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Perhaps the most interesting agents are mobile agents, which can themselves be intelligent or unintelligent. This narrower class of agents is the focus of this article. Unlike their static brethren, which are content to execute in the cozy confines of a single machine or address space, mobile agents have wheels. They migrate about the network, executing tasks at each waystation, potentially interacting with other agents that cross their paths.
To contrast static and mobile agents, consider a mobile and static version agent-based of e-mail delivery.
The dominant store-and-forward model in use today, in which a POP client communicates with an SMTP server, can be considered an application of static agents. The POP client, either at user request or on a preset timer, connects to a designated SMTP server and collects mail. The SMTP server receives and routes incoming mail, storing it into appropriate mailboxes for later retrieval. Both players in the transaction stay on their respective machines, using the network only for the transfer of message content.
A mobile agent design of the same transaction might define a mail carrier agent that travels about the network autonomously, handing messages to mail handler agents at each stop. These mail handler agents might themselves go mobile or spawn mobile children to distribute particular pieces of mail to other mailstops, perhaps to synchronize multiple mailboxes or to forward urgent messages to a paging service. In this model, the players in the transaction migrate between machines, bringing with them not just the message content but also their instructions on what to do when they arrive at each new destination.
Mobile agents are defined in formal terms by computer scientists as objects that have behavior, state, and location. A subset of behaviors of every agent is inherited from the model, notably those behaviors that define the means by which agents move from place to place. Mobile agent models almost always define a method of interagent messaging as well. Finally, a mobile agent model is not complete without defining a set of events that are of interest to the agent during its lifetime. For example, the event of arriving at a new location is a momentous one in the life of an agent and generally entails the invocation of one or more of its behaviors. The set of events varies a bit from model to model, but the following is a list of the most common ones:
For purposes of illustration, consider for a moment a mail carrier agent whose reason for living is to deliver our packages and late credit card payment notices. This mail carrier has a few behaviors, a couple of which are "Get mail from post office" and "Insert mail into mailbox." Another useful behavior might be "Elude attacking hound." Our trusty federal employee also has state -- specifically the contents of the mailbag. Finally, the mail carrier has location: either at the post office, in transit between mail drops, or at one of the stops along their route. As the mail carrier makes rounds, their location changes as the mail carrier migrates from point to point along their daily itinerary. State also changes as each bundle of parcels is safely deposited into the mailbox and outgoing mail is placed into that U.S. Postal Service regulation canvas bag.