This Addendum to Recommendation X.633 contains an SDL specification of the Network Fast Byte Protocol description. The Network Fast Byte Protocol applies to the provision of the OSI Connection-mode Network Service in end systems, and eliminates the roundtrip delay associated with the establishment and release of a network connection, and requires very low PCI overhead. The Network Fast Byte Protocol is intended for use in situations in which enhancements to the data link QoS are not required, and efficiency of operation (e.g. reduction of roundtrip delays on establishment and release) is of primary concern.