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Versions: (draft-mjkim-roll-routing-metrics) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 RFC 6551

Networking Working Group                                JP. Vasseur, Ed.
Internet-Draft                                        Cisco Systems, Inc
Intended status: Standards Track                             M. Kim, Ed.
Expires: April 29, 2010                  Future Tech Lab., Korea Telecom
                                                               K. Pister
                                                           Dust Networks
                                                                H. Chong
                                         Future Tech Lab., Korea Telecom
                                                        October 26, 2009


    Routing Metrics used for Path Calculation in Low Power and Lossy
                                Networks
                   draft-ietf-roll-routing-metrics-03

Status of this Memo

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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.

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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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Object Formats . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Node Metrics And Constraints Objects . . . . . . . . . . . . .  9
     3.1.  Node State And Attributes Object . . . . . . . . . . . . .  9
     3.2.  Node Energy Object . . . . . . . . . . . . . . . . . . . . 11
     3.3.  Hop-Count Object . . . . . . . . . . . . . . . . . . . . . 14
   4.  Link Metrics and Constraints Objects . . . . . . . . . . . . . 14
     4.1.  Throughput . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.2.  Latency  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  Link reliability . . . . . . . . . . . . . . . . . . . . . 17
       4.3.1.  The Link Quality Level Reliability Metric  . . . . . . 18
       4.3.2.  The Expected Transmission Count (ETX) Reliability
               Object . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.4.  Link Color Object  . . . . . . . . . . . . . . . . . . . . 21
       4.4.1.  Link Color Object Description  . . . . . . . . . . . . 21
       4.4.2.  Mode of Operation  . . . . . . . . . . . . . . . . . . 22
   5.  Computation of Dynamic Metrics and Attributes  . . . . . . . . 23
   6.  Objective Function . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 24
     7.1.  Other metric under consideration . . . . . . . . . . . . . 24
   8.  Metric Consistency . . . . . . . . . . . . . . . . . . . . . . 24
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     9.1.  Routing Objects  . . . . . . . . . . . . . . . . . . . . . 25
     9.2.  Routing Object Common Header . . . . . . . . . . . . . . . 25
     9.3.  NSA Object . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.4.  Hop-count Object . . . . . . . . . . . . . . . . . . . . . 26
     9.5.  Objective Code Point (OCP) Object  . . . . . . . . . . . . 27
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 27
     12.2. Informative References . . . . . . . . . . . . . . . . . . 27
     12.3. References . . . . . . . . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28















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1.  Introduction

   This document makes use of the terminology defined in
   [I-D.ietf-roll-terminology].

   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],
   [I-D.ietf-roll-home-routing-reqs] and
   [I-D.ietf-roll-building-routing-reqs].

   Historically, IGP such as OSPF ([RFC2328]) and IS-IS ([RFC1195]) have
   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
   (static) to compute constrained shortest paths for Traffic
   Engineering Label Switched Paths (TE LSPs).

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

   One of the prime objectives of this document is to propose 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 new routing metrics; new
   constraints could be defined in the future, as needed.

   RPL is a distance vector routing protocol that builds a Directed
   Acyclic Graph (DAG) based on routing metrics and constraints.  DAG
   formation rules are defined in [I-D.ietf-roll-rpl]:

   o  The DAG root 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 the path to only traverse
      nodes that are main 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 quality, also referred to as the path cost.  Link and
      nodes metrics are usually (but not always) additive: for example,
      the throughput or the reliability link metric can be used to
      evaluate the path cost.



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

   Routing metrics can be classified according to the following set of
   characteristics:

   o  Link versus Node metrics

   o  Qualitative versus quantitative

   o  Dynamic versus static

   It must be noted that the use of dynamic metrics is not new and has
   been experimented in ARPANET 2 [Khanna1989], with moderate success.
   The use of dynamic metrics is not trivial and very careful care must
   be given to the use of dynamic metrics since it may lead to potential
   routing instabilities.

   As pointed out in various routing requirements documents (see
   [RFC5673], [I-D.ietf-roll-home-routing-reqs] [RFC5548] and
   [I-D.ietf-roll-building-routing-reqs]), it must be possible to take
   into account a variety of node constraints/metrics during path
   computation.

   It is also worth mentioning that it is fairly common for links in
   LLNs to have fast changing node and link characteristics, which must
   be taken into account when specifying routing metrics.  For instance,
   in addition to the dynamic nature of wireless connectivity, nodes'
   resources such as residual energy and other link's charatacteristics
   such as the throughput are changing continuously and may have to be
   taken into account during the path computation.  Similarly, link
   attributes including throughput and reliability may drastically
   change over time due to multi-path interference.

   Very careful attention must be given when using dynamic metrics and
   attributes that affect routing decisions in order to preserve routing
   stability.  Routing metrics and constraints may either be static or
   dynamic.  When dynamic, a RPL implementation SHOULD make use of a
   multi-threshold scheme rather than fine granular metric updates so as
   to avoid constant routing changes.

   Furthermore, it is a time and energy consuming process to update
   dynamic metrics and recompute the routing tables on a frequent basis.
   Therefore, it may be desirable to use a set of discrete values to
   reduce computational overhead and bandwidth utilization.  Of course,
   this comes with a cost, namely, reduced metric accuracy.  Reliability
   is an example of qualitative parameters which may be used as a



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   routing metric by RPL.  Such qualitative parameters may be
   transformed to quantitative values.  In other cases, a set of flags
   may be defined to reflect a node state without having to define
   discrete values.

   Some link or node characteristics (e.g. link reliability flag, energy
   remaining on the node) may either be used by RPL as routing
   constraints or metric.  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 the maximum number of
   links with a specified reliability level (use as a metric).

   It is not required to use all the metrics and attributes specified in
   this document.  The set of routing metrics and constraints used by an
   RPL implementation is signalled along the Directed Acyclic Graph
   (DAG) computed by RPL via an Objective Function (OF) as described in
   [I-D.ietf-roll-rpl].  Indeed, RPL may be used to build DAGs with
   different characteristics.  For example, it may be desirable to build
   a DAG trying to maximize reliability by using the link reliability
   metric: in this case, the OF advertised by RPL in the Dag Information
   Option (DIO) message specifies an Objective Code Point (OCP)
   indicating that the link reliability metric is the metric used to
   compute the "best" path.  Another example might be to use the energy
   node characteristic (e.g. main powered versus battery operated) as a
   node constraint when building the DAG so as to avoid battery powered
   nodes in the DAG.  Links and nodes routing metrics and constraints
   are not exclusive.

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

   The specification of objective functions used to compute the DAG
   built by RPL is out of the scope of this document and will be
   specified in other documents.

   Some metrics are either aggregated or recorded.  In the former case,
   the metric is adjusted as the DIO message travels along the DAG.  For
   example, if the metric is the link latency, each node updates the
   latency metric along the DAG.  By contrast, metric may be recorded in
   which case each node adds a sub-object reflecting the local metric.
   For example, it might be desirable to record the link quality level
   along the path.  In this case, each visited node adds a sub-object
   reporting 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).  Upon
   receiving the DIO message from a set of parents, a node can decide
   which node to choose as a parent based on the maximum number of links
   with a specific link reliability level for example.



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   Notion of local versus global metric: some routing objects may have a
   local or a global significance.  In the former case, a metric may be
   transmitted to a neighbor to charaterize a link or a node as opposed
   to a path.  For example, a node may report information about its
   local energy without the need to propagate the energy level of all
   nodes along the path.  In contrast, other metrics such as link
   latency metrics are cumulative and global in the sense that they
   characterize a path cost using the latency as a metric.  In this
   particular example the delay is an aggregated global and cumulative
   link metric.


2.  Object Formats

   Routing metrics and constraints are carried within the DAG Metric
   Container object defined in [I-D.ietf-roll-rpl].

  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 5 6 7 8 9 0 1 2 3 4
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
 |     Type=2    |        Container Len          |  DAG Metric data    ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

        Figure 1: DAG Metric Container Format

   Routing metrics and constraints have a common format consisting of
   one or more 8-bit words 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-Ob-Type|Res|R|G| A |O|C|   Object Length (bytes)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                        (Object body)                        //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 2: Routing Metric/Constraint common header format

   The object body carries one or more sub-objects.

   Note that the routing metric objects defined in this document can
   appear in any order in the DAG Metric Container object with the
   exception of the Objective Function object that MUST appear first.

   Routing-Ob-Type: (Routing Object Type - 8 bits): the Routing Object
   Type field uniquely identifies each Routing object and is managed by



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   IANA.

   Res flags (2 bits).  Reserved field.  This field MUST be set to zero
   on transmission and MUST be ignored on receipt.

   o  C Flag.  When set, this indicates that the routing 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) and has no meaning for routing metrics.  When set,
      this indicates that the constraint is optional.  When cleared, the
      constraint is mandatory.

   o  A Field: The A field is used to indicate whether a routing metric
      is additive, reports a maximum or a minimum.

      *  A=0x00: The routing metric is additive

      *  A=0x01: The routing metric reports a maximum

      *  A=0x02: The routing metric reports a minimum

   o  G Flag: When set, the routing object is global.  When cleared it
      is local (see details below).

   o  R Fag: The R Flag is only relevant for global routing metric (C=0
      and G=1) and MUST be cleared for all other values of C and G. When
      set, this indicates that the routing metric is recorded along the
      path.  Conversely, when cleared the routing metric is aggregated.

   The A field has no meaning when the C Flag is set (i.e. the routing
   object refers to a routing constraint).

   Example 1: A DAG formed by RPL where all nodes must be main powered
   and the link metric is the link throughput.  In this case the DAG
   Metric container carries two routing objects: the link metric is the
   link throughput (C=0, O=0, A=00, G=1, R=0) and the node constraint is
   power (C=1, O=0, A=00, G=0, R=0).  Note that in this example, the
   link throughput is a global additive aggregated link metric.  An RPL
   implementation may use the metric to report a minimum (A=01).  In
   this case, when the link throughput metric is processed each node
   updates it is the link throughput is less than the value reported by
   the link throughput metric.

   Example 2: A DAG formed by RPL where the link metric is the link
   quality level and link quality levels must be recorded along the
   path.  In this case the DAG Metric container carries one routing



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   object: the link quality level (C=0, O=0, A=00, G=1, R=1).

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

   Type: 2 bytes
   Length: 2 bytes
   Value: variable

   A Routing metric TLV is comprised of 2 bytes for the type, 2 bytes
   specifying the TLV length, and a value field.

   The Length field defines the length of the value portion in bytes.

   Unrecognized TLVs MUST be ignored.

   IANA management of the routing metric objects identifier codespace is
   described in Section 9.


3.  Node Metrics And Constraints Objects

   It is fairly common for LLNs to be made of nodes with heterogeneous
   attributes and capabilities (e.g. nodes being battery operated or
   not, amount of memory, etc).  More capable and stable nodes may
   assist the most constrained ones for routing packets, which results
   in extension of network lifetime and efficient network operations.
   This is a typical use of constraint-based routing where the computed
   path may not be the shortest path according to some specified
   metrics.

3.1.  Node State And Attributes Object

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

   The NSA object MAY be present in the DAG Metric Container.  There
   MUST be only one NSA object per DAG Metric Container object.

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

   The NSA Routing Object Type is to be assigned by IANA (recommended
   value=1).







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   The format of the NSA object body is as follows:
     0                   1
     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 3: NSA Object format

   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, similarly to the "overload" bit used
   in routing protocols such as IS-IS.  Algorithms used to set the
   overload bit and to compute path to potentially avoid node with their
   overload bit set are outside the scope of this document.

   Data Aggregation Attribute: data fusion involves more complicated
   processing to improve accuracy of the output data while data
   aggregation mostly aims at reducing the amount of data.  This is
   listed as a requirement in Section 6.2 of [RFC5548].  Some
   applications may make use of the aggregation node attribute in their
   routing decision so as to minimize the amount of traffic on the
   network, thus potentially increasing its life time in battery
   operated environments.  Applications where high directional data flow
   is expected on a regular basis may take advantage of data aggregation
   supported routing.

   The following two bits of the NSA object are defined:

   o  O Flag: When set, this indicates that the node is overloaded and
      may not be able to process traffic.

   o  A Flag: When set, this indicates that the node can act as a
      traffic aggregator.  An implementation MAY decide to add optional
      TLVs (not currently defined) to further describe the node traffic
      aggregator functionality.

   The Flag field of the NSA Routing 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.






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3.2.  Node Energy Object

   Whenever possible, a node with low residual energy should not be
   selected as a router, thus the support for constrained-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.

   The routing engine may prefer a "longer" path that traverses mains-
   powered nodes or nodes equipped with energy scavenging, rather than a
   "shorter" path through battery operated nodes.

   Power and energy are clearly critical resources in LLNs.  As yet
   there are no simple abstractions which adequately cover the broad
   range of power sources and energy storage devices used in existing
   LLN nodes.  These include line-power, 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 4-20mA 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 which are
   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 like 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 LLN 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



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   constraints, this document defines three levels of fidelity in the
   solution.

   The simplest solution relies on a 2-bit field encoding three types of
   power sources: "powered", "battery", "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, H=P_in/P_out, in units of percent.  Nodes which are
   scavenging more power than they are consuming will register above
   100.  The time period for averaging power in this calculation is out
   of the scope of this document but something related to the discharge
   time of the energy storage device on the node is probably
   appropriate.  For battery powered devices, H is the current expected
   lifetime divided by the desired minimum lifetime.  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 H can be calculated as
   the ratio of desired max power (initial energy E_0 divided by desired
   lifetime T) to actual power H=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 H can be calculated as the total
   stored energy remaining versus the target energy remaining: H= E_bat
   / [E_0 (T-t)/T].

   An example OF is max(min(H)) for all battery operated nodes along the
   route, subject to the constraint that H>=100 for all scavengers along
   the route.

   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
   be only one NE object per DAG Metric Container object.

   The NE Routing Object Type is to be assigned by IANA (recommended
   value=2).










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   The format of the NE object body is as follows:
     0                   1
     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 4: NE Object format

   When used as a constraint, the NE object comprises only one NE sub-
   object.

   The format of the NE sub-object body is as follows:

     0                   1
     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 5: NE sub-object format

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

   The following flags are currently defined:

   o  T (node Type): 2-bit field indicating the node type.  When E=0x00,
      the node is main powered.  When E=0x01 is battery powered.  When
      E=0x02 the node is powered by a scavenger.

   o  I (Included): the I bit is only relevant when the node type is
      used as a constraint.  For example, the OF may indicate that the
      path must only traversed main powered node.  Conversely, the OF
      may indicate that battery operated node must be excluded.  The I
      bit is used to stipulate inclusion versus exclusion.  When set,
      this indicates that nodes of 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  E (Estimation): when set, the estimated percentage of remaining
      energy is indicated in the E-E 8-bit field.  When cleared, the
      estimated percentage of remaining energy is not provided.

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



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   No TLVs are currently defined.

   The most complex solution involves a half dozen TLV parameters
   representing energy storage, consumption, and generation capabilities
   of the node, as well as desired lifetime, and will appear in the next
   version of this document.

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
   be only one HP object per DAG Metric Container object.

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

   The HP routing metric object Type is to be assigned by IANA
   (recommended value=3)

   The HP routing metric object is a global routing object that
   characterizes a path.

   The format of the Hop Count object body is as follows:
     0                   1
     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 6: Hop Count Object format

   No Flags are currently defined.

   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.


4.  Link Metrics and Constraints Objects








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4.1.  Throughput

   Many LLNs support a wide range of throughputs, measured either in
   bits per second or packets per second.  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 sub-layer protocols
   which allow the effective bit rate and power consumption 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 be only one Throughput object per DAG Metric Container
   object.

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

   The Throughput object does not contain any additional TLV.

   The Throughput Object Type is to be assigned by IANA (recommended
   value=4)

   The Throughput Object is a global metric.
























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   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 7: Throughput Object Body Format


   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 8: Throughput Sub-object format
   Throughput: 32 bits.  The Throughput is encoded in 32 bits in IEEE
   floating point format (see [IEEE.754.1985]), expressed in bytes per
   second.  Refer to Section 3.1.2 of [RFC3471] for a table of commonly
   used values.

4.2.  Latency

   Similarly to throughput, the latency of many LLN MAC sub-layers can
   be varied over many orders of magnitude, again with a corresponding
   change in current consumption.  Some LLN MAC link layers will allow
   the latency to be adjusted globally on the subnet, or 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 be only one Latency object per DAG Metric Container object.

   The Latency object is made of Latency sub-objects and MUST as 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 TLV.

   The Latency object Type is to be assigned by IANA (recommended
   value=5)

   The Latency Object is a global metric or constraint.






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   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 9: Latency Object Body Format


    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 10: Latency Sub-object format

   Latency: 32 bits.  The Latency is encoded in 32 bits in IEEE floating
   point format (see [IEEE.754.1985]), expressed in milliseconds.  Refer
   to Section 3.1.2 of [RFC3471] for a table of commonly used values.

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

4.3.  Link reliability

   In LLNs, link reliability is degraded by external interference and
   multi-path interference.  Multipath typically affects both directions
   on the link equally, whereas external interference is sometimes uni-
   directional.  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.

   A change in link quality can affect network connectivity, thus, link
   quality may be taken into account as a critical routing metric.  Link
   quality metric should be applied to each directional link unless bi-
   directionality is one of routing metrics.

   A number of link reliability metrics could be defined reflecting
   several reliability aspects.  Two link reliability metrics are



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   defined in this document: the Link Quality Level (LQL) and the
   Expected Transmission count Metric (ETX).

   Note that an RPL implementation MAY either 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 3 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
   is implementation specific and outside of the scope of this document.

   The LQL is global and can either be used as a metric or a constraint.
   When used as a metric, the LQL metric can be recorded or aggregated.
   For example, the DAG may require to record the LQL for all traversed
   links.  Each node can then use the LQL to select the parent based on
   user defined rules (e.g. "select the path with the maximum number of
   links reporting a LQL value of 3").  By contrast the LQL link metric
   may be aggregated in which case, the sum of all LQL may be reported
   (additive metric) or the minimum value may be reported along the
   path.

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

   The LQL object MAY be present in the DAG Metric Container.  There
   MUST be only one LQL object with at least one LQL sub-object per DAG
   Metric Container object.

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

   The LQL object Type is to be assigned by IANA (recommended value=6)

   The LQL object is a global object that characterizes the path
   reliability.













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   The format of the LQL object body is as follows:
     0                   1
     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 11: LQL Object format

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

    Figure 12: LQL Type 1 sub-Object format

   Val: LQL value from 0 to 3 where 0 means undetermined and 1 indicates
   the lowest link quality.

   Counter: number of links.

   When the LQL metric is aggregated, the LQL object body comprises one
   LQL Type 2 sub-object:

   The format of the LQL Type 2 sub-object is as follows

     0                   1
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Aggregated LQL Value     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 13: LQL Type 2 sub-Object format

   Aggregated LQL Value: when used an an additive metric (A=0x00), the
   aggregated LQL value reports the sum of all the LQL values for all
   links along the path.  When used to report a minimum (A=0x02), the
   field reports the minimum LQL value of all links along the path.

4.3.2.  The Expected Transmission Count (ETX) Reliability Object

   The Expected Transmission Count (ETX) metric is the number of
   transmissions a node expects to make to a destination in order to



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   successfully deliver a packet.

   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 comput the ETX value.

   The ETX object MAY be present in the DAG Metric Container.  There
   MUST be only one ETX object per DAG Metric Container object.

   The ETX object is made of ETX sub-objects and MUST as least comprise
   one ETX sub-object.  Each ETX sub-object has a fixed length of 8
   bits.

   The ETX object does not contain any additional TLV.

   The ETX object Type is to be assigned by IANA (recommended value=7)

   The ETX object is a global 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 14: ETX Object Body Format


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

   Figure 15: ETX Sub-object format

   ETX: 8 bits.  The ETX is encoded in 8 bits in IEEE floating point
   format (see [IEEE.754.1985]).  Refer to Section 3.1.2 of [RFC3471]
   for a table of commonly used values.

   The ETX object may be used as a constraint or a path metric.  For
   example, an Objective Function may indicate that the ETX must not be
   below some specified value.  In this case, the ETX object common
   header indicates that the value relates to a constraint with a



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   minimum value.  In another example, the ETX object may be used as an
   aggregated additive metric where the value is updated along the path
   to reflect to path quality.

4.4.  Link Color Object

4.4.1.  Link Color Object Description

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

   The LC object can either be used 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.  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 be only one LC object with at least one LC
   sub-object per DAG Metric Container object.

   The LC routing object does not contain any additional TLV.

   The LC routing object Type is to be assigned by IANA (recommended
   value=8)

   The LC object may either be local or global.

   The format of the LC object body is as follows:
     0                   1
     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 16: LC Object format

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





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   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 17: 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        |I|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 18: LC Type 2 sub-object format

   I Bit: When cleared, this indicates that links with the specified
   color must be included.  When set, this indicates that links with the
   specified color must be excluded.

   The use of the LC object is outside of the scope of this document.

4.4.2.  Mode of Operation

   The Objective Function may use the link color as a constraint or a
   metric.

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

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







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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, a low-pass
   filter should be used to avoid rapid fluctuations of these values.
   Finally, although the specification of path computation algorithms
   using dynamic metrics are out the scope of this document, the
   objective function should be designed carefully 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 impact the
   traffic and power consumption in the network adversely.


6.  Objective Function

   The Objective Function (OF) is used by RPL to specify how the routing
   metric and constraints should be used to reach specific objectives.
   For example, the OF may specify that the objective is to find the
   constrained shortest path where the constraint is related to the node
   power mode and the metric is the ETX (e.g.  "Avoid battery operated
   links and compute the path that optimizes reliability").  As
   specified in [I-D.ietf-roll-rpl], the OF is used by a node to select
   its parent during the DAG building construction process.

   The OCP object is used to specify the OF and is carried within the
   DAG Metric Container object defined in [I-D.ietf-roll-rpl].

   There MUST be a single instance of the OCP object within the DAG
   Metric Container object.










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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
    |           OCP                 |     (TLVs)
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

           Figure 19: OCP Object Format

   The OCP object is not preceded by the common header specified for the
   routing and metric objects.

   The OCP object body may carry optional TLVs.  No TLVs are currently
   defined.

   OCP (Objective Code Point - 16 bits): the OCP field identifies the OF
   and is managed by IANA.


7.  Open Issues

   Other items to be addressed in further revisions of this document
   include:

7.1.  Other metric under consideration

   Metrics related to security (e.g. capability to avoid a node that has
   not been authorized or authenticated).


8.  Metric Consistency

   Since a set of metrics and constraints will be used for links and
   nodes in LLN, it is particularly critical to ensure the use of
   consistent metric calculation mechanisms for all links and nodes in
   the network.


9.  IANA Considerations

   IANA is requested to establish a new top-level registry to contain
   all Routing Objects codepoints and sub-registries.

   The allocation policy for each new registry is by IETF Consensus: new
   values are assigned through the IETF consensus 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).



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9.1.  Routing Objects

   IANA is requested to create a registry for Routing objects.  Each
   Routing object has a Routing object type value.

   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

9.2.  Routing Object Common Header

   IANA is requested to create a registry to manage the codespace of A
   field and the Flag field of the Routing Common header.

   Codespace of the A field (NSA Object)
    Value  Meaning                              Reference

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

   IANA is requested to create a registry to manage the Flag field of
   the Routing metric common header.

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

   o  Bit number

   o  Capability Description

   o  Defining RFC

   Several bits are defined for the Routing metric common header in this
   document.  The following values have been assigned:










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   Codespace of the Flag field (Routing common header)
     Bit      Description              Reference

      8       Constraint/metric        This document
      7       Optional Constraint      This document
      5-6     Additive/Max/Min         This document
      4       Global/Local             This document
      3       Recorded/Aggregated      This document

9.3.  NSA Object

   IANA is requested to create a registry to manage the codespace of the
   Flag field of the NSA Object.

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

   o  Bit number

   o  Capability Description

   o  Defining RFC

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

   Codespace of the Flag field (RP Object)
     Bit      Description              Reference

      14      Aggregator               This document
      15      Overloaded               This document

9.4.  Hop-count Object

   IANA is requested to create a registry to manage the codespace of the
   Flag field of the Hop-count Object.

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

   o  Bit number

   o  Capability Description

   o  Defining RFC

   No Flags are currently defined.




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9.5.  Objective Code Point (OCP) Object

   IANA is requested to create a registry to manage the codespace of the
   OCP field of the OCP object.

   No OCP codepoints are defined in this specification.


10.  Security Considerations

   Routing metrics should be handled in a secure and trustful manner.
   For instance, a malicious node can not advertise falsely that it has
   good metrics for routing and belong to the established path to have a
   chance to intercept packets.


11.  Acknowledgements

   The authors would like to acknowledge the contributions of Young Jae
   Kim, David Meyer, Mischa Dohler, Anders Brandt, Philip Levis, Pascal
   Thubert, Richard Kelsey, Jonathan Hui, Phil Levis and Tim Winter for
   their review and comments.


12.  References

12.1.  Normative References

   [I-D.ietf-roll-rpl]
              Winter, T., Thubert, P., and R. Team, "RPL: Routing
              Protocol for Low Power and Lossy Networks",
              draft-ietf-roll-rpl-03 (work in progress), October 2009.

   [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.

12.2.  Informative References

   [I-D.ietf-roll-building-routing-reqs]
              Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
              "Building Automation Routing Requirements in Low Power and
              Lossy Networks", draft-ietf-roll-building-routing-reqs-07
              (work in progress), September 2009.




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   [I-D.ietf-roll-home-routing-reqs]
              Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-08 (work in progress),
              September 2009.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-02 (work in
              progress), October 2009.

   [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.

   [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.

   [RFC3471]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Functional Description", RFC 3471,
              January 2003.

   [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.

12.3.  References

   [IEEE.754.1985]
              IEEE Standard 754, "Standard for Binary Floating-Point
              Arithmetic", August 1985.










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Authors' Addresses

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

   Email: jpv@cisco.com


   Mijeom (editor)
   Future Tech Lab., Korea Telecom
   17 Woomyeon-dong, Seocho-gu
   Seoul,   137-792
   Korea

   Email: mjkim@kt.com


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

   Email: kpister@dustnetworks.com


   Hakjin
   Future Tech Lab., Korea Telecom
   17 Woomyeon-dong, Seocho-gu
   Seoul,   137-792
   Korea

   Email: hjchong@kt.com















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