We have been introduced to the function calls socket, bind, sendto, and recvfrom to work with UDP sockets. Assume that the TCP sockets are not there. We know that UDP sockets are not reliable, meaning that there is no guarantee that a message sent using a UDP socket will reach the receiver. We want to implement our own socket type, called MRP (My Reliable Protocol) socket, that will guarantee that any message sent using a MRP socket is always delivered to the receiver. However, unlike TCP sockets, MRP sockets may not guarantee in-order delivery or exactly-once delivery of messages. Thus messages may be delivered more than once (duplicate messages) or out of order (later message delivered earlier).
MRP sockets guarantee reliable delivery using a simple approach. The sender sends a message using a UDP socket, and stores the message and the time it is sent in a table called the unacknowledged-message table. The receiver, on receiving the message, sends an ACK message back to the sender. The sender, on receiving the ACK for a message, removes that message from the unacknowledged-message table. If either the message or the ACK is lost, a timeout occurs at the sender, and the sender resends the message, and resets the sending time of that message in the unacknowledged-message table. This process repeats for each message.
To implement each MRP socket, we use the following:
-
One UDP socket through which all actual communication happen.
-
Two threads R and S. Thread R handles all messages received from the UDP socket, and thread S, handles the timeouts and retransmissions. More details of R and S are given later.
-
Two tables, unacknowledged-message table and received-message table. The first table contains the list of all messages that have been sent but not yet acknowledged by the receiver, along with the last sending time of the message. The second table contains all messages received in the socket.
Thus, if more than one MRP socket is created, one set of socket-thread-tables as above will be created for each MRP socket. The threads are killed and the data structures freed when the MRP socket is closed. You can assume that the maximum number of MRP sockets that can be opened is 25