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The Transport Layer of the OSI Reference Model | Protocols

The Transport Layer

In this article I am going to describe the detailed function, features and concepts of The Transport Layer of the OSI Reference Model’s Functions, Features and Concepts.

The Transport Layer in the OSI Reference Model

The Transport layer of the OSI Reference Model performs division or segment and reassembling of data into data stream by utilizing a method which is called multiplexing. It permits to join the information into the same data stream which is located in the upper layer applications and performs segment and reassemble. The Transport layer also offers nonstop data transmission services and creates a logical connection between the source and destination hosts on an internetwork. The Transport layer has two protocols “TCP & UDP”. TCP is a reliable protocol while UDP is not. However, when application developers have to work on TCP/IP, they have many choices because they have an option between the two protocols.

The Transport layer is also liable for offering the mechanisms for multiplexing upper layer applications, setting up sessions, and shutting down virtual circuits. This layer also providing transparent data transfer and reliable networking, which means the Transport layer uses the flow control, sequencing and acknowledgements. It also can be connection oriented or connectionless. Although, CCNA exam is focus on most likely to understanding you the connection oriented portion.

Flow Control

The main object of flow control is to give a way for the receiver to rule the amount of sending data by the source. Flow Control ensures the Data integrity at the Transport layer and permitting the users for reliable data communication between two hosts. Flow control prohibits a source on one side of the connection from swarming the barriers at the destination, an event that can result in lost data. Reliable transmission uses connection oriented communications session, and the protocols involvement results the following achievement:

  • The source host will respond back about the delivered segments.
  • Retransmitted segments are not acknowledged.
  • When segments are arrived on the destination they are sequenced in the proper form.
  • For preventing data loss, congestion and overloading, a controllable data flow is maintained.

Connection Oriented Communication

In reliable data transfer communication, a device creates a connection oriented transport with remote device by establishing a session. The source device first creates a connection oriented session with the destination, which is entitles as call setup or three way handshake. Then the data transmission is started and when finished, call expiration become active to shutdown the virtual circuit.

The following snapshot describe you a normal reliable session establishment between source and destination systems. You will find that both systems’ application programs started by informing their personal operating systems that a connection is about to be started. Both operating systems correspond by sending requests over the network verifying that the communication is granted and both sides are prepared to take place. Once this synchronization comes to pass, the connection is completely created and the communication starts. This method of virtual circuit is called overhead.

Establishing a Connection-Oriented Session

Congestion can be happen during transmission due to high performance processor is producing data traffic very faster than the network can handle transmission. A group of devices separately sending datagrams through a specific gateway can also damage things up as well. If a device gets an overflow of datagrams speedily, it stock up in the memory part which is called buffer. Buffering action corrects the issue of datagrams. Due to the transport utility, network overflow control methods work very well. If permitting data to be lost, the transport can release a “not ready” accuse to the source of the overflow. This method acts as kind of a stop-light signal to the source device to stop sending traffic to its overwhelmed host. Next host’ recipient processes the segments that were previously in memory pool its shock absorber it issue a “ready” transport accuse. And when the device ready to send information of its datagrams accepts the “go” accuse and continue the transmission.

Transmitting Segments With Flow Control

Datagrams sent to the destination in the definite way they were transmitted and sending will fail if this rule is violated, in Connection Oriented Transmission. And if the segment lost, duplicated or damaged a breakdown will transmit. These kinds of issues can be solved, if the destination host acknowledges the complete segment transmission to the source host.

A service is assumed as Connection Oriented in the following attributes:

  • If virtual circuit is available like three way handshake.
  • If sequencing used.
  • If acknowledgments used.
  • If flow control used.

Flow control has three types buffering, windowing, and congestion avoidance.


Preferably, information throughput occurs rapidly and proficiently. It would be slow if source device had to wait for an acknowledgment after transmitting each seg¬ment. But due to the available time after the source transmits the segment and before it ends processing acknowledgments from the destination, source uses the break as a prospect to send more information. Data segments always calculated in bytes. Transmitting machine permits the data segments to transmit without receiving an acknowledgment is called a window. Windows used to manage the values of unresolved & unacknowledged data segments. Some protocols measure the information by monitoring the quantity of packets, TCP/IP observes it by the number of bytes. As the following snapshot describes, there are window sizes one set to 1 and one set to 3.


If we configure window size to 1, the source waits for receiving an acknowledgment for every segment before transmitting another. But if we configure window size to 3, then it transmits three segments before an acknowledgment. And when host fails to receive any segment, the host will enhance its communication session by modifying the window size.


Integrity of data stream is ensured by the reliable data delivery, data stream drives from a device to other device through a useful data link. It is ensured that the data never be duplicated or lost. It is accomplished by positive acknowledgment with retransmission a method that needs a destination device to transmit with the transmitting source by forwarding acknowledgment message back to source when it receives data.

In the following snapshot, the source device sends out segments 1, 2, and 3. And the destination host acknowledged and confirmed by asking for segment 4. When it receives the acknowledgement, source device then send out segments 4, 5, and 6. If segment 5 does not reach to the next node, the destination node acknowledges it and transmits a request for segment 5 for resend. Then the source device resend that lost or damage segment and wait for acknowledgment that is compulsory to go ahead for transmitting segment 7.

Transport Layer Reliable Delivery

This was the little introduction to the Transport Layer of the OSI Reference Model with some function, features and concepts.

Filed in: Inter-Networking, Networking, Technology Updates

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