how much memory are needed when pppd connecting

hello, everyone!

how much memory are needed when the ppp link are connecting?

my pppd need about 300K memory before and after the ppp link connected.

but when the ppp link is build, the free memory will reduce about 1.2M. after the ppp link build ,the 1.2M memory will be free.

who can tell me the reason,is it normal?

thanks for all help.