Networking Working Group                                        P. Levis
Internet-Draft                                       Stanford University
Intended status: Informational                               A. Tavakoli
Expires: March 29, April 20, 2009                               S. Dawson-Haggerty
                                                             UC Berkeley
                                                      September 25,
                                                        October 17, 2008

Overview of Existing Routing Protocols for Low Power and Lossy Networks
                  draft-ietf-roll-protocols-survey-01
                  draft-ietf-roll-protocols-survey-02

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Abstract

   Networks of low power wireless devices introduce novel IP routing
   issues.  Low-power wireless devices, such as sensors, actuators and
   smart objects, have difficult constraints: very limited memory,
   little processing power, and long sleep periods.  As most of these
   devices are battery-powered, energy efficiency is critically
   important.  Wireless link qualities can vary significantly over time,
   requiring protocols to make agile decisions yet minimize topology
   change energy costs.  Routing over such low power and lossy networks
   has novel requirements that existing protocols may not address.  This
   document provides a brief survey of the strengths and weaknesses of
   existing protocols with respect to this class of networks.  From this
   survey it examines whether existing protocols as described in RFCs
   and mature drafts could be used without modification in these
   networks, or whether further work is necessary.

Requirements Language

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

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Methodology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Suitability Summary  . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Formal Definitions . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Table Scalability  . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Loss Response  . . . . . . . . . . . . . . . . . . . . . .  7
     4.4.  Control Cost . . . . . . . . . . . . . . . . . . . . . . .  8
     4.5.  Link and Node Cost . . . . . . . . . . . . . . . . . . . .  8  9
   5.  Routing Protocol Taxonomy  . . . . . . . . . . . . . . . . . .  9
   6.  Link State Protocols . . . . . . . . . . . . . . . . . . . . . 11 12
     6.1.  OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 12
     6.2.  OLSR . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.3.  TBRPF  . . . . . . . . . . . . . . . . . . . . . . . . . . 12 13
   7.  Distance Vector protocols  . . . . . . . . . . . . . . . . . . 13
     7.1.  RIP  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     7.2.  Ad-hoc On Demand Vector Routing (AODV) . . . . . . . . . . 13 14
     7.3.  DYMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.4.  DSR  . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 14 15
     8.1.  IPv6 Neighbor Discovery  . . . . . . . . . . . . . . . . . 14 15
     8.2.  MANET-NHDP . . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Issues  . . . . Considerations  . . . . . . . . . . . . . . . . . . . 15 16
   10. Manageability Issues . . . . . . . . . . . . . . . . . . . . . 15
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   12. Security Considerations  . . . . . . . . . . . . 16
   11. Acknowledgements . . . . . . . 15
   13. Acknowledgements . . . . . . . . . . . . . . . . 16
   12. Annex A - Routing protocol scalability analysis  . . . . . . . 15
   14. 16
   13. Annex A B - Routing protocol scalability analysis Logarithmic scaling of control cost  . . . . . . . 15
   15. . 19
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     15.1. 20
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     15.2. 20
     14.2. Informative References . . . . . . . . . . . . . . . . . . 19 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 22 23

1.  Terminology

   AODV: Ad-hoc On Demand Vector Routing

   DSR: Dynamic Source Routing

   DYMO: Dynamic Mobile On-Demand

   LLN: Low power and Lossy Network

   LSA: Link State Advertisement

   LSDB: Link State Database

   MANET: Mobile Ad-hoc Networks

   MAC: Medium Access Control

   MPLS: Multiprotocol Label Switching

   MPR: Multipoint Relays

   MTU: Maximum Transmission Unit

   OLSR: Optimized Link State Routing

   ROLL: Routing in Low power and Lossy Networks

   TDMA: Time Division Multiple Access

2.  Introduction

   Wireless is increasingly important to computer networking.  As
   Moore's Law has reduced computer prices and form factors, networking
   includes not only servers and desktops, but laptops, palmtops, and
   cellphones.  As computing device costs and sizes have shrunk, small
   wireless sensors, actuators, and smart objects have emerged as an
   important next step in inter-networking.  The sheer number of the
   low-power networked devices means that they cannot depend on human
   intervention (e.g., adjusting position) for good networking: they
   must have routing protocols that enable them to self-organize into
   multihop networks.

   Energy is a fundamental challenge in these devices.  Convenience and
   ease of use requires they be wireless and therefore battery powered.
   Correspondingly, low power operation is a key concern for this class
   of networked device.  Cost points and energy limitations cause these
   devices to have very limited resources: a few kB of RAM and a few MHz
   of CPU is typical.  As energy efficiency does not improve with
   Moore's Law, these limitations are not temporary.  This trend towards
   smaller, lower power, and more numerous devices has led to new low-
   power wireless link layers to support them.  In practice, wireless
   networks observe much higher loss rates than wired ones do, and low-
   power wireless is no exception.  Furthermore, many of these networks
   will include powered as well as energy constrained nodes.
   Nevertheless, for cost and scaling reasons, many of these powered
   devices will still have limited resources.

   These low power and lossy networks introduce constraints and
   requirements that other networks typically do not possess
   ([I-D.ietf-roll-home-routing-reqs] and
   [I-D.ietf-roll-indus-routing-reqs]).  As they were not designed with
   these requirements in mind, existing protocols may or may not work
   well in LLNs.  The first step to reaching consensus on a routing
   protocol for LLNs is to decide which of these two is true.  If an
   existing protocol can meet LLN requirements without any changes, then
   barring extenuating circumstances, it behooves us to use an existing
   standard.  However, if no current protocol can meet LLN's
   requirements, then further work will be needed to define and
   standardize with a protocol that can.  Whether or not such a protocol
   involves modifications to an existing protocol or a new protocol
   entirely is outside the scope of this document: this document simply
   seeks to answer the question: do LLNs require a new protocol
   specification document at all?

3.  Methodology

   To answer the question of LLNs require new protocol specification
   work, this document examines existing routing protocols and how well
   they can be applied to low power and lossy networks.  It provides a
   set of criteria with which to compare the costs and benefits of
   different protocol designs and examines existing protocols in terms
   of these criteria.

   The five criteria this document uses are derived from a set of drafts
   that describe the requirements of a few major LLN application
   scenarios.  The five criteria, presented in Section 3, are neither
   exhaustive nor complete.  Instead, they are one specific subset of
   high-level requirements shared across all of the application
   requirement drafts.  Because every application requirement draft
   specifies these criteria, then a protocol which does not meet one of
   them cannot be used without modifications or extensions.  However,
   because these criteria represent a subset of the intersection of the
   application requirements, any given application domain may impose
   additional requirements which a particular protocol may not meet.
   For this reason, these criteria are "necessary but not sufficient."
   A protocol that does not meet the criteria cannot be used as
   specified, but it is possible that a protocol meets the criteria yet
   is not able to meet the requirements of a particular application
   domain.  Nevertheless, a protocol that meets all of the criteria
   would be very promising, and deserve a closer look and consideration
   in light of LLN application domains.

   This document considers "existing routing protocols" to be protocols
   that are specified in RFCs or, in the cases of DYMO
   [I-D.ietf-manet-dymo] or OLSRv2 [I-D.ietf-manet-olsrv2] , a very
   mature draft which will most likely become an RFC.  This document
   does not seek to answer the question of whether there is any protocol
   anywhere which could meet LLN application requirements.  Rather, it
   seeks to answer whether protocols, as specified in current IETF
   standards documents, can meet such requirements.  If an existing
   protocol specification can be used unchanged, then writing additional
   protocol specifications is unnecessary.  For example, there are many
   academic papers and experimental protocol implementations available;
   while one or more of these may meet LLN requirements, if they are not
   specified in an RFC then a working group will need to write a new RFC
   for them to be a standard.  The question this document seeks to
   answer is not whether proposed, evaluated, theoretical or
   hypothetical protocol designs can satisfy LLN requirements: the
   question is whether existing IETF standards can.

   Whether a protocol meets these criteria was judged by thinking
   through each specification and considering the best implementation
   possible.  The judgement is based on what a specification allows,
   rather than any particular implementation of that specification.  For
   example, while many DYMO implementations use hopcount as a routing
   metric, the DYMO specification allows a hop to add more than one to
   the routing metric, so DYMO as a specification can support some links
   or nodes being more costly than others.

4.  Suitability Summary

   In this section, we present five important requirements for routing
   in low power and lossy networks, and evaluate protocols against them.
   This evaluation attempts to take a complicated and interrelated set
   of design decisions and trade-offs and condense them to a simple
   "pass", "fail", or "?".  As with any simplification, there is a risk
   of removing some necessary nuance.  However, we believe that being
   forced to take a position on whether or not these protocols are
   acceptable according to binary criterion will be constructive.

   We derive these criteria from existing documents that describe ROLL
   network application requirements.  These metrics do not encompass all
   application requirements.  Instead, they are a common set of routing
   protocol requirements that most applications domains share.
   Considering this very general and common set of requirements sets a
   minimal bar for a protocol to be generally applicable.  If a protocol
   cannot meet even these minimalist criteria, then it cannot be used in
   several major ROLL application domains and so is unlikely to be a
   good candidate for further analysis and examination.  Satisfying
   these minimal criteria is necessary but not sufficient: they do not
   represent the complete intersection of application requirements and
   applications introduce additional, more stringent requirements.  But
   this simplified view provides a first cut of the applicability of
   existing protocols, and those that do satisfy them might be
   reasonable candidates for further study.

   The five criteria are "table scalability", "loss response", "control
   cost", "link cost", and "node cost".  For each of these, the value
   "pass" indicates that a given protocol has satisfactory performance
   according to the metric.  The value "fail" indicates that the
   protocol does not have acceptable performance according to the
   metric, and that the RFC defining the protocol does not, as written,
   contain sufficient flexibility to alter the protocol to do so.
   Finally, "?" indicates that an implementation could exhibit
   satisfactory performance while following the RFC, but that the
   implementation descisions necessary to do so are not specified and
   may require some exploration.  In other words, a "fail" means a
   protocol would have to be modified so it is not compliant with its
   RFC in order to meet the criterion, while a "?" means a protocol
   would require a supplementary document further constraining and
   specifying how a protocol should behave.

4.1.  Formal Definitions

   To provide precise definitions of these metrics, we use formal big-O
   notation, where N refers to the number of nodes in the network, D
   refers to the number of unique destinations, and L refers to the size
   of a node's local, single-hop neighborhood (the network density).  We
   explain the derivation of each metric from application requirements
   in its corresponding section.

4.2.  Table Scalability

   Scalability support for large networks of sensors is highlighted as a
   key requirement by all three application requirements documents.
   Network sizes range from a minimum of 250 nodes in the home routing
   requirements [I-D.ietf-roll-home-routing-reqs] to very large networks
   of "tens of thousands to millions" of devices noted of the urban
   requirements [I-D.ietf-roll-urban-routing-reqs].  Networks are
   expected to have similar size in industrial settings, the
   requirements draft states that depths of up to 20 hops are to be
   expected [I-D.ietf-roll-indus-routing-reqs].  Given that network
   information maintained at each node is stored in routing and neighbor
   tables, along with the constrained memory of nodes, necessitates
   bounds on the size of these tables.

   This metric examines whether routing tables scale within reasonable
   memory resources of low-power nodes.  According to this metric,
   routing protocols that scale linearly with the size of the network or
   a node's neighborhood fail.  Scaling with the size of the network
   prevents networks from growing to reasonable size, while scaling with
   the network density precludes dense deployments.  However, as many
   low-power and lossy networks behave principally as data collection
   networks and principally communicate through routers to data
   collection points in the larger Internet, scaling with the number of
   such collection points is reasonable.  Protocols whose state scales
   with the number of destinations pass.

   More precisely, routing table size scaling with O(N) or O(L) fails.
   A table that scales O(D) (assuming no N or L) passes.

4.3.  Loss Response

   In low power and lossy networks, links routinely come and go due to
   being close to the SINR threshold.  It is important that link churn
   not trigger unnecessary responses by the routing protocol.  This
   point is stressed in all the application requirement documents,
   pointing to the need to localize response to link failures with no
   triggering of global network re-optimization, whether for reducing
   traffic or for maintaining low route convergence times
   ([I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-urban-routing-reqs], and
   [I-D.ietf-roll-indus-routing-reqs]).  The industrial routing
   requirements draft states that protocols must be able to "recompute
   paths based on underlying link characteristics which may change
   dynamically", as well as reoptimize when the device set changes to
   maintain service requirements.  The protocol should also "always be
   in the process of optimizing the system in response to changing link
   statistics."  Protocols with these properties should take care not to
   require global updates.

   A protocol which requires many link changes to propagate across the
   entire network fails.  Protocols which constrain the scope of
   information propagation to only when they affect routes to active
   destinations, or to local neighborhoods, pass.  Protocols which allow
   proactively path maintenance pass if the choice of which paths to
   maintain is user-specified.

   More precisely, loss responses that require O(N) transmissions fail,
   while responses that can rely on O(1) local broadcasts or O(D) route
   updates pass.

4.4.  Control Cost

   Battery-operated devices are a critical component of all three
   application spectrums, and as such special emphasis is placed on
   minimizing power consumption to achieve long battery lifetime,
   [I-D.ietf-roll-home-routing-reqs], with multi-year deployments being
   a common case [I-D.ietf-roll-indus-routing-reqs].  In terms of
   routing structure, any proposed L2N routing protocol ought to support
   the autonomous organization and configuration of the network at the
   lowest possible energy cost [I-D.ietf-roll-urban-routing-reqs].

   All routing protocols must transmit additional data to detect
   neighbors, build routes, transmit routing tables, or otherwise
   conduct routing.  As low-power wireless networks can have very low
   data rates, protocols which require a minimum control packet rate can
   have unbounded control overhead.  This is particularly true for
   event-driven networks, which only report data when certain conditions
   are met.  Regions of a network which never meet the condition can be
   forced to send significant control traffic even when there is no data
   to send.  For these use cases, hard-coded timing constants are
   unacceptable, because they imply a prior knowledge of the expected
   data rate.

   If

   Of course, protocols require the ability to send at least a very
   small amount of control traffic, in order to discover a topology.
   But this bootstrapping discovery and maintenance traffic should be
   small: communicating once an hour is unbounded by data traffic, far more reasonable than
   communicating once a protocol fails
   according to Control Cost metric.  Protocols which pass bound their second.  So while control traffic rate to their should be
   bounded by data traffic.  Protocols which pass do
   not use resources to maintain unused state.  More specifically, any
   protocol which traffic, it requires fixed-rate periodic control packets in the
   absence of data traffic fails.

4.5.  Link some leeway to bootstrap and Node Cost

   These two metrics specify how
   maintain a protocol chooses routes for data
   packets to take through the long-lived yet idle network.  Classical routing algorithms
   typically acknowledge

   In the differing costs case of paths control traffic, the communication rate (sum of
   transmissions and may use receptions at a
   shortest path algorithm to find paths.  This node) is a requirement better measure than the
   transmission rate (since energy is consumed for low
   power networks, both transmissions
   and receptions).  Controlling the transmission rate is insufficient,
   as links it would mean that the energy cost (sum of transmission and
   receptions) of control traffic could grow with O(L).

   A protocol fails the control cost criterion if its per-node control
   traffic (transmissions plus receptions) rate is not bounded by the
   data rate plus a small constant.  For example, a protocol using a
   beacon rate only passes if it can be turned arbitrarily low, in order
   to match the data rate.  Furthermore, packet losses necessitate that
   the control traffic may scale within a O(log(L)) factor of the data
   rate.  Meaning, if R is the data rate and e is the small constant,
   then a protocol's control traffic must be on the order of O(R log(L)
   + e) to pass this criteria.  The details of why O(log(L)) is
   necessary are in Annex B.

4.5.  Link and Node Cost

   These two metrics specify how a protocol chooses routes for data
   packets to take through the network.  Classical routing algorithms
   typically acknowledge the differing costs of paths and may use a
   shortest path algorithm to find paths.  This is a requirement for low
   power networks, as links must be evaluated as part of an objective
   function across various metric types, such as minimizing latency and
   maximizing reliability [I-D.ietf-roll-indus-routing-reqs].

   However, in low power networks it is also desirable to account for
   the cost of routing through particular routers.  Applications require
   node or parameter constrained routing, which takes into account node
   properties and attributes such as power, memory, and battery life
   that dictate a router's willingness or ability to route other
   packets.  Home routing requirements note that devices will vary in
   their duty cycle, and that routing protocols should prefer nodes with
   permanent power [I-D.ietf-roll-home-routing-reqs].  The urban
   requirements note that routing protocols may wish to take advantage
   of differing data processing and managment capabilities among network
   devices [I-D.ietf-roll-urban-routing-reqs].  Finally, industrial
   requirements cite differing lifetime requirements as an important
   factor to account for [I-D.ietf-roll-indus-routing-reqs].  Node cost
   refers to the ability for a protocol to incorporate router properties
   into routing metrics and use node attributes for constraint-based
   routing.

   A "pass" indicates that the protocol contains a mechanism allowing
   these considerations to be considered when choosing routes.

5.  Routing Protocol Taxonomy

   Routing protocols broadly fall into two classes: link-state and
   distance-vector.

   A router running a link-state protocol first establishes adjacency
   with its neighbors and then reliably floods the local topology
   information in the form of a Link State Advertisement packet.  The
   collection of LSAs constitutes the Link State Database (LSDB) that
   represents the network topology, and routers synchronize their LSDBs.

   Thus each node in the network has a complete view of the network
   topology.  Each router uses the LSDB to compute a routing table where
   each entry (reachable IP destination address) points to the next hop
   along the shortest path according to some metric.  Link state
   protocols (such as OSPF and IS-IS) support the concept of area
   (called "level" for IS-IS) whereby all the routers in the same area
   share the same view (they have the same LSDB) and areas are
   interconnected by border routers according to specific rules that
   advertise IP prefix reachability between areas.

   A distance vector protocol exchanges routing information rather than
   topological information.  A router running a distance vector protocol
   exchanges information with its "neighbors" with which it has link
   layer connectivity.  Tunneling and similar mechanisms can virtualize
   link layer connectivity to allow neighbors that are multiple layer 2
   hops away.  Rather than a map of the network topology from which each
   router can calculate routes, a distance vector protocol node has
   information on what routes its neighbors have.  Each node's set of
   available routes is the union of its neighbors routes plus a route to
   itself.  In a distance vector protocol, nodes may only advertise
   routes which are in use, enabling on-demand discovery.  In comparison
   to link state protocols, distance vector protocols have the advantage
   of only requiring neighbor routing information, but also have
   corresponding limitations which protocols must address, such as
   routing loops, count to infinity, split horizon, and slow convergence
   times.  Furthermore, routing constraints are difficult to enforce
   with distance vector protocols.

   Neighbor discovery is a critical component of any routing protocol.
   It enables a protocol to learn about which other nodes are nearby and
   which it can use as the next hop for routes.  As neighbor discovery
   is a key component of many protocols, several general protocols and
   protocol mechanisms have been designed to support it.  A protocol's
   neighbor set is defined by how many "hops" away the set reaches.  For
   example, the 1-hop neighbor set of a node is all nodes it can
   directly communicate with at the link layer, while the 2-hop neighbor
   set is its own 1-hop neighbor set and the 1-hop neighbor sets of all
   of its 1-hop neighbors.

   Because nodes often have very limited resources for storing routing
   state, protocols cannot assume that they can store complete neighbor
   information.  For example, a node with 4kB of RAM cannot store full
   neighbor state when it has 1000 other nodes nearby.  This means that
   ROLL protocols must have mechanisms to decide which of many possible
   neighbors they monitor as routable next hops.  For elements such as
   2-hop neighborhoods, these decisions can have a significant impact on
   the topology that other nodes observe, and therefore may require
   intelligent logic to prevent effects such as network partitions.

   Protocols Today

   Wired networks draw from both approaches.  OSPF or IS-IS, for
   example, are link-state protocols, while RIP is a distance-vector
   protocol.

   MANETs similarly draw from both approaches.  OLSR is a link-state
   protocol, while AODV and DYMO are distance vector protocols.  The
   general consensus in core networks is to use link state routing
   protocols as IGPs for a number of reasons: in many cases having a
   complete network topology view is required to adequately compute the
   shortest path according to some metrics.  For some applications such
   as MPLS Traffic Engineering it is even required to have the knowledge
   of the Traffic Engineering Database for constraint based routing.

   Furthermore link state protocols typically have superior convergence
   speeds (ability to find an alternate path in case of network element
   failure), are easier to debug and troubleshoot, and introduce less
   control packet overhead than distance vector protocols.  In contrast,
   distance vector protocols are simpler, require less computation, and
   have smaller storage requirements.  Most of these tradeoffs are
   similar in wireless networks, with one exception.  Because wireless
   links can suffer from significant temporal variation, link state
   protocols can have higher traffic loads as topology changes must
   propagate globally, while in a distance vector protocol a node can
   make local routing decisions with no effect on the global routing
   topology.  One major protocol, DSR, does not easily fit into one of
   these two classes.  Although it is a distance vector protocol, DSR
   has several properties that make it differ from most other protocols
   in this class.  We examine these differences in our discussion of
   DSR.

   The next two sections summarize several well established routing
   protocols.  This table shows, based on the criteria described above,
   whether these protocols meet ROLL criteria.  Annex A contains the
   reasoning behind each value in the table.

     Protocol      Table   Loss  Control   Link Cost  Node Cost
     OSPF          fail    fail    fail      pass       fail
     OLSRv2        fail    fail    fail      pass       pass
     TBRPF         fail    pass    fail      pass        ?
     RIP           pass    fail    fail       ?    pass       ?         fail
     AODV          pass    fail    pass      fail       fail
     DYMO[-low]    pass    fail    pass       ?         fail
     DSR           fail    pass    pass      fail       fail

6.  Link State Protocols

6.1.  OSPF

   OSPF (specified in [RFC2328] for IPv4 and in [RFC2740] for IPv6)) is
   a link state protocol designed for routing within an Internet
   Autonomous System (AS).  OSPF provides the ability to divide a
   network into areas, which can establish a routing hierarchy.  The
   topology within an area is hidden from other areas and IP prefix
   reachability across areas (inter-area routing) is provided using
   summary LSAs.  The hierarchy implies that there is a top-level
   routing area (the backbone area) which connects other areas.  Areas
   may be connected to the back-bone area through a virtual link.  OSPF
   maintains routing adjacencies by sending hello messages.  OSPF
   calculates the shortest path to a node using link metrics (that may
   reflect the link bandwidth, propagation delay, ...).  OSPF Traffic
   Engineering (OSPF-TE, [RFC3630]) extends OSPF to include information
   on reservable, unreserved, and available bandwidth.

6.2.  OLSR

   Optimized Link State Routing (OLSR) (see [RFC3626] and
   [I-D.ietf-manet-olsrv2]) is a link state routing protocol for
   wireless mesh networks.  OLSR nodes flood route discovery packets
   throughout the entire network, such that each node has a map of the
   mesh topology.  Because link variations can lead to heavy flooding
   traffic when using a link state approach, OLSR establishes a topology
   for minimizing this communication.  Each node maintains a set of
   nodes called its Multipoint Relays (MPR), which is a subset of the
   one-hop neighbors whose connectivity covers the two-hop neighborhood.
   Each node that is an MPR maintains a set called its MPR selectors,
   which are nodes that have chosen it to be an MPR.

   OLSR uses these two sets to apply three optimizations.  First, only
   MPRs generate link state information.  Second, nodes can use MPRs to
   limit the set of nodes that forward link state packets.  Third, an
   MPR, rather than advertise all of its links, can advertise only links
   to its MPR selectors.  Together, these three optimizations can
   greatly reduce the control traffic in dense networks, as the number
   of MPRs should not increase significantly as a network becomes
   denser.

   OLSR selects routes based on hop counts, and assumes an underlying
   protocol that determines whether a link exists between two nodes.
   OLSR's constrained flooding allows it to quickly adapt to and
   propagate topology changes.

   OLSR is closely related to clustering algorithms in the wireless
   sensor networking literature, in which cluster heads are elected such
   that routing occurs over links between cluster heads and all other
   nodes are leafs that communicate to a cluster head.

6.3.  TBRPF

   Topology Dissemination Based on Reverse Path Forwarding (see
   [RFC3684]) is another proactive link state protocol.  TBRPF computes
   a source tree, which provides routes to all reachable nodes.  It
   reduces control packet overhead by having nodes only transmit a
   subset of their source tree as well as by using differential updates.

   The major difference between TBRPF and OLSR is the routing data that
   nodes advertise and who chooses to aggregate information.  In OLSR,
   nodes select neighbors to be MPRs and advertise their link state for
   them; in TBRPF, nodes elect themselves to advertise relevant link
   state based on whether it acts as a next hop.

7.  Distance Vector protocols

7.1.  RIP

   The Routing Information Protocol (RIP) (defined in [RFC2453])
   predates OSPF.  As it is a distance vector protocol, routing loops
   can occur and considerable work has been done to accelerate
   convergence since the initial RIP protocols were introduced.  RIP
   measures route cost in terms of hops, and detects routing loops by
   observing a route cost approach infinity where "infinity" is referred
   to as a maximum number of hops.  RIP is typically not appropriate for
   situations where routes need to be chosen based on real-time
   parameters such as measured delay, reliability, or load or when the
   network topology needs to be known for route computation.

   "Triggered RIP" (defined in [RFC2091]) was originally designed to
   support "on-demand" circuits.  The aim of triggered RIP is to avoid
   systematically sending the routing database on regular intervals.

   Instead, triggered RIP sends the database when there is a routing
   update or a next hop adjacency change: once neighbors have exchanged
   their routing database, only incremental updates need to be sent.
   Because incremental updates cannot depend on periodic traffic to
   overcome loses, triggered RIP uses acknowledgment based mechanisms
   for reliable delivery.

7.2.  Ad-hoc On Demand Vector Routing (AODV)

   AODV (specified in [RFC3561]) is a distance vector protocol intended
   for mobile ad-hoc networks.  When one AODV node requires a route to
   another, it floods a request in the network to discover a route.  A
   depth-scoped flooding process avoids discovery from expanding to the
   most distant regions of the network that are in the opposite
   direction of the destination.  AODV chooses routes that have the
   minimum hop count.

   If an AODV route request reaches a node that has a route to the
   destination (this includes the destination itself), that node sends a
   reply along the reverse route.  All nodes along the reverse route can
   cache the route.  When routes break due to topology changes, AODV
   floods error messages and issues a new request.  Because AODV is on-
   demand it only maintains routes for active nodes.  When a link
   breaks, AODV issues a Route Error (RERR) and a new route request
   message (RREQ), with a higher sequence number so nodes do not respond
   from their route caches.  These packets can flood the entire network,
   giving loss response a fail.

7.3.  DYMO

   Dynamic Mobile On-Demand routing (DYMO) ([I-D.ietf-manet-dymo]) is an
   evolution of AODV.  The basic functionality is the same, but it has
   different packet formats, handling rules, and supports path
   accumulation.  Path accumulation allows a single DYMO route request
   to generate routes to all nodes along the route to that destination.
   Like AODV, DYMO uses hop counts as its routing metric, but links may
   have a cost >= 1, allowing DYMO to represent link costs.  Like AODV,
   on link breaks DYMO issues a new route request message (RREQ), with a
   higher sequence number so nodes do not respond from their route
   caches.  Correspondingly, a route request can flood the entire
   network.

7.4.  DSR

   Dynamic Source Routing ([RFC4728]) is a distance vector protocol, but
   a DSR packet source explicitly specifies the route for each packet.
   Because the route is determined at a single place -- the source --
   DSR does not require sequence numbers or other mechanisms to prevent
   routing loops, as there is no problem of inconsistent routing tables.
   Unlike AODV and DYMO, by pushing state into packet headers, DSR does
   not require per-destination routing state.  Instead, a node
   originating packets only needs to store a spanning tree of the part
   of the network it is communicating with.

8.  Neighbor Discovery

   A limit on maintained routing state (light footprint) prevents ROLL
   protocols from assuming they know all 1-hop, 2-hop, or N-hop
   neighbors.  For this reason, while protocols such as MANET-NHDP
   ([I-D.ietf-manet-nhdp]) and IPv6's neighbor discovery ([RFC4861])
   provide basic mechanisms for discovering link-layer neighbors, not
   all of their features are relevant.  This section describes these two
   protocols, their capabilities, and how ROLL protocols could leverage
   them.

8.1.  IPv6 Neighbor Discovery

   IPv6 neighbor discovery provides mechanisms for nodes to discover
   single-hop neighbors as well as routers that can forward packets past
   the local neighborhood.  There is an implicit assumption that the
   delegation of whether a node is a router or not is static (e.g.,
   based on a wired topology).  The fact that all routers must respond
   to a Router Solicitation requires that the number of routers with a
   1-hop neighborhood is small, or there will be a reply implosion.
   Furthermore, IPv6 neighbor discovery's support of address
   autoconfiguration assumes address provisioning, in that addresses
   reflect the underlying communication topology.  IPv6 neighbor
   discovery does not consider asymmetric links.  Nevertheless, it may
   be possible to extend and adapt IPv6's mechanisms to wireless in
   order to avoid response storms and implosions.

8.2.  MANET-NHDP

   The MANET Neighborhood Discovery Protocol (MANET-NHDP) provides
   mechanisms for discovering a node's symmetric 2-hop neighborhood.  It
   maintains information on discovered links, their interfaces, status,
   and neighbor sets.  MANET-NHDP advertises a node's local link state;
   by listening to all of its 1-hop neighbor's advertisements, a node
   can compute its 2-hop neighborhood.  MANET-NHDP link state
   advertisements can include a link quality metric.  MANET-NHDP's node
   information base includes all interface addresses of each 1-hop
   neighbor: for low-power nodes, this state requirement can be
   difficult to support.

9.  Security Issues

   TBD Considerations

   This document presents, considers, and raises no security
   considerations.

10.  Manageability Issues

   TBD

11.  IANA Considerations

   This document includes no request to IANA.

12.  Security Considerations

   TBD

13.

11.  Acknowledgements

14.

12.  Annex A - Routing protocol scalability analysis

   This aim of this Annex is to provide the details for the analysis
   routing scalability analysis.

   "OSPF"

   OSPF floods link state through a network.  Each router must receive
   this complete link set.  OSPF fails the table size criterion because
   it requires each router to discover each link in the network, for a
   total routing table size which is O(N * L).  This also causes it to
   fail the control cost criterion, since this information must be
   propagated.  Furthermore, changes in the link set require re-flooding
   the network link state even if the changed links were not being used.
   Since link state changes in wireless networks are often uncorrelated
   with data traffic and are instead caused by external (environmental)
   factors, this causes OSPF to fail both the control cost and loss
   response criteria.  OSPF routers can impose policies on the use of
   links and can consider link properties (Type of Service), as the cost
   associated with an edge is configurable by the system administrator
   [RFC2328], so receive a pass for link cost.  However, there is no way
   to associate metrics with routers (as costs are only applied to
   outgoing interfaces, i.e. edges) when computing paths, and so fails
   the node cost criteria.  While [RFC3630] discusses paths that take
   into account node attributes, it specifically states that no known
   algorithm or mechanism currently exists for incoporating this into
   the OSPF RFC.

   "OLSRv2"

   OLSRv2 is a proactive link state protocol, flooding this information
   through a set of multipoint relays (MPRs).  Routing state includes
   1-hop neighbor information for each node in the network, 1-hop and
   2-hop information for neighbors (for MPR selection), and a routing
   table (consisting of destination, and next hop), resulting in state
   proportional to network size and density (O(N*L + L^2)), and failing
   the table scalability criteria.

   Unacceptable control traffic overhead arises from flooding and
   maintenance.  HELLO messages are periodically broadcast local beacon
   messages, but TC messages spread topology information throughout the
   network (using MPRs).  As such, control traffic is proportional to
   O(N^2).  MPRs reduce this load to O(N^2 / L).  As the number of MPRs
   is inversely proportional to the density of the network and L is
   bounded by N, this means control traffic is at best proportional to
   O(N), and fails the control cost metric.

   Furthermore, changes in the link set require immediately re-flooding
   the network link state even if those links were not being used by
   routing, which fails the loss response metric.

   OLSR allows for specification of link quality, and also provides a
   'Willingness' metric to symbolize node cost, giving it a pass for
   both those metrics.

   "TBRPF"

   As a link state protocol where each node maintains a database of the
   entire network topology, TBRPF's routing table size scales with
   network size and density, leading to table sizes which are O(N * L)
   when a node receives disjoint link sets from its neighbors.  This
   causes the protocol to fail the table size criteria.  The protocol's
   use of differential updates should allow both fast response time and
   incremental changes once the distributed database of links has been
   established.  Differential updates are only used to reduce response
   time to changing network conditions, not to reduce the amount of
   topology information sent, since each node will periodically send
   their piece of the topology.  As a result, TBRPF fails the control
   overhead criteria.  However, its differential updates triggered by
   link failure do not immediately cause a global re-flooding of state
   (but only to affected routers) [RFC3684], leading to a pass for loss
   response.

   TBRPF has a flexible neighbor management layer which enables it to
   incorporate various types of link metrics into its routing decision
   by enabling a USE_METRIC flag [RFC3684].  As a result, it receives a
   pass for link cost.  It also provides a mechanism whereby routers can
   maintain multiple link metrics to a single neighbor, some of which
   can be advertised by the neighbor router [RFC3684].  Although the RFC
   does not specify a policy for using these values, developing one
   could allow TBRPF to satisfy this requirement, leading to a ? for the
   node cost requirement.

   "RIP"

   RIP is a distance vector protocol: all routers maintain a route to
   all other routers.  Routing table size is therefore O(N).  However,
   if destinations are known apriori, table size can be reduced to O(D),
   resulting in a pass for table scalability.  Each  While standard RIP
   requires each node broadcasts broadcast a beacon per period, and that updates
   must be propagated by affected nodes,
   irrespective of triggered RIP only sends
   updates when network conditions change in response to the data rate, failing path,
   so RIP passes the control cost metric.  Loss triggers updates, only
   propagating if part of a best route, but even if the route is not
   actively being used, resulting in a fail for loss response.  The rate
   of triggered updates is throttled, and these are only differential
   updates, yet this still doesn't account for other control traffic (or
   tie it to data rate) or prevent the triggered updates from being
   flooded along non-active paths.  [RFC2453]

   RIP receives a ? for link cost because while current implementations
   focus on hop count and that is the metric used in [RFC2453], the RFC
   also mentions that more complex metrics such as differences in
   bandwidth and reliability could be used.  However, the RFC also
   states that real-time metrics such as link-quality would create
   instability and the concept of node cost only appears as metrics
   assigned to external networks.  It also receives a ? because  While RIP has the concept of a
   network cost is introduced, which cost, it is added insufficient to link cost,
   but does not describe its use. node properties and so
   RIP fails the node cost criterion..

   "AODV"

   AODV table size is a function of the number of communicating pairs in
   the network, scaling with O(D).  This is acceptable and so AODV
   passes the table size criteria.  As an on-demand protocol, AODV does
   not generate any traffic until data is sent, and so control traffic
   is correlated with the data and so it receives a pass for control
   traffic.  When a broken link is detected, AODV will use a precursor
   list maintained for each destination to inform downstream routers
   (with a RERR) of the topology change.  However, the RERR message is
   forwarded by all nodes that have a route that uses the broken link,
   even if the route is not currently active, leading to a fail for loss
   response [RFC3561].

   AODV fails the link cost metric because the only metric used is hop
   count, and this is hardcoded in the route table entry, according to
   the RFC [RFC3561].  It fails the node cost requirement because there
   is no way for a router to indicate its [lack of] willingness to route
   while still adhering to the RFC.

   "DYMO/DYMO-low"
   The design of DYMO shares much with AODV, with some changes to remove
   precursor lists and compact various messages.  It still passes the
   table size criteria because it only maintains routes requested by
   RREQ messages, resulting in O(D) table size.  Control traffic (RREQ,
   RREP, and RREQ) are still driven by data, and hence DYMO passes the
   control cost criterion.  However, RERR messages are forwarded by any
   nodes that have a route using the link, even if inactive, leading to
   a fail of the loss reponse criteria [I-D.ietf-manet-dymo].

   While DYMO does indicate that the metric used for a link can vary
   from 1-65535, it specifically refers to this as distance, which is
   incremented by at least one at each hop [I-D.ietf-manet-dymo],
   leading to a ? in link cost.  While additional routing information
   can be added DYMO messages, there is no mention of node cost, leading
   to a fail in node cost.

   "DSR"

   DSR performs on-demand route discovery, and source routing of
   packets.  It maintains a source route for all destinations, and also
   a blacklist of all unidirectional neighbor links [RFC4728], leading
   to a total table size of O(D + L), failing the table size criterion.
   Control traffic is completely data driven, and so DSR receives a pass
   for this criteria.  Finally, a transmission failure only prompts an
   unreachable destination to be sent to the source of the message,
   passing the loss response criteria.

   DSR fails the link cost criterion because its source routes are
   advertised only in terms of hops, such that all advertised links are
   considered equivalent.  DSR also fails the node cost criterion
   because a node has no way of indicating its willingness to serve as a
   router and forward messages.

15.

13.  Annex B - Logarithmic scaling of control cost

   To satisfy the control cost criterion, a protocol's control traffic
   communication rate must be bounded by the data rate, plus a small
   constant.  That is, if there is a data rate R, the control rate must
   O(R + e), where e is a very small constant (epsilon).  Furthermore,
   the control rate may grow logarithmically with the size of the local
   neighborhood L. Note that this is a bound: it represents the most
   traffic a protocol may send, and good protocols may send much less.
   So the control rate is bounded by O(R log(L)) + e.

   The logarithmic factor comes from the fundamental limits of any
   protocol that maintains a communication rate.  For example, consider
   e, the small constant rate of communication traffic allowed.  Since
   this rate is communication, to maintain O(e), then only one in L
   nodes may transmit per time interval defined by e: that one node has
   a transmission, and all other nodes have a reception, which prevents
   them from transmitting.  However, wireless networks are lossy.
   Suppose that the network has a 10% packet loss rate.  Then if L=10,
   the expectation is that one of the nodes will drop the packet.  Not
   hearing a transmission, it will think it can transmit.  This will
   lead to 2 transmissions.  If L=100, then one node will not hear the
   first two transmissions, and there will be 3.  The number of
   transmissions, and the communication rate, will grow with O(log(L)).

   This logarithmic bound can be prevented through explicit coordination
   (e.g., leader election), but such approaches assumes state and
   control traffic to elect leaders.  As a logarithmic factor in terms
   of density is not a large stumbling or major limitation, allowing the
   much greater protocol flexibility it enables is worth its small cost.

14.  References

15.1.

14.1.  Normative References

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

15.2.

14.2.  Informative References

   [I-D.ietf-manet-dymo]
              Chakeres, I. and C. Perkins, "Dynamic MANET On-demand
              (DYMO) Routing", draft-ietf-manet-dymo-14 (work in
              progress), June 2008.

   [I-D.ietf-manet-nhdp]
              Clausen, T., Dearlove, C., and J. Dean, "MANET
              Neighborhood Discovery Protocol (NHDP)",
              draft-ietf-manet-nhdp-07 (work in progress), July 2008.

   [I-D.ietf-manet-olsrv2]
              Clausen, T., Dearlove, C., and P. Jacquet, "The Optimized
              Link State Routing Protocol version 2",
              draft-ietf-manet-olsrv2-07 (work in progress), July 2008.

   [I-D.ietf-roll-home-routing-reqs]
              Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirement in Low Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-03 (work in progress),
              September 2008.

   [I-D.ietf-roll-indus-routing-reqs]
              Networks, D., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low Power and Lossy
              Networks", draft-ietf-roll-indus-routing-reqs-01 (work in
              progress), July 2008.

   [I-D.ietf-roll-urban-routing-reqs]
              Dohler, M., Watteyne, T., Winter, T., Jacquenet, C.,
              Madhusudan, G., Chegaray, G., and D. Barthel, "Urban WSNs
              Routing Requirements in Low Power and Lossy Networks",
              draft-ietf-roll-urban-routing-reqs-01 (work in progress),
              July 2008.

   [RFC2091]  Meyer, G. and S. Sherry, "Triggered Extensions to RIP to
              Support Demand Circuits", RFC 2091, January 1997.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC2453]  Malkin, G., "RIP Version 2", STD 56, RFC 2453,
              November 1998.

   [RFC2740]  Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6",
              RFC 2740, December 1999.

   [RFC3561]  Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
              Demand Distance Vector (AODV) Routing", RFC 3561,
              July 2003.

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

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              September 2003.

   [RFC3684]  Ogier, R., Templin, F., and M. Lewis, "Topology
              Dissemination Based on Reverse-Path Forwarding (TBRPF)",
              RFC 3684, February 2004.

   [RFC4728]  Johnson, D., Hu, Y., and D. Maltz, "The Dynamic Source
              Routing Protocol (DSR) for Mobile Ad Hoc Networks for
              IPv4", RFC 4728, February 2007.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

Authors' Addresses

   Philip Levis
   Stanford University
   358 Gates Hall, Stanford University
   Stanford, CA  94305-9030
   USA

   Email: pal@cs.stanford.edu

   Arsalan Tavakoli
   UC Berkeley
   Soda Hall, UC Berkeley
   Berkeley, CA  94707
   USA

   Email: arsalan@eecs.berkeley.edu

   Stephen Dawson-Haggerty
   UC Berkeley
   Soda Hall, UC Berkeley
   Berkeley, CA  94707
   USA

   Email: stevedh@cs.berkeley.edu

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