CCNA RS_NB - Chapter 5

Chapter 5:
Transport Layer
Network Fundamentals
© 2008 Cisco Systems, Inc. All Presentation_ID rights reserved. Cisco Confidential 1

Chapter 5
5.1 Transport Layer Protocols
5.2 TCP and UDP
5.3 Summary
Presentation_ID © 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential 2

Chapter 5: 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

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
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

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

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
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.

Protocol Data Units??? Is this correct title??
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.

Protocol Data Units
TCP Reliability and Flow Control
Data Loss and Retransmission
 When TCP at the source host has not received an
acknowledgement after a predetermined amount of time, it will go
back to the last acknowledgement number that it received and
retransmit data from that point forward
Flow Control
 Uses the window size field in the TCP header to specify the
amount of data that can be transmitted before an acknowledgement
must be received.
 Adjusts the effective rate of data transmission to the maximum flow
that the network and destination device can support without loss.
 Manages the rate of transmission so that all data will be received
and retransmissions will be minimized.

Protocol Data Units
TCP Flow Control – Congestion Avoidance
Using Dynamic Window Sizes

TCP and UDP
TCP Reliability - Acknowledgements
 TCP uses a combination of sequence numbers and
acknowledgements to ensure all requested data has
been received.
 Can be inefficient when there is loss of one or more
segments
 Selective Acknowledgements (SACK) –
implementation of TCP that overcomes the inefficiency
of TCP acknowledgements

UDP Communication
UDP Low Overhead vs. Reliability
UDP
 Simple protocol that provides the basic transport layer function
 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 5: 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_NB - Chapter 5

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CCNA RS_NB - Chapter 5