An enumerated value which specifies the address type
of the source.
This object must be used along with the
crttMonIPEchoAdminSourceAddress
object as it identifies the address family of the
address specified by that object. If the value
of crttMonIPEchoAdminSourceAddress is a zero-length
string (e.g., because an IPv4 address is specified by
rttMonEchoAdminSourceAddress), this object contains
the value 'unknown'.