A method of transferring data packets across a multi-path network having at least one ingress port, a plurality of egress ports, and a plurality of network elements interconnected by network links, the method comprising: dynamically routing data packets across the multi-path network except where the data packet has ordering requirements matching a preceding data packet for which an exclusively assigned link exists, in which dynamic routing means that during its transit across the network a single data packet may be allocated previously selected links by a first group of network elements and may be allocated links dynamically by a second group of network elements; for each data packet arriving at an egress port of the multi-path network via a data packet path across the multi-path network, issuing a delivery acknowledgement from the egress port concerned, said data packet path connecting the egress port concerned with an ingress port of the multi-path network via one or more network elements, each network element having a network link connected therewith over which the data packet is transmitted from that network element; the method being characterized by: transmitting the delivery acknowledgement across the multi-path network following in reverse said data packet path taken by the data packet being acknowledged; at the or each network element in said data packet path and in response to receipt of such a delivery acknowledgement by that network element, updating a state of that network link connected with the network element which is in said data packet path taken by the data packet being acknowledged; assigning the or each network link in the data packet path of a first data packet being transmitted across the network individually exclusively to subsequent data packets having the same ordering requirement as said first data packet until a delivery acknowledgement of the most recent preceding data packet having the same ordering requirement is transmitted on such an exclusively assigned network link; and disabling dynamic routing of data packets by a network element in said data packet path of data packets that have ordering requirements matching at least one preceding data packet for which no acknowledgement has been received by the network element concerned. dynamically routing data packets across the multi-path network except where the data packet has ordering requirements matching a preceding data packet for which an exclusively assigned link exists, in which dynamic routing means that during its transit across the network a single data packet may be allocated previously selected links by a first group of network elements and may be allocated links dynamically by a second group of network elements; for each data packet arriving at an egress port of the multi-path network via a data packet path across the multi-path network, issuing a delivery acknowledgement from the egress port concerned, said data packet path connecting the egress port concerned with an ingress port of the multi-path network via one or more network elements, each network element having a network link connected therewith over which the data packet is transmitted from that network element; the method being characterized by: transmitting the delivery acknowledgement across the multi-path network following in reverse said data packet path taken by the data packet being acknowledged; at the or each network element in said data packet path and in response to receipt of such a delivery acknowledgement by that network element, updating a state of that network link connected with the network element which is in said data packet path taken by the data packet being acknowledged; assigning the or each network link in the data packet path of a first data packet being transmitted across the network individually exclusively to subsequent data packets having the same ordering requirement as said first data packet until a delivery acknowledgement of the most recent preceding data packet having the same ordering requirement is transmitted on such an exclusively assigned network link; and disabling dynamic routing of data packets by a network element in said data packet path of data packets that have ordering requirements matching at least one preceding data packet for which no acknowledgement has been received by the network element concerned. transmitting the delivery acknowledgement across the multi-path network following in reverse said data packet path taken by the data packet being acknowledged; at the or each network element in said data packet path and in response to receipt of such a delivery acknowledgement by that network element, updating a state of that network link connected with the network element which is in said data packet path taken by the data packet being acknowledged; assigning the or each network link in the data packet path of a first data packet being transmitted across the network individually exclusively to subsequent data packets having the same ordering requirement as said first data packet until a delivery acknowledgement of the most recent preceding data packet having the same ordering requirement is transmitted on such an exclusively assigned network link; and disabling dynamic routing of data packets by a network element in said data packet path of data packets that have ordering requirements matching at least one preceding data packet for which no acknowledgement has been received by the network element concerned.