1. © nCode 2000 Title of Presentation goes here - go to Master Slide to edit - Slide 1 Reliable Communication for Highly Mobile Agents ECE 7995: Term Paper November 20, 2001 Gowri V Pai, Graduate Student Computer Engineering Department Wayne State University

2. Reliable Communication for a Highly Mobile Agent Authors Amy L Murphy Gian Pietro Picco

3. Reliable Communication for a Highly Mobile Agent Introduction Mobile Agent Program that can migrate from machine to machine in a heterogeneous network

4. Reliable Communication for a Highly Mobile Agent Topic of Discussion Current Message Delivery Approaches Proposed Algorithm Application Interactive Close

5. Reliable Communication for a Highly Mobile Agent Need for Reliable Communication Asynchronous nature of message passing Asynchronous nature of agent migration Prevent any data loss

6. Reliable Communication for a Highly Mobile Agent Current Message Delivery Approaches Spanning Tree Broadcasting Spanning tree broadcasting  Concept Message broadcast using spanning tree of network nodes  Limitation Message delivery fails when the agent and the message cross in the channel

7. Reliable Communication for a Highly Mobile Agent Current Message Delivery Approaches Forwarding  Concept Home agent maintains a pointer to the mobile agent Upon migration, mobile agent informs the home agent  Limitation Message delivery fails when sent between migration and update, agent moved before message reached the destination Retransmission not fool proof Excessive communication

8. Reliable Communication for a Highly Mobile Agent Enabling Reliable Communication Filling in the gaps Spanning tree broadcasting Store Message at nodes until delivery is complete  Limitation This guarantees delivery, but nodes will have to store messages for arbitrary lengths of time Message is continually forwarded to new location, agent runs away and message never catches up

9. Reliable Communication for a Highly Mobile Agent Proposed Solution Phase I (Exactly once delivery)  Network of nodes and channels is known in advance  Only one message is present in the system at a time Phase II (At least once delivery)  Phase I results extended to concurrent multiple message delivery

10. Reliable Communication for a Highly Mobile Agent Proposed Solution Model –Nodes represent servers willing to host agents –Edges represent directional FIFO channels along which agents can migrate and messages passed –A path exists between every pair of nodes –A channel does not necessarily exist between every pair of nodes –Mobile agent server tracks hosting agent –Every agent has a globally unique identifier – direct message

11. Reliable Communication for a Highly Mobile Agent Delivery in a Static Network Graph Single Message Delivery –Algorithm uses ideas from Snapshot Algorithm [Chandy et al ] –Goal is to record the local state of the nodes and channels in order to construct a consistent global state –Initially all channels are “Open” –A “Flushed” state of the channel implies all agents on that channel ahead of the message have been forced out –When all incoming channels of a node are “Flushed”, the message copy at that node is deleted

12. Reliable Communication for a Highly Mobile Agent Delivery in a Static Network Graph Single Message Delivery Network partitioned into three regions  Finished Processing  Processing  Not Yet Processing

13. Reliable Communication for a Highly Mobile Agent Delivery in a Static Network Graph Multiple Message Delivery –Need for Concurrent flow of multiple messages To allow a source to transmit a burst of messages with out waiting for the delivery of the first one to complete To allow multiple sources to transmit at the same time –Message tagged with the identifier of the host that sent it –A sequence number and “Buffering” state to address the case of a message burst coming from a single source –A vector of states and buffers is maintained to address concurrent copies of message delivery by multiple sources.

14. Reliable Communication for a Highly Mobile Agent Delivery in a Dynamic Network Graph Limitations of Static Message Delivery –Necessitates knowing the network of neighbours –Insensitive to which nodes have been active Objectives of Dynamic Message Delivery –Flush channels and trap agents in regions of the network where the message will propagate –To allow network graph used for the delivery process to grow dynamically as the agents migrate

15. Reliable Communication for a Highly Mobile Agent Delivery in a Dynamic Network Graph Illustration - Destination ahead of source - X is the sender of all messages (only active node) - X sends burst of messages 1.. 4 to Y (X, Y active) - X sends a second burst of messages 5..8 - Before message 5 arrives at Y, if an agent is sent from Y to Z, it is put on hold - Only after messages 5..8 are received, these messages are delivered to the detained agent without broadcasting them to the neighboring nodes (X, Y, and Z active)

16. Reliable Communication for a Highly Mobile Agent Delivery in a Dynamic Network Graph Illustration - Source ahead of Destination - X is the sender of all messages (only active node) - X sends burst of messages 1.. 4 to Y (X, Y active) - X sends a second burst of messages 5..8 - Before message 5 arrives at Y, if an agent is sent from Z to Y, activation of channel (Z, Y) is delayed - Only after message 8 is processed at Y, channel (Z,Y) is activated

17. Reliable Communication for a Highly Mobile Agent Delivery in a Dynamic Network Graph Channel States ClosedChannels are not active in message delivery OpenChannel is waiting to participate in a message delivery FlushedCurrent message being delivered has arrived on this channel Buffering (j)Source is ahead of the destination Messages are buffered until j is processed Holding (j)Destination is ahead of the source Agents are held until j arrives.

18. Reliable Communication for a Highly Mobile Agent Delivery in a Dynamic Network Graph Multiple Message Sources –Nodes can deliver n messages concurrently, at most one for each node  Multiple instances of single message source algorithm superimposed on the network  A vector of size n represents the state of incoming channel  Channel state associated with the node where the message originated is used to process each message  All links are bi-directional to enable any node to originate a message

19. Reliable Communication for a Highly Mobile Agent Analysis of the Proposed Mechanism Implementation Issues –FIFO property must be maintained for every message and agent travelling through the channel –The mechanism assumes that the runtime support maintains some state about the network graph and the messages being exchanged Applicability –Generates considerable overall traffic (compared to a forwarding scheme)

20. Reliable Communication for a Highly Mobile Agent Analysis of the Proposed Mechanism Tradeoffs Guaranteed Delivery Multicast Capable Delivery Overhead Knowledge Maintained ForwardingNo One indirection Agent location BroadcastNoYes (no guarantees) One msg / spanning tree edge Spanning tree Static Snapshot Yes One msg / edge Neighbors (known in advance) Dynamic Snapshot Yes One msg / traversed edge Neighbors (discovered)

21. Reliable Communication for a Highly Mobile Agent Conclusions Mobility makes message delivery inherently difficult Limitations of available techniques –Do not guarantee message delivery –Constrain movement or connectivity of mobile agents Proposed solution –Uses distributed snapshot concept –Complements available techniques –Trades reliability for bandwidth consumption