Computer Networking Note2

8/13/2019 Computer Networking Note2

1/10By Dr. Mohammad Ali

Computer Networking

a Top-Down Approach Featuring the Internet

Chapter 2 Application Layer

Network applications are the raisons dtre of a computer network. A network application is the

reason the network exists. If there is no application, no need to provide the network protocol to support

him.

In this chapter, the first study including the application layer protocol, client / server model, processes,network applications such as transport layer interface, the basic concepts, and learning, including Web, e-

mail, DNS and P2P file-sharing and other network applications; Finally, we will learn web application

development, a simple client / server applications.

2.1 Principles of Network ApplicationsHow to create Web applications?

Web application development is the core of the preparation of client systems running on different

programs and implements the interaction between applications using the Internet to communicate. For

example, Web-based applications, including the need to communicate with each other two parts, one

running on the Host on the browser, the other is running on the Web Server Host Web Server process.

Developments of network applications, developers need to create more than one Host to run the softwareon, but without create on the run in Network-core software. Internet is a network application to provide a

communications infrastructure. Network core devices, including switches and routers, and did not realize

the transport layer and above the layer functions.

The greatest advantage of facilitating the deployment of applications, or you needs to create network

applications in the network core devices, so that almost impossible.

1. Application architecturesDevelopment of network applications, need to decide what kind of application architecture, the

application framework.

Note, the application of different structure and network architecture. From the application developer's

perspective, the network architecture is fixed and the application to provide specific services. Theapplication structure, application developers decided by the network, its function is to determine the

application on the client system in a different form of organization.

The current structure of commonly used applications, including: client / server, P2P, Hybrid of C / S

and P2P three.

1.1 Client-Server architectureThere is a client-server model, always-on host; called server (server) is called the client to provide

services for multiple Hosts that sent the request of clients to respond. Server generally has a permanent,

well-known address.

Client can intermittently start / connections. Client / Server system management benefits are easy, the

problem is the system bottleneck Server easily become a bottleneck.

1.2 Pure P2P architectureP2P, none of the C / S at the center of the Server, all the Host of equal status, called Peers, so this

system is also called Peer to Peer.

P2P is not to be always on the server, and peer could be replaced at any time their IP. Pure P2P

Gnutella is a good example.

The greatest benefit is the P2P system scalability (scalability) strong. Since each peer is both a Server

and Client, with the growing number of Peer system, the system's processing power is stronger.

P2P problem is manageable, because the system is completely decentralized, non-centered, and extremely

difficult to manage.

1.3 Hybrid of client-server and P2PNapster in early 1999 by a 18-year-old Northeastern University student Shawn Fanning developed by

the exchange of MP3 music, software and computer industries were set off in P2P computing technologies

(Peer-To-Peer Computing) of the wave. However, in December 1999, Napster was the traditional music

industry then rebound to copyright infringement and damages caused by the recording industry sued the



ground, music industry representatives, including Sony (Sony), Arts EMI (EMI), Warner (Warner),

Universal (Universal), Bertelsmann (BMG), and the American recording industry Association (Recording

Industry Association of America), in July 2000, District Court Judge Marilyn Patel of the sanctions, the

making of the Napster injunction to suspend Napster's service. In December 1999, including Warner,

BMG, one hundred Dai, Sony, Universal, including the five major record companies sued Napster for

copyright infringement record companies. ? Napster feature is that it addresses itself only MP3 files,

directories and indexes, all of the songs are actually stored in the user's personal computer.

2. Processes communicatingAs mentioned earlier, the network application is running on a different Host program and with the

communication between networks. Here we are discussing is how to achieve different communication

problems between the Host application.

Operating system is actually a process to communicate, and process refers to the host running a

program. In the same host in the communication between different processes by the operating system is

responsible for defining the IPC; such as: windows message mechanism; pipeline; semaphore; shared

storage area and so on.

Processes in different host communication between the content of our study is, inter-process

communication via message passing to achieve.

2. 1 Client and Server ProcessWeb application composition process by the pairs of these processes to send messages to each other

through the network (message). Usually a pair of mutual communication process, a process called a Server;

the other is called the Client process.

2.2 SocketsMost applications include more of the communication process by sending a message between each

other to communicate. Message passing between processes need to rely on the underlying network to

complete the process to send a message to the network, receiving messages from the network are achieved

through the Socket.

Socket transport layer is the application layer and the interface between, also known as API

(Application Programming Interface, Application Programming Interface). Process as a house, Socket

house like the door.The development of network applications, developers can layer on the side of the Socket in the

application full control, but only in the transport layer on the side of the Socket to a certain degree of

control, including the choice of transport protocol, to determine some of the transport layer parameters,

such as the maximum buffer, the maximum segment length and so on.

The early 1980s, the U.S. government's Advanced Research Projects agency (ARPA) to the

University of California, Berkeley, has provided funding to enable them to achieve the UNIX operating

system TCP / IP protocol. In this project, researchers for the TCP / IP network communications developed

an API (application programming interface). This API is called Socket interface (socket). Today, SOCKET

interface is a TCP / IP network of the most common API, but also on the INTERNET for the most common

application development API.

The early 1990s, the other by Microsoft co-developed a set of several companies under WINDOWS

common network programming interface, which Windows Sockets specifications. It is important Berkeley

Sockets expansion, mainly to increase the number of asynchronous functions, and adds features to meet the

Windows message-driven asynchronous network events selection mechanism. WINDOWSSOCKETS

specification is an open, multi-protocol support for network programming under Windows interface.

Version 1.0 from 1991 to 1995 2.0.8 version, through continuous improvement and Intel, Microsoft, Sun,

SGI, Informix, Novell and other companies with the full support of Windows network programming has

become the de facto standard. At present, the practical applications of the WINDOWSSOKCETS norms

are 1.1 and 2.0. The most important difference between the 1.1 versions only supports TCP / IP protocol,

and version 2.0 can support multiple protocols. Version 2.0 has a good backward compatibility, any use of

version 1.1 of the source code, binary files, and applications without modification to the 2.0 specification touse.

2.3 Addressing processesProcess addressing: the process of the message sender when sending a message to the process with

the purpose of the message identifier. Therefore, the process needs to be prepared.



Each Host Internet has a unique IP address, because each host may run more than one process; use

the IP address as the identifier of the process is not sufficient.

Process identifiers need to include two effects:

1, where the only run to determine the Process Host, use the IP address can be achieved.

2, the only running on to determine the Host Process. Port Number can be used to achieve.

Each message contains the port number used to identify the application process to receive messages.

3. App-layer protocolHost with each other through message passing between the communication between communicating

parties in order to be able to understand the meaning of the message, the message needs to be structured,

need to define the meaning of the message, need to define when to send the message, etc. What the

agreement. This is the agreement of three elements: syntax, semantics, and synchronization.

Therefore, the application layer protocol is responsible for defining the following:

1, the message type.

2, the message syntax: the structure of the message.

3, the message semantics: the meaning of the message.

4, when and how to send messages, and respond to the message.

4. What transport service does an app need?Application process and the transport layer socket is the interface between the client applications will

send a message through the socket to the transport layer, the receiver using socket transport layer message

to the application process. According to the network architecture, communication between the applications

depends on the underlying transport layer services. Transport layer provides a variety of services,

protocols, there are many, the application should be selected according to their needs when the service or

transport layer protocol. Overall, we will demand the application is divided into three areas: bandwidth,

data loss rate, delay.

5. Internet servicesThe Internet, there are two transport layer protocols, TCP and UDP, the development application

should first select one.

TCP provides services:

1, connection-oriented services: client and server processes in a formal transfer of data, you mustestablish a connection. After connection is established, the socket client and server processes exist between

Connections. TCP connection is full-duplex (full duplex), that client and server processes can

simultaneously send data.

Using connection-oriented services, applications knot Beam connections must be removed.

2, reliable transport services: TCP provides a reliable, accessible transport services in sequence. The

data source can certainly send sequence arrives, there will not be lost.

3, Congestion control: TCP congestion control is not necessarily provided for the inter-process

communication provides a number of benefits for the entire Internet is a greater significance.

The basic principle is that the network is congested or blocked the suppression of the data source to send.

TCP provides congestion control for the entire network is a good thing, but for some applications,

however, created The damage, especially audio and video applications, congestion control, these

applications may be less than the transmission speed of the minimum required bandwidth, thus causing

harm to the quality, and these should be Use of packet loss may be less sensitive. Therefore, these real-time

applications more suitable for use UDP, not TCP.

TCP problems:

1, cannot guarantee minimum bandwidth: Due to congestion control, the application of any rate there

is no way to send data, the rate may be sent to a lower rate to send data;

2, there is no delay to ensure that: TCP application data to ensure the final destination in sequence,

but the delay experienced by packets is no way to make any guarantee. WWW sometimes called World

Wide Wait.

UDP Services provided:UDP is a connectionless, lightweight transport protocol, providing most health services model. Not

connected, it should be intuitively more efficient than TCP.



1, no reliable data transmission: Data Push into the sender after the UDP Socket, UDP does not

guarantee that data will eventually reach the receiver, even if the arrival sequence is not guaranteed to

arrive;

2, there is no congestion control mechanism: the sender can order any rate to send data to the

network, regardless of network congestion conditions. But the data may be sent from reaching the final

recipient, production

Health and packet loss.

As the real-time applications in general can hold a certain percentage of packet loss within, so, UDP

is a better relative to TCP transport layer protocol.

2.2 The Web and HTTPEarly Internet (ARPANET) mainly for the military, education and scientific research fields, not for

the public to understand this situation until after the 1991 WWW appear to be completely changed. WWW

is the Internet to the public, making the Internet has become the world's largest, most influential data

network. WWW and E-mail together, profoundly changed the way we live and work. Therefore, WWW

and Email and become early Killer-Application. With telephone, radio and other communication

technologies different, WWW to provide a On-demand service, the user can decide what time to view

what. In addition, WWW's ease of use, strong, almost no learning can learn to use search engines to surf

between different sites for a variety of information.

Web is the core of HTTP (Hypertext Transfer Protocol, Hypertext Transfer Protocol). HTTPimplementation consists of two parts, namely client and server, the exchange between HTTP Message. As

mentioned earlier talking about the contents of the application layer protocol, as mentioned, HTTP main

provisions of the message structure and message of the client and server exchange method.

Web page contains one or more objects, the object can be a basic HTML file, JPEG image, Java

Applet, Audio / Video and so on. Base URL of the HTML document through an object to reference them.

1. Overview of HTTPHTTP protocol, the main provisions of the browser how to send the Server a request to a Web page

and the Server how to transfer the requested page to the Browser

Message structure and message delivery, as shown.

HTTP using a transport layer of the TCP protocol for data transmission.1) Browser first establishes a TCP connection with the Server

2) After the connection is established, browser and server on to / from the Socket to send / receive

HTTP messages.

With the TCP reliable data transfer, HTTP knows the message will definitely reach the other side,

this is the agreement the benefits of layering.

HTTP is a stateless (stateless) protocol; server does not save any state information to any client. If

the server in a very short period of time from the browser receives two requests for an object, server will

send the two responses.

2. HTTP connectionsHTTP uses TCP connections including Nonpersistent and Persistent types. By default, HTTP the

browser and server use persistent connections, but may also be configured to use non persistent

connections.

1, non-persistent connections: to establish a TCP connection, browser and the server through this

connection only transmits a request message and a respond message

2, persistent connection: establish a TCP connection, browser and the server through this

connection to transfer multiple request messages and respond more news

2.1 Non-Persistent HTTP: Response time modelingRTT (Round Trip Time, round-trip time): refers to the browser sends a small packet from the

beginning to the server to answer a little time to respond the message location. RTT, including: packet

propagation delay, queuing delay, processing delay, propagation delay and some other.

Note, here to calculate total time TCP connection is established using the piggyback (incidentally

confirmed), that is, establishing a TCP connection handshake with the third request message sent from the

Client together the.

Back-of-the-envelope estimation: a rough estimate, not precisely derived.



2.2 Persistent HTTP connectionOngoing work of HTTP, there are two types:

1, no assembly line method of continuous connection: that is not parallel persistent connections,

that is only a request to respond the current message is sent after receiving a request under

2, continuous line to connect the way: When the browser in the analysis of base HTML file, met on

the issue of a referenced object request message, send the request message regardless of the respond before

the message is reached. The default is to work HTTP/1.1 Persistent with pipelining.

3 HTTP Message format:3.1 request

HTTP protocol specification (RFC2616) message format for HTPP defined, the message there are

two types, namely Request and Respond message.

1, HTTP message is encoded as ASCII

2, HTTP message contains multiple lines, between lines with a Carriage Return + Line Feed Split

3, the first line is called the request line,

the other below the line is called Head line:

1) Request line: There are three fields: method field, URL field, and Version fields. Method can be

GET, POST and HEAD, usually as a GET. When an object request to the Server Browser usually use GET,

the object to the requested URL identifier. Field gives the version number used by the HTTP protocol

version number.

2) Head line:

Host line: specify where the object is requested Host Address / Domain Name: Because you can use

the Browse Proxy, Host line is necessary.

Connection line: If the value is close indicates Server in sending the requested object and close the

TCP connection, that connection nonpersistent of;

If the Keep-Alive Server using Persistent mode is instructed.

User-agent: Browser instructions to send the request message type. Mozilla is Netscape Navigator

browser start the internal name, derived from Mosaic and Godzilla

Composed of two words, meaning Mosaic Terminator. Take advantage of this Head Line, Server

Browser can provide the type of personalized service.Accept-language: Customers are more willing to receive the specified Object language version.

3.1 Method types supportedHTTP/1.0 only supports including GET, POST and HEAD, including three Method.

In addition to these three method HTTP/1.1, the PUT and DELETE added two methods:

1, PUT method: the request entity body of the message object contains more than upload to the server

specified by a URL where to go, usually used in Web publishing applications.

2. DELETE: Remove Server from the URL specified in the file / object.

3.2 HTTP Response MessageResponse message includes three parts:

1, status line: including version, status code, corresponding status information and other three

domains.

2, headlines:

1) Connection line: close refers to the Server in sending the requested object will close the TCP

connection keep-alive means sending the requested Server in the Object remain after a TCP connection

2) Data Bank: Server time creating respond message

3) Server line: Server basic information

4) Last-Modified: the sending object is created or last modified time, and the Web Cache

5) Content-Length: The length of the sending object, in units of byte

6) Content-type: the type of object being sent.

3, entity body: contains the requested object Respond to support the HTTP protocol there are many,many news Head Line

4 User-server Interaction: cookiesHTTP is a stateless protocol, which is conducive to the design and implementation of high-

performance Web server. However, sometimes Server hopes to identify the user and, based on user identity



to provide personalized services, or restrict access to users, hence the use of technology to the Cookie. The

use of Cookie, Server can track user actions.

Cookie defined as follows: Cookie is stored on a Web server on the user's hard drive section of the

text; Cookie allows a Web site stored on a user's computer to retrieve information and followed it. Pieces

of information to 'name / value' of the (name-value pairs) in the form of storage.

Keeping stateClient first visit to a Web Server, send the usual, the Request message does not contain a cookie;

Server for the client to create a cookie, in the response message using Set-cookie line to return to the client,

and record in their own back-end database;Client Server records in the file returned Cookie, in the subsequent request message sent to Server

contains the Cookie line.

The main purpose of the use of Cookie tracking users, including user authorization, online shopping,

product / advertising recommendations, user session state, etc.

Cookie is convenient, but also triggered a relatively strong opposition to an invasion of privacy.

Cookie may contain more personal information, including user names, passwords, surfing habits and other

very personal information that access by third parties may be used for commercial purposes after, resulting

in a violation of personal privacy. Such as the HTML page banner ads, may be images, animations, etc.,

need to use browser Request message from the server to read the company's advertising, advertising

company may be a large number of sites provide this service, so the advertising company may use the data

to user access to different sites to get used to / track information, and information involving personalprivacy.

5 Web CachingWeb Cache (Web Cache), also called a proxy server, and located between the client and server to

replace the Server responds to Client's Request. Client's Browser can be configured to use a proxy server to

the Server Request message is sent to all first shift Proxy Server. At this point, Browser and Proxy Server

to establish a TCP connection rather than the Server.

When the Proxy Server has the requested object, Proxy Server Response Client;

When Proxy Server is no object when requested, Proxy Server to request the original Server Object,

forwarded to the Client at the same time, a local by Cache.

Web Cache is both a Server, is Client. Upon receipt of Client's request, and the object beingrequested, to the Client sends a response, this time with a Server functionality; If the requested object, the

original Server needs to send a Request message, then Web Cache is the Client.

Server Web Cache closer than the Client, even if only from the delay will also reduce service

response time;

Web Cache installation locally, you can effectively reduce the access link through the Traffic (traffic).

Suppose, a user unit 100, each user access www.sina.com.cn. If Web Cache, the flow through the access

link is 100; using Proxy, Proxy, when the first user does not access the response object, Proxy request to

www.sina.com.cn, when another user requests has been saved Proxy www.sina.com.cn page, you no longer

need to request from the www.sina.com.cn, the flow through the access link becomes 1, the other 99 units

of flow in the internal network.

2.3 File Transfer ProtocolFTP

FTP: the file transfer protocolIn a typical FTP session, the user to use FTP to download files from a remote host or upload files to

the remote host. As shown, the user via FTP User Agent to interact with FTP.

Users need to provide the address of the remote host;

FTP Client to establish a TCP connection with the remote host;

Users and other authorized User Name and Password for the Server to verify the information;

Validated, the user can download files from Server to Server or upload files.

FTP: separate control, data connectionsLike FTP and HTTP is the File Transfer Protocol, runs on top of TCP. But has a very clear

distinction between the two:1, FTP Control, and Data using two connections to complete the file transfer.

1), Control Connection: Client and Server used to send control information between, such as user ID,

password, command, etc.

2), Data Connection: the actual file transfer is done through the Data Connection.



2, FTP in, Control connection Persistent, and is Non-Persistent Data connection, and can only

transfer a file.

FTP is a stateful protocol, that is, for each FTP Server to maintain session information, including the

current directory, and account number. Because Server need to maintain state, an FTP server supports

received a limited number of concurrent users.

Use a separate FTP Control connection to transmit control information of the method is called Out of

Band (band mechanism), and HTTP connection using a control information and data sharing in a way is

called In Band (in-band mechanism). Out-of-band using the benefits of control and data information

transfer as much as possible independently of each other.The establishment of data connections in two ways:

1, PASV (passive mode): Server informs client data ports open on their own information, passively

waiting for customers to connect

2, PORT (active): The client tells the server to open the data port on their own information, the server

actively connected client.

2.4 Electronic Mail in the Internet

Introduction2, Mail server: e-mail server is the core of the system.

Email account is the user's Mail Server in a Mailbox, mailbox stores to the user's information.

In addition, to maintain a mail server outgoing mail queue to temporarily store the mail is not sent.

3, SMTP: Simple Mail Transfer Protocol, e-mail application-layer protocol.

SMTP uses TCP at the sender's and recipient's Mail Server Mail Server to establish a connection

between the transmission e-mails. SMTP is also based on Client / Server structure, each mail server acting

as both a Client and Server roles. Send mail Server is called the SMTP Client, Mail Server to receive mail

called SMTP Server.

1. SMTP [RFC 2821]SMTP Mail Server in between sending and receiving e-mail using SMTP transport, SMTP is the Internet's

first application layer protocol and is therefore left with some archaic features such as message body and

message header with 7-bit ASCII encoding. Such a code for the communication link bandwidth shortage in

the online early is reasonable, but for now email support for multimedia messages, video messages,especially the use of binary 7-bit ASCII encoding is very unfavorable.

1. SMTP operationSMTP protocol at the sender and the recipient Mail Server Mail Server TCP connection established

between the ends, generally without resorting to the middle of the Mail Server. If the receiving Mail Server

does not start correctly, the mail message in Mail server of the sender waiting queue, rather than waiting in

the middle of a mail server.

Sample SMTP interactionSMTP transmission of a message as follows:

1, Client SMTP Server SMTP establish the Port: 25 TCP connections;2, Client SMTP Server SMTP between the application and the level of Handshaking operation, Client

Server describes the message to the sender and the recipient's email address;

3, the reliable transmission over TCP all messages sent to the Server

4, close the TCP connection

SMTP uses persistent TCP connection.

3.1 Mail message formatE-mail message format in the header line is part of the message, and the SMTP handshake command is

part of the agreement.

Constructed by the user e-mail message format, and delivers the message to your own mail server. Mail

server to extract relevant information from the mail, use SMTP handshake command associated with thereceiving mail server to establish a TCP connection to transfer messages.

3.2 multimedia extensions for non ASCII dataE-mail messages using SMTP requires 7-bit ASCII encoding, if the transmission of multimedia data via

e-mail, or binary data, this restriction problem. Therefore, in order to transmit multimedia information in



the mail, the sender needs to include in the message header in the other's head.

MIME multimedia messages in two important mail headers are:

1, Content-Type: User Agent receiving this message contains the type of information, such as image /

jpeg, etc., so that the recipient calls the appropriate procedures for processing;

2, Content-Transfer-Encoding: notify the recipient of a message has been encoded with the way the

ASCII code for the receiver to decode the message.

Base64 is currently the most widely used code; it will encode the binary data of letters and numbers.

Because the number of letters and numbers less than 256, so it is the 3 bytes of binary data into 4 bytes of

the letters and numbers. Base64 encoding is designed to put an arbitrary sequence of 8-bit byte cannoteasily be described as a form of direct recognition.

Base64 encoding requires the three 8-bit bytes (3 * 8 = 24) into four 6-bit bytes (4 * 6 = 24), followed

by six in front of each fill two 0, the formation of 8-bit the form of a byte (i.e. three bytes to four bytes,

high byte two of these four are 0), the new value of each byte in the form [0,63] between the total 64

different encoding, use base64 alphabet to find the value of this byte, each byte is encoded as the

corresponding character.

Why use Base64 encoding? The main problem of the designer to consider include: 1. is encrypted?

2. Encryption algorithm complexity and efficiency 3. How to deal with transmission? Encryption is yes,

but the purpose of encryption is not very safe to send Email. This encryption key is the "anti-anti-villain is

not a gentleman." That is to glance do not see the contents of the can. For this purpose the complexity of

the encryption algorithm and efficiency will not be too big and too low. And on a similar reason, MIMEprotocol and other protocols used to send Email address is how to send and receive Email, and not how to

safely send and receive Email. Therefore, the complexity of the algorithm is smaller, higher efficiency, or

because the Send Email and resource-intensive, way to go on a little crooked. ? In addition, because the

Email transmission process, due to historical reasons, Email only allowed to send ASCII characters, that is

an 8-bit byte of the lower 7 bits. So, if you send a letter with a non-ASCII characters (ie, the highest bit is 1

byte) of the Email through the "history problem" gateway may be a problem. Gateway will be the highest

position to 0! Therefore, in order to send a normal Email, this problem must be considered! Based on the

above some of the main reasons resulting Base64 encoding.

4. Mail access protocols

Using SMTP, messages are transmitted from the sender to the User Agent sender mail-server, sent fromthe sending mail server to send transmission to the receiver's mail-server, placed in the recipient's mail box,

mail box at the receiver side the mail server. Receiver use access protocol in the mail box from the server

to get mail.

Mail Access Protocol are:

1, Post Office Protocol Version 3 (POP3)

2, Internet Mail Access Protocol,

3, HTTP: This system is called Web-based e-mail, in essence, the use of HTTP User Agent to contact

with the Mail Server and send and receive mail.

4.1 POP3

The POP3 server listening on port 110. User Agent and mail server port 110 to establish a TCPconnection, the server first authenticates the user, including username, password, etc.; then, the user can

download, delete the message; the final out of the system, close the connection.

POP3 mode:

1, Download and Delete: Delete from server after downloading e-mail messages. Users can not access the

same from many places an email

2, Download and Keep: User Agent will download your mail to leave messages after the server has a copy,

you can visit many different places from the same message.

During a session in the session, POP3 Server will maintain some of the user's presence information, such as

which messages are marked for deletion. However, there is no different Session state transfer between, for

example, the mail is read from the PC, but in a notebook and do not see this message has been read.

4.2IMAPIMAP is a mail access protocol, but provides capabilities far more than POP3. IMAP, all messages are

stored on the server, allowing users to e-mail management, including creating the directory, move in a



different directory mail. In addition, IMAP in a different session to maintain state, as long as the message is

read, no matter from where the login server will be marked as read.

2.5 DNS- The Internet Directory Service

IntroductionIP address can identify the network host; you can also use the domain name (domain name) to

identify. Easy to understand and remember from the perspective of people prefer to use the domain name,for example, www.qassim.edu.sa; it is more easy to remember. However, Internet routing system uses the

IP address instead of domain name based on path selection. Therefore, the domain name to IP addressmapping. In the fourth chapter we will introduce the IP address, IP address is a 32-bit binary number, atotal of four bytes. Writing, between two adjacent bytes of dot (.) Segmentation, each byte with a decimalnumber, such as 202.194.86.87. This representation is called a "dotted decimal."DNS is responsible for the domain names into IP addresses. DNS (domain name service, domain nameservice).DNS is: 1, there are multiple DNS servers distributed hierarchical database system consisting of; 2, allowsthe user to use it for database query application layer protocol.DNS runs over UDP, the transport layer port is 53.DNS is the Internet's key features, the use of application layer to achieve the design reflects the Internet:"the complex process into the network edge to keep the network simple", which is the "end to endprinciple" (end to end)

1. DNS ServicesDNS services provided:1, to achieve the domain name to IP address resolution;2, the host alias: a complete domain name of the Host may have one or more aliases. Relative to the

complete alias, called standardization is easier to remember domain name. Such as the canonical name:relay1.west-coast.enterprise.com can have the form www.enterprise.com or enterprise.com other aliases.

3, mail server alias: with 2 similar. Mail application will normally be given first access to its canonicalname query aliases and the corresponding IP address.

4, load balancing: Load balancing can be achieved using DNS / balance / distribution. For some large-scale Web sites, may be formed by one or more servers to the outside of the Server Farm provides WWWservice. All of Server has the same name, but different IP address. When the user address resolution,DNS server will return all IP addresses is Client, but each time the IP address returned in a different order,so the request can be assigned to different WWW server whose IP address for processing, in order toachieve load balancing.

2. Overview of How DNS Works?Why use such a distributed DNS architecture?1, there is a single point of failure centralized structure of the problem;2, there are also focused on structural performance scalability problems, large number of requests

flooding the server;3, the centralized structure makes the server and the client's distance is large, the delay will be the

introduction of larger;

4, a single database will make the capacity of the database will be very big, frequent update operationwill also have management problems.Centralized structure of the poor scalability. Therefore, DNS from the outset with a distributed

architecture.

2.1 Local Name ServerIn addition to these three types of DNS servers, there is a more important on the DNS server system,

called the Local DNS Sever (local DNS server). Will be strict, the local DNS server does not belong to theDNS tree structure, but the DNS system is still very important.This server is the ISP provides you with the default DNS server (which can be easily by looking at TCP

/ IP Properties to see). Host when a DNS query, the query is first sent to the Local DNS Server. Therefore,Local DNS Server and DNS hierarchy is equivalent to the user between a Proxy.

2.1 Query: Iterative and RecursiveThere are two types of queries a DNS domain name:1, recursive query (recursive query): query mapping is not the DNS server queries other DNS Server,

DNS Server Each query will always get the results / failure and return the requested DNS server.


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2, iterative query (iteration query): If a DNS Server not responding mapping, then the DNS server returnsthe address of the next.

2.1 Query ExampleThis case there is an assumption that the TLD Server know gaia.cs.umass.edu authoritative DNS

server (dns.cs.umass.edu) address. But sometimes TLD server does not know the name of each hostaddress of the authoritative DNS server, but know that some know the Host name in the middle of theauthoritative DNS Server DNS server address. Assuming, University of Massachusetts DNS server for thedns.umass.edu; also assume that each department has its own DNS server, department has a DNS

server is a host Ben Xisuo Authoritative Domain Name Server. Therefore, when the middle of the DNSserver (dns.umass.edu) receives a query for the domain name, then returned to the host where theDepartment of the Ministry of the Authoritative name server address. Finally, a request to the AuthoritativeDNS server, complete the domain name to IP address resolution.

In this case the query method combines Recursive and Iterative two ways, Host sent to the local DNSServer Recursive queries are queries, and Local DNS Server Iterative methods belong to the query.

2.2 DNS: cachingDNS-Cache is an important feature of the DNS system. DNS Cache of thinking is very simple, when

the DNS Server received Host Name-> IP address mapping will be temporarily stored in their memory, thenext query results obtained from the local, which can improve performance.

3. DNS recordsRR is a quad, including: domain name, value, type, TTL. The TTL determines a record of the longest

in the Cache survival time, more than will be deleted.Quadruple in the meaning of the name and value varies with the type.1, Type = A when, Name of the host's domain name, value corresponding to this domain's IP address2, Type = NS time, Name for the domain, value domain responsible for address resolution for the

authorization server's domain name (hostname)3, Type = CNAME time, name of an alias, value for the alias name of the corresponding normalized4, Type = MX time, name of a mail server alias, value for the standardization of the domain name

server name. (foo.com, mail.bar.doo.com, MX)If a Hostname is the DNS server authoritative DNS server, it must contain resource records of type A

(Host corresponding to); if not authoritative DNS server, which will contain the domain name server Host arecord (type = NS), also will include a server in the domain name server Type = A records.

4. DNS protocol, messagesThere are two DNS protocol message, query, and reply, both the same format.

1 head (12 bytes):1) identification: 16 bit. DNS query initiated by the Client, Client to determine the identification,

server returns the results to the same identification also identifies reply.2) flags: 16 bits. Include:

1-bit of the query (0) / reply (1) sign1-bit of the authoritative DNS server, 1 - is1-bit sign of recursive queries, a recursive method that Host wants to use the query1-bit recursive queries can be a sign (reply message set)

3) Question parts: includes a name field, its value until the domain name; type field indicates thetype you want to query;

4) Reply messages Answer parts: the query results, RR, can have multiple results5) authority components: the authoritative DNS servers contain other information.

Documents

Computer Networking Note2