Lecture 8

1. Lecture 8 Advance Topics in Networking

2. Wireless Routing Protocols Overview Routing protocols are the engines or brains of mesh networking Routing protocols take care of ... Node discovery Border discovery Link metrics Route calculation Address management Uplink/backhaul management

3. Routing Protocols Types Proactive: OLSR (Optimized Link State Protocol) B.A.T.M.A.N. (Better Approach to Mobile Ad-Hoc Networking) Reactive: AODV (Ad-hoc on Demand Distance Vector) SrcRR (MIT Roofnet) Hybrid: HSLS (Hazy Sighted Link State Routing, CuWin) These are just some of the most relevant protocols in our context ... there are many other protocols!

4. Link State Routing

5. Generic Link State Routing • Each node monitors neighbors/local links and advertises them to the network • Usually state of local links is sent periodically • Must be re-sent because of non-reliable delivery and possible joins/merges • Each node maintains the full graph by collecting the updates from all other nodes • The set of all links forms the complete graph • Routing is performed using shortest path computations on the graph

6. Basic Hello Protocol Description • Used for neighbor discovery • Basic version • Each node sends a hello/beacon message periodically containing its id • Neighbors are discovered by hearing a hello from a previously un-heard from node • Neighbors are maintained by continuing to hear their periodic hello messages • Neighbors are considered lost if a number of their hello messages are no longer heard (typically two)

7. Basic Hello Protocol Properties • Very simple method of discovering neighbors • Generates constant overhead • Basic hello packets are very small • Not scalable in very dense networks where nodes have a large number of neighbors

8. Enhanced Hello Protocols • Hello message also carries neighbor list • Advantages • Enables symmetric link discovery • A symmetric link neighbor will include your id in its neighbor list • Enables two hop topology discovery • Useful for connected dominating set calculation • Used by OLSR for Multi-Point Relay (MPR) computation • Disadvantages • Significantly larger hello message size in dense networks • Differential hello protocol used in TBRPF to reduce overhead

9. Generic Link State Properties • Generic Link State Protocol • Basic hello protocol for neighbor/link discovery • Normal flooding for periodic link advertisement • Problems • Inefficient under mobility • Every link change causes two floods (from either endpoint) • Movement of a single node causes multiple link changes • Inefficient in static case • All links must be periodically advertised

10. Strategies for Optimizing Link State • Hello protocol enhancement (differential) • Flooding protocol enhancement • Reduces the cost of flooding by using knowledge of topology • Multi-Point Relay (OLSR) • Topology Based Reverse Path Forwarding (TBRPF) • Topology Filtering • Reduce the number of advertised links to less than the complete graph • Nearsighted Updates (Fisheye & HSLS) • Scoped flooding for advertisements • Update nearby nodes quickly & far nodes more slowly

11. OLSR • The Optimized Link State Routing Protocol (OLSR)[is an IP routing protocol which is optimized for mobile ad-hoc networks but can also be used on other wireless ad-hoc networks. • OLSR is a proactive link-state routing protocol which uses Hello and Topology Control (TC) messages to discover and then disseminate link state information throughout the mobile ad-hoc network. • Individual nodes use this topology information to compute next hop destinations for all nodes in the network using shortest hop forwarding paths • Link-state routing protocols such as OSPF and IS-IS elect a designated router on every link in order to perform flooding of topology information. • In wireless ad-hoc networks, there is different notion of a link, packets can and do go out the same interface; hence, a different approach is needed in order to optimize the flooding process. • Using Hello messages the OLSR protocol at each node discovers 2-hop neighbor information and performs a distributed election of a set of multipoint distribution relays (MPRs).

12. OLSR (Cont) • Nodes select MPRs such that there exists a path to each of its 2-hop neighbors via a node selected as an MPR. • These MPR nodes then source and forward TC messages. • This functioning of MPRs makes OLSR unique from other link state routing protocols in a few different ways: • The forwarding path for TC messages is not shared among all nodes but varies depending on the source, • Only a subset of nodes source link state information, not all links of a node are advertised but only those which represent MPR selections. • Since link-state routing requires the topology database to be synchronized across the network, OSPF and IS-IS perform topology flooding using a reliable algorithm. • Such an algorithm is very difficult to design for ad-hoc wireless networks, so OLSR doesn't bother with reliability; it simply floods topology data often enough to make sure that the database does not remain unsynchronized for extended periods of time.

13. OLSR (Cont) • Being a proactive protocol, routes to all destinations within the network are known and maintained before use. • Having the routes available within the standard routing table can be useful for some systems and network applications as there is no route discovery delay associated with finding a new route. • The routing overhead generated, while generally greater than that of a reactive protocol, does not increase with the number of routes being used. • Default and network routes can be injected into the system by HNA messages allowing for connection to the internet or other networks within the OLSR MANET cloud. Network routes are something which reactive protocols do not currently execute well. • Timeout values and validity information is contained within the messages conveying information allowing for differing timer values to be used at differing nodes.

14. OLSR (Cont) • The original definition of OLSR does not include any provisions for sensing of link quality; it simply assumes that a link is up if a number of hello packets have been received recently. This assumes that links are bi-modal (either working or failed), which is not necessarily the case on wireless networks, where links often exhibit intermediate rates of packet loss. Implementations such as the open source OLSRd have been extended with link quality sensing. • Being a proactive protocol, OLSR uses power and network resources in order to propagate data about possibly unused routes. While this is not a problem for wired access points, and laptops, it makes OLSR unsuitable for sensor networks which try to sleep most of the time. For small scale wired access points with low CPU power, the open source OLSRd project showed that large scale mesh networks can run with OLSRd on thousands of nodes with very little CPU power on 200MHz embedded devices. • Being a link-state protocol, OLSR requires a reasonably large amount of bandwidth and CPU power to compute optimal paths in the network. In the typical networks where OLSR is used (which rarely exceed a few hundreds of nodes), this does not appear to be a problem.

15. AODV • Ad hoc On-Demand Distance Vector (AODV) Routing is a routing protocol for mobile ad hoc networks (MANETs) and other wireless ad-hoc networks. • It was jointly developed in Nokia Research Center of University of California, Santa Barbara and University of Cincinnati by C. Perkins and S. Das. • AODV is capable of both unicast and multicast routing. • It is a reactive routing protocol, meaning that it establishes a route to a destination only on demand. • In contrast, the most common routing protocols of the Internet are proactive, meaning they find routing paths independently of the usage of the paths. • AODV is, as the name indicates, a distance-vector routing protocol. • AODV avoids the counting-to-infinity problem of other distance-vector protocols by using sequence numbers on route updates, a technique pioneered by DSDV.

16. AODV (Cont) • In AODV, the network is silent until a connection is needed. • At that point the network node that needs a connection broadcasts a request for connection. • Other AODV nodes forward this message, and record the node that they heard it from, creating an explosion of temporary routes back to the needy node. • When a node receives such a message and already has a route to the desired node, it sends a message backwards through a temporary route to the requesting node. • The needy node then begins using the route that has the least number of hops through other nodes. • Unused entries in the routing tables are recycled after a time. • When a link fails, a routing error is passed back to a transmitting node, and the process repeats. • The advantage of AODV is that it creates no extra traffic for communication along existing links. Also, distance vector routing is simple, and doesn't require much memory or calculation. • However AODV requires more time to establish a connection, and the initial communication to establish a route is heavier than some other approaches.

17. HSLS • The Hazy-Sighted Link State Routing Protocol (HSLS) is a wireless mesh network routing protocol being developed by the CUWiN Foundation. • This is an algorithm allowing computers communicating via wireless in a mesh network to forward messages to computers that are out of reach of direct radio contact. • Its network overhead is theoretically optimal,[utilizing both proactive and reactive link-state routing to limit network updates in space and time. • Its inventors believe it is a more efficient protocol to route wired networks as well. HSLS was invented by researchers at BBN Technologies. • HSLS was made to scale well to networks of over a thousand nodes, and on larger networks begins to exceed the efficiencies of the other routing algorithms. • This is accomplished by using a carefully designed balance of update frequency, and update extent in order to propagate link state information optimally. • Unlike traditional methods, HSLS does not flood the network with link-state information to attempt to cope with moving nodes that change connections with the rest of the network. • Further, HSLS does not require each node to have the same view of the network.

18. HSLS (Cont) • Link-state algorithms are theoretically attractive because they find optimal routes, reducing waste of transmission capacity. • The inventors of HSLS claim that routing protocols fall into three basically different schemes: proactive (such as OLSR), reactive (such as AODV), and algorithms that accept sub-optimal routings. • If one graphs them, they become less efficient as they are more purely any single strategy, as the network grows larger. The best algorithms seem to be in a sweet spot in the middle. • The routing information is called a "link state update." • The distance that a link-state is copied is the time to liveand is a count of the number of times it may be copied from one node to the next. • HSLS is said to optimally balance the features of proactive, reactive, and suboptimal routing approaches. • These strategies are blended by limiting link state updates in time and space. • By limiting the time to live the amount of transmission capacity is reduced. • By limiting the times when a proactive routing update is transmitted, several updates can be collected and transmitted at once, also saving transmission capacity. • .

19. HSLS (Cont) • Because HSLS sends distant updates infrequently, nodes do not have recent information about whether a distant node is still present. • This issue is present to some extent in all link state protocols, because the link state database may still contain an announcement from a failed node. • However, protocols like OSPF will propagate a link state update from the failed nodes neighbors, and thus all nodes will learn quickly of the failed node's demise (or disconnection). • With HSLS, one can't disambiguate between a node that is still present 10 hops away and a failed node until former neighbors send long-distance announcements. • Thus, HSLS may fail in some circumstances requiring high assurance.

20. Routing Protocol Evaluation

21. Routing Performance Metrics

22. Routing Protocol Evaluation Metrics • Four most common metrics • Delivery Ratio • Latency • Path Length Optimality • Control Overhead

23. Delivery Ratio • Number of packets successfully received by the destination / number sent by the source • Evaluated by setting up a number of “test” flows in the network • Commonly a number of constant bit rate (CBR) flows with a specified number of packets per second • Uses UDP so every dropped packet results in a reduction of the delivery ratio (no end-to-end retransmissions) • Congestion Sensitive • A large enough test load will result in reduced delivery ratio for ANY protocol due to congestion • Mobility Sensitive • If the routing protocol does not respond quickly to topology change, then packets sent on links that no longer exist will be lost

24. Delivery Ratio Examples Delivery Ratio vs. Test Load Delivery Ratio vs. Mobility

25. Latency • The time between the creation of a packet and its delivery to the destination • Usually measured using the same setup as delivery ratio • Congestion sensitive • Latency will drastically increase as the congestion limit is reached (due to waiting in large buffers) • Retransmission sensitive • Protocols that locally recover packets will achieve higher delivery ratio but will increase latency • On-demand sensitive • Protocols that setup routes after data is sent will have higher latency on the initial packets of a flow

26. Latency Example

27. Path Length Optimality • The difference between the length of the path used for sending packets in the protocol and the length of the best possible path • Measurement • Protocol path length observed for each packet using test flows • Best possible path computed offline using same mobility pattern • Measure of protocol’s ability to track good routes • Extra hops from non-optimal routes will result in increased congestion and medium utilization

28. Path Length Optimality Example

29. Control Overhead • Number/size of routing control packets sent by the protocol • Calculated using counters while simulating with test flows • Sometimes expressed as a ratio of control to data • Indication of how efficiently a routing protocol operates • High control overhead may adversely affect delivery ratio and latency under higher loads

30. Control Overhead Example

31. Mobility Models

32. Random Waypoint Mobility • Two parameters • Pause Time (Pt) • Max Speed (Vmax) • Each node starts at a random location • Executes loop • Pause for Pt seconds • Select a random destination (waypoint) • Move to that destination at a random speed (0,Vmax) • Repeat upon arrival

33. Random Waypoint Properties • Advantages • Easy to implement • Allows heterogeneous speeds and temporarily stationary nodes • Disadvantages • Non-uniform node distribution (tend towards center) • Un-stable instantaneous mobility (tends towards zero and oscillates)

34. Random Waypoint Properties (cont)

35. Random Waypoint Properties (cont)

36. Modified Random Waypoint • Narrow the random speed range • (.1 Vmax, .9 Vmax) instead of ( 0, Vmax ) • Pre-simulation mobility • Mobility properties stabilize before routing and data commences • Doesn’t fix non-uniform node distribution

37. Other Mobility Models • Billiard Model • Node selects a random direction, speed, and time • Moves in that direction at that speed for that time and then repeats (may have pause time as well) • Bounces off simulation boundary like a “billiard ball” • Maintains uniform node distribution, and uniform average speed • Group mobility patterns • Node mobility is sum of group mobility and individual mobility • Used by clustering based routing protocols (well suited for certain applications like the military movement) • Trace based mobility patterns • Record real life people/vehicle/etc. motion patterns • Requires location hardware such as GPS • Difficult to try variations or change “parameters”

38. Assignment • Write note on the words highlighted in Green in this lecture

39. The End Questions?