A data processing system comprising: first and second network ports (107, 108) each operable to support a network connection configured according to one or more of a predetermined set of physical layer protocols; and a processor configured to, on a network message being formed for transmission to a network endpoint accessible over either of the first and second network ports (107, 108): estimate the total time required to, for each of the predetermined set of physical layer protocols, negotiate a respective network connection and transmit the network message over that respective network connection; select the physical layer protocol having the lowest estimate of the total time required to negotiate a respective network connection and transmit the entire network message over that respective network connection; and configure at least one of the first and second network ports (107, 108) to use the selected physical layer protocol, the data processing system characterised in that at least some of the physical layer protocols of the predetermined set of physical layer protocols are of the same type of physical layer protocol but defined so as to operate with different compression schemes, and the estimate of the time required to, for each of the predetermined set of physical layer protocols, transmit the entire network message over that respective network connection includes the time required to decompress the network message were it compressed in accordance with the compression scheme of the respective physical layer protocol. first and second network ports (107, 108) each operable to support a network connection configured according to one or more of a predetermined set of physical layer protocols; and a processor configured to, on a network message being formed for transmission to a network endpoint accessible over either of the first and second network ports (107, 108): estimate the total time required to, for each of the predetermined set of physical layer protocols, negotiate a respective network connection and transmit the network message over that respective network connection; select the physical layer protocol having the lowest estimate of the total time required to negotiate a respective network connection and transmit the entire network message over that respective network connection; and configure at least one of the first and second network ports (107, 108) to use the selected physical layer protocol, estimate the total time required to, for each of the predetermined set of physical layer protocols, negotiate a respective network connection and transmit the network message over that respective network connection; select the physical layer protocol having the lowest estimate of the total time required to negotiate a respective network connection and transmit the entire network message over that respective network connection; and configure at least one of the first and second network ports (107, 108) to use the selected physical layer protocol, the data processing system characterised in that at least some of the physical layer protocols of the predetermined set of physical layer protocols are of the same type of physical layer protocol but defined so as to operate with different compression schemes, and the estimate of the time required to, for each of the predetermined set of physical layer protocols, transmit the entire network message over that respective network connection includes the time required to decompress the network message were it compressed in accordance with the compression scheme of the respective physical layer protocol.