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Networking Working Group                                        P. Levis
Internet-Draft                                       Stanford University
Intended status: Informational                               A. Tavakoli
Expires: October 17, 2009                             S. Dawson-Haggerty
                                                             UC Berkeley
                                                            Apr 15, 2009


Overview of Existing Routing Protocols for Low Power and Lossy Networks
                  draft-ietf-roll-protocols-survey-07

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on October 17, 2009.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Abstract

   Low-power wireless devices, such as sensors, actuators and smart



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   objects, present 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 introduces
   requirements that existing routing protocols may not fully address.
   Using existing application requirements documents, this document
   derives a minimal and not exhaustive set of criteria for routing in
   low-power and lossy networks.  It provides a brief survey of the
   strengths and weaknesses of existing protocols with respect to these
   criteria.  From this survey it examines whether existing and mature
   IETF protocols can be used without modification in these networks, or
   whether further work is necessary.  It concludes that no existing
   IETF protocol meets the requirements of this domain.




































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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  LLN Properties . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Question to Answer . . . . . . . . . . . . . . . . . . . .  6
   3.  Methodology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Protocols Considered . . . . . . . . . . . . . . . . . . .  6
     3.2.  Criteria . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Evaluation . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Formal Definitions . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Routing State  . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Loss Response  . . . . . . . . . . . . . . . . . . . . . . 10
     4.4.  Control Cost . . . . . . . . . . . . . . . . . . . . . . . 10
     4.5.  Link and Node Cost . . . . . . . . . . . . . . . . . . . . 11
   5.  Routing Protocol Taxonomy  . . . . . . . . . . . . . . . . . . 12
     5.1.  Protocols Today  . . . . . . . . . . . . . . . . . . . . . 13
   6.  Link State Protocols . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  OSPF & IS-IS . . . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  OLSR & OLSRv2  . . . . . . . . . . . . . . . . . . . . . . 15
     6.3.  TBRPF  . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Distance Vector protocols  . . . . . . . . . . . . . . . . . . 16
     7.1.  RIP  . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.2.  Ad-hoc On Demand Vector Routing (AODV) . . . . . . . . . . 16
     7.3.  DYMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.4.  DSR  . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  IPv6 Neighbor Discovery  . . . . . . . . . . . . . . . . . 18
     8.2.  MANET-NHDP . . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     13.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Routing protocol scalability analysis . . . . . . . . 21
   Appendix B.  Logarithmic scaling of control cost . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26











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1.  Terminology

   AODV: Ad-hoc On Demand Vector Routing

   DSR: Dynamic Source Routing

   DYMO: Dynamic Mobile On-Demand

   IS-IS: Intermediate System to Intermediate System

   OLSR: Optimized Link State Routing

   OSPF: Open Shortest Path First

   RIP: Routing Information Protocol

   TBRPF: Topology Dissemination Based on Reverse Path Forwarding

   LLN: Low power and Lossy Network

   LSA: Link State Advertisement

   LSDB: Link State Database

   MANET: Mobile Ad-hoc Network

   MAC: Medium Access Control

   MPLS: Multiprotocol Label Switching

   MPR: Multipoint Relays

   MTU: Maximum Transmission Unit

   ROLL: Routing in Low power and Lossy Networks

   TDMA: Time Division Multiple Access

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


2.  Introduction

   Wireless is increasingly important to computer networking.  As the
   technological progress behind Moore's Law has reduced computer prices



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   and form factors, networking has come to include 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.  The sheer
   number of such low-power networked devices means that they cannot
   depend on human intervention (e.g., adjusting position) for good
   connectivity: 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.
   Low power operation is a key concern for these sensors and actuators
   so as to allow them to function for months and years without
   interruption.  Cost points and energy limitations cause these devices
   to have very limited computational and storage 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; for
   instance, in addition to the constraints of limited resources and
   small power sources which constrain the amount of traffic a protocol
   may generate, these applications demand an embrace of heterogeneous
   node capabilities, and good support for specific traffic patterns
   ([I-D.ietf-roll-home-routing-reqs] and
   [I-D.ietf-roll-indus-routing-reqs]).

2.1.  LLN Properties

   In short, LLN routing protocols must operate under a set of
   constraints that traditional protocols typically do not consider.

   o  They need to operate with a hard, very small bound on state.

   o  In most cases, they optimize for preserving energy.

   o  Typical application data patterns are not simply unicast flows.

   o  They need to be effective with very small packet sizes, such as
      less than 127 octets.



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   o  They have to be very careful when trading off in-node efficiency
      for generality; most LLN nodes don't have computational or memory
      resources to waste.

2.2.  Question to Answer

   As existing protocols were not designed with all of these constraints
   in mind, they have made trade-offs which may or may not be
   appropriate for 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 such an existing protocol.  However, if no current protocol can
   meet LLN's requirements, then further work will be needed to define
   and standardize a protocol that can.  Whether or not such a protocol
   involves extensions to an existing protocol or developing a new
   protocol 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 whether 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.

3.1.  Protocols Considered

   This document does not seek to answer the question of whether there
   is any protocol outside the IETF which could meet LLN application
   requirements.  Rather, it seeks to answer whether any existing
   protocols developed and specified by the IETF can meet such
   requirements.  If an existing protocol specification can be used
   unchanged, then writing additional protocol specifications is
   unnecessary.  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 specify those in 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 protocols can.

   Therefore, this document considers "existing routing protocols" to be



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   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
   stable and mature draft that is a charter item of an active IETF
   working group.  The list of considered protocols is OSPF [RFC2328],
   IS-IS [RFC1142], RIP [RFC2453], OLSR [RFC3626], OLSv2
   [I-D.ietf-manet-olsrv2], TBRPF [RFC3684], AODV [RFC3561], DYMO
   [I-D.ietf-manet-dymo], and DSR [RFC4728].  This document also
   considers notable variants of these protocols, such as Triggered RIP
   [RFC2091].

   Some of these protocols are still works in progress, and so are
   changing over time.  To enable this document to be correct yet not
   dependent on this evolution, it document considers specifications as
   of a specific date: November 31, 2008.

   This document does not consider DTN bundles [RFC5050] or the DTN
   Licklider protocol [RFC5326] as suggested by the ROLL working group
   charter, because they are not routing protocols.  This document does
   not consider the DTN routing protocol PRoPHET
   [I-D.irtf-dtnrg-prophet] because its design is based on the non-
   randomness of node mobility, which is not common to LLN application
   domains.

3.2.  Criteria

   The five criteria this document uses are derived from a set of
   documents that describe the requirements of a few major LLN
   application scenarios.  The five criteria, presented in Section 4,
   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.

3.3.  Evaluation

   This document evaluates the above protocols by thinking through each
   specification and considering a hypothetical implementation that



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   performs as well as possible on the criteria.  The evaluation 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.  The analysis does not consider
   hypothetical extensions to protocols that require additional fields
   or message exchanges.


4.  Criteria

   This section presents five important criteria for routing in low
   power and lossy networks.  Later sections evaluate protocols against
   them.  The 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 criteria is constructive.

   We derive these criteria from existing documents that describe ROLL
   network application requirements [I-D.ietf-roll-home-routing-reqs]
   [I-D.ietf-roll-urban-routing-reqs]
   [I-D.ietf-roll-indus-routing-reqs].  These criteria do not encompass
   all application requirements.  Instead, they are a common set of
   routing protocol requirements that the applications domains in these
   documents share.  Considering this very general and common set of
   requirements sets a minimal bar for a protocol to be applicable for
   an LLN deployment.

   If a protocol cannot meet these minimalist criteria, then it cannot
   be used unchanged in several major ROLL application domains and so is
   unlikely to be a good candidate for use within the broader scope of
   all LLN application domains.  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 "routing state", "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 criterion.  The value "fail" indicates that the
   protocol does not have acceptable performance according to the



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   criterion, and that the document defining the protocol does not, as
   written, contain sufficient flexibility to allow the protocol to meet
   the criterion while conforming to the specification.  Finally, "?"
   indicates that an implementation could exhibit satisfactory
   performance while following the protocol specification, but that the
   implementation decisions 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
   specification 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.

   A protocol failing to meet one or more of the criteria does not
   exclude it from being used in LLNs.  Rather, it means that, in order
   to be used in LLNs, the protocol would need to be extended in ways
   that do not conform with the current specification document.

4.1.  Formal Definitions

   To provide precise definitions of these criteria, 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 criterion from application
   requirements in its corresponding section.

4.2.  Routing State

   This criterion indicates whether routing state scales reasonably
   within the memory resources of low-power nodes.  According to this
   criterion, routing state that scales 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 to the sizes necessary for
   many LLN applications when faced with the memory constraints devices
   in such applications exhibit.  Similarly, 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 state scaling with O(N) or O(L) fails.  State
   that scales O(D) (assuming no N or L) passes.






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4.3.  Loss Response

   In low power and lossy networks, links routinely come and go due to
   being close to the signal-to-noise threshold at the physical layer.
   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 LLN 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 an unbounded control overhead per data packet.  This is
   particularly true for event-driven networks, which only report data



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   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.

   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 far more reasonable than
   communicating once a second.  So while control traffic should be
   bounded by data traffic, it requires some leeway to bootstrap and
   maintain a long-lived yet idle network.

   In the case of control traffic, the communication rate (sum of
   transmissions and receptions at a node) is a better measure than the
   transmission rate (since energy is consumed for both transmissions
   and receptions).  Controlling the transmission rate is insufficient,
   as 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 Appendix B.

4.5.  Link and Node Cost

   These two criteria 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 forwarding 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



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   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 management 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 its 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



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   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, a typical amount
   for top-end microcontrollers, 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.

5.1.  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



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   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 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.  The table below shows, based on the criteria described
   above, whether these protocols meet ROLL criteria.  Appendix A
   contains the reasoning behind each value in the table.


     Protocol      State   Loss  Control   Link Cost  Node Cost
     OSPF/IS-IS    fail    fail    fail      pass       fail
     OLSRv2        fail     ?       ?        pass       pass
     TBRPF         fail    pass    fail      pass        ?
     RIP           pass    fail    pass       ?         fail
     AODV          pass    fail    pass      fail       fail
     DYMO          pass     ?      pass       ?          ?
     DSR           fail    pass    pass      fail       fail


                                 Figure 1


6.  Link State Protocols

6.1.  OSPF & IS-IS

   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



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   on reservable, unreserved, and available bandwidth.

   IS-IS (specified in [RFC1142]) is similar in many respects to OSPF,
   but as a descendent of the OSI protocol suite differs in some places
   such as the way areas are defined and used.  However, routing
   adjacencies are also maintained by local propagation of the LSDB, and
   a shortest path computation is used over a metric space which may
   measure delay, errors, or other link properties.

6.2.  OLSR & OLSRv2

   Optimized Link State Routing (OLSR) (see [RFC3626] and
   [I-D.ietf-manet-olsrv2]) is a link state routing protocol for MANETs.
   OLSR routers flood link state advertisement packets throughout the
   entire network, such that each node has a map of the network
   topology.  Because link variations can lead to heavy flooding traffic
   when using a link state approach, OLSR establishes a topology for
   minimizing this communication, imposes minimum time interval between
   two successive control transmissions by a router, and makes triggered
   updates optional.  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 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 optimized 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 leaves that communicate to a cluster head.

6.3.  TBRPF

   Topology Dissemination Based on Reverse Path Forwarding (see
   [RFC3684]) is another proactive link state protocol for MANETs.



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   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 for
   MANETs.  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



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   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.

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 a distance value as its routing metric which
   must be at least the hop count, but allows 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 for
   MANETs, 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.






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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 MANET router'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.  Conclusion

   Figure 1 shows that no existing IETF protocol specification meets the
   criteria described in Section 4.  Therefore, having a routing
   protocol for LLNs requires new protocol specification documents.
   Whether such documents describe modifications to existing protocols
   or new protocols it outside the scope of this document and warrants
   further discussion.  However, the results in Figure 1 may provide
   some insight or guidance in such a discusssion, indicating what
   protocol mechanisms may be better suited to LLNs than others.

   Such a discussion should not, however, be limited to the protocols
   listed in Figure Figure 1.  There are many existing protocols which
   are unsuitable as a general routing protocol but describe mechanisms
   that could be very useful in the context of LLNs.  Any such future
   discussion ought to consider how routing in LLNs may benefit from
   examining mechanisms from a broader suite of protocols than those
   listed in Figure Figure 1.



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10.  Security Considerations

   LLNs have security considerations.  These considerations vary greatly
   depending on application domain.  For example, deployers industrial
   monitoring networks may impose more stringent confidentiality
   requirements than home automation networks do.  Such requirements are
   an important consideration in protocol design, but their variety
   makes distilling them to a minimalist set of "necessary but not
   sufficient" criteria is of limited use.  The criteria in this
   document are not the only requirements and considerations in LLN
   protocols, and as such the omission of a security criterion should
   not be interpreted as a lack of a need for security in LLNs.


11.  IANA Considerations

   This document includes no request to IANA.


12.  Acknowledgements

   The authors would like to thank all the members of the ROLL working
   group for their valuable comments, and the chairs for their helpful
   guidance.

   We are also indebted to the Sensor Network Architecture group at
   Berkeley for contributing their helpful analysis: Prabal Dutta,
   Rodrigo Fonseca, Xiaofan Jiang, Jaein Jeong, Jorge Ortiz, and Jay
   Tanega.


13.  References

13.1.  Normative References

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

13.2.  Informative References

   [I-D.ietf-manet-dymo]
              Chakeres, I. and C. Perkins, "Dynamic MANET On-demand
              (DYMO) Routing", draft-ietf-manet-dymo-15 (work in
              progress), November 2008.

   [I-D.ietf-manet-nhdp]
              Clausen, T., Dearlove, C., and J. Dean, "MANET
              Neighborhood Discovery Protocol (NHDP)",



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              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]
              Porcu, G., "Home Automation Routing Requirements in Low
              Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-06 (work in progress),
              November 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-03 (work in
              progress), December 2008.

   [I-D.ietf-roll-urban-routing-reqs]
              Dohler, M., Watteyne, T., Winter, T., Barthel, D.,
              Jacquenet, C., Madhusudan, G., and G. Chegaray, "Urban
              WSNs Routing Requirements in Low Power and Lossy
              Networks", draft-ietf-roll-urban-routing-reqs-03 (work in
              progress), January 2009.

   [I-D.irtf-dtnrg-prophet]
              Lindgren, A. and A. Doria, "Probabilistic Routing Protocol
              for Intermittently Connected Networks",
              draft-irtf-dtnrg-prophet-01 (work in progress),
              November 2008.

   [RFC1142]  Oran, D., "OSI IS-IS Intra-domain Routing Protocol",
              RFC 1142, February 1990.

   [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,



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              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.

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

   [RFC5326]  Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
              Transmission Protocol - Specification", RFC 5326,
              September 2008.

   [chakeres.transparent]
              Chakeres, I. and E. Belding-Royer, "Transparent Influence
              of Path Selection in Heterogeneous Ad hoc Networks",
              Proceedings of the 15th IEEE International Symposium on
              Personal, Indoor and Mobile Radio Communications (PIMRC) ,
              September 2004.


Appendix A.  Routing protocol scalability analysis

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

   "OSPF & IS-IS"

   OSPF floods link state through a network.  Each router must receive
   this complete link set.  OSPF fails the routing state 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



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   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.

   IS-IS receives the same results as OSPF, because it maintains a
   consistent LSDB using similar mechanisms, and can account for link
   costs but not router costs in its shortest path computation.

   "OLSRv2"

   OLSRv2 is a proactive link state protocol, flooding link state
   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 routing state criterion.

   Unacceptable control traffic overhead may arise 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).

   Fisheye routing is a technique to reduce the frequency routing
   updates as the routing update propagates away from its source.  This
   has the potential to reduce the control overhead to acceptable
   levels, and it is possible to implement this technique without
   violating the specification because the specification does not
   require that all updates be sent with the same frequency.  However,
   there is no specification of how this should be accomplished.  Thus,
   OLSR receives a "?" for the control traffic criterion.  Fisheye
   routing does not alter the table size, so it does not modify OLSR's



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   result ("fail") on the routing state criterion.

   Furthermore, changes in the link set may require re-flooding the
   network link state even if those links were not being used by
   routing.  OLSR makes these triggered floods optional, but as sending
   no triggered updates will raise problems in topology consistency,
   OLSRv2 receives a '?' in the loss response criterion.

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

   "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 routing state criterion.  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 criterion.  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 routing state.  While standard RIP requires
   each node broadcast a beacon per period, and that updates must be
   propagated by affected nodes, triggered RIP only sends updates when



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   network conditions change in response to the data path, so RIP passes
   the control cost criterion.  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.  While RIP has the concept of a
   network cost, it is insufficient to describe 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 routing state criterion.  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 criterion 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"

   The design of DYMO shares much with AODV, with some changes to remove
   precursor lists and compact various messages.  It still passes the
   routing state criterion 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.  The DYMO specification places very few
   requirements on how nodes respond to route error RERR messages that



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   denote a broken route.  Therefore, while it is possible for a DYMO
   implementation to meet the loss response criterion, the specification
   is not clear on how to meet the criterion while still maintaining
   routes as link breaks .  This leads to a ? in loss repsonse
   [I-D.ietf-manet-dymo].

   DYMO indicates that the "distance" of a link can vary from 1-65535
   [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 properties.  There is great uncertainty whether DYMO can
   support them.  While there are no direct mechanisms in DYMO that can
   meet this criterion, some research results suggest that indirect
   mechanisms, such as packet timing, could [chakeres.transparent].
   Therefore, supporting node cost would require additional mechanisms
   or specifications, leading to a ? on the node cost criterion.

   "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 routing state
   criterion.  Control traffic is completely data driven, and so DSR
   receives a pass for this criterion.  Finally, a transmission failure
   only prompts an unreachable destination to be sent to the source of
   the message, passing the loss response criterion.

   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.


Appendix 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



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   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.


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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