1.5 Compare the implications of using connection-oriented communications with connectionless communications.

Connection-oriented communication establishes a session or circuit between two computers before transmission of data or voice can begin. Once set up, that circuit is dedicated to the transmission until the session is completed. Connection-oriented communication provides assurance of packet delivery. The telephone network and ATM are two examples of a connection-oriented network.

Connectionless communication includes in the packet the source and destination addresses. Hence an established session between two computers is not required. Ethernet, Token Ring and FDDI are examples of a connectionless network.

For the Networking Essentials exam, it is enough if you know that a connection-oriented communication provides assured delivery because it ‘shakes hands’ before the transmission. You can skip the following discussion.

Some authors consider the connection-oriented communication method reliable while they consider the connectionless communication method unreliable. However, this is inaccurate. Connection-oriented is always a reliable communication solution, therefore, it is a better choice than connectionless because of the assured delivery.

However, connectionless can be either reliable or unreliable. (You can think of an unreliable communication as sending a regular mail. You can not expect a response from the receiver. While a reliable communication can be thought as sending a register mail. You are notified when the letter reaches the receiver. ) Just like a register letter requires more processing works, a reliable protocol needs more overheads than an unreliable one.

Some people worry about unreliable communication methods. They think the system may crash if it doesn't know whether the communication error has occurred. In fact, in some applications such as playing video, losing several frames will not affect the communication. Moreover, protocols that operate at higher level of OSI model can provide an error correction mechanism, which is not implemented in unreliable protocols.

TCP provides a reliable, connection-oriented delivery service.

 

UDP provides the connectionless datagram service that offers unreliable delivery. UDP is primary used in multimedia application such as playing the video over the network. The connectionless transmission can prevent the client and server shake hands each time before the connection such as TCP/IP. It is not as reliable as TCP/IP but has fewer overheads. In those multimedia applications, losing one or two picture frame has little affect on the performance. Therefore, unreliable connectionless communication is acceptable.

 

IP provides an unreliable connectionless delivery. Please note either TCP or UDP is a transport layer protocol. They all use IP as the network layer protocol. Transport layer and network layer are two layers of ISO OSI model that we will discuss later. By now, you just need to know network layer is below the transport layer. Therefore, both TCP and UDP are based on IP. Because TCP uses IP as the lower layer protocol, we call it "TCP/IP" in convention. But for the UDP, in most cases, people just mention it "UDP" while not "UDP/IP".

 

Some people may have some confusion now. It is easy for them to understand that both UDP and IP are connectionless. But it is hard for them to grasp the idea that TCP is connection-oriented while IP is connectionless. In fact, TCP only adds the connection-oriented service to the IP protocol. IP only provides the routing information from the source host to the destination host and try to find the best the route between these two hosts. Because IP is unreliable, a packet might be lost in the process. Because UDP is also unreliable connectionless protocol like IP, UDP doesn’t do anything to prevent packet missing. However for the TCP, which also uses IP as the network layer protocol, it adds the connection-oriented service above the IP. Therefore, it guarantees that every packet delivers to the destination. Because it uses IP as a lower level protocol, some packets might be lost during the transmission. TCP always asks the destination host to send an acknowledgement (ACK) packet within a specific period. If TCP doesn’t receive any acknowledgement packet, it will retransmit the packet. Therefore, TCP is reliable connection-oriented.

NetBEUI provides both connection-oriented and connectionless data transfer services. If question ask you which protocol supports connectionless transmission, B should be also correct. However now, it ask which protocol ONLY supports connectionless transmission. Therefore, we ignore B in this question.

IPX transport handles datagram transmission to and from computers using the IPX protocol. A datagram is a connectionless packet whose delivery to a remote computer is not guaranteed. Therefore, IPX is also the correct answer.