1Version 3.0

Module 11TCP Application and

Transport

2Version 3.0

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.

3Version 3.0

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.

4Version 3.0

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.

5Version 3.0

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.

6Version 3.0

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.

7Version 3.0

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

8Version 3.0

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)

9Version 3.0

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.

10Version 3.0

Port Numbers

11Version 3.0

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

12Version 3.0

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

13Version 3.0

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

14Version 3.0

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.

15Version 3.0

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.

16Version 3.0

HTTP

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

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

17Version 3.0

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.

18Version 3.0

SMTP

19Version 3.0

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.

20Version 3.0

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.

21Version 3.0

SNMP

22Version 3.0

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.