Using Network Virtualization Techniques for Scalable Routing

1
Using Network Virtualization Techniques for
Scalable Routing

• Nick Feamster, Georgia TechLixin Gao, UMass
  AmherstJennifer Rexford, Princeton University

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Virtualization for Routing

• Path Splicing (Georgia Tech)
• Run multiple instances of routing protocol
• End system signals which instance to use at each
  hop
• Exponential diversity gain, modest complexity
• Underlay Fused with Overlays (Princeton/GT)
• Support overlay functions in routers
• Efficient forwarding and scalable monitoring
• HORN Hybrid Routing for Overlay Networks (UMass
  Amherst)
• Different nodes see different detailed subgraph
• Availability of link state with good scalability

3
Multipath Promise and Problems
t
s

• Bad If any link fails on both paths, s is
  disconnected from t
• Want End systems remain connected unless the
  underlying graph is disconnected

4
Path Splicing Main Idea
Compute multiple forwarding trees per
destination.Allow packets to switch slices
midstream.
s

• Step 1 Run multiple instances of the routing
  protocol, each with slightly perturbed versions
  of the configuration
• Step 2 Allow traffic to switch between instances
  at any node in the protocol

Feamster, Motiwala, and Vempala, Path Splicing
with Network Slicing
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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

Base Graph
3
3
t
s
3
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Network Slicing

• Goal Allow multiple instances to co-exist
• Mechanism Virtual forwarding tables

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Path Splicing in Practice

• Packet has shim header with routing 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

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Reliability Approaches that of Underlying Graph

• 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
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Open Questions and Deployment

• Can end hosts react quickly enough to recover?
• How does the end system find the alternate path?
• How does splicing perform for other topologies?
• Deployment Paths
• VINI
• Overlay
• Wireless
• Software Router (e.g., Quagga)

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Splicing Possible Applications

• Fast recovery from poorly performing paths
• Convergence-free routing
• Data transfer
• Security Consistency checking
• Spatial diversity in wireless networks

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Path Splicing High Points

• Simple Opaque 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

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Underlay Fused with Overlays

• Main idea Layered routing architecture
• Supporting overlay functions on routers
• Blur boundary between overlays and underlays
• Efficient forwarding
• Overlay forwarding on line cards
• Hosting the overlay control plane
• Scalable monitoring
• Registration of overlay links
• Notification of network events

Zhu, Rexford, Feamster, Bavier, UFO A Resilient
Layered Routing Architecture
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Efficient Forwarding

• Problem traffic must traverse bottleneck link
  both inbound and outbound
• Solution reflection points in routers

Upstream ISP
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Forwarding on Router Line Cards
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Scalable Monitoring

• Notification preserves overlay link abstractions
• Message (overlay source, overlay destination,
  event)
• Routers store state by explicit overlay
  registration
• Notification events that affect performance of
  overlay applications
• Physical failures of routers or links
• Reachability failures withdrawal, session
  failure
• Network congestion

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Notification of Network Events

• Register for unidirectional overlay links
• A-gtB
• B-gtC

(A,B) (A,C)
(A,B) (A,C)
(A,B)
(A,B)
2
3
1
(A,C)
(A,C)
4
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HORN Scalability vs. Convergence

• Link state routing
• Not scalable
• A virtual network could be as large as the
  Internet
• Distance vector routing
• Convergence delay
• Scales better
• Idea A tunable routing protocol?
• Trade scalability for better availability
• Each slice runs a fixed protocol with tunable
  parameter

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HORNHybrid rOuting for oveRlay Networks
A-B-C
C-H

A-B-C-H
A-B A-B-C A-D A-D-E
C-H
E-G-H
D-G-H
A knows a partial topology (adjacent sub-graph)
via link state protocol, and learns routes from
the border nodes of the sub-graph.
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Benefits and Challenges

• Benefits Availability
• Tailor to specific topology
• Accommodate underlying network constraints
• limiting scope of failure notification,
  geographical proximity
• Challenges
• Potential stretch on path
• Expose mapping between overlay and underlay

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Summary and Question

• Network virtualization to cheat on scalability
  tradeoffs
• Path diversity vs. scalability
• Efficiency vs. scalability
• Convergence vs. scalability
• What are the common abstractions, functions, etc.
  that the substrate should provide?
• Slicing
• Nesting
• Knobs for granularity control
• ?