Message Delivery Protocol

 

Message Delivery Protocol

·         A protocol is a set of rules and conventions shared by communicating entities .

·         Message passing can be blocking or non blocking .

·         In blocking protocol, a sender process is blocked till the message selivered to the . in this case , sender process has guarantee that the message sent by it ie delivered before it continues its execution .

A blocking protocol normally includes:

1.       Blocking send : the sender process is blocked until the message is received by the receiving process or by the mailbox.

2.       Blocking receive : the receiver nlocks until a message is available thus when both sender as well as receiver is blocked ,it is known as rebdevezevous . this combination allows for tight sysnchronization between process.

·         In non blockeing protocol, asender continues the execution after performing a send operation irrespective of whether the message is delivered or not

·         A non blocking protocol has the advantage that a sender is free to continue its execution immediately after sending a message

·         However , sender has no means of knoiwing when(and wheather )its message  is delivered to the receiver.

     

·         A non blocking send : the sending process sends the message and resumes operation.

·         Non blocking receive: the receiver retrieves either a valid message or a null.

 

Buffering

Wheather the communication is direct or indirect , message exchanged by communicating processes reside in a temporary queue.

There are  three different  ways of implementing such queue.

 

Zero capacity

·         Zero capacity queue cannot keep and message waiting in it , thus it has maximum length 0

·         For this , sending process must be blocked until the receiving process receives the message .

·         System with zero capacity queue is known as message system with no buffering.

 

Bounded capacity

·         Bounded capacity queue has finite length n. thus it can have n message.

waiting in it .

·         If the queue is not full , new message can be placed in the queue and the sending process is not blocked , it not blocked , it can continue execution without waiting.

·         If the link or queue is full , then the sender must be blocked until space is available in the queue.

·         This system is also known as automatic buffering .

 Unbounded capacity

·         Unbounded capacity queue has infinite length . thus any number of messages can wait in it .

In such a system , sending process is never blocked , can continue execution without waiting

Shared memory model

·         Normally , process use memory areas within the scope of virtual memory space . also memory management systems ensure that every process has a  well-defined and separate data and separate data and code area.

·         In shared memory model parts of private address space of two or more rae processes are mapped to same physical memory.

·         Such a memory areas are called shared memory regions.