1. Path Splicing with Network Slicing Nick Feamster Murtaza Motiwala Santosh Vempala

2. 2 Goals of Routing Reachability: allow endpoints to communicate with one another Scalability: scale to a large number of networks, destinations, routers, etc. High Diversity: expose paths to end hosts that survive failures –Capacity: the total available data rate between each source-destination pair should be high –Fault tolerance: the number of disjoint paths should be high, and the network should remain connected under failures Low Stretch: available paths should not be too circuitous Todays routing protocols do not exploit the diversity of the underlying network graph

3. 3 Multipath: 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 ts

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

5. 5 Contributions Two mechanisms for increasing path diversity Analytical and empirical analysis –only a few perturbations result in reliability that approaches that of the underlying graph Protocol design Preliminary deployment on VINI testbed

6. 6 Talk Outline Path Splicing –Mechanism #1: Perturbations –Mechanism #2: Network Slicing –Forwarding Definitions of path diversity –Connectivity –Expansion –Our metric: reliability Properties –High Reliability –Bounded Stretch Evaluation and Implementation status Extensions to splicing: BGP splicing

7. 7 Mechanism #1: Perturbations 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 ts 3 3 3 Base Graph ts 3.5 4 5 1.5 1.25 Perturbed Graph

8. 8 Perturbation Schemes Uniform: Perturbation is a function of the initial weight of the link Degree-based: Perturbation is a function of the degrees of the incident nodes –Intuition: Deflect traffic away from nodes where traffic might tend to pass through by default

9. 9 Mechanism #2: Network Slicing Goal: Allow multiple instances to co-exist Mechanism: Virtual forwarding tables a t c s b t a t c Slice 1 Slice 2 dstnext-hop

10. 10 Forwarding Traffic with Path Splicing Packet has shim header with forwarding bits Routers use lg(k) bits to index forwarding tables –Shift bits after inspection –Incremental deployment is trivial –Persistent loops cannot occur To access different (or multiple) paths, end systems simply change the forwarding bits

11. 11 Forwarding Operation End system sets forwarding bits in packet header Forwarding bits specify slice to be used at any hop Router: examines/shifts forwarding bits, and forwards t s Another idea: use both paths in parallel

12. 12 Definitions of Path Diversity Connectivity: Minimum number of edges whose failure disconnects the graph (min cut) Expansion: Intuitively, small cuts disconnect small groups of nodes from the graph

13. 13 A Definition Motivated by Reliability 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.

14. 14 Reliability Curve: Illustration Probability of link failure (p) Fraction of source-dest pairs disconnected Better reliability More edges available to end systems -> Better reliability

15. 15 Property: High Reliability GEANT (Real) and Sprint (Rocketfuel) topologies 1,000 trials p indicates probability edge was removed from base graph Reliability approaches optimal Average stretch is only 1.3 GEANT topology, degree-based perturbations

16. 16 Property: Bounded Stretch Stretch: How much longer is the path taken by packets over the optimal path? –Sub-concern: What about loops? Stretch is bounded in one slice by amount of perturbation …but what about the stretch of spliced paths? –As long as significant progress (a large fraction of the distance to d) is achieved for each hop, stretch is bounded Finite number of splicing bits limits the number of times a packet can switch slices –Eventually, packet ends up in a single slice

17. 17 Significant Novelty for Modest Stretch Novelty: difference in nodes in a perturbed shortest path from the original shortest path Example s d Novelty: 1 – (1/3) = 2/3 Fraction of edges on short path shared with long path

18. 18 Other Properties Scalable –Exponential increase in paths, linear increase in state Fast recovery from underlying failures Automatic tuning (e.g., for traffic engineering) –Perturbations achieve property of automatically spreading traffic across different links –Standard link-weight optimization is potentially brittle in the face of link failures Incrementally deployable

19. 19 Prototype Implementation Click and Quagga on PL-VINI –http://www.vini-veritas.net/ Control Plane Forwarding Table Daemon Classifier Control Plane Forwarding Table Daemon

20. 20 Variation: BGP Splicing Observation: Many routers already learn multiple alternate routes to each destination. Idea: Use the forwarding bits to index into these alternate routes at an ASs ingress and egress routers. Storing multiple entries per prefix Indexing into them based on packet headers Selecting the best k routes for each destination Required new functionality d default alternate Splice paths at ingress and egress routers

21. 21 Inter-AS Loops Problem: Potential for loops between ASes –AS-level loops can be longer than intra-AS loops Two possible approaches –Detection: routers mark packets and determine that packets have traversed the same AS twice –Prevention: Exploit common routing policies to ensure that packets are only deflected along valley- free paths

22. 22 Preventing Inter-AS Loops with Policy Observation: inter-AS loops inherently involve traversal that violates valley-free Constraints: 1. once a down deflection has occurred, do not deflect 2. only allow one across deflection Possible relaxation: allow a limited number of violations, specified by source

23. 23 Possible Application: Routing Security One Idea: Mitigating BGP route hijacks –End systems or routers learn multiple routes to each destination –Alternate paths at any intermediate point along the path can be tested by twiddling bits in the header Service that uses splicing to alternate paths to discover several valid ones

24. 24 Questions Can the end hosts react fast enough to recover from failures? –How does the end system find the alternate path? How does splicing perform for other topologies? –Other ISP networks –Overlay networks Interaction with traffic engineering?

25. 25 Related Work Pre-Computed Backup Paths –Multi-Topology Routing –Multiple Router Configuration –MPLS Fast Reroute End-Node Controlled Traffic –Source routing –Routing deflections Multipath routing (ECMP, MIRO, etc.) IGP link-weight optimization Measurement of path diversity and multihoming Layer-3 VPNs

26. 26 Possible Applications Fast recovery from poorly performing paths Fast data transfer with easy multi-path Security applications Overlay networks, CDNs, etc. Spatial diversity in wireless networks

27. 27 Summary Simple: Routing 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 No modifications to existing routing protocols http://www.cc.gatech.edu/~feamster/papers/path-splicing.pdf