newbie: $tcp set class_ 1

[Ken Laberteaux <klaberte@nd.edu>]

what is meant by setting class_ to some integer, e.g. 0 and 1 below, for
the UDP and TCP agent?

set udp0 [new Agent/UDP] ;# A UDP agent
$udp0 set class_ 0 ;# actually, the default, but. . .
#snip#
set tcp [new Agent/TCP]
$tcp set class_ 1