This is a simple connector that is placed on each link. It affixes
its label id to each packet that traverses it. The packet id is used
by the destination node incident on that link to identify the link by
which the packet reached it. The label id is configured by the Link
constructor. This object is an internal object, and is not designed
to be manipulated by user level simulation scripts. The object only
supports two methods:
assigns ifid that this object will affix to each packet.
returns the label that this object affixes to each packet.
The global class variable, ifacenum
_, specifies the next
available ifid number.