This field indicates the Transport type. This can be either: TP4 - OSI Transport Class 4 NSP - DECnet Phase IV Network Services Protocol If TP4 is specified, it can be followed by an optional T-Sel value as a series of hexadecimal digits after an equals sign. If an explicit T-Sel value is not specified, the DECnet default of "TP4=DEC0" is assumed. For example: TP4=AB12 - The T-Sel value is set to "AB12" TP4=AB1 - The T-Sel value is "AB10" (number of digits must be even) TP4 - The T-Sel value is assumed to be "DEC0" If a transport type is not specified, the default depends on the address. NSP is used if the address is specified using Phase IV format, or is an NSAP with an N-Sel value of "20". TP4 is used if the address is an NSAP with an N-Sel value other than "20".