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".