1. Peer to Peer Network Design Discovery and Routing algorithms
Nuno Santos
February 2, 2005

2. Overview Discovery and Routing in traditional networks
Existing P2P networks
Scalable, Pure P2P networks
Plaxton Mesh – Routing
Plaxton Mesh – Object Discovery

3. Discovery and Routing Traditional Networks
Discovery and routing – foundation of computer networks:
Discovery – Given a named object O, return the address of node X where O can be found
Routing – Given a message M and a destination node X, deliver M to X
Traditional networks have dedicated servers for these operations
Examples: DNS Servers, IP routers, directory servers
Generally static structure
Depend on human intervention
Single points of failure

4. Discovery and Routing P2P Networks
Desirable features of P2P networks
Dynamic:
Nodes join and leave at any time
Objects are inserted and removed often
Scalable
Most perform gracefully even with large numbers of nodes
Reliable
Must cope with arbitrary node failures
Consequences:
Central servers must be avoided
The network must adapt dynamically
Traditional discovery and routing algorithms not adequate

5. Discovery and Routing in P2P networks Real world examples
Napster – Single central server keeps a directory of nodes and objects.
Basically, a huge file catalog
Discovery and Routing is not really P2P
eDonkey, eMule – Based on multiple servers
Servers require special setup. Not ordinary nodes.
Clients connect to a server.
File list stored on server.
Servers communicate with each other for performing searches.
Improvement over Napster, no single server, but not pure P2P

6. Discovery and Routing in P2P networks Real world examples
GNUtella – Decentralized P2P network
Nodes can either be:
Peers – Normal nodes
Ultrapeers – Ad-hoc servers.
Peers get promoted to ultrapeers dynamically, based on their bandwidth.
Discovery based on broadcasts
Nodes send a search message to its ultrapeer
Ultrapeers forward message to all other peers
Repeat recursively until message reaches maximum hops (typically 7)
Searches generate a lot of traffic: O(n) complexity
Not scalable!

7. Current situation of P2P networks
Some are highly scalable – eMule, eDonkey
Rely on a static structure of servers
Some are pure P2P – Gnutella
Broadcast based searches generate too much traffic
Not scalable
Are scalable, pure P2P networks possible?

8. Scalable, Pure P2P networks
Active field of research for the last 5 to 7 years
Several research projects
Plaxton Mesh
Chord
Pastry
Tapestry
Common features
Pure P2P – No central servers
Scalable - Discovery and routing is O(log(n)). Routing tables size also O(log(n)).
Highly resilient
Adapt dynamically to node arrival, departure and failure
Applications based on research projects:
PAST – Distributed storage system
SCRIBE – Application level multicast
OceanStore – Distributed storage system with replica and redundancy support
Skype might also be in this class
No central server and scalable, but algorithm details unknown

9. Plaxton Mesh Overview
Plaxton Mesh – earliest known proposal for a scalable P2P network (1997)
Plaxton Mesh is static – no node arrival, departure or failure
Other projects based on same basic idea
Extensions supporting dynamic networks
More complex
Concepts in Plaxton
Nodes – Act as routers, clients and servers simultaneously.
Objects – Stored on servers, searched by clients.
Objects and nodes have unique, location independent names
Random fixed-length bit sequence in some base
Typically, hash of hostname or of object name
What Plaxton does:
Given message M and destination D, a Plaxton mesh routes M to the node whose name is numerically closest to D

10. Plaxton Mesh Routing Routing done incrementally, digit by digit
In each step, message sent to node with address one digit closer to destination.
Example - node 0325 sends M to node 4598
Possible route:
0325
B4F8
9098
7598
4598

11. Plaxton Mesh Routing Tables
Each node has a neighbor map
Used to find a node whose address has one more digit in common with the destination
Size of the map: B*L, where:
B – base used for the digits of the address. Eg. 8 in octal base
L – length of the addresses
4 digit addresses in octal base -> 8*4 tables.

12. Plaxton Mesh Routing Tables
Node 3642
x are wildcards. Can be filled with any address.
Each slot can have several addresses. Redundancy.
Example: node 3642 receives message for node 2342
Common prefix: XX42
Look into second column.
Must send M to a node one digit “closer” to the destination.
Any host with an address like X342.
Look in line 4

13. Plaxton Mesh Publishing and locating objects
Server S has an object O
Wants to make it known to clients
Strength of a Plaxton Mesh
No central directory
Directory distributed between nodes
How? Hash function
H: (object names) -> (node addresses)

14. Plaxton Mesh Publishing and locating objects
Server S wants to publish object O
S computes H(O), the object ID of O (hash of the name).
S sends a PUBLISH message to node H(O).
Node H(O) might not exist. No problem. Message forwarded deterministically to node X with address closest to A(O).
X - root node for object O
Nodes visited by the PUBLISH message associate A(O) with physical address of S (including root node)
These nodes are the tree of the object O

15. Plaxton Mesh Publishing and locating objects
A client C wants to find object O.
Evaluates H(O), using same algorithm as the server
Sends a QUERY message to H(O)
Forwarded to X, the object root.
The object root forwards the message to S.
Shortcut – Nodes in the tree of O also know location of O
If QUERY message reaches one of this nodes, it is forwarded directly to S.

16. Analysis of Plaxton Mesh
Advantages
Simple fault handling – Several possible routes.
Scalable – No central point, routing done with local information
Disadvantages
Global knowledge required to build routing tables
Static network
Root node vulnerability
More recent research overcomes most of disadvantages
Dynamic networks
Redundant storage

17. Conclusion Pure P2P networks are possible
Dynamic
Scalable
Highly resilient
Applications starting to appear
Skype (??)
Complex to build and understand
But very promising