CTI 2572 Technical Overview Manual
17
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.
