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Module 11TCP Application and

Transport

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Transport Layer

• The primary duties of the transport layer, Layer 4 of the OSI

model, are to transport and regulate the flow of information from

the source to the destination, reliably and accurately.

• End-to-end control and reliability are provided by sliding

windows, sequencing numbers, and acknowledgments.

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Flow Control

• Flow control avoids the problem of a transmitting host

overflowing the buffers in the receiving host.

• TCP provides the mechanism for flow control by allowing the

sending and receiving host to communicate.

• The two hosts then establish a data-transfer rate that is

agreeable to both.

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Three-Way Handshake

• TCP is a connection-oriented protocol. TCP requires connection establishment before data transfer begins

• For a connection to be established or initialized, the two hosts must synchronize. The synchronization requires each side to send its own initial sequence number and to receive a confirmation of exchange in an acknowledgment (ACK) from the other side.

This exchange is called the three-way handshake.

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Windowing

• Windowing is a flow-control mechanism.

• Windowing requires that the source device receive an

acknowledgment from the destination after transmitting a certain

amount of data.

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Acknowledgement• Positive acknowledgment

with retransmission is one technique that guarantees reliable delivery of data.

• Positive acknowledgment requires a recipient to communicate with the source and send back an acknowledgment message when the data is received.

• Segments that are not acknowledged within a given time period will result in a retransmission.

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TCP/IP• Transmission Control Protocol (TCP) is a connection-oriented

Layer 4 protocol

• TCP is responsible for breaking messages into segments,

reassembling them at the destination station, resending

anything that is not received, and reassembling messages from

the segments.

• The protocols that use TCP include:

– FTP (File Transfer Protocol)

– HTTP (Hypertext Transfer Protocol)

– SMTP (Simple Mail Transfer Protocol)

– Telnet

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UDP

• User Datagram Protocol (UDP) is the connectionless transport protocol

• UDP is a simple protocol that exchanges datagrams, without acknowledgments or guaranteed delivery

• UDP uses no windowing or acknowledgments so reliability, if needed, is provided by application layer protocols

• 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)

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TCP and UDP Port Numbers

• Both TCP and UDP use port (socket) numbers to pass information to the upper layers.

• Port numbers are used to keep track of different conversations crossing the network at the same time.

• Port numbers have the following assigned ranges:

– Numbers below 1024 are considered well-known ports numbers.

– Numbers above 1024 are dynamically assigned ports numbers.

– Registered port numbers are those registered for vendor-specific applications. Most of these are above 1024.

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Port Numbers

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TCP/IP ModelApplication Layer

• TCP/IP application layer

includes the session and

presentation layers from the

OSI model

• This means that issues of

representation, encoding, and

dialog control are handled in

the application layer rather

than in separate lower layers

as in the OSI model

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TCP/IP Protocols

• The TCP/IP protocols that support file transfer, e-mail, and

remote login are probably the most familiar to users of the

Internet.

• These protocols include the following applications:

– Domain Name System (DNS)

– File Transfer Protocol (FTP)

– Hypertext Transfer Protocol (HTTP)

– Simple Mail Transfer Protocol (SMTP)

– Simple Network Management Protocol (SNMP)

– Telnet

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DNS• The Domain Name System (DNS) is a system used on the

Internet for translating names of domains and their publicly advertised network nodes into IP addresses.

• A domain is a group of computers that are associated by their geographical location or their business type.

• There are more than 200 top-level domains on the Internet, examples include the following:

.us – United States .uk – United Kingdom

• There are also generic names, examples include the following: .edu – educational sites .com – commercial sites .gov – government sites .org – non-profit sites .net – network service

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FTP and TFTP

• FTP is a reliable, connection-oriented service that uses TCP to transfer files between systems that support FTP

• When files are copied from a server, FTP first establishes a control connection between the client and the server.

• Data transfer can occur in ASCII mode or in binary mode.

• After the file transfer has ended, the data connection terminates automatically.

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TFTP• TFTP is a connectionless service that uses User Datagram

Protocol (UDP).

• TFTP is used on the router to transfer configuration files and

Cisco IOS images

• TFTP is designed to be small and easy to implement.

• TFTP can read, write, or mail files to or from a remote server

but it cannot list directories and currently has no provisions for

user authentication

• It is useful in some LANs because it operates faster than FTP

and in a stable environment it works reliably.

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HTTP

• Hypertext Transfer Protocol (HTTP) works with the World Wide Web

• An address location is known as an Uniform Resource Locator (URL)

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SMTP

• Email servers communicate with each other using the Simple

Mail Transfer Protocol (SMTP) to send and receive mail.

• When a mail server receives a message destined for a local

client, it stores that message and waits for the client to collect

the mail.

• The most popular mail client protocols are POP3 and IMAP4,

which both use TCP to transport data.

• The SMTP protocol does not offer much in the way of security

and does not require any authentication.

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SMTP

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SNMP

• The Simple Network Management Protocol (SNMP) is an

application layer protocol that facilitates the exchange of

management information between network devices.

• SNMP enables network administrators to manage network

performance, find and solve network problems, and plan for

network growth.

• SNMP uses UDP as its transport layer protocol.

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SNMP• An SNMP managed network consists of the following three key

components:

– Network management system (NMS) – NMS executes

applications that monitor and control managed devices. The

bulk of the processing and memory resources required for

network management are provided by NMS.

– Managed devices –Managed devices, sometimes called

network elements, can be routers, access servers, switches,

and bridges, hubs, computer hosts, or printers.

– Agents –An agent has local knowledge of management

information and translates that information into a form

compatible with SNMP.

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SNMP

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Telnet

• Telnet client software provides the ability to login to a remote Internet host that is running a Telnet server application and then to execute commands from the command line.

• Telnet works at the application layer of the TCP/IP model. Therefore, Telnet works at the top three layers of the OSI model.