CCNA RS_ITN - Chapter 7

Chapter 7:
Transport Layer
Introduction to Networking
© 2008 Cisco Systems, Inc. All Presentation_ID rights reserved. Cisco Confidential 1

Chapter 7
7.1 Transport Layer Protocols
7.2 TCP and UDP
7.3 Summary
Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 2

Chapter 7: Objectives
 Describe the purpose of the transport layer in managing the
transportation of data in end-to-end communication.
 Describe characteristics of the TCP and UDP protocols, including
port numbers and their uses.
 Explain how TCP session establishment and termination
processes facilitate reliable communication.
 Explain how TCP protocol data units are transmitted and
acknowledged to guarantee delivery.
 Explain the UDP client processes to establish communication with
a server.
 Determine whether high-reliability TCP transmissions, or non-guaranteed
UDP transmissions, are best suited for common
applications.

Role of the Transport Layer

Transportation of Data
Role of the Transport Layer
The Transport Layer is responsible for establishing a temporary
communication session between two applications and delivering data
between them. TCP/IP uses two protocols to achieve this:
 Transmission Control Protocol (TCP)
 User Datagram Protocol (UDP)
Primary Responsibilities of Transport layer Protocols
 Tracking the individual communication between applications on the
source and destination hosts
 Segmenting data for manageability and reassembling segmented
data into streams of application data at the destination
 Identifying the proper application for each communication stream

Conversation Multiplexing
Segmenting the data
 Enables many different
communications, from many
different users, to be
interleaved (multiplexed) on
the same network, at the
same time.
 Provides the means to both
send and receive data when
running multiple applications.
 Header added to each
segment to identify it.

Transport Layer Reliability
Different applications have different transport reliability requirements
TCP/IP provides two transport layer protocols, TCP and UDP
Transmission Control Protocol (TCP)
 Provides reliable delivery ensuring that all of the data arrives at the
destination.
 Uses acknowledged delivery and other processes to ensure delivery
 Makes larger demands on the network – more overhead
User Datagram Protocol (UDP)
 Provides just the basic functions for delivery – no reliability
 Less overhead
TCP or UDP
 There is a trade-off between the value of reliability and the burden it
places on the network.
 Application developers choose the transport protocol based on the
requirements of their applications.

Introducing TCP and UDP
Introducing TCP
Transmission Control Protocol (TCP)
 RFC 793
 Connection-oriented – creating a session between source and
destination
 Reliable delivery – retransmitting lost or corrupt data
 Ordered data reconstruction – numbering and sequencing of
segments
 Flow control - regulating the amount of data transmitted
 Stateful protocol – keeping track of the session

Introducing UDP
User Datagram Protocol (UDP)
 RFC 768
 Connectionless
 Unreliable delivery
 No ordered data reconstruction
 No flow control
 Stateless protocol
Applications that use UDP:
 Domain Name System (DNS)
 Video Streaming
 Voice over IP (VoIP)

Separating Multiple Communications
Port Numbers are used by TCP and UDP to differentiate between
applications.
.

TCP and UDP Port Addressing

Netstat
 Used to examine TCP connections that are open and
running on a networked host

TCP Communication
TCP Server Processes

TCP Communication
TCP Connection, Establishment and
Termination
Three-Way Handshake
 Establishes that the destination device is present on the
network.
 Verifies that the destination device has an active
service and is accepting requests on the destination
port number that the initiating client intends to use for
the session.
 Informs the destination device that the source client
intends to establish a communication session on that
port number.

TCP Communication NEED New Graphic for this and next two slides
TCP Three-Way Handshake – Step 1
 Step 1: The initiating client requests a client-to-server
communication session with the server.

TCP Communication
 Step 2: The server acknowledges the client-to-server
communication session and requests a
server-to-client communication session.

TCP Communication
 Step 3: The initiating client acknowledges the
server-to-client communication session.

TCP Communication
TCP Session Termination

Reliability and Flow Control
TCP Reliability – Ordered Delivery
Sequence numbers used to reassemble segments into
original order

TCP Reliability – Acknowledgement and
Window Size
The sequence number and acknowledgement number
are used together to confirm receipt.
Window Size - The amount of data that a source can
transmit before an acknowledgement must be received.

TCP Reliability and Flow Control
Window Size and Acknowledgements

TCP Flow Control – Congestion Avoidance

TCP Reliability - Acknowledgements

UDP Communication
UDP Low Overhead vs. Reliability
UDP
 Simple protocol that provides the basic transport layer functions
 Used by applications that can tolerate small loss of data
 Used by applications that cannot tolerate delay
Used by
 Domain Name System (DNS)
 Simple Network Management Protocol (SNMP)
 Dynamic Host Configuration Protocol (DHCP)
 Trivial File Transfer Protocol (TFTP)
 IP telephony or Voice over IP (VoIP)
 Online games

UDP Communication
Datagram Reassembly

UDP Communication
UDP Server and Client Processes
 UDP-based server applications are assigned well-known
or registered port numbers.
 UDP client process randomly selects port number from
range of dynamic port numbers as the source port.

TCP or UDP
Applications that use TCP

TCP or UDP
Applications that use UDP

Chapter 7: Summary
 The role of the Transport layer is to provide three main
functions: multiplexing, segmentation and reassembly, and
error checking.
 These functions are necessary in order to address issues in
quality of service and security on networks.
 Knowing how TCP and UDP operate and which popular
applications use each protocol will allow the implementation
of quality of service and build more reliable networks.
 Ports provide a “tunnel” for data to get from the Transport
layer to the appropriate application at the destination.

CCNA RS_ITN - Chapter 7

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