Networking Working Group                                      O. Gnawali
Internet-Draft                                                  P. Levis
Intended status: Standards Track                     Stanford University
Expires: September 5, 7, 2012                                 March 4, 6, 2012

          The Minimum Rank with Hysteresis Objective Function
                draft-ietf-roll-minrank-hysteresis-of-05
                draft-ietf-roll-minrank-hysteresis-of-06

Abstract

   The Routing Protocol for Low Power and Lossy Networks (RPL) uses
   objective functions to construct routes that optimize or constrain
   the routes it selects and uses.  This specification describes the
   Minimum Rank Objective Function with Hysteresis (MRHOF), an objective
   function that selects routes that minimize a metric, while using
   hysteresis to reduce churn in response to small metric changes.
   MRHOF works with metrics that are additive along a route, and the
   metric it uses is determined by the metrics RPL Destination
   Information Object (DIO) messages advertise.

Status of this Memo

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The Minimum Rank Objective Function with Hysteresis  . . . . .  4
     3.1.  Computing the Path cost  . . . . . . . . . . . . . . . . .  4
     3.2.  Parent Selection . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  When Parent Selection Runs . . . . . . . . . . . . . .  6
       3.2.2.  Parent Selection Algorithm . . . . . . . . . . . . . .  6
     3.3.  Computing Rank . . . . . . . . . . . . . . . . . . . . . .  6  7
     3.4.  Advertising the Path Cost  . . . . . . . . . . . . . . . .  7  8
     3.5.  Working Without Metric Containers  . . . . . . . . . . . .  7  8
   4.  Using MRHOF for Metric Maximization  . . . . . . . . . . . . .  8
   5.  MRHOF Variables and Parameters . . . . . . . . . . . . . . . .  8  9
   6.  Manageability  . . . . . . . . . . . . . . . . . . . . . . . .  9 10
     6.1.  Device Configuration . . . . . . . . . . . . . . . . . . .  9 10
     6.2.  Device Monitoring  . . . . . . . . . . . . . . . . . . . . 10 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 11
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10 11
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 12
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 11 12
     10.2. Informative References . . . . . . . . . . . . . . . . . . 11 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 12

1.  Introduction

   An objective function specifies how RPL [I-D.ietf-roll-rpl] selects
   paths.  Objective functions can choose paths based on routing metrics
   or constraints.  For example, if an RPL instance uses an objective function
   that minimizes hop-count, RPL will select paths with minimum hop
   count.  The  RPL specification requires the use of a common OF by that all nodes in a network.  The possible network use of multiple OFs with a
   single network is for further common OF;
   relaxing this requirement may be a subject of future study.

   The nodes running RPL might use a number of metrics to describe a
   link or a node [I-D.ietf-roll-routing-metrics] and make it these metrics
   available for route selection.  These  RPL advertises metrics are advertised in RPL
   Destination Information Object (DIO) messages using with a Metric Container
   suboption.  An objective function can use these metrics to choose
   routes.  The
   only exception is the ETX metric, which is used without the metric
   container as described in Section 3.5.

   To decouple the details of an individual metric or objective function
   from forwarding and routing, RPL describes routes through a value
   called Rank.  Rank, roughly speaking, corresponds to the distance
   associated with a route.  RPL defines how nodes decide on paths based
   on Rank and advertise their Rank.  An objective function is responsible for
   computing a node's advertised Rank value defines how
   nodes calculate Rank, based on the Rank of its potential parents,
   metrics, and other network properties.

   This specification describes MRHOF, an objective function for RPL.
   MRHOF uses hysteresis while selecting the path with the smallest
   metric value.  The metric that MRHOF uses is determined by the
   metrics in the DIO Metric Container.  For example, the use of MRHOF
   with the latency metric allows RPL to find stable minimum-latency
   paths from the nodes to a root in the DAG instance.  The use of MRHOF
   with the ETX metric allows RPL to find the stable minimum-ETX paths
   from the nodes to a root in the DAG instance.  In the absence of a
   metric in the DIO Metric Container or the lack of a DIO Metric
   Container, MRHOF defaults to using ETX to compute Rank, as described
   in Section 3.5.

   Because MRHOF seeks to minimize path costs as described by metrics,
   it can only be used with an additive metric that must be minimized
   on the paths selected for routing. metrics.  MRHOF ignores metrics t

2.  Terminology

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

   This terminology used in this document is consistent with the
   terminologies described in [I-D.ietf-roll-terminology],

   [I-D.ietf-roll-rpl], and [I-D.ietf-roll-routing-metrics].

   This document introduces three terms:

   Selected metric:  The metric chosen by the network operator to use
         for path selection.  This metric can be any additive metric
         listed in [I-D.ietf-roll-routing-metrics].

   Path cost:  Path cost quantifies a property of an end-to-end path.
         Path cost is obtained by summing up the selected metric of the
         links or nodes along the path.  Path cost can be used by RPL to
         compare different paths.

   Worst parent:  The node in the parent set with the largest path cost.

3.  The Minimum Rank Objective Function with Hysteresis

   The Minimum Rank Objective Function with Hysteresis, Hysteresis Objective Function, MRHOF, is
   designed to find the paths with the smallest path cost while
   preventing excessive churn in the network.  It does so by finding the
   minimum cost path and switching to that path only if it is shorter
   (in terms of path cost) than the current path by at least a given
   threshold.

   MRHOF may be used with any additive metric listed in
   [I-D.ietf-roll-routing-metrics] as long the routing objective is to
   minimize the given routing metric.  Nodes MUST support at least one
   of these metrics: node energy, hop count, latency, link quality
   level, and ETX.  Nodes SHOULD support the ETX metric.  MRHOF does not
   support non-additive metrics.

3.1.  Computing the Path cost

   Nodes

   Root nodes (Grounded or Floating) set the variable cur_min_path_cost
   to MIN_PATH_COST.

   If a non-root node does not have metrics to compute the path cost for each
   through any of the candidate neighbor reachable on
   an interface.  The Path cost neighbors, it MUST join one of the
   candidate neighbors as a RPL Leaf.

   Otherwise, nodes compute the path cost for each candidate neighbor
   reachable on an interface.  The path cost of a neighbor represents
   the cost of the path, in terms of the selected metric, from a node to
   the root of the DODAG through the that neighbor.

   Root nodes (Grounded or Floating) set the variable cur_min_path_cost
   to MinHopRankIncrease.  A non-root node
   computes the path cost for a neighbor's path to the root through
   each candidate neighbor cost by adding these two components:

   1.  If the selected metric is a link metric, the selected metric for
       the link to a candidate neighbor.  If the selected metric is a
       node metric, the selected metric for the node.

   2.  The value of the selected metric in the metric container in the
       DIO sent by that neighbor.

   A node SHOULD compute the path cost for the path through each
   candidate neighbor reachable through an interface.  If a node cannot
   compute the path cost for the path through a candidate neighbor, the
   node MUST NOT select the candidate neighbor as its preferred parent,
   with one exception.  If the node does not have metrics to
   except if it cannot compute the path cost through any of the candidate neighbors, neighbor, in
   which case it MUST may join one of
   the candidate neighbors as a leaf node. as described above.

   If the selected metric is a link metric and the metric of the link to
   a neighbor is not available, the path cost for the path through that
   neighbor SHOULD be set to MAX_PATH_COST.  This cost value will
   prevent this path from being considered for path selection.

   If the selected metric is a node metric, and the metric is not
   available, the path cost through all the neighbors SHOULD be set to
   MAX_PATH_COST.

   The path cost corresponding to a neighbor SHOULD be re-computed each
   time:

   1.  The selected metric of the link to the candidate neighbor is
       updated.

   2.  If the selected metric is a node metric and the metric is
       updated.

   3.  A node receives a new metric advertisement from the candidate
       neighbor.

   This computation MAY also be performed periodically.  Too much delay
   in updating the path cost after the metric is updated or a new metric
   advertisement is received can lead to stale Rank or parent set. information.

3.2.  Parent Selection

   After computing the path cost for all the candidate neighbors
   reachable through an interface for the current DODAG iteration, a
   node selects the preferred parent.  This process is called parent
   selection.  To allow hysteresis, parent selection maintains a
   variable, cur_min_path_cost, which is the path cost of the current
   preferred parent.

3.2.1.  When Parent Selection Runs

   A MRHOF implementation SHOULD be performed perform Parent Selection each time:

   1.  The path cost for an existing candidate neighbor, including the
       preferred parent, changes.  This condition can be checked
       immediately after the path cost is computed.

   2.  A new candidate neighbor is inserted into the neighbor table.

   The parent selection MAY be deferred until a later time.  Deferring
   the parent selection can delay the use of better paths available in
   the network.

3.2.2.  Parent Selection Algorithm

   If the selected metric for a link is greater than MAX_LINK_METRIC,
   the node SHOULD exclude that link from consideration during parent
   selection.

   A node MUST select a candidate neighbor as its preferred parent if the path cost corresponding to that candidate neighbor is smaller than with the lowest path cost corresponding to the rest of the neighbors,
   as its preferred parent, except as indicated below:

   1.  If the smallest path cost for paths through the candidate
       neighbors is smaller than cur_min_path_cost by less than
       PARENT_SWITCH_THRESHOLD, the node MAY continue to use the current
       preferred parent.

   2.  If there are multiple paths with the smallest path cost and the
       smallest path cost is smaller than cur_min_path_cost by at least
       PARENT_SWITCH_THRESHOLD, a node MAY use a different objective
       function to select the preferred parent among the candidate
       neighbors on the path with the minimum cost.

   3.  A node MAY declare itself as a Floating root, and hence have no
       preferred parent, depending on the system configuration.

   4.  If the selected metric for a link is greater than
       MAX_LINK_METRIC, the node SHOULD exclude that link from
       consideration for parent selection.

   5.

   2.  If cur_min_path_cost is greater than MAX_PATH_COST, the node MAY
       declare itself as a Floating root.

   6.

   3.  If the smallest path cost for paths through the candidate
       neighbors is smaller than cur_min_path_cost by less than
       PARENT_SWITCH_THRESHOLD, the node MAY continue to use the current
       preferred parent.  This is the hysteresis component of MRHOF.

   4.  If ALLOW_FLOATING_ROOT is 0 and no neighbors are discovered, the
       node does not have a preferred parent, and MUST set
       cur_min_path_cost to MAX_PATH_COST.

   Except in the cases above, the candidate neighbor on the path with

   If there are multiple neighbors which share the smallest path cost is cost, a
   node MAY use a different objective function to select which of these
   neighbors should be considered to have the preferred parent. lowest cost.

   A node MAY include a
   total of PARENT_SET_SIZE up to PARENT_SET_SIZE-1 additional candidate
   neighbors in the its parent set.  The cost of path through the nodes in
   the parent set is smaller than or equal to the cost of the paths
   through any of the nodes that are not in the parent set.  If the cost
   of the path through the preferred parent and the worst parent is too
   large, a node MAY keep a smaller parent set. set than PARENT_SET_SIZE.

   Once the preferred parent is selected, the node sets its
   cur_min_path_cost variable to the path cost corresponding to the
   preferred parent.  The value of the cur_min_path_cost is carried in
   the metric container corresponding to the selected metric when DIO
   messages are sent.

3.3.  Computing Rank

   The

   DAG roots set their rank Rank to MinHopRankIncrease.

   Once a non-root node selects its parent set, it can use the following
   table to covert the path cost of the worst a parent (written as Cost in the
   table) to its Rank: a Rank value:

                    +--------------------+------------+
                    |  Node/link Metric  |    Rank    |
                    +--------------------+------------+
                    |     Node Energy    | 255 - Cost |
                    |      Hop-Count     |    Cost    |
                    |       Latency      | Cost/65536 |
                    | Link Quality Level |    Cost    |
                    |         ETX        |    Cost    |
                    +--------------------+------------+

                  Table 1: Conversion of metric to rank.

   Nodes MUST support at least

   Rank is undefined for these node/link metrics: Node state and
   attributes, throughput, and link color.  If the rank is undefined,
   the node must join one of the above metrics.  Nodes SHOULD
   support the ETX metric.

   If neighbors as a RPL Leaf node according
   to [I-D.ietf-roll-rpl].

   MRHOF uses this Rank calculation causes value to compute the increase in Rank between it associates with the
   path through each member of the parent set.  The Rank associated with
   a node
   and its worst path through a member of the parent to be less than MinHopRankIncrease, set is the maximum of the
   corresponding calculated Rank value from above and that node's
   advertised Rank plus MinHopRankIncrease.

   A node sets its Rank to the maximum of three values:

   1.  The Rank associated with the path through the preferred parent

   2.  The Rank of its worst the member of the parent set with the highest
       advertised Rank plus
   MinHopRankIncrease.

   Node rank is undefined for these node/link metrics: Node state and
   attributes, throughput, and link color.  If one

   3.  The Rank of the rank is undefined, path through the node must join one member of the neighbors as parent set with
       the highest path cost, minus MaxRankIncrease

   The first case is the Rank associated with the path through the
   preferred parent.  The second case covers requirement 4 of Rank
   advertisements in Section 8.2.1 of [I-D.ietf-roll-rpl].  The third
   case ensures that a leaf node according to
   [I-D.ietf-roll-rpl]. does not advertise a Rank which then
   precludes it from using members of its parent set.

3.4.  Advertising the Path Cost

   Once the preferred parent is selected, the node sets its
   cur_min_path_cost variable to the path cost corresponding to its
   preferred parent.  It then calculates the
   highest metric it will advertise in
   its metric container.  This value is the path cost element of the member of
   the parent set. set with the highest path cost.  Thus, while
   cur_min_path_cost is the cost of through the preferred parent, a node
   advertises the highest cost path from the node to the root through a
   member of the parent set.  The value of the cur_min_path_cost highest cost path is
   carried in the metric container corresponding to the selected metric
   when DIO messages are sent.

   If ETX is the selected metric, cur_min_path_cost is directly used as
   Rank and never advertised in a metric container.

   DODAG Roots advertise a metric value which computes to a cost of
   MIN_PATH_COST.

3.5.  Working Without Metric Containers

   In the absence of metric container, MRHOF uses ETX as its metric.  It
   locally computes the ETX of links to its neighbors and adds this
   value to their advertised Rank to compute the associated Rank of
   routes.  Once parent selection and rank computation is performed
   using the ETX metric, the node advertises a Rank equal to the ETX
   cost and MUST NOT include a metric container in its DIO messages.

4.  Using MRHOF for Metric Maximization

   MRHOF cannot be directly used for parent selection using metrics
   which require finding paths with maximum value of the selected
   metric, such as path reliability.  It is possible to convert such a
   metric maximization problem to a metric minimization problem for some
   metrics and use MRHOF provided:

      There is a fixed and well-known maximum metric value corresponding
      to the best path.  This is the path cost for the DAG root.  For
      example, the logarithm of the best link reliability has a value of
      0.

      The metrics in the maximization problem are all negative.  The
      logarithm of the link reliability is always negative.

   For metrics meeting the above conditions, the problem of maximizing
   the metric value is equivalent to minimizing the modified metric
   value, e.g., logarithm of link reliability.  MRHOF is not required to
   work with these metrics.

5.  MRHOF Variables and Parameters

   MRHOF uses the following variable:

      cur_min_path_cost: The cost of the path from a node through its
      preferred parent to the root computed at the last parent
      selection.

   MRHOF uses the following parameters:

      MAX_LINK_METRIC: Maximum allowed value for the selected link
      metric for each link on the path.

      MAX_PATH_COST: Maximum allowed value for the path metric of a
      selected path.

      MIN_PATH_COST: The minimum allowed value for the path metric of
      the selected path.

      PARENT_SWITCH_THRESHOLD: The difference between metric the cost of the
      path through the preferred parent and the minimum-metric minimum cost path in
      order to trigger the selection of a new preferred parent.

      PARENT_SET_SIZE: The number of candidate parents, including the
      preferred parent, in the parent set.

      ALLOW_FLOATING_ROOT: If set to 1, allows a node to become a
      floating root.

   The parameter values are assigned depending on the selected metric.
   The best values for these parameters should be experimentally
   determined.  The working group has long experience routing with the
   ETX metric.  Based on those experiences, these values are
   RECOMMENDED:

      MAX_LINK_METRIC: 10.  Disallow links with greater than 10 expected
      transmission count on the selected path.

      MAX_PATH_COST: 100.  Disallow paths with greater than 100 expected
      transmission count.

      MIN_PATH_COST: 0.  At root, the expected transmission count is 0.

      PARENT_SWITCH_THRESHOLD: 1.5. 172.  Switch to a new path only if it is
      expected to require at least 1.5 fewer transmission transmissions than the
      current path.  As ETX is encoded as number of transmissions times
      128, this is a threshold of 172.

      PARENT_SET_SIZE: 3.  If the preferred parent is not available, two
      candidate parents are still available without triggering a new
      round of route discovery.

      ALLOW_FLOATING_ROOT: 0.  Do not allow a node to become a floating
      root.

6.  Manageability

   Section 18 of [I-D.ietf-roll-rpl] depicts the management of RPL.
   This specification inherits from that section and its subsections,
   with the exception that metrics as specified in
   [I-D.ietf-roll-routing-metrics] are not used and do not require
   management.

6.1.  Device Configuration

   An implementation SHOULD allow the following parameters to be
   configured at installation time: MAX_LINK_METRIC, MAX_PATH_COST,
   MIN_PATH_COST, PARENT_SWITCH_THRESHOLD, PARENT_SET_SIZE, and
   ALLOW_FLOATING_ROOT.  An implementation MAY allow these parameters to
   be configured dynamically at run time once a network has been
   deployed.

   A MRHOF implementation SHOULD support the DODAG Configuration option
   as described in [I-D.ietf-roll-rpl] and apply the parameters it
   specifies.  Care should be taken in the relationship between the
   MRHOF PARENT_SWITCH_THRESHOLD parameter and the RPL MaxRankIncrease
   parameter.  For example, if MaxRankIncrease is smaller than
   PARENT_SWITCH_THRESHOLD, a RPL node using MRHOF could enter a
   situation where its current preferred parent causes the nodes Rank to
   increase more than MaxRankIncrease but MRHOF does not change
   preferred parents: this could cause the node to leave the routing
   topology even though there may be other members of the parent set
   which would allow the node's Rank to remain within MaxRankIncrease.

   Unless configured otherwise, a MRHOF implementation SHOULD use the
   default parameters as specified in Section 5.

   Because of the partially-coupled relationship between Rank and metric
   values, networks using MRHOF require care in setting
   MinHopRankIncrease.  A large MinHopRankIncrease will cause MRHOF to
   be unable to select paths with different hop counts but similar
   metric values.  If MinHopRankIncrease is large enough that its
   increment is greater than that caused by link cost, then metrics will
   be used to select a preferred parent but the advertised Rank will be
   a simple hopcount.  This behavior might be desirable, but it also
   might be unintended: care is recommended.

6.2.  Device Monitoring

   A MRHOF implementation should provide an interface for monitoring its
   operation.  At a minimum, the information provided should include:

      DAG information as specified in Section 6.3.1 of
      [I-D.ietf-roll-rpl], and including the DODAGID, the RPLInstanceID,
      the Mode of Operation, the Rank of this node, the current Version
      Number, and the value of the Grounded flag.

      A list of neighbors indicating the preferred parent.  The list
      should indicate, for each neighbor, the Rank, the current Version
      Number, the value of the Grounded flag, and associated metrics.

7.  Acknowledgements

   Thanks to Antonio Grilo, Nicolas Tsiftes, Matteo Paris, JP Vasseur,
   and Phoebus Chen for their comments.

8.  IANA Considerations

   This specification requires a value allocated from the "Objective
   Code Point (OCP)" sub-registry of the "Routing Protocol for Low Power
   and Lossy Networks (RPL)" registry.  A value of 1 is suggested.

9.  Security Considerations

   This specification makes simple extensions to RPL and so is
   vulnerable to and benefits from the security issues and mechanisms
   described in [I-D.ietf-roll-rpl] and
   [I-D.ietf-roll-security-framework].  This document does not introduce
   new flows or new messages, thus requires no specific mitigation for
   new threats.

   MRHOF depends on information exchanged in a number RPL protocol
   elements.  If those elements were compromised, then an implementation
   of MRHOF might generate the wrong path for a packet, resulting in it
   being misrouted.  Therefore, deployments are RECOMMENDED to use RPL
   security mechanisms if there is a risk that routing information might
   be modified or spoofed.

10.  References

10.1.  Normative References

   [I-D.ietf-roll-routing-metrics]
              Barthel, D., Vasseur, J., Pister, K., Kim, M., and N.
              Dejean, "Routing Metrics used for Path Calculation in Low
              Power and Lossy Networks",
              draft-ietf-roll-routing-metrics-19 (work in progress),
              March 2011.

   [I-D.ietf-roll-rpl]
              Brandt, A., Vasseur, J., Hui, J., Pister, K., Thubert, P.,
              Levis, P., Struik, R., Kelsey, R., Clausen, T., and T.
              Winter, "RPL: IPv6 Routing Protocol for Low power and
              Lossy Networks", draft-ietf-roll-rpl-19 (work in
              progress), March 2011.

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

10.2.  Informative References

   [I-D.ietf-roll-security-framework]
              Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
              Lozano, "A Security Framework for Routing over Low Power
              and Lossy Networks", draft-ietf-roll-security-framework-07
              (work in progress), January 2012.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-05 (work in
              progress), March 2011.

Authors' Addresses

   Omprakash Gnawali
   Stanford University
   S255 Clark Center, 318 Campus Drive
   Stanford, CA  94305
   USA

   Phone: +1 650 725 6086
   Email: gnawali@cs.stanford.edu

   Philip Levis
   Stanford University
   358
   412 Gates Hall, Stanford University
   Stanford, CA  94305
   USA

   Email: pal@cs.stanford.edu