Www.ciscopress.com Networking Basics CCNA 1 Chapter 11.

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Networking Basics CCNA 1Chapter 11

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The TCP/IP Transport Layer

• TCP/IP transport layer includes several protocols– Transmission Control Protocol (TCP)– User Datagram Protocol (UDP)– TCP has more functions– UDP has less overhead

• Main goal of the transport layer:– Provide the service of taking data from one

application process on one computer and delivering that data to the correct application process on another computer

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• Comparing transport and internet layers:– Internet layer delivers packets from one

computer to another, but is not concerned with which application sent the data or which application on the receiving computer needs the data

– If multiple instances of an application are running on the destination computer, the transport layer works to ensure the proper instance gets the data and not the others

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Comparing TCP and UDP

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Flow Control and Windowing• When a host sends data using TCP, the

receiving host can control how fast each TCP sender sends the data over time – this is called flow control

• Flow control is used for many reasons– Receiver needs time to process the data– Receiver has finite amount of memory, so if data

keeps arriving, it could run out of memory

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Flow Control Through Dynamic Sliding Windows• Receiver tells sender how many bytes the

sending host can send before it receives an acknowledgement – a value called a window

• When the sending host has sent an entire window’s worth of data, it must wait for an acknowledgement, slowing its rate of sending data

• The window size can be increased to allow a faster transfer rate

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Dynamic Windowing

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Flow Control Through Withholding Acknowledgements

• After a sending host has sent one window worth of bytes, it must wait to send more

• The receiver can wait to send acknowledgements, which prevents the sender from sending more data

• This process of withholding acknowledgements is sometimes known as start/stop flow control

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Withholding Acknowledgements

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Establishing and Terminating TCP Connections• TCP is a connection-oriented protocol

– Allows it to set initial values to the window and to initialize sequence number values

• TCP uses a process called a three-way handshake to create a new TCP connection– Uses three TCP segments that use two of the TCP

flags in the TCP header• The synchronize (SYN) flag• The acknowledge (ACK) flag

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Three-Way TCP Connection Establishment

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TCP Error Recovery (Reliability)• TCP is a reliable protocol (at least in a

networking perspective)– Performs error recovery– All data eventually gets to the destination,

even if some is initially lost in transport

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TCP Acknowledgements with no Error Recovery Needed

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TCP Error Recovery (Reliability)• TCP uses two header fields to signal to other

computer as to whether a segment was received– Sequence number

• Keeps track of all the bytes sent over a TCP connection

– Acknowledgement number• Lets the sending device know the number of the segment the

receiving device expects next• Practice of stating the next byte expected to be received is

called a forward acknowledgement or expectational acknowledgement

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TCP Error Recovery

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TCP Error Recovery (Reliability)• If a segment is lost or destroyed during

transmission, the receiving PC requests that it be retransmitted

• If an acknowledgement is not received by the sending PC within a certain time (a timer has been set), the sending PC resends the segment

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Segmentation, Reassembly, and In-Order Delivery• TCP segmentation refers to the process of

accepting a large chunk of data from the application protocol and breaking it into pieces that are small enough for transmission– The size of the segment is limited– Ethernet limits frames to 1500 data bytes in the data

field of a frame• IP and TCP headers are each 20 bytes long• Maximum data portion is therefore 1460 bytes

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Segmentation, Reassembly, and In-Order Delivery• TCP on the receiving computer reassembles

data into its original form• The data is put in the correct order

– If segments of a file are assembled out-of-order, the file is useless

– TCP provides a guarantee of in-order delivery

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Segmentation, Reassembly, and In-Order Delivery• Due to IP routing, a TCP receiver can receive

data out of order• If multiple routes exist between a source and a

destination, routers can load-balance over several routes

• Packets can arrive out of order

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TCP Providing In-Order Delivery

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TCP and UDP Header Reference• TCP and UDP use a header to hold information

for performing tasks– TCP needs ACK and SYN flags

• First two fields in TCP and UDP are identical– Both use port numbers to identify application

processes

• TCP has a longer header (20 bytes vs. 8 bytes)

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TCP and UDP Headers

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TCP Header Fields

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UDP Header Fields

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Identifying Application Processes Using Port Numbers

• Both TCP and UDP identify the specific application process that sends the data and the application process that needs to receive the data– To make this determination, TCP and UDP use port

numbers– Each application uses a different local port number

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Using Port Numbers to Identify the Correct Application Process

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• In previous slide, the application was assigned a dynamic port number by the host computer– A host typically dynamically allocates port numbers of

value 1024 (210) through 65,535 (216 - 1)– When a host starts a new application process, it

allocates a dynamic port number that is not already in use by another process

– By each process having its own port number, a PC can have multiple conversations with other PCs (sometimes called multiplexing)

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• Connection to Servers: Well-Known Ports– Most TCP/IP applications use a client/server

model for communications– Servers cannot use dynamic port numbers

because clients must know ahead of time what port numbers servers use

– Servers must wait and listen on certain port numbers for client requests

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• Connection to Servers: Well-Known Ports (continued)– For servers to work well, TCP/IP defines one or more

well-known ports, each reserved for use by a specific application protocol

• For example, HTTP connects to a server listening on port 80– A server might be a single, high-powered computer; in

this chapter it means “TCP/IP software application,” one that uses port numbers

– Multiple software server applications could be running on one physical server

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Client Connecting to Well-Known Port of a Web Server (80)

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Popular Applications and Their Well-Known Port Numbers

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• Comparing Well-Known, Dynamic, and Registered Ports– IANA assigns the values for well-known ports and for

registered port numbers• Registered ports are assigned to servers that the average

end-user can start, such as instant-messaging applications• Well-known ports are started by IT staff for applications such

as web servers, email, FTP servers

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Uses of Port Numbers

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• Comparing Well-Known, Dynamic, and Registered Ports (continued)– Both TCP and UDP use port numbers– To see the port numbers used on a computers

running a Microsoft OS, use the netstat –an command

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The TCP/IP Application Layer

• The TCP/IP application layer performs the same functions as the top three layers of the OSI model– Defines format of data being transferred– Performs encryption– Sets protocol rules for transferring data– Sets up combinations of messages, called dialogs or

sessions, that must be sent before a transaction can be considered complete

• Do not confuse the application layer with the applications themselves!

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Distinction Between an

E-Mail Application and E-Mail Application

Layer Protocols

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Application Protocols Used by End Users• Domain Name System (DNS) – resolves names

into IP addresses• Hypertext Transfer Protocol (HTTP) – transfers

files from web servers to web browsers• Simple Mail Transfer Protocol (SMTP) and Post

Office Protocol version 3 (POP3) –send and receive e-mail, respectively

• File Transfer Protocol (FTP) – stores and retrieves files

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Application Protocols Used by End Users

• Name Resolution Using DNS– Easier to remember names than IP addresses– A name like www.cisco.com is generically

called a hostname– The cisco.com part is the domain name– The last part (.com) is a top-level domain

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DNS Resolution After Inserting a URL into a Web Browser

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Common High-Level DNS Domains

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Application Protocols Used by End Users

• World Wide Web and HTTP– Web server software stores information that

the web server wants to make available to web browsers such as Internet Explorer and Firefox

– Web servers use HTTP to transfer the files that make up a web page

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Application Protocols Used by End Users• World Wide Web and HTTP (continued)

– HTTP transfer process:• The browser asks the web server to send one file that contains

instructions and displayable content• The browser displays the file’s contents• The browser looks for instructions inside the first file, which

might tell it to get more files• The browser asks the web server for additional files• The browser displays the additional content, which might

include instructions to download additional files• The process continues until all files are downloaded and

displayed• HTTP uses a GET request to ask for a file

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HTTP Transfers Three Files

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Application Protocols Used by End Users• World Wide Web and HTTP (continued)

– The term HTTP is from the first type of file supported by a web browser

• Hyper Text Markup Language (HTML)

– Web browsers needed to download files of this type, so HTTP was created as a protocol to handle HTML files

– Universal Resource Locators (URLs), commonly referred to as web addresses, identify web pages that need to be displayed, or a hyperlink can be clicked on

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Application Protocols Used by End Users• E-Mail Protocols: SMTP and POP3

– A PC sends mail to its SMTP server using its e-mail client software

– The SMTP server forwards the e-mail to the destination SMTP server

– The destination PC retrieves the e-mail with the POP3 protocol used by its e-mail client

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Process of Sending an E-Mail

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Application Protocols Used by End Users• E-Mail Protocols: SMTP and POP3 (continued)

– In some cases, the physical server sending the e-mail is a different server than the one used for checking e-mail

– POP3 is the most common protocol for an incoming e-mail server, but IMAP4 is sometimes used

– Most SMTP servers are configured to only accept e-mail from inside the same internetwork, due to security reasons

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Application Protocols Used by End Users• E-Mail Protocols: File Transfer Protocol

– File Transfer Protocol (FTP) supports file transfers• FTP clients store files on FTP servers and can retrieve files

from them

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Application Protocols Used by End Users• E-Mail Protocols: File Transfer Protocol

(continued)– FTP uses two separate TCP connections

• Control connection uses well-known port 21 on the FTP server and sends control information such as data transfer mode

• Connection on well-known port 20 is used for actual data transfer

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Application Protocols Often Used for Network Management

• Three primary network management protocols:1. Trivial File Transfer Protocol (TFTP): works similar to FTP, but

with some benefits when used on networking devices

2. Simple Network Management Protocol (SNMP): Allows management software (the client) to query networking devices (the server) to manage and control the networking devices

3. Telnet: allows a client to emulate a terminal; allows a user to sit at one computer (the Telnet client) and to use a remote computer (the Telnet server) through a text-based interface

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• Three primary network management protocols (continued):

1. TFTP• Performs some of the same functions as FTP• TFTP has some advantages: server software is small,

allows routers and switches to transfer files without consuming too much permanent storage space

• Most network engineers use a TFTP client package and a TFTP server package on their PCs

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2. SNMP• Allows a network engineer to monitor an internetwork’s

status• The Network Management System (NMS) software can

perform this function• Uses SNMP GET requests• The managed device (router, switch) must run an

SNMP agent to be able to reply to the GET requests

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NMS Using SNMP GETs to Retrieve Information from SNMP Agents

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3. Telnet• Allows a user to sit at one computer and access another IP host

by using Telnet client software (the other host must run Telnet server software)

• After connecting, the user at the Telnet client (called the local host) can issue commands that are executed on the other IP host (called the remote host)

• Telnet is the primary tool for remotely accessing routers and switches

• Cisco routers and switches come with built-in Telnet servers

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Telnet Client Accessing a Router

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Application Protocol Summary• Many other application protocols exist, but the ones

mentioned in this chapter are the mainstream• The protocols all share a common feature:

– They use a TCP transport layer protocol (either TCP or UDP– TCP: HTTP, SMTP, POP3, FTP, and Telnet– UDP: DNS, TFTP, SNMP

• UDP does not do error recovery; implements error recovery at the application layer

• DNS and SNMP use a timeout mechanism that allows them to resend a request or allows the user to repeat the action

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Summary

• Primary duty of the transport layer (OSI Layer 4), is to take data from the application process on one computer and deliver the data to the correct application process on another computer

• Both TCP and UDP identify data from upper-layer applications based on port number

• UDP provides only basic delivery of data and identification of applications by port number

• TCP provides additional functions, including error recovery (reliability), flow control, segmentation and re-assembly, and in-order data delivery

• TCP uses connection-oriented logic, connecting dynamically with a three-way handshake

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Summary

• TCP numbers the first byte of each segment with a sequence number

• TCP acknowledges the receipt of data by using an acknowledgement number

• This process allows a receiving host to tell the sender to resend data

• The sequence numbers allow the receiving host to put data in the correct order before giving it to the application

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Summary

• Flow control ensures a transmitting node does not overwhelm a receiving node

• The simplest method of flow control is when a receiving host sends a “not ready” signal by withholding acknowledgements

• TCP also uses dynamic windowing, which is a more efficient process

• Dynamic windowing allows the receiving host to vary the amount of data a sender can send before it has to receive an acknowledgement

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Summary

• Positive acknowledgement with retransmission refers to the process of explicitly acknowledging received data, with the sender resending any unacknowledged segments

• Connection-oriented TCP provides a wide range of functions, but UDP has some advantages– UDP has less overhead (an 8-byte header versus a

20-byte header for TCP)– UDP does not slow down because of flow control

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Summary

• Popular application layer protocols:– DNS: Used in IP networks to translate names of network nodes

into IP addresses– FTP: Transfers files between networks– HTTP: Delivers HTML documents to a client application, such as

a web browser– SMTP: Provides e-mail services– SNMP: Monitors and controls network devices and manages

configurations, statistics collection, performance and security– Telnet: Used to log in to a remote host that runs a Telnet server

application and then to execute commands from the command line

Www.ciscopress.com Networking Basics CCNA 1 Chapter 11.

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