Wireless Embedded Inter-Networking Foundations Ubiquitous Sensor Networks Self-Organized Network - Routing and Forwarding

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Wireless Embedded Inter-Networking Foundations
Ubiquitous Sensor Networks Self-Organized
Network - Routing and Forwarding

• David E. Culler
• University of California, Berkeley

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Embedded IP Architecture - Network
App
DHCPv6
Tran
UDP
TCP
6LoWPAN Adaptation
Network
Autoconf
ICMPv6
Forwarder
Router
Stateless Autoconf
Unicast
Multicast
Routing Algorithm
Forwarding Table Forwarding Table


Default
Queue



Buffer
Send Manager
Routing Table Routing Table


Send Manager
Link
Media Management Control
Data
Ack
Link Stats
Remote Media
Local Media
Neighbor Table Neighbor Table Neighbor Table Neighbor Table Neighbor Table Neighbor Table
Addr Period Phase Pending RSSI Success


Sample Period
Sample Phase
Phy
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The Basic Communication Primitive

• Transmit a packet
• Received by a set of nodes
• Dynamically determined
• Depends on physical environment at the time
• and what other communication is on-going
• And further constraints by the link layer
• Each selects whether to retransmit
• Potentially after modification
• And if so, when

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Wireless Multihop Communication

• Upon each transmission, one of the recipients
  retransmits
• determined by source, by receiver, by
• on the edge of the cell

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Why Multihop Communication?

• Power!
• to transmit D grows as D3 or worse
• to route distance D grows linearly
• Bandwidth (spatial multiplexing)
• With n nodes in a single cell, each gets at most
  1/n bandwidth
• Many small cells gt many simultaneous
  transmissions.
• Reliability (spatial diversity)
• Individual links experience interference,
  obstacles, and multipath effects
• Even short-range wireless wires require human
  nurturing
• IRDA, Bluetooth, WiFi, Cell
• Provides spatial diversity and receiver diversity
• rather than antenna diversity
• Protocol level reliability

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WSN Communication Requirements

• Local neighbor communication (1 to few)
• Dissemination (1 to many)
• Data Collection and Aggregation (many to 1)
• Point-to-point Transfers (1 to 1)
• Reliably over lossy links
• At low energy (E PT)
• Idle listening, management, monitoring
• Adapting to changing conditions
• With very light memory footprint

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Embedded Network Organization
IP Sensornet
Sensor Node
IP Network (powered)
Border Router
IP Device
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Network Architecture What forms the IP Link?

• IP Protocols assume certain link properties
• Many assume a full-broadcast domain
• Everyone can communicate with each other
• Reflexive and transitive reachability
• Not ad-hoc, wireless networks
• Examples
• IPv6 Neighbor Discovery
• IPv6 Address Autoconfiguration
• ICMPv6 Redirect
• DHCPv6

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Network Architecture PAN IP Link
Personal Area Network (PAN)
Single IP Link

• PAN ? IPv6 Link-Local Scope
• Emulate reflexive and transitive reachability
• Conceivable to run existing IP-based protocols
  unmodified
• No IP-level visibility into wireless topology
• Must define subnetwork functionality

mesh-under
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Network Architecture Local Link IP Link
Personal Area Network (PAN)
Multiple IP Links

• Radio Range ? IPv6 Link-Local Scope
• IP-level visibility into link topology
• Routing metrics across other link technologies
• Utilize functionality defined by IP
• Non-reflexive and non-transitive reachability

route-over
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Multi-Hop Communication
PAN

• Short-range radios Obstructions gt Multi-hop
  Communication is often required
• i.e. Routing and Forwarding
• That is what IP does!
• Mesh-under multi-hop communication at the link
  layer
• Still needs routing to other links or other PANs
• Route-over IP routing within the PAN
• 6LoWPAN supports both

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IP-Based Multi-Hop

• IP has always done multi-hop
• Routers connect sub-networks to one another
• The sub-networks may be the same or different
  physical links
• Routers utilize routing tables to determine which
  node represents the next hop toward the
  destination
• Routing protocols establish and maintain proper
  routing tables
• Routers exchange messages with neighboring
  routers
• Different routing protocols are used in different
  situations
• RIP, OSPF, IGP, BGP, AODV, OLSR,
• IP routing over 15.4 links does not require
  additional header information at 6LoWPAN layer
• Vast body of tools to support IP routing
• Diagnosis, visibility, tracing, management
• These need to be reinvented for meshing
• IP is widely used in isolated networks too
• Broad suite of security and management tools

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Terminology

• There is no single-hop routing!
• IP routing allows hosts in one network (IP Link)
  to communicate with hosts in another
• Topology Formation determining the connectivity
  graph, i.e., the network.
• Routing Protocols and Process for establishing
  what paths are used in communicating over that
  graph and setting up tables.
• Forwarding process of receiving messages on one
  interface, looking up the next hop, and
  transmitting them on that interface
• Meshing some combination of formation, routing,
  and forwarding that occurs at the link layer (L2)
  transparently to the network layer

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Meshing vs Routing

• Conventional IP link is a full broadcast domain
• Routing connects links (i.e, networks)
• Many IP links have evolved from a broadcast
  domain to a link layer mesh with emulated
  broadcast
• ethernet gt switched ethernet
• 802.11 gt 802.11s
• Utilize high bandwidth on powered links to
  maintain the illusion of a broadcast domain
• 802.15.4 networks are limited in bandwidth and
  power so the emulation is quite visible.
• Routing at two different layers may be in
  conflict
• On-going IETF work in ROLL working group
• Routing Over Low-Power and Lossy networks

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Classical View of Routing

• Connectivity between nodes defines the network
  graph.
• Topology formation
• A Routing algorithm determines the sub-graph that
  is used for communication between nodes.
• Route formation, path selection
• Packets are forwarded from source to destination
  over the routing subgraph
• At each node in the path, determine the recipient
  of the next hop
• The selection at each hop is made based on the
  information at hand
• Sender address, current address, destination
  address, information in the packet, information
  on the node.
• Table-driven, source based, algorithmic,
• Who knows the route? Do you determine it as you
  go?

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Basic Approaches

• Link state
• Nodes shout (send) and listen (receive) to
  determine neighbor connectivity.
• Each floods this information throughout (Link
  State Advertisement) so every node has a map
  (Link state data base) of the network.
• Any node can determine the path or the next hop.
• management protocol deals with changes in
  connectivity
• Classic Example OSPF
• Distance vector
• Nodes maintain routing information about
  distance and direction to destinations
• Choose next hop by comparing the cost of routing
  through neighbors
• Cost(dest D, neighbor b) linkCost(b)
  pathCost(b,D)
• Management propagates routing information
• Sequence numbers, etc.
• Classic Example RIP

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Whats different in WSN?

• There is no a priori network graph
• It is discovered by sending packets and seeing
  who receives them.
• The link relationship is not binary.
• pairs of nodes communicate with some probability
  that is determined by many of factors.
• It is not static.
• The embedding of the network in space is
  important.
• Need to get information to travel between
  particular physical places.
• But the communication range is not a simple
  function of distance.
• addressing naming
• Flat EUID? Hierarchical IP? Topologically
  meaningful? Spatially meaningful?

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Topology Formation

• Much of the paper protocols define connectivity
  graph with unit disk model
• Link(A,B) iff dist(A,B) R
• OK for rough calculations, but not for protocol
  design
• Nearby nodes may not be able to communicate.
• Far away nodes may be able to communicate.
• Nodes that communicated in the past may not be
  able to communicate in the future.
• Nodes may have intermittent communication
  depending on external factors.
• Connectivity is determined by communication
• If B receives packet reasonably reliably from A,
  then A ? B
• If A receives packet reasonably reliably from B,
  then A ? B
• And if both are true, A ?? B

R
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Wireless Routing Protocols

• Many wireless protocols in the IP context have
  been development in the IETF MANET (Mobile Ad Hoc
  Networking) working group in the context of
  802.11 links carrying traditional TCP/IP
  point-to-point traffic.
• AODV ad hoc on-demand distance vector
• OLSR Optimized link state Routing
• DSDV - Destination Sequenced Distance Vector
• DSR Dynamic Source Routing
• TDRPF - Topology Dissemination Based on
  Reverse-Path Forwarding
• Assume a fairly classic view of connectivity
• Naïve radio

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Neighbor Communication
0
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Fundamental Primitive

• Transmit to whatever receivers happen to hear it
• This is the fundamental primitive that is buried
  underneath complex protocols like Bluetooth, but
  not made available.
• It is what make it possible to build higher level
  protocols on the link, especially IP.
• To determine connectivity,
• Local broadcast
• Respond
• on-going protocol to estimate quality of the link
• Packet reliability (sequence numbers, acks)
• Note 802.15.4 acks only from a specific
  destination
• RSSI, LQI,

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Neighbor Communication in IPv6

• Route-over this is simply transmission to the
  Link Local All Nodes multicast address FF021
• Mesh-under the entire PAN is a single IP link
  (i.e., mesh under), this concept is lost.
• At the link layer, the broadcast address is
  well-defined
• But, in a low-power link, this is no longer the
  most basic communication primitive.
• Only receive if listening
• Link local broadcast requires either more wake up
  or more coordination than link local unicast
• Unicast/Multicast distinction remains

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Simple Address-Free Flooding Protocol

• Root broadcasts a new message to local
  neighborhood
• Each node performs a simple rule
• if (new incoming msg) then
• take local action
• retransmit modified msg
• No underlying routing structure required
• The connectivity over physical space determines
  it.

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Route-Free Flood
0
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Flooding

• Route free dissemination is extremely useful in
  its own right
• Disseminate information
• Router advertisements, solicitaitons,
• Network-wide discovery
• Join,
• It is also the network primitive that most ad
  hoc protocols used to determine a route
• Flood from source till destination is reached.
• Each node records the source of the flood packet
• This is the parent in the routing tree
• Reverse the links to form the path back

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Data Collection in concept
0
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The Problems

• Flood causes tremendous contention
• Many good links missed because of collistions
• Huge amount of noise
• Many links are not symmetric

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Flood Dynamics

• Experimental Setup
• 13x13 grid of nodes
• separation 2ft
• flat open surface
• Identical length antennas, pointing vertically
  upwards.
• Fresh batteries on all nodes
• Identical orientation of all nodes
• The region was clean of external noise sources.
• Range of signal strength settings
• Log many runs

Ganesan, Krishnamachari, Woo, Culler, Estrin and
Wicker, Complex Behavior at Scale An
Experimental Study of Low-Power Wireless Sensor
Networks , UCLA Computer Science Technical Report
UCLA/CSD-TR 02-0013
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Final Tree
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Factors

• Long asymmetric links are common
• Many children
• Nodes out of range may have overlapping cells
• hidden terminal effect
• Collisions gt these nodes hear neither parent
• become stragglers
• As the tree propagates
• folds back on itself
• rebounds from the edge
• picking up these stragglers.
• Redundancy
• Geometric overlap gt lt41 additional area

Ni, S.Y., Tseng, Y.C., Chen, Y.S., Sheu, J.P.
The broadcast storm problem in a mobile ad hoc
network. MobiCom'99
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Topology Reinforcement
0
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Network Layer
Network
Autoconf
ICMPv6
Forwarder
Router
Stateless Autoconf
Unicast
Multicast
Routing Algorithm
Forwarding Table Forwarding Table


Default
Queue



Buffer
Send Manager
Routing Table Routing Table


Send Manager

• Router
• Populate the routing table with candidates
• By listening to Router Advertisement (RA)
  messages
• In LoWPAN Link quality is advertised too
• Manages entries in the forwarding table
• Forwarder
• Recv datagram on interface, lookup next hop in
  FT, request transmission to that
• Default route
• In IPv6, topology reinforcement is by RA
• Plus whatever passive observation and link level
  piggy backing

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Routing Baseline

• Routing state very limited. Routing protocol
  messages very infrequent.
• Reduce problem by
• Egress routing (default routes)
• Ingress routing (source routing)
• Any-to-any through border router or scoped
  discovery

IP Network (powered)
Border Router
IP Device
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Routing Egress Routes

• Distance-vector protocol rooted at Border Router
• Piggyback on ICMPv6 Router Advertisements
• Hop-count or other routing metric
• Version number to clear routing state
• List of next-hop candidates
• Top candidate used to configure default routes
• Select a random next-hop if default route fails

fd004
fd001
fd003
fd002
fd005
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Routing Ingress Routes

• Record route sent to subnet router anycast
• Forward using IPv6 Routing extension header
• Use 16-bit short compressed form to minimize
  overhead

fd004
1 3 fd004
fd001
fd004
fd003
fd002
fd005
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Routing Any-to-any Routes

• Route through border router by default
• Utilize scoped discovery to for nearby neighbors
• Worst-case stretch (2D) with neighboring nodes
• New stretch 2D/(s1), s discovery scope

fd004
1 3 fd004
fd001
fd003
fd002
Worst-case Stretch
fd004
fd005
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Achieving Reliability in the Face of Uncertainty
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Abstracting Uncertainty
Distance
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Which node do you route through?
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What does this mean?

• Always routing through nodes at the hairy edge
• Wherever you set the threshold, the most useful
  node will be close to it
• Topology determination is a continuous process of
  discovery and validation
• and it must be done politely
• Connectivity is determined by communication
• If B receives packet reasonably reliably from A,
  then A ? B
• If A receives packet reasonably reliably from B,
  then A ? B
• And if both are true, A ?? B

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Reliability Techniques

• The IP end-to-end architecture assumes IP links
  are 99 reliable
• Best effort says dont try to push it to 99.999
  on every link, put E2E reliability protocols over
  it when you need it, i.e., TCP
• Not that every link should send and forget.
• Good Low-power links are often lt90 reliable
• 0.9h drops awfully quickly
• Need link-level acks
• 15.4 acks not even good enough
• Filters out asymmetric links
• Need hop-by-hop retransmission
• Piggy-back net acks on link level
• Retransmission and rerouting
• Provides the cross-layer visibility

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RoutingSelecting Bi-Directional Links and
Forming Routes
Prefix Next




Prefix Next



Default
Prefix Next




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High Quality Selection

• Sort by
• Link quality estimate confidence
• Path cost (including link quality)
• Link success rate requires state
• RSSI for insertion (no prior state)
• Link success computed by link for each
  transmission
• Dynamically alter default route to
• Refresh link quality estimates
• Discover lower path costs
• Re-routing naturally does this

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Building Neighborhoods Routes

• Node transmits to some unknown set
• Candidate nbrs are sources of incoming packets
• Estimate of inbound link reliability
• Occasionally announce inbound link states
• Provides reverse link estimate to outbound
  neighbors
• Basis for cost-based routing
• Cost-based Parent Selection
• depth(me) MIN nbr(me) depth(i)
• loss(me) MIN nbr(me) loss(i)est(me,i)
• trans(me) MIN nbr(me) trans(i)etrans(me,i)
• What about nbrs that dont fit in the table?
• FIFO, LRU, Frequency

Taming the Challenges of Reliable Multihop
Routing in Sensor Networks, Alec Woo and David
Culler,  SenSys. 2003.
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Pragmatics

• Maintain at least two (preferably three)
  candidate parents
• Link level retransmission and rerouting
• Use acks to determine quality of the link
• Throw in a new candidate from time to time.
• Do not record list of children
• Insufficient memory to build the table.
• Route update message on a trickle schedule

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Adding up the pieces - footprint
ROM RAM
CC2420 Driver 3149 272
802.15.4 Encryption 1194 101
Media Access Control 330 9
Media Management Control 1348 20
6LoWPAN IPv6 2550 0
Checksums 134 0
SLAAC 216 32
DHCPv6 Client 212 3
DHCPv6 Proxy 104 2
ICMPv6 522 0
Unicast Forwarder 1158 451
Multicast Forwarder 352 4
Message Buffers 0 2048
Router 2050 106
UDP 450 6
TCP 1674 50
24038 ROM
3598 RAM
Production implementation on TI msp430/cc2420
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and reliabilityApplication Power Model
Data Rate Sensitivity (Router)
Data Rate Sensitivity (Edge)
Deployment Duty Cycle
Deployment Reliability
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Discussion

• Upcoming
• Trickle protocol
• Extension of IPv6 to support this sort of routing