With an already functioning TFTP server and PXE boot environment for Debian 7 network install with files also being served over http from the same path as the TFTP server it is straightforward to boot UrBackup over the network. The same method applies to many Linux live cds.
Adding to pxelinux.cfg/default
Add the following to your default fiile in pxelinux.cfg
LABEL URbackup Menu Label ^UrBackup kernel vmlinuz append initrd=initrd.img boot=live config username=urbackup toram fetch=http://tftp/urbackup/filesystem.squashfs
Ensure you change the hostname to the ip or host of your tftp server and that you are serving the squashfs file over http. Note that the initrd and vmlinuz are from the debian network boot sources.