问题：

[多选] Which three of these statements about penultimate hop popping are true？（）

A . It is used only for directly connected subnets or aggregate routes.
B . It can only be used with LDP.
C . It is only used when two or more labels are stacked.
D . It enables the Edge LSR to request a label pop operation from its upstream neighbors.
E . It is requested through TDP using a special label value that is also called the implicit-null value.
F . It is requested through LDP using a special label value that is also called the implicit-null value.