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PROPOSED STANDARD
Errata Exist
Internet Engineering Task Force (IETF)                  JP. Vasseur, Ed.
Request for Comments: 6551                                 Cisco Systems
Category: Standards Track                                    M. Kim, Ed.
ISSN: 2070-1721                           Corporate Technology Group, KT
                                                               K. Pister
                                                           Dust Networks
                                                               N. Dejean
                                                              Elster SAS
                                                              D. Barthel
                                                   France Telecom Orange
                                                              March 2012


              Routing Metrics Used for Path Calculation in
                      Low-Power and Lossy Networks

Abstract

   Low-Power and Lossy Networks (LLNs) have unique characteristics
   compared with traditional wired and ad hoc networks that require the
   specification of new routing metrics and constraints.  By contrast,
   with typical Interior Gateway Protocol (IGP) routing metrics using
   hop counts or link metrics, this document specifies a set of link and
   node routing metrics and constraints suitable to LLNs to be used by
   the Routing Protocol for Low-Power and Lossy Networks (RPL).

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/rfc6551.

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



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   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.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................6
   2. Object Formats ..................................................7
      2.1. DAG Metric Container Format ................................7
      2.2. Use of Multiple DAG Metric Containers .....................10
      2.3. Metric Usage ..............................................10
   3. Node Metric/Constraint Objects .................................11
      3.1. Node State and Attribute Object ...........................11
      3.2. Node Energy Object ........................................12
      3.3. Hop Count Object ..........................................16
   4. Link Metric/Constraint Objects .................................17
      4.1. Throughput ................................................17
      4.2. Latency ...................................................18
      4.3. Link Reliability ..........................................19
           4.3.1. The Link Quality Level Reliability Metric ..........19
           4.3.2. The ETX Reliability Object .........................21
      4.4. Link Color Object .........................................22
           4.4.1. Link Color Object Description ......................22
           4.4.2. Mode of Operation ..................................24
   5. Computation of Dynamic Metrics and Attributes ..................24
   6. IANA Considerations ............................................25
      6.1. Routing Metric/Constraint Type ............................25
      6.2. Routing Metric/Constraint TLVs ............................25
      6.3. Routing Metric/Constraint Common Header Flag Field ........26
      6.4. Routing Metric/Constraint Common Header A Field ...........26
      6.5. NSA Object Flags Field ....................................26
      6.6. Hop-Count Object Flags Field ..............................27
      6.7. Node Type Field ...........................................27
   7. Security Considerations ........................................27
   8. Acknowledgements ...............................................28
   9. References .....................................................28
      9.1. Normative References ......................................28
      9.2. Informative References ....................................28









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1.  Introduction

   This document makes use of the terminology defined in [ROLL-TERMS].

   Low-power and Lossy Networks (LLNs) have specific routing
   characteristics compared with traditional wired or ad hoc networks
   that have been spelled out in [RFC5548], [RFC5673], [RFC5826], and
   [RFC5867].

   Historically, IGP, such as OSPF ([RFC2328]) and IS-IS ([RFC1195]),
   has used quantitative static link metrics.  Other mechanisms, such as
   Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) (see
   [RFC2702] and [RFC3209]), make use of other link attributes such as
   the available reserved bandwidth (dynamic) or link affinities (most
   of the time static) to compute constrained shortest paths for Traffic
   Engineering Label Switched Paths (TE LSPs).

   This document specifies routing metrics and constraints to be used in
   path calculation by the Routing Protocol for Low-Power and Lossy
   Networks (RPL) specified in [RFC6550].

   One of the prime objectives of this document is to define a flexible
   mechanism for the advertisement of routing metrics and constraints
   used by RPL.  Some RPL implementations may elect to adopt an
   extremely simple approach based on the use of a single metric with no
   constraint, whereas other implementations may use a larger set of
   link and node routing metrics and constraints.  This specification
   provides a high degree of flexibility and a set of routing metrics
   and constraints.  New routing metrics and constraints could be
   defined in the future, as needed.

   The metrics and constraints defined in this document are carried in
   objects that are OPTIONAL from the point of view of a RPL
   implementation.  This means that implementations are free to include
   different subsets of the functions (metric, constraint) defined in
   this document.  Specific sets of metrics/constraints and other
   optional RPL parameters for use in key environments will be specified
   as compliance profiles in applicability profile documents produced by
   the ROLL working group.  Note that RPL can even make use of no
   metric, for example, using the Objective Function defined in
   [RFC6552].

   RPL is a distance vector routing protocol variant that builds
   Directed Acyclic Graphs (DAGs) based on routing metrics and
   constraints.  DAG formation rules are defined in [RFC6550]:






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   o  The Destination-Oriented Directed Acyclic Graph (DODAG) root, as
      defined in [RFC6550], may advertise a routing constraint used as a
      "filter" to prune links and nodes that do not satisfy specific
      properties.  For example, it may be required for a path only to
      traverse nodes that are mains-powered or links that have at least
      a minimum reliability or a specific "color" reflecting a user-
      defined link characteristic (e.g., the link layer supports
      encryption).

   o  A routing metric is a quantitative value that is used to evaluate
      the path cost.  Link and node metrics are usually (but not always)
      additive.

   The best path is the path that satisfies all supplied constraints (if
   any) and that has the lowest cost with respect to some specified
   metrics.  It is also called the shortest constrained path (in the
   presence of constraints).

   Routing metrics may be categorized according to the following
   characteristics:

   o  Link versus node metrics

   o  Qualitative versus quantitative

   o  Dynamic versus static

   Routing requirements documents (see [RFC5673], [RFC5826], [RFC5548],
   and [RFC5867]) observe that it must be possible to take into account
   a variety of node constraints/metrics during path computation.

   Some link or node characteristics (e.g., link reliability, remaining
   energy on the node) may be used by RPL either as routing constraints
   or as metrics (or sometimes both).  For example, the path may be
   computed to avoid links that do not provide a sufficient level of
   reliability (use as a constraint) or as the path offering most links
   with a specified reliability level (use as a metric).  This document
   provides the flexibility to use link and node characteristics as
   constraints and/or metrics.

   The use of link and node routing metrics and constraints is not
   exclusive (e.g., it is possible to advertise a "hop count" both as a
   metric to optimize the computed path and as a constraint (e.g., "Path
   should not exceed n hops")).

   Links in LLN commonly have rapidly changing node and link
   characteristics; thus, routing metrics must be dynamic and techniques
   must be used to smooth out the dynamicity of these metrics so as to



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   avoid routing oscillations.  For instance, in addition to the dynamic
   nature of some links (e.g., wireless but also Power Line
   Communication (PLC) links), nodes' resources, such as residual
   energy, are changing continuously and may have to be taken into
   account during the path computation.

   It must be noted that the use of dynamic metrics is not new and has
   been experimented in ARPANET 2 (see [Zinky1989]).  The use of dynamic
   metrics is not trivial and great care must be given to the use of
   dynamic metrics since it may lead to potential routing instabilities.
   That being said, a lot of experience has been gained over the years
   on the use of dynamic routing metrics, which have been deployed in a
   number of (non-IP) networks.

   Very careful attention must be given to the pace at which routing
   metrics and attributes values change in order to preserve routing
   stability.  When using a dynamic routing metric, a RPL implementation
   should make use of a multi-threshold scheme rather than fine granular
   metric updates reflecting each individual change to avoid spurious
   and unnecessary routing changes.

   The requirements on reporting frequency may differ among metrics;
   thus, different reporting rates may be used for each metric.

   The set of routing metrics and constraints used by a RPL deployment
   is signaled along the DAG that is built according to the Objective
   Function (rules governing how to build a DAG) and the routing metrics
   and constraints are advertised in the DODAG Information Object (DIO)
   message specified in [RFC6550].  RPL may be used to build DAGs with
   different characteristics.  For example, it may be desirable to build
   a DAG with the goal to maximize reliability by using the link
   reliability metric to compute the "best" path.  Another example might
   be to use the energy node characteristic (e.g., mains-powered versus
   battery-operated) as a node constraint when building the DAG so as to
   avoid battery-powered nodes in the DAG while optimizing the link
   throughput.

   The specification of Objective Functions used to compute the DAG
   built by RPL is out of the scope of this document.  This document
   defines routing metrics and constraints that are decoupled from the
   Objective Function.  So a generic Objective Function could, for
   example, specify the rules to select the best parents in the DAG, the
   number of backup parents, etc., and it could be used with any routing
   metrics and/or constraints such as the ones specified in this
   document.






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   Some metrics are either aggregated or recorded.  An aggregated metric
   is adjusted as the DIO message travels along the DAG.  For example,
   if the metric is the number of hops, each node updates the path cost
   that reflects the number of traversed hops along the DAG.  By
   contrast, for a recorded metric, each node adds a sub-object
   reflecting the local valuation of the metric.  For example, it might
   be desirable to record the link quality level along a path.  In this
   case, each visited node adds a sub-object recording the local link
   quality level.  In order to limit the number of sub-objects, the use
   of a counter may be desirable (e.g., record the number of links with
   a certain link quality level), thus, compressing the information to
   reduce the message length.  Upon receiving the DIO message from a set
   of parents, a node might decide, according to the OF and local
   policy, which node to choose as a parent based on the maximum number
   of links with a specific link reliability level, for example.

   Note that the routing metrics and constraints specified in this
   document are not specific to any particular link layer.  An internal
   API between the Medium Access Control (MAC) layer and RPL may be used
   to accurately reflect the metrics values of the link (wireless,
   wired, PLC).

   Since a set of metrics and constraints will be used for links and
   nodes in a LLN, it is critical to ensure the use of consistent metric
   calculation mechanisms for all links and nodes in the network,
   similar to the case of inter-domain IP routing.

   There are many different permutations of options that may be
   appropriate in different deployments.  Implementations must clearly
   state which options they include, and they must state which are
   default and which are configurable as options within the
   implementation.  Applicability statements will be developed within
   the ROLL working group to clarify which options are applicable to the
   specific deployment scenarios indicated by [RFC5673], [RFC5826],
   [RFC5548], and [RFC5867].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].










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2.  Object Formats

2.1.  DAG Metric Container Format

   Routing metrics and constraints are carried within the DAG Metric
   Container object defined in [RFC6550].  Should multiple metrics
   and/or constraints be present in the DAG Metric Container, their use
   to determine the "best" path can be defined by an Objective Function.

   The Routing Metric/Constraint objects represent a metric or a
   constraint of a particular type.  They may appear in any order in the
   DAG Metric Container (specified in [RFC6550]).  They have a common
   format consisting of one or more bytes with a common header.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Routing-MC-Type|Res Flags|P|C|O|R| A   |  Prec | Length (bytes)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                        (object body)                        //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 1: Routing Metric/Constraint Object Generic Format

   The object body carries one or more sub-objects defined later in this
   document.  Note that an object may carry a TLV, which may itself
   comprise other TLVs.  A TLV carried within a TLV is called a TLV in
   this specification.

   Routing-MC-Type (Routing Metric/Constraint Type - 8 bits): the
   Routing Metric/Constraint Type field uniquely identifies each Routing
   Metric/Constraint object and is managed by IANA.

   Length (8 bits): this field defines the length of the object body,
   expressed in bytes.  It ranges from 0 to 255.

   Res Flags field (16 bits).  The Flag field of the Routing Metric/
   Constraint object is managed by IANA.  Unassigned bits are considered
   as reserved.  They MUST be set to zero on transmission and MUST be
   ignored on receipt.

   The following bits of the Routing Metric/Constraint Flag field object
   are currently defined:






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   o  'P' flag: the P field is only used for recorded metrics.  When
      cleared, all nodes along the path successfully recorded the
      corresponding metric.  When set, this indicates that one or
      several nodes along the path could not record the metric of
      interest (either because of lack of knowledge or because this was
      prevented by policy).

   o  'C' flag.  When set, this indicates that the Routing Metric/
      Constraint object refers to a routing constraint.  When cleared,
      the routing object refers to a routing metric.

   o  'O' flag: The 'O' flag is used exclusively for routing constraints
      ('C' flag is set).  When set, this indicates that the constraint
      specified in the body of the object is optional.  When cleared,
      the constraint is mandatory.  If the 'C' flag is zero, the 'O'
      flag MUST be set to zero on transmission and ignored on reception.

   o  'R' flag: The 'R' flag is only relevant for a routing metric (C=0)
      and MUST be cleared for C=1.  When set, this indicates that the
      routing metric is recorded along the path.  Conversely, when
      cleared, the routing metric is aggregated.

   A Field (3 bits): The A field is only relevant for metrics and is
   used to indicate whether the aggregated routing metric is additive,
   is multiplicative, reports a maximum, or reports a minimum.

   o  A=0: The routing metric is additive

   o  A=1: The routing metric reports a maximum

   o  A=2: The routing metric reports a minimum

   o  A=3: The routing metric is multiplicative

   The A field has no meaning when the 'C' flag is set (i.e., when the
   Routing Metric/Constraint object refers to a routing constraint) and
   is only valid when the 'R' bit is cleared.  Otherwise, the A field
   MUST be set to 0 and MUST be ignored on receipt.

   Prec field (4 bits): The Prec field indicates the precedence of this
   Routing Metric/Constraint object relative to other objects in the
   container.  This is useful when a DAG Metric Container contains
   several Routing Metric objects.  Its value ranges from 0 to 15.  The
   value 0 means the highest precedence.

   Example 1: A DAG formed by RPL where all nodes must be mains-powered
   and the best path is the one with lower aggregated expected
   transmission count (ETX).  In this case, the DAG Metric Container



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   carries two Routing Metric/Constraint objects: one is an ETX metric
   object with header (C=0, O=0, A=00, R=0) and the second one is a Node
   Energy constraint object with header (C=1, O=0, A=00, R=0).  Note
   that a RPL Instance may use the metric object to report a maximum
   (A=1) or a minimum (A=2).  If, for example, the best path is
   characterized by the path avoiding low quality links, then the path
   metric reports a maximum (A=1) (the higher the ETX, the lower the
   link quality): when the DIO message reporting the link quality metric
   (ETX) is processed by a node, each node selecting the advertising
   node as a parent updates the value carried in the metric object by
   replacing it with its local link ETX value if and only if the latter
   is higher.  As far as the constraint is concerned, the object body
   will carry a Node Energy constraint object defined in Section 3.1
   indicating that nodes must be mains-powered: if the constraint
   signaled in the DIO message is not satisfied, the advertising node is
   just not selected as a parent by the node that processes the DIO
   message.

   Example 2: A DAG formed by RPL where the link metric is the link
   quality level (defined in Section 4) and link quality levels must be
   recorded along the path.  In this case, the DAG Metric Container
   carries a Routing Metric/Constraint object: link quality level metric
   (C=0, O=0, A=00, R=1) containing multiple sub-objects.

   A Routing Metric/Constraint object may also include one or more
   additional type-length-value (TLV) encoded data sets.  Each Routing
   Metric/Constraint TLV has the same structure:

   Type: 1 byte
   Length: 1 byte
   Value: variable

   A Routing Metric/Constraint TLV is comprised of 1 byte for the type,
   1 byte specifying the TLV length, and a value field.  The TLV length
   field defines the length of the value field in bytes (from 0 to 255).

   Unrecognized TLVs MUST be silently ignored while still being
   propagated in DIOs generated by the receiving node.

   IANA manages the codepoints for all TLVs carried in routing
   constraint/metric objects.

   IANA management of the Routing Metric/Constraint objects identifier
   codespace is described in Section 6.







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2.2.  Use of Multiple DAG Metric Containers

   Since the length of RPL options is encoded using 1 octet, they cannot
   exceed 255 bytes, which also applies to the DAG Metric Container.  In
   the vast majority of cases, the advertised routing metrics and
   constraints will not require that much space.  However, there might
   be circumstances where larger space is required, should, for example,
   a set of routing metrics be recorded along a long path.  In this
   case, in order to avoid overflow, as specified in [RFC6550], routing
   metrics will be carried using multiple DAG Metric Container objects.

   In the rest of this document, this use of multiple DAG Metric
   Container objects will be considered as if they were actually just
   one long DAG Metric Container object.

2.3.  Metric Usage

   When the DAG Metric Container contains a single aggregated metric
   (scalar value), the order relation to select the best path is
   implicitly derived from the metric type.  For example, lower is
   better for Hop Count, Link Latency, and ETX.  Conversely, for Node
   Energy or Throughput, higher is better.

   An example of using such a single aggregated metric is optimizing
   routing for node energy.  The Node Energy metric (E_E field) defined
   in Section 3.2 is aggregated along paths with an explicit min
   function (A field), and the best path is selected through an implied
   Max function because the metric is Energy.

   When the DAG Metric Container contains several aggregated metrics,
   they are to be used as tiebreakers according to their precedence
   defined by their Prec field values.

   An example of such use of multiple aggregated metrics is the
   following: Hop Count as the primary criterion, Link Quality Level
   (LQL) as the secondary criterion, and Node Energy as the ultimate
   tiebreaker.  In such a case, the Hop Count, LQL, and Node Energy
   metric objects' Prec fields should bear strictly increasing values
   such as 0, 1, and 2, respectively.

   If several aggregated metrics happen to bear the same Prec value, the
   behavior is implementation dependent.









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3.  Node Metric/Constraint Objects

   Sections 3 and 4 specify several link and node metric/constraint
   objects.  In some cases, it is stated that there must not be more
   than one object of a specific type.  In that case, if a RPL
   implementation receives more than one object of that type, the second
   object MUST silently be ignored.

   In the presence of a constraint, a node MUST include a metric of the
   same type.  That metric is used to check whether or not the
   constraint is met.  In all cases, a node MUST not change the content
   of the constraint.

3.1.  Node State and Attribute Object

   The Node State and Attribute (NSA) object is used to provide
   information on node characteristics.

   The NSA object MAY be present in the DAG Metric Container.  There
   MUST NOT be more than one NSA object as a constraint per DAG Metric
   Container, and there MUST NOT be more than one NSA object as a metric
   per DAG Metric Container.

   The NSA object may also contain a set of TLVs used to convey various
   node characteristics.  No TLV is currently defined.

   The NSA Routing Metric/Constraint Type has been assigned value 1 by
   IANA.

   The format of the NSA object body 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 4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
    |     Res       |  Flags    |A|O|  Optional TLVs
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

                Figure 2: NSA Object Body Format

   Res flags (8 bits): Reserved field.  This field MUST be set to zero
   on transmission and MUST be ignored on receipt.

   Flags field (8 bits).  The following two bits of the NSA object are
   currently defined:

   o  'A' flag: data Aggregation Attribute.  Data aggregation is listed
      as a requirement in Section 6.2 of [RFC5548].  Some applications
      may make use of the aggregation node attribute in their routing



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      decision so as to minimize the amount of traffic on the network,
      thus, potentially increasing its lifetime in battery operated
      environments.  Applications where highly directional data flow is
      expected on a regular basis may take advantage of data aggregation
      supported routing.  When set, this indicates that the node can act
      as a traffic aggregator.  Further documents MAY define optional
      TLVs to describe the node traffic aggregator functionality.

   o  'O' flag: node workload may be hard to determine and express in
      some scalar form.  However, node workload could be a useful metric
      to consider during path calculation, in particular when queuing
      delays must be minimized for highly sensitive traffic considering
      Medium Access Control (MAC) layer delay.  Node workload MAY be set
      upon CPU overload, lack of memory, or any other node related
      conditions.  Using a simple 1-bit flag to characterize the node
      workload provides a sufficient level of granularity, similar to
      the "overload" bit used in routing protocols such as IS-IS.
      Algorithms used to set the overload bit and to compute paths to
      potentially avoid nodes with their overload bit set are outside
      the scope of this document, but it is RECOMMENDED to avoid
      frequent changes of this bit to avoid routing oscillations.  When
      set, this indicates that the node is overloaded and may not be
      able to process traffic.

   The unspecified flag fields MUST be set to zero on transmission and
   MUST be ignored on receipt.

   The Flags field of the NSA Routing Metric/Constraint object is
   managed by IANA.  Unassigned bits are considered as reserved.

3.2.  Node Energy Object

   It may sometimes be desirable to avoid selecting a node with low
   residual energy as a router; thus, the support for constraint-based
   routing is needed.  In such cases, the routing protocol engine may
   compute a longer path (constraint based) for some traffic in order to
   increase the network life duration.

   Power and energy are clearly critical resources in most LLNs.  As
   yet, there is no simple abstraction that adequately covers the broad
   range of power sources and energy storage devices used in existing
   LLN nodes.  These include mains-powered, primary batteries, energy
   scavengers, and a variety of secondary storage mechanisms.
   Scavengers may provide a reliable low level of power, such as might
   be available from a 4-20 mA loop; a reliable but periodic stream of
   power, such as provided by a well-positioned solar cell; or
   unpredictable power, such as might be provided by a vibrational
   energy scavenger on an intermittently powered pump.  Routes that are



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   viable when the sun is shining may disappear at night.  A pump
   turning on may connect two previously disconnected sections of a
   network.

   Storage systems, such as rechargeable batteries, often suffer
   substantial degradation if regularly used to full discharge, leading
   to different residual energy numbers for regular versus emergency
   operation.  A route for emergency traffic may have a different
   optimum than one for regular reporting.

   Batteries used in LLNs often degrade substantially if their average
   current consumption exceeds a small fraction of the peak current that
   they can deliver.  It is not uncommon for self-supporting nodes to
   have a combination of primary storage, energy scavenging, and
   secondary storage, leading to three different values for acceptable
   average current depending on the time frame being considered, e.g.,
   milliseconds, seconds, and hours/years.

   Raw power and energy values are meaningless without knowledge of the
   energy cost of sending and receiving packets, and lifetime estimates
   have no value without some higher-level constraint on the lifetime
   required of a device.  In some cases, the path that exhausts the
   battery of a node on the bed table in a month may be preferable to a
   route that reduces the lifetime of a node in the wall to a decade.

   Given the complexity of trying to address such a broad collection of
   constraints, this document defines two levels of fidelity in the
   solution.

   The simplest solution relies on a 2-bit field encoding three types of
   power sources: "powered", "battery", and "scavenger".  This simple
   approach may be sufficient for many applications.

   The mid-complexity solution is a single parameter that can be used to
   encode the energetic happiness of both battery-powered and scavenging
   nodes.  For scavenging nodes, the 8-bit quantity is the power
   provided by the scavenger divided by the power consumed by the
   application, E_E=P_in/P_out, in units of percent.  Nodes that are
   scavenging more power than they are consuming will register above
   100.  A good time period for averaging power in this calculation may
   be related to the discharge time of the energy storage device on the
   node, but specifying this is out of the scope of this document.  For
   battery-powered devices, E_E is the current expected lifetime divided
   by the desired minimum lifetime, in units of percent.  The estimation
   of remaining battery energy and actual power consumption can be
   difficult, and the specifics of this calculation are out of scope of
   this document, but two examples are presented.  If the node can
   measure its average power consumption, then E_E can be calculated as



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   the ratio of desired max power (initial energy E_0 divided by desired
   lifetime T) to actual power, E_E=P_max/P_now.  Alternatively, if the
   energy in the battery E_bat can be estimated, and the total elapsed
   lifetime, t, is available, then E_E can be calculated as the total
   stored energy remaining versus the target energy remaining: E_E=
   E_bat / [E_0 (T-t)/T].

   An example of an optimized route is max(min(E_E)) for all battery-
   operated nodes along the route, subject to the constraint that
   E_E>=100 for all scavengers along the route.

   Note that the estimated percentage of remaining energy indicated in
   the E_E field may not be useful in the presence of nodes powered by
   battery or energy scavengers when the amount of energy accumulated by
   the device significantly differ.  Indeed, X% of remaining energy on a
   node that can store a large amount of energy cannot be easily
   compared to the same percentage of remaining energy on a node powered
   by a tiny source of energy.  That being said, in networks where nodes
   have similar energy storage, such a percentage of remaining energy is
   useful.

   The Node Energy (NE) object is used to provide information related to
   node energy and may be used as a metric or as constraint.

   The NE object MAY be present in the DAG Metric Container.  There MUST
   NOT be more than one NE object as a constraint per DAG Metric
   Container, and there MUST NOT be more than one NE object as a metric
   per DAG Metric Container.

   The NE object Type has been assigned value 2 by IANA.

   The format of the NE object body 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 4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
    |     NE Sub-objects
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

      Figure 3: NE Sub-Object Format











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   The format of the NE sub-object body 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 4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
    | Flags |I| T |E|      E_E      |   Optional TLVs
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

            Figure 4: NE Sub-Object Format

   The NE sub-object may also contain a set of TLVs used to convey
   various nodes' characteristics.

   Flags field (8 bits).  The following flags are currently defined:

   o  I (Included): the 'I' bit is only relevant when the node type is
      used as a constraint.  For example, the path must only traverse
      mains-powered nodes.  Conversely, battery-operated nodes must be
      excluded.  The 'I' bit is used to stipulate inclusion versus
      exclusion.  When set, this indicates that nodes of the type
      specified in the node type field MUST be included.  Conversely,
      when cleared, this indicates that nodes of type specified in the
      node type field MUST be excluded.

   o  T (node Type): 2-bit field indicating the node type.  T=0
      designates a mains-powered node, T=1 a battery-powered node, and
      T=2 a node powered by an energy scavenger.

   o  E (Estimation): when the 'E' bit is set for a metric, the
      estimated percentage of remaining energy on the node is indicated
      in the E_E 8-bit field.  When cleared, the estimated percentage of
      remaining energy is not provided.  When the 'E' bit is set for a
      constraint, the E_E field defines a threshold for the inclusion/
      exclusion: if an inclusion, nodes with values higher than the
      threshold are to be included; if an exclusion, nodes with values
      lower than the threshold are to be excluded.

   E_E (Estimated-Energy): 8-bit unsigned integer field indicating an
   estimated percentage of remaining energy.  The E_E field is only
   relevant when the 'E' flag is set, and it MUST be set to 0 when the
   'E' flag is cleared.

   If the NE object comprises several sub-objects when used as a
   constraint, each sub-object adds or subtracts node subsets as the
   sub-objects are parsed in order.  The initial set (full or empty) is
   defined by the 'I' bit of the first sub-object: full if that 'I' bit
   is an exclusion, empty if that 'I' bit is an inclusion.




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   No TLV is currently defined.

   Future documents may define more complex solutions involving TLV
   parameters representing energy storage, consumption, and generation
   capabilities of the node, as well as desired lifetime.

3.3.  Hop Count Object

   The Hop Count (HP) object is used to report the number of traversed
   nodes along the path.

   The HP object MAY be present in the DAG Metric Container.  There MUST
   NOT be more than one HP object as a constraint per DAG Metric
   Container, and there MUST NOT be more than one HP object as a metric
   per DAG Metric Container.

   The HP object may also contain a set of TLVs used to convey various
   node characteristics.  No TLV is currently defined.

   The HP routing metric object Type has been assigned value 3 by IANA.

   The format of the Hop Count object body 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 4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
    |  Res  | Flags |   Hop Count   |  Optional TLVs
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

           Figure 5: Hop Count Object Body Format

   Res flags (4 bits): Reserved field.  This field MUST be set to zero
   on transmission and MUST be ignored on receipt.

   No Flag is currently defined.  Unassigned bits are considered
   reserved.  They MUST be set to zero on transmission and MUST be
   ignored on receipt.

   The HP object may be used as a constraint or a metric.  When used as
   a constraint, the DAG root indicates the maximum number of hops that
   a path may traverse.  When that number is reached, no other node can
   join that path.  When used as a metric, each visited node simply
   increments the Hop Count field.

   Note that the first node along a path inserting a Hop Count metric
   object MUST set the Hop Count field value to 1.





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4.  Link Metric/Constraint Objects

4.1.  Throughput

   Many LLNs support a wide range of throughputs.  For some links, this
   may be due to variable coding.  For the deeply duty-cycled links
   found in many LLNs, the variability comes as a result of trading
   power consumption for bit rate.  There are several MAC layer
   protocols that allow for the effective bit rate of a link to vary
   over more than three orders of magnitude with a corresponding change
   in power consumption.  For efficient operation, it may be desirable
   for nodes to report the range of throughput that their links can
   handle in addition to the currently available throughput.

   The Throughput object MAY be present in the DAG Metric Container.
   There MUST NOT be more than one Throughput object as a constraint per
   DAG Metric Container, and there MUST NOT be more than one Throughput
   object as a metric per DAG Metric Container.

   The Throughput object is made of throughput sub-objects and MUST at
   least comprise one Throughput sub-object.  The first Throughput sub-
   object MUST be the most recently estimated actual throughput.  The
   actual estimation of the throughput is outside the scope of this
   document.

   Each Throughput sub-object has a fixed length of 4 bytes.

   The Throughput object does not contain any additional TLVs.

   The Throughput object Type has been assigned value 4 by IANA.

   The format of the Throughput object body is as follows:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  (sub-object) .....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: Throughput Object Body Format











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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Throughput                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: Throughput Sub-Object Format

   Throughput: 32 bits.  The Throughput is encoded in 32 bits in
   unsigned integer format, expressed in bytes per second.

4.2.  Latency

   Similar to throughput, the latency of many LLN MAC sub-layers can
   vary over many orders of magnitude, again with a corresponding change
   in power consumption.  Some LLN MAC link layers will allow the
   latency to be adjusted globally on the subnet, on a link-by-link
   basis, or not at all.  Some will insist that it be fixed for a given
   link, but allow it to be variable from link to link.

   The Latency object MAY be present in the DAG Metric Container.  There
   MUST NOT be more than one Latency object as a constraint per DAG
   Metric Container, and there MUST NOT be more than one Latency object
   as a metric per DAG Metric Container.

   The Latency object is made of Latency sub-objects and MUST at least
   comprise one Latency sub-object.  Each Latency sub-object has a fixed
   length of 4 bytes.

   The Latency object does not contain any additional TLVs.

   The Latency object Type has been assigned value 5 by IANA.

   The Latency object is a metric or constraint.

   The format of the Latency object body is as follows:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  (sub-object) .....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 8: Latency Object Body Format







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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Latency                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 9: Latency Sub-Object Format

   Latency: 32 bits.  The Latency is encoded in 32 bits in unsigned
   integer format, expressed in microseconds.

   The Latency object may be used as a constraint or a path metric.  For
   example, one may want the latency not to exceed some value.  In this
   case, the Latency object common header indicates that the provided
   value relates to a constraint.  In another example, the Latency
   object may be used as an aggregated additive metric where the value
   is updated along the path to reflect the path latency.

4.3.  Link Reliability

   In LLNs, link reliability could be degraded for a number of reasons:
   signal attenuation, interferences of various forms, etc.  Time scales
   vary from milliseconds to days, and are often periodic and linked to
   human activity.  Packet error rates can generally be measured
   directly, and other metrics (e.g., bit error rate, mean time between
   failures) are typically derived from that.  Note that such
   variability is not specific to wireless link but also applies to PLC
   links.

   A change in link quality can affect network connectivity; thus, link
   quality may be taken into account as a critical routing metric.

   A number of link reliability metrics could be defined reflecting
   several reliability aspects.  Two link reliability metrics are
   defined in this document: the Link Quality Level (LQL) and the ETX
   Metric.

   Note that a RPL deployment MAY use the LQL, the ETX, or both.

4.3.1.  The Link Quality Level Reliability Metric

   The Link Quality Level (LQL) object is used to quantify the link
   reliability using a discrete value, from 0 to 7, where 0 indicates
   that the link quality level is unknown and 1 reports the highest link
   quality level.  The mechanisms and algorithms used to compute the LQL
   are implementation specific and outside of the scope of this
   document.




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   The LQL can be used either as a metric or a constraint.  When used as
   a metric, the LQL metric can only be recorded.  For example, the DAG
   Metric object may request all traversed nodes to record the LQL of
   their incoming link into the LQL object.  Each node can then use the
   LQL record to select its parent based on some user defined rules
   (e.g., something like "select the path with most links reporting a
   LQL value of 3 or less").

   Counters are used to compress the information: for each encountered
   LQL value, only the number of matching links is reported.

   The LQL object MAY be present in the DAG Metric Container.  There
   MUST NOT be more than one LQL object as a constraint per DAG Metric
   Container, and there MUST NOT be more than one LQL object as a metric
   per DAG Metric Container.

   The LQL object MUST contain one or more sub-object used to report the
   number of links along with their LQL.

   The LQL object Type has been assigned value 6 by IANA.

   The format of the LQL object body 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 4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
    |       Res     | LQL sub-object
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

      Figure 10: LQL Object Body Format

   Res flags (8 bits): Reserved field.  This field MUST be set to zero
   on transmission and MUST be ignored on receipt.

   When the LQL metric is recorded, the LQL object body comprises one or
   more LQL Type 1 sub-object.

   The format of the LQL Type 1 sub-object is as follows

     0
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    | Val | Counter |
    +-+-+-+-+-+-+-+-+

    Figure 11: LQL Type 1 Sub-Object Format





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   Val: LQL value from 0 to 7 where 0 means undetermined and 1 indicates
   the highest link quality.

   Counter: number of links with that value.

4.3.2.  The ETX Reliability Object

   The ETX metric is the number of transmissions a node expects to make
   to a destination in order to successfully deliver a packet.  In
   contrast with the LQL routing metric, the ETX provides a discrete
   value (which may not be an integer) computed according to a specific
   formula: for example, an implementation may use the following
   formula: ETX= 1 / (Df * Dr) where Df is the measured probability that
   a packet is received by the neighbor and Dr is the measured
   probability that the acknowledgment packet is successfully received.
   This document does not mandate the use of a specific formula to
   compute the ETX value.

   The ETX object MAY be present in the DAG Metric Container.  There
   MUST NOT be more than one ETX object as a constraint per DAG Metric
   Container, and there MUST NOT be more than one ETX object as a metric
   per DAG Metric Container.

   The ETX object is made of ETX sub-objects and MUST at least comprise
   one ETX sub-object.  Each ETX sub-object has a fixed length of 16
   bits.

   The ETX object does not contain any additional TLVs.

   The ETX object Type has been assigned value 7 by IANA.

   The format of the ETX object body is as follows:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  (sub-object) .....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 12: ETX Object Body Format

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ETX              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 13: ETX Sub-Object Format



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   ETX: 16 bits.  The ETX * 128 is encoded using 16 bits in unsigned
   integer format, rounded off to the nearest whole number.  For
   example, if ETX = 3.569, the object value will be 457.  If ETX >
   511.9921875, the object value will be the maximum, which is 65535.

   The ETX object may be used as a constraint or a path metric.  For
   example, it may be required that the ETX must not exceed some
   specified value.  In this case, the ETX object common header
   indicates that the value relates to a constraint.  In another
   example, the ETX object may be used as an aggregated additive metric
   where the value is updated along the path to reflect the path
   quality: when a node receives the aggregated additive ETX value of
   the path (cumulative path ETX calculated as the sum of the link ETX
   of all of the traversed links from the advertising node to the DAG
   root), if it selects that node as its preferred parent, the node
   updates the path ETX by adding the ETX of the local link between
   itself and the preferred parent to the received path cost (path ETX)
   before potentially advertising itself the new path ETX.

4.4.  Link Color Object

4.4.1.  Link Color Object Description

   The Link Color (LC) object is an administrative 10-bit link
   constraint (which may be either static or dynamically adjusted) used
   to avoid or attract specific links for specific traffic types.

   The LC object can be used either as a metric or as a constraint.
   When used as a metric, the LC metric can only be recorded.  For
   example, the DAG may require recording the link colors for all
   traversed links.  A color is defined as a specific set of bit values:
   in other words, that 10-bit field is a flag field, and not a scalar
   value.  Each node can then use the LC to select the parent based on
   user defined rules (e.g., "select the path with the maximum number of
   links having their first bit set 1 (e.g., encrypted links)").  The LC
   object may also be used as a constraint.

   When used as a recorded metric, a counter is used to compress the
   information where the number of links for each Link Color is
   reported.

   The Link Color (LC) object MAY be present in the DAG Metric
   Container.  There MUST NOT be more than one LC object as a constraint
   per DAG Metric Container, and there MUST NOT be more than one LC
   object as a metric per DAG Metric Container.

   There MUST be a at least one LC sub-object per LC object.




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   The LC object does not contain any additional TLVs.

   The LC object Type has been assigned value 8 by IANA.

   The format of the LC object body 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 4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
    |      Res      | LC sub-objects
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

      Figure 14: LC Object Format

   Res flags (8 bits): Reserved field.  This field MUST be set to zero
   on transmission and MUST be ignored on receipt.

   When the LC object is used as a recorded metric, the LC object body
   comprises one or more LC Type 1 sub-objects.

   The format of the LC Type 1 sub-object body is as follows:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Link Color     |  Counter  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 15: LC Type 1 Sub-Object Format

   When the LC object is used as a constraint, the LC object body
   comprises one or more LC Type 2 sub-objects.

   The format of the LC Type 2 sub-object body is as follows:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Link Color    |Reserved |I|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 16: LC Type 2 Sub-Object Format

   Reserved (5 bits): Reserved field.  This field MUST be set to zero on
   transmission and MUST be ignored on receipt.






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   'I' Bit: The 'I' bit is only relevant when the Link Color is used as
   a constraint.  When set, this indicates that links with the specified
   color must be included.  When cleared, this indicates that links with
   the specified color must be excluded.

   It is left to the implementer to define the meaning of each bit of
   the 10-bit Link Color Flag field.

4.4.2.  Mode of Operation

   The link color may be used as a constraint or a metric.

   o  When used as constraint, the LC object may be inserted in the DAG
      Metric Container to indicate that links with a specific color
      should be included or excluded from the computed path.

   o  When used as recorded metric, each node along the path may insert
      an LC object in the DAG Metric Container to report the color of
      the local link.  If there is already an LC object reporting a
      similar color, the node MUST NOT add another identical LC sub-
      object and MUST increment the counter field.

5.  Computation of Dynamic Metrics and Attributes

   As already pointed out, dynamically calculated metrics are of the
   utmost importance in many circumstances in LLNs.  This is mainly
   because a variety of metrics change on a frequent basis, thus,
   implying the need to adapt the routing decisions.  That being said,
   care must be given to the pace at which changes are reported in the
   network.  The attributes will change according to their own time
   scales.  RPL controls the reporting rate.

   To minimize metric updates, multi-threshold algorithms MAY be used to
   determine when updates should be sent.  When practical, low-pass
   filtering and/or hysteresis should be used to avoid rapid
   fluctuations of these values.  Finally, although the specification of
   path computation algorithms using dynamic metrics is out of the scope
   of this document, it is RECOMMENDED to carefully design the route
   optimization algorithm to avoid too frequent computation of new
   routes upon metric values changes.

   Controlled adaptation of the routing metrics and rate at which paths
   are computed are critical to avoid undesirable routing instabilities
   resulting in increased latencies and packet loss because of temporary
   micro-loops.  Furthermore, excessive route changes will adversely
   impact the traffic and power consumption in the network, thus,
   potentially impacting its scalability.




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6.  IANA Considerations

   IANA has established a new top-level registry, called "RPL Routing
   Metric/Constraint", to contain all Routing Metric/Constraint objects
   codepoints and sub-registries.

   The allocation policy for each new registry is by IETF review: new
   values are assigned through the IETF review process (see [RFC5226]).
   Specifically, new assignments are made via RFCs approved by the IESG.
   Typically, the IESG will seek input on prospective assignments from
   appropriate persons (e.g., a relevant working group if one exists).

   New bit numbers may be allocated only by an IETF Review action.  Each
   bit should be tracked with the following qualities:

   o  Bit number

   o  Capability Description

   o  Defining RFC

6.1.  Routing Metric/Constraint Type

   IANA has created a sub-registry, called "Routing Metric/Constraint
   Type", for Routing Metric/Constraint object types, which range from 0
   to 255.  Value 0 is unassigned.


   Value     Meaning                         Reference
     1       Node State and Attribute      This document
     2       Node Energy                   This document
     3       Hop Count                     This document
     4       Link Throughput               This document
     5       Link Latency                  This document
     6       Link Quality Level            This document
     7       Link ETX                      This document
     8       Link Color                    This document

6.2.  Routing Metric/Constraint TLVs

   IANA has created a sub-registry, called "Routing Metric/Constraint
   TLVs", used for all TLVs carried within Routing Metric/Constraint
   objects.  The Type field is an 8-bit field whose value is comprised
   between 0 and 255.  Value 0 is unassigned.  The Length field is an
   8-bit field whose value ranges from 0 to 255.  The Value field has
   value ranges depending on the Type; therefore, they are not defined
   here, since no Type is registered at this time.




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6.3.  Routing Metric/Constraint Common Header Flag Field

   IANA has created a sub-registry, called "Routing Metric/Constraint
   Common Header Flag field", to manage the 9-bit Flag field of the
   Routing Metric/Constraint common header.

   Several bits are defined for the Routing Metric/Constraint common
   header Flag field in this document.  The following values have been
   assigned:

   Codespace of the Flag field (Routing Metric/Constraint common header)

     Bit      Description              Reference

      8       Recorded/Aggregated      This document
      7       Optional Constraint      This document
      6       Constraint/Metric        This document
      5       P (Partial)              This document

   Bits 0-4 are currently reserved.

6.4.  Routing Metric/Constraint Common Header A Field

   IANA has created a sub-registry, called "Routing Metric/Constraint
   Common Header A field", to manage the codespace of the A field of the
   Routing Metric/Constraint common header.

   The A field is 3 bits in length, and it has values ranging from 0 to
   7.

   Codespace of the A field (Routing Metric/Constraint common header)
    Value  Meaning                              Reference

      0    Routing metric is additive           This document
      1    Routing metric reports a maximum     This document
      2    Routing metric reports a minimum     This document
      3    Routing metric is multiplicative     This document

6.5.  NSA Object Flags Field

   IANA has created a sub-registry, called "NSA Object Flag field", to
   manage the codespace of the 8-bit Flag field of the NSA object.

   Several bits are defined for the NSA Object Flag field in this
   document.  The following values have been assigned:






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   Codespace of the Flag field (NSA object)

     Bit      Description              Reference

      6      Aggregator               This document
      7      Overloaded               This document

   Bits 0-5 are reserved.

6.6.  Hop-Count Object Flags Field

   IANA has created a sub-registry, called "Hop-Count Object Flag
   field", to manage the codespace of the 4-bit Flag field of the Hop
   Count object.

   No Flag is currently defined.

6.7.  Node Type Field

   IANA has created a sub-registry, called "Node Type Field", to manage
   the codespace of the field of the Routing Metric/Constraint common
   header.

   The T field is 2 bits in length, and it has values ranging from 0 to
   3.

   Codespace of the T field (Routing Metric/Constraint common header)

   Value      Description                                    Reference
    0         a mains-powered node                         This document
    1         a battery-powered node                       This document
    2         a node powered by an energy scavenger        This document

7.  Security Considerations

   Routing metrics should be handled in a secure and trustful manner.
   For instance, RPL should not allow a malicious node to falsely
   advertise that it has good metrics for routing so as to be selected
   as preferred next-hop router for other nodes' traffic and intercept
   packets.  Another attack may consist of making intermittent attacks
   on a link in an attempt to constantly modify the link quality and
   consequently the associated routing metric, thus, leading to
   potential fluctuation in the DODAG.  Thus, it is RECOMMENDED for a
   RPL implementation to put in place mechanisms so as to stop
   advertising routing metrics for highly unstable links that may be
   subject to attacks.





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   Some routing metrics may also be used to identify some areas of
   weaknesses in the network (a highly unreliable link, a node running
   low in terms of energy, etc.).  Such information may be used by a
   potential attacker.  Thus, it is RECOMMENDED to carefully consider
   which metrics should be used by RPL and the level of visibility that
   they provide about the network state or to use appropriate the
   security measures as specified in [RFC6550] to protect that
   information.

   Since the routing metrics/constraints are carried within RPL message,
   the security routing mechanisms defined in [RFC6550] apply here.

8.  Acknowledgements

   The authors would like to acknowledge the contributions of Young Jae
   Kim, Hakjin Chong, David Meyer, Mischa Dohler, Anders Brandt, Philip
   Levis, Pascal Thubert, Richard Kelsey, Jonathan Hui, Alexandru
   Petrescu, Richard Kelsey, Mathilde Durvy, Phoebus Chen, Tim Winter,
   Yoav Ben-Yehezkel, Matteo Paris, Omprakash Gnawali, Mads Westergreen,
   Mukul Goyal, Joseph Saloway, David Culler, and Jari Arkko for their
   review and valuable comments.  Special thanks to Adrian Farrel for
   his thorough review.

9.  References

9.1.  Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5226]     Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26,
                 RFC 5226, May 2008.

   [RFC6550]     Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
                 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
                 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
                 Low-Power and Lossy Networks", RFC 6550, March 2012.

9.2.  Informative References

   [RFC1195]     Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
                 dual environments", RFC 1195, December 1990.

   [RFC2328]     Moy, J., "OSPF Version 2", STD 54, RFC 2328,
                 April 1998.





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RFC 6551          Routing for Path Calculation in LLNs        March 2012


   [RFC2702]     Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and
                 J. McManus, "Requirements for Traffic Engineering Over
                 MPLS", RFC 2702, September 1999.

   [RFC3209]     Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                 V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
                 LSP Tunnels", RFC 3209, December 2001.

   [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.

   [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.

   [RFC6552]     Thubert, P., Ed., "Objective Function Zero for the
                 Routing Protocol for Low-Power and Lossy Networks
                 (RPL)", RFC 6552, March 2012.

   [ROLL-TERMS]  Vasseur, JP., "Terminology in Low power And Lossy
                 Networks", Work in Progress, September 2011.

   [Zinky1989]   Zinky, J., Vichniac, G., and A. Khanna, "Performance of
                 the Revised Routing Metric for ARPANET and MILNET",
                 Military Communications Conference, MILCOM '89,
                 March 1989.
















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Authors' Addresses

   JP. Vasseur (editor)
   Cisco Systems
   11, Rue Camille Desmoulins
   Issy Les Moulineaux  92782
   France

   EMail: jpv@cisco.com


   Mijeom Kim (editor)
   Corporate Technology Group, KT
   17 Woomyeon-dong, Seocho-gu
   Seoul  137-792
   Korea

   EMail: mjkim@kt.com


   Kris Pister
   Dust Networks
   30695 Huntwood Ave.
   Hayward, CA  95544
   USA

   EMail: kpister@dustnetworks.com


   Nicolas Dejean
   Elster SAS
   Espace Concorde, 120 impasse JB Say
   Perols  34470
   France

   EMail: nicolas.dejean@coronis.com


   Dominique Barthel
   France Telecom Orange
   28 chemin du Vieux Chene, BP 98
   Meylan  38243
   France

   EMail: dominique.barthel@orange.com






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