packet header with variable length?

[Wei Sun <wsun@cis.ohio-state.edu>]

Hi,

I'm a new user of ns. I'm just wondering if we can implement packet with
variable length header in ns. The mannul says 

"BOB generally remains a fixed size thoughout a simulation, and the size
is recorded in the Packet::hdrlen_ member variable. This size is updated
during simulator configuration by OTcl."

and 

"It is not intended to be updated after configuration time."

Anybody has experience how to change the header size during the
simulation? 

Thanks in advance!

Wei Sun