P2P Group Meeting

(ICS/FORTH)

Monday, 28 March, 2005

A Scalable Content-Addressable NetworkSylvia Ratnasamy, Paul Francis, Mark Handley, Richard Karp,

Scott Shenker

In a nutshell...

CAN is a distributed hash table.

Consider a virtual world with specified geometry.

Add some nodes uniformly.

Each node holds information about its zone.

Each node holds information about its

neighbors.

Zone: a chunk of the entire hash table.

How it works?

We want to store a (key, data) pair.

Use a uniform hash function to map the key in a point P in the

virtual world.

P lies in a zone.

The node which is the owner of the zone now holds the (key,

data) pair.

How it works?

We want to retrieve the (key, value) pair.

Apply the same hash function to key.

You'll get P.

Now you can travel from your node to the node-owner of the zone

which P belongs to, using simple geometry.

Now, the horry details...CAN uses a virtual d-dimentional Cartesian coordinate

space on a d-torus.

The entire coordinate space is dynamically partitioned to

zones, owned by n nodes.

Add some black magic and you get:

Average Routing Path: (d/4)(n1/d) hops.

Nodes maintain: 2d neighbors.

(*If d=lgn/2, then we have O(lgn))

Design – ConstructionBootstrap process: via host caches.

After the initial connection the new node selects a random

point P and sends a JOIN request.

The node that owns P splits its zone and sets the new node

as a neighbor.

Periodically update messages are exchanged between nodes

located closed to the "neighborhood".

Node insertion is a local process and affects only

O(dimensions) existing nodes.

Node Departure

Explicit hand over: a node sends its zone to one of

its neighbors (the one with the smallest one), before

it leaves the system.

TAKEOVER: If a node doesn't receive an UPDATE

message from one of its neighbors for a long period,

then it takes the zone (recovery).

Design Improvements

Goal: decrease routing latency by adding some nice

features.

Feature addition tradeoff: Per node state and system

complexity is increased.

1. Multiple dimensions

2. Realities

Reality: An independent coordinate space with a

specific zone mapping.

Having multiple realities, a data object is replicated

to various locations in the system.

2. Realities

Realities vs Dimensions

3. RTT based routingEach node forwards a message to the neighbor with higher RTT

(round-trip-time) ratio.

4. Zone Overloading

Multiple peers (MAXPEERS~3,4) share the same zone in

the system.

Advantages

- Reduced path length. Zone overloading has the same

affect as reducing the nodes of the system.

- Reduced per-hop latency. Each node has multiple choices.

- Improved fault tolerance. A zone is less possible to be

vacant.

5. Multiple hash functions

6. Topology Forcing

Goal: apply an ordering based on RTT measures between

each node and a set of landmarks (well known machines,

i.e. DNS root names servers).

Stretch: the ratio of the latency on the CAN network to the

average latency on the IP network.

6. Topology Forcing

7. Uniform Partitioning

When a new node enters in the system, another node

must split its zone. If uniform partitioning is

enabled, the node with the largest zone volume will

be selected.

Uniform partitioning is a load balancing technique.

8. Hot Spot Management

Caching: Each node maintains a cash with keys of

popular data objects.

Replication: Each node may replicate popular data

objects to its neighbors.

Design Review

Results with a fixed system size

System Size = n18

Transit-Stub Topology Generator

TS topologies model networks using a 2-level hierarchy of

routing domains with transit domains that interconnect

lower level stub domains.

H(intra-transit, transit-stub, intra-stub)

R(low_limit, up_limit) {random values }

Example

H(100, 10, 1): A Transit-Stub topology with a hierarchical

link delay assignment of 100ms on intra -transit links, 10ms

on transit-stub links and 1ms on intra-stub links.

Main Result

In a system with approximately 260,000 peers, CAN

may achieve routing with a latency that is well

within a factor of two of the underlying network

latency.

Thank you for your time. :-)

Elias Athanasopoulos

elathan@ics.forth.gr

http://www.csd.uoc.gr/~elathan/