Compare the implications of using connection-oriented communications with connectionless communications.
The OSI Network layer determines the route a packet will take as it passes through a series of different LANs from the source PC to the destination PC. The complexity and versatility of Network layer addressing gives rise to two different communication modes for passing messages across the network, both of which are recognized under OSI:
connections without stopping all communications through the node. Another feature of connection-oriented communication is that internal nodes provide error correction at each link in the chain. Therefore, if a node detects an error, it asks the preceding node to retransmit. Connectionless mode does not provide these elaborate internal control mechanisms; instead, connectionless mode relegates all error correcting and retransmitting processes to the source and destination nodes. The end nodes acknowledge the receipt of packets and retransmit if necessary, but internal nodes do not participate in flow control and error correction (other than simply forwarding messages between the end nodes).
The advantage of connectionless mode is that connectionless communications can be processed more quickly and more simply because the internal nodes only forward data and thus don't have to track connections or provide retransmission or flow control. The differences between connection-oriented and connectionless modes of communication may be easier to understand by analogy Imagine talking to someone and then having her reafirm that she understood what you have told her after each sentence. Connectionless mode is like having a conversation with someone, but the speaker just carries on and assumes that the listener under- stands. Connection-oriented is slower, yet more reliable. Connectionless is faster, but has less capability to correct errors (misunderstandings in the conversation example) as they occur. Connectionless mode does have its share of disadvantages, however, including the following:
The OSI Network layer determines the route a packet will take as it passes through a series of different LANs from the source PC to the destination PC. The complexity and versatility of Network layer addressing gives rise to two different communication modes for passing messages across the network, both of which are recognized under OSI:
Connection-oriented mode. Error correction and ?ow control
are provided at internal nodes along the message path.
Connectionless mode. Internal nodes along the message path do
not participate in error correction and flow control. To understand the distinction between connection-oriented and connectionless communications, you must consider an important distinction between the OSI model's Data Link and Network layers. In theory, the Data Link layer facilitates the transmission of data across a single link between two nodes. The Network layer describes the process of routing a packet through a series of nodes to a destination on another link on the network. An example of this latter scenario is a message passing from a PC on one LAN segment through a series of routers to a PC on a distant part of the network. The internal nodes forwarding the packet also forward other packets between other end nodes. In connection-oriented mode, the chain of links between the source and destination nodes forms a kind of logical pathway connection. The nodes forwarding the data packet can track which packet is part of which connection. This enables the internal nodes to provide flow control as the data moves along the path. For example, if an internal node determines that a link is malfunctioning, the node can send a notification message backward, through the path to the source computer. Furthermore, because the internal node distinguishes among individual, concurrent connections in which it participates, this node can transmit (or forward) a "stop sending" message for one of itsconnections without stopping all communications through the node. Another feature of connection-oriented communication is that internal nodes provide error correction at each link in the chain. Therefore, if a node detects an error, it asks the preceding node to retransmit. Connectionless mode does not provide these elaborate internal control mechanisms; instead, connectionless mode relegates all error correcting and retransmitting processes to the source and destination nodes. The end nodes acknowledge the receipt of packets and retransmit if necessary, but internal nodes do not participate in flow control and error correction (other than simply forwarding messages between the end nodes).
The advantage of connectionless mode is that connectionless communications can be processed more quickly and more simply because the internal nodes only forward data and thus don't have to track connections or provide retransmission or flow control. The differences between connection-oriented and connectionless modes of communication may be easier to understand by analogy Imagine talking to someone and then having her reafirm that she understood what you have told her after each sentence. Connectionless mode is like having a conversation with someone, but the speaker just carries on and assumes that the listener under- stands. Connection-oriented is slower, yet more reliable. Connectionless is faster, but has less capability to correct errors (misunderstandings in the conversation example) as they occur. Connectionless mode does have its share of disadvantages, however, including the following:
Messages sometimes get lost due to an over?owing buffer or a
failed link along the pathway.
If a message gets lost, the sender doesn't receive noti?cation.
Retransmission for error correction takes longer because a
faulty transmission can't be corrected across an internal link.
It is important to remember that the OSI model is not a set of rules for communication; the OSI model is a framework in which models of communication are explained. As such, individual imple-mentations of connectionless protocols can attenuate some of the preceding disadvantages. It is also important to remember that connection-oriented mode, although it places much more emphasis on monitoring errors and controlling traf?c, doesn't always work either. Ultimately, the choice of connection-oriented or connectionless communications mode depends on interoperability with other systems, the premium for speed, and the cost of components.
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