LAYER 4 of OSI Model

The Transport Layer

Abstract

This article discusses how the transport layer is fundamental to the operation of the TCP/IP layered network architecture.

Introduction

When computers communicate with one another, certain rules, or protocols, are required to allow them to transmit and receive data in an orderly fashion. Throughout the world, the most widely adopted protocol suite is TCP/IP. Understanding how TCP/IP functions is important for a larger understanding of how data is transmitted in network environments.

The way in which IP delivers a packet of data across a network is a fundamental concept in the TCP/IP architecture used in large networks. Understanding how data is transmitted via IP is central to understanding how the TCP/IP suite of protocols functions overall. This, in turn, adds to an understanding of how data that is communicated across networks can be prioritized, restricted, secured, optimized, and maintained.

For the Internet and internal networks to function correctly, data must be delivered reliably. You can ensure reliable delivery of data through development of the application and by using the services provided by the network protocol. In the Open System Interconnection (OSI) and TCP/IP models, the transport layer manages the process of reliable data delivery. The transport layer hides details of any network-dependent information from the higher layers by providing transparent data transfer.

The TCP/IP User Datagram Protocol (UDP) and TCP protocols operate between the transport layer and the application layer to hide details of any network-dependent information from the application layer. In addition, learning how UDP and TCP function between the network layer and the transport layer provides a more complete understanding of how data is transmitted in a TCP/IP networking environment.

Transport Layer Functions

Residing between the application and network layers, the transport layer is fundamental to the operation of the TCP/IP layered network architecture.

The TCP/IP layered network architecture

The network layer routes information to its destination, but it cannot guarantee that the information arrives in the correct order, free of errors, or even that it will arrive at all. The transport layer provides two protocols, UDP and TCP, which provide communication services directly to the application process on the host. The "basic service" provided by the transport layer is session multiplexing, which is performed by both UDP and TCP. The "premium service" provided by the transport layer is ensuring reliable delivery, which is performed only by TCP.

The primary duty of the transport layer is the interconnection of application sessions to the network layer, which is provided by both UDP and TCP. If TCP is used, the transport layer has the further responsibilities of establishing end-to-end operations, segmentation, flow control, and applying reliability mechanisms.

An analogy of the UDP services is using the regular service by the post office to send your bill payments. You address each bill payment to the specific company address, stamp the envelope, and include your return address. The postal service guarantees its best effort to deliver each payment. The postal service does not guarantee delivery, and it is not responsible for letting you know that delivery was successful or unsuccessful. Like a regular mail service, UDP is a very simple protocol that provides only the most basic data transfer services.

An analogy for TCP services is sending certified mail through a postal service. Imagine that you live in San Francisco and you have a book that you'd like to send to your mother in New York. Then you discover that the postal service will only handle letters. You rip the pages out of the book, putting each page in a separate envelope.

To ensure that your mother reassembles the book correctly, you number each envelope. You address the envelopes and send the first one as "certified" mail. The postal service delivers it by any truck and any route but, because it is certified, upon delivery the carrier must get a signature from your mother and return a certificate of delivery to you.

Sending each page separately is tedious, so you send several envelopes together. The postal service again delivers each envelope by any truck and any route. Your mother signs a separate receipt for each envelope in the batch as she receives them.

If one envelope is lost in transit you would not receive a certificate of delivery for that numbered envelope and you would resend that page. After receiving all the envelopes, your mother puts the pages in the right order and pastes them back together to make the book.

Like certified mail, TCP is a complex protocol that offers precise and traceable data transfer services.

Session Multiplexing

Session multiplexing is an activity in which a single computer, with a single IP address, is able to have multiple sessions occur simultaneously. A session is created when a source machine needs to send data to a destination machine. Most often, this involves a reply, but a reply is not mandatory. The session is created and controlled within the IP network application, which contains the functionality of OSI Layers 5 through 7.

A best-effort session is very simple. The session parameters are sent to UDP. A best-effort session sends data to the indicated IP address using the port numbers provided. Each transmission is a separate event, and no memory or association between transmissions is retained.

When using the reliable TCP service, a connection must first be established between the sender and the receiver before any data can be transmitted. TCP opens a connection and negotiates connection parameters with the destination. During data flow, TCP maintains reliable delivery of the data and, when complete, closes the connection.

For example, you enter a URL for Yahoo into the address line in the Internet Explorer window, and the Yahoo site corresponding to the URL appears. With the Yahoo site open, you can open the browser again in another window and type in another URL (for example, Google). You can open another browser window and type the URL for Cisco.com and it will open.

Three sites are open using only one IP connection, because the session layer is sorting the separate requests based on the port number.

Segmentation

TCP takes data chunks from the application layers and prepares them for shipment onto the network. Each chunk is broken up into smaller segments which will fit the maximum transmission unit (MTU) of the underlying network layers. UDP, being simpler, does no checking or negotiating and expects the application process to give it data that will work.

Flow Control

If a sender transmits data faster than the receiver can receive it, the receiver will drop the data, requiring it to be retransmitted. Retransmission can waste time and network resources, which is why most flow control methods try to maximize the transfer rate while minimizing the requirements to retransmit.

In TCP, basic flow control is implemented by acknowledgment by the receiver of the receipt of data; the sender waits for this acknowledgment before sending the next part. However, if the round-trip time (RTT) is significant, the overall transmission rate may slow to an unacceptable level.

A mechanism called windowing increases network efficiency when combined with basic flow control. Windowing allows a receiving computer to advertise how much data it is able to receive before transmitting an acknowledgment to the sending computer.

Connection-Oriented Transport Protocol

Within the transport layer, a connection-oriented protocol, such as TCP, establishes a logical connection between sending and receiving peers before any data is exchanged. Once the connection exists, data is transmitted. Once the conversation between peers is over, the connection is terminated.

Reliability

TCP reliability has three main objectives:

• recognition and correction of data loss
• recognition and correction of duplicate or out-of-order data
• avoidance of congestion in the network

Reliability is not always necessary. For example, in a video stream, if a packet is dropped and then retransmitted, it will appear out of order. That would be frustrating and confusing to the audience and serve no useful purpose.

In real-time applications, such as voice and video streaming, dropped packets can be tolerated, as long as the overall percentage of dropped packets is low.

Summary

Computers communicate with each other using certain rules or protocols. The most widely used protocol is TCP/IP. In the TCP/IP suite the transport layer manages the process of reliable data delivery. The purpose of the transport layer is to hide the network requirements from the application layer.

The TCP/IP protocol suite handles processes like session multiplexing, data segmentation, flow control, and reliability.

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