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A Narrow Waist for Multipath
Routing
Murtaza Motiwala
Bilal Anwer, Mukarram bin Tariq
David Andersen, Nick Feamster
Many Threats to Availability
•
•
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•
•
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Natural disasters
Physical failures (node, link)
Router software bugs
Misconfiguration
Mis-coordination
Denial-of-service (DoS) attacks
Changes in traffic patterns
(e.g., flash crowd)
• …
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
– Complexity and 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”
Two Questions
• What is the appropriate mechanism for
achieving multiple paths?
– One example: Path Splicing
• What is the appropriate interface for allowing
end systems access to multiple paths?
– Path Bits: A “narrow waist” for Internet routing
Backup Paths: Promise and Problems
s
t
• 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
Path Splicing: Main Idea
Compute multiple forwarding trees per destination.
Allow packets to switch slices midstream.
s
t
• 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
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
Perturbed Graph
3
s
3
1.5
t
3
s
5
1.25
4
1.5
3.5
t
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
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 (2nd half of talk)
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
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.
Two Questions
• What is the appropriate mechanism for
achieving multiple paths?
– One example: Path Splicing
• What is the appropriate interface for allowing
end systems access to multiple paths?
– Path Bits: A “narrow waist” for Internet routing
Allow for Innovation Above & Below
• Different applications & uses for multipath
– Performance and load balancing
– Availability
• Different mechanisms
– Routing protocols: path splicing, ECMP, R-BGP,
NIRA, MIRO, …
– Implementation platforms: proprietary solutions, Click,
NetFPGA, OpenFlow
Need: Common interface for multipath routing.
A Narrow Waist for Multipath Routing
Decouple End Hosts from Protocols
• Easy access to multiple paths
• Consistent path selection per-host
– Same encoding should always yield the same path
• Interoperation among many different multipath
routing protocols
– Different switches and different networks may have
different implementations
Simple Interface and Implementation
• Ease of use at end systems
• Efficiently implementable in the network
• Scalable access to a large number of paths
– End systems should not have to store state to
represent all paths
– Routers should not have to store forwarding table
state for all paths
Path Bits: Three-Part Design
• Path bits interface
– Level of control
– Number of bits
• End system support
– Monitoring framework
– Packet interception
• Network control
– Indexing into forwarding tables
Decision: Level of Control
• Path bits are opaque
– Carry no explicit semantics
– Instead, provide the following:
Changing the bits, will, with high probability,
change the forwarding path
• Satisfies the two design goals
– End-host interface is decoupled from implementation
– Interface is simple: few changes required
Decision: Number of Bits
• A small number of bits can provide sufficient
flexibility.
– Example: Path splicing
• Possible to map many existing multipath
implementations to the interface
End-System Implementation
• Socket capture library
– Intercept connect() and
sendto()
• Kernel interface
– Click module
– Keeps track of active
flows & bits for those
flows
• Monitoring daemon
Network Implementations
Key Idea: Multiple forwarding table entires with bits to index.
Different routing protocols simply change what is in the tables
and how the bits are mapped to this interface.
• Software implementations in Click
– Splicing, Deflections, ECMP++
• Hardware implementations
– Intel IXP: Splicing (~30 lines of C++ plug-in)
– OpenFlow: kN entries per slice, ~20-25 lines at NOX
controller
– NetFPGA: Small additional overhead beyond base
router implementation
Path Splicing in Click
Eight lines of C++.
Routing Deflections in Click
Nine lines of C++.
NetFPGA Implementation
• Eight slices
• About 69% of
available BRAM
on NetFPGA
(base router uses
53% of available
BRAM)
NetFPGA Forwarding Performance
No noticeable performance penalty
with four tables.
Applications with Path Bits: Recovery
Single-Path Routing
(Baseline)
Path Splicing
ECMP++
Conclusion
• Many uses and applications for multipath routing
– Availability
– Performance
• Many implementations of protocols
– Path splicing: Simple, scalable, stable
– Routing deflections
• Path bits: narrow waist allows applications and
implementations to evolve independently
– Decouples end host request from network
implementation/protocol
– Affords very simple implementations
– Many environments: Data center, interdomain,
intradomain, etc.