A method of transmitting a data flow via a network performed by a negotiation module associated with the network, the network supporting transmission of data in accordance with a plurality of dynamically identified Quality of Service, QoS, models, each QoS model defining a Class of Service, CoS, that may be applied to traffic transported on the network, the method comprising: prior to transmission of the data flow, communicating with a client system associated with the data flow to configure a first class of service for the data flow based on a first of the QoS models; determining a requested QoS vector, wherein the requested QoS vector comprises a set of QoS metrics based on QoS requirements specified by the client system when configuring the first class of service; transmitting a first portion of the data flow through the network in accordance with the first class of service; determining a delivered QoS vector, wherein the delivered QoS vector comprises a set of QoS metrics based on the first QoS model and/or a based on a measured or forecast performance of the data flow; determining a plurality of available QoS vectors for a set of available QoS models; computing a first difference measure between the requested QoS vector and the delivered QoS vector; computing respective second difference measures between the requested QoS vector and each of the available QoS vectors; identifying one or more available QoS models in the set of available QoS models having QoS vectors closer to the requested QoS vector than the delivered QoS vector is to the requested QoS vector in accordance with the computed difference measures; detecting a renegotiation condition, wherein the renegotiation condition relates to a change in an available set of QoS models supported by the network and is detected in response to identifying one or more available QoS models having QoS vectors closer to the requested QoS vector than the delivered QoS vector; and in response to detecting the renegotiation condition: communicating with the client system to configure a second class of service for the data flow based on a second of the QoS models; and transmitting a subsequent portion of the data flow through the network in accordance with the second class of service. prior to transmission of the data flow, communicating with a client system associated with the data flow to configure a first class of service for the data flow based on a first of the QoS models; determining a requested QoS vector, wherein the requested QoS vector comprises a set of QoS metrics based on QoS requirements specified by the client system when configuring the first class of service; transmitting a first portion of the data flow through the network in accordance with the first class of service; determining a delivered QoS vector, wherein the delivered QoS vector comprises a set of QoS metrics based on the first QoS model and/or a based on a measured or forecast performance of the data flow; determining a plurality of available QoS vectors for a set of available QoS models; computing a first difference measure between the requested QoS vector and the delivered QoS vector; computing respective second difference measures between the requested QoS vector and each of the available QoS vectors; identifying one or more available QoS models in the set of available QoS models having QoS vectors closer to the requested QoS vector than the delivered QoS vector is to the requested QoS vector in accordance with the computed difference measures; detecting a renegotiation condition, wherein the renegotiation condition relates to a change in an available set of QoS models supported by the network and is detected in response to identifying one or more available QoS models having QoS vectors closer to the requested QoS vector than the delivered QoS vector; and in response to detecting the renegotiation condition: communicating with the client system to configure a second class of service for the data flow based on a second of the QoS models; and transmitting a subsequent portion of the data flow through the network in accordance with the second class of service. communicating with the client system to configure a second class of service for the data flow based on a second of the QoS models; and transmitting a subsequent portion of the data flow through the network in accordance with the second class of service.