FOUNDATION FOR INTELLIGENT PHYSICAL AGENTS

FIPA Contract Net Interaction Protocol Specification

Document title	FIPA Contract Net Interaction Protocol Specification
Document number	XC00029G	Document source	FIPA TC Communication
Document status	Experimental	Date of this status	2002/11/01
Supersedes	None
Contact	fab@fipa.org
Change history	See Informative Annex A — ChangeLog

Notice

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Foreword

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Contents

1 FIPA Contract Net Interaction Protocol 1

1.1 Explanation of the Protocol Flow. 2

1.2 Exceptions to Interaction Protocol Flow. 3

2 References. 5

3 Informative Annex A — ChangeLog. 6

3.1 2002/11/01 - version G by TC X2S. 6

1 FIPA Contract Net Interaction Protocol

The FIPA Contract Net Interaction Protocol (IP) is a minor modification of the original contract net IP pattern[1] in that it adds rejection and confirmation communicative acts. In the contract net IP, one agent (the Initiator) takes the role of manager which wishes to have some task performed by one or more other agents (the Participants) and further wishes to optimise a function that characterizes the task. This characteristic is commonly expressed as the price, in some domain specific way, but could also be soonest time to completion, fair distribution of tasks, etc. For a given task, any number of the Participants may respond with a proposal; the rest must refuse. Negotiations then continue with the Participants that proposed.

The representation of this IP is given in Figure 1 which is based on extensions to UML1.x. [Odell2001]. This protocol is identified by the token fipa-contract-net as the value of the protocol parameter of the ACL message.

Figure 1: FIPA Contract Net Interaction Protocol

1.1 Explanation of the Protocol Flow

The Initiator solicits m proposals from other agents by issuing a call for proposals (cfp) act (see [FIPA00037]), which specifies the task, as well any conditions the Initiator is placing upon the execution of the task. Participants receiving the call for proposals are viewed as potential contractors and are able to generate n responses. Of these, j are proposals to perform the task, specified as propose acts (see [FIPA00037]).

The Participant’s proposal includes the preconditions that the Participant is setting out for the task, which may be the price, time when the task will be done, etc. Alternatively, the i=n-j Participants may refuse (see [FIPA00037]) to propose. Once the deadline passes, the Initiator evaluates the received j proposals and selects agents to perform the task; one, several or no agents may be chosen. The l agents of the selected proposal(s) will be sent an accept-proposal act (see [FIPA00037]) and the remaining k agents will receive a reject-proposal act (see [FIPA00037]). The proposals are binding on the Participant, so that once the Initiator accepts the proposal, the Participant acquires a commitment to perform the task. Once the Participant has completed the task, it sends a completion message to the Initiator in the form of an inform-done or a more explanatory version in the form of an inform-result. However, if the Participant fails to complete the task, a failure message is sent.

Note that this IP requires the Initiator to know when it has received all replies. In the case that a Participant fails to reply with either a propose or a refuse act, the Initiator may potentially be left waiting indefinitely. To guard against this, the cfp act includes a deadline by which replies should be received by the Initiator. Proposals received after the deadline are automatically rejected with the given reason that the proposal was late. The deadline is specified by the reply-by parameter in the ACL message.

Any interaction using this interaction protocol is identified by a globally unique, non-null conversation-id parameter, assigned by the Initiator. The agents involved in the interaction must tag all of its ACL messages with this conversation identifier. This enables each agent to manage its communication strategies and activities, for example, it allows an agent to identify individual conversations and to reason across historical records of conversations.

In the case of 1:N interaction protocols or sub-protocols the Initiator is free to decide if the same conversation-id parameter should be used or a new one should be issued. Additionally, the messages may specify other interaction-related information such as a timeout in the reply-by parameter that denotes the latest time by which the sending agent would like to have received the next message in the protocol flow.

1.2 Exceptions to Interaction Protocol Flow

At any point in the IP, the receiver of a communication can inform the sender that it did not understand what was communicated. This is accomplished by returning a not-understood message. As such, Figure 1 does not depict a not-understood communication as it can occur at any point in the IP. The communication of a not-understood within an interaction protocol may terminate the entire IP and termination of the interaction may imply that any commitments made during the interaction are null and void. However, since this IP broadcasts to more than one Participant, multiple responses are also possible. Each response, then, must be evaluated separately – and some of these responses might be not-understood. However, terminating the entire IP in this case might not be appropriate, as other Participants may be continuing with their sub-protocols.

At any point in the IP, the initiator of the IP may cancel the interaction protocol by initiating the meta-protocol shown in Figure 2. The conversation-id parameter of the cancel interaction is identical to the conversation-id parameter of the interaction that the Initiator intends to cancel. The semantics of cancel should roughly be interpreted as meaning that the initiator is no longer interested in continuing the interaction and that it should be terminated in a manner acceptable to both the Initiator and the Participant. The Participant either informs the Initiator that the interaction is done using an inform-done or indicates the failure of the cancellation using a failure.

Figure 2: FIPA Cancel Meta-Protocol

This IP is a pattern for a simple interaction type. Elaboration on this pattern will almost certainly be necessary in order to specify all cases that might occur in an actual agent interaction. Real world issues such as the effects of cancelling actions, asynchrony, abnormal or unexpected IP termination, nested IPs, and the like, are explicitly not addressed here.

2 References

[FIPA00037] FIPA Communicative Act Library Specification. Foundation for Intelligent Physical Agents, 2000.
http://www.fipa.org/specs/fipa00037/

[Odell2001] Odell, James, Van Dyke Parunak, H. and Bauer, B., Representing Agent Interaction Protocols in UML. In: Agent-Oriented Software Engineering, Ciancarini, P. and Wooldridge, M., Eds., Springer, pp. 121-140, Berlin, 2001.
http://www.fipa.org/docs/input/f-in-00077/

3 Informative Annex A — ChangeLog

3.1 2002/11/01 - version G by TC X2S

Page 1, line 42: Reworked and expanded the section description of the IP

Page 2, Figure 1: The communication labeled inform-ref was changed to inform-result for clarity; the purpose of this communication is to inform the initiator of a result and inform-result implies inform-done

Page 2, Figure 1: The not-understood communication was removed

Page 2, Figure 1: To conform to UML 2, the protocol name was placed in a boundary, x is removed from the diamonds (xor is now the default) and the template box was removed

Page 2, line 72: Added a new section on Explanation of Protocol Flow

Page 2, line 72: Reworked and expanded the section on Exceptions of Protocol Flow to incorporate a meta-protocol for cancel

Page 2, line 72: Added a paragraph explaining the not-understood communication and its relationship with the IP