Specification
This document outlines the key elements of the Contract Negotiation Protocol. The used terms are described here.
1 Introduction
A Contract Negotiation (CN) involves two parties, a Provider that offers one or more Datasets under a usage contract and Consumer that requests Datasets. A CN is uniquely identified through an IRI. Each CN requires a newly generated IRI, which may not be used in a CN after a terminal state has been reached. A CN progresses through a series of states, which are tracked by the Provider and Consumer using messages. A CN transitions to a state in response to an acknowledged message from the counter-party. Both parties have the same state of the CN. In case the states differ, the CN is terminated and a new CN has to be initiated.
1.1 States
The CN states are:
TERMINATED: The Provider or Consumer has placed the CN in a terminated state. A termination message has been sent by either of the Participants and the other has sent an ACK response. This is a terminal state.
1.2 State Machine
The CN state machine is represented in the following diagram:
Transitions marked with C
indicate a message sent by the Consumer, transitions marked with P
indicate a Provider message. Terminal states are final; the state machine may not transition to another state. A new CN may be initiated if, for instance, the CN entered the TERMINATED
state due to a network issue.
2 Message Types
The CN state machine is transitioned upon receipt and acknowledgement of a message. This section details those messages as abstract message types.
Concrete wire formats are defined by the protocol binding, e.g., Contract Negotiation HTTPS Binding..
All Policy types (Offer, Agreement) must contain an unique identifier in the form of a URI. GUIDs can also be used in the form of URNs, for instance following the pattern urn:uuid:{GUID}.
An ODRL Agreement must have a target property containing the Dataset id.
2.1 Contract Request Message
The Contract Request Message is sent by a Consumer to initiate a CN or to respond to a Contract Offer Message sent by a Provider.
The Consumer must include an
offer
property, which itself must have a@id
property. If the message includes aproviderPid
property, the request will be associated with an existing CN and a Consumer Offer will be created using either theoffer
oroffer.@id
properties. If the message does not include aproviderPid
, a new CN will be created on Provider side using either theoffer
oroffer.@id
properties and the Provider selects an appropriateproviderPid
.Different to a Catalog or Dataset, the Offer inside a Contract Request Message must have an
odrl:target
attribute. However, it's contained Rules must not have anyodrl:target
attributes to prevent inconsistencies with the ODRL inferencing rules for compact policies.
2.2 Contract Offer Message
The Contract Offer Message is sent by a Provider to initiate a CN or to respond to a Contract Request Message sent by a Consumer.
Different to a Dataset (see DCAT Vocabulry Mapping), the Offer inside a ContractOfferMessage must have an
odrl:target
attribute. However, it's contained Rules must not have anyodrl:target
attributes to prevent inconsistencies with the ODRL inferencing rules for compact policies.
2.3 Contract Agreement Message
The Contract Agreement Message is sent by a Provider when it agrees to a contract. It contains the complete Agreement.
The message must contain a
consumerPid
and aproviderPid
.The message must contain an ODRL Agreement.
An Agreement must contain a
timestamp
property defined as an XSD DateTime type.An Agreement must contain an
assigner
andassignee
. The contents of these properties are a dataspace-specific unique identifier of the Agreement parties. Note that these identifiers are not necessarily the same as the identifiers of the Participant Agents negotiating the contract (e.g., Connectors).An Agreement must contain a
odrl:target
property. None of its Rules, however, must have anyodrl:target
attributes to prevent inconsistencies with the ODRL inferencing rules for compact policies.
2.4 Contract Agreement Verification Message
The Contract Agreement Verification Message is sent by a Consumer to verify the acceptance of an Agreement.
A Provider responds with an error if the contract cannot be validated or is incorrect.
The message must contain a
consumerPid
and aproviderPid
.
2.5 Contract Negotiation Event Message
When the Contract Negotiation Event Message is sent by a Provider with an eventType
property set to FINALIZED
, an Agreement has been finalized and the associated Dataset is accessible. The state machine is transitioned to the FINALIZED
state.
Other event types may be defined in the future.
A Consumer responds with an error if the contract cannot be validated or is incorrect.
The message must contain a
consumerPid
and aproviderPid
.When the message is sent by a Consumer with an
eventType
set toACCEPTED
, the state machine is placed in theACCEPTED
state.
Note that CN events are not intended for propagation of an Agreement state after a CN has entered a terminal state. It is considered an error for a Consumer or Provider to send an event after the CN state machine has entered a terminal state.
2.6 Contract Negotiation Termination Message
The Contract Negotiation Termination Message is sent by a Consumer or Provider indicating it has cancelled the CN sequence. The message can be sent at any state of a CN without providing an explanation. Nevertheless, the sender may provide a description to help the receiver.
The message must contain a
consumerPid
and aproviderPid
.If an error is received in response to the message, the sending party may choose to ignore the error.
Note that a CN may be terminated for a variety of reasons, for example, an unrecoverable error was encountered or one of the parties no longer wishes to continue. A Connector's operator may remove terminated CN resources after it has reached the terminated state.
3 Response Types
The ACK
and ERROR
response types are mapped onto a protocol such as HTTPS. A description of an error might be provided in protocol-dependent forms, e.g., for an HTTPS binding in the request or response body.
3.1 ACK - Contract Negotiation
The Contract Negotiation is an object returned by a Consumer or Provider indicating a successful state change happened.
3.2 ERROR - Contract Negotiation Error
The Contract Negotiation Error is an object returned by a Consumer or Provider indicating an error has occurred. It does not cause a state transition.
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