Transcript Ang's slides

Differential Provenance:
Better Network Diagnostics with Reference Events
Ang Chen
Yang Wu
Andreas Haeberlen
Wenchao Zhou+
University of Pennsylvania
Boon Thau Loo
Georgetown University+
Motivation: Finding the root cause of a symptom
Traffic arriving
at the wrong
server !?!
Overly specific flow entry
Internet
4.3.2.0/24
4.3.3.0/24
Bob
Web server 2
Web server 1
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DPI
Networks can (and frequently do!) have bugs
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Example: Software-defined networks
We need a good debugger!
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Debugging networks with provenance
C received packet
Packet P Packet P
B sent packet
A
B
C
B received packet
Rule match on B
A sent packet
A received packet
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Rule match on A
Typical debuggers tell us what happened:
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Rule installed by
controller
Incoming packet
at controller
NetSight: Packet histories
Y!: Network provenance
Key benefit: Rich explanation of what, when, and why.
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Problem: Explanation can be too big!
Rule 7:
Next-hop=port2
ot
ro
root
Root cause:
faulty rule
Packet arrives
at wrong server
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The problem: Finding the root cause in a large
provenance tree.
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Key insight: Use reference events!
S1
S2
S3
S4
S5
S6
Web server 2
Web server 1
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Bob
DPI
Remember that some packets were routed correctly.
The same things should have happened to all packets!
Key insight: If we have both a (bad) symptom and a (good)
reference, we only need to reason about the differences
between them!
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A new debugger
fault
Field 3 of config
entry 4 is wrong!
Bob
reference
Debugger
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Bob collects both a bad symptom and a good reference
Bob sends both events to the debugger
Debugger generates provenance, outputs difference
• Ideally, there is only one diff—the root cause!
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Outline
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Motivation: Network diagnostics
Background
Key insight
A new debugger
Differential provenance
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Are references typically available?
Strawman approach
Our approach
Initial results
- Conclusion
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Are references typically available?
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Survey:
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Posts on the ‘Outages’ mailing list in Sept-Dec 2014.
64 posts related to diagnostics.
42/64 (66%) posts involve both a fault and some reference.
Examples:
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Some DNS servers have stale records, but others are good
Probes sometimes fail, sometimes succeed
More examples in the paper
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Strawman solution
faulty rule
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root
root
= ?
new
root
Bad provenance
Reference provenance
root
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A strawman solution: Pick out different nodes in trees.
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Bad provenance: 201 nodes
Reference provenance: 156 nodes
Naïve diff: 278 nodes!
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Why does the strawman not work?
Faulty rule
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Observation: The diff can be larger than the individual trees.
Reason #1: Differences that “do not matter”
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E.g., timestamps, packet payloads, etc.
Reason #2: “Butterfly effect”
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A small difference can change later events drastically!
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Differential provenance
Output:
- Rule 7: change port
- Rule 9: change range
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Bad provenance
Reference provenance
Approach: Change past events, and think about what
could have happened.
• (1) Find some early ‘differences’ in the trees.
• (2) Change the faulty node to a correct equivalent.
• (3) Use replay to determine what would have happened.
• (4) Output the set of changes that align the trees.
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Technical challenges
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Challenge #1: Where do we start?
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Heuristics: Change early events, minimum changes…
E.g., prefer changing 1 event than 1000 events.
Challenge #2: How should we make the change?
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Approach: Think about what should have happened.
E.g., packet should go to switch 2, not 1.
Challenge #3: Irrelevant differences?
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Approach: Equivalence relations between events.
E.g., IPs 4.3.2.1 and 4.3.3.1
See paper for more details.
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Setup
Overly specific flow entry
Internet
Web server 1
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4.3.2.0/24
4.3.3.0/24
DPI
Setup
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Platform: RapidNet
SDN: 6 switches, 2 servers
The symptom: misrouted packets from 4.3.2.0/24
The reference: packets from 4.3.3.0/24
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Initial results
=
new
root
Fault: 201 nodes
Naïve diff
root
Reference: 156 nodes
=
Rule 7: next hop should
be port 1, not 2!
Differential provenance
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Differential provenance finds a single node (the faulty
rule) to be the root cause!
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Conclusion
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Debugging networks is hard
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Need good debuggers!
Provenance can find the causes of an event
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Problem: Explanation can be too detailed.
Idea: Use reference events
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Sufficient to find the (few) differences to the observed symptom
New debugger based on differential provenance
Result: Very precise diagnostics
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Ideally, can identify a single root cause!
Thanks!
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