Tài liệu Chapter 4 - OSI Transport Layer CCNA Exploration version 4.0 docx

Chapter 4 - OSI Transport Layer
CCNA Exploration version 4.0CCNA Exploration version 4.0
Overview
• 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
•
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• 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
Transport Layer Role and Services
• Explain the purpose of the Transport layer
• Enables multiple applications
to communicate over the
network at the same time on a
single device
• Ensures that, if required, all
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• Ensures that, if required, all
the data is received reliably
and in order by the correct
application.
• Employs error handling
mechanisms.
Transport Layer Role and Services
• Major functions of the transport layer and the role it plays in data

networks
– 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 the different applications.
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– Identifying the different applications.
Separating Multiple Communications
• Computer connected
to a network that is
simultaneously
receiving and sending
e-mail and instant
messages, viewing
websites, and
conducting a VoIP
phone call. Each of
these applications is
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these applications is
sending and receiving
data over the network
at the same time.
However, data from
the phone call is not
directed to the web
browser, and text from
an instant message
does not appear in an

e-mail.
Segmentation
• Without
segmentation, only
one application, the
streaming video for
example, would be
able to receive data.
You could not
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You could not
receive e-mails,
chat on instant
messenger, or view
web pages while
also viewing the
video.
Controlling the Conversation
• Segmentation
and Reassembly -
Most networks
have a limitation on
the amount of data
that can be
included in a single
PDU. The
Transport layer
divides application
• 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
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divides application
data into blocks of
data that are an
appropriate size. At
the destination, the
Transport layer
reassembles the
data before
sending it to the
destination
application or
service.
assigned an
address known as
a port so that the
Transport layer can
determine with
which application
or service the data
is identified.
Controlling the Conversation

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Transport Layer Role and Services
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• At the Transport layer the three basic operations of reliability are:
– Tracking transmitted data
– Acknowledging received data
– Retransmitting any unacknowledged data
Transport Layer Role and Services
• Transmission Control Protocol
(TCP)
– TCP is a connection-oriented
protocol, described in RFC
793. TCP incurs additional
overhead to gain functions.
Additional functions specified
by TCP are the same order
delivery, reliable delivery, and
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delivery, reliable delivery, and
flow control. 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
• UDP is a simple, connectionless protocol,
described in RFC 768. It has the advantage of
providing for low overhead data delivery,
datagrams are sent as "best effort" by this

Transport layer protocol.
Web, email, file transfer
DNS, Video streaming, VoIP
TCP Header format:
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TCP Header format: Port number
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• Source Port 16 bits.
• Destination Port 16 bits.
TCP Header format: Sequence
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• Sequence Number: 32 bits
– The sequence number of the
first data octet in this segment
(except when SYN is present).
TCP Header format: Acknowledgment
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• Acknowledgment Number: 32 bits
– This field contains the value of the
next sequence number the sender of
the segment is expecting to receive.
TCP Header format: Code bits
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• Control Bits: 8 bits
– ACK: Acknowledgment field significant
– RST: Reset the connection
– SYN: Synchronize sequence numbers
– FIN: No more data from sender
TCP Header format: Window
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• Window: 16 bits
– The number of data octets beginning
with the one indicated in the
acknowledgment field which the sender
of this segment is willing to accept.
User Datagram Protocol (UDP)
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• UDP is a simple protocol that exchanges datagrams, without
acknowledgments or guaranteed delivery. Error processing and
retransmission must be handled by higher layer protocols
• UDP is designed for applications that do not need to put
sequences of segments together
UDP header fields
• Source port – Number of the calling port
• Destination port – Number of the called port
• Length – Number of bytes including header and data
• Checksum – Calculated checksum of the header and data fields
• Data – Upper-layer protocol data
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• Data – Upper-layer protocol data
• The protocols that use UDP include:
- TFTP (Trivial File Transfer Protocol).
- SNMP (Simple Network Management Protocol).
- DHCP (Dynamic Host Control Protocol).
- DNS (Domain Name System).