Ef is the letter F.
An example of ef as a prefix is effigy, a form which is a representation of someone.
ef - Computer Definition
The Differentiated Services (DiffServ) protocol identifies two primary types of per-hop behaviors (PHBs), representing two service levels, or forwarding classes. Expedited Forwarding (EF) provides minimal delay, jitter, and loss. EF traffic exceeding the traffic profile, as defined by the Service Level Agreement (SLA), is discarded. Assured Forwarding (AF) comprises four classes, each of which contains three drop precedences and allocates certain amounts of buffer space and bandwidth. See also AF, bandwidth, buffer, delay, DiffServ, jitter, PHB, protocol, and SLA.