OSI Transport Layer
By Stephen Bucaro
The Transport layer (OSI layer 4) uses connection-oriented protocols to provide a
reliable end-to-end connection between the source computer and the destination computer.
Transmission Control Protocol (TCP) is a transport layer protocol that provides flow
control, multiplexing, error detection and recovery.
|Layer 2||Data Link|
At the transmitting end, the message is broken into smaller segments and each
segment is given a sequence number. At the receiving end the segments are checked
for errors. If the segments are received error free, they are reassembled in the
proper sequence and an acknowledgement is sent to the transmitting computer. If the
transmitting computer does not receive an acknowledgement, it resends the segments.
Other Transport Protocols
Sequenced packet Exchange (SPX) is a NetWare protocol that performs the Transport
layer functions for a NetWare IPX/SPX network. IPX/SPX was a popular protocol suite
for several years, but it has been replaced by the TCP/IP suite on most networks.
User Datagram Protocol (UDP) is a connectionless Transport layer protocol used to
send time sensitive data such as real-time audio and video. No error checking is
performed and no packet receipt acknowledgement is returned to the transmitting computer.
Since the data is being processed in real-time, it makes no sense to retransmit a packet
that has already been presented.
The Transport layer does not establish nor terminate a network connection. It is the
Session layer (OSI layer 5) that establishes, maintains, and terminates the connection.
The Transport layer is not involved in routing the packets from the source to the
destination. The Network layer (OSI layer 3) assigns logical addresses to the packets
and routes them through the network. Once the Session layer and the Network layer
establish a virtual circuit, the Transport layer provides reliable delivery of the data.
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