To extend NS2.26 to run BI_TCP, you need to do the following:

1. Download bitcp-code.tar.gz, unzip it, and then copy the 
   files into ns-allinone-2.26/ns-2.26/tcp directory. 
   (The original files will be overwritten.)

2. Add the following lines into file 
  ns-allinone-2.26/ns-2.26/tcl/lib/ns-default.tcl
   Agent/TCP set bs_beta_ 0.875
   Agent/TCP set bs_max_increment_ 32
   Agent/TCP set bs_min_increment_ 0.01
   Agent/TCP set bs_log_factor_ 4
   Agent/TCP set bs_fast_convergence_ 1

3. Make NS ("make clean", and then "make")

4. In your script, set the windowOption of a TCP connection 
   to 12 (BI-TCP). You can use bitcp-ex.tcl as an example.