CISCO CCNA1 Exploration - Network - Chapter - 4

OSI Transport Layer
Network Fundamentals – Chapter 4
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Chapter 1 © 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1
Objectives
 Explain the role of Transport Layer protocols and
services in supporting communications across data
networks
 Analyze the application and operation of TCP
mechanisms that support reliability
 Analyze the application and operation of TCP
mechanisms that support reassembly and manage
data loss.
 Analyze the operation of UDP to support
communicate between two processes on end devices
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Transport Layer Role and Services
Its primary responsibilities are:

 Tracking the individual communication between
applications on the source and destination hosts
 Segmenting data and managing each piece
 Reassembling the segments into streams of application
data
 Identifying different applications
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Cont…
 Tracking Individual Conversations - maintain the multiple

communication streams between different applications
running on the host.
 Segmenting Data – dividing data into manageable pieces
 Reassembling Segments- reconstructing a complete data
stream
 Identifying the Applications – using port numbers
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Separating Multiple Communications
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Controlling the conversation
The primary functions specified by all Transport layer protocols include:
 Segmentation and Reassembly
 Conversation Multiplexing - There may be many applications or services
running on each host in the network. Each of these applications or services is
assigned an address known as a port so that the Transport layer can
determine with which application or service the data is identified.
 In addition to using the information contained in the

headers, for the basic functions of data segmentation and
reassembly, some protocols at the Transport layer
provide:
–Connection-oriented conversations
–Reliable delivery
–Ordered data reconstruction
–Flow control
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Establishing a Session
 The Transport layer can provide this connection

orientation by creating a sessions between the
applications. These connections prepare the applications
to communicate with each other before any data is
transmitted. Within these sessions, the data for a
communication between the two applications can be
closely managed.
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Reliable Delivery
 For many reasons, it is possible for a piece of data to

become corrupted, or lost completely, as it is
transmitted over the network. The Transport layer can
ensure that all pieces reach their destination by having
the source device to retransmit any data that is lost.
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Same Order Delivery
 Because networks may provide multiple routes that can

have different transmission times, data can arrive in the
wrong order. By numbering and sequencing the
segments, the Transport layer can ensure that these
segments are reassembled into the proper order.
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Flow Control
 Network hosts have limited resources, such as memory

or bandwidth. When Transport layer is aware that these
resources are overtaxed, some protocols can request
that the sending application reduce the rate of data
flow. This is done at the Transport layer by regulating
the amount of data the source transmits as a group.
 Flow control can prevent the loss of segments on the
network and avoid the need for retransmission.
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Supporting Reliable communication
At the Transport layer the three basic operations of reliability are:
 tracking transmitted data

 acknowledging received data
 retransmitting any unacknowledged data
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 Supporting Reliable Communication
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TCP & UDP
 The two most common Transport layer protocols of

TCP/IP protocol suite are:
– Transmission Control Protocol (TCP) and
– User Datagram Protocol (UDP).
 Both protocols manage the communication of multiple

applications. The differences between the two are the
specific functions that each protocol implements.
 UDP is a simple, connectionless protocol

 TCP is a connection-oriented protocol.
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 Each TCP segment has 20 bytes of overhead in the
header encapsulating the Application layer data,
whereas each UDP segment only has 8 bytes of
overhead.
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 Identify how a port number is represented and describe
the role port numbers play in the TCP and UDP
protocols.
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Types of ports
 Well Known Ports (Numbers 0 to 1023) - These

numbers are reserved for services and applications.
 Registered Ports (Numbers 1024 to 49151) - These
port numbers are assigned to user processes or
applications.
 Dynamic or Private Ports (Numbers 49152 to 65535) -
Also known as Ephemeral Ports, these are usually
assigned dynamically to client applications when
initiating a connection. It is not very common for a client
to connect to a service using a Dynamic or Private Port
(although some peer-to-peer file sharing programs do).
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 Client sends TCP segment with:
–Destination Port: 23 (Well known port number)
–Source Port: 1028 (Dynamic Port assigned by client)
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 Server responds with TCP segment with:
–Destination Port: 1028 (Dynamic Port assigned by client)
–Source Port: 23 (Well known port number)
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Using NetStat
 Open a web browser.

 Open a command prompt window (Start->Run->cmd)
 Enter a URL of your choice.
 Type netstat –n in the command window.
 Questions:
–What is/are the source ports on your client?
–What is/are the destination ports on your client?
–What would be the source port(s) on the server?
–What would be the destination port(s) on the server?
–What application layer protocol is being used? How can you tell?
–What transport layer protocol is being used?
 Trying more at home:
–Use netstat to look at other networking applications such as FTP or Telnet.
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 Describe the role of segments in the transport layer and
the two principle ways segments can be marked for
reassembly.
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Application and Operation of TCP Mechanisms
 Trace the steps that show how the TCP reliability
mechanism works as part of a session
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 An individual server cannot have two services assigned to the same port number
within the same Transport layer services. A host running a web server application
and a file transfer application cannot have both configured to use the same port (for
example, TCP port 8080).
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TCP Connection Establishment and Termination
 In TCP connections, the host serving as a client initiates the session to the
server. The three steps in TCP connection establishment are:
 1. The initiating client sends a segment containing an initial sequence

value, which serves as a request to the server to begin a communications
session.
 2. The server responds with a segment containing an acknowledgement
value equal to the received sequence value plus 1, plus its own
synchronizing sequence value. The value is one greater than the
sequence number because the ACK is always the next expected Byte or
Octet. This acknowledgement value enables the client to tie the response
back to the original segment that it sent to the server.
 3. Initiating client responds with an acknowledgement value equal to the
sequence value it received plus one. This completes the process of
establishing the connection.
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Three-way Handshake
 The sequence is as follows:
 The sending host (A) initiates a connection by sending a SYN packet to the
receiving host (B) indicating its INS = X: A - > B SYN, seq of A = X
 B receives the packet, records that the seq of A = X, replies with an ACK of X + 1,
and indicates that its INS = Y. The ACK of X + 1 means that host B has received all
octets up to and including X and is expecting X + 1 next: B - > A ACK, seq of A = X,
SYN seq of B = Y, ACK = X + 1
 A receives the packet from B, it knows that the seq of B = Y, and responds with an
ACK of Y + 1, which finalizes the connection process: A - > B ACK, seq of B = Y,
ACK = Y + 1
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 Trace the steps in the handshake in the termination of
TCP sessions
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Managing TCP Sessions
 Describe how TCP sequence numbers are used to
reconstruct the data stream with segments placed in
the correct order
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 Describe the retransmission.remedy for lost data
employed by TCP (see 4.3.3)
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Flow control
 As the transport layer sends data segments, it tries to ensure that
data is not lost. Data loss may occur if a host cannot process data
as quickly as it arrives. The host is then forced to discard the data.
Flow control ensures that a source host does not overflow the
buffers in a destination host. To provide flow control, TCP allows
the source and destination hosts to communicate. The two hosts
then establish a data-transfer rate that is agreeable to both.
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 The amount of data that a source can transmit before

an acknowledgement must be received is called the
window size.
 Window Size is a field in the TCP header that enables
the management of lost data and flow control.
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Reducing Window size
 Describe the mechanisms in TCP that manage the
interrelationship between window size, data loss and
congestion during a session
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UDP Protocol
 Describe the characteristics of the UDP protocol and
the types of communication for which it is best suited
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UDP Datagram Reassembly
 Because UDP is connectionless, sessions are not

established before communication takes place as they
are with TCP. UDP is said to be transaction-based. In
other words, when an application has data to send, it
simply sends the data.
 Many applications that use UDP send small amounts of
data that can fit in one segment. However, some
applications will send larger amounts of data that must
be split into multiple segments. The UDP PDU is
referred to as a datagram, although the terms segment
and datagram are sometimes used interchangeably to
describe a Transport layer PDU.
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Cont…
 When multiple datagrams are sent to a destination,

they may take different paths and arrive in the wrong
order. UDP does not keep track of sequence numbers
the way TCP does. UDP has no way to reorder the
datagrams into their transmission order.
 Therefore, UDP simply reassembles the data in the
order that it was received and forwards it to the
application. If the sequence of the data is important to
the application, the application will have to identify the
proper sequence of the data and determine how the
data should be processed.
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UDP Protocol
 Describe in detail the process specified by the UDP
protocol to reassemble PDUs at the destination device
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UDP server process and requests
 Like TCP-based applications, UDP-based server applications are
assigned Well Known or Registered port numbers. When these
applications or processes are running, they will accept the data matched
with the assigned port number. When UDP receives a datagram destined
for one of these ports, it forwards the application data to the appropriate
application based on its port number.
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UDP Protocol
 The UDP client process randomly selects a port number from the dynamic
range of port numbers and uses this as the source port for the conversation.
The destination port will usually be the Well Known or Registered port
number assigned to the server process.
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OSI Transport Layer

Network Fundamentals – Chapter 4

Its primary responsibilities are:

 Tracking Individual Conversations - maintain the multiple

 In addition to using the information contained in the

 The Transport layer can provide this connection

 For many reasons, it is possible for a piece of data to

 Because networks may provide multiple routes that can

 Network hosts have limited resources, such as memory

At the Transport layer the three basic operations of reliability are:

 tracking transmitted data

 The two most common Transport layer protocols of

 Both protocols manage the communication of multiple

 UDP is a simple, connectionless protocol

 Well Known Ports (Numbers 0 to 1023) - These

 Open a web browser.

 1. The initiating client sends a segment containing an initial sequence

 The amount of data that a source can transmit before

 Because UDP is connectionless, sessions are not

 When multiple datagrams are sent to a destination,

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