This Addendum to Recommendation X.634 contains an SDL specification of the Transport Fast Byte Protocol description. The Transport Fast Byte Protocol eliminates the roundtrip delay associated with the establishment and release of a transport connection, and requires very low PCI overhead. The Transport Fast Byte Protocol is intended for use in situations in which enhancements to the network QoS are not required, and efficiency of operation (e.g. reduction of roundtrip delays on establishment and release) is of primary concern.