Peer-to-peer networks

Ant RowstronMicrosoft Research

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Talk overview

• Overview of Overlays/DHTs• Overview of Wireless Routing protocols• Description of Virtual Ring Routing– A routing protocol inspired by DHTs

2

Underlays (Networks)

3

Provides point-to-point communication A B

Overlay

4

A

B

Overlay and underlay

5

Unstructured overlays

• Unstructured overlays– No/weak constraint on neighbors

(for correctness)– Algorithmically simple – poor

performance

• Flood based techniques etc.

6Example: Gnutella, BitTorrent

Unstructured overlays

• Unstructured overlays– No/weak constraint on neighbors

(for correctness)– Algorithmically simple – poor

performance

• Flood based techniques etc.

7Example: Gnutella, BitTorrent

Structured overlays

• Structured overlays– Constraints on neighbors – Algorithmically more

complex• DHT API, KBR API

8Example: Distributed Hash Table (DHTs) - CAN, Chord, Pastry, Tapestry (2001)

Distributed Hash Tables (Pastry)

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root nodefor key

id space

nodeIdkey

• Large id space

• NodeIds picked randomly from space

• Keys picked randomly from space

• Key is managed by its root node:

• Live node with id closest to the key

Node routing state

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0* 1* 2* 3*

20* 21* 22* 23*

200* 201* 202* 203*

2030* 2031* 2032* 2033*

203231

• Topology aware routing table• NodeIds and keys in some base 2b (e.g., 4)• Prefix constraints on nodeIds for each slot• Pick closest node satisfying slot constraints

leaf set

nodeId

Routing

• Prefix matching: each hop resolves an extra key digit

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323310

323211

322021

313221

203231

nodeIdkey

route(m, 323310)

Using DHTs

root nodefor key

id space

nodeIdkey

• Keys picked randomly from space

• E.g. SHA1(“www.microsoft.com”)

• Generic building block– Application-level multicast– File Systems– Web Caches– File Sharing– Etc

• Generic building block– Application-level multicast– File Systems– Web Caches– File Sharing– Etc

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DHT experiences

• DHTs are:– Self-organizing– Decentralized– Fault-tolerant/churn tolerant– Scalable– Performance

• Question: – Can we apply lessons to underlay routing protocols?

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Wireless routing protocols

• Large number of wireless routing protocols• Numerous scenarios:– Mesh, Sensor, Mobile, VLAN, etc

• Protocols traditionally target a scenario

• Protocols use either:– Flooding– Coordinates

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Proactive routing

x

Nodes have complete topology map

Example: OLSR, DSDV 15

Reactive routing

Example: DSR, AODV 16

Coordinate-based routing

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(x,y)

y

x

q at (2,2)

p

lookup servicelookup service

lookup(p)p at (4,1)

at (4,1)

fixed identifier address

Example: GPSR, BVR

Previous work on wireless routing

• Flooding based algorithms scale poorly– Flood on topology changes or discover routes

• Geographical and landmark routing– Scale well but nodes have identifier and address• Some apps (sensor networks) may require just address

– DHT-like structure to translate between the two• Route setup delays• Additional maintenance overhead• Another target for attacks

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VRR: Virtual Ring Routing

• A fundamentally different approach

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8F690E

910

8F08E2

0FFFTopology-independentnode identifiers

Nodes organized into virtual ringby increasing identifier value

Each node maintainsa virtual neighbor set (vset)

VRR: The Virtual Ring

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VRR: Routing paths

physical network topology

8F6

8F6

Nodes only maintain routing paths to virtual neighbors:• Paths maintained proactively• Paths are bidirectional and typically multi-hop

VRR: Forwarding table

• Paths recorded in forwarding tables along path • Forwarding table contains

• Paths between node and vset members• Paths between other nodes that go through node• Paths to physical neighbors

endpointB pathIdnextA nextBendpointA

10E 10me F42 318F6 90E

910

forwarding table for node 8F6

8F6 me14A 140 F42 10E 28F6 F42 me F42 FF

14A

140

F42

10E

8F6

Physical network topology

VRR: Forwarding table state

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• Physical neighbors• VSet paths• Other VSet paths

VRR: Example routing

physical network topology

VRR: Example routing

physical network topology

VRR: Example routing

physical network topology

VRR: Example routing

physical network topology

VRR: Example routing

physical network topology

VRR: Routing summary

• Paths to virtual neighbors ensure correctness• Stretch empirically small• Many alternate paths to route around failures

VRR: Node joining

Network Topology

19A

164

8F6

16E

broadcast hellosto find physical neighborsSend setup request to 16E

VRR: Node joining

Network Topology

19A

164

8F6

16E

171

164 sends setup to 16E with its vset 16E sends setup requeststo nodes in received vset

16E adds node to vset when it receives setup

VRR: Handling failures

• Routing state is hard– No end-to-end heartbeats– Failures detected on missing acks or hellos– Local repair attempted first;– Otherwise, teardowns sent along all affected paths

• Two techniques to ensure consistency– Symmetric failure detection and acks on teardowns• If x marks y faulty, y is guaranteed to mark x faulty

– Lightweight optimistic transactions• If in doubt abort (teardown)

Evaluation

• Evaluated using:– Simulations (ns-2) • Mobility

– Testbeds• UCB Sensor Testbed with 68 mica2dot motes• MSRC 111 node 802.11a Wireless Testbed

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Sensor Mesh: VRR vs BVR

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Increasing packets

Increasing packets

BVR delivery ratio drops

BVR delivery ratio drops

Failures: VRR vs BVR

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Fail 10% nodesFail 10% nodes

BVR delivery ratio drops

BVR delivery ratio drops

Office Mesh: VRR vs LQSR

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Mobility: Simulation

• Simulation in ns-2– 802.11b MAC– Mobility: 0-20 m/s – CBR: 1 packet/sec

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Implementing Networking Services

• Implement “lookups” using key-based routing?

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Nodes organized into virtual ring.

VRR ARP:Flood-based ARP:

ARP: 192.169.0.45

Hash(192.169.0.45)

192.169.0.45192.169.0.45

Conclusions• Overlay routing at the network layer• VRR implements wireless routing– Scalable: no flooding– Single location-independent identifier per node• No translation from identifiers to addresses

– No route discovery or translation delays• VRR implements a DHT– Strong consistency– No additional overhead

Further information

• VRR Paper:– M. Caesar, M. Castro, E. Nightingale, G. O'Shea and A.

Rowstron, "Virtual Ring Routing: Network routing inspired by DHTs", Sigcomm 2006

http://research.microsoft.com/~antr/

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