Understanding the network.pdf - Back to Home

Layer 4 service. ICMP messages are wrapped and transported as IP datagrams in
the same way that TCP and UDP are transported.
Layer 4: TCP and UDP
Transmission Control Protocol (TCP) and User Datagram Protocol
(UDP) are the transport protocols (OSI Layer 4) for the TCP/IP
protocol suite. The transport layer, as you might recall, provides the
network-independent process connection services between host
client/server applications. These processes can be
connection-oriented (similar to making a phone call), an immediate
point-to-point communication, or connectionless, like sending a letter
through the mail. After it is sent, it might or might not arrive within a
reasonable time frame, and if it gets lost, you can always send
another.
Connection-oriented processes are point-to-point communication
streams. These connections are thought of as virtual circuits (VCs).
Each VC connection is distinct and used only for host-to-host
communication for a single process. In other words, a host with
multiple TCP communication sessions would have a separate VC for
each process. Connection-oriented transports are used for
interapplication communication processes that have the following
requirements:
• Dedicated connection setup and maintenance
• Delivery acknowledgment (by the destination host)
• Guaranteed byte-stream sequencing
Connectionless transport processes use standalone message units or
datagrams to transport interapplication data. Connectionless
transport is faster than connection-oriented transport because it does
not have the flow control and data integrity mechanisms.
Connectionless transport is used when the application is looking for
fast data transport and needs to control the transmission process,
such as with remote file and print services and fast query response
transactions. The hallmarks of connectionless transport are as
follows:
• Response-based, event-oriented messages
• No guaranteed delivery
• No data sequencing