Reliability of Message Delivery

One of the main requirements of TCPMPL is the reliability of the messages sent across the network by the processes. When a process sends a message of size to a destination process , the destination process must receive the message of size without the message getting lost. There are various ways by which reliability can be ensured:

• One way TCPMPL can provide reliable message delivery is to use an unreliable protocol and provide reliability at the message passing layer level. For providing reliability, TCPMPL should re-send messages if they are lost. To know whether the message has reached the destination or not, the destination process should send back an acknowledgement to the sender to indicate that the message was received successfully. TCPMPL should also buffer all the messages sent to the destination until it can ensure that the message has reached the destination. Once the messages has reached the destination, the buffered message can be discarded. TCPMPL should also maintain timers for each message that is sent across the network to the destination process. This timer is used for measuring the time to wait for the acknowledgement (ack) of a message. If the acknowledgement does not reach the sender process before the timer expires, the sender process retransmits that particular message. Duplicates can be generated due to the retransmission of messages to provide reliability. If the timer expires before the acknowledgement reaches the sender process, the sender process proceeds to retransmit the message, although the message has been received at the receiver process. This generates a duplicate of the original message. All these issues need to be addressed if one were to use an unreliable underlying protocol. One such unreliable protocol is the UDP/IP [4] protocol. This requires the message passing layer to have a server running on each machine which takes care of retransmission of messages to provide reliability since the message passing layer has to constantly see whether messages are received reliably at the destination.
• Another approach would be to use a reliable protocol provided by the system. Since the underlying protocol is reliable, TCPMPL does not need to provide the reliability of messages explicitly. One such reliable protocol is TCP which is built on top of the IP Protocol. TCP runs as a daemon on each machine. The services of TCP are available as part of the operating system services. The TCP protocol also takes care of reliability. Therefore, the message passing layer should use the TCP for reliable message delivery.

TCPMPL as the name indicates uses the TCP protocol to ensure reliability of message delivery.