1 / 33

Path Splicing

Path Splicing. Nick Feamster, Murtaza Motiwala, Megan Elmore, Santosh Vempala. Idea: Backup/Multipath. For intradomain routing IP and MPLS fast re-route Packet deflections [Yang 2006] ECMP, NotVia, Loop-Free Alternates [Cisco] For interdomain routing MIRO [Rexford 2006] Problem

aileen
Download Presentation

Path Splicing

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Path Splicing Nick Feamster, Murtaza Motiwala, Megan Elmore, Santosh Vempala

  2. Idea: Backup/Multipath • For intradomain routing • IP and MPLS fast re-route • Packet deflections [Yang 2006] • ECMP, NotVia, Loop-Free Alternates [Cisco] • For interdomain routing • MIRO [Rexford 2006] • Problem • Scale:Protecting against arbitrary failures requires storing lots of state, exchanging lots of messages • Control:End systems can’t signal when they think a path has “failed”

  3. Backup Paths: Promise and Problems • Bad: If any link fails on both paths, s is disconnected from t • Want:End systems remain connected unless the underlying graph has a cut s t

  4. t Path Splicing: Main Idea Compute multiple forwarding trees per destination.Allow packets to switch slices midstream. • Step 1 (Generate slices): Run multiple instances of the routing protocol, each with slightly perturbed versions of the configuration • Step 2 (Splice end-to-end paths): Allow traffic to switch between instances at any node in the protocol s

  5. Outline • Path Splicing for Intradomain Routing • Generating slices • Constructing paths • Forwarding • Recovery • Evaluation • Reliability and recovery • Stretch • Effects on traffic • Path Splicing for Interdomain Routing • Ongoing: Prototype and Deployment Paths

  6. Perturbed Graph 1.5 4 1.5 5 s t 1.25 3.5 Generating Slices • Goal: Each instance provides different paths • Mechanism: Each edge is given a weight that is a slightly perturbed version of the original weight • Two schemes: Uniform and degree-based “Base” Graph 3 3 s t 3

  7. How to Perturb the Link Weights? • Uniform: Perturbation is a function of the initial weight of the link • Degree-based:Perturbation is a linear function of the degrees of the incident nodes • Intuition: Deflect traffic away from nodes where traffic might tend to pass through by default

  8. a s t b dst next-hop c t a Slice 1 t c Slice 2 Constructing Paths • Goal: Allow multiple instances to co-exist • Mechanism: Virtual forwarding tables

  9. Forwarding Traffic • One approach: shim header with forwarding bits • Routers use lg(k) bits to index forwarding tables • Shift bits after inspection • To access different (or multiple) paths, end systems simply change the forwarding bits • Incremental deployment is trivial • Persistent loops cannot occur • Other variations are possible

  10. Alternate Approach • Use fewer bits per packet • Each router along the path uses the same set of random bits as an input to select the next hop • Advantages • Less per-packet overhead • Disadvantages • Less direct control over path • No explicit prevention of loops

  11. Recovery Mechanisms • End-system recovery • Switch slices at every hop with probability 0.5 • Network-based recovery • Router switches to a random slice if next hop is unreachable • Continue for a fixed number of hops until destination is reached 12

  12. Availability Evaluation: Two Aspects • Reliability: Connectivity in the routing tables should approach the that of the underlying graph • If two nodes s and t remain connected in the underlying graph, there is some sequence of hops in the routing tables that will result in traffic • Recovery:In case of failure (i.e., link or node removal), nodes should quickly be able to discover a new path

  13. Availability Evaluation • A definition for reliability • Does path splicing improve reliability? • How close can splicing get to the best possible reliability (i.e., that of the underlying graph)? • Can path splicing enable fast recovery? • Can end systems (or intermediate nodes) find alternate paths fast enough?

  14. Reliability Definition • Reliability:the probability that, upon failing each edge with probability p, the graph remains connected • Reliability curve:the fraction of source-destination pairs that remain connected for various link failure probabilities p • The underlying graph has an underlying reliability (and reliability curve) • Goal: Reliability of routing system should approach that of the underlying graph.

  15. Reliability Curve: Illustration Fraction of source-dest pairs disconnected Better reliability Probability of link failure (p) More edges available to end systems -> Better reliability

  16. Experimental Setup • Evaluation on two topologies • GEANT (Real) and Sprint (Rocketfuel) • Compute base graph by taking the union of k perturbed graphs • Remove an edge from the base graph with probability p • Compute number of pairs that could reach one another (average over 1,000 trials)

  17. Reliability Approaches Optimal • Sprint (Rocketfuel) topology • 1,000 trials • p indicates probability edge was removed from base graph Reliability approaches optimal Average stretch is only 1.3 Sprint topology,degree-based perturbations

  18. Simple Recovery Strategies Work Well • Which paths can be recovered within 5 trials? • Sequential trials: 5 round-trip times • …but trials could also be made in parallel Recovery approaches maximum possible Adding a few more slices improves recovery beyond best possible reliability with fewer slices.

  19. Significant Novelty for Modest Stretch • Novelty: difference in nodes in a perturbed shortest path from the original shortest path Fraction of edges on short path shared with long path Example s d Novelty: 1 – (1/3) = 2/3

  20. Evaluation Summary: Splicing Can Improve Availability • Reliability: Connectivity in the routing tables should approach the that of the underlying graph • Approach: Overlay trees generated using random link-weight perturbations. Allow traffic to switch between them • Result: Splicing ~ 10 trees achieves near-optimal reliability • Recovery:In case of failure, nodes should quickly be able to discover a new path • Approach: End nodes randomly select new bits • Result: Recovery within 5 trials approaches best possible.

  21. Does Splicing Create Loops? • Persistent loops are avoidable • In the simple scheme, path bits are exhausted from the header • Never switching back to the same • Transient loops can still be a problem because they increase end-to-end delay (“stretch”) • Longer end-to-end paths • Wasted capacity • Two-hop loops do occur (around 1 in 100 trials for k=2, more for higher values of k), but can be avoided with the mechanisms above

  22. Interactions with Traffic Maximum utilization unaffected

  23. Required new functionality • Storing multiple entries per prefix • Indexing into them based on packet headers • Selecting the “best” k routes for each destination Path Splicing for Interdomain Routing • Observation: Many routers already learn multiple alternate routes to each destination. • Idea: Use the bits to index into these alternate routes at an AS’s ingress and egress routers. default d alternate Splice paths at ingress and egress routers

  24. Interdomain Splicing Header • Intradomain bits function as before • Interdomain: Three sections • Ingress and egress • Policy: restrict “illegal” entries in the forwarding table

  25. Experimental Setup • 2,500-node policy-annotated AS graph • Use C-BGP to compute routes on base graph • Remove each inter-AS edge with probability p • Test connectivity between a random subset of AS pairs • Compute base reliability without policy restrictions

  26. Interdomain Splicing: Reliability 2-slice deployment approaches best possible

  27. Incremental Deployment Partial deployment provides some gains

  28. Comparison: Routing Deflections 29

  29. Effects on Traffic Sprint Abilene • Data: Abilene traces and synthetic Sprint traffic • Observations: • No adverse effects on traffic • Slightly balances traffic in the network • Due to longer paths, the overall utilization increases but is not significant ( ~ 4% for Abilene) 30

  30. Effects on Traffic Abilene Topology 31

  31. Ongoing Work • Software implementation • Click Element • PlanetLab/VINI deployment • Open questions • What API should the network layer provide? • How to perform monitoring/failure detection? • Extension to Cisco Multi-Topology Routing • In-progress IETF draft

  32. Open Questions and Ongoing Work • How does splicing interact with traffic engineering? Sources controlling traffic? • What are the best mechanisms for generating slices and recovering paths? • Can splicing eliminate dynamic routing?

  33. Conclusion • Simple: Forwarding bits provide access to different paths through the network • Scalable: Exponential increase in available paths, linear increase in state • Stable: Fast recovery does not require fast routing protocols http://www.cc.gatech.edu/~feamster/papers/splicing.pdf

More Related