Alternating Bit Protocol
========================

Protocol description :

A protocol for managing the re-transmission of lost messages.
Consider a sender and a receiver , and assume that the channel from the
sender to the receiver is initially empty. The alternating bit protocol
works as follows:

1. Each data message sent by Sender contains a protocol bit , 0 or 1.
2. When Sender sends a message , it send it repeatedly (with its
   corresponding bit) until receiving an acknowledgment (ACK) from
   Receiver that contains the same protocol bit as the message being
   sent. 
3. When Receiver receives a message , it sends an ACK to Sender and
   includes the protocol bit of the message received. The first time
   the message is received , the protocol delivers the message for
   processing. Subsequent messages with the same bit are simply
   acknowledged.
4. When Sender receives ACK containing the same bit as the mesage it
   is currently transmitting , it stops transmitting that message ,
   flips the protocol bit , and repeats the protocol for the next
   message.