OSI Transport Layer
Network Fundamentals – Chapter 4
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Objectives

Explain the need for the Transport layer.

Identify the role of the Transport layer as it provides
the end-to-end transfer of data between applications.

Describe the role of two TCP/IP Transport layer
protocols: TCP and UDP.

Explain the key functions of the Transport layer,
including reliability, port addressing, and
segmentation.

Explain how TCP and UDP each handle key
functions.

Identify when it is appropriate to use TCP or UDP and
provide examples of applications that use each
protocol.
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Course Index
 4.1 Roles of the Transport Layer
 4.2 The TCP Protocol – Communicating with Reliability
 4.3 Managing TCP Sessions
 4.4 The UDP Protocol - Communicating with Low
Overhead
 4.5 Lab Activities
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4.1 Roles of the Transport Layer
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4.1.1 Purpose of the Transport Layer
 Tracking Individual Conversations
 Segmenting Data
 Reassembling Segments
 Identifying the Applications
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4.1.2 Controlling the Conversations
 The Primary function of Transport layer protocols
Segmentation and Reassembly
Conversation Multiplexing
 Other function of Transport layer protocols
Connection-oriented conversations
Reliable delivery
Ordered data reconstruction
Flow control
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4.1.3 Supporting Reliable Communication
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4.1.4 TCP and UDP
 TCP
Connection-oriented
Reliable delivery
Flow control
Applications that use TCP:
Web Browsers; E-mail
File Transfers
 UDP
Simple
Connectionless
Low overhead
Best effort
Applications that use UDP:
Domain Name System (DNS); Video Streaming; Voice over IP (VoIP)
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4.1.5 Port Addressing
 Used for Identifying the Conversations
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4.1.5 Port Addressing
 Types of port numbers
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4.1.5 Port Addressing
 netstat Command
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4.1.6 Segmentation and Reassembly
 Ensures that data is transmitted within the limits of the media
 Ensures that data from different applications can be
multiplexed on to the media
 TCP and UDP Handle Segmentation Differently
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4.2 The TCP Protocol –
Communicating with Reliability
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4.2.1 TCP – Making Conversations Reliable
 TCP segment
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4.2.2 TCP Server Process
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4.2.3 TCP Connection Establishment and Termination
 Establishment of TCP sessions
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4.2.3 TCP Connection Establishment and Termination
 Termination of TCP sessions
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4.3 Managing TCP Sessions
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4.3.1 TCP Segment Reassembly
 Use the sequence number
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4.3.2 TCP Acknowledgement with Windowing
 Use the acknowledgement number
 It’s expectational acknowledgement
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4.3.3 TCP retransmission
 TCP usually only acknowledges data for contiguous
sequence bytes
 Selective Acknowledgements is an optional feature
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4.3.4 TCP Congestion Control – Minimizing Segment Loss
 Window size—flow and congestion control
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4.4 The UDP Protocol Communicating with Low Overhead
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4.4.1 UDP – Low Overhead vs. Reliability
 UDP
Provides the basic Transport layer functions
Lower overhead
It is not connection-oriented and does not provide the sophisticated
retransmission, sequencing, and flow control mechanisms
Application that use UDP
include:
Domain Name System (DNS)
Simple Network Management Protocol
(SNMP)
Dynamic Host Configuration Protocol (DHCP)
Routing Information Protocol (RIP)
Trivial File Transfer Protocol (TFTP)
Online games
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4.4.2 UDP Datagram Reassembly
 UDP protocol reassembles PDUs in the order that it
was received.
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4.4.3 & 4.4.4 UDP Processes
 Also use port numbers to identify a specified application
layer process and direct segments to the proper service
or application
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Summary
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