CPSC 689: Discrete Algorithms for Mobile and Wireless Systems

1
CPSC 689 Discrete Algorithms for Mobile and
Wireless Systems

• Spring 2009
• Prof. Jennifer Welch

2
Lecture 14

• Topic
• Routing in Mobile Wireless Networks
• Sources
• Karp. Multi-hop wireless networks.
• Vaidya, Chapter 6.
• Schiller, Chapter 8.
• Johnson, Maltz. Dynamic source routing in ad hoc
  wireless networks.
• Perkins, Royer. Ad hoc on-demand distance-vector
  routing.
• Chen, Murphy. Disconnected transitive
  communication.
• MIT 6.885 Fall 2008 slides

3
Point-to-Point Routing

• New problem Deliver messages from a particular
  source node S to a particular destination node D,
  in a mobile ad hoc network.
• Main issue S, and other nodes, dont
  (initially) know where D is.
• Similar to the basic IP routing service provided
  in the Internet send message to a particular IP
  address.
• Easier in Internet fixed infrastructure, with
  wires, routers, backbone networks,
• But still important in wireless setting.
• Very widely studied, proposals currently being
  put forth for practical routing methods.

4
Overview of Routing

• Internet-style approaches, not taking advantage
  of the wireless setting
• Destination-Sequenced Distance Vector (DSDV)
• Dynamic Source Routing (DSR)
• Ad-Hoc On-Demand Distance Vector Routing (AODV)
• A simple algorithm that tries to take advantage
  of mobility.
• Link-reversal algorithms
• Gafni, Bertsekas 81
• Temporally-Ordered Routing Algorithm (TORA)
• Location-free routing
• Routing using some substitute for geography.
• Geographical routing without location
  information, Gradient Routing (GLIDER), Beacon
  Vector Routing
• Location services
• Keeping track of geographical locations of mobile
  nodes.
• Awerbuch, Peleg 91, GRID, LLS
• Location-based routing
• Routing using actual geography.
• Geocasting, location-aided routing (LAR), compass
  routing, GPSR,

5
The Setting

• Wireless networks, no infrastructure, no
  hierarchical structure.
• Dynamic
• Stationary and/or mobile nodes.
• Failure/recoveries
• Scale Small or large, even Internet-scale.
• Typical examples
• Rooftop networks, of customer-owned radios
  stationary, large-scale.
• Floating sensors mobile, large-scale.
• Ad-hoc meetings, rescue workers in disaster
  areas, soldiers in battle

6
The Routing Problem

• Sender S anywhere in the network wants to send a
  message to a particular node D somewhere else in
  the network.
• Doesnt know where D is.
• Subproblem Set up a route (end-to-end path) to
  that node.
• Routes may need to change, in response to network
  changes.
• Some routing strategies don't set up routes, just
  send messages one step at a time.
• Real goal is delivering messages, not setting up
  routes.

7
Criteria

• Criteria
• Accommodate large scale, mobility
• Minimize communication overhead for sending data
  messages
• Maximize packet delivery success rate
• Minimize latency of packet delivery
• Minimize route length (e.g., number of hops)
• Minimize amount of state that must be maintained
  at each node
• Usual strategies for coping with scale
• Hierarchy Good when level boundaries are
  relatively fixed, can be chosen by administrative
  authority.
• Caching (e.g., for routes)
• But these don't work so well for dynamic or
  unstructured networks.

8
Reuse?

• Could use traditional protocols for wired
  networks in wireless networks
• Establish abstract network with point-to-point
  links.
• Define links using Neighbor Discovery protocol,
  based on periodic Hello messages.
• Run traditional protocols over the abstract
  network.
• But resulting protocols may not perform very
  well
• Links may be unreliable, failing and recovering
  often.
• May cause routes to break frequently, hurting
  performance.
• Point-to-point link abstraction doesnt fit
  wireless networks well, since they use local
  broadcast with interference.
• Need to adapt protocols, design new protocols.

9
Proactive vs. Reactive Protocols

• Proactive
• Maintain routes in the background, regardless of
  current data-sending activity.
• High overhead.
• Low latency for message forwarding.
• Reactive
• Establish and maintain routes only when needed
  for data-sending.
• Low overhead, if not too much data activity.
• May have high latency for message forwarding.

10
Proactive Protocols

• Main methods Link-State Routing,
  Distance-Vector Routing
• Adaptations of wired protocols, treating every
  node as a router.
• Try to establish and maintain routes with fewest
  hops (shortest paths).
• Use O(n) local state per node router.
• Require that, periodically, or in response to
  link changes, information must be propagated to
  all nodes.

11
Link-State Routing

• Used in Open Shortest Path First (OSPF) Internet
  protocol.
• Can adapt to use in wireless network, using local
  broadcast.
• Each node periodically broadcasts information
  about the status of its adjacent links,
  throughout the network.
• Every node maintains a complete picture of the
  network, to use for message routing.

12
Link-State Routing, More Details

• Each node periodically constructs a link state
  packet describing the status of all its adjacent
  links, up or down.
• Attaches sequence numbers, to keep track of which
  packets are more recent.
• Broadcasts the link state packets throughout the
  network, using a network-wide flooding algorithm.
• Each node collects all this information,
  assembles a complete picture of the network
  locally.
• Uses a centralized algorithm, e.g., Dijkstras
  algorithm, to determine shortest paths.
• Uses these shortest paths to fill in a next-hop
  routing table, indicating the next hop for a
  message for each destination D.

13
Link-State Routing, Optimizations

• Limit dissemination of link state packets (LSPs).
• Hazy-sighted LSP dissemination
• Propagate LSPs more frequently to shorter
  distances, less frequently to longer distances.
• Nodes keep more up-to-date about nearby links.
• Uncoordinated selective forwarding
• Propagate LSPs with some probability p.
• Dont propagate LSPs if overhear someone else
  sending it.
• Unreliable, since some neighbors might not have
  received it.
• Coordinated selective forwarding
• Decide cooperatively who should propagate.
• E.g., each node identifies a Relay Set, a subset
  of its neighbors such that each 2-hop neighbor is
  a neighbor of some node in the Relay Set.
• Propagate LSP to Relay Set nodes only.
• Requires Neighbor Discovery protocol to learn
  about 2-hop neighbors.

14
Distance-Vector Routing

• Bellman-Ford style
• Each node keeps track, for each possible
  destination D, of the shortest distance (number
  of hops) to D that it knows, RD.cost, together
  with the first hop along a shortest path,
  RD.nexthop.
• Each node periodically broadcasts to its
  neighboring nodes its distance vector, giving its
  current distances to all nodes.
• RD.cost for all D.
• Each node adjusts its distance estimates and
  first hops, based on information from its
  neighbors
• When Z receives distance d for D ? Z from
  neighbor X
• if RD.nexthop X then RD.cost d costZX
• else if d costZX lt RD.cost then
• RD.cost d costZX
• RD.nexthop X

15
Distance-Vector Routing

• When Z receives distance d for D ? Z from X
• if RD.nexthop X then RD.cost d costZX
• Update the cost estimate with newer information.
• else if d costZX lt RD.cost then
• RD.cost d costZX
• RD.nexthop X
• If a different neighbor provides a better path,
  update both the cost estimate and the nexthop.
• In addition, if Z does not hear from RD.nexthop
  for a while
• RD.cost ?
• RD.nexthop ?

16
Counting to Infinity

• Basic Distance-Vector algorithm allows counting
  to ?, if links fail
• With all links up, RAD.cost and RBD.cost
  stabilize to 1 and 2, respectively.
• Then link AD fails, setting RAD.cost ?.
• Then A hears from B, so RAD.cost 2 3 5,
  RAD.nexthop B (correct).
• Then link BD fails, setting RBD.cost ?.
• Then B hears from A, so RBD.cost 5 3 8,
  RBD.nexthop A (not correct).
• A hears from B, RAD.cost 8 3 11.
• B hears from A, RBD.cost 11 3 14.
• Etc.

17
DSDV

• Destination-Sequenced Distance-Vector Perkins,
  Bhagwat 94.
• Add mechanism to DV to prevent counting to ?.
• Source of problem in example
• As estimate of 5 was based on Bs old estimate
  of 2.
• Bs estimate is now ?.
• But B accepts As estimate.
• Mechanism to avoid using stale information in
  determining routes Associate a (destination
  sequence number, distance) tag with each routing
  table entry and each distance vector entry.
• (seqno1, dist1) better than (seqno2, dist2) if
  either
• seqno1 gt seqno2 (newer information), or
• seqno1 seqno2 and dist1 lt dist2 (better
  distance).

18
Some DSDV Details

• Somewhat complicated rules for manipulating tags,
  see Vaidya, Section 6.2.2. Roughly
• Node Z increases its own seqno (for Z itself) by
  2 each time it sends out a new distance vector.
• When node Z detects that a link to X has failed,
  Z increases its sequence number for X by 1.
• When processing a received distance vector from
  X, Z accepts new information for D only if its
  associated tag is better than the tag stored in
  RZD.
• Larger seqno, or same seqno and shorter distance.

19
DSDV Guarantees

• Valid route information is always associated with
  even seqnos.
• Tags are monotonic Starting from any node, and
  following nexthop entries for a particular D, the
  tags in the R tables increase monotonically until
  we either reach D or find RD.nexhop ?.
• Implies no routing loops.
• Avoids counting to ? In example, B would not
  accept the info from A because the tag is not
  better than the entry in RBD.

20
Proactive Algorithms, Summary

• Link-State Routing, Distance-Vector Routing
• Adaptations of wired protocols, treat every node
  as a router.
• Try to establish and maintain routes with fewest
  hops.
• High overhead
• Maintain routes in the background, regardless of
  current data-sending activity.
• O(n) local state per node.
• Require that, periodically, or in response to
  link changes, information must be propagated
  throughout the network.
• Frequent network changes result in instability,
  global updates.
• Suggests reactive algorithms might work better,
  in dynamic mobile ad hoc networks
• Establish and maintain routes only when needed
  for data-sending.
• Low overhead, if not too much data activity.

21
Reactive approaches

• Dynamic Source Routing (DSR) Johnson,
  Maltz 96
• Ad-Hoc On-Demand Distance Vector Routing (AODV)
  Perkins, Royer 99

22
DSR Overview

• Reactive, uses on-demand routing.
• When S has a message for D, it tries to find a
  good route to D.
• Performs route discovery, by flooding a query
  packet to find a route to D.
• S learns the entire route.
• Attaches the entire route to each data packet it
  sends (source routing).
• Caches the route so it doesn't have to search for
  new routes for each message (or each packet).
• But, if the network is large and or/changes
  rapidly, the routes tend to break frequently,
  which makes caching less effective.

23
DSR Performance Highlights

• Claim good performance
• Low communication overhead, high reliability and
  low latency of message delivery, and
  nearly-optimal routes, under a variety of
  conditions.
• During stable periods Little overhead, can
  reuse already-determined routes. Yields
  low-latency, low-communication reliable message
  delivery.
• When network changes Adapts reasonably quickly.
  Diagnoses broken routes and finds new ones.

24
DSR Assumptions

• Nodes are willing to participate fully
  forward traffic on behalf of others, etc.
• Network diameter
• Examples in paper are fairly small.
• Suggests that this may not scale well.
• Mobility
• Hosts may move at any time
• Speed much lower than time for packet
  transmission and local processing.
• Don't move so fast as to make route computation
  useless.
• Hosts have a promiscuous receive mode, that
  allows them to receive every packet they hear,
  not filtered by destination address.

25
DSR, Basic Operation

• Message forwarding Source routing
• Sender puts entire route in packet header,
  forwards packet to first host in the route,
  successive hosts forward.
• Each host maintains a route cache, caching some
  source routes it has learned, each with an
  expiration time.
• When sender S wants to send a packet to
  destination D
• S first checks its route cache for a route to D.
• If it finds one, it uses it.
• If not, uses route discovery protocol to try to
  find one.
• Processes other packets in the meantime.
• Route maintenance
• Source S monitors correct operation of its cached
  routes if a route isn't working, removes it from
  cache.
• If a currently-used route is discovered not to be
  working, then S may do route discovery again,
  resend on the new route.

26
DSR Route Discovery

• S broadcasts route request (RREQ) packet,
  containing id pair (S,D).
• Should result in route reply (RREP) containing
  full path.
• Route discovery works by simply flooding the
  packet, looking for a path that works.
• RREQ packet contains
• (S,D)
• request id, unique id for the request, allows
  pruning out duplicates
• route record, accumulates a record of the
  sequence of hops the packet takes during
  discovery
• Processing a RREQ
• Duplicate detection Discard if duplicate,
  according to request id.
• Loop detection If your own address already
  appears in the route record, this is a loop,
  discard.
• If you are D, the route has been found send RREP
  with the final route record back to S.
• Otherwise, append your own address and
  re-broadcast.

27
Sending the RREP from D to S

• If D has a route to S in its cache, sends the
  RREP on that.
• Else, if communication is bidirectional, D sends
  the RREP along the reverse of the route in the
  RREQ packet.
• Else, D initiates a route discovery for (D,S),
  flooding a new RREQ message, with the RREP
  piggybacked on it.
• We're not counting on this second route discovery
  to provide D with a route from D to S---just
  using the flooding facilities to send the RREP
  back to S.
• When S receives this new RREQ, it puts into its
  route cache the route from S to D that appears in
  the piggybacked RREP.
• This wouldnt work without piggybacking If D
  just did route discovery for (D,S) without
  piggybacking the RREP on the new RREQ message,
  then S would not have a path to use to return the
  new RREP to D!

28
DSR Route Maintenance

• Since DSR is reactive, it does not send routing
  updates continually
• Instead, while a route is actually being used to
  send a message, a route maintenance protocol
  monitors the operation of the route, informs
  sender of any problems.
• Some mechanisms used to diagnose the failure of
  individual hops along the path
• Link-level acks, with non-occurrence reported to
  higher layers.
• Passive acks, trying to overhear the next
  transmission by the next node along the path.
• Some application-level ack information, if
  available.
• Explicit acks added to the message-forwarding
  protocol.
• A node discovering such a local failure then
  sends a Route Error (RERR) message to the sender
  S of the current message, indicating the
  particular hop that failed.
• When S receives the RERR, it truncates all the
  paths in its cache at the failed hop.

29
DSR Route Maintenance

• Q How does the diagnosing node X get the RERR
  message back to the sender S?
• A As for the RREP
• If X has a route to S in its cache, sends the
  RERR on that.
• Or, reverse the route from the packet (if
  communication is bidirectional).
• Or piggyback RERR on a new route discovery.
• Another option X performs an entire new route
  discovery for (X,S), with no piggybacking, before
  sending the RERR message.
• Q Why is this not circular?
• A Because S should have a route to X in its
  cache.
• End-to-end route maintenance
• Use explicit ack for data packet, from
  destination D to source S.
• Gives coarser information---source could delete
  only one route, not truncate many routes as for
  single-hop diagnosis.

30
DSR Optimizations

• Storing routes in the route cache
• List of routes, indexed by destination.
• Tree of routes, to different destinations.
• When to add a route to the cache?
• As a result of a successful route discovery (of
  course).
• Maybe also
• When forwarding a data packet along a route, add
  the rest of the route.
• When forwarding an RREP containing a route, add
  the relevant portion.
• Some overheard routes.
• Use route cache to avoid propagating an RREQ
• If you already have a route to the intended
  target D of the RREQ (that would not introduce a
  loop), then just append your route to the route
  that already appears in the RREQ, to obtain a
  candidate route from S to D, and send this in a
  special RREP message back to S.
• Could also gather route info from neighbors
  caches.
• Set bound on route length, truncate search at
  that bound.

31
DSR Performance

• Packet-level simulation.
• Varied parameters in the protocol, number of
  nodes, pattern/speed of movement, distribution of
  nodes in space.
• Typically Mobile hosts in a large room, moving
  at walking speeds, with 3-meter
  transmission/reception range.
• Claim the results are also valid for vehicles,
  which move faster, over a larger region, because
  parameters scale appropriately (?).
• Random initial placements, random pause, random
  new locations, random speed.
• Results
• For all but the shortest pause times, the total
  number of packets transmitted is very close to
  optimal (for communicating actual data).
• Nearly all data packets are successfully
  delivered, (because route maintenance detects
  when routes are no longer working).
• The protocol finds and uses close to optimal
  routes.

32
Ad Hoc On-Demand Distance Vector (AODV) Routing

• Another reactive protocol, like DSR.
• But not a source routing algorithm
• Does not maintain the entire route anywhere, but
  distributes its representation among the nodes on
  the route.
• Like distance-vector algorithms.
• Nodes maintain distance-vector routing tables,
  route messages according to these tables.
• Tables populated on-demand Next hops added only
  as routes are needed, time out if not used, or if
  detected as broken.
• Gives usual advantages of reactive strategies
• Low communication overhead, small storage
• Lower local storage requirements than DSR, since
  only next hop information is stored, not full
  routes.
• Combination of performance advantages makes AODV
  more scalable than DSR and DSDV.

33
AODV Assumptions

• Basically the same as DSR
• Nodes are willing to participate fully.
• Mobility is slower than routing.
• Nodes in promiscuous receive mode.
• However, unlike for DSR, the network is not
  assumed to be small.
• AODV is intended to be scalable.

34
AODV, Basic Operation

• S wants to send a message to D, but has no entry
  for D in its routing table.
• S initiates route discovery by locally
  broadcasting a RREQ, containing
• (S,D)
• request_id
• source_seqno, dest_seqno
• hop_count
• (S, request_id) uniquely identify RREQ.

35
AODV, Basic Operation

• When a node receives an RREQ
• If has already seen it (same S, request_id),
  discard.
• If have relevant information, then reply.
• Else, increment hop_count, rebroadcast, and
  record
• (S,D), request_id, source_seqno
• Who forwarded the RREQ to you,
• Expiration time

36
AODV, Route Discovery

• Relevant information
• If you are the destination D, or have a route to
  D with the same or greater destination sequence
  number.
• Reply Next slide
• For now, assume no node knows any route to D.
• As the RREQ propagates through the network, the
  information nodes record about whom they received
  it from sets up temporary reverse pointers.
• Temporary because they get discarded after
  expiration time.

37
Replying to RREQs

• Node X with relevant information replies to an
  RREQ by generating a RREP, containing
• (S,D), dest_seqno
• hop_count, number of hops in route from X to D.
• lifetime
• This gets passed back along the reverse pointers,
  unicast fashion.
• RREP's handled separately from RREQs, no
  request_id.
• For a given (S,D), X passes RREP along the
  reverse pointer iff X hasn't already passed along
  such an RREP for a larger dest_seqno, or for the
  same dest_seqno with the same or smaller
  hop_count.
• Requires keeping track of RREPs handled for each
  (S,D).
• Assumes X has at most one reverse link for each
  (S,D)
• A reverse link should mean X is on an active path
  from S to D.
• Two reverse links would mean two active paths.
• If S sends out a new RREQ for D, it means the
  previous route was broken.
• So X should discard the old reverse link in favor
  of the new one.

38
Forward Path Setup

• As the RREP is passed back, each node that
  handles it sets up the appropriate forward
  pointer in its routing table entry for D.
• Each routing table entry holds, for each D
• Number of hops to D
• Next hop
• dest_seqno
• Active neighbors for this route (neighbors who
  have sent a message recently that used it)
• Expiration time for the route table entry (if not
  used for a while)

39
Local Connectivity Management

• Suppose D moves.
• Suppose A wants to send a message to D,
  broadcasts RREQ.
• A gets RREP's from S, B, and D chooses Ds
  because it has the largest dest_seqno.
• B overhears Ds RREP, changes its route table
  entry because this RREP has a larger dest_seqno.

40
Local Connectivity Management

• Q How does C determine its path to D is broken?
• A D is supposed to send Hello messages to C
  periodically.
• In general
• Nodes on active paths send Hello messages to
  their prececessors.
• If a predecessor doesnt receive one for a while,
  it assumes the successor is gone, propagates back
  a special RREP with hop_count ? and one greater
  dest_seqno.
• The effect Anyone who has not found a more
  recent route to the destination removes the
  route from its table.

41
Local Connectivity Management

• Here, C removes its entries for this route,
  propagates RREP back to B, who already has a
  better route.
• If D had moved far away, then C's infinite RREP
  might have propagated all the way back to S,
  forcing S to rediscover from scratch.

42
AODV Simulation Results

• Simulated AODV using PARSEC event-driven,
  packet-level simulator.
• Not very realistic
• Strict range cut-off.
• Perfect physical carrier sensing.
• Always a collision (losing both messages) if two
  neighbors broadcast at the same time.
• Ran experiments with 50, 100, 500, and 1000
  nodes, to test scalability.
• 1000 is very large compared to most MANET
  simulations.
• Delivery success rate
• For few nodes, very high success rate.
• For more nodes About 25 of messages lost.
• Main reason is collisions.
• Collisions are very important factor with MANET
  routing protocols.

43
AODV Real-World Experiments

• Dartmouth experiments
• Approximately 40 students carrying laptops,
  moving randomly.
• Playing field, 225 x 365m.
• Traffic generation 1200 byte packets, 5.5
  packets per stream, 3 seconds between packets, 15
  seconds between streams (numbers derived from a
  military application prototype).
• Around 425 bytes per second modest.
• Message delivery success rate 50
• Good latency, best compared to other algorithms
  tested (ODMRP, APRL, STARA).
• Second best for route costs (hop count).
• Smallest amount of overhead.
• Issues
• Collisions.
• Fluctuating links you might think you are
  somones neighbor because you got a message, but
  the link quality is bad.
• Control traffic Aggressively seeking new routes
  can make things worse.

44
Disconnected Transitive Communication Chen,
Murphy

• Routing algorithms weve discussed assume a
  network that stays connected, though the network
  topology can change.
• Now assume the network may not always be
  connected.
• Nodes in clusters, clusters intermittently
  connected.
• For applications that can tolerate highly
  asynchronous communication.
• Minutes or hours delay, not milliseconds or
  seconds.
• Don't use to run a ssh connection.
• But perhaps to update two military camps about a
  change in the plans for a battle coming up in a
  few days.

45
Disconnected Transitive Communication Chen,
Murphy

• Routing algorithms weve discussed assume a
  network that stays connected, though the network
  topology can change.
• Now assume the network may not always be
  connected.
• Nodes in clusters, clusters intermittently
  connected.
• For applications that can tolerate highly
  asynchronous communication.
• Minutes or hours delay, not milliseconds or
  seconds.
• Don't use to run a ssh connection.
• But perhaps to update two military camps about a
  change in the plans for a battle coming up in a
  few days.

46
The Basic Idea

• Algorithm maintains no long-term or medium-term
  state, not even on-demand routes.
• Just pass the message on, hop-by-hop.
• Pass it to the node in your current cluster that
  is closest to the destination.
• Figuring out who is closest is the main trick
  might require application-layer information.

47
More Detail

• If X has a message to send or pass on to D
• X broadcasts probe throughout its component
  (multi-hop).
• Recipients test their utility for D respond if
  high enough.
• Both broadcasting and response use low-level
  protocol like DSR or AODV, within the cluster.
• X passes message on to node with highest utility.

48
Utility Measures

• History-based utility measures
• Most Recently Noticed Higher utility if you
  have noticed (encountered) D recently.
• Most Frequently Noticed Higher utility if you
  have noticed D frequently.
• Future-based
• Future Plans Assumes the application at your
  node will tell you next time it expects to see D
  (e.g., by reading a calendar).
• Status-based
• Power More power, better utility.
• Rediscovery Interval Smaller RDI means a better
  chance of discovering D or someone closer to D.
• Weighted combinations of these.
• Sender can choose its own metric, based on
  special scenario-specific information.

49
Rediscovery

• Choosing a good rediscovery interval (RDI) is
  important.
• If its too big, you may miss potential
  connections.
• Too small leads to too much traffic.
• Their solution
• Double RDI after each unsuccessful probe for new
  connections.
• If new node discovered, reset RDI to initial
  value.
• Discover new nodes using Hello messages.