Alternating bit and go-back-n network emulator: version 1.1 j. f.kurose

this code should be used for pa2, unidirectional or bidirectional

data transfer protocols (from a to b. bidirectional transfer of data

is for extra credit and is not required). network properties:

- one way network delay averages five time units (longer if there

are other messages in the channel for gbn), but can be larger

- packets can be corrupted (either the header or the data portion)

or lost, according to user-defined probabilities

- packets will be delivered in the order in which they were sent

(although some can be lost).