A method of operating an optical transport network having the steps of: sending a traffic flow along a route through nodes of the optical transport network from a source node to a destination node over one or more optical links, the traffic flow comprising data grouped into a number of resizable data units, the resizable data units having a structure according to G.709 ODUflex, each data unit having a data unit overhead, at least some of the nodes having switches arranged to route the data units, a number of the data units being encapsulated by optical transport frames, each having a frame overhead associated with the optical link carrying the traffic flow, and characterised by changing a rate of the traffic flow by: at the source node, changing the data unit overhead to indicate a resize of the data units to any of the switches along the route, changing at the source node the frame overhead to indicate a change in rate of an optical signal carrying the traffic flow to any nodes along the route which terminate an optical link, the change in rate corresponding to the resize of the data units, at the source node, grouping the data of the traffic flow into the resized data units, encapsulating the resized data units with optical transport frames and tuning an optical transmission part to transmit the frames in an optical signal at the new rate, detecting at any intermediate nodes along the route which terminate the optical signal, the indication of the change in rate in the frame overhead and tuning an optical transmission part of that intermediate node to transmit the traffic flow on a next optical link along the route at the new rate according to the indication of the change in rate. sending a traffic flow along a route through nodes of the optical transport network from a source node to a destination node over one or more optical links, the traffic flow comprising data grouped into a number of resizable data units, the resizable data units having a structure according to G.709 ODUflex, each data unit having a data unit overhead, at least some of the nodes having switches arranged to route the data units, a number of the data units being encapsulated by optical transport frames, each having a frame overhead associated with the optical link carrying the traffic flow, and characterised by changing a rate of the traffic flow by: at the source node, changing the data unit overhead to indicate a resize of the data units to any of the switches along the route, changing at the source node the frame overhead to indicate a change in rate of an optical signal carrying the traffic flow to any nodes along the route which terminate an optical link, the change in rate corresponding to the resize of the data units, at the source node, grouping the data of the traffic flow into the resized data units, encapsulating the resized data units with optical transport frames and tuning an optical transmission part to transmit the frames in an optical signal at the new rate, detecting at any intermediate nodes along the route which terminate the optical signal, the indication of the change in rate in the frame overhead and tuning an optical transmission part of that intermediate node to transmit the traffic flow on a next optical link along the route at the new rate according to the indication of the change in rate. at the source node, changing the data unit overhead to indicate a resize of the data units to any of the switches along the route, changing at the source node the frame overhead to indicate a change in rate of an optical signal carrying the traffic flow to any nodes along the route which terminate an optical link, the change in rate corresponding to the resize of the data units, at the source node, grouping the data of the traffic flow into the resized data units, encapsulating the resized data units with optical transport frames and tuning an optical transmission part to transmit the frames in an optical signal at the new rate, detecting at any intermediate nodes along the route which terminate the optical signal, the indication of the change in rate in the frame overhead and tuning an optical transmission part of that intermediate node to transmit the traffic flow on a next optical link along the route at the new rate according to the indication of the change in rate.