CS 260: Seminar in Computer Science Topic: Mobile Ad Hoc Networks Instructor: Srikanth Krishnamurthy Tuesday, Thursday 2

1. CS 260: Seminar in Computer Science Topic: Mobile Ad Hoc Networks Instructor: Srikanth Krishnamurthy Tuesday, Thursday 2:10 p.m. – 3:30 p.m.

2. Info • My E-Mail: krish@cs.ucr.edu • Office Hours: Wednesdays 10 a.m. to 11 a.m. • No class on Thursday 4th October. • RECOMMENDED BOOK: • AD HOC NETWORKING by Charlie Perkins • Covers only Routing • A good reference. • Collection of papers – you might find them on web.

3. Course Details • Discussions based • Evaluation by means of Project • 60 % for Report • 30 % for Presentation • 10 % for Participation/ Discussion • Report Due Date: November 19th 2001 • Presentations by Students From Nov 22nd to Dec 6th. 15 Minute Presentations + 5 Minutes for Questions.

4. ABSOLUTELY NO EXTENSIONS

5. What do I expect in the project? • Could be Study Oriented • It should be excellent ! • Simulations • New stuff though! • If you are doing simulations, no point in repeating other peoples work. • Implementations • Difficult – need not be original • Lack of resource may be a problem. • Ultimately your enthusiasm is what counts. • You can do joint projects but individual presentations.

6. What I expect to Cover • Discussion on what is happening in the area of Mobile Ad Hoc Networks. • I expect great class participation. • Nothing etched in stone – open opportunities for research and innovation. • No consensus yet on the way things ought to be done. • What I leave out you might cover in your presentations. • Attendance for “your” colleague’s presentations is used as a criteria for points on “class participation”.

7. Topics • Ad Hoc Networks – An Introduction • Routing in Ad Hoc Networks • Table Driven Routing • Destination Sequenced Distance Vector (DSDV) • Source Tree Adaptive Routing (STAR) • On Demand Routing • Temporally Ordered Routing Algorithm (TORA) • Dynamic Source Routing (DSR) • Ad hoc On-demand Distance Vector (AODV)

8. Topics • Routing in Ad Hoc Networks • Hybrid Routing – Zone Routing Protocol • Location Aided Routing (LAR, DREAM) • Clustering • Open Issues – Heterogeneity, Scalability • Medium Access Control • The 802.11 Standard, What are the Issues ? • Open Problems

9. Topics • Multicasting • On Demand Multicast Routing Protocol (ODMRP) • Core Assisted Mesh Protocol (CAMP) • Wireless Ad Hoc Real-Time Multicast (WARM) • TCP for Ad hoc Networks • Challenges • Explicit Failure Notifications • Split TCP and use of Proxies • Power Management in Ad Hoc Networks – How ? Why? • Security in Ad Hoc Networks.

10. Motivation TO MAKE A DIFFERENCE 

11. Ad Hoc Networks • Ad Hoc Networks consist of peer to peer communicating nodes (possibly mobile) – no infrastructure. • Topology of the network changes dynamically – links appear and disappear dynamically. • Find application in military deployments, rescue operations, electronic classrooms etc. • Can be interfaced with the Internet. • Easy to deploy but difficult to maintain.

12. Node X Range of Node X

13. Differences from Wire-line networks • Links are unstable – break and form often • Routing Methods need to be changed. • Multicasting is more challenging • TCP may not work well. • A large number of users trying to access a “broadcast” channel. • No Centralized Controller such as a base station in cellular networks • Medium Access control is a challenge. • More difficult to make it secure !

14. Routing Preview • DISTANCE VECTOR ROUTING • Based on Bellman Ford Method • RIP is based on this ! • Let us consider an example !

15. D A Link 2 Link 4 C Link 1 Link 6 B Link 3 Link 5 E • Consider D • Initially nothing in routing table. • When it receives an update from C and E, it notes that these nodes are one hop away. • Subsequent route updates allow D to form its routing table.

16. D A Broken Link 4 Link 2 C Link 1 Link 6 Broken B Link 3 Link 5 E Network partitions into two isolated islands • Disadvantage of Distance Vector Routing is formation of loops. • Let Link 2 break, and after some time let link 3 break.

17. After Link 2 is broken, Node A routes packets to C, D, and E through Node B. • Node B detects that Link 3 is broken. • It sets the distance to nodes C, D and E to be infinity. • Let Node A in the meantime transmit a update saying that it can reach nodes C, D, and E with the appropriate costs that were existing before – i.e., via Node B. • Node B thinks it can route packets to C, D, and E via Node A. • Node A thinks it can route packets to C, D, and E, via Node B. • A routing loop is formed – Counting to Infinity problem. • Methods that were proposed to overcome this – Split Horizon and Split Horizon with poisonous reverse.

18. Destination Sequenced Distance Vector (DSDV) • Invented by Charlie Perkins and Pravin Bhagwat • Why ? • Loops are caused more often. • Split-Horizon or Split Horizon with Poisonous reverse require interface info. • In Wireless there is usually only one interface ! – Channel is broadcast in nature. • Other methods (see book/paper) require co-ordination among nodes that would be difficult if topological changes are frequent. • DSDV does not require nodes to be synchronized.

19. MAIN IDEA: Each routing table entry is tagged with a “sequence number” that is originated by the corresponding destination node in that entry. • -- Did you get it ? --- • This ensures that the “freshest” update is maintained. • In our previous example what happens ?

20. D A Broken Link 4 Link 2 C Link 1 Link 6 Broken B Link 3 Link 5 E Network partitions into two isolated islands • Node A’s update is stale !!! Sequence number indicated for nodes C,D, and E is lower than the sequence number maintained at B. Looping avoided !

21. DSDV : Nitty Gritty Details • In DSDV, each routing table update will contain for each destination node X: • The address of the destination node X • The number of hops required to reach node X • The latest sequence number information received with regards to node X • This sequence number would have “originated” at node X. • Of all the paths with the same sequence number the one with the minimal cost is chosen as the route.

22. When a link is found broken, the cost to any destination that is routed via this hop is set to infinity. • A new broadcast routing packet is created. • For each such destination a new sequence number is assigned. • This sequence number is greater than the latest sequence number generated by the destination by “one”. • When a node receives a message with an infinite cost metric, if it has an entry indicating a sequence number equal to or greater than the one advertised, it triggers a new broadcast with that sequence number.

23. This enables the node (the one which sent the initial broadcast message) to know of an alternate path to the destination. • If indeed an alternate path exists, it is found ! • It is possible that a node receives multiple updates with the same sequence number for a destination. • It is also possible due to timing skews etc. that a route with a “bad metric” is obtained before the route with “the best” metric. • Do we send an update for every metric ? • Could lead to congestion in the network !! • Remember – wireless bandwidth is scarce ---- why ?

24. Thus, each node waits for sometime before propagating an update. – hopefully an update that gives the route with the best metric is obtained before this “time”. • Thus, if a new routing update is received and the cost indicated in the update is higher than the cost of the path to that destination, currently stored then: • The new route is used for sending packets. • But the new route is not advertised until a pre-determined time. • Thus, the node maintains two routing tables : one for routing and the other to be included in advertisements. • The time up to which the node has to wait is called the “damping time”.

25. Routing Information Packets • Two types • Full Dump  Carries the entire routing table. • This is transmitted infrequently. • Incremental update  Carries only the information with regards to changes in routing tables. • Broadcasted more often.

26. An Example • In this example node X moves from its first position where it is a neighbor of node B to a new position where it is a neighbor of node E.

27. Routing Tables at Node A prior to Node X’s migration

28. The sequence number entry indicates the last sequence number received from that destination. • When Node X moves, it would generate a new route update that is sent to Node E. • Node E would propagate this to Node C, which in turn propagates this to the other nodes. • Note: In the interim many intermediate changes would have occurred (e.g. Node B detects that the link to Node X is broken) and this would trigger other route updates. • Thus, sequence numbers associated with other destinations would have changed as well.

29. Routing Tables at Node A prior to Node X’s migration

30. Purging Route Table Entries • In order to ensure that routing table entries don’t become stale, they will have to be purged if no updates are received. • An additional column is included in the routing table to indicate the time at which the previous update was received (or to indicate when the entry is to be purged). • If the entry is not refreshed in a timely fashion, the entry is purged and the corresponding cost to that destination is set to infinity. • A route update is generated to indicate this.

31. Complexity • Each node’s update has to reach every other node in the network. • Thus, if there are “n” nodes in the network, the number of messages grows as O(n2). • The memory storage at each node is O(n).

32. Advantages / Disadvantages • Simple to implement • Fairly good overhead/ storage • It may take time before a node gets to know of a change. • Frequent updates may be necessary if the topology of the network changes often. • Not scalable – limited to small / medium sized networks. • Optimal values for purge interval, interval between updates etc. may be dynamic – difficult to estimate.

33. References • Chapter 3 of Reference book. • C.Perkins and P.Bhagwat, “Highly Dynamic Destination-Sequenced Distance-Vector (DSDV) Routing for Mobile Computers”, Computer Communication Review, pp 234-244, October 1994. • http://citeseer.nj.nec.com/perkins94highly.html

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