Connectionless service is a self-contained action and does not include establishment, maintenance and releasing a connection.
• Each message carries the full destination address and is treated and routed independently of all other messages.
• There is no prior negotiation between sender and receiver of data.
• Connectionless service does not even preserve the order of delivery of messages. As a result the different messages may arrive out of order at the destination.
• Connectionless service is modeled after a postal system, in which we· drop a letter in a mailbox by specifying the destination address. In such a system two letters dropped in the same mailbox going to the same destination may likely take different routes and arrive out of order.
Both the services discussed above can be reliable or unreliable depending upon the fact whether or not the receiver acknowledges the receipt of each message.
1. The service is said to be reliable when the receiver acknowledges the receipt of each message so that the sender is sure that data is received. However, such an acknowledgement sometimes results in overhead and delays.
• Reliable connection oriented service is used for file transfer and has two different implementations. One preserves the message boundaries (reliable message stream) and the other treating data as stream of bytes with no message boundaries (reliable byte stream).
• Reliable message stream service is used for transferring the sequence of pages where it is important to preserve the message boundaries.
• On the other hand reliable byte stream service is used for remote login. In this case, when a user logs into a remote server, a byte stream from user’s computer is transferred to remote server.
• Another class of connection oriented service called unreliable connection oriented service does not require acknowledgements to be sent to the source. Such a service is used where the use of acknowledgements introduce delays. For example: In case of digitized voice over telephone, it is preferable to hear a bit of noise on the line from time to time than to experience a delay waiting for acknowledgements.
2. The service is said to be unreliable when the receiver does not acknowledge the receipt of the message.
• Unreliable connectionless service is also known as unreliable datagram service. Such a service is commonly used for junk e-mails or simply junk-mails. The sender of such junk-mails does not establish or release the connection to send these mails nor does he require 100 percent reliable delivery. Unreliable datagram service is similar to telegram service, which also does not return an acknowledgement to the sender.
3. There can also be reliable connectionless service called acknowledged diagram service.
Such a service may not require connection to be established for sending one short message but essentially requires reliability. This is similar to sending a registered letter and requiring a return receipt. When a receipt comes back, the sender is sure that the letters was delivered to the destination.
• Another kind of service provided is the request-reply service. In this service the sender transmits a single datagram containing a request and receiver transmits a reply that contains the answer. Request-reply service is used in client-server model where client issues a request and server responds to it. For example: a query to a database for specific information.