Transport Layer (Layer 4) tutorials

Data received from Session Layer 5 is formatted into "Segments" to be transmitted to the Network Layer 3.

  • Provides reliable transport method, Acknowledgement, error correction and retransmission of data when necessary.

  • It makes sure that sender and receiver communicate at speed they both can handle called flow control

  • Layer 4 is responsible for end-to-end connections and data delivery between two hosts.

  • The ability to segment and reassemble data is a key functionality of this layer.

  • When one system is sending data to another system, that data can be segmented into smaller data blocks and transmitted across the network.

  • The receiving system can then reassemble the segmented data blocks at the Transport layer.

  • Transmissions occur via logical connectivity between the sender and destination.


Layer 4 provides the following functionality

  • Fault detection

  • Error recovery

  • Establishing, maintaining, and tearing down virtual circuits


Acknowledgments, Sequencing, and Flow control.

  • Acknowledgments : Delivered segments are acknowledged to the sender. If they are not acknowledged, the sender will retransmit.

  • Sequencing : Data segments are sequenced into their original order when they arrive at the destination.

  • Flow Control : Provides buffer controls that prevent packet flooding to the destination host. Buffers store bursts of data for processing when the transmission is complete.


  • Layer 4 protocols

  • Transmission Control Protocol (TCP)

  • User Datagram Protocol (UDP)

  • Sequenced Packet Exchange (SPX is communications protocol created by Novell NetWare )