A method of configuring a set of service nodes on a host computer to provide a set of attribute-based middlebox services to data compute nodes, 'DCNs', (105, 205) on the host computer, the method comprising: on the host computer: collecting a first set of attributes used to define attribute-based middlebox service rules processed by the set of middlebox service nodes on the host computer; collecting a second set of attributes associated with at least one data message flow of a DCN (105, 205); comparing (320, 325, 330) the first and second sets of attributes to identify a subset of the second set of attributes that are in the first set of attributes used to define the middlebox service rules, and to generate a service tag that represents the identified subset of the second set of attributes; and associating (340, 345) the service tag with the data message flow for the set of middlebox service nodes to subsequently use to retrieve the identified subset of the second set of attributes and to use the identified subset of the second set of attributes to process the attribute-based service rules for data messages of the data message flow, wherein said comparing of the first and second sets of attributes and the generation and association of the service tag with the data message flow allow collected attributes in the second set of attributes to be discarded when the collected attributes are not used to define the middlebox service rules. on the host computer: collecting a first set of attributes used to define attribute-based middlebox service rules processed by the set of middlebox service nodes on the host computer; collecting a second set of attributes associated with at least one data message flow of a DCN (105, 205); comparing (320, 325, 330) the first and second sets of attributes to identify a subset of the second set of attributes that are in the first set of attributes used to define the middlebox service rules, and to generate a service tag that represents the identified subset of the second set of attributes; and associating (340, 345) the service tag with the data message flow for the set of middlebox service nodes to subsequently use to retrieve the identified subset of the second set of attributes and to use the identified subset of the second set of attributes to process the attribute-based service rules for data messages of the data message flow, collecting a first set of attributes used to define attribute-based middlebox service rules processed by the set of middlebox service nodes on the host computer; collecting a second set of attributes associated with at least one data message flow of a DCN (105, 205); comparing (320, 325, 330) the first and second sets of attributes to identify a subset of the second set of attributes that are in the first set of attributes used to define the middlebox service rules, and to generate a service tag that represents the identified subset of the second set of attributes; and associating (340, 345) the service tag with the data message flow for the set of middlebox service nodes to subsequently use to retrieve the identified subset of the second set of attributes and to use the identified subset of the second set of attributes to process the attribute-based service rules for data messages of the data message flow, wherein said comparing of the first and second sets of attributes and the generation and association of the service tag with the data message flow allow collected attributes in the second set of attributes to be discarded when the collected attributes are not used to define the middlebox service rules.