communication network notes

Common Internet Applicationspp

W b B i (HTTP) o Web Browsing (HTTP)

o File Transfer Protocol (FTP)

El t ni M il (SMTP)o Electronic Mail (SMTP)

o Domain Name Service (DNS)

o VoIP Internet Telephonyo VoIP, Internet Telephony

o Peer-to-peer Services

(This set of slides are mainly from the lecture notes of Prof. S. K. Bose)

Client Server Paradigmg

Typical network application has two pieces:

client and server interconnected by the network

Network

query/request response/replyquery/request response/reply

Client Server Paradigmg

applicationtransportnetwork

Client: i iti t s t t ith s networkdata link

physical initiates contact with server

(speaks first) typically requests service from

s request

server, Web: client implemented in

browser; e-mail: in mail readerl

applicationtransportnetwork

l

reply

Server: provides requested service to client data link

physical provides requested service to client e.g., Web server sends requested Web

page, mail server delivers e-mail

Addressing Processes

Identifier used to uniquely identify processes (both sending or receiving) is

< IP Address, Port Number >

IP Address of the host Port Number identifies the IP Address of the host where the process is

runningprocess in the host, e.g. HTTP Server: 80, Mail Server: 25, Telnet Service: 23

Well Known Port Numbers dedicated to particular applications In a host supporting such an application, the service provided

ill l s b il bl t th sp ifi d ll kn n p t numbwill always be available at the specified well known port number

What kind of transport service does an application need?application need?

Data loss BandwidthData loss some apps (e.g., audio) can

tolerate some loss other apps (e.g., file transfer,

some apps (e.g., multimedia) require minimum amount of bandwidth to be effectivepp ( g , f f ,

telnet) require 100% reliable data transfer

Timing

other apps (elastic apps) make use of whatever bandwidth they get Timing

some apps (e.g., Internet telephony, interactive games) require low delay to

y g

g ) q ybe effective

Internet Transport Protocol Servicesp

TCP service UDP serviceTransmission Control Protocol (Connection-Oriented Stream Service)

Universal Datagram Protocol (Best-Effort, Connection-Less Datagram Service)

TCP service behaves like a virtual data pipe! Whatever you push in (bywriting to the corresponding TCP socket) at one end of the pipeeventually comes out in sequence from the other end of the pipe (oneventually comes out, in sequence, from the other end of the pipe (onthe corresponding socket) !

UDP is more uncertain. Whatever data you give to UDP must be given asa packet and UDP will try its best to deliver it at the other end. Itp ydoes not give any guarantees on this or even that it will deliver the datapackets in sequence.

Internet Transport Protocols Servicesp

TCP service: UDP service: connection-oriented: setup

required between client and server processes

unreliable data transfer between sending and receiving processes

reliable transport between sending and receiving process

flow control: sender wont

does not provide: connection setup, reliability, flow control,

l foverwhelm receiver

congestion control: throttle sender when network

congestion control, timing, or bandwidth guarantee

Application m st first set p aoverloaded does not providing: timing,

minimum bandwidth

Application must first set-up aTCP connection and then useit.

The connection must also beguarantees The connection must also beterminated after use

Web and HTTP Hyper Text Transfer Protocol Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio

filefile, Web page consists of base HTML-file which includes several

referenced objects Each object is addressable by a URL Each object is addressable by a URL Example URL:

http://www.iitg.ernet.in/ece/fac.htmp g

host name path namewhere to find the object in the given hostwhere to find the object in the given host

Try View Source in IE to see the HTML file for a web page. Note that this isalways a text file with links/URLs for all the objects reference by the web pagey j y p g

How does HTTP operate?

HTTP is the Application LayerProtocol used in the World WideWebWeb

HTTP follows a Client Servermodel with the client sending

PC runningExplorer

model with the client sendingrequests to the server andgetting files/objects from it

Server running

WebServer program like

Apache

Client: Browser program (e.g. IE,Netscape, Mozilla, Opera etc.) thatrequests, receives, and displaysWeb pages/objects/files

Mac runningNavigator

Web pages/objects/files

Server: Web server program (e.g.Apache) sends objects in responseto requests

HTTP Overview

Uses TCP as follows -

client initiates TCP

HTTP is a stateless protocol Servers do not keep record of

previous client requests client initiates TCP connection (creates socket) to server, port 80

server accepts TCP

previous client requests

server accepts TCP connection from client

HTTP messages (application-layer protocol messages)

Protocols that maintain state are complex!layer protocol messages)

exchanged between browser (HTTP client) and Web server (HTTP server)

are complex! past history (state) must be

maintained if server/client crashes, their

f b server (HTTP server) TCP connection closed

views of state may be inconsistent, must be reconciled

HTTP connections

Non-persistent HTTP Persistent HTTP

At most one object is sent over a TCP connection.

/

Multiple objects can be sent over single TCP

ti b t li t HTTP/1.0 uses non-persistent HTTP

connection between client and server.

HTTP/1.1 uses persistent connections in default connections in default mode

Persistent HTTP would be more efficient as multiple objects can betransferred in one TCP connection, instead of requiring a separate TCPconnection for each objectconnection for each object

Types of HTTP Connectionsyp

Non persistent P i t t P sist t ith Non-persistent Persistent without

Pipelining

Persistent with Pipelining

p g

Authorization used to control access to server content using name and passwordg p

Client asks for resource which needs authorization

Server asks client to th ti t ithclient serverauthenticate with username and password to get access to requested resource

Client asks for the resource again and includes username and password with request

Clients Browser remembers the username/password It has to repeat the username/password for subsequent requests for It has to repeat the username/password for subsequent requests forobjects on the server

Cookies: Used by server to keep state information on visiting users/clients

Many major Web sites use cookies e g Yahoo Amazon

on visiting users/clients

Many major Web sites use cookies e.g. Yahoo, Amazon, and most major portals and e-commerce sites.

Four Components of Cookie Technology :

1) cookie header line in the HTTP response message2) cookie header line in HTTP request message3) cookie file kept on users host and managed by users 3) cookie file kept on user s host and managed by user s

browser4) back-end database at Web site

Cookies: Advantages and Controversiesg

Cookie Supported Featuresh i i

Cookie Controversies cookies permit sites to authorization

shopping carts recommendations

cookies permit sites to learn a lot about you

you may supply name and e-mail to sites

user session state (Web e-mail)mail to sites

search engines use redirection & cookies to learn yet morey

advertising companies obtain info across sites

Conditional GET: client-side cachingg(faster operation eliminating redundant accesses)

Client caches (i e stores temporarily) files/objects that it Client caches (i.e. stores temporarily) files/objects that ithas got from the server earlier

For subsequent accesses to the server for a cached object,th i d t d l d th bj t i if ththere is no need to download the object once again if thecached copy is not any older than the copy of the object withthe server

Eliminates redundant accesses and makes downloads moreefficient by downloading only those objects which are either

(a) not already there in clients cache(a) not already there in client s cache

or (b) for which the server has a more recent copy

Using a HTTP Proxy (Proxy Server)g y ( y )

Server AClient 1 Proxy

Server

Server A

Server B

Client NClient N

Local Network DomainDomain Internet

Using a HTTP Proxy (Proxy Server)g y ( y )

Client makes all its HTTP requests to its Proxy Server Proxy Server gets the response (requested file etc ) from the Proxy Server gets the response (requested file etc.) from the corresponding server and forwards it to the requesting client

I t d fil i

ProxyServer

Client 1Server A

In case requested file is already in the Proxys cache, then it can forward the cached copy to the Client Server Internet Hosts only see the Proxy Server and cannot see the individual clients. Proxy also

Server By

serves as a security barrier (e.g. a firewall) to protect the clients from the Internet

Client N

File Transfer and Access Methods

FTP: File Transfer Protocol

Most common file transfer protocol used to transfer files over a network

TFTP: Trivial File Transfer Protocol

Simpler file transfer method which does not phave all the functionalities of FTP

NFS N t k Fil S tNFS: Network File System

An on-line shared file access that transparentlyinterconnects the file systems of individual hosts toymake it look like a commonly accessible, large, sharedfile system

FTP: File Transfer Protocol

f l ffile transfer FTPserver

FTPuser

interface

FTPclient

l l fil remote fileuser

transfer file to/from remote host

local filesystem

remote filesystem

at host

client/server model client: side that initiates transfer (either to/from

remote) server: remote host server: remote host

ftp: may be between two programs or with a human user on the client side

ftp server: port 21 (well known port)f p p ( p )

Schematic Representation of FTPp

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Client System Server System

Data Transfer

Control Process

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Control Process

Data Transfer

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Operating System

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TCP/IP Internet

TCP Control Connection and TCP DataConnection between FTP Client and FTP ServerConnection between FTP Client and FTP Server

FTP Control and Data Connections

Control Connection established when the FTP session startsand stays for the whole session; Client establishes connectionywith port 21 on the Server side; FTP Server is alwayslistening at port 21 for Clients who want to set up FTPconnections with it

Client uses the Control Connection to do commands likeset/change directory, read directory contents or makenew directory at the Servery

Client also uses the Control Connection to request Server for(a) downloading specified file from Server

or (b) uploading specified file to Serveror (b) uploading specified file to Server

For the actual data transfer for upload/download, theServer sets up the Data Connection as needed.

Data connection taken down after the transfer

FTP: Separate Control & Data Connections

FTP client contacts FTP server at port 21, specifying TCP as transport protocol

TCP control connectionport 21TCP as transport protocol

Client obtains authorization over control connection, login required for authentication FTP FTP

TCP data connectionport 20required for authentication

(guest login may be done) Client browses remote

directory by sending

client serverp

Server opens a second TCP data connection to transfer directory by sending

commands over control connection.

When server receives a

data connection to transfer another file.

Control connection: out of band When server receives a

command for a file transfer, the server opens a TCP data connection to client

band FTP server maintains state:

current directory, earlier authentication

After transferring one file, server closes connection.

authentication

Electronic Mail outgoing message queue

Three major components:user mailbox

message queue

useragent

user agents mail servers simple mail transfer protocol:

useragent

mailserver

agent

SMTPSMTP

User Agent

mailserver user

agent

SMTP

SMTP mail reader for composing, editing and

reading mail messagesuseragent

mailserver

SMTP

e.g., Eudora, Outlook, elm, Netscape Messenger

outgoing, incoming messages st d s

useragent

userstored on server agent

Electronic Mail

Mail Servers useragent

mailbox contains incoming messages for user

message queue of outgoing (to

useragent

mailserver

agent

SMTP message queue of outgoing (to be sent) mail messages SMTP protocol between mail

servers to send email messages

mailserver user

agent

SMTP

SMTP client: sending mail server server: receiving mail

serveruseragent

mailserver

SMTP

server

useragent

useruseragent

Electronic Mail: The SMTP Protocol

uses TCP to reliably transfer email message from client to server port 25to server, port 25

direct transfer: sending server to receiving server three phases of transfer

handshaking (greeting) transfer of messages closure closure

command/response interaction commands: ASCII text response: status code and phrase

messages must be in 7-bit ASCII (MIME extension to support non-ASCII contents described later)pp

SMTP: Some Details

SMTP uses persistent connections (like persistent HTTP, all p pfiles are transferred in the same connection)

SMTP requires message (header & body) to be in 7-bit ASCII legacy of original email service which was only meant to legacy of original email service which was only meant to handle text

SMTP requires special extensions for non-ASCII content (multimedia binary files programs etc ) Non ASCII content (multimedia, binary files, programs etc.). Non-ASCII content encoded into the form of ASCII text at the sender and decoded at the receiver (e.g. base 64)

P d f d d dd l f SMTP modified to provide some additional security features as well (these dont work too well, spamming and viruses still get spread)

SMTP: Comparisons with HTTPp

SMTP persistent like persistent HTTP/1.0p p HTTP: pull (client picks up content from web server) SMTP: push (client pushes content to other server)

Both HTTP and SMTP have ASCII command/response interaction and status codes

SMTP requires every thing in 7-bit ASCII (unlike HTTP) and special encoding needed for non-text content

HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in one multipart msg

Mail Message Formatg

Standard Format for Text Message:

header lines, e.g.,

headerblankline

, g , To: From: Subject:

body Subjectdifferent from SMTP

commands! bodyy

the message, ASCII characters only

Mail Message Format with Multimedia Extensionsg

MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content

type

From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0method used

MIME version

MIME Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg b 64 d d d t

multimedia datatype, subtype,

to encode data

base64 encoded data ..... ......................... ......base64 encoded data

yp , yp ,parameter declaration

encoded data

MIME typesContent Type: type/subtype; parametersContent-Type: type/subtype; parameters

Text Video example subtypes: plain,

html example subtypes: mpeg, quicktime

Image example subtypes: jpeg,

gifApplication other data that must be

Audio example subtypes: basic

processed by reader before viewable

example subtypes: example subtypes: basic(8-bit mu-law encoded), 32kadpcm (32 kbps coding)

p ypmsword, octet-stream

How exactly would users interact with their mail server to send/receive mail?their mail server to send/receive mail?

useruserSMTP

agent

senders mail

useragent

receivers mail Alice Bob

server server

In early days of email, the typical procedure would be

1 Alice logs into her mail server to write mail for Bob 1. Alice logs into her mail server to write mail for Bob. (Use mail, elm, pine etc. for this.)

2. Alices mail server uses SMTP to forward the mail to Bobs mail serverBob s mail server.

3. Bobs mail server puts the mail in Bobs mail box

4 Bob (when he wants to read his email) logs into his 4. Bob (when he wants to read his email) logs into his mail server and reads his mail. (Use mail, elm, pine etc. for this.)

This is generally not a preferred implementation today The current trend is for the sender and receiver to run th i ti U A t t it d d/h dl th their respective User Agents to write and read/handle the mail

The User Agents interact with the corresponding mail g p gserver to get the required job done

Some advantages of this are -

1. Bobs mail server can be down when Alice writes her mail,2. Alice and Bob can handle multimedia mail and do not have to

be restricted to the boring text mail of early daysbe restricted to the boring text mail of early days

3. Alice and Bob can choose to keep their mail in their owncomputers and not on the mail server

Mail access protocols

useragentuseragent

SMTP SMTP Mail AccessP t l

senders mail server

Protocolreceivers mail

server

Sender (Alice) can have her User Agent use SMTPto transfer mail from her computer to her MailServer same style of operation as before withAlices computer acting as the SMTP clientAlice s computer acting as the SMTP client

Bobs User Agent cannot use SMTP to get mail fromhis Mail Server remember, SMTP is a PUSH and nota PULL type of protocol

Solution:

Bob runs a Mail a PULL type of protocol Bobs computer will also not allow his mail server topush mail to him because of security concerns hence,Bobs mail server cannot use SMTP to deliver the mail

Bob runs a Mail Access Protocolto get/read the mail from his Bob s mail server cannot use SMTP to deliver the mail

to Bob server

Mail access protocols

useragentuseragent

SMTP SMTP Mail AccessProtocol

senders mail server

Protocolreceivers mail

server

Some Mail Access Protocols that Bob can typically run are

1. POP3 Post Office Protocol (Version 3)( )

2. IMAP Internet Mail Access Protocol

P b B d b b d E l3. HTTP Using Web Browsers and Web-based Email(like Hotmail, Yahoo etc.) to read/write mail directly at the mail server

POP3 Protocol (A simple protocol for Mail Access tomail servers from user machinesmail servers from user machines,commonly supported by most servers )

Three phases of POP3 protocol are Three phases of POP3 protocol are

1. Authorization: User agent telnets to port 110 of mail server and sends login/password to server and sends login/password to authenticate the user downloading the mail

2. Transaction: List messages at server, Retrieve f h M k/ k messages from the server, Mark/unmark

messages for deletion

User (client) issues QUIT command to end ( ) QPOP3 session with the server

3. Update: After user logs out, the server deletes the messages marked for deletionmessages marked for deletion

POP3 Optionsp

Two options are possible -Two options are possible

1. Download-&-Delete

Automatically download received mails from mailbox inu m y w m f m mserver to users own computer and DELETE them from theserver

2 Download & Keep2. Download-&-Keep

Automatically download received mails from mailbox inserver to users own computer but KEEP a copy in the

til it i li itl d l t d b thserver until it is explicitly deleted by the user

POP3 Optionsp

Download-&-Keep is useful for a user who wants to accesshis/her mail from different computers at different times,pe.g. home PC, office workstation, personal laptop

With Download-&-Delete, mails downloaded by the user fromone computer will no longer be accessible if user accessesone computer will no longer be accessible if user accessesthe mail server from another computer

Options of Download-&-Delete and Download-&-Keep areoptions that may be selected when setting up the POP3options that may be selected when setting up the POP3access system between the users computer (user agent &mail access software) and the mail server. Can also bemodified later by the usermodified later by the user

POP3 is simple to implement because it is stateless across sessions,even though it does maintain some state information (i.e. whichgmails to delete) when a session terminates

POP3 and IMAP

With POP3 access, user downloads mails to his/her localhi d th l llmachine and can then locally

(a) organize them into appropriate folders in his own machine

(b) move mails between folders or(b) move mails between folders or

(c) search for mail by sender name or subject

This poses a problem for nomadic users as these folders and This poses a problem for nomadic users as these folders andusage options are not accessible to the same user at adifferent machine

IMAP (Internet Mail Access Protocol) is more powerful than IMAP (Internet Mail Access Protocol) is more powerful thanPOP3 and allows these features. However, it is a morecomplicated protocol to support, both at the clients and atthe serverthe server

IMAP (Internet Mail Access Protocol)( )

Keep all messages in one place: the serverMay be accessed by the user from different computers atMay be accessed by the user from different computers atdifferent times

Allows user to organize messages in foldersN il h d f l INBOX f ldNew mail enters the default INBOX folder, user cansubsequently create folders and move received mails tothese foldersIMAP k i IMAP keeps user state across sessions:Names of folders and mappings between message IDs andfolder names need to be remembered for each user

Has commands to allow access to components of a message(e.g. message header or only parts of a multipart MIMEmessage)Useful for limited access over low-bandwidth connections

Web-Based Email using Web Browsersg

Web-based Email started with Hotmail who offered it free Free Web-based Email now provided by almost every major

portal site. Service financed by advertisements and may alsosupport an instant messenger/chat service

Web interface to their normal email service is also providedby most organizations (universities, companies) to theiremployees and studentsp y

Advantage: True Nomadic Mobility as the electronic mailboxcan be accessed from anywhere

Di d i b Disadvantage: Security may be a concern

Web-Based Email using Web Browsersg

Alice sends email to Bob where both use web-based email fromdifferent service providersdifferent service providers

1. Alice uses her Web Browser as the User Agent. She usesHTTP to write her email and send it to her Mail Server

2. Alices Mail Server uses SMTP to transfer the email to Bobsemail server

3 When he wants to read his email Bob uses his Web Browser3. When he wants to read his email, Bob uses his Web Browseras the User Agent and HTTP as the protocol to access hismailbox at the mail server and read/write email

User can run his Web Browser from anywhere and on any machine User can create and manage his/her own folders at the Web-based

Email Service providers mail server (just as in IMAP)Email Service provider s mail server (just as in IMAP)

DNS: Domain Name Systemy

distributed database implemented in hierarchy of many nameservers

application-layer protocols (e.g. HTTP, SMTP, FTP etc.) useDNS services to translate user-supplied host names to IPppaddresses (i.e. resolve names)

application hosts, routers, and name servers to communicateto make this work

DNS is an application layer protocol which provides a coreInternet function to allow other applications to work moreeasily with host names rather than IP addresseseasily with host names rather than IP addresses

Make DNS Query to resolve name of target host

DNS The Domain Name S tSystem

Th DNS N S The DNS Name Space Resource Records Name Servers

The DNS Name SpaceThe DNS Name Space

A portion of the Internet domain name A portion of the Internet domain name space.

Hierarchical Operation of DNSp

UserUser Application DNS Query

DNS other, hierarchically organized name servers

User Host

Response

Local Name

organized name servers on the Internet who can help answer the query

Server Hosts DNS Query sent first to Local Name Server

Local Name Server replies if it knows; otherwise, it passes the query on to other Name Servers on the Internet

Eventually if answer obtained then Local Name Server replies with a Eventually, if answer obtained then Local Name Server replies with a DNS Response to the User Host


Types of Name Servers

1 L l N S

No server has all the name-to-IP

1. Local Name Server

2. Root Name Server

3 Authoritative Name Server

address mappings Cooperatively, they can usually find one who does 3. Authoritative Name Server

4. Intermediate Name Server

find one who does have this

Hosts sends DNS query only to its own Local Name Server Local Name Server may query others to get the requiredinformation if it does not have this itself Caching is used for greaterinformation if it does not have this itself. Caching is used for greaterefficiency and response speed. Cache is also timed out

(Query used may be Iterative or Recursive)

It ti if DNS i t th DNS t it b h lfIterative if a DNS passes queries to other DNS to query on its behalf.Recursive if it finds higher order DNS and contacts them directly


Root Name Server

Target Hosts Domain

Local N

Server

Authoritative Name ServerName

ServerName Server

Requesting Host

Target Host for DNS gQuery

Hierarchical Operation of DNS

Local Name Servers

Each ISP company has Local (default) Name Server i e Each ISP, company has Local (default) Name Server , i.e.for its local domain (reachable within only a few routers)

For a Host, the IP address of its Local Name Server isusually configured in it when the Host is set upusually configured in it when the Host is set up.

Some times, multiple local name servers may be set up, e.g.as Primary Name Server and Secondary Name Server.

In some networks, Hosts may be configured to get theaddress of the DNS automatically (e.g. from a DomainController))

Hosts DNS query first goes to Local Name Server. DNSresponse will also eventually come back to the Local NameServer even though it may have actually obtained theServer even though it may have actually obtained theinformation from elsewhere


A DNS Query which cannot be answered by the Local Name Serverwill be referred up to another hierarchically higher name server Thewill be referred up to another, hierarchically higher, name server. TheLocal Name Sever acts as the DNS Client and queries the next higherDNS that it knows about

A Host in the EEE domain will first ask the EEE DNS.A Host in the EEE domain will first ask the EEE DNS.

If the EEE DNS does not know then it will refer thequery to the next higher CITS DNS

E ntu ll n un ns d DNS qu ill ch R t N m S Eventually, an unanswered DNS query will reach a Root Name Serverof the Internet

The Internet has a few (about a dozen or so) very large Root NameServers. These know more about the Internet mappings than any otherserver and all unanswered DNS queries will eventually be sent to one ofq ythese.

Documents

communication network notes