If you are using ftpto download files into a public ftp site, you probably don't require that the site prove its identity, even if it requires proof of your own. On the other hand, if you are providing a password or credit card number to an application, you probably want to be sure of the receiver's bona fides. In these cases, mutual authentication is required — that is, both the context initiator and the acceptor must prove their identities.
A context initiator can request mutual authentication by setting gss_init_sec_context()'s req_flags argument to the value
GSS_C_MUTUAL_FLAG. If mutual authentication has been authorized, the function indicates authorization by setting the ret_flags argument to this value. If mutual authentication is requested but not available, it is the initiating application's responsibility to respond accordingly — the GSS-API will not terminate a context for this reason. Some mechanisms will perform mutual authentication regardless of whether it has been requested.