1. Routing in Mobile Ad Hoc Networks (from Ad Hoc Networking by Charles Perkins) Thanks to Prof. Yu at Cleveland State Univ.

2. 2 Contents Network layer solutions in infrastructure-less networks  What is MANET: Mobile ad hoc (multihop) networks?  Routing algorithms for MANETs  Flooding  Dynamic Source Routing (DSR)  Ad-hoc On-Demand Distance Vector (AODV)

3. 3 Mobile Ad Hoc Networks (MANETs)  A collection of mobile hosts may form a temporary network without the aid of any established infrastructure or centralized administration (base station)  Routes between nodes may potentially contain multiple hops, thus also called multihop networks  Better space utilization than direct communication  Requires a longer latency as for a single communication pair due to more number of hops  But the overall performance improves because simultaneous transmissions are allowed as long as they are separated in space  Better energy utilization than direct communication

4. 4 Mobile Ad Hoc Networks A C B EE C B A  Multihop communication  May need to traverse multiple links to reach a destination  C is not reachable from A but can communicate via B  Major difficulty  Mobility causes route changes A graph can be used to model a MANET

5. 5 Why Ad Hoc Networks ?  Advantages  Ease of deployment  Speed of deployment  Decreased dependence on infrastructure  Applications  Personal area networking  cell phone, laptop, ear phone, wrist watch  Military environments  soldiers, tanks, planes  Civilian environments  taxi cab network  meeting rooms  sports stadiums  boats, small aircraft  Emergency operations  search-and-rescue  policing and fire fighting

6. 6 Characteristics of MANET  Characteristics  Dynamic topology  Broadcast transmission: overhearing is possible  Bidirectional connection  Limited resources: bandwidth, CPU, battery  Higher link error rate  Assumptions  Moderate movement with respect to packet transmission latency  Promiscuous receive mode  Enough number of mobile hosts: no network partitioning

7. 7 Hierarchical architecture Internet AP Router Router/ Gateway Router/ Gateway Mobile Path Handoff Router AP/ Router Wireless Mesh Backhaul New Route Old Route 802.11b/g (2.4GHz) 802.11a (5GHz)

8. 8 Contents Network layer solutions in infrastructure-less networks  What is MANET: Mobile ad hoc (multihop) networks?  Routing algorithms for MANETs  Flooding  Dynamic Source Routing (DSR)  Ad-hoc On-Demand Distance Vector (AODV)

9. 9 Flooding of Control Packets  How to reduce the scope of the route request flood ?  LAR (Localized Area Routing)  Query localization  How to reduce redundant broadcasts ?  The Broadcast Storm Problem

10. 10 Flooding for Data Delivery  Sender S broadcasts data packet P to all its neighbors  Each node receiving P forwards P to its neighbors  Sequence numbers used to avoid the possibility of forwarding the same packet more than once  Packet P reaches destination D provided that D is reachable from sender S  Node D does not forward the packet

11. 11 Flooding for Data Delivery B A S E F H J D C G I K Represents transmission of packet P Z Y Broadcast transmission M N L

12. 12 Flooding for Data Delivery B A S E F H J D C G I K Node H receives packet P from two neighbors: potential for collision Z Y M N L

13. 13 Flooding for Data Delivery B A S E F H J D C G I K Node C receives packet P from G and H, but does not forward it again, because node C has already forwarded packet P once Z Y M N L

14. 14 Flooding for Data Delivery B A S E F H J D C G I K Z Y M Nodes J and K both broadcast packet P to node D Since nodes J and K are hidden from each other, their transmissions may collide  Packet P may not be delivered to node D at all, despite the use of flooding N L

15. 15 Flooding for Data Delivery B A S E F H J D C G I K Z Y Node D does not forward packet P, because node D is the intended destination of packet P M N L

16. 16 Flooding for Data Delivery B A S E F H J D C G I K Flooding completed Nodes unreachable from S do not receive packet P (e.g., node Z) Nodes for which all paths from S go through the destination D also do not receive packet P (example: node N) Z Y M N L

17. 17 Flooding for Data Delivery B A S E F H J D C G I K Flooding may deliver packets to too many nodes (in the worst case, all nodes reachable from sender may receive the packet) Z Y M N L

18. 18 Flooding for Data Delivery: Advantages & Disadvantages  Advantages  Simplicity  May be more efficient than other protocols when rate of information transmission is low enough  Potentially higher reliability of data delivery  Because packets may be delivered to the destination on multiple paths

19. 19 Flooding for Data Delivery: Advantages & Disadvantages  Disadvantages  Potentially, very high overhead  Data packets may be delivered to too many nodes who do not need to receive them  Potentially lower reliability of data delivery  Flooding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead Broadcasting in IEEE 802.11 MAC is unreliable (no ACK)  In our example, nodes J and K may transmit to node D simultaneously, resulting in loss of the packet in this case, destination would not receive the packet at all

20. 20 Dynamic Source Routing (DSR)  Source determines the entire path to the destination when it has data packet to send  On-demand or reactive protocols  No periodic advertisements  Three control packets: RREQ, RREP, RERR  Procedures (1) Route discovery procedure (2) Route maintenance procedure (3) DSR Optimizations (Route cache)

21. 21 Assumption: Cooperative nodes Moderate speed of node Relatively small network diameter (5-10 hops) Detectable packet error Unidirectional or bidirectional link A single IP address Promiscuous mode (optional)

22. 22 A B C E D G H F A A A-B A-C A-C-E A-B-DA-B-D-G Route Discovery Route Request (RREQ) Route Reply (RREP) Initiator ID Initiator seq# Target ID Partial route RREQ FORMAT - Each node appends own identifier when forwarding RREQ - RREQ is broadcast, while RREP is unicast

23. 23 Route Discovery: at source A A needs to send to G Lookup Cache for route A to G Route found? Start Route Discovery Protocol Continue normal processing Route Discovery finished Packet in buffer? Send packet to next-hop don e Buffer packet no Write route in packet header yes no wait

24. 24 Route Discovery: At an intermediate node Accept route request packet in recently seen requests list? Discard route request yes no Host’s address already in partial route Discard route request yes Store in list Broadcast packet Send route reply packet done myAdd r=targ et no Append myAddr to partial route no yes

25. 25 Route Reply in DSR  Route Reply can be sent by reversing the route in Route Request (RREQ) only if links are guaranteed to be bi-directional  To ensure this, RREQ should be forwarded only if it received on a link that is known to be bi-directional  If unidirectional (asymmetric) links are allowed, then RREP may need a route discovery for A from node G  Unless node G already knows a route to node A  If a route discovery is initiated by G for a route to A, then the Route Reply is piggybacked on the Route Request from G.  If IEEE 802.11 MAC is used to send data, then links have to be bi-directional (since Ack is used)

26. 26 Now, Route is Discovered  Node A on receiving RREP, caches the route included in the RREP  When node A sends a data packet to G, the entire route is included in the packet header  hence the name source routing  Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded

27. 27 A B C E D G H F Route Maintenance G RERR Route Cache (A) G: A, B, D, G G: A, C, E, H, G F: C, E, F

28. 28 Additional feature #1: Caching Overheard Routes ABCDE VWXYZ Node A Cache E: A, B, C, D, E Node C Cache E: C, D, E E: C, D, E A: C, B, A E: C, D, E A: C, B, A Z: C, X, Y, Z V: C, X, W, V

29. 29 Route Caching: Beware!  When sending a RREQ  Send first with TTL=1 hoping that nearby hosts have an entry: non-propagating route request  If no reply for some time, send RREQ with maximum TTL  This is called “Expand Ring Search”  Beware  Stale caches can adversely affect performance  With passage of time and host mobility, cached routes may become invalid  A sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route  Adverse impact on TCP

30. 30 Additional feature #2: RREP with Cached Routes A B C E D H F G RERR Route Cache (A) G: A, B, D, G F: C, E, F RREQ Route Cache (C) G: C, E, H, G RREP G:A,C,E,H,G Route Cache (C) G: C, E, D, G RREQ (! D-G) RREQ (! D-G) RREQ (! D-G)

31. 31 A B C E D G H F Additional feature #3: Packet Salvage G RERR Route Cache (D) G: D, E, H, G Caution: No double salvage allowed !!!

32. 32 A Summary of DSR Entirely on-demand, potentially zero control message overhead Trivially loop-free with source routing Conceptually supports unidirectional links as well as bidirectional links High packet delays/jitters associated with on-demand routing Space overhead in packets and route caches Promiscuous mode operations consume excessive amount of power Network partition problem - Message explosion problem (large number of route request packets)  Use exponential backoff to limit the rate at which new route discoveries may be initiated for the same target

33. 33 Contents Network layer solutions in infrastructure-less networks  What is MANET: Mobile ad hoc (multihop) networks?  Routing algorithms for MANETs  Flooding  Dynamic Source Routing (DSR)  Ad-hoc On-Demand Distance Vector (AODV)

34. 34 Ad Hoc On-Demand Distance Vector Routing (AODV)  DSR vs DSDV  DSR typically outperforms DSDV because periodic control messages are not needed  A main drawback of DSR is the large packet header (entire route information)  AODV = DSR + DSDV  Routes are discovered only when there are packets (on- demand) as in DSR  Nodes main distance and next node vectors as in DSDV to reduce the header size * Explained very well in C.E.Perkins’ Ad Hoc Networking book

35. 35 A B C E D G H F A A A-B A-C A-C-E A-B-DA-B-D-G Route Discovery in DSR Route Request (RREQ) Route Reply (RREP) Initiator ID Initiator seq# Target ID Partial route RREQ FORMAT - Each node appends own identifier when forwarding RREQ

36. 36 A B C E D G H F A, G, 1 A, G, 2 A, G, 3 G, A, 3 G, A, 2 G, A, 1 Route Discovery in AODV Route Request (RREQ) Route Reply (RREP) Initiator ID Target ID Hop count RREQ FORMAT - Each node remembers where the packet came from (reverse path) during RREQ. - It set up forward path during RREP.

37. 37 Reverse Path Setup in AODV B A S E F H J D C G I K Z Y M N L

38. 38 Forward Path Setup in AODV B A S E F H J D C G I K Z Y M N L Forward links are setup when RREP travels along the reverse path Those paths are not included in the header of each data packet but stored in each node as a routing table or DV

39. 39 Link State Routing  LSR  Each node periodically floods status of its links  Each node re-broadcasts link state information received from its neighbor  Each node keeps track of link state information received from other nodes  Each node uses above information to determine next hop to each destination  Direct application of LSR in a MANET is difficult due to a large control overhead  Due to mobility, broadcast period cannot be reduced as in wired network

40. 40 Routing in MANET (Unicast)  Many protocols have been proposed  Some have been invented specifically for MANET  Others are adapted from previously proposed protocols for wired networks  No single protocol works well in all environments  some attempts made to develop adaptive protocols  Proactive protocols (or called “Table-based”)  Determine routes independent of traffic pattern  Traditional LS and DV routing protocols are proactive  Reactive protocols (or called “On-demand”)  Maintain routes only if needed (DSR)  Hybrid protocols

41. 41 Reactive Routing Protocols  Flooding  Data packets are flooded to the destination  But also to all other nodes => much overhead but more reliable  DSR (Dynamic Source Routing)  First, small control packet is flooded to discover a route  Then, unicast data packets along the discovered route  But each data packet includes the information on the entire path in its packet header => larger packet size  AODV (Ad-hoc On-demand Distance Vector)  Route discovery as in DSR but each node keeps a routing table as in proactive algorithm to reduce the packet size  Reactive + Proactive

42. 42 Trade-Off between Proactive and Reactive Protocols  Latency of route discovery  Proactive protocols may have lower latency since routes are maintained at all times  Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y  Overhead of route discovery/maintenance  Reactive protocols may have lower overhead since routes are determined only if needed  Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating  Which approach achieves a better trade-off depends on the traffic and mobility patterns