperable Implementations
This section summarizes basic interoperability and references
normative text for RPL implementations operating in one of three
major modes. Implementations are expected to support either no
Downward routes, Non-Storing mode only, or Storing mode only. A
fourth mode, operation as a leaf, is also possible.
Implementations conforming to this specification may contain
different subsets of capabilities as appropriate to the application
scenario. It is important for the implementer to support a level of
interoperability consistent with that required by the application
scenario. To this end, further guidance may be provided beyond this
specification (e.g., as applicability statements), and it is
understood that in some cases such further guidance may override
portions of this specification.
16.1. Common Requirements
In a general case, the greatest level of interoperability may be
achieved when all of the nodes in a RPL LLN are cooperating to use
the same MOP, OF, metrics, and constraints, and are thus able to act
as RPL routers. When a node is not capable of being a RPL router, it
may be possible to interoperate in a more limited manner as a RPL
leaf.
All RPL implementations need to support the use of RPL Packet
Information transported within data packets (Section 11.2). One such
mechanism is described in [RFC6553].
RPL implementations will need to support the use of Neighbor
Unreachability Detection (NUD), or an equivalent mechanism, to
maintain the reachability of neighboring RPL nodes (Section 8.2.1).
Alternate mechanisms may be optimized to the constrained capabilities
of the implementation, such as hints from the link layer.
This specification provides means to obtain a PIO and thus form an
IPv6 address. When that mechanism is used, it may be necessary to
perform address resolution and duplicate address detection through an
external process, such as IPv6 ND [RFC4861] or 6LoWPAN ND
[6LOWPAN-ND].
Winter, et al. Standards Track [Page 109]
RFC 6550 RPL March 2012
16.2. Operation as a RPL Leaf Node (Only)
o An implementation of a leaf node (only) does not ever participate
as a RPL router. Interoperable implementations of leaf nodes
behave as summarized in Section 8.5.
o Support of a particular MOP encoding is not required, although if
the leaf node sends DAO messages to set up Downward routes, the
leaf node should do so in a manner consistent with the mode of
operation indicated by the MOP.
o Support of a particular OF is not required.
o In summary, a leaf node does not generally issue DIO messages, it
may issue DAO and DIS messages. A leaf node accepts DIO messages
though it generally ignores DAO and DIS messages.
16.3. Operation as a RPL Router
If further guidance is not available then a RPL router implementation
MUST at least support the metric-less OF0 [RFC6552].
For consistent operation a RPL router implementation needs to support
the MOP in use by the DODAG.
All RPL routers will need to implement Trickle [RFC6206].
16.3.1. Support for Upward Routes (Only)
An implementation of a RPL router that supports only Upward routes
supports the following:
o Upward routes (Section 8)
o MOP encoding 0 (Section 20.3)
o In summary, DIO and DIS messages are issued, and DAO messages are
not issued. DIO and DIS messages are accepted, and DAO messages
are ignored.
16.3.2. Support for Upward Routes and Downward Routes in Non-Storing
Mode
An implementation of a RPL router that supports Upward routes and
Downward routes in Non-Storing mode supports the following:
o Upward routes (Section 8)
Winter, et al. Standards Track [Page 110]
RFC 6550 RPL March 2012
o Downward routes (Non-Storing) (Section 9)
o MOP encoding 1 (Section 20.3)
o Source-routed Downward traffic ([RFC6554])
o In summary, DIO and DIS messages are issued, and DAO messages are
issued to the DODAG root. DIO and DIS messages are accepted, and
DAO messages are ignored by nodes other than DODAG roots.
Multicast is not supported through the means described in this
specification, though it may be supported through some alternate
means.
16.3.3. Support for Upward Routes and Downward Routes in Storing Mode
An implementation of a RPL router that supports Upward routes and
Downward routes in Storing mode supports the following:
o Upward routes (Section 8)
o Downward routes (Storing) (Section 9)
o MOP encoding 2 (Section 20.3)
o In summary, DIO, DIS, and DAO messages are issued. DIO, DIS, and
DAO messages are accepted. Multicast is not supported through the
means described in this specification, though it may be supported
through some alternate means.
16.3.3.1. Optional Support for Basic Multicast Scheme
A Storing mode implementation may be enhanced with basic multicast
support through the following additions:
o Basic Multicast Support (Section 12)
o MOP encoding 3 (Section 20.3)
16.4. Items for Future Specification
A number of items are left to future specification, including but not
limited to the following:
o How to attach a non-RPL node such as an IPv6 host, e.g., to
consistently distribute at least PIO material to the attached
node.
Winter, et al. Standards Track [Page 111]
RFC 6550 RPL March 2012
o How to obtain authentication material in support if authenticated
mode is used (Section 10.3).
o Details of operation over multiple simultaneous instances.
o Advanced configuration mechanisms, such as the provisioning of
RPLInstanceIDs, parameterization of Objective Functions, and
parameters to control security. (It is expected that such
mechanisms might extend the DIO as a means to disseminate
information across the DODAG).
17. RPL Constants and Variables
The following is a summary of RPL constants and variables:
BASE_RANK: This is the Rank for a virtual root that might be used to
coordinate multiple roots. BASE_RANK has a value of 0.
ROOT_RANK: This is the Rank for a DODAG root. ROOT_RANK has a value
of MinHopRankIncrease (as advertised by the DODAG root), such
that DAGRank(ROOT_RANK) is 1.
INFINITE_RANK: This is the constant maximum for the Rank.
INFINITE_RANK has a value of 0xFFFF.
RPL_DEFAULT_INSTANCE: This is the RPLInstanceID that is used by this
protocol by a node without any overriding policy.
RPL_DEFAULT_INSTANCE has a value of 0.
DEFAULT_PATH_CONTROL_SIZE: This is the default value used to
configure PCS in the DODAG Configuration option, which dictates
the number of significant bits in the Path Control field of the
Transit Information option. DEFAULT_PATH_CONTROL_SIZE has a
value of 0. This configures the simplest case limiting the
fan-out to 1 and limiting a node to send a DAO message to only
one parent.
DEFAULT_DIO_INTERVAL_MIN: This is the default value used to configure
Imin for the DIO Trickle timer. DEFAULT_DIO_INTERVAL_MIN has a
value of 3. This configuration results in Imin of 8 ms.
DEFAULT_DIO_INTERVAL_DOUBLINGS: This is the default value used to
configure Imax for the DIO Trickle timer.
DEFAULT_DIO_INTERVAL_DOUBLINGS has a value of 20. This
configuration results in a maximum interval of 2.3 hours.
Winter, et al. Standards Track [Page 112]
RFC 6550 RPL March 2012
DEFAULT_DIO_REDUNDANCY_CONSTANT: This is the default value used to
configure k for the DIO Trickle timer.
DEFAULT_DIO_REDUNDANCY_CONSTANT has a value of 10. This
configuration is a conservative value for Trickle suppression
mechanism.
DEFAULT_MIN_HOP_RANK_INCREASE: This is the default value of
MinHopRankIncrease. DEFAULT_MIN_HOP_RANK_INCREASE has a value
of 256. This configuration results in an 8-bit wide integer
part of Rank.
DEFAULT_DAO_DELAY: This is the default value for the DelayDAO Timer.
DEFAULT_DAO_DELAY has a value of 1 second. See Section 9.5.
DIO Timer: One instance per DODAG of which a node is a member.
Expiry triggers DIO message transmission. A Trickle timer with
variable interval in [0,
DIOIntervalMin..2^DIOIntervalDoublings]. See Section 8.3.1
DAG Version Increment Timer: Up to one instance per DODAG of which
the node is acting as DODAG root. May not be supported in all
implementations. Expiry triggers increment of
DODAGVersionNumber, causing a new series of updated DIO message
to be sent. Interval should be chosen appropriate to
propagation time of DODAG and as appropriate to application
requirements (e.g., response time versus overhead).
DelayDAO Timer: Up to one timer per DAO parent (the subset of DODAG
parents chosen to receive destination advertisements) per
DODAG. Expiry triggers sending of DAO message to the DAO
parent. See Section 9.5
RemoveTimer: Up to one timer per DAO entry per neighbor (i.e., those
neighbors that have given DAO messages to this node as a DODAG
parent). Expiry may trigger No-Path advertisements or
immediately deallocate the DAO entry if there are no DAO
parents.
18. Manageability Considerations
The aim of this section is to give consideration to the manageability
of RPL, and how RPL will be operated in an LLN. The scope of this
section is to consider the following aspects of manageability:
configuration, monitoring, fault management, accounting, and
performance of the protocol in light of the recommendations set forth
in [RFC5706].
Winter, et al. Standards Track [Page 113]
RFC 6550 RPL March 2012
18.1. Introduction
Most of the existing IETF management standards are MIB modules (data
models based on the Structure of Management Information (SMI)) to
monitor and manage networking devices.
For a number of protocols, the IETF community has used the IETF
Standard Management Framework, including the Simple Network
Management Protocol [RFC3410], the Structure of Management
Information [RFC2578], and MIB data models for managing new
protocols.
As pointed out in [RFC5706], the common policy in terms of operation
and management has been expanded to a policy that is more open to a
set of tools and management protocols rather than strictly relying on
a single protocol such as SNMP.
In 2003, the Internet Architecture Board (IAB) held a workshop on
Network Management [RFC3535] that discussed the strengths and
weaknesses of some IETF network management protocols and compared
them to operational needs, especially configuration.
One issue discussed was the user-unfriendliness of the binary format
of SNMP [RFC3410]. In the case of LLNs, it must be noted that at the
time of writing, the CoRE working group is actively working on
resource management of devices in LLNs. Still, it is felt that this
section provides important guidance on how RPL should be deployed,
operated, and managed.
As stated in [RFC5706]:
A management information model should include a discussion of what
is manageable, which aspects of the protocol need to be
configured, what types of operations are allowed, what protocol-
specific events might occur, which events can be counted, and for
which events an operator should be notified.
These aspects are discussed in detail in the following sections.
RPL will be used on a variety of devices that may have resources such
as memory varying from a few kilobytes to several hundreds of
kilobytes and even megabytes. When memory is highly constrained, it
may not be possible to satisfy all the requirements listed in this
section. Still it is worth listing all of these in an exhaustive
fashion, and implementers will then determine which of these
requirements could be satisfied according to the available resources
on the device.
Winter, et al. Standards Track [Page 114]
RFC 6550 RPL March 2012
18.2. Configuration Management
This section discusses the configuration management, listing the
protocol parameters for which configuration management is relevant.
Some of the RPL parameters are optional. The requirements for
configuration are only applicable for the options that are used.
18.2.1. Initialization Mode
"Architectural Principles of the Internet" [RFC1958], Section 3.8,
states: "Avoid options and parameters whenever possible. Any options
and parameters should be configured or negotiated dynamically rather
than manually". This is especially true in LLNs where the number of
devices may be large and manual configuration is infeasible. This
has been taken into account in the design of RPL whereby the DODAG
root provides a number of parameters to the devices joining the
DODAG, thus avoiding cumbersome configuration on the routers and
potential sources of misconfiguration (e.g., values of Trickle
timers, etc.). Still, there are additional RPL parameters that a RPL
implementation should allow to be configured, which are discussed in
this section.
18.2.1.1. DIS Mode of Operation upon Boot-Up
When a node is first powered up:
1. The node may decide to stay silent, waiting to receive DIO
messages from DODAG of interest (advertising a supported OF and
metrics/constraints) and not send any multicast DIO messages
until it has joined a DODAG.
2. The node may decide to send one or more DIS messages (optionally,
requesting DIO for a specific DODAG) as an initial probe for
nearby DODAGs, and in the absence of DIO messages in reply after
some configurable period of time, the node may decide to root a
floating DODAG and start sending multicast DIO messages.
A RPL implementation SHOULD allow configuring the preferred mode of
operation listed above along with the required parameters (in the
second mode: the number of DIS messages and related timer).
18.2.2. DIO and DAO Base Message and Options Configuration
RPL specifies a number of protocol parameters considering the large
spectrum of applications where it will be used. That said,
particular attention has been given to limiting the number of these
parameters that must be configured on each RPL router. Instead, a
Winter, et al. Standards Track [Page 115]
RFC 6550 RPL March 2012
number of the default values can be used, and when required these
parameters can be provided by the DODAG root thus allowing for
dynamic parameter setting.
A RPL implementation SHOULD allow configuring the following routing
protocol parameters. As pointed out above, note that a large set of
parameters is configured on the DODAG root.
18.2.3. Protocol Parameters to Be Configured on Every Router in the LLN
A RPL implementation MUST allow configuring the following RPL
parameters:
o RPLInstanceID [DIO message, in DIO Base message]. Although the
RPLInstanceID must be configured on the DODAG root, it must also
be configured as a policy on every node in order to determine
whether or not the node should join a particular DODAG. Note that
a second RPLInstanceID can be configured on the node, should it
become root of a floating DODAG.
o List of supported Objective Code Points (OCPs)
o List of supported metrics: [RFC6551] specifies a number of metrics
and constraints used for the DODAG formation. Thus, a RPL
implementation should allow configuring the list of metrics that a
node can accept and understand. If a DIO is received with a
metric and/or constraint that is not understood or supported, as
specified in Section 8.5, the node would join as a leaf node.
o Prefix Information, along with valid and preferred lifetime and
the 'L' and 'A' flags. [DIO message, Prefix Information Option].
A RPL implementation SHOULD allow configuring if the Prefix
Information option must be carried with the DIO message to
distribute the Prefix Information for autoconfiguration. In that
case, the RPL implementation MUST allow the list of prefixes to be
advertised in the PIO along with the corresponding flags.
o Solicited Information [DIS message, in Solicited Information
option]. Note that a RPL implementation SHOULD allow configuring
when such messages should be sent and under which circumstances,
along with the value of the RPLInstance ID, 'V'/'I'/'D' flags.
o 'K' flag: when a node should set the 'K' flag in a DAO message
[DAO message, in DAO Base message].
o MOP (Mode of Operation) [DIO message, in DIO Base message].
Winter, et al. Standards Track [Page 116]
RFC 6550 RPL March 2012
o Route Information (and preference) [DIO message, in Route
Information option]
18.2.4. Protocol Parameters to Be Configured on Every Non-DODAG-Root
Router in the LLN
A RPL implementation MUST allow configuring the Target prefix [DAO
message, in RPL Target option].
Furthermore, there are circumstances where a node may want to
designate a Target to allow for specific processing of the Target
(prioritization, etc.). Such processing rules are out of scope for
this specification. When used, a RPL implementation SHOULD allow
configuring the Target Descriptor on a per-Target basis (for example,
using access lists).
A node whose DODAG parent set is empty may become the DODAG root of a
floating DODAG. It may also set its DAGPreference such that it is
less preferred. Thus, a RPL implementation MUST allow configuring
the set of actions that the node should initiate in this case:
o Start its own (floating) DODAG: the new DODAGID must be configured
in addition to its DAGPreference.
o Poison the broken path (see procedure in Section 8.2.2.5).
o Trigger a local repair.
18.2.5. Parameters to Be Configured on the DODAG Root
In addition, several other parameters are configured only on the
DODAG root and advertised in options carried in DIO messages.
As specified in Section 8.3, a RPL implementation makes use of
Trickle timers to govern the sending of DIO messages. The operation
of the Trickle algorithm is determined by a set of configurable
parameters, which MUST be configurable and that are then advertised
by the DODAG root along the DODAG in DIO messages.
o DIOIntervalDoublings [DIO message, in DODAG Configuration option]
o DIOIntervalMin [DIO message, in DODAG Configuration option]
o DIORedundancyConstant [DIO message, in DODAG Configuration option]
In addition, a RPL implementation SHOULD allow for configuring the
following set of RPL parameters:
Winter, et al. Standards Track [Page 117]
RFC 6550 RPL March 2012
o Path Control Size [DIO message, in DODAG Configuration option]
o MinHopRankIncrease [DIO message, in DODAG Configuration option]
o The DODAGPreference field [DIO message, DIO Base object]
o DODAGID [DIO message, in DIO Base option] and [DAO message, when
the 'D' flag of the DAO message is set]
DAG root behavior: in some cases, a node may not want to permanently
act as a floating DODAG root if it cannot join a grounded DODAG. For
example, a battery-operated node may not want to act as a floating
DODAG root for a long period of time. Thus, a RPL implementation MAY
support the ability to configure whether or not a node could act as a
floating DODAG root for a configured period of time.
DAG Version Number Increment: a RPL implementation may allow, by
configuration at the DODAG root, refreshing the DODAG states by
updating the DODAGVersionNumber. A RPL implementation SHOULD allow
configuring whether or not periodic or event triggered mechanisms are
used by the DODAG root to control DODAGVersionNumber change (which
triggers a global repair as specified in Section 3.2.2).
18.2.6. Configuration of RPL Parameters Related to DAO-Based Mechanisms
DAO messages are optional and used in DODAGs that require Downward
routing operation. This section deals with the set of parameters
related to DAO messages and provides recommendations on their
configuration.
As stated in Section 9.5, it is recommended to delay the sending of
DAO message to DAO parents in order to maximize the chances to
perform route aggregation. Upon receiving a DAO message, the node
should thus start a DelayDAO timer. The default value is
DEFAULT_DAO_DELAY. A RPL implementation MAY allow for configuring
the DelayDAO timer.
In a Storing mode of operation, a storing node may increment DTSN in
order to reliably trigger a set of DAO updates from its immediate
children, as part of routine routing table updates and maintenance.
A RPL implementation MAY allow for configuring a set of rules
specifying the triggers for DTSN increment (manual or event-based).
When a DAO entry times out or is invalidated, a node SHOULD make a
reasonable attempt to report a No-Path to each of the DAO parents.
That number of attempts MAY be configurable.
Winter, et al. Standards Track [Page 118]
RFC 6550 RPL March 2012
An implementation should support rate-limiting the sending of DAO
messages. The related parameters MAY be configurable.
18.2.7. Configuration of RPL Parameters Related to Security Mechanisms
As described in Section 10, the security features described in this
document are optional to implement and a given implementation may
support a subset (including the empty set) of the described security
features.
To this end, an implementation supporting described security features
may conceptually implement a security policy database. In support of
the security mechanisms, a RPL implementation SHOULD allow for
configuring a subset of the following parameters:
o Security Modes accepted [Unsecured mode, Preinstalled mode,
Authenticated mode]
o KIM values accepted [Secure RPL control messages, in Security
section]
o Level values accepted [Secure RPL control messages, in Security
section]
o Algorithm values accepted [Secure RPL control messages, in
Security section]
o Key material in support of Authenticated or Preinstalled key
modes.
In addition, a RPL implementation SHOULD allow for configuring a
DODAG root with a subset of the following parameters:
o Level values advertised [Secure DIO message, in Security section]
o KIM value advertised [Secure DIO message, in Security section]
o Algorithm value advertised [Secure DIO message, in Security
section]
18.2.8. Default Values
This document specifies default values for the following set of RPL
variables:
DEFAULT_PATH_CONTROL_SIZE
DEFAULT_DIO_INTERVAL_MIN
DEFAULT_DIO_INTERVAL_DOUBLINGS
DEFAULT_DIO_REDUNDANCY_CONSTANT
Winter, et al. Standards Track [Page 119]
RFC 6550 RPL March 2012
DEFAULT_MIN_HOP_RANK_INCREASE
DEFAULT_DAO_DELAY
It is recommended to specify default values in protocols; that being
said, as discussed in [RFC5706], default values may make less and
less sense. RPL is a routing protocol that is expected to be used in
a number of contexts where network characteristics such as the number
of nodes and link and node types are expected to vary significantly.
Thus, these default values are likely to change with the context and
as the technology evolves. Indeed, LLNs' related technology (e.g.,
hardware, link layers) have been evolving dramatically over the past
few years and such technologies are expected to change and evolve
considerably in the coming years.
The proposed values are not based on extensive best current practices
and are considered to be conservative.
18.3. Monitoring of RPL Operation
Several RPL parameters should be monitored to verify the correct
operation of the routing protocol and the network itself. This
section lists the set of monitoring parameters of interest.
18.3.1. Monitoring a DODAG Parameters
A RPL implementation SHOULD provide information about the following
parameters:
o DODAG Version number [DIO message, in DIO Base message]
o Status of the 'G' flag [DIO message, in DIO Base message]
o Status of the MOP field [DIO message, in DIO Base message]
o Value of the DTSN [DIO message, in DIO Base message]
o Value of the Rank [DIO message, in DIO Base message]
o DAOSequence: Incremented at each unique DAO message, echoed in the
DAO-ACK message [DAO and DAO-ACK messages]
o Route Information [DIO message, Route Information Option] (list of
IPv6 prefixes per parent along with lifetime and preference]
o Trickle parameters:
* DIOIntervalDoublings [DIO message, in DODAG Configuration
option]
Winter, et al. Standards Track [Page 120]
RFC 6550 RPL March 2012
* DIOIntervalMin [DIO message, in DODAG Configuration option]
* DIORedundancyConstant [DIO message, in DODAG Configuration
option]
o Path Control Size [DIO message, in DODAG Configuration option]
o MinHopRankIncrease [DIO message, in DODAG Configuration option]
Values that may be monitored only on the DODAG root:
o Transit Information [DAO, Transit Information option]: A RPL
implementation SHOULD allow configuring whether the set of
received Transit Information options should be displayed on the
DODAG root. In this case, the RPL database of received Transit
Information should also contain the Path Sequence, Path Control,
Path Lifetime, and Parent Address.
18.3.2. Monitoring a DODAG Inconsistencies and Loop Detection
Detection of DODAG inconsistencies is particularly critical in RPL
networks. Thus, it is recommended for a RPL implementation to
provide appropriate monitoring tools. A RPL implementation SHOULD
provide a counter reporting the number of a times the node has
detected an inconsistency with respect to a DODAG parent, e.g., if
the DODAGID has changed.
When possible more granular information about inconsistency detection
should be provided. A RPL implementation MAY provide counters
reporting the number of following inconsistencies:
o Packets received with 'O' bit set (to Down) from a node with a
higher Rank
o Packets received with 'O' bit cleared (to Up) from a node with a
lower Rank
o Number of packets with the 'F' bit set
o Number of packets with the 'R' bit set
18.4. Monitoring of the RPL Data Structures
18.4.1. Candidate Neighbor Data Structure
A node in the candidate neighbor list is a node discovered by the
same means and qualified to potentially become a parent (with high
enough local confidence). A RPL implementation SHOULD provide a way
Winter, et al. Standards Track [Page 121]
RFC 6550 RPL March 2012
to allow for the candidate neighbor list to be monitored with some
metric reflecting local confidence (the degree of stability of the
neighbors) as measured by some metrics.
A RPL implementation MAY provide a counter reporting the number of
times a candidate neighbor has been ignored, should the number of
candidate neighbors exceed the maximum authorized value.
18.4.2. Destination-Oriented Directed Acyclic Graph (DODAG) Table
For each DODAG, a RPL implementation is expected to keep track of the
following DODAG table values:
o RPLInstanceID
o DODAGID
o DODAGVersionNumber
o Rank
o Objective Code Point
o A set of DODAG parents
o A set of prefixes offered Upward along the DODAG
o Trickle timers used to govern the sending of DIO messages for the
DODAG
o List of DAO parents
o DTSN
o Node status (router versus leaf)
A RPL implementation SHOULD allow for monitoring the set of
parameters listed above.
18.4.3. Routing Table and DAO Routing Entries
A RPL implementation maintains several information elements related
to the DODAG and the DAO entries (for storing nodes). In the case of
a non-storing node, a limited amount of information is maintained
(the routing table is mostly reduced to a set of DODAG parents along
with characteristics of the DODAG as mentioned above); whereas in the
case of storing nodes, this information is augmented with routing
entries.
Winter, et al. Standards Track [Page 122]
RFC 6550 RPL March 2012
A RPL implementation SHOULD allow for the following parameters to be
monitored:
o Next Hop (DODAG parent)
o Next Hop Interface
o Path metrics value for each DODAG parent
A DAO Routing Table entry conceptually contains the following
elements (for storing nodes only):
o Advertising Neighbor Information
o IPv6 address
o Interface ID to which DAO parents has this entry been reported
o Retry counter
o Logical equivalent of DAO Content:
* DAO-Sequence
* Path Sequence
* DAO Lifetime
* DAO Path Control
o Destination Prefix (or address or Mcast Group)
A RPL implementation SHOULD provide information about the state of
each DAO Routing Table entry states.
18.5. Fault Management
Fault management is a critical component used for troubleshooting,
verification of the correct mode of operation of the protocol, and
network design; also, it is a key component of network performance
monitoring. A RPL implementation SHOULD allow the provision of the
following information related to fault managements:
o Memory overflow along with the cause (e.g., routing tables
overflow, etc.)
o Number of times a packet could not be sent to a DODAG parent
flagged as valid
Winter, et al. Standards Track [Page 123]
RFC 6550 RPL March 2012
o Number of times a packet has been received for which the router
did not have a corresponding RPLInstanceID
o Number of times a local repair procedure was triggered
o Number of times a global repair was triggered by the DODAG root
o Number of received malformed messages
o Number of seconds with packets to forward and no next hop (DODAG
parent)
o Number of seconds without next hop (DODAG parent)
o Number of times a node has joined a DODAG as a leaf because it
received a DIO with a metric/constraint that was not understood
and it was configured to join as a leaf node in this case (see
Section 18.6)
It is RECOMMENDED to report faults via at least error log messages.
Other protocols may be used to report such faults.
18.6. Policy
Policy rules can be used by a RPL implementation to determine whether
or not the node is allowed to join a particular DODAG advertised by a
neighbor by means of DIO messages.
This document specifies operation within a single DODAG. A DODAG is
characterized by the following tuple (RPLInstanceID, DODAGID).
Furthermore, as pointed out above, DIO messages are used to advertise
other DODAG characteristics such as the routing metrics and
constraints used to build to the DODAG and the Objective Function in
use (specified by OCP).
The first policy rules consist of specifying the following conditions
that a RPL node must satisfy to join a DODAG:
o RPLInstanceID
o List of supported routing metrics and constraints
o Objective Function (OCP values)
A RPL implementation MUST allow configuring these parameters and
SHOULD specify whether the node must simply ignore the DIO if the
advertised DODAG is not compliant with the local policy or whether
the node should join as the leaf node if only the list of supported
Winter, et al. Standards Track [Page 124]
RFC 6550 RPL March 2012
routing metrics and constraints, and the OF is not supported.
Additionally, a RPL implementation SHOULD allow for the addition of
the DODAGID as part of the policy.
A RPL implementation SHOULD allow configuring the set of acceptable
or preferred Objective Functions (OFs) referenced by their Objective
Code Points (OCPs) for a node to join a DODAG, and what action should
be taken if none of a node's candidate neighbors advertise one of the
configured allowable Objective Functions, or if the advertised
metrics/constraint is not understood/supported. Two actions can be
taken in this case:
o The node joins the DODAG as a leaf node as specified in
Section 8.5.
o The node does not join the DODAG.
A node in an LLN may learn routing information from different routing
protocols including RPL. In this case, it is desirable to control,
via administrative preference, which route should be favored. An
implementation SHOULD allow for the specification of an
administrative preference for the routing protocol from which the
route was learned.
Internal Data Structures: some RPL implementations may limit the size
of the candidate neighbor list in order to bound the memory usage; in
which case, some otherwise viable candidate neighbors may not be
considered and simply dropped from the candidate neighbor list.
A RPL implementation MAY provide an indicator on the size of the
candidate neighbor list.
18.7. Fault Isolation
It is RECOMMENDED to quarantine neighbors that start emitting
malformed messages at unacceptable rates.
18.8. Impact on Other Protocols
RPL has very limited impact on other protocols. Where more than one
routing protocol is required on a router, such as an LBR, it is
expected for the device to support routing redistribution functions
between the routing protocols to allow for reachability between the
two routing domains. Such redistribution SHOULD be governed by the
use of user configurable policy.
Winter, et al. Standards Track [Page 125]
RFC 6550 RPL March 2012
With regard to the impact in terms of traffic on the network, RPL has
been designed to limit the control traffic thanks to mechanisms such
as Trickle timers (Section 8.3). Thus, the impact of RPL on other
protocols should be extremely limited.
18.9. Performance Management
Performance management is always an important aspect of a protocol,
and RPL is not an exception. Several metrics of interest have been
specified by the IP Performance Monitoring (IPPM) working group: that
being said, they will be hardly applicable to LLN considering the
cost of monitoring these metrics in terms of resources on the devices
and required bandwidth. Still, RPL implementations MAY support some
of these, and other parameters of interest are listed below:
o Number of repairs and time to repair in seconds (average,
variance)
o Number of times and time period during which a devices could not
forward a packet because of a lack of a reachable neighbor in its
routing table
o Monitoring of resources consumption by RPL in terms of bandwidth
and required memory
o Number of RPL control messages sent and received
18.10. Diagnostics
There may be situations where a node should be placed in "verbose"
mode to improve diagnostics. Thus, a RPL implementation SHOULD
provide the ability to place a node in and out of verbose mode in
order to get additional diagnostic information.
19. Security Considerations
19.1. Overview
From a security perspective, RPL networks are no different from any
other network. They are vulnerable to passive eavesdropping attacks
and, potentially, even active tampering when physical access to a
wire is not required to participate in communications. The very
nature of ad hoc networks and their cost objectives impose additional
security constraints, which perhaps make these networks the most
difficult environments to secure. Devices are low-cost and have
limited capabilities in terms of computing power, available storage,
and power drain; it cannot always be assumed they have a trusted
computing base or a high-quality random number generator aboard.
Winter, et al. Standards Track [Page 126]
RFC 6550 RPL March 2012
Communications cannot rely on the online availability of a fixed
infrastructure and might involve short-term relationships between
devices that may never have communicated before. These constraints
might severely limit the choice of cryptographic algorithms and
protocols and influence the design of the security architecture
because the establishment and maintenance of trust relationships
between devices need to be addressed with care. In addition, battery
lifetime and cost constraints put severe limits on the security
overhead these networks can tolerate, something that is of far less
concern with higher bandwidth networks. Most of these security
architectural elements can be implemented at higher layers and may,
therefore, be considered to be out of scope for this specification.
Special care, however, needs to be exercised with respect to
interfaces to these higher layers.
The security mechanisms in this standard are based on symmetric-key
and public-key cryptography and use keys that are to be provided by
higher-layer processes. The establishment and maintenance of these
keys are out of scope for this specification. The mechanisms assume
a secure implementation of cryptographic operations and secure and
authentic storage of keying material.
The security mechanisms specified provide particular combinations of
the following security services:
Data confidentiality: Assurance that transmitted information is only
disclosed to parties for which it is intended.
Data authenticity: Assurance of the source of transmitted information
(and, hereby, that information was not modified in transit).
Replay protection: Assurance that a duplicate of transmitted
information is detected.
Timeliness (delay protection): Assurance that transmitted
information was received in a timely manner.
The actual protection provided can be adapted on a per-packet basis
and allows for varying levels of data authenticity (to minimize
security overhead in transmitted packets where required) and for
optional data confidentiality. When nontrivial protection is
required, replay protection is always provided.
Replay protection is provided via the use of a non-repeating value
(CCM nonce) in the packet protection process and storage of some
status information (originating device and the CCM nonce counter last
received from that device), which allows detection of whether this
particular CCM nonce value was used previously by the originating
Winter, et al. Standards Track [Page 127]
RFC 6550 RPL March 2012
device. In addition, so-called delay protection is provided amongst
those devices that have a loosely synchronized clock on board. The
acceptable time delay can be adapted on a per-packet basis and allows
for varying latencies (to facilitate longer latencies in packets
transmitted over a multi-hop communication path).
Cryptographic protection may use a key shared between two peer
devices (link key) or a key shared among a group of devices (group
key), thus allowing some flexibility and application-specific trade-
offs between key storage and key maintenance costs versus the
cryptographic protection provided. If a group key is used for peer-
to-peer communication, protection is provided only against outsider
devices and not against potential malicious devices in the key-
sharing group.
Data authenticity may be provided using symmetric-key-based or
public-key-based techniques. With public-key-based techniques (via
signatures), one corroborates evidence as to the unique originator of
transmitted information, whereas with symmetric-key-based techniques,
data authenticity is only provided relative to devices in a key-
sharing group. Thus, public-key-based authentication may be useful
in scenarios that require a more fine-grained authentication than can
be provided with symmetric-key-based authentication techniques alone,
such as with group communications (broadcast, multicast) or in
scenarios that require non-repudiation.
20. IANA Considerations
20.1. RPL Control Message
The RPL control message is an ICMP information message type that is
to be used carry DODAG Information Objects, DODAG Information
Solicitations, and Destination Advertisement Objects in support of
RPL operation.
IANA has defined an ICMPv6 Type Number Registry. The type value for
the RPL control message is 155.
20.2. New Registry for RPL Control Codes
IANA has created a registry, RPL Control Codes, for the Code field of
the ICMPv6 RPL control message.
New codes may be allocated only by an IETF Review. Each code is
tracked with the following qualities:
o Code
Winter, et al. Standards Track [Page 128]
RFC 6550 RPL March 2012
o Description
o Defining RFC
The following codes are currently defined:
+------+----------------------------------------------+-------------+
| Code | Description | Reference |
+------+----------------------------------------------+-------------+
| 0x00 | DODAG Information Solicitation | This |
| | | document |
| | | |
| 0x01 | DODAG Information Object | This |
| | | document |
| | | |
| 0x02 | Destination Advertisement Object | This |
| | | document |
| | | |
| 0x03 | Destination Advertisement Object | This |
| | Acknowledgment | document |
| | | |
| 0x80 | Secure DODAG Information Solicitation | This |
| | | document |
| | | |
| 0x81 | Secure DODAG Information Object | This |
| | | document |
| | | |
| 0x82 | Secure Destination Advertisement Object | This |
| | | document |
| | | |
| 0x83 | Secure Destination Advertisement Object | This |
| | Acknowledgment | document |
| | | |
| 0x8A | Consistency Check | This |
| | | document |
+------+----------------------------------------------+-------------+
RPL Control Codes
20.3. New Registry for the Mode of Operation (MOP)
IANA has created a registry for the 3-bit Mode of Operation (MOP),
which is contained in the DIO Base.
New values may be allocated only by an IETF Review. Each value is
tracked with the following qualities:
o Mode of Operation Value
Winter, et al. Standards Track [Page 129]
RFC 6550 RPL March 2012
o Capability description
o Defining RFC
Four values are currently defined:
+----------+------------------------------------------+-------------+
| MOP | Description | Reference |
| value | | |
+----------+------------------------------------------+-------------+
| 0 | No Downward routes maintained by RPL | This |
| | | document |
| | | |
| 1 | Non-Storing Mode of Operation | This |
| | | document |
| | | |
| 2 | Storing Mode of Operation with no | This |
| | multicast support | document |
| | | |
| 3 | Storing Mode of Operation with multicast | This |
| | support | document |
+----------+------------------------------------------+-------------+
DIO Mode of Operation
The rest of the range, decimal 4 to 7, is currently unassigned.
20.4. RPL Control Message Options
IANA has created a registry for the RPL Control Message Options.
New values may be allocated only by an IETF Review. Each value is
tracked with the following qualities:
o Value
o Meaning
o Defining RFC
Winter, et al. Standards Track [Page 130]
RFC 6550 RPL March 2012
+-------+-----------------------+---------------+
| Value | Meaning | Reference |
+-------+-----------------------+---------------+
| 0x00 | Pad1 | This document |
| | | |
| 0x01 | PadN | This document |
| | | |
| 0x02 | DAG Metric Container | This Document |
| | | |
| 0x03 | Routing Information | This Document |
| | | |
| 0x04 | DODAG Configuration | This Document |
| | | |
| 0x05 | RPL Target | This Document |
| | | |
| 0x06 | Transit Information | This Document |
| | | |
| 0x07 | Solicited Information | This Document |
| | | |
| 0x08 | Prefix Information | This Document |
| | | |
| 0x09 | Target Descriptor | This Document |
+-------+-----------------------+---------------+
RPL Control Message Options
20.5. Objective Code Point (OCP) Registry
IANA has created a registry to manage the codespace of the Objective
Code Point (OCP) field.
No OCPs are defined in this specification.
New codes may be allocated only by an IETF Review. Each code is
tracked with the following qualities:
o Code
o Description
o Defining RFC
20.6. New Registry for the Security Section Algorithm
IANA has created a registry for the values of the 8-bit Algorithm
field in the Security section.
Winter, et al. Standards Track [Page 131]
RFC 6550 RPL March 2012
New values may be allocated only by an IETF Review. Each value is
tracked with the following qualities:
o Value
o Encryption/MAC
o Signature
o Defining RFC
The following value is currently defined:
+-------+------------------+------------------+---------------+
| Value | Encryption/MAC | Signature | Reference |
+-------+------------------+------------------+---------------+
| 0 | CCM with AES-128 | RSA with SHA-256 | This document |
+-------+------------------+------------------+---------------+
Security Section Algorithm
20.7. New Registry for the Security Section Flags
IANA has created a registry for the 8-bit Security Section Flags
field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
No bit is currently defined for the Security Section Flags field.
20.8. New Registry for Per-KIM Security Levels
IANA has created one registry for the 3-bit Security Level (LVL)
field per allocated KIM value.
For a given KIM value, new levels may be allocated only by an IETF
Review. Each level is tracked with the following qualities:
o Level
o KIM value
Winter, et al. Standards Track [Page 132]
RFC 6550 RPL March 2012
o Description
o Defining RFC
The following levels per KIM value are currently defined:
+-------+-----------+---------------+---------------+
| Level | KIM value | Description | Reference |
+-------+-----------+---------------+---------------+
| 0 | 0 | See Figure 11 | This document |
| | | | |
| 1 | 0 | See Figure 11 | This document |
| | | | |
| 2 | 0 | See Figure 11 | This document |
| | | | |
| 3 | 0 | See Figure 11 | This document |
| | | | |
| 0 | 1 | See Figure 11 | This document |
| | | | |
| 1 | 1 | See Figure 11 | This document |
| | | | |
| 2 | 1 | See Figure 11 | This document |
| | | | |
| 3 | 1 | See Figure 11 | This document |
| | | | |
| 0 | 2 | See Figure 11 | This document |
| | | | |
| 1 | 2 | See Figure 11 | This document |
| | | | |
| 2 | 2 | See Figure 11 | This document |
| | | | |
| 3 | 2 | See Figure 11 | This document |
| | | | |
| 0 | 3 | See Figure 11 | This document |
| | | | |
| 1 | 3 | See Figure 11 | This document |
| | | | |
| 2 | 3 | See Figure 11 | This document |
| | | | |
| 3 | 3 | See Figure 11 | This document |
+-------+-----------+---------------+---------------+
Per-KIM Security Levels
20.9. New Registry for DODAG Informational Solicitation (DIS) Flags
IANA has created a registry for the DIS (DODAG Informational
Solicitation) Flags field.
Winter, et al. Standards Track [Page 133]
RFC 6550 RPL March 2012
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
No bit is currently defined for the DIS (DODAG Informational
Solicitation) Flags field.
20.10. New Registry for the DODAG Information Object (DIO) Flags
IANA has created a registry for the 8-bit DODAG Information Object
(DIO) Flags field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
No bit is currently defined for the DIS (DODAG Informational
Solicitation) Flags.
20.11. New Registry for the Destination Advertisement Object (DAO)
Flags
IANA has created a registry for the 8-bit Destination Advertisement
Object (DAO) Flags field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
Winter, et al. Standards Track [Page 134]
RFC 6550 RPL March 2012
The following bits are currently defined:
+------------+------------------------------+---------------+
| Bit number | Description | Reference |
+------------+------------------------------+---------------+
| 0 | DAO-ACK request (K) | This document |
| | | |
| 1 | DODAGID field is present (D) | This document |
+------------+------------------------------+---------------+
DAO Base Flags
20.12. New Registry for the Destination Advertisement Object (DAO)
Acknowledgement Flags
IANA has created a registry for the 8-bit Destination Advertisement
Object (DAO) Acknowledgement Flags field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
The following bit is currently defined:
+------------+------------------------------+---------------+
| Bit number | Description | Reference |
+------------+------------------------------+---------------+
| 0 | DODAGID field is present (D) | This document |
+------------+------------------------------+---------------+
DAO-ACK Base Flags
20.13. New Registry for the Consistency Check (CC) Flags
IANA has created a registry for the 8-bit Consistency Check (CC)
Flags field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
Winter, et al. Standards Track [Page 135]
RFC 6550 RPL March 2012
o Defining RFC
The following bit is currently defined:
+------------+-----------------+---------------+
| Bit number | Description | Reference |
+------------+-----------------+---------------+
| 0 | CC Response (R) | This document |
+------------+-----------------+---------------+
Consistency Check Base Flags
20.14. New Registry for the DODAG Configuration Option Flags
IANA has created a registry for the 8-bit DODAG Configuration Option
Flags field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
The following bits are currently defined:
+------------+----------------------------+---------------+
| Bit number | Description | Reference |
+------------+----------------------------+---------------+
| 4 | Authentication Enabled (A) | This document |
| 5-7 | Path Control Size (PCS) | This document |
+------------+----------------------------+---------------+
DODAG Configuration Option Flags
20.15. New Registry for the RPL Target Option Flags
IANA has created a registry for the 8-bit RPL Target Option Flags
field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
Winter, et al. Standards Track [Page 136]
RFC 6550 RPL March 2012
o Defining RFC
No bit is currently defined for the RPL Target Option Flags.
20.16. New Registry for the Transit Information Option Flags
IANA has created a registry for the 8-bit Transit Information Option
(TIO) Flags field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
The following bits are currently defined:
+------------+--------------+---------------+
| Bit number | Description | Reference |
+------------+--------------+---------------+
| 0 | External (E) | This document |
+------------+--------------+---------------+
Transit Information Option Flags
20.17. New Registry for the Solicited Information Option Flags
IANA has created a registry for the 8-bit Solicited Information
Option (SIO) Flags field.
New bit numbers may be allocated only by an IETF Review. Each bit is
tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
Winter, et al. Standards Track [Page 137]
RFC 6550 RPL March 2012
The following bits are currently defined:
+------------+--------------------------------+---------------+
| Bit number | Description | Reference |
+------------+--------------------------------+---------------+
| 0 | Version Predicate match (V) | This document |
| | | |
| 1 | InstanceID Predicate match (I) | This document |
| | | |
| 2 | DODAGID Predicate match (D) | This document |
+------------+--------------------------------+---------------+
Solicited Information Option Flags
20.18. ICMPv6: Error in Source Routing Header
In some cases RPL will return an ICMPv6 error message when a message
cannot be delivered as specified by its source routing header. This
ICMPv6 error message is "Error in Source Routing Header".
IANA has defined an ICMPv6 "Code" Fields Registry for ICMPv6 Message
Types. ICMPv6 Message Type 1 describes "Destination Unreachable"
codes. The "Error in Source Routing Header" code is has been
allocated from the ICMPv6 Code Fields Registry for ICMPv6 Message
Type 1, with a code value of 7.
20.19. Link-Local Scope Multicast Address
The rules for assigning new IPv6 multicast addresses are defined in
[RFC3307]. This specification requires the allocation of a new
permanent multicast address with a link-local scope for RPL nodes
called all-RPL-nodes, with a value of ff02::1a.
21. Acknowledgements
The authors would like to acknowledge the review, feedback, and
comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
Yoav Ben-Yehezkel, Phoebus Chen, Quynh Dang, Mischa Dohler, Mathilde
Durvy, Joakim Eriksson, Omprakash Gnawali, Manhar Goindi, Mukul
Goyal, Ulrich Herberg, Anders Jagd, JeongGil (John) Ko, Ajay Kumar,
Quentin Lampin, Jerry Martocci, Matteo Paris, Alexandru Petrescu,
Joseph Reddy, Michael Richardson, Don Sturek, Joydeep Tripathi, and
Nicolas Tsiftes.
The authors would like to acknowledge the guidance and input provided
by the ROLL Chairs, David Culler and JP. Vasseur, and the Area
Director, Adrian Farrel.
Winter, et al. Standards Track [Page 138]
RFC 6550 RPL March 2012
The authors would like to acknowledge prior contributions of Robert
Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Boot,
Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas
Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Moon,
Jim Bound, Yanick Pouffary, Henning Rogge, and Arsalan Tavakoli, who
have provided useful design considerations to RPL.
RPL Security Design, found in Section 10, Section 19, and elsewhere
throughout the document, is primarily the contribution of the
Security Design Team: Tzeta Tsao, Roger Alexander, Dave Ward, Philip
Levis, Kris Pister, Rene Struik, and Adrian Farrel.
Thanks also to Jari Arkko and Ralph Droms for their attentive
reviews, especially with respect to interoperability considerations
and integration with other IETF specifications.
22. Contributors
Stephen Dawson-Haggerty
UC Berkeley
Soda Hall
Berkeley, CA 94720
USA
EMail: stevedh@cs.berkeley.edu
23. References
23.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences
and More-Specific Routes", RFC 4191, November 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
Winter, et al. Standards Track [Page 139]
RFC 6550 RPL March 2012
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 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.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J.
Ko, "The Trickle Algorithm", RFC 6206, March 2011.
[RFC6275] Perkins, C., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, July 2011.
[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean,
N., and D. Barthel, "Routing Metrics Used for Path
Calculation in Low-Power and Lossy Networks", RFC 6551,
March 2012.
[RFC6552] Thubert, P., Ed., "Objective Function Zero for the
Routing Protocol for Low-Power and Lossy Networks
(RPL)", RFC 6552, 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.
[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.
23.2. Informative References
[6LOWPAN-ND] Shelby, Z., Ed., Chakrabarti, S., and E. Nordmark,
"Neighbor Discovery Optimization for Low Power and
Lossy Networks (6LoWPAN)", Work in Progress,
October 2011.
[FIPS180] National Institute of Standards and Technology, "FIPS
Pub 180-3, Secure Hash Standard (SHS)", US Department
of Commerce , February 2008,
<http://www.nist.gov/itl/upload/fips180-3_final.pdf>.
Winter, et al. Standards Track [Page 140]
RFC 6550 RPL March 2012
[Perlman83] Perlman, R., "Fault-Tolerant Broadcast of Routing
Information", North-Holland Computer Networks,
Vol.7: p. 395-405, December 1983.
[RFC1958] Carpenter, B., "Architectural Principles of the
Internet", RFC 1958, June 1996.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic",
RFC 1982, August 1996.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management
Information Version 2 (SMIv2)", STD 58, RFC 2578,
April 1999.
[RFC3307] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC 3307, August 2002.
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for
Internet-Standard Management Framework", RFC 3410,
December 2002.
[RFC3535] Schoenwaelder, J., "Overview of the 2002 IAB Network
Management Workshop", RFC 3535, May 2003.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter
with CBC-MAC (CCM)", RFC 3610, September 2003.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and
L. Wood, "Advice for Internet Subnetwork Designers",
BCP 89, RFC 3819, July 2004.
[RFC4101] Rescorla, E. and IAB, "Writing Protocol Models",
RFC 4101, June 2005.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
February 2008.
Winter, et al. Standards Track [Page 141]
RFC 6550 RPL March 2012
[RFC5184] Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K.
Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3
(L3)-Driven Fast Handover", RFC 5184, May 2008.
[RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
"Routing Requirements for Urban Low-Power and Lossy
Networks", RFC 5548, May 2009.
[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
[RFC5706] Harrington, D., "Guidelines for Considering Operations
and Management of New Protocols and Protocol
Extensions", RFC 5706, November 2009.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks",
RFC 5826, April 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power
and Lossy Networks", RFC 5867, June 2010.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding
Detection (BFD) for IPv4 and IPv6 (Single Hop)",
RFC 5881, June 2010.
[RFC6130] Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
Network (MANET) Neighborhood Discovery Protocol
(NHDP)", RFC 6130, April 2011.
[ROLL-TERMS] Vasseur, J., "Terminology in Low power And Lossy
Networks", Work in Progress, September 2011.
Winter, et al. Standards Track [Page 142]
RFC 6550 RPL March 2012
Appendix A. Example Operation
This appendix provides some examples to illustrate the dissemination
of addressing information and prefixes with RPL. The examples depict
information being distributed with PIOs and RIOs and the use of DIO
and DAO messages. Note that this appendix is not normative, and that
the specific details of a RPL addressing plan and autoconfiguration
may vary according to specific implementations. RPL merely provides
a vehicle for disseminating information that may be built upon and
used by other mechanisms.
Note that these examples illustrate use of address autoconfiguration
schemes supported by information distributed within RPL. However, if
an implementation includes another address autoconfiguration scheme,
RPL nodes might be configured not to set the 'A' flag in PIO options,
though the PIO can still be used to distribute prefix and addressing
information.
A.1. Example Operation in Storing Mode with Node-Owned Prefixes
Figure 32 illustrates the logical addressing architecture of a simple
RPL network operating in Storing mode. In this example, each Node,
A, B, C, and D, owns its own prefix and makes that prefix available
for address autoconfiguration by on-link devices. (This is conveyed
by setting the 'A' flag and the 'L' flag in the PIO of the DIO
messages). Node A owns the prefix A::/64, Node B owns B::/64, and so
on. Node B autoconfigures an on-link address with respect to Node A,
A::B. Nodes C and D similarly autoconfigure on-link addresses from
Node B's prefix, B::C and B::D, respectively. Nodes have the option
of setting the 'R' flag and publishing their address within the
Prefix field of the PIO.
Winter, et al. Standards Track [Page 143]
RFC 6550 RPL March 2012
+-------------+
| Root |
| |
| Node A |
| |
| A::A |
+------+------+
|
|
|
+------+------+
| A::B |
| |
| Node B |
| |
| B::B |
+------+------+
|
|
.--------------+--------------.
/ \
/ \
+------+------+ +------+------+
| B::C | | B::D |
| | | |
| Node C | | Node D |
| | | |
| C::C | | D::D |
+-------------+ +-------------+
Figure 32: Storing Mode with Node-Owned Prefixes
A.1.1. DIO Messages and PIO
Node A, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Set
'R' flag: Clear
Prefix Length: 64
Prefix: A::
Node B, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Set
'R' flag: Set
Prefix Length: 64
Prefix: B::B
Winter, et al. Standards Track [Page 144]
RFC 6550 RPL March 2012
Node C, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Set
'R' flag: Clear
Prefix Length: 64
Prefix: C::
Node D, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Set
'R' flag: Set
Prefix Length: 64
Prefix: D::D
A.1.2. DAO Messages
Node B will send DAO messages to Node A with the following
information:
o Target B::/64
o Target C::/64
o Target D::/64
Node C will send DAO messages to Node B with the following
information:
o Target C::/64
Node D will send DAO messages to Node B with the following
information:
o Target D::/64
A.1.3. Routing Information Base
Node A will conceptually collect the following information into its
Routing Information Base (RIB):
o A::/64 connected
o B::/64 via B's link local
o C::/64 via B's link local
o D::/64 via B's link local
Node B will conceptually collect the following information into its
RIB:
o ::/0 via A's link local
o B::/64 connected
o C::/64 via C's link local
o D::/64 via D's link local
Winter, et al. Standards Track [Page 145]
RFC 6550 RPL March 2012
Node C will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o C::/64 connected
Node D will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o D::/64 connected
A.2. Example Operation in Storing Mode with Subnet-Wide Prefix
Figure 33 illustrates the logical addressing architecture of a simple
RPL network operating in Storing mode. In this example, the root
Node A sources a prefix that is used for address autoconfiguration
over the entire RPL subnet. (This is conveyed by setting the 'A'
flag and clearing the 'L' flag in the PIO of the DIO messages.)
Nodes A, B, C, and D all autoconfigure to the prefix A::/64. Nodes
have the option of setting the 'R' flag and publishing their address
within the Prefix field of the PIO.
Winter, et al. Standards Track [Page 146]
RFC 6550 RPL March 2012
+-------------+
| Root |
| |
| Node A |
| A::A |
| |
+------+------+
|
|
|
+------+------+
| |
| Node B |
| A::B |
| |
+------+------+
|
|
.--------------+--------------.
/ \
/ \
+------+------+ +------+------+
| | | |
| Node C | | Node D |
| A::C | | A::D |
| | | |
+-------------+ +-------------+
Figure 33: Storing Mode with Subnet-Wide Prefix
A.2.1. DIO Messages and PIO
Node A, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Clear
Prefix Length: 64
Prefix: A::
Node B, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Set
Prefix Length: 64
Prefix: A::B
Winter, et al. Standards Track [Page 147]
RFC 6550 RPL March 2012
Node C, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Clear
Prefix Length: 64
Prefix: A::
Node D, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Set
Prefix Length: 64
Prefix: A::D
A.2.2. DAO Messages
Node B will send DAO messages to Node A with the following
information:
o Target A::B/128
o Target A::C/128
o Target A::D/128
Node C will send DAO messages to Node B with the following
information:
o Target A::C/128
Node D will send DAO messages to Node B with the following
information:
o Target A::D/128
A.2.3. Routing Information Base
Node A will conceptually collect the following information into its
RIB:
o A::A/128 connected
o A::B/128 via B's link local
o A::C/128 via B's link local
o A::D/128 via B's link local
Node B will conceptually collect the following information into its
RIB:
o ::/0 via A's link local
o A::B/128 connected
o A::C/128 via C's link local
o A::D/128 via D's link local
Winter, et al. Standards Track [Page 148]
RFC 6550 RPL March 2012
Node C will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o A::C/128 connected
Node D will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o A::D/128 connected
A.3. Example Operation in Non-Storing Mode with Node-Owned Prefixes
Figure 34 illustrates the logical addressing architecture of a simple
RPL network operating in Non-Storing mode. In this example, each
Node, A, B, C, and D, owns its own prefix, and makes that prefix
available for address autoconfiguration by on-link devices. (This is
conveyed by setting the 'A' flag and the 'L' flag in the PIO of the
DIO messages.) Node A owns the prefix A::/64, Node B owns B::/64,
and so on. Node B autoconfigures an on-link address with respect to
Node A, A::B. Nodes C and D similarly autoconfigure on-link
addresses from Node B's prefix, B::C and B::D, respectively. Nodes
have the option of setting the 'R' flag and publishing their address
within the Prefix field of the PIO.
Winter, et al. Standards Track [Page 149]
RFC 6550 RPL March 2012
+-------------+
| Root |
| |
| Node A |
| |
| A::A |
+------+------+
|
|
|
+------+------+
| A::B |
| |
| Node B |
| |
| B::B |
+------+------+
|
|
.--------------+--------------.
/ \
/ \
+------+------+ +------+------+
| B::C | | B::D |
| | | |
| Node C | | Node D |
| | | |
| C::C | | D::D |
+-------------+ +-------------+
Figure 34: Non-Storing Mode with Node-Owned Prefixes
A.3.1. DIO Messages and PIO
The PIO contained in the DIO messages in the Non-Storing mode with
node-owned prefixes can be considered to be identical to those in the
Storing mode with node-owned prefixes case (Appendix A.1.1).
A.3.2. DAO Messages
Node B will send DAO messages to Node A with the following
information:
o Target B::/64, Transit A::B
Node C will send DAO messages to Node A with the following
information:
o Target C::/64, Transit B::C
Winter, et al. Standards Track [Page 150]
RFC 6550 RPL March 2012
Node D will send DAO messages to Node A with the following
information:
o Target D::/64, Transit B::D
A.3.3. Routing Information Base
Node A will conceptually collect the following information into its
RIB. Note that Node A has enough information to construct source
routes by doing recursive lookups into the RIB:
o A::/64 connected
o B::/64 via A::B
o C::/64 via B::C
o D::/64 via B::D
Node B will conceptually collect the following information into its
RIB:
o ::/0 via A's link local
o B::/64 connected
Node C will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o C::/64 connected
Node D will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o D::/64 connected
A.4. Example Operation in Non-Storing Mode with Subnet-Wide Prefix
Figure 35 illustrates the logical addressing architecture of a simple
RPL network operating in Non-Storing mode. In this example, the root
Node A sources a prefix that is used for address autoconfiguration
over the entire RPL subnet. (This is conveyed by setting the 'A'
flag and clearing the 'L' flag in the PIO of the DIO messages.)
Nodes A, B, C, and D all autoconfigure to the prefix A::/64. Nodes
must set the 'R' flag and publish their address within the Prefix
field of the PIO, in order to inform their children which address to
use in the transit option.
Winter, et al. Standards Track [Page 151]
RFC 6550 RPL March 2012
+-------------+
| Root |
| |
| Node A |
| A::A |
| |
+------+------+
|
|
|
+------+------+
| |
| Node B |
| A::B |
| |
+------+------+
|
|
.--------------+--------------.
/ \
/ \
+------+------+ +------+------+
| | | |
| Node C | | Node D |
| A::C | | A::D |
| | | |
+-------------+ +-------------+
Figure 35: Non-Storing Mode with Subnet-Wide Prefix
A.4.1. DIO Messages and PIO
Node A, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Set
Prefix Length: 64
Prefix: A::A
Node B, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Set
Prefix Length: 64
Prefix: A::B
Winter, et al. Standards Track [Page 152]
RFC 6550 RPL March 2012
Node C, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Set
Prefix Length: 64
Prefix: A::C
Node D, for example, will send DIO messages with a PIO as follows:
'A' flag: Set
'L' flag: Clear
'R' flag: Set
Prefix Length: 64
Prefix: A::D
A.4.2. DAO Messages
Node B will send DAO messages to Node A with the following
information:
o Target A::B/128, Transit A::A
Node C will send DAO messages to Node A with the following
information:
o Target A::C/128, Transit A::B
Node D will send DAO messages to Node A with the following
information:
o Target A::D/128, Transit A::B
A.4.3. Routing Information Base
Node A will conceptually collect the following information into its
RIB. Note that Node A has enough information to construct source
routes by doing recursive lookups into the RIB:
o A::A/128 connected
o A::B/128 via A::A
o A::C/128 via A::B
o A::D/128 via A::B
Node B will conceptually collect the following information into its
RIB:
o ::/0 via A's link local
o A::B/128 connected
Node C will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o A::C/128 connected
Winter, et al. Standards Track [Page 153]
RFC 6550 RPL March 2012
Node D will conceptually collect the following information into its
RIB:
o ::/0 via B's link local
o A::D/128 connected
A.5. Example with External Prefixes
Consider the simple network illustrated in Figure 36. In this
example, there are a group of routers participating in a RPL network:
a DODAG root, Nodes A, Y, and Z. The DODAG root and Node Z also have
connectivity to different external network domains (i.e., external to
the RPL network). Note that those external networks could be RPL
networks or another type of network altogether.
RPL Network +-------------------+
RPL::/64 | |
| External |
[RPL::Root] (Root)----------+ Prefix |
| | EXT_1::/64 |
| | |
| +-------------------+
[RPL::A] (A)
:
:
:
[RPL::Y] (Y)
| +-------------------+
| | |
| | External |
[RPL::Z] (Z)------------+ Prefix |
: | EXT_2::/64 |
: | |
: +-------------------+
Figure 36: Simple Network Example
In this example, the DODAG root makes a prefix available to the RPL
subnet for address autoconfiguration. Here, the entire RPL subnet
uses that same prefix, RPL::/64, for address autoconfiguration,
though in other implementations more complex/hybrid schemes could be
employed.
The DODAG root has connectivity to an external (with respect to that
RPL network) prefix EXT_1::/64. The DODAG root may have learned of
connectivity to this prefix, for example, via explicit configuration
or IPv6 ND on a non-RPL interface. The DODAG root is configured to
announce information on the connectivity to this prefix.
Winter, et al. Standards Track [Page 154]
RFC 6550 RPL March 2012
Similarly, Node Z has connectivity to an external prefix EXT_2::/64.
Node Z also has a sub-DODAG underneath of it.
1. The DODAG root adds a RIO to its DIO messages. The RIO contains
the external prefix EXT_1::/64. This information may be repeated
in the DIO messages emitted by the other nodes within the DODAG.
Thus, the reachability to the prefix EXT_1::/64 is disseminated
down the DODAG.
2. Node Z may advertise reachability to the Target network
EXT_2::/64 by sending DAO messages using EXT_2::/64 as a Target
in the Target option and itself (Node Z) as a parent in the
Transit Information option. (In Storing mode, that Transit
Information option does not need to contain the address of Node
Z). A non-storing root then becomes aware of the 1-hop link
(Node Z -- EXT_2::/64) for use in constructing source routes.
Node Z may additionally advertise its reachability to EXT_2::/64
to nodes in its sub-DODAG by sending DIO messages with a PIO,
with the 'A' flag cleared.
Winter, et al. Standards Track [Page 155]
RFC 6550 RPL March 2012
Authors' Addresses
Tim Winter (editor)
EMail: wintert@acm.org
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
France
Phone: +33 497 23 26 34
EMail: pthubert@cisco.com
Anders Brandt
Sigma Designs
Emdrupvej 26A, 1.
Copenhagen DK-2100
Denmark
EMail: abr@sdesigns.dk
Jonathan W. Hui
Arch Rock Corporation
501 2nd St., Suite 410
San Francisco, CA 94107
USA
EMail: jhui@archrock.com
Richard Kelsey
Ember Corporation
Boston, MA
USA
Phone: +1 617 951 1225
EMail: kelsey@ember.com
Winter, et al. Standards Track [Page 156]
RFC 6550 RPL March 2012
Philip Levis
Stanford University
358 Gates Hall, Stanford University
Stanford, CA 94305-9030
USA
EMail: pal@cs.stanford.edu
Kris Pister
Dust Networks
30695 Huntwood Ave.
Hayward, CA 94544
USA
EMail: kpister@dustnetworks.com
Rene Struik
Struik Security Consultancy
EMail: rstruik.ext@gmail.com
JP. Vasseur
Cisco Systems
11, Rue Camille Desmoulins
Issy Les Moulineaux 92782
France
EMail: jpv@cisco.com
Roger K. Alexander
Cooper Power Systems
20201 Century Blvd., Suite 250
Germantown, MD 20874
USA
Phone: +1 240 454 9817
EMail: roger.alexander@cooperindustries.com
Winter, et al. Standards Track [Page 157]