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PRoPHET Update

DTN Research Group @ IETF 78 27 March 2010 Elwyn Davies elwynd@folly.org.uk Avri Doria Samo Grasič Anders Lindgren avri@psg.com samo@grasic.net dugdale@gmail.com. PRoPHET Update. Progress with PRoPHET New Version http://tools.ietf.org/html/draft-irtf-dtnrg-prophet-06.

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PRoPHET Update

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  1. DTN Research Group @ IETF 78 27 March 2010 Elwyn Davies elwynd@folly.org.uk Avri Doria Samo Grasič Anders Lindgren avri@psg.comsamo@grasic.netdugdale@gmail.com PRoPHET Update

  2. PRoPHET Update - IRTF 78 Progress with PRoPHETNew Versionhttp://tools.ietf.org/html/draft-irtf-dtnrg-prophet-06 • PRoPHET in use in N4C testbeds • Slovenia (continuous use for almost 2 years) • Arctic Sweden (Summer testing) • Simulation work by Samo and Elwyn • Draft update includes • Addressimg IRSG review comments • Improvements resulting from recent experiments • Additional capabilities to fix long standing issue

  3. PRoPHET Update - IRTF 78 Experiments in Progress - 1 • Slovenia • Gradually extending network of unattended environmental sensors (meteo and radiation) • Data mules • Active nodes in cars • Symbiotic (Parasitic) nodes – just a USB memory stick • Using PRoPHET routing • Analysis of results has found problem with transitive update algorithm

  4. PRoPHET Update - IRTF 78 Experiments in Progress - 2 • Arctic Sweden • See reports – previous DTNRG slides! • GSM to edge of area, Helicopter mules • Two systems running in parallel • 'Village Router' hot spot system - static routing/DTN2See http://village.n4c.eu - need to set HTTP proxy for best effect! Email and web caching proxy system • PRoPHET routing nomadic systemSee http://dtn.n4c.eu for more info, NSIM, web cam, logs, etc • Two months testing during reindeer marking/ hiking season in Padjelanta

  5. PRoPHET Update - IRTF 78 Refresher: Key Equations • Notation: P(A,B): Delivery Predictability in Node A for delivering a bundle to Node B. • Eqn 1: On A encountering B • P_(A,B) = P_(A,B)_old + ( 1 - P_(A,B)_old ) * P_encounter • Eqn 2: If A hasn't encountered D for a while: • P_(A,D) = P_(A,D)_old * gamma^K • Eqn 3: Does A meeting B mean that A is a better bet for delivering to C? Transitivity... • P_(A,C) = P_(A,C)_old + ( 1 - P_(A,C)_old ) * P_(A,B) * P_(B,C) * beta

  6. PRoPHET Update - IRTF 78 Simulation Work - 1 • Samo working to reproduce and validate results from RAPID paper comparison • PRoPHET came out very poorly in this work • Slow progress due to various problems • Competing demands of test bed deployment • Acquisition of simulation code used by RAPID team • Found RAPID PRoPHET implementation didn't match specification • Recently got comparative results using ONE sim • #1 conclusion: match proto to scenario • Otherwise random routing will work better!

  7. PRoPHET Update - IRTF 78 Simulation Work 2 • Elwyn has been working on visualizing the evolution of delivery predictability • Simple Excel Spreadsheet simulator • Python program + Excel display using mobility files generated by Anders original prpogram. • Using these to check how DPs evolve and ensure that they reflect the expected future encounters. • Evaluated proposed alternative evolution equations • Selected replacements now in field trials

  8. PRoPHET Update - IRTF 78 Main Problems Identified • Inappropriate evolution of delivery predictabilities (DPs) • Repeated encounters of '3rd party' nodes leads to jammed bundles . • Parking Lot Problem – When is an encounter not a (new) encounter? • Totally random encounters => noise in DPs • Deriving a relationship between the various PRoPHET parameters • Need guidelines for setting parameters appropriately

  9. PRoPHET Update - IRTF 78 Bundles Jammed in 3rd Parties • Scenario (at least 4 nodes: A - D): • Node B meets Node A & (separately) C occasionally. • Nodes C and D meet each other repeatedly between meetings of Nodes B.and C • DP for Node A increases steadily in Nodes C & D until bigger than DP for Node A in Node B • Result: Bundles for A not transferred from C/D to B; hence jam in C and are never delivered. • Identified in long term Slovenian experiment • Fix: Improved transitivity update algorithm

  10. PRoPHET Update - IRTF 78 Parking Lot Problem • Scenario: • Multiple Wi-FI nodes milling around in a small area • Multiple 'encounters' e.g. due to Wi-Fi reconnections. • Can result in • DP update 'loops' – changes propagated round loop • One actual encounter being treated as several • Identified from theoretical considerations • Fix: Allow nodes to suppress DP updates

  11. PRoPHET Update - IRTF 78 Noise in DPs from Random Encounters • Scenarios in which a node encounters lots of nodes that it will never meet again • Typical of some simulation scenarios • Can lead to random routing outperforming PRoPHET (and other algorithms) if there is little pattern to encounters • Make sure you are using PRoPHET in an appropriate scenario! • Fix: Set P_(A,B) to a lower value on 1st meeting • Could avoid applying transitivity on first encounter

  12. PRoPHET Update - IRTF 78 Too Many Degrees of Freedom? • PRoPHET has a lot of (apparently) independent parameters. • There wasn't a good story on how to choose a consistent set of parameters. • Fix: Describe how they each relate to the 'characteristic time period' of the PRoPHET zone. • and this also relates to the approptiate bundle lifetime and custody timer settings

  13. PRoPHET Update - IRTF 78 Improvements: Equation 1 • Reduce the effect of random, one-off encounters • On first encounter set DP to a lower value • Suggest 0.25 rather than the 0.75 in old version • Do the same if the DP has decayed below a threshold because last encounter was a long time ago. • Use existing equation 1 on subsequent encounters when P_(A,B) not below threshold

  14. PRoPHET Update - IRTF 78 Improvements: Equation 1 • Avoid DP actually reaching 1 ('Almost Sure') on repeated encounters • New equation: • P_(A,B) = P_(A,B)_old + ( 1 - delta - P_(A,B)_old ) * P_encounter • Delta is a small value that stops the value getting right to 1.0

  15. PRoPHET Update - IRTF 78 Improvements: Equation 3 • Avoid DP increasing inappropriately due to repeated encounters between 3rd parties that haven't actually met suitable intermediaries. • The improved equation is intended to converge the DPs for C in A and B after repeated encounters BUT only to the highest value that either of them started with • P_(A,C) = MAX( P_(A,C)_old, P_(A,B) * P_(A,C) * beta ) • Beta is a 'slowing factor' – reduces convergence rate

  16. PRoPHET Update - IRTF 78 Characteristic Time Period • Concept: A ProPHET zone has a characteristic time period (CTP) that is determined by the expected time intervals between encounters • Bundle expiry times should be set so that bundles will be around sufficiently long to be delievered but not hang around for too long • DP decay should reduce a DP to essentially zero after 3 to 5 multiples of CTP • An encounter should at least undo the expected decay between encounters • Treat closely spaced encounters as one

  17. PRoPHET Update - IRTF 78 Improvements: Suppress DP Recalc • Added a flag into the Hello TLV that requests suppression of DP recalculation phase. • Has to be agreed by both parties in an encounter • Otherwise recalculation is done on both sides • Nodes can choose to suppress recalculation if they have met very recently and they don't have new information to convey • Very recently means (say) 5% of CTP

  18. PRoPHET Update - IRTF 78 Progress towards RFC • RG Last Call completed • Various clarifications (especially thanks to Kevin) • Currently in IRSG Review process • Somer additional clarifications added in latest version (-06) • Some significant improvements since review • May need to repeat last call/review

  19. PRoPHET Update - IRTF 78 New Protocol Version • The protocol has been taken to version 2 as the changes to the equations and Hello TLV are not fully interoperable.

  20. PRoPHET Update - IRTF 78 Next Steps • Pass the improvements to IRSG reviewer • Check performance of improvements in the field • Do we need another Last Call? • There have been significant updates

  21. PRoPHET Update - IRTF 78 Thank You

  22. PRoPHET Update - IRTF 78 DTN2 Functional Spec • Product of N4C work – Elwyn and Avri Doria • Nearly 200 pages!! • Currently accessible athttp://www.n4c.eu/Download/n4c-wp2-023-dtn-infrastructure-fs-12.pdf • Contains: • Spec of API • A complete specification of the TCL command interface • Detailed operation of the core specification • Missing – Sessions and External Router/CL

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