[ns] drop pkt because of CBK?

[=?GB2312?B?s8K3vQ==?=]

hi,all

recently i'm working on a wireless network simulation with ns.
when i look into the .tr file, i found that there are a lot of pkt droped by
the src node by the reason of
CBK. i refered to the <ns manual>. it explain CBK as DROP_RTR_MAC_CALLBACK.

i don't what the CBK means and other reasons of droping pkt.
could anyone tell me?
it could be a big help.

then i also found that the dropped pkt was received by the dest node.

these confused me.

any reply will be helpful.

thanks, guys.