Path Splicing Nick Feamster, Murtaza Motiwala, Megan Elmore, Santosh Vempala
Jan 18, 2016
Path Splicing
Nick Feamster, Murtaza Motiwala, Megan Elmore, Santosh Vempala
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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”
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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
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Path Splicing: Main Idea
• 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
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Compute multiple forwarding trees per destination.Allow packets to switch slices midstream.
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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
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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
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“Base” Graph
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1.5
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Perturbed Graph
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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
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Constructing Paths
• Goal: Allow multiple instances to co-exist• Mechanism: Virtual forwarding tables
a
t
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s b
t a
t c
Slice 1
Slice 2
dst next-hop
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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
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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
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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
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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
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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?
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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.
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Reliability Curve: Illustration
Probability of link failure (p)
Fraction of source-dest pairs disconnected
Better reliability
More edges available to end systems -> Better reliability
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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)
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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
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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.
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Significant Novelty for Modest Stretch
• Novelty: difference in nodes in a perturbed shortest path from the original shortest path
Example
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Novelty: 1 – (1/3) = 2/3
Fraction of edges on short path shared with long path
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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.
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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
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Interactions with Traffic
Maximum utilization unaffected
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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.
• Storing multiple entries per prefix • Indexing into them based on packet headers• Selecting the “best” k routes for each destination
Required new functionality
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alternate
Splice paths at ingress and egress routers
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Interdomain Splicing Header
• Intradomain bits function as before• Interdomain: Three sections
– Ingress and egress– Policy: restrict “illegal” entries in the forwarding table
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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
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Interdomain Splicing: Reliability
2-slice deployment approaches best possible
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Incremental Deployment
Partial deployment provides some gains
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Comparison: Routing Deflections
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Effects on Traffic• 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)
Abilene Sprint
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Effects on Traffic
Abilene Topology
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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
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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?
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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