CS计算机代考程序代写 dns database DHCP cache FTP Lecture 7

Lecture 7
Application-Layer Protocols
Dr Vicky Liu
v. .au

7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Data Link
1. Physical

HTTP
DNS
Telnet
SMTP
SSH
DHCP
FTP
…

Outline
Domain Name System (DNS)
Hypertext Transfer Protocol (HTTP)
Simple Mail Transfer Protocol (SMTP)
File Transfer Protocol (FTP)
Telnet and Secure Socket Shell (SSH)
Dynamic Host Configuration Protocol (DHCP)

DNS
The service converts a human readable name into an IP address on a network.
It can be thought of as a White Pages or Yellow Pages directory service. DNS is like the phone book of the Internet.
DNS is a name-to-address resolution protocol that keeps a list of computer names and their IP addresses.
Using DNS, a user can use a computer’s name instead of using its IP address.
Applications that use DNS include World Wide Web (WWW), email, and instant messages.
4

Visiting a Web Site
When you enter library.qut.edu.au in your web browser, the DNS client service contacts the DNS server specified in the IP configuration of your operating system and requests that the name be resolved to an IP address
Once the IP address for the website is returned, your computer can contact the web server to request a web page.

DNS Structure – Root Servers

The DNS is a hierarchical naming system.
The top of that hierarchy is the root domain.
The root domain contains all top-level domains of the Internet.
Root servers are a network of hundreds of servers for redundancy in many countries around the world.
6

They are configured in the DNS root zone as 13 named authorities.
This does not mean that there are only 13 physical servers; each operator uses redundant computer equipment to provide reliable service
As of 2020-07-21, the root server system consists of 1086 instances operated by the 12 independent root server operators

DNS Structure – Root Servers (cont.)
7

DNS Structure – TLDs

When you put all the names of a branch together, separated by periods, you have the fully qualified domain name (FQDN).
The top of that hierarchy is the root domain.
The root domain contains all top-level domains (TLDs) of the Internet.
The two main types of TLDs
Country-code TLDs (ccTLDs)
Generic TLDs (gTLDs)

DNS Structure – ccTLDs

Specify with two-letter country code for countries
Australia (.au),
New Zealand (.nz),
United Kingdom (.uk)
…
9

DNS Structure – gTLDs

gTLDs represent with a set of three or more letter code the categories of the organisation
government (.gov)
education (.edu)
commercial (.com)
military (.mil)
organisation (.org)
network (.net)
.…
10

How do you choose between
ccTLD and gTLD?

DNS Structure – 2nd level domains
Second-level domains are usually the name of a company or institution.
The subdomain level is optional and can consist of names separated by a period.
The host level represents individual computers hosting network services.
For example, in www.qut.edu.au, ‘au’ is the top-level domain name, ‘edu’ is the second-level domain, ‘qut’ is the subdomain, and ‘www’ is the hostname.
12

DNS Server
DNS servers are composed of the following:
DNS zones – a database of primarily hostname and IP address pairs
Resource records – the unit of information entry in DNS zone files
Cache – results of queries are cached so that if the same query occurs again, the local DNS server can respond without having to contact another server
Root hints – file containing a list of all IP addresses of Internet root servers
DNS Server service – runs in the background and listens for DNS queries on UDP port 53

DNS Client
The DNS client is responsible for communicating with a DNS server to resolve computer and domain names to IP addresses
The DNS client is referred to as a “resolver”
An operating system (OS) must be configured at least one address of a DNS server that it can query

DNS Client

In Windows, the first DNS server configured is called the preferred DNS server and the second one is the alternate DNS server
15

Authoritative and non-authoritative answers
An authoritative answer is when the DNS server hosting the DNS record responses to the DNS query.
An authoritative DNS server is used by domain name owners to store DNS records. It provides authoritative answers to DNS resolvers.
A DNS server provides answers for which they are not authoritative, the queried DNS record is not in its zone file. This is known as a non-authoritative answer.

Iterative and recursive query in DNS
Recursive query
A query that demands a resolution or the answer
The initial request the resolver makes to the local server is a recursive query. The local DNS server must provide the information requested by the resolver
Iterative query
A query that does not demand resolution
When the local server issues queries to other servers, the other servers only provide information if they have it
17

Local DNS Server
DNS Server
1. Query for www.microsoft.com
8. The destination address is 203.0.19.190
2. Query root server
3. Try one of TLD servers

DNS Server
microsoft.com

TLD Server
.com

Root
4. Query .com TLD server
5 Try one of microsoft.com servers
6. Query Microsoft.com DNS server
7. The address is 203.0.19.190

Web Server
www.microsoft.com
Visit www.microsoft.com

Name Resolution Process
18

Query the IP address for www.microsoft.com
Query to root server:
If local DNS server does not recognize the domain name, then it queries the root server.
Try one of these .com TLD servers
Query .com TLD server:
Contact one of the microsoft.com DNS servers
Query to the microsoft.com DNS server
The query is resolved
Then the resolver returns the resolved IP address to the requester

Summary of DNS
Functionality of DNS
Hierarchical structure of DNS
Root, TLDs, subdomain-level domains
Roles for DNS clients and servers
Basic process of a DNS name resolution
20

WWW
The most popular service on the Internet
Immense collection of Web pages and other resources that can be downloaded across the Internet and displayed on a web browser
The main concept is hypertext
WWW is the set of linked hypertext documents that can be viewed on web browsers

Web Client
To communicate with a web server, using Hypertext Transfer Protocol (HTTP),
Firefox, IE, Chrome, etc.
To submit an HTTP request to the server

Web Server
To provide response message to the client
To provide web content that can be accessed through the Internet
Proprietary: Microsoft’s Internet Information Services (IIS)
Open source: Apache developed by Apache Software Foundation

WWW’s Major functional components
HyperText Markup Language (HTML)
Hypertext Transfer Protocol (HTTP)
Uniform Resource Locator (URI)
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HyperText Markup Language (HTML)
Is the language used to create webpages
Describes the visual appearance of a document to be displayed by an Internet browser
Defines the structure and layout of a Web document by using a variety of tags and attributes
Title, lists, paragraph
Fonts, colors, positioning using CSS (Cascading Style Sheets)
Linking to another document or to a file
26

HTML (cont.-)
27

HTTP: Protocol of the World Wide Web
Originally, its main purpose was to transfer static web pages written in HTML
Now, it is also used for general file transfer and downloading/displaying multimedia files
The standard (and default) port for HTTP (Web) servers to listen on is 80, though they can use any port.
HTTP is an application-layer protocol based on TCP as its Transport-layer protocol
Functions as a request-response protocol in the client-server computing model
Identifies and locates network resources by a Uniform Resource Locator (URL)

HTTP Encapsulation
Example of how the layers work together:
You start your Web browser with a Web site address
The web browser formats a request for your home page by using the Application layer protocol HTTP
The request looks something like:

HTTP Encapsulation (cont.-)
Example continued:
The Application-layer protocol HTTP passes the request down to the TCP
TCP adds a header to the request
The unit of information the Transport layer works with is called a segment
TCP passes the segment to the Internetwork layer protocol (IP)

HTTP Encapsulation (cont.-)
Example continued:
IP places its header on the segment:

The unit of information is now called a packet
The packet is passed down to the Network access layer, where the NIC operates
A frame header and trailer are added

The frame is delivered to the network medium as bits
on its way to the Web server
The Web server processes it and returns a Web page

IP header
TCP header
Get the Website’s home page
IP header
TCP header
Get the Website’s home page
Frame header
Frame Trailer
31

Locating a Resource Object on the Internet
Every object on the Internet has a unique Uniform Resource Locator (URL)
All URLs consist of four parts:
Service type
Host or domain name
Directory or subdirectory information
Filename
http://www.qut.edu.au/study/example.htm
ftp://opensaurce.com/public/utilities/installer.exe
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Web Client/Server Communication
The browser first contacts to a DNS for resolving the target’s IP address
The DNS replies with the corresponding IP address for the web server.
The web browser connects to the web server sending an HTTP request for the target website with a TCP 3-way handshake.
The web server receives the request and checks for the request message. If the requested page exists, then the web server replies it;
else it sends an HTTP 404 error message.

Source: https://web.stanford.edu/class/msande91si/www-spr04/readings/week1/InternetWhitepaper.htm
Client
Web
server
DNS
server
1
2
3
4
5
6
7
8
The Client initiates a contact with the Web server to request for a service. The user types a URL into a Web browser:

Web Client/Server Communication (cont.)
The web browser receives the requested page and then the connection is closed.
The browser then parses through the web page information and looks for other page elements it needs to complete the web page.
For each element needed, the browser makes additional connections and HTTP requests to the server for each element.
When the browser has finished loading all images, info, etc. the page will be completely loaded in the browser window.
Source: https://web.stanford.edu/class/msande91si/www-spr04/readings/week1/InternetWhitepaper.htm
Client
Web
server
DNS
server
1
2
3
4
5
6
7
8
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HTTP Summary
Roles for HTTP servers and clients
WWW’s major functional components
HTTP, HTML, URL
Web client/server communication process

Three E-mail protocols
Post Office Protocol version 3 (POP3)
Internet Message Access Protocol version 4 (IMAP4)
Simple Mail Transfer Protocol (SMTP)
37

Post Office Protocol version 3 (POP3)
Email clients use the POP3 to download incoming messages from an e-mail server to their local desktops
POP3 clients download e-mail from the mail server running at the user’s ISP, and these messages are then deleted from the server
POP3 uses TCP port 110
38

Internet Message Access Protocol version 4 (IMAP4)
IMAP4 has advanced message controls:
The capability to manage messages locally yet store them on a server
IMAP4 downloads only e-mail headers initially and then downloads the message body and attachments when the message is selected
IMAP4 uses TCP port 143
39

Simple Mail Transfer Protocol (SMTP)
It is the standard protocol for sending email over the Internet.
POP3 is used to retrieve e-mail and SMTP is used to send it.
SMTP uses TCP port 25

Summary of E-mail Protocols
All three email protocols are TCP-based protocols to ensure reliable delivery of email messages.
POP3 and IMAP are for mail retrievals and SMPT is for sending email messages
IMAP is designed to store messages on the mail server. It stores incoming and outgoing messages on the server in folders.
POP3 is designed to store messages on the local device.

FTP
FTP is a client/server protocol used to transfer files and manage files across a network:
FTP uses TCP ports 20 and 21
Port 21 is for users sending control commands
Port 20 is for transferring file data
It is not a secure protocol, as user credentials and data are sent in plaintext
Uses URL for an FTP site begins with ftp://…
The 3 most common ways to access an FTP site:
Through a browser
Using a FTP client program
Issuing FTP commands at a text-based command prompt

Is FTP Still Used?
FTP was a revolutionary development when it was first introduced in the 1970s.
Within the last 40+ years, FTP has served as the foundation for various methods of sending data
FTP is outdated and insecure.
44

Telnet and SSH
Telnet and Secure Shell (SSH)
allows a user log into another computer remotely
Example: use to connect to a managed switch or router
Uses a well-known port: 23 on the server side
Is not a secure protocol
SSH
Uses TCP port 22
Provides an encrypted channel between the client and server

PuTTY
PuTTY is a client program that supports Telnet along with SSH and Rlogin (remote login) network protocols.

Remote Desktop Software
Remote desktop software
Remote Desktop Protocol (RDP)
Uses Graphic User Interface (GUI) to manage/access Windows computers remotely
Independent Computing Architecture (ICA)
Virtual network computing (VNC)

DHCP
DHCP is used to automatically assign IP addresses as needed within a network.
When a computer is turned on, it requests an address from a DHCP server that is configured as a DHCP server
The DHCP server assigns an address to this client for a specific amount of time (called a lease)
DHCP Servers listen on UDP port 67 for IP address releases.
DHCP clients use UDP port 68 for IP address requests

DHCP Server
A DHCP server is composed of the following:
IP address scope – a range of IP addresses the server leases to clients
Scope options – IP settings:
The default gateway, DNS servers, and other options
Reservations – an IP address tied to a particular MAC address
When the client’s MAC address matches an address specified by a reservation, the reserved IP is leased to the client instead of getting it form the scope
Exclusions – one or more IP addresses excluded from the IP address range, e.g.
If the scope ranges from 192.168.1.1 to 192.168.1.100, you can exclude addresses 192.168.1.1-192.168.1.10 for static IP assignment
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DHCP Lease Process

During the boot process, a DHCP client broadcasts a DHCPDISCOVER message that it’s looking for a DHCP server
The DHCP server reserves an IP address for the client and makes a lease offer by sending a DHCPOFFER message to the client via unicast.
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DHCP Lease Process (cont.)
The client responds with a DHCPREQUEST message via broadcast to accept the offered IP address.
If several DHCP servers respond to the request, the client accepts the first offer that it receives.
The DHCP server whose offer was accepted responds with a DHCPACK message via unicast.
It acknowledges the lease acceptance and contains the client’s IP address lease and other IP addressing information.

DHCP Lease Renewal Process
After an address is leased
A record of the lease is stored in a database, including a lease expiration time
When 50% of the lease time has elapsed, the computer attempts to renew the lease from the same DHCP server that originally responded
If no response, the computer waits until the lease reaches 87.5%, a broadcast DHCP renewal request is sent
If no response when lease expires, the computer broadcasts a DCHP request for a new IP address

DHCP Server
Benefit of using DHCP
In a large network, it can keep track of assigned addresses and to which machine they are assigned.
Computers can easily be moved and requested new IP configuration from a DHCP server on the network.
IP lease time can be controlled.
IP addresses can be reusable for other computer.
DHCP uses UDP
DHCP servers are usually located on the same network and DHCP messages are short.
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Summary of DHCP
DHCP functionality
DHCP server elements
DHCP lease process
DHCP renewal process

End of Lecture

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