A method of communicating a protocol data unit (400) between a first process (214) and a second process (215) comprises locating the first process at a source node (102) in a communications network and locating the second process at the source node (102). The first process (214) is arranged to send the protocol data unit (400) to the second process (215) at an address corresponding to an, at least notional, target node (114, D). The source node (102) comprises a protocol stack arranged to adapt the protocol data unit (400) to include a data structure definition (406) that supports routing path selection, the data structure definition (406) being used to identify a first address corresponding to the source node (102) as a final destination for the protocol data unit (400), the round-trip path including a first intermediate routing address. Subsequently, the adapted protocol data unit (400) leaves the source node (102) and follows a round-trip path back to the source node (102) via an intermediate node (114, D) corresponding to the first intermediate routing address for processing by the second process (215).