Does coreos.live.rootfs_url support any other hosting protocol other than http and https?

Issue

• According to Red Hat OpenShift Container Platform 4.6 documentation , ftp should be supported for downloading .img and .ign files. But, the installation failed and got error as below.

coreos.live.rootfs_url= supports HTTP and HTTPS only
Please fix your PXE configuration

• The Documentation example however shows ftp example as follows.

rd.neednet=1 console=ttysclp0 coreos.inst.install_dev=dasda coreos.live.rootfs_url=ftp://
cl1.provide.example.com:8080/assets/rhcos-live-rootfs.s390x.img
coreos.inst.ignition_url=ftp://cl1.provide.example.com:8080/ignition/bootstrap.ign
ip=172.18.78.2::172.18.78.1:255.255.255.0:::none nameserver=172.18.78.1
rd.znet=qeth,0.0.bdf0,0.0.bdf1,0.0.bdf2,layer2=1,portno=0 zfcp.allow_lun_scan=0 cio_ignore=all,
!condev rd.dasd=0.0.3490