What is a forwarded clock? I'm reading the overview book of the Intel QuickPath Interconnect and it says there:

The physical connectivity of each interconnect link is made up of twenty differential signal pairs plus a differential forwarded clock.

I can't find what is a forwarded clock, does anyone know what it is and if there is some material for reading about it?
According to lecture notes of Sam Palermo, there are three common I/O clocking architectures:

Common clock (synchronous)
Forward clock (source synchronous)
Embedded clock (clock recovery)