SkipNet: A Scalable Overlay Network with Practical Locality Properties

1
SkipNet A Scalable Overlay Network with
Practical Locality Properties

• Nick Harvey, Mike Jones,
• Stefan Saroiu, Marvin Theimer, Alec Wolman
• Presented by Qingqing Yuan

2
Overlay Networks

• Overlays have achieved several goals
• Scalable and decentralized infrastructure
• Uniform and random load and data distribution
• But, at the price of data controllability
• Data may be stored far from its users
• Data may be stored outside its domain
• Local accesses leave local organization

3

• Basic trade-off data controllability vs. data
  uniformity
• SkipNet
• Traditional overlay functionality
• Provides an abstraction to control this
  trade-off
• Constrained load balancing (CLB)

4
Outline

• Basic SkipNet design
• SkipNet locality properties
• Performance evaluation
• Conclusions

5
Outline

• Basic SkipNet design
• SkipNet locality properties
• Performance evaluation
• Conclusions

6
Why not a DHT?

• Controlling Data Location is not the goal of a
  DHT
• DHTs provide load balancing at the price of
  where data is stored
• May be stored far away
• May be stored out of the domain
• Destroy Locality
• Discard useful application-specific information

7
Structure of SkipNet Perfect Skip Lists
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SkipNet

• Key property two address spaces
• Name ID space nodes are sorted by their names
  (e.g. DNS names)
• Numeric ID space nodes are randomly distributed
• Combining both spaces achieves
• Content Path locality
• Scalable peer-to-peer overlay network
• O(log N) routing performance in both spaces
• O(log N) routing state per node

9
SkipNet Ring

• Pointers at level h skip over 2h nodes
• Nodes are ordered by names

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SkipNet Ring

• Pointers at level h skip over 2h nodes
• Nodes are ordered by names

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SkipNet Ring

• Pointers at level h skip over 2h nodes
• Nodes are ordered by names

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SkipNet Global View
L 3
L 2
L 1
M
D
O
Root Ring
Level L 0
T
A
V
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X
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Two Address Spaces

• SkipNet can route efficiently in both address
  spaces
• Name ID space (e.g. DNS names)
• Numeric ID space

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Routing by Name ID
Ring 000
Ring 001
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Ring 100
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L 3
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L 1
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• Example route from A to V
• Simple Rule Forward the message to node that is
  closest to dest, without going too far.

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Routing by Name ID
Ring 000
Ring 001
Ring 010
Ring 011
Ring 100
Ring 101
Ring 110
Ring 111
O
M
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L 3
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M
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Ring 01
Ring 00
Ring 10
Ring 11
A
T
L 2
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M
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Ring 1
Ring 0
T
L 1
A
Z
V
X
M
D
O
Root Ring
T
A
Level L 0
V
Z
X

• Example route from A to V
• Simple Rule Forward the message to node that is
  closest to dest, without going too far.

16
Routing by Name ID
Ring 000
Ring 001
Ring 010
Ring 011
Ring 100
Ring 101
Ring 110
Ring 111
O
M
D
L 3
T
A
V
X
Z
M
O
D
Ring 01
Ring 00
Ring 10
Ring 11
A
T
L 2
V
Z
X
M
O
D
Ring 1
Ring 0
T
L 1
A
Z
V
X
M
D
O
Root Ring
T
A
Level L 0
V
Z
X

• Example route from A to V
• Simple Rule Forward the message to node that is
  closest to dest, without going too far.

17
Routing by Name ID
Ring 000
Ring 001
Ring 010
Ring 011
Ring 100
Ring 101
Ring 110
Ring 111
M
O
D
L 3
T
A
V
X
Z
M
O
D
Ring 01
Ring 00
Ring 10
Ring 11
A
T
L 2
V
Z
X
M
O
D
Ring 1
Ring 0
T
L 1
A
Z
V
X
M
D
O
Root Ring
T
A
Level L 0
V
Z
X

• Example route from A to V
• Simple Rule Forward the message to node that is
  closest to dest, without going too far.

18
Routing by Name ID
Ring 000
Ring 001
Ring 010
Ring 011
Ring 100
Ring 101
Ring 110
Ring 111
O
M
D
L 3
T
A
V
X
Z
M
O
D
Ring 01
Ring 00
Ring 10
Ring 11
A
T
L 2
V
Z
X
M
O
D
Ring 1
Ring 0
T
L 1
A
Z
V
X
M
D
O
Root Ring
T
A
Level L 0
V
Z
X

• Example route from A to V
• Simple Rule Forward the message to node that is
  closest to dest, without going too far.

19
Routing by Name ID
Ring 000
Ring 001
Ring 010
Ring 011
Ring 100
Ring 101
Ring 110
Ring 111
O
M
D
L 3
T
A
V
X
Z
M
O
D
Ring 01
Ring 00
Ring 10
Ring 11
A
T
L 2
V
Z
X
M
O
D
Ring 1
Ring 0
T
L 1
A
Z
V
X
M
D
O
Root Ring
T
A
Level L 0
V
Z
X

• Example route from A to V
• Simple Rule Forward the message to node that is
  closest to dest, without going too far.

20
Routing by Numeric ID

• Provides the basic DHT primitive
• To store file Foo.c
• Hash(Foo.c) ? a random numeric ID
• Find highest ring matching that numeric ID
• Store file on node in that ring
• Log N routing efficiency

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DHT Example
Ring 000
Ring 001
Ring 010
Ring 011
Ring 100
Ring 101
Ring 110
Ring 111
O
M
D
L 3
T
A
V
Z
X
M
O
D
Ring 01
Ring 00
Ring 10
Ring 11
A
T
L 2
V
Z
X
M
O
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Ring 1
Ring 0
T
L 1
A
Z
V
X
M
D
O
Root Ring
T
A
Level L 0
V
Z
X

• Store file Foo.c from node A
• Hash(Foo.c) 101
• Route from A to V in numeric space

22
Talk Outline

• Basic SkipNet design
• SkipNet locality properties
• Performance evaluation
• Conclusions

23
Key Locality Properties

• In practice, two properties are important
• Content Locality ability to explicitly place
  data
• Placement on a single node or on a set of nodes
• Path Locality ability to guarantee that local
  traffic remains local
• One abstraction is important CLB

24
Constrained Load Balancing (CLB)

• Multiple DHTs with differing scopes using a
  single SkipNet structure
• A result of the ability to route in both address
  spaces
• Divide data object names into 2 partsusing the
  ! special character
• CLB Domain
  CLB Suffix
• microsoft.com!skipnet.htm
  l

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CLB Example
com.microsoft
com.sun
gov.irs
edu.ucb

• To read file com.microsoft!skipnet.html
• Route by name ID to com.microsoft
• Route by numeric ID to Hash(skipnet.html)within
  the com.microsoft constraint

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SkipNet Path Locality
com.microsoft
com.sun
gov.irs
edu.ucb

• Organizations correspond to contiguous SkipNet
  segments
• Internal routing by NameID remains internal
• Nodes have left / right pointers

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Fault Tolerance

• Many failures occur along organizational
  boundaries
• Gateway/firewall failure, BGP misconfig, physical
  network cut,
• SkipNet handles organizational disconnect
  gracefully
• Results in two well-connected, partitioned
  SkipNets
• Efficient remerging algorithms
• Node independent failures
• Same resiliency as systems such as Chord and
  Pastry

28
Primary Security Benefit Weakness

• SkipNet name access control mechanism
• Content locality ensures that content stays
  within organization
• Path locality prevents
• malicious forwarders
• analysis of internal traffic
• external tampering
• Easier to target organizations
• Someone creates one million nodes with name
  prefixes microsofa.com and microsort.com
• Most traffic to/from Microsoft will go through a
  microsofa / microsort intermediate node

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Talk Outline

• Basic SkipNet design
• SkipNet locality properties
• Performance evaluation
• Conclusions

30
Routing by Name ID Performance
Benefits come at no extra cost
31
Surviving Organizational Disconnect
Disconnected Org Size 15 of all nodes
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Conclusions

• SkipNet
• Traditional overlay functionality
• Explicit control of data placement
• Constrained load balancing
• Content Path Locality are basic ingredients to
• Data controllability
• Manageability
• Security
• Data availability
• Performance

33
Questions?

• Thank you!