TFTP is by far the most common way to deliver a boot image in a IP-based network (possibly after using bootp or DHCP).

RBoot (short for Remote Boot) is mostly used by early HP PA-RISC machines. Later firmware releases may also use bootp and TFTP, but I'm unsure on this...

MOP is one of the oldest boot protocols and used on early VAXstations as well as DECstations. It's not only about booting. You can also remotely control VAXen's and DECstation's consoles with this protocol suite, but matching client applications seem to be missing under Linux...