Lecture -9 Client Server Concepts DNS,Telnet,Ftp

Lecture -9 Client Server Concepts DNS,Telnet,Ftp

In this lecture I would like to touch up on
a very important concept which is used widely in the internet. Nowadays you must have heard
about the term client-server concept. Client server programming in fact most of the internet
applications that we see today, that we use today, they are based this kind of client-server
programming. So first I try to explain what this client-server programming concept is
all about. Then we shall be specifically looking at some of the applications of the internet.
So client server concepts and some of the applications are the topic of our discussion
today. So first see what the client server model
is all about. This is a standard model for developing network applications today. Now
as the name implies there is the notion of a client, there is the notion of a server.
The basic idea is that a server is a program or which is sometimes called a process. A
program in execution is called a process. A server is a process which is continuously
running and it wants to offer some services to clients. A client in contrast is another
process which is requesting the service from a server. Well a simple example the server
may simply return with the date and time; the current date and current time. The client
can ask the server what is the date and time. Please tell me the server can return back
with the date and time. This can be a very simple example. Now in general a server can
provide any kind of service and a client is somebody who is asking for that service. This
is the difference between client and server and in general since we are talking about
networking environment the server and client may not be running on the same machine. They
may be on different computers altogether. Typically server is the program which starts
first and server waits for requests from clients. So as and when the requests come from client
the server will service and will again go back and wait for some more request from the
clients. So pictorially this looks like this. There
can be one server; there can be several clients which need service. And we shall see later
there are different ways of handling these requests. In case there are simultaneous requests
coming. Typical scenario as I had mentioned that the
server should be the first process to start. In the typical scenario, first the server
process must start on some system, the server process must initialize itself and then it
goes to sleep waiting for a client request. See sleep is a terminology which is used in
operating system. It means that it basically goes to the waiting state. It is not executing
anything right now. But it is waiting for a request from a client. If that event occurs
that means a request from a client comes then the server program will wake up and it will
start executing. So if you remember or recall the typical state transition diagram of a
process that are ready running block, then possibly the server process will be in the
block state. It will be waiting for a request from the client to come in order to move into
ready then a client process will start. Well it can be in the same system, it can be on
some other system and it can send a request to the server. So the server will obviously process the request.
So when the server has finished providing the server can go back to sleep waiting for
the next client request and this process will repeat indefinitely. As long as you do not
explicitly shut down the server this is a simple scenario. Server is waiting clients
are sending requests, the server will be servicing the requests, it will be sending back the
response and this will go on. Now typically the roles of the client and
server processes are asymmetric. See asymmetric in terms of roles, as well as when you write
the actual client and server programs. This program you may be writing in a language like
C or C++ or maybe in Java or any language. You will find that the way you write the client
program, the way you write the server program, there are some places where they differ that
is they are called asymmetric. They are not symmetrical if the client and server program
look at exactly the same. Then they would have said to be symmetrical but they are asymmetric.
Now in in terms of the exact way the requests are processed by the server servers can be
categorized as either Iterative or concurrent. Now let us see what these two mean. A server is said to be Iterative when the
first thing is that the server process knows in advance how long it takes to handle each
request. Typically these kinds of server are used only when the service time is fairly
small. If the request service time can be fairly long this is not a good solution, this
is not a good alternative. Typically, iterative servers are used in cases where the service
times for the client requests are fairly small in time. So single copy of the server will
run at all times and the server will process one client request at a time. The server will
handle each request itself. Since it is a single copy then only one request can be handled
at a time. So if there are five clients which are sending requests at the same time, the
first one that the server receives will be handled first the other four have to wait.
As soon as the server is finished with the first is ready with the next. The next requesting
queue will come and the others will wait. So in this way it will go on. So one request
is being serviced or processed at a time and all other clients which have also sent request
will be waiting. This is how it works. So the client may have to wait if the server
is busy processing other requests. Now in contrast, concurrent servers in a different
ways. There are typically used when the amount of work required to handle a request is unknown.
So you really do not know how long the client process will engage the server in providing
the result of the request. Some request may take much longer time. Say I am giving a simple
example, you can think of an application like a railway reservation system. Suppose whenever
an operator turns on the system and connects to the server, say after three hours he shuts
down the connection. Maybe for this entire 3 hour the client remains connected to the
server and whatever request is going, the server is servicing immediately. So this,
a long term connection or you can say request processing scenario. Here you cannot handle
the request in an iterative way. But here what is done is that whenever a client
request comes, the server starts another process to handle each request. Which means that a
copy of the server caters to a clients request in a dedicated fashion as many copies of server
as there are client requests, see if there are, and see here this scenario is like this.
There originally there was one server which was waiting for client request say client
A sends a request. So what this server will do server will make a copy of itself and will
allocate that copy to cater to the request of client A in a dedicated fashion and the
server itself will go back and wait for another request. Now a request from client B comes
the server makes a copy of it and asks that copy to service request from B and so on. So in this way the thing will go on, if there
are ten requests coming simultaneous, there will be 10 copies of the server. Now this
may be seems to be a little complicated. That means how these copies are created. But if
you know some of the details about process management, in operating system will know
that almost in all operating systems there are some functions which you can use to create
network processes. Like for example if you are using the UNIX or the Linux system there
is a function called that is a system call. Suppose you are writing a program in C, if
there is a function called in the C program, so whenever that is executed there is another
copy of the process that gets created. So effective concurrent servers uses that Fork
kind of calls to create multiple copies of itself. Now whatever we have said is the basic concept
behind client and server. Now how it actually works? Well the client server communication
can use either TCP or UDP depending on your requirement. Now if it is TCP, then before
start communication a connection has to establish. But if it is UDP of course you do not see
when we talk about a connection, we must first understand what we mean by a connection. See
suppose there are two computers on two side; one out here, one out here, this is your source
and this is your destination. And you want to establish a connection between S and D,
while this can be a two way communication in general. Now there are a few things you
need to know about this communication, like which protocol you are using are you using,
TCP are you using, UDP or something else, some new protocol. What is the IP address of the source? What
is the IP address of the destination and also as I had that in both the source and destination
sides there can be multiple programs which are running. So the port number specifies
exactly which program the packet is destined to or is coming from. So you can have the
source port number and destination port number also. So when you have all these 5 components
known to you, you know exactly what is going is on, you know exactly what protocol is being
used, you know exactly which program out here is talking to which program out here. Now
you can say that a connection has exactly been established actually been established
between these two programs right. So these 5 components will constitute a connection. Now typically when you develop a network application,
it always advisable to use some standard and well accepted protocols. Because if you follow
some you will often get support from the software tools that you use in developing the software.
Now here I am showing a very typical scenario. Say whenever one wants to develop an internet
application, usually at the data link layer level, you have the Ethernet protocol in most
of the LAN’s. We see in the network layer it has to be IP at transport layer it can
be TCP. And we you are developing the application. If you are using TCP and IP then there are
some standard application programming interfaces or API’s which are available one very popular
API is called the so called Berkley Socket Interface. Now if you are using Berkley Socket
Interface, you will be having access to some functions which if you call using that you
can actually establish a connection between a client and a server, you can actually send
a packet, you can receive a packet, and you can do whatever you want. So you will have
to understand what these functions are and how to use them. Once you know that you can
very easily, write such an application. Now the basic concept behind the Berkeley
socket which is part of the Berkeley socket distribution or BST or Berkeley standard distribution
is called the socket. Socket is the basic concept behind inter-process communication
over the network between client and server. Basically the concept of socket allows one
process to communicate with another. Now this one process can be the client the other process
can be the server. They maybe on the same machine they can be on different machines.
Now analogy is like the telephone line you can establish a connection over the telephone
to allow one person to talk to another. Similarly socket is an analogy the telephone connection
using socket you can establish the connection then client can send something. The server
receive the server can send back something the client will receive this way a two communication
can start. Now from the socket point of view, there is
a term called full association which refers to that same 5 tuple I referred to earlier
Protocol, Local IP address, Local port number, Remote IP address, Remote port number. All
these 5 things taken together, is called an association. And technically speaking socket is defined
as a so called half-association. Like you have the two sides; one is the source, other
is the destination. Now if we have complete information about one of these sides that
is called a socket. Like either if you have the protocol local IP address, local port
number. That is also socket on one side on the side of the source. If you have protocol
remote IP address, remote port numbers, then you have the socket on the side of destination.
So the source opens a socket; the destination opens a socket, then there are some functions
using which they communicate and establish a full-association among the sockets. This
is how it actually works but in order to actually understand how it works you will have to look
into this function in depth. And you will actually study how this client server programming
can be done. But I am not into details of the coding and
details of the function because these are slightly out of the scope of the present structure.
But if you are interested you can certainly into the details of the Berkeley socket interface.
What kind of functions are available and how you can utilize, those functions to write
client and server programs. Both in the connection oriented, that means using TCP or in the connection-less,
which means the UDP environments. So my purpose of telling all these things was to give you
a rough idea about what this client server concept is all about and I have told you all
the internet applications that you see they are all based on the client server concept.
Now I will start with one of the most important applications which will in itself is not an
independent application. But is used by many other practical applications, that is, the
so called Domain Name System or DNS. This is you may have heard, this is a very
important service which is provided in the internet in LAN’s in different areas. Basically the domain name system is a global
database system for internet addressing mail. Other see, here this need not be restricted
to internet and mail it can refer to World Wide Web, Telnet, FTP, any kind of application
you use today. You typically use a name you do not always use an address. The purpose
of the DNS essentially is to convert from a name into an address. So this is how it
works. Now the advantage of using DNS is that see the using names rather than addresses
is much easier to use easier to memorize. Like for example I am giving you, one practical
I can say example, say the mail server of the computer science and engineering department
of IIT Kharagpur. That can be referred to by a name cse, the full name is cse.iitkgp.ernet.in.
Well you are familiar with names of this type. Now this kind of a name is easier to memorize,
but I also know that the IP address of the cse machine is Of course you cannot remember many such addresses
with numbers you may tend to forget. But using names is much more easier because it is more
English like more easy to memorize. Now in the domain name system there is a concept
domains and sub domains actually domain refers to a set of computers which are related in
some way. There are some computers which are related either by their location. Location
means I am talking about geographical location, they may be I can say all computers in India.
All computers in IIT Kharagpur, so these are some examples. These are some categorization
of sub domains based on their location and I can also specify these computers based on
the way they are used or their function. Like I can say that well all computers which are
in the educational institutions all computers which refer to commercial organizations. These
are example of distinction from the functional point of view. So as I had mentioned that DNS servers basically
translate domain names to IP addresses. This is the basic task and this thing is done in
a distributed system because of this simple reason. Because one server in one place cannot
have information about all the domain names in the whole world. There are millions and
millions of domain names and the corresponding addresses you cannot expect that there will
be one single server available in one place where you can get all the information. Rather
this has to be some kind of distributed, you can say database, but information is spread
across a number of different computers. So just using some systematic algorithm these
computers can interact among themselves to get means whatever you can say result you
want to get. Given a DNS, a name, says a name you can get an IP address by systematic query
of these multiple server in a particular order. We shall see how these orders would be shortly. But before that let us talk about something
called Top Level Domains. Well you may be aware that the addresses you write for example
yahoo.com. Com say something, .gov something, .edu stanford .edu. In refers to India say
ernet.in. See these are called Top Level Domain when you write an address for example when
I write say www.yahoo.com, the last part that I write that is called
the Top Level Domain. Now Top Level Domain can be one of many I only shown a few out
of them. Some of these are based on functional classification, com means Commercial or Commercial
organization org means non-profit, net means Network server provider gov, gov traditionally
stands for US government, mil Military, edu Educational. Now in addition to this there
are two letter domains for each and every country of the world. For example pk stands
for Pakistan, there is a code for each and every country, this in stands for India, this
nz stands for New Zealand, tw stands for Taiwan and so on. Now the domain name structure is a hierarchy
I am giving an example. So I talked about the address of our mail server in the computer
science and engineering department. Since address has four different parts; cse iitkgp
ernet in. Well if you look from the other side from the side of in, this in is a sub
domain. This in refers to all computers in India, this ernet is a sub class inside India.
Now within there can be other sub classes there can be nic. There can be VSNL there
can be others also. So ernet is a sub domain that is inside India. Again inside the ernet
you have iitkgp. So within ernet you have iitkgp IIT Bombay you can have others as well
I have not shown. All I have shown only two, then inside iitkgp you can have computer science
engineering, electronics communication engineering, mechanical engineering. Then within IIT Bombay you can have again
have computer science engineering, math and so on. So you can see that we you give a domain
name this is an example of a so called fully qualified domain name. Fully qualified means
starting from the top level domain I am specifying the entire path of the sub domains that you
should traverse to arrive at the particular place that you want to come I want to refer
to cse I specify in ernet iitkgp and then cse. So these are examples of domains and
sub domains. So I had mentioned that domain distribution is or domain name management
is not in one palace. It is distributed for example iitkgp domain server can only keep
information about the names that are under iitkgp like cse, ece, mech and so on. Domain name system of ernet can only keep
track of all the institutions that are under ernet like iitkgp IIT Bombay, IIT Delhi and
other institutions. Similarly domain name India can they can keep track of all those
ISP’s that are inside India like ernet nic VSNL and so on. So in the each of these tables
you have only those entries that are immediately under it, there is a big domain, there are
smaller sub domains under it and each of these sub domains. Again they will be having smaller
sub domains under them. So each of these domains will contain information only about the sub
domain that are immediately under it. This is how the domain name server information
is distributed. So effectively DNS server will store the domain name and IP address
they will store information only about the computers that are immediately under them. Just have a quick look at statistics. This
is only up to the year 1995 you can just imagine how it has grown since. These are the figures
in 1000s, the number of computers in the dot com domain in 1991 it was about 2 lakhs. In
1995 it is “2.4” million. Similarly educational institutions from 250 1000. It has gone to
about “1.7” million. So you can just imagine the rate in which the number of computers
are growing. So the number of such domain sub domain and these names are growing. Now DNS server converts an IP address it converts
domain name into an IP address. Let us see how this is actually done and how this domain
name servers are called or where they are kept. Now the most common domain name server
is Berkeley Internal Name Domain in short it is called BIND, B I N D. For instance if
you are using a UNIX or a Linux system you will find that there is a process whose name
is named the Berkeley Internal Name. Domain server runs as a process called named. This
is the actually name server process. When an application it can be telnet, ftp, web
mail, anything when it wants to get some information from the server. It can invoke the so called
DNS name resolver. See the BIND is already running, that is the server program. The client
program is separate the client program will be using to be used to send a request to the
server program. So there is one popular client program. This is called nslookup. So if you run nslookup on any system, it can
be a Window system it can be Linux system. Whatever so that if you run this nslookup
then what will happen? It will ask you to type in the domain name well it has to be
the fully qualified domain name starting from the top level. And this nslookup will be actually
contacting the name server and it will get translated into the IP address and the IP
address will be returned back to you. So either in the UNIX or Linux scenario or in the Windows
environment wherever if you give a command nslookup, say if you are using the Windows
environment you give it, from that run prompt or through a command prompt. Then we will
find that it will be asking you to type a domain name. You type a domain name www.yahoo.com
return you will find that after some time it will return you back with the IP address
of that yahoo.com. This is how it works. But as I had mentioned that the name server
that is running, it cannot have all the information. Well and in general it will not have the information
locally. In that case every name server has a connection with a so called primary name
server which means if you have a name server out here this is your name server. If it does
not have information then it will contact a primary name server. So you can configure
that which will be the primary name server for your case and in case the primary name
server is not available. You can also specify some secondary name server, some other name
server also specify secondary name server. So if the primary is not accessible you can
contact the secondary name server. But the point to note is that well whether you contact
the primary or secondary that is not important. The important thing is that information is
available in one place you have to look into other places if you do not have the information. For example in your primary name server I
am giving an example in your institute. There is one primary name server available. So in
all our computers I have we have a set means I have also set it in my computer set the
primary name server to be that particular machine. So whenever we type in some name,
some domain name, that domain name gets transferred to that primary name server with a request
for the IP address. If it available in its local table it immediately sends back. Otherwise
it sends back to its primary name server. Because if you look at that primary name server
that is also not knowing everything. So with respect to that particular machine there will
be some other primary name server who, it will ask if it does not have the information
there. So everyone will be having a link to some other name server till you reach the
absolute top level name server. From there it can again come down, so I will give a small
example. This diagram gives you the so called hierarchy
of name server. See at the top you have a virtual root level name server say I have
a taken some example. I have taken two educational institutions Stanford and Georgia tech and
two commercial sites rediff and yahoo, stanford and gatech will be connected to a name server
under edu. They will be connected with a name server under com, if you have Indian companies
they will be under the India in name server. If you have Australian companies they will
be having under au so on. So this is how name servers are organised. This root will contain
information about all the top domains. This edu will contain information about all the
universities under this edu. This com will contain all the dot com companies. This is
how the domain name information are distributed. Now let us see how this name resolution takes
place. Well here let us take an example. Suppose here we have a client and we want to get the
IP address of yahoo.com say. And the client is a machine which is connected to the Stanford.edu
site. So the client will first send a request to the Stanford.edu name server. This Stanford.edu
name server will find that the information is not available with Stanford.edu. So it
will be sending in sending the request up to the edu name server. Well edu will also
find that well yahoo.com is not in my table. So this edu web server will also send it one
level up to the root name server. So root name server will tell well yes I know what
com is, com is somewhere here. So it will forward the request to the com name server.
Finally com name server will know well yes I know yahoo.com; yahoo dot com is here. So
from here you can get back the actual IP address. Once the IP address is obtained this is the
so called recursive name resolution the respond that means the actual IP address which we
have found out. This will be following the exact reverse path
with respect to the original request. So you see that so many message exchanges are required
to get back the IP address of the name that you have requested for. This is the requested
name resolution process and here say with respect to the client if you send out one
request exactly one response is been sent back. So just if I am the client if I am if
I am a client means, I am on a computer, I am typing some name, say www.yahoo.com. So
from my computer exactly one request will go out and exactly once response will come
back. So beyond that what is happening that I am unable to see that I am not seeing? So
but if it is recursive name resolution then there are lot of message transmission that
are going on from one name server to other. In order to resolve the name. There is another alternative, it is called
iterative; iterative says that the client will sequentially send queries to DNS server.
Like for example, this is was the client. The client first sends a request to the say
Stanford.edu name server. Stanford.edu name server will
be sending back a response saying that well I do not have the information. But you can
contact edu. So it will send back another request to the edu name server. The edu name
server will be sending back another request saying that well I do not have root maybe
having. So it will be sending another request to the root. So in this way the client has
the responsibility of sending the requests getting the sending next request and so on.
So the responses that the client is getting back, in this case will contain which name
server I can contact next or in the final case I have got the IP address finally. So this iterative name server is easier to
implement, that is why most of the systems they implement this iterative name server
process. So the client sequentially send request. So the response is negative it is not found,
then the corresponding DNS server will be sending back the, which server to query next.
So unlike recursive name server where only one response is sent back to the client. Here
you can have multiple requests going out and multiple responses coming into the client.
Now this name resolution or this DNS protocol is central to any internet application. We
now talk about some of the internet applications which maybe using DNS for their use. Two of
the protocols we would be discussing as part of today’s lecture. The first is the so called protocol. Telnet
is one of the oldest protocols that have been in use. Even before the internet took its
shape this telnet command or the telnet facility was available. Subsequently telnet has become
an internet standard. The idea of telnet is that it allows a person sitting on one computer
to work on another computer. Like for example I am sitting on this computer A and I want
to start using computer B over the network. What I do? I give a command from A the command
looks like this telnet followed by the address of the computer in which I want to work. Well
here I can either specify the name or if I remember the IP address I can also specify
the IP address. Now once I do a telnet from A to B. Well obviously I am trying to start
a remote session and usually there is some authentication where I am if I am sitting
at A I will have to supply a valid login name and password corresponding to machine B to
gain entry. So sitting machine A I give a telnet B and
on the screen I find there are some username password coming typing those things I gain
entry into machine B. Now whatever I see is this screen corresponding to machine B, if
I give a directory listing I can see the files which are there on machine B. If I run a prom
if I say for example run a command like C compiler or anything if I run a command. Command
is not running on my computer rather the command is running on computer B or what is happening.
Whatever is the output of that command somehow that output is coming on my screen. So as
if I am having an illusion that I am actually sitting on the computer B, this is a very
useful facility particularly in a network environment where you do not have everything
on your computer something maybe available on some other computer you can do a telnet
and use it. So some typical applications, telnet can be
used to use software which is only available on a central server. For example, another
application suppose you have a printer attached to one computer. So you want taken some printout,
so you can do a logarithm you can do a telnet to that remote machine, remote host and then
you can do a print command. So a typical scenario in a laboratory environment is that many users
who are typically students do a telnet to a remote server and work there. Server is
actually a bigger and faster computer. Now the advantage of this is that instead of loading
the software all these students are wanting to use on all their computers, you are loading
them in only one central place. It also simples a lot the maintenance of the software. That
means how they are using, what they are using and if you want change the software to a new
version you need to this change only in one place. So telnet is a very useful command. Now by default the telnet server runs on port
number 23 which means whenever you give a command like telnet, something telnet, say
some machine xyz, then a TCP connection is established to the machine TCP with the machine
xyz over port number 23. 23 is a standard for telnet which is the default port number
which is used. But in fact this telnet command can be used to specify any other port number
as well as how like this. I can do a telnet to any machine by specifying the name. But
at the end as an optional parameter I can also specify the port number. Now you will
learn later that this port number 25 is actually port number which is used by the mail servers.
So when I do a telnet to this machine over port number 25, what I am trying to do is
that I am actually trying to contact the mail server directly. There are many cases are
many application where you may want to do this. Well we shall be talking about this
in more detail. When you talk about the mail email electronic mail system later there we
will see that we can actually do our telnet to connect to a mail server and you can actually
send and receive mails through that facility. Just to show you a sample telnet session,
this is an example which we have taken from a Linux system. This was the original Linux
prompt SUNNY. This is the command which was given telnet cse. See what this telnet cse
was given, cse was a name. So automatically the domain name system was contacted and the
names cse was translated into the corresponding IP address. But as a user I do not see that
happening. It is happening in the background automatically. So even if I have given telnet
cse, I am seeing that some IP address is coming trying to contact This is the
IP address of the cse. So finally after connection is done some message is coming connected to
cse. This is the message connect which is coming from the telnet server. On the cse
side some other message is coming you note these things. Some Linux version etcetera
coming. Then a prompt is coming asking me to supply the login name. So I supply the
login name here, a prompt is coming to supply the password. Well I supply the password here, but this
is not visible because passwords are normally not visible on screen. So if my user name
and password, so if my user and password are correct some welcome messages come right here.
Right here last login this Linux, this you have new mail and this is prompt, which is
the prompt of this cse machine. So now I can give some commands where the response will
be coming from this cse machine and not from this SUNNY machine. So this is just a simple
example that I have shown that how we can use telnet to connect to a new or remote machine.
We talk about another protocol called FTP, see using telnet we are able to logon to another
system and use some facility out there. But what we cannot do in telnet is that we cannot
physically transfer a file from one machine to another, we cannot bring a file from that
machine to my machine or I cannot send a file from my machine to that machine. For that
purpose you need to use a protocol called FTP or File Transfer Protocol. So let us see
what FTP is. Now in FTP allows a person to transfer file
as I had said between two machines. Just exactly like telnet to provide authentication you
need to supply login name and password. Because if this was not, so then anybody can transfer
any file across any machines. That is not what you always want. There has to be some
security also, some files you do not want anyone to access. Only persons who are having
a valid account login name password can only access. So this kind of login name and password
authentication has to be there and the command is very similar to telnet, ftp name or ftp
ip address. Anonymous FTP is a variation of FTP which
had become very popular in the early days of the internet. Now see the conventional
FTP requires the user to supply a user name and a password. But suppose on my machine
I have kept a number of information useful files which many people may want. But how
many people have access to my machine, they do not have a valid username and password
many of them. In fact so this anonymous FTP is a mechanism using which anyone who is sitting
anywhere in the world can come to my machine and take some files from me. Here the idea
is very simple whenever the system asks for the login name you type. This string anonymous
instead of a valid login name and when it asks for the password you type your email
address. Then whenever you type the login name as anonymous
and your password as your email address then you get logged in as an anonymous user in
that FTP server. There may be a lot there may be a whole lot of files available for
the anonymous user for downloading. So you can have a look at the files, you can list
it, you can get the files that you need. In fact this anonymous FTP is in the early days
of the internet was one reason that internet become so popular. There are many good peoples
who wanted to share their information. They kept a lot of information on their servers
and using that they allowed other people to share. So the huge amount of resources were
available in anonymous FTP sites. Now the FTP process model looks like this.
Suppose this is a client which gives the FTP command and this is server on which the FTP
server is work working. Now in fact FTP uses two port numbers; port number 21 is standard
and this 22 can be in fact this can be anything. But port number 21 is must. There are two
connections which are established; one is called the control connection through the
control connect. Control connection is established over port number 21. So the FTP connection
gets established after the control connection. Whenever you want to transfer a data, so a
data connection will be established. Every time you transfer a data and that data connection
can be opened over any port number. Here as an example I have shown 22. But it can 1000,
2000, 3000 anything. But this port number 21is standard. So the idea is that if you
do a telnet and if I want to transfer 5 files, there will be 5 data connections opened and
closed one after the other. For every single file transfer data connection will be opened
and after the transfer is complete it will again be closed. So this is what I have said, 2 connections
is established. One is the control connection over port 21 and control connection remains
all throughout a session until or unless you logout of that FTP. And the other port number
is a temporary port number used for every file being transferred. As I have said for
every file transfer you establish new connection. And some typical FTP command, there are many
commands. I have shown only a few put or mput. Put means send one file, mput means multiple
put, get or mget receive a file or multiple file, hash if before put or mput. You give
the hash command; you can see the progress of the file transfer some symbols are getting
displayed one after the other. Now each symbol typically dis display shows either 4 kilo
bytes or 8 kilo bytes of data transfer. So you can see the hashes coming one after the
other. You can see that well my file transfer is in progress prompt. It prompt the user
before the transfer whether you really want to transfer yes, no like that. It asks you
whether the file you want to transfer is a binary file or a text file ASCII file, exit
means it terminates exits the FTP server. Well some example session, let us say, suppose
here again we are sitting on a machine called SUNNY, from there I want to do a FTP into
another machine whose name is say cseindigo. So connected to cseindigo, the connection
over the control connection is established. Cs, see some response codes or you can say
220, some I say 331, 230. These are some intermediate response codes followed by the actual response.
See when this connection is done, then this 220 means that the connection is successful.
Then I ask for the username then ask for password. This 331 anything which ends with one, this
means this is an intermediate completion reply. So this username password thing is not yet
complete, it is half way through. So once this authentication is done it response to
230 it ends with 0. Means it is the end of one step. So now you see a prompt FTP comes.
Now I can give the FTP commands. Like once small example I am giving. If I
give a dir command, it means directory. So I can see a list of all the files which are
there in the other computer. So I can see the list of all the files their sizes, how
big they are and so on. So if I choose to transfer any one of these files I can give
a command. Say I want to transfer this file lan-cse fig
get this file name. So it opens a new connection, it also tells you that 671 bytes received
transfer complete. Then I can terminate the connection by this quit. So connection gets
terminated. So this shows the sample. FTP session how you can use it, so this brings
me to the end of today’s lecture. Before ending today let me have a quick look at the
solutions to the quiz questions of last lecture. First question was: What are the four types
of BGP messages? They are OPEN, UPDATE, KEEPALIVE and NOTIFICATION. How is a BGP connection between two routers
maintained? As I had mentioned by periodically exchanging
KEEPALIVE messages. What are the main functional procedures in
BGP? Neighbour acquisition network acquisition
network reachability. Why is IPv4 not suitable for supporting real-time
traffic? Because they are based on datagram, they do
not guarantee that packets will follow the same path. So there will unequal delays in
the packet and may also they can also get out-of-order delivery for which real-time
response is not possible. How are the headers in IPv6 packet kept track
of? So all extension headers after the base header
are chained in a link list. This I had mentioned. How does IPv6 attempt to support real-time
traffic? IPv6 allows a pair of stations to establish
a path before sending packets. So this is an option you have in IPv6 to support real-time
traffic. What does the Payload length field in the
IPv6 header signify? This specify the length of the extension headers
and the transport level protocol data in an IPv6 packet. What does the Next Header field mean?
The type of information that immediately follows the current header. The current header, so
there can be multiple headers or the PDU. So the Next Header will contain what is following
the present header. In IPv6 how are the fragments kept track of
in case the packet is fragmented? First thing is that base header is copied
into all the fragments and if there is a fragment header in one packet it means that it is fragmented.
So there has to be fragment header in all the fragments. What is anycast addressing?
Packet will deliver to exactly one of a set of computers. How can we use tunnelling to send an IPv6
over an IPv4? The entire IPv6 packet is encapsulated as
payload and IPv4 header will be added to it. So now some questions from today’s lecture. What is an iterative sever?
What is a concurrent server? Which of client or server must start first?
What are the components of a socket? What is a domain?
What are named and nslookup? How does recursive name resolution works? How can you connect to a server xyz.com over
port number 1234? Why does ftp use more than one port numbers
for communication? What are the functions of the hash and bin
commands in ftp? So with that we come to the end of today’s
lecture. Thank you.

35 thoughts on “Lecture -9 Client Server Concepts DNS,Telnet,Ftp

  1. Prof.I.Sengupta thank you so much for this video and all the other ones you have posted here on YouTube, as 1 patron has said already you are "the man". Great work.

  2. at 32:24 you are describing about primary name server and secondary name servers. that part doesn't looks write to me. can u explain me is primary is same as master and secondary is slave ?

  3. you don't understand because you don't understand english well. what is the meaning of "dawn"?

    definittion of dawn

    dawn (dôn)
    1. The time each morning at which daylight first begins.
    2. A first appearance; a beginning: the dawn of history. See Synonyms at beginning.
    intr.v. dawned, dawn·ing, dawns
    1. To begin to become light in the morning.
    2. To begin to appear or develop; emerge.
    3. To begin to be perceived or understood: Realization of the danger soon dawned on us.

  4. The presentation is very informative & well explained. I guess by mistake port 22 was used for ftp data port. Port 22 is used for SSH, the data port should be 20 (rfc959). Even in ftp over ssh we use port 21 for data connection.
    Port 20 is generally used in Active FTP, where server intiate data transfer fusing source port 20.

  5. Really good video lecturer is very clear and explains things in a very straight forward manner that is easy to understand. Helped my revision so much!

  6. Excellent explanations, excellent video, excellent professor. Very helpful small comments or examples that you don't find when you just read literature about this

  7. I got placement in campus drive because of the concept of DNS. That u taught at 35:00 in this lecture.
    Interviewer asked exact working of DNS.
    He impressed so much..

    Thank u so much sir !!!!!

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