application layer chapter-9

© Oxford University Press 2011

Computer NetworksComputer Networks

Bhushan Trivedi, Director, MCA Programme, at the GLS Institute of

Computer Technology, Ahmadabad


Chapter 9

Computer NetworksThe Application Layer


The application layer

• A layer interacting with the user• Applications run at application layer• Use TCP or UDP to get their job done• SOCKET API is used for interaction• Varieties of applications to deal with• Users expect similar behavior for Internet

based applications like browsers


The DNS


Domain namespace requirements

• Efficient• Continue working despite break downs• Should not introduce additional overheads• Information available from the nearby servers• Contain a few additional information• A single server can manage multiple

networks.• It should be secure enough (not provided)


Labels and domains


Domains and

hierarchy:

lower level

domains are part of higher

level domains


Domain names

• Partially and fully qualified names• Resolver• Resolution process


Advantages of hierarchy

• Space constraint • Processing time constraint• Congestion near the server• Organized form of information• Efficient resolution of domain names


Advantages of hierarchy

• A comparatively static database is better to be organized this way

• Clear cut controlling infrastructure• Delegation of authority is clearly defined• Autonomous control


Distributed database

• Most of the queries in DNS are resolved within the local server’s scope

• It reduces the network traffic • No single point of failure• Easy maintenance• Can be easily managed


Distributed databases

Database for ‘in’ac Address of acmil Address of miledu Address of edu… …

Database for ‘ac’A Address of ABC… …

Database for ‘ABC’ict Address of ictibm Address of ibmica Address of ica

Table 9.4 Database for ‘ict’Lara Resource records

for LaraPonting Resource records

for PontingVishwanath Resource records

for Vishwanath


Iterative Name

Resolution


Recursive name resolutio

n


Zones


The registration process


Domain Name

servers, root,

TLD and Local

servers


Type Meaning ValueA IPv4 address 32-bit value

AAAA IPv6 address 128-bit valueCNAME Canonical Name Alias name for host

PTR Pointer IP address (IPv4 or IPv6)NS Name Server Name server(s)

SOA Start of Authority Zone name etcMX Mail Exchanger mail server/exchanger

HINFO Host Info Optional informationSPF Sender Policy

FrameworkIdentify mail servers

TXT Text Text informationDNSKEY DNS Key Public key of domain

RRSIG R R Signature Resource Record Signature


Domain Time-to-live

Class Type Value

207.118.130.194 / 24

172800 IN PTR abcict.org.

ftp.abcict.org.

172800 IN CNAME

Vishwanath.abcict.org.

abcict.org. 172800 IN NS ns2.webmastersindia.com.

abcict.org. 172800 IN NS ns1.webmastersindia.com.

abcict.org. 172800 IN A 207.118.130.194

https://www.glsict.org:8443/plesk/client@5/domain@70/dns/resource-record@965/properties/?previous_page=dns






abcict.org. 172800 IN MX (10) mail.abcict.org.172800 IN MX (20) Vishwanath.abcic

t.org172800 IN MX (30) Ponting.abcict.org

mail.abcict.org.

172800 IN A 207.118.130.194

www.abcict.org.

172800 IN CNAME Ponting.abcict.org

www.abcict.org.

172800 IN CNAME abcict.org.


Ponting.abcict.org.

172800 IN A 208.118.130.194

Ponting.abcict.org.

172800 IN HINFO HP laptop with RHEL 9

Vishwanath.abcict.org.

172800 IN A 209.118.130.194

172800 IN HINFO HP I3laser.abcict.org

172800 IN A 210.118.130.194

172800 IN HINFO The laser printer


Different types of resource records

DNS records for a big website with multiple serversDomain Time to

liveClass Type Value

www.TheBigWebsite.com

172800 IN A 1.2.3.4


172800 IN A 2.3.4.5


172800 IN A 3.4.5.6


172800 IN A 4.5.6.7


172800 IN A 5.6.7.8


Name serversabcict.org. 172800 IN NS ns2.webmastersindia.com.abcict.org. 172800 IN NS ns1.webmastersindia.com.


Load balancing in name servers

DomainTime to

liveClass Type Value

oracle.com. 172800 IN NS u-ns2.oracle.com.oracle.com. 172800 IN NS u-ns3.oracle.com.oracle.com. 172800 IN NS u-ns4.oracle.comoracle.com. 172800 IN NS u-ns5.oracle.comoracle.com. 172800 IN NS u-ns6.oracle.com.oracle.com. 172800 IN NS ns1.oracle.com.oracle.com. 172800 IN NS ns4.oracle.com.oracle.com. 172800 IN NS Ns1.oracle.com.


CNAME to help

• www.gogol.com. 172800 IN CNAMEwww.google.com

• www.goggle.com. 172800 IN CNAME www.google.com

• abcict.org. 172800 IN CNAME www.abcict.org

• www.abcict.org. 172800 IN CNAME abcict.org


Partially qualified domain nameswww 172800 IN CNAME Ponting ;canonical name

is Ponting.abcict.orgftp 172800 IN CNAME Vishwanath ;canonical

name is Vishwanath.abcict.org

ftp.abcict

172800 IN CNAME Vishwanath.abcict.org ;exactly as above

Resource records with blank namesabcict.org. 172800 IN MX (10) mail.abcict.org.

172800 IN MX (20) Vishwanath.abcict.org


DNS query and resource record in response


Dynamic DNS


DNS Sec

• The DNS records are kept encrypted using the private key of the DNS. It can only be decrypted using the public key of the DNS itself.

• DNS resource records also contain a digital signature

• Name servers contain public keys for the zones they are responsible for


Browser and server interacting


The HTTP request exampleGET http://www.oup.co.in/category.php?cat_id=43

HTTP/1.1Host: www.oup.co.inUser-Agent: Mozilla/5.0; Accept: text/html,application/xhtml+xml,

application/xml;q=0.9,*/*;q=0.8Accept-Language: en-us,en;q=0.5Accept-Encoding: gzip,deflateCookie:

PHPSESSID=c3a1082780a2c6621126fa3bd8ad11f1


HTTP ResponseHTTP/1.0 200 OKDate: Mon, 19 Apr 2010 10:50:00 GMTServer: Apache/1.3.27 (Unix) PHP/4.2.3

mod_ssl/2.8.12 OpenSSL/0.9.7gX-Powered-By: PHP/4.2.3Expires: Thu, 19 Nov 2010 08:52:00 GMTCache-Control: no-store, no-cache, must-

revalidate, post-check=0, pre-check=0Content-Type: text/htmlConnection: close


HTTP request and response


The structure of request and response


Components of request and response


Complete request structure


Complete response structure


Method FunctionGET Retrieve a page from the server. A few

simple inputs can be provided. Body is always empty.

POST Provide information to the server. Nonempty body part contains the information.

HEADER Same as GET except the body of the page is not retrieved.

PUT Place the data at the user- specified URL.DELETE Remove an object from the web server.CONNECT Helps converting an HTTP request into

HTTPS.


The persistent connection

• Often , multiple requests are sent and responses are sought from a single connection

• When the connection does not need to be established afresh, the response time improves

• Pipelining also helps


Persistent connection


Cookies


Session variables

• if (username == “ ”),• //no username provided • Jump to login page• else if (user name != administrator)• Display “this page is restricted” message•


Conditional download: request

GET http://www.glsict.org/mcaregular.htm HTTP/1.1

Host: www.glsict.orgIf-modified-since: Thu, 29 Mar 2010 4:35:10

GMT


Conditional download: Two responses

Response-1HTTP/1.1 304 Not Modified

Response-2HTTP/1.1 200 OKDate: Mon, 19 Apr 2010 10:50:00 GMTServer: ApacheLast-Modified: 15 Apr 2010 3:34:05 GMT


Conditional Download-1


Conditional download-2


Conditional download-3


Proxies as intermediaries

• Reduce load on web servers• Reduced response time• Traffic monitoring• Extending Internet access to others • Max-forwards header


How Proxy works


Dynamic Web, client and server side


The email sending process


The email receiving process


Components of the email system

• The first component is known as the user agent.

• It is what the users interact with to send and receive mails.

• The second component is known as the message transfer agent which navigates the mails to their intended recipients.

• The third component is the mail itself.


The complete mailing process


Mail transfer using SMTP client and server


Alias expansion while sending


Alias expansion while recceiving


Alias expansion in

web mail


conventional mail

Example with the Email format

From: Lara BrianABC ICT AhmedabadTo:Gayle ChrisOBS

Content description

Example of content Name of content

Sender’s andReceiver’s mail address

[email protected]@OBS.com

Envelope

From: Lara BrianTo:Gayle Chris

Sender’s andReceiver’s name

Prof. Lara BrianGayle Chris

Header The message

Dear Gayle,Here is second bookRegardsLara

The content of the message

Dear Gayle,Here is second book RegardsLara

Body


SMTP


Client Server Communication SMTP

S: 220 OBS.com SMTP server readyC: HELO abcict.orgS: 250 Hello OBS.com, I

am glad to meet youC: MAIL FROM:[email protected]: 250 OkC: RCPT TO:[email protected]: 250 OkC: RCPT

TO:[email protected]: 250 OkC: RCPT TO:[email protected]


Continue…

S: 550 No such user hereC: DATAS: 354 End data with <CR><LF>.<CR><LF>C: From: "LARA Brian" < [email protected] >C: To: “Gayle Chris” [email protected]: Cc: [email protected]: Subject: The Second BookC:


Continue…

C: Hello Gayle.C: I will soon send you the second Book.C: regardsC: LaraC: .S: 250 Ok: C: QUITS: 221 Bye {The server closes the connection}


Internet Media Types

From: [email protected] To: [email protected]: Thu, 29 APR 2010 16:02:43 -0500MIME-Version: 1.0Content-Type: image/jpegContent-Transfer-Encoding: base64< a blank line>………..binary data for the image in text form………


Type and meaning

Content Type Meaningtext Text information, i.e. a notepad file.

image A photograph or graphicsaudio Recorded sound.video Recorded video.

application A binary file like a Word doc. multipart Message divided in multiple parts, each

with different type of datamessage A forwarded email or a reference linkX–…. Non-standard header.


SMTP and HTTP comparison• Both of them connects and transfer files• Considering HTTP 1.1, they use persistent

connection. • use headers and values in their messages• HTTP does not convert non-text data into text,

SMTP converts non-text data into 7-bit ASCII. • HTTP needs separate fetch for each object.

SMTP converts the non-text portion into text and send together


SMTP and HTTP comparison

• In HTTP, the client fetches the data, while in SMTP, the client sends the data.

• As SMTP needs to covert other types of data into 7-bit ASCII, it needs MIME.

• HTTP does not require such service, as it transfers every object individually.


Base 64 encoding

6-bit sequence

010100 010010 101010 101000 011100

Converted character

U S w o c

ASCII value

85 83 119 111 99

Value in binary

01010101

01010011

01110111

01101111

01100011


Quoted printable encoding0000 0010

0000 0111

1100 1011

0100 1001

0101 0011

2 7 Non-ASCII

I S

0000 0010

0000 0111

00111101

01000011

(ASCII of C)

1000010

(ASCII of B)

0100 1001

0101 0011

2 7 = 1100, i.e. C (Hex)

1011, i.e. B (Hex)

I S


Intermediaries in mailing


Intermediary in web mail


POP3 and IMAP

• Delete mail is default in POP3• IMAP can have multiple mailboxes• IMAP allows partial mail download• IMAP allows access by features and content• IMAP allows message status to be informed• IMAP is more complicated• Webmails provide everything IMAP provides


Filters and SPAM

• Differentiating incoming mails– subject line– sender’s name– some specific words in the body

• Yahoo! And Gmail also provides filters• SPAM is one popular type of filter


FTP Features

• Authentication is provided• Separates its control messages from the

actual data• interactive and providing format

specification• it is stateful


FTP process


Conventional FTP

connection


Control connection open


Data connection open


The Passive

FTP connection


Data connection open in Passive FTP


Transfer between heterogeneous computers: control characters

Command Decimal value

Action

NUL 0 No operationBEL 7 bell-like sound.BS 8 Move left by one and deleteCR 13 Move to left margin


The GUI version of FTP


Secure FTP, SSL-FTP

• FTP sends and receives everything in plaintext

• SFTP is FTP over Secure TCP or SSH• Secure FTP is from IETF. This also uses

SSH• FTP based on SSL or FTPS is also popular• For simpler copy operations SCP also is

possible to be used


BluetoothPiconets

and scatternet


Bluetooth

• Architecture• Pairing• Applications• Profiles• Protocol stack


Applications/ Profiles Application layer

Audio

OtherRFComm

.

Telephon

y

Serviced discovery

Control

Middleware layer

LLC

Data link layer

Logical link control adaptation protocol

Link ManagerBaseband Physical

layerPhysical Radio802.15 protocol stack


Most common frame format


Header which is repeated three times