Lecture delivered as part of Masters-level Software Agents course.

1. CSM13: Software AgentsCSM13: Software Agents
Agents and P2PAgents and P2P
James Salter
j.salter@surrey.ac.uk
19 March 2004

2. OutlineOutline
 What is Peer-to-Peer?
 Relationship to Agents
 Locating resources in P2P Networks
 Problems with Napster and Gnutella
 Solutions
 Other Issues with P2P & Agents

3. What is “P2P Computing”?What is “P2P Computing”?
 Everybody seems to have a different
idea
 File sharing
 Distributed computing
 Inter-connected communities of users
 Hideouts for virus writers and associated
criminals
 Opposite of client/server network
 All nodes can function as clients and servers
 These are all valid definitions

4. Network ArchitecturesNetwork Architectures
Client/Server Pure Peer-to-Peer
 Clients send requests
to/via a central server
 Small #messages
 Single point of failure
 No central server
 Clients connected to
one or more peers
 Resilient
 Large #messages

5. Hybrid Peer-to-Peer NetworksHybrid Peer-to-Peer Networks
 Some nodes act as group managers/routers
 More robust than client/server
 Potentially lower #messages than pure P2P

6. Characteristics of a P2P NetworkCharacteristics of a P2P Network
 Nodes can act as clients and servers
 No centralised server/authority
 In Pure P2P Networks
 Network is highly dynamic
 Nodes join and leave regularly
 Large-scale
 Potentially millions of nodes
 Nodes are autonomous
 But co-operate to share/retrieve resources

7. The Agent RelationshipThe Agent Relationship
 How does this relate to agents?
 P2P network platform for mobile agents
 Access agents over P2P network
 Agents administer P2P network
 P2P Networks are useless if agents,
users and applications cannot find
resources they require
 Queries can be represented as agents

8. Searching for ResourcesSearching for Resources
Napster
 Hybrid P2P
 Queries sent to
central server
 Nodes upload list of
files they hold
 Server returns IPs of
nodes hosting
matching files
 Direct connection
between nodes for
transferring files File
Transfer

9. Searching for ResourcesSearching for Resources
Gnutella
 Pure P2P Network
 Queries flooded from
node to node
 Query has Time-To-
Live (TTL) counter
 Decremented each
time visits a node
 Query discarded when
TTL=0
 Prevents endless
loops

10. The ProblemThe Problem
 Scalability
 Napster: central server is a bottleneck
 Gnutella: large volumes of query traffic
 Number of Messages
 Gnutella: query must be sent to every node
 Fault Tolerance
 Napster: single point of failure
 Ability to Find Resources
 Gnutella: query horizon (TTL limit)

11. SolutionSolution
Combine benefits of Gnutella and Napster
 Scalability and Number of Messages
some form of structure to avoid visiting every node
but more than one point of contact
 Fault Tolerance
structure to avoid central server
 Ability to Find Resources
provide alternative structure to remove TTL
Introduce Structure

12. Hash TablesHash Tables
 Hash functions map data keys to buckets
 Keys are stored in buckets in index table
 Example:
Distributed Hash Tables
 Each node hosts part of the index (one
or more buckets)
0
1
2
3
4
5
f(x) = x mod 6
f(15) = 15 mod 6 = 3
12, 24, 30
19, 61
20, 26, 56
3, 9
10, 16, 28, 34
23, 35, 65
f(36) = 36 mod 6 = 0

13. Chord: ArchitectureChord: Architecture
 Nodes hosting part of the index table
organised in a ring 1
41
7
22
28
34
48
52
70
73
 Each node is given
an identifier
 Hash function
determines node
host for a key(word)
 Nodes provide
pointers to machines
hosting resources

14. Chord: QueryingChord: Querying
 Queries routed around the ring following
finger table information
 Finger table stores
more information
about nodes closer
to it than those
further away
 Routing ~ O(log N)
 Search space
halved every hop
1
41
7
22
28
34
48
52
70
73

15. QueriesQueries
 Queries may be a list of keywords:
“ford AND fiesta”
 We may need more complex queries,
specifying keyword-value pairs:
“make=ford AND model=fiesta AND
mileage=20000”
 Boolean Operators: AND, OR, NOT
keyword
keyword value

16. A Ring of Chord RingsA Ring of Chord Rings
 Create a ring for every keyword
 Super Ring stores pointers to Keyword
Rings
Super Ring
Keyword
Ring
Keyword
Ring
Keyword
Ring
Keyword
Ring
Keyword
Ring

17. A Ring of Chord RingsA Ring of Chord Rings
 Support for name-value queries
 Each physical machine can take part in
multiple rings
 Each node in super ring runs an agent
responsible for keyword ring organisation
and maintenance
 Agents on each node handle incoming
queries
 Queries can be spawned as agents
 Multiple keyword queries (CPU=1.8 AND RAM=128)
 More complex queries?

18. Further ImprovementsFurther Improvements
 Record address of keyword rings
 Can route queries for previous keywords
directly to keyword ring
 Cache previous search results
 Direct mapping between providers and
consumers
 Cache becomes Preference List
 Ordered list of resource providers most likely
to provide the required resource

19. Other Issues in P2P/AgentsOther Issues in P2P/Agents
 P2P applications cannot (usually)
communicate with each other
 Agent collaboration over P2P networks
 Enhancements to existing P2P protocols
using agents
 Enhance agent co-ordination schemes
using P2P co-ordination mechanisms
 Self organisation of P2P networks
 Micro-payments for resources/brokers

20. SummarySummary
 P2P can be used for sharing resources
 Removes need for central management
 Large scale, dynamic networks
 P2P can enhance agent systems
 Agents can enhance P2P
 Resource Discovery research area
 Agents and P2P can be closely related