IBM CTI 2572 manual Network Protocols, Connectionless vs. Connection Oriented Protocols

CHAPTER 4. NETWORK PROTOCOLS

4.1 Connectionless vs. Connection Oriented Protocols

In most networking discussions you will find references to connectionless and connection- oriented services. The following analogy may help you understand the features and advantages of each.

Connectionless Protocols

Connectionless services may be compared to the standard U.S. mail. You place a letter in an envelope and mail it. There is no need to notify the recipient that you are mailing the letter and no requirement that the post office confirm that the letter was delivered (although they may notify you eventually if the letter could not be delivered). You may ask the recipient to respond. If they do not respond within a period of time, you can take some followup action (like sending another letter). If you mail several letters to the same recipient, there is no guarantee by the post office that they will be delivered in the same sequence that they were mailed or that all of the group will be delivered.

Connectionless network protocols work the same way. The user software sends a packet (datagram) to another node on the network. There is no requirement to establish communications with the recipient prior to sending the packet nor does the network protocol confirm delivery (although the protocol may notify the user if the datagram could not be delivered). The application software may expect or request a reply from the recipient. If no reply is received within a specified period of time (timeout), the application software may re-try. If several datagrams are sent, there is no guarantee by the network that they will arrive in the same sequence that they were sent or that all will be delivered.

Connectionless protocols are frequently described in network terminology as unreliable. Despite the emotional baggage that this term carries, it simply means that the protocol used at this level will not guarantee that packets will be delivered or that notification will be given if a packet cannot be delivered. A higher level protocol or the application program itself may perform this function.

Connectionless protocols provide several advantages: they make efficient uses of network resources, are much simpler to implement, and require less processor overhead. A connectionless protocol would typically be used to send a command to a PLC to read data from memory.

CTI 2572 Technical Overview Manual 17