Networking Working Group P. Levis
Internet-Draft Stanford University
Intended status: Informational A. Tavakoli
Expires: January 14, 2009 S. Dawson-Haggerty
UC Berkeley
July 13, 2008
Overview of Existing Routing Protocols for Low Power and Lossy Networks
draft-levis-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 requirements that existing mesh protocols only partially address.
This document provides a brief survey of the strengths and weaknesses
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of existing protocols with respect to this class of networks. It
provides guidance on how lessons from existing and prior efforts can
be leveraged in future protocol design.
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. Suitability Summary . . . . . . . . . . . . . . . . . . . . . 4
3.1. Formal Definitions . . . . . . . . . . . . . . . . . . . . 5
3.2. Table Scalability . . . . . . . . . . . . . . . . . . . . 5
3.3. Loss Response . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Control Cost . . . . . . . . . . . . . . . . . . . . . . . 6
3.5. Link and Node Cost . . . . . . . . . . . . . . . . . . . . 7
4. Routing Protocol Taxonomy . . . . . . . . . . . . . . . . . . 7
5. Link State Protocols . . . . . . . . . . . . . . . . . . . . . 9
5.1. OSPF . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. OLSR . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.3. TBRPF . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Distance Vector protocols . . . . . . . . . . . . . . . . . . 11
6.1. RIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. DSDV . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. Ad-hoc On Demand Vector Routing (AODV) . . . . . . . . . . 12
6.4. DYMO . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.5. DSR . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 13
7.1. IPv6 Neighbor Discovery . . . . . . . . . . . . . . . . . 13
7.2. MANET-NHDP . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Security Issues . . . . . . . . . . . . . . . . . . . . . . . 13
9. Manageability Issues . . . . . . . . . . . . . . . . . . . . . 14
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
11. Security Considerations . . . . . . . . . . . . . . . . . . . 14
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
13. Annex A - Routing protocol scalability analysis . . . . . . . 14
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
14.1. Normative References . . . . . . . . . . . . . . . . . . . 18
14.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Terminology
AODV: Ad-hoc On Demand Vector Routing
DSDV: Destination Sequenced Distance Vector
DSR: Dynamic Source Routing
DYMO: Dynamic Mobile On-Demand
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
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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] and
[I-D.ietf-roll-indus-routing-reqs]). This document examines existing
routing protocols and how well they can be applied to low power and
lossy networks. It provides a basic framework with which to compare
the costs and benefits of different protocol designs and examines
existing protocols within this framework. From these observations it
provides guidance on how existing solutions can be leveraged in
future protocol design.
3. Suitability Summary
In this section, we present five important requirements for routing
in low power and lossy networks, and evaluate protocols against them.
Our 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 metrics 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
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reasonable candidates for further study.
The five metrics 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 appear to
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 design
considerations necessary to do so are not specified.
3.1. 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 single-hop local neighborhood (the network density). We
explain the derivation of each metric from application requirements
in its corresponding section.
3.2. Table Scalability
Scalability support for large networks of sensors is highlighted as a
key requirement by the application requirements documents mentioned
above, with size ranging from a minimum of 250 nodes
([I-D.brandt-roll-home-routing-reqs]) to very large networks
([I-D.dohler-roll-urban-routing-reqs]), and network depths of up to
20 hops ([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.
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3.3. 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], and
[I-D.ietf-roll-indus-routing-reqs]).
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.
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. 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]), with multi-year deployments
being a common case ([I-D.ietf-roll-indus-routing-reqs]). In terms
of routing structure, any proposed L2N routing protocol ought to
support the autonomous organization and configuration of the network
at the lowest possible energy cost
([I-D.dohler-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 data traffic, a protocol
fails according to Control Cost metric. Protocols which pass bound
their control traffic rate to their data traffic. Protocols which
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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. Link and Node Cost
These two metrics specify how a protocol chooses routes for data
packets to take through the network. Classical routing algorithms
typically acknowledge the differing costs of paths and may use a
shortest path algorithm to find paths. This is a requirement for low
power networks, as links must be evaluated as part of an objective
function across various metric types, such as minimizing latency and
maximizing reliability ([I-D.ietf-roll-indus-routing-reqs]).
However, in low power networks it is also desirable to account for
the cost of routing through particular routers. Applications require
node or parameter constrained routing, which takes into account node
properties abd 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]). 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.
4. 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.
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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, and DYMO are distance vector protocols.
The general consensus in core networks is to use link state routing
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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.
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 fail fail fail ? fail
AODV pass ? pass fail fail
DSDV fail fail fail ? fail
DYMO[-low] pass fail pass fail fail
DSR fail ? pass fail ?
5. Link State Protocols
5.1. 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
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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. 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.
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5.3. 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.
6. Distance Vector protocols
6.1. 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.
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6.3. 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, it only maintains routes for active nodes. When a link
breaks, AODV 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.
6.4. DYMO
Dynamic Mobile On-Demand routing (DYMO) (([I-D.ietf-manet-dymo]) is
an evolution of AODV. The basic functionality is the same, but it
has different packet formats, handling rules, and supports path
accumulation. Path accumulation allows a single DYMO route request
to generate routes to all nodes along the route to that destination.
Like AODV, DYMO uses hop counts as its routing metric. 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. 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, 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.
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7. 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. 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. 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.
8. Security Issues
TBD
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9. Manageability Issues
TBD
10. IANA Considerations
This document includes no request to IANA.
11. Security Considerations
TBD
12. Acknowledgements
13. 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 will utilize multiple routes to a
destination if they exist, and so receives a pass for the link cost
requirement. However, there is no way to associate metrics with
routers when computing paths, and so fails the node cost criteria.
"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, and a routing table, resulting in
state proportional to network size and density (O(N*L + L^2)), and
failing the table scalability criteria.
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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 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 routing table size scales with
network size, leading to table sizes which are O(N * L) when a node
receives disjoint link sets from its neighbors. However, 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 node
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broadcasts a beacon per period, and updates must be propagated by
affected nodes, irrespective of data rate, failing the control cost
metric. Loss triggers updates, only propagating if part of a best
route, even if the route is not actively being used, resulting in a
fail for loss response.
RIP receives a ? for link cost, because while current implementations
focus on hop count, any number can theoretically be used to advertise
link quality. There is no concept of router quality, and so it
receives a fail for the node cost criteria.
"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. This information is not sent
as a flood, leading to an acceptable of traffic for a loss.
Furthermore, the router encountering a broken link may initiate local
repair via a scoped flood. However, link churn may also result in
RREQ and RREP messages being flooded to the entire network.
Depending on these and other implementation decisions like the use of
scoped RREQs, AODV may provide satisfactory loss response; however,
many of these settings are not specified in the RFC. Therefore, AODV
receives a ? for loss response.
AODV allows the source router to wait and collect a number of RREP
messages before choosing which route is to be used. This allows that
router to evaluate the link cost of the different alternatives,
although it is not clear how this should be done. AODV fails the
link cost requirement because it does not appear that that a design
goal was to choose paths which are minimal under some metric. 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.
"DSDV"
DSDV is another distance vector protocol, similar to RIP, with the
only main difference being in the usage of destination-based sequence
numbers to prevent routing loops. As such the following analysis
applies, which is the same as RIP's.
DSDV receives a pass for scalability because table size can be
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limited to O(D) if all destinations are known apriori. Control cost
is a fail, because periodic beaconing, irrespective of data rate, is
required, with updates propagating throughout the network. Loss
response is also a fail since although loss only results in
propagation to routers that utilize that link in their routing
tables, there is no mechanism for restricting 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 in the protocol description, and node cost is a fail as
there is no provision for router quality.
"DYMO/DYMO-low"
The design of DYMO shares much with AODV, with some changes to remove
precursor lists and compact various messages. Table size is somewhat
smaller, including entries for neighboring DYMO routers and routes
passing through the router. Control overhead has been reduced
somewhat, and DYMO does not generate spurious RERRs; these messages
are only generated when a forwarding failure occurs. DYMO includes
mechanisms to add additional routing information (potentially
including router willingness), but does not define explicit policy
mechanisms for choosing links and routers along a path.
"DSR"
DSR performs source 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 is used. Routes are selected
based on hop count, with no mechanism for differentiating between
"routers".
DSR fails the table criterion because it maintains a blacklist of
neighbors with whom connectivity is not bidirectional: this scales
with O(L). DSR receives a ? on the loss criterion because some 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 the
control criterion because all control traffic is on-demand, and so is
bound to data traffic rates. 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
the node cost criterion because sources decide on whom to send
packets through, and nodes cannot announce their capabilities in this
regard. However, a new route reply option could possibly achieve
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this goal.
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.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-13 (work in
progress), April 2008.
[I-D.ietf-manet-nhdp]
Clausen, T., Dearlove, C., and J. Dean, "MANET
Neighborhood Discovery Protocol (NHDP)",
draft-ietf-manet-nhdp-06 (work in progress), March 2008.
[I-D.ietf-manet-olsrv2]
Clausen, T., Dearlove, C., and P. Jacquet, "The Optimized
Link State Routing Protocol version 2",
draft-ietf-manet-olsrv2-06 (work in progress), June 2008.
[I-D.ietf-roll-indus-routing-reqs]
Networks, D. and P. Thubert, "Industrial Routing
Requirements in Low Power and Lossy Networks",
draft-ietf-roll-indus-routing-reqs-00 (work in progress),
April 2008.
[RFC2091] Meyer, G. and S. Sherry, "Triggered Extensions to RIP to
Support Demand Circuits", RFC 2091, January 1997.
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[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
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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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