T Computer Networks II Delay Tolerant Networking (DTN)

Transcription

1 T Computer Networks II Delay Tolerant Networking (DTN) Matti Siekkinen Large part based on slides by: Thomas Plagemann: Message Routing & Event Notification in Sparse Mobile Networks Mostafar Ammar: Keynote talk at Co-Next 2005 Joerg Ott: S slides 1

2 Outline What is Delay Tolerant Networking? Mobility DTN routing protocols Epidemic Routing Message Ferrying Applications of DTN 2

3 Traditional Wired Networks IP Link+PHY layers Application TCP / UDP IP Link+PHY layers endsystem (source) router endsystem (destination) Separation between endsystems and routers Routers responsible for finding stable path 3

4 Traditional Wired Networks Network is connected Cables link routers to each other There is a path from any source to any destination What if link goes down? E.g. cable cut or router crash Redundant paths with links to several routers (backup links) Routing protocols figure out a new route Usually short temporary disconnection at most 4

5 Store and Forward paradigm Store: receive whole packet before transmitting any bits Need to process it: error control, prefix lookup, Forward: transmit packet immediately after receiving and processing it All routers and switches usually behave in this way Implications Need some but limited amount of memory to buffer necessary amount of bits Routers forget about packets right away 5

6 Traditional Mobile Ad-hoc Wireless Networks (MANET) node = endsystem + router node (source) node (destination) No separation between endsystems and routers Nodes responsible for finding stable path 6

7 Traditional Mobile Ad-hoc Wireless Networks (MANET) node (source) node (destination) Application TCP / UDP Nodes may move Routing layer responsible for reconstructing (repairing) stable paths when movement occurs IP Link+PHY layers 7

8 Traditional MANET Paradigm Network is still connected There exists a (possibly multi-hop) path from any source to any destination The path exists for a long-enough period of time to allow meaningful communication If path is disrupted it can be repaired in short order E.g. node moves or dies MANET routing protocols take care of finding new path Store and forward is used here too Can disconnected networks also exist? 8

9 The Rise of Sparse Networks 9

10 Sparse Wireless Networks Disconnected By necessity By design (e.g. for power considerations) Mobile With enough mobility to allow for some connectivity over time Data paths may not exist at any one point in time but do exist over time 10

11 Mobility-Assisted Data Delivery: Store-Carry-Forward Mobility used for connectivity New Forwarding Paradigm Store Carry for a while Forward Special nodes: Transport entities that are not sources or destinations à Delay/Disruption Tolerant Networks (DTN) 11

12 Do DTNs use basic Internet protocols? TCP requires a connection State maintained at both ends Connection timed out Also basic IP assumes existing routes No route to host, Network unreachable Need new solutions at least for transport layer and above IP addressing can be used Alternatively, possible to add a layer between TCP/IP and applications IETF Bundle protocol and application-level support WWW phone..." SMTP HTTP SIP..." TCP UDP " " IP" " Eth PPP WiFi 3GPP " copper fiber radio OFDM FHSS..." 12

13 Building blocks of DTN Store-carry-forward packet switching Mobility Models are important here Routing protocols Different strategies can be used Depends a lot on mobility and resources available Security mechanisms New challenges for security Out of scope for this lecture 13

14 Outline What is Delay Tolerant Networking? Mobility DTN routing protocols Epidemic Routing Message Ferrying Applications of DTN 14

15 Role of mobility DTNs rely on some level of mobility Cannot form new connections in sparse networks otherwise There are many kinds of mobility People, vehicles, animals, robots, Urban, rural, Mobility models are useful Evaluate solutions and compare them using simulators Don t have to do field trials every time 15

16 Space vs. Space/Time path Space path: A multi-hop path where all the links are active at the same time Space/Time path: A multi-hop path that exists over time NOTE: S path is a special case of S/T path 16

17 Example S D Space Paths 17

18 Example S D No Path 18

19 Example S D Space Time Path 19

20 The Mobile Wireless Space High Relative mobility Hybrid environments Space/Time paths No paths Low Space paths High Node Density Low 20

21 Mobility: What is on the move? Planet, satellite Vehicle Airplane, car, train, bus, ship, spaceship People Pedestrian, cyclist, passenger, driver Individually or in groups Animals Individually or in groups Robot 21

22 Mobility traces vs. models Mobility traces from the real world Cannot measure all possible scenarios, obviously Provide useful insights for building models Many challenges Technical issues People don t like to be tracked Mobility models Based on known or measured behavior Real world traces Possible to tune parameters to match desired scale and dimensions (e.g., #nodes) May be very complex to build very realistic models 22

23 Types of mobility models Mobility models Random Temporal dependency Geographic restriction Spatial dependency Random walk Gauss-Markov Pathway Reference point group model Random waypoint Smooth random Obstacle Pursue Random direction Column Other variations Community 23

24 Random mobility models All models consider fixed area (e.g. a room) Random walk, a.k.a. drunken sailor Erratic unpredictable movement Randomly choose direction and speed Move for fixed amount of time or distance Nodes won t make progress in distance Random waypoint Draw a random destination point Move to destination at randomly chosen speed Random direction Node picks a random direction and speed Stop at the border of area These (esp. waypoint) have been applied a lot Do they capture realistic scenarios..? e Time 24

25 Dependency models Temporal dependency Movement is affected by the history E.g., consider that nodes often do not change directions in completely random way change speed very abruptly Geographic restriction Movement is bounded by streets, freeways, obstacles Spatial dependency Preferred locations Nodes often move together (group models) Manhattan model 25

26 Predicting mobility Is it possible? Yes, in a scenario behaving according to known model Difficult in practice What s the point? Can improve a lot performance of DTN Don t forward messages to nodes that we know will not be able to deliver them 26

27 Outline What is Delay Tolerant Networking? Mobility DTN routing protocols Epidemic Routing Message Ferrying Applications of DTN 27

28 MANET routing Roughly two main approaches Proactive routing Like routing protocols in wired networks Exchange information with neighbors non-stop Continuously maintain routing table up to date Short delay but uses resources all the time (bandwidth, energy, CPU) Reactive routing On-demand routing Compute route when have data to send Longer delay but less resources consumed Cannot use either of these for DTNs Cannot compute a path to destination when space paths don t exist! 28

29 DTN routing protocols Problem: How to get packets from source to destination given that we have no pure space paths? Routing protocol (together with mobility) largely determines Resource consumption Message delivery probability and delay à Crucial component Many protocols have been proposed in literature We look at a few selected ones in detail Probably one-size-fits-all doesn t make sense Too diverse application scenarios 29

30 How would you design a DTN routing protocol? Take 5-10 mins to think about how to route packets in DTNs What kinds of strategies could be used? What are the different tradeoffs? Discuss with person(s) sitting next to you! 30

31 Classifying DTN routing protocols Replication vs. forwarding Some solutions rely on replicating packets to many neighbors E.g. Epidemic Routing and Spray and Wait Others maintain at most one copy of packet in the network Tradeoff between resource consumption and message delivery probability/delay Assisted vs. unassisted Some mobility is imperative in DTNs We can add mobility if none exists naturally E.g. Message Ferrying 31

32 Classifying DTN routing protocols Resource constraints Unlimited vs. finite storage, bandwidth, energy Some protocols explicitly consider finite resources E.g. RAPID Leverage mobility information Probabilistic (prediction) Historic meeting information E.g. MaxProb 32

33 Outline What is Delay Tolerant Networking? Mobility DTN routing protocols Epidemic Routing Message Ferrying Applications of DTN 33

34 Epidemic Routing By Vahdat and Becker in 2000 Representative simple example of replicating routing protocol Utilize natural physical movement of devices to transport data Store-carry-forward paradigm Nodes buffer and carry data when disconnected Nodes exchange data when met Data is replicated throughout the network 34

35 Epidemic Routing: The Idea 35

36 Epidemic Routing: The Idea 36

37 Epidemic Routing: The Idea 37

38 Epidemic Routing: The Idea message is delivered 38

39 Epidemic Routing protocol Epidemic Routing: Basic Elements Each message contains Unique message ID Hop count Ack request (optional) Each node has ID Message buffer Hash table indexing the message buffer Key is message ID Summary vector (SV) Bit vector, describes which entries in hash table are set List of last seen nodes Avoid unnecessary connections Tradeoff: buffer size vs. message delivery probability and latency 39

40 Epidemic Routing: Evaluation Implementation in ns-2 50 mobile nodes Area 1500m x 300m Random waypoint Speed 0 20 m/s (uniformly distributed) Message size 1 KB 45 message sources/sinks Each one sends one message to each other one Total of 45x44 = 1980 messages Each second 1 new message initiated All messages initiated after 1980 seconds 40

41 Epidemic Routing: Evaluation Range has a huge impact on delivery latency Need to wait for neighbors to come to range Messages take many hops If range too short, cannot deliver all messages No way to reach destination host Simulation time matters MANET routing no longer works (space-time paths) Infinite buffer size? [Vahdat & Becker, TechReport 2000] 41

42 Epidemic Routing: Evaluation Limiting max number of hops affects latency Dramatically longer delivery latency with less than 3 hops Why longer delay and not just less packets delivered? When hop count reaches 1, message only delivered to destination, not dropped 50m range Infinite buffer size Infinite buffer size [Vahdat & Becker, TechReport 2000] 42

43 Epidemic Routing: Evaluation Too small buffer size reduces message delivery probability a lot Longer delivery latency means more packets in the network More packets in network means higher probability for full buffers 5%-25% of originated messages is enough for good performance 50m range [Vahdat & Becker, TechReport 2000] 43

44 Summary of ER Simple scheme to cope with space-time paths Robust to disconnections Leverage natural mobility Limitations Potentially high-failure rate Message duplication consumes nodal resources Basic scheme can be improved with contact probability information Mobility models 44

45 Outline What is Delay Tolerant Networking? Mobility DTN routing protocols Epidemic Routing Message Ferrying Applications of DTN 45

46 Message Ferrying By Zhao, Ammar, Zegura in 2004 Captures general properties of assisted protocols Exploit non-randomness in device movement to deliver data A set of nodes called ferries responsible for carrying data for all nodes in the network Store-carry-forward paradigm to accommodate disconnections Ferries act as a moving communication infrastructure for the network 46

47 Message Ferrying The Idea MF S M S MF M D D 47

48 MF variations Ferry mobility Task-oriented, e.g., bus movement Trajectories are due to other tasks Messaging-oriented, e.g., robot movement Trajectories aim specifically to optimize message delivery Regular node mobility Stationary Mobile: task-oriented or messaging-oriented Number of ferries Level of coordination between Ferries Regular nodes Ferry designation Switching roles as ferry or regular node 48

49 MF example with mobile nodes Nodes are mobile and limited in resources, e.g., buffer, energy Single ferry is used Not limited in buffer or energy Trajectory of the ferry is known to all nodes Data communication in messages Application layer data unit Message timeout 49

50 Four approaches Non-Proactive ( = Messaging-Specific) mobility Only rely on inherent movement Nodes meet ferry by chance Ferrying without Epidemic Routing Ferrying with Epidemic Routing Regular nodes also replicate packets when meeting each other Proactive mobility Nodes modify their trajectories for communication purposes Node-Initiated MF(NIMF) Nodes move to meet ferry Ferry-Initiated MF (FIMF) Ferry moves to meet nodes 50

51 Node-Initiated Message Ferrying Meet the ferry? If no, keep OK working Working 51

52 Node-Initiated Message Ferrying Go to Ferry 52

53 Node-Initiated Message Ferrying Send/Recv Go to Work 53

54 Node-Initiated Message Ferrying Go to Work 54

55 Mode Transition WORKING GO TO FERRY Not planned Intentional GO TO WORK SEND/RECEIVE 55

56 Ferry Operations in NIMF 1. Move according to a ferry route; 2. Broadcast Hello messages periodically; 3. On reception of an Echo message from a node: Exchange messages with the node; 56

57 Node Trajectory Control When should node move to meet the ferry? Goal: minimize message drops and reduce proactive movement Msgs are dropped when regular node s buffer fills up or msgs time out à Have to go meet ferry frequently enough to prevent this Going to meet ferry requires node to move proactively à Reduces time node can work on assigned tasks (e.g. in emergency operation) Go to ferry if Work-time percentage > threshold and Estimated message drop percentage > threshold Msg drop rate can be estimated knowing generation rate, timeout, buffer size, ferry drop rate 57

58 Ferry-Initiated MF Ferry takes proactive movement to meet up with nodes Long range (lr) radio is used to transmit control messages Ferry tells about its location Nodes send service requests and location updates Short range (sr) radio is used to exchange messages Msg forwarding, device discovery, and msg drop computation is same as in NIMF 58

59 Ferry-initiated MF example (a) Ferry follows def. route, broadcasts its location (lr radio) Node sends service request (lr radio) with location Ferry Location N2 1: Ferry Location Ferry F Location 2: Service Location Request Update S default ferry route (a) F N1 default ferry route S (b) N2 N1 (b) Ferry moves to meet node Node guides ferry using location update (lr radio) default ferry route default ferry route F (c) Ferry and node exchange packets using sr radio when they are close enough N2 F message exchange S N1 N2 S N1 (d) Ferry goes back to its default route (c) (d) ferry route movement radio transmission ferry node before movement after movement 59

60 Heuristics for Ferry Control How does ferry decide its trajectories given many service requests? Goal: minimize msg drops Finding optimal trajectory is NP hard problem à Need to use heuristics: Nearest neighbor (NN) Always visit closest next node after finished meeting with a node Traffic aware (TA) Consider both location and known msg drop information 60

61 Evaluation Simulate using NS with MAC and default energy model 40 nodes in 5km x 5km area 25 random (source, destination) pairs Node mobility is random-waypoint with max speed 5m/s Message timeout: 8000 sec Single ferry with speed 15m/s and rectangle ferry route 61

62 Message Delivery Rate Ferry considerably improves delivery rate Further, proactive movement helps a lot Two reasons: MF improves connectivity MF reduces buffer contention problem caused by flooding in ER Mes s age delivery rate FIMF NIMF F w/er 0.3 ER Epidemic Routing E pidemic R outing (w/ ferry) 0.2 NIMF 0.1 FIMF- NN FIMF- TA Node buffer size (messages) 62

63 Message Delay Delay increases with buffer size in all schemes Nodes buffer longer before meeting with Ferry Delay much shorter with ER than MF MF explicitly delays packets by batching them in nodes and ferry ER w/ Ferry strikes middle ground Mes s age delay (s ec ) FIMF NIMF F w/er 1500 ER 1000 Epidemic Routing E pidemic R outing (w/ ferry) NIMF 500 FIMF- NN FIMF- TA Node buffer size (messages) 63

64 Impact of node mobility pattern Mobility Model Scheme Delivery Rate Delay (sec) Energy efficiency (KB/J) Random Waypoint Limited Random Waypoint NIMF FIMF ER(w/ ferry) ER NIMF FIMF ER(w/ ferry) ER Node mobility pattern has much smaller impact on MF (with proactive mobility) than ER performance Logical since ER relies purely on natural movement of nodes MF uses proactive movement à less affected by natural movement 64

65 Where Does MF Fit? High MF is applicable for the entire space Space/Time Paths MF is necessary here Mobility Hybrid Environments Low Space Paths Traditional MANET solutions apply here No (Space/Time) Paths High Node Density Low 65

66 Outline What is Delay Tolerant Networking? Mobility DTN routing protocols Epidemic Routing Message Ferrying Applications of DTN 66

67 SWIM: Shared Wireless Infostation Model SWIM infostation Biological information acquisition system Tracking and studying whales E.g., learn behavior and response to human disturbances When whales surface, they Exchange data with each other (kind of epidemic routing) Offload data to SWIM station when getting close Alternative to systems that rely on satellite communication 67

68 Vehicles on Highways Networks Destination Source 68

69 Vehicles on Highways Networks Destination Source 69

70 Vehicles on Highways Networks Destination Source 70

71 DakNet MIT project, 2004 Commercial: First Mile Solutions, later United Villages, Inc. Leverage existing transportation infrastructure to provide rural connectivity Kind of message ferrying approach Voice mail, , other elastic applications Much cheaper compared to wireless cellular infrastructure or landline service 71

72 Summary DTNs are solutions for disconnected networks Basic components Store-carry-forward Mobility Routing protocols Security Lots of different routing protocols exist No one size fits all Suitability highly dependent on target scenario Many application scenarios exist Mostly niche applications, obviously 72