wdiff draft-ietf-roll-protocols-survey-00.txt draft-ietf-roll-protocols-survey-01.txt

Networking Working Group P. Levis
Internet-Draft Stanford University
Intended status: Informational A. Tavakoli
Expires: February 6, March 29, 2009 S. Dawson-Haggerty
UC Berkeley
August 5,
September 25, 2008

Overview of Existing Routing Protocols for Low Power and Lossy Networks
draft-ietf-roll-protocols-survey-00
draft-ietf-roll-protocols-survey-01

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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 mesh protocols only partially may not address. This
document provides a brief survey of the strengths and weaknesses of
existing protocols with respect to this class of networks. It
provides guidance on how lessons from From this
survey it examines whether existing protocols as described in RFCs
and prior efforts can mature drafts could be leveraged used without modification in future protocol design. 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 . . . . . . . . . . . . . . . . . . . . . 4
3.1. 5
4.1. Formal Definitions . . . . . . . . . . . . . . . . . . . . 5
3.2. 6
4.2. Table Scalability . . . . . . . . . . . . . . . . . . . . 5
3.3. 6
4.3. Loss Response . . . . . . . . . . . . . . . . . . . . . . 6
3.4. 7
4.4. Control Cost . . . . . . . . . . . . . . . . . . . . . . . 6
3.5. 8
4.5. Link and Node Cost . . . . . . . . . . . . . . . . . . . . 7
4. 8
5. Routing Protocol Taxonomy . . . . . . . . . . . . . . . . . . 7
5. 9
6. Link State Protocols . . . . . . . . . . . . . . . . . . . . . 9
5.1. 11
6.1. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. 11
6.2. OLSR . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.3. 12
6.3. TBRPF . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. 12
7. Distance Vector protocols . . . . . . . . . . . . . . . . . . 11
6.1. 13
7.1. RIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. DSDV . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. 13
7.2. Ad-hoc On Demand Vector Routing (AODV) . . . . . . . . . . 12
6.4. 13
7.3. DYMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.5. 14
7.4. DSR . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. 14
8. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 13
7.1. 14
8.1. IPv6 Neighbor Discovery . . . . . . . . . . . . . . . . . 13
7.2. 14
8.2. MANET-NHDP . . . . . . . . . . . . . . . . . . . . . . . . 13
8. 15
9. Security Issues . . . . . . . . . . . . . . . . . . . . . . . 13
9. 15
10. Manageability Issues . . . . . . . . . . . . . . . . . . . . . 14
10. 15
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
11. 15
12. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. 15
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
13. 15
14. Annex A - Routing protocol scalability analysis . . . . . . . 14
14. 15
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
14.1. 19
15.1. Normative References . . . . . . . . . . . . . . . . . . . 18
14.2. 19
15.2. Informative References . . . . . . . . . . . . . . . . . . 18 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21 22

1. Terminology

AODV: Ad-hoc On Demand Vector Routing

DSDV: Destination Sequenced Distance Vector

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

LSA: Link State Advertisement

LSDB: Link State Database

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.brandt-roll-home-routing-reqs]
([I-D.ietf-roll-home-routing-reqs] and
[I-D.ietf-roll-indus-routing-reqs]). This document examines As they were not designed with
these requirements in mind, existing
routing protocols and how may or may not work
well they can be applied in LLNs. The first step to low power and
lossy networks. It provides reaching consensus on a basic framework with which routing
protocol for LLNs is to compare
the costs and benefits decide which of different protocol designs and examines
existing protocols within this framework. From these observations it
provides guidance on how 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 solutions
standard. However, if no current protocol can meet LLN's
requirements, then further work will be leveraged in
future 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 design.
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. Suitability Summary

In Methodology

To answer the question of LLNs require new protocol specification
work, this section, we present five important requirements for document examines existing routing
in protocols and how well
they can be applied to low power and lossy networks, and evaluate protocols against them.
Our evaluation attempts to take networks. It provides a complicated and interrelated
set of
design decisions criteria with which to compare the costs and trade-offs benefits of
different protocol designs and condense them to a simple "pass",
"fail", or "?". As with any simplification, there is a risk examines existing protocols in terms
of
removing some necessary nuance. However, we believe that being
forced to take a position on whether or not these protocols criteria.

The five criteria this document uses are
acceptable according to binary criterion will be constructive.

We derive these metrics derived from existing documents a set of drafts
that describe ROLL
network application requirements. These metrics do not encompass all the requirements of a few major LLN application requirements.
scenarios. The five criteria, presented in Section 3, are neither
exhaustive nor complete. Instead, they are a common set one specific subset of routing
protocol
high-level requirements that most applications domains share.
Considering this very general and common set shared across all of requirements sets a
minimal bar for the application
requirement drafts. Because every application requirement draft
specifies these criteria, then a protocol to be generally applicable. If a protocol
cannot which does not meet even these minimalist criteria, then it one of
them cannot be used in
several major ROLL application domains and so is unlikely to be a
good candidate for further analysis and examination. Satisfying without modifications or extensions. However,
because these minimal criteria is necessary but not sufficient: they do not represent a subset of the complete intersection of the
application requirements, any given application domain may impose
additional requirements and
applications introduce additional, more stringent requirements. But
this simplified view provides which a first cut of the applicability of
existing protocols, and those that do satisfy them might be
reasonable candidates for further study.

The five metrics are "table scalability", "loss response", "control
cost", "link cost", and "node cost". particular protocol may not meet.
For each of these, this reason, these criteria are "necessary but not sufficient."
A protocol that does not meet the value
"pass" indicates criteria cannot be used as
specified, but it is possible that a given protocol has satisfactory performance
according to the metric. The value "fail" indicates that meets the
protocol does criteria yet
is not have acceptable performance according able to meet the
metric, and requirements of a particular application
domain. Nevertheless, a protocol that meets all of the RFC defining 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 protocol 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 appear to
contain sufficient flexibility seek to alter answer the question of whether there is any protocol to do so.
Finally, "?" indicates that an implementation
anywhere which could exhibit
satisfactory performance while following the RFC, but that design
considerations necessary 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.

3.1. 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 local neighborhood (the network density). We
explain the derivation of each metric from application requirements
in its corresponding section.

3.2.

4.2. Table Scalability

Scalability support for large networks of sensors is highlighted as a
key requirement by the all three application requirements documents mentioned
above, with size ranging documents.
Network sizes range from a minimum of 250 nodes
([I-D.brandt-roll-home-routing-reqs]) in the home routing
requirements [I-D.ietf-roll-home-routing-reqs] to very large networks
([I-D.dohler-roll-urban-routing-reqs]), and network
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 ([I-D.ietf-roll-indus-routing-reqs]). 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.

3.3.

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.brandt-roll-home-routing-reqs],
[I-D.dohler-roll-urban-routing-reqs],
([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.

3.4.

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.brandt-roll-home-routing-reqs]),
[I-D.ietf-roll-home-routing-reqs], with multi-year deployments being
a common case ([I-D.ietf-roll-indus-routing-reqs]). [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.dohler-roll-urban-routing-reqs]). [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 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 control traffic is uncorrelated with unbounded by data traffic, a protocol fails
according to Control Cost metric. Protocols which pass bound their
control traffic rate to their data traffic. Protocols which pass do
not use resources to maintain unused state. More specifically, any
protocol which requires fixed-rate periodic control packets in the
absence of data traffic fails.

3.5.

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]). [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 abd and attributes such as power, memory, and battery life
that dictate a router's willingness or ability to route other
packets([I-D.brandt-roll-home-routing-reqs],
[I-D.dohler-roll-urban-routing-reqs]).
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, DSDV, 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. Later sections consider the properties of these protocol
with respect to ROLL requirements. 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 ? fail ?
AODV pass fail pass fail fail
DSDV pass fail fail ? fail
DYMO[-low] pass fail pass ? ? fail
DSR fail ? pass pass fail ?

5. fail

6. Link State Protocols

5.1.

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.

5.2.

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.

5.3.

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

6.1.

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.

6.2. DSDV

Destination Sequenced Distance Vector Routing uses distance vectors
to continuously maintain routes throughout a network. Unlike RIP,
DSDV uses per-node sequence numbers to provide a total ordering on
route information age in order to prevent loops. In DSDV, each node
maintains a route to each other node.

6.3.

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 does not maintain routes to all nodes as DSDV does;
instead,
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.

6.4.

7.3. DYMO

Dynamic Mobile On-Demand routing (DYMO) (([I-D.ietf-manet-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.

6.5.

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 DSDV, AODV, 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.

7.1.

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.

7.2.

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

10. Manageability Issues

TBD

10.

11. IANA Considerations

This document includes no request to IANA.

11.

12. Security Considerations

TBD

12. Acknowledgements

13. Acknowledgements

14. 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, neighbors (for MPR selection), and a routing table,
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). As L is bounded by N, this means control traffic is
proportional to O(N). 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, TBRPF protocol where each node maintains a database of the
entire network topology, TBRPF's routing table size scales with
network size, size and density, leading to table sizes which are O(N * L)
when a node receives disjoint link sets from its neighbors. However, this 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 are triggered
by a link failure do not immediately cause a global re-flooding of
state, leading to a pass for loss response.

TBRPF has a flexible neighbor management layer which enables it to
incorporate various link metrics into its routing decision. As a
result, it receives a pass for link cost. It also provides a
mechanism whereby routers are able to advertise their "willingness to
route." 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, with all routers maintaining a
route to all other routers, and as such table size being O(N).

However, if destinations are known apriori, table size can be reduced
to O(D), resulting in a pass for table scalability. Each changing network conditions, not to reduce the amount of
topology information sent, since each node
broadcasts a beacon per period, and updates must be propagated by
affected nodes, irrespective will periodically send
their piece of data rate, failing the control cost
metric. Loss triggers updates, only propagating if part of topology. As a best
route, even if result, TBRPF fails the route is control
overhead criteria. However, its differential updates triggered by
link failure do not actively being used, resulting in immediately cause a
fail global re-flooding of state
(but only to affected routers) [RFC3684], leading to a pass for loss
response.

RIP receives

TBRPF has a ? for link cost, because while current implementations
focus on hop count, any number can theoretically be used flexible neighbor management layer which enables it to advertise
link quality. There is no concept
incorporate various types of router quality, and so it
receives a fail for the node cost criteria.

"AODV"

AODV table size is link metrics into its routing decision
by enabling 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. USE_METRIC flag [RFC3684]. 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 a result, it receives a
pass for control
traffic. When a broken link is detected, AODV will use cost. It also provides a precursor
list maintained for each destination to inform downstream mechanism whereby routers
(with can
maintain multiple link metrics to a RERR) single neighbor, some of which
can be advertised by the topology change. This information is neighbor router [RFC3684]. Although the RFC
does not sent
as specify a flood, policy for using these values, developing one
could allow TBRPF to satisfy this requirement, leading to an acceptable of traffic for a loss.
Furthermore, ? for the router encountering
node cost requirement.

"RIP"

RIP is a broken link may initiate local
repair via distance vector protocol: all routers maintain a scoped flood. route to
all other routers. Routing table size is therefore O(N). However, link churn may also result in
RERR messages being flooded
if destinations are known apriori, table size can be reduced to the entire network. Therefore, AODV
receives O(D),
resulting in a fail pass for loss response.

AODV allows the source router to wait and collect table scalability. Each node broadcasts a number of RREP
messages before choosing which route is to
beacon per period, and updates must be used. This allows that
router to evaluate propagated by affected nodes,
irrespective of data rate, failing the link control cost metric. Loss
triggers updates, only propagating if part of a best route, but even
if the different alternatives,
although it route is not clear how 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 should be done. AODV fails 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 requirement because it does not appear while current implementations
focus on hop count and that is the metric used in [RFC2453], the RFC
also mentions that a design
goal was to choose paths which are minimal under some metric. It
fails 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 requirement only appears as metrics
assigned to external networks. It also receives a ? because there is no way for the
concept of a router network cost is introduced, which is added to indicate link cost,
but does not describe its [lack of] willingness to route while still adhering
to the RFC.

"DSDV"

DSDV use.

"AODV"

AODV table size is another distance vector protocol, similar to RIP, with a function of the
only main difference being number of communicating pairs in
the usage of destination-based sequence
numbers to prevent routing loops. As such network, scaling with O(D). This is acceptable and so AODV
passes the following analysis
applies, which 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 same as RIP's.

DSDV data and so it receives a pass for scalability because table size can be
limited to O(D) if all destinations are known apriori. Control cost control
traffic. When a broken link is detected, AODV will use a fail, because periodic beaconing, irrespective precursor
list maintained for each destination to inform downstream routers
(with a RERR) of data rate, the topology change. However, the RERR message is
required, with updates propagating throughout
forwarded by all nodes that have a route that uses the network. Loss
response broken link,
even if the route is also not currently active, leading to a fail since although for loss only results in
propagation to routers that utilize that
response [RFC3561].

AODV fails the link in their routing
tables, there cost metric because the only metric used is no mechanism for restricting hop
count, and this propagation to
active routes, rather than all routes. Link cost is a ? since
theoretically distance metrics other than hops could be used, but are
not covered hardcoded in the protocol description, and route table entry, according to
the RFC [RFC3561]. It fails the node cost is a fail as requirement because there
is no provision way for a router quality. 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. Table It still passes the
table size is somewhat
smaller, including entries for neighboring DYMO routers and criteria because it only maintains routes
passing through the router. requested by
RREQ messages, resulting in O(D) table size. Control overhead has been reduced
somewhat, traffic (RREQ,
RREP, and RREQ) are still driven by data, and hence DYMO does not generate spurious RERRs; these passes the
control cost criterion. However, RERR messages are only generated when forwarded by any
nodes that have a forwarding failure occurs. Nevertheless,
these RERRs can flood route using the entire network, imposing an O(N) cost.
DYMO includes mechanisms to add additional routing information
(potentially including router willingness), but does not define
explicit policy mechanisms for choosing routers along a path. Its
extensible packet format could allow router properties link, even if inactive, leading to be
specified in headers, giving it
a ? on node cost. Rather than rely
solely on hopcount, fail of the loss reponse criteria [I-D.ietf-manet-dymo].

While DYMO allows links to have costs in does indicate that the range of metric used for a link can vary
from 1-65535, but does not specify how these might be used, giving 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 ?
on in link cost.

"DSR"

DSR performs source While additional routing of packets, discovering packets through a
route discovery mechanism. Only table entries needed by the data are
maintained, which is equivalent to the number of unique next hops
needed to access all desired destinations. Control traffic is only
initiated when a new route is being discovered, or when an existing
route fails, and as such is proportional to the data rate. Loss does
not trigger updates, unless the path information
can be added DYMO messages, there is used. Routes are selected
based on hop count, with no mechanism for differentiating between
"routers". mention of node cost, leading
to a fail in node cost.

"DSR"

DSR fails the table criterion because it performs on-demand route discovery, and source routing of
packets. It maintains a source route for all destinations, and also
a blacklist of
neighbors with whom connectivity is not bidirectional: this scales
with O(L). DSR receives all unidirectional neighbor links [RFC4728], leading
to a ? on the loss criterion because some total table size of
its mechanisms, such as automatic route shortening and route cache
suggest that it may be able to meet the loss criterion, but exactly
how these are implemented will affect its efficiency. DSR passes O(D + L), failing the
control criterion because all control table size criterion.
Control traffic is on-demand, completely data driven, and so is
bound DSR receives a pass
for this criteria. Finally, a transmission failure only prompts an
unreachable destination to data traffic rates. 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 has a ? on also fails the node cost criterion
because sources decide on whom a node has no way of indicating its willingness to send
packets through, and nodes cannot announce their capabilities in this
regard. However, serve as a new route reply option could possibly achieve
this goal.

14.
router and forward messages.

15. References

14.1.

15.1. Normative References

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

14.2.

15.2. Informative References

[I-D.brandt-roll-home-routing-reqs]
Brandt, A., "Home Automation Routing Requirement in Low
Power and Lossy Networks",
draft-brandt-roll-home-routing-reqs-01 (work in progress),
May 2008.

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

[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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