Discovery of Infrastructure in MultiAgent Systems

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Discovery of Infrastructurein Multi-Agent Systems

• Brent K. Langley, Massimo Paolucci, Katia Sycara
• blangley, paolucci, katia _at_cs.cmu.edu
• Carnegie Mellon University, Pittsburgh, PA, USA
• Intelligent Software Agents Group _at_ The Robotics
  Institute

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RETSINA Architecture
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Initial Motivations

• Challenges
• Dynamic Environment/Infrastructure Availability
• Transitory Execution Environment for Agent
  Application
• Undependable Infrastructure
• Systems
• Networks/Connectivity
• Services

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Even-more Motivations

• Goals
• Automatic / Self Configuring Agents and
  Infrastructure
• Fault Tolerance and Dynamic Reconfiguration
• Scalability in Quantity, Loading, anddispersing
  into groups or over WANs

(while still allowing systems to find each
other)
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Agent Discovery Mechanisms

• Static - Preloaded or specifically initialized
• Dynamic
• Local SSDP Multicast-based Local (or limited
  distance) Discovery by Service Type
• Wide-Area A2A Peer-to-Peer network-based, with
  Agent Community Affinity and Aggregation
• Learned
• Informed Infrastructure facilitated (Look-Up
  Service, Matchmakers, Brokers, UDDI)
• Overheard A2A, not requested but useful
• Extracted garnered from plans or coordinated
  multi-agent operations

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Local Dynamic Discovery

• Simple Service Discovery Protocol (SSDP) from the
  Universal Plug-n-Play (UPnP) initiative
• Multicast search requests to populate lists of
  infrastructure service provider alternatives
• Receive Multicast Alive Byebye messages to
  automatically update discovered-provider lists.
• Service-specific reactions to Discovery Events
• Register with one or more services and optionally
  automatically register with newly alive systems
• Auto fail-over and pruning of services lists to
  maintain list of viable service providers

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SSDP Communications
Announcement of Availability
Discontinuance of Availability
Alive
MulticastwithLimited TTL
Byebye
MulticastwithLimited TTL
HTTP NOTIFY(GENA)
HTTP NOTIFY(GENA)
Search Request
MulticastwithLimited TTL
Query Contains Service Type or
wildcard for all Host/Port for response
HTTP M-Search
Search for Available Services
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They always want more

• We had reliable dynamic discovery on local
  subnets
• We implemented static hierarchical and mesh
  connectivity between remote infrastructure
  devices that lived in dispersed islands of
  discovery
• However, we wanted the same dynamic discovery,
  self configuration self-healing in wide-area
  systems

- BUT -
The multicast protocol used by SSDP is limited
by policies, systems, networks, routers, and
firewalls
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Discovery Over the Internet

• Problem How to quickly and broadly implement a
  solution to allow discovery and lookup
  communication between widely dispersed systems
• Possible Solutions
• Build, test, rollout, and support new platform
  over the wide area network / Internet
• Utilize Existing Service
• Re-purpose Extend Existing Service

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Discovery Over the Internet

• Problem How to quickly and broadly implement a
  solution to allow discovery and lookup
  communication between widely dispersed systems
• Our Solution Piggy-back on top of an existing
  non-proprietary and popular (widely utilized)
  communications framework that provides global
  connectivity - Gnutella.

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Gnutella Connectivity

• Last November there were typically 2,000 Gnutella
  Servents online at any instant
• In May 2001, there are typically over 40,000
  Servents online at any instant
• If all Servents keeps 5 peer connections open,
  and use a time-to-live of 7 when sending
  messages, a single query could reach up to
  109,225 other systems.
• Some Servents are more beneficial to be
  connected-to than others.

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Agent-to-AgentEnhancements
Categorize Task Prime Alt Local Home Cache Bad Use
d New
Confidence Querys Seen Q-Hits Seen Activity
Level Bad Packets Dup Queries Dup Msgs Dup
Pkts Repeat Pkt Type Sequential Pkts
Query Modification Min SpeedEncoded Task
Identifier - Quick Check see if worthwhile to
check - Hide from standard Gnutella
Servents Prefix Query with Task Identifier -
Absolute Check (appropriate to process) Encrypt
Query and Responses - Further Hide Query from
Gnutella - Protect conversation Task-Structu
re - Query QueryHit Packets escalated to A2A
Task Objects for Handling
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Gnutella Communications
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Evolving from P2P to A2A

• Gnutella Peer-to-Peer (P2P) networks provide
  random connectivity to a large range of other
  systems over a wide-area the Internet.
• Agent-to-Agent (A2A) adds task identifiers to
  messages which classify hosts into communities
  with specific interests.
• A2A prefers connectivity partners that share the
  same interests. This affinity causes
  congregations, or communities, of Agents in P2P
  networks to be formed.
• A2A attempts to maintain certain levels of
  connectivity to related Agents, and to hosts that
  have a higher confidence of providing needed
  information (directly or through their peers)
• A2A reduces random P2P connectivity, in favor of
  these links with higher probability for
  satisfying directed or background discovery and
  lookup processes.

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Gnutella Peer-to-Peer (P2P) vsCMUs
Agent-to-Agent (A2A)
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3 Basic Objects That A2A Apps Use

• A Task object identifies the A2A community that
  will interact via this communication portal and
  controls its related I/O.
• A Question object is created when you ask a
  question to the Task object or when a query comes
  in from another peer.
• An Answer object is an individual response (one
  of possibly many) associated with the specific
  Question object that it applies to.

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Agent Use of A2A Objects

• An Agent creates a Task object to converse with
  similar Peers.
• The Agent asks the Task a query and receives a
  Question object.
• Incoming replies will be stored in the
  appropriate Question object.
• When the Agent retrieves a reply it is given an
  Answer object.
• Incoming Questions are held in a limited-size
  circular queue.
• Agents can get new Question objects from the Task
  and compose single or multi-part replies.
• When the Agent is ready, it can send the reply to
  the P2P network.

Agent
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!
!
!
?
?
?
?
?
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ANS Client Task
Match Maker Client Task
Peer Activity Client Task
A2A Management Process
Peer-to-Peer Network
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A2A Discovery

• When a Task object is created, it can be marked
  as a Discoverable task.
• Discoverable Tasks automatically create a
  discovery Question object to periodically attempt
  to discover other Agents that support the same
  Task type.
• Discoverable Tasks create an Auto-Answer that
  watches incoming Questions to see if they are
  discovery queries. If so, the Agents location
  is automatically replied.
• Discovery replies are maintained by the Task as a
  list of known community peers, and utilized by
  the A2A state management process to ensure
  adequate connectivity to each peer community.

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A2A Discovery Startup Sequence

• Initial connectivity from preferences, previous
  state information, or user specification
• Supplement with Multicast discovery of local
  hosts and utilize Internet P2P host cache servers
• Follow normal P2P host discovery Ping/Pong
  processing to supplement connectivity resources,
  and track confidence in these systems
• Further tune connectivity with A2A by
  categorizing discovered local (near-by) and
  home-base hosts.
• Send task-specific discovery lookup
  requests to P2P network to facilitate communal
  connectivity

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Query Integration

• Each Gnutella Query has an Integer value a
  String. The Integer represents a minimum speed
  requested for downloads, and the String is the
  query.
• When a new Agent-to-Agent Task is created, a
  String representing the Tasks Community is
  specified.
• This String is encoded into an Integer
  representing a value greater than 10 million
  bits/second.
• The encoded integer is inserted into Gnutella
  Queries as the minimum download speed, and the
  Community Identification String is prefixed to
  each of the Tasks actual Query message strings.

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Query Filtering

• Gnutella clients will ignore the A2A query since
  they wont be able to provide the desired
  minimum speed.
• A2A clients will bypass normal Gnutella queries
  since the minimum speed will typically be too
  low, and not match any valid encoded Task
  Identifier.
• A2A clients that dont have a Task object
  identified by the same encoded integer will also
  bypass the message.
• If A2A detects it is hosting a Task that matches
  the encoded integer value, it verifies that the
  Gnutella Query String actually begins with the
  Tasks Community Identification String.
• If so, the Task creates a Question object for the
  Query and links it to the Task for processing.

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Questions and Answers

• Applications can create Auto-Answers to
  automatically reply with the Agents location
  upon arrival of a matching query.
• Applications can check a previously created
  Question object to see if it has any new Answers
• Applications can block, waiting for an Answer to
  a Question.
• Applications can create a call-back type of
  process to be automatically called when an Answer
  arrives for a Question.
• Applications can check to see if new Questions
  have arrived.
• Applications can block, waiting for a Question to
  arrive.
• Applications can create a call-back process to
  automatically be called by a Task when a specific
  Question arrives.
• Catch-all Questions can see all (including
  Gnutella) queries.

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Agents in a Peer-to-Peer World
A2A Community
A2A Community
A2A Community
A2A Community
Gnutella P2P Network
A2A Community
A2A Community
A2A Community
A2A Community
A2A Community
A2A Community
A2A Community
A2A Community
A2A Community
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Interactions Direct and via Peers
Other Agent Environments
Other Resources
Other Agent Services
Gnutella Network
Agent Name Services
Brokers
MatchMakers
Auctions
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Take-home Lessons

• Infrastructure Services Support Robust Agent
  Activities
• Discovery allows dynamic configuration and
  adaptation to changing and evolving environments
• Multicast-based discovery is fast and reliable,
  but limited to localized networks
• Wide Area Discovery implemented over a P2P
  architecture (such as Gnutella) provides dynamic
  reconfiguration of globally dispersed Agents and
  their Infrastructure Services

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A2A Directions

• P2P resident Infrastructure services
• P2P Client Application as a platform for
  Agent-based Applications
• Customized auction and yard-sale agents
• Network security agents
• SETI_at_home style processor sharing
• Distributed process or application dispersal
• Personal Agent background interest coordinator
• Agent Mobility
• Scalability of agent communications

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A2A Architecture
Task
Logger Task
Match Maker Task
Agent Name Service Task
Question
Task
Ask question
Answer
A2A Connectivity Management Layer
Gnutella
Gnutella Query
Response
TCP/IP Network
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In-band/Out-of-band

• Gnutella uses two protocols
• Gnutella protocol as for p2p coordination and a
  lookup service (in-band), and
• HTTP protocol for file transfer (out-of-band)
• Agent Infrastructure needs to delineate
• Discovery/Service Lookup can use Gnutella
  Protocol
• Agent-to-Agent or Agent-to-Middle-Agent
  (Infrastructure) conversations should they be
  in-band, or a separate out-of-band protocol.
• Guidelines for using In-band Gnutella Protocol
• Asynchronous Query/Reply format
• Short messages in queries and replies
• Quick replies to queries can be generated
• Desire fewer maintained connections to multiple
  services

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Out-of-Band Protocol Drivers

• Use an Out-of-Band Protocol if Application
• Requires synchronous communications
• Requires transactions and state
• Requires strict message ordering
• Has Short time-outs for reply reception
• Takes Long time to process request and form reply
• Might have packets intentionally discarded by
  other Gnutella clients for protocol violations
• Require private conversations

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Agent Name Service Clients on A2A/P2P

• A Discoverable Task to automatically form
  communities with other retsinaAgentNameService
  peers.
• register agent-name commands create Auto-Answers
  for future lookup agent-name and listall queries
• unregister agent-name commands remove local
  Auto-Answers of appropriate lookup and listall
  queries
• Agents can lookup other Agents without the use of
  an ANS server.
• Agents can cache previously found lookup
  information and facilitate lookups by other peers.

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Agent Name Service Servers

• Basic Implementation as ANS Clients with with
  large caches, that are not only populated by
  local lookup requests, but also by monitoring
  other ANS Client register messages.
• Secure Implementation might include registration
  and/or lookup security screening, and
  unregistration verification.
• Conversations could also be encrypted.

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A2A Middle Agentsfor Gnutella

• Agents/Middle-Agents monitor all Gnutella Queries
  replies that pass on the peer-to-peer network
• Middle-Agents can model, Traffic Loads, Consumers
  (Requestors), Producers (Repliers), Quality of
  Peers
• If a Middle-Agent sees a request for a service
  provider, it could learn or broker that service
• A Middle-Agent that sees a request to implement a
  plan, could break it into sub-plans and
  redistribute to peers
• If a Middle-Agent is going to respond to a bid,
  but observes other related bid opportunities, it
  can provide a combined bid option (a user asks an
  agent specifically for a bid on auto-insurance,
  but agent observes that other bids have been
  asked from the same companys home, health, life
  insurance agents a possible lower price for a
  combined policy could be proposed.)

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Middle-Agents on Gnutella

• Middle-Agents can service more clients with fewer
  Sockets
• When a Middle-Agents connectivity limits are
  reached, new clients could be connected to
  current peers and still access the service.
• Clients could overhear answer and avoid
  repeated asking of popular/common questions
• Middle-Agents are typically online longer and
  less transient than normal Agents. Longer
  connectivity to the p2p network allows a peer to
  find more, and better categorize other peers (and
  communities) to increase their level and quality
  of connectivity.
• Many Middle-Agents work in concert with other
  types of Middle-Agents to facilitate Agent
  activities. An ANS that gets a MatchMaker
  Discovery Query may not be able to respond
  directly, but in relaying the message to the
  ANSs peers, there is a high probability that one
  of the Agents talking to the ANS is also talking
  to a MatchMaker.