Lesson 14: Connection-Oriented vs. Connectionless

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Connection-oriented communication includes the steps of setting up a call from one computer to another, transmitting/receiving data, and then releasing the call, just like a voice phone call.  However, the network connecting the computers is a packet switched network, unlike the phone system's circuit switched network.  Connection-oriented communication is done in one of two ways over a packet switched network:  with and without virtual circuits.

Without virtual circuits: This is what TCP does in the Internet.  The only two machines in the Internet that are aware a connection is established are the two computers at the endpoints.  The Internet itself--its routers and links--have no information about the presence of a connection between the two computers.  This means that all of the packets flowing between the two computers can follow different routes.  One benefit of establishing the connection is that the flow of packets from the source to the destination can be slowed down if the Internet is congested and speeded up when congestion disappears. Another benefit is that the endpoints can anticipate traffic between them, and agree to cooperate to ensure the integrity and continuity of the data transfers. This allows the network to be treated as a "stream" of data, as we will study later.

Virtual circuit:  This is not used in the Internet, but is used in other types of networks (eg. the "X.25" protocol, still popular in Europe).  The routers within the network route all packets in one connection over the same route.  The advantage is that video and voice traffic are easier to carry, because routers can reserve memory space to buffer the transmission.


Connectionless communication is just packet switching where no call establishment and release occur.  A message is broken into packets, and each packet is transferred separately.  Moreover, the packets can travel different route to the destination since there is no connection. Connectionless service is typically provided by the UDP (User Datagram Protocol), which we will examine later.  The packets transferred using UDP are also called datagrams

Click here to view a the concepts of connection-oriented, and connectionless communication... (Flash animation)

Comparison between Connection-oriented and Connectionless Communication


Feature Connectionless Connection-oriented
How is data sent? one packet at a time as continuous stream of packets
Do packets follow same route? no virtual circuit:  yes
without virtual circuit:  no
Are resources reserved in network? no virtual circuit:  yes
without virtual circuit:  no
Are resources reserved in communicating hosts? no yes
Can data sent can experience variable latency? yes yes
Is connection establishment done? no yes
Is state information stored at network nodes? no virtual circuit:  yes
without virtual circuit:  no
What is impact of node/switch crash? only packets at node are lost all virtual circuits through node fail
What addressing information is needed on each packet? full source and destination address virtual circuit:  a virtual circuit number
without virtual circuit:  full source and destination address
Is it possible to adapt sending rate to network congestion? hard to do virtual circuit:  easy if sufficient buffers allocated
without virtual circuit:  harder to do

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Permission to use this lesson as part of CS1204 and CS1604 was granted by the Institute for Distance and Distributed Learning, Virginia Tech and Marc Abrams, Ph.D., Copyright © 2000. This lesson was originally developed for use in CS5244 at Virginia Tech.