chapter 9 network services and applications. introduction look at: –understanding network services...

Chapter 9

Network Services and Applications

Introduction

• Look at:– Understanding Network Services (9.1)– File Transfer Protocol (FTP) (9.2)– Telnet (9.3)– Trivial File Transfer Protocol (TFTP) (9.4)– Domain Name System (DNS) (9.5)

Introduction

• Look at:– Dynamic Host Configuration Protocol

(DHCP) (9.6)– Simple Network Management Protocol

(SNMP) (9.7)– Electronic Mail (E-mail) (9.8)– World Wide Web (9.9)– Remote Procedure Call (RPC) and

Middleware (9.10)

Understanding Network Services

• Services help the operating system and applications communicate with each other

• This is done through TCP and UDP acting as port managers for the applications and services that are in the top layer


• In order to establish a connection, a machine needs to know the IP address and port number on which the application communicates

• The destination port number is placed in the header and is used to pass traffic to the correct application

• There are 65,535 ports that can be accessed on a machine


• The well-known ports are those from 0 through 1023

• These can be used only by system processes

• Ports 1024 through 49151 are registered

• Ports 49152 through 65535 are dynamic or private

File Transfer Protocol (FTP)

• File Transfer Protocol (FTP) allows a person to transfer files between two computers

• This is usually a client and a server, while being connected to the Internet

• FTP makes it possible to move one or more files between computers with security and data integrity controls appropriate for the Internet


• FTP is a TCP-based service that utilizes a data port and a control port

• Traditionally these are port 21 for the command port and port 20 for the data port

• In active mode, the FTP client doesn't make the actual connection to the data port of the server; it simply states what port it is listening on and the server connects to the specified port on the client


• In passive mode, the client initiates all connections to the server

• The client opens two random unprivileged ports locally

• This is useful when trying to provide FTP connections through firewalls

• Most browsers only support passive mode


• In standard routing, the route table is consulted every time a frame is received, and so plays a fundamental role in the proper delivery of data

• A routing table only maintains the best possible route to a destination, not all possible routes


• FTP is used with your Web browser or through a command line prompt

• Programs such as Fetch, Cute FTP, and WS_FTP also are used for transferring and managing files

• TRICKLE provides an alternative to FTP • It distributes files upon request or by

subscription

Telnet

• Telecommunications Network or Telnet is a protocol that provides a way for clients to connect to servers on the Internet

• The Telnet application is built over TCP/IP and provides the local machine with the means to emulate a terminal session compatible with the remote computer

• It allows the user to create a connection and send commands and instructions interactively to the remote machine

Telnet

• The Telnet command is similar to the FTP command

• Telnet has no graphical user interface (GUI). • The Telnet TCP connection is established

between a random unprivileged port on the client and port 23 on the server

• Because a TCP connection is full-duplex and identified by the pair of ports, the server can engage in many simultaneous connections involving its port 23 and different random unprivileged ports on the client

Trivial File Transfer Protocol (TFTP)

• Trivial File Transfer Protocol (TFTP) is a simple form of the File Transfer Protocol that uses the User Datagram Protocol (UDP)

• It is often used for booting or loading programs on diskless workstations

• It does not guarantee delivery and provides no security features

Trivial File Transfer Protocol (TFTP)

• TFTP provides its own reliable delivery using a simple stop-and-wait acknowledgment system

• Its services run at port 69 • TFTP issues read and write requests to the

remote machine• It can be implemented within the firmware on

network devices that do not contain hard drive

Domain Name System (DNS)

• Domain Name System (DNS) takes the names we type into a Web browser and resolves them to a proper network address

• DNS consists of name servers and resolvers • Domain name servers store authoritative data

about sections of a distributed database and respond to browser requests by supplying name-to-address conversions


• There are several implementations of DNS• One the most popular is called Berkeley

Internet Name Domain (BIND)• BIND is an Internet name server for Unix

operating systems • Because most of the development on the

DNS protocols is based on this code, the BIND name server is the most widely used on the Internet


• The last portion of a host name, such as .com, is the top-level domain to which the host belongs

• Within every top-level domain there is a second-level domain, such as novell.com

• The fully qualified domain name (FQDN) consists of the host name appended to the computer’s domain


• Name servers do not have complete information, so often it is necessary to obtain information from more than one server to resolve a query

• If the name server is unfamiliar with the domain name, the resolver will ask a server further up the tree

• It will continue to forward up until it finds one that knows the information


• At the top of the DNS database tree are root name servers, which contain pointer records to master name servers for each of the top-level domains

• Each name server manages a group of records called a zone

• Zones are set up to help resolve names more easily and for replication purposes


• DNS zones specify the domain name boundary in which a DNS server has authority to perform name translations

• The .arpa domain maintains a reverse list of IP addresses to Internet addresses

• The IP addresses in the .arpa domain are listed in reverse order

• You can either administer your own DNS servers or have an Internet service provider (ISP) do it for you

Dynamic Host Configuration Protocol (DHCP)

• DHCP is an extension of the Bootstrap Protocol (BOOTP)

• DHCP has capabilities for assigning clients a network address for a fixed period of time

• It can allow for reassignment of network addresses to different clients

• DHCP provides the means for a client to acquire all of the IP configuration parameters that it needs in order to operate


• The most important piece of data distributed by DHCP is the IP address

• DHCP supports three methods of IP address allocation:– Manual– Automatic– Dynamic

• Dynamic addressing simplifies network administration


• In dynamic addressing, the IP addresses are kept track of by the software rather than an administrator

• It is the only one of the three methods that allows the server to automatically reuse an address that is no longer needed

• It is useful for assigning an address to a client that will be connected to the network only temporarily


• DHCP is not supported by all operating systems

• It can only work with TCP/IP• It cannot work with AppleTalk or IPX/SPX

because it is tied to IP• These protocols have no need for DHCP

because they have their own automated mechanisms for assigning network addresses


• When a DHCP device attaches itself to the network for the first time, it broadcasts a DHCPDISCOVER packet using UDP on port 67

• All DHCP servers on the local segment will broadcast a DHCPOFFER packet that contains proper configuration for the client based on parameters that are specified in the DHCP server on port 68

• The client may receive multiple DHCPOFFER packets from any number of servers


• The client then broadcasts a DHCPREQUEST packet that identifies the server address (siaddr) and IP address (yiaddr) offer that it has selected

• The server then returns a DHCPACK that sends the client all the requested parameters

• Once the client has the lease, it must be renewed prior to the expiration

• Generally, a client attempts to renew its lease halfway through the lease process

Simple Network Management Protocol (SNMP)

• SNMP is part of the TCP/IP protocol suite • It is an Application layer protocol that is used

to exchange management information between network devices

• SNMP enables network administrators to manage network performance, find and solve network problems, and plan for network growth


• SNMP management infrastructure consists of three main components:– SNMP managed node – SNMP agent – SNMP network management station

• Three versions of SNMP exist • SNMPv3 addresses major security and

authentication concerns of SNMPv1 and SNMPv2


• All agents and management stations must belong to an SNMP community

• SNMP and management stations that belong to the same community can accept messages from each other

• The Remote Monitoring (RMON) specification can be considered an extension to the SNMP standard

• Cisco Systems includes SNMP and RMON functionality in its software

Electronic Mail (E-mail)

• Electronic mail (e-mail) was one of the first Internet applications

• E-mail uses a store-and-forward method of transmission

• The messages are stored in an electronic mailbox

• When a user logs on, the messages are downloaded onto the workstation


• Windows, Linux, and NetWare all have their own versions of e-mail software

• Besides message delivery, many e-mail products offer:– address books for storing contact

information – filtering software for eliminating junk mail – the ability to make distribution lists


• Multipurpose Internet Mail Extensions (MIME) is the standard that defines the format of text messages

• The basic idea behind this standard is that the content of e-mail messages is logically divided into two pieces:– the header– the body


• Several different formats can be chosen for the e-mail body besides basic text formatting:– HTML supports text formatting, color and

background images, horizontal lines, alignments, HTML styles, and Web pages

– MIME HTML (MHTML) enables full Web pages to be sent inside e-mail messages

– Plain Text does not contain any formatting


• Several different formats can be chosen for the e-mail body besides basic text formatting:– Rich Text supports text formatting, bullets,

color, and alignment– S/MIME helps ensure the security of e-mail

by enabling users to digitally encrypt and sign messages

– Pretty Good Privacy (PGP) allows messages to be digitally signed and encrypted


• The standard protocols used for sending Internet e-mail are:– Simple Mail Transfer Protocol (SMTP)– and Post Office Protocol (POP)

• Post Office Protocol (POP) is used to retrieve e-mail from a mail server

• IMAP4 deals strictly with the client-side handling of e-mail


• IMAP4 allows client computers to work with messages stored in mailboxes on remote mail servers

• SMTP works above the TCP/IP layer on port 25

• SMTP is used as a transport protocol for sending e-mail server-to-server

World Wide Web

• The Web consists of:– Your computer– Web browser software – A connection to an ISP– Servers that host data– Routers or switches that direct the flow of

information

• Based on a client/server architecture

World Wide Web

• The language used to format pages on the Web is called the Hypertext Markup Language (HTML)

• HTML is a document markup language that includes a set of tags for defining the format and style of documents

• Web pages are written in HTML so that Web browsers can understand them

World Wide Web

• Web clients and servers use Hypertext Transfer Protocol (HTTP) to communicate with each other

• HTTP is an application-level stateless protocol

• It only defines what the browser and Web server say to each other

• Each command is executed independently

World Wide Web

• A Web browser is the client software that allows you to access and view any document on the Web

• A Web page is accessed by typing a Uniform Resource Locator (URL) into the address bar of the browser

• Every Web site and every Web page has a unique URL

World Wide Web

• In addition to HTML the following markup languages exist: – SGML– XML– XHTML – DHTML – RDF – DAML

Remote Procedure Call (RPC) and Middleware

• Remote Procedure Call (RPC) is a protocol that a program can use to request a service from a program located on another computer in a network

• It uses the client/server model• The requesting program is a client and the

service program is the server• The remote procedure call is intended to act

across the network transparently


• RPC is transport independent • It allows the application to use a variety

of transports• RPC does not care how a message is

passed from one process to another• RPC deals only with specification and

interpretation of messages


• Middleware is software that connects applications, allowing them to exchange data

• It is a general term for any programming that provides messaging services so that two separate, and often already existing applications, can communicate

• It is software that consists of a set of services that allow multiple processes running on one or more machines to interact across a network

chapter 9 network services and applications. introduction look at: –understanding network services...

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