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OSI Transport Layer

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 7Application
Layer 6Presentation
Layer 5Session
Layer 4Transport
Layer 3Network
Layer 2Data Link
Layer 1Physical

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 IPX/SPX was a popular protocol suite for several years, but it has been replaced by the TCP/IP suite on most networks.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.

More Networking Protocols and Standards:
• VTP (VLAN Trunking Protocol)
• IPv6 Payload Length Field and Jumbograms
• IP Addressing and Subnetting
• IEEE 802.11.x Wireless Standards
• Free eBook: Introduction to 802.11 Wireless
• Video - Transport Layer (Layer 4) of OSI Networking Model
• IGRP (Interior Gateway Routing Protocol)
• Networking Routing Believability and Administrative Distance
• The OSI Application Layer
• T-Carrier - A Complete and Comprehensive Guide

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