Transcript The Transport Layer

Review of Chapters 12, 13 & 14
Transport Layer
Session Layer
Presentation Layer
Chapter 12
Transport Layer
Layers 1 - 3
 Layer 1 allows bit streams to be created and
to travel.
 Layer 2 packages those data packets into
frames to be converted to bit streams and
makes LAN delivery possible.
 Layer three packages data from upper layers
in packets and makes routing and WAN
delivery possible.
Layer 4 - Transport Layer
But we have made no provision for
assuring our data reliably travels
end-to-end across the often vast network
path. Layer 4 performs multiple functions to
provide this "quality of service.”
Quality of Service
The phrase "quality of service" is often used
to describe the purpose of Layer 4 - the
transport layer. Its primary duties are to
transport and regulate the flow of
information from source to destination,
reliably and accurately.
TCP & UDP
The emphasis of this curriculum is on
TCP/IP Ethernet networks. The TCP/IP
protocol of the OSI model Layer 4
(transport layer) has two protocols - TCP
and UDP.
TCP
TCP supplies a virtual circuit between end-user
applications. These are its characteristics:
 connection-oriented
 reliable
 divides outgoing messages into segments
 reassembles messages at the destination station
 re-sends anything not received
 reassembles messages from incoming segments.
UDP
UDP transports data unreliably between hosts.
Following are the characteristics of UDP:
 connectionless
 unreliable
 transmit messages (called user datagrams)
 provides no software checking for message
delivery (unreliable)
 does not reassemble incoming messages
 uses no acknowledgements
 provides no flow control
TCP Segment Fields
Following are the definitions of some of the fields
in the TCP segment:
 source port -- number of the calling port
 destination port -- number of the called port
 sequence number -- number used to ensure
correct sequencing of the arriving data
 acknowledgment number - next expected TCP
octet
 window -- number of octets that the sender is
willing to accept
TCP & UDP Port Numbers
Both TCP and UDP use port (or socket)
numbers to pass information to the upper
layers.
 Numbers below 255 - are for public
applications
 Numbers from 255-1023 - are assigned to
companies for marketable applications
 Numbers above 1023 - are unregulated
3-Way Handshake

TCP hosts establish a connectionoriented session with one another
using a three-way handshake.

A three-way handshake
synchronizes a connection at both
ends before user data is exchanged.
Window Size
Window size determines the amount of data
that you can transmit at one time before
receiving an acknowledgment from the
destination.
Sequencing
 TCP provides sequencing of segments with
a forward reference acknowledgment.
 If a sequence number is missing in the
series, that segment is re-transmitted.
Chapter 13
The Session Layer
The Session Layer
The session layer establishes, manages, and
terminates sessions between applications.
Dialog Control
 When the session layer decides whether to
use 2-way simultaneous or 2-way alternate
communication it is called dialogue control.
 The session layer uses two major forms of
dialog control:
2-way alternate communication
2-way simultaneous communication
2-Way Alternate and 2-Way
Simultaneous Communication.
 2-way alternate communication avoids
interrupts by taking turns.
 2-way simultaneous communication allows
2 messages to cross during a conversation.
Dialogue Separation
Dialogue separation is the orderly initiation,
termination, and managing of
communication.
Layer 5 Protocols
Layer 5 has a number of important
protocols:
 Network File System (NFS)
 Structured Query Language (SQL) Remote
Procedure Call (RPC)
 X-Window System AppleTalk Session Protocol
(ASP)
 Digital Network Architecture Session Control
Protocol (DNA SCP)
Chapter 14
The Presentation Layer
Layer 6 - The Presentation Layer
 The presentation layer is responsible for
presenting data in a form that the receiving
device can understand.
 The presentation layer has 3 main functions:
data formatting
data compression
data encryption
Data Formatting
To understand how data formatting works,
imagine two dissimilar systems. The first system
uses Extended Binary Coded Decimal Interchange
Code (EBCDIC) to format text. The second
system uses American Standard Code for
Information Interchange (ASCII) to format text.
Layer 6 provides the translation between these two
different types of codes.
Graphic File Formats
The Internet uses two binary file formats to
display images:
 Graphic Interchange Format (GIF)
 Joint Photographic Experts Group (JPEG).
Any computer with a reader for the GIF and
JPEG file formats can read these file types,
regardless of the type of computer.
Multimedia File Format
 The multimedia file format is another type
of binary file, which stores sounds, music,
and video.
 Sound files may be completely downloaded,
first, and then played, or they may
download while they are playing. The latter
method is referred to as streaming audio.
Markup Language
Another type of file format is markup
language. This format acts as a set of
directions that tell a Web browser how to
display and manage documents. Hypertext
Markup Language (HTML) is the language
of the Internet.
Encryption & Compression
 Layer 6 is responsible for data encryption.
Data encryption protects information during
its transmission.
 The presentation layer is also responsible
for the compression of files.
Conclusion
This concludes our review of chapters 12,
13 and 14. You are now ready to take the
online exam!