1. Ad-hoc Network Routing

2. Ad-hoc Networks
Infrastructure-less wireless networks dynamically formed using only mobile hosts (no routers)
Network topology dynamic as all hosts are mobile!
Mobile hosts themselves double up as routers!!
Multi-hop paths …
Highly resource constrained
Extreme case of network mobility…

3. Applications … Military – digital battlefield Disaster relief
Sensor networks
Areas with no cellular coverage

4. Illustration A B C D

5. Routing Protocols Proactive approaches Reactive approaches
DSDV (destination sequenced distance vector), LSR (link state routing), OLSR (optimized link state routing)
Reactive approaches
DSR (dynamic source routing), AODV (ad-hoc on-demand distance vector)
Hybrid approaches
ZRP (zone routing protocol), CEDAR (Core extraction distributed ad-hoc routing)

6. Proactive routing protocols…
Unsuitable for such a dynamic n/w
For example, consider link-state routing that sends out network-wide floods for every link-state change …
Even in the absence of any existing connections, considerable overhead spent in maintaining “network state”

7. Goals Low overhead route computation
Ability to recover from frequent failures at low-cost
Scalable (with respect to mobility and number of hosts)
Robust

8. Reactive (On-demand) protocols
Compute routes only when needed
Even if network state changes, any re-computation done only when any existing connections are affected
Example:
Dynamic source routing (DSR)
Ad-hoc On-demand Distance Vector (AODV)

9. Dynamic Source Routing
Based on source routing
On-demand
Route computation performed on a per-connection basis
Source, after route computation, appends each packet with a source-route
Intermediate hosts forward packet based on source route

10. DSR – Basic Operation
When higher layer gives DSR a packet to transmit, and there exists no route in the route-cache, route computation is performed
Source S floods the network with a RREQ (route request) packet for the destination D
Every host I that receives the RREQ packet checks to see if I=D. If not, I adds its identifier to the RREQ packet header and forwards the packet

11. DSR – Basic Operation (contd.)
Each forward is a local broadcast (unlike in a wireline network where a flood would entail multiple local transmissions)
When D receives packet, the RREQ message contains the list of identifiers it traversed through
D responds to the sender S with an RREP (route reply) message containing the route in the RREQ – How?

12. DSR – RREP propagation
If MAC layer assumes bi-directionality (e.g.?), D has to use the same route (reversed) that is being conveyed to S
If not, and if network can have asymmetric links (why?), D has to use an explicitly computed route from D to S
Infinite loop possible?

13. DSR (Contd.)
When S receives the RREP message, it starts using it from thereon
Every packet from S to D has the route in its header
Intermediate hosts merely forward based on the source route

14. DSR – Route Maintenance
Consider path S-X-Y-Z-D
Let Y-Z link break (why?)
Y initiates a route error (RERR) message back to S conveying the link failure
When S receives the RERR, it switches to an alternate route (or re-issues another RREQ if none available)

15. Additional Mechanisms - 1
Caching overheard routes
Every host caches routes that it overhears (how?)
Such routes are later used if needed
Cached routes also used in the next mechanism

16. Additional Mechanisms - 2
Replying to route requests using cached routes
When an intermediate host I receives a route request for D, and has a cached route for D, it replies to the source with the cached route+the route thus far from S-I
Reduces route computation latency
Provides source with multiple route options
Cons?

17. Additional Mechanisms - 3
Preventing route reply storms
Too many route replies can be generated if reply-from-cache is enabled
Each node waits for a period t (proportional to the hop-count of cached path) before it replies
Reply suppressed if source starts using new route

18. Additional Mechanisms - 4
Expanding ring search
Instead of network wide flood, send RREQ with TTL=1
If no reply, send RREQ with TTL=2, and so on …
Reduces route computation overhead
Trade-offs?

19. Additional Mechanisms - 5
Packet salvaging
If Y-Z fails in S-X-Y-Z-D, Y sends back RERR
Y also tries to salvage packets by using any cached route it might have for D

20. Additional Mechanisms - 6
Route shortening
Consider path S-X-Y-Z-D
Let Y move within range of S (and still remains connected to Z & D)
Y issues an unsolicited route reply to S conveying shorter route (S-Y-Z-D)

21. AODV Ad-hoc on-demand distance vector
Hop-by-hop routing as opposed to source routing
On-demand
When RREQ propagates, routing tables updated at intermediate nodes (for route to source of RREQ)
When RREP is sent by destination, routing tables updated at intermediate nodes (for route to destination), and propagated back to source
Trade-offs?

22. Ad hoc Networks CHARACTERISTICS Wireless multihop network 
No wired backbone 
Dynamic Topology
(0-20m/s) 
Mobile nodes with scarce
resources 
Low bandwidth channels
(1-10 Mbps) 
Broadcast Medium
(IEEE MAC protocol) 

23. Routing Protocols Proactive approaches Reactive approaches
DSDV (destination sequenced distance vector), LSR (link state routing), OLSR (optimized link state routing)
Reactive approaches
DSR (dynamic source routing), AODV (ad-hoc on-demand distance vector)
Hybrid approaches
ZRP (zone routing protocol), CEDAR (Core extraction distributed ad-hoc routing)

24. Comparison Link state Local Low maintenance overhead
Low update overhead
High access overhead
Dynamic networks
Local
Low maintenance overhead
Low access overhead
High update overhead
Static networks
Link state

25. Broadcast Storms
When a packet is flooded in an ad-hoc network, the flood is performed through a series of local broadcasts
A series of local broadcasts results in the broadcast storm problem
Broadcast storms result in lowered throughput performance or utilization

26. Broadcast storms (contd.)
When C Forwards the flood message:
Redundant transmissions
41% additional (useful) coverage for first time re-broadcast
19% for second time
Contention with other neighbors
Collisions with transmissions of neighbors A B C D E

27. Proactive/Reactive (contd.)
Local
Link state
Low maintenance overhead
Low update overhead
High access overhead
Dynamic networks
Low maintenance overhead
Low access overhead
High update overhead
Static networks
Need for a low-cost communication infrastructure that balances access and update overheads.

28. Goals of a Communication Infrastructure
No wired backbone 
Dynamic Topology
(0-20m/s)
Mobile nodes with scarce
resources
Low bandwidth channels
(1-10 Mbps)
Broadcast Medium
(IEEE MAC protocol)
CHARACTERISTICS
GOALS
Participation of Mobile hosts
Robust and self-configuring infrastructure
Involve as few nodes as possible
Low-cost communication mechanisms
Avoid series of local broadcasts

29. CEDAR: a three tiered approach
Extract a subset of the nodes in the network to perform state management: The CORE
Support an efficient and low-cost flooding protocol for the core that avoids the broadcast storm problem: The Core Broadcast mechanism
Support a novel state propagation mechanism that uses the core broadcast: The WAVES based State Propagation

30. The Core Approximates the minimum dominating set of the network
“minimum”  small number of core nodes
dominating set  low update and access overhead
Self configuring mesh structure
mesh  robust (unlike a tree or the spine)
self-configuring  adaptive to topology change

31. The Core: Illustration
CHANNELS
NETWORK NODES

32. Core Computation Algorithm
periodically, node u broadcasts a beacon
(u,d*(u),d(u),dom(u))
if u does not have dominator, then it sets dom(u) v,
where v is the node in N1(u) with the largest value for
(d*(v),d(v),v) in lexicographic order.
u then sends v a unicast message including the following
information: (u, {(w,dom(w))|w  N1’(u)}). V then
increments d*(v).
If d*(u) > 0, then u joins core.

33. Properties of the Core
Each core node has knowledge about every core node in its 3 hop neighborhood and how to reach those nodes
Property 1: Every core node will have atleast one more core node in its 3 hop neighborhood
Property 2: A virtual graph consisting of core nodes and links between each pair of core nodes that are within 3 hops of each other will be connected

34. Core Broadcast
Mechanism for flooding on the core using only unicast transmissions
Unicast  eliminates broadcast storms
Extensions to the MAC protocol used to snoop messages, cache message IDs, and send negative clear to sends (NCTS)
reduced overhead for flooding

35. Core Broadcast: Illustration
RTS1
RTS1
CTS1 A
DATA1
CTS1
RTS1 B
NCTS
NCTS
CTS1
CTS1
Snooped
C got DATA1
RTS1
RTS1
Snooped DATA1 C

36. State Propagation
Convey non-local information to core nodes in an intelligent manner
Restrict the propagation of stale information to very few nodes
Restrict the number of core nodes to which state information is propagated based on the amount of resource
Allows CEDAR to dynamically adapt point of operation based on network dynamics

37. State Propagation using Waves
LINK GOES UP
LINK GOES DOWN

38. The Add/Delete Waves Add waves Delete waves
carries information about addition of link
travels slowly (delayed at each of its hops)
Delete waves
carries information about deletion of link
travels fast (preferential transmission)
When a delete wave catches up with an add wave, it kills the add wave
Allows CEDAR to dynamically adapt point of operation based on network dynamics

39. CEDAR: Summary Core Core broadcast Waves
involves small number of nodes
reduces update and access overhead
robust, self configuring, and low-cost maintenance
Core broadcast
eliminates broadcast storms
reduces message overhead
no global computation
Waves
intelligent propagation of non-local state
allows CEDAR to have a dynamic point of operation

40. Ad hoc Routing Protocols Revisited
DSR[CMU] & AODV[Nokia/UCSB/Sun]
DSR: source routing, AODV: distance vector
involve all nodes in the routing process
flood request in the network, destination replies back with the path traversed by the request
use series of local broadcasts to perform flood
SOLUTION: Operate DSR and AODV over CEDAR

41. Enhancing Ad hoc Routing Protocols using CEDAR
DSRCEDAR & AODVCEDAR
involve only core nodes
use core broadcast for flooding among core nodes
core broadcast does not use local broadcasts: no broadcast storms
use core for fault tolerance in the event of route failures

42. Performance Results
>15%
>25%
<50%
<40%
DSRcedar AODVcedar DSR AODV x
SCENARIO: 2200x600m, 100 nodes, 20 flows, 4Kbps-20Kbps
MOBILITY: 20 metres/second, Pause time 0, Waypoint model

43. Puzzle Monty Hall Problem
You are a contestant on a game show. In front of you are three closed doors. The game show host informs you that behind one of these doors is the motor car of your dreams, but behind the other two doors lies a peanut (which you're allergic to anyway!).
The quiz-master asks you to select a door. After you have selected, he then opens one of the other two doors that does not contain the car. He does this every week to build up the suspense for the watching millions. He asks if you would like to open the door you originally selected and take home that prize, or switch to the remaining door and go home with that prize. 
Is it in your best interests to switch, or to remain with your original selection ?