what is $class in ns?

[Ian Liu <ijhl@MIT.EDU>]

I am trying to source a modified version of ns-route.tcl and
route-proto.tcl into my own code. It gives me error like:

can't read "preference_": no such variable
...
"$class set preference_"
 (procedure "_o308" line 11)
 (Agent/rtProto/Direct init line 11)
...

Can anyone explain why ns is doing that? How can I mix it?

Thanks in advance,

Ian LIU