1. A method for performing a measurement on data traffic at a node of a communication network, the node including a number n>2 of bidirectional ports, each port comprising an input interface and an output interface, the input interface of each port being connected to the output interface of the other (n−1) ports, the method comprising: at the input interface of each port, receiving first data units, including a first marking bit and a second marking bit, the first data units being addressed to the output interface of the other (n−1) ports and a) for each received first data unit, checking whether the first marking bit of the first data unit is set to a value indicating that the first data unit is marked and, in the affirmative, checking whether the second marking bit of the first data unit is set to any value selected from a first value and a second value; b) in response to the first marking bit of the first data unit being set to the value indicating that the first data unit is marked, updating a global input parameter when the second marking bit of the first data unit is set to the first value, independently of the output interface to which the data unit is addressed; and c) when the second marking bit of the first data unit is set to the first value and when the first data unit is addressed to the output interface of one of the other (n−1) ports, updating a partial input parameter associated with that input interface and output interface; the method further comprising, at the output interface of each port, receiving, from the (n−1) input interfaces of the other (n−1) ports, second data units including a first marking bit and a second marking bit, the second data units comprising at least part of the first data units, and d) for each received second data unit, checking whether the first marking bit of the second data unit is set to a value indicating that the second data unit is marked and, in the affirmative, checking whether the second marking bit of the second data unit is set to any value selected from the first value and the second value; e) in response to the first marking bit of the second data unit being set to the value indicating that the second data unit is marked, when the second marking bit of the second data unit is set to the first value and when the second data unit is received from the input interface of one of the other (n−1) ports, updating a partial output parameter associated with that input interface and output interface; and f) updating a global output parameter when the second marking bit of the second data unit is set to the first value, independently of the input interface from which the second data unit is received; the method further comprising g) performing a link measurement on the data traffic based on a difference between the global input parameter and the global output parameter and performing a node measurement on the data traffic based on a difference between the partial input parameter and the partial output parameter, wherein the link measurement and the node measurement are performed at predetermined block periods.