Ad-Hoc Networks

1. Ad-Hoc Networks Establishing node-to-node communication with no infrastructure needed Đorđe Trifunović, djole@europemail.com Nikola Milanović, nikola99@EUnet.yu Prof. Dr. Veljko Milutinović, vm@etf.bg.ac.yu Authors:

2. What will you learn from this tutorial? This tutorial will guide you through the following sections: • Introduction • Mobile networks • Routing in ad-hoc networks • Security in ad-hoc networks • Bluetooth • The IEEE/UB ad-hoc multihop sensor network and Bluetooth; lessons learned from the research by: • Gvozden Marinković (mgvozden@eunet.yu) • Aleksandar Radovanović (biblbroks@sezampro.yu) • Aleksandar Berić (alberic@eunet.yu) • Branislav Čukanović (chuka@beotel.yu) • Nikola Milanović (nikola99@eunet.yu)

3. Introduction to Ad-Hoc Networks Evolution of network communications – A new stage…

4. Introduction • Two basic groups of ad-hoc networks: • Networks of mobile computers handled by users • Wireless sensor networks • Basic characteristic: ability to establish network communication between hosts without any infrastructure needed. • The most significant advance compared to classic fixed systems • Reveals a very large scale of new possibilities

5. Introduction • Ad-hoc networks may be considered as a new stage in evolution of network communications • Fixed computer networks: • Concepts and mechanisms educed and amended for long time… • Lot of experiences acquired… • A useful base for origin and development of mobile networks...

6. Introduction • Wireless communication has its peculiarities. • Taking the already developed solutions is not possible. • In the beginning: modifications and adaptations of existing mechanisms. • Later: more and more of new ideas, relieved from the ballast of obsolete concepts.

7. Mobile Networks Meet the family…

8. Mobile Networks • Not long time ago, mobile networks were treatedjust as extensions of fixed networks. • Actors are: • Fixed hosts (FH) • Mobile hosts (MH) • Base stations (also known as mobility support routers – MSR)

9. MSR MH MH MH Mobile Networks Mobility Support Router (MSR) • Every MSR supports the area limited by its range (wireless cell). • MSR can communicate with MHs currently located in its cell. • MHs directly communicate only with MSRs • MHs can freely move from one cell to another

10. FH FH FH FH MSR MH MH Mobile Networks Sending packets from FH to MH • MSRs are bridges between the wired network and mobile hosts. • When some fixed host (FH) wants to send a packet to a mobile host (MH), communication is divided into two parts: • Standard communication inside the fixed network,from FH to the proper MSR; • Wireless communication between MSR and MH.

11. FH FH FH FH MSR MH MH Mobile Networks Hiding mobility • Mediation of MSR is entirely transparent to FH. • For this kind of communication, indirect protocols were developed, with the purpose of hiding mobility from immobile hosts.

12. Mobile Networks One-hop wireless communication • Direct communication between mobile hosts does not exist • Mobility is very limited and dependant on the existing wired infrastructure: • MHs can move only within areas determined by the range of MSR, as well as by the range of its own transmitter; • Mobility is limited to only one hopbetween mobile host and fixed network • Anyhow, a step towards real, multihop wireless networks. • Nature of wireless communications had to be considered • Later was of great benefit for development of ad-hoc networks

13. Mobile Networks Nature of wireless communications • Wireless vs wired links: • Slower • Less reliable • Prone to loss of signal due to noise and fading • With much more limited bandwidth • With much more frequent occurrence of asymmetric quality of communication • Mobile hosts are often disconnected from the fixed network for short or long periods of time: • Moving out of range • Exhausted battery • …

14. Mobile Networks Realizations and usage possibilities • Wireless Local Area Network (WLAN); • Connecting mobile and portable computers to existing widely used fixed networks like the Internet; • …

15. Mobile Networks Drawbacks and limitations • Infrastructure is needed… • Requires large investments • Time consuming installation • Communication cannot be always established where needed • Expensive maintenance

16. Mobile Networks • In many cases it is necessary to establish a connection even if infrastructure does not exist, or is damaged. • Typical example: alarming rescuers in case of earthquake, flood, war destruction… • Communication must be established without any preliminary setup (ad-hoc).

17. Mobile Networks Ad-Hoc Networks • Mobile hosts can communicate between each other on much greater distances than covered by their ranges. • That is practicable thanks to presence of other mobile hosts that can be reached by the source host, and that are willing to retransmit its packets further on • Thus, propagating from one MH to another, packets are conveyed to the destination • That is how multihop wireless communication through a temporally formed ad-hoc network is realized.

18. Routing in Ad-Hoc Networks How to find the right way?

19. Routing in Ad-Hoc Networks • Efficient routing of packets • In conventional networks, the most widely used routing algorithms are such as distant vector or link state • Periodical broadcast, with the purpose of keeping routing tables up-to-date • In some cases those algorithms were adapted to be used in ad-hoc networks • We will just mention two representatives: • Destination-Sequenced Distance-Vector (DSDV) • Wireless Routing Protocol (WRP) • Benefit: Route to every host in the network is always known. But…

20. Routing in Ad-Hoc Networks • Drawbacks of adapted conventional routing algorithmsseem to be of much more significance than the benefits: • Large bandwidth overhead • Batteries quickly become exhausted • Significantly reduced scalability • Unneeded cumulation of redundant routes • Often not able to quickly enough respond to dynamics of changes in systems in which the hosts can move

21. Routing in Ad-Hoc Networks On-demand routing protocols • Because of specified constraints of said solutions,we are going to pay more attention on another approach,which is fundamental for the so-called on demandrouting protocols. • We will shortly describe three of those protocols, which attack the problem from different standpoints,introducing different assumptions and diversely prioritising problems that are to be solved: • Dynamic Source Routing (DSR) • Ad-Hoc On-Demand Distance Vector Routing (AODV) • Temporally Oriented Routing Algorithm (TORA)

22. Routing in Ad-Hoc Networks • All the proposed solutions contribute to the apparent reclamation of performance, compared to classic algorithms, which work much better in the stationary environment for which they were designed in the first place. • Algorithms that will be presented here are just specimens of a large number of solutions developed by now (ietf.org). • New algorithms are still being developed and evolved.

23. Routing Protocols - DSR 1. Dynamic Source Routing (DSR) • Based on the concept of source routing: • Sender provides the sequence of nodes through which the packets will be sent • Sequences are held in route cachethat every host must maintain for itself • Route is determined dynamically, when it is needed: • There are no periodical advertisements of routers • Instead, every host initiates route discoverywhen it needs to send a packet to another host for which initiator does not have the associated route in its cache

24. 5 3 1 1,3,4 1,3 1,3,4,5 1 4 src 1,2 1 1,3,4 6 2 dst Routing Protocols - DSR Route Discovery – route request • Initiated by sending a route request packet: • Propagates through the network until it reaches the destinationhost (if the route exists); • On its way, it collects addresses of all visited hosts, and stores them into its route record;

25. 5 (1,3,4,6) 3 (1,3,4,6) 1 4 (1,3,4,6) 6 2 Routing Protocols - DSR Route Discovery – route reply • The first route request packet that arrives to destination is accepted, its routerecord is copied and returned to the initiator using the route reply packet. • Destination host returns the route reply to the initiator of route discovery, using the route from its own cache.

26. Routing Protocols - DSR Route Discovery – route reply (2) • If destination host does not have a route to the source host in its cache, there are two options: • Route reply is returned using inverse route that was found by the routerequest packet; • Destinationhost initiates routediscovery to find a route to the original initiator. • First option requires symmetric links: • Transfer quality must be the same in both directions; • But that is often not the fact in mobile communications.

27. Routing Protocols - DSR Route Discovery – route reply (3) • Second opportunity (inverse route discovery, from destination to source) is more significant: • Providing support for non-symmetric links (very important merit of this algorithm). • I that case, the original route reply must be sent together with new route request, i.e. attached to it (that is called piggybacking)

28. Routing Protocols - DSR Route Maintenance • Implemented by acknowledgements and route error packets. • Acknowledgements may be: • hop-by-hop – links must be symmetric • end-by-end – important when links are not symmetric

29. err err ack ack Routing Protocols - DSR Route Maintenance (2) • When using hop-by-hop acknowledgement: • Host which did not get acknowledgement for its retransmissionsends route error packet with information about hop that broke down; • Upon that error packet, source host truncates routing treebeing held in its cache, at the point of that hop: • When using end-by-end acknowledgement: • Information about the point of breakage does not exist; • Sourcehost may only assume that the last hop is broken. ?

30. Routing Protocols - DSR Modifications / Optimisations • Various modifications and amendments of this algorithm are feasible. • We’ll mention just one of them – capability of working in the so-called promiscuous receive mode: • Host auscultate packets that were sent to other hosts, and updates its own cache according to the information thus received; • This, however, causes more power to be used and more rapid battery discharge .

31. Routing Protocols - DSR Summary – DSR merits • Ability to work with asymmetric links. • No periodical routing advertisement: • Enables bandwidth and energy conservation; • Overhead does not exist when there are no changes in the network. • Can be easily improved to become able for providing multiple routes: • That way, it is not always necessary to initiate new route discovery when some link breaks.

32. Routing Protocols - DSR Summary – DSR drawbacks Caused by the nature of source routing. • Large bandwidth overhead: • Route request packets rapidly grow as they propagate through the network(in their route records they store information about every host over which they passed); • That causes potential huge route reply packets; • Also larger message packets, because addressing demands the whole route to be specified. • Scalability problems – acceptable size of the network is limited: • Diameter of the network(the largest number of hops needed for communication between any two hosts in the network)directly refers to bandwidth overhead.

33. Routing Protocols - DSR Summary • Dynamic Source Routing protocol is suitable for appliance in ad-hoc networks: • with moderate numbers of mobile hosts; • which move with moderate velocities.

34. src Routing Protocols - AODV 2. Ad-Hoc On-Demand Distance Vector Routing (AODV) • New route is discovered in a manner that looks similar to route discovery by DSR: • Source host (src) broadcasts route request (RREQ) to all of its neighbours when needs to discover route to some destination host (dst); • Then, it waits for route reply (RREP). • But similarity is discontinued at this point. ?

35. Routing Protocols - AODV Route Request • Sequence number • Number that every host generates for itself. • It is incremented every time when something is changed in adjacency (e.g., when some link breaks). • For every route, destination sequence number (DSN) is stored in the routing table • Last DSN that src earlier knew for any route to dst, is sent in RREQ, together with current sequence number of srcand other information needed:RREQ (src, dst, srcSN, dstDSN, … )

36. RREQ RREP route tosrc route todst Routing Protocols - AODV • RREQ does not contain the route record: • Does not collect information about hosts through which it propagates; • Remembers only the number of hops. • Instead, the host through which RREQ propagates adds inverse route (towards src) to its routing table: • Stores, together with other relevant information, the address of the neighbour (n1) that sent RREQ to it; • If that host later receives relevant RREP, it will automatically know that reply should be transferred to the neighbour (n1); • In that case, it also records the address of the neighbour (n2) that sent RREP, thus establishing route towards dst. n1 n2

37. DSN 6: 4,6 6: 3,4,6 5 5 6: 6 3 3 AODV 1 1 4 4 6: 4 6 6 6: 3 2 2 6: 6 Routing Protocols - AODV • Instead of recording the whole route, as with DSR applied, host here keeps only next hop (among other relevant information about some destination), i.e. address of its neighbour to which it transfers packets addressed to the destination:

38. DSN=10 DSN=10 DSN=10 DSN=12 Routing Protocols - AODV Route Reply • When RREQ reaches a host that has a route to dst, comparison of DSNs from the packet and from the routing table is made: • If DSN from RREQ is greater  the host’s route to dst is not recent enough  the host rebroadcasts the request; • Otherwise, the host returns RREP to src, with the calculated information about the discovered route (total hop count, lifetime that remains…), among which more recent DSN, copied from the routing table of the host. DSN(dst)=8 DSN(dst)=12

39. Routing Protocols - AODV • RREQ may reach dst itself, and then dst returns RREP to src. • Anyway, RREP is returned using inverse route formed by intermediate hosts during the propagation of RREQ. dst src

40. Routing Protocols - AODV Route Maintenance • For every route that a host is acquainted with, it maintains the list of neighbours that use that route, so that it is able to notice them about eventual link breakage on the route. • Link breakage is detected by the absence of hellomessages, which must be emitted by every host after the specified time interval expires.

41. Routing Protocols - AODV Summary – Advantages of AODV over DSR • Significantly smaller network bandwidth overhead: • Both control and message packets are smaller; • The reason is the requirement of only two addresses when routing (destination and next hop), instead of the whole route as with sequenced routing; • This is good for scalability, because the size of a packet does not depend on the network diameter. • Provides support for multicasting.

42. Routing Protocols - AODV Summary – AODV drawbacks • Works only with symmetric links. • Hosts must periodically advertise hello messages: • Increased bandwidth overhead; • Reduced possibility of energy conservation by remaining in the sleep mode. • Does not support multi path routing(offers only one route per destination): • Every time when some link on the route breaks, new route must be discovered; • Increased probability of congestion.

43. Routing Protocols - TORA 3. Temporally OrientedRouting Algorithm (TORA) • Offers an interesting approach to problem solution. • Conceived as link-reversal algorithm. • The idea is to define topology of a network using a directed acyclic graph (DAG): • Hosts represented as nodes and with directed links; • Direction of link is realized by assigning height to every node, so that the link is directed from the node with greater height to the node with lower height.

44. Routing Protocols - TORA General idea • The destination node should have the minimal height in the graph. • Other nodes get greater and greater height as the distance from the destination grows. • Packets may be sent only from “higher” to “lower” nodes, i.e., only via downstream links.

45. Routing Protocols - TORA DAG Forming • Starts when node that does not have downstream links wants to send a packet to a destination node. • Initially, all nodes in the graph have undetermined height (NULL), except the destination node that has the height of ZERO (which is considered less even from NULL). • Sourcenode then broadcasts QRY packet to its neighbours. • QRY packet propagates through the network, marking every node over which it passes as “interested for route discovery” by setting its route request flag.

46. Routing Protocols - TORA • When QRY packet arrives to a node that has at least one downstream link, the node then emits the UPD packet. • UPD propagates back through the network, setting the height to all nodes with the routerequestflag set,at the same time resetting those flags. • Every further node gets greater height then the precedent one on the path of the UPD propagation. dst src

47. Routing Protocols - TORA • Many downstream links can lead to the same destination. • Algorithm enables multiple path routing.

48. Routing Protocols - TORA • In case of link break: • If the node still has downstream links left, no action is performed • Otherwise, the node broadcasts a UPD packet, thus recovering DAG • Recovering is a one-pass process, except in the case of network partitioning

49. Routing Protocols - TORA Advantages of TORA: • Fast route discovery • Multiple path routing • Recovering is localised • Multicast support • Lightweight Adaptive Multicast (LAM) algorithm

50. Routing Protocols - TORA Downsides of TORA: • Requires external timing mechanism (GPS…) • DAG becomes less optimal as the time passes • Can be solved using refresh packets