CPSC 689: Discrete Algorithms for Mobile and Wireless Systems

1. CPSC 689: Discrete Algorithms for Mobile and Wireless Systems Spring 2009 Prof. Jennifer Welch

2. Lecture 14 • Topic: • Routing in Mobile Wireless Networks • Sources: • Karp. Multi-hop wireless networks. • Vaidya, Chapter 6. • Schiller, Chapter 8. • Johnson, Maltz. Dynamic source routing in ad hoc wireless networks. • Perkins, Royer. Ad hoc on-demand distance-vector routing. • Chen, Murphy. Disconnected transitive communication. • MIT 6.885 Fall 2008 slides Discrete Algs for Mobile Wireless Sys

3. Point-to-Point Routing • New problem: Deliver messages from a particular source node S to a particular destination node D, in a mobile ad hoc network. • Main issue: S, and other nodes, don’t (initially) know where D is. • Similar to the basic IP routing service provided in the Internet: send message to a particular IP address. • Easier in Internet: fixed infrastructure, with wires, routers, backbone networks,… • But still important in wireless setting. • Very widely studied, proposals currently being put forth for practical routing methods. Discrete Algs for Mobile Wireless Sys

4. Overview of Routing • Internet-style approaches, not taking advantage of the wireless setting: • Destination-Sequenced Distance Vector (DSDV) • Dynamic Source Routing (DSR) • Ad-Hoc On-Demand Distance Vector Routing (AODV) • A simple algorithm that tries to take advantage of mobility. • Link-reversal algorithms • [Gafni, Bertsekas 81] • Temporally-Ordered Routing Algorithm (TORA) • Location-free routing • Routing using some substitute for geography. • Geographical routing without location information, Gradient Routing (GLIDER), Beacon Vector Routing • Location services • Keeping track of geographical locations of mobile nodes. • [Awerbuch, Peleg 91], GRID, LLS • Location-based routing • Routing using actual geography. • Geocasting, location-aided routing (LAR), compass routing, GPSR,… Discrete Algs for Mobile Wireless Sys

5. The Setting • Wireless networks, no infrastructure, no hierarchical structure. • Dynamic: • Stationary and/or mobile nodes. • Failure/recoveries • Scale: Small or large, even Internet-scale. • Typical examples: • Rooftop networks, of customer-owned radios; stationary, large-scale. • Floating sensors: mobile, large-scale. • Ad-hoc meetings, rescue workers in disaster areas, soldiers in battle… Discrete Algs for Mobile Wireless Sys

6. The Routing Problem • Sender S anywhere in the network wants to send a message to a particular node D somewhere else in the network. • Doesn’t know where D is. • Subproblem: Set up a route (end-to-end path) to that node. • Routes may need to change, in response to network changes. • Some routing strategies don't set up routes, just send messages one step at a time. • Real goal is delivering messages, not setting up routes. Discrete Algs for Mobile Wireless Sys

7. Criteria • Criteria: • Accommodate large scale, mobility • Minimize communication overhead for sending data messages • Maximize packet delivery success rate • Minimize latency of packet delivery • Minimize route length (e.g., number of hops) • Minimize amount of state that must be maintained at each node • Usual strategies for coping with scale: • Hierarchy: Good when level boundaries are relatively fixed, can be chosen by administrative authority. • Caching (e.g., for routes) • But these don't work so well for dynamic or unstructured networks. Discrete Algs for Mobile Wireless Sys

8. Reuse? • Could use traditional protocols for wired networks in wireless networks: • Establish abstract network with point-to-point links. • Define links using Neighbor Discovery protocol, based on periodic Hello messages. • Run traditional protocols over the abstract network. • But resulting protocols may not perform very well: • “Links” may be unreliable, failing and recovering often. • May cause routes to break frequently, hurting performance. • Point-to-point link abstraction doesn’t fit wireless networks well, since they use local broadcast with interference. • Need to adapt protocols, design new protocols. Discrete Algs for Mobile Wireless Sys

9. Proactive vs. Reactive Protocols • Proactive: • Maintain routes in the background, regardless of current data-sending activity. • High overhead. • Low latency for message forwarding. • Reactive: • Establish and maintain routes only when needed for data-sending. • Low overhead, if not too much data activity. • May have high latency for message forwarding. Discrete Algs for Mobile Wireless Sys

10. Proactive Protocols • Main methods: Link-State Routing, Distance-Vector Routing • Adaptations of wired protocols, treating every node as a router. • Try to establish and maintain routes with fewest hops (“shortest paths”). • Use O(n) local state per node = router. • Require that, periodically, or in response to link changes, information must be propagated to all nodes. Discrete Algs for Mobile Wireless Sys

11. Link-State Routing • Used in Open Shortest Path First (OSPF) Internet protocol. • Can adapt to use in wireless network, using local broadcast. • Each node periodically broadcasts information about the status of its adjacent links, throughout the network. • Every node maintains a complete picture of the network, to use for message routing. Discrete Algs for Mobile Wireless Sys

12. Link-State Routing, More Details • Each node periodically constructs a link state packetdescribing the status of all its adjacent links, up or down. • Attaches sequence numbers, to keep track of which packets are more recent. • Broadcasts the link state packets throughout the network, using a network-wide flooding algorithm. • Each node collects all this information, assembles a complete picture of the network locally. • Uses a centralized algorithm, e.g., Dijkstra’s algorithm, to determine shortest paths. • Uses these shortest paths to fill in a next-hop routing table,indicating the next hop for a message for each destination D. Discrete Algs for Mobile Wireless Sys

13. Link-State Routing, Optimizations • Limit dissemination of link state packets (LSPs). • Hazy-sighted LSP dissemination: • Propagate LSPs more frequently to shorter distances, less frequently to longer distances. • Nodes keep more up-to-date about nearby links. • Uncoordinated selective forwarding: • Propagate LSPs with some probability p. • Don’t propagate LSPs if overhear someone else sending it. • Unreliable, since some neighbors might not have received it. • Coordinated selective forwarding: • Decide cooperatively who should propagate. • E.g., each node identifies a Relay Set, a subset of its neighbors such that each 2-hop neighbor is a neighbor of some node in the Relay Set. • Propagate LSP to Relay Set nodes only. • Requires Neighbor Discovery protocol to learn about 2-hop neighbors. Discrete Algs for Mobile Wireless Sys

14. Distance-Vector Routing • Bellman-Ford style • Each node keeps track, for each possible destination D, of the shortest distance (number of hops) to D that it knows, R[D].cost, together with the first hop along a shortest path, R[D].nexthop. • Each node periodically broadcasts to its neighboring nodes its distance vector, giving its current distances to all nodes. • R[D].cost for all D. • Each node adjusts its distance estimates and first hops, based on information from its neighbors: • When Z receives distance d for D  Z from neighbor X: • if R[D].nexthop = X then R[D].cost := d + costZX • else if d + costZX < R[D].cost then • R[D].cost := d + costZX • R[D].nexthop := X Discrete Algs for Mobile Wireless Sys

15. Distance-Vector Routing • When Z receives distance d for D  Z from X: • if R[D].nexthop = X then R[D].cost := d + costZX • Update the cost estimate with newer information. • else if d + costZX < R[D].cost then • R[D].cost := d + costZX • R[D].nexthop := X • If a different neighbor provides a better path, update both the cost estimate and the nexthop. • In addition, if Z does not hear from R[D].nexthop for a while: • R[D].cost :=  • R[D].nexthop :=  Discrete Algs for Mobile Wireless Sys

16. 1 5 5 8 2 2  1  2 2 D D D 3 3 3  A A A B B B Counting to Infinity • Basic Distance-Vector algorithm allows “counting to ”, if links fail: • With all links up, RA[D].cost and RB[D].cost stabilize to 1 and 2, respectively. • Then link AD fails, setting RA[D].cost := . • Then A hears from B, so RA[D].cost := 2 + 3 = 5, RA[D].nexthop := B (correct). • Then link BD fails, setting RB[D].cost := . • Then B hears from A, so RB[D].cost := 5 + 3 = 8, RB[D].nexthop := A (not correct). • A hears from B, RA[D].cost := 8 + 3 = 11. • B hears from A, RB[D].cost := 11 + 3 = 14. • Etc. Discrete Algs for Mobile Wireless Sys

17. DSDV • Destination-Sequenced Distance-Vector [Perkins, Bhagwat 94]. • Add mechanism to DV to prevent counting to . • Source of problem in example: • A’s estimate of 5 was based on B’s old estimate of 2. • B’s estimate is now . • But B accepts A’s estimate. • Mechanism to avoid using stale information in determining routes: Associate a (destination sequence number, distance) tag with each routing table entry and each distance vector entry. • (seqno1, dist1)better than(seqno2, dist2)if either: • seqno1 > seqno2 (newer information), or • seqno1 = seqno2 and dist1 <dist2 (better distance). Discrete Algs for Mobile Wireless Sys

18. Some DSDV Details • Somewhat complicated rules for manipulating tags, see [Vaidya, Section 6.2.2]. Roughly: • Node Z increases its own seqno (for Z itself) by 2 each time it sends out a new distance vector. • When node Z detects that a link to X has failed, Z increases its sequence number for X by 1. • When processing a received distance vector from X, Z accepts new information for D only if its associated tag is better than the tag stored in RZ[D]. • Larger seqno, or same seqno and shorter distance. Discrete Algs for Mobile Wireless Sys

19. DSDV Guarantees • Valid route information is always associated with even seqnos. • Tags are monotonic: Starting from any node, and following nexthop entries for a particular D, the tags in the R tables increase monotonically until we either reach D or find R[D].nexhop = . • Implies no routing loops. • Avoids counting to : In example, B would not accept the info from A because the tag is not better than the entry in RB[D]. Discrete Algs for Mobile Wireless Sys

20. Proactive Algorithms, Summary • Link-State Routing, Distance-Vector Routing • Adaptations of wired protocols, treat every node as a router. • Try to establish and maintain routes with fewest hops. • High overhead: • Maintain routes in the background, regardless of current data-sending activity. • O(n) local state per node. • Require that, periodically, or in response to link changes, information must be propagated throughout the network. • Frequent network changes result in instability, global updates. • Suggests reactive algorithms might work better, in dynamic mobile ad hoc networks: • Establish and maintain routes only when needed for data-sending. • Low overhead, if not too much data activity. Discrete Algs for Mobile Wireless Sys

21. Reactive approaches • Dynamic Source Routing (DSR) [Johnson, Maltz 96] • Ad-Hoc On-Demand Distance Vector Routing (AODV) [Perkins, Royer 99] Discrete Algs for Mobile Wireless Sys

22. DSR Overview • Reactive, uses on-demand routing. • When S has a message for D, it tries to find a good route to D. • Performs route discovery, by flooding a query packet to find a route to D. • S learns the entire route. • Attaches the entire route to each data packet it sends (source routing). • Caches the route so it doesn't have to search for new routes for each message (or each packet). • But, if the network is large and or/changes rapidly, the routes tend to break frequently, which makes caching less effective. Discrete Algs for Mobile Wireless Sys

23. DSR Performance Highlights • Claim good performance: • Low communication overhead, high reliability and low latency of message delivery, and nearly-optimal routes, under a variety of conditions. • During stable periods: Little overhead, can reuse already-determined routes. Yields low-latency, low-communication reliable message delivery. • When network changes: Adapts reasonably quickly. Diagnoses broken routes and finds new ones. Discrete Algs for Mobile Wireless Sys

24. DSR Assumptions • Nodes are willing to “participate fully”: forward traffic on behalf of others, etc. • Network diameter: • Examples in paper are fairly small. • Suggests that this may not scale well. • Mobility: • Hosts may move at any time • Speed much lower than time for packet transmission and local processing. • Don't move so fast as to make route computation useless. • Hosts have a “promiscuous receive mode”, that allows them to receive every packet they hear, not filtered by destination address. Discrete Algs for Mobile Wireless Sys

25. DSR, Basic Operation • Message forwarding: Source routing • Sender puts entire route in packet header, forwards packet to first host in the route, successive hosts forward. • Each host maintains a route cache, caching some source routes it has learned, each with an expiration time. • When sender S wants to send a packet to destination D: • S first checks its route cache for a route to D. • If it finds one, it uses it. • If not, uses route discovery protocol to try to find one. • Processes other packets in the meantime. • Route maintenance: • Source S monitors correct operation of its cached routes; if a route isn't working, removes it from cache. • If a currently-used route is discovered not to be working, then S may do route discovery again, resend on the new route. Discrete Algs for Mobile Wireless Sys

26. DSR Route Discovery • S broadcasts “route request” (RREQ) packet, containing id pair (S,D). • Should result in “route reply” (RREP) containing full path. • Route discovery works by simply flooding the packet, looking for a path that works. • RREQ packet contains: • (S,D) • request id, unique id for the request, allows pruning out duplicates • route record, accumulates a record of the sequence of hops the packet takes during discovery • Processing a RREQ: • Duplicate detection: Discard if duplicate, according to request id. • Loop detection: If your own address already appears in the route record, this is a loop, discard. • If you are D, the route has been found; send RREP with the final route record back to S. • Otherwise, append your own address and re-broadcast. Discrete Algs for Mobile Wireless Sys

27. Sending the RREP from D to S • If D has a route to S in its cache, sends the RREP on that. • Else, if communication is bidirectional, D sends the RREP along the reverse of the route in the RREQ packet. • Else, D initiates a route discovery for (D,S), flooding a new RREQ message, with the RREP piggybacked on it. • We're not counting on this second route discovery to provide D with a route from D to S---just using the flooding facilities to send the RREP back to S. • When S receives this new RREQ, it puts into its route cache the route from S to D that appears in the piggybacked RREP. • This wouldn’t work without piggybacking: If D just did route discovery for (D,S) without piggybacking the RREP on the new RREQ message, then S would not have a path to use to return the new RREP to D! Discrete Algs for Mobile Wireless Sys

28. DSR Route Maintenance • Since DSR is reactive, it does not send routing updates continually • Instead, while a route is actually being used to send a message, a route maintenance protocol monitors the operation of the route, informs sender of any problems. • Some mechanisms used to diagnose the failure of individual hops along the path: • Link-level acks, with non-occurrence reported to higher layers. • Passive acks, trying to overhear the next transmission by the next node along the path. • Some application-level ack information, if available. • Explicit acks added to the message-forwarding protocol. • A node discovering such a local failure then sends a Route Error (RERR) message to the sender S of the current message, indicating the particular hop that failed. • When S receives the RERR, it truncates all the paths in its cache at the failed hop. Discrete Algs for Mobile Wireless Sys

29. DSR Route Maintenance • Q: How does the diagnosing node X get the RERR message back to the sender S? • A: As for the RREP: • If X has a route to S in its cache, sends the RERR on that. • Or, reverse the route from the packet (if communication is bidirectional). • Or piggyback RERR on a new route discovery. • Another option: X performs an entire new route discovery for (X,S), with no piggybacking, before sending the RERR message. • Q: Why is this not circular? • A: Because S should have a route to X in its cache. • End-to-end route maintenance: • Use explicit ack for data packet, from destination D to source S. • Gives “coarser” information---source could delete only one route, not truncate many routes as for single-hop diagnosis. Discrete Algs for Mobile Wireless Sys

30. DSR Optimizations • Storing routes in the route cache: • List of routes, indexed by destination. • Tree of routes, to different destinations. • When to add a route to the cache? • As a result of a successful route discovery (of course). • Maybe also: • When forwarding a data packet along a route, add the rest of the route. • When forwarding an RREP containing a route, add the relevant portion. • Some overheard routes. • Use route cache to avoid propagating an RREQ: • If you already have a route to the intended target D of the RREQ (that would not introduce a loop), then just append your route to the route that already appears in the RREQ, to obtain a candidate route from S to D, and send this in a special RREP message back to S. • Could also gather route info from neighbors’ caches. • Set bound on route length, truncate search at that bound. Discrete Algs for Mobile Wireless Sys

31. DSR Performance • Packet-level simulation. • Varied parameters in the protocol, number of nodes, pattern/speed of movement, distribution of nodes in space. • Typically: Mobile hosts in a large room, moving at walking speeds, with 3-meter transmission/reception range. • Claim the results are also valid for vehicles, which move faster, over a larger region, because parameters scale appropriately (?). • Random initial placements, random pause, random new locations, random speed. • Results: • For all but the shortest pause times, the total number of packets transmitted is very close to optimal (for communicating actual data). • Nearly all data packets are successfully delivered, (because route maintenance detects when routes are no longer working). • The protocol finds and uses close to optimal routes. Discrete Algs for Mobile Wireless Sys

32. Ad Hoc On-Demand Distance Vector (AODV) Routing • Another reactive protocol, like DSR. • But not a source routing algorithm: • Does not maintain the entire route anywhere, but distributes its representation among the nodes on the route. • Like distance-vector algorithms. • Nodes maintain distance-vectorrouting tables, route messages according to these tables. • Tables populated on-demand: Next hops added only as routes are needed, time out if not used, or if detected as broken. • Gives usual advantages of reactive strategies: • Low communication overhead, small storage • Lower local storage requirements than DSR, since only next hop information is stored, not full routes. • Combination of performance advantages makes AODV morescalablethan DSR and DSDV. Discrete Algs for Mobile Wireless Sys

33. AODV Assumptions • Basically the same as DSR: • Nodes are willing to “participate fully”. • Mobility is slower than routing. • Nodes in promiscuous receive mode. • However, unlike for DSR, the network is not assumed to be small. • AODV is intended to be scalable. Discrete Algs for Mobile Wireless Sys

34. H I G S A F E B C D AODV, Basic Operation • S wants to send a message to D, but has no entry for D in its routing table. • S initiates route discovery by locally broadcasting a RREQ, containing: • (S,D) • request_id • source_seqno, dest_seqno • hop_count • (S, request_id) uniquely identify RREQ. Discrete Algs for Mobile Wireless Sys

35. H I G S A F E B C D AODV, Basic Operation • When a node receives an RREQ: • If has already seen it (same S, request_id), discard. • If have relevant information, then reply. • Else, increment hop_count, rebroadcast, and record: • (S,D), request_id, source_seqno • Who forwarded the RREQ to you, • Expiration time Discrete Algs for Mobile Wireless Sys

36. H I G S A F E B C D AODV, Route Discovery • Relevantinformation: • If you are the destination D, or have a route to D with the same or greater destination sequence number. • Reply: Next slide… • For now, assume no node knows any route to D. • As the RREQ propagates through the network, the information nodes record about whom they received it from sets up temporary reverse pointers. • Temporary because they get discarded after expiration time. Discrete Algs for Mobile Wireless Sys

37. Replying to RREQs • Node X with relevant information replies to an RREQ by generating a RREP, containing: • (S,D), dest_seqno • hop_count, number of hops in route from X to D. • lifetime • This gets passed back along the reverse pointers, unicast fashion. • RREP's handled separately from RREQs, no request_id. • For a given (S,D), X passes RREP along the reverse pointer iff X hasn't already passed along such an RREP for a larger dest_seqno, or for the same dest_seqno with the same or smaller hop_count. • Requires keeping track of RREPs handled for each (S,D). • Assumes X has at most one reverse link for each (S,D): • A reverse link should mean X is on an active path from S to D. • Two reverse links would mean two active paths. • If S sends out a new RREQ for D, it means the previous route was broken. • So X should discard the old reverse link in favor of the new one. Discrete Algs for Mobile Wireless Sys

38. H I G S A F E B C D Forward Path Setup • As the RREP is passed back, each node that handles it sets up the appropriate forward pointer in its routing table entry for D. • Each routing table entry holds, for each D: • Number of hops to D • Next hop • dest_seqno • Active neighbors for this route (neighbors who have sent a message recently that used it) • Expiration time for the route table entry (if not used for a while) Discrete Algs for Mobile Wireless Sys

39. H I G S D A F E B C Local Connectivity Management • Suppose D moves. • Suppose A wants to send a message to D, broadcasts RREQ. • A gets RREP's from S, B, and D; chooses D’s because it has the largest dest_seqno. • B overhears D’s RREP, changes its route table entry because this RREP has a larger dest_seqno. Discrete Algs for Mobile Wireless Sys

40. H I G S D A F E B C Local Connectivity Management • Q: How does C determine its path to D is broken? • A: D is supposed to send Hello messages to C periodically. • In general: • Nodes on active paths send Hello messages to their prececessors. • If a predecessor doesn’t receive one for a while, it assumes the successor is gone, propagates back a special RREP with hop_count =  and one greater dest_seqno. • The effect: Anyone who has not found a more recent route to the destination removes the route from its table. Discrete Algs for Mobile Wireless Sys

41. H I G S D A F E B C Local Connectivity Management • Here, C removes its entries for this route, propagates RREP back to B, who already has a better route. • If D had moved far away, then C's infinite RREP might have propagated all the way back to S, forcing S to rediscover from scratch. Discrete Algs for Mobile Wireless Sys

42. AODV Simulation Results • Simulated AODV using PARSEC event-driven, packet-level simulator. • Not very realistic: • Strict range cut-off. • Perfect physical carrier sensing. • Always a collision (losing both messages) if two neighbors broadcast at the same time. • Ran experiments with 50, 100, 500, and 1000 nodes, to test scalability. • 1000 is very large compared to most MANET simulations. • Delivery success rate: • For few nodes, very high success rate. • For more nodes: About 25% of messages lost. • Main reason is collisions. • Collisions are very important factor with MANET routing protocols. Discrete Algs for Mobile Wireless Sys

43. AODV Real-World Experiments • Dartmouth experiments: • Approximately 40 students carrying laptops, moving randomly. • Playing field, 225 x 365m. • Traffic generation: 1200 byte packets, 5.5 packets per stream, 3 seconds between packets, 15 seconds between streams (numbers derived from a military application prototype). • Around 425 bytes per second – modest. • Message delivery success rate: 50% • Good latency, best compared to other algorithms tested (ODMRP, APRL, STARA). • Second best for route costs (hop count). • Smallest amount of overhead. • Issues: • Collisions. • Fluctuating links: you might think you are somone’s neighbor because you got a message, but the link quality is bad. • Control traffic: Aggressively seeking new routes can make things worse. Discrete Algs for Mobile Wireless Sys

44. E B D F A C Disconnected Transitive Communication [Chen, Murphy] • Routing algorithms we’ve discussed assume a network that stays connected, though the network topology can change. • Now assume the network may not always be connected. • Nodes in clusters, clusters intermittently connected. • For applications that can tolerate highly asynchronous communication. • Minutes or hours delay, not milliseconds or seconds. • Don't use to run a ssh connection. • But perhaps to update two military camps about a change in the plans for a battle coming up in a few days. Discrete Algs for Mobile Wireless Sys

45. E B D F C A Disconnected Transitive Communication [Chen, Murphy] • Routing algorithms we’ve discussed assume a network that stays connected, though the network topology can change. • Now assume the network may not always be connected. • Nodes in clusters, clusters intermittently connected. • For applications that can tolerate highly asynchronous communication. • Minutes or hours delay, not milliseconds or seconds. • Don't use to run a ssh connection. • But perhaps to update two military camps about a change in the plans for a battle coming up in a few days. Discrete Algs for Mobile Wireless Sys

46. The Basic Idea • Algorithm maintains no long-term or medium-term state, not even on-demand routes. • Just pass the message on, hop-by-hop. • Pass it to the node in your current cluster that is “closest” to the destination. • Figuring out who is closest is the main trick; might require application-layer information. Discrete Algs for Mobile Wireless Sys

47. More Detail • If X has a message to send or pass on to D: • X broadcasts probe throughout its component (multi-hop). • Recipients test their utility for D; respond if high enough. • Both broadcasting and response use low-level protocol like DSR or AODV, within the cluster. • X passes message on to node with highest utility. Discrete Algs for Mobile Wireless Sys

48. Utility Measures • History-based utility measures: • Most Recently Noticed: Higher utility if you have noticed (encountered) D recently. • Most Frequently Noticed: Higher utility if you have noticed D frequently. • Future-based: • Future Plans: Assumes the application at your node will tell you next time it expects to see D (e.g., by reading a calendar). • Status-based: • Power: More power, better utility. • Rediscovery Interval: Smaller RDI means a better chance of discovering D or someone closer to D. • Weighted combinations of these. • Sender can choose its own metric, based on special scenario-specific information. Discrete Algs for Mobile Wireless Sys

49. Rediscovery • Choosing a good rediscovery interval (RDI) is important. • If it’s too big, you may miss potential connections. • Too small leads to too much traffic. • Their solution: • Double RDI after each unsuccessful probe for new connections. • If new node discovered, reset RDI to initial value. • Discover new nodes using Hello messages. Discrete Algs for Mobile Wireless Sys