Networking Working Group P. Levis
Internet-Draft Stanford University
Intended status: Informational A. Tavakoli
Expires: July 16, 2009 S. Dawson-Haggerty
UC Berkeley
January 12, 2009
Overview of Existing Routing Protocols for Low Power and Lossy Networks
draft-ietf-roll-protocols-survey-03
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Abstract
Networks of low power wireless devices introduce novel IP routing
issues. Low-power wireless devices, such as sensors, actuators and
smart objects, have difficult constraints: very limited memory,
little processing power, and long sleep periods. As most of these
devices are battery-powered, energy efficiency is critically
important. Wireless link qualities can vary significantly over time,
requiring protocols to make agile decisions yet minimize topology
change energy costs. Routing over such low power and lossy networks
has novel requirements that existing protocols may not address. This
document provides a brief survey of the strengths and weaknesses of
existing protocols with respect to this class of networks. From this
survey it examines whether existing protocols as described in RFCs
and mature drafts could be used without modification in these
networks, or whether further work is necessary.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Suitability Summary . . . . . . . . . . . . . . . . . . . . . 7
4.1. Formal Definitions . . . . . . . . . . . . . . . . . . . . 8
4.2. Table Scalability . . . . . . . . . . . . . . . . . . . . 8
4.3. Loss Response . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Control Cost . . . . . . . . . . . . . . . . . . . . . . . 9
4.5. Link and Node Cost . . . . . . . . . . . . . . . . . . . . 10
5. Routing Protocol Taxonomy . . . . . . . . . . . . . . . . . . 11
5.1. Protocols Today . . . . . . . . . . . . . . . . . . . . . 12
6. Link State Protocols . . . . . . . . . . . . . . . . . . . . . 13
6.1. OSPF & IS-IS . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. OLSR & OLSRv2 . . . . . . . . . . . . . . . . . . . . . . 14
6.3. TBRPF . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7. Distance Vector protocols . . . . . . . . . . . . . . . . . . 15
7.1. RIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.2. Ad-hoc On Demand Vector Routing (AODV) . . . . . . . . . . 15
7.3. DYMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.4. DSR . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 16
8.1. IPv6 Neighbor Discovery . . . . . . . . . . . . . . . . . 17
8.2. MANET-NHDP . . . . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
12. Annex A - Routing protocol scalability analysis . . . . . . . 18
13. Annex B - Logarithmic scaling of control cost . . . . . . . . 21
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
14.1. Normative References . . . . . . . . . . . . . . . . . . . 22
14.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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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
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 internetworking. The sheer number of the low-
power networked devices means that they cannot depend on human
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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 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 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]).
As they were not designed with these requirements in mind, existing
protocols may or may not work well in LLNs. The first step to
reaching consensus on a routing protocol for LLNs is to decide which
of these two is true. If an existing protocol can meet LLN
requirements without any changes, then barring extenuating
circumstances, it behooves us to use an existing standard. However,
if no current protocol can meet LLN's requirements, then further work
will be needed to define and standardize with a protocol that can.
Whether or not such a protocol involves modifications to an existing
protocol or a new protocol entirely is outside the scope of this
document: this document simply seeks to answer the question: do LLNs
require a new protocol specification document at all?
3. Methodology
To answer the question of LLNs require new protocol specification
work, this document examines existing routing protocols and how well
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they can be applied to low power and lossy networks. It provides a
set of criteria with which to compare the costs and benefits of
different protocol designs and examines existing protocols in terms
of these criteria.
The five criteria this document uses are derived from a set of drafts
that describe the requirements of a few major LLN application
scenarios. The five criteria, presented in Section 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.
This document considers "existing routing protocols" to be protocols
that are specified in RFCs or, in the cases of DYMO
[I-D.ietf-manet-dymo] or OLSRv2 [I-D.ietf-manet-olsrv2], a very
mature draft which will most likely become an RFC. We do 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.
We do not examine the Network Mobility Basic Support Protocol (NEMO
RFC 3963 [RFC3963]) because it is not a routing protocol. We do not
examine hierarchical NEMO [I-D.thubert-tree-discovery] as a candidate
because it only maintains a default route and so is insufficient for
general routing. Although NEMO itself is not a suitable routing
solution to LLNs, some of its mechanisms, such as loop-free tree
formation, might be useful in an LLN routing protocol.
This document does not seek to answer the question of whether there
is any protocol anywhere which could meet LLN application
requirements. Rather, it seeks to answer whether protocols, as
specified in current IETF standards documents, can meet such
requirements. If an existing protocol specification can be used
unchanged, then writing additional protocol specifications is
unnecessary. For example, there are many academic papers and
experimental protocol implementations available; while one or more of
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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 a hypothetical
implementation which took advantage of the specification so as to
perform as well as possible on the metrics. 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
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"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 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 RFC in
order to meet the criterion, while a "?" means a protocol would
require a supplementary document further constraining and specifying
how a protocol should behave.
4.1. Formal Definitions
To provide precise definitions of these metrics, we use formal big-O
notation, where N refers to the number of nodes in the network, D
refers to the number of unique destinations, and L refers to the size
of a node's local, single-hop neighborhood (the network density). We
explain the derivation of each metric from application requirements
in its corresponding section.
4.2. Table Scalability
Scalability support for large networks of sensors is highlighted as a
key requirement by all three application requirements documents.
Network sizes range from a minimum of 250 nodes in the home routing
requirements [I-D.ietf-roll-home-routing-reqs] to very large networks
of "tens of thousands to millions" of devices noted of the urban
requirements [I-D.ietf-roll-urban-routing-reqs]. Networks are
expected to have similar size in industrial settings, the
requirements draft states that depths of up to 20 hops are to be
expected [I-D.ietf-roll-indus-routing-reqs]. Given that network
information maintained at each node is stored in routing and neighbor
tables, along with the constrained memory of nodes, necessitates
bounds on the size of these tables.
This metric examines whether routing tables scale within reasonable
memory resources of low-power nodes. According to this metric,
routing protocols that scale linearly with the size of the network or
a node's neighborhood fail. Scaling with the size of the network
prevents networks from growing to reasonable size, while scaling with
the network density precludes dense deployments. However, as many
low-power and lossy networks behave principally as data collection
networks and principally communicate through routers to data
collection points in the larger Internet, scaling with the number of
such collection points is reasonable. Protocols whose state scales
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with the number of destinations pass.
More precisely, routing table size scaling with O(N) or O(L) fails.
A table that scales O(D) (assuming no N or L) passes.
4.3. Loss Response
In low power and lossy networks, links routinely come and go due to
being close to the SINR threshold. It is important that link churn
not trigger unnecessary responses by the routing protocol. This
point is stressed in all the application requirement documents,
pointing to the need to localize response to link failures with no
triggering of global network re-optimization, whether for reducing
traffic or for maintaining low route convergence times
([I-D.ietf-roll-home-routing-reqs],
[I-D.ietf-roll-urban-routing-reqs], and
[I-D.ietf-roll-indus-routing-reqs]). The industrial routing
requirements draft states that protocols must be able to "recompute
paths based on underlying link characteristics which may change
dynamically", as well as reoptimize when the device set changes to
maintain service requirements. The protocol should also "always be
in the process of optimizing the system in response to changing link
statistics." Protocols with these properties should take care not to
require global updates.
A protocol which requires many link changes to propagate across the
entire network fails. Protocols which constrain the scope of
information propagation to only when they affect routes to active
destinations, or to local neighborhoods, pass. Protocols which allow
proactively path maintenance pass if the choice of which paths to
maintain is user-specified.
More precisely, loss responses that require O(N) transmissions fail,
while responses that can rely on O(1) local broadcasts or O(D) route
updates pass.
4.4. Control Cost
Battery-operated devices are a critical component of all three
application spectrums, and as such special emphasis is placed on
minimizing power consumption to achieve long battery lifetime,
[I-D.ietf-roll-home-routing-reqs], with multi-year deployments being
a common case [I-D.ietf-roll-indus-routing-reqs]. In terms of
routing structure, any proposed 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
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neighbors, build routes, transmit routing tables, or otherwise
conduct routing. As low-power wireless networks can have very low
data rates, protocols which require a minimum control packet rate can
have unbounded control overhead. This is particularly true for
event-driven networks, which only report data when certain conditions
are met. Regions of a network which never meet the condition can be
forced to send significant control traffic even when there is no data
to send. For these use cases, hard-coded timing constants are
unacceptable, because they imply a prior knowledge of the expected
data rate.
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.
The control cost metric is a necessary but not sufficient condition
for a protocol to be a viable routing protocol for LLNs. Protocols
not meeting this bound are unacceptable for use in this environment;
however, there may be protocols which receive a "pass" for this
metric and yet are also unsuitable.
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 Annex B.
4.5. Link and Node Cost
These two metrics specify how a protocol chooses routes for data
packets to take through the network. Classical routing algorithms
typically acknowledge the differing costs of paths and may use a
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shortest path algorithm to find paths. This is a requirement for low
power networks, as links must be evaluated as part of an objective
function across various metric types, such as minimizing latency and
maximizing reliability [I-D.ietf-roll-indus-routing-reqs].
However, in low power networks it is also desirable to account for
the cost of routing through particular routers. Applications require
node or parameter constrained routing, which takes into account node
properties and attributes such as power, memory, and battery life
that dictate a router's willingness or ability to route other
packets. Home routing requirements note that devices will vary in
their duty cycle, and that routing protocols should prefer nodes with
permanent power [I-D.ietf-roll-home-routing-reqs]. The urban
requirements note that routing protocols may wish to take advantage
of differing data processing and 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 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
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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.
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
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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 protocol, DSR, does not easily fit into one of these
two classes. Although it is a distance vector protocol, DSR has
several properties that make it differ from most other protocols in
this class. We examine these differences in our discussion of DSR.
The next two sections summarize several well established routing
protocols. This table shows, based on the criteria described above,
whether these protocols meet ROLL criteria. Annex A contains the
reasoning behind each value in the table.
Protocol Table Loss Control Link Cost Node Cost
OSPF/IS-IS fail fail fail pass fail
OLSRv2 fail fail ? pass pass
TBRPF fail pass fail pass ?
RIP pass fail pass ? fail
AODV pass fail pass fail fail
DYMO[-low] pass fail pass ? fail
DSR fail pass pass fail fail
6. Link State Protocols
6.1. OSPF & 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
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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.
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
wireless mesh networks. OLSR nodes flood link state advertisement
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.
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6.3. TBRPF
Topology Dissemination Based on Reverse Path Forwarding (see
[RFC3684]) is another proactive link state protocol. TBRPF computes
a source tree, which provides routes to all reachable nodes. It
reduces control packet overhead by having nodes only transmit a
subset of their source tree as well as by using differential updates.
The major difference between TBRPF and OLSR is the routing data that
nodes advertise and who chooses to aggregate information. In OLSR,
nodes select neighbors to be MPRs and advertise their link state for
them; in TBRPF, nodes elect themselves to advertise relevant link
state based on whether it acts as a next hop.
7. Distance Vector protocols
7.1. RIP
The Routing Information Protocol (RIP) (defined in [RFC2453])
predates OSPF. As it is a distance vector protocol, routing loops
can occur and considerable work has been done to accelerate
convergence since the initial RIP protocols were introduced. RIP
measures route cost in terms of hops, and detects routing loops by
observing a route cost approach infinity where "infinity" is referred
to as a maximum number of hops. RIP is typically not appropriate for
situations where routes need to be chosen based on real-time
parameters such as measured delay, reliability, or load or when the
network topology needs to be known for route computation.
"Triggered RIP" (defined in [RFC2091]) was originally designed to
support "on-demand" circuits. The aim of triggered RIP is to avoid
systematically sending the routing database on regular intervals.
Instead, triggered RIP sends the database when there is a routing
update or a next hop adjacency change: once neighbors have exchanged
their routing database, only incremental updates need to be sent.
Because incremental updates cannot depend on periodic traffic to
overcome loses, triggered RIP uses acknowledgment based mechanisms
for reliable delivery.
7.2. Ad-hoc On Demand Vector Routing (AODV)
AODV (specified in [RFC3561]) is a distance vector protocol intended
for mobile ad-hoc networks. When one AODV node requires a route to
another, it floods a request in the network to discover a route. A
depth-scoped flooding process avoids discovery from expanding to the
most distant regions of the network that are in the opposite
direction of the destination. AODV chooses routes that have the
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minimum hop count.
If an AODV route request reaches a node that has a route to the
destination (this includes the destination itself), that node sends a
reply along the reverse route. All nodes along the reverse route can
cache the route. When routes break due to topology changes, AODV
floods error messages and issues a new request. Because AODV is on-
demand it only maintains routes for active nodes. When a link
breaks, AODV issues a Route Error (RERR) and a new route request
message (RREQ), with a higher sequence number so nodes do not respond
from their route caches. These packets can flood the entire network,
giving loss response a fail.
7.3. DYMO
Dynamic Mobile On-Demand routing (DYMO) ([I-D.ietf-manet-dymo]) is an
evolution of AODV. The basic functionality is the same, but it has
different packet formats, handling rules, and supports path
accumulation. Path accumulation allows a single DYMO route request
to generate routes to all nodes along the route to that destination.
Like AODV, DYMO uses 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, 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
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protocols, their capabilities, and how ROLL protocols could leverage
them.
8.1. IPv6 Neighbor Discovery
IPv6 neighbor discovery provides mechanisms for nodes to discover
single-hop neighbors as well as routers that can forward packets past
the local neighborhood. There is an implicit assumption that the
delegation of whether a node is a router or not is static (e.g.,
based on a wired topology). The fact that all routers must respond
to a Router Solicitation requires that the number of routers with a
1-hop neighborhood is small, or there will be a reply implosion.
Furthermore, IPv6 neighbor discovery's support of address
autoconfiguration assumes address provisioning, in that addresses
reflect the underlying communication topology. IPv6 neighbor
discovery does not consider asymmetric links. Nevertheless, it may
be possible to extend and adapt IPv6's mechanisms to wireless in
order to avoid response storms and implosions.
8.2. MANET-NHDP
The MANET Neighborhood Discovery Protocol (MANET-NHDP) provides
mechanisms for discovering a node's symmetric 2-hop neighborhood. It
maintains information on discovered links, their interfaces, status,
and neighbor sets. MANET-NHDP advertises a node's local link state;
by listening to all of its 1-hop neighbor's advertisements, a node
can compute its 2-hop neighborhood. MANET-NHDP link state
advertisements can include a link quality metric. MANET-NHDP's node
information base includes all interface addresses of each 1-hop
neighbor: for low-power nodes, this state requirement can be
difficult to support.
9. Security Considerations
This document presents, considers, and raises no security
considerations.
10. IANA Considerations
This document includes no request to IANA.
11. 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
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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.
12. Annex A - Routing protocol scalability analysis
This aim of this Annex 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 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.
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
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failing the table scalability criteria. Fisheye routing does not
alter the table size.
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 impliment 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, or a
guarentee that implementations with different algorithms for deciding
how frequently to age and retransmit topology updates. Thus, OLSR
receives a "?" for the control traffic metric.
Furthermore, changes in the link set may require immediately re-
flooding the network link state even if those links were not being
used by routing, which fails the loss response metric.
OLSR allows for specification of link quality, and also provides a
'Willingness' metric to symbolize node cost, giving it a pass for
both those metrics.
"TBRPF"
As a link state protocol where each node maintains a database of the
entire network topology, TBRPF's routing table size scales with
network size and density, leading to table sizes which are O(N * L)
when a node receives disjoint link sets from its neighbors. This
causes the protocol to fail the table size criteria. The protocol's
use of differential updates should allow both fast response time and
incremental changes once the distributed database of links has been
established. Differential updates are only used to reduce response
time to changing network conditions, not to reduce the amount of
topology information sent, since each node will periodically send
their piece of the topology. As a result, TBRPF fails the control
overhead criteria. However, its differential updates triggered by
link failure do not immediately cause a global re-flooding of state
(but only to affected routers) [RFC3684], leading to a pass for loss
response.
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TBRPF has a flexible neighbor management layer which enables it to
incorporate various types of link metrics into its routing decision
by enabling a USE_METRIC flag [RFC3684]. As a result, it receives a
pass for link cost. It also provides a mechanism whereby routers can
maintain multiple link metrics to a single neighbor, some of which
can be advertised by the neighbor router [RFC3684]. Although the RFC
does not specify a policy for using these values, developing one
could allow TBRPF to satisfy this requirement, leading to a ? for the
node cost requirement.
"RIP"
RIP is a distance vector protocol: all routers maintain a route to
all other routers. Routing table size is therefore O(N). However,
if destinations are known apriori, table size can be reduced to O(D),
resulting in a pass for table scalability. 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 network conditions change in response to the data path,
so RIP passes the control cost metric. Loss triggers updates, only
propagating if part of a best route, but even if the route is not
actively being used, resulting in a fail for loss response. The rate
of triggered updates is throttled, and these are only differential
updates, yet this still doesn't account for other control traffic (or
tie it to data rate) or prevent the triggered updates from being
flooded along non-active paths. [RFC2453]
RIP receives a ? for link cost because while current implementations
focus on hop count and that is the metric used in [RFC2453], the RFC
also mentions that more complex metrics such as differences in
bandwidth and reliability could be used. However, the RFC also
states that real-time metrics such as link-quality would create
instability and the concept of node cost only appears as metrics
assigned to external networks. 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 table size criteria. As an on-demand protocol, AODV does
not generate any traffic until data is sent, and so control traffic
is correlated with the data and so it receives a pass for control
traffic. When a broken link is detected, AODV will use a precursor
list maintained for each destination to inform downstream routers
(with a RERR) of the topology change. However, the RERR message is
forwarded by all nodes that have a route that uses the broken link,
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even if the route is not currently active, leading to a fail for loss
response [RFC3561].
AODV fails the link cost metric because the only metric used is hop
count, and this is hardcoded in the route table entry, according to
the RFC [RFC3561]. It fails the node cost requirement because there
is no way for a router to indicate its [lack of] willingness to route
while still adhering to the RFC.
"DYMO/DYMO-low"
The design of DYMO shares much with AODV, with some changes to remove
precursor lists and compact various messages. It still passes the
table size criteria because it only maintains routes requested by
RREQ messages, resulting in O(D) table size. Control traffic (RREQ,
RREP, and RREQ) are still driven by data, and hence DYMO passes the
control cost criterion. However, RERR messages are forwarded by any
nodes that have a route using the link, even if inactive, leading to
a fail of the loss reponse criteria [I-D.ietf-manet-dymo].
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, leading to a fail in node cost.
"DSR"
DSR performs on-demand route discovery, and source routing of
packets. It maintains a source route for all destinations, and also
a blacklist of all unidirectional neighbor links [RFC4728], leading
to a total table size of O(D + L), failing the table size criterion.
Control traffic is completely data driven, and so DSR receives a pass
for this criteria. Finally, a transmission failure only prompts an
unreachable destination to be sent to the source of the message,
passing the loss response criteria.
DSR fails the link cost criterion because its source routes are
advertised only in terms of hops, such that all advertised links are
considered equivalent. DSR also fails the node cost criterion
because a node has no way of indicating its willingness to serve as a
router and forward messages.
13. Annex B - Logarithmic scaling of control cost
To satisfy the control cost criterion, a protocol's control traffic
communication rate must be bounded by the data rate, plus a small
constant. That is, if there is a data rate R, the control rate must
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O(R + e), where e is a very small constant (epsilon). Furthermore,
the control rate may grow logarithmically with the size of the local
neighborhood L. Note that this is a bound: it represents the most
traffic a protocol may send, and good protocols may send much less.
So the control rate is bounded by O(R log(L)) + e.
The logarithmic factor comes from the fundamental limits of any
protocol that maintains a communication rate. For example, consider
e, the small constant rate of communication traffic allowed. Since
this rate is communication, to maintain O(e), then only one in L
nodes may transmit per time interval defined by e: that one node has
a transmission, and all other nodes have a reception, which prevents
them from transmitting. However, wireless networks are lossy.
Suppose that the network has a 10% packet loss rate. Then if L=10,
the expectation is that one of the nodes will drop the packet. Not
hearing a transmission, it will think it can transmit. This will
lead to 2 transmissions. If L=100, then one node will not hear the
first two transmissions, and there will be 3. The number of
transmissions, and the communication rate, will grow with O(log(L)).
This logarithmic bound can be prevented through explicit coordination
(e.g., leader election), but such approaches assumes state and
control traffic to elect leaders. As a logarithmic factor in terms
of density is not a large stumbling or major limitation, allowing the
much greater protocol flexibility it enables is worth its small cost.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
14.2. Informative References
[I-D.ietf-manet-dymo]
Chakeres, I. and C. Perkins, "Dynamic MANET On-demand
(DYMO) Routing", draft-ietf-manet-dymo-16 (work in
progress), December 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
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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.thubert-tree-discovery]
Thubert, P., Bontoux, C., Montavont, N., and B. McCarthy,
"Nested Nemo Tree Discovery",
draft-thubert-tree-discovery-07 (work in progress),
August 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,
July 2003.
[RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol (OLSR)", RFC 3626, October 2003.
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[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.
[RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
Thubert, "Network Mobility (NEMO) Basic Support Protocol",
RFC 3963, January 2005.
[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.
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
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Stephen Dawson-Haggerty
UC Berkeley
Soda Hall, UC Berkeley
Berkeley, CA 94707
USA
Email: stevedh@cs.berkeley.edu
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