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Slide #1IETF 75 – Roll WG – July 2009 RPL (pronounced ripple) Routing Protocol for Low Power and Lossy Networks IETF 75 status draft-dt-roll-rpl-01.txt.

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Presentation on theme: "Slide #1IETF 75 – Roll WG – July 2009 RPL (pronounced ripple) Routing Protocol for Low Power and Lossy Networks IETF 75 status draft-dt-roll-rpl-01.txt."— Presentation transcript:

1 Slide #1IETF 75 – Roll WG – July 2009 RPL (pronounced ripple) Routing Protocol for Low Power and Lossy Networks IETF 75 status draft-dt-roll-rpl-01.txt Anders Brandt Thomas Heide Clausen Stephen Dawson-Haggerty Jonathan W. Hui Kris Pister Pascal Thubert Tim Winter

2 RPL Status New draft draft-dt-roll-rpl-01 From DT named by chairs Topics –Why Distance Vector? –Why Directed Acyclic Graphs? –What is the Objective Function? –Why detach / merge? Why handle loops this way? –Why route stretching? Currently discussed issues Slide #2IETF 75 – Roll WG – July 2009

3 Slide #3IETF 75 – Roll WG – July 2009 Why Distance Vector? Trade-off between capabilities and resources for highly constrained devices

4 Why Distance Vector? Trade off between capabilities and resources for highly constrained devices Distance Vector is widely used in WSN Simplicity => more control cost over time Need to handle classical DV issues Improvements proposed –Trickle, loop detection, dominating set Slide #4IETF 75 – Roll WG – July 2009

5 DV protocol in IP Networks RIP –Unfit for large, unstable & complex topologies –Sync effect, transient loops, count to infinity –Partial fixes by split horizon (poison reverse) More recent protocols –Trade RIPs loops for temporary black holes –Path hold-down then route poisoning –Quarantine path for which hop counts goes up Slide #5IETF 75 – Roll WG – July 2009

6 DV protocol in IP Networks (2) Distance Increase Condition (as proven by Jaffe and Moss) –One cannot create a loop by adopting a lower cost path upon a decrease information or the lowest path cost ever at any time Current Successor or Source Node cond. (as proven by Garcia-Luna-Aceves) –One cannot create a loop by adopting a next hop thats nearer than either the current next hop or self ever was. Slide #6IETF 75 – Roll WG – July 2009

7 DV protocol in IP Networks (3) In short, it is safe to pick a shortest path upon a neighbor distance decrease or if that yields the shortest path ever for either self or a next hop. This translates into picking a parent thats shallower then self ever was is safe whereas picking a parent thats deeper is not safe. Question is how to restart ever Slide #7IETF 75 – Roll WG – July 2009

8 Predictable routing behavior? Unpredictable network characteristics –variable metrics, –Unpredictable reachability Goal: –acceptable result, –most time for most nodes Many ways this can be arranged –None perfect –None permanent Slide #8IETF 75 – Roll WG – July 2009

9 Slide #9IETF 75 – Roll WG – July 2009 Why Directed Acyclic Graphs? DAGs enable (re)route around transient loss

10 DAG As opposed to trees –Enable (re)route around loss/interference –Adapt to transient changes –A MUST in requirement drafts Route along the DAG –Adapted for P2MP and MP2P patterns –Enables P2P as well Can be extended for optimum P2P Slide #10IETF 75 – Roll WG – July 2009

11 The perfect DAG? Since there no perfect wires with constant properties theres hardly a perfect DAG Conditions change, batteries deplete Need to arbiter between conflicting metrics and goals Build a structure that is satisfactory for all Maintain with probably a slow evolution Slide #11IETF 75 – Roll WG – July 2009

12 Slide #12IETF 75 – Roll WG – July 2009 What is the Objective funtion? Thats a plugin that generalizes the notion of depth

13 Can one scalar metric fits it all? Parent selection is a case specific logic –Different objectives for different networks –E.g. compare battery (levels?) then compare RSSI then ETX then bandwidth (available?)… That Objective Function accounts for –statistical information (ETX, use, loss) –boolean information (battery operated) –physical information (max bandwidth) –configuration (preference, security level…) Slide #13IETF 75 – Roll WG – July 2009

14 The Objective Function Varies per use case. –The idea is to specify a few ones –Will move to metrics draft if this is adopted –Let SDOs and suppliers define alternates RA-DIO {{Metrics}, Objective Function, Depth computation} –OFs should prefer a matching DAG. –Loop avoidance is generic based on depth IN: information from potential parents OUT: depth metric, preferred parent list Slide #14IETF 75 – Roll WG – July 2009

15 The abstract depth metric Stable and Scalar for simple comparison Strictly monotonous to orient the parent links and sort out siblings Universal thus very abstract Coarse grained to enable multiple parents and siblings Constrained in range to count rapidly to infinity if that can happen in the protocol Slide #15IETF 75 – Roll WG – July 2009

16 Slide #16IETF 75 – Roll WG – July 2009 Why detach / merge for ? Why handle loops this way? This implement advanced DV concepts (from DUAL) to avoid some loops and Count to Infinity

17 The DAG ID The network is split is drainage systems identified by DAG ID. This enables to limit the consequences of a sink loss Also enables to mark a frozen subgraph The protocol enables a smooth DAG split and merge using the Dag hop timer Slide #17IETF 75 – Roll WG – July 2009

18 Loop avoidance vs. detection RPL can not afford the extra cost of ack and/or Reliable Multicast. Because of radio conditions, nodes might be temporarily unreachable anyway So the choice is this: positive assurance of a new path OR reactive loop detection We need both if the positive assurance is optionally executed. Slide #18IETF 75 – Roll WG – July 2009

19 Say we loose D->B D can not reattach deeper But it can attach to a different tree Remember the DUAL freezing: we need to identify which nodes are impacted and cut them off Thats when ever starts in the shortest path ever Slide #19 IETF 75 – Roll WG – July 2009 What is that detaching business? A LBR-1 GH F 7 E BC D I 2 3 3

20 Say we loose D->B D does not have a parent anymore If D could move deeper it might select I and be lucky Or it might select H and create a loop D->H->F->D Remember the distance increase condition: only reducing the depth is safe Slide #20 IETF 75 – Roll WG – July 2009 Why not reattach deeper? A LBR-1 GH F 7 E BC D I 2 3 3

21 Say we loose D->B Then D detaches and we want to reattach Without Dag Hop Timer, F would attach to A and maybe even H to F. Then F would have to move to D and H to I, causing churn in the sub-DAG. DHT makes D jump first because it ends up shallower, then H and F, then G Slide #21 IETF 75 – Roll WG – July 2009 What is the DAG Hop Timer for? A LBR-1 GH F 7 E BC D I 2 3 3

22 Split horizon in RIP prevents a one hop feedback loop by not advertizing a destination to a corresponding next hop This draft prevents all RADIO feedback loops by forcing all nodes to ignore RADIOs from deeper FOR STABILITY. All nodes look towards the sink and are not impacted by changes in their back. Slide #22IETF 75 – Roll WG – July 2009 Why Ignore RAs from deeper? G H F Depth n Depth n G Depth n+2 1 H 1 Depth n+3

23 Slide #23IETF 75 – Roll WG – July 2009 Route stretching? Acceptable trade-off Inherent with constrained states.

24 Non shortest path ? Usually Very small stretch Avoid wasting invested resoures to get the packet half way. Slide #24IETF 75 – Roll WG – July 2009

25 Missed anything? Why Distance Vector? => DV Enables decent trade off between capabilities and resources for highly constrained devices Why Directed Acyclic Graphs => DAGs Enable (re)route around transient loss What is the Objective Function? => Thats a plugin that generalizes the notion of depth Why detach / merge? Why handle loops this way? => This implement advanced DV concepts (from DUAL) to avoid some loops and Count to Infinity Why route stretching? => Thats Acceptable trade-off - Inherent with constrained states. Slide #25IETF 75 – Roll WG – July 2009

26 Slide #26IETF 75 – Roll WG – July 2009 Issues under discussion

27 Source Routing Needs to be supported Needs fleshing out in the draft Address compression Slide #27IETF 75 – Roll WG – July 2009

28 Binding to Neighboring protocol RPL binds to IPv6 ND for a number of reasons: –Its there –Its reactive to traffic Potential for other bindings –Hello, NHDP … –Thomas to propose a more generic view though ND still preferred for reasons above Slide #28IETF 75 – Roll WG – July 2009

29 RA-DIO loss Might cause a child to ignore that a parent has detached Might end up in a loop unless a positive assurance of a loop free path –Proposal in the draft: a sequence number –Alt: test the path (probe sink via candidate) Also possible: a reliable multicast for RAs Note: EIGRP acks update as and queries Slide #29IETF 75 – Roll WG – July 2009


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