“Data over Time” Delay Tolerant Networks

1. “Data over Time”Delay Tolerant Networks Joy Ghosh LANDER cse@buffalo

2. Overview • Challenged Internets (CI) • Shortfalls of TCP/IP in CI • Concept of DTN • Message-oriented • Overlay architecture • Classes of Service (CoS) • Routing Issues • Conclusion

3. Challenged Internets (CI) • Some causes: • Host / Router mobility extremes • Power management • Interference • Examples • Terrestrial mobile nets: partitions • Sensor / Actuator: power saving • Military ad-hoc: jamming

4. Paths & Links • High end-to-end latency • Transmission time • Propagation delay • Processing time • Queuing time • Low asymmetric data rates • Return channel may be even absent • Avoid chatty conversation • Data carousels / de-fragmentation

5. Paths & Links • Non-faulty Disconnection • Due to motion • Predictable (satellites, busses) • Opportunistic (node coming in range) • End user or router motion • Third entity motion causing block • Due to low-duty-cycle • Predictable • Low capability devices save power • Usually triggered by events - sensors • Routing • lack of reach is normal – not a fault • Pre-schedule data based on outage info

6. Paths & Links • Long Queuing Times • Next hop unavailable • Alternative next hops • Persistent storage

7. Network Architecture • Interoperability considerations • Most protocols only use LL & MAC • Simple & local in scope • May use application specific framing formats • May fail to implement reliability, congestion control, and security • Not well matched for Internet Protocols • Minimal assumptions of underlying protocol stack capabilities for interoperation

8. Network Architecture • Security • Exotic media  forwarding costly • Protection of forwarding service • Access control matrix for CoS • Authentication at critical points • End-to-end security not suitable • Too much delay for key exchange • Drop unwanted traffic ASAP

9. End System Characteristics • Limited longevity • Hostile environments • Power exhasution • Delegation of reliable delivery • Low duty cycle • Save power / event triggered • Limited resources • Limited memory & processing

10. Overview • Challenged Internets (CI) • Shortfalls of TCP/IP in CI • Concept of DTN • Message-oriented • Overlay architecture • Classes of Service (CoS) • Routing Issues • Conclusion

11. TCP/IP basics • Expectations of TCP/IP • Map: IP packet format, domain names  physical network • Fundamental service • Fully-connected, best-effort, unicast datagram transport • Suffers when: • Links change radically (wireless, ATM) • Service enhanced significantly (IP multicast, QoS)

12. TCP/IP applied directly to CI • Transport Layer (TCP, SCTP, UDP) • RTT too high for slow start • Waste of bandwidth • False congestion indication • MSL in RFC793 is 2 mins  Low

13. TCP/IP applied directly to CI • Network Layer (IP) • Lossy network  smaller frames • Frequent fragmentation • No fragment retransmission • Maximum TTL is 225  Low

14. TCP/IP applied directly to CI • Application Layer (Routing) • Delayed soft-state refreshes • Label links as non-operating (e.g., RIP, BGP) • Lack of response to HELLO messages (e.g., OSPF) • Application Layer (Others) • Delay cause application timeout • Delay may outlive application life

15. Application Development Practice • Network based applications • Application level timeouts • Lack of failover – no auto route • Synchronous programming • Chatty applications protocols (FTP requires 6 RTT) • New network API required

16. Proxies & Protocol Boosters • Performance Enhancing Proxies (PEPs), Protocol Boosters - Link repair approaches • “fool” TCP/IP based end stations to operate more efficiently over paths with poor links • Different levels of transparency • Local connection termination • Modification of TCP ACK stream • Retransmit lost TCP packets w/o end user input • Use of PEPs discouraged due to fragility • Confound end-to-end diagnostics • Hurt security below transport (IPSEC) • Alternative: application layer proxies • Internet to special network name mapping and protocol translation

17. Use of Electronic Mail • Pros • Asynchronous message delivery • Delay tolerance • Flexible name/address semantics • Cons • Lack of dynamic routing • SMTP is a chatty protocol • Mostly reliable with likely failure notification

18. Overview • Challenged Internets (CI) • Shortfalls of TCP/IP in CI • Concept of DTN • Message-oriented • Overlay architecture • Classes of Service (CoS) • Routing Issues • Conclusion

19. Delay Tolerant Networkmessage-oriented overlay architecture • Interoperability among CI & Internet • Overlaid above different protocol stacks in various networks • Store & forward message delivery for non-interactive traffic • a-priori knowledge of communication capacity needs (intermediate storage, retransmission bandwidth) • Virtual circuits unsuitable due to pre-establishment of network • Favors asynchronous I/O programming

20. Network of Regional Networks • Similar protocol stacks in one region • DTN Gateways – Metanet Waypoint • Provides entry to a region (protocol translation) • Name mapping from global tuple to locally resolved name • Enforces policy and control • Persistent storage for reliable delivery

21. Delay Isolation via Transport Layer Termination Persistent storage

22. Bundle Encapsulation • Bundles consist of 3 things: • Source application’s user data • Control information from source application to destination application (how to handle data) • Bundle header

23. DTN Region Naming

24. Name Tuples (Late Binding)

25. InterPlaNetary (IPN) Internet

26. Class of Service (CoS) • Challenged Internet  limited resources • Priority based resource allocation • Subset of types of service by USPS

27. Class of Service (CoS) • Challenged Internet  limited resources • Priority based resource allocation • USPS has simple but elaborate services • Subset of types of service by USPS • DTN CoS • Custody Transfer • Return Receipt • Custody-Transfer Notification • Bundle-Forwarding Notification • Priority of Delivery: Bulk, Normal, Expedited • Authentication

28. DTN Class of Service (CoS)

29. Custody Transfer

30. Path Selection & Scheduling • No end-to-end path • Routes are a cascade of time-dependent contacts • Contact parameters • Start & end times • Capacity • Latency • End points • Direction • Predictable vs. Opportunistic contacts • Multi-commodity flow optimization problem • “flows over time” optimization problem – NP-hard • More under “Routing Issues”

31. Protocol Translation &Convergence Layers • Bundle forwarder may augment underlying protocol stack • Reliable delivery • Connection setup with notification • Flow control • Congestion control • Message boundaries • May fall back to retransmission timers in case of failed connections

32. Structure of Bundle Forwarder

33. Time Synchronization • Required for scheduling, path selection, and congestion management • Devices in challenged environments often collect data / position w.r.t. time • Pre-programmed control instructions to be executed in future time • Reliability of post-facto data analysis • Removal of expired pending messages • Protocols like NTP are used

34. Security User certificate • Per-hop security to avoid delay • Drop unwanted packets ASAP • Only edge routers need per user certificate • Not secure against router compromise

35. Congestion & Flow Control • Flow • DTN forwarder uses available underlying region-local transport protocol • If not available, its constructed at the DTN convergence layer • Congestion • Long absence of contacts leads to massive storage of data • Data for which custody has been accepted cannot be discarded • Priority queue for custody storage • Large messages are denied custody • Messages are spooled FCFS based on priority • Potential problems • Priority invasion (lower custodially recvd blocks high) • Head-of-line blocking (custodially recvd w/o contact) • Proactive : admission control • Reactive: reservation, rejection, etc.

36. Overview • Challenged Internets (CI) • Shortfalls of TCP/IP in CI • Concept of DTN • Message-oriented • Overlay architecture • Classes of Service (CoS) • Routing Issues • Conclusion

37. Network model for Routing

38. Routing Issues • No end-to-end path • Both proactive and reactive fail • Capacity is time dependent • Long buffering of messages • Multiple contacts • Might be more optimal to wait for future contact than send on available one

39. Routing Objective • Maximize probability of message delivery • Prevent buffer overflow • Speed up the release of resources • Minimize the end-to-end delay • Use knowledge of future topology dynamics in path selection • Per-hop routing preferred over Source routing • Flexibility to change next hop • Message splitting is allowed • Fragmentation of large messages • Each fragment routed independently

40. Knowledge Oracles • Contacts Summary Oracle • Time-invariant / aggregate characteristics of contacts • Contacts Oracle • Time-varying DTN multi-graph • Queuing Oracle • Buffer occupancy at any node at any time • Traffic Demand Oracle • Future traffic demand

41. Routing Algorithm Classes • Zero Knowledge • No oracles / minimal extreme • Complete Knowledge • All oracles / LP formulation • Partial Knowledge • One or more oracles • Practical algorithms

42. Knowledge vs. Performance

43. Zero Knowledge Routing • First Contact (FC) • Forward to first available contact • No sense of direction / may oscillate • Requires only local knowledge • Trivial implementation • Improvements • Incorporate sense of trajectory • Use path vector to prevent loops

44. Partial Knowledge Routing • Assigns cost to edges • Costs reflect estimated delay on edge • Queuing time: time till contact available • Transmission delay: time to inject into edge • Propagation delay: time to travel on edge • Computes minimum cost path • Conveniently uses different oracles • Computationally efficient distributed algorithms already exist for shortest-path based routing problems • Finds however single path from source to destination – no optimal splitting

45. Partial Knowledge Routing • Cost function: ω (e, t) • If message arrives at node ‘u’ at time ‘t’, and if edge ‘e’ (between ‘u’ & ‘v’) is chosen, message will reach node ‘v’ at time ‘t + ω (e, t)’ • FIFO: if t1 < t2, t1 + ω (e, t1) <= t2 + ω (e, t2) • Algorithm with Time-invariant Costs • Use modified Dijkstra’s algorithm • If L[v] > (L[u] + ω (e, L[u] + T)) then L[v]  (L[u] + ω (e, L[u] + T)) (T  start time) • Minimum Expected Delay (MED) • Oracles: Contacts Summary • Edge cost = avg. wait time + prop delay + trx delay • Proactive routing is used for time-invariant cost • Fixed routes for all messages • Minimizes avg. waiting time but doesn’t optimize path • Improvement • Dynamically make use of superior contacts per-hop • Multiple disjoint paths to balance load

46. Partial Knowledge Routing • Cost function: ω’ (e, t, m, s) • Edge ‘e’, time ‘t’, message size ‘m’, node assigning cost ‘s’ • ω’ (e, t, m, s) = t’ (e, t, m, s) – t + d (e, t’) where, • c (e, t)  capacity of edge ‘e’ at time ‘t’ • Q (e, t, s)  queue size at source of edge ‘e’, at time ‘t’ as predicted by node ‘s’ • t’  earliest time queues data at ‘e’ and message can be injected into the edge • Integral  volume of data through ‘e’ in interval [t, t’’] • d (e, t’)  propagation delay seen by message

47. Partial Knowledge Routing • Algorithms with Time-varying Costs • Earliest Delivery (ED) • Contacts Oracle • Q (e, t, s) = 0 • Source routed • Buffer overflow  cascaded delay • Earliest Delivery with Local Queuing (EDLQ) • Contacts Oracle • Q (e, t, s) = data queued for ‘e’ at ‘t’ if e = (s , *) = 0 otherwise • Per-hop routed  path vector to avoid loops • Earliest Delivery with All Queues (EDAQ) • Contacts + Queuing Oracles • Q (e, t, s) = data queued for ‘e’ at ‘t’ at node s • Source routed • Reservation of edge capacity along computed path

48. Complete Knowledge Routing

49. Complete Knowledge Routing

50. Complete Knowledge Routing