Movatterモバイル変換

Internet Engineering Task Force (IETF)                  JP. Vasseur, Ed.Request for Comments: 6551                                 Cisco SystemsCategory: 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 2012Routing Metrics Used for Path Calculation inLow-Power and Lossy NetworksAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date ofVasseur, et al.              Standards Track                    [Page 1]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 Contents1. Introduction ....................................................31.1. Requirements Language ......................................62. Object Formats ..................................................72.1. DAG Metric Container Format ................................72.2. Use of Multiple DAG Metric Containers .....................102.3. Metric Usage ..............................................103. Node Metric/Constraint Objects .................................113.1. Node State and Attribute Object ...........................113.2. Node Energy Object ........................................123.3. Hop Count Object ..........................................164. Link Metric/Constraint Objects .................................174.1. Throughput ................................................174.2. Latency ...................................................184.3. Link Reliability ..........................................194.3.1. The Link Quality Level Reliability Metric ..........194.3.2. The ETX Reliability Object .........................214.4. Link Color Object .........................................224.4.1. Link Color Object Description ......................224.4.2. Mode of Operation ..................................245. Computation of Dynamic Metrics and Attributes ..................246. IANA Considerations ............................................256.1. Routing Metric/Constraint Type ............................256.2. Routing Metric/Constraint TLVs ............................256.3. Routing Metric/Constraint Common Header Flag Field ........266.4. Routing Metric/Constraint Common Header A Field ...........266.5. NSA Object Flags Field ....................................266.6. Hop-Count Object Flags Field ..............................276.7. Node Type Field ...........................................277. Security Considerations ........................................278. Acknowledgements ...............................................289. References .....................................................289.1. Normative References ......................................289.2. Informative References ....................................28Vasseur, et al.              Standards Track                    [Page 2]

RFC 6551          Routing for Path Calculation in LLNs        March 20121.  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]:Vasseur, et al.              Standards Track                    [Page 3]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 toVasseur, et al.              Standards Track                    [Page 4]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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.Vasseur, et al.              Standards Track                    [Page 5]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 inRFC 2119 [RFC2119].Vasseur, et al.              Standards Track                    [Page 6]

RFC 6551          Routing for Path Calculation in LLNs        March 20122.  Object Formats2.1.  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:Vasseur, et al.              Standards Track                    [Page 7]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 ContainerVasseur, et al.              Standards Track                    [Page 8]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 inSection 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 inSection 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 inSection 6.Vasseur, et al.              Standards Track                    [Page 9]

RFC 6551          Routing for Path Calculation in LLNs        March 20122.2.  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   inSection 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.Vasseur, et al.              Standards Track                   [Page 10]

RFC 6551          Routing for Path Calculation in LLNs        March 20123.  Node Metric/Constraint Objects   Sections3 and4 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 inSection 6.2 of [RFC5548].  Some applications      may make use of the aggregation node attribute in their routingVasseur, et al.              Standards Track                   [Page 11]

RFC 6551          Routing for Path Calculation in LLNs        March 2012      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 areVasseur, et al.              Standards Track                   [Page 12]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 asVasseur, et al.              Standards Track                   [Page 13]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 FormatVasseur, et al.              Standards Track                   [Page 14]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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.Vasseur, et al.              Standards Track                   [Page 15]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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.Vasseur, et al.              Standards Track                   [Page 16]

RFC 6551          Routing for Path Calculation in LLNs        March 20124.  Link Metric/Constraint Objects4.1.  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 FormatVasseur, et al.              Standards Track                   [Page 17]

RFC 6551          Routing for Path Calculation in LLNs        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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       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 FormatVasseur, et al.              Standards Track                   [Page 18]

RFC 6551          Routing for Path Calculation in LLNs        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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         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.Vasseur, et al.              Standards Track                   [Page 19]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 FormatVasseur, et al.              Standards Track                   [Page 20]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 FormatVasseur, et al.              Standards Track                   [Page 21]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 Object4.4.1.  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.Vasseur, et al.              Standards Track                   [Page 22]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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.Vasseur, et al.              Standards Track                   [Page 23]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   '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.Vasseur, et al.              Standards Track                   [Page 24]

RFC 6551          Routing for Path Calculation in LLNs        March 20126.  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 RFC6.1.  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 document6.2.  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.Vasseur, et al.              Standards Track                   [Page 25]

RFC 6551          Routing for Path Calculation in LLNs        March 20126.3.  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 document6.5.  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:Vasseur, et al.              Standards Track                   [Page 26]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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 document7.  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.Vasseur, et al.              Standards Track                   [Page 27]

RFC 6551          Routing for Path Calculation in LLNs        March 2012   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.  References9.1.  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.Vasseur, et al.              Standards Track                   [Page 28]

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.Vasseur, et al.              Standards Track                   [Page 29]

RFC 6551          Routing for Path Calculation in LLNs        March 2012Authors' 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.comVasseur, et al.              Standards Track                   [Page 30]