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Internet Engineering Task Force (IETF)                        O. GnawaliRequest for Comments: 6719                         University of HoustonCategory: Standards Track                                       P. LevisISSN: 2070-1721                                      Stanford University                                                          September 2012The Minimum Rank with Hysteresis Objective FunctionAbstract   The Routing Protocol for Low-Power and Lossy Networks (RPL)   constructs routes by using Objective Functions that optimize or   constrain the routes it selects and uses.  This specification   describes the Minimum Rank with Hysteresis Objective Function   (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 additive metrics along a route, and   the metrics it uses are determined by the metrics that the RPL   Destination Information Object (DIO) messages advertise.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/rfc6719.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 of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document mustGnawali & Levis              Standards Track                    [Page 1]

RFC 6719                          MRHOF                   September 2012   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 . . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .33.  The Minimum Rank with Hysteresis Objective Function  . . . . .43.1.  Computing the Path Cost  . . . . . . . . . . . . . . . . .43.2.  Parent Selection . . . . . . . . . . . . . . . . . . . . .53.2.1.  When Parent Selection Runs . . . . . . . . . . . . . .63.2.2.  Parent Selection Algorithm . . . . . . . . . . . . . .63.3.  Computing Rank . . . . . . . . . . . . . . . . . . . . . .73.4.  Advertising the Path Cost  . . . . . . . . . . . . . . . .83.5.  Working without Metric Containers  . . . . . . . . . . . .84.  Using MRHOF for Metric Maximization  . . . . . . . . . . . . .95.  MRHOF Variables and Parameters . . . . . . . . . . . . . . . .96.  Manageability  . . . . . . . . . . . . . . . . . . . . . . . .106.1.  Device Configuration . . . . . . . . . . . . . . . . . . .106.2.  Device Monitoring  . . . . . . . . . . . . . . . . . . . .117.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .128.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .129.  Security Considerations  . . . . . . . . . . . . . . . . . . .1210. References . . . . . . . . . . . . . . . . . . . . . . . . . .1210.1. Normative References . . . . . . . . . . . . . . . . . . .1210.2. Informative References . . . . . . . . . . . . . . . . . .131.  Introduction   An Objective Function specifies how RPL [RFC6550] selects paths.  For   example, if an RPL instance uses an Objective Function that minimizes   hop count, RPL will select paths with a minimum hop count.  RPL   requires that all nodes in a network use a common Objective Function;   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 [RFC6551] and make these metrics available for route   selection.  RPL advertises metrics in RPL Destination Information   Object (DIO) messages with a Metric Container suboption.  An   Objective Function can use these metrics to choose routes.   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 basedGnawali & Levis              Standards Track                    [Page 2]

RFC 6719                          MRHOF                   September 2012   on Rank and advertise their Rank.  An Objective Function defines how   nodes calculate Rank, based on the Rank of its potential parents,   metrics, and other network properties.   This specification describes the Minimum Rank with Hysteresis   Objective Function (MRHOF), an Objective Function for RPL, which 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 Directed Acyclic Graph (DAG) instance [RFC6550].   The use of MRHOF with the Expected Transmission Count (ETX) metric   [RFC6551] 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 of a DIO Metric Container, MRHOF defaults   to using ETX to compute Rank, as described inSection 3.5.   Because MRHOF seeks to minimize path costs as described by metrics,   it can only be used with additive metrics.  MRHOF does not support   metrics that are not additive.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 inRFC2119 [RFC2119].   The terminologies used in this document are consistent with the   terminologies described in [ROLL-TERM] and [RFC6551].   The terminologies used in this document are also consistent with the   terminologies described in [RFC6550], except the term Rank.  In this   document, Rank refers to the value of the Rank field, not DAGRank as   in [RFC6550].   This document introduces three terms:   Selected metric:  The metric chosen for path selection by the network      operator.  MRHOF supports using a single metric for path      selection.  The decision to use a metric (other than ETX) as the      selected metric is indicated by the presence of the chosen metric      in the DIO Metric Container.  The selection of the ETX metric is      indicated by the absence of the Metric Container, in which case      ETX is advertised as Rank.Gnawali & Levis              Standards Track                    [Page 3]

RFC 6719                          MRHOF                   September 2012   Path cost:  Path cost quantifies a property of an end-to-end path.      Path cost is obtained by each node summing up the selected link      metric to the path cost advertised by the parent.  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 with Hysteresis Objective Function   The Minimum Rank with 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 using two   mechanisms.  First, it finds the minimum cost path, i.e., path with   the minimum Rank.  Second, it switches to that minimum Rank path only   if it is shorter (in terms of path cost) than the current path by at   least a given threshold.  This second mechanism is called   "hysteresis".   MRHOF may be used with any additive metric listed in [RFC6551] as   long as the routing objective is to minimize the given routing   metric.  Nodes MUST support at least one of these metrics: hop count,   latency, or ETX.  Nodes SHOULD support the ETX metric.  MRHOF does   not support non-additive metrics.3.1.  Computing the Path Cost   Root nodes (Grounded or Floating) set the variable cur_min_path_cost   to the metric value that computes to a Rank of MinHopRankIncrease.   If a non-root node does not have metrics to compute the path cost   through any of the candidate 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 Destination-Oriented DAG (DODAG) through that   neighbor.  A non-root node computes a neighbor's path cost by adding   two components:   1.  If the selected metric is a link metric, the selected metric for       the link to the candidate neighbor.  If the selected metric is a       node metric, the selected metric for the node.Gnawali & Levis              Standards Track                    [Page 4]

RFC 6719                          MRHOF                   September 2012   2.  The value of the selected metric in the Metric Container in the       DIO sent by that neighbor.  In case the Metric Container is       empty, ETX is the selected metric -- use the Rank advertised by       that neighbor as the second component.  SeeSection 3.5 for       details on how an ETX metric is used in MRHOF.   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.   However, if the node cannot compute the path cost through any   neighbor, it may join the candidate neighbor as a Leaf, 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 recomputed each   time any of the following conditions are met:   1.  The selected metric of the link to the candidate neighbor is       updated.   2.  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 SHOULD also be performed periodically.  While it is   harmless to delay this computation up to a minimum Trickle interval   [RFC6550], longer delays in updating the path cost after the metric   is updated or a new metric advertisement is received can lead to   stale information.3.2.  Parent Selection   After computing the path cost for all the candidate neighbors   reachable through an interface for the current DODAG iteration   [RFC6550], 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.Gnawali & Levis              Standards Track                    [Page 5]

RFC 6719                          MRHOF                   September 20123.2.1.  When Parent Selection Runs   A MRHOF implementation SHOULD 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.   If, despite the above, it is necessary to defer the parent selection   until a later time (e.g., up to the Trickle minimum interval   [RFC6550]), note that doing so 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 the candidate neighbor with the lowest path cost   as its preferred parent, except as indicated below:   1.  A node MAY declare itself as a Floating root, and hence have no       preferred parent, depending on system configuration.   2.  If cur_min_path_cost is greater than MAX_PATH_COST, the node MAY       declare itself as a Floating root.   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.   If there are multiple neighbors that share the smallest path cost, a   node MAY use a different selection criteria to select which of these   neighbors should be considered to have the lowest cost.   A node MAY include up to PARENT_SET_SIZE-1 additional candidate   neighbors in its parent set.  The cost of the 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 costGnawali & Levis              Standards Track                    [Page 6]

RFC 6719                          MRHOF                   September 2012   of the path through the preferred parent and the worst parent is too   large, a node MAY keep a smaller parent 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   DAG roots set their Rank to MinHopRankIncrease.   Once a non-root node selects its parent set, it can use the following   table to covert the path cost of a parent (written as Cost in the   table) to a Rank value:                     +------------------+------------+                     | Node/link Metric |    Rank    |                     +------------------+------------+                     |     Hop-Count    |    Cost    |                     |      Latency     | Cost/65536 |                     |        ETX       |    Cost    |                     +------------------+------------+                   Table 1: Conversion of Metric to Rank   If MRHOF is used with other metrics, the Rank is undefined.  If the   Rank is undefined, the node must join one of the neighbors as a RPL   Leaf node according to [RFC6550].   MRHOF uses this Rank value to compute the Rank it associates with the   path through each member of the parent set.  The Rank associated with   a path through a member of the parent set is the maximum of two   values.  The first is the corresponding Rank value calculated with   the table above, the second is that nodes' advertised Rank plus   MinHopRankIncrease.   A node sets its Rank to the maximum of three values:   1.  The Rank calculated for the path through the preferred parent.   2.  The Rank of the member of the parent set with the highest       advertised Rank, rounded to the next higher integral Rank, i.e.,       to MinHopRankIncrease * (1 + floor(Rank/MinHopRankIncrease)).   3.  The largest calculated Rank among paths through the parent set,       minus MaxRankIncrease.Gnawali & Levis              Standards Track                    [Page 7]

RFC 6719                          MRHOF                   September 2012   The first case is the Rank associated with the path through the   preferred parent.  The second case covers requirement 5 of Rank   advertisements inSection 8.2.1 of [RFC6550].  The third case ensures   that a node does not advertise a Rank, which then precludes it from   using members of its parent set.   Note that the third case means that a node advertises a conservative   Rank value based on members of its parent set.  This conservative   value might be significantly higher than the Rank calculated for the   path through the preferred parent.  Accordingly, picking a parent set   whose paths have a large range of Ranks will likely result in   subptimal routing: nodes might not choose good paths because they are   advertised as much worse than they actually are.  The exact selection   of a parent set is an implementation decision.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 metric it will advertise in   its metric container.  This value is the path cost of the member of   the parent set with the highest path cost.  Thus, while   cur_min_path_cost is the cost 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 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, a node MUST NOT advertise it in a   metric container.  Instead, a node MUST advertise an approximation of   its ETX in its advertised Rank value, following the rules described   inSection 3.3.  If a node receives a DIO with a Metric Container   holding an ETX metric, MRHOF MUST ignore the ETX metric value in its   Rank calculations.   DODAG Roots advertise a metric value that computes to a Rank value of   MinHopRankIncrease.3.5.  Working without Metric Containers   In the absence of a 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 the Rank and MUST NOT   include a metric container in its DIO messages.  While assigning Rank   in this case, use the representation of ETX described in [RFC6551],   i.e., assign Rank equal to ETX * 128.Gnawali & Levis              Standards Track                    [Page 8]

RFC 6719                          MRHOF                   September 20124.  Using MRHOF for Metric Maximization   MRHOF cannot be directly used for parent selection using metrics that   require finding paths with a 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.      PARENT_SWITCH_THRESHOLD: The difference between the cost of the      path through the preferred parent and the 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.Gnawali & Levis              Standards Track                    [Page 9]

RFC 6719                          MRHOF                   September 2012   The parameter values are assigned depending on the selected metric.   The best values for these parameters are determined by the   requirement of the specific RPL deployment.  For instance, if we use   ETX as the selected metric and UDP as the transport protocol, we   should use a small MAX_LINK_METRIC (e.g., ETX of 1.1) so that link-   layer retransmissions are sufficient to provide a good chance of end-   to-end reliability.   The working group has extensive experience routing with the ETX   metric [Hui08b].  Based on those experiences, the following values   are RECOMMENDED when ETX is the selected metric:      MAX_LINK_METRIC: 512.  Disallow links with greater than 4 expected      transmission counts on the selected path.      MAX_PATH_COST: 32768.  Disallow paths with greater than 256      expected transmission counts.      PARENT_SWITCH_THRESHOLD: 192.  Switch to a new path only if it is      expected to require at least 1.5 fewer transmissions than the      current path.      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.  ManageabilitySection 18 of [RFC6550] depicts the management of RPL.  This   specification inherits from that section and its subsections, with   the exception that metrics as specified in [RFC6551] 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,   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 MUST support the DODAG Configuration option as   described in [RFC6550] and apply the parameters it specifies.  Care   should be taken in the relationship between the MRHOF   PARENT_SWITCH_THRESHOLD parameter and the RPL MaxRankIncreaseGnawali & Levis              Standards Track                   [Page 10]

RFC 6719                          MRHOF                   September 2012   parameter.  For example, if MaxRankIncrease is smaller than   PARENT_SWITCH_THRESHOLD, a RPL node using MRHOF could enter a   situation in which its current preferred parent causes the node's   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   that would allow the node's Rank to remain within MaxRankIncrease.   Unless configured otherwise, a MRHOF implementation SHOULD use the   default parameters as specified inSection 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 hop count.  This behavior might be desirable, but it also   might be unintended; care is recommended.   With ETX as the selected metric, RPL's Rank advertisement rules can   require a DODAG Root to advertise a Rank higher than its   corresponding ETX value, as a DODAG Root advertises a Rank of   MinHopRankIncrease.  Because all DODAG Roots within a DODAG Version   advertise the same Rank, this constant value typically does not   affect route selection.  Nevertheless, it means that if a DODAG   Version has a MinHopRankIncrease of M and a path has an advertised   ETX of E, then the actual ETX of the path is likely closer to a value   of E-M than a value of E.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 inSection 6.3.1 of [RFC6550],      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.Gnawali & Levis              Standards Track                   [Page 11]

RFC 6719                          MRHOF                   September 20127.  Acknowledgements   Thanks to Antonio Grilo, Nicolas Tsiftes, Matteo Paris, JP Vasseur,   and Phoebus Chen for their comments.  Thanks to Barry Leiba, Brian   Haberman, Martin Stiemerling, Ralph Droms, Robert Sparks, Russ   Housley, Stephen Farrell, Wesley Eddy, Miguel A. Garcia, Mukul Goyal,   and Michael Richardson for their feedback during the publication   phase of this document.8.  IANA Considerations   Per this document, IANA has allocated value 1 from the "Objective   Code Point (OCP)" sub-registry of the "Routing Protocol for Low Power   and Lossy Networks (RPL)" registry.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 [RFC6550] and [ROLL-SEC].  This document does not   introduce new flows or new messages, and thus requires no specific   mitigation for new threats.   MRHOF depends on information exchanged in a number of 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.  References10.1.  Normative References   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC6550]   Winter, T., Thubert, P., 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.   [RFC6551]   Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.               Barthel, "Routing Metrics Used for Path Calculation in               Low-Power and Lossy Networks",RFC 6551, March 2012.Gnawali & Levis              Standards Track                   [Page 12]

RFC 6719                          MRHOF                   September 201210.2.  Informative References   [Hui08b]    Hui, J. and D. Culler, "IP is dead, long live IP for               wireless sensor networks", Proceedings of the 6th ACM               Conference on Embedded Networked Systems SenSys 2008,               November 2008,               <http://portal.acm.org/citation.cfm?id=1460412.1460415>.   [ROLL-SEC]  Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.               Lozano, "A Security Framework for Routing over Low Power               and Lossy Networks", Work in Progress, January 2012.   [ROLL-TERM] Vasseur, J., "Terminology in Low power And Lossy               Networks", Work in Progress, September 2011.Authors' Addresses   Omprakash Gnawali   University of Houston   PGH 577, University of Houston   Houston, TX  77204   USA   Phone: +1 713 743 3356   EMail: gnawali@cs.uh.edu   Philip Levis   Stanford University   412 Gates Hall, Stanford University   Stanford, CA  94305   USA   EMail: pal@cs.stanford.eduGnawali & Levis              Standards Track                   [Page 13]