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RFC 7185 - Link Metrics for the Mobile Ad Hoc Network (MANET) Ro


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Internet Engineering Task Force (IETF)                       C. Dearlove
Request for Comments: 7185                               BAE Systems ATC
Category: Informational                                       T. Clausen
ISSN: 2070-1721                                 LIX, Ecole Polytechnique
                                                              P. Jacquet
                                                Alcatel-Lucent Bell Labs
                                                              April 2014

                 Rationale for the Use of Link Metrics
    in the Optimized Link State Routing Protocol Version 2 (OLSRv2)

Abstract

   The Optimized Link State Routing Protocol version 2 (OLSRv2) includes
   the ability to assign metrics to links and to use those metrics to
   allow routing by other than minimum hop count routes.  This document
   provides a historic record of the rationale for, and design
   considerations behind, how link metrics were included in OLSRv2.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7185.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   than English.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................5
   3. Applicability ...................................................5
   4. Motivational Scenarios ..........................................5
   5. Link Metrics ....................................................7
      5.1. Link Metric Properties .....................................7
      5.2. Link Metric Types ..........................................8
      5.3. Directional Link Metrics ..................................10
      5.4. Reporting Link and Neighbor Metrics .......................10
      5.5. Defining Incoming Link Metrics ............................12
      5.6. Link Metric Values ........................................12
   6. MPRs with Link Metrics .........................................14
      6.1. Flooding MPRs .............................................14
      6.2. Routing MPRs ..............................................16
      6.3. Relationship between MPR Sets .............................19
   7. Security Considerations ........................................21
   8. Acknowledgements ...............................................21
   9. Informative References .........................................21
   Appendix A.  MPR Routing Property .................................23

1.  Introduction

   The Optimized Link State Routing Protocol version 1 (OLSRv1)
   [RFC3626] is a proactive routing protocol for mobile ad hoc networks
   (MANETs) [RFC2501].  OLSRv1 finds the shortest, defined as minimum
   number of hops, routes from a router to all possible destinations.

   Using only minimum hop routes may result in what are, in practice,
   inferior routes.  Some examples are given in Section 4.  Thus, one of
   the distinguishing features of the Optimized Link State Routing
   Protocol version 2 (OLSRv2) [RFC7181] is the introduction of the
   ability to select routes using link metrics other than the number of
   hops.

   During the development of OLSRv2, the working group and authors
   repeatedly discussed how and why some choices were made in the
   protocol specification, particularly at the metric integration level.
   Some of the issues may be non-intuitive, and this document is
   presented as a record of the considerations and decisions to provide
   informational discussion about motivation and historic design
   choices.  This document is intended to be useful as a reference if
   those questions arise again.

   Use of the extensible message format [RFC5444] by OLSRv2 has allowed
   the addition, by OLSRv2, of link metric information to the HELLO
   messages defined in the MANET Neighborhood Discovery Protocol (NHDP)
   [RFC6130] as well as inclusion in the Topology Control (TC) messages
   defined in [RFC7181].

   OLSRv2 essentially first determines local link metrics from 1-hop
   neighbors, these being defined by a process outside OLSRv2, then
   distributes required link metric values in HELLO messages and TC
   messages, and then finally forms routes with minimum total link
   metric.  Using a definition of route metric other than number of hops
   is a natural extension that is commonly used in link state protocols.

   A metric-based route selection process for OLSRv2 could have been
   handled as an extension to OLSRv2.  However, were this to have been
   done, OLSRv2 routers that did not implement this extension would not
   recognize any link metric information and would attempt to use
   minimum hop-count routes.  This would have meant that, in effect,
   routers that did implement and routers that did not implement this
   extension would differ over their valuation of links and routes.
   This would have led to the fundamental routing problem of "looping".
   Thus, if metric-based route selection were to have been considered
   only as an extension to OLSRv2, then routers that did implement and
   routers that did not implement this extension would not have been

   able to interoperate.  This would have been a significant limitation
   of such an extension.  Link metrics were therefore included as
   standard in OLSRv2.

   This document discusses the motivation and design rationale behind
   how link metrics were included in OLSRv2.  The principal issues
   involved when including link metrics in OLSRv2 were:

   o  Assigning metrics to links involved considering separate metrics
      for the two directions of a link, with the receiving router
      determining the metric from transmitter to receiver.  A metric
      used by OLSRv2 may be either of:

      *  A link metric, the metric of a specific link from an OLSRv2
         interface of the transmitting router to an OLSRv2 interface of
         the receiving router.

      *  A neighbor metric, the minimum of the link metrics between two
         OLSRv2 routers, in the indicated direction.

      These metrics are necessarily the same when these routers each
      have a single OLSRv2 interface but may differ when either has
      more.  HELLO messages may include both link metrics and neighbor
      metrics.  TC messages include only neighbor metrics.

   o  Metrics as used in OLSRv2 are defined to be dimensionless and
      additive.  The assignment of metrics, including their relationship
      to real parameters such as data rate, loss rate, and delay, and
      the management of the choice of metric, is outside the scope of
      [RFC7181], which simply uses these metrics in a consistent manner.
      Within a single MANET, including all components of a temporarily
      fragmented MANET, a single choice of link metric is used.  By use
      of a registry of metric types (employing extended types of a
      single Address Block TLV type), routers can be configured to use
      only a subset of the available metric types.

   o  Node metrics were not included in OLSRv2.  Node metrics can be
      implemented by the addition of the corresponding value to all
      incoming link metrics by the corresponding router.

   o  The separation of the two functions performed by multipoint relays
      (MPRs) in OLSRv1, optimized flooding and reduced topology
      advertisement for routing, into separate sets of MPRs in OLSRv2
      [RFC7181], denoted "flooding MPRs" and "routing MPRs".  Flooding
      MPRs can be calculated as in [RFC3626], but the use of link
      metrics in OLSRv2 can improve the MPR selection.  Routing MPRs
      need a metric-aware selection algorithm.  The selection of routing
      MPRs guarantees the use of minimum distance routes using the

      chosen metric, while using only symmetric 2-hop neighborhood
      information from HELLO messages and routing MPR selector
      information from TC messages.

   o  The protocol Information Bases defined in OLSRv2 include required
      metric values.  This has included additions to the protocol
      Information Bases defined in NHDP [RFC6130] when used by OLSRv2.

2.  Terminology

   All terms introduced in [RFC5444], including "message" and "TLV"
   (type-length-value), are to be interpreted as described there.

   All terms introduced in [RFC6130], including "MANET interface",
   "HELLO message", "heard", "link", "symmetric link", "1-hop neighbor",
   "symmetric 1-hop neighbor", "2-hop neighbor", "symmetric 2-hop
   neighbor", "symmetric 2-hop neighborhood", and the symbolic constants
   SYMMETRIC and HEARD, are to be interpreted as described there.

   All terms introduced in [RFC7181], including "router", "OLSRv2
   interface", "willingness", "multipoint relay (MPR)", "MPR selector",
   "MPR flooding", and the TLV type LINK_METRIC, are to be interpreted
   as described there.

3.  Applicability

   The objective of this document is to retain the design considerations
   behind how link metrics were included in [RFC7181].  This document
   does not prescribe any behavior but explains some aspects of the
   operation of OLSRv2.

4.  Motivational Scenarios

   The basic situation that suggests the desirability of use of routes
   other than minimum hop routes is shown in Figure 1.

                            A ----- X ----- B
                             \             /
                              \           /
                               Y ------- Z

                                 Figure 1

   The minimum hop route from A to B is via X.  However, if the links A
   to X and X to B are poor (e.g., have low data rate or are unreliable)
   but the links A to Y, Y to Z, and Z to B are better (e.g., have
   reliable high data rate), then the route A to B via Y and Z may be
   preferred to that via X.

   There are other situations where the use of some links should be
   discouraged, even if the avoidance of them does not show immediately
   obvious benefits to users.  Consider a network with many short-range
   links and a few long-range links.  Use of minimum hop routes will
   immediately lead to heavy use of the long-range links.  This will be
   particularly undesirable if those links achieve their longer range
   through reduced data rate or through being less reliable.  However,
   even if the long-range links have the same characteristics as the
   short-range links, it may be better to reserve usage of the long-
   range links for when this usage is particularly valuable -- for
   example, when the use of one long-range link saves several short-
   range links, rather than the single link saving that is needed for a
   minimum hop route.

   A related case is that of a privileged relay.  An example is an
   aerial router in an otherwise ground-based network.  The aerial
   router may have a link to many, or even all, other routers.  That
   would lead to all routers attempting to send all their traffic (other
   than to symmetric 1-hop neighbors and some symmetric 2-hop neighbors)
   via the aerial router.  It may, however, be important to reserve that
   capacity for cases where the aerial router is actually essential,
   such as if the ground-based portion of the network is not connected.

   Link metrics provide a possible solution to these scenarios.  For
   example, in Figure 1, the route A to Y to Z to B could be preferred
   to A to X to B by making the metrics on the former path 1 and those
   on the latter path 2.  The aerial privileged relay could be used only
   when necessary by giving its links maximal metric values, with much
   smaller other metric values or, if the aerial link is to be preferred
   to N ground links, by giving the ground links metric values of 1
   while making the sum of the aerial node uplink and downlink metrics
   equal to N.

   Other cases may involve attempts to avoid areas of congestion,
   attempts to route around insecure routers, and attempts by routers to
   discourage being used as relays due to, for example, limited battery
   power.  OLSRv2 does have another mechanism to aid in this: a router's
   willingness to act as an MPR.  However, there are cases where that
   cannot help but where use of non-minimum hop routes could.

   Similarly, note that OLSRv2's optional use of link quality (through
   its use of [RFC6130]) is not a solution to these problems.  Use of
   link quality as specified in [RFC6130] allows a router to decline to
   use a link, not only on its own, but on all routers' behalf.  It does
   not, for example, allow the use of a link otherwise determined to be
   too low quality to be generally useful as part of a route where no
   better links exist.  These mechanisms (link quality and link metrics)
   solve distinctly different problems.

   It should also be noted that the loop-free property of OLSRv2 applies
   strictly only in the static state.  When the network topology is
   changing and when messages can be lost, it is possible for transient
   loops to form.  However, with update rates appropriate to the rate of
   topology change, such loops will be sufficiently rare.  Changing link
   metrics is a form of network topology change and should be limited to
   a rate slower than the message information update rate (defined by
   the parameters HELLO_INTERVAL, HELLO_MIN_INTERVAL, REFRESH_INTERVAL,
   TC_INTERVAL, and TC_MIN_INTERVAL).

5.  Link Metrics

   This section describes the required and selected properties of the
   link metrics used in OLSRv2, followed by implementation details
   achieving those properties.

5.1.  Link Metric Properties

   Link metrics in OLSRv2 are:

   o  Dimensionless.  While they may, directly or indirectly, correspond
      to specific physical information (such as delay, loss rate, or
      data rate), this knowledge is not used by OLSRv2.  Instead,
      generating the metric value is the responsibility of a mechanism
      external to OLSRv2.

   o  Additive, so that the metric of a route is the sum of the metrics
      of the links forming that route.  Note that this requires a metric
      where a low value of a link metric indicates a "good" link and a
      high value of a link metric indicates a "bad" link, and the former
      will be preferred to the latter.

   o  Directional, the metric from router A to router B need not be the
      same as the metric from router B to router A, even when using the
      same OLSRv2 interfaces.  At router A, a link metric from router B
      to router A is referred to as an incoming link metric, while a
      link metric from router A to router B is referred to as an
      outgoing link metric.  (These are, of course, reversed at router
      B.)

   o  Specific to a pair of OLSRv2 interfaces, so that if there is more
      than one link from router A to router B, each has its own link
      metric in that direction.  There is also an overall metric, a
      "neighbor metric", from router A to router B (its 1-hop neighbor).
      This is the minimum value of the link metrics from router A to
      router B, considering symmetric links only; it is undefined if
      there are no such symmetric links.  A neighbor metric from one
      router to another is always equal to a link metric in the same

      direction between OLSRv2 interfaces of those routers.  When
      referring to a specific OLSRv2 interface (for example, in a Link
      Tuple or a HELLO message sent on that OLSRv2 interface), a link
      metric always refers to a link on that OLSRv2 interface to or from
      the indicated 1-hop neighbor OLSRv2 interface, while a neighbor
      metric may be equal to a link metric to and/or from another OLSRv2
      interface.

5.2.  Link Metric Types

   There are various physical characteristics that may be used to define
   a link metric.  Some examples, which also illustrate some
   characteristics of metrics that result, are:

   o  Delay is a straightforward metric; as it is naturally additive,
      the delay of a multi-link route is the sum of the delays of the
      links.  This does not directly take into account delays due to
      routers (such as due to router queues or transition of packets
      between router interfaces) rather than links, but these delays can
      be divided among incoming and outgoing links.

   o  Probability of loss on a link is, as long as probabilities of loss
      are small and independent, approximately additive.  (A slightly
      more accurate approach is using a negatively scaled logarithm of
      the probability of not losing a packet.)  If losses are not
      independent, then this will be pessimistic.

   o  Data rates are not additive.  They even have the wrong
      characteristic of being good when high and bad when low; thus, a
      mapping that inverts the ordering must be applied.  Such a mapping
      can, at best, only produce a metric that is acceptable to treat as
      additive.  Consider, for example, a preference for a route that
      maximizes the minimum data rate link on the route and then prefers
      a route with the fewest links of each data rate from the lowest.
      If links may be of three discrete data rates, "high", "medium",
      and "low", then this preference can be achieved, on the assumption
      that no route will have more than 10 links, with metric values of
      1, 10, and 100 for the three data rates.

      If routes can have more than 10 links, the range of metrics must
      be increased; this was one reason for a preference for a wide
      "dynamic range" of link metric values.  Depending on the ratios of
      the numerical values of the three data rates, the same effect may
      be achieved by using a scaling of an inverse power of the
      numerical values of the data rates.  For example, if the three
      data rates were 2, 5, and 10 Mbit/s, then a possible mapping would
      be the fourth power of 10 Mbit/s divided by the data rate, giving
      metric values of 625, 16, and 1 (good for up to 16 links in a

      route).  This mapping can be extended to a system with more data
      rate values, for example, giving a 4 Mbit/s data rate a metric
      value of about 39.  This may lose the capability to produce an
      absolutely maximal minimum data rate route but will usually
      produce either that, or something close (and at times maybe
      better, is a route of three 5 Mbit/s links really better than one
      of a single 4 Mbit/s link?).  Specific metrics will need to define
      the mapping.

   There are also many other possible metrics, including using physical-
   layer information (such as signal-to-noise ratio and error-control
   statistics) and information such as packet-queuing statistics.

   In a well-designed network, all routers will use the same metric
   type.  It will not produce good routes if, for example, some link
   metrics are based on data rate and some on path loss (except to the
   extent that these may be correlated).  How to achieve this is an
   administrative matter, outside the scope of OLSRv2.  In fact, even
   the actual physical meanings of the metrics is outside the scope of
   OLSRv2.  This is because new metrics may be added in the future, for
   example, as data rates increase, and may be based on new, possibly
   non-physical, considerations, for example, financial cost.  Each such
   type will have a metric type number.  Initially, a single link metric
   type zero is defined as indicating a dimensionless metric with no
   predefined physical meaning.

   An OLSRv2 router is instructed which single link metric type to use
   and recognize, without knowing whether it represents delay,
   probability of loss, data rate, cost, or any other quantity.  This
   recognized link metric type number is a router parameter and subject
   to change in case of reconfiguration or possibly the use of a
   protocol (outside the scope of OLSRv2) permitting a process of link
   metric type agreement between routers.

   The use of link metric type numbers also suggests the possibility of
   use of multiple link metric types and multiple network topologies.
   This is a possible future extension to OLSRv2.  To allow for that
   future possibility, the sending of more than one metric of different
   physical types, which should otherwise not be done for reasons of
   efficiency, is not prohibited, but types other than that configured
   will be ignored.

   The following three sections assume a chosen single link metric type,
   of unspecified physical nature.

5.3.  Directional Link Metrics

   OLSRv2 uses only "symmetric" (bidirectional) links, which may carry
   traffic in either direction.  A key decision was whether these links
   should each be assigned a single metric, used in both directions, or
   a metric in each direction, noting that:

   o  Links can have different characteristics in each direction.  Use
      of directional link metrics recognizes this.

   o  In many (possibly most) cases, the two ends of a link will
      naturally form different views as to what the link metric should
      be.  To use a single link metric requires a coordination between
      the two that can be avoided if using directional metrics.  Note
      that if using a single metric, it would be essential that the two
      ends agree as to its value; otherwise, it is possible for looping
      to occur.  This problem does not occur for directional metrics.

   Based on these considerations, directional metrics are used in
   OLSRv2.  Each router must thus be responsible for defining the metric
   in one direction only.  This could have been in either direction,
   i.e., a router is responsible for either incoming or outgoing link
   metrics, as long as the choice is universal.  The former (incoming)
   case is used in OLSRv2 because, in general, receiving routers have
   more information available to determine link metrics (for example,
   received signal strength, interference levels, and error-control
   coding statistics).

   Note that, using directional metrics, if router A defines the metric
   of the link from router B to router A, then router B must use router
   A's definition of that metric on that link in that direction.
   (Router B could, if appropriate, use a bad mismatch between
   directional metrics as a reason to discontinue use of this link,
   using the link quality mechanism defined in [RFC6130]; note that this
   is a distinct mechanism from the use of link metrics.)

5.4.  Reporting Link and Neighbor Metrics

   Links, and hence link metrics, are reported in HELLO messages.  A
   router must report incoming link metrics in its HELLO messages in
   order for these link metrics to be available at the other end of the
   link.  This means that, for a symmetric link, both ends of the link
   will know both of the incoming and outgoing link metrics.

   As well as advertising incoming link metrics, HELLO messages also
   advertise incoming neighbor metrics.  These are used for routing MPR
   selection (see Section 6.2), which requires use of the lowest metric

   link between two routers when more than one link exists.  This
   neighbor metric may be using another OLSRv2 interface, and hence, the
   link metric alone is insufficient.

   Metrics are also reported in TC messages.  It can be shown that these
   need to be outgoing metrics:

   o  Router A must be responsible for advertising a metric from router
      A to router B in TC messages.  This can be seen by considering a
      route connecting single OLSRv2 interface routers P to Q to R to S.
      Router P receives its only information about the link from R to S
      in the TC messages transmitted by router R, which is an MPR of
      router S (assuming that only MPR selectors are reported in TC
      messages).  Router S may not even transmit TC messages (if no
      routers have selected it as an MPR and it has no attached networks
      to report).  So any information about the metric of the link from
      R to S must also be included in the TC messages sent by router R;
      hence, router R is responsible for reporting the metric for the
      link from R to S.

   o  In a more general case, where there may be more than one link from
      R to S, the TC message must, so that minimum metric routes can be
      constructed (e.g., by router P), report the minimum of these
      outgoing link metrics, i.e., the outgoing neighbor metric from R
      to S.

   In this example, router P also receives information about the
   existence of a link between Q and R in the HELLO messages sent by
   router Q.  Without the use of metrics, this link could be used by
   OLSRv2 for 2-hop routing to router R, using just HELLO messages sent
   by router Q.  For this property (which accelerates local route
   formation) to be retained (from OLSRv1), router P must receive the
   metric from Q to R in HELLO messages sent by router Q.  This
   indicates that router Q must be responsible for reporting the metric
   for the outgoing link from Q to R.  This is in addition to the
   incoming link metric information that a HELLO message must report.
   Again, in general, this must be the outgoing neighbor metric, rather
   than the outgoing link metric.

   In addition, Section 6.1 offers an additional reason for reporting
   outgoing neighbor metrics in HELLO messages, without which metrics
   can properly affect only routing, not flooding.

   Note that there is no need to report an outgoing link metric in a
   HELLO message.  The corresponding 1-hop neighbor knows that value; it
   specified it.  Furthermore, for 2-hop neighborhood use, neighbor
   metrics are required (as these will, in general, not use the same
   OLSRv2 interface).

5.5.  Defining Incoming Link Metrics

   When a router reports a 1-hop neighbor in a HELLO message, it may do
   so for the first time with link status HEARD.  As the router is
   responsible for defining and reporting incoming link metrics, it must
   evaluate that metric and attach that link metric to the appropriate
   address (which will have link status HEARD) in the next HELLO message
   reporting that address on that OLSRv2 interface.  There will, at this
   time, be no outgoing link metric available to report, but a router
   must be able to immediately decide on an incoming link metric once it
   has heard a 1-hop neighbor on an OLSRv2 interface for the first time.

   This is because, when receiving a HELLO message from this router, the
   1-hop neighbor seeing its own address listed with link status HEARD
   will (unless the separate link quality mechanism indicates otherwise)
   immediately consider that link to be SYMMETRIC, advertise it with
   that link status in future HELLO messages, and use it (for MPR
   selection and data traffic forwarding).

   It may, depending on the physical nature of the link metric, be too
   early for an ideal decision as to that metric; however, a choice must
   be made.  The metric value may later be refined based on further
   observation of HELLO messages, other message transmissions between
   the routers, or other observations of the environment.  It will
   probably be best to over-estimate the metric if initially uncertain
   as to its value, to discourage, rather than over-encourage, its use.
   If no information other than the receipt of the HELLO message is
   available, then a conservative maximum link metric value, denoted
   MAXIMUM_METRIC in [RFC7181], should be used.

5.6.  Link Metric Values

   Link metric values are recorded in LINK_METRIC TLVs, defined in
   [RFC7181], using a compressed (lossy) representation that occupies 12
   bits.  The use of 12 bits is convenient because, when combined with 4
   flag bits of additional information, described below, this results in
   a 2-octet value field.  However, the use of 12 bits, and thus the
   availability of 4 flag bits, was a consequence of a design to use a
   modified exponent/mantissa form with the following characteristics:

   o  The values represented are to be positive integers starting 1, 2,
      ...

   o  The maximum value represented should be close to, but less than
      2^24 (^ denotes exponentiation in this section).  This is so that
      with a route limited to no more than 255 hops, the maximum route
      metric is less than 2^32, i.e., can be stored in 32 bits.  (The
      link metric value can be stored in 24 bits.)

   A representation that is modified from an exponent/mantissa form with
   e bits of exponent and m bits of mantissa and that has the first of
   these properties is one that starts at 1, then is incremented by 1 up
   to 2^m, then has a further 2^m increments by 2, then a further 2^m
   increments by 4, and so on for 2^e sets of increments.  This means
   that the represented value is never in error by more than a half (if
   rounding) or one (if truncating) part in 2^m, usually less.

   The position in the increment sequence, from 0 to 2^m-1, is
   considered as a form of mantissa and denoted a.  The increment
   sequence number, from 0 to 2^e-1, is considered as a form of exponent
   and denoted b.

   The value represented by (b,a) can then be shown to be equal to
   (2^m+a+1)2^b-2^m.  To verify this, note that:

   o  With fixed b, the difference between two values with consecutive
      values of a is 2^b, as expected.

   o  The value represented by (b,2^m-1) is (2^m+2^m)2^b-2^m.  The value
      represented by (b+1,0) is (2^m+1)(2^(b+1))-2^m.  The difference
      between these two values is 2^(b+1), as expected.

   The maximum represented value has b = 2^e-1 and a = 2^m-1 and is
   (2^m+2^m)(2^(2^e-1))-2^m = 2^(2^e+m)-2^m.  This is slightly less than
   2^(2^e+m).  The required 24-bit limit can be achieved if 2^e+m = 24.
   Of the possible (e,m) pairs that satisfy this equation, the pair e =
   4, m = 8 was selected as most appropriate and is that used by OLSRv2.
   It uses the previously indicated e+m = 12 bits.  An algorithm for
   converting from a 24-bit value v to a 12-bit pair (b,a) is given in
   Section 6.2 of [RFC7181].

   As noted above, the 12-bit representation then shares two octets with
   4 flag bits.  Putting the flag bits first, it is then natural to put
   the exponent bits in the last four bits of the first octet and to put
   the mantissa bits in the second octet.  The 12 consecutive bits,
   using network byte order (most significant octet first), then
   represent 256b+a.  Note that the ordering of these 12-bit
   representation values is the same as the ordering of the 24-bit
   metric values.  In other words, two 12-bit metrics fields can be
   compared for equality/ordering as if they were unsigned integers.

   The four flag bits each represent one kind of metric, defined by its
   direction (incoming or outgoing) and whether the metric is a link
   metric or a neighbor metric.  As indicated by the flag bits set, a
   metric value may be of any combination of these four kinds of metric.

6.  MPRs with Link Metrics

   MPRs are used for two purposes in OLSRv2.  In both cases, it is MPR
   selectors that are actually used, MPR selectors being determined from
   MPRs advertised in HELLO messages.

   o  Optimized Flooding.  This uses the MPR selector status of
      symmetric 1-hop neighbor routers from which messages are received
      in order to determine if these messages are to be forwarded.  MPR
      selector status is recorded in the Neighbor Set (defined in
      [RFC6130] and extended in [RFC7181]) and determined from received
      HELLO messages.

   o  Routing.  Non-local link information is based on information
      recorded in this router's Topology Information Base.  That
      information is based on received TC messages.  The neighbor
      information in these TC messages consists of addresses of the
      originating router's advertised (1-hop) neighbors, as recorded in
      that router's Neighbor Set (defined in [RFC6130] and extended in
      [RFC7181]).  These advertised neighbors include all of the MPR
      selectors of the originating router.

   Metrics interact with these two uses of MPRs differently, as
   described in the following two sections.  This leads to the
   requirement for two separate sets of MPRs for these two uses when
   using metrics.  The relationship between these two sets of MPRs is
   considered in Section 6.3.

6.1.  Flooding MPRs

   The essential detail of the "flooding MPR" selection specification is
   that a router must select a set of MPRs such that a message
   transmitted by a router and retransmitted by all its flooding MPRs
   will reach all of the selecting router's symmetric 2-hop neighbors.

   Flooding MPR selection can ignore metrics and produce a solution that
   meets the required specification.  However, that does not mean that
   metrics cannot be usefully considered in selecting flooding MPRs.
   Consider the network in Figure 2, where numbers are metrics of links
   in the direction away from router A, towards router D.

                                    3
                                A ----- B
                                |       |
                              1 |       | 1
                                |       |
                                C ----- D
                                    4

                                 Figure 2

   Which is the better flooding MPR selection by router A: B or C?  If
   the metric represents probability of message loss, then clearly
   choosing B maximizes the probability of a message sent by A reaching
   D.  This is despite C having a lower metric in its connection to A
   than B does.  (Similar arguments about a preference for B can be made
   if, for example, the metric represents data rate or delay rather than
   probability of loss.)

   However, neither should only the second hop be considered.  If this
   example is modified to that in Figure 3, where the numbers still are
   metrics of links in the direction away from router A, towards router
   D, then it is possible that, when A is selecting flooding MPRs,
   selecting C is preferable to selecting B.

                                    3
                                A ----- B
                                |       |
                              1 |       | 3
                                |       |
                                C ----- D
                                    4

                                 Figure 3

   If the metrics represent scaled values of delay or the probability of
   loss, then selecting C is clearly better.  This indicates that the
   sum of metrics is an appropriate measure to use to choose between B
   and C.

   However, this is a particularly simple example.  Usually, it is not a
   simple choice between two routers as a flooding MPR, each only adding
   one router coverage.  When considering which router to next add as a
   flooding MPR, a more general process should incorporate the metric to
   that router and the metric from that router to each symmetric 2-hop
   neighbor as well as the number of newly covered symmetric 2-hop
   neighbors.  Other factors may also be included.

   The required specification for flooding MPR selection is in
   Section 18.4 (also using Section 18.3) of [RFC7181], which may use
   the example MPR selection algorithm in Appendix B of [RFC7181].
   However, note that (as in [RFC3626]) each router can make its own
   independent choice of flooding MPRs, and flooding MPR selection
   algorithm, and still interoperate.

   Also note that the references above to the direction of the metrics
   is correct: for flooding, directional metrics outward from a router
   are appropriate, i.e., metrics in the direction of the flooding.
   This is an additional reason for including outward metrics in HELLO
   messages, as otherwise a metric-aware MPR selection for flooding is
   not possible.  The second-hop metrics are outgoing neighbor metrics
   because the OLSRv2 interface used for a second-hop transmission may
   not be the same as that used for the first-hop reception.

6.2.  Routing MPRs

   The essential detail of the "routing MPR" selection specification is
   that a router must, per OLSRv2 interface, select a set of MPRs such
   that there is a 2-hop route from each symmetric 2-hop neighbor of the
   selecting router to the selecting router, with the intermediate
   router on each such route being a routing MPR of the selecting
   router.

   It is sufficient, when using an additive link metric rather than a
   hop count, to require that these routing MPRs provide not just a
   2-hop route but a minimum distance 2-hop route.  In addition, a
   router is a symmetric 2-hop neighbor even if it is a symmetric 1-hop
   neighbor, as long as there is a 2-hop route from it that is shorter
   than the 1-hop link from it.  (The property that no routes go through
   routers with willingness WILL_NEVER is retained.  Examples below
   assume that all routers are equally willing, with none having
   willingness WILL_NEVER.)

   For example, consider the network in Figure 4.  Numbers are metrics
   of links in the direction towards router A, away from router D.
   Router A must pick router B as a routing MPR, whereas for minimum hop
   count routing, it could alternatively pick router C.  Note that the
   use of incoming neighbor metrics in this case follows the same
   reasoning as for the directionality of metrics in TC messages, as
   described in Section 5.4.

                                    2
                                A ----- B
                                |       |
                              1 |       | 1
                                |       |
                                C ----- D
                                    3

                                 Figure 4

   In Figure 5, where numbers are metrics of links in the direction
   towards router A and away from router C, router A must pick router B
   as a routing MPR, but for minimum hop count routing, it would not
   need to pick any MPRs.

                                    1
                                  A - B
                                   \  |
                                  4 \ | 2
                                     \|
                                      C

                                 Figure 5

   In Figure 6, where numbers are metrics of links in the direction
   towards router A and away from routers D and E, router A must pick
   both routers B and C as routing MPRs, but for minimum hop count
   routing, it could pick either.

                               D        E
                               |\      /|
                               | \ 3  / |
                               |  \  /  |
                             1 |   \/   | 1
                               |   /\   |
                               |  /  \  |
                               | / 2  \ |
                               |/      \|
                               B        C
                                \       |
                                 \     /
                                3 \   / 2
                                   \ /
                                    A

                                 Figure 6

   It is shown in Appendix A that selecting routing MPRs according to
   this definition and advertising only such links (plus knowledge of
   local links from HELLO messages) will result in selection of lowest
   total metric routes, even if all links (advertised or not) are
   considered in the definition of a shortest route.

   However, the definition noted above as sufficient for routing MPR
   selection is not necessary.  For example, consider the network in
   Figure 7, where numbers are metrics of links in the direction towards
   router A, away from other routers; the metrics from B to C and C to B
   are both assumed to be 2.

                                1
                            A ----- B
                             \     /
                            4 \   / 2
                               \ /
                                C ----- D ----- E
                                    3       5

                                 Figure 7

   Using the above definition, A must pick both B and C as routing MPRs,
   in order to cover the symmetric 2-hop neighbors C and D,
   respectively.  (C is a symmetric 2-hop neighbor because the route
   length via B is shorter than the 1-hop link.)

   However, A only needs to pick B as a routing MPR, because the only
   reason to pick C as a routing MPR would be so that C can advertise
   the link to A for routing -- to be used by, for example, E.  However,
   A knows that no other router should use the link C to A in a shortest
   route because routing via B is shorter.  So, if there is no need to
   advertise the link from C to A, then there is no reason for A to
   select C as a routing MPR.

   This process of "thinning out" the routing MPR selection uses only
   local information from HELLO messages.  Using any minimum distance
   algorithm, the router identifies shortest routes, whether one, two,
   or more hops, from all routers in its symmetric 2-hop neighborhood.
   It then selects as MPRs all symmetric 1-hop neighbors that are the
   last router (before the selecting router itself) on any such route.
   Where there is more than one shortest distance route from a router,
   only one such route is required.  Alternative routes may be selected
   so as to minimize the number of last routers -- this is the
   equivalent to the selection of a minimal set of MPRs in the non-
   metric case.

   Note that this only removes routing MPRs whose selection can be
   directly seen to be unnecessary.  Consequently, if (as is shown in
   Appendix A) the first approach creates minimum distance routes, then
   so does this process.

   The examples in Figures 5 and 6 show that use of link metrics may
   require a router to select more routing MPRs than when not using
   metrics and even require a router to select routing MPRs when,
   without metrics, it would not need any routing MPRs.  This may result
   in more, and larger, messages being generated and forwarded more
   often.  Thus, the use of link metrics is not without cost, even
   excluding the cost of link metric signaling.

   These examples consider only single OLSRv2 interface routers.
   However, if routers have more than one OLSRv2 interface, then the
   process is unchanged; other than that, if there is more than one
   known metric between two routers (on different OLSRv2 interfaces),
   then, considering symmetric links only (as only these are used for
   routing) the smallest link metric, i.e., the neighbor metric, is
   used.  There is no need to calculate routing MPRs per OLSRv2
   interface.  That requirement results from the consideration of
   flooding and the need to avoid certain "race" conditions, which are
   not relevant to routing, only to flooding.

   The required specification for routing MPR selection is in
   Section 18.5 (also using Section 18.3) of [RFC7181], which may use
   the example MPR selection algorithm in Appendix B of [RFC7181].
   However, note that (as in [RFC3626]) each router can make its own
   independent choice of routing MPRs, and routing MPR selection
   algorithm, and still interoperate.

6.3.  Relationship between MPR Sets

   It would be convenient if the two sets of flooding and routing MPRs
   were the same.  This can be the case if all metrics are equal, but in
   general, for "good" sets of MPRs, they are not.  (A reasonable
   definition of this is that there is no common minimal set of MPRs.)
   If metrics are asymmetrically valued (the two sets of MPRs use
   opposite direction metrics) or routers have multiple OLSRv2
   interfaces (where routing MPRs can ignore this but flooding MPRs
   cannot), this is particularly unlikely.  However, even using a
   symmetrically valued metric with a single OLSRv2 interface on each
   router, the ideal sets need not be equal, nor is one always a subset
   of the other.  To show this, consider these examples, where all
   lettered routers are assumed equally willing to be MPRs, and numbers
   are bidirectional metrics for links.

   In Figure 8, A does not require any flooding MPRs.  However, A must
   select B as a routing MPR.

                                    1
                                  A - B
                                   \  |
                                  4 \ | 2
                                     \|
                                      C

                                 Figure 8

   In Figure 9, A must select C and D as routing MPRs.  However, A's
   minimal set of flooding MPRs is just B.  In this example, the set of
   routing MPRs serves as a set of flooding MPRs, but a non-minimal one
   (although one that might be better, depending on the relative
   importance of number of MPRs and flooding link metrics).

                                      2
                                   C --- E
                                  /     /
                               1 /     / 1
                                /  4  /
                               A --- B
                                \     \
                               1 \     \ 1
                                  \     \
                                   D --- F
                                      2

                                 Figure 9

   However, this is not always the case.  In Figure 10, A's set of
   routing MPRs must contain B but need not contain C.  A's set of
   flooding MPRs need not contain B but must contain C. (In this case,
   flooding with A selecting B rather than C as a flooding MPR will
   reach D but in three hops rather than the minimum two that MPR
   flooding guarantees.)

                                   2   1
                                 B - C - D
                                 |  /
                               1 | / 4
                                 |/
                                 A

                                 Figure 10

7.  Security Considerations

   An attacker can have an adverse impact on an OLSRv2 network by
   creating apparently valid messages that contain incorrect link
   metrics.  This could take the form of influencing the choice of
   routes or, in some cases, producing routing loops.  This is a more
   subtle, and likely to be less effective, attack than other forms of
   invalid message injection.  These can add and remove other and more
   basic forms of network information, such as the existence of some
   routers and links.

   As such, no significantly new security issues arose from the
   inclusion of metrics in OLSRv2.  Defenses to the injection of invalid
   link metrics are the same as to other forms of invalid message
   injection, as discussed in the Security Considerations section of
   [RFC7181].

   There are possible uses for link metrics in the creation of security
   countermeasures to prefer the use of links that have better security
   properties, including better availability, to those with poorer
   security properties.  This, however, is beyond the scope of both this
   document and [RFC7181].

8.  Acknowledgements

   The authors would like to gratefully acknowledge the following people
   (listed alphabetically) for intense technical discussions, early
   reviews, and comments on the documents and its components: Brian
   Adamson (NRL), Alan Cullen (BAE Systems), Justin Dean (NRL), Ulrich
   Herberg (Fujitsu), Charles Perkins (Huawei), Stan Ratliff (Cisco),
   and Henning Rogge (FGAN).

   Finally, the authors would like to express their gratitude to (listed
   alphabetically) Benoit Claise, Adrian Farrel, Stephen Farrell, and
   Suresh Krishnan for their reviews and comments on the later draft
   versions of this document.

9.  Informative References

   [RFC2501]  Corson, S. and J. Macker, "Mobile Ad hoc Networking
              (MANET): Routing Protocol Performance Issues and
              Evaluation Considerations", RFC 2501, January 1999.

   [RFC3626]  Clausen, T. and P. Jacquet, "Optimized Link State Routing
              Protocol (OLSR)", RFC 3626, October 2003.

   [RFC5444]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
              "Generalized Mobile Ad Hoc Network (MANET) Packet/Message
              Format", RFC 5444, February 2009.

   [RFC6130]  Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
              Network (MANET) Neighborhood Discovery Protocol (NHDP)",
              RFC 6130, April 2011.

   [RFC7181]  Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
              "The Optimized Link State Routing Protocol Version 2", RFC
              7181, April 2014.

Appendix A.  MPR Routing Property

   In order for routers to find and use shortest routes in a network
   while using the minimum reduced topology supported by OLSRv2 (that a
   router only advertises its MPR selectors in TC messages), routing MPR
   selection must result in the property that there are shortest routes
   with all intermediate routers being routing MPRs.

   This appendix uses the following terminology and assumptions:

   o  The network is a graph of nodes connected by arcs, where nodes
      correspond to routers with willingness not equal to WILL_NEVER
      (except possibly at the ends of routes).  An arc corresponds to
      the set of symmetric links connecting those routers; the OLSRv2
      interfaces used by those links are not relevant.

   o  Each arc has a metric in each direction, being the minimum of the
      corresponding link metrics in that direction, i.e., the
      corresponding neighbor metric.  This metric must be positive.

   o  A sequence of arcs joining two nodes is referred to as a path.

   o  Node A is an MPR of node B if corresponding router A is a routing
      MPR of router B.

   The required property (of using shortest routes with reduced
   topology) is equivalent to the following property: for any pair of
   distinct nodes X and Z, there is a shortest path from X to Z, X - Y1
   - Y2 - ... - Ym - Z such that Y1 is an MPR of Y2, ..., Ym is an MPR
   of Z.  Call such a path a routable path, and call this property the
   routable path property.

   The required definition for a node X selecting MPRs is that for each
   distinct node Z from which there is a two-arc path, there is a
   shorter, or equally short, path that is either Z - Y - X where Y is
   an MPR of X or is the one-arc path Z - X.  Note that the existence of
   locally known, shorter paths that have more than two arcs, which can
   be used to reduce the numbers of MPRs, is not considered here.  (Such
   reductions are only when the remaining MPRs can be seen to retain all
   necessary shortest paths and therefore retain the required property.)

   Although this appendix is concerned with paths with minimum total
   metric, not number of arcs (hop count), it proceeds by induction on
   the number of arcs in a path.  Although it considers minimum metric
   routes with a bounded number of arcs, it then allows that number of
   arcs to increase so that overall minimum metric paths, regardless of
   the number of arcs, are considered.

   Specifically, the routable path property is a corollary of the
   property that for all positive integers n and all distinct nodes X
   and Z, if there is any path from X to Z of n arcs or fewer, then
   there is a shortest path, from among those of n arcs or fewer, that
   is a routable path.  This may be called the n-arc routable path
   property.

   The n-arc routable path property is trivial for n = 1 and directly
   follows from the definition of the MPRs of Z for n = 2.

   Proceeding by induction, assuming the n-arc routable path property is
   true for n = k, consider the case that n = k+1.

   Suppose that X - V1 - V2 - ... - Vk - Z is a shortest k+1 arc path
   from X to Z.  We construct a path that has no more than k+1 arcs, has
   the same or shorter length (hence has the same, shortest, length
   considering only paths of up to k+1 arcs, by assumption), and is a
   routable path.

   First, consider whether Vk is an MPR of Z.  If it is not, then
   consider the two-arc path Vk-1 - Vk - Z.  This can be replaced either
   by a one-arc path Vk-1 - Z or by a two-arc path Vk-1 - Wk - Z, where
   Wk is an MPR of Z, such that the metric from Vk-1 to Z by the
   replacement path is no longer.  In the former case (replacement one-
   arc path), this now produces a path of length k, and the previous
   inductive step may be applied.  In the latter case, we have replaced
   Vk by Wk, where Wk is an MPR of Z.  Thus, we need only consider the
   case that Vk is an MPR of Z.

   We now apply the previous inductive step to the path X - V1 - ... -
   Vk-1 - Vk, replacing it by an equal length path X - W1 - ... Wm-1 -
   Vk, where m <= k, where this path is a routable path.  Then, because
   Vk is an MPR of Z, the path X - W1 - ... - Wm-1 - Vk - Z is a
   routable path and demonstrates the n-arc routable path property for n
   = k+1.

   This thus shows that for any distinct nodes X and Z, there is a
   routable path using the MPR-reduced topology from X to Z, i.e., that
   OLSRv2 finds minimum length paths (minimum total metric routes).

Authors' Addresses

   Christopher Dearlove
   BAE Systems Advanced Technology Centre
   West Hanningfield Road
   Great Baddow, Chelmsford
   United Kingdom

   Phone: +44 1245 242194
   EMail: chris.dearlove@baesystems.com
   URI:   http://www.baesystems.com/

   Thomas Heide Clausen
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex
   France

   Phone: +33 6 6058 9349
   EMail: T.Clausen@computer.org
   URI:   http://www.thomasclausen.org/

   Philippe Jacquet
   Alcatel-Lucent Bell Labs

   Phone: +33 6 7337 1880
   EMail: philippe.jacquet@alcatel-lucent.com

 

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